**3. Adaptation of endogenous host tissue to neural transplantation**

Although adult neurogenesis was overlooked in the past and neuroscientists were convinced that new neurons were not produced beyond the early stages of development, it has now been demonstrated conclusively that there are select regions in the brain that regularly accept new neurons into established circuits that are derived from precursor neuroblasts that retain mitotic capacity throughout our lives, and divide asymmetrically to produce new neurons as daughter cells (for review see [68]). Two key areas that benefit from this neurogenesis would be the hippocampus and olfactory bulb, and the degree of this natural new neuron incorporation depends on the activity levels generated in these regions. Specific functions that rely upon neurogenesis include new learning for the hippocampus [69], and rich olfactory sensory experience for the olfactory bulb [70]. As this occurs regularly in an activity-dependent manner already, it stands to reason that neuronal precursors transplanted into these regions would be more likely to receive signals encouraging their incorporation into either the hippocampus or the olfactory bulb, and in fact, this seems to be the case [71, 72]. Yet in other regions such as the striatum (the main input structure of the basal ganglia), the capacity of endogenous neuronal progenitors to become neurons seems reduced compared to exogenous transplantations [73]. The answer to why this distinction exists has been a top priority among those of us who foresee more successful replacement therapies. While the whole picture is not available, what seems clear is that there is an interactive relationship between the endogenous host cells and transplanted cells at the center. The so-called "neurogenic" regions (hippocampus and olfactory bulb) where replacement happens regularly as part of the natural progression throughout our lives would likely not be a useful target region for clinical transplantation for any key neuronal disorders, given that their potential for reconstructive replacement remains high. Yet we might initially presume that the mechanisms encouraging incorporation, such as the guidance molecules used and trophic factors encouraging survival as connections are established, or the afferent connections grown into and onto the transplant cells as the afferent component, may follow rules similar to transplant events elsewhere.

contribution may suffice, at least initially, in bolstering the circuit in question, though release would need to take place in key areas to be effective. Dopaminergic inputs seem to exhibit both of these characteristics ("open" and "closed"). This concept, commonly overlooked in the

Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell…

http://dx.doi.org/10.5772/intechopen.71790

13

Another aspect of transplant-related plasticity is the extent to which neurons are transplanted into a typical or atypical host environment for the neuron type that is their intended end-point for the current therapy. It is clear that during early fetal periods, useful progenitor populations can naturally undergo sufficient prior developmental modification to become predisposed to becoming a certain common neural type and can be found in regionally distinct populations in the fetus. For example, cells from the lateral ganglionic eminence show a high propensity to become GABAergic striatal neurons [78], or cells from the fetal mesencephalic region show a high propensity to become dopaminergic type neurons (e.g., [79]). Specific developmental trajectory predispositions can also be coaxed from progenitor cell populations *in vitro*, where the approximate mitogens, epigenetic cues, and morphogenic signals are maintained and progressively modified to encourage specific phenotype development trajectories [6, 80, 81]. Once neural developmental predisposition is established, it is perhaps fair to suggest that there are some host regions in which predisposed neurons would thrive as they would be placed into a "familiar" environment (i.e., a *homotypic* host region; e.g., GABAergic medium spiny predisposed neurons transplanted into the striatum) and some host regions that would *not* represent environments that might foster familiarity (i.e., an *ectopic* host region; e.g., dopaminergic destined neurons transplanted into the striatum). Precedent for this homotypic versus ectopic distinction has been set [82, 83]. Keep in mind that this distinction is made to capture the regional relations of predis-

posed neurons for certain locations, and functional benefit concerns are secondary.

For years, neuroscientists have been studying the anatomy of neuronal populations of various types that produce different neurochemical compositions throughout the brain and the corresponding afferent connections that grow into and drive activity in these different regions. Establishing appropriate afferent drive onto the neurons that are transplanted would be a clear sign that the circuit into which the transplanted cells need to merge has accepted them as part of the equation. Clearly then, when placed into a homotypic host region this sort of acceptance would be more likely based on the proximity of the transplanted cells to appropriate afferent input that such cells need to be driven properly by the host brain architecture. The prime example of this sort of transplant that has shown considerable acceptance into the host circuit and was extensively characterized by Klas Wictorin in 1992 is the intrastriatal transplant of striatal-predisposed precursor cells obtained from the embryonic day 14–15 fetal lateral ganglionic eminence following an excitotoxic lesion of the host striatum [84]. The extensive host innervation of this transplant along with the extensive growth and integration of the transplant with the host in the context of circuit re-establishment was dramatic, long-lasting, and seemed to contribute considerable support to the lesioned circuit as seen by neurobehavioral improvement. Wictorin indicated that the initial destructive lesion to destroy local endogenous striatal neurons is crucial for enabling the sort of host integration seen, as the absence of such a lesion (i.e., transplantation into an intact striatum) yielded far less integration [85]. It stands to reason this would occur because afferent inputs would find greater ease in filling an open void or niche so long as it maintains a general presence after

clinic, will be expanded upon later.

When precision is required in the placement of axon terminals, it would seem that the parameters for what might be considered functional success would be correspondingly more restrictive or demanding. Here it is appropriate to briefly describe how establishing a wide range of general chronic dopamine can provide considerable benefit in Parkinson's disease, and the distinction between "open" diffusion-capable release mechanisms versus "closed" synaptic connections circumscribed by glial borders. Due to the chronic widespread levels of dopamine persisting in extracellular space, simple diffusion-based neurotransmitter delivery is often discussed without emphasizing the more nuanced details of precise release. To illustrate, in the case of dopamine loss in Parkinson's disease, the standard drug levodopa promotes endogenous release to higher global levels without significant dependence on direct synaptic integration of the remaining endogenous dopamine neurons, as a large majority of these are gone when this treatment is prescribed (presumably after at least 70% of the endogenous innervation deteriorates). Also, dopamine-lesioned experimental model animals have been improved by treatment with synthetic slow-release nanoparticles [74] or transplantation of genetically modified fibroblasts [75] that likely neither need, nor have the capacity to respond to, afferent control. In this context these treatments, as well as the dopamine systems considered, are seen as utilizing volume transmission or "open" synapses that tend to increase release levels over larger areas, based either on simple diffusion mechanisms or low-level chronic stimulation. By contrast, there are systems that rely on comparatively local transmission or "closed" synapses that are locally circumscribed by glial cells to certain synaptic junctions, and usually depend much more heavily on the timing of inputs for their function. Systems utilizing volume transmission would, by this definition, present an ambiguity to whether they necessitate as much acceptance into the network [76, 77]. So long as they provide the requisite compound this contribution may suffice, at least initially, in bolstering the circuit in question, though release would need to take place in key areas to be effective. Dopaminergic inputs seem to exhibit both of these characteristics ("open" and "closed"). This concept, commonly overlooked in the clinic, will be expanded upon later.

daughter cells (for review see [68]). Two key areas that benefit from this neurogenesis would be the hippocampus and olfactory bulb, and the degree of this natural new neuron incorporation depends on the activity levels generated in these regions. Specific functions that rely upon neurogenesis include new learning for the hippocampus [69], and rich olfactory sensory experience for the olfactory bulb [70]. As this occurs regularly in an activity-dependent manner already, it stands to reason that neuronal precursors transplanted into these regions would be more likely to receive signals encouraging their incorporation into either the hippocampus or the olfactory bulb, and in fact, this seems to be the case [71, 72]. Yet in other regions such as the striatum (the main input structure of the basal ganglia), the capacity of endogenous neuronal progenitors to become neurons seems reduced compared to exogenous transplantations [73]. The answer to why this distinction exists has been a top priority among those of us who foresee more successful replacement therapies. While the whole picture is not available, what seems clear is that there is an interactive relationship between the endogenous host cells and transplanted cells at the center. The so-called "neurogenic" regions (hippocampus and olfactory bulb) where replacement happens regularly as part of the natural progression throughout our lives would likely not be a useful target region for clinical transplantation for any key neuronal disorders, given that their potential for reconstructive replacement remains high. Yet we might initially presume that the mechanisms encouraging incorporation, such as the guidance molecules used and trophic factors encouraging survival as connections are established, or the afferent connections grown into and onto the transplant cells as the afferent

component, may follow rules similar to transplant events elsewhere.

12 Neuroplasticity - Insights of Neural Reorganization

When precision is required in the placement of axon terminals, it would seem that the parameters for what might be considered functional success would be correspondingly more restrictive or demanding. Here it is appropriate to briefly describe how establishing a wide range of general chronic dopamine can provide considerable benefit in Parkinson's disease, and the distinction between "open" diffusion-capable release mechanisms versus "closed" synaptic connections circumscribed by glial borders. Due to the chronic widespread levels of dopamine persisting in extracellular space, simple diffusion-based neurotransmitter delivery is often discussed without emphasizing the more nuanced details of precise release. To illustrate, in the case of dopamine loss in Parkinson's disease, the standard drug levodopa promotes endogenous release to higher global levels without significant dependence on direct synaptic integration of the remaining endogenous dopamine neurons, as a large majority of these are gone when this treatment is prescribed (presumably after at least 70% of the endogenous innervation deteriorates). Also, dopamine-lesioned experimental model animals have been improved by treatment with synthetic slow-release nanoparticles [74] or transplantation of genetically modified fibroblasts [75] that likely neither need, nor have the capacity to respond to, afferent control. In this context these treatments, as well as the dopamine systems considered, are seen as utilizing volume transmission or "open" synapses that tend to increase release levels over larger areas, based either on simple diffusion mechanisms or low-level chronic stimulation. By contrast, there are systems that rely on comparatively local transmission or "closed" synapses that are locally circumscribed by glial cells to certain synaptic junctions, and usually depend much more heavily on the timing of inputs for their function. Systems utilizing volume transmission would, by this definition, present an ambiguity to whether they necessitate as much acceptance into the network [76, 77]. So long as they provide the requisite compound this Another aspect of transplant-related plasticity is the extent to which neurons are transplanted into a typical or atypical host environment for the neuron type that is their intended end-point for the current therapy. It is clear that during early fetal periods, useful progenitor populations can naturally undergo sufficient prior developmental modification to become predisposed to becoming a certain common neural type and can be found in regionally distinct populations in the fetus. For example, cells from the lateral ganglionic eminence show a high propensity to become GABAergic striatal neurons [78], or cells from the fetal mesencephalic region show a high propensity to become dopaminergic type neurons (e.g., [79]). Specific developmental trajectory predispositions can also be coaxed from progenitor cell populations *in vitro*, where the approximate mitogens, epigenetic cues, and morphogenic signals are maintained and progressively modified to encourage specific phenotype development trajectories [6, 80, 81]. Once neural developmental predisposition is established, it is perhaps fair to suggest that there are some host regions in which predisposed neurons would thrive as they would be placed into a "familiar" environment (i.e., a *homotypic* host region; e.g., GABAergic medium spiny predisposed neurons transplanted into the striatum) and some host regions that would *not* represent environments that might foster familiarity (i.e., an *ectopic* host region; e.g., dopaminergic destined neurons transplanted into the striatum). Precedent for this homotypic versus ectopic distinction has been set [82, 83]. Keep in mind that this distinction is made to capture the regional relations of predisposed neurons for certain locations, and functional benefit concerns are secondary.

For years, neuroscientists have been studying the anatomy of neuronal populations of various types that produce different neurochemical compositions throughout the brain and the corresponding afferent connections that grow into and drive activity in these different regions. Establishing appropriate afferent drive onto the neurons that are transplanted would be a clear sign that the circuit into which the transplanted cells need to merge has accepted them as part of the equation. Clearly then, when placed into a homotypic host region this sort of acceptance would be more likely based on the proximity of the transplanted cells to appropriate afferent input that such cells need to be driven properly by the host brain architecture. The prime example of this sort of transplant that has shown considerable acceptance into the host circuit and was extensively characterized by Klas Wictorin in 1992 is the intrastriatal transplant of striatal-predisposed precursor cells obtained from the embryonic day 14–15 fetal lateral ganglionic eminence following an excitotoxic lesion of the host striatum [84]. The extensive host innervation of this transplant along with the extensive growth and integration of the transplant with the host in the context of circuit re-establishment was dramatic, long-lasting, and seemed to contribute considerable support to the lesioned circuit as seen by neurobehavioral improvement. Wictorin indicated that the initial destructive lesion to destroy local endogenous striatal neurons is crucial for enabling the sort of host integration seen, as the absence of such a lesion (i.e., transplantation into an intact striatum) yielded far less integration [85]. It stands to reason this would occur because afferent inputs would find greater ease in filling an open void or niche so long as it maintains a general presence after the lesion. A transplant without deteriorative or destructive loss would also be unnecessary because, as stated, the transplant is meant to restore the lost contribution. Wictorin describes a considerable ingrowth of afferent inputs from the host brain into the transplant with cortical, thalamic, nigral dopaminergic, and serotonergic inputs from the raphe that showed extensive yet differential degrees of penetration into the graft [84]. Also relevant was the point made about how regions that did not appear "striatum-like" seemed far less capable of inducing dopaminergic ingrowth and that specific transplants of cerebellar or cortical tissue into the excitotoxin-lesioned striatum of adult rats yielded no such dopaminergic innervation. Electrophysiological experiments demonstrated that these innervations of the graft were synaptically functional, supporting host-originating cortical drive [86, 87], and functional dopaminergic modification of GABA release from transplanted cells into the globus pallidus and the substantia nigra reticulata [88].

cable for light stimulation in the present study) had clearly shown that rats with intact striata exhibit largely low levels of spontaneous activity [93]. Thus this technique that uses pulled 4-barrel glass electrodes (recording via 3 M NaCl, and iontophoresis of 0.25 M glutamate with a 0.25 M NaCl balance barrel and a narrow fiber-optic cable delivering blue light through the final barrel, wiring and fiber-optic cable connected to a combined swivel apparatus above the chamber) took advantage of a movable electrode holder allowing multiple exploratory passes through the dorsomedial striatum while monitoring the extracellular field for potential signs of single unit activity. Iontophoresis of glutamate was our global stimulant capable of activating any striatal neuron in proximity whether it originated endogenously or from the

Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell…

http://dx.doi.org/10.5772/intechopen.71790

15

We went in expecting a low yield of photosensitive units given established findings that transplanted neurons tend not to incorporate well in an intact striatum (e.g., [84]). In our experiments, each animal received approximately 40,000 neuronal stem cells as 8–10 neurospheres per animal except "controls." The distribution of behaviorally responsive units was similar to previous work with similar electrodes employed without light [93] among controls. Among our main findings was that none of the units that responded to light or light/glutamate combinations also responded to the behavior of the animal. This suggests that although glutamatergic inputs from both cortex and thalamus synapsed functionally with grafts placed into the striatum following an ibotenic acid lesion [84], we saw little evidence of the corresponding freelymoving-animal incorporation that likely would have generated behavior-induced responses in our localized and verified transplanted cells. Of the 40,000 potential contributors placed directly into the pathway of our electrodes, we recorded from approximately 20 light-sensitive cells per animal at 2 weeks and then found a far smaller number (approximately 2-3) at 4 weeks. However it was not the case that the neurons derived from transplants were unable to form connections, as on multiple occasions with the subjects tested at 2 weeks post-transplantation we witnessed responses of what we predicted were spontaneously active endogenous units that were clearly inhibited during light stimulation (see **Figure 1**). None of these light-induced inhibitions were found at 4 weeks, in part because spontaneous activity was also harder to find at this later date. However, this finding and the fact that 2–3 out of the average 20 light stimulation events yielded this sort of response at 2 weeks but not at 4 weeks also suggests the possibility of temporary local synaptic connections being formed and then lost between the periods explored. While it is possible that a certain lack of drive (evidenced by the lack of behavioral drive on light-activated units) may have contributed to their demise as well as inflammatory or immune responses, another finding from this study was particularly intriguing. During the search for EYFP-expressing units at the final stages of these experiments, rats euthanized after explorations at 4 weeks contained far higher levels of fluorescent units merging into the olfactory-destined rostral migratory stream (see **Figure 2**). Newly produced neurons from the subventricular zone (where our neural stem cells were originally harvested) initially follow the edges of the lateral ventricles and then proceed ventrally into the stream headed for the olfactory bulb [94]. Fluorescent cells were consistently found to be incorporated into this system in a more rounded and presumably migratory state that, while still potentially responsive to both light and glutamate stimulation, would not be expected to have incorporated host glutamatergic drive that has been associated with driving CAM kinase II responses and the corresponding cessation of

transplant.

migration and synaptic arbor development [95].

Although speculative, it is possible that even if the neurons transplanted into the striatum that became GABAergic did not grow extensively into the host tissue and participate more fully in the host basal ganglia circuitry, some circuit support might be established by enhancing only local GABA release from these neurons with limited incorporation as exclusively interneurons. It has become clear that in the context of the Huntington's disease condition, even prior to the major deterioration of striatal cells, there is considerable and abnormal spontaneous activity within the dorsal striatum [89, 90], and it appears that overactive glutamate release or diminished reuptake transport of glutamate is at least partially to blame for this [91, 92]. Under these circumstances, a considerable disruption of striatal function might arise due largely to this main input region being considerably noisier than normal (electrophysiologically speaking), and under such circumstances the proper selection of outputs would necessarily become challenged. If transplanted cells were simply driven by locally increased glutamatergic inputs or the ambient glutamate levels, after which they proceeded to feed back onto local medium spiny projection neurons in a manner that minimized this noise, some presumed information processing capacity might be restored, despite the lack of full integration.

To highlight the behavioral relevance of induced electrophysiology on transplanted cells, our laboratory initiated a project that involved preliminary transduction of transplant-destined neuronal precursor cells harvested from the subventricular zone of neonatal rats (P1 to P2) with Channelrhodopsin-2 (ChR2). This receptor construct allowed for rapid and exclusive optogenetic stimulation of these cells as they became functional neurons activated by blue light. The construct also contained a transgene with a synapsin promoter as well as code for enhanced yellow fluorescent protein (EYFP) for visualization post-euthanasia. It was our interest to explore the propensity of these transplanted cells to incorporate with the circuitry of the otherwise intact dorsal striatum in a manner that would allow a movable skull-mounted iontophoresis/single-unit electrophysiology electrode with fiber optic light incorporation to locate transplanted cells by slowly moving dorsoventrally across the striatum of a freelymoving rat and searching for units that would respond to various local stimulations. There were three stimulation types that could be generated: iontophoresis of glutamate, stimulation with 473 nm blue light, and behavioral stimulation. The advantage of this strategy was that only cells transduced with ChR2 (i.e., the cells to be transplanted) would show photosensitivity to blue light. Several interesting findings arose from this work. Previous work with a similar iontophoresis electrode (modified merely to allow the inclusion of a narrow fiber optic cable for light stimulation in the present study) had clearly shown that rats with intact striata exhibit largely low levels of spontaneous activity [93]. Thus this technique that uses pulled 4-barrel glass electrodes (recording via 3 M NaCl, and iontophoresis of 0.25 M glutamate with a 0.25 M NaCl balance barrel and a narrow fiber-optic cable delivering blue light through the final barrel, wiring and fiber-optic cable connected to a combined swivel apparatus above the chamber) took advantage of a movable electrode holder allowing multiple exploratory passes through the dorsomedial striatum while monitoring the extracellular field for potential signs of single unit activity. Iontophoresis of glutamate was our global stimulant capable of activating any striatal neuron in proximity whether it originated endogenously or from the transplant.

the lesion. A transplant without deteriorative or destructive loss would also be unnecessary because, as stated, the transplant is meant to restore the lost contribution. Wictorin describes a considerable ingrowth of afferent inputs from the host brain into the transplant with cortical, thalamic, nigral dopaminergic, and serotonergic inputs from the raphe that showed extensive yet differential degrees of penetration into the graft [84]. Also relevant was the point made about how regions that did not appear "striatum-like" seemed far less capable of inducing dopaminergic ingrowth and that specific transplants of cerebellar or cortical tissue into the excitotoxin-lesioned striatum of adult rats yielded no such dopaminergic innervation. Electrophysiological experiments demonstrated that these innervations of the graft were synaptically functional, supporting host-originating cortical drive [86, 87], and functional dopaminergic modification of GABA release from transplanted cells into the globus pallidus and

Although speculative, it is possible that even if the neurons transplanted into the striatum that became GABAergic did not grow extensively into the host tissue and participate more fully in the host basal ganglia circuitry, some circuit support might be established by enhancing only local GABA release from these neurons with limited incorporation as exclusively interneurons. It has become clear that in the context of the Huntington's disease condition, even prior to the major deterioration of striatal cells, there is considerable and abnormal spontaneous activity within the dorsal striatum [89, 90], and it appears that overactive glutamate release or diminished reuptake transport of glutamate is at least partially to blame for this [91, 92]. Under these circumstances, a considerable disruption of striatal function might arise due largely to this main input region being considerably noisier than normal (electrophysiologically speaking), and under such circumstances the proper selection of outputs would necessarily become challenged. If transplanted cells were simply driven by locally increased glutamatergic inputs or the ambient glutamate levels, after which they proceeded to feed back onto local medium spiny projection neurons in a manner that minimized this noise, some presumed information

To highlight the behavioral relevance of induced electrophysiology on transplanted cells, our laboratory initiated a project that involved preliminary transduction of transplant-destined neuronal precursor cells harvested from the subventricular zone of neonatal rats (P1 to P2) with Channelrhodopsin-2 (ChR2). This receptor construct allowed for rapid and exclusive optogenetic stimulation of these cells as they became functional neurons activated by blue light. The construct also contained a transgene with a synapsin promoter as well as code for enhanced yellow fluorescent protein (EYFP) for visualization post-euthanasia. It was our interest to explore the propensity of these transplanted cells to incorporate with the circuitry of the otherwise intact dorsal striatum in a manner that would allow a movable skull-mounted iontophoresis/single-unit electrophysiology electrode with fiber optic light incorporation to locate transplanted cells by slowly moving dorsoventrally across the striatum of a freelymoving rat and searching for units that would respond to various local stimulations. There were three stimulation types that could be generated: iontophoresis of glutamate, stimulation with 473 nm blue light, and behavioral stimulation. The advantage of this strategy was that only cells transduced with ChR2 (i.e., the cells to be transplanted) would show photosensitivity to blue light. Several interesting findings arose from this work. Previous work with a similar iontophoresis electrode (modified merely to allow the inclusion of a narrow fiber optic

processing capacity might be restored, despite the lack of full integration.

the substantia nigra reticulata [88].

14 Neuroplasticity - Insights of Neural Reorganization

We went in expecting a low yield of photosensitive units given established findings that transplanted neurons tend not to incorporate well in an intact striatum (e.g., [84]). In our experiments, each animal received approximately 40,000 neuronal stem cells as 8–10 neurospheres per animal except "controls." The distribution of behaviorally responsive units was similar to previous work with similar electrodes employed without light [93] among controls. Among our main findings was that none of the units that responded to light or light/glutamate combinations also responded to the behavior of the animal. This suggests that although glutamatergic inputs from both cortex and thalamus synapsed functionally with grafts placed into the striatum following an ibotenic acid lesion [84], we saw little evidence of the corresponding freelymoving-animal incorporation that likely would have generated behavior-induced responses in our localized and verified transplanted cells. Of the 40,000 potential contributors placed directly into the pathway of our electrodes, we recorded from approximately 20 light-sensitive cells per animal at 2 weeks and then found a far smaller number (approximately 2-3) at 4 weeks. However it was not the case that the neurons derived from transplants were unable to form connections, as on multiple occasions with the subjects tested at 2 weeks post-transplantation we witnessed responses of what we predicted were spontaneously active endogenous units that were clearly inhibited during light stimulation (see **Figure 1**). None of these light-induced inhibitions were found at 4 weeks, in part because spontaneous activity was also harder to find at this later date. However, this finding and the fact that 2–3 out of the average 20 light stimulation events yielded this sort of response at 2 weeks but not at 4 weeks also suggests the possibility of temporary local synaptic connections being formed and then lost between the periods explored. While it is possible that a certain lack of drive (evidenced by the lack of behavioral drive on light-activated units) may have contributed to their demise as well as inflammatory or immune responses, another finding from this study was particularly intriguing. During the search for EYFP-expressing units at the final stages of these experiments, rats euthanized after explorations at 4 weeks contained far higher levels of fluorescent units merging into the olfactory-destined rostral migratory stream (see **Figure 2**). Newly produced neurons from the subventricular zone (where our neural stem cells were originally harvested) initially follow the edges of the lateral ventricles and then proceed ventrally into the stream headed for the olfactory bulb [94]. Fluorescent cells were consistently found to be incorporated into this system in a more rounded and presumably migratory state that, while still potentially responsive to both light and glutamate stimulation, would not be expected to have incorporated host glutamatergic drive that has been associated with driving CAM kinase II responses and the corresponding cessation of migration and synaptic arbor development [95].

safe and likely permanently established is not supported; yet it seems to underlie the persistent sentiment that after certain periods of time, the mere existence of neuronally integrated and transplant-derived neurons in the host tissue represents a *successful* fix for the destruction or deterioration of host tissue. Such reasoning is fallacious and ignores the intricacies and con-

Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell…

http://dx.doi.org/10.5772/intechopen.71790

17

Others who have transplanted into the intact striatum using lateral ganglionic eminencederived cells witnessed a correspondingly diminished interaction with the striatal circuit. In fact, following such transplantations, Magavi and Lois [96] found a greater degree of growth into and synaptic integration with orbitofrontal cortex and the claustrum than with either striatal or nigral connections, indicating also the inability of the striatum to attract connections into the homotypic basal ganglia circuit. It is standard procedure that experiments investigating the degree of host incorporation depict dendritic arbors and other signs of likely synaptic input following transplantation, but it would be misleading to indicate that whatever snapshot taken in post-experimentation histology is a fixed and permanent condition. It would run contrary to what we know about natural endogenous synaptic plasticity to believe that any fixed depiction of synaptic status remains a permanent or "set in stone" phenomenon, as we know endogenous synapses are constantly dancing with each other, exchanging connections regularly due to competitive interactions [97]. To compete and participate in this drawn-out request for a place in the circuit, it would be important that sufficient drive is established and, after the driving elements (e.g., corticostriatal or thalamostriatal inputs) are relieved of their targets by prior lesions, there would be a likely increase in terminals seeking destinations, and this is lacking in the intact striatum. It stands to reason that neurons without such drive might continue to migrate until they can position themselves to receive it. Clearly a considerable effort is engaged by both corticostriatal and thalamostriatal afferents to synaptically integrate with striatal grafts that follow target-destructive excitotoxic lesions as well as transplanted neurons that grow far

As mentioned above, extensive dopaminergic ingrowth occurs from grafts of fetal progenitors into a lesioned striatum, indicating that not only does glutamatergic host innervation likely drive this population, but this population is also modulated by dopamine in a hostcontrolled manner. Despite this capacity, thus far, most experiments exploring the viability of transplanting cells as a treatment for Parkinson's disease have targeted cells predisposed to become dopaminergic ectopically into the striatum rather than the homotypic substantia nigra. The rationale behind striatal transplantation of these dopaminergic-destined cells rather than transplanting the cells into the substantia nigra region is largely because of the expectation that neurons transplanted into the substantia nigra region would not be able to grow axonal extensions sufficiently through the relatively inhospitable terrain of the adult brain to deliver the needed dopamine into the striatum. Also, striatal transplants would likely provide comparatively more dopamine in the target region. This concept was formulated by Anders Björklund and his collaborators [98, 99] as the idea of dopaminergic tissue transplants for Parkinson's disease was initially proposed. Previously described limitations to extensive axon growth through the adult CNS would clearly support this notion. Thus, the large majority of the experiments exploring replacement transplantations for Parkinsonian circumstances targets the dorsal striatum and would fall into the category of ectopic host destinations.

stantly-shifting nature of even seemingly well-established intact neural circuitry.

more extensive integrations into the basal ganglia circuitry [84].

**Figure 1.** Spontaneous unit activity inhibited by light. Here a spontaneously active unit from within the striatum was inhibited by light. We predict that the connectivity was such that a transplanted (thus light-responsive) unit had adopted a GABAergic transmitter type and when it became activated by local light in sufficient proximity was induced to synaptically suppress the unit it had synapsed upon as depicted in the insert. *Insert Diagram*: Pulled glass electrode depicted descending from top. Darker sphere at electrode tip represents fiber-optic cable-derived blue light stimulation emanating from electrode tip. Small interneuron to electrode right depicts EYFP-expressing transplanted cell presumably sufficiently close to be excited by blue light but not to contribute to recorded *activation* response typical of "direct" stimulation. The recorded medium spiny cell, juxtaposed to the electrode tip, represents the spontaneously-active neuron providing recorded activity that was otherwise insensitive to the blue light barring GABA influence elicited from the sensitive transplanted cell connected to it.

From this study we concluded that when transplants are placed into the less-hospitable intact striatum it is possible that far fewer neurons make synapses with the local host and that when they do at earlier stages, these synaptic connections are far from permanent. It is likely that the unique proximity of the electrode, spontaneously active cell, and photosensitive cell depicted in **Figure 1**, that we hypothesize would elicit the witnessed inhibitory responses, would be serendipitous under the most opportune conditions, particularly considering the scarcity of spontaneously active striatal units within the intact brain (harder to find in general; [93]). Yet the consistency of the findings at 2 weeks and the complete lack at 4 weeks suggests that at least some transplanted units made only temporary synapses that disappeared later either due to cell death or resumption of migration. This may occur when the signals indicating the propensity for circuit interaction by the endogenous host cells are weaker, and there is subsequent reduced effort to formulate more robust and permanent interactions. Therefore, the presumption that once neurons form a synaptic relationship, the newly transplanted cells are somehow

**Figure 2.** Fluorescent migrating cells at 4 weeks. Shown is a clear mass of cells that had migrated along the ventricles toward the bottom portion of the ventricle seen in both bright field (A) and the fluorescent images of B and C showing migrating transplant-origin cells fluorescing brightly in this location, outside of the recording area of the dorsal striatum. Line segments in each image represent 100 µm. C represents a different region of this clustering from a separate, similarly-treated animal focused at a deeper level of the rostral migratory stream. Bottom of ventricle not seen in C but is just above the upper left corner of image.

safe and likely permanently established is not supported; yet it seems to underlie the persistent sentiment that after certain periods of time, the mere existence of neuronally integrated and transplant-derived neurons in the host tissue represents a *successful* fix for the destruction or deterioration of host tissue. Such reasoning is fallacious and ignores the intricacies and constantly-shifting nature of even seemingly well-established intact neural circuitry.

Others who have transplanted into the intact striatum using lateral ganglionic eminencederived cells witnessed a correspondingly diminished interaction with the striatal circuit. In fact, following such transplantations, Magavi and Lois [96] found a greater degree of growth into and synaptic integration with orbitofrontal cortex and the claustrum than with either striatal or nigral connections, indicating also the inability of the striatum to attract connections into the homotypic basal ganglia circuit. It is standard procedure that experiments investigating the degree of host incorporation depict dendritic arbors and other signs of likely synaptic input following transplantation, but it would be misleading to indicate that whatever snapshot taken in post-experimentation histology is a fixed and permanent condition. It would run contrary to what we know about natural endogenous synaptic plasticity to believe that any fixed depiction of synaptic status remains a permanent or "set in stone" phenomenon, as we know endogenous synapses are constantly dancing with each other, exchanging connections regularly due to competitive interactions [97]. To compete and participate in this drawn-out request for a place in the circuit, it would be important that sufficient drive is established and, after the driving elements (e.g., corticostriatal or thalamostriatal inputs) are relieved of their targets by prior lesions, there would be a likely increase in terminals seeking destinations, and this is lacking in the intact striatum. It stands to reason that neurons without such drive might continue to migrate until they can position themselves to receive it. Clearly a considerable effort is engaged by both corticostriatal and thalamostriatal afferents to synaptically integrate with striatal grafts that follow target-destructive excitotoxic lesions as well as transplanted neurons that grow far more extensive integrations into the basal ganglia circuitry [84].

From this study we concluded that when transplants are placed into the less-hospitable intact striatum it is possible that far fewer neurons make synapses with the local host and that when they do at earlier stages, these synaptic connections are far from permanent. It is likely that the unique proximity of the electrode, spontaneously active cell, and photosensitive cell depicted in **Figure 1**, that we hypothesize would elicit the witnessed inhibitory responses, would be serendipitous under the most opportune conditions, particularly considering the scarcity of spontaneously active striatal units within the intact brain (harder to find in general; [93]). Yet the consistency of the findings at 2 weeks and the complete lack at 4 weeks suggests that at least some transplanted units made only temporary synapses that disappeared later either due to cell death or resumption of migration. This may occur when the signals indicating the propensity for circuit interaction by the endogenous host cells are weaker, and there is subsequent reduced effort to formulate more robust and permanent interactions. Therefore, the presumption that once neurons form a synaptic relationship, the newly transplanted cells are somehow

**Figure 2.** Fluorescent migrating cells at 4 weeks. Shown is a clear mass of cells that had migrated along the ventricles toward the bottom portion of the ventricle seen in both bright field (A) and the fluorescent images of B and C showing migrating transplant-origin cells fluorescing brightly in this location, outside of the recording area of the dorsal striatum. Line segments in each image represent 100 µm. C represents a different region of this clustering from a separate, similarly-treated animal focused at a deeper level of the rostral migratory stream. Bottom of ventricle not seen in C but

connected to it.

16 Neuroplasticity - Insights of Neural Reorganization

is just above the upper left corner of image.

**Figure 1.** Spontaneous unit activity inhibited by light. Here a spontaneously active unit from within the striatum was inhibited by light. We predict that the connectivity was such that a transplanted (thus light-responsive) unit had adopted a GABAergic transmitter type and when it became activated by local light in sufficient proximity was induced to synaptically suppress the unit it had synapsed upon as depicted in the insert. *Insert Diagram*: Pulled glass electrode depicted descending from top. Darker sphere at electrode tip represents fiber-optic cable-derived blue light stimulation emanating from electrode tip. Small interneuron to electrode right depicts EYFP-expressing transplanted cell presumably sufficiently close to be excited by blue light but not to contribute to recorded *activation* response typical of "direct" stimulation. The recorded medium spiny cell, juxtaposed to the electrode tip, represents the spontaneously-active neuron providing recorded activity that was otherwise insensitive to the blue light barring GABA influence elicited from the sensitive transplanted cell

> As mentioned above, extensive dopaminergic ingrowth occurs from grafts of fetal progenitors into a lesioned striatum, indicating that not only does glutamatergic host innervation likely drive this population, but this population is also modulated by dopamine in a hostcontrolled manner. Despite this capacity, thus far, most experiments exploring the viability of transplanting cells as a treatment for Parkinson's disease have targeted cells predisposed to become dopaminergic ectopically into the striatum rather than the homotypic substantia nigra. The rationale behind striatal transplantation of these dopaminergic-destined cells rather than transplanting the cells into the substantia nigra region is largely because of the expectation that neurons transplanted into the substantia nigra region would not be able to grow axonal extensions sufficiently through the relatively inhospitable terrain of the adult brain to deliver the needed dopamine into the striatum. Also, striatal transplants would likely provide comparatively more dopamine in the target region. This concept was formulated by Anders Björklund and his collaborators [98, 99] as the idea of dopaminergic tissue transplants for Parkinson's disease was initially proposed. Previously described limitations to extensive axon growth through the adult CNS would clearly support this notion. Thus, the large majority of the experiments exploring replacement transplantations for Parkinsonian circumstances targets the dorsal striatum and would fall into the category of ectopic host destinations.

The question remains largely open how such cells, when they become dopaminergic neurons, will integrate into circuitry, given that they are typically stimulated at their cell body level by glutamatergic signals entering into the substantia nigra. In fact, the substantia nigra seems to receive active control inputs originating from the subthalamic nucleus and the somatosensory/ motor cortex, both of which are touted as being capable of initiating rapid responses to salient events [100]. By comparison the cortical drive on striatal neurons tends to be highly converged on any individual medium spiny target from across relatively wide regions of the cortex, an organization that would require high collaboration between multiple regions to activate any single striatal neuron [101]. This common convergence is likely to be partially responsible for the relative silence observed within the striatum while otherwise intact animals are at quiet rest. The chances that cortical input into the striatum would even closely approximate that obtained from the cortical and subthalamic input to the substantia nigra is therefore low to begin with, let alone the serendipity that would be necessary to result in all such transplanted dopaminergic neurons stimulated by the same subset of glutamatergic afferents. This would severely challenge the precision of drive on these transplanted neurons, with terminations more haphazard across the transplanted population, resulting in a much more *constant* degree of afferent stimulation.

for processing transplants as only a very small boost in dopamine-producing capacity (such as 100–200 surviving transplanted neurons) seems sufficient to eliminate amphetamine-induced rotations by providing both chronic amphetamine-driven dopamine in general to the dorsal striatum [83]. Keeping this in mind, the ongoing adjustments in postsynaptic, and potentially presynaptic responses as well, are likely to reduce the dynamic responsiveness of the transplantestablished dopamine system as seen by researchers, cautioning us about the inadequacy of drug-

Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell…

http://dx.doi.org/10.5772/intechopen.71790

19

The insufficiency of phasic release restoration also likely underlies the inability of Parkinson's patients on L-Dopa replacement therapy to rapidly adjust to ongoing motoric demands [108]. To accomplish a relative surge in dopamine release at a critical behavioral juncture such that the presence of dopamine provides sufficient ongoing support during more emergency situations, such as the need to escape from entrapment or predation, or falling into a body of water and needing to swim, phasic firing of nigrostriatal neurons occurs. The fact that gap junction connections have been found between nigral dopaminergic neurons and that electrophysiological behavior of the nigral population as a whole maintains consistency indicative of such electrotonic coupling [109, 110] suggests that wide ranging locations within the anterior striatal targets receive temporally consistent bursts as a result, in time with the events necessitating dopaminergic modulation. This phasic firing was recorded by Wolfram Schultz from dopamine neurons in the primate ventral tegmental area in his famous experiments that showed the cues responsible for generating increased drive on these mesolimbic neurons shifted from the initial pure reinforcement toward environmental cues *predictive* of the reinforcement and/or the risk associated with the reinforcement [111–113]. It is likely that what engages phasic drive among those neurons that support proper motivation in the arena of learning conditioning is driven in a manner somewhat distinct from the phasic drive on dopamine neurons that serve movement-related calculations within the dorsal part of the striatum. As we approximate human viability of transplantation, it is perhaps fair to mention that the borders of these two dopamine systems within the broader striatum of primates may not be as simple as their general projection parameters, and there is considerable overlap between the ascending dopamine systems (as described in [114]); there has nevertheless been a relatively consistent distinction made in the functional attributes of the projections. The apparent overlap may support the anecdotal events we have heard of where an immobile Parkinson's patient can initiate movement toward the exit of a building should this patient hear warnings

of "*fire*" being exclaimed locally, though a stress-related release would be phasic.

Striatal cholinergic interneurons of the large aspiny variety are more likely to be tonically active for larger proportions of time than the main population of medium spiny GABAergic neurons, so their contributions to the ongoing processing within the region can be described as dynamic. These interneurons are controlled in a complicated way by dopamine, glutamate, and local GABA signals. Upon deeper scrutiny, the dopaminergic control of these large aspiny cholinergic neurons has been shown to involve differential employment of glutamate co-released from dopaminergic terminals along with dopamine between dorsal and ventral striata, rendering these regions distinct in how acetylcholine is driven [115]. The common understanding of the interaction between dopamine and acetylcholine in the striatum is that it is inverse, such that phasic bursts of dopamine lead to phasic pauses in ongoing activity among the large aspiny

induced rotation in capturing underlying recovery dynamics [107].

Explorations of the corticostriatal input into dopamine-predisposed grafts of fetal mesencephalic tissue have yielded mixed results from inputs that appear to take on the morphologic appearance of nigral cortical input (thick fibers giving off thin collaterals; [102]). Another point raised more recently by Braak and del Tredici [103] is that striatal medium spiny neurons tend to lose their spines over time during the ongoing pathology of Parkinson's disease and that dopaminergic inputs in the intact striatum of otherwise healthy subjects seem to interact in a complicated modulatory manner on spine shafts while the corticostriatal terminals engage their tips. As this arrangement is progressively lost, the ability of dopaminergic grafts to successfully interact with the main projection medium spiny efferents may also be jeopardized. If the drive on grafted dopaminergic neurons in the striatum is not well controlled, the ability to duplicate distinct periods of phasic release that mark events of behavioral significance may be missing from the grafted dopaminergic neurons.

Most synapses engage plastic mechanisms that adopt diminished responses to non-dynamic and unchanging levels of drive in a manner similar to the way sensory systems habituate to consistency. We know that rats given large unilateral 6-OHDA-induced lesions of dopaminergic input to the striatum tend to respond within days to apomorphine stimulation in a manner that depends upon postsynaptic modifications that establish "supersensitivity," and that there are modifications of dopamine receptors related to this occurring for extended periods following the lesion [104]. However, it has also been shown using equivalent lesions in mice that their rotation intensity diminishes over time when their lesioned hemisphere is continuously treated with apomorphine using an osmotic pump, suggesting that these modifications that support the supersensitivity *compensation* are reversible when sufficient dopaminergic stimulation remains persistent [105]. Processing the degree of postsynaptic responsivity to dopamine levels with a behavioral assay is common, since it is likely that adjustments in dopamine receptor sensitivity are continuously occurring in response to the degree of stimulation in a manner that stabilizes responses over time (e.g., [106]). It is interesting to note that, although very popular in the literature, recording diminishing rotation in response to transplantation has been deemed more distinctly inadequate for processing transplants as only a very small boost in dopamine-producing capacity (such as 100–200 surviving transplanted neurons) seems sufficient to eliminate amphetamine-induced rotations by providing both chronic amphetamine-driven dopamine in general to the dorsal striatum [83]. Keeping this in mind, the ongoing adjustments in postsynaptic, and potentially presynaptic responses as well, are likely to reduce the dynamic responsiveness of the transplantestablished dopamine system as seen by researchers, cautioning us about the inadequacy of druginduced rotation in capturing underlying recovery dynamics [107].

The question remains largely open how such cells, when they become dopaminergic neurons, will integrate into circuitry, given that they are typically stimulated at their cell body level by glutamatergic signals entering into the substantia nigra. In fact, the substantia nigra seems to receive active control inputs originating from the subthalamic nucleus and the somatosensory/ motor cortex, both of which are touted as being capable of initiating rapid responses to salient events [100]. By comparison the cortical drive on striatal neurons tends to be highly converged on any individual medium spiny target from across relatively wide regions of the cortex, an organization that would require high collaboration between multiple regions to activate any single striatal neuron [101]. This common convergence is likely to be partially responsible for the relative silence observed within the striatum while otherwise intact animals are at quiet rest. The chances that cortical input into the striatum would even closely approximate that obtained from the cortical and subthalamic input to the substantia nigra is therefore low to begin with, let alone the serendipity that would be necessary to result in all such transplanted dopaminergic neurons stimulated by the same subset of glutamatergic afferents. This would severely challenge the precision of drive on these transplanted neurons, with terminations more haphazard across the transplanted population, resulting in a much more *constant* degree of afferent stimulation.

Explorations of the corticostriatal input into dopamine-predisposed grafts of fetal mesencephalic tissue have yielded mixed results from inputs that appear to take on the morphologic appearance of nigral cortical input (thick fibers giving off thin collaterals; [102]). Another point raised more recently by Braak and del Tredici [103] is that striatal medium spiny neurons tend to lose their spines over time during the ongoing pathology of Parkinson's disease and that dopaminergic inputs in the intact striatum of otherwise healthy subjects seem to interact in a complicated modulatory manner on spine shafts while the corticostriatal terminals engage their tips. As this arrangement is progressively lost, the ability of dopaminergic grafts to successfully interact with the main projection medium spiny efferents may also be jeopardized. If the drive on grafted dopaminergic neurons in the striatum is not well controlled, the ability to duplicate distinct periods of phasic release that mark events of behavioral significance may be missing from the grafted

Most synapses engage plastic mechanisms that adopt diminished responses to non-dynamic and unchanging levels of drive in a manner similar to the way sensory systems habituate to consistency. We know that rats given large unilateral 6-OHDA-induced lesions of dopaminergic input to the striatum tend to respond within days to apomorphine stimulation in a manner that depends upon postsynaptic modifications that establish "supersensitivity," and that there are modifications of dopamine receptors related to this occurring for extended periods following the lesion [104]. However, it has also been shown using equivalent lesions in mice that their rotation intensity diminishes over time when their lesioned hemisphere is continuously treated with apomorphine using an osmotic pump, suggesting that these modifications that support the supersensitivity *compensation* are reversible when sufficient dopaminergic stimulation remains persistent [105]. Processing the degree of postsynaptic responsivity to dopamine levels with a behavioral assay is common, since it is likely that adjustments in dopamine receptor sensitivity are continuously occurring in response to the degree of stimulation in a manner that stabilizes responses over time (e.g., [106]). It is interesting to note that, although very popular in the literature, recording diminishing rotation in response to transplantation has been deemed more distinctly inadequate

dopaminergic neurons.

18 Neuroplasticity - Insights of Neural Reorganization

The insufficiency of phasic release restoration also likely underlies the inability of Parkinson's patients on L-Dopa replacement therapy to rapidly adjust to ongoing motoric demands [108]. To accomplish a relative surge in dopamine release at a critical behavioral juncture such that the presence of dopamine provides sufficient ongoing support during more emergency situations, such as the need to escape from entrapment or predation, or falling into a body of water and needing to swim, phasic firing of nigrostriatal neurons occurs. The fact that gap junction connections have been found between nigral dopaminergic neurons and that electrophysiological behavior of the nigral population as a whole maintains consistency indicative of such electrotonic coupling [109, 110] suggests that wide ranging locations within the anterior striatal targets receive temporally consistent bursts as a result, in time with the events necessitating dopaminergic modulation. This phasic firing was recorded by Wolfram Schultz from dopamine neurons in the primate ventral tegmental area in his famous experiments that showed the cues responsible for generating increased drive on these mesolimbic neurons shifted from the initial pure reinforcement toward environmental cues *predictive* of the reinforcement and/or the risk associated with the reinforcement [111–113]. It is likely that what engages phasic drive among those neurons that support proper motivation in the arena of learning conditioning is driven in a manner somewhat distinct from the phasic drive on dopamine neurons that serve movement-related calculations within the dorsal part of the striatum. As we approximate human viability of transplantation, it is perhaps fair to mention that the borders of these two dopamine systems within the broader striatum of primates may not be as simple as their general projection parameters, and there is considerable overlap between the ascending dopamine systems (as described in [114]); there has nevertheless been a relatively consistent distinction made in the functional attributes of the projections. The apparent overlap may support the anecdotal events we have heard of where an immobile Parkinson's patient can initiate movement toward the exit of a building should this patient hear warnings of "*fire*" being exclaimed locally, though a stress-related release would be phasic.

Striatal cholinergic interneurons of the large aspiny variety are more likely to be tonically active for larger proportions of time than the main population of medium spiny GABAergic neurons, so their contributions to the ongoing processing within the region can be described as dynamic. These interneurons are controlled in a complicated way by dopamine, glutamate, and local GABA signals. Upon deeper scrutiny, the dopaminergic control of these large aspiny cholinergic neurons has been shown to involve differential employment of glutamate co-released from dopaminergic terminals along with dopamine between dorsal and ventral striata, rendering these regions distinct in how acetylcholine is driven [115]. The common understanding of the interaction between dopamine and acetylcholine in the striatum is that it is inverse, such that phasic bursts of dopamine lead to phasic pauses in ongoing activity among the large aspiny striatal cholinergic neurons. The inverse responsivity to reward in the striatal systems driven by the dopaminergic input akin to what Schultz recorded from was clearly demonstrated by Morris and colleagues [116] in their work that looked at the dopamine responses simultaneously with the cholinergic responses. The complexities of that interaction and the manner in which it may in fact capture an extensive range of guidance information has been thoroughly described (e.g., [117]). Acetylcholine is unique compared with other transmitters in that the enzyme acetylcholinesterase that is abundantly expressed externally breaks the molecule down rapidly and restricts the domain of effectiveness to localized regions. This is relevant to our story because acetylcholine modulates dopamine release at the terminal level [118], adjusting the release that may otherwise be driven by afferent stimulation at the cell body level. In fact this level of local cholinergic control is likely to be driven differently by thalamostriatal versus corticostriatal origins of glutamatergic drive [119], providing those two differential systems unique access to this key transmitter input. On top of this, it has become clear that glutamatergic input also controls local dopamine terminal release in a receptor-dependent manner such that dopamine release may well be modified by both local striatal (reviewed in [120]) and distal nigral/VTA mechanisms. As described before, these inputs find their way in and around the spines expressed by the medium spiny neurons such that glutamatergic inputs to the spine tips get molded by dopaminergic, cholinergic, and GABAergic inputs engaging spine or dendrite shafts (see **Figure 3**) to maintain the proper final output signal that proceeds through the basal ganglia. To be effective, the timing of release would need to be carefully controlled as well as proximity. It is clear that these temporal dynamics contribute to the utility of each differential transmitter contribution, and to the overall effectiveness of the projecting efferents carrying signals to further destinations in the basal ganglia loops, as this aspect of basal ganglia function has been reviewed and explored extensively [121–123].

In the context of the present review, it is relevant to pause and ask ourselves if ectopic transplants of dopaminergic neurons into the striatum, during the course of or following extensive destruction due to Parkinson's disease, can approach the dynamic control present in the otherwise intact system. These transplanted dopaminergic neurons would likely be connected to rather haphazardly by corticostriatal or thalamostriatal glutamatergic, local cholinergic or GABAergic interneurons, and already lack the over-arching control of phasic release that is typically driven at the substantia nigra. The local control features described above have led investigators to suspect that dopamine release within the striatum might be *considerably independent* of nigral control [124], further supported by both anatomical evidence of striatalderived fibers growing into grafts [125], and electrophysiological evidence showing approximately 50% of grafted mesencephalic cells being activated by frontal cortex stimulation [126]. However, none of these supporting findings suggest any clear resumption of the temporal dynamics of dopamine modulation in a manner that might be expected to fully restore behavioral versatility. Not only this, but such ectopic transplantations would likely lead to a dispersion of neuronal soma due to migration throughout the striatum that, while often seen as a positive attribute given the likely corresponding breadth of dopamine contribution, would also render the temporal release control much more regionally distinct. The previous point about gap junction connectivity between dopaminergic neurons suggests that at least initially, electrophysiological phasic firing is driven in a more unified manner that is more likely to be behavioral-event-related rather than based on local control, with local control as a secondary mechanism. As diffusion of the released dopamine occurs between such contributing neurons, this differential release control would likely establish a new consistent ambient chronic level, lacking a temporal relationship with sensorimotor events that dopamine is meant to modify, providing progressively decreasing phasic relevance to the circuit over time. At this point, if the host system has not adapted in ways that reduce dynamic sensitivities, such dopamine contributions may elicit disruptive effects, such as dyskinesias [127, 128], though other research calls this into question [80, 129]. Certainly the diminished temporal control of striatal dopamine release would be a reason for diminished support of dynamic behavioral control,

directly or indirectly through their converging influence on the medium spiny striatal efferent.

**Figure 3.** Simplified standard medium spiny dendrite arrangement. The prominent central aspect represents a medium spiny cell dendrite expressing standard dendritic spines. Onto this, many narrow glutamatergic inputs are depicted as synapsing onto the tips of dendritic spines, while dopaminergic input travels upward juxtaposing en-passant varicosities onto spine or dendrite shafts, modulating receptive states in a coordinated manner. Dopaminergic inputs are temporally enhanced by coincident phasic stimulation at the level of the nigra along with electrotonic coupling of nigral neurons. Proximity of cholinergic and GABAergic inputs not shown but each transmitter contribution influences the other either

Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell…

http://dx.doi.org/10.5772/intechopen.71790

21

and might be argued to underlie the longevity of a graft's therapeutic effectiveness.

**4. Adaptation of transplanted neural cells to the endogenous host** 

It is nearly impossible to distinguish between host-to-transplant and transplant-to-host communication because the interactions between both populations are so intimate. However, for the flow of this review, we decided to first address mechanisms of concern regarding modifications engaged by local host cells such as their growth into the transplant and efforts to exert control. Now, we turn our attention to the manner in which transplanted cells likely

**tissue**

striatal cholinergic neurons. The inverse responsivity to reward in the striatal systems driven by the dopaminergic input akin to what Schultz recorded from was clearly demonstrated by Morris and colleagues [116] in their work that looked at the dopamine responses simultaneously with the cholinergic responses. The complexities of that interaction and the manner in which it may in fact capture an extensive range of guidance information has been thoroughly described (e.g., [117]). Acetylcholine is unique compared with other transmitters in that the enzyme acetylcholinesterase that is abundantly expressed externally breaks the molecule down rapidly and restricts the domain of effectiveness to localized regions. This is relevant to our story because acetylcholine modulates dopamine release at the terminal level [118], adjusting the release that may otherwise be driven by afferent stimulation at the cell body level. In fact this level of local cholinergic control is likely to be driven differently by thalamostriatal versus corticostriatal origins of glutamatergic drive [119], providing those two differential systems unique access to this key transmitter input. On top of this, it has become clear that glutamatergic input also controls local dopamine terminal release in a receptor-dependent manner such that dopamine release may well be modified by both local striatal (reviewed in [120]) and distal nigral/VTA mechanisms. As described before, these inputs find their way in and around the spines expressed by the medium spiny neurons such that glutamatergic inputs to the spine tips get molded by dopaminergic, cholinergic, and GABAergic inputs engaging spine or dendrite shafts (see **Figure 3**) to maintain the proper final output signal that proceeds through the basal ganglia. To be effective, the timing of release would need to be carefully controlled as well as proximity. It is clear that these temporal dynamics contribute to the utility of each differential transmitter contribution, and to the overall effectiveness of the projecting efferents carrying signals to further destinations in the basal ganglia loops, as this aspect of basal ganglia function

In the context of the present review, it is relevant to pause and ask ourselves if ectopic transplants of dopaminergic neurons into the striatum, during the course of or following extensive destruction due to Parkinson's disease, can approach the dynamic control present in the otherwise intact system. These transplanted dopaminergic neurons would likely be connected to rather haphazardly by corticostriatal or thalamostriatal glutamatergic, local cholinergic or GABAergic interneurons, and already lack the over-arching control of phasic release that is typically driven at the substantia nigra. The local control features described above have led investigators to suspect that dopamine release within the striatum might be *considerably independent* of nigral control [124], further supported by both anatomical evidence of striatalderived fibers growing into grafts [125], and electrophysiological evidence showing approximately 50% of grafted mesencephalic cells being activated by frontal cortex stimulation [126]. However, none of these supporting findings suggest any clear resumption of the temporal dynamics of dopamine modulation in a manner that might be expected to fully restore behavioral versatility. Not only this, but such ectopic transplantations would likely lead to a dispersion of neuronal soma due to migration throughout the striatum that, while often seen as a positive attribute given the likely corresponding breadth of dopamine contribution, would also render the temporal release control much more regionally distinct. The previous point about gap junction connectivity between dopaminergic neurons suggests that at least initially, electrophysiological phasic firing is driven in a more unified manner that is more likely to be

has been reviewed and explored extensively [121–123].

20 Neuroplasticity - Insights of Neural Reorganization

**Figure 3.** Simplified standard medium spiny dendrite arrangement. The prominent central aspect represents a medium spiny cell dendrite expressing standard dendritic spines. Onto this, many narrow glutamatergic inputs are depicted as synapsing onto the tips of dendritic spines, while dopaminergic input travels upward juxtaposing en-passant varicosities onto spine or dendrite shafts, modulating receptive states in a coordinated manner. Dopaminergic inputs are temporally enhanced by coincident phasic stimulation at the level of the nigra along with electrotonic coupling of nigral neurons. Proximity of cholinergic and GABAergic inputs not shown but each transmitter contribution influences the other either directly or indirectly through their converging influence on the medium spiny striatal efferent.

behavioral-event-related rather than based on local control, with local control as a secondary mechanism. As diffusion of the released dopamine occurs between such contributing neurons, this differential release control would likely establish a new consistent ambient chronic level, lacking a temporal relationship with sensorimotor events that dopamine is meant to modify, providing progressively decreasing phasic relevance to the circuit over time. At this point, if the host system has not adapted in ways that reduce dynamic sensitivities, such dopamine contributions may elicit disruptive effects, such as dyskinesias [127, 128], though other research calls this into question [80, 129]. Certainly the diminished temporal control of striatal dopamine release would be a reason for diminished support of dynamic behavioral control, and might be argued to underlie the longevity of a graft's therapeutic effectiveness.
