**8. Effects on the glutamatergic system**

Glutamate is the main excitatory neurotransmitter of the central nervous system [31]. Two glutamatergic circuits are particularly important in the pathophysiology of hepatic encephalopathy: (1) an yet unproven hypothetic pathway that would descend from the frontal lobe and (2) the perforant pathway originated in the entorhinal cortex.

It is believed that the frontal descending pathway (**Figure 1**) originates in layer V pyramidal neurons and projects to the centers of other neurotransmitters in the brainstem. There, it performs synapses with dopaminergic neurons of the ventral tegmental area and the substantia nigra, the serotonergic neurons of raphe nuclei and noradrenergic neurons of the locus coeruleus, influencing their activity [37]. If this hypothesis is correct, glutamatergic hyperactivity would act as a final pathway common to the changes induced by hyperammonemia and neuroinflammation, disturbing other neurotransmission systems, in steps that would invoke neuropsychiatric symptoms, and, in more severe cases, cause coma [4]. In addition, the frontal descending pathway would act as a "brake" for the dopaminergic pathway that leaves the ventral tegmental area toward the accumbens nucleus (located between the putamen and the caudate nucleus), influencing its activity through inhibitory GABAergic interneurons in the brainstem. This would result in tonic inhibition of dopamine release, with important consequences for executive and motor functions [37].

The perforant pathway (**Figure 2**) originates in the medial portion of the temporal cortex, called the entorhinal cortex, and projects to the granular cells of the dentate gyrus. The axons of these cells form a pathway of mossy fibers, which goes to the *Cornu Ammonis* (CA) or Ammon's horn, more precisely to the pyramidal cells of the CA3 region. Then, the pyramidal cells emit excitatory collaterals, the Schaffer collaterals, that go to the pyramidal cells of the CA1 region. A brief discharge of high-frequency stimuli in any of these three components of the perforant pathway increases the excitatory postsynaptic potentials in hippocampal neurons, which can last for hours, days, or even weeks. This facilitation is called long-term potentiation and, in addition to the hippocampus, also occurs in the amygdala, striatum (putamen and caudate nucleus), and cerebellar Purkinje cells, being essential for the formation of new traces of memory and learning [29, 32].

#### **Figure 1.**

*The frontal descending pathway would originate in the frontal cortex and influence directly or indirectly (through inhibitory interneurons) the activity of the neurotransmitter centers of the brainstem. AN: accumbens nucleus, GP: globus pallidus, RN: raphe nucleus, S: striatum, SN: substantia nigra, and T: thalamus.*

#### **Figure 2.**

*The perforant pathway originates in the entorhinal cortex (EC) and extends to the dentate gyrus (DG), from which neurons establish synapses with the CA3 and CA1 regions of the hippocampus, being involved with memory formation.*

Glutamate receptors are classified as metabotropic (coupled to G protein) and ionotropic (bound to ion channels). There are at least eight subtypes of metabotropic receptors and three classes of ionotropic receptors named according to agonists that selectively bind to them: NMDA (N-methyl-D-aspartate), AMPA (α-hydroxy-5-methyl-4-isoxazolepropionic acid), and kainate [37]. The first two have a particular relevance in hepatic encephalopathy, since the accumulation of glutamate in the synaptic clefts causes its hyperactivation, with excessive calcium influx [12]. This constant opening (tonic) of the ionotropic channels results in greater production of free radicals, with consequent neuronal apoptosis [26, 31]. The development of this process in the perforant pathway is a possible explanation for the episodic memory deficits presented by cirrhotics [12, 32]. Ammonia also induces apoptosis as a result of overproduction of nitric oxide [12], and this could explain why in some individuals such deficits become irreversible.

### **9. Effects on the GABAergic system**

Cortical neurons are also modulated by GABA-secreting neighboring interneurons, the main inhibitory neurotransmitter of the central nervous system [31]. Such cells organize themselves so that they can project their axons directly onto pyramidal cells, inhibiting glutamatergic neurotransmission, or extending their axons to other GABAergic interneurons that influence pyramidal cells, inhibiting the inhibition (and therefore, disinhibiting) of glutamatergic activity.

There are three main types (GABAA, GABAB, and GABAC) and numerous subtypes of GABA receptors. GABAA and GABAC receptors are ionic channels sensitive to ligands and are part of a macromolecular complex that forms an inhibitory chlorine channel, whereas GABAB receptors are members of a different class, bound to protein G (metabotropic receptors). Depending on the composition of their subunits, GABAA receptors may be sensitive to benzodiazepines [37]. Nonbenzodiazepine-sensitive subtypes are located outside the synapses, capturing not only GABA that diffuses beyond it but also locally released neuroactive steroids as a consequence of microglial

**47**

*The Neurobiology of Hepatic Encephalopathy DOI: http://dx.doi.org/10.5772/intechopen.86320*

greater considerations.

quently, the GABAergic tone [28].

**10. Effects on the dopaminergic system**

pathway, and (3) the nigrostriatal pathway.

activation [31]. Nonbenzodiazepine-sensitive extrasynaptic GABAA receptors promote tonic inhibition of postsynaptic neurons, as opposed to phasic inhibition induced by benzodiazepine-sensitive GABAA receptors. In addition, GABAA receptors bind effectively to other modulators, such as alcohol and neurosteroids, in a different

Experimental studies have shown that, in chronic hepatic encephalopathy, increased GABAergic tone in the cerebellar cortex results in motor incoordination [28, 32]. Several theories have been proposed throughout the history to explain the elevation of the activity of this neurotransmission pathway: (1) increased GABA synthesis, (2) increased expression of GABAA receptors in postsynaptic terminals, (3) modulation of GABAA receptors by neuroactive steroids, and (4) reversion of the action of astrocytic GABA transporters [26, 28]. Most studies, however, show with confidence that: (1) although glutamine is a precursor for GABA, GABA synthesis is not increased in hepatic encephalopathy and (2) GABAA receptor expression does not change in chronic liver insufficiency [12, 21, 26]. Therefore, hypotheses (3) and (4) regarding the modulation of GABAA receptors by neurosteroids and reversion of the action of astrocytic transporters are those that require

Experimental studies with acute hepatic failure demonstrate that neurosteroids synthesized locally by microglial cells from cholesterol participate in the modulation of GABAA receptor activity. Such neuroactive steroids may have an inhibitory effect (e.g., pregnenolone), functioning as positive allosteric modulators of GABAA, or excitatory receptors (e.g., allopregnanolone and tetrahydrodeoxycorticosterone), functioning as negative allosteric modulators of GABAA. It is believed that under the influence of hyperammonemia, both have their synthesis increased, but it is difficult to understand what emerges from the elevation of these two classes of hormones, which have antagonic actions [26]. However, the current body of evidence supports the exploration of GABAA receptors as potential treatment targets (e.g., pregnenolone sulfate and bicuculline) in chronic hepatic encephalopathy [29].

On the other hand, some of the effects of GABA are terminated by the action of the GABA transporter (GAT), which acts reuptaking it at the presynaptic neuron terminal [37]. Although there is disagreement over the exact location of the four subtypes of GABA transporters (GAT1–4) in pre- and postsynaptic neurons and glial cells, it is clear that a key transporter in hepatic encephalopathy is GAT3 [38]. It is found on the surface of astrocytes and microglial cells, and its action can be reversed both in the presence of chronic hyperammonemia and/or glutamatergic hyperactivity, increasing the availability of GABA in the synaptic cleft and, conse-

The main dopaminergic projections originate predominantly in the neurotransmission centers of the brainstem, especially the ventral tegmental area and substantia nigra. They are modulated by glutamatergic and GABAergic neurons and, among other functions, regulate movements, reward, and cognition [37]. Three dopaminergic circuits are particularly important in the pathophysiology of chronic hepatic encephalopathy: (1) the mesocortical pathway, (2) the striatal-thalamic-cortical

The mesocortical pathway (**Figure 3**) originates in the cellular bodies of the ventral tegmental area and extends to the prefrontal cortex, where it regulates executive functions [37]. The latter correspond to a set of abilities that, in an integrated way, allow the individual to direct behaviors to goals, to evaluate the efficiency and

location than GABA agonists, the so-called allosteric sites [37].

#### *The Neurobiology of Hepatic Encephalopathy DOI: http://dx.doi.org/10.5772/intechopen.86320*

*Liver Disease and Surgery*

**Figure 2.**

*memory formation.*

Glutamate receptors are classified as metabotropic (coupled to G protein) and ionotropic (bound to ion channels). There are at least eight subtypes of metabotropic receptors and three classes of ionotropic receptors named according to agonists that selectively bind to them: NMDA (N-methyl-D-aspartate), AMPA (α-hydroxy-5-methyl-4-isoxazolepropionic acid), and kainate [37]. The first two have a particular relevance in hepatic encephalopathy, since the accumulation of glutamate in the synaptic clefts causes its hyperactivation, with excessive calcium influx [12]. This constant opening (tonic) of the ionotropic channels results in greater production of free radicals, with consequent neuronal apoptosis [26, 31]. The development of this process in the perforant pathway is a possible explanation for the episodic memory deficits presented by cirrhotics [12, 32]. Ammonia also induces apoptosis as a result of overproduction of nitric oxide [12], and this could

*The perforant pathway originates in the entorhinal cortex (EC) and extends to the dentate gyrus (DG), from which neurons establish synapses with the CA3 and CA1 regions of the hippocampus, being involved with* 

explain why in some individuals such deficits become irreversible.

the inhibition (and therefore, disinhibiting) of glutamatergic activity.

Cortical neurons are also modulated by GABA-secreting neighboring interneurons, the main inhibitory neurotransmitter of the central nervous system [31]. Such cells organize themselves so that they can project their axons directly onto pyramidal cells, inhibiting glutamatergic neurotransmission, or extending their axons to other GABAergic interneurons that influence pyramidal cells, inhibiting

There are three main types (GABAA, GABAB, and GABAC) and numerous subtypes of GABA receptors. GABAA and GABAC receptors are ionic channels sensitive to ligands and are part of a macromolecular complex that forms an inhibitory chlorine channel, whereas GABAB receptors are members of a different class, bound to protein G (metabotropic receptors). Depending on the composition of their subunits, GABAA receptors may be sensitive to benzodiazepines [37]. Nonbenzodiazepine-sensitive subtypes are located outside the synapses, capturing not only GABA that diffuses beyond it but also locally released neuroactive steroids as a consequence of microglial

**9. Effects on the GABAergic system**

**46**

activation [31]. Nonbenzodiazepine-sensitive extrasynaptic GABAA receptors promote tonic inhibition of postsynaptic neurons, as opposed to phasic inhibition induced by benzodiazepine-sensitive GABAA receptors. In addition, GABAA receptors bind effectively to other modulators, such as alcohol and neurosteroids, in a different location than GABA agonists, the so-called allosteric sites [37].

Experimental studies have shown that, in chronic hepatic encephalopathy, increased GABAergic tone in the cerebellar cortex results in motor incoordination [28, 32]. Several theories have been proposed throughout the history to explain the elevation of the activity of this neurotransmission pathway: (1) increased GABA synthesis, (2) increased expression of GABAA receptors in postsynaptic terminals, (3) modulation of GABAA receptors by neuroactive steroids, and (4) reversion of the action of astrocytic GABA transporters [26, 28]. Most studies, however, show with confidence that: (1) although glutamine is a precursor for GABA, GABA synthesis is not increased in hepatic encephalopathy and (2) GABAA receptor expression does not change in chronic liver insufficiency [12, 21, 26]. Therefore, hypotheses (3) and (4) regarding the modulation of GABAA receptors by neurosteroids and reversion of the action of astrocytic transporters are those that require greater considerations.

Experimental studies with acute hepatic failure demonstrate that neurosteroids synthesized locally by microglial cells from cholesterol participate in the modulation of GABAA receptor activity. Such neuroactive steroids may have an inhibitory effect (e.g., pregnenolone), functioning as positive allosteric modulators of GABAA, or excitatory receptors (e.g., allopregnanolone and tetrahydrodeoxycorticosterone), functioning as negative allosteric modulators of GABAA. It is believed that under the influence of hyperammonemia, both have their synthesis increased, but it is difficult to understand what emerges from the elevation of these two classes of hormones, which have antagonic actions [26]. However, the current body of evidence supports the exploration of GABAA receptors as potential treatment targets (e.g., pregnenolone sulfate and bicuculline) in chronic hepatic encephalopathy [29].

On the other hand, some of the effects of GABA are terminated by the action of the GABA transporter (GAT), which acts reuptaking it at the presynaptic neuron terminal [37]. Although there is disagreement over the exact location of the four subtypes of GABA transporters (GAT1–4) in pre- and postsynaptic neurons and glial cells, it is clear that a key transporter in hepatic encephalopathy is GAT3 [38]. It is found on the surface of astrocytes and microglial cells, and its action can be reversed both in the presence of chronic hyperammonemia and/or glutamatergic hyperactivity, increasing the availability of GABA in the synaptic cleft and, consequently, the GABAergic tone [28].
