**3.5 LTP in other regions of CNS**

Although LTP was first described at the perforant path synapses on the neurons of the DG, subsequently most of the work on the mechanism of LTP is performed on the Schaffer collateral synapses on the CA1 pyramidal neurons.

In the CA1 region, NMDAR-mediated and NMDAR-independent LTP have been described and they are expressed mainly as postsynaptic mechanisms. Presynaptic LTP was also discovered in hippocampus and cerebellum. In hippocampus, presynaptic LTP can be observed in mossy fiber pathway. There is no need for calcium influx in the postsynaptic compartment for eliciting this form of LTP and this is NMDAR-independent. In the induction of presynaptic LTP, presynaptic calcium release is essential. R-type calcium channels are voltage dependent calcium channels that can mediate presynaptic calcium release. This calcium influx can activate several signaling pathways needed for the induction of mossy fiber LTP (MF-LTP). Both pharmacological and genetic analyses indicate that a rise in presynaptic cAMP is a crucial component. The cAMP level is enhanced by the activation of Ca2+/CaM activated adenyl cyclase 1 (AC1) and leads to the activation of PKA. This PKA activation regulates key molecules needed for the enhanced neurotransmitter release (Nicoll, 2005).

### **3.5.1 Cerebellar LTP**

In cerebellum, parallel fibres (PF) of cerebellar granule cells form synapse with Purkinje cells (PC) of Purkinje cell layer (Fig. 5). When the PF is stimulated at 4-8 Hz for 15 s, a presynaptic form of LTP is induced at PF. This shows a similar molecular mechanism as that of the mossy fiber pathway in the hippocampus. A postsynaptically expressed form of LTP was more recently described and is observed as a reversal of PF-LTD. PF–LTP can be induced by repetitive PF stimulation (1 Hz for 5 minutes) without concomitant CF activation and requires a lower calcium transient for its induction than PF-LTD (Vogt & Canepari, 2010). PF-LTP generally depends on the activation of phosphatases such as PP1, PP2A, and PP2B and is independent of activity of kinases such as CaMKII and PKC (Jorntell & Hansel, 2006). In cerebellar PCs, GluR1 expression is weak, and the majority of AMPA receptors consist of

were applied to chemically induce LTP (Chem-LTP). Chem-LTP is an alternative to high frequency stimulation and has the advantage that it can activate all the cells in the culture.

Forskolin/rolipram-induced LTP was predominantly used in slice cultures; it can also be applied for dissociated hippocampal neuronal cultures. This form of chemically induced, highly sensitive plasticity state is based on the increase of intracellular cAMP levels by the application of the adenylyl cyclase activator forskolin (50 µM) and the phosphodiesterase inhibitor rolipram (0.1 µM) in Mg2+ and 2-Cl-adenosine free artificial cerebrospinal fluid for 16 min (Otmakhov, 2004). This induction procedure is bypassing the need for synaptic activation, and by raising cAMP concentration directly activates PKA and signaling pathways that underlie synaptic plasticity. However, froskolin/rolipram-LTP still require NMDAR activation

Although LTP was first described at the perforant path synapses on the neurons of the DG, subsequently most of the work on the mechanism of LTP is performed on the Schaffer

In the CA1 region, NMDAR-mediated and NMDAR-independent LTP have been described and they are expressed mainly as postsynaptic mechanisms. Presynaptic LTP was also discovered in hippocampus and cerebellum. In hippocampus, presynaptic LTP can be observed in mossy fiber pathway. There is no need for calcium influx in the postsynaptic compartment for eliciting this form of LTP and this is NMDAR-independent. In the induction of presynaptic LTP, presynaptic calcium release is essential. R-type calcium channels are voltage dependent calcium channels that can mediate presynaptic calcium release. This calcium influx can activate several signaling pathways needed for the induction of mossy fiber LTP (MF-LTP). Both pharmacological and genetic analyses indicate that a rise in presynaptic cAMP is a crucial component. The cAMP level is enhanced by the activation of Ca2+/CaM activated adenyl cyclase 1 (AC1) and leads to the activation of PKA. This PKA activation regulates key molecules needed for the enhanced neurotransmitter release

In cerebellum, parallel fibres (PF) of cerebellar granule cells form synapse with Purkinje cells (PC) of Purkinje cell layer (Fig. 5). When the PF is stimulated at 4-8 Hz for 15 s, a presynaptic form of LTP is induced at PF. This shows a similar molecular mechanism as that of the mossy fiber pathway in the hippocampus. A postsynaptically expressed form of LTP was more recently described and is observed as a reversal of PF-LTD. PF–LTP can be induced by repetitive PF stimulation (1 Hz for 5 minutes) without concomitant CF activation and requires a lower calcium transient for its induction than PF-LTD (Vogt & Canepari, 2010). PF-LTP generally depends on the activation of phosphatases such as PP1, PP2A, and PP2B and is independent of activity of kinases such as CaMKII and PKC (Jorntell & Hansel, 2006). In cerebellar PCs, GluR1 expression is weak, and the majority of AMPA receptors consist of

and involve the recruitment of CaMKII to dendritic spines (Molnar, 2011).

One example of Chem-LTP is mentioned below.

**3.4.3.1 Forskolin/rolipram-induced LTP** 

**3.5 LTP in other regions of CNS** 

(Nicoll, 2005).

**3.5.1 Cerebellar LTP** 

collateral synapses on the CA1 pyramidal neurons.

GluR2–GluR3 heteromeric complexes. An activity dependent synaptic delivery of GluR2 has been shown during the induction of LTP in PF-PC synapses. This activity driven process involves NO-mediated binding of *N*-ethylmaleimide sensitive factor (NSF) to GluR2. In PF LTP, GluR2 synaptic delivery is also facilitated by dephosphorylation of GluR2 at Ser880.

Fig. 5. Cellular anatomy of the cerebellum. Adapted from Ramnani, 2006

### **3.5.2 LTP in spinal cord**

Spinal LTP has been demonstrated in different areas of the spinal cord. The ventral and the superficial dorsal horn, Wide Dynamic Range (WDR) neurons and superficial neurons in the spinal cord that project to the parabrachial area in the brain stem are some of the sites where LTP has been demonstrated. It has been suggested that the generation of LTP in spinal cord may be one mechanism, whereby acute pain may be transformed into a chronic pain state. LTP in superficial spinal dorsal horn involves simultaneous activation of multiple receptors like the NMDAR, the Neurokinin 1 (NK-1) receptor for substance P and mGluRs. This LTP is likely to occur in both the sensory and the affective pain pathways. LTP in deep spinal WDR neurons have a pivotal role in transmission of painful inputs. As with LTP in the superficial spinal cord, activation of the ionotrophic glutamate receptors (AMPA and NMDA subtypes) and the NK1 receptor seems crucial for the induction of LTP in deep WDR neurons (Rygh et al, 2005).

Molecular Mechanisms in Synaptic Plasticity 311

Therefore instead of kinases, phosphatases get activated as they require comparatively lower Ca2+ concentrations for activation. The protein phosphatase activated is PP2B or calcineurin. PP2B can in turn activate PP1. PP2B dephosphorylates and inactivates Inhibitor-1. This relieves

In hippocampus, plasticity is mediated by conductance changes of AMPARs which are in turn regulated by phosphorylation. The majority of AMPARs at hippocampal synapses are GluR1/GluR2 and GluR2/GluR3 heteromers. The trafficking of GluR1 plays a dominant role in plasticity. Activated PP1 brings about dephosphorylation of GluR1 at Ser845 (Fig. 6) and promotes AMPAR internalization (Lee et al., 2000). Inhibition of PP2B blocks GluR1 internalization and thereby LTD, suggesting the importance of phosphatase activity in LTD (Beattie et al., 2000). Targeting PP1 precisely to synapses upon NMDAR activation is crucial for LTD expression and is facilitated by PP1 binding proteins like spinophilin, neurabin, etc. (Morshita et al., 2001). In hippocampus, AMPARs are stabilized on the membrane by NSF and clathrin adaptor protein AP2, which bind to the NSF binding site on GluR2. During NMDAR-LTD, AP2 replaces NSF and this initiates AMPAR endocytosis. Clathrin mediated endocytosis of AMPARs is triggered by a neuronal calcium sensor known as hippocalcin. Upon activation, hippocalcin translocates to the plasma membrane, where it forms a complex with AP2 and GluR2 and initiates clathrin mediated AMPAR endocytosis. Protein interacting with C-kinase 1 (PICK1) is another protein that binds directly to GluR2 and it can also bind to PKC. PICK1 competes with AMPAR binding protein (ABP) and glutamate receptor interacting protein (GRIP) for binding to C-terminal of GluR2 and promotes internalization. PICK1 also helps in modifying neuronal architecture by interacting with F-actin. PP2B interacts with A-kinase anchor protein-150 (AKAP-150) which in turn interacts with PSD-95. PSD-95 further interacts with NMDAR thereby positioning PP2B near NMDAR (Bhattacharya et al., 2009). This helps in the activation of PP2B by Ca2+ influx through NMDARs. Activated PP2B can mediate the NMDAR-induced endocytosis of AMPARs that underlies one major form of LTD. Disruption of the interaction between PSD-95 and AKAP-150 strongly inhibited NMDAR-dependent endocytosis of AMPARs (Bhattacharya et al., 2009). Phosphorylation of Ser295 of PSD-95 occurs *in vivo*, and it enhances the ability of PSD-95 to accumulate in the PSD, to recruit surface AMPA receptors, and to strengthen synaptic transmission. During LTD, PSD-95 is dephosphorylated at Ser295 facilitating its removal from PSD. This mechanism also plays a role

the inhibition of PP1 by Inhibitor-1 thereby activating it (Mulkey et al., 1993).

in the NMDAR-dependent endocytosis of AMPAR (Kim et al., 2007).

resulting in depression of synaptic transmission.

Although a major form of LTD is mediated by NMDARs, the ultimate direction of change in synaptic efficacy is brought about by changes in AMPAR function (Collingridge et al., 2010). Calcium influx through the NMDAR is central to the induction of both LTP and LTD because intracellular application of calcium chelators, such as BAPTA or EGTA, prevents induction of plasticity. Since induction of LTP and LTD are controlled by the postsynaptic NMDAR, any presynaptic component of expression requires a retrograde messenger that can signal to the presynaptic terminal that coincidence has occurred. Two candidates are nitric oxide (NO) and endocannabinoids (eCB) (Bliss & Cook, 2011). *N*arachidonylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) are two major eCBs that activate type I cannabinoid receptors (CB1) receptors on the presynaptic neuron in the brain (Di Marzo et al., 1998). Upon stimulation, eCBs are released from postsynaptic neurons and travel across the synaptic cleft to activate CB1 on presynaptic terminals,
