Induction
LTP and LTD can be induced in vitro by different protocols that rely on presynaptic stimulation alone or combined stimulations of the pre and postsynaptic partners. All protocols lead to the depolarization of the postsynaptic cell that is necessary for the activation of NMDARs and to the subsequent increase in calcium concentration within the dendritic spine. Presynaptic stimulation alone is performed with a tetanus38 or a theta-burst39 that certainly drive LTP through the sustained NMDARs activation. Conversely, the stimulation of presynaptic afferents at low frequency40 triggers LTD probably because of the weak activation of NMDARs. Synchronous stimulations of pre and postsynaptic partners trough a pairing protocol41 or a spike timing-dependent induction42 trigger LTP, while asynchronous stimulations with an unpaired protocol43 or modified spike timing-dependent induction44 trigger LTD. The mechanism that determines the polarity of changes toward LTP or LTD is rather uncertain but should rely again on the differential activation of NMDARs. As Hebb postulated, coincident activity in pre and postsynaptic cells when experimental stimuli are
Introduction! Plasticity conditions
36 Acute slices
37 Slice cultures
38 100 Hz for 1 sec
39 5 bursts made of 4 pulses at 100 Hz during 1 sec
40 1 Hz for 10 min
41 Presynaptic stimulations paired with postsynaptic depolarizations that make the postsynaptic cell to fire
42 Presynaptic stimulation just before induction of a backpropagating AP in the postsynaptic cell
43 Presynaptic stimulation out of phase with postsynaptic depolarization
44 Postsynaptic backpropagating AP precedes the presynaptic stimulus
paired conduct to a higher NMDARs activation than when they are desynchronized (Cavazzini et al., 2005).
Expression
LTP and LTD at CA1 synapses depend on calcium increase induced by NMDARs activation.
Injection of calcium chelators in CA1 neurons blocks LTP or LTD induction (Lynch et al., 1983) (Mulkey and Malenka, 1992) while the photo-release of caged calcium into these cells induces LTP or LTD (Malenka et al., 1988) (Yang et al., 1999). Calcium influx into spines is principally
mediated by NMDARs that are twenty times more permeable to calcium than AMPARs (Garaschuk et al., 1996). Use of D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), a selective blocker of NMDARs prevents LTP or LTD induction (Harris et al., 1984) (Dudek and Bear, 1992). Counter-intuitively, NMDAR activity is also subject to negative feedback by calcium entry. The mechanism could lay on NR1 subunit modifications by calcium-calmodulin that reduces NMDARs open’s probability (Ehlers et al., 1996). Thus, calcium acts also as regulator of its own synaptic entry. SER calcium stores are necessary for synaptic plasticity (Harvey and Collingridge, 1992). They are present in half of the general spine population and up to 80% of mature mushroom spines (Harris et al., 1992). Calcium can be released from the SER by activation of ryanodine receptor (RyR) in hippocampal pyramidal neurons. Activation of RyRs is complex and unclear but shows to be
sensitive to increase in cytosolic calcium as well as other endogenous cofactors like cAMP (Fill and Copello, 2002).
Given that LTP and LTD both depend on calcium buffering by calmodulin, a distinction must be done between these two conditions to determine the polarity of changes. Low levels of calcium first activate phosphatases that drive LTD because they have a higher affinity for calcium than kinases that drive LTP (Cormier et al., 2001). Moreover, a long or short increase in calcium concentration tends to promote LTD or LTP respectively (Mizuno et al., 2001).
In summary, it seems that the kinetic and level of calcium increase dictate the polarity of changes trough a competition between kinases and phosphatases. Fast and high increase leads to LTP while small and prolonged signal triggers LTD (Ismailov et al., 2004). Additionally, the subunit composition of NMDARs that is regulated by activity45 (Barria and Malinow, 2002) is likely to control also the propensity for LTP or LTD as different subunits confer distinct biophysical channel properties (Monyer et al., 1992) and can recruit different intracellular signaling molecules (Leonard et al., 1999).
Introduction! Plasticity conditions
45 Metaplasticity
When LTP is induced, there is a peak increase of calcium within milliseconds, principally through NMDARs, that reaches 50 to 100 nM above the baseline of 100 nM and that decays over 20 to 200 ms. Calcium can also enter the spine through voltage-gated calcium channels (VGCCs) that show high amplitude and rapid kinetic. VGCCs can be activated by backpropagating action potentials (bAP) that invade dendritic spines (Stuart and Sakmann, 1994). Presumably, when bAP are paired with synaptic events, the augmented depolarization enhance the relieve of the magnesium blockade of NMDARs and this property is used by spike timing-dependent protocol for LTP induction.
Although VGCCs are considered as contributing little to NMDARs-dependent LTP, a form of VGCC-dependent LTP can be obtained by a strong neuronal stimulation in the presence of the NMDARs blocker AP5 (Grover and Teyler, 1990).
Intracellular calcium binds to Calmodulin that activates adenylate cyclase46 and CaMKII that becomes then autonomously active for a long time via autophosphorylation (Giese et al., 1998).
Inhibition of CaMKII blocks LTP induction but not maintenance, confirming its key function in LTP initiation. (Otmakhov et al., 1997). The only exception is during early postnatal development, when early expression of LTP doesn’t rely on CaMKII but on the activation of PKA (Yasuda et al., 2003). Other kinases should play a role even if their function is still uncertain: PKC, mitogen-activated protein kinase (MAPK) cascade, phosphatidyl-inositol-3-phosphate (PI3) kinase and Src kinase. Finally, kinases activation induces translocation of supplementary AMPARs to the synapse jointly to the phosphorylation of GluR1 that enhances AMPARs single channel conductance.
Phosphorylation of ser-831 on the C-tail of GluR1 is performed by CaMKII and PKC without any change in ser-84547 (Lee et al., 2000). Together these events increase the amplitude of EPSPs (Malenka and Nicoll, 1999).
For LTD, calcium released from ER is important in addition to calcium entering through NMDARs, during suboptimal stimulation protocols (Nakano et al., 2004). LTD expression depends on calcium activation of phosphatases PP1 and Calcineurin48 (Kirkwood and Bear, 1994) and is correlated with dephosphorylation of PKA substrates (Kameyama et al., 1998) but not PKC and CaMKII substrates (Lee et al., 1998). Dephosphorylation of ser-845 on the C-tail of GluR1 is probably performed by PP1/Calcineurin and decreases AMPAR channels open probability (Banke et al., 2000). In parallel, a physical loss of AMPARs is induced via dynamin and clathrin-dependent mechanisms initiated by the recruitment of AP2 on GluR2 (Carroll et al., 1999). Further, NMDARs transmission is also depressed during LTD but postsynaptic mechanisms that differ from AMPARs depression are still largely unknown (Morishita et al., 2005).
Maintenance
Like all long-lasting cell biological phenomena, LTP and LTD require new protein synthesis and gene transcription for their maintenance even if the time since when they are required is difficult to precise (Fonseca et al., 2006a). Signaling molecules that could link activity at the synapse to transcription in the nucleus could be PKA, CaMKIV and MAPK that activate the transcription factor cAMP response element-binding (CREB). Activated synapses seem to be tagged to allow translocation of newly synthesized AMPARs into the right place (Matsuo et al., 2008). Translation
Introduction! Plasticity conditions
46 cAMP production
47 PKA substrate
48 Also known as PP2B
of mRNA can also occur locally within dendrites or even spines but nothing is clear about the identity of the newly synthesized proteins absolutely required to maintain LTP. Additionally, protein degradation by the ubiquitin/proteasome pathway is also a crucial modulator of the glutamatergic synapse and therefore necessary for the long-term maintenance of LTD and LTP (Colledge et al., 2003) (Fonseca et al., 2006b).
In summary, functional modifications that occur at the synapse reinforce or depress transmission strength when activity at both synaptic sides is congruent or not. Activation of NMDARs leads to a postsynaptic rise in calcium that activates kinases for LTP or phosphatases for LTD. These mediator proteins affect differentially AMPARs expression at the synapse, reinforcing or diminishing the postsynaptic sensitivity for presynaptically released glutamate in the case of LTP or LTD respectively.