Zafar I. Bashir

Differential Roles of NR2A and NR2B-Containing NMDA Receptors in Cortical Long-Term Potentiation and Long-Term Depression

It is widely believed that long-term depression (LTD) and its counterpart, long-term potentiation (LTP), involve mechanisms that are crucial for learning and memory. However, LTD is difficult to induce in adult cortex for reasons that are not known. Here we show that LTD can be readily induced in adult cortex by the activation of NMDA receptors (NMDARs), after inhibition of glutamate uptake. Interestingly there is no need to activate synaptic NMDARs to induce this LTD, suggesting that LTD is triggered primarily by extrasynaptic NMDA receptors. We also find that de novo LTD requires the activation of NR2B-containing NMDAR, whereas LTP requires activation of NR2A-containing NMDARs. Surprisingly another form of LTD, depotentiation, requires activation of NR2A-containing NMDARs. Therefore, NMDARs with different synaptic locations and subunit compositions are involved in various forms of synaptic plasticity in adult cortex.

Long-term depression: multiple forms and implications for brain function

Long-term potentiation (LTP) and long-term depression (LTD) remain widely accepted vertebrate models for the cellular and molecular mechanisms that underlie synaptic changes during learning and memory. Although LTD is a phenomenon that occurs in many regions of the CNS, it is clear that the mechanisms recruited in its induction and expression can vary, depending on many factors, including brain region and developmental time point. LTD in the hippocampus and cerebellum is probably the best characterized, although there are also other brain areas where mechanisms of LTD are well understood, and where it is thought to have a functional role.

Long-term depression: a cascade of induction and expression mechanisms.

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

Differential roles of NR2A and NR2B-containing NMDA receptors in LTP and LTD in the CA1 region of two-week old rat hippocampus

The role of NMDA receptors in the induction of long-term potentiation (LTP) and long-term depression (LTD) is well established but which particular NR2 subunits are involved in these plasticity processes is still a matter of controversy. We have studied the effects of subtype selective NMDA receptor antagonists on LTP induced by high frequency stimulation (100 Hz for 1s) and LTD induced by low frequency stimulation (1 Hz for 15 min) in the CA1 region of hippocampal slices from 14 day old Wistar rats. Against recombinant receptors in HEK293 cells NVP-AAM077 (NVP) was approximately 14-fold selective for NR2A vs NR2B receptors, whilst Ro 25-6981 (Ro) was highly selective for NR2B receptors. On NMDA receptor-mediated EPSCs from Schaffer collaterals in CA1 neurones, NVP and Ro both reduced the amplitude but differentially affected the time constant of decay. The data are compatible with the selective effect of NVP (0.1 microM) and Ro (4 microM) on native NR2A and NBR2B receptors, respectively. NVP reduced both LTP and LTD whereas Ro reduced only LTP. Thus, LTP was reduced by 63% at 0.1 microM NVP and almost completely at 0.4 microM whereas 5 microM Ro reduced LTP by 45%. These data are consistent with a role for both NR2A and NR2B in the induction of LTP, under our experimental conditions. In comparison, LTD was unaffected by Ro (5 microM) even in the presence of a glutamate uptake inhibitor threo-beta-benzylaspartic acid (TBOA) to increase the concentration of glutamate at NR2B containing receptors. NVP (0.2-0.4 microM), however, produced a concentration dependent inhibition of LTD which was complete at 0.4 microM. The lack of effect of 0.1 microM NVP on LTD contrasts with its marked effect on LTP and raises the possibility that different NVP-sensitive NR2 subunit-containing NMDA receptors are required for LTP and LTD in this preparation.

Cholinergic neurotransmission is essential for perirhinal cortical plasticity and recognition memory.

We establish the importance of cholinergic neurotransmission to both recognition memory and plasticity within the perirhinal cortex of the temporal lobe. The muscarinic receptor antagonist scopolamine impaired the preferential exploration of novel over familiar objects, disrupted the normal reduced activation of perirhinal neurones to familiar compared to novel pictures, and blocked production of long-term depression (LTD) but not long-term potentiation (LTP) of synaptic transmission in perirhinal slices. The consistency of these effects across the behavioral, systems, and cellular levels of analysis provides strong evidence for the involvement of cholinergic mechanisms in synaptic plastic processes within perirhinal cortex that are necessary for recognition memory.

Different forms of LTD in the CA1 region of the hippocampus: role of age and stimulus protocol

In this study, we have investigated the developmental range over which different stimulus protocols induce long‐term depression (LTD). Low‐frequency stimulation (LFS; 900 stimuli, 1 Hz) produced LTD in hippocampal slices from rats younger than ∼ 40 days old, but not in animals aged between ∼ 40 days and 16 weeks. We demonstrate, however, that different stimulus protocols can result in LTD in the adult hippocampus. Whilst one paired‐pulse low‐frequency stimulus protocol [PP‐LFS; 50 ms paired‐pulse interval (PPI), 900 pairs of stimuli] produced N‐methyl‐ d‐aspartate (NMDA) receptor‐independent LTD, another PP‐LFS protocol (200 ms PPI; 900 pairs) produced NMDA receptor‐dependent LTD. Furthermore, the saturation of NMDA receptor‐dependent LTD did not prevent the induction of further NMDA receptor‐independent LTD. This lack of occlusion suggests that different mechanisms of expression may underlie each of the above forms of LTD in the adult hippocampus. In contrast to the adult hippocampus, NMDA receptor‐dependent LTD was induced by both LFS and PP‐LFS (50 ms PPI) in slices from young animals (12–20 days). Although they share a common induction mechanism, LTD induced by PP‐LFS may be expressed through other mechanisms in addition to those underlying LFS‐induced LTD in the young hippocampus. In conclusion, the results in this study demonstrate that mechanisms of long‐term synaptic depression within the hippocampus can alter radically with development of the central nervous system and with the use of different induction protocols.

An experimental test of the role of postsynaptic calcium levels in determining synaptic strength using perirhinal cortex of rat

1 We have investigated the prediction of a relationship between the magnitude of activity‐dependent increases in postsynaptic calcium and both the magnitude and direction of synaptic plastic change in the central nervous system. Activity‐dependent increases in calcium were buffered to differing degrees using a range of concentrations of EGTA and the effects on synaptic plasticity were assessed. 2 Activity‐dependent synaptic plasticity was induced during whole‐cell recording in rat perirhinal cortex in vitro. In control conditions (0.5 mm EGTA) low frequency stimulation (LFS; 200 stimuli) delivered to neurones held at ‐40 or ‐70 mV induced long‐term depression (LTD) or, at ‐10 mV, induced long‐term potentiation (LTP). 3 The relationship between EGTA concentration (0.2 to 10 mm) and the magnitude of LTD was examined. This relationship described a U‐shaped curve, as predicted by models of synaptic plasticity. This provides strong evidence that the magnitude of LTD is determined by the magnitude of the increase in intracellular calcium concentration. 4 LFS paired with depolarisation to ‐10 mV induced LTD, no change or LTP as activity‐dependent postsynaptic calcium levels were allowed to increase progressively by the use of progressively lower concentrations of buffer (10 to 0.2 mm EGTA). 5 We investigated if the lack of plasticity that occurs at the transition between LTD and LTP is due to induction of both of these processes with zero net change, or is due to neither LTD nor LTP being induced. These experiments were possible as LTP but not LTD was blocked by the protein kinase inhibitor staurosporine while LTD but not LTP was blocked by the mGlu receptor antagonist MCPG. At the transition between LTD and LTP, blocking LTP mechanisms did not uncover LTD whilst blocking LTD mechanisms did not uncover LTP. This suggests that the transition between LTD and LTP is due to the lack of induction of both of these processes and also suggests that these two processes are induced independently of one another.

Tyrosine Phosphatases Regulate AMPA Receptor Trafficking during Metabotropic Glutamate Receptor-Mediated Long-Term Depression

Two forms of long-term depression (LTD), triggered by activation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), respectively, can be induced at CA1 synapses in the hippocampus. Compared with NMDAR-LTD, relatively little is known about mGluR-LTD. Here, we show that protein tyrosine phosphatase (PTP) inhibitors, orthovanadate and phenylarsine oxide, selectively block mGluR-LTD induced by application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG-LTD), because NMDAR-LTD is unaffected by these inhibitors. Furthermore, DHPG-LTD measured using whole-cell recording is similarly blocked by either bath-applied or patch-loaded PTP inhibitors. These inhibitors also block the changes in paired-pulse facilitation and coefficient of variation that are associated with the expression of DHPG-LTD. DHPG treatment of hippocampal slices was associated with a decrease in the level of tyrosine phosphorylation of GluR2 AMPA receptor (AMPAR) subunits, an effect blocked by orthovanadate. Finally, in dissociated hippocampal neurons, orthovanadate blocked the ability of DHPG to reduce the number of AMPA receptor clusters on the surface of dendrites. Again, the effects of PTP blockade were selective, because NMDA-induced decreases in surface AMPAR clusters was unaffected by orthovanadate. Together, these data suggest that activation of postsynaptic PTP results in tyrosine dephosphorylation of AMPARs and their removal from the synapse.

A new form of long-term depression in the perirhinal cortex

We demonstrate a form of long-term depression (LTD) in the perirhinal cortex that relies on interaction between different glutamate receptors. Group II metabotropic glutamate (mGlu) receptors facilitated group I mGlu receptor-mediated increases in intracellular calcium. This facilitation plus NMDA receptor activation may be necessary for induction of LTD at resting membrane potentials. However, depolarization enhanced NMDA receptor function and removed the requirement of synergy between group I and group II mGlu receptors: under these conditions, activation of only NMDA and group I mGlu receptors was required for LTD. Such glutamate receptor interactions potentially provide new rules for synaptic plasticity. These forms of LTD occur in the perirhinal cortex, where long-term decreases in neuronal responsiveness may mediate recognition memory.

Induction of LTD in the adult hippocampus by the synaptic activation of AMPA/kainate and metabotropic glutamate receptors

It has been suggested that the induction of long-term depression (LTD) may be developmentally regulated since LTD can be readily induced by LFS in slices from young but not adult animals. However, we have recently reported that paired pulse low frequency stimulation (PP-LFS) can reliably induce LTD in the CA1 region of adult hippocampal slices. We now describe the role of glutamate receptors in the induction of LTD in adult hippocampus. The induction of LTD was prevented by the combined application of AMPA/kainate and metabotropic glutamate (mGlu) receptor antagonists (CNQX and LY341495). However, LTD was not blocked by the co-application of NMDA and mGlu receptor antagonists nor by the co-application of NMDA and AMPA/kainate receptor antagonists. Taken together, the above results suggest that activation of either AMPA/kainate or mGlu receptors is sufficient to induce LTD. Therefore, these results suggest that PP-LFS can efficiently activate AMPA/kainate and mGlu receptors in order to induce long-lasting synaptic depression in the CA1 region of the adult hippocampus in vitro.