Andrew J Doherty

(RS)-2-Chloro-5-Hydroxyphenylglycine (CHPG) Activates mGlu5, but not mGlu1, Receptors Expressed in CHO Cells and Potentiates NMDA Responses in the Hippocampus

A new phenylglycine derivative, (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), has been synthesized and shown to selectively activate mGlu5a receptors, compared to mGlu1 alpha receptors, when expressed in CHO cells. This selective mGlu5 receptor agonist also potentiates NMDA-induced depolarizations in rat hippocampal slices. CHPG may be a useful tool for studying the role of mGlu5 receptors in the central nervous system.

The role of GSK-3 in synaptic plasticity.

Glycogen synthase kinase‐3 (GSK‐3), an important component of the glycogen metabolism pathway, is highly expressed in the CNS. It has been implicated in major neurological disorders including Alzheimers disease, schizophrenia and bipolar disorders. Despite its central role in these conditions it was not known until recently whether GSK‐3 has neuronal‐specific functions under normal conditions. However recent work has shown that GSK‐3 is involved in the regulation of, and cross‐talk between, two major forms of synaptic plasticity, N‐methyl‐D‐aspartate receptor (NMDAR)‐dependent long‐term potentiation (LTP) and NMDAR‐dependent long‐term depression (LTD). The present article summarizes this recent work and discusses its potential relevance to the treatment of neurological disorders.

The potent mGlu receptor antagonist LY341495 identifies roles for both cloned and novel mGlu receptors in hippocampal synaptic plasticity

Understanding the roles of metabotropic glutamate (mGlu) receptors has been severely hampered by the lack of potent antagonists. LY341495 (2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-y l)propanoic acid) has been shown to block group II mGlu receptors in low nanomolar concentrations (Kingston, A.E., Ornstein, P.L., Wright, R.A., Johnson, B.G., Mayne, N.G., Burnett, J.P., Belagaje, R., Wu, S., Schoepp, D.D., 1998. LY341495 is a nanomolar potent and selective antagonist at group II metabotropic glutamate receptors. Neuropharmacology 37, 1-12) but can be used in higher concentrations to block all hippocampal mGlu receptors, identified so far by molecular cloning (mGlu1-5,7,8). Here we have further characterised the mGlu receptor antagonist activity of LY341495 and have used this compound to investigate roles of mGlu receptors in hippocampal long-term potentiation (LTP) and long-term depression (LTD). LY341495 competitively antagonised DHPG-stimulated PI hydrolysis in AV12-664 cells expressing either human mGlu1 or mGlu5 receptors with Ki-values of 7.0 and 7.6 microM, respectively. When tested against 10 microM L-glutamate-stimulated Ca2+ mobilisation in rat mGlu5 expressing CHO cells, it produced substantial or complete block at a concentration of 100 microM. In rat hippocampal slices, LY341495 eliminated 30 microM DHPG-stimulated PI hydrolysis and 100 microM (1S,3R)-ACPD-inhibition of forskolin-stimulated cAMP formation at concentrations of 100 and 0.03 microM, respectively. In area CA1, it antagonised DHPG-mediated potentiation of NMDA-induced depolarisations and DHPG-induced long-lasting depression of AMPA receptor-mediated synaptic transmission. LY341495 also blocked NMDA receptor-independent depotentiation and setting of a molecular switch involved in the induction of LTP; effects which have previously been shown to be blocked by the mGlu receptor antagonist (S)-MCPG. These effects may therefore be due to activation of cloned mGlu receptors. In contrast, LY341495 did not affect NMDA receptor-dependent homosynaptic LTD; an effect which may therefore be independent of cloned mGlu receptors. Finally, LY341495 failed to antagonise NMDA receptor-dependent LTP and, in area CA3, NMDA receptor-independent, mossy fibre LTP. Since in the same inputs these forms of LTP were blocked by (S)-MCPG, a novel type of mGlu receptor may be involved in their induction.

The role of JAK-STAT signaling within the CNS

JAK-STAT is an efficient and highly regulated system mainly dedicated to the regulation of gene expression. Primarily identified as functioning in hematopoietic cells, its role has been found critical in all cell types, including neurons. This review will focus on JAK-STAT functions in the mature central nervous system. Our recent research suggests the intriguing possibility of a non-nuclear role of STAT3 during synaptic plasticity. Dysregulation of the JAK-STAT pathway in inflammation, cancer and neurodegenerative diseases positions it at the heart of most brain disorders, highlighting the importance to understand how it can influence the fate and functions of brain cells.

A novel, competitive mGlu5 receptor antagonist (LY344545) blocks DHPG-induced potentiation of NMDA responses but not the induction of LTP in rat hippocampal slices

We have investigated the pharmacological properties of LY344545, a structurally related epimer of the broad spectrum competitive metabotropic glutamate receptor antagonist, LY341495. We have found that LY344545 also antagonizes competitively nearly all mGlu receptor subtypes, but with a wide spectrum of activity. The order of potency for the human receptor isoforms was mGlu5a (IC50 of 5.5±0.6 μM)>mGlu2=mGlu3>mGlu1α=mGlu7>mGlu6=mGlu8. No significant mGlu4 receptor antagonist activity was detected at the highest concentration used (100 μM). 100 μM LY344545 displaced 50±5% of [3H]‐CGP39653 binding, but less than 30% of [3H]‐kainate or [3H]‐AMPA in radioligand binding assays. LY344545 antagonized L‐glutamate stimulated Ca2+ release in CHO cells transfected with mGlu receptors in a concentration dependent manner with a 10 fold higher affinity for the rat mGlu5a receptor (Ki=2.1±0.6 μM) compared to the rat mGlu1α receptor (Ki=20.5±2.1 μM). 50 μM (1S, 3R)‐ACPD‐induced Ca2+ rises in hippocampal CA1 neurones were also antagonized (IC50=6.8±0.7 μM). LY344545 antagonized 10 μM (S)‐3,5‐DHPG‐induced potentiation of NMDA depolarizations in CA1 neurones (EC50=10.6±1.0 μM). At higher concentrations (100 μM), LY344545 was an NMDA receptor antagonist. LY344545 also blocked the induction, but not the expression, of LTP at CA3 to CA1 synapses with an IC50>300 μM. This effect is consistent with its weak activity at NMDA receptors. These results demonstrate that the binding of ligands to mGlu receptor subtypes is critically dependent on the spatial orientation of the same molecular substituents within a given chemical pharmacophore. The identification of LY344545 as the first competitive antagonist to show selectivity towards mGlu5 receptors supports the potential to design more selective and potent competitive antagonists of this receptor. These results further indicate that mGlu receptor‐mediated potentiation of NMDA responses is not essential for the induction of LTP.

Neuronal calcium sensors and synaptic plasticity

Calcium entry plays a major role in the induction of several forms of synaptic plasticity in different areas of the central nervous system. The spatiotemporal aspects of these calcium signals can determine the type of synaptic plasticity induced, e.g. LTP (long-term potentiation) or LTD (long-term depression). A vast amount of research has been conducted to identify the molecular and cellular signalling pathways underlying LTP and LTD, but many components remain to be identified. Calcium sensor proteins are thought to play an essential role in regulating the initial part of synaptic plasticity signalling pathways. However, there is still a significant gap in knowledge, and it is only recently that evidence for the importance of members of the NCS (neuronal calcium sensor) protein family has started to emerge. The present minireview aims to bring together evidence supporting a role for NCS proteins in plasticity, focusing on emerging roles of NCS-1 and hippocalcin.

ACET is a highly potent and specific kainate receptor antagonist: Characterisation and effects on hippocampal mossy fibre function

Kainate receptors (KARs) are involved in both NMDA receptor-independent long-term potentiation (LTP) and synaptic facilitation at mossy fibre synapses in the CA3 region of the hippocampus. However, the identity of the KAR subtypes involved remains controversial. Here we used a highly potent and selective GluK1 (formerly GluR5) antagonist (ACET) to elucidate roles of GluK1-containing KARs in these synaptic processes. We confirmed that ACET is an extremely potent GluK1 antagonist, with a Kb value of 1.4+/-0.2 nM. In contrast, ACET was ineffective at GluK2 (formerly GluR6) receptors at all concentrations tested (up to 100 microM) and had no effect at GluK3 (formerly GluR7) when tested at 1 microM. The X-ray crystal structure of ACET bound to the ligand binding core of GluK1 was similar to the UBP310-GluK1 complex. In the CA1 region of hippocampal slices, ACET was effective at blocking the depression of both fEPSPs and monosynaptically evoked GABAergic transmission induced by ATPA, a GluK1 selective agonist. In the CA3 region of the hippocampus, ACET blocked the induction of NMDA receptor-independent mossy fibre LTP. To directly investigate the role of pre-synaptic GluK1-containing KARs we combined patch-clamp electrophysiology and 2-photon microscopy to image Ca2+ dynamics in individual giant mossy fibre boutons. ACET consistently reduced short-term facilitation of pre-synaptic calcium transients induced by 5 action potentials evoked at 20-25Hz. Taken together our data provide further evidence for a physiological role of GluK1-containing KARs in synaptic facilitation and LTP induction at mossy fibre-CA3 synapses.

Antagonist activity of α-substituted 4-carboxyphenylglycine analogues at group I metabotropic glutamate receptors expressed in CHO cells

We have investigated the antagonist properties of 6 α‐substituted phenylglycine analogues based on the structure of 4‐carboxyphenylglycine (4‐CPG) for group I metabotropic glutamate receptors (mGlu1α and mGlu5a) permanently expressed in CHO cells. (S)‐4‐CPG and (S)‐MCPG were the most selective mGlu1α receptor antagonists. Longer chain α‐carbon substitutions resulted in a progressive loss of antagonist affinity at mGlu1α receptors but not at mGlu5a receptors. Thus mGlu1α receptor antagonists require small aliphatic groups at the α‐position. α‐cyclopropyl‐4‐CPG showed a tendency towards mGlu5a selectivity, suggesting that bulky groups at this position may favour mGlu5a receptor antagonism. We demonstrate that the mGlu5a receptor displays agonist‐dependent antagonism. L‐glutamate‐induced Ca2+ release in mGlu5a receptor expressing cells was more susceptible to antagonism by cyclic α‐carbon derivatives than (S)‐3,5‐dihydroxyphenylglycine (DHPG)‐induced Ca2+ release in the same cell line. The data presented suggests that mGlu1α and mGlu5a receptors have different steric and/or conformational requirements for the binding of antagonists and different amino acids which could interact with agonists. These phenylglycine analogues could provide leads for the development of subtype selective antagonists.

Promiscuous Interactions between AMPA-Rs and MAGUKs

What controls the number of AMPA receptors at excitatory synapses? MAGUKs are known to play a critical role in this process, but which ones are involved and when has been contentious. In this issue of Neuron, Elias et al. have elucidated the roles of three MAGUKs, PSD-95, PSD-93, and SAP-102, in the targeting of AMPA receptors to synapses in hippocampal neurons.

The use of the hippocampal slice preparation in the study of Alzheimer's disease

In the present article we show how studying synaptic mechanisms in hippocampal slice preparations provides information that may be useful in, firstly, the understanding of the aetiology of Alzheimers disease and, secondly, in the development of novel therapies for dementia. We use several examples, drawn from our own work: (i) The identification of the function of AMPA receptors and NMDA receptors in synaptic transmission and synaptic plasticity. (ii) The discovery of mechanisms that can regulate the activation of NMDA receptors. (iii) The use of transgenic models of Alzheimers disease. (iv) The identification of a mechanism that can account for the cognitive enhancing effects of the NMDA receptor antagonist memantine. (v) The discovery of a role of glycogen synthase kinase-3beta (tau kinase) in synaptic plasticity.