Characterisation of the effects of ATPA, a GLU(K5) kainate receptor agonist, on GABAergic synaptic transmission in the CA1 region of rat hippocampal slices.

Kainate receptors are implicated in a variety of physiological and pathological processes in the CNS. Previously we demonstrated that (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA), a selective agonist for the GLU(K5) subtype of kainate receptor, depresses monosynaptically evoked inhibitory postsynaptic potentials (IPSPs) in the CA1 region of the rat hippocampus. In the current study, we provide a more detailed characterisation of this effect. Firstly, our data demonstrate a rank order of potency of domoate>kainate>ATPA>alpha-amino-3-(3-hydroxy-5-methyl-4-isoxalolyl)propionic acid Secondly, we confirm that the effects of ATPA are not mediated indirectly via the activation of gamma-aminobutyric acid receptors (i.e. either GABA(A) or GABA(B)). Thirdly, we show that the small increase in conductance induced by ATPA is insufficient to account for the depression of monosynaptic inhibition. Fourthly, we show that the effects of ATPA on IPSPs are antagonised by the GLU(K5)-selective antagonist (3S, 4aR, 6S, 8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884). However, LY382884 is less potent as an antagonist of the effects of ATPA on IPSPs compared to its depressant effect on EPSPs.

Characterisation of the effects of ATPA, a GLU(K5) receptor selective agonist, on excitatory synaptic transmission in area CA1 of rat hippocampal slices.

Kainate receptors are involved in a variety of synaptic functions in the CNS including the regulation of excitatory synaptic transmission. Previously we described the depressant action of the GLU(K5) selective agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA) on synaptic transmission in the Schaffer collateral-commissural pathway of rat hippocampal slices. In the present study we report several new features of the actions of ATPA at this synapse. Firstly, the effectiveness of ATPA is developmentally regulated. Secondly, the effects of ATPA decline during prolonged or repeated applications. Thirdly, the effects of ATPA are not mediated indirectly via activation of GABA(A), GABA(B), muscarinic or adenosine A(1) receptors. Fourthly, elevating extracellular Ca(2+) from 2 to 4 mM antagonises the effects of ATPA. Some differences between the actions of ATPA and kainate on synaptic transmission in the Schaffer collateral-commissural pathway are also noted.

Synaptic activation of a presynaptic kainate receptor facilitates AMPA receptor-mediated synaptic transmission at hippocampal mossy fibre synapses.

The development of GluR5-selective kainate receptor ligands is helping to elucidate the functions of kainate receptors in the CNS. Here we have further characterised the actions of a GluR5 selective agonist, ATPA, and a GluR5 selective antagonist, LY382884, in the CA3 region of rat hippocampal slices. In addition, we have used LY382884 to study a novel synaptic mechanism. This antagonist substantially reduces frequency facilitation of mossy fibre synaptic transmission, monitored as either AMPA or NMDA receptor-mediated EPSCs. This suggests that GluR5-containing kainate receptors on mossy fibres function as autoreceptors to facilitate the synaptic release of L-glutamate, in a frequency-dependent manner.

Increased seizure susceptibility in mice lacking metabotropic glutamate receptor 7.

To study the role of mGlu7 receptors (mGluR7), we used homologous recombination to generate mice lacking this metabotropic receptor subtype (mGluR7(-/-)). After the serendipitous discovery of a sensory stimulus-evoked epileptic phenotype, we tested two convulsant drugs, pentylenetetrazole (PTZ) and bicuculline. In animals aged 12 weeks and older, subthreshold doses of these drugs induced seizures in mGluR7(-/-), but not in mGluR7(+/-), mice. PTZ-induced seizures were inhibited by three standard anticonvulsant drugs, but not by the group III selective mGluR agonist (R,S)-4-phosphonophenylglycine (PPG). Consistent with the lack of signs of epileptic activity in the absence of specific stimuli, mGluR7(-/-) mice showed no major changes in synaptic properties in two slice preparations. However, slightly increased excitability was evident in hippocampal slices. In addition, there was slower recovery from frequency facilitation in cortical slices, suggesting a role for mGluR7 as a frequency-dependent regulator in presynaptic terminals. Our findings suggest that mGluR7 receptors have a unique role in regulating neuronal excitability and that these receptors may be a novel target for the development of anticonvulsant drugs.

Kainate receptors are involved in synaptic plasticity.

The ability of synapses to modify their synaptic strength in response to activity is a fundamental property of the nervous system and may be an essential component of learning and memory. There are three classes of ionotropic glutamate receptor, namely NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid) and kainate receptors; critical roles in synaptic plasticity have been identified for two of these. Thus, at many synapses in the brain, transient activation of NMDA receptors leads to a persistent modification in the strength of synaptic transmission mediated by AMPA receptors. Here, to determine whether kainate receptors are involved in synaptic plasticity, we have used a new antagonist, LY382884 ((3S, 4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydro isoquinoline-3-carboxylic acid), which antagonizes kainate receptors at concentrations that do not affect AMPA or NMDA receptors. We find that LY382884 is a selective antagonist at neuronal kainate receptors containing the GluR5 subunit. It has no effect on long-term potentiation (LTP) that is dependent on NMDA receptors but prevents the induction of mossy fibre LTP, which is independent of NMDA receptors. Thus, kainate receptors can act as the induction trigger for long-term changes in synaptic transmission.

Kainate receptors: subunits, synaptic localization and function.

Although it is well established that kainate receptors constitute an entirely separate group of proteins from AMPA receptors, their physiological functions remain unclear. The molecular cloning of subunits that form kainate receptors and the ability to study recombinant receptors is leading to an increased understanding of their functional properties. Furthermore, the development of kainate receptor-selective agonists and antagonists over the past few years is now allowing the physiological roles of these receptors and, in some cases, specific subunits to be investigated. As a consequence, the synaptic activation of postsynaptic kainate receptors and the presence of presynaptic kainate receptors that serve to regulate excitatory and inhibitory synaptic transmission have been described, and will be discussed in this article by Ramesh Chittajallu, Steven Braithwaite, Vernon Clarke and Jeremy Henley.

The GluR5 subtype of kainate receptor regulates excitatory synaptic transmission in areas CA1 and CA3 of the rat hippocampus.

Activation of kainate receptors depresses excitatory synaptic transmission in the hippocampus. In the present study, we have utilised a GluR5 selective agonist, ATPA [(RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid], and a GluR5 selective antagonist, LY294486 [(3SR,4aRS,6SR,8aRS)-6-([[(1H-tetrazol-5-y l)methyl]oxy]methyl)-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3 -carboxylic acid], to determine whether GluR5 subunits are involved in this effect. ATPA mimicked the presynaptic depressant effects of kainate in the CA1 region of the hippocampus. It depressed reversibly AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor-mediated field excitatory postsynaptic potentials (field EPSPs) with an IC50 value of approximately 0.60 microM. The dual-component excitatory postsynaptic current (EPSC) and the pharmacologically isolated NMDA (N-methyl-D-aspartate) receptor-mediated EPSC were depressed to a similar extent by 2 microM ATPA (61 +/- 7% and 58 +/- 6%, respectively). Depressions were associated with an increase in the paired-pulse facilitation ratio suggesting a presynaptic locus of action. LY294486 (20 microM) blocked the effects of 2 microM ATPA on NMDA receptor-mediated EPSCs in a reversible manner. In area CA3, 1 microM ATPA depressed reversibly mossy fibre-evoked synaptic transmission (by 82 +/- 10%). The effects of ATPA were not accompanied by any changes in the passive properties of CA1 or CA3 neurones. However, in experiments where K+, rather than Cs+, containing electrodes were used, a small outward current was observed. These results show that GluR5 subunits comprise or contribute to a kainate receptor that regulates excitatory synaptic transmission in both the CA1 and CA3 regions of the hippocampus.

A hippocampal GluR5 kainate receptor regulating inhibitory synaptic transmission.

The principal excitatory neurotransmitter in the vertebrate central nervous system, L-glutamate, acts on three classes of ionotripic glutamate receptors, named after the agonists AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid), NMDA (N-methyl-D-aspartate) and kainate. The development of selective pharmacological agents has led to a detailed understanding of the physiological and pathological roles of AMPA and NMDA receptors. In contrast, the lack of selective kainate receptor ligands has greatly hindered progress in understanding the roles of kainate receptors. Here we describe the effects of a potent and selective agonist, ATPA ((RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid) and a selective antagonist, LY294486 ((3SR, 4aRS, 6SR, 8aRS)-6-((((1H-tetrazol-5-yl) methyl)oxy)methyl)-1, 2, 3, 4, 4a, 5, 6, 7, 8, 8a-decahydroisoquinoline-3-carboxylic acid), of the GluR5 subtype of kainate receptor. We have used these agents to show that kainate receptors, comprised of or containing GluR5 subunits, regulate synaptic inhibition in the hippocampus, an action that could contribute to the epileptogenic effects of kainate.

NMDA receptor dependence of mGlu-mediated depression of synaptic transmission in the CA1 region of the rat hippocampus.

1. The depression of synaptic transmission by the specific metabotropic glutamate receptor (mGlu) agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylate ((1S,3R)-ACPD) was investigated in area CA1 of the hippocampus of 4-10 week old rats, by use of grease-gap and intracellular recording techniques. 2. In the presence of 1 mM Mg2+, (1S,3R)-ACPD was a weak synaptic depressant. In contrast, in the absence of added Mg2+, (1S,3R)-ACPD was much more effective in depressing both the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components of synaptic transmission. At 100 microM, (1S,3R)-ACPD depressed the slope of the field excitatory postsynaptic potential (e.p.s.p.) by 96 +/- 1% (mean +/- s.e.mean; n = 7) compared with 23 +/- 4% in 1 mM Mg(2+)-containing medium (n = 17). 3. The depressant action of 100 microM (1S,3R)-ACPD in Mg(2+)-free medium was reduced from 96 +/- 1 to 46 +/- 6% (n = 7) by the specific NMDA receptor antagonist (R)-2-amino-5-phosphonopentanoate (AP5; 100 microM). 4. Blocking both components of GABA receptor-mediated synaptic transmission with picrotoxin (50 microM) and CGP 55845A (1 microM) in the presence of 1 mM Mg2+ also enhanced the depressant action of (1S,3R)-ACPD (100 microM) from 29 +/- 5 to 67 +/- 6% (n = 6). 5. The actions of (1S,3R)-ACPD, recorded in Mg(2+)-free medium, were antagonized by the mGlu antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG). Thus, depressions induced by 30 microM (1S,3R)-ACPD were reversed from 48 +/- 4 to 8 +/- 6% (n = 4) by 1 mM (+)-MCPG. 6. In Mg(2+)-free medium, a group I mGlu agonist, (RS)-3, 5-dihydroxyphenylglycine (DHPG; 100 microM) depressed synaptic responses by 74 +/- 2% (n = 18). In contrast, neither the group II agonists ((2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine; L-CCG-1; 10 microM; n = 4) and ((2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine; DCG-IV; 100 nM; n = 3) nor the group III agonist ((S)-2-amino-4-phosphonobutanoic acid; L-AP4; 10 microM; n = 4) had any effect. 7. The depolarizing action of (1S,3R)-ACPD, recorded intracellularly, was similar in the presence and absence of Mg(2+)-AP5 did not affect the (1S,3R)-ACPD-induced depolarization in Mg(2+)-free medium. Thus, 50 microM (1S,3R)-ACPD induced depolarizations of 9 +/- 3 mV (n = 5), 10 +/- 2 mV (n = 4) and 8 +/- 2 mV (n = 5) in the three respective conditions. 8. On resetting the membrane potential in the presence of 50 microM (1S,3R)-ACPD to its initial level, the e.p.s.p. amplitude was enhanced by 8 +/- 3% in 1 mM Mg2+ (n = 5) compared with a depression of 37 +/- 11% in the absence of Mg2+ (n = 4). Addition of AP5 prevented the (1S,3R)-ACPD-induced depression of the e.p.s.p. (depression of 4 +/- 5% (n = 5)). 9. It is concluded that activation by group 1 mGlu agonists results in a depression of excitatory synaptic transmission in an NMDA receptor-dependent manner.

Pharmacology of postsynaptic metabotropic glutamate receptors in rat hippocampal CA1 pyramidal neurones.

1. Activation of metabotropic glutamate receptors (mGluRs) in hippocampal CA1 pyramidal neurones leads to a depolarization, an increase in input resistance and a reduction in spike frequency adaptation (or accommodation). At least eight subtypes of mGluR have been identified which have been divided into three groups based on their biochemical, structural and pharmacological properties. It is unclear to which group the mGluRs which mediate these excitatory effects in hippocampal CA1 pyramidal neurones belong. We have attempted to address this question by using intracellular recording to test the effects of a range of mGluR agonists and antagonists, that exhibit different profiles of subtype specificity, on the excitability of CA1 pyramidal neurones in rat hippocampal slices. 2. (2S, 1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (L-CCG1) caused a reduction in spike frequency adaptation and a depolarization (1-10 mV) associated with an increase in input resistance (10-30%) at concentrations (> or = 50 microM) that have been shown to activate mGluRs in groups I, II and III. Similar effects were observed with concentrations (50-100 microM) of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) and (1S,3S)-ACPD that exhibit little or no activity at group III mGluRs but which activate groups I and II mGluRs. 3. Inhibition of the release of endogenous neurotransmitters through activation of GABAB receptors, by use of 200 microM (+/-)-baclofen, did not alter the effects of (1S,3R)-ACPD (50-100 microM), (1S,3S)-ACPD (100 microM) or L-CCG1 (100 microM). This suggests that mGluR agonists directly activate CA1 pyramidal neurones. 4. Like these broad spectrum mGluR agonists, the racemic mixture ((SR)-) or resolved (S)-isomer of the selective group I mGluR agonist 3,5-dihydroxyphenylglycine ((SR)-DHPG (50-100 microM) or (S)-DHPG (20-50 microM)) caused a reduction in spike frequency adaptation concomitant with postsynaptic depolarization and an increase in input resistance. In contrast, 2S,1'R,2'R,3'R-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV; 100 microM) and (S)-2-amino-4-phosphonobutanoic acid (L-AP4; 100-500 microM), which selectively activate group II mGluRs and group III mGluRs, respectively, had no effect on the passive membrane properties or spike frequency adaptation of CA1 pyramidal neurones. 5. The mGluR antagonists (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG; 1000 microM) and (S)-4-carboxyphenylglycine ((S)-4CPG; 1000 microM), which block groups I and II mGluRs and group I mGluRs, respectively, had no effect on membrane potential, input resistance or spike frequency adaptation per se. Both of these antagonists inhibited the postsynaptic effects of (1S,3R)-ACPD (50-100 microM), (1S,3S)-ACPD (30-100 microM) and L-CCG1 (50-100 microM). (+)-MCPG also reversed the effects of (SR)-DHPG(75 gM). (The effect of (S)-4CPG was not tested.) Their action was selective in that both antagonists did not reverse the reduction in spike frequency adaptation induced by carbachol (1 microM) or noradrenaline(10 microM) whereas atropine (10 microM) and propranolol (100 microM) did.6 From these data it is concluded that the mGluRs in CAl pyramidal neurones responsible for these excitatory effects are similar to the mGluRs expressed by non-neuronal cells transfected with cDNA encoding group I mGluRs.

Pharmacological evidence for an involvement of group II and group III mGluRs in the presynaptic regulation of excitatory synaptic responses in the CA1 region of rat hippocampal slices.

The actions of four mGluR antagonists, (+)-MCPG, MAP4, MCCG and (S)-4CPG, were evaluated against agonist-induced depressions of synaptic transmission at the Schaffer collateral-commissural pathway in rat hippocampal slices. (+)-MCPG (1 mM) reversed very effectively depressions of field EPSPs induced by (1S,3R)-ACPD and (1S,3S)-ACPD but had weak and variable effects on depressions induced by L-AP4. It had no effect on depressions induced by either (-)-baclofen or carbachol. In contrast, MAP4 (500 microM) reversed very effectively depressions induced by L-AP4 without affecting depressions induced by (1S,3S)-ACPD. MCCG (1 mM) had the opposite activity; it antagonized depressions induced by (1S,3S)-ACPD but not those induced by L-AP4. Finally, (S)-4CPG (1 mM) reversed small depressions of field EPSPs induced by high concentrations (50-100 microM) of (1S,3R)- and (1S,3S)-ACPD, but not L-AP4, whilst having no effect on large depressions induced by 10 microM (1S,3S)-ACPD in voltage-clamped cells. These results confirm and extend the effectiveness and selectivity of (+)-MCPG as an mGluR antagonist. The divergent effects of the group I antagonist, (S)-4CPG, can be explained by an indirect action on postsynaptic receptors which is manifest when high agonist concentrations are used in non-voltage-clamp experiments. The action of MCCG and MAP4 indicates that two pharmacologically-distinct mGluRs, belonging to classes II and III, can regulate synaptic transmission in the CA1 region via presynaptic mechanisms.