Elsevier

Neuropharmacology

Volume 47, Issue 3, September 2004, Pages 363-372
Neuropharmacology

Characterisation of the effects of ATPA, a GLUK5 kainate receptor agonist, on GABAergic synaptic transmission in the CA1 region of rat hippocampal slices

https://doi.org/10.1016/j.neuropharm.2004.05.004Get rights and content

Abstract

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 GLUK5 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>α-amino-3-(3-hydroxy-5-methyl-4-isoxalolyl)propionic acid (AMPA). Secondly, we confirm that the effects of ATPA are not mediated indirectly via the activation of γ-aminobutyric acid receptors (i.e. either GABAA or GABAB). 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 GLUK5—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.

Introduction

l-glutamate mediates the majority of fast excitatory neurotransmission in the central nervous system (CNS). Pharmacological studies first demonstrated the existence of kainate receptors (Davies et al., 1979, Davies and Watkins, 1979, McLennan and Lodge, 1979, Agrawal and Evans, 1986). Molecular cloning then identified five members of the kainate receptor family (GluR5, GluR6, GluR7, KA1, KA2; referred here according to IUPHAR nomenclature as GLUK5–7, GLUK1–2), which can exist in various homomeric and heteromeric assemblies (Bettler and Mulle, 1995). The subsequent development of kainate receptor knockout animals and selective pharmacological tools (Bleakman and Lodge, 1998) has enabled progress to be made in elucidating the functions of kainate receptors in the CNS (Chittajallu et al., 1999, Huettner, 2003, Lerma, 2003).

The activation of kainate receptors can regulate the level of synaptic inhibition in the hippocampus (Sloviter and Damiano, 1981, Westbrook and Lothman, 1983, Fisher and Alger, 1984, Kehl et al., 1984), possibly via a direct action on evoked GABAergic transmission (Davies and Collingridge, 1989, Clarke et al., 1997, Rodríguez-Moreno et al., 1997). However, the extent to which this represents a distinct effect in the vicinity of presynaptic terminals or arises as an indirect consequence of the increase in interneurone excitability is controversial (Cossart et al., 1998, Frerking et al., 1998, Bureau et al., 1999, Frerking et al., 1999, Rodríguez-Moreno et al., 2000).

In a previous work, we characterised an agonist (ATPA) and antagonist (LY294486) selective for GLUK5 receptors and used these, together with kainate, to identify a role of GLUK5-containing kainate receptors in the regulation of monosynaptically evoked GABAergic synaptic transmission in area CA1 of the hippocampus (Clarke et al., 1997). Although our experiments suggested that the effects of ATPA were directly on GABAergic neurones, it was subsequently proposed that kainate receptor activation leads to depression of inhibition indirectly; the hypothesis was that excitation of interneurones leads to GABA release which acts on GABAA receptors to shunt IPSPs and on GABAB receptors to cause inhibition of GABA release via presynaptic GABAB receptors (Frerking et al., 1999).

One purpose of the present work was to re-investigate this possibility. We find no evidence for an indirect effect via GABA release. In addition, we determined the sensitivity of the effects of ATPA to the more selective GLUK5 kainate receptor antagonist LY382884 (Bortolotto et al., 1999). Our results suggest that this antagonist displays a differential sensitivity at antagonising the GLUK5 containing kainate receptors that regulate GABA and glutamate release.

Section snippets

Materials and methods

Wistar rats in the age range 10–16 weeks were either anaesthetised with halothane (3.5%) and decapitated or killed according to schedule 1, in accordance with UK Home Office legislation. The brain was removed rapidly and placed in ice-chilled, oxygenated artificial cerebrospinal fluid (aCSF) comprising of (in mM): NaCl 124, KCl 3, NaHCO3 26, CaCl2 2, MgSO4 1, d-glucose 10, NaH2PO4 1.25 saturated with 95% O2, 5% CO2. Parasaggital slices (400 μm thick) containing the hippocampal region were

Effects of ATPA on intracellular properties and evoked excitatory transmission

Initially we examined the effects of ATPA on evoked EPSPs and passive membrane properties (Fig. 1). At a concentration of 3 μM, ATPA caused a hyperpolarisation followed by a slower repolarisation to a level close to the initial baseline membrane potential, with an associated decrease in input resistance. EPSP amplitude was depressed by 42±9% (p<0.05), whilst input resistance was reduced by 8±1% (n=3; Fig. 1A). At a concentration of 10 μM, ATPA caused a larger hyperpolarisation (ca. 3–4 mV), and

Discussion

The purpose of the present study was to characterise in more detail the effects of ATPA on monosynaptically evoked GABAergic synaptic transmission in area CA1 of the adult rat hippocampus. Although ATPA can activate AMPARs, the use of GYKI53655 restricted its actions to kainate receptors; this was confirmed by the lack of the effects of AMPA on monosynaptically evoked IPSPs. GYKI53655 eliminated the depolarising action of ATPA, demonstrating that this was due to the activation of AMPA

Acknowledgements

This work has been supported by the MRC.

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