Regulation of synaptic transmission in the mossy fibre-granule cell pathway of rat cerebellum by metabotropic glutamate receptors

doi:10.1016/S0028-3908(99)00003-9

Neuropharmacology

Volume 38, Issue 6, 15 June 1999, Pages 805-815

https://doi.org/10.1016/S0028-3908(99)00003-9 Get rights and content

Abstract

The role of metabotropic glutamate receptors (mGluRs) in the mossy fibre-granule cell pathway in rat cerebellum was studied using slice preparations and electrophysiological techniques. Application of the group I selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked, in a concentration-dependent manner (EC₅₀=33 μM), a depolarising/hyperpolarising complex response from granule cells which was preferentially inhibited by the group I selective antagonist (S)-4-carboxyphenylglycine (4CPG). The group III selective agonist l-amino-4-phosphonobutyrate (AP4) evoked a hyperpolarising response (EC₅₀=10 μM) which was inhibited by the group II/III selective antagonist (S)-α-methyl-4-phosphonophenylglycine (MPPG). The group II agonist (2S,2′R,3′R)-2-(2′,3′-dicarboxylcyclopropyl)glycine (DCG-IV) elicited no measurable voltage change. The amplitude of the synaptically-mediated mossy fibre response in granule cells was unaffected during application of AP4, was reduced by DHPG and was enhanced by DCG-IV (EC₅₀=80 nM). These effects were inhibited by the group selective antagonists 4CPG and (2S,1′S,2′S,3′R)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine (PCCG-4), respectively. Further investigation using patch-clamp recording revealed that DCG-IV potently inhibited spontaneous GABAergic currents. We conclude that group I and III (but not group II) mGluRs are functionally expressed by granule cells, whereas unexpectedly group II or III mGluRs do not appear to be present presynaptically on mossy fibre terminals. Group II mGluRs are located on Golgi cell terminals; when activated these receptors cause disinhibition, a function which may be important for gating information transfer from the mossy fibres to the granule cells.

Introduction

The family of metabotropic glutamate receptors (mGluRs) presently consists of eight members, which have been divided into three groups (Pin and Duvoisin, 1995, Conn and Pin, 1997). When expressed heterologously, group I mGluRs (mGluR1 and 5) are positively coupled to phosphoinositide hydrolysis, while group II receptors (mGluR2 and 3) and group III receptors (mGluR4, 6-8) are negatively coupled to cyclic AMP. The manner in which these receptors function at central synapses in normal and disease states is unclear. In the cerebellum, at the synapse between parallel fibres and Purkinje cells, mGluRs are present presynaptically (Crepel et al., 1991), postsynaptically (Batchelor and Garthwaite, 1997) and they play an essential role in the induction of one form of synaptic plasticity, long-term depression (Hartell, 1994). In contrast, little is known about the role of mGluRs at the upstream synapse formed between mossy fibres and granule cells. Glutamatergic mossy fibres, which provide more than 80% of the input to the cerebellar cortex, synapse onto granule cells in structures termed glomeruli. Granule cells, via their axons, the parallel fibres, synapse with Purkinje cells and inhibitory interneurones, including Golgi cells whose axons project into the glomerulus in close proximity to mossy fibre-granule cell synapses (Eccles et al., 1967). Recently, mGluRs have been postulated to be involved in long-term potentiation at this synapse (Rossi et al., 1996), but which mGluR subtype(s) are involved and their location(s) are unclear.

In situ hybridisation studies (Abe et al., 1992, Shigemoto et al., 1992, Ohishi et al., 1993a, Tanabe et al., 1993) indicate the existence of mRNA coding for subtypes of mGluRs in granule cells (group I/III) and in Golgi cells (group I/II). The distribution of the receptors within these cell types is unclear although there has been one immunocytochemical study at the electron microscopic level, which indicated that group II mGluRs are present on Golgi cell terminals, but not on granule cell dendrites (Ohishi et al., 1994). Whether or not mGluRs are present on mossy fibre terminals is not known, except they do not appear to express group II receptors (Ohishi et al., 1994).

Studies of the physiology of mGluRs has been hampered by a lack of selective ligands. Recently, however, compounds have become available that exhibit selectivity between the three groups as assessed using receptors expressed in cell lines; however they have not been well characterised in neuronal tissue.

The aim of this study was to identify the functional significance of mGluRs in the mossy fibre-granule cell pathway using electrophysiological recording techniques and group selective agonists and antagonists.

Section snippets

Slice preparation and electrophysiological recording techniques

We used three types of cerebellar slice in this study: sagittal slices for patch-clamp recording of individual granule cells; pial slices for grease-gap recording of the effects of exogenous agonists on a population of granule cells, and biplanar slices, which are hybrid sagittal-pial slices, allowing grease-gap recording of the synaptic mossy fibre response in a population of granule cells. Slices were cut from the vermis of 3–4-week-old (grease-gap) or 11–14-day-old (patch-clamp) Wistar rats

Voltage changes elicited by mGluR ligands on granule cells

Application of the group I mGluR agonist DHPG for 45 s evoked a reversible and concentration-dependent (EC₅₀=33 μM; Fig. 1a, d) response in pial slices which comprised two components: an initial depolarisation, which desensitised during drug application (at 20–30 s), followed by a hyperpolarisation (Fig. 1a). The relative amplitude of the two components varied between preparations but it was constant for a particular slice. In contrast, the group III mGluR agonist AP4 caused a reversible,

Discussion

In this study, the actions of reportedly group selective mGluR agonists and antagonists were characterised in the mossy fibre pathway of rat cerebellar slices. The grease-gap recording technique (Harvey and Collingridge, 1993, Garthwaite and Batchelor, 1996) is well suited for such studies, since recordings are stable, thus allowing generation of concentration-response curves from a single slice, the population recording eliminates cell-to-cell variation, and lastly the technique is relatively

Conclusions

In this study, group selective mGluR ligands were characterised in rat cerebellar slices. Their actions provide evidence for the existence of group I (probably mGluR1) and group III mGluRs in granule cells. Synaptic transmission in the mossy fibre pathway was depressed by activation of group I mGluRs but was unaffected by AP4 and DCG-IV (in the presence of bicuculline), suggesting that both group III and group II mGluRs are absent from mossy fibre terminals. This is unusual for a glutamatergic

Acknowledgements

This study was supported by a Wellcome Trust project grant (JG and AMB) and a Wellcome Trust 4-year Neuroscience Prize Studentship to PV.

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Regulation of synaptic transmission in the mossy fibre-granule cell pathway of rat cerebellum by metabotropic glutamate receptors

Abstract

Introduction

Section snippets

Slice preparation and electrophysiological recording techniques

Voltage changes elicited by mGluR ligands on granule cells

Discussion

Conclusions

Acknowledgements

J. Biol. Chem.

Neuropharmacology

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Neurosci. Lett.

Eur. J. Pharmacol.

J. Neurosci. Methods

Neuroscience

Neuroscience

Neuroscience

Neuron

Neurosci. Res.

Neuropharmacology

Neuropharmacology

Neuron

Neuropharmacology

Neurosci. Lett.

Eur. J. Pharmacol. Mol. Pharmacol.

Eur. J. Pharmacol.

Metabotropic glutamate receptors coupled to phospholipase D

Intersynaptic diffusion of neurotransmitter

Trends Neurosci.

Frequency detection and temporally dispersed synaptic signal association through a metabotropic glutamate receptor pathway

Nature

Development of a tonic form of synaptic inhibition in rat cerebellar granule cells resulting from persistent activation of GABAA receptors

J. Physiol.

Pharmacological antagonism of the actions of group II and III mGluR agonists in the lateral perforant path of rat hippocampal slices

Br. J. Pharmacol.

The metabotropic glutamate receptor types 2/3 inhibit L-type calcium channels via a pertussis toxin-sensitive G-protein in cultured cerebellar granule cells

J. Neurosci.

Facilitatory coupling between a glutamate metabotropic receptor and dihydropyridine-sensitive calcium channels in cultured cerebellar granule cells

J. Neurosci.

Development of a tonic form of synaptic inhibition in rat cerebellar granule cells resulting from persistent activation of GABA_A receptors