Presynaptic group II metabotropic glutamate receptors reduce stimulated and spontaneous transmitter release in human dentate gyrus

Abstract

Metabotropic glutamate receptors (mGluRs) control excitatory neurotransmission as inhibitory autoreceptors at many synapses throughout the CNS. Since pharmacological activation of mGluRs potently depresses excitatory transmission, anticonvulsive effects were found in a number of experimental epilepsies. However, although native rodent mGluRs and heterologously expressed human mGluRs have so far been investigated in great detail, our knowledge about native human mGluRs in situ is limited. Here we used acute human hippocampal slices prepared from hippocampi surgically removed for the treatment of temporal lobe epilepsy in order to investigate the modulation of glutamatergic transmission by human mGluRs at the perforant path-granule cell synapse. The broad spectrum mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) profoundly and reversibly reduced field EPSPs (fEPSPs) with an EC₅₀ of 30±7.4 μM. Paired-pulse depression of fEPSPs was converted into strong facilitation. The inhibition of fEPSPs by ACPD was mimicked by the specific group II mGluR agonist (2S, 2′R, 3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV), while the specific group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) was ineffective. The effect of ACPD was blocked by group II antagonist (2S,3S,4S)-2methyl-2-(carboxycyclopropyl)glycine (MCCG) but was not changed by coapplication of the specific group III antagonist (S)2 amino2methyl4phosphonobutanoic acid (MAP4). ACPD reduced pharmacologically isolated intracellular EPSPs in granule cells to the same extent as fEPSPs, whereas a specific group III agonist had no effect on EPSPs. Whole-cell recordings from morphologically identified granule cells revealed that DCG-IV significantly reduced the frequency of miniature EPSCs (mEPSCs) in granule cells while the mean amplitude of mEPSCs was not affected. We conclude that in human dentate gyrus mGluR2/3 can almost completely depress glutamate release by a presynaptic mechanism which acts downstream of presynaptic voltage gated calcium-entry and most likely involves a direct modulation of the release machinery.

Introduction

From studies on rodent hippocampus it is known that group II metabotropic glutamate receptors (mGluRs) are located on perforant path terminals contacting hippocampal granule cell dendrites in the molecular layer (Shigemoto et al., 1997). At this synapse agonists of group II mGluRs acutely depress glutamatergic transmission (Macek et al., 1996, Dietrich et al., 1997) and are also involved in various forms of long-term synaptic plasticity (O’Leary and O’Connor, 1998, Kulla et al., 1999, Huang et al., 1999). The acute and potent depressant effect on excitatory neurotransmission by drugs activating group II mGluRs is considered responsible (Conn and Pin, 1997) for their anticonvulsant effect in animal models of chronic temporal lobe epilepsy (Attwell et al., 1998a, Attwell et al., 1998b). Recently, it has been demonstrated in rat dentate gyrus that brief presynaptic activity leads to activation of group II mGluRs via accumulation of glutamate (Kew et al., 2001). Thus, rodent data suggest that group II mGluRs play an important role in regulating synaptic excitation of granule cells and that group II mGluR agonists could turn out to be potential candidates for clinical trial in epilepsy. It is therefore of great importance to know, whether group II mGluRs have a comparable pharmacological profile and physiological role in human tissue. Human group II mGluRs have been cloned and pharmacologically characterised in expression systems, but up to now there is no data on the pharmacology and physiology of native human group II mGluRs in situ available. Despite the strong similarities in pharmacology and in amino acid sequence between recombinant human and native rodent mGluRs (Conn and Pin, 1997) there could be species dependent differences in the expression pattern and coupling of the receptors to signalling cascades. Moreover, human receptor function can be altered by neurological disorders: in a previous study we observed a loss of the depressing effect of group III mGluRs in epileptic patients suffering from Ammon’s horn sclerosis (Dietrich et al., 1999b). In the present study we took advantage of the unique opportunity to investigate human hippocampi resected for the treatment of temporal lobe epilepsy and analysed the modulation of synaptic transmission at the perforant path-granule cell synapse by human group II mGluRs.