Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The group III mGlu receptor agonist ACPT-I exerts anxiolytic-like but not antidepressant-like effects, mediated by the serotonergic and GABA-ergic systems

Our earlier studies have demonstrated that (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid ACPT-I, a group III mGlu receptor agonist, produced anxiolytic-like and antidepressant-like actions after central administration. Here we describe the anxiolytic-like effects of ACPT-I after intraperitoneal administration in the stress-induced hyperthermia (SIH), elevated plus-maze (PMT) tests in mice and in the Vogel test in rats. However, the compound did not produce antidepressant-like effects in the tail suspension test (TST) or in the forced swim test (FST) in mice. The potential anxiolytic effect of ACPT-I (20 mg/kg) in the SIH test was inhibited by the benzodiazepine receptor antagonist flumazenil (given i.p., 10 mg/kg), and by a 5-HT(1A) receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl) cyclohexane-carboxamide (WAY100635) (0.1 mg/kg s.c.). At the same time, ritanserin (0.5 mg/kg i.p.), the 5-HT2A/C receptor antagonist, did not change the anxiolytic-like effects of ACPT-I. The results of these studies indicate that the GABA-ergic and serotonergic systems are involved in the potential anxiolytic action of ACPT-I.

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

Neurochemistry and pharmacology of the major hippocampal transmitter systems: synaptic and nonsynaptic interactions

Hippocampus plays a crucial role in important brain functions (e.g. memory, learning) thus in the past two decades this brain region became a major objective of neuroscience research. During this period large number of anatomical, neurochemical and electrophysiological data have been accumulated. While excellent reviews have been published on the anatomy and electrophysiology of hippocampal formation, the neurochemistry of this area has not been thoroughly surveyed. Therefore the aim of this review is to summarize the neurochemical and pharmacological data on the release of the major neurotransmitters found in the hippocampal region: glutamate (GLU), gamma-amino butyric acid (GABA), acetylcholine (ACh), noradrenaline (NA) and serotonin (5-HT). In addition, this review analyzes the synaptic and nonsynaptic interactions between hippocampal neuronal elements and overviews how auto- and heteroreceptors are involved in the presynaptic modulation of transmitter release. The presented data clearly show that transmitters released from axon terminals without synaptic contact play an important role in the fine tuning of communication between neurons within a neuronal circuit.

Release of [3H]GABA evoked by glutamate agonists from hippocampal slices: effects of dithiothreitol and glutathione

The effects of dithiothreitol (DTT) and, reduced (GSH) and oxidized (GSSG), glutathione on the release of [3H]GABA evoked by glutamate and its agonists were studied in rat hippocampal slices. DTT had no effect on the basal release of [3H]GABA but it enhanced and prolonged the glutamate agonist-evoked release. This effect was abolished by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801), a noncompetitive NMDA antagonist, and blocked by Mg2+ ions. It was only slightly attenuated by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA receptor antagonist, and not affected by L-(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. The effect of DTT on the NMDA-evoked release of GABA was only slightly affected by extracellular Ca2+ but completely blocked by verapamil even in the absence of Ca2+. GSH and GSSG attenuated or abolished the effects of DTT on the agonist-induced release of [3H]GABA. The results imply that the enhanced and prolonged release of GABA evoked by the coexistence of DTT and excitatory amino acids and attenuated by endogenous GSH and GSSG is a consequence of sustained activation of the NMDA receptor-governed ionophores, which contain functional thiol groups. DTT, GSH and GSSG may regulate the redox state and accessibility of these groups. In addition to the influx of extracellular Ca2+, DTT mobilizes Ca2+ from intracellular pools distinct from those regulated by metabotropic glutamate receptors.