CHARACTERIZATION OF THE METABOTROPIC GLUTAMATE RECEPTORS (MGLURS) WHICH MODULATE GABA-MEDIATED INHIBITION IN THE VENTROBASAL THALAMUS

The ventrobasal thalamus (VB) relays and processes somatosensory infor mation ascending to the cerebral cortex. Several types of mGluR are kn own to be present in VB, and we have previously shown that Group II an d Group III mGluR agonists can reduce inhibitory synaptic transmission by acting at presynaptic receptors on GABAergic terminals in this str ucture. We have tested the action of several antagonists against the d isinhibitory action of the Group II agonist CCG-I [(2S,3S,4S)-alpha-(c arboxycyclopropyl)-glycine] and the Group III agonist L-AP4 [L-2-amino -4-phosphonobutyrate] in the VB of anaesthetized rats using extracellu lar single-neurone recording techniques and iontophoretic applications of mGluR antagonists and agonists. The antagonists MAP4 [alpha-methyl -L-AP4] and MPPG [(+/-)-alpha-methyl-4-phosphonophenylglycine] reduced the disinhibitory actions of L-AP4 whilst having little effect on the disinhibitory action of CCG-I. In contrast, MCCG [alpha-methyl-CCG-I] and MCPG [(+)-alpha-methyl-4-carboxyphenylglycine] antagonized CCG-I, whilst having less effect against L-AP4 responses. These results supp ort the hypothesis that GABAergic inhibitory transmission in VB can be modulated by at least two types of mGluR, belonging to Group II and G roup III. Furthermore, the novel antagonists appear to be useful tools for the future study of the physiological role of these receptors in thalamic sensory processing. Copyright (C) 1996 Elsevier Science Ltd.