ROLE OF N-METHYL-D-ASPARTATE AND METABOTROPIC GLUTAMATE RECEPTORS IN CORTICOTHALAMIC EXCITATORY POSTSYNAPTIC POTENTIALS IN-VIVO

The ventrobasal thalamus is the principal somatosensory thalamic relay nucleus, and it receives two major sources of excitatory input: first ly an input from ascending sensory afferents, and secondly a descendin g projection from the primary somatosensory cortex.(16) There is consi derable anatomical evidence to suggest that both of these projections utilise the excitatory amino acid L-glutamate as their neurotransmitte r.(4,8,9,17) Previous work from this laboratory has shown that the sen sory input to the rat ventrobasal thalamus in vivo is mediated by iono tropic excitatory amino acid receptors of both the N-methyl-D-aspartat e and non-N-methyl-D-aspartate type.(27,28) These findings are consist ent with data from other studies in various thalamic relay nuclei.(15, 33,36) In contrast, there are considerably less data available concern ing the synaptic pharmacology of the corticothalamic projection althou gh there have been both speculation and studies concerning the functio nal significance of this pathway.(18,34) There is some evidence to sug gest an involvement of N-methyl-D-aspartate receptors(10,30) and metab otropic glutamate receptors.(21) The aim of this study was to determin e which excitatory amino acid receptors might mediate cortically-elici ted excitatory postsynaptic potential in the ventrobasal thalamus in v ivo. Intracellular recordings were made, and neurotransmitter antagoni sts were applied on to rat ventrobasal thalamus neurons by microiontop horesis, Cortically-elicited excitatory postsynaptic potentials were r educed by the N-methyl-D-aspartate antagonist /-)-2-carboxy-piperazin- 4-yl]propyl-1-phosphonate, or the Group I metabotropic antagonist (S)- 4-carboxyphenylglycine. These data indicate that both N-methyl-D-aspar tate receptors and Group I(possibly metabotropic glutamate receptors t ype 1) metabotropic receptors are involved in the mediation of cortico thalamic transmission. Such a transmitter mechanism would allow a modu latory system that could selectively enhance other excitatory inputs. Some of these data have been reported in abstract form.(13) (C) 1996 I BRO. Published by Elsevier Science Ltd.