IMMUNOCYTOCHEMICAL VISUALIZATION OF THE MGLUR1A METABOTROPIC GLUTAMATE-RECEPTOR AT SYNAPSES OF CORTICOTHALAMIC TERMINALS ORIGINATING FROM AREA-17 OF THE RAT

Pre-embedding immunogold histochemistry was combined with Phaseolus vu lgaris leucoagglutinin anterograde tract tracing in order to analyse t he relationship between the subcellular localization of the mGluR1a me tabotropic glutamate receptors and the distribution of corticothalamic synapses in the dorsal lateral geniculate nucleus (dLGN) and the late ral posterior nucleus (LP) of the rat. The injection of the tracer int o area 17 labelled two types of corticothalamic terminals: (i) the sma ll boutons constituting the majority of the labelled fibres which form asymmetrical synapses both in the dLGN and LP; and (ii) the giant ter minals typically participating in glomerulus-like synaptic arrangement s and found exclusively in the lateral posterior nucleus, The small co rticothalamic terminals often established synapses with mGluR1a-immuno positive dendrites, with immunometal particles concentrated at the per iphery of their postsynaptic membranes. In contrast, the synapses form ed by giant boutons in the lateral posterior nucleus were always mGluR 1a-immunonegative. We conclude that the corticothalamic fibres forming the small synaptic terminals are the most likely candidates for the p ostulated mGluR-mediated modulation of visual information flow by cort icothalamic feedback mechanisms.