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 vulgaris leucoagglutinin anterograde tract tracing in order to analyse the relationship between the subcellular localization of the mGluR1a metabotropic glutamate receptors and the distribution of corticothalamic synapses in the dorsal lateral geniculate nucleus (dLGN) and the lateral posterior nucleus (LP) of the rat. The injection of the tracer into area 17 labelled two types of corticothalamic terminals: (i) the small boutons constituting the majority of the labelled fibres which form asymmetrical synapses both in the dLGN and LP; and (ii) the giant terminals typically participating in glomerulus-like synaptic arrangements and found exclusively in the lateral posterior nucleus. The small corticothalamic terminals often established synapses with mGluR1a-immunopositive dendrites, with immunometal particles concentrated at the periphery of their postsynaptic membranes. In contrast, the synapses formed by giant boutons in the lateral posterior nucleus were always mGluR1a-immunonegative. We conclude that the corticothalamic fibres forming the small synaptic terminals are the most likely candidates for the postulated mGluR-mediated modulation of visual information flow by corticothalamic feedback mechanisms.