EVIDENCE FOR GLUTAMATE AS NEUROTRANSMITTER IN TRIGEMINOTHALAMIC TRACTAND SPINOTHALAMIC TRACT TERMINALS IN THE NUCLEUS-SUBMEDIUS OF CATS

The nucleus submedius in the medial thalamus of cats is an important t ermination site for lamina I trigemino- and spinothalamic tract (TSTT) neurons, many of which are nociceptive-specific, and the nucleus subm edius has been proposed to be a dedicated nociceptive substrate involv ed in the affective aspect of pain. In the present study, the distribu tion of glutamate was examined by immunocytochemical methods in order to evaluate the possible role of this amino acid as a neurotransmitter in TSTT terminals in the nucleus submedius. TSTT terminals were ident ified by anterograde transport of horseradish peroxidase and wheatgerm agglutinin-horseradish peroxidase conjugate from the spinal cord or t he medullary dorsal horn. Quantitative analysis of immunogold labellin g revealed that TSTT terminals contain about twice the tissue average of glutamate-like immunoreactivity, A strong positive correlation was found between the density of synaptic Vesicles and the density of gold particles in these terminals, whereas no relationship was seen betwee n these variables in GABAergic presynaptic dendrites, Enrichment of gl utamate-like immunoreactivity (similar to 250% of the tissue average) was also observed in terminals of presumed cortical origin. Presynapti c dendrites and neuron cell bodies in the nucleus submedius were found to contain relatively low levels of glutamate-like immunoreactivity, at or below the tissue average, These observations provide evidence th at glutamate is a neurotransmitter in lamina I TSTT terminals in the n ucleus submedius. The findings also suggest glutamatergic neurotransmi ssion between cortical afferents and nucleus submedius neurons, Glutam ate is therefore likely to be an important mediator of nociceptive pro cessing in the medial thalamus.