GABA-LIKE IMMUNOREACTIVITY IN THE THALAMIC NUCLEUS SUBMEDIUS OF THE CAT

The distribution of GABAergic elements and their synaptic contacts in the nucleus submedius, a specific nociceptive relay in the medial thal amus of the cat, was studied using light and electronmicroscopic poste mbedding immunohistochemical method. About one-fourth of the neurons i n nucleus submedius were GABA immunoreactive. These neurons were gener ally smaller than the unlabeled neurons and are probably local circuit neurons. Electron microscopy showed GABA immunoreactivity in two type s of vesicle-containing profiles, F-terminals and presynaptic dendrite s. F-terminals formed simple synapses with the dendrites of presumed t halamocortical relay cells. Presynaptic dendrites were involved in mor e complex synaptic arrangements that included ascending trigemino:hala mic and spinothalamic tract terminals and thalamocortical relay cell d endrites. Analysis of single sections showed that about 40% of the tri geminothalamic and spinothalamic tract terminals, identified by antero grade transport of horseradish peroxidase, were presynaptic to GABAerg ic presynaptic dendrites. These results show that GABAergic neurons ar e frequent in nucleus submedius and that the GABAergic elements make s ynaptic connections similar to those described fcr other sensory relay nuclei, including the somatosensory ventroposterior nucleus. This sug gests that GAbAergic mechanisms play an important role in the processi ng of nociceptive and thermoreceptive information. Copyright (C) 1996 IBRO.