GABA immuno-electron microscopic study of the nucleus of the optic tract in the rabbit

The pretectal nucleus of the optic tract (NOT) was investigated immunocytochemically with an antiserum against gamma aminobutyric acid (GABA) employing the pre-embedding peroxidase antiperoxidase technique at the light microscopic level and the postembedding colloidal gold technique at the electron microscopic level. GABA immunoreactivity was observed in cell bodies of different sizes and as punctate structures in the neuropil. In the electron microscope, besides immunoreactive dendrites, four different types of terminals were found to be GABA-immunopositive; three types of terminals with clustered and flattened vesicles (F-profile) and a fourth type with pleomorphic vesicles, presumably of dendritic origin (P-profile). Both P- and F-profiles formed symmetrical synapses with dendritic profiles arranged in clusters ensheathed by glial elements. GABA-immunopositive terminals were observed in synaptic contact with somata and retinal terminals (R-profiles) that were always GABA-immunonegative. Some GABA-immunopositive somata showed presynaptic contacts with dendrites. The presence of GABA in numerous distinct elements in the NOT and the diversity in labeled somata and terminals demonstrate the importance of the inhibitor neurotransmitter in the NOT and suggest that its function is not limited to interneurons.