Immunohistochemical studies on the GABAergic system in the rat supraoptic nucleus using the PAP method with an application of electron microscopy.

GABAergic neuronal profiles in the supraoptic nucleus of the rat were immunohistochemically identified by using a purified GABA antibody with the peroxidase-antiperoxidase method. The localization of GABA-like immunoreactivity in nerve terminals on the neurosecretory neurons was examined electron microscopically. A few small GABAergic neurons were found inside the supraoptic nucleus while only a very few medium-sized ones were detected in the perinuclear zone. Intrinsic, non-GABAergic small neurons covered by GABAergic neuropil were also detected. The neuropil with GABAergic axo-somatic synapses evenly encompassed unlabeled neurosecretory perikarya throughout the supraoptic nucleus. The GABAergic system seems to inhibit both vasopressin and oxytocin cells. In this area, glia cells showed clear outlines of unlabeled somata around counter-stained nuclei. Blood capillaries in the supraoptic nucleus were only slightly covered with a GABAergic neuropil. Electron microscopic observations demonstrated the presence of GABAergic axo-somatic symmetrical and axo-dendritic asymmetrical synapses on the neurosecretory neurons. GABA-like immunoreactivity was localized on the membranes of microtubules and synaptic vesicles, on the external membranes of the mitochondria, and on the inner leaf of the presynaptic sites. Numerous pairs of non-immunoreactive synapses were arranged along these immunoreactive synapses.