Light- and electron-microscopic immunocytochemistry of glutamic acid decarboxylase (GAD) in the basal hypothalamus: morphological evidence for neuroendocrine gamma-aminobutyrate (GABA).
GABAergic cells and axon terminals were localized in the basal hypothalamus of differnet species (rat, mouse and cat), by means of an immunocytochemical approach using a specific and well-characterized antiserum to the GABA biosynthetic enzyme, glutamate decarboxylase. Light-microscopic visualization was performed with an indirect immunofluorescence method and electron-microscopic observations were made on material with pre-embedding staining and use of the peroxidase-antiperoxidase procedure. At the light-microscopic level, a dense immunofluorescent plexus was observed over both the medial and lateral parts of the external layer of the median eminence. The labelling extended from the rostal part of the median eminence up to the pituitary stalk. Over the subependymal and internal layers only a few immunoreactive dots were visible, except around the blood vessels where they appeared more concentrated. Immunoreactive varicosities could be found following the outlines of the capillary loops and lining tanycyte processes, especially in the median eminance midportion. At the electron-microscopic level, the immunolabelling was exclusively found over neuronal profiles in the median eminence. The latter represented a small fraction of the total number of varicosities visible on the same section. Labelled profiles typically contained numerous small clear synaptic vesicles and only a few or no dense-core vesicles. In the subependymal and internal layers, rare labelled endings were found close to ependymal cells or among transversally cut fibers, respectively. In the palisadic zone, elongated positive boutons were visible intermingled with bundles of unlabelled axons and glial or ependymal processes. In the neurohemal contact zone, immunoreactive endings were observed  among unlabelled neurosecretory endings in close vicinity to fenestrated capillary perivascular space. Small moderately intense immunofluorescent varicosities were observed all over the hypothalamus.  The density of the glutamate decarboxylase-positive network was higher than in most diencephalic regions. Intraventricular or topical injection of colchicine allowed the visualization of small lightly immunoreactive cells in the diffusion area of colchicine. In the arcuate nucleus labelled axonal endings containing small pleomorphic synaptic vesicles and sometimes a few dense-core vesicles were observed at the electron-microscopic level. Typical synaptic junctions were commonly found between positive endings and unlabelled perikarya, or more frequently, unlabelled dendrites. These findings show that glutamate decarboxylase-containing endings are localized ed in several strategic sites for potential GABAergic neuroendocrine regulations. The GABAergic endings found among neurosecretory endings in the neurohemal contact zone may provide the morphological support for the release of gamma-aminobutyrate into the portal blood flow as an hypothalamic hypophysiotropic hormone.
D000818:Animals; D001369:Axons / Q000201:enzymology; D002262:Carboxy-Lyases / Q000378:metabolism*; D005968:Glutamate Decarboxylase / Q000378:metabolism*; D007031:Hypothalamus / Q000201:enzymology*; D008297:Male; D008473:Median Eminence / Q000201:enzymology; D051379:Mice; D008807:Mice, Inbred BALB C; D008854:Microscopy, Electron; D009412:Nerve Fibers / Q000201:enzymology; D009474:Neurons / Q000201:enzymology; D010286:Paraventricular Hypothalamic Nucleus / Q000201:enzymology; D010902:Pituitary Gland / Q000201:enzymology; D051381:Rats; D011919:Rats, Inbred Strains; D013572:Synaptic Vesicles / Q000201:enzymology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D005680:gamma-Aminobutyric Acid; D002262:Carboxy-Lyases; D005968:Glutamate Decarboxylase