The effects of hypothalamic cuts at various rostro-caudal levels on the GABAergic innervation of the neurointermediate lobe of the pituitary gland have been studied. The GABAergic innervation was visualized through glutamate-decarboxylase (GAD) immunocytochemistry. Caudal hypothalamic cuts which transected the pituitary stalk completely abolished the GAD immunoreactive plexus. Rostral cuts which separated about one-third of the median eminence and arcuate nucleus from the pituitary gland decreased the GAD-immunoreactive network in the intermediate lobe but did not affect the neural lobe significantly. Although the precise location of the cell bodies giving rise to the GABAergic innervation of the neurointennediate lobe remains unknown, our findings indicate that their projections are descending ones. They are severed by rostral hypothalamic cuts and show a rostrocaudal arrangement. It is likely that the GABAergic endings of the intermediate lobe originate in the rostral hypothalamus, probably in the rostral part of the arcuate nucleus and/or in the anterior periventricular area. The GABAergic fibers in the neural lobe have a more caudal origin than those innervating the intermediate lobe.
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