GAP-43-Immunoreactive Nerve Fibers in the Rat Pituitary: Modulation after 6-Hydroxydopamine-Induced Degeneration and Regeneration.

This study demonstrates the presence of the growth-associated protein, GAP-43, in nerve fibers innervating the adult rat neurointermediate pituitary. Previous studies demonstrated that innervation to the proopiomelanocortin cells of the intermediate pituitary could undergo degeneration after chemical denervation with 6-hydroxydopamine (6-OHDA), followed by recovery/regeneration of terminals 3 to 4 weeks after drug treatment. Here, we investigated the modulation of GAP-43 levels following chemical denervation of the pituitary. For these studies, pituitary tissue sections were stained with antibody to GAP-43 and analyzed by light and electron microscopic techniques. Using these methods for immunolocalization of GAP-43, staining was observed only in nerve fibers of the intermediate and neural lobes. Additional groups of animals were studied for GAP-43 protein content in the pituitary. One week after 6-OHDA injections to adult male rats, reduced amounts of GAP-43 were observed by Western blot analysis, compared to control tissue, and sections showed little or no immunostaining in the intermediate lobe after exposure to the drug. Three weeks after drug treatment, the levels of GAP-43 were closer to controls by Western blot analysis, in agreement with the reinnervation of the pituitary as seen in immunostained sections. The loss of GAP-43 immunostaining from denervated tissue, followed by its recovery during reinnervation, supports the hypothesis that nerve fibers innervating this adult neuroendecrine area are capable of undergoing synaptic plasticity.