Alterations in GAP-43-immunoreactive innervation in the aging rat pituitary.
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The levels and distribution of the growth-associated protein, GAP-43, were examined in the pituitary glands of young and aging Sprague-Dawley rats, using immunohistochemical techniques on tissue sections and Western blot analyses. GAP-43-immunoreactive innervation was observed in sections in the intermediate and neural lobes of animals aged 8-15 months, while in the oldest rats studied (17 months), stained fibers were observed mainly in the neural, but not the intermediate lobe. Western blots revealed reduced levels of GAP-43 in samples from 15 month old animals, as compared to 12 month old rats, in the neurointermediate lobes. There was no immunoreactivity for GAP-43 in the anterior lobes in the tissue sections or in the blots in any of the glands examined. A diminished level of GAP-43 in pituitary innervation in aged animals suggests a reduced ability for nerve terminals to undergo 'plastic' changes in their relationship to target endocrine cells. Since GAP-43 has also been suggested to modulate neurotransmitter release, a reduction in the protein in aging nerve terminals may diminish availability of transmitters at presynaptic sites.