AXONS CONTAINING THE GROWTH-ASSOCIATED PROTEIN GAP-43 SPECIFICALLY TARGET RAT CORTICOTROPHS FOLLOWING ADRENALECTOMY

An extensive network of nerve fibers immunoreactive for the neuronal g rowth associated protein GAP-43 (GAP-43-IR) is present within the ante rior pituitary (AP) of the rat, and the density of these fibers has be en reported to increase 4 days after adrenalectomy (ADX). in the prese nt study, we employed confocal dual-label immunofluorescence microscop y to determine whether GAP-43-IR fibers are specifically associated wi th corticotrophs at various intervals after ADX, A dramatic increase i n the density of GAP-43-IR was apparent 4 days after ADX, and this inc rease was sustained at 7 and 14 days post-ADX, The percentage of corti cotrophs in apparent contact with GAP-43-IR axons was 87% at 4 days af ter ADX and 92% at 14 days. In addition, fewer than 15% of GAP-43-IR t erminals were associated with cells other than corticotrophs in either group. This highly specific targeting of corticotrophs during a perio d in which these cells are undergoing both hypertrophy and hyperplasia indicates that axonal sprouting is occurring in response to ADX. Whil e the less intense GAP-43-IR in the AP of intact rats precluded precis e quantitative analysis, the majority of corticotrophs also appeared t o be selectively innervated in these animals. The observations that GA P-43-IR axons selectively contact corticotrophs, and that both the spe cificity and thoroughness of innervation are maintained by targeted gr owth of GAP-43-IR axons following ADX, strongly suggest that these fib ers are of functional significance.