Adrenal vasoactive intestinal peptide participates in neonatal corticosteroid production in the rat

Neonatal rats (3-14 days old) exhibit a period of adrenal hyporesponsiveness characterized by blunted corticosterone (B) responses to stress and reduced adrenal sensitivity to adrenocorticotropic hormone (ACTH). Several adrenomedullary peptidergic systems like vasoactive intestinal peptide (VIP) are postulated to influence cortical function. VIP is known to stimulate corticosterone secretion in vitro and to be released from the adrenal medulla following splanchnic nerve stimulation. Here, we tested whether 1) accelerated sympathetic innervation of the adrenal gland by daily L-thyroxine (T4) treatment modified the ontogeny of adrenal VIP and 2) an increase in VIP synthesis could prematurely increase adrenal sensitivity and corticosteroid output during neonatal life. Immunohistochemical VIP staining revealed a different ontogenetic pattern between adrenal regions from days 2-18 and different sensitivities to T4 treatment. Capsular staining was most abundant at all ages and increased with T4 treatment, whereas medullary staining was seen by day 18 and was not affected by T4. Throughout development, VIP receptors were detected mostly in the capsular region, but not in the adrenal cortex. Although receptor levels were not modified by T4 injections, T4 significantly enhanced VIP mRNA levels in the whole adrenal at all ages. In vivo administration of VIP (0.1-2.0 mg/kg body wt ip) to 9- to 12-day-old neonates increased pituitary ACTH, adrenal B, and aldosterone secretion significantly. Corticotropin-releasing factor immunoneutralization before VIP injection diminished VIP-induced ACTH release but still produced small but significant B and aldosterone secretion. Our results show that 1) VIP innervation of the adrenal capsule is present soon after birth and is increased by sympathetic activity whereas VIP appears only much later in the medulla and does not coincide with the onset of splanchnic innervation and 2) exogenous VIP stimulates ACTH, B, and aldosterone release during development and the effect of VIP on steroidogenic secretion is occurring through ACTH secretion, but also, at least in part, directly at the level of the adrenal gland.