Quantification of vasoactive intestinal peptide immunoreactivity in the anterior pituitary glands of intact male and female, ovariectomized, and estradiol benzoate-treated rats.

There are considerable data suggesting that vasoactive intestinal peptide (VIP) is involved in the regulation of PRL secretion; however, the role and cell of origin of anterior pituitary VIP remain to be determined. Immunocytochemical (ICC) studies have generally failed to detect VIP-immunoreactive (IR) cells in the pituitary of the untreated rat, although VIP-IR cells have been observed in the pituitaries of hypothyroid or estrogen-treated rats. This study was designed to examine the cellular distribution and tissue content of VIP in the anterior pituitary gland of rats under selected endocrine conditions known to alter the rates of PRL and VIP synthesis and secretion. To this end, anterior pituitary VIP and PRL content (ICC and RIA) and serum PRL levels were determined in ovariectomized (OVX) and OVX rats 3 days after treatment with 7 or 70 micrograms estradiol benzoate (EB). For comparison, pituitary VIP and PRL content (ICC and RIA) and serum PRL levels in untreated male and diestrous female rats were determined. Immunostaining for VIP was accomplished using a newly developed primary antiserum. Significant numbers of VIP-IR cells per 5-microns section were found in the anterior pituitary glands of all animals examined (275 +/- 33 in diestrous to 481 +/- 103 cells in male rats). VIP was not colocalized with PRL in any of the pituitaries regardless of steroid treatment or sex. Furthermore, the number of VIP-IR cells per pituitary gland was not significantly correlated with sex or EB treatment. Treatment with 70 micrograms, but not 7 micrograms, EB significantly increased the pituitary content of VIP and serum PRL levels compared to those after ovariectomy. However, both EB treatments resulted in a significant increase in pituitary PRL content compared to that in untreated OVX rats. Pituitaries from male rats had several-fold more VIP and less PRL content than pituitaries from diestrous rats. These data show that 1) in contrast to previous ICC studies, VIP-IR cells are readily detected in the anterior pituitary of intact male and female and OVX as well as EB-treated rats; 2) VIP is localized to cells other than lactotrophs, regardless of the steroid background; and 3) marked changes in anterior pituitary VIP content are not accompanied by changes in VIP-IR cell number.