Vasoactive intestinal peptide levels in the hypothalamus and anterior pituitary do not change in association with the pre-ovulatory prolactin surge in the rat

Abstract

We examined whether vasoactive intestinal peptide (VIP) is involved in the preovulatory prolactin (PRL) surge in the rat. Ovariectomized adult rats implanted sc with an estradiol capsule 2 days previously were decapitated at hourly intervals between 1100–1800 h. The plasma PRL and luteinizing hormone showed an evident surge at 1700 h and 1800 h, respectively. The anterior pituitary PRL content was significantly lower at and after 1600 h than at 1100 h. However, there was no significant change in the VIP content in the suprachiasmatic nucleus, paraventricular nucleus, median eminence or anterior pituitary throughout the entire period of observation. Since all of these regions are known to be involved in the VIP synthesis and/or release, the present results appear to cast some doubt on previous studies reporting a significant role for VIP in the generation of the pre-ovulatory PRL surge.