Pineal vasoactive intestinal peptide is reduced during the proestrous stage of the rat estrous cycle.

Immunoreactive levels of vasoactive intestinal peptide (IR-VIP) in the rat pineal gland were examined during the estrous cycle. IR-VIP was shown to be identical to the synthetic porcine material by its similarity in competitive binding studies, and by both gel filtration and high pressure liquid chromatography. Pineal IR-VIP decreased at early proestrus (0300 h, dark), partially as a function of the rise in serum estradiol levels. Although the functional role of VIP in the pineal remains to be clearly elucidated, these results suggest that it might be involved in the reproductive function of the female rat, and that estrogens could partially modulate its pineal concentration.