Light and agonist alter vasoactive intestinal peptide binding and intracellular accumulation of adenosine 3',5'-monophosphate in the rat pineal gland

We examined the effects of environmental light and prior treatment with an agonist on vasoactive intestinal polypeptide (VIP) binding and VIP stimulation of cAMP accumulation in the rat pineal gland. VIP binding to pinealocytes and cAMP accumulation in response to VIP were significantly increased in animals kept exposed to constant light compared to those in animals experiencing a dark night before the experiments. Scatchard analysis of [125I]VIP binding indicated the presence of two classes of binding sites: high affinity, low capacity sites and low affinity, high capacity sites. The increased VIP binding to pinealocytes in rats maintained in constant light was attributed to an increase in the number of available VIP receptors at both high and low affinity sites. The affinity of VIP binding to cells was not affected by exposure of the animals to light. VIP stimulation of cAMP accumulation was not inhibited by d,l-propranolol. Prior treatment of pinealocytes with VIP decreased [125I]VIP binding by reducing the number of receptors and significantly inhibited subsequent VIP stimulation of cAMP accumulation. Prior treatment with norepinephrine did not alter the number of VIP receptors. Our results strongly suggest that VIP is a neuromodulator of pineal function.