Evidence for vasoactive intestinal polypeptide (VIP) altering the firing rate of preoptic, septal and midbrain central gray neurons

The effect of the iontophoretic application of vasoactive intestinal polypeptide (VIP) on the extracellular electrical activity (neuronal firing rate) of 91 neurons localized in the preoptic (PO), septal (S) region and midbrain central gray (MCG) was studied in urethane-anesthetized female rats. When applied in minute quantities, VIP induced both excitatory ( N = 14) and inhibitory ( N = 8) changes in the membrane excitability of PO and S neurons (total N = 58), while only inhibitory ( N = 9) changes were observed in the MCG neurons (total N = 33; thus 24 MCG neurons were found to be unresponsive to VIP). The latency and duration of the VIP-induced response was, for the most part, characterized by a rapid onset and persisted for the duration of the ejecting pulse. However, five out of the 58 PO and S neurons and one out of the 33 MCG neurons did show responses that were longer and more variable in latency and duration. Of 26 PO neurons recorded and tested with VIP, only five neurons were determined to be antidromically identified (AI) as having their axons in the median eminence. The application of VIP increased the neuronal firing rate in two AI PO neurons, decreased the activity in one, and was ineffective in altering the activity in two other AI PO neurons. The VIP-induced changes in the neuronal firing rate appear to be specific and reproducible, and not related to the ejecting current nor pH of the solution. The results suggest that VIP, a gastrointestinal hormone that is also localized in the brain, can alter the neuronal firing rate of hypothalamic and midbrain neurons, thus providing additional evidence for its possible influence on brain and neuroendocrine function.