Time-dependent effects of intrahypothalamic injection of vasoactive intestinal peptide on spontaneous motor activity...

In male Wistar rats, microinjection cannulae were stereotaxically implanted in the ventromedial and paraventricular hypothalamic nuclei. After control recordings of spontaneous motor activity for 4-6 days, vasoactive intestinal peptide (VIP) was injected in a dose of 1.0 microgram and a volume of 2.0 microliters into these nuclei at 1100 or 1900 h, and the activity during the 3-h period following was compared with that on the control days in individual animals. VIP injection produced an increase when injected at 1100 h, and a decrease when injected at 1900 h. Thus, a time-dependent fluctuation in the VIP effect on the hypothalamic neuronal activity was suggested.