Time course of feeding induced by pentobarbital-injections into the rat's VMH.

Chemical blockade of the ventromedial hypothalamus (VMH) by single microinjections of pentobarbital in ad lib fed male Wistar rats induced a meal that did not outlast a 10-min postinjection period. When food was presented only after this period, feeding was nearly absent. Repeating the injection after 10 and 20 min led each time to eating, although the amount consumed decreased. Scanning behaviour was drastically depressed during the 4-min postinjection period but then recovered within 6-11 min. Sniffing into the litter was enhanced for about the same interval (8-10 min). Fixed-ratio lever pressing for food upon pentobarbital-blockade of the VMH also tended to disappear after this period, whether many (low ratios) or few (high ratios) food pellets were obtained. The limited meal size after VMH-blockade therefore presumably resulted from the waning of the anaesthetic effect (e.g. diffusion) in this brain site.