Time course and transition analysis of the behavioural effects of microinjection of pentobarbital and noradrenaline into the ventromedial hypothalamus of the rat.

Noradrenaline or the aspecific neural blocker pentobarbital were injected into the ventromedial hypothalamus (VMH) of male Wistar rats fed ad lib. Both drugs promptly induced eating behaviour which lasted for about 4-10 min. Feeding occurred equally well, either with food pellets scattered throughout the cage or when the pellets were concentrated in a small food cup. Compared with noradrenaline or with food deprivation, pentobarbital markedly depressed scanning for at least 12 min after injection, and decreased food-transport and undirected locomotion. When food pellets were scattered all over the floor, pentobarbital enhanced sniffing into the litter. This enhancement occurred just after the injection but before the break-through of feeding itself, and also after the fade-out of feeding but before the recovery of scanning. As a rule, behaviour transitions leading to sniffing-litter were more preferred after pentobarbital than after either noradrenaline or food deprivation. Compared with pentobarbital and food deprivation, noradrenaline enhanced digging. With regard to the sequencing of behaviours, noradrenaline induced random chaining between behaviours, while pentobarbital yielded a most rigid chaining; food deprivation produced an intermediate result. The data are matched against expectations from Veening's funnel model of behaviour sequencing in VMH-blocked rats. The relation between the enhancement of sniffing-litter and of feeding after pentobarbital is discussed.