Effect of prior sensitization of stereotypy on the development of tolerance to amphetamine-induced hypophagia.

The purpose of this experiment was to explore the relation between amphetamine-induced stereotyped movements and the development of tolerance to the initial hypophagic effect of the drug. If stereotyped movements contribute to hypophagia, then prior sensitization of stereotypy should exaggerate the initial hypophagic effect and retard the development of tolerance. To test this hypothesis, one group of rats was given intermittent injections of amphetamine (2.5 mg/kg) to induce sensitization, and another group was given saline as a control. Sensitization was characterized by increased head scanning movements and decreased stationary activity, locomotion and sniffing. However, dose-response tests revealed that sensitized rats did not show increased hypophagia in a milk drinking task. Subsequently, subgroups from each group received daily access to milk after injections of either amphetamine (2 mg/kg), to induce tolerance, or saline. Contrary to the hypothesis, rats that had previously been sensitized developed more tolerance than nonsensitized rats. It is suggested that tolerance may involve the channeling of one form of stereotyped movement (head scanning) into another form (licking of the drinking tube). Such channeling represents a constrained form of instrumental learning that is conceptually similar to the behavioral plasticity described in rats receiving electrical brain stimulation or tail pinch.