Tolerance to amphetamine hypophagia: a microstructural analysis of licking behavior in the rat.

The development of tolerance to amphetamine-induced hypophagia was assessed by recording changes in lick parameters in rats given chronic administration of the drug (2 mg/kg) and access to sweetened milk. Although licking and milk intake gradually recovered, the volume of milk ingested per lick remained suppressed. Amphetamine had no effect on the interlick interval or the force per lick. In contrast, the drug caused a sustained increase in the number of lick bursts (defined by pause criteria of 0.5-2.0 s) and a decrease in the number of licks per burst (but only at pause criteria of 0.5 and 1.0 s). These results suggest that tolerant rats frequently interrupt licking, resulting in less efficient capture of milk.