Effects of Response-Independent Food Pellet Delivery on the Development of Tolerance to the Rate-Decreasing Effect of d-Amphetamine in Rats

Abstract

The extent to which initial drug administration decreases the ability of an organism to obtain reinforcement can affect the development of behavioral tolerance. Environmental enrichment affects many drug-related phenomena, but its effects on the development of tolerance are not clear. The present study examined how enriching the environment by providing non-contingent food pellets during experimental sessions affects the development of tolerance to a dose of damphetamine that produces a loss in reinforcement. Lever pressing for food pellets was maintained under a multiple schedule consisting of two variable-interval (VI) 60-s components. One component was enriched by providing food pellets non-contingently under a variable-time (VT) 120-s schedule for rats in the Less-Enriched Group (n = 6) and a VT 30-s schedule for rats in the More-Enriched Group (n = 6). Effects of a range of doses of d-amphetamine (0.1 to 3.0 mg/kg) were assessed before (acute) and during (chronic) repeated injections of a dose that reduced the number of food pellets earned by at least 50% from sessions in which saline was tested acutely. There was a dose-dependent decrease in lever pressing, and relatively large doses were required to decrease the number of food pellets earned. Tolerance developed to a similar extent between components with and without non-contingent food pellets for rats in both groups. These results indicate that enriching the environment by providing non-contingent food pellets during experimental sessions does not differentially affect the development of tolerance.