Effects of novelty and methamphetamine on conditioned and sensory reinforcement

Abstract

BackgroundLight onset can be both a sensory reinforcer (SR) with intrinsic reinforcing properties, and a conditioned reinforcer (CR) which predicts a biologically important reinforcer. Stimulant drugs, such as methamphetamine (METH), may increase the reinforcing effectiveness of CRs by enhancing the predictive properties of the CR. In contrast, METH-induced increases in the reinforcing effectiveness of SRs, are mediated by the immediate sensory consequences of the light. MethodsThe effects of novelty (on SRs) and METH (on both CRs and SRs) were tested. Experiment 1: rats were pre-exposed to 5s light and water pairings presented according to a variable-time (VT) 2min schedule or unpaired water and light presented according to independent, concurrent VT 2min schedules. Experiment 2: rats were pre-exposed to 5s light presented according to a VT 2min schedule, or no stimuli. In both experiments, the pre-exposure phase was followed by a test phase in which 5s light onset was made response-contingent on a variable-interval (VI) 2min schedule and the effects of METH (0.5mg/kg) were determined. ResultsNovel light onset was a more effective reinforcer than familiar light onset. METH increased the absolute rate of responding without increasing the relative frequency of responding for both CRs and SRs. ConclusionNovelty plays a role in determining the reinforcing effectiveness of SRs. The results are consistent with the interpretation that METH-induced increases in reinforcer effectiveness of CRs and SRs may be mediated by immediate sensory consequences, rather than prediction.