Rate-dependent effects of monoamine releasers on intracranial self-stimulation in rats: implications for abuse liability assessment

Rate dependency' in the discipline of behavioral pharmacology describes a phenomenon wherein the effect of a drug on the rate of a behavior varies systematically as a function of the baseline, predrug rate of that behavior. Historically, rate-dependency studies have compared drug effects on different baseline rates of behavior maintained either by different schedules of reinforcement or during sequential segments of a fixed-interval schedule. The current experiment generated different baseline rates of behavior by altering frequency of electrical stimulation in an intracranial self-stimulation assay. Amphetamine and 10 other monoamine releasers were analyzed for their ability to produce rate-dependent effects in this assay. There were three main findings. First, all compounds produced rate-dependent effects at some dose. Second, one parameter of rate-dependency plots (peak Y-intercept of the regression line) correlated with in-vitro neurochemical data on selectivity of these compounds to release dopamine versus serotonin (P<0.025, R-2=0.50). Lastly, a correlation between peak Y-intercept and breakpoints under a progressive-ratio procedure in nonhuman primates was also significant (P<0.05, R-2=0.64). Overall, these results extend the rate-dependent effects of monoamine releasers to behavior maintained under intracranial self-stimulation and suggest that, at least for monoamine releasers, the Y-intercept parameter of rate-dependency plots might be a useful metric of drug reward and predictor of drug self-administration measures of drug reinforcement.