Paradoxical tolerance to cocaine after initial supersensitivity in drug-use-prone animals

Abstract

There is great interest in outlining biological factors and behavioral characteristics that either predispose or predict vulnerability to substance use disorders. Response to an inescapable novel environment has been shown to predict a "drug-use-prone" phenotype that is defined by rapid acquisition of cocaine self-administration. Here, we showed that response to novelty can also predict the neurochemical and behavioral effects of acute and repeated cocaine in rats. We used cocaine self-administration under a fixed-ratio 1 schedule followed by fast-scan cyclic voltammetry in brain slices to measure subsecond dopamine (DA) release and uptake parameters in drug-use-prone and -resistant phenotypes. Despite no significant differences in stimulated release and uptake, animals with high responses to a novel environment had DA transporters that were more sensitive to cocaine-induced uptake inhibition, which corresponded to greater locomotor activating effects of cocaine. These animals also acquired cocaine self-administration more rapidly and, after 5 days of extended access cocaine self-administration, high-responding animals showed robust tolerance to DA uptake inhibition by cocaine. The effects of cocaine remained unchanged in animals with low novelty responses. Similarly, the rate of acquisition was negatively correlated with DA uptake inhibition by cocaine after self-administration. Thus, we showed that tolerance to the cocaine-induced inhibition of DA uptake coexists with a behavioral phenotype that is defined by increased preoccupation with cocaine as measured by rapid acquisition and early high intake.

Keywords: dopamine; dopamine transporter; individual differences; rat; self-administration; voltammetry.