Cocaine euphoria, dysphoria, and tolerance assessed using drug-induced changes in brain-stimulation reward

The time course of cocaine-induced changes in self-stimulation thresholds were used to evaluate cocaine euphoria and dysphoria as a function of the chronicity of drug treatment, dosage level, and the spacing of injections. It was assumed that cocaine-induced decreases in thresholds were indicative of cocaine euphoria, while increases in thresholds reflected rebound dysphoric responses to cocaine administration. Three experiments were performed using self-stimulating rats implanted with ventral tegmental area electrodes. Cocaine's threshold-lowering effects were evident 15 min postinjection (IP) with thresholds returning to baseline by approximately 3.0 h after treatment. Little evidence for cocaine-induced increases in thresholds was observed during periods of chronic cocaine treatment. However, thresholds were slightly elevated upon withdrawal from chronic cocaine treatment in Experiments 2 and 3. No evidence of tolerance or sensitization to cocaine-induced shifts in thresholds was noted with single daily injections, while multiple daily injections produced tolerance to cocaine's threshold-lowering effects. It is concluded that cocaine's ability to enhance brain-stimulation reward is highly reliable and robust, while decreases in brain-stimulation reward associated with chronic cocaine treatment are less reliable and difficult to demonstrate. The possible influence of drug dosage on the induction of cocaine dysphoria and the ability of various self-stimulation procedures to measure dysphoric effects are discussed.