Gradual tolerance of metabolic activity is produced in mesolimbic regions by chronic cocaine treatment, while subsequent cocaine challenge activates extrapyramidal regions of rat brain

Abstract

Acute administration of cocaine is known to enhance extracellular dopamine levels in the striatum and to activate immediate-early gene expression in striatal neurons. Regional cerebral metabolic rate for glucose (rCMR(glc)) reportedly increases in extrapyramidal and mesolimbic brain regions in response to acute cocaine treatment. However, chronic administration attenuates the cocaine-induced enhancement of regional dopamine response and the induction of immediate-early gene expression in these regions. Chronic treatment also produces tolerance to cocaine's reinforcing effects. Thus, differential responses to cocaine occur with increasing length of treatment. Therefore, we examined the time course of effects of repeated daily cocaine treatment on rCMR(glc) in rat brain. Acute administration of 10 mg/kg cocaine slightly increased rCMR(glc) in mesolimbic and extrapyramidal regions. However, no significant effects were observed until more than 7 d of treatment, whereupon rCMR(glc) was reduced compared to saline treatment in the infralimbic portion of the medial prefrontal cortex, nucleus accumbens, olfactory tubercle, habenula, amygdala, and a few other brain regions. In contrast, after 13 d of 10 mg/kg cocaine treatment, challenge with 30 mg/kg cocaine increased rCMR(glc) in the striatum, globus pallidus, entopeduncular nucleus, subthalamus, substantia nigra pars reticulata, and a few other regions without affecting limbic or mesolimbic regions. Thus, repeated daily treatment with a low dose of cocaine gradually decreased metabolic activity particularly in mesolimbic regions. Subsequent treatment with a higher dose produced metabolic activation mostly in extrapyramidal regions. This effect of chronic treatment could represent tolerance to the initial metabolic response, which can be replicated thereafter but only by increasing the drug dose. These results suggest that tolerance to the metabolic effects of cocaine in selective mesolimbic circuits may contribute to the development of behavioral dependence with repeated exposure.