Abstract
The mesoaccumbens dopamine system is intricately involved in the locomotor stimulation produced by cocaine and sensitization of this effect following repeated cocaine administration. The mechanisms responsible for the expression of sensitized locomotion appear to involve alterations in both presynaptic (increased dopamine release) and postsynaptic (increased responsiveness of dopamine D1 receptors) aspects of dopamine neurotransmission within the nucleus accumbens. The present experiments used behavioral and single-cell electrophysiological techniques to determine the persistence of sensitization and of enhanced postsynaptic responses to cocaine within the nucleus accumbens following various periods of withdrawal from repeated cocaine treatment (10 mg/kg i.p., twice daily, 14 d). Behavioral sensitization to the locomotor stimulant effects of cocaine was evident after 1 d, 1 week, and 1 month, but not 2 months of withdrawal. A similar time course was observed for the enhanced efficacy of cocaine-induced inhibition of nucleus accumbens neurons, whether cocaine was administered systemically or locally by microiontophoresis. Nucleus accumbens neurons also exhibited sensitized inhibitory responses to iontophoretically applied GABA after 1 d of withdrawal, but not later times. These findings suggest that cocaine sensitization is relatively persistent, but not necessarily permanent, and support the hypothesis that expression of behavioral sensitization to cocaine involves actions within the NAc, particularly those mediated by dopamine D1 receptors.
