COCAINE-INDUCED CONDITIONED LOCOMOTION - ABSENCE OF ASSOCIATED INCREASES IN DOPAMINE RELEASE

The potent reinforcing effects of cocaine can readily become associated with salient environmental stimuli that acquire secondary reinforcing properties. This phenomenon is of considerable significance as intense craving can be evoked by stimuli previously associated with the effects of cocaine. It has been proposed that the reinforcing properties of these conditional stimuli are due to their ability to elicit neural events that are similar to those produced by the drug itself. Given the large body of evidence that implicates the mesolimbic dopaminergic projection in the unconditioned behavioural properties of cocaine, the present study used in vivo microdialysis to determine whether stimuli paired with cocaine elicit increases in interstitial dopamine in the nucleus accumbens that are similar to the unconditioned effects of this drug. When administered acutely, cocaine (10 mg/kg, i.p.) produced a potent unconditioned increase in interstitial dopamine concentrations (300% of basal values) in the nucleus accumbens. The results from two separate experiments indicate that the administration of cocaine (10 mg/kg for seven days) in association with a specific environment produced significant locomotion in that environment. Compared to subjects that received saline in both settings, rats that received cocaine in their home cage (pseudoconditioned group) did not exhibit increased locomotion on the test day. Although repeated pairing of cocaine with a specific environment produced conditioned locomotion, there was no concomitant conditional increase in dopamine release. Specifically, the modest increase in dopamine (10-15% above basal values) observed after exposure to the conditional environment was equal in the conditioned and pseudoconditioned groups. Moreover, increases in the interstitial concentrations of the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid were not significantly different between the conditioned and pseudoconditioned subjects. These data do not support the hypothesis that stimuli paired with cocaine produce their behavioural effects by eliciting similar neurochemical effects as cocaine.