Specific reductions of striatal prodynorphin and D₁ dopamine receptor messenger RNAs during cocaine abstinence

It is well established that the opioid neuropeptide and dopamine systems are altered following the use of cocaine. However very little information is available about their possible involvement during cocaine abstinence. In the present study, the mRNA expression of the dopamine receptors, D₁ and D₂, and the opioid peptides, prodynorphin and proenkephalin, were analyzed in the rat striatum using in situ hybridization histochemistry. Saline or cocaine (30 mg/kg, i.p.) were administered to rats once daily for 1 or 10 days. To examine cocaine abstinence, animals were treated for 10 days as described followed by a 10-day drug free period. Acute and intermittent cocaine administration elevated the prodynorphin mRNA expression in the dorsal striatum, consistent with previous reports, while the abstinent phase resulted in a significant reduction of prodynorphin mRNA levels in the ventrorostral striatum. The D₁-receptor mRNA was decreased in the caudorostral striatum during cocaine withdrawal, a finding opposite to the increase observed following a single injection of the drug. Proenkephalin and the D₂-receptor mRNAs were not altered during cocaine abstinence, though proenkephalin was elevated following acute but not repeated cocaine administration. These results show long-term suppression on prodynorphin and D₁-receptor systems in specific striatal populations localized mainly in rostral areas during withdrawal from cocaine.