Temporal upregulation of prodynorphin mRNA in the primate striatum after cocaine self‐administration

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Abstract
Several human and rat studies suggest that the striatal dynorphin system is important for neuroadaptation following cocaine exposure. In the current study, prodynorphin (PDYN) mRNA expression was examined in monkeys at initial and chronic phases of cocaine self‐administration. Adult Rhesus monkeys were trained to self‐administer food (banana flavoured pellets) or cocaine (0.03 or 0.3 mg/kg per injection) on a fixed interval 3‐min schedule for 5 or 100 sessions. Each session ended after 30 reinforcers were delivered. The PDYN mRNA expression was analysed in the precommissural striatum using in situ hybridization histochemistry. We found a specific activation of PDYN mRNA expression in the limbic‐innervated patch/striosome compartment of the dorsal caudate and dorsal putamen during the initial (i.e. 5 day) phase of the high dose cocaine self‐administration. After 100 days of the high dose exposure, the patch/striosome compartment remained activated, but an increase in PDYN mRNA levels was also evident in the sensorimotor‐connected matrix compartment of the caudate. Neither self‐administration phase resulted in significant changes in the corresponding striatal regions of the low dose cocaine‐exposed primates. Moreover, cocaine self‐administration failed to alter the PDYN mRNA expression in high‐ or low‐expressing PDYN cell populations in the nucleus accumbens during any condition studied. These results demonstrate the vulnerability of the dorsal striatum (in particular the caudate) to neuroadaptations following long‐term high dose cocaine self‐administration. In addition, the temporal nature of the changes in PDYN gene expression within the striatal compartments could reflect a change in drug responsivity that occurs during the transition to drug dependence.