Shifted Balance Between Ventral Striatal Prodynorphin and Proenkephalin Biases Development of Cocaine Place Avoidance.

Evidence from human self-report and rodent models indicate that cocaine can induce a negative affective state marked by panic and anxiety, which may reduce future cocaine use or promote co-use with opiates. Dynorphin-mediated signalling within the striatum is associated with negative affect following cocaine withdrawal and stress-induced cocaine seeking. Here, we used a trace conditioning procedure to first establish the optimum parameters to capture this transient cocaine negative affective state in wild-type mice, and then we investigated striatal opioid peptides as a substrate mediating cocaine conditioned place avoidance (CPA). Previous reports indicate that trace conditioning, where drug administration occurs after removal from the conditioning chamber, results in CPA to ethanol, nicotine and amphetamine. We tested different cocaine doses, conditioning session lengths and apparatus types to determine which combination yields the best cocaine CPA. Cocaine CPA was moderately larger at the highest cocaine dose (25 mg/kg), but this did not generalize across apparatus types and the effect was transient; thus, data were collapsed across all parameters. Cocaine conditioning scores were variable but also became more polarized across conditioning, with approximately equal proportions developing preference and avoidance. We then correlated cocaine CPA with striatal gene expression levels of the opioid peptides enkephalin (Penk) and dynorphin (Pdyn) using qPCR. Cocaine CPA was correlated with low Pdyn levels and a low Pdyn:Penk ratio in the ventral, but not dorsal, striatum. Consistent with this, mice with higher striatal Pdyn relative to Penk were more resistant to developing cocaine CPA compared with littermate controls. This approach revealed a subset of subjects sensitive to the aversive state immediately following cocaine administration. Our findings suggest that striatal dynorphin has opposing roles in mediating the aversion associated with acute cocaine intoxication versus cocaine withdrawal.