Role of serotonin in the regulation of the dynorphinergic system by a kappa-opioid agonist and cocaine treatment in rat CNS

It has been shown that chronic cocaine increases prodynorphin mRNA in the caudate putamen and decreases it in the hypothalamus. In addition, treatment with a kappa-opioid receptor agonist produced the opposite effect on prodynorphin gene expression in these brain regions and also evoked a decrease in the hippocampus. It is already known that kappa-opioid receptor agonists decrease the development of sensitization to some of the behavioral effects of cocaine. The serotonin system has also been shown to regulate dynorphin gene expression and a continuous infusion of fluoxetine induced prodynorphin gene expression in the same pattern as the kappa-opioid agonist (+)(5a,7a,8b)-N-methyl-N-[7-(1-pyrrolidinyl)-1 oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) in the brain regions investigated. It is interesting to note that treatment with a continuous infusion of cocaine produced different effects on this parameter. To determine whether serotonin plays a role in the regulation of prodynorphin mRNA by kappa-opioid agonists or cocaine, rats were treated with the serotonin depleter parachloroamphetamine (PCA). Beginning 24 h later, rats were treated with the selective kappa-opioid agonist U-69593 for 5 days or continuously with cocaine for 7 days and prodynorphin mRNA was measured. Prodynorphin mRNA was decreased significantly in the hypothalamus, caudate putamen, and hippocampus of rats treated with a single injection of PCA. Subsequent to PCA administration the effects of U-69593 or cocaine on prodynorphin mRNA were differentially affected across brain regions. Prodynorphin gene expression was still increased by U-69593 treatment in the hypothalamus and decreased in the caudate putamen. Cocaine treatment still produced a decrease in this parameter in the hypothalamus and an increase in the caudate putamen. In contrast, in the hippocampus, the decrease in prodynorphin mRNA produced by U-69593 was no longer evident after PCA and cocaine, which previously had no effect, now increased it in the serotonin-depleted group. These findings suggest that serotonin is necessary to maintain normal levels of dynorphin mRNA in all of the investigated brain areas and that the regulation of prodynorphin mRNA expression by chronic treatment with a kappa-opioid receptor agonist or cocaine requires serotonin in the hippocampus, but not in the hypothalamus or caudate putamen.