Rationale: Kappa agonists can attenuate reinstatement of cocaine-seeking behavior induced by cocaine priming. The mechanisms underlying this effect have not been characterized fully but may have a serotonergic component as kappa agonists also increase the release of serotonin (5-hydroxytryptamine, 5-HT).
Objectives: This study investigated the role of kappa opioid receptor and 5-HT mechanisms in kappa agonist-induced attenuation of cocaine priming in monkeys.
Methods: Squirrel monkeys were trained to self-administer cocaine (0.18-0.3 mg/kg/injection) under a second-order schedule in which drug seeking was maintained jointly by cocaine injections and a cocaine-paired visual stimulus. In extinction sessions, saline was substituted for cocaine, and the cocaine-paired stimulus was omitted. During test sessions, only saline was available for self-administration, and response-contingent presentations of the cocaine-paired stimulus were restored.
Results: Priming injections of cocaine (0.1-1.0 mg/kg) induced reinstatement of drug seeking. Maximal levels of responding were similar to those maintained by active cocaine self-administration. Pretreatment with the kappa agonists enadoline (0.01 mg/kg) and spiradoline (0.3 mg/kg) or the 5-HT transport inhibitors fluoxetine (5.6 mg/kg) and citalopram (10.0 mg/kg) attenuated the priming effects of cocaine, shifting the cocaine dose-response function rightward and downward. Inhibition of cocaine-induced reinstatement of drug seeking by spiradoline and fluoxetine was reversed by R(+)8-hydroxy-2-(di-n-propylamino)tetralin (0.03 mg/kg), a 5HT(1A) agonist that inhibits 5-HT release. The effects of spiradoline also were reversed by the kappa antagonist nor-binaltorphimine (10.0 mg/kg).
Conclusions: Results suggest that the capacity of kappa opioid agonists to increase extracellular 5-HT levels may at least partially underlie kappa agonist-induced modulation of cocaine seeking.