Self-administration of fentanyl, cocaine and ketamine: effects on the pituitary-adrenal axis in rhesus monkeys.

Rationale: Drugs of abuse can affect the functioning of the hypothalamic-pituitary-adrenal (HPA) axis. Acute administration of drugs that serve as reinforcers have been observed to stimulate the rat HPA axis, leading to the suggestion that these stimulatory effects may contribute to the development of drug-maintained behaviors.

Objectives: To determine whether reinforcing drugs that are dissimilar with respect to their mechanisms of action have similar effects on HPA axis activity at doses that are self-administered. Rhesus monkeys were randomly assigned to self-administer the mu-opioid agonist fentanyl, the psychomotor stimulant cocaine, or the NMDA antagonist ketamine.

Methods: Each monkey was trained to press a lever in order to receive an intravenous injection of drug or saline. Blood samples were obtained before, during, and after the self-administration sessions and assayed for ACTH and cortisol by radioimmunoassay.

Results: Fentanyl, cocaine, and ketamine were each self-administered across a range of doses. However, the three drugs differed in their effects on ACTH and cortisol. Cocaine stimulated ACTH and cortisol secretion, a finding that is consistent with previous rat and primate studies. Self-administration of both fentanyl and ketamine inhibited HPA axis activity. HPA inhibition by fentanyl is consistent with other monkey and human studies, and contrasts with the stimulatory effects of mu-opioids in rodents. The inhibitory effect of ketamine on ACTH and cortisol secretion contrasts with findings in the few primate studies that have evaluated NMDA antagonists. Neither fentanyl nor cocaine, at doses that maintained maximum rates of responding, produced significant changes in ACTH and cortisol levels.

Conclusions: There appears to be little commonality between different classes of abused drugs and their effects on the HPA axis, which calls into question the necessity for HPA axis stimulation in the reinforcement of drug-maintained behavior.