Kappa opiate agonists modulate the hypothalamic-pituitary-adrenocortical axis in the rat

Abstract

Systemic injections of opiate agonists were made in male rats to elucidate the involvement of multiple opioid receptors in the stress response. As an index of activity in the hypothalamic-pituitary-adrenocortical axis, plasma corticosterone was measured by radioimmunoassay. Rats were injected with ethylketocyclazocine (EKC), U50488H, MR2034, bremazocine or tifluadom and sacrificed 1 hr later. These kappa agonists produced potent, dose-dependent, stereospecific increases in plasma corticosterone levels at doses far below those needed to elicit analgesia. These effects were reversed by opiate antagonists, naloxone or Win 44441-3, which by themselves caused dose-dependent decreases in plasma corticosterone. Animals made tolerant to the prototype kappa agonist, U50488H, showed an attenuated response to an acute injection of the drug. However, when animals made tolerant to morphine were injected acutely with U50488H, the drug caused a dramatic increase in corticosterone levels. In hypophysectomized rats, U50488H and L-EKC did not increase plasma corticosterone. The agonist/antagonists, butorphanol and cyclazocine, when injected, behaved like kappa agonists and increased plasma corticosterone levels potently. The mu opiates, morphine and etorphine, also had similar effects but were less potent and efficacious than the kappa agonists. The delta agonist D-Ala-D-Leu enkephalin showed similar results, confirming a mu and delta opioid input into the hypothalamic-pituitary-adrenocortical axis. There were concomitant increases in plasma adrenocorticotropin in morphine-, D-Ala-D-Leu enkephalin-, L-EKC- and U50488H-treated rats which were also seen in adrenalectomized rats. D-EKC and D-cyclazocine, which bind to sigma sites, had no effect on corticosterone.(ABSTRACT TRUNCATED AT 250 WORDS)