Opiate inhibition of growth hormone secretion in young chickens

Plasma concentrations of growth hormone (GH) were decreased following the intravenous administration of morphine sulfate. Maximum inhibition of GH secretion was observed 40 min after morphine sulfate challenge. At this time, doses of morphine sulfate (at 5 mg and 50 mg/kg) reduced the GH concentrations by 86 and 90%, respectively, in comparison with those in the vehicle-injected controls. An opiate antagonist, naloxone, had no stimulatory effect on basal GH concentrations, but attenuated the GH response to morphine. Neither morphine nor naloxone had any significant effect on circulating luteinizing hormone (LH) levels. These results indicate an inhibitory opiate pathway in the control of GH release and demonstrate effects on GH and LH secretion contrary to those observed in mammalian species.