The effects of opiate agonists on growth hormone and prolactin release in rats

Various opioid receptor agonists, including Met5-enkephalin amide, Leu5-enkephalin amide, [D-Ala]2-Met5-enkephalin amide, [D-Ala]2-Leu5-enkephalin amide, morphine sulfate, d-methadone hydrochloride, and l-methadone hydrochloride were administered to adult male rats by subcutaneous injection. All opioid receptor agonists except Leu5-enkephalin amide significantly stimulated growth hormone and prolactin release. Naloxone and naltrexone blocked the hormone stimulatory effects of the opioids and both naloxone and naltrexone, when administered alone, significantly reduced serum growth hormone and prolactin concentrations. The dopaminergic agonist apomorphine, but not the alpha-adrenergic agonist clonidine, blocked opiate stimulation of prolactin. Morphine sulfate caused growth hormone release in rats pretreated with alpha-methyl-p-tryosine, a catecholamine synthesis inhibitor. Cholinergic agonists, physostigmine and pilocarpine, antagonized the growth hormone and prolactin release induced by morphine sulfate. The data suggest that the opiates stimulate prolactin via an interaction with catecholaminergic neurons controlling prolactin release and stimulate growth hormone via a mechanism independent of alpha-adrenergic or general catecholaminergic influence. The mechanism through which cholinergic agonists act to inhibit opiate agonist stimulation of growth hormone is presently unknown.