Effects of Endorphins on Prolactin and Growth Hormone Secretion in Rats

Abstract

Summary Intraventricular injection of β-endorphin and α-endorphin, in doses of 10 ng to 10 μg per rat, resulted in a significant and dose-related increase in plasma prolactin (PRL) levels in urethane-anesthetized male rats. β-endorphin (350 ng and 1 μg) and α-endorphin (10 μg) also caused a significant increase in plasma growth hormone (GH), although smaller doses of these peptides had no significant effect. The increases in plasma PRL and GH induced by β-endorphin were larger than those induced by α-endorphin when compared on a molar basis. Plasma PRL and GH responses to β-endorphin (1 μg) and α-endorphin (10 μg) were both significantly blunted by naloxone (250 μg), an opiate receptor blocking agent, when it was simultaneously injected intravenously. Met⁵-enkephalin (10 ng, 1 μg and 10 μg) and Leu⁵-enkephalin (1 μg and 10 μg) also significantly elevated plasma PRL levels, but less so than did β- and α-endorphins, whereas plasma GH concentrations were not changed at the doses examined. These results suggest that endorphins stimulate PRL and GH secretion in a different manner in the rat.