a Departments of Pharmacology (1st Chair), School of Medicine, Italy
b Biological Sciences, University of Milan, Milano 20129, Italy
Abstract:
The effect of Leu5-enkephalin on growth hormone (GH) and prolactin (PRL) release was studied in the infant rat and compared to that of morphine. In 10 day-old pups, intracerebroventricular injection of Leu5-enkephalin (50, 75 and 100 μg) resulted in a dose-related increase in plasma GH; morphine was active as GH releaser at the dose of 5 and 10 μg, but not at 2.5 μg. Pretreatment with naloxone (2 mg/kg ip) suppressed the GH-releasing effect of either Leu5-enkephalin (100 μg) or morphine (10 μg). Leu5-enkephalin (75 and 100 μg) induced a rise in plasma PRL which was neither dose-related nor antagonized by naloxone; morphine (5 and 10 μg) was active as PRL releaser and its effect was antagonized by naloxone. These results indicate that: 1) Leu5-enkephalin stimulates both GH and PRL release; 2) the release of GH by Leu5-enkephalin but likely not that of PRL involves specific opiate receptors; 3) morphine releases GH and PRL through specific opiate receptors.