EFFECTS OF MORPHINE AND NALOXONE ON PROLACTIN AND GROWTH-HORMONE GENE-EXPRESSION IN THE MALE-RAT PITUITARY-GLAND

It is generally admitted that opioids can stimulate the release of bot h prolactin (PRL) and growth hormone (GH). In order to investigate the role of opioids in the regulation of PRL and GH gene expression in th e rat pituitary, we studied the effects of chronic administration of t he opioid drug morphine and an opiate receptor antagonist naloxone on both PRL and GH gene expression as measured by in situ hybridization. Four-day treatment with morphine (40 mg/kg/day) produced a 12% increas e in PRL mRNA levels. Conversely, naloxone (4 mg/kg/day) decreased the autoradiographic reaction by 10%. The concomitant administration of m orphine and naloxone induced no significant changes in PRL gene expres sion. On the other hand, treatment with morphine produced a 22% decrea se in GH mRNA levels, an effect which was prevented by the concomitant administration of naloxone. When injected alone, naloxone did not mod ify the hybridization signal. These results clearly indicate that opio ids are involved not only in the regulation of GH and PRL release but also in the gene expression of the two hormones. The discordance obser ved between the acute effects of morphine on GH release and the effect of the opioid drug on mRNA levels remains to be clarified.