ROLE OF SEROTONIN IN THE REGULATION OF PROLACTIN GENE-EXPRESSION IN THE MALE-RAT AS EVALUATED BY IN-SITU HYBRIDIZATION

It is well documented that serotonin (5-HT) is involved in the regulat ion of prolactin (PRL) release as a stimulator. To evaluate the role o f 5-HT on PRL gene expression, we have investigated the effects of rep eated administration (during 2 days) of 5-HT, the 5-HT1+2 receptor ant agonist methysergide, the 5-HT2 receptor antagonist ketanserin, and a combination of 5-HT and methysergide on PRL mRNA levels as measured by in situ hybridization in the adult male rat. The treatment with 5-HT induced a 14% increase in the hybridization signal, although the admin istration of methysergide produced a small reduction (5%) in PRL mRNA levels. On the other hand, ketanserin had no effect on the hybridizati on signal. The stimulatory effect of 5-HT was completely prevented by the concomitant administration of methysergide. The present results, t ogether with previous data, indicate that 5-HT exerts a positive tonic influence not only on the release of PRL but on the biosynthesis of t he hormone as evaluated by mRNA level measurements. They also strongly suggest that the role of 5-HT in the regulation of PRL secretion is m ediated via activation of 5-HT1 receptors, although a dopaminergic eff ect of methysergide on PRL secretion cannot be totally excluded.