DIFFERENT SEROTONIN RECEPTOR TYPES PARTICIPATE IN 5-HYDROXYTRYPTOPHANINDUCED GONADOTROPINS AND PROLACTIN-RELEASE IN THE FEMALE INFANTILE RAT

Serotonin (5-HT) receptors can be classified into at least three, poss ibly up to seven, classes of receptors. They comprise the 5-HT1, 5-HT2 , and 5-HT3 classes, the 'uncloned' 5-HT4 receptor and the recombinant receptors 5-ht(5), 5-ht(6) and 5-ht(7). We investigated the role of d ifferent serotonin receptor types in a neuroendocrine response to the activation of the serotonergic system. Female immature rats were chose n as an experimental model as it has been shown that during the 3rd we ek of life, and not at later developmental stages, 5-hydroxytryptophan (5-HTP, a serotonin precursor) induces gonadotropin release in female s and not in males. Besides, at this age, serotonin releases prolactin in both sexes. 5-HTP (50 mg/kg) released prolactin, luteinizing hormo ne (LH) and follicle-stimulating hormone (FSH) as expected. Ketanserin (5-HT2A antagonist) and methysergide (5-HT2C antagonist) blocked 5-HT P-induced prolactin release, but did not block the LH or FSH responses . Ondansetron (5-HT3 receptor antagonist) did not modify prolactin res ponse to 5-HTP, whereas it blocked 5-HTP-induced LH and FSH release. P ropranolol (5-HT1 and beta-adrenergic antagonist) blocked prolactin, L H and FSH release induced by 5-HTP. The 5-HT2C agonist 1-(3-chlorophen yl)piperazine dihydrochloride released prolactin, without modifying LH or FSH release. Methyl-quipazine and phenylbiguanide (5-HT3 agonists) increased both LH and FSH levels, without altering prolactin secretio n. The present experiments indicate that serotonin acting at the 5-HT3 receptor mediates LH and FSH release in infantile female rats, wherea s 5-HT2C or (2A) receptor types participate in the release of prolacti n at this age. 5-HT1 receptor type may be involved in the release of t he three hormones, though a (b)eta-adrenergic component of the respons e cannot be discarded.