Comparative Maturation of the Regulation of Prolactin and Thyrotropin by Serotonin and Thyrotropin-Releasing Hormone in Male and Female Rats

The aim of this study was to compare the effects of TRH, serotonin, and haloperidol on the secretion of PRL and TSH in male and female rats from birth to puberty. Serum PRL in males and females was low from birth to 20 days; it then increased gradually until puberty. TSH did not change significantly throughout the period studied. The PRL-releasing effect of serotonin became evident at 12 days, and at all times when the response occurred, it was greater in males than in females. This was also observed in adult rats in which 2.5 mg/kg ip serotonin caused an increase in serum PRL in males but not in diestrous females. Serotonin did not modify TSH at any age. On the other hand, TRH induced the release of TSH and PRL from the first day of life and no sex difference was observed. Haloperidol caused release of PRL from birth, and the effect increased significantly with age. The PRL increase induced with haloperidol was greater than that obtained with TRH used in supramaximal doses. A clear sexual difference became evident on day 20, the PRL-releasing effect of haloperidol being more pronounced in females than in males. Haloperidol did not modify TSH values. It is concluded that the mechanisms by which TRH and dopamine regulate PRL secretion mature earlier than those influenced by serotonin. Sexual differences in the manifestation of these mechanisms were observed, the male being more sensitive to serotonin and the female to haloperidol. TRH causes release of TSH from birth; the serotoninergic and dopaminergic pathways are not of paramount importance in the regulation of TSH in the prepubertal rat.