In vitro pituitary prolactin, growth hormone and follicle stimulating hormone secretion during sexual maturation of female rats primed with melatonin

The influence of in vivo melatonin administration on in vitro pituitary fol licle stimulating hormone (FSH), growth hormone (GH) and prolactin secretio n, as well as the possible influence of dopamine (DA) were evaluated in pre pubertal (31-day-old), pubertal (33-day-old) and adult female rats at diest rus phase of the sexual cycle. The in vitro pituitary hormone secretions we re evaluated at basal rate for the first hour of incubation only, in Krebs Ringer phosphate (KRP) (I-1) and after a second hour of incubation with KRP (I-2) or with KRP+DA (I-2 plus DA). I1PRL secretion was significantly high er in 33-day-old control and melatonin treated (MEL) rats as compared to I- 2 periods. However, in 31-day-old rats I-1 secretion was higher than in the I-2 or I2+DA periods, in MEL rats. In vitro GH secretion was significantly higher at I-1 than during I-2 periods in the control 31- and 33-day-old gr oups, but not in MEL rats. The only significant effect of DA was the elevat ion of GH in prepubertal MEL rats. In vitro FSH release was increased by me latonin in 31- and 33-day-old female rats. No differences in PRL, GH and FS H secretion were found in adult rats. In conclusion, the results show that melatonin effects upon in vitro pituitary gland activity are reproductive-s tage-dependent modifying the secretory capacity of the lactotrop, gonadotro p and somatotrop during prepubertal and pubertal ages but not in adult rats studied at a quiescent phase of the sexual cycle.