Hypothalamic tyrosine hydroxylase activity and plasma gonadotropin and prolactin levels in ovariectomized-steroid treated rats.

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase (TH) activity were evaluated in ovariectomized (OVX) estradiol benzoate (EB) or progesterone (P) treated rats. Single injection of 10 micrograms or daily injection of 5 micrograms EB/rat for 7 days significantly lowered gonadotropin levels in OVX animals and elevated PRL levels. Single injection of 2 mg or daily injection of 200 micrograms P/rat for 7 days increased gonadotropin and PRL levels. Hypothalamic TH activity was significantly elevated by estradiol. Single injection of 2 mg P suppressed TH activity in contrast to the elevation in enzyme activity following chronic treatment. These results indicate that hypothalamic noradrenergic as well as dopaminergic neurons participate in the stimulatory or inhibitory feedback effects of ovarian hormones on gonadotropin and PRL secretion.