Plasma luteinizing hormone and prolactin levels and hypothalamic catecholamine synthesis in steroid-treated ovariectomized rats.

The effects of estrogen (E) and progesterone (P) on synthesis rates and endogenous levels of hypothalamic norepinephrine (NE) and dopamine (DA) were determined in individual ovariectomized (OVX) rats. 3H-tyrosine (3H-T) was injected intra-arterially and the rate of its incorporation into 3H-DA and 3H-NE was determined at 10, 15, 30, and 45 min. Steroid treatment for 2 days effectively decreased plasma LH levels and elevated plasma prolactin (Prl) levels while endogenous levels of NE and DA were not affected. NE synthesis was not affected by steroid treatment. A significant increase in DA synthesis was correlated with low plasma LH levels, suggesting that DA inhibits LH release. The elevated plasma Prl levels seen in steroid-treated rats were correlated with increased DA synthesis in the hypothalamus. This may indicate that elevated plasma Prl levels stimulate dopaminergic neurons in a short-loop negative feedback fashion.