Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity in rats during estrous cycle, after overiectomy and after blockade of catecholamine biosynthesis.

ABSTRACT

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity were evaluated at 0900, 1200 and 1700 h during diestrus, proestrus and estrus, ovariectomized and after systemic administration of reserpine or α-methyl p-tyrosine, which interfere with catecholamine biosynthesis, in rats. Gonadotropin and prolactin levels showed peak values during the afternoon of proestrus, while hypothalamic tyrosine hydroxylase activity was markedly lowered at 1200 on proestrus. Gonadotropin levels were slightly lowered whereas prolactin concentrations and hypothalamic tyrosine hydroxylase activity were significantly increased by reserpine. Depletion of hypothalamic dopamine by reserpine apparently resulted in significant elevation of prolactin levels which inturn induce tyrosine hydroxylase. Gonadotropin levels and hypothalamic tyrosine hydroxylase activity were significantly suppressed after the administration of α-methyl p-tyrosine. Prolactin levels, however, were elevated significantly. These results indicate that catecholamines are involved in the control of gonadotropin and prolactin release during estrous cycle and inhibition of catecholamines biosynthesis by α-methyl p-tyrosine could result in suppression of gonadotropin levels, whereas removal of tonic inhibition of hypothalamic dopamine by α- methyl-p-tyrosine elevate prolactin levels.