Negative Feedback Effects of Progesterone Correlated with Changes in Hypothalamic Norepinephrine and Dopamine Turnover Rates, Median Eminence Luteinizing Hormone-Releasing Hormone, and Peripheral Plasma Gonadotropins

Progesterone (P) amplifies and advances gonadotropin surges when administered to estradiol (E₂)-treated ovariectomized rats. While daily rhythmic LH surges continue to occur in E₂-treated rats, they are extinguished in E₂- and P (E₂P)-treated animals 24 h after P treatment. We examined whether this negative feedback effect on P affects catecholamine activity within the median eminence, medial preoptic nucleus, arcuate nucleus, and suprachiasmatic nucleus and also the changes which occur in median eminence LHRH concentrations. Twenty-four hours after P exposure, LH and FSH surges in E₂Ptreated rats are extinguished, and the magnitude of the PRL surge is reduced. In E₂-treated rats, there is an increase in medial preoptic nucleus, suprachiasmatic nucleus, and median eminence NE turnover rates from 1000–1200 to 1500–1700 h, but these changes in NE activity do not occur in E₂-treated rats which received P 24 h earlier. Rather, such E₂P-treated animals have markedly elevated medial preoptic nucleus, arcuate nucleus, and median eminence DA turnover rates during the period that plasma gonadotropin levels are suppressed. No differences in median eminence LHRH concentrations in E₂- or E₂P-treated rats were detected. P may exert its negative feedback action in suppressing LH, FSH, and PRL release by blocking activation of the hypothalamic noradrenergic system and by increasing dopaminergic activity within the tuberoinfundibular system.