j. Neural Transmission 40, 235--244 (1977) ,/ourno2or Neur~ T~ms.~n 9 by Springer-Verlag 1977 Effects of Estrogen on Prolactin and Tuberoinfundibular Dopaminergic Neurons D. C. Eikenburg, A. J. Ravitz, G. A. Gudelsky, and K. E. Moore Department of Pharmacology, Michigan State University, East Lansing, Michigan, U.S.A. With 2 Figures Received November 28, 1976 Summary Serum prolactin concentrations and dopamine turnover in the striatum and median eminence were studied in male rats aRer the administration of estradiol benzoate. The a-methyltyrosine-induced reduction of dopamine concentrations in these brain regions was used to evaluate relative rates of turnover. Steady state dopamine concentrations in the median eminence and striatum were not altered by 1, 3 or 5 days of estradiol treatment. However, 3 or 5 days of estradiol administration enhanced dopamine turnover in the median eminence but not in the striatum. Estradiol treatment failed to alter dopamine turnover in the median eminence of hypo- physectomized rats. Estradiol increased serum prolactin concentrations at all of the times examined. Although a-methyhyrosine also increased serum prolactin, this increase was further enhanced in estradiol-treated rats. The increased prolactin response to a-methyhyrosine and increased dopamine turnover in the median eminence of estradiol-treated rats suggests that tuberoinfundibular dopaminergic neurons may be part of a hormonal- neuronal negative feedba& loop which functions to regulate prolactin secretion. Introduction In recent years there has been a growing body of evidence suggesting that the tuberoinfundibular dopamine system is involved in a neuronal-hormonal negative feedback loop which modulates Journal of Neural Transmission40/4 17