Turnover Changes in the Tubero-infundibular Dopamine Neurons During the Ovarian Cycle of the Rat1

Abstract

The amine turnover in the tuberoinfundibular DA neurons and the LH pituitary contents of the same group of rats have been studied during the various stages of the rat ovarian cycle. An estimation of the amine turnover has been obtained by studying the decline of DA stores in the median eminence following tyrosinehydroxylase inhibition using the histochemical fluorescence analysis of DA. The LH pituitary levels were determined with the help of a bioassay based on the fact that in vitro addition of LH to isolated prepubertal rat ovaries quantitatively stimulates the ovarian lactate production. The results show that the DA turnover was selectively decreased in the median eminence in proestrus and early estrus compared to other stages of the ovarian cycle. The decrease in DA turnover was associated with a drop in the pituitary LH level. No changes in the amine turnover occurred in the nigro-neostriatal DA neurons during the ovarian cycle. The results are interpreted to indicate that the tubero-infundibular DA neurons act to inhibit LHRF secretion from the median eminence by means of making axo-axonic contacts with LHRF-containing nerve terminals. It is proposed that removal of this dopaminergic inhibitory input is a necessary event for ovulation to take place in the normally cycling rat, facilitating discharge of LHRF induced via the preoptic tubero tracts. (Endocrinology88: 1415, 1971)