Effect of constant light and androgen-sterilization on the amine turnover of the tubero-infundibular dopamine neurons: blockade of cyclic activity and induction of a persistent high dopamine turnover in the median eminence.

Abstract

ABSTRACT The dopamine (DA) turnover in the median eminence and in the neostriatum in the rat has been studied during the normal ovarian cycle, in rats exposed to constant light and in androgen-sterilized rats treated with the tyrosine hydroxylase inhibitor α-methyl-tyrosine-methylester. The rate of depletion of DA provides an estimation of the turnover and this can be evaluated semi-quantitatively by means of the highly sensitive and specific histochemical fluorescence method of Falck and Hillarp for the demonstration of catecholamines (CA). The results reveal cyclic turnover changes in the tubero-infundibular DA neurons during the ovarian cycle but not in the neostriatal DA neurons. The turnover is decreased during pro-oestrus and early oestrus. In the persistent oestrous rats, whether induced by constant light or by neonatal steroid treatment, no cyclic changes in DA turnover in the median eminence occurred. The turnover remained at a constant high level in these rats, similar to that found in dioestrus of normal cycling rats. The high oestrogen secretion in persistent oestrous rats was probably responsible for the high DA turnover, since in these rats castration caused a marked reduction in the DA turnover. Testosterone and oestrogen caused a dosedependent increase in DA turnover in the median eminence in androgensterilized castrated rats. The DA neurons of the androgen-sterilized castrated rats were found to be somewhat less sensitive to treatment with oestrogen than those of normal castrated rats. The present results support the view that the tubero-infundibular DA neurons participate in the inhibition of the cyclic release of LHRF and FSHRF from the median eminence and that the inhibitory feed-back action of oestrogen and testosterone is partly mediated via an increase in DA turnover and release in the median eminence, resulting in inhibition of LHRF and-FSHRF release.