Prolactin stimulates [ 3H]dopamine release from dispersed rat tubero-infundibular dopaminergic neurons and dopamine decreases gonadotropin-releasing hormone release induced by calcium ionophore

Abstract

stimulates [!H]dopamine release from dispersed rat tubero-infundibular dopaminergic neurons and dopamine decreases gonadotropin-releasing hormone release induced by calcium ionophore. Acta Endocrinol 1993;129:548-53. ISSN 0001-5598 In order to investigate the involvement of prolactin-dopamine and dopamine-gonadotropin interactions in the hypothalamo-pituitary axis of hyperprolactinemia, in vitro studies were performed using primary cultures of dispersed rat hypothalamic heterogeneous cells containing tubero-infundibular dopaminergic neurons or gonadotropin-releasing hormone (GnRH) neurons. We observed that prolactin caused dose-dependent stimulation of [3H]dopamine release after a 16-h incubation. Staurosporin (10 nmol/l), an inhibitor of protein kinase C, significantly reduced the [3H]dopamine release induced by prolactin (1 mg/l). Incubation of tubero-infundibular dopaminergic neurons with prolactin (1 mg/l) had no effect on intracellular cyclic adenosine monophosphate accumulation. Dopamine (1 \g=m\mol/l)significantly (p < 0.01) reduced the release of GnRH induced by 50 \g=m\mol/l calcium ionophore from dispersed hypothalamic cells from the preoptic area, while prolactin had no effect on GnRH release. These data support the hypothesis that the antigonadotropic effect of prolactin on the hypothalamus is mediated by an inhibitory effect of dopamine on GnRH release.Hypersécrétion of prolactin (PRL) in women is fre¬ quently associated with anovulation, but the mechan¬ ism by which PRL inhibits cyclic pituitary-ovarian activity is poorly understood. Bohnet et al. (1) reported that pulsatile LH secretion does not occur in most patients with hyperprolactinemia, and Bergh et al. (2) also reported that the mid-cycle LH surge is usually greatly diminished or absent in hyperprolactinemic women. Moreover, an impaired positive feedback effect of estrogen on LH release in patients with hyperprolacti¬ nemia has been reported by us (3) and others (4). In addition, reduction of circulating PRL levels by surgical removal of a PRL-secreting adenoma or by bromocriptine treatment led to restoration of the LH pulsatility (1), a return of the LH positive feedback response to estrogen (5) and ovulations (6). Tresguerres andEsquifino (7) also confirmed this effect of PRL in a hyperprolactinemic rat model. These findings in humans and animals suggest that PRL may inhibit LH release via a central action including the pituitary gland.Several studies in animals (8,9) have shown that PRL stimulates dopamine (DA) turnover in the hypothala¬ mus and that increased dopaminergic tone seems to be of physiological significance in the inhibition of LH secre¬ tion (10). The findings of other investigators (11-15) are consistent with the view that DA neurons in the lateral palisade zone of the median eminence mediate the inhibitory effect of PRL on gonadotropin secretion. Thus, PRL-DA and DA-gonadotropin interactions may play a role in the functional disturbances occurring in the hypothalamus of human subjects.We developed recently a system for p...