Role of dopamine in the regulation of gonadotropin-releasing hormone in the male rat brain as studied by in situ hybridization.

Abstract

The effects of the dopaminergic antagonist haloperidol (HAL) as well as the D2 dopamine receptor agonist bromocriptine (BRO) on GnRH messenger RNA (mRNA) levels in the male rat were investigated by quantitative in situ hybridization. Since it had already shown that androgens could induce a decrease in GnRH mRNA levels in castrated rats, the involvement of the dopaminergic system in the inhibitory effect of dihydrotestosterone (DHT) was also investigated. In situ hybridization was performed on paraformaldehyde-fixed cryostat sections through an area extending from the preoptic area to the anterior hypothalamus using a 35S-labeled 48-base oligodeoxynucleotide complementary to the GnRH coding region of the GnRH DNA. The corresponding mRNA levels were assessed by counting the number of silver grains overlying labeled neurons. In intact animals, a 14-day treatment with BRO increased by 67% the number of silver grains per neuron while HAL decreased by 31% the value of this parameter. Hypophysectomy which induced a 22% decrease in the hybridization signal could not prevent the effects of BRO or HAL. Gonadectomy performed 14 days earlier increased by 54% the mean number of silver grains while a 14-day treatment of gonadectomized animals with DHT decreased the hybridization signal by 48%. On the other hand, the concomitant administration of HAL and DHT did not prevent the inhibitory effect of DHT but rather resulted in a decrease of the hybridization signal which was more marked than that induced by DHT or HAL alone. The present data clearly demonstrate that GnRH mRNA levels are positively regulated by dopamine and that the effects of BRO and HAL on GnRH mRNA are not mediated by variations in pituitary hormone secretion. Moreover, our results suggest that the effect of DHT on GnRH gene expression is probably not mediated by the dopaminergic system.