Alterations in pituitary GnRH and dopamine receptors associated with the seasonal variation and regulation of gonadotropin release in the goldfish ( Carassius auratus)

Abstract

Seasonal variations in the serum concentrations of gonadotropin (GtH) and the serum GtH response to intraperitoneal injection of domperidone, a specific dopamine receptor antagonist, were examined in goldfish. In addition, the effects of in vivo treatment of goldfish with a superactive analog of salmon gonadotropin-releasing hormone (sGnRH-A) and domperidone on the binding parameters of pituitary GnRH and dopamine receptors were investigated in goldfish. Serum concentrations of GtH and the maximum GtH response to domperidone increased in correlation with advancing gonadal maturation; values increased from those in sexually regressed fish in January to maximal levels observed in fish in late stages of gonadal recrudescence in March, followed by a decrease with gonadal regression. At all stages, injection of domperidone increased serum concentrations of GtH in a dose-related manner; however, the ED50 of domperidone did not vary significantly over the course of the reproductive cycle. Multiple injections of sGnRH-A caused a progressively increasing and more prolonged serum GtH response; as well, multiple sGnRH-A treatment significantly potentiated the serum GtH response to domperidone without altering the ED50 of domperidone. sGnRH-A treatment caused a significant increase in the number of dopamine/neuroleptic receptors in the goldfish pars distalis, accompanied by a nonsignificant increase in dopamine/neuroleptic receptors in the neurointermediate lobe, and significantly increased the number of high-affinity GnRH receptors in the goldfish pituitary. Treatment with domperidone also significantly increased pituitary high-affinity GnRH receptor numbers. Receptor affinities were not significantly altered by either sGnRH-A or domperidone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)