Testosterone regulation of gonadotropin production in goldfish.

Gonadotropin (GTH) production in teleosts is primarily regulated by complex interrelationships between the sex steroids, neurotransmitters, and GTH-releasing hormone (GnRH). Both steroids and GnRH individually regulate GTH secretion and subunit (alpha and beta) gene expression. Steroids may exert their actions directly at the level of pituitary, or indirectly by affecting brain GnRH and other neurotransmitters, causing stimulation or inhibition of GTH production. This paper reviews studies concerning the effects of testosterone on basal and GnRH-induced GTH production in the goldfish pituitary. Overall, the findings demonstrate that testosterone and GnRH individually exert stimulatory actions on GTH production, whereas a combination of testosterone and GnRH treatments reduce GTH subunit mRNA levels in the goldfish pituitary. The secretion of GTH hormones, however, is stimulated by GnRH, as well as by combined testosterone and GnRH treatments. The contributing factors that could explain the observed inhibitory effects include decreased transcription, increased posttranscriptional degradation, or increased translation of GtH subunit mRNA in the goldfish pituitary. At the present time, however, we are not able to distinguish between these possibilities.