Seasonal Effect of Gonadotrophin Inhibitory Hormone on Gonadotrophin‐Releasing Hormone ‐induced Gonadotroph Functions in the Goldfish Pituitary

We have shown that native goldfish gonadotrophin inhibitory hormone (g G n IH ) differentially regulates luteinsing hormone (LH)‐β and follicle‐stimulating hormone (FSH)‐β expression. To further understand the functions of g G n IH , we examined its interactions with two native goldfish gonadotrophin‐releasing hormones, salmon gonadotrophin‐releasing hormone (s G n RH) and chicken (c) G n RH ‐II in vivo and in vitro . Intraperitoneal injections of g G n IH alone reduced serum LH levels in fish in early and mid gonadal recrudescence; this inhibition was also seen in fish co‐injected with either s G n RH or c G n RH ‐II during early recrudescence. Injection of g G n IH alone elevated pituitary LH ‐β and FSH ‐β m RNA levels at early and mid recrudescence, and FSH ‐β m RNA at late recrudescence. Co‐injection of g G n IH attenuated the stimulatory influences of s G n RH on LH ‐β in early recrudescence, and LH ‐β and FSH ‐β m RNA levels in mid and late recrudescence, as well as the c G n RH ‐II‐elicited increase in LH ‐β, but not FSH ‐β, m RNA expression at mid and late recrudescence. s G n RH and c G n RH ‐II injection increased pituitary g G n IH ‐R m RNA expression in mid and late recrudescence but g G n IH reduced g G n IH ‐R m RNA levels in late recrudescence. g G n IH did not affect basal LH release from perifused pituitary cells and continual exposure to g G n IH did not alter the LH responses to acute applications of G n RH . However, a short 5‐min G n IH treatment in the middle of a 60‐min G n RH perifusion selectively reduced the c G n RH ‐II‐induced release of LH . These novel results indicate that, in goldfish, g G n IH and G n RH modulate pituitary G n IH ‐ R expression and g G n IH differentially affects s G n RH and c G n RH ‐II regulation of LH secretion and gonadotrophin subunit m RNA levels. Furthermore, these actions are manifested in a reproductive stage‐dependent manner.