Temperature, but not photoperiod, influences gonadotropin-releasing hormone binding in the pituitary of the three-spined stickleback, Gasterosteus aculeatus.

Gonadotropin-releasing hormone (GnRH) binding characteristics in pituitaries of stickle-backs under different physiological conditions were studied using D-Arg6-Pro9-salmonGnRH-NEt as labeled ligand. Both males and females displayed marked seasonal changes in the capacity of high-affinity GnRH binding sites; there was a high content in the breeding season (summer) (800-1500 pmol/pituitary) and no detectable high affinity (< 150 pmol) binding in late winter-early spring. The binding capacity was lower in postbreeding fish (ca. 400 pmol/pituitary in females, ca. 900 pmol in males) than in breeding fish (females: ca. 1850, males ca. 1400 pmol). GnRH binding sites were also studied in fish exposed to long and short photoperiod in combination with high and low temperature in winter. Only long photoperiod in combination with high temperature stimulated sexual maturation. The capacity of the GnRH binding sites was similar in fish exposed to long (females 1550 pmol, males 1000 pmol) and short (females 1800, males 900) photoperiod in combination with high temperature. In fish exposed to low temperature, binding was nondetectable irrespective of the photoperiod.