The effects of season and temperature on reproductive neuroendocrine function : in particular the neurotransmitter GABA in the goldfish, Carassius auratus
A series of experiments were undertaken to further characterise the GABAergic system in goldfish, and its role in reproduction. In addition, experiments were performed to examine the effects of water temperature on neuroendocrine systems. As reproduction is a seasonal event in fish, the GABAeric system was examined at two different times of the year. In both the female and male goldfish GABA synthesis rates in the telencephalon were decreased when serum testosterone (T) levels were elevated. In contrast, increased in GABA synthesis rates in the hypothalamus were detected when serum estradiol (E2) levels were elevated. In female goldfish, GVG-injection stimulated GTH-II release in January (mid to late gonadal recrudescence) but not in July (sexually regressed). In contrast, in males GVG-injection stimulated GTH-II release in fish in July but not in October (early recrudescent fish). In goldfish, temperature is an important reproductive cue therefore, the effects of three differing water temperatures (11°C, 18°C and 24°C) on the GABAergic system was examined. GABA synthesis rates significantly increased in the telencephalon and hypothalamus of goldfish maintained at 24°C, compared with 11°C and 18°C. In the pituitary significant increases in GABA synthesis rates were seen between 11°C and 18°C and 18° and 24°C. The possibility is that the increases in GABA synthesis rates was due to increases in the GABA synthesising mRNA expression in the brain of female goldfish. Two forms of GAD have been identified in goldfish, GAD65 and GAD67.Water temperature had no effect on GAD65 mRNA expression in either the telencephalon or hypothalamus however, GAD67 mRNA expression significantly increased in the telencephalon of fish held at 18°C compared with those held at 11°C. In goldfish, increasing water temperature increases blood GTH-II levels.