Changes in brain levels of gonadotropin-releasing hormone and serum levels of gonadotropin and growth hormone in goldfish during spawning

Abstract

Changes in serum levels of gonadotropin (GtH) and growth hormone (GH) and in levels of gonadotropin-releasing hormone (GnRH) in the pituitary and discrete brain areas were studied in male goldfish during spawning with spontaneously ovulating females. Spontaneous ovulation in females was induced by raising water temperature from 12 to 20 °C and providing spawning substrate of artificial floating vegetation. Spawning occurs naturally in sexually mature male goldfish in the presence of ovulating females. Serum GtH levels in spawning male goldfish exposed to ovulatory females increased markedly in synchrony with the ovulatory GtH surge in females. There was also a significant increase in serum GH levels in the spawning males. GnRH levels in the olfactory bulbs, telencephalon, hypothalamus, and pituitary of the spawning males showed marked decreases at the same time as serum levels of both GtH and GH peaked; these events in the males corresponded to the approximate time of ovulation, and similar changes in serum GtH and GH levels and brain GnRH levels, in the females. This temporal correlation between changes in GnRH levels in the brain and pituitary and increases in serum levels of GtH and GH in males as well as females supports the idea that activation of the GnRH neuronal system may be a common pathway for the stimulation of pituitary GtH and GH secretion in goldfish during spawning.