Steroid and thyroid hormone profiles following a single injection of partly purified salmon gonadotropin or GnRH analogues in male and female sea lamprey.

The effects of exogenous administration of gonadotropin-releasing hormone (GnRH) analogues or of a partly purified salmon gonadotropin extract (GTH) on the duration of steroid and thyroid hormone levels were determined in female and male sea lampreys, Petromyzon marinus, tested under differing temperature and reproductive status. Plasma estradiol levels, but not androgens, were significantly elevated in response to the GnRH analogues or GTH injection compared to controls in female and male lampreys. Higher temperature and/or advance in time of maturation appeared to be inversely related to plasma estradiol levels. These data provide further evidence of hypothalamic control over reproductive function in lampreys. Plasma thyroxine was significantly elevated after female lampreys were treated with GTH, GnRHa (10 micrograms/lamprey) or GnRHa (1 microgram/lamprey) compared to controls. The present study is the first to demonstrate that the GnRH analogue stimulated in some way the pituitary-thyroid axis. These data suggest that a GnRH activity may activate both gonado- and thyrotropic secretion or that the endogenous hormone may itself have both functions in one of the most primitive vertebrates, the sea lamprey.