Dose–response relationship of 15α-hydroxylated sex steroids to gonadotropin-releasing hormones and pituitary extract in male sea lampreys (Petromyzon marinus)

The sea lamprey (Petromyzon marinus) is one of the earliest extant vertebrates for which the hypothalamic-pituitary-gonadal (HPG) axis has been shown to control and regulate reproduction in a similar fashion to gnathostome vertebrates. While the two forms of gonadotropin-releasing hormones in the sea lamprey (GnRH I and GnRH III) have been studied extensively, their in vivo effect on synthesis of 15α-hydroxytestosterone (15α-T) and 15α-hydroxyprogesterone (15α-P) have only been partially characterized. In the present study, plasma concentrations of 15α-T and 15α-P were measured in prespermiating sea lampreys that were given a single injection of either GnRH I or GnRH III in doses ranging from 5 to 100 μg/kg, or of pituitary extract (as a source of gonadotropin). Plasma was sampled at 1–6 h and 6–48 h post-injection, in separate experiments, in order to characterize the peak and duration of responses. 15α-T plasma concentrations increased slightly in response to all three treatments, but not in a dose-dependent manner, and the timing of peak concentrations varied between doses. However, 15α-P plasma concentrations showed a greater range of response (between 1 and 100 ng/ml) and were clearly correlated with the injection dose. Plasma concentrations of 15α-P also responded to far lower doses of GnRH I and GnRH III than any other steroid previously investigated in lampreys. The plasma concentrations of 15α-P peaked at 6 h after injection for all three treatments, and levels reached a mean of 53.1 ng/ml. In female lampreys that were injected twice with 50 μg/ml GnRH I or III, 15α-T concentrations did not exceed 0.5 ng/ml and 15α-P concentrations did not exceed 1 ng/ml. These results lend further support to the hypothesis that 15α-P plays an important role in the reproductive endocrinology of male lampreys.