Use of a mammalian gonadotropin-releasing hormone (GnRH) agonist to characterize pituitary GnRH receptors in white sturgeon (Acipenser transmontanus Richardson)

Summary Aim of the present study was to characterize the pituitary GnRH receptor in white sturgeon, Acipenser transmontanus, using a superagonist analog of mammalian GnRH [D-Ala6, des-Gly10–Pro9]-ethylamide and to investigate the possible effect of estradiol-17β treatment on the concentration and affinity of the GnRH receptor in immature white sturgeon. The binding of 125I-GnRH-A to sturgeon pituitary receptors was rapid and saturable at 4°C and 20°C. However, maximal binding at 20°C was almost two-fold greater than the highest binding noted at 4°C. Specific binding of radioligand was directly related to the amount of tissue included in the assay system over the range of 5–20 mg fresh tissue equivalents per ml. The binding capacity of 125I-GnRH-A with sturgeon pituitary tissue was much greater than radiolabeled GnRH. Administration of E2 to immature sturgeon caused an almost two-fold increase in GnRH-A binding capacity (E2 treated: Bmax = 2.87 fmoles 3 mg−1 FTE; control: Bmax = 1.70 fmoles 3 mg−1), and did not affect GnRH-A binding affinity (E2 treated: Ka = 0.13 × 1011 m−1; control: Ka = 0.15 × 1011 m−1). Overall, the study provides evidence that the GnRH analog is effective for characterizing the GnRH receptor in white sturgeon; however, more experimentation is necessary to determine whether E2 administration to immature white sturgeon can increase the GnRH receptor capacity.