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Title: Role of GnRH-II and its receptor in testicular function

item DESAULNIERS, AMY - University Of Nebraska
item CEDERBERG, REBECCA - University Of Nebraska
item MILLS, GINGER - University Of Nebraska
item FORD, JOHNNY - Retired ARS Employee
item Lents, Clay
item WHITE, BRETT - University Of Nebraska

Submitted to: Society for the Study of Reproduction Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 3/28/2013
Publication Date: 7/1/2013
Citation: Desaulniers, A.T., Cederberg, R.A., Mills, G.A., Ford, J.J., Lents, C.A., White, B.R. 2013. Role of GnRH-II and its receptor in testicular function [abstract]. Biology of Reproduction Supplement (46th Annual Meeting of the Society for the Study of Reproduction). pp. 367-368 (Abstract #820).

Interpretive Summary:

Technical Abstract: The highly conserved, second mammalian isoform of gonadotropin-releasing hormone (GnRH-II) regulates the interaction between energy balance and reproductive behavior in females, as well as exhibits anti-proliferative effects on cancer cells. Furthermore, GnRH-II is an inefficient modulator of gonadotropin secretion, being only 10% as effective at stimulating LH release as the native form of GnRH (GnRH-I). Despite this evidence, not all species maintain a functional receptor specific to this ligand (GnRHR-II). Instead, GnRH-II exerts its function via the GnRH-I rather than the GnRHR-II receptor in these species. Uniquely, a functional GnRHR-II has been discovered in pigs and recent evidence suggests that GnRH-II may be directly involved in testosterone (T) production, as boars immunized against GnRH-II displayed reduced plasma concentrations of T, independent of changes in luteinizing hormone (LH) levels. Our laboratory has detected GnRH-II but not GnRH-I receptors on the plasma membrane of Leydig cells in boar testes. Consistent with this, the objective of this study was to identify the function of GnRH-II and its receptor within swine testes. Towards this end, adult Chinese Meishan boars (n = 7) were surgically fitted with jugular cannulas. Blood plasma samples were collected every 20 min for 2 h prior to treatment with either D-Ala6 GnRH-I or D-Ala6 GnRH-II (150 ng/kg of body weight). Blood was collected every 10 min for 30 min and then every 20 min thereafter for 4.5 h. In a crossover design, opposite treatments were given to the same animals 1 wk later. We observed a treatment x time interaction (P < 0.0001) for changes in LH concentrations. After I.V. administration of D-Ala6 GnRH-I, LH concentrations increased 2-fold over pretreatment levels within 20 min, reaching maximal levels (4-fold) by 90 min, and remained elevated (3-fold) for the duration of sampling. Similarly, LH levels increased 2-fold by 20 min following D-Ala6 GnRH-II treatment. However, unlike the response to D-Ala6 GnRH-I, LH concentrations steadily declined to pretreatment values throughout the remainder of the trial. Interestingly, there was no treatment x time interaction for T levels. Both treatments stimulated a 2-fold increase in T production compared to pretreatment hormone levels (P < 0.0001). Therefore, GnRH-II has the capacity to stimulate T production as effectively as GnRH-I despite significantly reduced LH production. In a second experiment, testicular tissue from mature Chinese Meishan boars (n = 7) was incubated in TC199 medium containing human chorionic gonadotropin (hCG; 1 I.U.), GnRH-II (1 µM), GnRH-II plus hCG or vehicle. Media were collected hourly for 3 h and T levels determined by RIA. After sampling, tissue was snap frozen for immunoblotting utilizing primary antibodies directed against GnRHR-II or LH receptor. Protein loading was normalized using ß-actin. Treatment with hCG increased T secretion compared to the control (P < 0.001). Notably, GnRH-II and GnRH-II plus hCG increased steroidogenesis to similar levels as hCG alone (P > 0.05). Quantification of immunoblots revealed no differences in either GnRHR-II or LH receptor protein levels. These data suggest that GnRH-II directly stimulates T production without upregulating the LH receptor. Thus, GnRH-II may directly impact T production via binding to the GnRHR-II on Leydig cells, bypassing the hypothalamic-pituitary-gonadal axis in the boar. USDA is an equal opportunity provider and employer.