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Title: LH independent testosterone production is mediated by the interaction between GnRH-II and its receptor in the boar testis

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

Submitted to: Biology of Reproduction Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 7/1/2014
Publication Date: 7/19/2014
Citation: Desaulniers, A.T., Cederberg, R.A., Mills, G.A., Ford, J.J., Lents, C.A., White, B.R. 2014. LH independent testosterone production is mediated by the interaction between GnRH-II and its receptor in the boar testis [abstract]. In: Society for the Study of Reproduction, 47th annual meeting, 19-23 July 2014, Grand Rapids, MI. Abstract 81.

Interpretive Summary:

Technical Abstract: The second mammalian isoform of gonadotropin-releasing hormone (GnRH-II) functions quite differently from the classical form (GnRH-I), being an ineffective modulator of gonadotropin release. Not all species that produce GnRH-II maintain a full length GnRH-II receptor (GnRHR-II). Instead, GnRH-II can mediate its action via the GnRH-I receptor (GnRHR-I). In contrast, a functional GnRHR-II has been identified in pigs and our research indicated its presence on the plasma membrane of Leydig cells. We also showed that GnRH-II can stimulate testosterone (T) production as effectively as GnRH-I, despite eliciting reduced luteinizing hormone (LH) secretion. Therefore, the objectives of this study were to characterize GnRH-II and its receptor in reproductive tissues and further examine ligand-receptor interactions in vivo. First, immunoblotting of tissues from mature boars (n = 5) revealed 6-fold more GnRHR-II protein in the testis than anterior pituitary gland (P < 0.0001). Further, GnRH-II levels detected by ELISA were highest in the testis (1,321 pg/ml), intermediate in the pituitary (393 pg/ml) and lowest in the hypothalamus (220 pg/ml; P < 0.0001). Next, we sought to abolish the interaction of GnRH-II with the GnRHR-I by utilizing a GnRH-I antagonist (SB-75) prior to GnRH analog delivery in adult boars (n = 5/trt) fitted with jugular cannulae. Basal blood samples were collected and then SB-75 (5 µg active compound/kg BW) was given intramuscularly. Approximately 7.5 h later, either D-Ala**6 GnRH-I or -II (150 ng/kg BW) was administered intravenously (I.V.), blood samples were collected regularly for 270 min and LH and T levels were measured by RIA. We observed a treatment x time interaction (P < 0.0001) for changes in LH release. After I.V. infusion of D-Ala**6 GnRH-I, LH levels increased within 10 min, remained high (3-fold) for 30 min and returned to baseline by 110 min. Conversely, D-Ala**6 GnRH-II treatment did not elevate LH levels (P > 0.05). Strikingly, we did not observe a treatment x time interaction for T response, as T levels increased similarly for GnRH-I and -II treatments. These results indicate that SB-75 attenuated LH, but not T, release in GnRH-II treated boars by blocking the GnRHR-I. Lastly, basal blood samples were collected from cannulated boars (n = 6/trt) prior to sedation (sodium pentothal, 6.6 mg/kg BW). Boars were subjected to intratesticular injection of saline or a low dose (300 ng) of D-Ala**6 GnRH-I or -II. Blood samples were collected recurrently for 240 min. We observed an effect of treatment for LH and T production (P < 0.0001). GnRH-I induced an increase in LH secretion compared to control and GnRH-II treated animals (P < 0.01). Like LH, GnRH-I stimulated significantly greater T production than GnRH-II or saline (P < 0.01). Notably, GnRH-II induced T secretion that was also higher than controls (P < 0.001), indicating that T was produced in the absence of an increase in LH secretion. Together these data indicate that GnRH-II and its receptor are involved in localized T synthesis, independent from LH secretion in the boar. Partial support from NIFA (2011-67015; CAL).