The Role of GnRH-II and its Receptor in Testicular Function

The second mammalian isoform of GnRH (GnRH-II) has been linked to regulation of cell proliferation, feed intake, and the interaction between energy balance and reproductive behavior. In contrast to the native form of GnRH (GnRH-I), GnRH-II is an inefficient modulator of gonadotropin secretion. Unlike many species, a functional receptor (GnRHR-II) specific to this ligand has been discovered in the pig that may be directly involved in testosterone production. Therefore, our objective was to identify the role of GnRH-II and its receptor in testicular function. First, there was 6-fold more GnRHR-II protein in the testis than anterior pituitary gland of boars. GnRH-II levels determined by ELISA were highest in testis (1,321 pg/ml), intermediate in pituitary (393 pg/ml) and lowest in hypothalamus (220 pg/ml) tissue homogenates. Immunohistochemistry indicated that the GnRHR-II was located on the plasma membrane of Leydig cells within the interstitium as well as germ cells. Second, immunocytochemistry demonstrated that GnRHR-IIs were localized to the neck region of spermatozoa. Consistent with this, GnRH-II was also detected in seminal plasma (225 pg/ml). Third, testicular tissue explants exposed to GnRH-II secreted testosterone similarly to hCG treated tissues, without influencing GnRHR-II or LH receptor protein levels. Finally, 4 in vivo experiments were performed on boars surgically fitted with jugular cannulae. In Exp. 1, testosterone concentrations were elevated following treatment with either aGnRH-I (D-ala6 GnRH-I) or GnRH-II (D-ala6 GnRH-II) agonist, although GnRH-II induced changes in LH levels were not correlated with testosterone production. In Exp. 2, treatment with a GnRH-I (SB-75) or GnRH-II (Trp-1) antagonist suppressed testosterone, but not LH, secretion. In Exp. 3, testosterone concentrations increased after treatment with SB-75 followed by infusion of eitherGnRH-I or -II, despite the inability of GnRH-II to induce LH secretion. In Exp. 4, intratesticular injection with GnRH-I, GnRH-II or saline resulted in GnRH-II-induced stimulation of testosterone production without increasing LH levels. Thus, GnRH-II may directly impact testosterone production via binding to the GnRHR-II on Leydig cells, bypassing LH production by the anterior pituitary gland. Advisor: Brett R. Whit