Young Scholar Presentation: Primary Hypogonadism in Gonadotropin-Releasing Hormone II Receptor Knockdown Boars

Paradoxically, the second mammalian GnRH isoform (GnRH-II) and its receptor (GnRHR-II) are not physiological regulators of gonadotropin secretion. Instead, our data suggests that both are abundantly produced in the porcine testis and mediate testosterone secretion, independent of luteinizing hormone (LH). To further study the role of this system, our laboratory generated a knockdown (KD) swine line with 70% lower testicular GnRHR-II mRNA levels compared with littermate controls. During pubertal development, testosterone concentrations tended to be reduced in transgenic versus littermate control boars (P < 0.06), yet LH concentrations were unaffected (P > 0.10). In adults, the diurnal secretory patterns of testosterone and basal circulating concentrations of 9 other gonadal steroids were evaluated using animals fit with indwelling jugular cannulae. Testosterone concentrations were constitutively reduced in GnRHR-II KD compared with littermate control boars (P < 0.05). Pulse analysis indicated that transgenic boars tended to produce fewer pulses of testosterone than littermate controls (P < 0.10). Amplitude of pulses was reduced in transgenic boars (P < 0.05) but pulse duration was unaffected (P > 0.10). GnRHR-II KD boars also tended to have lower minimum and maximum concentrations of testosterone (P < 0.10). Mass spectrometry revealed that gonadal steroids were dramatically impacted by GnRHR-II KD; concentrations of steroids derived from the testis were either significantly decreased (7 hormones) or tended to be reduced (3 hormones) in transgenic boars. Next, the sensitivity of GnRHR-II KD and littermate control boars to GnRH analogues was assessed. Transgenic males produced less testosterone (P < 0.05) in response to treatment with a GnRHR antagonist (SB-75), known to bind GnRHR-II. Compared with littermate control boars, transgenic males were also less responsive to GnRH-II and human chorionic gonadotropin (P < 0.05). In order to determine if reduced testosterone secretion affected semen quality, ejaculates were subjected to computer-assisted semen analysis. Both sperm motility and the number of artificial insemination doses produced per ejaculate tended to be reduced in GnRHR-II KD boars (P < 0.10). At euthanasia, transgenic boars tended to have smaller testes than littermate controls (P < 0.10) and produced less testosterone per gram of testicular tissue (P < 0.05). Ultimately, these data demonstrate that GnRH-II and its receptor are critical modulators of steroidogenesis within porcine Leydig cells and may represent novel targets to enhance boar fertility. Partially supported by USDA/NIFA AFRI ELI predoctoral fellowship (2017-67011-26036; ATD) and AFRI (2017-67015-26508; BRW) funds. USDA is an equal opportunity provider and employer.