Reduced endogenous GnRH-II receptor expression leads to decreased 17ß-estradiol secretion despite larger follicular diameter in cyclic gilts

Authors: ROSS, CAITLIN - University Of Nebraska; CEDERBERG, REBECCA - University Of Nebraska; CHOAT, FINA - University Of Nebraska; ELSKEN, DOROTHY - University Of Nebraska; KURZ, SCOTT - University Of Nebraska; MILLS, GINGER - University Of Nebraska; Lents, Clay; WHITE, BRETT - University Of Nebraska

Submitted to: Society for the Study of Reproduction Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 5/1/2020
Publication Date: 7/12/2020
Citation: Ross, C.E., Cederberg, R.A., Choat, F.H., Elsken, D.H., Kurz, S.G., Mills, G.A., Lents, C.A., White, B.R. 2020. Reduced endogenous GnRH-II receptor expression leads to decreased 17ß-estradiol secretion despite larger follicular diameter in cyclic gilts [abstract]. Society for the Study of Reproduction Annual Meeting, Virtual. p. 46-47. Abstract 1991. Available: https://www.ssr.org/news-events/past-meetings/ssrvirtual2020

Interpretive Summary:

Technical Abstract: Pigs are the only livestock species encoding a functional protein for both the second isoform of gonadotropin-releasing hormone (GnRH-II) and its cognate receptor (GnRHR-II). Unlike the classical GnRH system, GnRH-II and GnRHR-II are expressed in reproductive and non-reproductive tissues. To examine the role of GnRH-II and its receptor in reproductive function, we produced a swine line with reduced endogenous levels of GnRHR-II (GnRHR-II KD). Our laboratory demonstrated that GnRH-II binding to its receptor on Leydig cells stimulates LH-independent testosterone secretion in porcine testes. However, the role of the GnRH-II/GnRHR-II system has not been elucidated in female pigs. Therefore, the objectives of this study were to characterize 17ß-estradiol secretion and compare morphometric criteria of mature GnRHR-II KD (n = 4) and littermate control (n = 4) gilts during the follicular phase of the estrous cycle. Prepubertal animals were monitored daily for behavioral estrus beginning at 180 d of age. Once all females exhibited their third behavioral estrus, they were individually fed 15 mg of the progestogen, altrenogest, for 14 consecutive days to synchronize estrus. During this time, indwelling jugular catheters were surgically placed. At 48 h after the final altrenogest feeding, blood samples for 17ß-estradiol were collected from each animal every 4 h until 24 h following the onset of estrus (0 h), determined by twice daily estrous detection. Serum was obtained and 17ß-estradiol quantified by radioimmunoassay. Animals were euthanized during proestrus (Day 18 to 20) of the following estrous cycle, and body, ovarian, uterine, oviductal, and right kidney weights determined. Next, follicle diameter was measured using calipers and antral follicles (= 6 mm) counted. Statistical analyses were performed using the MIXED procedure of SAS. The model for 17ß-estradiol concentrations included line, time and line x time as fixed effects, litter as a random effect, and time as the repeated measure (subject = gilt x line). The model for morphometric data included line as a fixed effect and litter as a random effect. During the follicular phase, a tendency for a line x time interaction (P = 0.0745) was detected for 17ß-estradiol levels; additionally, circulating 17ß-estradiol concentrations tended to be reduced approximately 20% in GnRHR-II KD (18.7 ± 2.5 pg/mL) vs. control (23.2 ± 2.5 pg/mL) females (P = 0.0760). Furthermore, total area under the curve (AUC) was lower for GnRHR-II KD females compared to controls (P = 0.05) and AUC at peak 17ß-estradiol levels (-44 to -8 h relative to the onset of estrus) tended to be reduced in transgenics (P = 0.0996). Finally, morphometric weights and antral follicle counts were not different between lines (P > 0.10). However, follicles tended to be larger in GnRHR-II KD (7.7 ± 0.3 mm) than control (6.9 ± 0.3 mm) females (P = 0.0602). Thus, these data indicate that the GnRH-II/GnRHR-II system regulates 17ß-estradiol secretion and follicular dynamics in mammalian females, representing a potential avenue for future reproductive therapies. Supported by USDA/NIFA AFRI (2017-67015-26508) and Hatch Multistate (NEB-26-244) funds.