Effects of GnRH agonist immunization on vaginal electrical resistance, FSH, LH, and ovaries in prepubertal female sheep

The study is aimed at the investigation of the effects of GnRH agonist (alarelin) immunization on the vaginal electrical resistance (VER) and secretion of FSH and LH in sheep. Forty-two prepubertal female sheep were assigned to six groups (n = 7). Animals in experimental groups I (EG-I), II (EG-II), and III (EG-III) were twice subcutaneously injected with 200, 300, and 400 μg of alarelin antigens, respectively. Animals in experimental groups IV (EG-IV) and V (EG-V) were four times subcutaneously injected with 200 and 300 μg of alarelin antigens, respectively. Animals in the control group (CG) were subcutaneously injected with 2.0 ml of control liquid. Serum concentrations of FSH and LH were detected using ELISA. VER was measured by the electrical resistance detector. The follicle vertical diameter, follicle transverse diameter, theca folliculi externa thickness (FET), theca folliculi interna thickness (FIT), and follicle wall thickness (FWT) were calculated through microscope images using ImagePlus software. The results showed that the serum FSH concentration in EGs started to increase on day 14 and reached peak levels on day 35, with a maximum in EG-V. Serum LH concentration in EGs decreased from day 7 and reached the minimum levels (P < 0.05) on day 21 or 28. Serum FSH in EG-III–EG-V was higher than that in EG-I (P < 0.05) and CG (P < 0.01) in days 28–60. VER decreased after injection of alarelin antigen, the values reached the minimum levels on day 28 in EG-I and EG-II, on day 35 in EG-III–EG-V (P < 0.05), respectively. In days 45–60, VER values in EG-III–EG-V were lower than those in CG (P < 0.01). Ovarian weights in EGs were higher than those in CG; the values of FET, FIT, FWT, follicle vertical diameter (FVD), and follicle transverse diameter (FTD) in EG-III and EG-V were greater than those in CG, EG-I, EG-II, and EG-IV. In conclusion, alarelin active immunization can decrease VER, induce the synthesis and secretion of FSH, and promote follicle development in prepubertal female sheep. VER had a positive correlation with serum LH, and a negative correlation with FSH.