Suppression of ovarian activity in the gilt and reversal by exogenous gonadotrophin administration

The aim of the current experiment was to study the regulation of follicle d evelopment in the pig using a potent GnRH agonist (GnRH-A) to initially sup press follicle development. Large-White hybrid gilts (n = 8) were treated d uring the luteal phase with GnRH-A. Four of these GnRH-A treated gilts and four control gilts were given a GnRH bolus on days 14 and 28 after GnRH-A a dministration or during the luteal phase in control gilts. Blood samples we re collected for 10 h for; FSH and LH, after which 1500 IU PMSG were admini stered and the ovaries and uteri recovered 72 h later. A further four GnRH- A treated gilts and four control gilts were slaughtered either 28 days afte r GnRH-A administration or during the luteal phase respectively, and all fo llicles greater than or equal to 1 mm diameter were dissected. The mean bas al plasma FSH level was lower (P < 0.01) in GnRH-A treated than control gil ts and showed no response to the GnRH challenge although levels increased ( P < 0.01) in control gilts, The mean basal plasma LH levels were similar (P > 0.1) in GnRH-A treated and control gilts. Whilst in GnRH-A treated gilts plasma LH levels showed no response to the GnRH challenge, plasma LH level s were increased (P < 0.01) in control gilts. Pulsatile LH secretion was ab olished in GnRH-A treated but not in control gilts. Plasma oestradiol level s were lower (P < 0.001) in GnRH-A treated gilts than in control gilts, but nevertheless both GnRH-A treated and control gilts responded to PMSG with increased plasma oestradiol levels. Treatment with GnRH-A reduced both the mean (2.1 vs. 2.7 mm; P < 0.01) and the maximal follicle diameter (4 vs. 6 mm) and reduced (P < 0.01) the total number of follicles greater than or eq ual to 2 mm diameter compared with control gilts. Administration of PMSG in creased both mean follicle diameter (5.1 vs. 4.4 mm; P < 0.01) and maximal follicle diameter (7 vs. 9 mm) and caused a reduction(P < 0.001) in the tot al number of follicles greater than or equal to 2 mm diameter-in both GnRH- A treated and control gilts. In summary, this study has demonstrated, for t he first time in the pig, that the inhibition of follicle development as a result of pituitary down regulation/desensitisation can be reversed by exog enous gonadotrophin treatment. This model will be a powerful tool with whic h to investigate the precise regulation of follicle development in the pig. (C) 1999 Elsevier Science B.V. All rights reserved.