[Application of GnRH-agonists in young sows affects cyclic gonadotropin release]

Gonadotropin-releasing hormone (GnRH) controls by pulsatile secretion the synthesis and release of LH and FSH. In contrast to the endogenous peptide a long-term treatment of GnRH agonists (GnRH-A) initiates a continuous stimulation. The aim of the present study was to determine the influence of a GnRH-A on peripheral LH, FSH and steroid concentrations, measured by specific RIA methods, in cyclic gilts. Therefore blood samples were drawn via i.v. catheters once daily (8 h) and frequently every second day for six hours (8-14 h) in 20 min intervals. The experiment was started on day 10 of the oestrus cycle. Four gilts were injected with 7.5 mg triptorelin (D-Trp6-LHRH, Decapeptyl) and three gilts injected with vehicle served as control. The administration of GnRH-A induced a release of FSH and LH within the first one or two hours lasting 2-6 h. After this initial peak gonadotropin levels remained in a low range. In control gilts LH and FSH remained at basal level and increased between day 20-21. There was only a small number of LH pulses (n = 5) or no FSH pulses after GnRH-A during the intensive sampling periods. In untreated gilts the mean LH pulse frequency increased from 0.67 +/- 0.47 on day 12 to 2.0 +/- 1.63 to 1.5 +/- 0.5 on day 16-20. Differences in pulse amplitudes were observed during the oestrus cycle. Defined FSH pulses were not detected. The basal LH and FSH levels were not affected by GnRH-A treatment. Follicular development and ovulation were down-regulated following GnRH-A. The results indicate that GnRH agonists in a longtime active depot formulation induce their suppressive effects on ovarian function in gilts by marked decrease of pulsatile gonadotropin release.