Abstract   

    Pituitary LH responses to long-term pulsatile and continuous infusion of GnRH were monitored in oestradiol-17 β-implanted ovariectomized ewes during seasonal anoestrous (April and May). The experiment was performed in 2 replicates (8 ewes per replicate). Half of the animals in each replicate were infused continuously with GnRH (175 ng/h),  while the other half were given a pulsatile injection of 350 ng GnRH every 2h for a period of 20 days. GnRH administration was carried out via indwelling jugular vein catheter. Blood samples for LH determination were collected at 15-min intervals, from 6 h before until 24 h after the start of treatment, and then at 8-h intervals on days 3, 6, 10, 15, and 20 of the treatment. The 8-h bleed on day 20 was immediately followed by a 12-h bleed once the treatment had ended. Before the start of GnRH treatment. plasma LH concentrations rose immediately in infused animals. However, after an initial significant elevation on day 1, LH values were not different from mean pre-treatment concentrations f()r the rest of the treatment period. In contrast, injected ewes (350 ng GnRH) responded to each GnRH injection throughout the 20-day treatment period. The results suggested that the pituitary gland remains responsive to pulsatile but not to continuous GnRH administration for longer time periods.    

Key words: Ovariectomized ewe, GnRH, Estradial-17β Implant,  Pituitary Response.