The induction of ovulation and luteal function in seasonally anoestrous ewes treated with small-dose multiple injections of Gn-RH

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Abstract
Seasonally anestrous ewes were injected i.v. with 250, 500 or 100 ng GnRH [gonadotropin releasing hormone] at 2 h intervals for 8 days (2 sheep/treatment). Each injection of 250 or 500 ng GnRH resulted in a transient rise in plasma LH [luteinizing hormone] concentrations. Treatment with 1000 ng GnRH per injection resulted in a more sustained rise in plasma LH concentrations in 1 of 2 sheep during the early part of the treatment period. A preovulatory-type LH peak occurred 17-48 h after the start of treatment in all ewes, with a 2nd preovulatory-type peak 106-133 h later in those ewes receiving 500 or 1000 ng GNRH per injection. Ovulation, with subsequent normal luteal function, occurred in all sheep. The rise in plasma progesterone concentrations appeared to be delayed in those ewes treated with 500 or 1000 ng GnRH compared to ewes treated with 250 ng GnRH. The absence of ovulation during seasonal anestrus apparently is due to an inadequate pattern of episodic LH secretion.