A negative feedback role for the ovaries of the mare on tonic LH secretion before the first ovulation of the breeding season.

The research examines a possible feedback mechanism in mare ovaries that influences LH (luteinizing hormone) secretion prior to the first round of breeding. It suggests that one or more factors originating from the ovaries restrain the levels of circulating LH during the transition into the breeding phase.

Methodology

• Eleven anoestrous (not in heat) mares were arbitrarily grouped into two: Group A – intact (6 mares) and Group B – ovariectomized (ovaries removed) in January (5 mares).
• During the months of February to April, blood samples were drawn from the jugular vein.
• Ovarian activity was gauged through ultrasound and ovulation was affirmed through progesterone analysis.
• Intact mares (Group A) ovulated anywhere between 2 and 28 April.

Results

• During periods 1 and 2 (14 or 15 February and 3 or 4 March), the LH concentrations were similar in both groups.
• In intact mares (Group A), the concentration of LH in the bloodstream remained low during periods 3 and 4.
• While in the ovariectomized group (Group B), LH concentrations started to heighten in period 3 and drastically increased by period 4.
• LH levels in Group B during this period were significantly higher than in Group A.
• Increased LH levels in the ovariectomized mares during periods 3 and 4 were related to well-defined pulsatile profiles.
• Mares in Group A showcased infrequent and low-amplitude pulses during the same timeframe.
• The absence of LH pulses in intact mares during periods 3 and 4 might have been due to low pulse frequency or a decrease in amplitude leading to pulses going undetected.

Interpretation and Hypotheses

• The study indicates that in intact mares, low LH concentrations during the transition into the breeding season may hence, be influenced by a factor of ovarian origin.
• It remains unknown whether this presumed hindrance of LH production encounters a seasonal change in effectiveness and contributes to regulating the annual breeding cycle of the mare.