Prolactin and LH release in response to LH-RH and TRH in ewes during dioestrus, pregnancy and post partum

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Abstract
With advancing pregnancy in the ewe there was a marked decline in plasma LH [luteinizing hormone] concentrations and pituitary LHRH responsiveness (integrated LH release) and a marked increase in plasma prolactin values and pituitary TRH responsiveness (integrated prolactin release). In lactating ewes plasma LH levels and pituitary LHRH responsiveness had returned to values found in the luteal phase of the normal cycle by 21 days post partum; at 42 days post partum prolactin levels were still high. No interaction between TRH and LHRH on prolactin and LH release in diestrous ewes was detected. In non-pregnant ewes plasma prolactin levels were significantly higher in June than in Jan. but TRH responsiveness was similar. In sheep, pituitary LH secretion apparently recovers more rapidly from the chronic negative feedback effect of estrogens and progesterone in pregnancy than prolactin secretion recovers from the chronic positive feedback effects of estrogens. This finding may be a contributory factor in the resistance to resumption of breeding activity.