Direct effect of prolactin, induced by TRH injection, on ovarian oestradiol secretion in the ewe

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Abstract
The effect of sustained high plasma levels of prolactin, induced by repeated 2-h i.v. injections of TRH (20 .mu.g), on ovarian estradiol secretion and plasma levels of LH [luteinizing hormone] and FSH was investigated during the preovulatory period in the ewe. Plasma levels of progesterone declined at the same rate after prostaglandin-induced luteal regression in control and TRH-treated ewes. TRH treatment results in a signficant increase in plasma levels of LH and FSH compared to controls from 12 h after luteal regression until 5-6 h before the start of the preovulatory surge of LH. In spite of this, and a similar increase in pulse frequency of LH in control and TRH-treated ewes, ovarian estradiol secretion was significantly suppressed in TRH-treated ewes compared to that in control ewes. The preovulatory surge of LH and FSH, the 2nd FSH peak and subsequent luteal function in terms of plasma levels of progesterone were not significantly different between control and TRH-treated ewes. TRH treatment, presumably by maintaining elevated plasma levels of prolactin, results in suppression of estradiol secretion by a direct effect on the ovary in the ewe.