Plasma Cortisol Is Increased during the Inhibition of LH Secretion by Central LHRH in the Ewe

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Abstract
The delayed and sustained suppression of LH secretion induced by the administration of LHRH into the third cerebral ventricle of the ovariectomized ewe suggests the activation of a neuroendocrine mechanism involving components separate from the LHRH system. Endogenous opioid peptides are involved in regulating LHRH secretion but our recent work shows they do not mediate this inhibition. There is, however, clear evidence for a role for each of the components of the hypothalamopituitary-adrenal axis in the suppression of LHRH/LH secretion. Thus, the relationship between the central administration of LHRH, changes in plasma cortisol and LH secretion was investigated. Injection of LHRH (21 pmol) into the third cerebral ventricle of ovariectomized ewes caused a significant and rapid rise in plasma cortisol to a maximum of 4–5 times pre-injection values, followed by a delayed but sustained reduction in LH secretion. There was a high correlation (r = -0.902) between the increase in cortisol and the reduction in LH. Both the stimulatory effect of central LHRH on plasma cortisol and the inhibitory effect on plasma LH were blocked by prior central treatment with an LHRH antagonist. Intravenous infusion of cortisol, to reach levels observed after central LHRH administration, reduced LH secretion (although not to the levels which followed central LHRH) due in part to a reduction in pituitary responsiveness to LHRH. These experiments provide evidence that cortisol, either alone or in combination with another component(s) of the hypothalamopituitary-adrenal axis, may play a role in the LHRH-induced inhibition of LHRH/LH secretion in the sheep.

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