Effect of Hypothalamic Deafferentation on Secretion of Luteinizing Hormone in the Ewe

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Abstract
These experiments investigated the effect of frontal hypothalamic deafferentation (HD) on basal LH levels and on estrogen-induced LH release in the ewe. HD was performed with a Halász knife positioned with the aid of x-ray and stereotaxic apparatus. In ovariectomized ewes, serum LH levels were determined in samples collected by jugular venipuncture at 20-min intervals for 3 h, once prior to HD and again approximately 4 weeks after HD. The ability of an injection of 50 µg of estradiol benzoate (EB) to elicit an LH surge also was determined both before and after HD of ovariectomized ewes. Bilateral cuts, 8 mm in diameter, placed at the anterior edge of the arcuate nucleus, reduced mean basal LH from 8.2 ± 2.0 ng/ml to nearly nondetectable levels and obliterated both pulsatile and EB-induced LH release. Bilateral cuts placed between the arcuate nucleus and the suprachiasmatic nuclei reduced mean basal LH levels from 7.5 ± 1.0 to 1.8 ± 0.3 ng/ml, but did not alter the pulsatile pattern and blocked EB-induced LH release. Bilateral cuts placed through the suprachiasmatic nuclei did not reduce mean LH levels or alter the pulsatile pattern, however, EB-induced LH release was either blocked or greatly reduced. In ovariectomized ewes subjected to sham HD or to posterior HD, LH secretion was unaltered. Frontal HD of intact ewes blocked ovulation. These data suggest that in the ewe, neural input from the suprachiasmatic or preoptic regions is not required for maintenance of basal LH levels, but is required for occurrence of an LH surge. Input from both the retrochiasmatic region and the MBH appears necessary for the maintenance of normal basal LH levels in ovariectomized ewes.