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Effect of Implantation of Anti-LH Serum Into Median Eminence on Rat Pituitary and Serum LH 1

Summary A specific rabbit anti-sheep LH serum (A/S) was implanted into the rat hypothalamic median eminence (ME) to investigate the mode of action of a “short” feedback of endogenous LH by neutralization of this hormone at its presumed site of action in the brain. Rats were ovariectomized 7 or 14 days before A/S implantation and sacrificed 2 or 6 days later. The pituitary and serum LH levels were measured by a radioimmunoassay technique. Implantation of A/S resulted in a significant decrease in pituitary content of LH but no change in concentration of serum LH. Implantation of sheep LH in ME of ovariectomized rats also reduced pituitary LH content significantly with no effect on serum LH. LH A/S implantation did not alter gross weight of the anterior pituitary or the target organs of its tropic hormones. These data suggests that while an unphysiologically large amount of exogenous LH implanted directly into ME exhibits a negative effect on pituitary content of LH, endogenous LH appears to exert a positive short feedback action on the hypothalamo-pituitary axis. The authors thank the Endocrine Study Section, NIH for the generous supply of ovine LH and the National Institute of Arthritis and Metabolic Diseases, Rat Pituitary Hormone Program for Rat LH assay kit. (prepared by Dr. A. F. Parlow).