Hypothalamic control of the post-castration rise in serum LH concentration in rams.

Sexually mature rams were left intact, castrated (wethers), castrated and implanted with testosterone, or castrated, implanted with testosterone and pulse-infused every hour with LHRH. Serum concentrations of LH increased rapidly during the first week after castration and at 14 days had reached values of 13.1 +/- 2.2 ng/ml (mean +/- s.e.m.) and were characterized by a rhythmic, pulsatile pattern of secretion (1.6 +/- 0.1 pulses/h). Testosterone prevented the post-castration rise in serum LH in wethers (1.0 +/- 0.5 ng/ml; 0 pulses/h), but a castrate-type secretory pattern of LH was obtained when LHRH and testosterone were administered concurrently (10.7 +/- 0.8 ng/ml; 1.0 pulse/h). We conclude that the hypothalamus (rather than the pituitary) is a principal site for the negative feedback of androgen in rams and that an increased frequency of LHRH discharge into the hypothalamo-hypophysial portal system contributes significantly to the post-castration rise in serum LH.