Orchidectomy unleashes pulsatile luteinizing hormone secretion in the rat

We charted the development of pulsatile luteinizing hormone (LH) secretion as a function of the time elapsed after removal of the testes. On seven occasions between the moment of castration and 80 days afterwards, we obtained consecutive blood samples at frequent (2.5- to 5-min) intervals from cannulated male rats. Orchidectomy increased both the amplitude and frequency of LH release within 1 day after surgery. Amplitude: From 19 h through 80 days postcastration, peak LH levels rose steadily, and LH pulses grew progressively more pronounced in nadir-to-peak amplitude. Frequency: Our findings offer new evidence establishing an increase in LH pulse frequency from less than 1 per h to 2-3 per h within 1 day after orchidectomy. Once deprived of testicular influences, the frequency of pulsatile LH discharges remained static through 80 days. The sudden onset (less than 1 day after castration) and temporal uniformity of high-frequency LH pulses demonstrate that LH release is governed by an intrinsic, 20- to 30-min neural periodicity in castrate rats. Most important, these findings imply that the testes mask or modulate the expression of an intrinsic, 20- to 30-min neural generator directing the periodic discharge of LH in the intact male rat.