NEGATIVE FEEDBACK ON LUTEINIZING-HORMONE SECRETION REQUIRES INFREQUENT EXPOSURE TO TESTICULAR ANDROGENS

The present study tested the hypothesis that a minimum frequency of te stosterone pulses is required to exert negative feedback control of lu teinizing hormone (LH), Effect of gonadal steroids on endogenous LH re lease in rams was investigated after injecting ovine LH (NIADDK-oLH-26 , i.v.; 5 mu g 100kg(-1) BW) to induce testosterone pulses while rams were undergoing windows of LH suppression during the influence of a LH releasing hormone (LHRH) antagonist (SE-75; Bajusz et al., 1988, Int. J. Peptide Protein Res., 32: 425-435). Patterns of LH release were ch aracterized as rams recovered from LH inhibition. Rams were exposed to artificial photoperiods of 12 weeks of long days (16L:8D) followed by short days (8L:16D) to initiate reproductive activity. The study bega n during Week 5 of short days. Serial plasma samples were collected fr om six postpubertal rams (five Suffolks and one Hampshire) at 15-min i ntervals for 69.5 h. All rams were given LHRH antagonist (10 mu g kg(- 1) BW, s,c.) at Hour 24 during each of three collection periods which took place on alternating weeks in a Latin square design. Rams were gi ven a different exogenous oLH treatment (no oLH, one dose at 12 h afte r antagonist or three doses at 6-h intervals with initial dose 6 h aft er antagonist) during each collection period. Exogenous oLH decreased the number of LH pulses during the recovery period (Hours 48 to 72 of experiment; 24 to 48 h after antagonist; P < 0.01). Exogenous oLH admi nistered at 6-h intervals (P < 0.01) or at 12 h after the antagonist ( P < 0.05) decreased the number of endogenous LH pulses during the reco very period compared with controls, Characteristics of endogenous puls es of testosterone during recovery were not affected by exogenous oLH, Imposed testosterone releases were intended to provide negative feedb ack, and thus, decrease the number of pulses of LH. It was expected th at at least three pulses of testosterone in 18 h would be required to provide negative feedback control of LH secretion. However, imposed pu lses of testosterone as infrequent as once in 18 h resulted in a decre ase in LH secretion.