Figure 1.

Study protocol diagram. Five healthy male participants attended for eight study visits at least a week apart and received a different treatment combination (A–H) in random order at each visit. After 1 hour of baseline blood sampling every 15 minutes, the treatments were initiated at t = 0 hour: naltrexone (single 50 mg tablet) swallowed with water or 8-hour infusions of vehicle, 2.56 nmol/kg · h NKB, or 0.1 nmol/kg · h kisspeptin-54 administered alone or in combinations (A–H below) in random order. Oral naltrexone is rapidly absorbed with peak plasma levels achieved after 1 hour and a half-life of 12 hours; thus, stable levels were achieved for most of the study period (26, 27). Ten-minute blood sampling for gonadotropins and T and hourly blood sampling for estradiol was commenced from the start of the treatment at t = 0 hour and continued for the duration of the 8-hour study period, after which the infusions were stopped and the study ended. Upward black arrows represent frequent blood sampling during the study.