Reflex Release of Gonadotropin and Increased Plasma Testosterone Concentration in Male Rabbits During Copulation

Abstract

Peripheral plasma from sexually mature male New Zealand rabbits contains only 45 ± 7.7 mμg/100 ml testosterone (T) and 23 ± 5.1 mμg/100 ml androstenedione (A). These steroids were measured with double isotope derivative techniques using 35S-thiosemicarbazide as reagent and 3H-steroid indicator for calculation of recovery. In contrast to these low basal androgen levels in peripheral plasma, human chorionic gonadotropin (HCG) or human menopausal gonadotropin caused a 25-fold and 13.8-fold rise, respectively, in plasma testosterone within 30 min after iv injection. Minimal changes in androstenedione values were noted with these preparations. The act of copulation or presence of a female resulted in a striking rise in plasma testosterone within 30 min. The change in the T/A ratio with this stimulus was similar to the changes resulting from HCG administration. These studies suggest that there is a neuroendocrine reflex in the male rabbit, analogous to reflex ovulation in the female, which probably involves visual or olfactory stimuli, release of gonadotropin, stimulation of Leydig cells, and rapid secretion of testosterone which may be facilitatory to the reproductive act. (Endocrinology82: 627, 1968)