The effect of testosterone on serum gonadotropins of castrated rats kept under different lighting conditions.

Testosterone feedback sensitivity was measured as the ability of testosterone propionate to decrease serum LH and FSH of long-term castrated (4 wk) rats under four different lighting conditions: periodic light (12L:12D), constant light (LL), constant darkness (DD), and dim night illumination (1 lx) with a 12L:12D photoperiod. Rats were exposed to the different lighting conditions for 1 wk, during which they received daily testosterone propionate (125 micrograms or 250 micrograms s.c.). At the end of the experiment the rats were decapitated at 1100 h, and serum gonadotropin levels were measured by RIA. Serum LH of the rats kept under LL was reduced to the level of the intact rats with the smaller testosterone dose (125 micrograms/day). Under all other lighting conditions only the large dose (250 micrograms/day) was able to restore the serum LH concentration to the level of the intact rats. Serum FSH was restored only partially, and the effect was the same with both doses and similar under all lighting conditions. We conclude that the increase in testosterone negative feedback sensitivity was not caused by the lack of periodicity of illumination alone, but that sufficient intensity of lighting throughout the 24 h was needed as well.