Objectives. To elucidate the effect of testosterone on penile nerve supply.

Methods. Three groups of 10 rats each were assessed; two groups were castrated and the third underwent a sham operation (control). After castration, one group received subcutaneous injection of testosterone while the others received sesame oil. At 8 weeks, the rats underwent a functional analysis. The evaluation included a subcutaneous injection with apomorphine to study centrally mediated erection, and cavernous nerve electrostimulation and papaverine injection to study peripherally mediated erection. At death, a penile midshaft specimen was taken for nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase staining.

Results. In the apomorphine study, castrated rats had no erections, but the erectile function of those receiving testosterone was restored to the level of the controls. The mean numbers of NADPH-diaphorase-stained nerve fibers in the copora cavernosa and both dorsal nerves of castrated rats, at 165.8 ± 20.0 and 271.3 ± 21.1, respectively, were significantly lower than those of the controls, at 271.7 ± 14.6 and 471.2 ± 27.6, respectively. Those of the testosterone replacement group, at 290.7 ± 10.1 and 500.7 ± 23.9, respectively, recovered to the control level. The intracavernosal pressure decreased significantly in the absence of testosterone, both after electrostimulation and intracavernosal papaverine injection, and recovered to the control level after testosterone replacement.

Conclusions. Our results indicate that testosterone acts on the nervous system to mediate erection. When it is absent, there may be downregulation of both the production and activity of nitric oxide (NO), thereby decreasing the response to peripheral stimulation via the NO pathway. Testosterone replacement may upregulate NO activity to the control level.