Delayed testosterone replacement restores nitric oxide synthase-containingnerve fibres and the erectile response in rat penis

Objective To elucidate the effect of testosterone on penile innervation. Materials and methods Three groups of six rats each were assessed; two grou ps (1 and 2) were castrated and the third (group 3) underwent a sham operat ion (control). Eight weeks after castration, group 2 received a subcutaneou s injection with testosterone. At 8 weeks, the rats in group 1 and 3 underw ent a final functional analysis while those in group 2 did so at 12 weeks. The evaluation included a subcutaneous injection with apomorphine to study centrally mediated erection, and cavernosal nerve electrostimulation and pa paverine injection to study peripherally mediated erection. At death a peni le mid-shaft specimen was taken for NADPH-diaphorase staining. Results In the apomorphine study, castration resulted in significantly fewe r yawns and erections than in the control, and those in group 2 significant ly better central erectile function than in the controls. The mean (sem) nu mber of nitric oxide synthase (NOS)-containing nerve fibres in the corpora cavernosa and both dorsal nerves of castrated rats, at 46.2 (9.1) and 203 ( 32.1), respectively, were significantly lower than in rats in group 2, at 8 4.1 (11.2) and 300.6 (17.1), and than in the controls, at 88.6 (10.9) and 3 06.3 (22.9), respectively. The intracavernosal pressure decreased significa ntly in the absence of testosterone, both after electrostimulation and intr acavernosal papaverine injection. However, there was no difference between the control and group 2 rats in either the number of NOS-containing nerve f ibres or in the peripheral erectile functional study. Conclusions Testosterone acts on the nervous system to mediate erection; wh en it is absent there may be down-regulation of both the production and act ivity of NO, thereby decreasing the response to peripheral stimulation via the NO pathway. The restoration of erectile function seen in rats in group 2 supports this phenomenon. Delayed testosterone replacement has no detrime ntal effect on the restoration of the erectile mechanism after castration.