Background: Multiple pathways have been proposed for the course of adrenergic fibers to the penis and, although it is generally recognized that the pudendal nerve (PudN) is the most important, there is little quantitative information available.
Methods: We used image analysis of catecholamine histofluorescence to quantify the effect of various nerve lesions on the adrenergic innervation of the rat penis. In addition to the denervation studies and as a direct test of whether penile adrenergic fibers traversed the pelvic plexus, penile neurons in the sympathetic chain were first labeled with a retrograde dye placed in the penis. The cavernous nerve of these animals was later exposed to another dye with different spectral characteristics.
Results: Interruption of the sensory branch of the PudN reduced adrenergic innervation of cavernosal smooth muscle by 86% (+/- 2.5%). Vascular fibers of the deep penile and helicine arteries were also severely reduced but not entirely eliminated. Interruption of the motor branch of the PudN had a lesser and more variable effect on penile adrenergic innervation: a 21.2% (+/- 6.8%) decrease in cavernosal muscle innervation but no obvious affect on vasomotor fibers. Combining the nerve lesions with phenol degeneration of perivascular fibers of the pudendal vessels further reduced but did not entirely eliminate adrenergic fibers in the cavernosal muscle and penile vessels.
Conclusions: The dramatic reduction of adrenergic innervation of the penis after section of the PudN, especially the sensory branch, and the absence of double-labeled neurons in the sympathetic chain suggest that the PudN nerve is the major, if not the exclusive, pathway by which adrenergic fibers reach penile erectile tissue of the rat.