Nitric oxide-synthesizing neurons originating at several different levels innervate rat penis.

While the crucial role of neurally produced nitric oxide in mediating penile erection is well established, the understanding of the peripheral neuroanatomy of the nitric oxide-ergic pathways is still incomplete. This study was designed to elucidate further the distribution of nitric oxide synthase, and its relation to the distribution of neuropeptides and tyrosine hydroxylase in all penis-projecting neural pathways. A triple-labelling technique was employed, with the retrograde tracer Fluoro Gold combined with neuropeptide immunohistochemistry and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, a marker of nitric oxide synthase. The presence within the penis of scattered nerve cell bodies exhibiting NADPH-diaphorase activity was revealed. Most (76%) of the penis-projecting neurons in the major pelvic ganglion exhibited NADPH-diaphorase activity and immunoreactivity to vasoactive intestinal peptide, while none of them contained tyrosine hydroxylase. Sympathetic paravertebral postganglionic neurons, in turn, contained tyrosine hydroxylase, but did not exhibit NADPH-diaphorase activity. In the afferent, sensory neurons projecting to the penis from the dorsal root ganglia, NADPH-diaphorase activity coexisted with immunoreactivity to both substance P (8%) and calcitonin gene-related peptide (26%). Preganglionic neurons originating in the spinal cord intermediolateral column at the thoracolumbar level T11-L3 terminated, not only in the major pelvic ganglion, but also within the penis. The majority (81%) of the penis-projecting neurons exhibited NADPH-diaphorase activity. The results indicate that the rat penis receives several different nitric oxide-ergic neural projections. It is therefore possible that nitric oxide affects penile erection at several neuronal levels.