Innervation of the fibro-elastic type of the penis: An immunohistochemical study in the male pig

The occurrence and colocalization of several biologically active neuropeptides, catecholamine-, acetylcholine- or nitric oxide-synthesizing enzymes tyrosine hydroxylase (TH), dopamine-β-hydroxylase (DβH), choline acetyltransferase (ChAT) and nitric oxide synthase (NOS I), respectively, as well as the vesicular acetylcholine transporter (VAChT) were investigated in the penile glans (GP), corpus and crura (CP), as well as in the retractor penis muscle (RPM) of juvenile and adult boars. Immunohistochemistry revealed that nerves immunoreactive (IR) to TH, DβH, vasoactive intestinal polypeptide (VIP) and somatostatin (SOM) were the most numerous, followed (in decreasing order of density) by nerves IR to NOS, neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP), galanin (GAL), Leu⁵-enkephalin (LENK) and ChAT/VAChT. The CP contained the largest number of nerve fibres followed by the RPM, GP and corpus. Enzyme/peptide-containing nerves were associated with both the vascular and non-vascular penile structures. However, differences existed for their density and intrapenile distribution. Nerve terminals IR for different combinations of VIP, GAL or SOM were more frequent than those IR for NOS or CGRP in the non-vascular penile structures while the vasculature and the RPM received a prominent TH/DβH-, VIP-, SOM- or NOS-IR nerve input. The present data indicate that the porcine penis receives nerve fibres that exhibit diverse chemical codes and that differences in the chemical coding of the nerve fibres may depend on their penile target-structure.