Calcitonin gene-related peptide (CGRP)-immunoreactive nerve terminals in the whole mount preparations of the dog urethra.

To visualize the entire shape of the intraepithelial nerve fibers, whole mount preparations of the dog urethra were produced and immunostained with an antiserum against CGRP, one of the predominant substances contained in the nerves. The immunoreactive nerves in the lamina propria were smooth (non-beaded) in appearance and weak in immunoreaction. Within the epithelium, they displayed typical beaded profiles and were intense in immunoreaction. The intraepithelial fibers branched and wound into an extensive network with wide meshes ("reticular terminal"). The bead-like swellings included large ones resembling Herring bodies in hypophyseal neurosecretory fibers. Another type of nerve terminal, consisting of fine and weakly immunopositive fibers, was also found in the epithelium. These branched in dendritic or in dense bouquet-like fashion, occupying smaller areas ("bouquet-like terminals). Vesicular swellings often characterized these terminals, though they were smaller and more uniform in size and far less in their amount of immunoreactive substance than were the swellings in the reticular terminals. Both types of nerve terminals originated from the same nerve trunk. The connection between the reticular and bouquet-like terminals, which may presumably represent secretory and receptive parts, respectively, morphologically supports the possible occurrence of an axon reflex in the urethral CGRP neurons. Our whole mount preparations, when doubly stained with CGRP and serotonin antibodies, further revealed the CGRP-positive reticular terminals being closely associated with serotonin- or CGRP-immunoreactive paraneurons dispersed in the epithelium.