Sensory innervation of the porcine urethral muscle.

The urethral muscle (UM) is a pelvic striated muscle involved in the voluntary control of micturition, that needs complex interactions between autonomic and somatic afferent and efferent pathways. The afferent fibres travelling in the pudendal nerves are both myelinated and unmyelinated and can modulate bladder activity being sensitive to urine flow, contraction of the muscle itself and painful stimuli (1‐2). For a better understanding of visceral nociception and excretory mechanisms we studied the site, morphology and neurochemical characteristics of primary sensory neurons projecting to the UM of the pig, considered an important animal model for biomedical studies. After injection of 50 μl of Fast Blue (FB) in the ventral side of the UM of two 40‐Kg boars, the bilateral T14‐ Ca1 spinal ganglia (SG) were collected, cryosectioned and immunohistochemically processed to assess the co‐existence of the vanilloid receptor (VR1) with substance P (SP) and neuronal nitric oxide‐synthase (nNOS) within FB positive (FB+) neurons. We observed a thousand FB+ neurons bilaterally in S2‐S4 SG. Their area ranged from 300 to 2000 μm2. The majority (~69%) of the FB+ cells tested was VR1‐immunoreactive (VR1‐IR). Part of them (~12%) was also nNOS‐IR, while only ~4% was also SP‐IR. nNOS‐IR and SP‐IR neurons were found in ~16% and ~6% of the FB+ neurons tested. The presence of FB+ neurons almost exclusively in the S2‐S4 suggests that afferent fibres from the pig UM travel in the pudendal nerves. The VR1‐IR small/medium sized neurons could be the source of the unmyelinated nociceptive fibres responding to noxious chemical and mechanical stimuli, already documented in the lower urinary tract (3). Activation of these afferents triggers painful sensations as well as body defence mechanisms such as inflammation and bladder hyperactivity that eliminate infectious or irritating, potentially injurious, agents from the urinary tract. nNOS‐IR neurons were observed in a small number of cells, but it is known that their number increases as a consequence of pathological lesions. They synthesize NO, a gaseous neurotransmitter that should act in a retrograde manner both in SG and in the spinal cord modulating multisynaptic local circuits that process nociceptive inputs (4). SP‐IR neurons were scarce and most of them co‐expressed VR1. They could trigger central autonomic reflex and peripheral axon reflexes which modulate smooth muscle activity and facilitate transmission in autonomic ganglia in an anterograde manner. In conclusion, our results indicate an important role of nociception among afferent inputs from UM that facilitate the micturition reflex and promote complete bladder emptying.