The contribution of capsaicin-sensitive sensory nerves to xylene-induced visceral pain in conscious, freely moving rats.

1. Intravesical instillation of xylene (10-100%, dissolved in silicone oil) through a catheter implanted into the bladder of conscious, freely-moving rats produced behavioural effects (licking of lower abdomen or perineal region) suggestive of intense visceral pain, not mimicked by topical application of the irritant on the urethral outlet. 2. The xylene-induced visceral pain was prevented, to the same extent, by systemic desensitization to capsaicin (50 mg/kg s.c.) performed in either adult or newborn rats, as well as by extrinsic bladder denervation (pelvic ganglionectomy), thus indicating the involvement of primary afferents in the bladder wall. 3. Other behavioural responses induced by xylene instillation into the bladder (hind limb hyperextension, grooming) were not affected by systemic capsaicin desensitization in either adult or newborn rats, but were abolished by bladder denervation. 4. Systemic capsaicin desensitization produced an almost complete depletion of substance P-, neurokinin A-like and calcitonin gene-related peptide-like immunoreactivity in the rat urinary bladder. 5. These findings indicate that, in addition to their role in activating reflex micturition, the neuropeptides-containing capsaicin-sensitive sensory nerves of the rat bladder are involved in chemogenic visceral pain.