A model for the study of visceral pain states: chronic inflammation of the chronic decerebrate rat urinary bladder by irritant chemicals

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Abstract
Normal healthy visceral tissue is largely insensitive to many kinds of tissue damaging stimuli, such as cutting, crushing or burning. When inflamed, however, visceral tissue frequently and easily elicits pain. Here we have produced inflammatory resonses in urinary bladders of chronically decerebrated rats with irritant chemicals, as a model of a visceral pain state. The irritants used were 25% turpentine, 2.5% mustard oil, 2% croton oil. All produced a strong inflammatory response, as judged by protein extravasation and oedema, and invasion of the tissue with leucocytes, which started within hours of treatment. The inflammatory states were associated with hyperexcitable bladder reflexes; baseline pressures were larger at given volumes, and large micturition contractions occurred at lower volumes. These changes persisted for up to 48 h after a single treatment. The behaviour of the animals and their responsiveness to noxious stimulation were also assessed. In general animals were hypersensitive to noxious stimuli applied to the tail or caudal abdomen, with small or no changes in responses to hind limb or rostral abdominal stimulation. These irritants, particularly turpentine, applied to chronic decerebrate rats may thus provide useful models for studies of the neuronal processes which contribute to visceral nociception.