Long term alterations in the excitability of the flexion reflex produced by peripheral tissue injury in the chronic decerebrate rat.

Chronic decerebrate rats have been prepared by the aspiration of all the cranial contents rostral to the mesencephalon. With careful nursing, body temperature control and orogastric tube feeding the rats survive for up to 4 months. The decerebrate rats have intact brain stem and spinal reflexes including locomotion, righting reflexes and grooming. Mild noxious cutaneous thermal and mechanical stimuli of an intensity which does not produce tissue damage evokes flexion withdrawal reflexes, vocalization, orientation to the site of the injury and generalized escape or attack responses, from the animals. These effects are transient and only occur during the application of the stimulus. The rats do not exhibit stress reactions, aggression or vocalization when handled before or after application of such noxious stimuli. The production of localized tissue injury by thermal or chemical means results in long term alterations (up to 6 weeks) in the thresholds and excitability of the flexor reflex, both ipsilateral and contralateral, to the site of the injury. It is proposed that the reduction in the flexor reflex threshold is analogous to the hyperalgesia and allodynia that follows tissue injury in man and that the long term alterations in the flexor reflex in the chronic decerebrate rat provide a model for the study of chronic pain, without the ethical problems associated with such studies in intact animals.