Studies in suppression of nocifensive reflexes measured with tail flick electromyograms and using intrathecal drugs in barbiturate anesthetized rats.

We report on the effects of somatic conditioning stimuli on the reflexive nocifensive tail flick electromyograms (TFEMGs) in the rat anesthetized with continuous pentobarbital infusions. Rather than using a reflexive tail movement as a measure of noxious responses, TFEMGs were recorded from the base of the rat's tail as this gave more reliable results in the anesthetized state. In order to demonstrate the compatibility of this model with previous tail flick studies, we demonstrated an inhibition of TFEMGs by intrathecal morphine which was reversed by intravenous naloxone. The TFEMG latencies were then shown to be increased by electroacupuncture. This effect was antagonized by intrathecal naltrexone pretreatment. All of these results under barbiturate anesthesia resembled those observed previously in awake rats. The constant pentobarbital infusion maintained a stable baseline state, as reflected by TFEMG latencies and blood pressure recordings during the prolonged time-course of each experiment (approximately 1 h). We feel that this method of studying electroacupuncture suppression of nocifensive reflexes is valuable for its relative simplicity and reliability. It also has the virtue of avoiding the pitfalls of such studies in awake animals.