Comparison of escape and tail flick thresholds in the rat: a psychophysical analysis of morphine hypoalgesia.

The present study describes a wheel turn/tail flick paradigm that was designed to simultaneously assess nociceptive thresholds of responses organized at spinal and supraspinal levels of the CNS in the rat. The paradigm involves training rats to perform an operant wheel turn response in order to escape current applied to the tail. Thresholds for the supraspinally organized escape response and the spinally organized tail flick reflex were determined via the psychophysical method of constant stimuli. Response latencies for wheel turn escape and tail flick were recorded to determine whether changes in nociceptive thresholds were confounded with changes in motor performance. The systemic administration of 3 mg/kg morphine sulfate elevated thresholds for both responses, but escape thresholds were elevated to a greater degree than tail flick thresholds. Because response latencies at threshold were not affected by morphine treatment, it appears that performance deficits did not contribute to the increase in thresholds. Advantage of these psychophysical procedures in assessing nociceptive responding in animals are discussed.