RAT TAIL-FLICK REFLEX - MAGNITUDE MEASUREMENT OF STIMULUS-RESPONSE FUNCTION, SUPPRESSION BY MORPHINE AND HABITUATION

1. To quantitatively investigate a nocifensive behavioral response, we developed a method to measure the magnitude of the rat's tail flick r eflex and its modulation. A radial array of force transducers measured forces of tail flicks (in rostral, horizontal, and vertical planes) e licited by graded noxious radiant thermal stimulation of the conscious rat's tail, from which the overall movement vector was calculated. 2. The rostrally directed component of tail flicks was always larger tha n dorsal or horizontal components; the latter was usually in a preferr ed (left or right) direction regardless of which side of the tail was heated. Tail flick force vectors increased from 40 to 46-52-degrees-C and then leveled off. Stimulus-response functions were reproducible wi thin and across rats and were fitted by second-order polynomial functi ons, whose correlation coefficients were similar when the left or righ t side of the tail was stimulated in separate sessions (r2 = 0.408 and 0.45 1, respectively). The inverse latency of tail flicks also increa sed with temperature in a manner fitted by a second-order polynomial ( r2 = 0.707, 0.553 for left and right side, respectively). 3. Systemic administration of morphine (1 or 2 mg/kg ip) usually suppressed tail f licks in an all-or-none manner; i.e., flicks at all stimulus temperatu res were either totally abolished (n = 7) or unaffected (n = 5) after morphine. In three rats, 1 mg/kg morphine suppressed tail flick magnit ude subtotally, reducing the slope of the linear portion of the stimul us-response function. Morphine effects were reversed by the opiate ant agonist naloxone. 4. Tail flick magnitude decreased over repeated tria ls of 44-degrees-C heat stimuli delivered to one tail site, recovered after a 15-min rest period, and decremented more quickly with subseque nt stimulus repetition. The decrement was less at long (2 or 4 min) th an at short (1 min) interstimulus intervals, and high (50-degrees-C) t han at low (44-degrees-C) stimulus intensities. The reflex decrement t ransferred to a nearby stimulus site in some rats, and was ''dishabitu ated'' after a noxious tail pinch. These observations are consistent w ith habituation of the tail flick reflex. 5. This method, therefore, p rovides a quantitative and reproducible measure of tail flick reflex m agnitude that is sensitive to morphine. The underlying neural circuitr y of the tail flick reflex is discussed in relation to limb withdrawal reflexes.