Intrathecal administration of substance P in the rat: spinal transection or morphine blocks the behavioural responses but not the facilitation of the tail flick reflex.

Substance P (10 micrograms) was administered intrathecally (i.t.) via a chronic catheter to the L5 vertebral level in rats with spinal cords transected at the T10-12 level (n = 11) and in intact rats pretreated 25 min earlier with morphine (5 micrograms i.t., n = 5). In both experimental paradigms, substance P decreased the reaction time in the tail flick test at 1 min after injection; the magnitude of this effect resembled that which occurs in intact rats given substance P alone. In both paradigms, substance P failed to produce the behavioural responses typically observed in untreated rats given substance P. These results indicate that substance P-induced facilitation of the tail flick reflex may occur via spinal mechanisms alone. The results also suggest that in the spinal cord morphine preferentially blocks the relay of substance P-induced information to supraspinal structures. Finally, as the behavioural response is blocked by the analgesic and by spinal transection, this is interpreted as further support for the proposal that this behavioural response indicates that the animal has perceived a noxious stimulus.