The nociceptive withdrawal response evoked by electrical stimulation in intact unanesthetized rats
The nociceptive withdrawal response (NWR) is characterized by withdrawal of a limb to avoid noxious stimuli. While the NWR has been studied in humans and spinalized animals, there is a significant lack of research in intact, unanesthetized non-human mammals using electrical stimulation. The aim of our research was to determine if there is a dependence on stimulus location when evoking the NWR using electrical stimulation in intact, unanesthetized rats. Previous research suggests a hypothesis that stimulation of the front of the foot would produce dorsiflexion whereas stimulation to the back of the foot would produce plantar flexion, as well as a lack of stimulation. Rats were stimulated at five locations on the plantar surface of the hind foot using brief, electrical current pulses. The resulting movement of the NWR in the sagittal plane with high speed video (500 fps) to determine the time course of changes in joint angle in the ankle, knee, and hip, and a conventional video camera was placed underneath the rat to determine the initial and final location of the foot in the frontal plane. Our results show the NWR occurred in two phases: an early flexion phase, in which the foot was withdrawn from the stimulus, and a late extension phase, in which the foot was rapidly replaced on the surface. The early flexion phase was stereotyped, while the late extension phase varied with the initial position of the foot preceding stimulation. Surprisingly, compared to similar research using heat stimuli, the replacement of the foot was always in the caudal direction and there was a lack of initial extension at the ankle, knee, and hip joints, in contrast to similar studies with heat stimuli. Together, our results suggest that the NWR is designed to maintain posture rather than accurately match the response to the location of the stimulus. Posture, more broadly, is an important component in modulating reflexes.