Dependence of the nociceptive withdrawal response on stimulus location in the intact, unanesthetized rat

Brady Simpson

Abstract

The nociceptive withdrawal response (NWR) is a critical movement characterized by the avoidance of noxious or otherwise unwanted stimuli. The response is mediated in mammals by withdrawal of the hind limb. Although the NWR has been studied in humans and spinalized animals, it is unclear whether there is a dependence on stimulus location in intact, unanesthetized, non-human animals. The specific aim of my research was to use high speed video of the intact, unanesthetized rat’s limb to determine if there was an effect of stimulus location on hind limb movement during the NWR evoked by heat stimuli. I hypothesized, based on previous work in this laboratory, that there may be only a limited dependence on stimulus location. The rat hind foot plantar surface and lower leg were stimulated in multiple locations with a laser-based heat stimulus and the resulting NWR movement was recorded using high speed video to determine if the time course of joint angle (ankle, knee, hip) changes were influenced by stimulus location. Results show that there were three distinct sequential phases of movement: early extension, rapid flexion and rapid extension. There was limited effect of stimulus location on the three phases of response but a clear dependence on the initial location of the foot prior to stimulation. Overall, the NWR appears designed to preserve postural stability rather than accurately direct withdrawal movements away from noxious stimuli.