Enhanced responses of spinal dorsal horn neurons to heat and cold stimuli following mild freeze injury to the skin

The effects of a mild freeze injury to the skin on responses of nociceptive dorsal horn neurons to cold and heat stimuli were examined in anesthetized rats. Electrophysiological recordings were obtained from 72 nociceptive sp inal neurons located in the superficial and deep dorsal horn. All neurons h ad receptive fields (RFs) on the glabrous skin of the hindpaw, and neurons were functionally divided into wide dynamic range (WDR) and high-threshold (HT) neurons. Forty-four neurons (61%) were classified as WDR and responded to both innocuous and noxious mechanical stimuli (mean mechanical threshol d of 12.8 +/- 1.6 mN). Twenty-eight neurons (39%) were classified as HT and were excited only by noxious mechanical stimuli (mean mechanical threshold of 154.2 +/- 18.3 mN). Neurons were characterized for their sensitivity he at (35 to 51 degreesC) and cold (28 to -12 degreesC) stimuli applied to the ir RF. Among WDR neurons, 86% were excited by both noxious heat and cold st imuli, while 14% responded only to heat. For HT neurons, 61% responded to h eat and cold stimuli, 32% responded only to noxious heat, and 7% responded only to noxious cold. Effects of a mild freeze injury (-15 degreesC applied to the RF for 20 s) on responses to heat and cold stimuli were examined in 30 WDR and 22 HT neurons. Skin freezing was verified as an abrupt increase in skin temperature at the site of injury due to the exothermic reaction a ssociated with crystallization. Freezing produced a decrease in response th resholds to heat and cold stimuli in most WDR and HT neurons. WDR and HT ne urons exhibited a mean decrease in response threshold for cold of 9.0 +/- 1 .3 degreesC and 10.0 +/- 1.6 degreesC, respectively. Mean response threshol ds for heat decreased 4.0 +/- 0.4 degreesC and 4.3 +/- 1.3 degreesC in WDR and HT neurons, respectively. In addition, responses to suprathreshold cold and heat stimuli increased. WDR and HT neurons exhibited an 89% and a 192% increase in response across all cold stimuli, and a 93 and 92% increase in responses evoked across all heat stimuli, respectively. Our results demons trate that many spinal neurons encode intensity of noxious cold as well as noxious heat over a broad range of stimulus temperatures. Enhanced response s of WDR and HT neurons to cold and heat stimuli after a mild freeze injury is likely to contribute to thermal hyperalgesia following a similar freeze injury in humans.