Most of the endogenous pain modulation (EPM) involves the spinal dorsal horn (SDH). EPM including diffuse noxious inhibitory controls have been extensively described in oligoneuronal electrophysiological recordings but less attention had been paid to responses of the SDH neuronal population to heterotopic noxious stimulation (HNS). Spinal somatosensory-evoked potentials (SEP) offer the possibility to evaluate the neuronal network behavior, reflecting the incoming afferent volleys along the entry root, SDH interneuron activities and the primary afferent depolarization. SEP from de lumbar cord dorsum were evaluated during mechanical heterotopic noxious stimuli. Sprague-Dawley rats (n = 12) were Laminectomized (T10-L3). The sural nerve of the left hind paw was electrically stimulated (5 mA, 0.5 ms, 0.05 Hz) to induce lumbar SEP. The HNS (mechanic clamp) was applied sequentially to the tail, right hind paw, right forepaw, muzzle and left forepaw during sural stimulation. N wave amplitude decreases (-16.6 %) compared to control conditions when HNS was applied to all areas of stimulation. This effect was more intense for muzzle stimulation (-23.5 %). N wave duration also decreased by -23.6 %. HNS did not change neither the amplitude nor the duration of the P wave but dramatically increases the dispersion of these two parameters. The results of the present study strongly suggest that a HNS applied to different parts of the body is able to reduce the integrated electrical response of the SDH, suggesting that not only wide dynamic range neurons but many others in the SDH are modulated by the EPM.

Despite the frequent clinical hyper- or hypothermia cases, thermal-dependence of the endogenous pain modulation system at the spinal cord is not well understood. We evaluate spinal dorsal horn neuronal network responses during mechanical heterotopic noxious stimuli (HNS) at three different body temperatures (34; 37 or 40°C) by measuring lumbar cord dorsum potentials activated by electrical stimulation of the ipsilateral sural nerve in adult thiopental anesthetized rats. A noxious clamp was applied randomly to the tail, right hindpaw, right forepaw, muzzle and left forepaw. HNS induced a decrease of the N wave amplitude and duration at 37°C. This effect was reduced at 40°C for both amplitude (-18.2% for 37-40°C; P<0.0005) and duration (-16.4% for 37-40°C; P<0.0001). P wave did not show neither amplitude nor duration changes at neither 3 tested temperatures. Clinical range changes of temperature could modify pain sensation, moreover, hyperthermia increases nociceptive sensory input to dorsal horn, and could exacerbate pain sensation in individuals with fever.