Separate groups of dorsal horn neurons transmit spontaneous activity and mechanosensitivity one day after plantar incision

Dorsal horn neuron (DHN) sensitization can be induced and maintained by nociceptor activation. In previous studies, only a small increase in ongoing DHN activity was present immediately after plantar incision; yet, powerful activation of nociceptors was prominent 1 day after incision. In the present study, rats underwent plantar incision or sham surgery as control. One day later pain behaviors were measured; then DHN activity and response properties to mechanical stimulation were recorded in vivo. In some neurons with spontaneous activity, the incision was anesthetised using bupivacaine. Spontaneous activity was identified in 9 of 27 neurons in the control group and in 16 of 26 neurons (P < 0.05) in the incision group; the spontaneous activity in incised animals (19.1 ± 8.5 imp/s) was also greater than in controls (10.6 ± 6.1 imp/s, P < 0.05). For neurons without spontaneous activity, the responses to mechanical stimuli of the incision group were greater (P < 0.01); for neurons with spontaneous activity, there was no such difference. After bupivacaine injection, no difference was present in the amount of spontaneous activity between the two groups. This study demonstrates that 1 day after incision, DHN sensitization manifests in markedly increased spontaneous activities, enhanced responses to mechanical stimuli and expanded receptive fields (RFs). Separate groups of neurons appear to transmit spontaneous activity and enhanced responses to mechanical stimuli. Inhibition of spontaneous activity by blockade of afferent input indicates that the prolonged spinal hyperactivity remains largely dependent on the ongoing primary afferent activity.