Alteration of medullary dorsal horn neuronal activity following inferior alveolar nerve transection in rats

The effects of inferior alveolar nerve (IAN) transection on escape behavior and MDH neuronal activity to noxious and nonnoxious stimulation of the fac e were precisely analyzed. Relative thresholds for escape from mechanical s timulation applied to the whisker pad area ipsilateral to the transection w ere significantly lower than that for the contralateral and sham-operated w hisker pad until 28 days after the transection, then returned to the preope rative level at 40 days after transection. A total of 540 neurons were reco rded from the medullary dorsal horn (MDH) of the nontreated naive rats [low -threshold mechanoreceptive (LTM), 27; wide dynamic range (WDR), 31; nocice ptive specific (NS), 11] and sham-operated rats with skin incision (LTM, 34 ; WDR, 30; NS, 23) and from the ipsilateral (LTM, 82; WDR, 82; NS, 31) and contralateral MDH relative to the IAN transection (LTM, 77; WDR, 82; NS, 33 ). The electrophysiological properties of these neurons were precisely anal yzed. Background activity of WDR neurons on the ipsilateral side relative t o the transection was significantly increased at 2-14 days after the operat ion as compared with that of naive rats. Innocuous and noxious mechanical-e voked responses of LTM and WDR neurons were significantly enhanced at 2-14 days after IAN transection. The mean area of the receptive fields of WDR ne urons was significantly larger on the ipsilateral MDH at 2-7 days after tra nsection than that of naive rats. We could not observe any modulation of th ermal responses of WDR and NS neurons following IAN transection. Also, no M DH neurons were significantly affected in the rats with sham operations. Th e present findings suggest that the increment of neuronal activity of WDR n eurons in the MDH following IAN transection may play an important role in t he development of the mechano-allodynia induced in the area adjacent to the area innervated by the injured nerve.