Mechanical and thermal hyperalgesia and ectopic neuronal discharge after chronic compression of dorsal root ganglia

Chronic compression of the dorsal root ganglion (CCD) was produced in adult rats by implanting a stainless steel rod unilaterally into the interverteb ral foramen, one rod at L-4 and another at L-5. Two additional groups of ra ts received either a sham surgery or an acute injury consisting of a transi ent compression of the ganglion. Withdrawal of the hindpaw was used as evid ence of a nocifensive response to mechanical and thermal stimulation of the plantar surface. In addition, extracellular electrophysiological recording s of spontaneous discharges were obtained from dorsal root fibers of former ly compressed ganglia using an in vitro nerve-DRG-dorsal root preparation. The mean threshold force of punctate indentation and the mean threshold tem perature of heating required to elicit a 50% incidence of foot withdrawal i psilateral to the CCD were significantly lower than preoperative values thr oughout the 35 days of postoperative testing. The number of foot withdrawal s ipsilateral to the CCD during a 20-min contact with a temperature-control led floor was significantly increased over preoperative values throughout p ostoperative testing when the floor was 4 degrees C (hyperalgesia) and, to a lesser extent, when it was 30 degrees C (spontaneous pain). Stroking the foot with a cotton wisp never elicited a reflex withdrawal before surgery b ut did so in most rats tested ipsilateral to the CCD during the first 2 pos toperative weeks. In contrast, the CCD produced no changes in responses to mechanical or thermal stimuli on the contralateral foot. The sham operation and acute injury produced no change in behavior other than slight, mechani cal hyperalgesia for similar to 1 day, ipsilateral to the acute injury. Ect opic spontaneous discharges generated within the chronically compressed gan glion and, occurring in the absence of blood-borne chemicals and without,an intact sympathetic nervous system, were recorded from neurons with intact, conducting, myelinated or unmyelinated peripheral nerve fibers. The incide nce of spontaneously active myelinated fibers was 8.61% for CCD rats versus 0.96% for previously nonsurgical rats. We hypothesize that a chronic compr ession of the dorsal root ganglion after certain injuries or diseases of th e spine may produce, in neurons with intact axons, abnormal ectopic dischar ges that originate from the ganglion and potentially contribute to low back pain, sciatica, hyperalgesia, and tactile allodynia.