Enhanced excitability of sensory neurons in rats with cutaneous hyperalgesia produced by chronic compression of the dorsal root ganglion

Subthreshold membrane potential oscillations of type A neurons in injured DRG

An abundance of subthreshold membrane potential oscillations (SMPOs) at resting potential was observed in the neurons of chronically compressed dorsal root ganglia (DRG) using intracellular recording in vivo. Out of 386 neurons, 63 type A neurons displayed SMPOs. Three types of SMPOs were distinguished based on their characterizations of oscillation: (1) A regular pattern of SMPO emerged consistently with a mean frequency of 86 Hz and mean amplitudes of 3.3 mV. (2) A spindle-like pattern of SMPO was denominated by a spindle alteration of its amplitude. (3) An irregular pattern of SMPO had no rule on its change of amplitude and frequency. Compared with normal DRG neurons and injured DRG neurons but without SMPO, the injured DRG neurons with SMPO had the lowest spike rheobase, in accordance with the detection of spike accommodation. No significant differences among the three groups can be found in either membrane potential or input resistance. Further observation showed that the spontaneous discharge of hyperexcitable neurons usually occurred on the depolarizing phase of oscillations. In addition, the regular pattern of SMPO was based on the period and integer multiple patterns of spontaneous discharges. The spindle-like pattern of SMPO contributed to spontaneous bursting discharge. The irregular pattern of SMPO had a striking relation with irregular spontaneous discharge. The results show that neurons with SMPO in injured DRG have a higher excitability than those without SMPO, and that the SMPO underlie the patterns of spontaneous discharges, suggesting that SMPO is the basic electrophysiological change of hyperexcitable neurons.

Contribution of Na(v)1.8 sodium channels to action potential electrogenesis in DRG neurons

C-type dorsal root ganglion (DRG) neurons can generate tetrodotoxin-resistant (TTX-R) sodium-dependent action potentials. However, multiple sodium channels are expressed in these neurons, and the molecular identity of the TTX-R sodium channels that contribute to action potential production in these neurons has not been established. In this study, we used current-clamp recordings to compare action potential electrogenesis in Na(v)1.8 (+/+) and (-/-) small DRG neurons maintained for 2-8 h in vitro to examine the role of sodium channel Na(v)1.8 (alpha-SNS) in action potential electrogenesis. Although there was no significant difference in resting membrane potential, input resistance, current threshold, or voltage threshold in Na(v)1.8 (+/+) and (-/-) DRG neurons, there were significant differences in action potential electrogenesis. Most Na(v)1.8 (+/+) neurons generate all-or-none action potentials, whereas most of Na(v)1.8 (-/-) neurons produce smaller graded responses. The peak of the response was significantly reduced in Na(v)1.8 (-/-) neurons [31.5 +/- 2.2 (SE) mV] compared with Na(v)1.8 (+/+) neurons (55.0 +/- 4.3 mV). The maximum rise slope was 84.7 +/- 11.2 mV/ms in Na(v)1.8 (+/+) neurons, significantly faster than in Na(v)1.8 (-/-) neurons where it was 47.2 +/- 1.3 mV/ms. Calculations based on the action potential overshoot in Na(v)1.8 (+/+) and (-/-) neurons, following blockade of Ca(2+) currents, indicate that Na(v)1.8 contributes a substantial fraction (80-90%) of the inward membrane current that flows during the rising phase of the action potential. We found that fast TTX-sensitive Na(+) channels can produce all-or-none action potentials in some Na(v)1.8 (-/-) neurons but, presumably as a result of steady-state inactivation of these channels, electrogenesis in Na(v)1.8 (-/-) neurons is more sensitive to membrane depolarization than in Na(v)1.8 (+/+) neurons, and, in the absence of Na(v)1.8, is attenuated with even modest depolarization. These observations indicate that Na(v)1.8 contributes substantially to action potential electrogenesis in C-type DRG neurons.

Topical application of acidic bupivacaine to the lumbar ganglion induces mechanical hyperalgesia in the rat

To investigate the neurologic mechanisms of acidic local anesthetic-induced low back pain in humans, we administered bupivacaine and buffered saline at acidic or alkalinized pH at the L5 dorsal root ganglion (DRG) of rats via a hole drilled through the transverse process covering the DRG. Behavioral changes were tested before and after bupivacaine or saline administration. Results indicate that acute single-dose infusion of the DRG with bupivacaine (0.5%) at acidic pH (5.5) induced ipsilateral mechanical hyperalgesia that lasted for 7 days. Acute infusion of alkalinized bupivacaine (pH 7.2), however, caused only minor hyperalgesia that lasted <3 days. Similar results were obtained when bupivacaine was replaced with saline. Alternatively, chronic delivery of acidic saline to the DRG via a subcutaneously implanted osmotic pump resulted in a significant decrease in the withdrawal threshold on the ipsilateral hind paw that lasted for 10 days. In rats receiving chronic treatment of the DRG with alkalinized saline, mechanical hyperalgesia lasted for only 3 days. The results demonstrated that acidic bupivacaine deposited at the DRG causes pain and hyperalgesia when the effects of the local anesthetic have dissipated. These findings may explain the limited therapeutic effects of some acidic local anesthetics used for management of cancer-related and chronic back pain.

IMPLICATIONS

Acidic bupivacaine administered at the L5 lumbar ganglion causes pain and hypersensitivity of the hind paw in the rat. These findings may explain the limited therapeutic effects of some acidic local anesthetics used for treatment of cancer-related and chronic back pain.

Perfusion of the mechanically compressed lumbar ganglion with lidocaine reduces mechanical hyperalgesia and allodynia in the rat

The rat L(5) dorsal root ganglion (DRG) was chronically compressed by inserting a hollow perforated rod into the intervertebral foramen. The DRG was constantly perfused through the hollow rod with either lidocaine or normal saline delivered by a subcutaneous osmotic pump. Behavioral evidence for neuropathic pain after DRG compression involved measuring the incidence of hindlimb withdrawals to both punctate indentations of the hind paw with mechanical probes exerting different bending forces (hyperalgesia) and to light stroking of the hind paw with a cotton wisp (tactile allodynia). Behavioral results showed that for saline-treated control rats: the withdrawal thresholds for the ipsilateral and contralateral paws to mechanical stimuli decreased significantly after surgery and the incidence of foot withdrawal to light stroking significantly increased on both ipsilateral and contralateral hind paws. Local perfusion of the compressed DRG with 2% lidocaine for 7 days at a low flow-rate (1 microl/h), or for 1 day at a high flow-rate (8 microl/h) partially reduced the decrease in the withdrawal thresholds on the ipsilateral foot but did not affect the contralateral foot. The incidence of foot withdrawal in response to light stroking with a cotton wisp decreased significantly on the ipsilateral foot and was completely abolished on the contralateral foot in the lidocaine treatment groups. This study demonstrated that compression of the L(5) DRG induced a central pain syndrome that included bilateral mechanical hyperalgesia and tactile allodynia. Results also suggest that a lidocaine block, or a reduction in abnormal activity from the compressed ganglia to the spinal cord, could partially reduce mechanical hyperalgesia and tactile allodynia.

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 intervertebral foramen, one rod at L(4) and another at L(5). Two additional groups of rats received either a sham surgery or an acute injury consisting of a transient compression of the ganglion. Withdrawal of the hindpaw was used as evidence of a nocifensive response to mechanical and thermal stimulation of the plantar surface. In addition, extracellular electrophysiological recordings of spontaneous discharges were obtained from dorsal root fibers of formerly compressed ganglia using an in vitro nerve-DRG-dorsal root preparation. The mean threshold force of punctate indentation and the mean threshold temperature of heating required to elicit a 50% incidence of foot withdrawal ipsilateral to the CCD were significantly lower than preoperative values throughout the 35 days of postoperative testing. The number of foot withdrawals ipsilateral to the CCD during a 20-min contact with a temperature-controlled floor was significantly increased over preoperative values throughout postoperative 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 but did so in most rats tested ipsilateral to the CCD during the first 2 postoperative 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, mechanical hyperalgesia for approximately 1 day, ipsilateral to the acute injury. Ectopic spontaneous discharges generated within the chronically compressed ganglion 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 incidence of spontaneously active myelinated fibers was 8.61% for CCD rats versus 0.96% for previously nonsurgical rats. We hypothesize that a chronic compression of the dorsal root ganglion after certain injuries or diseases of the spine may produce, in neurons with intact axons, abnormal ectopic discharges that originate from the ganglion and potentially contribute to low back pain, sciatica, hyperalgesia, and tactile allodynia.