Spinal Nerve Injury Enhances Subthreshold Membrane Potential Oscillations in DRG Neurons: Relation to Neuropathic Pain

Abstract

Primary sensory neurons with myelinated axons were examined in vitro in excised whole lumbar dorsal root ganglia (DRGs) taken from adult rats up to 9 days after tight ligation and transection of the L(5) spinal nerve (Chung model of neuropathic pain). Properties of subthreshold membrane potential oscillations, and of repetitive spike discharge, were examined. About 5% of the DRG neurons sampled in control DRGs exhibited high-frequency, subthreshold sinusoidal oscillations in their membrane potential at rest (V(r)), and an additional 4.4% developed such oscillations on depolarization. Virtually all had noninflected action potentials (A(0) neurons). Amplitude and frequency of subthreshold oscillations were voltage sensitive. A(0) neurons with oscillations at V(r) appear to constitute a population distinct from A(0) neurons that oscillate only on depolarization. Axotomy triggered a significant increase in the proportion of neurons exhibiting subthreshold oscillations both at V(r) and on depolarization. This change occurred within a narrow time window 16-24 h postoperative. Axotomy also shifted the membrane potential at which oscillation amplitude was maximal to more negative (hyperpolarized) values, and lowered oscillation frequency at any given membrane potential. Most neurons that had oscillations at V(r), or that developed them on depolarization, began to fire repetitively when further depolarized. Spikes were triggered by the depolarizing phase of oscillatory sinusoids. Neurons that did not develop subthreshold oscillations never discharged repetitively and rarely fired more than a single spike or a short burst, on step depolarization. The most prominent spike waveform parameters distinguishing neurons capable of generating subthreshold oscillations, and hence repetitive firing, was their brief postspike afterhyperpolarization (AHP) and their low single-spike threshold. Neurons that oscillated at V(r) tended to have a more prolonged spike, with slower rise- and fall-time kinetics, and lower spike threshold, than cells that oscillated only on depolarization. The main effects of axotomy were to increase spike duration, slow rise- and fall-time kinetics, and reduce single-spike threshold. Tactile allodynia following spinal nerve injury is thought to result from central amplification ("central sensitization") of afferent signals entering the spinal cord from residual intact afferents. The central sensitization, in turn, is thought to be triggered and maintained in the Chung model by ectopic firing originating in the axotomized afferent neurons. Axotomy by spinal nerve injury enhances subthreshold membrane potential oscillations in DRG neurons, augments ectopic discharge, and hence precipitates neuropathic pain.

Burst Discharge in Primary Sensory Neurons: Triggered by Subthreshold Oscillations, Maintained by Depolarizing Afterpotentials

“…Rather than their discharge being triggered by the classical rhythmogenic process whereby a sustained depolarizing current repeatedly draws the membrane potential toward spike threshold, firing in these cells is caused by sinusoidal oscillations in the membrane potential. Spikes arise when peaks of normally subthreshold sinusoids reach threshold (Amir et al, 1999;C.-N. Liu et al, 2000a). We now confirm that this also applies in vivo.…”

“…Moreover, even when artificially depolarized, few DRG neurons are capable of firing more than a brief spike burst. The slow, tonic depolarizations normally caused by physiological stimuli rarely evoke any spikes at all (Villiere and McLachlan, 1996;Amir et al, 1999;C.-N. Liu et al, 2000a). How, then, does ectopic afferent discharge occur?…”

“…Axotomy enhances firing by increasing the number of oscillating neurons and shifting the membrane potential at which oscillations occur closer to rest (V r ) (Amir et al, 1999;C.-N. Liu et al, 2000a). Additional amplification results from replacement of the usual slow intermittent firing pattern with high-frequency bursts.…”

“…Oscillation sinusoids typically remained phase-locked with a pure sine wave for only a few tens of cycles, before drifting by up to several Hertz. Defining a stable mean frequency requires sampling over epochs of several seconds (Amir et al, 1999;C.-N. Liu et al, 2000a). Oscillation amplitude was voltage sensitive, with a peak of 3-6 mV at membrane potentials of Ϫ55 to Ϫ30 mV.…”

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy

“…We also suggest that the prolonged nature of the afterdischarge in this model is due to continued peripheral drive even after the pinch stimulus is ended. Thus, based on our findings we propose that the hyperalgesia and dysesthesia experienced by people with peripheral neuropathy might be at least partially due to sustained repetitive discharge of primary afferents (Amir et al, 1999;Liu et al, 2000a) and prolongation of excitation of primary afferent neurons in response to tactile or noxious stimulation of the skin.…”