Tactile allodynia in the absence of C-fiber activation: altered firing properties of DRG neurons following spinal nerve injury

Abstract

We examined the relation between ectopic afferent firing and tactile allodynia in the Chung model of neuropathic pain. Transection of the L5 spinal nerve in rats triggered a sharp, four- to six-fold increase in the spontaneous ectopic discharge recorded in vivo in sensory axons in the ipsilateral L5 dorsal root (DR). The increase, which was not yet apparent 16 h postoperatively, was complete by 24 h. This indicates rapid modification of the electrical properties of the neurons. Only A-neurons, primarily rapidly conducting A-neurons, contributed to the discharge. No spontaneously active C-neurons were encountered. Tactile allodynia in hindlimb skin emerged during precisely the same time window after spinal nerve section as the ectopia, suggesting that ectopic activity in injured myelinated afferents can trigger central sensitization, the mechanism believed to be responsible for tactile allodynia in the Chung model. Most of the spike activity originated in the somata of axotomized DRG neurons; the spinal nerve end neuroma accounted for only a quarter of the overall ectopic barrage. Intracellular recordings from afferent neuron somata in excised DRGs in vitro revealed changes in excitability that closely paralleled those seen in the DR axon recordings in vivo. Corresponding changes in biophysical characteristics of the axotomized neurons were catalogued. Axotomy carried out at a distance from the DRG, in the mid-portion of the sciatic nerve, also triggered increased afferent excitability. However, this increase occurred at a later time following axotomy, and the relative contribution of DRG neuronal somata, as opposed to neuroma endings, was smaller. Axotomy triggers a wide variety of changes in the neurochemistry and physiology of primary afferent neurons. Investigators studying DRG neurons in culture need to be alert to the rapidity with which axotomy, an inevitable consequence of DRG excision and dissociation, alters key properties of these neurons. Our identification of a specific population of neurons whose firing properties change suddenly and synchronously following axotomy, and whose activity is associated with tactile allodynia, provides a powerful vehicle for defining the specific cascade of cellular and molecular events that underlie neuropathic pain.

Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

“…We selected medium-sized neurons since they include nociceptors, and because they constitute a physiologically relevant cellular population that develops increased excitability after nerve injury [58,80]. Several studies have indicated that this neuronal population exhibits the most significant phenotypic changes after nerve injury [11,23,53,55,56,63,80,98]. Our observations on iberiotoxin-sensitive current in small neurons demonstrates that injury-related effects on primary afferent neurons are not unique to the medium size group.…”

“…Inhibition of K (Ca) channels by norepinephrine has been also implicated in the membrane hyper-excitability and hyperalgesia after chronic constriction sciatic nerve injury [37]. Spontaneous firing develops also in injured myelinated afferents after SNL [53], thus our findings that axotomized myelinated -but not adjacent uninjured afferents-develop increased excitability, and axotomized medium-sized fibers lose I K(Ca) , contribute additional support to the argument that injured (and not uninjured) myelinated afferents mediate neuropathic pain behavior after SNL [6]. Because BK channels control neurotransmitter release [86] and inter-neuronal glutaminergic synaptic efficacy [72], it is also likely that loss of BK currents in small and medium sized neurons after axotomy may enhance synaptic transmission of afferent signaling, as well.…”

“…We therefore chose to focus on this representative population. Several studies have indicated that medium sized somata with axons of Aδ-fiber caliber exhibit significant phenotypic changes after nerve injury, and may be an important generator of ectopic discharges [11,23,53,55,56,63,98]. In addition, a previous study from our lab showed that Aδ fibers exhibited the most pronounced electrophysiological changes after SNL, and in particular increased AP duration, decreased AHP amplitude and duration, and increased repetitive firing during sustained depolarization, manifestations that may explain increased neuronal excitability and hyperalgesia in neuropathic pain states [80].…”

Increased Sensitivity of Injured and Adjacent Uninjured Rat Primary Sensory Neurons to Exogenous Tumor Necrosis Factor-α after Spinal Nerve Ligation

“…The spinal nerve (stump) was stimulated with a suction electrode. Action potentials were evoked with a fixed latency to a minimum of three stimuli before they were classified according to their conduction velocity: A-␤, Ͼ14 m/sec; A-␦, 1-14 m/sec; C fibers, Ͻ1 m/sec (Waddell et al, 1989;Liu et al, 2000). The neural signal was differentially amplified, filtered (Neurolog, Digitimer, Hertfordshire, UK), displayed on a digital oscilloscope, and digitized as waveforms at a rate of 20 kHz via the Spike 2 data acquisition system (Cambridge Electronic Design, Cambridge, UK) on a Pentium III PC.…”

Imbalance of ionic conductances contributes to diverse symptoms of demyelination

“…Regardless of the precise etiology, clinical presentation often involves negative (loss-of-function) symptoms, including loss of motor control (i.e., paresis) and sensory deficits such as blindness and numbness, as well as positive (gain-of-function) symptoms, including muscle spasms, tactile allodynia, and chronic pain that is constant or paroxysmal (1,(4)(5)(6)(7). Which muscles or sensory modalities are affected depends on where in the nervous system the demyelinating lesions develop, but changes in conduction velocity alone are clearly insufficient to explain the breadth of symptoms observed, especially the positive ones.…”