Spontaneous activity of axotomized afferent neurons after L5 spinal nerve injury in rats

Sympathetic-sensory coupling after L5 spinal nerve lesion in the rat and its relation to changes in dorsal root ganglion blood flow

Transection of the L5 spinal nerve in rats results in allodynia- and hyperalgesia-like behavior to mechanical stimulation which are thought to be mediated by ectopic activity arising in lesioned afferent neurons mainly in the dorsal root ganglion (DRG). It has been suggested that the neuropathic pain behavior is dependent on the sympathetic nervous system. In rats 3-56 days after L5 spinal nerve lesion, we tested responses of axotomized afferent fibers recorded in the dorsal root of the lesioned segment to norepinephrine (NE, 0.5 microg/kg) injected intravenously and to selective electrical stimulation of the lumbar sympathetic trunk (LST). In some experiments we measured blood flow in the DRG by laser Doppler flowmetry. The majority of lesioned afferent fibers with spontaneous activity responded to neither LST stimulation (82.4%) nor NE (71.4%). In those which did react to LST stimulation, responses occurred only at high stimulation frequencies (likely to be above the physiological range), and they could be mimicked by non-adrenergic vasoconstrictor drugs (angiotensin II, vasopressin). Excitatory responses to LST stimulation were closely correlated with the stimulation-induced phasic vasoconstrictions in the DRG. We therefore hypothesized that the activation of lesioned afferents might be brought about indirectly by an impaired blood supply to the DRG. To test this hypothesis we induced a strong and sustained baseline vasoconstriction in the DRG by blocking endothelial nitric oxide synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME) applied systemically. L-NAME enhanced baseline vascular resistance in the DRG about threefold and also increased stimulation-induced vasoconstrictions. After L-NAME, the majority of axotomized neurons with spontaneous activity were activated by LST stimulation (76%) or NE (75%). Again, activations closely followed stimulation-induced phasic vasoconstrictions in the DRG provided that a critical level of vasoconstriction was exceeded. In the present study, inhibitory responses to LST stimulation were generally rare and could be reversed to activation by prolonged stimulation or after L-NAME. These results show that sympathetic-sensory coupling occurs only in a minority of axotomized afferents after L5 spinal nerve injury. Like previous studies, they cast doubt on the notion that the L5 spinal nerve lesion is a good model for sympathetically maintained pain. Since responses of lesioned afferent neurons to LST stimulation and NE could be provoked with high reliability after inducing vasoconstriction in the DRG, and since they mirrored stimulation-induced vasoconstrictions in the DRG, it appears that in this model the association of sympathetic activity with afferent discharge occurs mainly when perfusion of the DRG is impaired.

Dorsal root section elicits signs of neuropathic pain rather than reversing them in rats with L5 spinal nerve injury

Mechanical allodynia- and hyperalgesia-like behavior which develops in rats after L5 spinal nerve lesion has been suggested to be due to ectopic activity in the lesioned afferent neurons originating at the lesion site and/or in the dorsal root ganglion because it is eliminated by section of the dorsal root. Here we reevaluated the effect of a dorsal rhizotomy in rats after L5 spinal nerve lesion. Using calibrated von Frey hairs, paw withdrawal threshold to single stimuli and paw withdrawal incidence to repetitive stimulation were tested before and after nerve section. Neuropathic pain behavior of similar time course and magnitude also developed after cutting the L5 dorsal root, and L5 spinal nerve lesion-induced abnormal behavior could not be reversed by dorsal rhizotomy. The neuropathic pain behavior elicited by dorsal root section also developed when impulse conduction in the dorsal root axons was blocked during rhizotomy by a local anesthetic, i.e. when the immediate injury discharge was prevented from reaching the spinal cord. These results challenge the widely accepted idea that neuropathic pain behavior developing after spinal nerve lesion is dependent on ectopic activity in the lesioned afferent neurons. However, the present results do not rule out the possibility that after the two nerve lesions the mechanisms generating neuropathic pain behavior are different. After dorsal rhizotomy neuropathic pain behavior may be related to deafferentation whereas after spinal nerve lesion it may be caused by ectopic activity.

Axotomized and intact muscle afferents but no skin afferents develop ongoing discharges of dorsal root ganglion origin after peripheral nerve lesion

After peripheral nerve lesions, some axotomized afferent neurons develop ongoing discharges that originate in the dorsal root ganglion (DRG). We investigated in vivo which functional types of afferent neurons contributed to this ectopic activity. Six to twelve days after the gastrocnemius soleus (GS) nerve supplying skeletal muscle and the sural (SU) nerve supplying skin had been transected (experimental group E1), 20.4% of afferent neurons with myelinated axons projecting into the GS nerve produced ongoing discharges of irregular or bursting pattern. In contrast, all SU neurons were silent. Additional transection of peroneal and tibial nerves (group E2) induced ongoing activity in a similar percentage of GS neurons (22.1%), but their mean discharge frequency was higher (6.0 vs 2.7 Hz), and more of them exhibited bursting discharges (63 vs 17%). When the GS nerve had been left intact while tibial, peroneal, and SU nerve had been transected (group E3), 18.8% of unlesioned GS neurons developed ongoing discharges at a mean frequency of 6.1 Hz; most of them exhibited a bursting pattern. Without a preceding nerve lesion, almost no GS neuron (1.1%) fired spontaneously. Most afferent neurons with ongoing activity had an axonal conduction velocity of 5-30 m/sec indicating that some of these neurons may have had nociceptive function. These findings provide the first evidence that after peripheral nerve injury both axotomized as well as intact afferent neurons supplying skeletal muscle but not skin afferents generate ongoing activity within the DRG, probably because of a yet unknown signal in the DRG triggered by axotomy.

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

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.