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

Abstract

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

Fig. 1

Effect of peripheral nerve block, administered after mechanical cutaneous stimulation (pinch), on the afterdischarge of a WDR dorsal horn neuron in a rat 14 days after peripheral neuropathy. (A) The top ratemeter record shows that pinch stimulation (21 N for 3 s) of the cutaneous receptive field of the cuff-implanted hind paw produced an initial discharge, high frequency activity which lasted only for the duration of the stimulus, and a long lasting slowly decaying afterdischarge, increased activity which persisted approximately 20 min. The vertical axis shows the firing frequency (each bin represents the rate of discharge of this neuron in spikes/s) and the horizontal axis represents time (both axis are linear). The time and duration of the pinch stimulus are shown by the narrow rectangle below the ratemeter histogram. Exposing the sciatic nerve to 0.9% saline (lasting 4 min followed by saline flush, indicated by the gray and white bars, respectively, shown below the ratemeter histogram) had no effect on the rate of neural discharge. The inset to the left shows the cutaneous receptive field to touch stimulation, depicted by the shaded area. The inset to the right shows the area subjected to pinch stimulation. The neuron was 740 μm from the dorsal surface of the spinal cord and ipsilateral to the cuff-implanted sciatic nerve. The extracellular record inset shows single unit activity at a time in the ratemeter histogram indicated by the dotted lines. (B) Ratemeter histogram and single unit activity of the same neuron in (A) 60 min later showing that mechanical cutaneous stimulation (identical to that used in A) of the cutaneous receptive field (shown in inset) produced an initial discharge which lasted only for the duration of the stimulus and was similar in magnitude to that shown in (A). This second pinch stimulus also induced a slowly decaying afterdischarge with an onset similar to that seen in (A) but only up to the time of administration of lidocaine to the cuff-implanted sciatic nerve after which the firing rate was decreased and remained considerably less than that before lidocaine administration for the remainder of the recording. Exposure of the cuff-implanted sciatic nerve to 2% lidocaine, began 4 min after the pinch stimulus, lasted 4 min, and was followed by saline flush, indicated by the gray and white bars, respectively, shown below the ratemeter histogram.

Fig. 2

Histogram summarizing the effect of peripheral nerve block, administered after the pinch stimulus, on the magnitude of the pinch-induced afterdischarge of 5 ipsilateral, single WDR dorsal horn neurons from 5 rats 14 days after cuff implantation. The vertical axis shows the mean±SEM number of spikes for on-going discharge (normalized to the number of spikes per 60 s) before pinch stimulation, and the number of spikes per pinch-evoked fast initial discharge and slowly decaying afterdischarge. The horizontal axis shows the magnitude of the slowly decaying afterdischarge of the 5 WDR neurons before peripheral nerve block (dark gray bar at left; 0.9% saline applied to the cuff-implanted sciatic nerve) and magnitude of the slowly decaying afterdischarge of the same dorsal horn neurons after nerve block (dark gray bar at right; 2% lidocaine applied to the cuff-implanted sciatic nerve). Comparison of the grouped afterdischarge data reveals a significant decrease in the number of spikes in the afterdischarge of dorsal horn neurons in which lidocaine was applied to the cuff-implanted sciatic nerve (***P<0.001). Note that the magnitude of the on-going discharge and the magnitude of the initial discharge for WDR neurons in which saline was administered to the sciatic nerve during the afterdischarge (black and light gray bars, respectively, at left) are similar to those in which the cuff-implanted sciatic nerve was exposed to lidocaine during the afterdischarge (black and light gray bars, respectively, shown at right).

Fig. 3

Histogram showing the depressant effect of nerve block of the cuff-implanted sciatic nerve on spontaneous on-going discharge of WDR dorsal horn neurons in rats 14 days after cuff implantation. The vertical axis represents the mean±SEM percent inhibition of spontaneous on-going discharge. The horizontal axis shows the effect of application of lidocaine to the cuff-implanted sciatic nerve, distal (black bar; n=18) or proximal (gray bar; n=19) to the cuff, on the magnitude of the on-going discharge (measured 5 min after the end of application of lidocaine). ***P<0.001 vs. effect on dorsal horn activity of the cuff-implanted sciatic nerve exposed to saline (0% inhibition). Note that the percent inhibition of the magnitude of on-going discharge with the sciatic nerve exposed to lidocaine proximal to the cuff is greater (⁺P<0.05) than that with the sciatic nerve exposed to lidocaine distal to the cuff.

Fig. 4

Histogram showing the depressant effect of nerve block of the cuff-implanted sciatic nerve on the magnitude of on-going discharge of WDR dorsal horn neurons in rats 42 days after cuff implantation. The vertical axis represents the mean±SEM percent inhibition of spontaneous on-going discharge. The horizontal axis shows the effect of application of lidocaine to the cuff-implanted sciatic nerve, distal (black bar; n=10) or proximal (gray bar; n=10) to the cuff, on the magnitude of on-going discharge (measured 5 min after the end of application of lidocaine). ***P<0.001 vs. effect on dorsal horn activity of the cuff-implanted sciatic nerve exposed to saline (0% inhibition).