Electroacupuncture improves thermal and mechanical sensitivities in a rat model of postherpetic neuralgia

Abstract

Background: Electroacupuncture (EA) is effective in relieving pain in patients with postherpetic neuralgia (PHN). However, the mechanism underlying the therapeutic effect of EA in PHN is still unclear. Systemic injection of resiniferatoxin (RTX), an ultrapotent analog of TRPV1 agonist, in adult rats can reproduce the clinical symptoms of PHN by ablating TRPV1-expressing sensory neurons. In this study, we determined the beneficial effect of EA and the potential mechanisms in this rat model of PHN.

Methods: PHN was induced in rats by a single injection of RTX. Thermal hyperalgesia was tested with a radiant heat stimulus, and mechanical allodynia was quantified with von Frey filaments. TRPV1 receptors were shown by using immunofluorescence labeling. The ultrastructural changes of the sciatic nerve were assessed by electron microscopic examination. The sprouting of myelinated primary afferent terminals into the spinal dorsal horn was mapped by using the transganglionic tracer cholera toxin B-subunit (CTB).

Results: RTX injection diminished thermal sensitivity and gradually induced tactile allodynia within 3 weeks. EA applied to GB30 and GB34 at 2 and 15 Hz, but not 100 Hz, significantly increased the thermal sensitivity 4 weeks after treatment and decreased the tactile allodynia 2 weeks after treatment in RTX-treated rats. EA treatment at 2 and 15 Hz recovered the loss of TRPV1-positive dorsal root ganglion neurons and their central terminals of afferent fibers in the spinal superficial dorsal horn of RTX-treated rats. Moreover, EA significantly reduced the loss of unmyelinated fibers and the damage of the myelinated nerve fibers of RTX-treated rats. Furthermore, EA at 2 and 15 Hz inhibited the sprouting of myelinated primary afferent terminals into the spinal lamina II of RTX-treated rats.

Conclusions: EA treatment improves thermal perception by recovering TRPV1-positive sensory neurons and nerve terminals damaged by RTX. EA Also reduces RTX-induced tactile allodynia by attenuating the damage of myelinated afferent nerves and their abnormal sprouting into the spinal lamina II. Our study provides new information about the mechanisms of the therapeutic actions of EA in the treatment of PHN.

Figure 1

Time course of the effects of EA on RTX-induced thermal hypoalgesia and mechanical allodynia. A, Time course of the paw withdrawal latency to a noxious heat stimulus in vehicle- and RTX-treated rats. B, Effects of 2, 15, or 100 Hz EA, and sham EA on thermal withdrawal threshold in response to a heat stimulus applied to the left hindpaw of RTX-treated rats. EA was administered for 30 min, once every other day for 5 weeks, starting from 1 week after RTX injection, as indicated by arrows. C, Time course of mechanical withdrawal threshold in response to von Frey filaments in vehicle- and RTX-treated rats. D, Effects of 2, 15, or 100 Hz EA and sham EA on mechanical withdrawal threshold in response to von Frey filaments applied to the left hindpaw of RTX treated rats. Data are expressed as means ± SEM (n = 10 rats in each group). * P < 0.01, compared with the vehicle group; # P < 0.05, compared with the sham EA group.

Figure 2

Effect of EA on RTX-induced deletion of TRPV1-immunoreactive neurons in the DRG. A, Representative images showing TRPV1-immunoreactive neurons in the lumbar DRG of vehicle (a), RTX (b), RTX plus 2 Hz EA (c), RTX plus 15 Hz EA (d), RTX plus 100 Hz EA (e), and RTX plus sham EA (f) groups. Scale bar, 50 μm. B, Summary data show the number of TRPV1 immunoreactive neurons in different groups. Data are expressed as means ± SEM (n = 6 rats in each group). *P < 0.05, compared with the vehicle group; # P < 0.05, compared with the sham EA group.

Figure 3

Effect of EA on RTX-induced deletion of TRPV1 immunoreactive central terminals in the spinal dorsal horn. A, Representative images showing TRPV1 immunoreactive central terminals of afferent fibers in the spinal dorsal horn of vehicle (a), RTX (b), RTX plus 2 Hz EA (c), RTX plus 15 Hz EA (d), RTX plus 100 Hz EA (e), and RTX plus sham EA (f) groups. Scale bar, 50 μm. B, Summary data show the area of TRPV1 immunoreactive central terminals in different groups. Data are expressed as means ± SEM (n = 6 rats in each group). *P < 0.05, compared with the vehicle group; # P < 0.05, compared with the sham EA group.

Figure 4

Effects of EA on RTX-induced ultrastructural changes of myelinated fibers and the number of unmyelinated fibers in the sciatic nerve. A, Representative electron photomicrographs show ultrastructural changes of myelinated and unmyelinated fibers in the sciatic nerve of vehicle (a), RTX (b), RTX plus 2 Hz EA (c), RTX plus 15 Hz EA (d), RTX plus 100 Hz EA (e), and RTX plus sham EA (f) groups. Scale bar, 5 μm. Asterisk, myelinated fibers; arrow, unmyelinated fibers. B, Summary data show the area of myelin sheath of myelinated fibers in the sciatic nerve in different groups. C, Summary data show the number of unmyelinated fibers in the sciatic nerve section in different groups. Data are expressed as means ± SEM (n =3 rats in each group). *P < 0.05, compared with the vehicle group; # P < 0.05, compared with the sham EA group.

Figure 5

Effect of EA on RTX-induced sprouting of myelinated afferent fibers in the spinal lamina II. A, Representative images show CTB immunoreactive myelinated fibers in the spinal dorsal horn of vehicle (a), RTX (b), RTX plus 2 Hz EA (c), RTX plus 15 Hz EA (d), RTX plus 100 Hz EA (e), and RTX plus sham EA (f) groups. Right side is the medial side of dorsal horn in all the images. Scale bar, 50 μm. Dotted line marks the division separating lamina II and III. Asterisk, sprouting of myelinated fibers in lamina II. B, Summary data show the area of CTB-labeled afferent terminals in lamina II in different groups. Data are expressed as means ± SEM (n = 3 rats in each group). *P < 0.05, compared with the vehicle group; # P < 0.05, compared with the sham EA group.