Contribution of alpha(2) receptor subtypes to nerve injury-induced pain and its regulation by dexmedetomidine

Abstract

There is evidence that noradrenaline contributes to the development and maintenance of neuropathic pain produced by trauma to a peripheral nerve. It is, however, unclear which subtype(s) of alpha adrenergic receptors (AR) may be involved. In addition to pro-nociceptive actions of AR stimulation, alpha(2) AR agonists produce antinociceptive effects. Here we studied the contribution of the alpha(2) AR subtypes, alpha(2A), alpha(2B) and alpha(2C) to the development of neuropathic pain. We also examined the antinociceptive effect produced by the alpha(2) AR agonist dexmedetomidine in nerve-injured mice. The studies were performed in mice that carry either a point (alpha(2A)) or a null (alpha(2B) and alpha(2C)) mutation in the gene encoding the alpha(2) AR. To induce a neuropathic pain condition, we partially ligated the sciatic nerve and measured changes in thermal and mechanical sensitivity. Baseline mechanical and thermal withdrawal thresholds were similar in all mutant and wild-type mice; and, after peripheral nerve injury, all mice developed comparable hypersensitivity (allodynia) to thermal and mechanical stimulation. Dexmedetomidine reversed the allodynia at a low dose (3 microg kg(-1), s.c.) and produced antinociceptive effects at higher doses (10 - 30 microg kg(-1)) in all groups except in alpha(2A) AR mutant mice. The effect of dexmedetomidine was reversed by intrathecal, but not systemic, injection of the alpha(2) AR antagonist RS 42206. These results suggest that neither alpha(2A), alpha(2B) nor alpha(2C) AR is required for the development of neuropathic pain after peripheral nerve injury, however, the spinal alpha(2A) AR is essential for the antinociceptive effects of dexmedetomidine.

Figure 1

Effect of nerve injury in (a) α_2A, (b) α_2B and (c) α_2C AR mutant (−/−) and wild-type (+/+) mice on thermal response latencies of the injured (ipsi) and the non-injured (contra) paw. Nerve injury produced comparable thermal allodynia in the wild-type and mutant mice. Data are presented as the mean±s.e.mean; n=10 per group.

Figure 2

Effect of nerve injury in (a) α_2A, (b) α_2B and (c) α_2C AR mutant (−/−) and wild-type (+/+) mice on mechanical paw withdrawal thresholds of the injured (ipsi) and the non-injured (contra) paw. Nerve injury produced comparable mechanical allodynia in the wild-type and mutant mice. Data are presented as the mean±s.e.mean; n=10 per group.

Figure 3

Effect of 50 mg kg⁻¹ guanethidine on thermal allodynia of the injured paw in (a) α_2A, (b) α_2B and (c) α_2C AR mutant (−/−) and wild-type (+/+) mice. Thermal paw withdrawal latencies were determined before (PRE) and 24 h after the injection of guanethidine (POST). Data are presented as the mean±s.e.mean. The asterisks indicate that guanethidine produced a significant increase in paw withdrawal latencies (comparing PRE vs PST, paired t-test; ^*P<0.05, ^**P<0.01). The dotted line indicates the baseline latency before the nerve-injury surgery was performed.

Figure 4

Effect of 50 mg kg⁻¹ guanethidine on mechanial allodynia of the injured paw in (a) α_2A, (b) α_2B and (c) α_2C AR mutant (−/−) and wild-type (+/+) mice. Mechanical thresholds were determined before (PRE) and 24 h after the injection of guanethedine (POST). Data are presented as the mean±s.e.mean. The asterisks indicate a significant decrease in mechanical sensitivity (Wilcoxon signed rank test; ^*P<0.05) comparing PRE and POST. The dotted line indicates the baseline latency before the nerve injury was performed.

Figure 5

Effect of dexmedetomidine on thermal allodynia of the injured paw in (a) α_2A, (b) α_2B and (c) α_2C AR mutant and wild-type mice. Ten to 14 days after the nerve transection, we established post-injury baseline (b) latencies. The animals then received a s.c. injection of dexmedetomidine and 20 – 40 min later we again measured thermal sensitivity. The asterisks indicate significant increases in paw withdrawal latencies (^***P<0.001, ANOVA followed by Student Neuman-Keuls post hoc test) compared to post-injury baseline (indicated by ‘B' in the figure) values. The dotted line indicates the ‘normal' paw withdrawal latencies before the nerve injury was performed; an increase in latency above this value is considered to be antinociceptive. Data are presented as the mean±s.e.mean; n=5 – 6 per dose.

Figure 6

Effect of dexmedetomidine on mechanical allodynia of the injured paw in (a) α_2A, (b) α_2B and (c) α_2C AR mutant and wild-type mice. Ten to 14 days after the nerve injury, we established post-injury baseline (b) mechanical withdrawal thresholds. The animals then received a s.c. injection of dexmedetomidine and 20 – 40 min later we again measured mechanical thresholds. The asterisks indicate significant increases in paw withdrawal thresholds (^*P<0.05, Friedman test followed by Wilcoxon signed rank test) compared to post-injury baseline (indicated by ‘B' in the figure) values. The dotted line indicates mechanical thresholds of the paw before the nerve injury was performed. Data are presented as the mean±s.e.mean; n=5 – 6 per dose.

Figure 7

The effect of RS 42206 (0.3 mg kg⁻¹, i.p.) on all anti-allodynic dose of dexmedetomidine (3.0 μg kg⁻¹, s.c.) in nerve injured mice. Data are presented as the mean±s.e.mean for (a) thermal paw withdrawal latency and (b) mechanical paw withdrawal thresholds of the injured (ipsi) and the non-injured (contra) side. N=6 per group. The asterisks indicate significant increases (a: ^**P<0.01, t-test, b: ^*P<0.05, Wilcoxon signed rank test) comparing latencies or thresholds before and after drug treatment.

Figure 8

The effect of RS 42206 (0.3 mg kg⁻¹, i.p.) on an antinociceptive dose of dexmedetomidine (30 μg kg⁻¹, s.c.) in nerve-injured mice. Data are presented as the mean±s.e.mean for (a) thermal paw withdrawal latency and (b) mechanical paw withdrawal thresholds of the injured (ipsi) and the non-injured (contra) paws. N=6 per group. The asterisks indicate significant increases (a: ^***P<0.001, t-test, b: ^*P<0.05, Wilcoxon signed rank test) comparing latencies or thresholds before and after drug treatment.

Figure 9

The effect of intrathecal injection of 30 μg RS 42206 on an analgesic dose of dexmedetomidine (30 μg kg⁻¹, s.c.) in nerve-injured mice. Data are presented as the mean±s.e.mean (a) thermal paw withdrawal latency and (b) mechanical paw withdrawal thresholds of the inured (ipsi) and the non-injured (contra) paws. There was no significant difference (a: P>0.05, t-test, b: P>0.05, Wilcoxon signed rank test) between pre and post thermal latencies or mechanical thresholds indicating that RS 42206 reversed the effect of dexmedetomidine. N=6 per group.