Role of descending noradrenergic system and spinal alpha2-adrenergic receptors in the effects of gabapentin on thermal and mechanical nociception after partial nerve injury in the mouse

Abstract

1. To gain further insight into the mechanisms underlying the antihyperalgesic and antiallodynic actions of gabapentin, a chronic pain model was prepared by partially ligating the sciatic nerve in mice. The mice then received systemic or local injections of gabapentin combined with either central noradrenaline (NA) depletion by 6-hydroxydopamine (6-OHDA) or alpha-adrenergic receptor blockade. 2. Intraperitoneally (i.p.) administered gabapentin produced antihyperalgesic and antiallodynic effects that were manifested by elevation of the withdrawal threshold to a thermal (plantar test) or mechanical (von Frey test) stimulus, respectively. 3. Similar effects were obtained in both the plantar and von Frey tests when gabapentin was injected intracerebroventricularly (i.c.v.) or intrathecally (i.t.), suggesting that it acts at both supraspinal and spinal loci. This novel supraspinal analgesic action of gabapentin was only obtained in ligated neuropathic mice, and gabapentin (i.p. and i.c.v.) did not affect acute thermal and mechanical nociception. 4. In mice in which central NA levels were depleted by 6-OHDA, the antihyperalgesic and antiallodynic effects of i.p. and i.c.v. gabapentin were strongly suppressed. 5. The antihyperalgesic and antiallodynic effects of systemic gabapentin were reduced by both systemic and i.t. administration of yohimbine, an alpha2-adrenergic receptor antagonist. By contrast, prazosin (i.p. or i.t.), an alpha1-adrenergic receptor antagonist, did not alter the effects of gabapentin. 6. It was concluded that the antihyperalgesic and antiallodynic effects of gabapentin are mediated substantially by the descending noradrenergic system, resulting in the activation of spinal alpha2-adrenergic receptors.

Figure 1

Gabapentin exhibits antihyperalgesic and antiallodynic effects. Thermal hyperalgesia and tactile allodynia were assessed using the plantar and von Frey tests, respectively. Gabapentin (30 and 100 mg kg⁻¹) was administered i.p. at time zero. Each point represents the mean±s.e.m. of seven separate experiments. Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin application. Open diamond in each graph shows the mean of pooled PWLs (left) or 50% thresholds (right) obtained before ligation in the three groups of mice. The asterisks indicate data points for which a significant difference between the control (open circle) and gabapentin-treated groups (closed triangle and square) was observed, as determined by two-tailed nonparametric Bonferroni-type multiple comparisons following the Kruskal–Wallis test (two comparisons in three groups, ^*P<0.05).

Figure 2

Effects of a local injection of gabapentin. Both (a, i.c.v.) and (b, i.t.) administration of gabapentin (30 and 100 μg) ameliorated the symptoms of thermal hyperalgesia and tactile allodynia, indicating that gabapentin has both supraspinal and spinal actions. Gabapentin was injected at time zero. Each point represents the mean±s.e.m. of six or seven separate experiments. Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin injection. Open diamond in each graph shows the mean of pooled PWLs (left in a and b) or 50% thresholds (right in (a) and (b)) obtained before ligation in the three groups of mice. The asterisks indicate data points for which a significant difference between the control (open circle) and gabapentin-treated groups (closed triangle and square) was observed, as determined by two-tailed nonparametric Bonferroni-type multiple comparisons following the Kruskal–Wallis test (two comparisons in three groups, ^*P<0.05).

Figure 3

Gabapentin administered i.c.v. does not affect locomotor activity. Mice developing thermal hyperalgesia and tactile allodynia were injected i.c.v. with either saline (vehicle control) or gabapentin (gbp, 100 μg), and the assessment of locomotor activities was carried out for 1 h postinjection (n=9–10). Ordinates: locomotion measured in 5-min periods (left) and between 0–30 and 30–60 min after injection (right). Gabapentin was administered at time zero.

Figure 4

Gabapentin does not produce antinociceptive effects against acute thermal and mechanical nociception in nonligated mice. Acute thermal and mechanical nociception were assessed by the plantar and paw pressure tests, respectively. Note that an intensity of radiant heat higher than that used in ligated animals was applied in the study on acute thermal nociception. Gabapentin (30 and 100 mg kg⁻¹, i.p. in (a) and 100 μg, i.c.v. in (b)) was administered at time zero. Each point represents the mean±s.e.m. of six or seven separate experiments. Ordinates: mean PWLs (plantar test; left) and nociceptive threshold (paw pressure; right). Abscissae: time in minutes after gabapentin injection.

Figure 5

Depletion of descending NA. 6-OHDA was injected intracisternally immediately before ligation of the sciatic nerve. After the assessment of thermal hyperalgesia and tactile allodynia (see Figures 6 and 9), the contents of NA, 5-HT, and DA in the brain stem (B) and spinal cord (S) obtained from vehicle control (open columns, n=17) and 6-OHDA-treated mice (hatched columns, n=49–53) were measured using reverse-phase high-performance liquid chromatography with electrochemical detection.

Figure 6

Depletion of descending NA levels strongly reduces the antihyperalgesic and antiallodynic effects of systemically and i.c.v. administered gabapentin. 6-OHDA was injected intracisternally immediately before ligation of the sciatic nerve. Gabapentin (30 and 100 mg kg⁻¹, i.p. in (a), and 30 and 100 μg, i.c.v. in (b)) was administered at time zero. Each point represents the mean±s.e.m. of six separate experiments. Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin administration. Open diamond in each graph shows the mean of pooled PWLs (left in (a) and (b)) or 50% thresholds (right in (a) and (b)) obtained before ligation in the four groups of mice. The asterisks indicate data points for which a significant difference between the control (open circle) and gabapentin-treated groups (closed triangle and closed square) was observed, as determined by two-tailed nonparametric Bonferroni-type multiple comparisons following the Kruskal–Wallis test (two comparisons in three groups, ^*P<0.05). Pretreatment with vehicle (ascorbic acid) alone did not affect the antihyperalgesic and antiallodynic actions of gabapentin (100 mg kg⁻¹, i.p. in (a) and 100 μg, i.c.v. in (b)), as shown superimposed in the graphs (n=5–6, open square).

Figure 7

The α₂-adrenergic receptor antagonist yohimbine reduces the antihyperalgesic and antiallodynic effects of gabapentin. Yohimbine HCl (yoh) was administered either i.p. ((a); 0.3 and 1 mg kg⁻¹) or i.t. ((b); 1 and 3 μg) 15 min before the administration of gabapentin (gbp, 100 mg kg⁻¹, i.p., administered at time zero). Each point represents the mean±s.e.m. of six or seven separate experiments. Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin application. Open diamond in each graph shows the mean of pooled PWLs (left in (a) and (b)) or 50% thresholds (right in (a) and (b)) obtained before ligation in the three groups of mice. The asterisks indicate data points for which a significant difference between the gabapentin-only (open circle) and yohimbine-treated groups (closed triangle and square) was observed, as determined by two-tailed nonparametric Bonferroni-type multiple comparisons following the Kruskal–Wallis test (two comparisons in three groups, ^*P<0.05).

Figure 8

The α₁-adrenergic receptor antagonist prazosin does not affect the antihyperalgesic and antiallodynic effects of gabapentin. Prazosin HCl (pra) was administered either i.p. ((a); 1 mg kg⁻¹) or i.t. ((b); 3 μg) 15 min before the administration of gabapentin (gbp, 100 mg kg⁻¹, i.p., administered at time zero). Each point represents the mean±s.e.m. of six or seven separate experiments (open circle; gabapentin-only, closed triangle; prazosin-treated). Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin application. Open diamond in each graph shows the mean of pooled PWLs (left in (a) and (b)) or 50% thresholds (right in (a) and (b)) obtained before ligation in the two groups of mice.

Figure 9

Depletion of descending NA levels or blockade of spinal α₂-adrenergic receptors partly reduces the antihyperalgesic and antiallodynic effects of i.t. administered gabapentin. 6-OHDA was injected intracisternally immediately before ligation of the sciatic nerve. Gabapentin (gbp, 30 and 100 μg, i.t. in (a) and (b)) was administered at time zero. Yohimbine HCl (yoh, 1 and 3 μg, i.t. in (b)) was administered 15 min before the administration of gabapentin. Each point represents the mean±s.e.m. of seven separate experiments. Ordinates: mean PWLs (plantar test; left) and 50% thresholds (von Frey test; right). Abscissae: 7 days before (pre-ope) and time in minutes after gabapentin administration. Open diamond in each graph shows the mean of pooled PWLs (left in (a) and (b)) or 50% thresholds (right in (a) and (b)) obtained before ligation in the four (a) and three (b) groups of mice. In (a), pretreatment with vehicle (ascorbic acid) alone did not affect the antihyperalgesic and antiallodynic actions of gabapentin (100 μg, i.t.), as shown superimposed in the graphs (n=6, open square). In (a) and (b), the asterisks indicate data points for which a significant difference between (a) the control (open circle) and gabapentin-treated groups (closed triangle and closed square) or (b) the gabapentin-only (open circle) and yohimbine-treated groups (closed triangle and square) was observed, as determined by two-tailed nonparametric Bonferroni-type multiple comparisons following the Kruskal–Wallis test (two comparisons in three groups, ^*P<0.05).