Nicotinic acetylcholine receptors expressed in the ventralposterolateral thalamic nucleus play an important role in anti-allodynic effects

Abstract

Background and purpose: Much interest is currently being focused on the anti-nociceptive effects mediated by nicotinic acetylcholine (nACh) receptors, including their location and mechanism of action. The purpose of this study was to elucidate these issues using 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), a nACh receptor agonist, and [(125)I]5IA.

Experimental approach: We partially ligated the sciatic nerve of Sprague-Dawley rat to induce neuropathic pain [Seltzer's partial sciatic nerve ligation (PSL) model]. We then examined the changes in nACh receptor density in the CNS using [(125)I]5IA autoradiography and the involvement of nACh receptors in anti-nociceptive effects in the region where changes occurred.

Key results: Autoradiographic studies showed that the accumulation of [(125)I]5IA and the number of nACh receptors in the thalamus of PSL rats were increased about twofold compared with those in the sham-operated rats. No change was observed in other brain regions. Rats injected in the ventral posterolateral thalamic nucleus (VPL) with 5IA demonstrated a significant and dose-dependent anti-allodynic effect and this effect was completely antagonized by mecamylamine, injected with 5IA, into the VPL. The blockade of nACh receptors in the VPL by mecamylamine decreased by 70% the anti-allodynic effect of 5IA, given i.c.v. Moreover, mecamylamine given intra-VPL by itself, induced significant hyperalgesia.

Conclusions and implications: Our findings suggest that the nACh receptors expressed in the VPL play an important role in the anti-allodynic effects produced by exogenous and endogenous agonists.

Figure 1

(A) Effect of i.c.v. administered 5IA on neuropathic tactile allodynia. Data are presented as a percentage of the maximum possible effect (% MPE). Each point represents the mean ± SEM of the ligated paws of five to six animals per group. *P < 0.05 versus vehicle at the same time point. (B) Effect of mecamylamine (MEC, 5 mg·kg⁻¹, s.c., 30 min prior) on the anti-allodynic effect induced by 5IA (i.c.v.). Data are presented as a percentage of the maximum possible effect. Each point represents the mean ± SEM of the ligated paws of five to six animals per group. **P < 0.01 versus vehicle, #P < 0.05, ##P < 0.01 versus MEC alone, and †P < 0.05, ††P < 0.01 versus 5IA alone at the same time point. 5IA, 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine; MPE, maximal possible effect.

Figure 2

(A) Representative autoradiograms of brain regions from sham-operated and PSL rats given [¹²⁵I]5IA, i.v. (B) Changes in the regional accumulation of [¹²⁵I]5IA in Seltzer's PSL model of neuropathic pain. A significant increase was observed only in the thalamus, and no detectable change occurred in other regions. Each column represents the mean ± SEM of four to seven animals per group. *P < 0.05 versus sham. PSL, partial sciatic nerve ligation; 5IA, 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine; Frn, frontal cortex; Str, striatum; Hip, hippocampus; Th, thalamus; PPTg, pedunculopontine tegmental nucleus; NRM, nucleus raphe magnus; LC, locus coeruleus; Cer, cerebellum.

Figure 3

(A) Saturation curves of [¹²⁵I]5IA binding to sham-operated and PSL rat thalamus determined ex vivo. Each point represents the mean ± SEM of five to seven animals per group. The B_max values estimated by nonlinear regression analysis of these mean data were 12.5, 19.9 and 21.2 fmol/mg protein in sham-operated, 2 week post-PSL, and 1 month post-PSL group respectively. (B) Changes in nACh receptor density in brain regions of rats with Seltzer's PSL model of neuropathic pain. A significant increase was observed only in the thalamus, and no detectable change occurred in other regions. Each column represents the mean ± SEM of five to seven animals per group. *P < 0.05, **P < 0.01 versus sham. PSL, partial sciatic nerve ligation; 5IA, 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine; Frn, frontal cortex; Str, striatum; Hip, hippocampus; Th, thalamus; PPTg, pedunculopontine tegmental nucleus; NRM, nucleus raphe magnus; LC, locus coeruleus; Cer, cerebellum.

Figure 4

(A) Effect of 5IA injected intra-VPL on neuropathic tactile allodynia. Data are presented as a percentage of the maximum possible effect (% MPE). Each point represents the mean ± SEM of the ligated paws of five to six animals per group. *P < 0.05, **P < 0.01 versus vehicle, and #P < 0.05 versus 1 nmol of 5IA at the same time point. (B) Effect of mecamylamine (MEC, 10 nmol in 0.5 µL) administered concurrently with 5IA in an intra-VPL manner on the anti-allodynic effect produced by 5IA (10 nmol in 0.5 µL, intra-VPL). Data are presented as a percentage of the maximum possible effect. Each point represents the mean ± SEM of the ligated paws of five to six animals per group. *P < 0.05, **P < 0.01 versus vehicle, #P < 0.05, ##P < 0.01 versus MEC alone, and †P < 0.05, ††P < 0.01 versus 5IA alone at the same time point. (C) Effect of mecamylamine (MEC, 10 nmol in 0.5 µL, intra-VPL, 5 min prior) on the anti-allodynic effect produced by 5IA (10 nmol in 5 µL, i.c.v.). Data are presented as a percentage of the maximum possible effect. Each point represents the mean ± SEM of the ligated paws of four to seven animals per group. *P < 0.05, **P < 0.01 versus vehicle/vehicle, #P < 0.05, ##P < 0.01 versus MEC/vehicle, and †P < 0.05 versus vehicle/5IA at the same time point. 5IA, 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine; MPE, maximal possible effect; VPL, ventral posterolateral thalamic nucleus.

Figure 5

(A) Effect of mecamylamine injected intra-VPL on neuropathic tactile allodynia. Data are presented as a percentage of the maximum possible effect (% MPE). Each point represents the mean ± SEM of the ligated paws of five to six animals per group. *P < 0.05 versus vehicle at the same time point. (B) Dose-response curve of mecamylamine-induced hyperalgesia. The open symbol represents the AUC of the vehicle-treated group, and the closed symbols represent that of each concentration of the mecamylamine-treated group. *P < 0.05, **P < 0.01 versus vehicle. AUC, area under the curves; MPE, maximal possible effect; VPL, ventral posterolateral thalamic nucleus.