The Analgesic Effects of (5R,6R)6-(3-Propylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1] Octane on a Mouse Model of Neuropathic Pain

BACKGROUND: Both pharmacologic and genetic approaches have been used to study the involvement of the muscarinic acetylcholine system in the regulation of chronic pain. Previous studies suggest that the M2 and M4 subtypes of muscarinic acetylcholine receptors (mAChRs) are important targets for the development of chronic pain. (5R,6R)6-(3-Propylthio-1,2,5-thiadiazol-4-yl)-1-azabicyclo[3.2.1] octane (PTAC) has agonist effects on muscarinic M2 and M4 receptors and antagonist effects on muscarinic M1, M3, and M5 receptors. However, its analgesic effects have been less studied. METHODS: Male C57B L/6 mice were anesthetized, and left common peroneal nerve (CPN) ligation was performed to induce neuropathic pain. Before and after the application of PTAC systemically or specifically to the anterior cingulate cortex (ACC), the withdrawal thresholds to mechanical stimulation and static weight balance were measured, and the effects of PTAC on the conditioned place preference (CPP) were further evaluated. Western blotting was used to examine the expression of M1 and M2 in the striatum, ACC, and ventral tegmental area. RESULTS: The application of PTAC ([i.p.] intraperitoneal injection) increased the paw withdraw threshold in both the early (0.05 mg/kg, mean difference [95% confidence interval, CI]: 0.19 [0.05–0.32]; 0.10 mg/kg: mean difference [95% CI]: 0.34 [0.22–0.46]) and the late phases (0.05 mg/kg: mean difference [95% CI]: 0.45 [0.39–0.50]; 0.1 mg/kg: mean difference [95% CI]: 0.44 [0.37–0.51]) after nerve injury and rebalanced the weight distribution on the hind paws of mice (L/R ratio: before, 0.56 ± 0.03. 0.05 mg/kg, 1.00 ± 0.04, 0.10 mg/kg, 0.99 ± 0.03); however, it failed to induce place preference in the CPP (0.05 mg/kg, 2-way analysis of variance, P > .05; 0.2 mg/kg, 2-way analysis of variance, P > .05,). At the same doses, the analgesic effects at D3–5 lasted longer than the effects at D14–16. This may be due to the down-regulation of the M2 and M1 in tested brain regions. CONCLUSIONS: These observations suggested that PTAC has analgesic effects on the neuropathic pain induced by nerve injury.