Low dose of donepezil improves gabapentin analgesia in the rat spared nerve injury model of neuropathic pain: single and multiple dosing studies

The use of cholinergic drugs, either alone or in combination with other drugs, has been suggested as an approach to improve treatment outcome for patients suffering from neuropathic pain. The present study was undertaken in the rat spared nerve injury model of neuropathic pain to evaluate the effect of the cholinesterase inhibitor donepezil when administered (1) alone and (2) as low-dose in combination with the first-line recommendation gabapentin. The co-administration studies were performed following single and multiple dosing. Single, parenteral dosing of donepezil (1, 1.5 and 3 mg/kg s.c.) produced a dose-dependent reversal of the neuropathic pain behaviour. Co-administration of a sub-effective dose of donepezil (0.5 mg/kg s.c.) and low doses of gabapentin (10 and 30 mg/kg s.c.) resulted in a three- to fourfold increase of the analgesic effect, in comparison with gabapentin administered alone. Following multiple, oral dosing, gabapentin (25 mg/kg p.o.) was administered once daily over 20 days. Addition of donepezil (1.5 mg/kg p.o.) from day 11 to day 20 resulted in improved analgesia during the period of combination therapy, in comparison with the gabapentin monotherapy period. Furthermore, the treatment effects were stable in both the mono- and the combination therapy period, indicating that tolerance development does not occur within the studied time frame. In conclusion, the results from this preclinical study support the use of donepezil as adjunctive to gabapentin to improve the therapeutic outcome in the management of neuropathic pain.