Amitriptyline prolongs the antihyperalgesic effect of 2 Hz electroacupuncture in mononeuropathic rats

Amitriptyline and electroacupuncture (EA) have been used in the management of neuropathic pain. This study examines whether the use of an intraperitoneal injection of amitriptyline changes the effectiveness of EA to reduce neuropathic pain in rats. Changes in the nociceptive threshold of intraperitoneal saline (0.8 ml/kg)- or amitriptyline (0.8 mg/kg)-treated rats were evaluated using the tail-flick test and a model of neuropathic pain before, during and after a 20-min period of 2 Hz EA applied to the Zusanli and Sanynjiao acupoints. The experiments were conducted two or seven days after a sham or chronic constriction injury (CCI) of the sciatic nerve. Amitriptyline had no effect against thermal or neuropathic pain in sham EA-stimulated rats. Two Hz EA increased the tail-flick latency and reduced the mechanical hyperalgesia evoked by the CCI and was more effective against mechanical hyperalgesia when used at the earlier phase of the neuropathic pain than at the later phase. These effects of EA were significantly increased in intensity in the tail-flick test and lasted longer in neuropathic rats after intraperitoneal amitriptyline. The duration of the analgesic effect of 2 Hz EA in the operated hind paws of saline- or amitriptyline-treated rats was longer at two than at seven days after nerve injury. The analgesic effect of EA and its combination with amitriptyline were detected in both sham and CCI rats. The combined effect of amitriptyline and EA may represent an alternative for prolonging the efficacy of EA in the management of neuropathic pain.