Periaqueductal gray stimulation suppresses spontaneous pain behavior in rats

Methods for evaluating analgesic effect for spontaneous pain are increasingly important because it is reported by most patients with neuropathic pain. The present study assessed the analgesic effects of periaqueductal gray (PAG) stimulation in the spared nerve injury (SNI) model of neuropathic pain of the rat. Spontaneous rapid paw withdrawal movements were used as the index of spontaneous pain. Deep-brain stimulation in the PAG was performed in rats 3 weeks after SNI. Significant analgesic effects on spontaneous pain behavior were observed at the same stimulation parameter that reversed the reduced mechanical threshold of the von Frey test. Both analgesic effects lasted 30-40min beyond the 3min stimulation period. In summary, PAG stimulation was effective in alleviating spontaneous pain and mechanical allodynia in the SNI rat. The frequency of spontaneous paw lifting, a behavioral index of spontaneous pain used in this study, will be useful for future testing of therapeutic methods.