Potential sex differences in activation of pain-related brain regions in nonhuman primates with a unilateral spinal nerve ligation.

The lack of truly robust analgesics for chronic pain is owed, in part, to the lack of an animal model that reflects the clinical pain state and of a mechanism-based, objective neurological indicator of pain. The present study examined stimulus-evoked brain activation with functional magnetic resonance imaging in male and female cynomolgus macaques following unilateral L7 spinal nerve ligation and the effects of clinical analgesics pregabalin, duloxetine, and morphine on brain activation in these macaques. A modified straight leg raise test was used to assess pain severity in awake animals and to evoke regional brain activation in anesthetized animals. The potential effects of clinical analgesics on both awake pain behavior and regional brain activation were examined. Following spinal nerve ligation, both male and female macaques showed significantly decreased ipsilateral straight leg raise thresholds, suggesting the presence of radicular-like pain. Morphine treatment increased straight leg raise thresholds in both males and females whereas duloxetine and pregabalin did not. In male macaques, the ipsilateral straight leg raise activated contralateral insular and somatosensory cortex (Ins/SII), and thalamus. In female macaques, the ipsilateral leg raise activated cingulate cortex and contralateral insular and somatosensory cortex. Straight leg raises of the contralateral, unligated leg did not evoke brain activation. Morphine reduced activation in all brain regions in both male and female macaques. In males, neither pregabalin nor duloxetine decreased brain activation compared with vehicle treatment. In females, however, pregabalin and duloxetine decreased the activation of cingulate cortex compared with vehicle treatment. The current findings suggest a differential activation of brain areas depending on sex following a peripheral nerve injury. Differential brain activation observed in this study could underlie qualitative sexual dimorphism in clinical chronic pain perception and responses to analgesics. Future pain management approaches for neuropathic pain will need to consider potential sex differences in pain mechanism and treatment efficacy.