Activation of mu opioid receptor inhibits the excitatory glutamatergic transmission in the anterior cingulate cortex of the rats with peripheral inflammation

Emerging evidence recently indicates that the anterior cingulate cortex is critically involved in the central processing and modulation of noxious stimulus, although the neuroadaptation in the anterior cingulate cortex has not been well documented in the conditions of chronic pain. Meanwhile, the cellular mechanism underlying opiate analgesia in the anterior cingulate cortex remains unclear. To address these issues, the present study was undertaken to explore the adaptation of excitatory glutamatergic transmission and mu opioid receptor-mediated modulation of glutamatergic transmission in the anterior cingulate cortex slices from the complete Freund′s adjuvant (CFA)-inflamed rats. The results demonstrated that glutamatergic paired-pulse facilitation was decreased in the anterior cingulate cortex neurons from the CFA-inflamed rats, indicating an enhanced presynaptic glutamate release. In addition, activation of mu opioid receptor significantly inhibited the glutamatergic excitatory postsynaptic currents (EPSCs) in the anterior cingulate cortex neurons, which was attained through the suppression of presynaptic glutamate release. Taken together, these findings provided the evidence for the functional adaptation of central glutamatergic transmission induced by peripheral inflammation, and elucidated the cellular mechanism underlying opiate analgesia in the anterior cingulate cortex.