Paradoxical increases in anterior cingulate cortex activity during nitrous oxide–induced analgesia reveal a signature of pain affect

The general consensus is that increases in neuronal activity in the anterior cingulate cortex (ACC) contribute to pain's negative affect. In this study, using in vivo imaging of calcium dynamics of ACC neurons in mice, we report that nitrous oxide, an inhaled analgesic anesthetic that reduces pain affect in patients, paradoxically, significantly increases ACC activity. This nitrous oxide-induced increase in ACC activity was confirmed by increased expression of the immediate early gene, c-fos . In parallel studies comparing reflexive vs affective-motivational behavioral responses with noxious stimuli delivered by an infrared laser, we established that nitrous oxide also reduces pain affect in mice. To investigate how increases in ACC activity could reduce pain affect, we next assessed the effect of nitrous oxide on nociceptive processing by neurons of the ACC. Unexpectedly, nitrous oxide did not alter the absolute level of laser-evoked ACC activity. However, because nitrous oxide increases prestimulus baseline activity, the relative magnitude of noxious laser-evoked ACC activation, as compared with the control condition, was significantly reduced in mice inhaling nitrous oxide. Importantly, this reduction correlated with nitrous oxide-induced reductions in affective-motivational, but not reflexive behaviors. We suggest that this relative change in activity represents a neural signature of the affective pain experience in mice.