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Abstract

Oral administration of sweet stimuli to newborns is effective at alleviating pain during injections. In adults, administration of sweet stimuli prolongs the latency in perceiving experimentally induced pressure stimuli as painful. However, experimental results for adults are mixed, with some negative reports suggesting that adults do not experience analgesic effects from sweet stimuli. On the other hand, we have investigated the effects of olfactory stimulation on analgesia using near-infrared spectroscopy (NIRS), which measures changes in cerebral blood flow after olfactory stimulation. The olfactory stimulation reduced pain ratings, and a significant difference in blood flow increase in the right dorsolateral prefrontal cortex (rDLPFC) was observed when pain stimulation alone was compared with pain stimulation combined with olfactory stimulation. Furthermore, previous studies have shown that sweet stimuli increase cerebral blood flow in pain-free adults. Therefore, based on the hypothesis that sweet stimuli are involved in pain relief, the present study used NIRS to examine whether sweet stimuli affect cerebral blood flow changes targeting the rDLPFC. Pain stimulation was delivered via transcutaneous electrical stimulation. The results demonstrated that sweet taste stimulation is associated with pain relief by activating the rDLPFC, even though the score of visual analog scale used as the pain rating scale to measure the intensity of pain decreased following water stimulation. Our present study uses NIRS to measure changes in cerebral blood flow—thereby representing the intensity of pain through physiologically relevant changes—and demonstrates the effectiveness of sweet stimuli for pain relief in adults.