[Understanding of the psychiatry in palliative care: dysfunction of the rewarding system under the pain state associated with exacerbating pain].

Recent human brain imaging studies have examined differences in activity in the nucleus accumbens (N.Acc.) in response to heat stimuli between controls and patients with chronic pain, and have revealed that the N.Acc. plays a role in predicting the value of a noxious stimulus and its offset, and in the consequent changes in the motivational state. Nevertheless, the molecular mechanisms of change in the circuitry involved in emotion and motivation in response to chronic pain stimuli were not fully explored. On the other hand, it has been considered that micro RNAs (miRNAs) play important roles as key modulators of post-transcriptional gene expression. We have reported that changes in miRNAs are associated with predicted changes in gene expression of candidate targets in the N.Acc. under neuropathic pain. Therefore, we have introduced a new insight into an epigenetic dysfunction of "mesolimbic motivation/valuation circuitry" under a neuropathic pain-like state. These findings raise intriguing possibilities that miRNA-modulating cellular events along with epigenetic modifications may be associated with neural plasticity and neuronal adaptive responses in mesolimbic motivation/valuation circuitry under which the neuropathic pain may induce negative emotions, exacerbating pain.