Lack of endogenous opioid release during sustained visceral pain: a [11C]carfentanil PET study

Opioidergic neurotransmission in the central nervous system is involved in somatic pain, but its role in visceral pain remains unknown. We aimed to quantify endogenous opioid release in the brain during sustained painful gastric distension. Therefore, 2 dynamic [11C]carfentanil positron emission tomography scans were performed in 20 healthy subjects during 2 conditions: sustained (20 minutes) painful proximal gastric balloon distension at predetermined individual discomfort threshold (PAIN) and no distension (NO PAIN), in counterbalanced order. Pain levels were assessed during scanning using visual analogue scales and after scanning using the McGill Pain Questionnaire. Emotional state was rated after scanning using the Positive and Negative Affect Schedule. Distribution volume ratios in 21 volumes of interest in the pain matrix were used to quantify endogenous opioid release. During the PAIN compared to the NO PAIN condition, volunteers reported a significantly higher increase in negative affect (5.50±1.29 versus 0.10±1.08, P=.0147) as well as higher pain ratings (sensory: 74.05±9.23 versus 1.50±0.95, P<.0001; affective: 91.42±8.13 versus 4.33±6.56, P<.0001). No difference in endogenous opioid release was demonstrated in any of the volumes of interest. Thus, contrary to its somatic counterpart, no opioid release is detected in the brain during sustained visceral pain, despite similar pain intensities. Endogenous opioids may play a less important role in visceral compared to somatic pain.