The temporal expression of circulating microRNAs after acute experimental pain in humans.

BACKGROUND
MicroRNAs (miRNAs) can modulate several biological systems, including the pain system. This study aimed to evaluate the temporal expression of circulating miRNAs in the plasma of healthy volunteers as a marker for epigenetic changes before and after an acute, experimental, pain provocation by intramuscular hypertonic saline injection.


METHODS
Twenty volunteers were randomly allocated into two groups and received either hypertonic (pain) or isotonic (control) saline injection in the first dorsal interosseous muscle of their dominant hand. Pain intensity was continuously recorded for 20 minutes after injection on a VAS scale from 0 to 100 (0 indicates no pain and 100 the worst imaginable pain). Blood samples were taken at baseline, 30 minutes, 3 hours, and 24 hours post-injection, and plasma was separated. MiRNA extracts were used for RNA sequencing with the Illumina NextSeq platform. MiRNA transcripts were compared between the pain and the no-pain, control group at every time point. Significant differences were considered when folds were >2 and the False Discovery Rate was p<0.05.


RESULTS
After 30 minutes, 4 miRNAs were significantly altered in the pain group compared to controls, which increased to 24 after 3 hours and to 42 after 24 hours from baseline (p<0.0001). Two miRNAs were consistently upregulated through the experiment. Enrichment analysis showed significant miRNAs involved brain perception of pain, brain signaling, and response to stimuli.


CONCLUSIONS
This exploratory study is the first to report on the temporal expression of circulating miRNAs after an acute, human experimental muscle pain model.