Acupuncture triggers an increase in the extracellular concentration of ATP, ADP, AMP and adenosine. (a) Representative HPLC chromatograms before, during and after acupuncture. The samples were collected by a microdialysis probe implanted in close proximity to the Zusanli point. Standards of adenosine, AMP, ADP and ATP (0.3 μM each) are shown on top. (b) Time course of purine release in response to acupuncture. (c) Histogram summarizing the mean concentrations of adenosine, AMP, ADP and ATP during baseline nonstimulated conditions, as well as during and following acupuncture (*P < 0.05, **P < 0.01, paired t test compared to 0 min, n = 8). Error bars indicate s.e.m.