Electroacupuncture (EA) is commonly applied in clinical practice and research, as well as in experimental investigations into the mechanisms of acupuncture. This study investigates publication trends in clinical and experimental studies of EA (1975-2011) for pain and non-pain research; EA use for different clinical conditions (1974-2012); and the relation of EA research, including stimulation frequency, to opioid peptide mechanisms. Applicable PubMed ‘all fields’ searches were conducted, identified studies were classified using PubMed filters and manually, and data extracted into tables. A total of 2916 clinical studies were located, of which 19% involved EA. In addition, 3344 animal studies were located, of which 48% involved EA. The publication rate of EA studies per year has increased over time, but the percentage of studies of pain has fallen from 60% to 25%. The conditions most commonly treated with EA are musculoskeletal, neurological, obstetric and gastrointestinal, along with intraoperative and postoperative analgesia. EA studies, particularly with low frequency stimulation, are more likely to support the role of endogenous opioid mechanisms than manual acupuncture studies, and opioid release is more likely in the central nervous system than the circulation. EA is increasingly eployed in clinical and especially experimental research, particularly for non-pain conditions.
Acupuncture does release endogenous opioids, but this probably depends on ‘dosage’, with the evidence more consistent and convincing for EA than for manual acupuncture. Different frequencies of EA seam to activate different endogenous opioid mechanisms.