Acupuncture Analgesia: A Review of Its Mechanisms of Actions

Acupuncture has been used for thousands of years for pain relief, but the mechanisms by which it produces this analgesic effect are still the subject of intense scientific investigation. This review, conducted by investigators from China Medical University in Taiwan, offers a comprehensive analysis of three decades of research on how acupuncture works in the brain and body to reduce pain perception.

Scientific interest in the mechanisms of acupuncture analgesia intensified beginning in the 1970s, when researchers began to systematically investigate how this ancient therapeutic practice could produce measurable effects in pain relief. The authors analyzed hundreds of studies performed in animal and human models, focusing mainly on electroacupuncture, a modern version of traditional acupuncture that combines needle insertion with controlled electrical stimulation. This approach allows greater standardization of experiments, since it is possible to precisely control the frequency, voltage, and duration of the applied stimulus.

The research revealed that acupuncture activates multiple systems in the body to produce analgesia. The best-established mechanism involves the release of natural body substances called endorphins and their chemical relatives, known as endogenous opioids. These include beta-endorphin, enkephalin, endomorphin, and dynorphin, which are the "natural analgesics" of our body. The studies demonstrated that different frequencies of electrical stimulation activate different types of these substances: lower frequencies (around 2 Hz) primarily stimulate the release of beta-endorphin, enkephalin, and endomorphin, which bind to mu- and delta-type opioid receptors in the nervous system.

Higher frequencies (100 Hz), on the other hand, stimulate the release of dynorphin, which acts on kappa-type opioid receptors.

In addition to endogenous opioids, the investigators identified another important mechanism involving serotonin, a neurotransmitter that plays a key role in pain regulation. Acupuncture activates a descending pain inhibition pathway from the brainstem to the spinal cord, mediated by serotonin. This pathway works as a natural pain "brake": when activated by acupuncture, neurons in the nucleus raphe magnus, located in the brainstem, release serotonin that descends to the spinal cord. There, serotonin activates interneurons that release enkephalin, blocking the transmission of pain signals before they reach the brain.

A particularly interesting finding was the discovery that acupuncture works differently in normal versus inflammatory and chronic pain states. In healthy animals, different frequency types produce distinct effects through different opioid receptors. However, in animal models with inflammation and hyperalgesia (excessive pain sensitivity), both low and high frequencies appear to work through the same mu and delta receptors, suggesting that acupuncture adapts its mechanisms of action according to the patient's condition. This may explain why acupuncture is frequently more effective in people who actually have pain than in healthy volunteers.

The clinical implications of these findings are significant for both patients and health care professionals. For patients, these studies provide a solid scientific basis for the use of acupuncture in the treatment of pain, demonstrating that its effects are not placebo but result from real and measurable neurobiologic mechanisms. The research suggests that acupuncture may be particularly effective for inflammatory conditions and chronic pain, offering an alternative or complement to conventional medications. For professionals, understanding these mechanisms allows a more rational application of the technique, including the choice of the most appropriate stimulation frequency for different types of pain and the combination of acupuncture with other therapeutic modalities.

The review also highlighted the emerging role of the autonomic nervous system and the inflammatory reflex in the action of acupuncture. Recent research suggests that acupuncture can modulate the body's immune and inflammatory response through activation of the parasympathetic nervous system, providing an explanation for its beneficial effects in a variety of conditions beyond pain. The hypothalamus, a brain region that integrates the nervous and hormonal systems, appears to play a central role in these effects, coordinating responses through the hypothalamic-pituitary-adrenal axis.

Despite the significant advances in understanding the mechanisms of acupuncture, the review acknowledges several important limitations. Many studies were performed in animal models, and it is not always possible to directly extrapolate these results to humans. In addition, most research focused on electroacupuncture rather than traditional manual acupuncture, raising questions about the applicability of the findings to actual clinical practice. Differences in experimental protocols, including point location, treatment duration, and evaluation methods, make it difficult to compare results across studies and reach definitive conclusions.

After three decades of intensive research, although much has been discovered about how acupuncture produces analgesia, many questions remain unanswered. The mechanisms are clearly complex and multifaceted, involving multiple neurobiologic systems that work together. This complexity may in fact be a therapeutic advantage, providing multiple pathways for pain relief and explaining why acupuncture can be effective for a wide range of conditions. Future research will likely continue to reveal new aspects of these mechanisms, potentially leading to more precise and effective applications of this ancient healing art.