Opioids and exercise. An update.

A number of endogenously produced opioid peptides interact with centrally and peripherally located specific receptors to form a widespread neuroendocrine system with many implications for human function. It is becoming increasingly evident that moderately high and high intensity exercise stimulate the release of the opioid peptide beta-endorphin to the circulation and this event may be subject to considerable intra- and interindividual variation. Moreover, endorphin levels probably remain elevated for 15 to 60 minutes following exercise. The duration of exertion does not seem to be critical, and low or moderate (less than 75% VO2max) intensity efforts do not stimulate this response. It also appears (mostly from animal model research) that exercise might elicit central opioid effects, but there is conflicting evidence on this topic. Physical training may encourage adapted opioid system function (e.g. altered peptide response to exercise or receptor number), but these adaptations are not well elucidated by the few existing studies. The significance of peripherally released opioid peptides during exercise has frequently been questioned. Exercise-induced affective response (e.g. mood enhancement), analgesia, food intake suppression and reproductive dysfunction are often mentioned as potentially controlled by an opioid mediated mechanism. While most of these events are normally considered under central control, it is time we begin entertaining the notion of peripheral effects (e.g. altered catecholamine release) and afferent input affecting central function in some of these phenomena. Additionally, evidence exists to suggest peripherally released enkephalins may cross the blood-brain barrier, but this is probably not true for endorphins. A number of other reported exercise-related events could possibly involve an underlying opioid mechanism. Exercise-associated metabolic regulation, immunosuppression, and cardiovascular function are areas for future opioid research.