Naloxone enhances motion sickness: endorphins implicated.

This study evaluated the time course to Malaise III in human subjects given naloxone and placebo with a double-blind cross-over protocol in the prevention of motion sickness induced by exposure to coriolis stimulation in a rotating chair. During naloxone tests, subjects reached the designated level of sickness sooner than during the placebo testing (significance greater than 0.05) and their discomfort lingered for up to 3 d--a feature not seen with the placebo. This implicates endogenous opiates with an endogenous protective or adaptive role in the control of motion sickness. It is suggested that when subjects experience endogenous opioid withdrawal, such as post exercise, they could be in a state of neuron hypersensitivity, and thus more prone to any form of exogenous emetic stimuli. Greater tolerance to motion stresses could be experienced in subjects whose endorphins were repeatedly elevated, thus avoiding a hypersensitivity state from endogenous opiate withdrawal. Subjects whose endorphins have not been elevated in the first instance cannot secondarily suffer opioid abstinence.