Magnetic fields as environmental specific cues for morphine-induced analgesia: interactions with tolerance development.

It has been previously demonstrated that weak fluctuating magnetic fields can act as environmental specific cues for the development of tolerance to morphine-induced analgesia in mice. During the course of this tolerance development the basal nociceptive response of the animals preexposed to magnetic fields are also increased. The magnetic field-induced increase in the basal nociceptive response can be blocked by the opiate antagonist, naloxone, suggesting an opioid-mediated stress effect of the magnetic stimuli. It is suggested that part of the actions of the magnetic stimuli as environmental specific stimuli may arise through their actions as stressors during the tolerance acquisition phase, as well as from their effects on opioid systems.