356 Brain Research, 418(1987) 356-36(3 Elsevier BRE 22437 Morphine-induced aede m m Iow-inter, magnetic inhibition of nocturnal inmice is a fun on ofr c Klaus-Peter Ossenkopp 1 and Martin Kavaliers 2 tDepartment of Psychology and 2Division of Oral Biology, Faculty of Dentistry, The University of Western Ontario. London. Ont. ¢Canada) (Accepted 12 May 1987) Key words: Analgesia; Morphine: 60-Hz magnetic field: Mouse: Power line frequency: Health effect In 2 experiments male CF-1 mice were exposed for 60 min, during the mid-dark period of the day-night cycle, to low-intensity (0.5-1.5 gauss, rrns) 60-Hz magnetic fields and then tested for levels of analgesia induced by morphine (10 mg/kg) injections. The magnetic field exposures inhibited the degree of morphine-induced analgesia in a field intensity-dependent manner in both experi- ments (P < 0.01) with the largest inhibitory effect after exposure to the t .5-gauss field. Analysis of the combined data from the two ex- periments revealed a significant (P < 0.001) linear relationship between level of analgesia and magnetic field intensity, Thus. these data demonstrated a functional relationship between the behavioral effects of morphine in mice and the strength of the 60-Hz magnet- ic field. Possible mechanisms underlying these effects are discussed. A variety of biological changes produced by expo- sure to magnetic fields have been documented 1722 and these findings have led to a range of speculations as to the possible modes of action of magnetic fields on biological systems 1°'2°'22. A growing body of evi- dence now indicates that time-varying magnetic fields, especially those in the extremely low frequen- cy (ELF) range (0.1-100 Hz), can alter an animal's endogenous opioid systems and responses to exoge- nous opiates 25. A marked attenuation in the degree of analgesia and hyperactivity induced in mice by ad- ministration of the opiate, morphine, has been ob- served after exposure to different types of magnetic fields 12.18,21 . Similar types of attenuation in morphine- induced analgesia have been noted following natural disturbances in the geomagnetic field ~6. Other stud- ies have shown that exposure to fluctuating magnetic fields can alter the development of tolerance to mor- phine-induced analgesia m mice ~3 and that part of this effect is the result of conditioning processes in which the magnetic field cues act as conditioned stim- uli and show stimulus control over the expression of analgesia. More recent experiments have shown that ELF magnetic fields selectively influence the effects of various classes of opiates. The analgesic effects of specific U, 6 and ~c-opiate receptor-directed ligands are inhibited, but those of a-opiates are unaffected by exposure to the magnetic fields tS. Stress-induced opioid analgesia and hyperactivity are inhibited by exposure to ELF magnetic fields as well 16'17 indicat- mg that these magnetic fields also influence the en- dogenous opioid systems. The mechanism(s) where- by the magnetic fields influence the endogenous opi- oid systems are poorly understood, but results of ex- periments with calcium chelators and ionophores have shown that at least some of these effects of ELF magnetic fields are compatible with actions on calci- um and possibly other divalent ions 14. There is also some indirect evidence that magnetic field-induced inhibition of the activity of the pineal gland 35 may al- ter the functions of the endogenous opioid systems H. Concern about health effects due to exposure to high-voltage transmission lines has been expressed 19 with numerous reports documenting biological el- Correspondence: K.-P. Ossenkopp, Department of Psychology, University of Western Ontario. London. Ont., Canada. N6A 5C2. 0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)