. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MAGNETIC FIELDS AND OPIOID SYSTEMS 305 in invertebrates. The effects in vertebrates have included retardation in embryological development, changes in behavioral activity levels and inhibition of chemically and electrically kindled seizures. These effects are compatible with alterations in the functioning of endogenous opioid systems (review in Ossenkopp and Kavaliers, 1988). This speculation of opioid involvement is encouraged by the observation that acute (30-min) exposure to low intensity 60 Hz magnetic fields markedly re
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MAGNETIC FIELDS AND OPIOID SYSTEMS 305 in invertebrates. The effects in vertebrates have included retardation in embryological development, changes in behavioral activity levels and inhibition of chemically and electrically kindled seizures. These effects are compatible with alterations in the functioning of endogenous opioid systems (review in Ossenkopp and Kavaliers, 1988). This speculation of opioid involvement is encouraged by the observation that acute (30-min) exposure to low intensity 60 Hz magnetic fields markedly reduces morphine-in- duced analgesia levels in mice, with a functional relation- ship between magnetic field intensity and the degree of inhibition of analgesia (Ossenkopp and Kavaliers, 1987). Recently, it was observed that exposure of C'cpaca to low intensity ( gauss, rms) 60 Hz magnetic fields in a Helmholtz coil apparatus, as shown in Figure 2, also re- sulted in an attenuation of morphine-induced analgesia (Kavaliers et 1990). Various durations of exposure (, 2, 12, 48, or 120 h) to the 60 Hz fields reduced the levels of morphine-induced analgesia in both the light and dark periods of a 12 h light: 12 h dark cycle, with the magnetic stimuli having significantly greater inhibitory effects in the dark period. The inhibitory effects of the magnetic fields were reversible. Twenty-four hours after exposure, the levels of morphine-induced analgesia were not significantly different from pre-exposure levels (Kav- aliers et al., 1990). These effects in C'cpaca are consistent with the day-night rhythms in the inhibitory effects of naloxone and 60 Hz and rotating magnetic fields on mor- phine-induced analgesia in nocturnal rodents (Kavaliers and Ossenkopp, 1984; Ossenkopp and Kavaliers, 1987). The weak 60 Hz magnetic fields also significantly reduced the levels of the elevated naloxone-sensitive dark period nociceptive response latencies in Cepaea, while not af- fecting th
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