BLOCKADE OF SENSORY NEURON ACTION-POTENTIALS BY A STATIC MAGNETIC-FIELD IN THE 10 MT RANGE

To characterize the inhibitory effect of a static magnetic field, acti on potentials (AP) were elicited by intracellular application of 1 ms depolarizing current pulses of constant amplitude to the somata of adu lt mouse dorsal root ganglion neurons in monolayer dissociated cell cu lture. During the control period, <5% of stimuli failed to elicit AP. During exposure to an similar to 11 mT static magnetic field at the ce ll position produced by an array of four permanent center-charged neod ymium magnets of alternating polarity (MAG-4A), 66% of stimuli failed to elicit AP. The number of failures was maximal after about 200-250 s in the field and returned gradually to baseline over 400-600 s. A dir ect or indirect effect on the conformation of AP generating sodium cha nnels could account for these results because 1) failure was preceded often by reduction of maximal rate of rise, an indirect measure of sod ium current; 2) recovery was significantly prolonged in more than one- half of neurons that were not stimulated during exposure to the MAG-4A field; and 3) resting membrane potential, input resistance, and chron axie were unaffected by the field. The effect was diminished or preven ted by moving the MAG-4A array along the X or Z axis away from the neu ron under study and by increasing the distance between magnets in the XY plane. Reduction of AP firing during exposure to the similar to 0.1 mT field produced by a MAG-4A array of micromagnets was about the sam e as that produced by a MAG-4A array of the large magnets above. The s imilar to 28 mT field produced at cell position by two magnets of alte rnating polarity and the similar to 88 mT field produced by a single m agnet had no significant effect on AP firing. These findings suggest t hat field strength alone cannot account for AP blockade. (C) 1995 Wile y-Liss, Inc.