October 24, 2006
EFFECTS OF STEADY MAGNETIC FIELDS ON ACTION POTENTIALS OF SENSORY NEURONS IN VITRO
Michael J. McLean, M.D., Ph.D., Robert R. Holcomb, M.D., Ph.D., Artur W. Wamil, M.D., Ph.D., Joel D. Pickett, M.D.
ABSTRACT
Exposure to a static field (10 milliTesla) produced by an array of four permanent magnets of alternating polarity (side, facing neuron under study) reduced or blocked action potential (AP) firing by adult mouse dorsal root ganglion neurons in monolayer disassociated cell culture.
The effect was reversible with slow recovery of firing over several minutes. Arrays of four magnets of like polarity (all positive or all negative poles; 32-35 milliTesla) also reduced firing, but APs returned within seconds after removal of these arrays. An alternating dipolar array (13.7 milliTesla) had no effect. These findings suggest that the configuration of magnets and gradients within the field may be more important than field strength in determining biological effects.
Devices controlling such magnetic fields could be used for the treatment of chronic, medication-resistant pain. static magnetic fields, magnetic field gradient, action potentials (AP), cell culture, dorsal root ganglion cells.
INTRODUCTION
At this time of heightened public concern about the health impact of magnetic fields, there is increasing use of magnetic devices in the practice of clinical medicine. Examples include magnetic resonance imaging of body structures; the use of SQUID (semiconductive quantum interference device) probes to detect magnetic fields produced by cardiac and neural tissue; the use of pulsed magnetic fields to enhance bone healing8; and, the historical use of magnets to treat pain.
Yet, understanding of cellular effects of magnetic fields is in its infancy. Theoretical studies have indicated that homogeneous fields of 25-100 Tesla (T: SI unit of magnetic field density) would be required to affect ionic currents of nerve processes.10,11 Studies of effects of homogeneous fields (up to 10 T) on non-mammalian preparations have yielded mixed results. Some studies reported effects on aspects of neuronal excitability, such as chronaxie, action potential firing and/or response to neurotransmitters.
Others showed no effects and identified technical flaws in previous studies interpreted as positive. Thus, the question of whether magnetic fields alter membrane function remains unresolved. Adult mammalian sensory neurons can be maintained in vitro and provide a model system in which to test the biological effects of magnetic fields in a controlled environment.
Other investigators have recorded from cell bodies of neurons in sensory (dorsal root) ganglia of anesthetized animals. Neurons with stimulus-response characteristics of pain sensitive neurons and fibers with slow conduction velocities, a criterion used clinically to segregate nerve fibers subserving different sensory functions, had long duration action potentials. Mechanosensitive neurons had fast conduction velocities and brief APs.
In vitro, some sensory neurons responded to pain-producing, substances. Under voltage-clamp conditions using the patch clamp technique, two different sodium currents were observed in dorsal root ganglion neurons. We have identified functional subtypes of neurons in vitro by a combination of AP waveform and firing pattern, chemosensitivity and the type(s) of sodium current generating the AP upstroke.
One subtype, the LD neurons, had long-duration (2-5 msec at takeoff potential) APs which fired repetitively during 400 msec intracellularly applied depolarizing current pulses and which were resistant to tetrodotoxin (TTX), a marine toxin known to block fast sodium channels of nerve and muscle. The LID neurons were excited by endogenous (bradykinin, histamine) and exogenous (capsaicin) irritants known to produce pain in man.
Properties of LD neurons resembled those of nociceptive neurons in vivo. Other neurons (SD) had short duration (0.5-2.0 msec) APs which fired once or a few times during 400 msec depolarizations and were blocked reversibly by TXX. The SD neurons were not excited by irritants. Properties of SD neurons24 were more like those of mechanoceptive neurons in vivo. Pressure application of solution in which sodium was replaced with choline reversibly blocked APs of both LD and SD neurons, i.e. both types of APs were sodium-dependent.
Using patch clamp techniques we found some neurons had TTX-resistant sodium current, TTX-sensitive sodium current, or a mixture of the two. By analogy based on effects of TTX, LID neurons APs, and hence those of nociceptive neurons, seem to be generated by the slow, TTX-resistant current.
Action potentials of SD, hence mechanoceptive, neurons are generated by the fast TTX-sensitive current typically seen in central neurons and muscle. In this symposium, we summarize findings which show that inhomogeneous magnetic fields (<20 mT) produced by arrays of permanent magnets reduced AP firing by adult mouse sensory neurons, both LD and SD, in monolayer dissociated cell culture. Preliminary accounts of this work have been published.