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EFFECTS OF STATIC MAGNETIC FIELDS ON
ISOLATED NEURONS
Maria Azanza, A. del Moral
To cite this version:
Maria Azanza, A. del Moral.
EFFECTS OF STATIC MAGNETIC FIELDS ON
JOURNAL DE PHYSIQUE
Colloque C8, Supplement au no 12, Tome 49, ddcembre 1988
EFFECTS OF STATIC MAGNETIC FIELDS ON ISOLATED NEURONS
Maria J. Azanza (I) and A. del Moral (2)
(I) Biologia, Facultad de Medicina, Universidad de Zaragoza, 50009 Zaragoza, Spain
(2) Departamento de Fisica de la Materia Condensada e ICMA, Universidad de Zaragoza y CSIC. 50009
Zaragoza, Spain
Abstract. - We have applied static magnetic fields (SMF) of 1 160 and 2 600 gauss t o isolated neurons. Under SMF the ion kinetics across the membrane is disturbed. c a 2 + ions are removed from their stores and activate either the K+
membrane conductance or the CAN currents being modified in turn the membrane excitable properties.
The effect of steady magnetic fields (SMF) on the spontaneous activity of isolated neurons is described. To our howledge these are the first reported direct measurements showing the influence of SMF on the ca2+ kinetics and the subsequent electrophysiological membrane changes. ~ a ~ n e t i c fields induce alterations in the levels of intra and extracellular calcium and also in the rates of cellular calcium efflux [l-31. We have considered that a direct observation of the SMF effects on isolated neurons activity could reveal the electro- physiological responses concomitant with the presum- ably underlying changes in ca2+ ions kinetics following exposure.
Our experiments were carried out on neurons of the brain of "Helix aspersa"
.
The brains were mounted in a bath and immersed in Ringer solution (NaC180 mM; KC1 4 mM; CaC12 7 mM; MgCl, 5 mM; Tris-HC1 buffer 5 mM; pH = 8). Intracellular recordings were made using glass micro-electrodes filled with 1 M potassium acetate, tip resistance 2-20 MO. SMF, ranging from 30 to 2 600 gauss, were generated with an Electromagnet. The brain chamber was kept between the pole pieces of the magnet and fields of 1 160 and 2 600 gauss (similar to the ones used in NMR imaging techniques 141) were applied.We have studied 50 cells. An 86 per cent of the studied neurons are inhibited and the 14 per cent are excited by the SMF exposure. The inhibition is recorded as membrane hyperpolarizations. We could associate the hyperpolarization with the sorting of
K+ ions out of the cell. In order to check the role played by the K+ ions we performed a series of ex- periments in Ringer K+ free and in Ringer with K+
20 mM (five times the original concentration, Fig. 1). The sorting of K+ ions is favoured in Ringer K+ free (Fig. lc) and is blocked when the outsider concen- tration (K$) increases (Fig. Id). Removal of K+ from the bathing fluid blocks the sodium/potassium pump. When 20 mM K+ is added to the Ringer so- lution the pump is restored, but since the exchange is 3 ~ a + / 2 ~ + , the membrane resting potential (Em) de- creases (Fig. Id (*)). Nevertheless the concentration
f
~ i n g e r K +free .5mV 1 min -l--
Fig. 1. - a) Spontaneous cell activity. After 30 min the cell stops firing and becomes silent. b) 1 150 gauss SMF induces negative spikes (19 mV) and slower hyperpolariza- tions (5 mV, 5 sec).
gradient for K+ favours the influx of these ions into the cell and Em goes up back to its normal value.
In previous experiments applying SMF on 25 addi- tional cells we had observed no changes either in the amplitude or the duration of the.spikes nor in the ve- locity of discharge. We had then to discard a direct effect of SMF on the K+ kinetics. The neurons un- der consideration were silent when tested under SMF and, since they did not fired spikes, we could discard the K+ channels activated by the entering ca2+ dur- ing depolarization [5]. The experiment shown in fig- ure 1 (d) reveals a dependence of K+ conductance ( g ~ ) with ca2+ intracellular ions. The apparently anoma- lous increased hyperpolarizations are due to concomi- tant changes in ca2+ kinetics. As K$ concentration
increases voltage regulated c a 2 + channels are opened [6] and these ions move inside the cell down their electrochemical gradient. ca2+-dependent-K+ chan- nels are then opened and g~ increases [7]. We tested then the influence of c a 2 + ions on the inhibitory re-
C8 - 2060 JOURNAL DE PHYSIQUE sponse under different c a 2 + concentrations in the su-
perfusate: Ringer ca2' free and Ringer with five times the original c a 2 + concentration. In c a 2 + free Ringer no changes were observed but in rich
ca2+
the hy- perpolarization duration notably increased (Fig. 2).Fig. 2. - Under SMF, 35 mM ca2+ Ringer, increases the hyperpolarization duration.
We had now t o separate the Kt-channels subpopula- tions which could be involved in the hyperpolarization response t o SMF application. Tetraethylammonium (TEA) is a well known blocker of t h e voltage activated Kt-channels. When TEA (5 mM) was added t o the su- perfusate (Fig. 3c) the hyperpolarization was not abol- ished what means that K+ ions are probably moving through a ca2+-dependent-channels population.
c 5mM TEA
Fig. 3. - a) Spontaneous cell activity, 2 600 gauss SMF is applyed. b) the hyperpolarizations elicited by SMF are potentiated in 0 mM K+ Ringer. c) The spikes recorded shown that the voltage-dependent-K+-channels have been blocked by TEA.
As a preliminary hypothesis we could think of an increase in intracellular c a 2 + free ions due t o SMF exposure. In turn this would induce the activation of the ca2+-dependent-K+-channels and thereby the Kf -conductance membrane activation. The increment in the intracellular concentration of c a 2 + ions under SMF could also explain the excitation recorded in the 14 per cent of the cells studied (Fig. 4). We have ob- served that in neurons excited under SMF [8] when either the Nat ions are removed or verapamil (a po- tent blocker of the c a 2 + channels) is added t o the
Fig. 4. - a) Spontaneous cell activity. b) 1 150 Gauss SMF
excites the cell. d) 80 seconds after removing SMF stimuli the cell firing activity decays and finally stops.
superfusate the spikes are not completely abolished. These cells are partially Na2+-independent and c a 2 + ions are required t o carry the inward current of the action potential activating nonspecific membrane cur- rents (CAN currents [9]).
We have shown elsewhere [8] that SMF mimics the inhibitory and excitatory response t o caffeine on neu- rons, effects which are c a 2 + dependent. Caffeine mov- ilizes c a 2 + ions from the endoplasmic reticulum, where from do SMF movilizes c a 2 + ions? A possibility t o be tested are the neuronal c a 2 + high-affinity cytosolic soluble proteins like calmoduline. Inasmuch as mag- netic dipolar orientational processes could be involved in the c a 2 + liberation susceptibility measurements of calmoduline are envisaged.
Acknowledgment
M. J . Azanza is indebted t o the Spanish CAICYT (grant no P B 851391) and t o the DGA of Arag6n (grant no 14/86) for financial support.
[I] Dixey, R. and Rein, G., Nature 296 (1982) 253. [2] Blackman, C. F., Benane, S. G., Rabinowitz,
J. R., House, D. E. and Joines, W. T., Bioelec- tromagnetics 6 (1985) 327.
[3] Kavaliers, M. and Ossenkopp, K. P., Brain Res. 379 (1986) 30.
[4] Edelman, R., Discuss. Neurosc. 1 (1984)l. [5] Moreton, R. B., J. Exp. Biol. 57 (1972) 513. [6] Blaustein, M. P., J. Physiol. 247 (1975) 617. [7] Meech, R. W., Ann. Rev. Biophys. Bioen. 7
(1978) 1.
[8] Azanza, M. J. t o be published.
