Effect of Electrical Fields on Neuronal Growth and Regeneration

Wise Young

I INTRODUCTION

Cells utilize electrical signals to communicate with each other. Neurons, in particular, not only respond to electrical fields but also generate electrical fields. These characteristics of neurons have naturally led many scientists to believe that exogenously imposed electrical fields influence neuronal activity, growth, and function. Since central neurons do not regenerate effectively, the possibility of applying electrical fields to stimulate and maintain neuronal regeneration has long been an area of intense speculation and exploration. Despite many years of concerted research, the subject has remained controversial. I review below some of the data that support electrical field effects on neuronal growth and regeneration, discuss why such bioelectrical manipulation of regeneration has not achieved widespread acceptance, and briefly discuss ionic mechanisms that may underlie observed effects of weak electromagnetic fields on neuronal growth and regeneration.

II NEUROREGENERATIVE EFFECTS OF ELECTRICAL FIELDS

Some evidence indicates that electromagnetic fields, particularly those arising from dc electrical currents, influence neuronal growth. The evidence falls into several categories. First, explanted dorsal root sensory ganglia have shown remarkable tendencies to migrate toward anodal poles of applied dc fields. Second, neuroblasts and other cultured cells also grow toward the negative (anodic) pole of applied dc electrical fields. Third, dc electrical currents and pulsed radiofrequency electromagnetic fields have been reported to increase the rate and extent of peripheral nerve regeneration. Finally, dc electrical currents were observed to reduce axonal dieback and facilitate axonal regrowth after transection of the spinal cords. These are described in sequence below.

A Chick Dorsal Root Sensory Ganglion Explants

Early studies (D. Ingvar, 1947; S. Ingvar, 1920) of electrical field effects on explanted sensory ganglia were flawed by inattention to the direction and intensity of currents delivered. In 1946, Marsh and Beam provided one of the first convincing reports that 100 mV/mm fields suppressed chick neurite growth at the anode (negative) but accelerated and deflected neurite outgrowth at the cathode (positive) side of the explant. Other workers have claimed positive effects (Sisken and Smith, 1975) but did not rule out the possibility of electrode products contaminating the cultures. Using wick electrodes, Jaffe and Poo (1979) showed that dorsal root ganglion explants were exquisitely responsive to 30–100 mV/mm fields. The ganglionic neurons migrated toward the anode, exhibiting a well-known tendency of cells called galvanotaxis (Ambrose, 1965; Erickson and Nucitelli, 1984). Higher fields of 70 mV/mm enhanced neurite outgrowth in the direction of the cathode. Reversal of the field inhibited growth but did not deflect neurite growth toward the cathode.

B Cultured Neuroblasts

Hinkle et al. (1981) studied the effects of imposed electrical fields on neurite outgrowth of neuroblasts from frog (Xenopus) eggs. They found that electrical fields markedly enhanced neurite outgrowth and direction of growth toward the cathode, while neurites on the anodal side showed retarded growth. Patel and Poo (1982, 1984) found that neurites would change directions of growth toward the cathode. McCaig (1986a,b) observed two- to threefold increases in neurite growth toward the cathode and decreased growth rate and neurite absorption on the anodal side (McCaig, 1987). Other investigators (DeBoni and Anderchek, 1986), however, did not find these results under similar experimental conditions.

C Peripheral Nerves

Weak dc currents (100–200 nA) have been reported to enhance peripheral nerve regeneration in frogs (Borgens et al., 1977, 1979). Axon sprouting increased in mammalian nerves exposed to such currents (Pomeranz et al., 1984; Pomeranz, 1986). Similarly weak currents (10 μA/cm2 at 100 mV/cm) increased reinnervation of hindpaw muscles after cut and suture did not crush lesions of the sciatic nerve (McDevitt et al., 1987). Other investigators have reported positive results (Politis et al., 1988; Zanakis, 1988). Raji (1984; Raji and Bowden, 1983) reported that pulsed radiofrequency electromagnetic fields increased the rate of peripheral nerve regeneration but did not rule out the possibility that these fields heated the nerves. More recently, McGinnis and Murphy (1992) were unable to show a quantitative increase in the number or rate of regenerating axons in mammalian sciatic nerves subjected to crush injury.

D Spinal Cord

Stronger dc currents (7–10 μA) increased both the rate and extent of giant axon regeneration in lamprey (Borgens, 1982; Borgens et al., 1981) and inhibited dieback of these axons (Roederer et al., 1983). These observations were extended to guinea pigs after dorsal hemisections (Borgens et al., 1986a) in studies showing that dorsal column axons not only grow back to the lesion site but course around the lesion in remarkable hairpin turns (Borgens et al., 1986b). The regeneration was associated with significant neurophysiological and behavioral evidence of recovered function (Borgens et al., 1987, 1990). Other investigators have reported similar cathode-oriented dorsal column axonal regeneration in contused rat spinal cord (Politis and Zanakis, 1988) and that dc currents (Wallace et al., 1987b) but not ac currents (Wallace et al., 1987a) enhanced behavioral recovery in rats after clip compression injuries of the spinal cord.

III CAVEATS AND A NEED FOR SKEPTICISM

Despite the numerous reports of electrically enhanced regeneration, many scientists have remained skeptical of electromagnetic effects on neuronal systems for one principle reason. The mechanisms of weak electromagnetic bioelectric effects are poorly understood. In contrast, the effects of strong electromagnetic fields on nervous tissues are well known and accepted. For example, electrical stimulation forms the basis of much of neurophysiology. Electrical stimulation of neurons requires currents or fields on the order of 1–10 mA/cm2 or 1–10 V/mm (Chan et al., 1988). Very intense magnetic fields are required to activate neurons (Cohen et al., 1991). Sensory detection of high-frequency electromagnetic fields usually occurs only when fields heat the tissue (Schwan, 1957). The electrical currents and fields reported to have biological effects are several orders of magnitude smaller, in the range of microamperes per square centimeter and millivolts per milliliter or smaller.

Exogenously applied currents and fields claimed to have bioelectric effects are in fact frequently smaller than endogenous currents and fields generated by the cells. While some investigators (Borgens and McCaig, 1989) have regarded this as an argument in favor of “natural” electrical currents being a cause of growth and healing, it is nevertheless difficult to attribute cause to fields that are “vanishingly small in comparison with potentials needed to yield significant membrane responses” (Schwan, 1982). For instance, most living cells normally have a transmembrane dc potential of about − 60 mV. An imposed electrical field of 100 mV/mm will induce less than 0.1 mV shifts of membrane potentials in intact cells, at the level of noise fluctuations. Likewise, exogenously applied dc electrical currents of 1–100 μA tend to pass around cells in the extracellular space and minimally penetrate cells (Schwan, 1984). Bioelectric effects from weak electromagnetic fields cannot be attributed to classical membrane ionic channels, transport, and receptor mechanisms.

Unfortunately, difficult-to-test hypotheses (e.g., Jacobson, 1991) and expansive claims have long demonated bioelectromagnetic studies. For centuries, many physicians and charlatans have attributed miraculous health effects to electrotherapy. Even well-established and institutionally approved medical applications of electrical stimulation, such as healing of nonunion bony fractures (Brighton, 1981), have seldom been put to rigorous clinical testing. In fact, the only double-blind randomized trial of pulsed electromagnetic field effects for bone healing was negative (Barker et al., 1984). Failures to replicate experimental findings, even in well-controlled in vitro situations, are common. The application of even dc electrical currents is fraught with pitfalls (Robinson, 1989), and the problems attending high-frequency electromagnetic field application are legend. Finally, regeneration is a difficult phenomenon to demonstrate and quantify. Combined, these problems have led to continued skepticism and a failure of bioelectromagnetic field studies to join mainstream neurobiology.

The ability of electromagnetic fields to promote neuronal regeneration nevertheless deserves careful and critical consideration for several reasons. First, the phenomenon of...