Objective. To propose a mechanistic model that can explain the effects of fields on the order of 1 μV/m on animal cells, and to present experimental evidence in support of the proposed mechanism using cells in electrosensitive fish.
Methods. Glass catfish (Kryptopterus bicirrhis) were immobilized and electric current was applied to electroreceptor organs by means of electrodes located on either side of the anal fin. Antibodies against plasma membrane fragments from electroreceptor cells were produced using standard methods. The spike frequency of the electroreceptor nerve was measured in the presence and absence of the antibodies and in the presence and absence of an applied electric field (DC–20 Hz). The electric field inside the electroreceptor organ was calculated in two stages, using Femlab.
Results. We hypothesized that the field-sensitive element was a glycoprotein associated with the gate of a Na channel (Fig. 1). Application of an electric field exerts a force F on the molecule, thereby mechanically opening the gate (Fig. 1). Each glycoprotein molecule contained many negative charges (q = −eZ, where e is the elementary charge and Z is the number of charges per molecule). Consequently, even a weak electric field (E) could cause a mechanical force (qE) great enough to overcome the potential-energy barrier (U) between closed and opened channel states. U ≥ kT/2, where k is Boltzman’s constant, T is temperature, and kT/2 is the thermal energy associated with one degree of freedom. Therefore |qEΔx| > U > kT/2, where Δx is the displacement of the channel gate (Fig. 1), assumed to be 6 nm. Calculations showed that a glycoprotein sphere with a diameter of about 15 μm could switch a Na+ channel at E = 1 μV/m.
A glycoprotein gel is present in the electroreceptor organ of Kryptopterus, and therefore that species was used to test the model. Response sensitivity was a maximum at about 10 Hz (Fig. 2). At that frequency, the fish could detect the field produced by an applied current of 1 μA (Fig. 2). Calculations showed that the corresponding electric field at the apical membrane of the cell was 1 μV/m (Fig. 3). Assumed conductivities: water and gel, 24 mS/m; tissue, 100 mS/m; cell interior, 100 mS/m; cell membrane, 0.07 μS/m; skin, 0.8 mS/m.
Immunoglobulin containing antibodies raised against membrane fragments from electroreceptor cells blocked field-induced changes in spike frequency of the afferent neuron that innervated the electroreceptor organ (Fig. 4).
Discussion. Taken together, this evidence indicated that electroreceptor cell transduced a 10-Hz field of 1 μV/m at the apical surface, and that the effect could be explained by the proposed model.
|Fig. 1. Proposed model for electroreception. An applied electric field exerts a force F on a negatively charged glycoprotein molecule thereby mechanically opening the gate of a sodium channel in the apical membrane. Back to text|
|Fig. 2. Sensitivity (change in spike frequency per unit field) as a function of stimulus intensity and frequency (insert). The baseline spike frequency (zero stimulus) did not change during the measurements (about 2 hours). Back to text|
|Fig. 3. Calculated electric field inside an electroreceptor organ corresponding to the application of 1 nA. Contour-line resolution, 1 μV/m (First contour (*), 0.75 μV/m). Fields in tissue and cytoplasm set to zero. Back to text|
|Fig. 4. Effect of antibodies on the response of electroreceptor nerve activity in Kryptopterus to fields. B, baseline activity. S1, 10 μV/m, 5 Hz; S2, 10 μV/m, 10 Hz; S3, 10 μV/m, 20 Hz. EA, electroreceptor-cell antibodies. CA, control antibodies. *P < 0.05, compared with electric field alone. Back to text|