Dorsal root potentials and changes in extracellular potassium in the spinal cord of the frog

Abstract

1. In the present study changes in extracellular potassium, ([K]e), were recorded in the isolated spinal cord of the frog with glial cell recordings and K-selective micro-electrodes to test the hypothesis that elevations in [K]e during neuronal activity produce the dorsal root potential (d.r.p.). 2. Sucrose gap recording from the dorsal root (d.r.) was used to record responses to root stimulation and to exogenously applied K+. 3. Stimulation of the ventral root, which elicits a d.r.p. in the frog spinal cord, was not associated with any change in [K]e, suggesting that d.r.p.s produced by ventral root stimulation are not due to changes in [K]e. 4. The largest change in [K]e observed following single stimuli to the dorsal root was 0.4 mM. Such a change in [K]e, if evenly distributed, would depolarize the dorsal root by about 1 mV and yet the simultaneously recorded d.r.p. evoked by stimulating an adjacent dorsal root (d.r.-d.r.p.) was over 10 mV. 5. The time-to-peak of the glidal cell responses was 10 times that of the d.r.-d.r.p. Low frequency (1-10 Hz) d.r. stimulation caused a decremental summation of glial cell responses, while there was no summation in the d.r.-d.r.p. These results suggest that the d.r.p. produced by single d.r. stimulation is generated in large part by a mechanism other than a change in [K]e. 6. During high frequency d.r. stimulation, which evoked 6-8 mM increases in [K]e, the adjacent d.r. was depolarized to a greater extent than that produced by single stimuli. The magnitude of this depolarization was similar to that produced by applying a [K]e equivalent to that observed in the spinal cord during high frequency stimulation. Thus, a substantial component of the sustained d.r. depolarization during high frequency d.r. stimulation may result from changes in [K]e. 7. In the presence of magnesium, high frequency d.r. stimulation evoked a picrotoxin resistant depolarization of an adjacent d.r. whose magnitude correlated well with the changes in [K]e recorded in the spinal cord. 8. In the presence of picrotoxin a slow, long duration depolarization of the d.r. occurred following single stimuli to the adjacent d.r. and the appearance and time course of this response correlated well with the time course of changes in [K]e. 9. Addition of K+ to the Ringer solution in concentrations up to 12 mM had a facilitatory action on reflex activity in the frog spinal cord. 10 The present results suggest that although changes in [K]e play a relatively minor role in generating d.r.p.s. elicited by single d.r. stimulation, the sustained dorsal root depolarization evoked either by high frequency stimulation or by single stimuli in the presence of picrotoxin may be due to a considerable extent to [K]e.