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

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Abstract
Changes in extracellular K ([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 (DRP). Sucrose gap recording from the dorsal root (DR) recorded responses to root stimulation and to exogeneously applied K+. Stimulation of the ventral root, which elicits a DRP in the frog spinal cord, was not associated with any change in [K]e, suggesting that DRP produced by ventral root stimulation are not due to changes in [K]e. 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 DRP evoked by stimulating an adjacent dorsal root (DR-DRP) was over 10 mV. The time-to-peak of the glial cell responses was 10 times that of the DR-DRP. Low frequency (1-10 Hz) DR stimulation caused a decremental summation of glial cell responses, while there was no summation in the DR-DRP. The DRP produced by single DR stimulation is generated in large part by a mechanism other than a change in [K]e. During high frequency DR stimulation, which evoked 6-8 mM increases in [K]e, the adjacent DR 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. A substantial component of the sustained DR depolarization during high frequency DR stimulation may result from changes in [K]e. In the presence of Mg, high frequency DR stimulation evoked a picrotoxin resistant depolarization of an adjacent DR whose magnitude correlated well with the changes in [K]e recorded in the spinal cord. In the presence of picrotoxin a slow, long duration depolarization of the DR occurred following single stimuli to the adjacent DR and the appearance and time course of this response correlated well with the time course of changes in [K]e. 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. Although changes in [K]e play a relatively minor role in generating DRP elicited by single DR stimulation, the sustained DRP 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.