Effects of divalent cations on the frequency of spontaneous action potentials from the lateral line organ of Xenopus laevis.

The effect of superfusion of the internal surface of the skin of Xenopus laevis with saline containing Co2+, Ca2+, Mg2+, or Ba2+, on the frequency of spontaneous action potentials of the lateral line nerve, was studied to investigate the role of extracellular Ca2+ in spontaneous neural activity. Addition of divalent cations to frog saline, either singly or as a mixture of two different ions, produced concentration-dependent suppression of spontaneous rate. The rank order of potency for suppression by each ion, perfused alone, was Co2+ greater than Ca2+ greater than Mg2+ greater than Ba2+. Suppression by combinations of Mg2+ and Co2+, or of Ca2+ and Co2+, was approximated by the sum of the suppressive effects of each cation. Ca2+ was more suppressive than Mg2+ when each of these ions was paired with the same amount of Co2+, while Ca2+ was approximately as suppressive as Co2+ when similarly paired with Mg2+. One interpretation of the suppression by Ca2+ invokes the hypothesis that divalent cations suppress spontaneous activity by charge screening of voltage-sensitive Na+ channels on afferent dendrites and that release of neurotransmitter by the influx of extracellular Ca2+ through voltage-sensitive Ca2+ channels of hair cells may not be the sole mechanism for generation of spontaneous activity in the lateral line. These results quantify the relative suppressive potency of common divalent cations in the lateral line, and serve as a caveat to investigators who interpret a blockade of action potentials by high concentrations of Co2+ or Mg2+ as sufficient evidence for dependence of neurotransmission upon extracellular Ca2+, particularly in acousticolateralis systems.