Figure 3. Effects of changes in K⁺ on spontaneous activity of CA3 and CA1 cells.

A, simultaneous records with one electrode located in CA3 and the other in CA1. Spikes were evident in the CA3 record at both 2 and 6 mM external K⁺, while the record from the CA1 region was largely silent until K⁺ was increased to 6 mM. B, changes in spike frequency measured from CA3 (upper trace) and from CA1 (lower trace) as K⁺ was reduced from 4 to 2 mM and then increased to 8 mM in steps of 2 mM. C, histograms of spike amplitudes from the CA3 recording electrode in the presence of 2 and 8 mM K⁺. At 2 mM K⁺, peaks were apparent. At 8 mM K⁺, the spike frequency was increased sixfold and the amplitude histogram was smooth. The amplitude distribution (N(A)) at 8 mM K⁺ was fitted to an equation of the form, N(A) = cA^−b, assuming a uniform cell packing density and discharge frequency (see Appendix). The optimal fit was obtained with constants b = 1.505 and c = 990, and the Kolmogorov-Smirnov test gave a value of P = 0.59 (with n = 3081). The fit diverges from the observed distribution for spikes of large amplitude, corresponding to signals from cells close to the recording electrode.