The effects of moderate changes of extracellular K+ and Ca2+ on synaptic and neural function in the CA1 region of the hippocampal slice.

The effects of moderate changes of the concentration of ions on the function of mammalian central nervous tissue have not exactly been determined. We placed tissue slices from rat hippocampal formation in an interface chamber for study in vitro. Extracellular potentials were recorded in stratum radiatum and stratum pyramidale in response to stimuli of varying intensity applied to the Schaffer collateral bundle. The overall input-output relationship of excitatory synaptic transmission was gauged by expressing postsynaptic population spike amplitude as a function of presynaptic volley amplitude. The components of the transmission process were also examined by plotting the maximal rate of rise (slope) of the focally recorded synaptic potential (fEPSP) as a function of presynaptic volley amplitude, and the population spike amplitude as a function of the fEPSP slope. Raising the concentration of K+ from the normal level of 3.5 mM to 5 mM caused an average increase of 48% in the population spike evoked by a given presynaptic volley. This was due to an increased electrical excitability of pyramidal cells, as indicated by an increase of the population spike evoked by a given magnitude of fEPSP. Conversely, lowering [K+]o from 3.5 to 2 mM caused a decrease of the population spike relative to a given magnitude of either the presynaptic volley or the fEPSP. Changing [K+]o within these limits caused no significant change of the fEPSP evoked by a given presynaptic volley. Raising [Ca2+]o from 1.2 to 1.8 mM caused a 35% increase in both the fEPSP and the population spike evoked by a given presynaptic volley, and lowering [Ca2+]o to 0.8 mM caused a decrease of both these functions. The amplitude of the population spikes evoked by given fEPSPs changed surprisingly little (but consistently) when [Ca2+]o was varied within these limits. We conclude that moderate changes of [K+]o influence mainly the electric excitability of hippocampal pyramidal cells, with little effect on transmitter release or on the response of the postsynaptic membrane to transmitter, while moderate changes of [Ca2+]o affect the release of excitatory synaptic transmitter more than they affect postsynaptic membrane function.