Bistability dynamics in simulations of neural activity in high-extracellular-potassium conditions

Modulation of extracellular potassium concentration ([K](o)) has a profound impact on the excitability of neurons and neuronal networks. In the CA3 re gion of the rat hippocampus synchronized epileptiform bursts occur in condi tions of increased [K](o). The dynamic nature of spontaneous neuronal firin g in high [K](o) is, therefore of interest. One particular interest is the potential presence of bistable behaviors such as the coexistence of stable repetitive firing and fixed rest potential states generated in individual c ells by the elevation of [K](o). The dynamics of repetitive activity genera ted by increased [K](o) is investigated in a 19-compartment hippocampal pyr amidal cell (HPC) model and a related two-compartment reduced HPC model. Re sults. are compared wi th those for the Hodgkin-Huxley equations in similar conditions. For neural models, [K](o) changes are simulated as a shift in the potassium reversal potential (E-K). Using phase resetting and bifurcati on analysis techniques, all three models are shown to have specific regions of E-K that result in bistability. For activity in bistable parameter regi ons, stimulus parameters are identified that switch high-potassium. model b ehavior from repetitive firing to a quiescent state. Bistability in the HPC models is limited to a very small parameter region. Consequently, our resu lts suggest that it is likely some HPCs in networks exposed to high [K](o) continue to burst such that a stable, quiescent network state does not exis t. In [K](o) ranges where HPCs are not bistable, the population may still e xhibit bistable behaviors where synchronous population events are reversibl y annihilated by phase resetting pulses, suggesting the existence of a nons ynchronous network attractor.