The biophysical bases of synchronous activity in the hippocampal formation

For the last few years, our laboratory has used experimental and computational techniques to study the biophysical underpinnings of both normal and abnormal synchronous activity in the hippocampal formation and other regions of the brain. Among our findings: 1) Robust synchronous activity occurs most readily when one or more biophysical processes (e.g., the dynamics of particular voltage- or ligand-gated ion channels) serve as a rate-limiting step, with a time scale that matches the rhythm in question. Specifically, we have shown that slow voltage-gated channels in neurons of the entorhinal cortex may contribute to the theta rhythm (White et al., 1995); and that the time constant with which chemical inhibition decays is crucial for stabilizing the gamma rhythm within its particular range of frequencies (Chow et al., 1998, White et al., 1998). 2) Electrical noise from flicker in the conductance states of voltage-gated ion channels interacts with intrinsic cellular nonlinear dynamics to provide novel mechanisms for synchronization (Acker, 2000; White et al., 1998; White et al., 2000). 3) Ionotropic chemical inhibition within the hippocampus is known to have at least two time scales (Banks et al., 2000). Modeling studies (White et al., 2000) indicate that these two time scales of inhibition may contribute to the mixed gamma and theta rhythms seen in vivo.