Hippocampal rhythm generation: Gamma-related theta-frequency resonance in CA3 interneurons

During different behavioral states different population activities are pres ent in the hippocampal formation. These activities are not independent: sha rp waves often occur together with high-frequency ripples, and gamma-freque ncy activity is usually superimposed on theta oscillations. There is both e xperimental and theoretical evidence supporting the notion that gamma oscil lation is generated intrahippocampally, but there is no generally accepted view about the origin of theta waves. Precise timing of population bursts o f pyramidal cells may be due to a synchronized external drive. Membrane pot ential oscillations recorded in the septum are unlikely to fulfill this pur pose because they are not coherent enough. We investigated the prospects of an intrahippocampal mechanism supplying pyramidal cells with theta frequen cy periodic inhibition, by studying a model of a network of hippocampal inh ibitory interneurons. As shown previously, interneurons are capable of gene rating synchronized gamma-frequency action potential oscillations. Exciting the neurons by periodic current injection, the system could either be entr ained in an oscillation with the frequency of the inducing current or exhib it in-phase periodic changes at the frequency of single cell (and network) activity. Simulations that used spatially inhomogeneous stimulus currents s howed antiphase frequency changes across cells, which resulted in a periodi c decrease in the synchrony of the network. As this periodic change in sync hrony occurred in the theta frequency range, our network should be able to exhibit the theta-frequency weakening of inhibition of pyramidal cells, thu s offering a possible mechanism for intrahippocampal theta generation.