Laminar distribution of hippocampal rhythmic slow activity (RSA) in the behaving rat: current-source density analysis, effects of urethane and atropine.

This study investigated the laminar distribution of rhythmic slow wave activity (RSA) in the dorsal hippocampus of the rat during running. Depth analyses of field EEG were performed by stepping the recording electrode in 82.5 micron increments and sampling RSA at each depth. One-dimensional current-source density (CSD) was calculated from the RSA profiles to enhance spatial resolution of current sources and sinks. Laminar analysis of power, coherence, and phase of RSA with respect to a stationary electrode in the stratum oriens of CA1 was performed with spectral methods. RSA waves in the CA1-dentate axis had power maxima at about the hippocampal fissures, hilus, outer molecular layer of the endal leaf of dentate gyrus and stratum oriens of CA1, in that order. A gradual shift of phase occurred in stratum radiatum of CA1. Large phase-shifts were found in both the endal and ectal leaves of the fascia dentata. A null zone and associated sudden phase-reversal of RSA were observed in stratum lucidum of CA3. Multiunit activity showed phase-locked modulation with RSA in the granule cell layer of the dentate gyrus and pyramidal cell layer of CA1, CA3, and subiculum. CSD analysis in the CA1-dentate axis revealed multiple source-sink pairs. The sinks and sources showed cyclic changes with RSA, and were attributed to the rhythmic, but time-shifted, activity of hippocampal afferents from the septum and entorhinal cortex. The gradual phase-shift in CA1, and the configurational changes of RSA waves with depth, are explained by the summation of extracellular currents produced by time-delayed sink-source pairs (RSA dipoles). When the cholinergic septohippocampal path was blocked by atropine a null zone in the middle of stratum radiatum of CA1 occurred and the phase-shift of RSA became steeper. Under urethane anesthesia a null zone was present in the inner stratum radiatum associated with a sudden phase-reversal of RSA. Urethane reduced the power of RSA in the hilus and decreased the firing rate of the granule cells. It is suggested that field RSA is produced by several rhythmical dipoles along the somadendritic surface of pyramidal cells and granule cells and the spatiotemporal relations of the individual dipoles determine the actually observed extracellular RSA.