Faculty Bibliography

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

Wave shape patterns and spectral properties of hippocampal slow wave activity (RSA) were studied in behaving rats equipped with stationary recording/stimulating electrodes and a movable microelectrode. RSA waves had maximum power at about the hippocampal fissure, and two minima just below the pyramidal cells of CA1 and the inner molecular layer of the dentate gyrus, respectively. The phase profile of RSA was gradual during both running and lever pressing, but the two profiles showed phase differences in the stratum radiatum of CA1 and the hilus. Averaged RSA waves consisted of fast rising and slow decaying components, giving a saw-tooth like pattern. RSA waves were more asymmetric during running than during lever pressing. The slow component showed a sudden polarity reversal below the pyramidal layer of CA1. The fast component of RSA showed a gradual shift and change of the slope with depth. An additional small amplitude wave riding on the slow component ('notch') was present during running. The amplitude increase of the 'notch' occasionally caused frequency doubling of RSA and consequent high power of the second harmonic. The gradual shift and change of the fast component are explained by the hypothesis that somatic inhibitory and dendritic excitatory RSA dipoles in CA1 and dentate gyrus are active at different times of the RSA cycle

Rats equipped with stimulating and recording electrodes in the hippocampus were trained to lap water on a variable interval schedule. High amplitude EEG sharp waves ( SPW ) were recorded from the CA1 region during drinking. High frequency stimulation of the commissural/associational system enhanced both the incidence and amplitude of the drinking associated SPWs up to 48 h. These findings indicate that strong, synchronous activation of an input pathway may induce long-term alteration of the excitability of the intra-hippocampal circuitry

An overview of the current literature reveals a richness and complexity of anatomical, pharmacological and physiological features of the input systems to the archicortex. Evidence is cited to demonstrate that several afferent paths terminate on and directly excite hippocampal formation interneurons ('non-principal' cells) besides their contacts with pyramidal and granule cells (principal cells). Since all interneurons are thought to be inhibitory, afferent excitation results in a dual effect: direct excitation of principal cells is coupled with concurrent disynaptic feed-forward inhibition. Interneuron activation generally precedes principal cell activation when both are driven by a given afferent path. At least some interneurons take a part in both feed-back and feed-forward inhibition. It is suggested that most of the major inputs to the hippocampal formation dually innervate both interneurons and principal cells and that the excitability of the principal cells depends upon the relative strengths of the inputs to these two cell types. The hypothesis of dual innervation appears powerful in resolving existing anatomical and physiological controversies

Sampling and analysis programs are implemented on a microcomputer for the computation of autopower, coherence and phase spectra of unit train and EEG. Application to the hippocampus reveals that both theta and fast activity in the EEG are correlated with different phase relationships with unit activity. The estimate of coherence and phase spectra of unit-EEG relationship has not been reported before and is unique for the present method

Rats implanted with recording and stimulating electrodes were trained to run in an activity wheel for a water reward. Unitary discharges and slow activity were recorded by a movable tungsten microelectrode and by fixed electrodes. Single cells were classified according to their spontaneous and evoked response properties as pyramidal cells, granule cells and interneurons. Unit activity, EEG and their interrelations were studied by spectral and spike-triggered averaging methods. Gradual phase-shifts of RSA were observed both in CA1 and the dentate gyrus. Movement-related RSA was correlated with a decrease in firing rate of pyramidal cells and an increase in the firing of both interneurons and granule cells. In the CA1 region pyramidal cells and interneurons fired preferentially on the negative and positive phases of the locally derived RSA, respectively. In the dentate gyrus both granule cells and interneurons discharged mainly on the positive portion of the local RSA waves, about 90 degrees before the CA1 pyramidal cells. Fourier analysis of the spike trains of interneurons and granule cells showed high power at RSA frequency, coherent with the concurrent EEG. Phase relations between discharges of interneurons and RSA remained unchanged following urethane anesthesia. In waking rats, atropine administration resulted in a decreased discharge of interneurons at RSA frequency, and reduced coherence with RSA. Lesions of the septum or the fimbria-fornix abolished RSA and the rhythmic discharges of the interneurons. Isolation of the entorhinal cortex (EC) from its cortical inputs did not change either EEG or neuronal firing. However, in such a preparation atropine completely abolished RSA and related rhythmicity of interneurons. During drinking and immobility but not during walking, sharp waves (SPW) of about 40-100 ms duration appeared in the EEG. SPWs were invariably accompanied by synchronous discharges of several pyramidal cells and interneurons. CA3 pyramidal cells also discharged in synchronous bursts but without local SPWs. Laminar profiles of SPWs and the field potentials evoked by stimulation of Schaffer collaterals were essentially identical. The behavior-dependent occurrence of SPWs was retained following atropine administration, septal lesion or EC isolation but was lost after fimbria-fornix-neocortex lesion or following atropine administration in EC isolated rats. In addition to relations to RSA and SPWs, interneurons were phase-locked to the fast EEG pattern (25-70 Hz). This relationship was preserved following lesions of the septum or the fimbria-fornix complex.(ABSTRACT TRUNCATED AT 400 WORDS)

The correlation between cell firing and hippocampal theta activity was studied with the spike-triggered averaging method in rats anesthetized with urethane. Projection cells in the CA1 region and the dentate gyrus fired with highest probability on the negative phase of the theta waves recorded from the corresponding regions. CA1 interneurons discharged mainly on the positive phase. In the dentate gyrus about half of the interneurons fired on the negative phase, while the remaining half discharged preferentially on the positive phase of the locally derived theta waves. It was suggested that septal theta 'pacemaker' cells directly excite hippocampal interneurons which in turn rhythmically inhibit a large number of projection cells