Library

Notes: Summary The participation of physiologically identified hippocampal neurons in spontaneous and hypothalamically induced theta activity was studied in rabbits lightly anaesthetized with urethane. Dentate granule cells were identified by their orthodromic response to perforant path stimulation, CA1 and CA3 pyramids by antidromic activation from the alveus and Schaffer collaterals, respectively, and basket cells by their response to increasing orthodromic activation. The discharges of many hippocampal cells were grossly correlated to the pattern of slow wave activity. Few cells were spontaneously active during irregular slow wave activity. With the appearance of rhythmical slow wave activity of 4–6 Hz, the unit discharges also increased in frequency. Dentate granule cells had the lowest threshold for activation and also a longer duration of the increased discharge frequency, compared to other cell types. There was a characteristic pattern of transition for dentate granule cells and CA1 pyramidal cells from a silent to an active state. The cell discharges paralleled the changes in amplitude, regularity, and frequency of theta slow waves. Large-amplitude, high-frequency theta was correlated with rhythmic burst discharges of up to 2–3 spikes per burst. As theta amplitude and frequency decreased, the number of spikes per burst reduced until only regular single spikes occurred. When theta activity was replaced by irregular slow wave activity, the cell discharges became irregular and sometimes ceased entirely. At high levels of activation, CA1 pyramids often showed clusters of high-frequency discharges with declining amplitude (complex spikes). For each cell a cycle histogram was constructed, placing the discharges in one of 20 bins according to their time relation to the simultaneously recorded slow theta waves. In addition, by Fourier transformation of the cycle histograms, the technique allowed a quantitative description of the degree and type of rhythmicity. The analysis indicated that virtually all dentate granule cells and CA1 pyramidal cells were phaselocked to the negative portion of the theta waves recorded from the corresponding region. The cells differed in their degree of coupling, as expressed by the modulation index of their cyclic histograms. Dentate granule cells had higher modulation indices than the CA1 pyramids. There was a suggestion that basket cells and CA3 cells had smaller modulation indices, but the low number of cells recorded mitigate against any strong conclusions. The results are interpreted as corroborating earlier findings that the dentate granule region and the CA1 pyramidal region are the main generators of hippocampal theta activity. A “size principle” was proposed to explain the role of synaptic depolarizing pressure in the rhythmic activation of hippocampal neurons and the fact that small neurons (dentate granules and CA1 pyramids) were better driven than larger neurons.