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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.

Notes: Summary 1. The spatial organization of the efferent projections of CA1 and CA3 hippocampal pyramids has been studied using recordings of fibre volleys, orthodromic and antidromic population spikes and synaptic field potentials, following microelectrode stimulation of the fimbria, CA1 alveus, or subiculum. 2. Only CA3 pyramidal cells were found to send their axons into the fimbria. In the septal two thirds of the hippocampus the CA1 pyramidal cells project in a caudal direction to the pyramidal part of the subiculum. The temporal third was not explored for technical reasons. 3. Fimbrial fibres are arranged in a strictly parallel fashion, the rostro-medial CA3 cells distributing their axons near to the hippocampus, while those located at the temporal extreme distribute their axons to the outer edge of the fimbria. The organization of the Schaffer collaterals and the projections of the CA1 cells consisted of parallel lamellae, oriented nearly transversely to the longitudinal axis of the hippocampus in rabbits (more obliquely in cats). The findings indicate that CA3 cell discharge via the Schaffer collaterals represents a major input driving the CA1 cells. 4. The dichotomy with regard to hippocampal output suggests that the CA3 and CA1 regions of the hippocampus may subserve different functions, thus probably participating differentially in various behavioural situations. 5. This organization makes it possible to study the behaviour of animals with selective and regional de-efferentation of the CA3 or of the CA1 regions by making discrete lesions in the fimbria and alveus, respectively. Alternatively, recording the fibre volley from the fimbria may provide a useful monitor of the output of the CA3 region during different behaviours.

Notes: Abstract Alcaptonuria is a rare, hereditary disorder of amino acid metabolism, secondary to lack of homogentisic acid oxydase. As a consequence, there is ex accumulation of homogentisic acid, which is excreted in the urine and deposited in the connective tissues. This deposition results in ochronotic pigmentation and arthropathy, of which some characteristic radiological findings are demonstrated.