Circadian variations in the amplitude of field potentials elicited by stimulation of perforant path axons and recorded from the dentate gyrus were observed in rats with chronically implanted electrodes entrained to a 12:12 h (07.00 On, 19.00 Off) light-dark cycle. Population spike input-output curves, constructed daily at five 2 h intervals (14.00, 16.00, 18.00, 20.00, 22.00), showed maximum spike amplitude at 16.00 during the light period and a decrease to minimum amplitude at 20.00 during the dark period of the cycle in each animal tested. Decreases in spike amplitude at 20.00 relative to 16.00 were also observed (1) in the laminar profile of the granule cell layer field potential; (2) on selected days when the light period was extended from 19.00 to 21.00; and (3) following bilateral adrenalectomy. In addition the circadian variation in population spike amplitude was shown not to be related to a change in behavioural state of the animal, and was not accompanied by significant variations in the dendritically located perforant path synaptic current. It is concluded that the observed changes in population spike amplitude reflect a true circadian entrainment of granule cell sensitivity to perforant path synaptic input which is not a function of the presence or absence of light, the level of behavioural arousal, or circulating levels of corticosterone. The results are discussed in relation to other reports in terms of firstly, the need for control over important physiological and behavioural parameters in assessing circadian variations in granule cell response to perforant path input and, secondly, possible mechanisms which might be responsible for such circadian variations.
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