Circadian rhythms in hippocampal responsiveness to perforant path stimulation and their relation to behavioral state

Male rats were chronically implanted with electrodes to record the monosynaptic granule cell response in the dentate gyrus of hippocampus to perforant path stimulation while the animals were unrestrained. Samples of the dentate response were collected every 15 min for up to 14 days under various lighting conditions including constant light (LL) and constant darkness (DD). Circadian rhythms in the slope of evoked population EPSPs were found in all records. EPSP rhythms reached peak values during subjective night. Circadian rhythms in the height of the population spike were found in 8 out of the 12 animals studied. Spike rhythms reached peak values during the subjective day at times when EPSPs were at a daily minimum. Rhythms were confirmed statistically by periodogram. Rhythms followed changes in the LD cycle and persisted under LL or DD. The behavioral state associated with each sample was determined by EEG and video records of 4 animals. Day-night differences in EPSPs were found within the active awake, still alert, slow-wave and paradoxical sleep states. Day-night differences in the spike were detected during the still alert and paradoxical sleep states.