Repeated sequences of neural activity are a pervasive feature of neural networks in vivo and in vitro. In the hippocampus, sequential firing of many neurons over 100-300 ms periods re-occurs during behavior and during periods of quiescence. However, it is not known whether the hippocampus produces longer sequences of activity or whether such sequences are restricted to specific network states. Furthermore, whether long repeated patterns of activity are transmitted to single cells downstream is unclear.
To answer these questions, we recorded intracellularly from hippocampal CA1 of awake, behaving male mice, to examine both subthreshold activity and spiking output in single neurons. In 8/9 recordings, we discovered long (900 ms) reoccurring subthreshold fluctuations or “repeats”. Repeats generally were high-amplitude, non-oscillatory events reoccurring with 10 millisecond precision. Using statistical controls, we determined that repeats occurred more often than would be expected from unstructured network activity (e.g. by chance). Most spikes occurred during a repeat and when a repeat contained a spike, the spike reoccurred with precision on the order of 20 ms or less, showing that long repeated patterns of subthreshold activity are strongly connected to spike output. Unexpectedly, we found that repeats occurred independently of classic hippocampal network states like theta oscillations or sharp-wave ripples. Together, these results reveal surprisingly long patterns of repeated activity in the hippocampal network that occur non-stochastically, are transmitted to single downstream neurons, and strongly shape their output. This suggests that the timescale of information transmission in the hippocampal network is much longer than previously thought.
SIGNIFICANCE STATEMENT
We found long (900 ms or more) repeated subthreshold patterns of activity in CA1 of awake, behaving mice. These repeated patterns (‘repeats’) occurred more often than expected by chance and with 10 millisecond precision. Most spikes occurred within repeats and reoccurred with a precision on the order of 20 ms. Surprisingly, there was no correlation between repeat occurrence and classical network states such as theta oscillations and sharp-wave ripples. These results provide strong evidence that long patterns of activity are repeated and transmitted to downstream neurons, suggesting that the hippocampus can generate longer sequences of repeated activity than previously thought.