Evidence for Long-Timescale Patterns of Synaptic Inputs in CA1 of Awake Behaving Mice

• Published in: The Journal of neuroscience 2018-02, Vol.38 (7), p.1821-1834
• Main Authors: Kolb, Ilya, Talei Franzesi, Giovanni, Wang, Michael, Kodandaramaiah, Suhasa B, Forest, Craig R, Boyden, Edward S, Singer, Annabelle C
• Format: Article
• Language: English
• Subjects: Algorithms; Animals; Behavior, Animal - physiology; CA1 Region, Hippocampal - physiology; Downstream; Firing pattern; Hippocampus; Information processing; Kinetics; Male; Membrane Potentials - physiology; Mice; Mice, Inbred C57BL; Mouse devices; Nerve Net - cytology; Nerve Net - physiology; Neural networks; Neurons; Neurons - physiology; Patch-Clamp Techniques; Synapses - physiology; Synaptic Transmission - physiology; Theta rhythms; Time; Variations; Wakefulness - physiology
• ISSN: 0270-6474; 1529-2401
• DOI: 10.1523/JNEUROSCI.1519-17.2017

Repeated sequences of neural activity are a pervasive feature of neural networks and In the hippocampus, sequential firing of many neurons over periods of 100-300 ms reoccurs 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 eight of nine recordings, we discovered long (900 ms) reoccurring subthreshold fluctuations or "repeats." Repeats generally were high-amplitude, nonoscillatory events reoccurring with 10 ms 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, 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 nonstochastically, 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. We found long (≥900 ms), 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 ms 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.