Cholinergic dynamics in the septo-hippocampal system provide phasic multiplexed signals for spatial novelty and correlate with behavioral states.

In the hippocampal formation, cholinergic modulation from the medial septum/diagonal band of Broca (MSDB) is known to correlate with the speed of an animal's movements at sub-second timescales and also supports spatial memory formation. Yet, the extent to which sub-second cholinergic dynamics, if at all, align with transient behavioral and cognitive states supporting the encoding of novel spatial information remains unknown. In this study, we used fiber photometry to record the temporal dynamics in the population activity of septo-hippocampal cholinergic neurons at sub-second resolution during a hippocampus-dependent object location memory task using ChAT-Cre mice of both sexes. Using a linear mixed-effects model, we quantified the extent to which cholinergic dynamics were explained by changes in movement speed, behavioral states such as locomotion, grooming, and rearing, and hippocampus-dependent cognitive states such as recognizing a novel location of a familiar object. The data show that cholinergic dynamics contain a multiplexed code of fast and slow signals i) coding for the logarithm of movement speed at sub-second timescales, ii) providing a phasic spatial novelty signal during the brief periods of exploring a novel object location, and iii) coding for recency of environmental change at a seconds-long timescale. Furthermore, behavioral event-related phasic cholinergic activity demonstrates that fast cholinergic transients correlate with a switch in cognitive and behavioral states. These findings enhance understanding of the mechanisms by which cholinergic modulation contributes to the coding of movement speed and encoding of novel spatial information. Acetylcholine is well known as a neuromodulator of cognitive functions and behavior, and computational models suggest an important role in the encoding of new memories. However, whether cholinergic dynamics are fast enough to serve as a spatial novelty signal is unknown. Here, we demonstrate that cholinergic signaling in the septo-hippocampal circuitry of mice exhibits multiple timescales of activity, where fast signals reflect the detection of novel object locations, encode the logarithm of movement speed, and correlate with behavioral state transitions. At longer timescales, cholinergic transients encode recency of environmental change. These findings provide important insights into the mechanisms by which acetylcholine contributes to encoding and retrieval dynamics and the acquisition of spatial memories during exploratory behavior and memory-guided navigation.