Optogenetic frequency scrambling of hippocampal theta oscillations dissociates working memory retrieval from hippocampal spatiotemporal codes

Abstract

The precise temporal coordination of activity in the brain is thought to be fundamental for memory encoding and retrieval. Pacemaker GABAergic neurons in the medial septum (MS) provide the largest source of innervation to the hippocampus and play a major role in controlling hippocampal theta (~8 Hz) oscillations. While pharmacological inhibition of the MS is known to disrupt memory, the exact role of MS inhibitory neurons and theta frequency rhythms in hippocampal representations and memory is not fully understood. Here, we dissociate the role of theta rhythms in spatiotemporal coding and memory using an all-optical interrogation and recording approach in freely behaving mice. We propose a novel paradigm to dissociate encoding of space, time and distance in freely moving mice and apply complementary optogenetic stimulation paradigms of MS GABAergic neurons to either pace or abolish theta altogether while recording large hippocampal cell assemblies using calcium imaging conjointly. We first show that optogenetic frequency scrambling of MS GABAergic neuron activity abolished theta rhythms and modulated the activity of a subpopulation of CA1 neurons. Such stimulation led to decreased memory retrieval in both a delayed non-match to sample task, a novel place object recognition task, as well as spontaneous cue-guided linear alternation. Strikingly, scrambled stimulations were not associated with disrupted encoding of place, time, distance, or multiplexed information. Our study suggests that theta rhythms play a specific and essential role in supporting working memory retrieval and maintenance while not being necessary for hippocampal spatiotemporal codes.