Characteristics of place field formation in the hippocampal CA1 and CA3 regions

Hippocampal pyramidal cells (PCs) express spatially tuned activity (i.e., ‘place cells’ exhibiting ‘place fields’, PF) to support navigation. However, the cellular mechanisms of novel PF formation are elusive. Behavioral time scale synaptic plasticity (BTSP), recently described in mouse CA1PCs, is a putative mechanism of PF formation (Bittner et al., 2015). It starts with a silent PC exhibiting a large Ca2+ plateau in dendrites accompanied by somatic bursting, which initiates the emergence of tuning near the location of induction. Consequently, activity is strongest during formation and lower during subsequent visits. BTSP in CA1PCs has an asymmetric, seconds-long kernel giving rise to a backward shift in tuning after PF formation. The properties of BTSP-formed PFs in CA1 are still incompletely understood, with even less known about BTSP in CA3. Here we recorded CA1PCs and CA3PCs using two-photon Ca2+ imaging in head-fixed Thy1-GCaMP6s mice navigating in two randomly alternating virtual environments, and classified PFs as either newly formed or established. Inspired by recent work (Priestley et al., 2022), we calculated PF formation gain (i.e., relatively strong formation activity) and initial shift (i.e., backward shift in tuning) that capture the proposed properties of BTSP-formed PFs in CA1. We show that newly formed PFs in CA1PCs exhibit higher gain and larger shift than established PFs. In contrast, newly formed PFs in CA3 do not differ from established PFs in terms of shifting. This suggests that BTSP is either less prevalent in CA3, or it manifests differently.