2025 Theses Doctoral

Illuminating Subicular Dynamics through Multiphoton Holography

O'Neil, Darik Andrew

The subiculum has a poorly-defined, but necessary, role in the neurobiology of memory, despite a rich tradition of research investigating hippocampal contributions to memory. While traditional models of memory retrieval highlight the reactivation of a specific engram formed during learning, emerging evidence suggests the subiculum is necessary for the retrieval of trace fear memories without being necessary for their initial encoding.

This presents a central paradox: how can a brain region be critical for retrieving a memory it wasn't necessary to form? Given the subiculum’s unique, recurrent architecture, its diverse array of projections, and a broad implication in the etiology of epilepsy, we hypothesized that the subiculum may be facilitating engram reactivation through the amplification of CA1 output. To test this hypothesis, we conducted multiphoton holographic calcium imaging of the subiculum and subicular-hippocampal border region during an ethologically-grounded, trace-fear conditioning task. In this task, mice learned to associate one of two tones (CS+, CS-) with an aversive stimulus despite being separated by a 10-second stimulus-free trace period. Conditioned fear expression took the form of burrowing: head-fixed mice retract a lightweight “burrow” that slides along a frictionless rail.

Surprisingly, we observed robust activity in response to both the CS+ and CS-. We observed rapid, cue-onset evoked responses specific to both the CS+ and CS-. In contrast, subicular activity exhibited reciprocal changes at cue-offset. Unexpectedly, the offset of CS- tone triggered an unexpected, immediate burst of activity. Meanwhile, we observed a substantial reduction of activity in specific neurons during CS+ trials, which peaked near the anticipated US delivery. Our result suggests that the subiculum behaves less like a conveyor belt for a canonical replay of a fixed engram and more like a signal processing hub that projects dense, high-dimensional information from which downstream regions can flexibly extract information without requiring substantial network reconfiguration.