Journal Article

Noncanonical projections to the hippocampal CA3 regulate spatial learning and memory by modulating the feedforward hippocampal trisynaptic pathway

Nitz, Douglas A., Amalraj, Michelle, Xu, Xiangmin, Lin, Xiaoxiao, Holmes, Todd C., Avila, Brenda, Blanton, Crisylle

2021

Overview

The hippocampal formation (HF) is well documented as having a feedforward, unidirectional circuit organization termed the trisynaptic pathway. This circuit organization exists along the septotemporal axis of the HF, but the circuit connectivity across septal to temporal regions is less well described. The emergence of viral genetic mapping techniques enhances our ability to determine the detailed complexity of HF circuitry. In earlier work, we mapped a subiculum (SUB) back projection to CA1 prompted by the discovery of theta wave back propagation from the SUB to CA1 and CA3. We reason that this circuitry may represent multiple extended noncanonical pathways involving the subicular complex and hippocampal subregions CA1 and CA3. In the present study, multiple retrograde viral tracing approaches produced robust mapping results, which supports this prediction. We find significant noncanonical synaptic inputs to dorsal hippocampal CA3 from ventral CA1 (vCA1), perirhinal cortex (Prh), and the subicular complex. Thus, CA1 inputs to CA3 run opposite the trisynaptic pathway and in a temporal to septal direction. Our retrograde viral tracing results are confirmed by anterograde-directed viral mapping of projections from input mapped regions to hippocampal dorsal CA3 (dCA3). We find that genetic inactivation of the projection of vCA1 to dCA3 impairs object-related spatial learning and memory but does not modulate anxiety-related behaviors. Our data provide a circuit foundation to explore novel functional roles contributed by these noncanonical hippocampal circuit connections to hippocampal circuit dynamics and learning and memory behaviors.

• Publisher: Public Library of Science,Public Library of Science (PLoS)
• Subject: Adenoviruses / Analysis / Animals / Back propagation / Behavioral assessment / Biology and Life Sciences / Brain - physiology / Brain Mapping - methods / CA1 Region, Hippocampal - physiology / CA3 Region, Hippocampal - metabolism / CA3 Region, Hippocampal - physiology / Circuits / Complexity / Cortex (perirhinal) / Cortex (subicular) / Cortex (temporal) / Forecasting / Gene expression / Gene mapping / Genetic aspects / Genomes / Hippocampus / Hippocampus - physiology / Identification and classification / Inactivation / Labeling / Learning / Male / Medicine and Health Sciences / Memory / Memory - physiology / Methods / Mice / Mice, Inbred C57BL / Mice, Transgenic / Neural networks / Neural Pathways - physiology / Neurons / Parahippocampal region / Perirhinal Cortex - physiology / Physiological aspects / Rabies / Research and Analysis Methods / Social Sciences / Spatial discrimination learning / Spatial Learning - physiology / Spatial memory / Subiculum / Temporal lobe / Tracing / Wave propagation