Sublayer‐ and pathway‐specific control of CA1 input processing by VIP interneurons

Vasoactive intestinal peptide-expressing interneurons (VIP-INs) are prominent regulators of inhibition in hippocampal circuits, yet how their dual inhibitory and disinhibitory actions shape pathway-specific processing across pyramidal cell sublayers remains unclear. Here, we combined pathway-specific electrophysiology with optogenetic and chemogenetic manipulations and semi-quantitative immunohistochemistry to examine how VIP-INs control excitation-inhibition balance in superficial (sPCs) and deep (dPCs) CA1 pyramidal neurons. Optogenetic activation revealed direct inhibitory synaptic input from VIP-INs onto both sublayers with comparable strength, despite distinct bouton composition, with cannabinoid receptor-positive VIP terminals enriched in superficial strata and cannabinoid receptor-negative boutons more prominent in deep layers. Chemogenetic silencing of VIP-INs uncovered sublayer- and pathway-specific disinhibitory motifs. During Schaffer collateral activation, VIP-INs preferentially reduced inhibition onto dPCs, indicating selective disinhibitory control of deep pyramidal neurons. In contrast, during temporoammonic input processing, VIP-IN silencing increased excitatory responses in sPCs while enhancing inhibitory drive in dPCs, consistent with differential regulation of distal dendritic integration. Selective optogenetic activation of lateral entorhinal cortex inputs further revealed a shared disinhibitory role of VIP-INs, as silencing increased inhibitory synaptic responses in both sublayers, with a stronger effect in sPCs. Together, these findings identify VIP-INs as pathway- and sublayer-specific modulators that dynamically gate excitation-inhibition balance in CA1, supporting parallel processing of hippocampal inputs rather than uniform network disinhibition. KEY POINTS: Interneurons that release vasoactive intestinal peptide (VIP cells) are often viewed as simple disinhibitory 'gatekeepers', but their role in regulating superficial and deep CA1 pyramidal cell sublayers along the hippocampal radial axis has remained unclear. We show that VIP cells regulate excitation and inhibition through distinct circuit motifs linked to superficial and deep pyramidal neurons rather than through uniform network disinhibition. VIP cells provide direct inhibition to pyramidal neurons while also suppressing other inhibitory cells, revealing a dual mechanism that depends on the active input pathway. During Schaffer collateral input, VIP cells preferentially reduce inhibition of deep neurons, whereas during temporoammonic input, including lateral entorhinal cortex projections, they shape dendritic excitation and inhibition in a sublayer-specific manner. These findings identify VIP cells as pathway- and layer-specific modulators that help the hippocampus balance stable and flexible information processing.