Subregion-Specific Input Organization of Prefrontal-Projecting Basal Forebrain Cholinergic Neurons and Weakened Striatal–NBM Inhibitory Transmission in 5xFAD mice

Basal forebrain cholinergic neurons regulate cortical activity and cognition and are vulnerable in Alzheimer’s disease (AD). However, the upstream circuits controlling projection-defined basal forebrain cholinergic populations remain incompletely understood. Here, we used projection-specific rabies-mediated monosynaptic tracing to map whole-brain inputs to medial prefrontal cortex (mPFC)-projecting cholinergic neurons in the nucleus basalis of Meynert (NBM) and horizontal limb of the diagonal band of Broca (HDB). mPFC-projecting NBM and HDB cholinergic neurons received broad but distinct input patterns. NBM cholinergic neurons received prominent striatal input, including input from D1-expressing medium spiny neurons, whereas HDB cholinergic neurons showed proportionally weaker striatal input and broader non-striatal contributions. Optogenetic electrophysiology confirmed that striatal inputs formed monosynaptic GABAergic inhibitory synapses onto NBM cholinergic neurons. This inhibitory transmission was weakened in 5xFAD mice, indicating impairment of a striatal–NBM inhibitory circuit in an AD mouse model. Together, these findings reveal subregion-specific input organization of mPFC-projecting basal forebrain cholinergic neurons and identify a vulnerable striatal–NBM circuit in AD.