Subtype-resolved norepinephrine imaging with an engineered α₁D-adrenergic receptor indicator

Genetically encoded fluorescent indicators enable optical measurement of neuromodulator dynamics, but available norepinephrine (NE) sensors draw from a limited subset of adrenergic receptor scaffolds. Here, we developed Alpha1DLight, an NE indicator engineered from the human α₁D-adrenergic receptor. In HEK293T cells, it showed large NE-evoked fluorescence responses (ΔF/F₀ up to 612%), nanomolar apparent affinity for NE (EC₅₀ = 76 nM), rapid activation (τ_ON = 193 ms), slow deactivation (τ_OFF = 6.75 s), and no coupling to downstream signaling. Pharmacological benchmarking against published α₁D-AR reference data indicated preservation of key α₁D-like ligand-response properties, supporting receptor-informed optical pharmacology. Alpha1DLight also reported reversible NE signals in cultured neurons, cerebellar slices, and behaving mice during fiber photometry, including endogenous release associated with arousal-related behavior and optogenetic activation of locus coeruleus projections. Together, these results establish an α₁D-derived, subtype-informed approach to NE imaging that complements existing sensors and is well suited for probing sustained noradrenergic engagement.