Monitoring norepinephrine release in vivo using next-generation GRAB NE sensors
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Abstract
Norepinephrine (NE) is an essential biogenic monoamine neurotransmitter, yet researches using prototype NE sensors were limited by their low sensitivities. Here, we developed next-generation versions of GPCR activation-based NE sensors (GRAB NE2m and GRAB NE2h ) with a superior response, high sensitivity and selectivity to NE both in vitro and in vivo . Notably, these sensors can detect NE release triggered by either optogenetic or behavioral stimuli in freely moving mice, producing robust signals in the locus coeruleus and hypothalamus. With the development of a novel transgenic mouse line, we recorded both NE release and calcium dynamics with dual-color fiber photometry throughout the sleep-wake cycle; moreover, dual-color mesoscopic imaging revealed cell type‒specific spatiotemporal dynamics of NE and calcium during sensory processing and locomotion. Thus, these new GRAB NE sensors are valuable tools for monitoring the precise spatiotemporal release of NE in vivo , providing new insights into the physiological and pathophysiological roles of NE.
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selectivity
It is very likely that the selectivity claimed here for GRAB-NE2 sensors is not accurate, see our work on GRAB-NE1 sensors: https://www.nature.com/articles/s41592-023-01959-z Titrations should be performed with more datapoints at high concentrations of ligands to ensure a correct curve fit and EC50 estimation.
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Next, to measure cell type‒specific noradrenergic and calcium signaling in response to tactile stimuli, we crossed our dual-NECa mouse line with mice expressing CaMKIIα-Cre or GFAP-Cre to drive the expression of both GRABNE2m and jRGECO1a in excitatory neurons and astrocytes, respectively; we then performed mesoscopic imaging and measured the change in NE and calcium in response to unilateral whisker stimulation (Figure 5E).
It's very exciting to see the transgenic mouse available, and to compare activity between NE and neurons/astrocytes. A question about crossing to the GFAP-Cre line: Did you confirm that expression was limited to astrocytes, or is it a mixed cell-type population? In others' hands, GFAP-Cre has lead to expression in both astrocytes and neurons, and the inducible Ald1h1-CreER line may be a cleaner option.
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