Inferring norepinephrine dynamics from partial observations reveals the temporal structure of elevations during...

Area of Science:

• Neuroscience
• Biomedical Engineering
• Signal Processing

Background:

• Hemodynamic artifacts complicate two-photon fluorescence imaging of genetically encoded reporters, especially when biological signals and vascular dynamics occur on similar timescales.
• Existing correction methods like isobestic recording or repeated experiments are often impractical for real-time neuromodulator studies.
• Accurate measurement of neuromodulator dynamics, such as norepinephrine (NE), is crucial for understanding brain function.

Purpose of the Study:

• To introduce a flexible, tiered framework for correcting hemodynamic artifacts in norepinephrine (NE) imaging.
• To enable accurate inference of NE dynamics across different recording scenarios, from ideal to data-limited.
• To investigate the temporal relationship between locus coeruleus (LC) noradrenergic axonal activity and extracellular NE release in the cortex.

Main Methods:

• Verification of dual-channel recording with an inert fluorescent reporter for direct hemodynamic correction.
• Development and application of a Long Short-Term Memory (LSTM)-based model for post-hoc artifact removal using NE signals and behavioral data.
• Recovery of NE dynamics features from behavioral variables alone when fluorescence recordings are unavailable.
• Simultaneous two-photon imaging of LC noradrenergic axons and extracellular NE, coupled with behavioral monitoring.

Main Results:

• Dual-channel recording effectively corrects hemodynamic artifacts in real-time.
• The LSTM model successfully predicts and removes hemodynamic contributions from NE signals.
• Key features of NE dynamics can be estimated from behavioral data alone, offering a proxy for neuromodulatory state.
• Cortical NE signals exhibit graded responses to behavioral intensity, correlating with locomotion and pupil dilation.
• Extracellular NE levels integrate LC output over time, peaking later than axonal activity and remaining elevated post-activity.

Conclusions:

• Accurate hemodynamic correction is essential for reliable interpretation of norepinephrine dynamics.
• The developed tiered framework provides robust methods for NE signal analysis across diverse experimental conditions.
• Extracellular NE signaling reflects a temporal integration of locus coeruleus output, rather than instantaneous firing.
• These findings offer a clearer understanding of the temporal structure of cortical norepinephrine signaling and its relation to behavior.