Ultrafast neuronal imaging of dopamine dynamics with designed genetically encoded sensors

JUNE 29, 2018

Imaging dopamine release in the brain

Neuromodulator release alters the function of target circuits in poorly known ways. An essential step to address this knowledge gap is to measure the dynamics of neuromodulatory signals while simultaneously manipulating the elements of the target circuit during behavior. Patriarchi et al. developed fluorescent protein–based dopamine indicators to visualize spatial and temporal release of dopamine directly with high fidelity and resolution. In the cortex, two-photon imaging with these indicators was used to map dopamine activity at cellular resolution.

Science, this issue p. eaat4422

Structured Abstract

INTRODUCTION

Neuromodulators, such as dopamine, norepinephrine, or serotonin, exert powerful control over neural circuit dynamics that give rise to diverse neural function and behavior. Altered neuromodulator signaling is a key feature of virtually all human neurological and psychiatric disorders, including Parkinson’s disease, schizophrenia, depression, and addiction. Hence, drugs that mimic or block neuromodulators have become important components in the treatment of these disorders. Much work is devoted to determining exactly what information neuromodulatory neurons represent, but very little is known about how these signals alter the function of their target circuits.

RATIONALE

To address this problem, scientists need to be able to monitor the spatiotemporal dynamics of neuromodulatory signals in target circuits while also measuring and manipulating the elements of the circuit during natural behavior. However, existing technologies for detecting neuromodulators, such as analytic chemical or cell-based approaches, have limited spatial or temporal resolution, thus preventing high-resolution measurement of neuromodulator release in behaving animals. We recognized the potential of combining genetically encoded indicators based on fluorescent proteins with modern microscopy to support direct and specific measurement of diverse types of neuromodulators with needed spatial and temporal resolution.

RESULTS

We report the development and validation of dLight1, a novel suite of intensity-based genetically encoded dopamine indicators that enables ultrafast optical recording of neuronal dopamine dynamics in behaving mice. dLight1 works by directly coupling the conformational changes of an inert human dopamine receptor to changes in the fluorescence intensity of a circularly permuted green fluorescent protein. The high sensitivity and temporal resolution of dLight1 permit robust detection of physiologically or behaviorally relevant dopamine transients. In acute striatum slices, dLight1 faithfully and directly reports the time course and concentration of local dopamine release evoked by electrical stimuli, as well as drug-dependent modulatory effects on dopamine release. In freely moving mice, dLight1 permits deep-brain recording of dopamine dynamics simultaneously with optogenetic stimulation or calcium imaging of local neuronal activity. We were also able to use dLight1 to chronically measure learning-induced dynamic changes within dopamine transients in the nucleus accumbens at subsecond resolution. Finally, we show that two-photon imaging with dLight1 revealed a high-resolution (cellular level) dopamine transient map of the cortex showing spatially distributed, functionally heterogeneous dopamine signals during a visuomotor learning task.

CONCLUSION

To overcome the major barriers of current methods and permit high-resolution imaging of dopamine dynamics in the mammalian brain, we developed and applied a new class of genetically encoded indicators. This work validates our sensor design platform, which could also be applied to developing sensors for other neuromodulators, including norepinephrine, serotonin, melatonin, and opioid neuropeptides. In combination with calcium imaging and optogenetics, our sensors are well poised to permit direct functional analysis of how the spatiotemporal coding of neuromodulatory signaling mediates the plasticity and function of target circuits.

High-resolution dopamine imaging in vivo.

dLight1 permits robust detection of physiologically and behaviorally relevant dopamine (DA) transients with high sensitivity and spatiotemporal resolution, including dynamic learning-induced dopamine changes in the nucleus accumbens (bottom) and task-specific dopamine transients in the cortex (top).

Neuromodulatory systems exert profound influences on brain function. Understanding how these systems modify the operating mode of target circuits requires spatiotemporally precise measurement of neuromodulator release. We developed dLight1, an intensity-based genetically encoded dopamine indicator, to enable optical recording of dopamine dynamics with high spatiotemporal resolution in behaving mice. We demonstrated the utility of dLight1 by imaging dopamine dynamics simultaneously with pharmacological manipulation, electrophysiological or optogenetic stimulation, and calcium imaging of local neuronal activity. dLight1 enabled chronic tracking of learning-induced changes in millisecond dopamine transients in mouse striatum. Further, we used dLight1 to image spatially distinct, functionally heterogeneous dopamine transients relevant to learning and motor control in mouse cortex. We also validated our sensor design platform for developing norepinephrine, serotonin, melatonin, and opioid neuropeptide indicators.

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