Synthetic nanosensors for imaging neuromodulators

Abstract

Neuromodulation plays a critical role in regulating brain function and its dysregulation is implicated in the pathogenesis of numerous neurological and psychiatric disorders. However, only in the last few years have optical tools become available to probe the spatial and temporal profiles of neuromodulator signaling, including dopamine, with the requisite resolution to uncover mechanisms of neuromodulation. In this review, we summarize the current state of synthetic nanomaterial-based optical nanosensors for monitoring neurotransmission with high spatial and temporal resolution. Specifically, we highlight how synthetic nanosensors can elucidate the spatial distribution of neuromodulator release sites and report the temporal dynamics and spatial diffusion of neuromodulator release. Next, we outline advantages and limitations of currently available nanosensors and their recent application to imaging endogenous dopamine release in brain tissue. Finally, we discuss strategies to improve nanosensors for in vivo use, with implications for translational applications.

Keywords: Dopamine; Near infrared imaging; Neuromodulation; Single-walled carbon nanotubes (SWNTs).

Fig. 1.. SWNT-based catecholamine nanosensors.

(a) Schematic depicting single-walled carbon nanotubes and amphiphilic polymers self-assembling into a dopamine-sensitive nIR nanosensor after sonication. (b) nIR fluorescence emission from nIRCat nanosensors before and after addition of 0.1 mM dopamine in vitro shows a strong turn-on response. (c) Titration curves of fluorescence response of nIRCat nanosensors to dopamine and norepinephrine show their dynamic range falls within the range relevant to endogenous release. (d) Nanosensors can distribute in the extracellular space and report changes in dopamine concentrations around synapses such as those in the striatum. DA: dopamine, MSN: GABAergic medium spiny neuron, Glu: glutamatergic neuron. (e) Representative field of view in striatal brain slice showing nIRCat fluorescence modulation (ΔF/F) in response to a single electrical pulse (0.5 mA, 3 ms). Because the striatum lacks other catecholaminergic innervation (e.g. norepinephrine), nIRCat fluorescence modulation reports changes in extracellular dopamine in response to electrical stimulation. Data in (b) and (c) were previously published in Beyene et al. (2018). Mean and standard deviation error bands (b) or bars (c) from n = 3 independent measurements are shown with data (circles) fit with Hill equation (solid line) in (c). Data in (e) were previously published in Beyene et al. (2019).

Fig. 2.. Imaging acute brain slice and cell cultures using SWNT-based nanosensors.

(a) Schematic depicting nIRCat nanosensors for nIR imaging of dopamine release from acute striatal brain slice. After incubation, an image of nIRCat’s nIR fluorescence response to evoked dopamine release is captured with a microscope objective. The right panel shows a single frame from a time series gathered in the striatum showing the entire field of view, overlaid with regions of interest (ROIs) identified using per-pixel ΔF/F stack projections of nIRCat fluorescence modulation. (b) Fluorescence imaging of nIRCat captures the pharmacological effects of DAT blocker nomifensine on dopamine reuptake and clearance. Data are presented as field of view mean traces with standard deviation error bands; n = 3 biological replicates. (c) Frequency histogram of ROI sizes depicted in (a), revealing a median ROI size of 2 μm ‘hotspots’ of dopamine turn-on response. (d) Inset: ΔF/F of individual ROIs before and after application of 1 μM sulpiride. Scatter plot shows post-to-pre sulpiride response (ΔF/F) ratio vs. pre-drug ΔF/F for individual ROIs shown in inset. (e) Schematic depicting differentiated PC12 cells cultured on top of an optical nanosensor microarray for measuring dopamine efflux. (f) A nIR image showing the extracellular dopamine concentrations around the perimeter of a cell when vesicle release was triggered by potassium at t₀. (g,h) Identification of regional hotspots were mapped onto the perimeter of each cell to compare magnitude of dopamine release to cell morphology and local membrane curvature. Data presented in (a-d) previously published in Beyene et al. (2019); data from (e-h) previously published in Kruss et al. (2017).