Near-Infrared Carbon Nanotube Tracking Reveals the Nanoscale Extracellular Space around Synapses

Abstract

We provide evidence of a local synaptic nanoenvironment in the brain extracellular space (ECS) lying within 500 nm of postsynaptic densities. To reveal this brain compartment, we developed a correlative imaging approach dedicated to thick brain tissue based on single-particle tracking of individual fluorescent single wall carbon nanotubes (SWCNTs) in living samples and on speckle-based HiLo microscopy of synaptic labels. We show that the extracellular space around synapses bears specific properties in terms of morphology at the nanoscale and inner diffusivity. We finally show that the ECS juxta-synaptic region changes its diffusion parameters in response to neuronal activity, indicating that this nanoenvironment might play a role in the regulation of brain activity.

Local diffusion in the extracellular space of the brain

“…Diffusion permeability inside and outside of the synaptic milieu has been a topic of great interest for years, due to its implications for synaptic transmission and neurotransmitter spillover and uptake (Barbour, 2001;Rusakov et al, 2011). More recently, nanoscopic approaches, such as single-molecule tracking, have been used to provide maps of ECS diffusivity in local environments, for example in the vicinity of the synapse (Paviolo et al, 2022). Here, researchers determined that in the immediate area around the synaptic cleft, within 500 nm of the postsynaptic density (termed "juxta-synaptic" space), molecules diffuse 10 times faster than outside this space.…”

Near‐Infrared Fluorescence Lifetime Imaging of Biomolecules with Carbon Nanotubes**

“…Detection of dopamine and other neurotransmitters with a high spatiotemporal resolution is therefore a major but challenging goal to gain deeper insights in intercellular communication. In contrast to other approaches such as electrochemical or genetically encoded sensors [50–58] imaging of neurotransmitters with SWCNT‐based sensors offers a very high spatial and temporal resolution [42, 46, 59–61] …”

“…In contrast to other approaches such as electrochemical or genetically encoded sensors [50][51][52][53][54][55][56][57][58] imaging of neurotransmitters with SWCNT-based sensors offers a very high spatial and temporal resolution. [42,46,[59][60][61] First surface chemistry screening approaches showed that SWCNTs coated with single stranded (ss)DNA respond to dopamine and other catecholamines by a reversible increase in fluorescence intensity. [43] Molecular dynamics simulations revealed that an interaction between polar groups from dopamine and the phosphate backbone of the DNA moves the backbone closer to the SWCNT resulting in an altered electrostatic potential at the SWCNT surface.…”

Nahinfrarot Fluoreszenz‐Lebensdauer Mikroskopie von Biomolekülen mit Kohlenstoffnanoröhren**

“…Der Nachweis von Dopamin und anderen Neurotransmittern mit einer hohen räumlichen und zeitlichen Auflösung ist daher ein wichtiges, aber anspruchsvolles Ziel, um tiefere Einblicke in die interzelluläre Kommunikation zu gewinnen. Im Gegensatz zu anderen Methoden wie elektrochemischen oder genetisch kodierten Sensoren [50–58] bietet die Bildgebung von Neurotransmittern mit SWCNT‐basierten Sensoren eine sehr hohe räumliche und zeitliche Auflösung [42, 46, 59–61] …”

“…Im Gegensatz zu anderen Methoden wie elektrochemischen oder genetisch kodierten Sensoren [50][51][52][53][54][55][56][57][58] bietet die Bildgebung von Neurotransmittern mit SWCNT-basierten Sensoren eine sehr hohe räumliche und zeitliche Auflösung. [42,46,[59][60][61] Erste Screening-Ansätze zur Oberflächenchemie von SWCNTs zeigten, dass SWCNTs, die mit einzelsträngiger (single-stranded, ss)-DNA beschichtet sind, auf Dopamin und andere Katecholamine mit einem reversiblen Anstieg der Fluoreszenzintensität reagieren. [43] Moleküldynamik-Simulationen ergaben, dass die Wechselwirkung zwischen den polaren Gruppen des Dopamins und dem Phosphatrückgrat der DNA das Rückgrat näher an die Nanoröhre heranbringt, was zu einem veränderten elektrostatischen Potenzial an der SWCNT-Oberfläche führt.…”