In the nervous system, synapses are where the action is. There, across a narrow gap between adjacent cells, neurons talk to one another through the dynamic exchange of chemical and electrical signals. Molecules known as neurotransmitters and neuromodulators induce or inhibit action potentials — spikes in voltage across neuronal cell membranes that trigger the release of other molecules. This cross-talk ultimately enables the production of emotions, thoughts, behaviours — everything that makes the brain what it is.
A new way to capture the brain’s electrical symphony
To decode these conversations, researchers have relied on various tools. These include electrophysiology, in which electrodes are inserted into the brain or individual cells (in the case of patch-clamp recording) to measure the voltage changes linked to action potentials; microdialysis, in which some of the fluid surrounding neurons is extracted and analysed; and fast-scan cyclic voltammetry, which uses implanted electrodes to detect certain signalling molecules.
But these methods have limitations. For instance, electrophysiology can precisely measure action potentials, but scientists can’t pinpoint the exact neurotransmitters or neuromodulators (collectively called neurochemicals) that drive them. Microdialysis can identify specific molecules, but it lacks the spatial and temporal resolution to pinpoint exactly when and where these neurochemicals are released, and voltammetry often struggles to distinguish molecules that are similar to each other.
The development of genetically encoded sensors over the past two decades has offered a way for neuroscientists to circumvent these issues. Such sensors were initially developed to identify action potentials in cells by tracking changes in calcium ions, but in recent years, researchers have expanded the toolbox to detect key neurochemicals.
The next generation of these sensors now enables scientists to ask questions such as: how much of a specific neurochemical is released in response to the firing of an action potential? How many action potentials are required to release a given neurochemical? And how long does that molecule stick around? “All these kinds of questions, for the vast majority of molecules — we’re talking about dopamine, serotonin, acetylcholine and many others — we know virtually nothing about,” says Nicolas Tritsch, a neuroscientist at McGill University in Montreal, Canada. “This new class of genetically encoded sensors has really opened up this world.”
Lighting up cells
The revolution in genetically encoded neurochemical sensing began with calcium. Action potentials activate specialized channel proteins on the neuron’s cell membrane and allow calcium to enter, changing the calcium concentration. By fusing the calcium-binding protein calmodulin with a fluorescent protein and genetically targeting the hybrid molecule to specific populations of cells, researchers have developed sensors that light up in response to calcium fluctuations — a proxy for neuronal activity.
One popular variety of genetically encoded calcium indicators, known as GCaMPs, has been around since 2001. Researchers have since optimized the sensors’ speed and sensitivity — and they are now mainstays of neuroscience research. “They’re so ubiquitous that most papers that use GCaMPs stopped citing the relevant papers,” says Loren Looger, a neuroscientist at the University of California, San Diego (UCSD), whose team has been developing these sensors.
Genetically encoded sensors offer several advantages (see ‘Tracking neurochemicals’). Researchers can express them at specific times in particular cells, then pair them with techniques such as optogenetics to cause them to fire in response to light. But they also require genetic manipulation and might change the biology of cells in unexpected ways.
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