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A new paper from the Akerman group, published in the journal Nature Communications this week, explains the development of a new ‘ORCHID’ biosensor to measure the driving force acting on inhibitory receptors in the brain.

ORCHID: all-Optical Reporting of CHloride Ion Driving force
(A) The novel approach combines a light-activated chloride channel with a genetically-encoded fluorescent voltage reporter. (B) Activation of the chloride channel with blue light drives the membrane potential (Vm; black trace) towards the reversal potential for chloride (ECl; green dashed line). The resulting Vm shift is measured via the genetically-encoded fluorescent voltage reporter, providing an estimate of the driving force for chloride (DFCl) that is effective across different ECl and Vm values
ORCHID: all-Optical Reporting of CHloride Ion Driving force (A) The novel approach combines a light-activated chloride channel with a genetically-encoded fluorescent voltage reporter. (B) Activation of the chloride channel with blue light drives the membrane potential (Vm; black trace) towards the reversal potential for chloride (ECl; green dashed line). The resulting Vm shift is measured via the genetically-encoded fluorescent voltage reporter, providing an estimate of the driving force for chloride (DFCl) that is effective across different ECl and Vm values

Ionic driving forces generate the transmembrane ion fluxes through membrane receptors, channels and transporter proteins, which are essential for cellular functions and represent targets for pharmacological treatments of disease. Driving forces depend upon both the ion’s concentration gradient across the membrane and the voltage difference across the membrane. Despite their importance in cell biology, the only methods available to measure ionic driving forces have been intracellular electrophysiological recording techniques. Unfortunately, these techniques are resource-intense, difficult to perform in small cellular compartments, and often disturb the underlying ion gradients that they are intended to measure. To address these limitations and open up new areas of research, a collaboration between colleagues at the University of Cape Town, Janelia Farm, and the Akerman Group in the Department of Pharmacology, has developed the first all-optical biosensor for measuring ionic driving forces.

The new biosensor is named “ORCHID”, which stands for all-Optical Reporting of CHloride Ion Driving force. It is shown that ORCHID is able to accurately measure the driving force acting upon inhibitory synaptic receptors in the brain, which are mediated by chloride permeable GABA-A receptors. ORCHID’s all-optical design avoids the aforementioned issues associated with electrophysiological recordings and the paper demonstrates ORCHID's ability to provide accurate, high-throughput measurements of resting and dynamic driving forces for GABA-A receptors in genetically-targeted cell types over a range of timescales. ORCHID is shown to confirm theoretical predictions about the biophysical mechanisms that establish driving forces for GABA-A receptors, reveals novel differences in chloride driving forces between neurons and astrocytes, and provides the first in vivo estimates of intact chloride driving forces. This new study therefore extends our understanding of inhibitory synaptic transmission and demonstrates the potential for all-optical methods to assess ionic driving forces.

The paper is first authored by Josh Selfe and is published this week in Nature Communications. The full paper is entitled “All-optical reporting of inhibitory receptor driving force in the nervous system” and is available to read here:

https://www.nature.com/articles/s41467-024-53074-y