New Technique Enables Simultaneous Measurement of Neuronal Signals in Awake Animals

Researchers at Kyushu University have developed an innovative, non-invasive technique to measure two crucial signals—membrane voltage and intracellular calcium levels—simultaneously in the neurons of awake animals. Published in Communications Biology on September 16, 2024, the study provides new insights into how these signals encode different types of information related to sensory stimuli.

Neurons, the brain's primary cells, transmit information through electrical signals. When a neuron receives a stimulus, changes in membrane voltage (the electrical charge across the neuron's membrane) trigger the neuron to activate, sending an electrical signal along its length. These voltage changes also affect the levels of intracellular calcium within the neuron.

Traditionally, membrane voltage was measured using invasive electrode-based methods, while calcium activity was monitored using fluorescent proteins sensitive to calcium ions, offering an indirect indication of neuron activity. However, these methods have typically been used separately, making it difficult to study how these two signals interact in real-time.

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Recent advances in fluorescent proteins that respond to membrane voltage have now made it possible to simultaneously measure both calcium and membrane voltage signals in neurons.

"Simultaneous measurement of intracellular calcium ions and membrane voltage can help us to understand how neurons encode information for sensory processing in neuronal circuits," explained senior author Professor Takeshi Ishihara from Kyushu University's Faculty of Science.

In collaboration with Kyushu Institute of Technology’s Faculty of Computer Science and Systems Engineering, Ishihara and his team developed a technique that allows for high-speed simultaneous measurement of both signals in living animal neurons. By capturing 250 frames per second under the microscope and employing advanced image processing, the team detected rapid fluctuations in the fluorescent intensity of both calcium and membrane voltage sensors.

Focusing on olfactory neurons in Caenorhabditis elegans—a commonly used model organism—the researchers discovered that exposure to odors caused both membrane voltage and intracellular calcium levels to change. Crucially, they found that these two signals encode separate information: membrane voltage indicated the presence of an odor, while intracellular calcium levels corresponded to the odor's concentration. This simultaneous measurement provided a clearer understanding of how the brain processes and differentiates sensory input.

The team also identified two ion channels essential for the membrane voltage changes triggered by sensory stimulation. Additionally, they discovered that a protein called ODR-3, which is involved in neuronal signal transmission, plays a key role in stabilizing membrane voltage, preventing unwanted neuron firing, and regulating responses to odors.

Looking ahead, this method could be applied to more complex animals and different types of neurons, potentially offering valuable insights into information coding in neural circuits.

Ishihara concluded, "These high-speed simultaneous measurements reveal the distinct functions of membrane voltage and intracellular calcium ion signals induced by sensory stimuli. This research can help improve our understanding of sensory processing in both simple model organisms and higher organisms."