Advances in Fluorescent Probes to Illuminate Chloride Dynamics in the Brain

Abstract Chloride ions (Cl−) play an important role in regulating neuronal excitability, inhibitory neurotransmission, cell volume control, and ionic homeostasis in the brain. Precise control of the intracellular chloride concentration is essential for normal neuronal and glial function, while its dysregulation has been linked to neurological disorders such as epilepsy, neurodevelopmental disorders, schizophrenia, and neurodegenerative diseases. Therefore, understanding chloride dynamics has become an essential step in neuroscience research. To further support the observation of chloride dynamics, fluorescence imaging has emerged as a powerful approach for monitoring intracellular chloride in living cells, offering high spatial and temporal resolution that complements conventional electrophysiological techniques. This review provides an overview of the significance of chloride dynamics in the nervous system and discusses fluorescence-based strategies for investigating chloride homeostasis. The development of fluorescent chloride probes is summarized, from early small-molecule dyes to genetically encoded fluorescent proteins with emphasis on their sensing mechanisms, advantages, limitations, and applications. Finally, current challenges and future directions in probe development, including improved chloride selectivity, reduced pH sensitivity, and subcellular targeting, are discussed.