Lessons from the use of in vivo cellular calcium imaging in primary sensory neurons and spinal cord.

Primary somatosensory neurons, glial cells in the peripheral ganglia, and neural circuits in the spinal cord function as dynamic network circuits that transmit information to the brain. Although a variety of methods and techniques have been used to study individual neurons or tissue explants, the number of neurons that can be monitored is limited. Imaging intact primary sensory neurons, such as those in the dorsal root ganglion and trigeminal ganglia, and the spinal cord in vivo using fluorescent calcium markers helps overcome the limitations of previous methods and techniques by allowing researchers to monitor tens to thousands of cells simultaneously. This allows researchers to conduct experiments to elucidate somatosensory mechanisms and responses to axonal injury that were previously difficult or impossible to observe. Using this approach, researchers have studied dynamic neural network circuits, connectivity, responses to soft and deep touch, heat, cold, chemicals, inflammation, and injury, and they have repeatedly imaged individual neurons over long periods of time. Approaches include using calcium-sensitive fluorescent dyes and genetically encoded markers, performing terminal exposure surgeries, using chambers designed to monitor large numbers of cells or repeatedly imaging small numbers of cells, and imaging animals with or without anesthesia. This review discusses the advantages and disadvantages of in vivo calcium imaging for studying somatosensory and axonal injury in peripheral sensory ganglia and the dorsal spinal cord, as well as anticipated future directions.