Fluorescent sensors for intracellular signaling in the brain: imaging neurons, glia, and vascular cells

Optical imaging has increasingly become the go-to technique for studying brain activity. The advancement of such approaches, which typically assess brain activity by monitoring the release or activity of second messengers, neurotransmitters, or electrical signals, has been entirely dependent on the development of sensors. Given advances in the field, a review of sensor development, including the latest sensors, is both timely and important for understanding their application in optical imaging. We seek to provide an overview of the sensors most commonly used by investigators to study brain function through optical imaging, including <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mi>Ca</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> </mml:math> , voltage, and cAMP sensors, highlighting their developmental trajectory, applications, and relative strengths and weaknesses. We systematically reviewed the most recent publications that describe either the development or use of optical sensors in the context of brain imaging. We evaluated technical specifications and performance in real-life applications of these biosensors. We identified and highlighted sensors that have been characterized and widely adopted in various applications. We discussed their utility, kinetics, and practical advantages and disadvantages. Because of their more advanced development, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mi>Ca</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> </mml:math> sensors receive more extensive consideration in our discussion. Overall, we reveal a plethora of available sensors that allow investigators to examine brain activity based on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mi>Ca</mml:mi> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> </mml:math> dynamics, cAMP activity, and electrical activity. Although further development is needed, the substantial progress in optical imaging, which is critically enabled by advances in sensor technology, is evident. These tools collectively provide researchers with powerful new capabilities to visualize and dissect the complex dynamics of brain function.