Two-Photon Excitation of di-4-ANEPPS for Optical Recording of Action Potentials in Rabbit Heart

Optical mapping of contracting hearts

Optical mapping is a widely used tool to record and visualize the electrophysiological properties in a variety of myocardial preparations such as Langendorff-perfused isolated hearts, coronary-perfused wedge preparations, and cell culture monolayers. Motion artifact originating from the mechanical contraction of the myocardium creates a significant challenge to performing optical mapping of contracting hearts. Hence, to minimize the motion artifact, cardiac optical mapping studies are mostly performed on non-contracting hearts, where the mechanical contraction is removed using pharmacological excitation-contraction uncouplers. However, such experimental preparations eliminate the possibility of electromechanical interaction, and effects such as mechano-electric feedback cannot be studied. Recent developments in computer vision algorithms and ratiometric techniques have opened the possibility of performing optical mapping studies on isolated contracting hearts. In this review, we discuss the existing techniques and challenges of optical mapping of contracting hearts.

Two-photon excitation of FluoVolt allows improved interrogation of transmural electrophysiological function in the intact mouse heart

BACKGROUND & AIMS

Two-photon excitation of voltage sensitive dyes (VSDs) can measure rapidly changing electrophysiological signals deep within intact cardiac tissue with improved three-dimensional resolution along with reduced photobleaching and photo-toxicity compared to conventional confocal microscopy. Recently, a category of VSDs has emerged which records membrane potentials by photo-induced electron transfer. FluoVolt is a novel VSD in this category which promises fast response and a 25% fractional change in fluorescence per 100 mV, making it an attractive optical probe for action potential (AP) recordings within intact cardiac tissue. The purpose of this study was to characterize the fluorescent properties of FluoVolt as well as its utility for deep tissue imaging.

METHODS

Discrete tissue layers throughout the left ventricular wall of isolated perfused murine hearts loaded with FluoVolt or di-4-ANEPPS were sequentially excited with two-photon microscopy.

RESULTS

FluoVolt loaded hearts suffered significantly fewer episodes of atrio-ventricular block compared to di-4-ANEPPS loaded hearts, indicating comparatively low toxicity of FluoVolt in the intact heart. APs recorded with FluoVolt were characterized by a lower signal-to-noise ratio and a higher dynamic range compared to APs recorded with di-4-ANEPPS. Although both depolarization and repolarization parameters were similar in APs recorded with either dye, FluoVolt allowed deeper tissue excitation with improved three-dimensional resolution due to reduced out-of-focus fluorescence generation under two-photon excitation.

CONCLUSION

Our results demonstrate several advantages of two-photon excitation of FluoVolt in functional studies in intact heart preparations, including reduced toxicity and improved fluorescent properties.

2-Photon excitation fluorescence microscopy enables deeper high-resolution imaging of voltage and Ca(2+) in intact mice, rat, and rabbit hearts

We describe a novel two-photon (2P) laser scanning microscopy (2PLSM) protocol that provides ratiometric transmural measurements of membrane voltage (Vm ) via Di-4-ANEPPS in intact mouse, rat and rabbit hearts with subcellular resolution. The same cells were then imaged with Fura-2/AM for intracellular Ca(2+) recordings. Action potentials (APs) were accurately characterized by 2PLSM vs. microelectrodes, albeit fast events (<1 ms) were sub-optimally acquired by 2PLSM due to limited sampling frequencies (2.6 kHz). The slower Ca(2+) transient (CaT) time course (>1ms) could be accurately described by 2PLSM. In conclusion, Vm - and Ca(2+) -sensitive dyes can be 2P excited within the cardiac muscle wall to provide AP and Ca(2+) signals to ∼400 µm.

Comparison of two voltage-sensitive dyes and their suitability for long-term imaging of neuronal activity

One of the key approaches for studying neural network function is the simultaneous measurement of the activity of many neurons. Voltage-sensitive dyes (VSDs) simultaneously report the membrane potential of multiple neurons, but often have pharmacological and phototoxic effects on neuronal cells. Yet, to study the homeostatic processes that regulate neural network function long-term recordings of neuronal activities are required. This study aims to test the suitability of the VSDs RH795 and Di-4-ANEPPS for optically recording pattern generating neurons in the stomatogastric nervous system of crustaceans with an emphasis on long-term recordings of the pyloric central pattern generator. We demonstrate that both dyes stain pyloric neurons and determined an optimal concentration and light intensity for optical imaging. Although both dyes provided sufficient signal-to-noise ratio for measuring membrane potentials, Di-4-ANEPPS displayed a higher signal quality indicating an advantage of this dye over RH795 when small neuronal signals need to be recorded. For Di-4-ANEPPS, higher dye concentrations resulted in faster and brighter staining. Signal quality, however, only depended on excitation light strength, but not on dye concentration. RH795 showed weak and slowly developing phototoxic effects on the pyloric motor pattern as well as slow bleaching of the staining and is thus the better choice for long-term experiments. Low concentrations and low excitation intensities can be used as, in contrast to Di-4-ANEPPS, the signal-to-noise ratio was independent of excitation light strength. In summary, RH795 and Di-4-ANEPPS are suitable for optical imaging in the stomatogastric nervous system of crustaceans. They allow simultaneous recording of the membrane potential of multiple neurons with high signal quality. While Di-4-ANEPPS is better suited for short-term experiments that require high signal quality, RH795 is a better candidate for long-term experiments since it has only minor effects on the motor pattern.