Monitoring membrane potential with second-harmonic generation

Functional interrogation of neural circuits with virally transmitted optogenetic tools

The vertebrate brain comprises a plethora of cell types connected by intertwined pathways. Optogenetics enriches the neuroscientific tool set for disentangling these neuronal circuits in a manner which exceeds the spatio-temporal precision of previously existing techniques. Technically, optogenetics can be divided in three types of optical and genetic combinations: (1) it is primarily understood as the manipulation of the activity of genetically modified cells (typically neurons) with light, i.e. optical actuators. (2) A second combination refers to visualizing the activity of genetically modified cells (again typically neurons), i.e. optical sensors. (3) A completely different interpretation of optogenetics refers to the light activated expression of a genetically induced construct. Here, we focus on the first two types of optogenetics, i.e. the optical actuators and sensors in an attempt to give an overview into the topic. We first cover methods to express opsins into neurons and introduce strategies of targeting specific neuronal populations in different animal species. We then summarize combinations of optogenetics with behavioral read out and neuronal imaging. Finally, we give an overview of the current state-of-the-art and an outlook on future perspectives.

Temporal focusing microscopy

Axial localization of multiphoton excitation to a single plane is achieved by temporal focusing of an ultrafast pulsed excitation. We take advantage of geometrical dispersion in an extremely simple experimental setup, where an ultrashort pulse is temporally stretched and hence its peak intensity is lowered outside the focal plane of the microscope. Using this strategy, out-of-focus multiphoton excitation is dramatically reduced, and the achieved axial resolution is comparable to line-scanning multiphoton microscopy for wide-field excitation and to point-scanning multiphoton microscopy for line excitation. In this introduction, we provide a detailed description of the considerations in choosing the experimental parameters, as well as the alignment of a temporal focusing add-on to a multiphoton microscope. We also review current advances and applications for this technique.

Second Harmonic Imaging of Membrane Potential

The non-linear optical effect known as second harmonic generation (SHG) has been recognized since the earliest days of the laser. But it has only been in the last 20 years that it has begun to emerge as a viable microscope imaging contrast mechanism for visualization of cell and tissue structure and function. This is because only small modifications are required to equip a standard laser scanning 2-photon microscope for second harmonic imaging microscopy (SHIM). SHG signals from certain membrane-bound dyes are highly sensitive to membrane potential, indicating that SHIM may become a valuable probe of cell physiology. However, for the current generation of dyes and microscopes, the small signal size limits the number of photons that can be collected during the course of a fast action potential. Better dyes and optimized microscope optics could ultimately lead to the ability to image neuronal electrical activity with SHIM.