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Thermogenetics is a promising neuromodulation technique based on the use of heat-sensitive ion channels. However, on the way to its clinical application, a number of questions have to be addressed. First, to avoid immune response in future human applications, human ion channels should be studied as thermogenetic actuators. Second, heating levels necessary to activate these channels in vivo in brain tissue should be studied and cytotoxicity of these temperatures addressed. Third, the possibility and safety of chronic neuromodulation has to be demonstrated. In this study, we present a comprehensive framework for thermogenetic neuromodulation in vivo using the thermosensitive human ion channel hTRPV1. By targeting hTRPV1 expression to excitatory neurons of the mouse brain and activating them within a non-harmful temperature range with a fiber-coupled infrared laser, we not only induced neuronal firing and stimulated locomotion in mice, but also demonstrated that thermogenetics can be employed for repeated neuromodulation without causing evident brain tissue injury. Our results lay the foundation for the use of thermogenetic neuromodulation in brain research and therapy of neuropathologies., (© 2024. The Author(s).)

Cutting-edge methods of laser microscopy combined with fluorescent protein engineering and spectral analysis provide a unique resource for high-resolution neuroimaging, enabling a high-fidelity, high-contrast detection of fine structural details of neural cells and intracellular compartments. In addition to their extraordinary imaging abilities in real space, such methods can help resolve the neural states in a multidimensional space of neural responses whereby individual neurons and neural populations encode information on external stimuli. This study shows, however, that laser-induced biochemical processes in neural cells can give rise to an uncertainty of neural states, setting an upper bound on the information that optical measurements can provide on neural states, neural encodings, and neural dynamics. Comparison of absorbed laser power with the native biochemical energy budget of neuronal firing suggests that each readout photon in optical recording comes at a cost of precision of neural encoding and a loss of information encoded by the neural response. A quantitative measure for such a measurement-induced neural uncertainty can be defined, as this study shows, in terms of the Fisher information, relating the lower bound of this uncertainty to the loss of the Shannon information capacity of neural states., Competing Interests: Declaration of Competing Interest The author declares no financial or commercial conflict of interest., (Copyright © 2020 Elsevier B.V. All rights reserved.)

We demonstrate a reconnectable implantable ultraslim fiber-optic microendoscope that integrates a branching fiber bundle (BFB) with gradient-index fiber lenses, enabling a simultaneous fluorescence imaging of individual cells in distinctly separate brain regions, including brain structures as distant as the neocortex and hippocampus. We show that fluorescence images of individual calcium-indicator-expressing neurons in the brain of freely moving transgenic mice can be recorded, via the implanted BFB probe, in parallel with time- and cell-resolved traces of calcium signaling, thus enabling correlated circuit-dynamics studies at -multiple sites within the brain of freely moving animals., (© 2020 Wiley-VCH GmbH.)

Optical coupling between a single, individually addressable neuron and a properly designed optical fiber is demonstrated. Two-photon imaging is shown to enable a quantitative in situ analysis of such fiber-single-neuron coupling in the live brain of transgenic mice. Fiber-optic interrogation of single pyramidal neurons in mouse brain cortex is performed with the positioning of the fiber probe relative to the neuron accurately mapped by means of two-photon imaging. These results pave the way for fiber-optic interfaces to single neurons for a stimulation and interrogation of individually addressable brain cells in chronic in vivo studies on freely behaving transgenic animal models, as well as the integration of fiber-optic single-neuron stimulation into the optical imaging framework., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A bundle of individually addressable optical fibers is shown to enable three-dimensional optical readout from single neurons in live brain, as well as in intact brain extracted from transgenic mice. With individual fibers in the bundle being only a few microns in diameter, single neurons are readily resolved in brain images transmitted by the fiber bundle. The third dimension is added by scanning the fiber in the longitudinal direction, with the fluorescence return read out from only one of the fibers in the bundle., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Thermogenetics is a promising innovative neurostimulation technique, which enables robust activation of neurons using thermosensitive transient receptor potential (TRP) cation channels. Broader application of this approach in neuroscience is, however, hindered by a limited variety of suitable ion channels, and by low spatial and temporal resolution of neuronal activation when TRP channels are activated by ambient temperature variations or chemical agonists. Here, we demonstrate rapid, robust and reproducible repeated activation of snake TRPA1 channels heterologously expressed in non-neuronal cells, mouse neurons and zebrafish neurons in vivo by infrared (IR) laser radiation. A fibre-optic probe that integrates a nitrogen-vacancy (NV) diamond quantum sensor with optical and microwave waveguide delivery enables thermometry with single-cell resolution, allowing neurons to be activated by exceptionally mild heating, thus preventing the damaging effects of excessive heat. The neuronal responses to the activation by IR laser radiation are fully characterized using Ca 2+ imaging and electrophysiology, providing, for the first time, a complete framework for a thermogenetic manipulation of individual neurons using IR light.

Specialty fiber probes are used for in vivo depth-resolved mapping of neuron activity through the optical detection of fluorescent-protein reporters expressed inside the living brain of anesthetized transgenic mice. Supercontinuum radiation produced by highly nonlinear photonic-crystal fibers is employed to demonstrate a simultaneous multicolor interrogation of several biomarkers in a model aqueous solution system, thus suggesting the way toward a multiplex mapping of various types of neuron dynamics inside the living brain.

The Hodgkin-Huxley treatment of the dynamics of a nerve impulse on a cell membrane is combined with a phenomenological description of molecular hyperpolarizabilities to develop a closed-form model of an action-potential-sensitive second-harmonic response of membrane-bound chromophores. This model is employed to understand the key properties of the map between the action potential and modulation of the second harmonic from a cell membrane stained with hyperpolarizable chromophore molecules.

Dual-cladding photonic crystal fibers (PCFs) with two zero-dispersion points are used to enhance the two-photon excited luminescence (TPL) response from fluorescent protein biomarkers and neuron activity reporters in dye-cell experiments and in in vivo work on transgenic mice and tadpoles. The soliton transmission of ultrashort pulses through a PCF suppresses dispersion-induced temporal pulse spreading, maintaining a high level of field intensity needed for efficient TPL excitation. The soliton self-frequency shift, stabilized against laser power fluctuations by a specific PCF dispersion design, is employed to accurately match the wavelength of the soliton PCF output with the two-photon absorption spectrum of dye or fluorescent protein biomarker molecules, enhancing their TPL response and allowing the laser damage of biotissues to be avoided.