Significance: Infrared neural stimulation (INS) utilizes pulsed infrared light to selectively elicit neural activity without exogenous compounds. Despite its versatility in a broad range of biomedical applications, no comprehensive comparison of factors pertaining to the efficacy and safety of INS such as wavelength, radiant exposure, and optical spot size exists in the literature.
Aim: Here, we evaluate these parameters using three of the wavelengths commonly used for INS, 1450 nm, 1875 nm, and 2120 nm.
Approach: In an in vivo rat sciatic nerve preparation, the stimulation threshold and transition rate to 100% activation probability were used to compare the effects of each parameter.
Results: The pulsed diode lasers at 1450 nm and 1875 nm had a consistently higher (∼1.0 J / cm2) stimulation threshold than that of the Ho:YAG laser at 2120 nm (∼0.7 J / cm2). In addition, the Ho:YAG produced a faster transition rate to 100% activation probability compared to the diode lasers. Our data suggest that the superior performance of the Ho:YAG is a result of the high-intensity microsecond spike at the onset of the pulse. Acute histological evaluation of diode irradiated nerves revealed a safe range of radiant exposures for stimulation.
Conclusion: Together, our results identify measures to improve the safety, efficacy, and accessibility of INS technology for research and clinical applications.
Infrared neural modulation is an optical technique to stimulate or inhibit neural activity with pulsed infrared light (IR). This study focuses on effects of IR and heat on microglial calcium signaling, a main indicator of physiological changes within the immune-like cell. Widefield fluorescence microscopy quantified the intracellular calcium fluxes, cell death, and early apoptosis fluorescent indicators in BV2 immortalized microglia cell culture after irradiation. We have found that unique cellular responses are highly associated with different temperature increases, and the irradiance thresholds for these responses do not drive cells to Caspase-3 apoptosis nor necrotic cellular death.
Schwann cells are the most abundant glial cell in the peripheral nervous system. Apart from their important role on axonal myelination they exert a key role on the regulation of axonal excitability and metabolism, nerve repair, immunological response and pain sensation. However, we still need to fully understand their complete role in the peripheral nervous system. Infrared pulsed stimulation is a label-free technique that proved useful to stimulate neural cells. We present for the first time the use of this method to modulate Schwann cells physiology and the versatility of the method to induce different responses with different stimulation parameters.
Astrocytes play a critical role in regulating brain physiology at ionic, synaptic and whole-organ level. Their function is mainly controlled by ionic signalings, in particular Ca2+ signaling, that mediate cell-cell communication. Infrared pulsed stimulation is a label-free tool that has been previously used to modulate neuronal firing. Here we present the use of infrared pulsed light to stimulate ionic astrocytic signaling in order to modulate astroglial physiology. We present infrared pulsed stimulation as a powerful technique to understand astrocytic function and dysfunction.
Stimulated Raman scattering (SRS) offers a drastic speed advantage over conventional vibrational spectroscopic imaging techniques – making it ideal for studying fast biochemical dynamics. We developed an experimental paradigm that applies spectral stimulated Raman scattering (SRS) imaging to study the mechanisms of infrared (IR) photostimulation of neuronal cells. Infrared neural stimulation (INS) is a label-free optical neuromodulation technique with high spatial and temporal precision. Using SRS, changes in lipid and water vibrational signatures in live cells during INS were observed, suggesting that lipid membrane deformation accompanies IR exposure. The speeds afforded by SRS enables unprecedented observation of fast cellular biophysical dynamics.
We have shown previously that astrocytes are independently sensitive to IR exposure in the context of infrared neural stimulation (INS). More recently, our advances have identified some physiological basis of astrocytic sensitivity to INS. Single pulse INS evokes intracellular calcium signaling primarily mediated through inositol-triphosphate receptor (IP¬3R) activation, rather than thermally sensitive transient receptor protein (TRP) pathways. Interestingly, different dosing strategies for IR appear to evoke different physiology. Our findings suggest that tailoring irradiation parameters with INS may be used for preferentially drive separate physiological responses in astrocytes, and possibly other cell types.
Preterm birth (PTB), when defined as labor before 37 weeks of gestation, affects approximately 1 out of every 10 births in the United States, leading to high rates of mortality. Complete understanding of the mechanism of PTB requires non-invasive, multi-modal techniques that can provide information about the cascade of labor onset. This study compares the cervical remodeling in wild-type term and induced preterm mouse models using Raman spectroscopy. This study demonstrates the potential of Raman spectroscopy as a non-invasive, real-time in-vivo modality to understand cervix remodeling, thus guiding future studies to improve reproductive and neonatal outcomes.
Infrared neural stimulation (INS) has the potential to serve as a spatially precise tool for neuroscientists and clinicians. Because of difficulties comparing various INS techniques in the current literature, we present a study comparing the stimulation threshold and transition rate to 100% activation probability for diode and Ho:YAG lasers. The stimulation threshold was lower for the Ho:YAG than the diode lasers across all examined spot sizes. The Ho:YAG also exhibited a nearly binary transition to 100% activation that can be mimicked with diode lasers by decreasing the pulse width. These results will help maximize the performance of laser diodes for INS.
Infrared neural stimulation (INS) is a promising neuromodulation technique capable of exciting neural tissue without the need for exogeneous agents or genetic modification. Due to its high spatial specificity, INS could improve upon traditional methods of selective neural stimulation in both the laboratory and the clinic. As of yet, no study has compared the efficacy and safety of using different INS parameters such as spot size and wavelength. Moreover, differences in the methods of determining energy deposition and laser spot size make it difficult to compare stimulation parameters used in the current literature. Here, we present results comparing INS efficacy using 1450nm and 1875nm light over a range of spot sizes and radiant exposures. Stimulation thresholds were determined using recorded compound muscle action potentials (CMAPs) and visible muscle contractions in an in vivo rat sciatic nerve model. Overall, 1450nm light required lower radiant exposures to achieve threshold activation as compared to 1875nm. While radiant exposures remained relatively constant across different spot sizes when using 1450nm, the threshold radiant exposures for 1875nm exposures increased with spot size suggesting deeper nerves fibers tend to be activated. Moreover, exposures using a flat-top beam profile yielded less variability in the stimulation threshold than those using a Gaussian profile. As in previous studies, histology confirmed that damaging radiant exposures are several times greater than the stimulation threshold for both 1450nm and 1875nm. Our results provide valuable insight for future studies involving INS and for further developing INS as both a research and clinical tool.
In mammals, neurons are accompanied by numerous cell types, including microglia, oligodendrocytes, and astrocytes. These cells work cohesively to stabilize the neuronal microenvironments and ensure cellular functionality is unperturbed. Studies involving infrared neural stimulation (INS) have been focused on neuronal-type cellular responses, while the effects on non-neuronal cells have been largely unexplored. Astrocytes, in particular, play a key role in maintaining extracellular ion and neurotransmitter concentrations, maintaining osmotic balance, preserving synaptic connectivity, and in mediating neurovascular coupling responses in the brain. Previous work has speculated whether astrocytes are affected by INS in the brain. Here, we characterize the effect of INS on astrocytes using calcium dynamics in response to varying optical parameters and explore whether astrocytic response is driven by neuronal activity or directly by transient thermal energy deposition. Using video-rate calcium-fluorescence multiphoton imaging and nonlinear microscopy of cultured cells, we found that calcium activity in astrocytes is sensitive to infrared light independent of neuronal presence. These cells exhibited similar time-dependent behavior as reported in previous in vivo studies. Success in eliciting calcium responses from a cell, as well as response amplitude, appears to correlate with radiant exposure. The duration and shape of such responses are variable from cell to cell, with some responses aligning with previous observations. Our results suggest that collateral activation of non-neuronal cells, particularly in astrocytes, may play a role when applying INS in vivo.
Infrared neural stimulation (INS) provides optical control of neural excitability using near to mid-infrared (mid-IR) light, which allows for spatially selective, artifact-free excitation without the introduction of exogenous agents or genetic modification. Although neural excitability is mediated by a transient temperature increase due to water absorption of IR energy, the molecular nature of IR excitability in neural tissue remains unknown. Current research suggests that transient changes in local tissue temperature give rise to a myriad of cellular responses that have been individually attributed to IR mediated excitability. To further elucidate the underlying biophysical mechanisms, we have begun work towards employing a novel multimodal nonlinear imaging platform to probe the molecular underpinnings of INS. Our imaging system performs coherent anti-Stokes Raman scattering (CARS), stimulated Raman scattering (SRS), two-photon excitation fluorescence (TPEF), second-harmonic generation (SHG) and thermal imaging into a single platform that allows for unprecedented co-registration of thermal and biochemical information in real-time. Here, we present our work leveraging CARS and SRS in acute thalamocortical brain slice preparations. We observe the evolution of lipid and protein-specific Raman bands during INS and electrically evoked activity in real-time. Combined with two-photon fluorescence and second harmonic generation, we offer insight to cellular metabolism and membrane dynamics during INS. Thermal imaging allows for the coregistration of acquired biochemical information with temperature information. Our work previews the versatility and capabilities of coherent Raman imaging combined with multiphoton imaging to observe biophysical phenomena for neuroscience applications.
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