Matarazzo Jerico V, Ajay Elise A, Payne Sophie C, Trang Ella P, Thompson Alex C, Marroquin Jason B, Wise Andrew K, Fallon James B, Richardson Rachael T
Bionics Institute, East Melbourne, VIC, Australia.
Department of Engineering, University of Melbourne, Parkville, VIC, Australia.
Front Neurosci. 2023 Jun 8;17:1190662. doi: 10.3389/fnins.2023.1190662. eCollection 2023.
Electrical stimulation offers a drug-free alternative for the treatment of many neurological conditions, such as chronic pain. However, it is not easy to selectively activate afferent or efferent fibers of mixed nerves, nor their functional subtypes. Optogenetics overcomes these issues by controlling activity selectively in genetically modified fibers, however the reliability of responses to light are poor compared to electrical stimulation and the high intensities of light required present considerable translational challenges. In this study we employed a combined protocol of optical and electrical stimulation to the sciatic nerve in an optogenetic mouse model to allow for better selectivity, efficiency, and safety to overcome fundamental limitations of electrical-only and optical-only stimulation.
The sciatic nerve was surgically exposed in anesthetized mice ( = 12) expressing the ChR2-H134R opsin the parvalbumin promoter. A custom-made peripheral nerve cuff electrode and a 452 nm laser-coupled optical fiber were used to elicit neural activity utilizing optical-only, electrical-only, or combined stimulation. Activation thresholds for the individual and combined responses were measured.
Optically evoked responses had a conduction velocity of 34.3 m/s, consistent with ChR2-H134R expression in proprioceptive and low-threshold mechanoreceptor (Aα/Aβ) fibers which was also confirmed immunohistochemical methods. Combined stimulation, utilizing a 1 ms near-threshold light pulse followed by an electrical pulse 0.5 ms later, approximately halved the electrical threshold for activation ( = 0.006, = 5) and resulted in a 5.5 dB increase in the Aα/Aβ hybrid response amplitude compared to the electrical-only response at equivalent electrical levels ( = 0.003, = 6). As a result, there was a 3.25 dB increase in the therapeutic stimulation window between the Aα/Aβ fiber and myogenic thresholds ( = 0.008, = 4).
The results demonstrate that light can be used to prime the optogenetically modified neural population to reside near threshold, thereby selectively reducing the electrical threshold for neural activation in these fibers. This reduces the amount of light needed for activation for increased safety and reduces potential off-target effects by only stimulating the fibers of interest. Since Aα/Aβ fibers are potential targets for neuromodulation in chronic pain conditions, these findings could be used to develop effective strategies to selectively manipulate pain transmission pathways in the periphery.
电刺激为许多神经系统疾病（如慢性疼痛）的治疗提供了一种无药物的替代方法。然而，选择性激活混合神经的传入或传出纤维及其功能亚型并非易事。光遗传学通过在基因改造的纤维中选择性地控制活动来克服这些问题，然而与电刺激相比，对光反应的可靠性较差，并且所需的高强度光带来了相当大的转化挑战。在本研究中，我们在光遗传学小鼠模型中对坐骨神经采用了光刺激和电刺激相结合的方案，以实现更好的选择性、效率和安全性，从而克服单纯电刺激和单纯光刺激的基本局限性。
在表达ChR2-H134R视蛋白并由小白蛋白启动子驱动的麻醉小鼠（n = 12）中，手术暴露坐骨神经。使用定制的外周神经袖带电极和452 nm激光耦合光纤，通过单纯光刺激、单纯电刺激或联合刺激来引发神经活动。测量个体反应和联合反应的激活阈值。
光诱发反应的传导速度为34.3 m/s，这与本体感觉和低阈值机械感受器（Aα/Aβ）纤维中ChR2-H134R的表达一致，免疫组织化学方法也证实了这一点。联合刺激采用1 ms接近阈值的光脉冲，随后0.5 ms后施加电脉冲，与同等电刺激水平下的单纯电刺激相比，激活的电阈值大约减半（p = 0.006，n = 5），并且Aα/Aβ混合反应幅度增加了5.5 dB（p = 0.003，n = 6）。结果，Aα/Aβ纤维与肌源性阈值之间的治疗刺激窗口增加了3.25 dB（p = 0.008，n = 4）。
结果表明，光可用于使光遗传学改造的神经群体处于接近阈值的状态，从而选择性降低这些纤维中神经激活的电阈值。这减少了激活所需的光量，提高了安全性，并通过仅刺激感兴趣的纤维减少了潜在的脱靶效应。由于Aα/Aβ纤维是慢性疼痛状态下神经调节的潜在靶点，这些发现可用于制定有效策略，以选择性地操纵外周的疼痛传递途径。
