Stimulating Ca<sup>2+</sup> photoactivation of nerve cells by near-infrared light

Calcium ions (Ca²⁺) play a key role of the nerve cells generating universal intracellular signals and controlling important functions. Ca²⁺ activation is of great significance for explaining the subcellular-level biological process. Light stimulated nerve cells to control intracellular signals and membrane activities has become a main method in neuroscience, and the photoactivation is one of the main ways to study intracellular Ca²⁺ transmission. Nerve cells can be directly stimulated by light to produce action potentials, but such techniques are inaccurate in the delivered light energy. To improve this, here in this work we show that gold nanorods (GNRs) can be conjugated to ligands to bound to human neuroblast cells (SH-SY5Y), and introduce an optical method of stimulating and monitoring Ca²⁺ signal in nerve cells in which the plasmonic excitation of GNRs is used. In this paper, we use confocal microscopy to display the 488 nm continuous wave laser irradiating SH-SY5Y cells with Ca²⁺ indicator (Fluo-4, AM) to check fluorescence. Near-infrared pulsed light at the plasmon resonance absorption peak of GNRs is used to stimulate Ca²⁺ signal transduction in SH-SY5Y labeled with GNRs, and Fluo-4, AM is used for two-photon excited fluorescence imaging. In addition, we use the pulsed laser with power of 0.5 mW and a wavelength of 800 nm. The Ca²⁺ activation can be achieved in 10 s on average. The release rate of Ca²⁺ from SH-SY5Y cells labeled with GNRs is 6 times that without GNRs. Next, in order to determine the source of changes in Ca²⁺, we use the BPATA to deplete the intracellular Ca²⁺, after 5 min, 200 μmol/L Ca²⁺ solution is added, and its Δ<i>F</i>/<i>F</i> is found to be more than 1.5 times that without GNRs. Thus, we believe that GNRs could enhance photoactivation through local surface plasmon resonance induced membrane depolarization and generate an action potential. The results prove the feasibility of using GNRs to enhance the activation of Ca²⁺ in nerve cells, and provide an optical means of lower photodamage and more precise for studying nerve cell ion channels. Our study demonstrates that enhancing photoactivation by GNRs could provide an outlook of basic research in neuroscience.