LSO:Ce Inorganic Scintillators Are Biocompatible With Neuronal and Circuit Function

Abstract

Optogenetics is widely used in neuroscience to control neural circuits. However, non-invasive methods for light delivery in brain are needed to avoid physical damage caused by current methods. One potential strategy could employ x-ray activation of radioluminescent particles (RPLs), enabling localized light generation within the brain. RPLs composed of inorganic scintillators can emit light at various wavelengths depending upon composition. Cerium doped lutetium oxyorthosilicate (LSO:Ce), an inorganic scintillator that emits blue light in response to x-ray or ultraviolet (UV) stimulation, could potentially be used to control neural circuits through activation of channelrhodopsin-2 (ChR2), a light-gated cation channel. Whether inorganic scintillators themselves negatively impact neuronal processes and synaptic function is unknown, and was investigated here using cellular, molecular, and electrophysiological approaches. As proof of principle, we applied UV stimulation to 4 μm LSO:Ce particles during whole-cell recording of CA1 pyramidal cells in acute hippocampal slices from mice that expressed ChR2 in glutamatergic neurons. We observed an increase in frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), indicating activation of ChR2 and excitation of neurons. Importantly, LSO:Ce particles did not affect survival of primary mouse cortical neurons, even after 24 h of exposure. In extracellular dendritic field potential recordings, no change in the strength of basal glutamatergic transmission was observed during exposure to LSO:Ce microparticles. However, the amplitude of the fiber volley was slightly reduced with high stimulation. Additionally, there was a slight decrease in the frequency of sEPSCs in whole-cell voltage-clamp recordings from CA1 pyramidal cells, with no change in current amplitudes. The amplitude and frequency of spontaneous inhibitory postsynaptic currents were unchanged. Finally, long term potentiation (LTP), a synaptic modification believed to underlie learning and memory and a robust measure of synaptic integrity, was successfully induced, although the magnitude was slightly reduced. Together, these results show LSO:Ce particles are biocompatible even though there are modest effects on baseline synaptic function and long-term synaptic plasticity. Importantly, we show that light emitted from LSO:Ce particles is able to activate ChR2 and modify synaptic function. Therefore, LSO:Ce inorganic scintillators are potentially viable for use as a new light delivery system for optogenetics.

Feasibility of cerium-doped LSO particles as a scintillator for x-ray induced optogenetics

“…ChR2 has been shown to be activated by wavelengths between 350 to 525 nm, with a peak at 470 nm (Lin et al, 2009;Bartley et al, 2019). Previously, we demonstrated that radioluminescent LSO:Ce microparticles are biocompatible with neuronal health and function (Bartley et al, 2019), and thus potentially useful for X-ray optogenetics.…”

“…ChR2 has been shown to be activated by wavelengths between 350 to 525 nm, with a peak at 470 nm (Lin et al, 2009;Bartley et al, 2019). Previously, we demonstrated that radioluminescent LSO:Ce microparticles are biocompatible with neuronal health and function (Bartley et al, 2019), and thus potentially useful for X-ray optogenetics. LSO:Ce has been widely studied as a scintillator, and has been shown to emit visible light in response to X-ray (Spurrier et al, 2007;Valais et al, 2008;Burdette et al, 2019).…”

“…Several photosensitive proteins can be activated by X-rays exposure (Dawson and Wiederwohl, 1965;Pande et al, 2016), especially ones that respond to UV light (Cannon et al, 2019). Channelrhodopsin-2 (ChR2), a light-gated cation channel, can be activated by wavelengths in the UV range (Bartley et al, 2019), as well as visible light (Boyden et al, 2005). However, it is unknown if ChR2 can be directly activated by acute X-ray exposure.…”

“…One potential strategy for less invasive light delivery to the brain is the use of X-rays to activate radioluminescent materials (Shuba, 2014;French et al, 2018;Bartley et al, 2019;. Radioluminescence is produced by exposing scintillating material to ionizing radiation, thereby causing generation of visible light.…”

“…LSO:Ce material has a high light output (Melcher and Schweitzer, 1991;Roy et al, 2013), and its X-ray activation has been shown to emit wavelengths of light needed to activate channelrhodopsin-2 (ChR2) (Melcher and Schweitzer, 1991). Previously, we showed that LSO:Ce particles had minimal effects on neuronal function and synaptic transmission (Bartley et al, 2019). Together, this makes it an ideal material to determine if radioluminescence from RLPs generated by X-ray exposure can activate ChR2 to modulate synaptic circuits.…”