Deep Brain Optogenetic Stimulation Using Bessel Beam

Abstract

Optogenetics is emerging as a unique method to stimulate and probe in-vivo neural circuits with high cellular specificity achieved by genetic targeting; and precise temporal resolution provided by interaction of light-gated ion-channel Channelrhodopsin-2 (ChR2) with blue stimulation beam. Since the biological tissue exhibits higher attenuation due to absorption and scattering in the blue activation spectrum, it leads to lowering of the intensity and spatial resolution as the light travels to further depths. Further, diverging blue light emanating from optical fiber or LED, used for in-vivo stimulation, requires placement of the light source near the stimulation region implying deep surgical implantation. While we demonstrated higher resolution and depth of stimulation by two-photon microbeam, effective in-vivo activation of targeted cells over large area necessitates use of single-photon beam. In order to achieve deep-brain optogenetic stimulation by single-photon, here we propose use of non-diffracting Bessel beam instead of Gaussian beam. For generating and delivering the Bessel beam via optical fiber, a micro-axicon was fabricated at the tip. Free-space propagation of Gaussian beam from the cleaved fiber was compared to that of the Bessel beam. The large propagation distance, characteristics of Bessel beam is better suited for in-depth single as well as two-photon optogenetic stimulation of the ChR2 sensitized cells. To validate this in neuronal tissue, theoretical simulations were conducted using Bessel and Gaussian beam, based on Monte Carlo photon transport method applied on layered mouse brain geometry. In contrast to Bessel beam, Gaussian beam due to its divergence coupled with the inherent tissue optical properties was found to travel very limited depth in the tissue and the intensity below few mW/mm2 not being sufficient to stimulate the ChR2 sensitized cells. We will present results of both theoretical simulations and validation experiments.