Figure 2 |. Integrated in vivo optical and physiological system for control of the lateral hypothalamus in the setting of behavior alanalysis.

a, Schematic of the behavioural set-up used for in vivo deep-brain photostimulation in mice. Magnification (inset) shows the EEG/EMG connector used for sleep recording and the cannula guide used for lateral hypothalamus light delivery through an optical fibre. b, Schematic of experimental set-up, showing relationship between the optical fibre, brain tissue and attenuating light. r, optical fibre radius; z, tissue depth from fibre end; q_div, the half-angle of divergence. LH, lateral hypothalamus. c, Normalized light intensity(mW mm⁻²) as a function of lateral hypothalamus tissue depth z. Values were experimentally determined as previously described¹⁴ by measuring the light intensity after transmission through a given tissue thickness and dividing by the intensity of the light emanating from the optical fibre tip. Tissue of different thickness was prepared in the form of acute brain slices from adult C57BL/6 mice. Error bars indicate one standard deviation from the mean. Sample size: 0.2 mm, n = 8; 0.4mm, n = 6; 1mm, n = 2. Fits were produced using the Kubelka–Munk model of light transmission through diffuse scattering media as previously described¹⁴. We estimate that by placing the tip of the optical fibre at the upper limit of the lateral hypothalamus, at least 1 mW mm⁻² of light, which is sufficient to activate ChR2¹⁰, reaches the entire Hcrt field (0.75 mm (medial-lateral) × 0.75mm (dorso-ventral) × 1mm (rostro-caudal)).