1.6 μm gold particles

Mice were anesthetized by isoflurane and euthanized by cervical dislocation. Eyes were removed and placed in oxygenated mouse ACSF (pH 7.4), containing the following (in mm): 119 NaCl, 2.5 KCl, 1.3 MgCl₂*6H₂O, 2.5 CaCl₂*2H2O, 1 NaHPO₄, 11 glucose, 20 HEPES. Retinas were isolated from the eyecup under a dissection microscope and mounted onto nitrocellulose filter paper (Millipore). DNA-coated gold particles were prepared by coating 12.5 mg of 1.6 μm gold particles (Bio-Rad) with 20 μg of CMV:CFP and 7 μg of CMV:PSD95-YFP plasmids. A Helios gene gun (Bio-Rad) was used to biolistically deliver plasmid-coated gold particles to whole-mounted retinas. A suspension of DNA-coated gold particles in ethanol was precipitated onto the inner surface of Teflon tubing (Bio-Rad) and subsequently cut into short segments (12 mm long). Gold particles were propelled onto the tissue using helium gas at 40 psi. Retinas were then transferred to an oxygenated and humidified chamber and maintained for 26 h at 32°C, allowing fluorescent protein to be expressed sufficiently for subsequent imaging.