br Acknowledgments Research was supported by NIH grant DP

Acknowledgments
Research was supported by NIH grant 1DP1OD008267-01, The Curing Kids Fund, Foundation for Retina Research, Foundation Fighting Blindness (Individual Investigator Award and Center Grant), Research to Prevent Blindness and Institutional Funds. LHV is an inventors on patents related to AAV gene therapy. LHV has served as a consultant and is inventor on technologies licensed to biotechnology and pharmaceutical industry. LHV is co-founder and consultant to GenSight Biologics.

Introduction
Acquired outer retinal degeneration associated with age-related macular degeneration is the leading cause of blindness in the United States and most of the developed world, while outer retinal degeneration associated with retinitis pigmentosa is the leading inherited cause of blindness. Blindness in both conditions results from irreversible loss of the rod and cone photoreceptors from the outer retina. The remaining retinal cell types – bipolar cells, horizontal cells, amacrine cells, Muller cells, and retinal ganglion Q-VD-Oph manufacturer – remain viable (although significantly rewired (Marc & Jones, 2003)).
In theory, if one could recapitulate the native temporal firing pattern of each retinal ganglion cell in response to a dynamic visual scene, one could ‘restore’ vision to a retina devoid of rods and cones. Progress has been made on several approaches to this goal. Embryonic or inducible progenitor stem cells can be differentiated to a retinal fate and transplanted; this approach restores some light sensitivity in mouse models of retinitis pigmentosa (Lamba, Gust, & Reh, 2009). Second, external stimulation of retinal ganglion cells by opto-electronic prosthetics has also advanced significantly in the past decade, with Food and Drug Administration approval of the Argus II device, capable of restoring some visual phenomena when implanted in profoundly blind individuals (Ahuja et al., 2011; Humayun et al., 2012). Third, virally-mediated gene therapy approaches using optogenetics, including channelrhodopsin and halorhodopsin to excite and suppress retinal cells, respectively, have advanced in the past decade, and have also been shown to restore vision-like function to mice with advanced outer retinal degeneration (Ivanova, Roberts, et al., 2010; Lagali et al., 2008; Tomita et al., 2007).
Over the past 8years, a new approach has been advanced for vision restoration. Compounds such as tetra-ethyl-ammonium which block voltage-gated potassium channels have been known for over a half century to induce neuronal cell firing when administered extracellularly (George & Johnson, 1961). By linking these agents to a photo-isomerizable moiety (azobenzene), novel compounds have been produced that can activate neurons in a light-dependent fashion. When applied to retinas with outer retinal degeneration, these compounds are capable of inducing light-dependent firing of remaining retinal ganglion cells. Such activation may form the basis for reconstitution of vision in eyes with outer retinal degeneration.

Azobenzenes
The light-isomerizable moiety utilized in these compounds is azobenzene (Fig. 1). This class of compounds was among the first described in organic chemistry, having been characterized by Eilhard Mitscherlich in 1834. Azobenzenes are produced by reduction of nitrobenzene; originally this was accomplished with iron filings as catalyst (more recent methods utilize zinc). Azo dyes were extensively used throughout the late 19th and early 20th century in industry to color clothing and foodstuffs.
The property of azobenzene and its derivatives that renders it useful for photopharmacology is the photoisomerization of cis and trans isomers. The isomers can be interchanged with specific wavelengths of light. The trans-to-cis conversion utilizes short wavelength light (typically ultraviolet for unsubstituted azobenzene), while the cis-to-trans isomerization occurs under longer wavelength light (blue light for the unsubstituted azobenzene). The cis isomer is less stable than the trans and so cis-azobenzene will thermally convert back to the trans in dark. Photoisomerization of trans to cis azobenzene is extremely rapid (on the order of picoseconds), while the reversion in dark of cis to trans takes hours for unsubstituted azobenzene. However, substitution of electron-withdrawing groups can alter both the thermal stability of the cis-form, as well as the action spectrum for isomerization (Banghart, Volgraf, & Trauner, 2006). Because of these properties azobenzene derivatives have been used in a variety of nanotechnology applications ranging from surface holography, liquid crystal displays, and optoelectronics (Brehmer et al., 1997; Choi et al., 2007; Matharu, Jeeva, & Ramanujam, 2007; Nakatsuji, 2004; Oliveira et al., 2005).