, 2010) by which heterologous membrane expression of novel microbial opsins for optogenetics in neuroscience may be achieved. Moreover, diverse opportunities to develop or discover new optogenetic tools exist given the large diversity of microbial opsin genes in nature, and since 2008 screens of genomic data have led to identification of many additional tools (e.g., Zhang et al., 2008, Chow et al., 2010, Gradinaru et al.,

Talazoparib datasheet 2010 and Yizhar et al., 2011a). The microbial (type I) opsin genes described above encode strictly ion flow modulators, which control the excitability of a neuron by directly manipulating its membrane potential—either bringing the membrane potential nearer to or above the threshold for generating Rucaparib concentration an action potential or hyperpolarizing the cell and thereby inhibiting spiking. While this approach has advantages of speed and

precision, in some experimental protocols temporally precise modulation of intracellular processes may be necessary. Vertebrate rhodopsin (such as the light-sensing protein in the mammalian eye) is both an opsin (type II), in that it is covalently bound to retinal (in the cis configuration) with function modulated by the absorption of photons, and a G protein-coupled receptor (GPCR), in that it is coupled on the intracellular side to G protein signaling. Expressing vertebrate rhodopsins alone can confer light sensitivity, which can be observed as a slow inhibitory ( Li et al., 2005) or excitatory ( Melyan et al., 2005) modulation. Since these heterologous expression experiments are conducted in the absence of the native G protein (e.g., transducin), the rhodopsin must engage in novel interactions with unknown G proteins not normally linked to rhodopsin that are present in the host cell, and effects on cellular properties may therefore depend on the specific G protein pathways present in each host cell type. Optogenetic recruitment of well-defined biochemical signaling events can be achieved in generalizable fashion by constructing chimeras ( Kim et al., 2005) and between vertebrate rhodopsin

and conventional ligand-gated GPCRs that can serve as single-component neural control tools ( Airan et al., 2009 and Oh et al., 2010), such as the dopaminergic, serotonergic, and adrenergic receptors that play important roles in neurotransmission and neuromodulation. This type II approach can capitalize upon the retinoids present within vertebrate tissues, as identified in the course of microbial (type I) opsin work ( Deisseroth et al., 2006 and Zhang et al., 2006). When used as optogenetic tools these type II fusion proteins are referred to as optoXRs, which allow for optically controlled intracellular signaling with temporal resolution suitable for modulating behavior in freely moving mice ( Airan et al., 2009).