Optogenetics enables the controlled excitation of defined neuron populations in animals that have been genetically sensitized to light. The ReaChR channelrhodopsin variant used in such studies incorporates portions of VChR1 and VChR2 (channelrhodopsins from Volvox carteri), ChIEF (the transmembrane domains of Chlamydomonas reinhardtii channelrhodopsins 1 and 2 modified with an Ile to Val mutation), and a point mutation (L171I) to enhance activation by red light (~590 nm). Axons expressing the protein become selectively inactivated during continuous excitation. Although ReaChR viral vector models for neurological circuit mapping exist, they demonstrate some variability in expression around the injection site. To achieve more uniform expression, ReaChR tagged with fluorescent protein mCitrine was integrated into the mouse genome. Conditional expression from the Gt(ROSA)26Sor promoter is enabled upon excision of a single loxP-flanked stop cassette. In vitro recording of coronal sections from Flp-excised animals infected with AAV-Syn-iCre (virus expressing iCre from a synapsin promoter) reveal uniform current responses from L2/3 pyramidal neurons stimulated with 590 nm light. Applications for cortical circuit mapping with dual photostimulation (2CRACM) have been demonstrated.&nbsp;

ReaChR protein (a red-shifted channelrhodopsin) was tagged with mCitrine fluorescent protein and placed under the conditional control of the Gt(ROSA)26Sor promoter. Excision of a single loxP- flanked stop cassette enables fluorescent expression. As an optogenetic tool, this strain is useful for the selective activation and mapping of neuron populations.

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B6.Cg-Gt(ROSA)26Sor<tm2.2Ksvo>/J Frozen Embryo