A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits This study introduces a bistable inhibitory optoGPCR, PdCO, derived from the ciliary opsin of Platynereis dumerilii, which enables efficient and precise suppression of synaptic transmission in mammalian neurons. PdCO exhibits high temporal precision and is well-suited for spectral multiplexing with other optogenetic tools. It can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, enabling detailed synapse-specific functional circuit mapping. The study evaluates multiple bistable opsins for optogenetic applications and finds that PdCO is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. The study demonstrates that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping. The study also shows that PdCO can be used in conjunction with other optogenetic tools for spectral multiplexing, allowing for the simultaneous activation of multiple optogenetic actuators and reporters. The study also shows that PdCO can be used in vivo to modulate mouse behavior by inhibiting dopaminergic projections from the substantia nigra to the dorsomedial striatum, a neural pathway that plays an important role in animal locomotion. The study further shows that PdCO can be used to inhibit specific synapses in vivo, such as those projecting from the nucleus accumbens to the ventral tegmental area, and impact behavior. The study concludes that PdCO is a promising tool for optogenetic control of neural circuits, offering high temporal precision, spectral multiplexing capabilities, and the ability to suppress synaptic transmission in a projection-specific manner. The study also highlights the potential of PdCO for use in non-neuronal tissue and in two invertebrate neuronal model systems.A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits This study introduces a bistable inhibitory optoGPCR, PdCO, derived from the ciliary opsin of Platynereis dumerilii, which enables efficient and precise suppression of synaptic transmission in mammalian neurons. PdCO exhibits high temporal precision and is well-suited for spectral multiplexing with other optogenetic tools. It can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, enabling detailed synapse-specific functional circuit mapping. The study evaluates multiple bistable opsins for optogenetic applications and finds that PdCO is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. The study demonstrates that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping. The study also shows that PdCO can be used in conjunction with other optogenetic tools for spectral multiplexing, allowing for the simultaneous activation of multiple optogenetic actuators and reporters. The study also shows that PdCO can be used in vivo to modulate mouse behavior by inhibiting dopaminergic projections from the substantia nigra to the dorsomedial striatum, a neural pathway that plays an important role in animal locomotion. The study further shows that PdCO can be used to inhibit specific synapses in vivo, such as those projecting from the nucleus accumbens to the ventral tegmental area, and impact behavior. The study concludes that PdCO is a promising tool for optogenetic control of neural circuits, offering high temporal precision, spectral multiplexing capabilities, and the ability to suppress synaptic transmission in a projection-specific manner. The study also highlights the potential of PdCO for use in non-neuronal tissue and in two invertebrate neuronal model systems.