Optogenetic silencing of neurotransmitter release at Schaffer collateral synapses with a naturally occurring invertebrate rhodopsin

Mauro Pulin¹, Mathias Mahn², Inbar Saraf-Sinik², Pritish Patil², Felicitas Bruentgens³, Eyal Bitton², Shaked Palgi², Asaf Gat², Julien Dine², Jonas Wietek², Ido Davidi², Rivka Levy², Fangmin Zhou¹, Kathrin Sauter¹, Peter Soba¹, Dietmar Schmitz³, Benjamin Rost³, Ofer Yizhar², J. Simon Wiegert¹

¹ Center for Molecular Neurobiology Hamburg, Falkenried 94, 20251 Hamburg, Germany
² Department of Neurobiology, Weizmann Institute of Science, Rehovot 76100 Israel
³ Charité-Universitätsmedizin Berlin, Corporate member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Neuroscience Research Center, 10117, Berlin, Germany

Optogenetic silencing is a powerful tool for dissecting neuronal circuits and understanding the contribution of defined neuronal populations to behavioral processes. However, silencing of long-range axonal projections has posed a formidable challenge for modern neuroscience. Existing optogenetic tools suffer from low efficacy and off-target effects when applied to presynaptic terminals, while chemogenetic approaches for synaptic silencing lack spatiotemporal precision. Here we present eOPN3, a targeting-enhanced type-II mosquito rhodopsin that can effectively suppress presynaptic neurotransmitter release through activation of the Gi/o signaling pathway, as demonstrated in autaptic hippocampal neuronal cultures. In behaving mice, eOPN3-mediated suppression of auditory afferents to the basolateral amygdala leads to impaired recall of auditory-cued fear. Here we show that expression of eOPN3 in CA3 pyramidal cells of organotypic hippocampal slices yields efficient membrane targeting and trafficking to distal axons projecting to area CA1. Activation of eOPN3 in the somatodendritic compartment of CA3 cells leads to GIRK-channel mediated hyperpolarization and reduces action potential firing. In contrast, activation of eOPN3 at synaptic terminals reduces the amplitude of EPSCs between pairs of CA3 and CA1 cells without inhibiting presynaptic CA3-cell action potential firing. Two-photon calcium imaging at individual presynaptic boutons revealed a decrease in action potential-evoked Ca²⁺ influx upon eOPN3 activation, which was independent of GIRK-channel activation, indicating a direct, Gi/o-mediated presynaptic action of eOPN3. We therefore conclude that eOPN3 can be used to selectively suppress neurotransmitter release at synaptic terminals with high spatiotemporal precision, opening new avenues for functional interrogation of long-range neuronal circuits in vivo.