Photoactivation of individual synapses in vivo with covalent photoswitches targeting endogenous glutamate receptors

Glutamate receptors (GluRs) play key roles in neurotransmission at excitatory synapses and in the regulation of synaptic plasticity. We have developed a targeted covalently-attached photoswitch (TCP, Izquierdo-Serra et al., 2016) that allows the remote control of endogenous ionotropic GluRs using light. We here combined this photopharmacological effector with genetic and chemical calcium sensors to demonstrate all-optical reversible control of GluRs at multiple levels of spatial resolution in the brain: we achieved the photoactivation of multiple neurons, individual neurons, and single synapses in rat hippocampal slices and in intact Xenopus laevis brain in vivo, which is challenging using other methods. We show that this compound selectively targets AMPA and kainate receptors. Labeled receptors remained functional for long periods of time (>8 hours). This allowed us to longitudinally track endogenous receptor physiology during events of synaptic plasticity, such as long-term depression (LTD). We could monitor the loss of functionality of AMPA/kainate receptors during NMDAR-dependent LTD in hippocampal neurons. TCPs are, therefore, a unique optical tool to label, photo-control and functionally track endogenous receptors in brain tissue without genetic manipulation.