Tetanus on demand! (not for bioweapons, but for neuroscience research) Tetanus toxin induces paralysis by using a protease to cleave VAMP2, a protein required for synaptic vesicle fusion and neurotransmitter release. To harness this toxicity and use it as a beneficial tool for neuroscience research, we fused a light-sensitive LOV domain from oat plants into tetanus’ protease domain and made its activity controllable by light. LATeNT (for light-activated tetanus neurotoxin) can be expressed in specific cell populations, and then turned on with a brief pulse of blue light. Synaptic inhibition is potently inhibited only in those cells expressing LATeNT while neighboring cells are unaffected. We used LATeNT in the mouse brain to discover a new cell population (SST+ interneurons in the hippocampus) that modulates anxiety. 24 hours after shutting off LATeNT by taking away the light, anxiety levels returned to normal. We are excited for neuroscientists to try this new tool! The inhibitory effect is stronger and more sustained than with other tools like halorhodopsin and DREADDs, but the timing and location of the effect are completely under your control. Preprint here: Very proud of amazing protein engineer @heegwangroh, our fantastic neuroscience collaborator Ji Won Um, co-first author Dongwook Kim from the Um lab, and our other wonderful co-authors. Image credit: painting by Sir Charles Bell depicting opisthotonos, which can be caused by tetanus (1809)