We developed a novel strategy for conditional silencing of synaptic transmission in specific neuronal types in transgenic animals. We generated a recombinant protein termed immuno-tetanus toxin (ITet), which contains a monoclonal antibody variable region for human interleukin-2 receptor alpha-subunit (IL-2Ralpha) fused to tetanus toxin light chain. ITet was designed to transiently suppress transmitter release from target neurons genetically engineered to express human IL-2Ralpha via proteolytic cleavage of vesicle-associated membrane protein-2 (VAMP-2). The in vivo actions of ITet were investigated by using mutant mice that express IL-2Ralpha in striatal neurons under the control of the gene encoding dopamine D(2) receptor. Unilateral ITet injection into the striatum induced rotational behavior in the mutant mice and the rotations gradually reversed to the normal level. The behavioral alteration was accompanied by a transient decrease in the striatal VAMP-2 level and depolarization-evoked transmitter release in synaptic target region. However, ITet injection caused no structural change in striatal cells and nerve terminals in the mutants. These data indicate that ITet acts on striatal neurons bearing human IL-2Ralpha and temporally reduces their VAMP-2 content, thereby causing the blockade of transmitter release. Our ITet technology provides a useful approach for inducible and reversible control of synaptic transmission in specific neuronal types in the brain.