Although accumulating evidence suggests that serotonin (5-HT) controls emotional behaviors and cognition via acting on its receptors, it is not clear that the receptor subtype responsible for the specific function. Fourteen pharmacologically distinct 5-HT receptor subtypes have been identified, but some of them lack any selective ligands because of their similarity. It is expensive and impractical to obtain knock-out mice for all the subtypes. Therefore we attempted to knock out 5-HT receptor genes by injecting virus vectors expressing sgRNA to adult Cas9 knock-in mice. This approach would provide easier and low-cost knock out of 5-HT receptor genes in specific cell types and brain regions. First, we targeted 5-HT1A receptors in the dorsal raphe nucleus (DRN) because the phenotype of 5-HT1A knock-out mice has been clarified. A 5-HT1A agonist, 8-OH-DPAT induced hypothermia is attenuated by the injection of viral vector expressing sgRNA for 5-HT1A gene into DRN of Cas9 knock-in mice. DNA sequencing and mismatch cleavage assay detected the target gene editing. Immunohistochemical analysis revealed that most of the virally induced Cas9 were expressed in GAD-positive GABAergic neurons.