Gomez, J.L. et al. Science 357, 503–507 (2017).

Chemogenetic manipulation of neural activity is an alternative to optogenetics, especially for manipulations at longer timescales. In this technology, CNO (clozapine N-oxide) is thought to act on DREADDs (designer receptors exclusively activated by designer drugs) to induce a G-protein signaling cascade. However, Gomez et al. now show that CNO does not bind to DREADDs in cell culture or in brain slices. Instead, clozapine exhibits high affinity for DREADDs. Furthermore, the researchers find that, unlike clozapine, CNO does not cross the blood–brain barrier in mice. As CNO is known to convert to clozapine in vivo, the researchers propose that CNO, which has been widely used in chemogenetic experiments, acts on DREADDS via its clozapine metabolite. Indeed, they find that clozapine itself is more potent in mice than CNO is. Thus, they recommend using clozapine in chemogenetic applications.