Although Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) represent a technical revolution in integrative neuroscience, the first ligands used were not as selective as expected. Compound 21 (C21) was recently proposed as an alternative, but in vivo characterization of its properties is not sufficient yet. Here, we evaluated its potency to selectively modulate the activity of nigral dopaminergic (DA) neurons through the canonical DREADD receptor hM4Di using TH-Cre rats. In males, 1 mg.kg -1 of C21 strongly increased nigral neurons activity in control animals, indicative of a significant off-target effect. Reducing the dose to 0.5 mg.kg -1 circumvented this aspecific effect, while activated the inhibitory DREADDs and selectively reduced nigral neurons firing. In females, 0.5 mg.kg -1 of C21 induced a transient and residual off-target effect that may mitigated the inhibitory DREADDs-mediated effect.This study raises up the necessity to test selectivity and efficacy of chosen ligands for each new experimental condition.

DREADDs (designer receptors exclusively activated by designer drugs) are a powerful and tremendous new technique for selectively manipulating a specific neuronal (or non-neuronal) subpopulation. Recent studies indicate, however, that ligands used for DREADDs, such as clozapine-N-oxide or its parent compound clozapine, are not as selective as expected, even at reasonable concentrations. Although the new generation of ligands specifically developed for DREADDs or alternative chemogenetic receptors may present some improvements, the absence of potential off-target effects remains to be fully demonstrated. Together, indications from the recent literature on DREADDs should warn current and future users about some weaknesses of this expanding technique in the field of integrative neuroscience and encourage them to take some specific precautions to avoid important pitfalls with DREADDs, which remain a promising and complementary approach to optogenetics with the relevant controls.