Chemogenetic manipulation of CX3CR1⁺ cells transiently induces hypolocomotion independent of microglia

Chemogenetic approaches using Designer Receptors Exclusively Activated by Designer Drugs (DREADD, a family of engineered GPCRs) were recently employed in microglia. Here, we used Cx3cr1 ^CreER/+ :R26 ^hM4Di/+ mice to express Gi-DREADD (hM4Di) on CX3CR1⁺ cells, comprising microglia and some peripheral immune cells, and found that activation of hM4Di on long-lived CX3CR1⁺ cells induced hypolocomotion. Unexpectedly, Gi-DREADD-induced hypolocomotion was preserved when microglia were depleted. Consistently, specific activation of microglial hM4Di cannot induce hypolocomotion in Tmem119 ^CreER/+ :R26 ^hM4Di/+ mice. Flow cytometric and histological analysis showed hM4Di expression in peripheral immune cells, which may be responsible for the hypolocomotion. Nevertheless, depletion of splenic macrophages, hepatic macrophages, or CD4⁺ T cells did not affect Gi-DREADD-induced hypolocomotion. Our study demonstrates that rigorous data analysis and interpretation are needed when using Cx3cr1 ^CreER/+ mouse line to manipulate microglia.