Selective manipulation of spinal microglia by chemogenetics: Implications for allodynia and inflammatory signaling

The absence of selective pharmacological tools is a major barrier to the in vivo study of microglia. To address this issue, we developed a Gq- and Gi-coupled Designer Receptor Exclusively Activated by a Designer Drug (DREADD) to enable selective stimulation or inhibition of microglia, respectively. DREADDs under a CD68 (microglia/macrophage) promoter were intrathecally transfected via an AAV9 vector. Naive rats intrathecally transfected with Gq (stimulatory) DREADDs exhibited significant allodynia following intrathecal administration of the DREADD-selective ligand clozapine-N-oxide (CNO), which was abolished by intrathecal IL-1ra. Chronic constriction injury-induced allodynia was attenuated by intrathecal CNO in rats intrathecally transfected with Gi (inhibitory) DREADDs. To explore mechanisms, BV2 cells were stably transfected with Gq or Gi DREADDs in vitro . CNO treatment induced pro-inflammatory mediator production per se from cells expressing Gq-DREADDs, and inhibited lipopolysaccharide-induced pro-inflammatory mediator production from cells expressing Gi-DREADDs. These studies are the first to manipulate microglia using DREADDs, which allow the role of glia in pain to be conclusively demonstrated, unconfounded by neuronal off-target effects that exist for all other glial inhibitors. Hence, these studies are the first to conclusively demonstrate that in vivo stimulation of resident spinal microglia in intact spinal cord is (a) sufficient for allodynia, and (b) necessary for allodynia induced by peripheral nerve injury. DREADDs are a unique tool to selectively explore the physiological and pathological role of microglia in vivo .