Abstract

Emerging evidence indicates the involvement of GPR55 and its proposed endogenous ligand, lysophosphatidylinositol (LPI), in nociception, yet their role in central pain processing has not been explored. Using Ca²⁺ imaging, we show here that LPI elicits concentration-dependent and GPR55-mediated increases in intracellular Ca²⁺ levels in dissociated rat periaqueductal gray (PAG) neurons, which express GPR55 mRNA. This effect is mediated by Ca²⁺ release from the endoplasmic reticulum via inositol 1,4,5-trisphosphate receptors and by Ca²⁺ entry via P/Q-type of voltage-gated Ca²⁺ channels. Moreover, LPI depolarizes PAG neurons and upon intra-PAG microinjection, reduces nociceptive threshold in the hot-plate test. Both these effects are dependent on GPR55 activation, because they are abolished by pretreatment with ML-193 [N-(4-(N-(3,4-dimethylisoxazol-5-yl)sulfamoyl)-phenyl)-6,8-dimethyl-2-(pyridin-2-yl)quinoline-4-carboxamide], a selective GPR55 antagonist. Thus, we provide the first pharmacological evidence that GPR55 activation at central levels is pronociceptive, suggesting that interfering with GPR55 signaling in the PAG may promote analgesia.