Reduction of Microglial Reactivity by Cannabidiol: Preliminary Data Obtained in an Astrocyte–Microglia Co-Culture Model of Inflammation

Background/Objectives: Glia-mediated inflammation contributes to a wide range of central nervous system (CNS) disorders, including epilepsy. Pregabalin (PGB) and cannabidiol (CBD) are CNS-acting drugs prescribed for various neuropsychiatric conditions, especially seizures. This study investigated the effects of PGB and CBD on glial properties in an astrocyte–microglia co-culture model of inflammation. Methods: Physiological (M5, containing 5–10% microglia) and pathological inflammatory astrocyte–microglia co-cultures (M30, containing 30–40% microglia) were collected from the postnatal brain hemispheres of Wistar rats (P0-P2) according to an established protocol and treated with different concentrations of PGB (3, 10, 30 and 60 µg/mL) for 24 h or CBD (50, 500 and 1000 ng/mL) for 1 h or 24 h. Metabolic activity was assessed by the MTT assay. Microglial phenotypes and astroglial connexin (Cx)43 expression were detected by immunocytochemistry. Results: In M5 co-cultures, short-term incubation (1 h) with high concentrations (1000 ng/mL) of CBD significantly reduced glial viability (p < 0.05), while no significant changes were observed in M30 co-cultures. After 24 h of incubation, M5 cultures exhibited a significant increase in metabolic activity at 50 ng/mL (p < 0.0001) and 500 ng/mL (p < 0.05), and a reduction at 1000 ng/mL (p < 0.05), suggesting impaired viability at high concentrations under physiological conditions. The distribution of microglial phenotypes in physiological M5 co-cultures incubated with CBD remained unchanged. In M30 co-cultures, CBD incubation for 1 and 24 h significantly reduced microglial activation and promoted a shift from reactive, phagocytic to homeostatic, ramified microglial phenotype (p < 0.05, p < 0.01, p < 0.0001). In contrast, PGB did not affect glial cell viability, microglial phenotypes or Cx43 expression in physiological and inflammatory co-cultures, indicating that the mechanisms of action of PGB probably do not include modulation of glial cells in vitro. Conclusions: The inhibition of microglial reactivity by CBD suggests potential positive effects on the neuroinflammatory component involved in the pathogenesis of CNS disorders such as epilepsy.