Unlike alcohol, scant evidence links microglia to drug use disorders (DUDs) (see Table II). Microglia express opioid receptors that inhibit chemotaxis (Chao et al. 1997) and induce apoptosis when stimulated (Hu et al. 2002). On a behavioural level, increased rodent maternal handling inhibits reinstatement of morphine-induced CPP in adulthood by increasing microglial expression of the anti-inflammatory IL-10 in the rat NAcc (Schwarz et al. 2011). Furthermore, microglial p38 (a mitogen activated protein kinase/extracellular signal-related kinase pathway intermediate) in the NAcc also mediates the acquisition and maintenance of morphine-induced CPP (Zhang et al. 2012). Microglial TLRs, e.g., TLR2 (Zhang et al. 2011), TLR4 (Watkins et al. 2009), and TLR9 (He et al. 2011), also modulate the neurochemical and behavioural effects of opioids in rodents.
