Background and Aim: Glial cells are activated at multiple sites along the pain pathway following peripheral trauma or inflammation, thus inducing production and release of several pro-inflammatory cytokines which act in a autocrine/paracrine fashion, drive pain amplification and maintain chronic pain states like neuropathic pain. Toll-like receptor 4 (TLR4) engagement mediates the initial release of pro-inflammatory cytokines, such as TNF-, which in turn sustain prolonged glial activation. TLR4-mediated modulation of glial activity as well as opioid-induced glial activation have been extensively investigated; however, mu-opioid receptor (MOR) expression and function in glial cells have been little explored. Aim of this study has been to investigate MOR expression upon exposure of U87-MG human glial cells and rat primary microglia to TNF- 24 – 72 h), and to evaluate any effect elicited by different opioid agonists on MOR intracellular signalling. Results: We found that TNF- determined a significant, concentration- and time-dependent up-regulation of MOR mRNA and protein levels; as expected, morphine and DAMGO induced ERK phosphorilation in untreated glial cells, but not in TNE--pre-treated glial cells. Our findings show that MOR expression and function in glial cells are significantly altered following prolonged exposure to TNF-, as MORmediated activation of MAPK pathway in glial cells seems to be desensitized after TNF- treatment, albeit the significant up-regulation of MOR expression. Conclusions: These results suggest that MOR may play a relevant role in glial cells which is altered by prolonged exposure to TNF-: any influence on glial modulation of neuronal functions and opioid effectiveness in chronic pain states needs further investigations.