Blockade of kappa-opioid receptors amplifies microglia-mediated inflammatory responses.

Brain kappa-opioid receptors (KORs) are implicated in the pathophysiology of depressive and anxiety disorders, stimulating interest in the therapeutic potential of KOR antagonists. Research on KOR function has tended to focus on KOR-expressing neurons and pathways such as the mesocorticolimbic dopamine system. However, KORs are also expressed on non-neuronal cells including microglia, the resident immune cells in the brain. The effects of KOR antagonists on microglia are not understood despite the potential contributions of these cells to overall responsiveness to this class of drugs. Previous work in vitro suggests that KOR activation suppresses proinflammatory signaling mediated by immune cells including microglia. Here, we examined how KOR antagonism affects microglia function in vivo, together with its effects on physiological and behavioral responses to an immune challenge. Pretreatment with the prototypical KOR antagonist JDTic potentiates levels of proinflammatory cytokines (IL-1β, IL-6) in blood following administration of lipopolysaccharide (LPS), an immune-activating agent, without triggering effects on its own. Using magnetic-activated cell sorting (MACs), we found that KOR antagonism potentiates LPS-induced cytokine expression within microglia. This effect is accompanied by potentiation of LPS-induced hyperthermia, although reductions in body weight and locomotion were not affected. Histological analyses confirm that LPS produces visible changes in microglia morphology consistent with activation, but this effect is not altered by KOR antagonism. Considering that inflammation is increasingly implicated in depressive and anxiety disorders, these findings raise the possibility that KOR antagonist actions on microglia may detract from actions on neurons that contribute to their therapeutic potential.