Loss of M1 Acetylcholine Receptor-mediated Orexinergic Activity Contributes to Immune Dysfunction in Experimental Sepsis

Background : Sepsis (life-threatening organ dysfunction caused by a dysregulated host response to infection) causes millions of deaths worldwide annually. Sepsis-induced changes in brain regulatory functions remain understudied. Previous work demonstrated that cecal ligation and puncture (CLP, a murine model of sepsis) affected physiologic variables and serum cytokines and hormone levels. Correction of decreased activity in the orexinergic nervous system or administration of the M1 muscarinic acetylcholine receptor (M1mAChR) agonist xanomeline reversed some of these findings. We hypothesized that these CLP - induced changes resulted, in part, from a loss of M1mAChR-mediated orexinergic nervous system activation . Main Body : Xanomeline reversed CLP-induced loss of orexinergic activity and restored physiologic variables and hormone levels to baseline; these corrections were eliminated by addition of the orexin receptor antagonist almorexant. To examine the effects of system reactivation we developed a transgenic mouse whose orexinergic neurons could be depolarized via a Designer Receptor Exclusively Activated by Designer Drugs (DREADD) and its ligand, clozapine-N-oxide (CNO). Orexinergic re-activation or xanomeline administration reversed CLP-induced changes in TNFa and IL-1b levels; almorexant eliminated xanomeline effects. CNO reversed the effects of CLP on serum levels of IL-6 and KC; this effect was not present after xanomeline administration. G-CSF, a colony stimulating factor, was not affected by either CNO or xanomeline. Both orexinergic activation and xanomeline administration reversed CLP-induced increase in the number of splenic macrophages and monocyte-derived dendritic cells (DCs); almorexant did not affect the response to xanomeline. CLP-induced decreases in the numbers of central DCs, CD4 ⁺ or CD8 ⁺ T cell numbers in the spleen; this response was not altered by either CNO or xanomeline. Conclusion : Decreased orexinergic activity mediates some post-CLP immunologic changes, identifying a previously unrecognized proximal pathogenic mechanism in sepsis. Some, but not all, of these changes result from a loss of M1mAChR-mediated stimulation of orexinergic neurons. These findings suggests that disruption of orexin’s central coordinating function is a key, and perhaps causative, component of the dysregulated host response that is the defining characteristic of sepsis.