TNF activates astrocytes and catecholaminergic neurons in the solitary nucleus: implications for autonomic control

Tumor necrosis factor [TNF] produces a profound anorexia associated with gastrointestinal stasis. Our work suggests that the principal site of action of TNF to cause this change in gastric function is via vagal afferents within the nucleus of the solitary tract [NST]. Excitation of these afferents presumably causes gastric stasis by activating downstream NST neurons that, in turn, suppress gastric motility via action on neurons in the dorsal motor nucleus of the vagus that project to the stomach. Results from our parallel studies on gastric vago-vagal reflexes suggest that noradrenergic neurons in the NST are particularly important to the generation of reflex gastroinhibition. Convergence of these observations led us to hypothesize that TNF action in the NST may preferentially affect putative noradrenergic neurons. The current study confirms our observations of a dose-dependent TNF activation of cells [as indicated by cFOS production] in the NST. The phenotypic identity of these TNF-activated neurons in the NST was approximately 29% tyrosine hydroxylase [TH]-positive [i.e., presumably noradrenergic neurons]. In contrast, less than 10% of the nitrergic neurons were activated after TNF exposure. Surprisingly, another 54% of the cFOS-activated cells in the NST were phenotypically identified to be astrocytes. Taken together with previous observations, the present results suggest that intense or prolonged vagal afferent activity [induced by visceral pathway activity, action of gut hormones or cytokines such as TNF] can alter local astrocyte immediate early gene expression that, in turn, can provoke long-term, perhaps permanent changes in the sensitivity of vagal-reflex circuitry.