The presence of cell surface Ca2+ permeable alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainate (Ca-A/K) channels on subsets of central neurons influences both normal physiological function and vulnerability to excitotoxicity. Factors that regulate the formation and membrane insertion of Ca-A/K channels, however, are poorly understood. Recently, the cytokine tumor necrosis factor-alpha (TNF-alpha) was shown to increase the cell surface expression of an AMPA receptor (AMPAR) subunit (GluR1) and to potentiate vulnerability to AMPAR-mediated injury. In this study, we examined the possibility that TNF-alpha might also increase numbers of functional Ca-A/K channels. In acute hippocampal slice preparations, TNF-alpha appeared to increase Ca-A/K channel numbers in pyramidal neurons (HPNs), as assessed using a histochemical stain based on kainate-induced uptake of Co2+ ions (Co2+ labeling). In dissociated hippocampal neuronal cultures, TNF-alpha exposure (6 nM, 15 min) induced a rapid increase in cell surface levels not only of GluR1, but also of the AMPAR subunit GluR2, on most neurons, without evident new protein synthesis. Furthermore, consistent with the slice studies, fluorescence Ca2+ imaging techniques revealed an increase in numbers of Ca-A/K channels on what appeared to be a subset of HPNs. These observations are the first to provide evidence for the rapid upregulation of neuronal Ca-A/K channels in response to a cytokine or any other soluble factor, and provide a novel mechanism through which TNF-alpha may modulate both synaptic function and neuronal vulnerability.