Nitric oxide subserves diverse physiologic roles in the nervous system. NO is produced from at least three different NO synthase (NOS) isoforms: neuronal NOS (nNOS), endothelial NOS, and immunologic NOS (iNOS). We show that nNOS is the predominant isoform constitutively expressed in glia. NO derived from nNOS in glia inhibits the transcription factor nuclear factor B (NFB) as NOS inhibitors enhance basal NFB activation. Pyrrolidine dithiocarbamate (PDTC) is an inhibitor of NFB in most cells; however, we show that PDTC is also a potent scavenger of NO through formation of mononitrosyl iron complexes with PDTC. In Jurkat cells, a human T-cell lymphoma cell line, tumor necrosis factor-␣ (TNF-␣) induces NFB activation that is inhibited by PDTC. Contrary to the results in Jurkat cells, PDTC did not inhibit tumor necrosis factor-␣-induced NFB activation in astrocytes; instead PDTC itself induces NFB activation in astrocytes, and this may be related to scavenging of endogenously produced NO by the PDTC iron complex. In astrocytes PDTC also dramatically induces the NFBdependent enzyme, iNOS, supporting the physiologic relevance of endogenous NO regulation of NFB. NFB activation in glia from mice lacking nNOS responds more rapidly to PDTC compared with astrocytes from wild-type mice. Our data suggest that nNOS in astrocytes regulates NFB activity and iNOS expression, and indicate a novel regulatory role for nNOS in tonically suppressing central nervous system, NFBregulated genes.Nitric oxide is a potent messenger molecule with diverse physiologic activities, including regulation of vascular tone, neurotransmission, and killing of microorganisms and tumor cells (1-3). NO is produced from L-arginine (L-Arg) by the enzyme NO synthase (NOS). A family of related NOS proteins are the products of different genes and include neuronal NOS (nNOS, type 1), immunologic NOS (iNOS, type 2), and endothelial NOS (eNOS, type 3) (3). nNOS occurs in discreet neuronal populations in the brain and also is localized to the sarcoplasmic reticulum of skeletal muscle (4). eNOS primarily has endothelial cell localizations, but also is localized to a variety of other tissue types, including CA1 pyramidal cells of the hippocampus (5). Both nNOS and eNOS are constitutively expressed and are calcium-calmodulin-dependent enzymes (3, 4). iNOS is expressed in response to cytokines, lipopolysaccharide (LPS), and a host of other agents (6, 7). iNOS has been localized to a variety of cell types upon appropriate immunologic stimulation (6, 7). The key to regulation of NO production by iNOS is through regulation of transcription (8, 9).Characterization of the promoter region of the gene for iNOS reveals a complex pattern of regulation (8-12). Upstream from the transcription start site are distinct regulatory regions, including LPS-related response elements, binding sites for NFB, and ␥-interferon motifs (8-11). Recent studies indicate that NO transcriptionally inhibits iNOS mRNA expression in astrocytes (13). However, the mechanism by which NO transc...