Pivotal role of arachidonic acid in the regulation of neuronal nitric oxide synthase activity and inducible nitric oxide synthase expression in activated astrocytes

Astrocytes respond to agents leading to progressively greater increases in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) with a linear release of arachidonic acid (ARA), due to activation of cytosolic phospholipase A(2), and with a bell-shaped curve of nitric oxide (NO) release, due to Ca(2+)-dependent activation/inhibition of neuronal NO synthase (nNOS). Inhibition of nNOS is mediated by a signaling driven by ARA, either extensively released at high [Ca(2+)](i) or supplemented to the cultures at nanomolar levels. Proinflammatory factors, as bacterial lipopolysaccharide/interferon-gamma, cause rapid ARA-dependent nNOS inhibition, critical for the delayed expression of nuclear factor-kappaB (NF-kappaB)-dependent genes as inducible NOS. We therefore propose that the onset of the neuroinflammatory response is strictly regulated by the relative amounts of NO and ARA produced by their constitutive enzymes. In particular, the inflammatory product ARA initiates the inflammatory response via inhibition of nNOS, thereby allowing NF-kappaB activation. Astrocytes contribute to the regulation of this process by producing both constitutive NO and ARA, as well as by expressing NF-kappaB-dependent genes.