Abstract
Nitric oxide (NO), an unstable radical formed via oxidative deamination of L-arginine by NO synthase, has the activity to evoke release of several neurotransmitters including acetylcholine, catechol-amines and neuroactive amino acids. N-methyl-D-aspartate (NMDA) receptor stimulation also causes neurotransmitter release through NO formation, which is supported by the data that extracellular hemoglobin ompletely abolishes the stimulatory effect of NMDA on neurotransmitter release. In addition, NO formed by NMDA receptor activation exhibits its stimulatory action on neurotransmitter release extracellularly. A product formed by the reaction of NO and superoxide, peroxynitrite, is also considered to be partly involved in NO-evoked neurotransmitter release. The removal of Ca2+ or Na+ significantly reduces the release of GABA evoked by an NO generator, S-nitroso-N-acetylpenicillamine (SNAP), and simultaneous withdrawal of these ions completely abolishes the SNAP-induced release of GABA. Either in the presence or absence of Ca2+, GABA transport inhibitors such as nipecotic acid dose-dependently suppress the SNAP-induced GABA release in the presence of Na+. These results indicate that NO-evoked neurotransmitter release is mediated by two distinct release systems, a Ca2+-dependent system and the reverse process of a Na+-dependent carrier-mediated transport system.
