Evidence for nitric oxide-generator cells in the brain.

Nitric oxide (NO) is a free radical that has been recently recognized as a neuronal messenger molecule. In order to understand the way in which NO functions in the central nervous system (CNS), it is important to identify the NO-generator and NO-target cells in the brain. I measured firstly the distribution of NO synthase in the brain, which catalyzes L-arginine to form NO, by the measurement of citrulline formation that is also synthesized from L-arginine together with NO in equal molar bass. In the brain of adult rat, the most potent activity of NOS was apparent in the cerebellum, next in the olfactory bulb and medium in the cerebrum. Further, in the presence of NADPH and Ca2+, NOS activity was detected in the neuron cultures derived from the cerebrum of fetal rat. Astrocytes, one type of glia, prepared also from the cerebrum of fetal rat, appeared to have a small but significant NOS activity. As astrocytes possess a high amount of cytosolic guanylate cyclase that is known to be activated by NO, the changes in the intracellular cGMP levels in the astrocytes were measured as another index of NO formation. The treatment of astrocytes with NOS inhibitor caused the suppression of the intracellular cGMP levels. These results indicate that NO is definitely produced by astrocytes. In addition, in the blood vessel system of the brain, although NOS has been thought to be localized in the endothelial cells of only larger vessels, NOS activity was also observed in the microvessel endothelial cells of the cerebrum of both adult and fetal pig. These data suggest that neuronal cells may be the major site of NO generation in the brain, and that the NOS is a constitutive type. The data also suggest that astrocytes can also express constitutive NOS, although the potency is not so large. Microvessel endothelial cells of the brain are also one of the sources of NO. The NO produced by these cells increases the cGMP levels in the astrocytes and may affect some physiological and/or pathophysiological events in the brain.