EXPRESSION OF THE NEURAL FORM OF NITRIC-OXIDE SYNTHASE BY CA1 HIPPOCAMPAL-NEURONS AND OTHER CENTRAL-NERVOUS-SYSTEM NEURONS

Nitric oxide can act as a neurotransmitter and a retrograde modulator of synaptic transmission, but uncontrolled nitric oxide synthase activ ity has been associated with neural degeneration. Although earlier stu dies using immune histochemistry, in situ hybridization, and NADPH-dia phorase staining had suggested that nitric oxide synthase is not expre ssed in the CA1 neurons of the hippocampus, we have recently demonstra ted that NADPH-diaphorase activity can be detected in CA1 neurons of t he hippocampus. To confirm that this diaphorase activity reflects nitr ic oxide synthase, we have developed a more sensitive in situ hybridiz ation procedure, and an RNase protection assay to detect message for c onstitutive nitric oxide synthase, the form constitutively expressed i n many neurons. Message for constitutive nitric oxide synthase is expr essed in the hippocampus, and it is localized to neural cell layers CA 1, CA3, the dentate gyrus and some displaced neurons, but not to CA2. Expression of constitutive nitric oxide synthase message in the CA1 re gion was lost when pyramidal neurons died due to transient forebrain i schemia, supporting the conclusion that CA1 pyramidal cells express co nstitutive nitric oxide synthase. Although constitutive nitric oxide s ynthase message is strongly expressed in CA3 and the dentate gyrus, th ere is little diaphorase activity in these cells, suggesting that ther e map be post-transcriptional controls that limit constitutive nitric oxide synthase expression in some cells. Message for constitutive nitr ic oxide synthase is also present in a number of other regions, includ ing the amygdala, several hypothalamic nuclei, the cerebellum the olfa ctory bulb, two distinct regions of the perirhinal cortex, the subthal amic nuclei, a neuronal layer in the retrosplenial granular cortex, th e lateral geniculate nucleus, the presubiculum, the inferior colliculu s, the superior colliculus, the pedunculopontine tegmental nucleus, an d scattered individual neurons in the cortex, hippocampus and brainste m. These studies support a role for nitric oxide in multiple regions o f the central nervous system. In particular, nitric oxide synthase, th e enzyme responsible for the synthesis of nitric oxide, is expressed i n the CA1 region of the hippocampus, where there is evidence that nitr ic oxide may play a major role in long-term potentiation. CA1 hippocam pal neurons are an example of a population of neurons that express con stitutive nitric oxide synthase but are very sensitive to excitotoxici ty and ischemic insults.