Nitric oxide imaging in living neuronal tissues using fluorescent probes

Nitric oxide (NO) is a major modulator of neural functions. Since NO is a gaseous molecule with very short half-life, the spatial distribution of NO and its relationship to neuronal activity are difficult to resolve. Non-invasive and direct visualization of NO in neuronal tissues had been hampered by the lack of a suitable method to identify NO directly. A fluorescent indicator, which directly detects NO under physiological conditions, would be advantageous. Several indicators for direct detection of NO have been developed, which react with NO by forming a fluorescent complex. However, some of these dyes have cytotoxic properties or have been found to be rather unspecific under certain conditions. Fortunately, some of the indicators, which change their fluorescent pattern in the presence of NO, appear to be promising for the visualization of NO. Since little is known about the spatial spread and the temporal aspects of NO release after a specific stimulus, the use of the specific and non-toxic fluorescent NO indicators could provide a potentially powerful tool to study these aspects of NO release in neuronal tissues in vitro and in vivo. Such measurements, especially in combination with electrophysiological recordings, would greatly further NO research. In addition, based on their fluorescent pattern, these NO-sensitive dyes can be distinguished from the calcium-sensitive dye Fura-2, which allows NO-imaging together with calcium-imaging. This article summarizes recent advances and current trends in the visualization of NO in living neuronal tissues.