SEMIQUANTITATIVE ESTIMATION OF FORMALDEHYDE-INDUCED FLUORESCENCE OF NORADRENALINE IN CENTRAL NORADRENALINE NERVE TERMINALS

The fluorescence-concentration relationship of noradrenaline (NA) in NA nerve terminals of rat cerebral cortex and hypothalamus demonstrated with the Falck-Hillarp formaldehyde fluoresence method has been investigated. Comparisons were made between experimentally induced changes in NA concentrations as recorded by chemical-analytical assay and fluorescence intensities of NA nerve terminals as estimated subjectively in the fluorescence microscope. The results obtained show that in cerebral cortex there was a close correlation between NA concentration and fluorescence intensity up to the normal endogenous NA level, indicating a linear relationship between the two parameters. In hypothalamus, however, a correlation between fluorescence intensity and NA concentration was observed only when the nerves contained less than 50% of the endogenous NA level. Above this value, there was no further proportional increase in fluorescence intensity, probably because of a concentration-dependent quenching of the fluorescence. Essentially the same results were obtained when investigating previously emptied NA nerve terminals refilled with various amounts of 3 H-NA. The data presented clearly show that a good NA quantitation of cortical nerve terminals can be made by careful subjective estimation of the fluorescence intensity.