FLUORESCENCE METHODS FOR THE HISTOCHEMICAL DEMONSTRATION OF MONOAMINES: 4. HISTOCHEMICAL DIFFERENTIATION BETWEEN DOPAMINE AND NORADRENALINE IN MODELS

If primary catecholamines e.g. dopamine and noradrenaline in a dried protein layer are exposed to gaseous formaldehyde, they are converted to intensely fluorescent 6,7-dihydroxy-3,4-dihydroisoquinolines. This reaction can be used for the histochemical demonstration of these amines. The fluorescent products of dopamine and noradrenaline have practically identidal activation and emission spectra. The 4,6,7-trihydroxy-3,4-dihydroisoquinoline formed from noradrenaline has, however, a labile hydroxy group at 4-position which can be easily split off. This dehydration to the fully aromatic 6,7-dihydroxyisoquinoline was found to take place readily in a dried protein layer by treatment with thionyl chloride gas at 50°C. The isoquinoline shows other flurorescence characteristics than the 3,4-dihydroisoquinolines and is not reduced by sodium borohydride. A direct histochemical differentiation between dopamine and noradrenaline is thus possible.