HISTOCHEMICAL IDENTIFICATION OF NORADRENALINE IN FLUORESCENCE MICROSCOPY BY BOROHYDRIDE-PERIODIC ACID SEQUENCE

The catecholamines—dopa, dopamine, adrenaline and noradrenaline—are known to produce fluorescing 3,4-dihydroisoquinoline derivatives on condensation with formaldehyde. These dihydro products could be converted to 1,2,3,4-tetrahydroisoquinoline derivatives by borohydride reduction and in turn again to dihydro forms by re-exposing to formaldehyde vapor. Periodic acid was found to cleave the —C—C—bond at 3—4 position of tetrahydroisoquinoline derivative of noradrenaline only. This prevented the cyclitization of heterocyclic ring of tetrahydro form to give fluorescing dihydro product. This reaction can be utilized for the localization of noradrenaline in histochemical fluorescent preparations. Noradrenaline is prevented from regaining its fluorescence on re-exposure to formaldehyde by imposing periodic acid oxidation after borohydride bleaching.