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The histochemical fluorescence method of Falck and Hillarp for the demonstration of biogenic monoamines is based on the finding that the amines can be condensed with formaldehyde to yield strongly fluorescent compounds, provided that they are enclosed in a dried protein layer, as in freeze-dried or air-dried tissues. This review deals mainly with certain principal features of the method: its chemistry, sensitivity, specificity and possibilities for histochemical differentiation between the various amines. Some comments are made on certain of the results obtained with this method.

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It has been shown in previous papers that catecholamines and 5-hydroxytryptamine can under certain conditions be converted to highly fluorescent 6,7 - dihydroxy - 3,4 - dihydroisoquinolines and 6-hydroxy-3,4-dihydro-β-carboline respectively, and that this can be used as a highly sensitive and specific method for the histochemical demonstration of the monoamines at the cellular level. In the present paper it is shown that the fluorescent compounds are very readily reduced by sodium borohydride to the corresponding, non-fluorescent 1,2,3,4-tetrahydro-compounds—even if they are present in a non-extractable state in dried serum albumin spots or in tissue sections—and that the fluorescence can be regenerate by renewed formaldehyde treatment. The non-specific fluorescence ( e. g. autofluorescence) in tissue sections was never observed to undergo any changes on borohydride treatment. On the basis of these findings a very simple histochemical test has been worked out to check directly in the tissue section whether or not an observed fluorescence is due to the presence of the reacting monoamines.

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