Recent developments in aldehyde-induced monoamine fluorescence: the aluminum-formaldehyde (ALFA) method applied to immature and adult central nervous tissue.

In this review, the new aluminum-formaldehyde (ALFA) histofluorescence method for the highly sensitive visualization of monoamine-containing neurones in adult and immature central nervous tissue is summarized. Animals are first perfused with a buffer containing high concentrations of aluminum ions and the brains are then freeze-dried, reacted with formaldehyde vapour and further processed according to the Falck-Hillarp fluorescence method. The ALFA technique applied to adult brains visualizes all known catecholamine neurone systems with a sensitivity comparable to, and for certain noradrenergic systems higher than, that of the previously published glyoxylic acid-Vibratome method. The catecholamine systems in immature brains are demonstrated with a sensitivity clearly superior to that of any other available method. If the ALFA method is combined with systemic injections of alpha-methylnoradrenaline into young animals (less than one week old), there is a dramatic increase in the intensity and number of catecholaminergic fibres. Many catecholaminergic systems which have too low concentrations of transmitter to be visualized in the untreated animal even with the ALFA method, can be demonstrated after administration of alpha-methylnoradrenaline. The use of freeze-dried, paraffin-embedded tissue in the ALFA method makes possible convenient storage and parallel processing of many specimens. This mode of processing also allows en bloc reaction, which is the only way by which consistent and reproducible fluorescence yields can be obtained throughout large series of sections and parallel-processed specimens. In animals pretreated with L-tryptophan and monoamine oxidase-inhibitor, the technique is also useful for studies on central indolamine-containing systems.