On the possible existence of a new intraneuronal monoamine in the spinal cord of the rat.

Intraneuronal monoamines have been studied with the histochemical fluorescence method of Falck and Hillarp in the rat spinal cord. The formaldehyde-inducedintrancuronal fluorophores have been analyzed with microspectrofluorometry in combination with drugs which deplete monoamines from the central nervous system. Three types of intraneuronal fluorescence were found. 1) A green fluorescence, with spectra identical with those of the catecholamine fluorophores, which was depleted by reserpine and α-methyl- m -tyrosine but not by p -chlorophenylalanine, was found. This fluorescence was most likely derived from Primary catecholamines. 2) A yellow fluorescence, with spectra identical with those of the 5-hydroxytryptamine fluorophore, which was depleted by reserpine and p -chlorophenylalanine but not by α-methyl- m -tyrosine, was found. This fluorescence was probably derived from 5-hydroxytryptamine. 3) A yellow to brownish-yellow fluorescence, with spectra clearly deviating from those of the catecholamine and the 5-hydroxytryptamine fluorophores, which was depleted by reserpine but not by p -chlorophenylalanine or α-methyl- m -tyrosine, was found. Moreover, this fluorescenceshowed upon ultraviolet irradiation a photodecomposition rate, which was markedly slower than that of the 5-hydroxytryptamine fluorophore. These results strongly indicate that this intraneuronal fluorogenic substance is not identical with a catecholamine or 5-hydroxytryptamine. The spectral characteristics of this third type of fluorescence showed general features characteristic for many indolykthylamines.