Serotonin and substance P coexist in neurons of the rat's central nervous system

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Abstract
5-Hydroxytryptamine (serotonin)-containing neurons in the rat''s medullary raphe and interfascicularis hypoglossi cell groups were identified by autoradiography following prolonged intraventricular administration of 5-hydroxy[3H]tryptamine, fluorescence histochemistry for the demonstration of endogenous 5-hydroxytryptamine, and microspectrofluorimetric analysis of excitation and emission spectra. Immunocytochemical methods (the unlabeled primary antibody-peroxidase antiperoxidase and indirect immunofluorescence methods) were applied with antisera to substance P in order to localize immunoreactivity in these medullary neurons. The raphe nuclei and the interfascicularis hypoglossi nucleus were heterogeneous cell groups that contained neurons that displayed an uptake-storage capacity for 5-hydroxy[3H]tryptamine and a formaldehyde-induced fluorescence with spectral characteristics identical to those of the 5-hydroxytryptamine fluorophor; (these cells exhibited hight to low fluorescence intensities without detectable substance P-like immunoreactivity), neurons with various 5-hydroxytryptamine fluorescence intensities and intense to low degrees of substance P-like immunoreactivity; and neurons with various degrees of substance P-like immunoreactivity without detectable 5-hydroxytryptamine fluorescence or 5-hydroxy[3H]tryptamine uptake and storage capacity. Apparently some neurons contain high or low levels of only 5-hydroxytryptamine or substance P, whereas other neurons contain 5-hydroxytryptamine and substance P in various proportions. The present findings demonstrated the presence of 2 putative transmitters, a biogenic amine and a polypeptide, within the same neuron in the mammalian CNS.