Atypical indolamine-immunoreactive cell groups in the dorsal myelencephalon of the rat

The presence of atypical indolamine-immunoreactive (IAI) neurones in the dorsal myelencephalon of the rat was demonstrated by means of peroxidase-antiperoxidase (PAP) immunocytochemistry. Besides the area postrema, two other regions, viz. the solitary complex and the superficial rostral cuneate fascicle, were found to contain neuronal perikarya displaying a normally weak staining which was markedly enhanced after monoaminoxidase (MAO) inhibition. In contrast to immunoreactive cells of the periaqueductal central gray, the dorsal myelencephalic IAI neurons were undetectable after serotonin synthesis inhibition with p-chlorophenylalanine (PCPA), as were immunoreactive terminal neuropils in most brainstem areas. However, sequential treatment with the MAO inhibitor, iproniazid, completely reversed the PCPA-induced suppression of perikaryal immunoreactivity and partially restored axonal staining. None of the atypical cell groups displayed a detectable formaldehyde-induced specific histofluorescence. Since brain levels of tryptamine are likely to increase significantly after MAO inhibitor/PCPA treatment, and furthermore tryptamine can be assumed to cross-react with serotonin, it is suggested that the observed atypical IAI neurons may represent either 1. (1) a subpopulation of serotoninergic neurons; 2. (2) previously postulated true tryptamine neurons; or 3. (3) non-indolaminergic neurons endowed with a selective uptake mechanism for serotonin or tryptamine. These results corroborate the view that different types of indolamine neurons exist in the rat brainstem. Moreover, they underscore the need for cautious interpretation of serotonin neuron mapping studies involving the use of MAO inhibitors.