An immunocytochemical study on the GABA-ergic and serotonin-ergic neurons in rat locus ceruleus with special reference to possible existence of the masked indoleamine cells.

Alternate consecutive cryostat sections of the rat locus ceruleus (LC) were immunostained for either GABA or serotonin by the unlabelled peroxidase-antiperoxidase method. By identifying the same neurons in adjacent sections of this series, we confirmed that the GABA- and serotonin-ergic neurons are different cell groups. Serial sections of the LC were alternately stained either by the immunocytochemical method for serotonin or with cresyl violet for neuroanatomical observations, under normal and various experimental conditions using pharmacological manipulations. By using both a monoamine oxidase (MAO) inhibitor (pargyline) and a serotonin precursor, 5-hydroxytryptophan rather than L-tryptophan, with or without an inhibitor of serotonin synthesis, p-chlorophenylalanine, we have demonstrated that many LC neurons have the capacity to accumulate serotonin and not other indoleamines. It is suggested that the LC has numerous masked indoleamine cells that contain large amounts of MAO in the cytoplasm, so that under physiological conditions, the serotoninimmunoreactivity of these cells is masked.