A topography and ultrastructural characterization of in vivo 5,7-dihydroxytryptamine-labeled serotonin-containing neurons in the central nervous system of Aplysia californica.

1. Several weeks after administration of 5,7-dihydroxytryptamine (5,7-DHT) to Aplysia, a dark pigmentation appears in serotonin-containing neurons, and this pigmentation allows visual identification of serotonergic neurons but does not appear to alter their physiology. 2. We have determined the distribution of labeled nerve cell bodies in the various ganglia of Aplysia and have characterized the pigment containing structures in both control and labeled neurons. 3. All neurons in this preparation, whether or not they utilize serotonin as a transmitter, contain pigment granules, and three types of pigment granules can be distinguished. After 5,7-DHT a new type of granule appears in serotonergic neurons, probably reflecting lysosomes that have accumulated serotonergic synaptic vesicles that contain the oxidized 5,7-DHT. 4. It remains unclear why this substance does not cause neurotoxicity in mollusks as it does in mammalian preparations.