Specific association of neurotransmitter with somatic lysosomes in an identified serotonergic neuron of Aplysia californica.

Injection of [3H]serotonin directly into the cell body of the giant metacerebral neuron (GCN), an identified serotonergic cell in the Aplysia cerebral ganglion, revealed a striking association of the labeled transmitter with large lysosomes. Most of the [3H]serotonin in the neuron is sequestered by lysosomes, which may function as storage depots for the transmitter. This localization is specific: [3H]serotonin was not found in similar lysosomes in the perikaryon of R2, an identified Aplysia cholinergic neuron. [3H]Dopamine, [3H]histamine, and [3H]-N-acetylgalactosamine injected into the serotonergic neuron were not localized in lysosomes. Because of this specificity, it may be possible to use lysosomal binding of labeled transmitter as a radioautographic method for identifying the transmitter type of a nerve cell body. These organelles are lysosomal in origin because they contain acid phosphatase activity. Moreover, examination of isolated GCN cell bodies by fluorescence microscopy revealed the presence of abundant autofluorescent yellow pigment characteristic of lipofuscin. We suggest that [3H]serotonin labels organelles containing membranes that once were integral components of the serotonergic vesicle. Lysosomes might accumulate the transmitter with a high degree of specificity because they contain membranes, perhaps recycled from the neuron's synaptic terminals, that retain their ability to concentrate and bind serotonin.