Cellular specificity of serotonin storage and axonal transport in identified neurones of Aplysia californica.

1. [(3)H]D,L-5-hydroxytryptophan ([(3)H]5HTP) was injected under pressure into cell bodies of identified cholinergic and serotonergic neurones in the central nervous system of the marine mollusc, Aplysia californica.2. Both serotonergic and cholinergic neurones converted [(3)H]5HTP to [(3)H]5-hydroxytryptamine ([(3)H]5HT).3. The fate of [(3)H]5HT in the two types of neurones differed. In serotonergic cells, 5HT was present primarily in particulate form; the transmitter readily moved from cell bodies into nerves by selective transport. In contrast, 5HT remained free in the cytoplasm of the cholinergic neurone, and was not transported from the cell body.4. Treatment of Aplysia with reserpine decreased the proportion of [(3)H]5HT associated with particulate material, and also decreased the amount of [(3)H]5HT recovered.5. Serotonergic neurones possess specific mechanisms for the storage and axonal transport of 5HT which are absent in cholinergic cells.