CHEMICAL, ENZYMATIC AND ULTRASTRUCTURAL CHARACTERIZATION OF 5‐HYDROXYTRYPTAMINE‐CONTAINING NEURONS FROM THE GANGLIA OF APLYSIA CALIFORNICA AND TRITONIA DIOMEDIA

— Several identified neurons in Aplysia and Tritonia ganglia were shown to contain measurable quantities (4–6 pmol/cell body) of 5‐hydroxytryptamine (5‐HT). A metabolic correlate for the limited distribution of 5‐HT among the neurons of Tritonia is provided by the finding that the enzyme, aromatic acid decarboxylase (AAD), is 500 times more active in nerve cells containing 5‐HT than in neurons devoid of the amine. Although all Aplysia neurons have some AAD activity, 5‐HT cell bodies in this species are 10‐fold more active than cell bodies which do not contain 5‐HT. The cytoplasm of 5‐HT cell bodies in Aplysia and Tritonia characteristically contains granules that have minimum diameters of approx. 1000 Å and eccentric opaque cores. This type of granule was not found in somata which did not contain measurable 5‐HT. These data illustrate the metabolic and morphological specialization in 5‐HT‐containing neurons of molluscs.