THE CHARACTERIZATION OF HISTIDINE DECARBOXYLASE AND ITS DISTRIBUTION IN NERVES, GANGLIA AND IN SINGLE NEURONAL CELL BODIES FROM THE CNS OF APLYSIA CALIFORNICA

Histamine (HA) is present in substantial quantities in all ganglia of Aplysia californica. Within the cerebral ganglia this amine is known to be concentrated in at least two identified neurons designated C-2 neurons. In this study a combination of chemical and enzymatic analyses was employed to provide evidence for the existence of a biochemical pathway for HA synthesis in ganglia and individual neurons of this marine mollusk. Examination of extracts of individual neurons dissected from ganglia organ-cultured in the presence of ³H]histidine showed that every neuron accumulated labelled histidine, but only the HA-containing C-2 neurons synthesized and stored labelled HA suggesting that the formation of HA in Aplysia could be catalyzed by the enzyme histidine decarboxylase (HDC). HDC activity was studied with a new microradiometric assay. Many of the properties of the molluscan HDC studied were found to correspond to the vertebrate enzyme. Enzyme activity was inhibited by α-hydrazino-histidine but unaffected by concentrations of α-methyldopa or by 5-(3,4-dihydroxycinnamoyl) salicylic acid which produced nearly complete inhibition of aromatic amino acid decarboxylase activity. HDC was measurable in nervous but not other Aplysia tissues assayed. All 5 major ganglia contained HDC activity which spanned a 15-fold range between the least and most active ganglia. Only 4 of the 13 nerve trunks assayed yielded measurable enzymic activity; these active nerves were associated with the cerebral ganglia which has the highest HDC activity of all measured ganglia. Of the numerous individual neurons assayed for HDC, only the C-2 cells showed measurable enzyme activity, about 25 pmol/cell/h or 70 μmol/g protein/h. Since the activity of HDC in the HA-containing neurons was at least three orders of magnitude larger than all other neurons assayed in the cerebral and other ganglia, these data appear to provide a direct metabolic basis for the selective presence of HA in these cells, and they indicate that the cellular presence of HDC provides a useful biochemical marker for the location of HA-rich neurons in Aplysia.