Uptake of [3H]serotonin and [3H]dopamine has been investigated in the central nervous system of the marine mussel, Mactra stultorum L. by means of light and electron microscopic autoradiography. It was established that among the axon profiles of the neuropil of the ganglia, axons containing dense-core vesicles with 700-1500 Å diameter play the primary role in the uptake of both serotonin and dopamine. Although the nerve cell bodies in the cortical layer were not extensively labelled, glial cells and processes in this layer took up large amounts of the labelled amines. This autoradiographic study shows that transmitter re-uptake is a possible means of transmitter inactivation in the central nervous system of marine mussels and that glial elements might also participate in the uptake and inactivation of transmitter.
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