Early development of the serotonergic and dopaminergic nervous system in Spisula solidissima (surf clam) larvae.

We have defined the development of the serotonergic and dopaminergic components of the central nervous system in the early Spisula solidissima (surf clam) embryo using HPLC and immunocytochemistry. HPLC analysis reveals norepinephrine, dopamine, and serotonin are present at 24 h post-fertilization. Immunocytochemistry shows that the serotonergic nervous system emerges during the late trochophore stage with the development of a single serotonergic cell, C/A1, in the cerebral/apical ganglion. After 48 h, a second serotonergic cell forms, C/A2, which is connected to C/A1 by two serotonergic processes, and a single serotonergic cell emerges in the visceral ganglion, V1. At 72 h, a new serotonergic cell body develops in the cerebral/apical ganglion, C/A3. After 96 h, the cerebral/apical ganglion and visceral ganglion are connected by a serotonergic process. Expression of the dopamine receptor, D2, begins by 24 h with a generalized expression in the region of the developing gut. D2 expression in the gut ceases by 48 h. At 48 h, a network of fibers forms dorsolateral to the mouth. By 72 h, D2 expressing projections emerge from this network.