Development of catecholaminergic neurons in the pond snail, Lymnaea stagnalis: I. Embryonic development of dopamine-containing neurons and dopamine-dependent behaviors

The embryonic development of the catecholaminergic system of the pond snail , Lymnaea stagnalis, was investigated by using chromatographic and histoche mical methods. High performance liquid chromatography suggested that dopami ne was the only catecholamine present in significant concentrations through out the embryonic development of Lymnaea. Dopamine first became detectable at about embryonic stage (E) 15 (15% of embryonic development) and then inc reased in amount during early development to reach about 120-140 fmol per a nimal by around E40. Dopamine content remained stable during mid-embryogene sis (E40-65), increased slowing for the next couple of days, and then incre ased rapidly to culminate at about 400 fmol per animal by hatching. The det ection of aldehyde- and glyoxylate-induced fluorescence and of tyrosine hyd roxylaselike immunoreactivity indicated that the first catecholaminergic ce lls appeared in the late trochophore or early veliger stage of embryonic de velopment (E32-35). The paired perikarya of these transient apical catechol aminergic (TAC) neurons were located beneath the apical plate, remained out side of the central ganglia during embryogenesis, and no longer contained d etectable catecholamines close to hatching. TAC neurons bore cilia on the e nds of short processes that penetrated the overlying epithelium; their long processes branched repeatedly under the ciliated apical plate. Several sma ller catecholaminergic cells first appeared in the anterior margin of the f oot at a stage when the embryos began to metamorphose from the veliger form (E55). Similar bipolar cells later appeared in the tentacle and lips. The axone of all of these small peripheral cells projected centrally and termin ated within the neuropil of different central ganglia. Central catecholamin ergic neurons, including RPeD1, differentiated only after metamorphosis was complete (E75). Development of locomotor, respiratory, and feeding behavio rs correlated with maturation of catecholaminergic neurons, as indicated by histology and chromatography. (C) 1999 Wiley-Liss, Inc.