Figure 5.

Serotonin (5-hydroxytryptamine; 5HT) transporter (5HTT) and 5HT synthesis enzyme (TryMO) are expressed in serotonergic neurons of the adult ganglionic nervous system inside the larva as early as prehatching. 5HT is an important signaling molecule in the larval nervous system, as several studies from molluscs and other animals with larval development have shown (see Fig. 3). We identified 5HTT, TryMO, and 5HT receptor sequences in the Aplysia transcriptome and analyzed their relative expression patterns during development (A). We then analyzed the expression patterns qualitatively using MWISH in trochophore and prehatching veliger stages. Note that the stages used for in situ hybridizations are indicated in panel A by a dashed box. 5HTT (B–E) and TryMO (F–I) both localize in cells of the adult nervous system of early and late veliger stages (green staining). These cells have been previously shown to be serotonergic (Marois and Carew, ’97b). Stages are as follows: B/F Trochophore–Veliger transition (Fig. 1J); C/G early veliger stage (Fig. 1K); D/H later veliger stage; and E/I veliger stage at hatching (Fig. 1L). Inserts are three dimensional reconstructions from confocal microscope stacks of serotonergic cells shows in the DIC-FITC merge. Negative controls (sense) probe were performed and did not result in any specific staining pattern, with the exception of the ubiquitous background stain in the stomach region. (data not shown). Also note that only a subset of cells express both TryMO and 5HTT. Ac, anal cells; F, foot; M, mouth; Pv, prevelum; St, stomach; Vl, velar lobe. Scale bars: B: 42 μm; C: 45 μm; D: 50 μm; E: 80 μm; F: 40 μm; G: 45 μm; H: 50 μm; I: 75 μm.