Figure 7.
Proposed mechanism of stage dependent 5-HT receptor mediated regulation of larval development in H. trivolvis. (A) At premetamorphic stage, Gs-coupled serotonin receptors (such as 5-HT4R and 5-HT7R) and their down-stream effectors are highly expressed and their relative amount exceeds that of 5-HT1 receptor and coupled Gi-protein. Thus, the ligand activation of different 5-HT receptor subtypes results in overall elevation of cAMP level (red arrow in square brackets), which leads to decreased rate of cell divisions and finally results in developmental retardation. (B) At metamorphic stage, 5-HT4R, 5-HT7R, and coupled Gs-protein become down-regulated, while expression of 5-HT1R and Gi-protein increases. Relative amount of 5-HT1R now exceeds that of 5-HT4R and 5-HT7R, and ligand stimulation results in overall reduction of cAMP level (green arrow in square brackets), and subsequent developmental acceleration. According to the proposed mechanism, endogenous serotonin released from the ASO neurons in respond to external cues induces opposite effects at premetamorphic and metamorphic stages.