Developmental expression of tryptophan hydroxylase gene in Ciona intestinalis.

To describe the serotonergic system in a tunicate larva, we cloned a gene encoding for tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin synthesis, in the ascidian Ciona intestinalis and studied its expression pattern during development. Ci-TPH expression was found from tailbud stage in the precursor cells of the visceral ganglion and in the tail. In the larva, TPH-expressing neurons formed two clusters in the anterior central nervous system at the level of the visceral ganglion. Moreover, we found Ci-TPH expression at the level of the muscle cells of the tail and suggested that this localisation might be at the level of neuro-muscular junctions. Moreover, we discussed the involvement of serotonin in the control of larval locomotory activity.