The freshwater planarian Dugesia japonica has a relatively well-organized central nervous system (CNS) consisting of a brain in the anterior region and a pair of ventral nerve cords (VNCs).1,2) The VNCs are located as a distinct structure in a more ventral region than the brain.1) In addition, planarians can morphologically and functionally regenerate their whole body, including the CNS. This regenerative ability is supported by pluripotent stem cells that are distributed in the mesenchymal space throughout the body.3,4) The events and molecules involved in regeneration have been extensively investigated, and the findings suggest that the molecular mechanism underlying planarian brain regeneration is similar to that underlying mammalian brain development. 5,6) Recently, planarians have begun to be used in the field of neuropharmacology to analyze the efficiency of several pharmacological drugs in a simple but quantitative manner.7-10) Thus, planarians have contributed to the fields of neuroscience and neuropharmacology. A large number of neural-related genes were isolated by expressed sequence tag (EST) and genome database analyses.11,12) Furthermore, a variety of neuralrelated genes are expressed in restricted domains in the planarian CNS, and play important roles in neural functions.
13)Recently, we identified serotonergic, dopaminergic, gaminobutyric acid (GABA)ergic and octopaminergic neurons, and described the distribution of the neural networks by immunofluorescence using specific antibodies against the rate-limiting enzymes for the biosynthesis of the respective neurotransmitters.14-18) In order to characterize the neural projections and connections between several domains in the planarian CNS, it is important to determine the distribution and functions of these receptors. The receptors for neurotransmitters are classified into ion-gated channel receptors and metabotropic receptors, which are characterized by coupling with G-proteins. 19,20) 5-Hydroxytryptamine (5-HT; also known as serotonin) receptors are classified into seven types (5-HT 1 -5HT 7 receptors) in mammals.21) Six of them (i.e., 5-HT 1 , 5-HT 2 , 5-HT 4 -5-HT 7 receptors) are G-protein-coupled receptors (GPCRs) that are characterized by seven transmembrane domains. 5-HT 3 receptor is a ligand-gated channel type receptor (ionotropic receptor).22) Previously, four serotonin receptor-like planarian receptors (5HTLpla1-4) were identified by ligand binding assays. 23,24) In this study, we isolated a 5-HT receptor, DjSER-7 (Dugesia japonica serotonin receptor type 7) from an EST database, 11) and identified the ligand by electrophysiological analysis using Xenopus oocytes.
MATERIALS AND METHODS
AnimalsClonal asexual strain SSP of the planarian Dugesia japonica was maintained in autoclaved tap water at 22-24°C and starved at least for 7 d before experiments.Isolation of DjSER-7 cDNA In order to determine the 5Ј-end sequence of DjSER-7, 5Ј rapid amplification of cDNA ends (5Ј rapid amplification of cDNA ends (RACE)) was carried out using a SMART RAC...