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First published online January 17, 2007
Journal of Experimental Biology 210, 403-412 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02666

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Serotonin stimulates [Ca²⁺]_i elevation in ciliary ectodermal cells of echinoplutei through a serotonin receptor cell network in the blastocoel

Hideki Katow^*, Shunsuke Yaguchi and Keiichiro Kyozuka

Research Center for Marine Biology, Graduate School of Life Sciences, Tohoku University, Asamushi, Aomori, Aomori 039-3501, Japan

^* Author for correspondence (e-mail: hkatow{at}mail.tains.tohoku.ac.jp)

Accepted 23 November 2006

A full-length serotonin receptor mRNA from the 5Hthpr gene was sequenced from larvae of the sea urchin, Hemicentrotus pulcherrimus. The DNA sequence was most similar to 5HT-1A of the sea urchin Strongylocentrotus purpuratus found by The Sea Urchin Genome Project, and the protein sequence predicted the presence of seven transmembrane domains. Immunohistochemistry with anti-5HThpr antibodies indicated that the protein was expressed on blastocoelar cells that comprised the major blastocoelar network (serotonin receptor cell network). These network cells inserted their processes into the ectoderm in various regions, including the ciliary band region. Serotonin injected into the blastocoel stimulated a transient elevation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) in the ectoderm, as detected by Oregon-Green dextran, injected earlier in development. The calcium transient propagated as a wave at about 175 µm s^–1, but was not detectable in the serotonin receptor-positive cell network. In larvae treated with p-chlorophenylalanine, a potent and irreversible serotonin synthesis inhibitor, serotonin application did not stimulate [Ca²⁺]_i, the serotonin receptor cell network did not develop properly, and the swimming behavior of the larvae was abnormal. However, formation of a different nervous system comprising synaptotagmin-possessed neurites was not affected by p-chlorophenylalanine treatment. These results imply that serotonin secreted from the apical ganglion into the blastocoel stimulates the elevation of [Ca²⁺]_i in the larval ectodermal cells through the serotonin receptor cell network.

Key words: 5Hthpr, p-chlorophenylalanine, sea urchin