The ontogeny of synaptophysin expression patterns on the GABAergic ciliary band-associated strand during larval development of the sea urchin, Hemicentrotus pulcherrimus A. Agassiz, 1864

Abstract

The swimming activity of sea urchin larvae depends on the ciliary beating primarily generated at the circumoral ciliary band (CB) and is regulated by several neurotransmitters, including γ-aminobutyric acid (GABA). Although GABA is primarily localized in the CB, its synthetase glutamate decarboxylase (GAD) is not expressed in the CB but in the ciliary band-associated strand (CBAS). The CBAS expresses synaptophysin (Syp), a major synaptic vesicle component that is detected in the GAD-expressing puncta. In this study, we analyzed the ontogeny of the spatiotemporal expression pattern of Syp. Syp was initially detected in the cytoplasm of small patches of ectodermal cells from the mesenchyme blastula stage, then along with GABA from the mid-gastrula stage. At the prism stage, the blastocoelar cells also expressed Syp and GABA. In larvae, GABA was detected in the CBAS and the CB. The latter also expressed the GABA(A) receptor. A GAD inhibitor, 3-mercaptopropionic acid, inhibited GABA expression in the CBAS and the CB. During and after the 4-arm pluteus stage (4aPL), the CBAS completed the encircling of the oral ectoderm region, which, however, left the CBAS-absent upper oral lobe region. The present study indicated close localization of GABA and Syp in the puncta of the CBAS and that of GABA and GABA(A)R in the CB, and that, for the first time in the sea urchin nervous system, implicates the Syp-mediated efferent GABA transmission from the CBAS to the CB.