First description of a histamine receptor of class 2 (HRH2) in a protochordate: expression during blastogenesis and role in regulation of ciliary beat frequency

Histaminergic receptors belong to the family of seven-transmembrane α-helix domain receptors classified in mammals into four distinct classes. Despite being widely studied in vertebrates, few data are available on the invertebrate receptors, with only predicted H1 and H2 sequences for non-chordate deuterostomes. We report the first transcript evidence of an H2 receptor for histamine in the colonial ascidian Botryllus schlosseri showing a high degree of conservation with HRH2 mammalian and other vertebrate orthologous proteins. The transcript and protein localisation during blastogenic development through in situ hybridisation and immunohistochemistry has been described. The mRNA expression appears first in the ciliary tissues of the alimentary system in filter-feeding adults and the buds, with a particular intensity in the pharynx. Transcription is activated very early, beginning from the inner layer of the disc of the secondary bud. From one generation to the next, the transcript signals become more and more intense at the level of the emergence of primordia of the branchial and peribranchial chambers and, finally, in the cells bordering the stigmata, dorsal lamina, and non-glandular ciliated zones of the endostyle. The translated H2 receptor appears as soon as the primordia of branchial and peribranchial chambers form in the secondary bud, and, in the primary buds, is found mainly in the protostigmata before the two layers of branchial and peribranchial epithelial tissue perforate to form the stigmata. In the adult zooid, the H2 receptor is expressed by ciliated mucous cells involved in food progression throughout the whole length of the alimentary canal. The observation of the effects of histamine and histamine-receptor antagonist (ranitidine) and agonist (dimaprit) drugs on explanted branchial tissue has provided confirmation concerning the receptor class and its role in regulating the ciliary beat frequency. The involvement in the local regulation of ciliary activity is of particular concern for evolutionary considerations because HRH2 seems to have been conserved in the pharynx and its developmental derivatives (e.g. upper respiratory tract and middle ear of mammals) during the evolution of chordates.