Abstract
The neurohypophyseal hormone oxytocin (Oxt) has been shown to stimulate prolactin (Prl) synthesis and release from the adenohypophysis in rats. However, little is known about the functional roles of Oxt-like neuropeptides in the adenohypophysis of non-mammalian vertebrates. In this study, cDNAs encoding ricefield eel oxytocin-like receptors (Oxtlr), namely isotocin (Ist) receptor 1 (Istr1) and 2 (Istr2), were isolated and specific antisera were generated, respectively. RT-PCR and Western blot analysis detected the presence of both Istr1 and Istr2 in the brain and pituitary, but differential expression in some peripheral tissues, including the liver and kidney, where only Istr1 was detected. In the pituitary, immunoreactive Istr1 and Istr2 were differentially distributed, with the former mainly in adenohypophyseal cell layers adjacent to the neurohypophysis, whereas the latter in peripheral areas of the adenohypophysis. Double immunofluorescent images showed that immunostaining of Istr1, but not Istr2 was localized to growth hormone (Gh) cells, but neither of them was expressed in Prl cells. Ist inhibited Gh release in primary pituitary cells of ricefield eels and increased Gh contents in the pituitary gland of ricefield eels at 6 h after in vivo administration. Ist inhibition of Gh release is probably mediated by cAMP, PKC/DAG, and IP3/Ca2+ pathways. In contrast, Ist did not affect either prl gene expression or Prl contents in primary pituitary cells. Results of this study demonstrated that Ist may not be involved in the regulation of Prl, but inhibit Gh release via Istr1 rather than Istr2 in ricefield eels, and provided evidence for the direct regulation of Gh cells by oxytocin-like neuropeptides in the pituitary of non-mammalian vertebrates.
Highlights
In mammals, oxytocin (Oxt), an nonapeptide neurohormone, is produced in hypothalamic neurons and transported via axons to the neurohypophysis, from where Oxt is secreted into the systemic circulation [1]
In the phylogenetic tree (Figure 1) generated by the neighbor-joining method, teleost Istrs are categorized as Ist receptor 1 (Istr1) and Ist receptor 2 (Istr2) branches, into which ricefield eel Istr1 and Istr2 are clustered correspondingly
Synteny analysis showed that genes around teleost istr2, but not istr1 are highly conserved as compared to those around oxtlr genes in tetrapods (Figure 2)
Summary
Oxytocin (Oxt), an nonapeptide neurohormone, is produced in hypothalamic neurons and transported via axons to the neurohypophysis, from where Oxt is secreted into the systemic circulation [1]. All mammals have a second neurohypophysial hormone, arginine vasopressin (AVP), which differs from Oxt by two amino acids and is believed to have arisen from a gene duplication event in evolution [2]. The classical roles of Oxt are to regulate uterine contractility [3], and mediate milk ejection in response to suckling during lactation [4]. In the pituitary of rat, the Oxt receptor (Oxtr) was shown to be localized to the anterior and posterior lobes [5]. The concentrations of Oxt in the pituitary portal blood are 15–50 times higher than those in peripheral plasma [6]. These lines of evidence suggest a possible role for
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
