Different pituitary action of NK3Ra and NK3Rb in grass carp

TAC3 regulates GnRH/gonadotropin synthesis in female chickens

Neurokinin B (NKB), a peptide encoded by the tachykinin 3 (TAC3), is critical for reproduction in all studied species. However, its potential roles in birds are less clear. Using the female chicken (c-) as a model, we showed that cTAC3 is composed of five exons with a full-length cDNA of 787 bp, which was predicted to generate the mature NKB peptide containing 10 amino acids. Using cell-based luciferase reporter assays, we demonstrated that cNKB could effectively and specifically activate tachykinin receptor 3 (TACR3) in HEK293 cells, suggesting its physiological function is likely achieved via activating cTACR3 signaling. Notably, cTAC3 and cTACR3 were predominantly and abundantly expressed in the hypothalamus of hens and meanwhile the mRNA expression of cTAC3 was continuously increased during development, suggesting that NKB-TACR3 may emerge as important components of the neuroendocrine reproductive axis. In support, intraperitoneal injection of cNKB could significantly promote hypothalamic cGnRH-Ι, and pituitary cFSHβ and cLHβ expression in female chickens. Surprisingly, cTAC3 and cTACR3 were also expressed in the pituitary gland, and cNKB treatment significantly increased cLHβ and cFSHβ expression in cultured primary pituitary cells, suggesting cNKB can also act directly at the pituitary level to stimulate gonadotropin synthesis. Collectively, our results reveal that cNKB functionally regulate GnRH/gonadotropin synthesis in female chickens.

Sequences analysis and pituitary actions of tachykinins in Chinese sturgeon (Acipenser sinensis)

Tachykinins belong to a large, evolutionarily conserved family of brain/gut peptides that are involved in a variety of physiological functions in mammals, such as reproductive regulation. However, little information was available about tachykinins in ancient fish lineage. In the present study, we firstly identified three tachykinin genes (named tac1, tac3 and tac4) and three neurokinin receptors (named nk1r, nk2r and nk3r) from Chinese sturgeon brain and pituitary. Sequence analysis showed that tac1 encoded substance P (SP) and neurokinin A (NKA), tac3 encoded neurokinin B (NKB) and NKB-related peptide (NKBRP), and tac4 encoded hemokin 1 (HK-1) and hemokin 2 (HK-2), respectively. The luciferase reporter assay results showed that NK1R preferentially selected asSP, NK2R preferentially selected asNKA, and NK3R preferentially selected asNKB. Tissue expression analysis showed that the three tac genes were highly detected in the telencephalon and hypothalamus, whereas nkr genes were widely expressed in peripheral tissues. Spatio-temporal expression analysis showed that all three tac genes were highly expressed in unknown sex individuals. Intraperitoneal injection experiments showed that both asSP and asNKB could stimulate luteinizing hormone (LH) release in Chinese sturgeon serum. At the transcriptional level, asSP and asNKB could significantly reduce pituitary follicle-stimulating hormone beta (fshβ) mRNA expression, but induce pituitary growth hormone (gh) mRNA expression. In addition, estradiol (E2) could stimulate tac3 mRNA expression in hypothalamus. Taken together, this study provided information on the tachykinin family in Chinese sturgeon and demonstrates that asNKB and asSP could be involved in reproductive and growth regulation in pituitary.

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.