The evolution of tachykinin/tachykinin receptor (TAC/TACR) in vertebrates and molecular identification of the TAC3/TACR3 system in zebrafish (Danio rerio)

An atlas and database of neuropeptide gene expression in the adult zebrafish forebrain

Zebrafish is a useful model organism in neuroscience; however, its gene expression atlas in the adult brain is not well developed. In the present study, we examined the expression of 38 neuropeptides, comparing with GABAergic and glutamatergic neuron marker genes in the adult zebrafish brain by comprehensive in situ hybridization. The results are summarized as an expression atlas in 19 coronal planes of the forebrain. Furthermore, the scanned data of all brain sections were made publicly available in the Adult Zebrafish Brain Gene Expression Database (https://ssbd.riken.jp/azebex/). Based on these data, we performed detailed comparative neuroanatomical analyses of the hypothalamus and found that several regions previously described as one nucleus in the reference zebrafish brain atlas contain two or more subregions with significantly different neuropeptide/neurotransmitter expression profiles. Subsequently, we compared the expression data in zebrafish telencephalon and hypothalamus obtained in this study with those in mice, by performing a cluster analysis. As a result, several nuclei in zebrafish and mice were clustered in close vicinity. The present expression atlas, database, and anatomical findings will contribute to future neuroscience research using zebrafish.

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.

The Effects of Tachykinin1 Gene Products on Prepubertal Dabry's Sturgeon (Acipenser dabrynus) Pituitary Hormone Secretion and Gene Expression

As an ancient and endangered species unique to the Yangtze River in China, the wild population of the Dabry's sturgeon has become scarce. Due to the long time till the first sexual maturity of Dabry's sturgeon, the population of artificially bred Dabry's sturgeon recovered slowly. As a member of the tachykinin family, TAC1 has been reported to have a variety of functions in mammals such as pain control, smooth muscle contraction and reproductive cycle regulation, but the function of Tac1 in fish has been rarely reported. In this study, we synthesized two tac1 gene products, Substance P (SP) and neurokinin A (NKA), and further verified the effect of two tac1 gene products on the secretion of related hormones in the pituitary of Dabry's Sturgeon by intraperitoneal injection and co-culture of primary cells. Expression studies revealed that the newly cloned tac1 were mainly distributed in the hypothalamus and pituitary tissue of the brain. In prepubertal Dabry's sturgeon, this study showed that the two gonadotropins' mRNA levels in pituitary tissue can be significantly increased by SP and NKA through intraperitoneal injection, and the LH protein level in serum was also increased. Further study showed that both NKA and SP could promote the two gonadotropins' mRNA expression in pituitary cells of Dabry's sturgeon. In addition, we explored the optimal dose and time of SP and NKA on pituitary cells is 24 h and over 10 nM. These results, as a whole, suggested that tac1 gene products play an important role in gonadotropin release and gonadal development in prepubertal Dabry's sturgeon.

Role of Neurokinin B in gametogenesis and steroidogenesis of freshwater catfish, Clarias batrachus

Neurokinin B (NKB), a recently discovered neuropeptide, plays a crucial role in regulating the kiss-GnRH neurons in vertebrate's brain. NKB is also characterized in gonadal tissues; however, its role in gonads is poorly understood. Therefore, in the present study, the effects of NKB on gonadal steroidogenesis and gametogenesis through in vivo and in vitro approaches using NKB antagonist MRK-08 were evaluated. The results suggest that the NKB antagonist decreases the development of advanced ovarian follicles and germ cells in the testis. In addition, MRK-08 further reduces the production of 17β-estradiol in the ovary and testosterone in the testis under both in vivo and in vitro conditions in a dose-dependent manner. Furthermore, the in vitro MRK-08 treatment of gonadal explants attenuated the expression of steroidogenic marker proteins, i.e., StAR, 3β-HSD, and 17β-HSD dose-dependently. Moreover, the MAP kinase proteins, pERK1/2 & ERK1/2 and pAkt & Akt were also downregulated by MRK-08. Thus, the study suggests that NKB downregulates steroidogenesis by modulating the expressions of steroidogenic marker proteins involving ERK1/2 & pERK1/2 and Akt/pAkt signalling pathways. NKB also appears to regulate gametogenesis by regulating gonadal steroidogenesis in the catfish.

Response of Pituitary Cells and Tissues to Neurokinin B and F in the Nile tilapia

Neurokinin B (NKB) is a neuropeptide involved in the regulation of reproductive endocrine system of vertebrate animals, including fish. However, the pathway of NKB action in fish has not been clearly elucidated. In order to clarify the effect of NKB and NKF (neurokinin F) on gonadotropic hormone (GTH) gene expression in the pituitary, we studied the changes of LHβ and FSHβ gene expressions by using two different pituitary culture methods (whole pituitary culture or dispersed pituitary cell culture). Pituitaries were removed from mature female and male Nile tilapia. Changes of LHβ and FSHβ gene expressions were measured and compared after the treatment with NKB or NKF peptides at concentrations 0 to 1,000 nM. Expression of GTH genes in the whole pituitary cultures treated with NKB or NKF peptides did not show significant difference except in female at one concentration when treated with NKF. On the contrary, there were significant changes of GTH gene expressions in the dispersed pituitary cell cultures when treated with NKB and NKF peptides. These results suggest that dispersed pituitary cell culture is more relevant than whole pituitary culture in studying the function of pituitary, and that NKB and NKF could act directly on the pituitary to regulate the expression of GTH genes.

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.

TAC3/TACR3 System Function in the Catadromous Migration Teleost, Anguilla japonica

Neurokinin B (NKB), a member of the tachykinin (TAC) family, plays important roles in mammalian neuropeptide secretion in related to reproduction. However, its potential role in spawning migration teleost is less clear. In the present study, Japanese eel (Anguilla japonica) was employed to study the performance of NKB in regulating reproduction. Results showed that two tac3 and one tacr3 genes were identified in Japanese eel. Sequence analysis showed that two tac3 transcripts, tac3a and tac3b, encode four NKBs: NKBa-13, NKBa-10, NKBb-13, and NKBb-10. However, compared with other species, a mutation caused early termination of TACR3 protein was confirmed, leading to the loss of the 35 amino acid (aa) C-terminal of the receptor. Expression analysis in different tissues showed that both tac3a and tac3b mRNAs were highly expressed in the brain. In situ hybridization localized both tac3a and tac3b mRNAs to several brain regions, mainly in the telencephalon and hypothalamus. Because of the mutation in TACR3 of Japanese eel, we further analyzed whether it could activate the downstream signaling pathway. Luciferase assay results showed the negative regulation of cAMP Response Element (CRE) and Sterol Response Element (SRE) signal pathways by Japanese eel NKBs. Intraperitoneal injection of four different NKB mature peptides at 100 ng/g had negative effect on either gnrh or gth gene expression. However, the high concentration of NKBa-10 and NKBb-13 (1,000 ng/g) upregulated mgnrh and fshb or lhb expression level significantly, which may be mediated by other receptors. In general, the NKBs/NK3Rs system has important functions in regulating eel puberty onset.

Multiple tachykinins and their receptors characterized in the gastropod mollusk Pacific abalone: Expression, signaling cascades, and potential role in regulating lipid metabolism

Tachykinin (TK) families, including the first neuropeptide substance P, have been intensively explored in bilaterians. Knowledge of signaling of TK receptors (TKRs) has enabled the comprehension of diverse physiological processes. However, TK signaling systems are largely unknown in Lophotrochozoa. This study identified two TK precursors and two TKR isoforms in the Pacific abalone Haliotis discus hannai (Hdh), and characterized Hdh-TK signaling. Hdh-TK peptides harbored protostomian TK-specific FXGXRamide or unique YXGXRamide motifs at the C-termini. A phylogenetic analysis showed that lophotrochozoan TKRs, including Hdh-TKRs, form a monophyletic group distinct from arthropod TKRs and natalisin receptor groups. Although reporter assays demonstrated that all examined Hdh-TK peptides activate intracellular cAMP accumulation and Ca²⁺ mobilization in Hdh-TKR-expressing mammalian cells, Hdh-TK peptides with N-terminal aromatic residues and C-terminal FXGXRamide motifs were more active than shorter or less aromatic Hdh-TK peptides with a C-terminal YXGXRamide. In addition, we showed that ligand-stimulated Hdh-TKRs mediate ERK1/2 phosphorylation in HEK293 cells and that ERK1/2 phosphorylation is inhibited by PKA and PKC inhibitors. In three-dimensional in silico Hdh-TKR binding modeling, higher docking scores of Hdh-TK peptides were consistent with the lower EC50 values in the reporter assays. The transcripts for Hdh-TK precursors and Hdh-TKR were highly expressed in the neural ganglia, with lower expression levels in peripheral tissues. When abalone were starved for 3 weeks, Hdh-TK1 transcript levels, but not Hdh-TK2, were increased in the cerebral ganglia (CG), intestine, and hepatopancreas, contrasting with the decreased lipid content and transcript levels of sterol regulatory element-binding protein (SREBP). At 24 h post-injection in vivo, the lower dose of Hdh-TK1 mixture increased SREBP transcript levels in the CG and hepatopancreas and accumulative food consumption of abalone. Higher doses of Hdh-TK1 and Hdh-TK2 mixtures decreased the SREBP levels in the CG. When Hdh-TK2-specific siRNA was injected into abalone, intestinal SREBP levels were significantly increased, whereas administration of both Hdh-TK1 and Hdh-TK2 siRNA led to decreased SREBP expression in the CG. Collectively, our results demonstrate the first TK signaling system in gastropod mollusks and suggest a possible role for TK peptides in regulating lipid metabolism in the neural and peripheral tissues of abalone.

Novel Pituitary Actions of TAC4 Gene Products in Teleost

Tachykinin 4 (TAC4) is the latest member of the tachykinin family involved in several physiological functions in mammals. However, little information is available about TAC4 in teleost. In the present study, we firstly isolated TAC4 and six neurokinin receptors (NKRs) from grass carp brain and pituitary. Sequence analysis showed that grass carp TAC4 could encode two mature peptides (namely hemokinin 1 (HK1) and hemokinin 2 (HK2)), in which HK2 retained the typical FXGLM motif in C-terminal of tachyinin, while HK1 contained a mutant VFGLM motif. The ligand-receptor selectivity showed that HK2 could activate all 6 NKRs but with the highest activity for the neurokinin receptor 2 (NK2R). Interestingly, HK1 displayed a very weak activation for each NKR isoform. In grass carp pituitary cells, HK2 could induce prolactin (PRL), somatolactin α (SLα), urotensin 1 (UTS1), neuromedin-B 1 (NMB1), cocaine- and amphetamine-regulated transcript 2 (CART2) mRNA expression mediated by NK2R and neurokinin receptor 3 (NK3R) via activation cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA), phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP3)/protein kinase C (PKC) and calcium²⁺ (Ca²⁺)/calmodulin (CaM)/calmodulin kinase-II (CaMK II) cascades. However, the corresponding stimulatory effects triggered by HK1 were found to be notably weaker. Furthermore, based on the structural base for HK1, our data suggested that a phenylalanine (F) to valine (V) substitution in the signature motif of HK1 might have contributed to its weak agonistic actions on NKRs and pituitary genes regulation.

Different pituitary action of NK3Ra and NK3Rb in grass carp

In mammals, NK3R is the specific receptor for NKB, which played an important role in reproduction. Recently, two NK3R isoforms, namely NK3Ra and NK3Rb, have been identified in fish. However, little is known about the pituitary actions of the two NK3R isoforms in fish. In this study, both NK3Ra and NK3Rb were isolated from grass carp pituitary. Although their sequence similarity was only 61.6%, the two NK3R isoforms displayed similar ligand selectivity and binding affinity to TAC3 gene products (NKBa, NKBRPa and NKBRPb). In addition, both NK3Ra and NK3Rb displayed similar signaling pathways, including PKA, PKC, MAPK and Ca²⁺ cascades. Tissue distribution indicated that both NK3Ra and NK3Rb were highly detected in grass carp pituitary. Further study found that NK3Ra was mainly located in pituitary LHβ cells, while NK3Rb was only detected in pituitary SLα cells. Furthermore, NK3Ra and NK3Rb activation could induce LHβ and SLα promoter activity, respectively. These results suggested that the two NK3R isoforms displayed different pituitary actions in fish. Using grass carp pituitary cells as model, we found that PACAP could significantly reduce NK3Ra, but induce NK3Rb mRNA expression coupled with cAMP/PKA and PLC/PKC pathways. Interestingly, PACAP could also significantly inhibit LHβ, but stimulate SLα mRNA expression in grass carp pituitary cells. Furthermore, NK3R antagonist could not only inhibit LHβ mRNA expression, but also block PACAP-induced SLα mRNA expression in grass carp pituitary cells. These results suggested that NK3Ra and NK3Rb could mediate PACAP-reduced LHβ and -induced SLα mRNA expression in grass carp pituitary, respectively.

Knockout of tac3 genes in zebrafish shows no impairment of reproduction

Neurokinin B (NKB) plays a pivotal role in the regulation of reproduction in vertebrates. However, whether this neuropeptide is dispensable for reproduction in teleosts remains unknown. In order to reveal its authentic functions in fish, in this study, two tachykinin 3 (tac3) genes encoding Nkbs were functional mutated in zebrafish using the Transcription Activator-like Effector Nucleases (TALEN) technology. We established tac3a^-/-, tac3b^-/- and tac3a^-/-;tac3b^-/- mutant lines, and investigated their reproductive performance and ontogeny. According to our study, spermatogenesis and folliculogenesis were not impaired in tac3a^-/-, tac3b^-/- and tac3a^-/-;tac3b^-/- mutant lines, but changes in the expression of genes related to reproductive axis were observed after loss of Tac3, suggesting that possible compensatory response was activated to maintained the reproductive function in zebrafish. In summary, our results indicate that mutation of tac3 genes do not disrupt the reproduction in zebrafish unlike in mammals, revealing the plasticity of reproductive neuroendocrine system in the brain of zebrafish.

Tacr3/NK3R: Beyond Their Roles in Reproduction

The Tacr3 gene encodes tachykinin receptor 3 (NK3R), which belongs to the tachykinin receptor family. This family of proteins includes typical G protein-coupled receptors and belongs to the rhodopsin subfamily. NK3R functions by binding to its high-affinity ligand, neurokinin B(NKB). The role of Tacr3/NK3R in growth and reproduction has been extensively studied, but Tacr3/NK3R is also widely expressed in the nervous system from the spinal cord to the brain and is involved in both physiological and pathological processes in the nervous system, including mood disorders, chronic pain, learning and memory deficiencies, Alzheimer's disease, Parkinson's disease, addiction-related processes, hypoxic-ischemic encephalopathy, body fluid management, neural development, and schizophrenia. Here, we summarize the structure of NK3R/NKB and its cellular signaling as well as the expression of Tacr3/NK3R in the nervous system, and we provide a comprehensive summary of the role of Tacr3/NK3R in neurological diseases, including reproduction-related disorders and other neurological diseases. At the end of this review, we propose the hypothesis that Tacr3/NK3R mediates a variety of brain functions by affecting the excitability of different neurons with specific functions. On the basis of this "excited or not" hypothesis, more studies related to Tacr3 should be carried out in other nervous system diseases in order to better understand the biological roles of Tacr3.

The gonadotropin-releasing hormones: Lessons from fish

Fish have been of paramount importance to our understanding of vertebrate comparative neuroendocrinology and the mechanisms underlying the physiology and evolution of gonadotropin-releasing hormones (GnRH) and their genes. This review integrates past and recent knowledge on the Gnrh system in the fish model. Multiple Gnrh isoforms (two or three forms) are present in all teleosts, as well as multiple Gnrh receptors (up to five types), which differ in neuroanatomical localization, pattern of projections, ontogeny and functions. The role of the different Gnrh forms in reproduction seems to also differ in teleost models possessing two versus three Gnrh forms, Gnrh3 being the main hypophysiotropic hormone in the former and Gnrh1 in the latter. Functions of the non-hypothalamic Gnrh isoforms are still unclear, although under suboptimal physiological conditions (e.g. fasting), Gnrh2 may increase in the pituitary to ensure the integrity of reproduction under these conditions. Recent developments in transgenesis and mutagenesis in fish models have permitted the generation of fish lines expressing fluorophores in Gnrh neurons and to elucidate the dynamics of the elaborate innervations of the different neuronal populations, thus enabling a more accurate delineation of their reproductive roles and regulations. Moreover, in combination with neuronal electrophysiology, these lines have clarified the Gnrh mode of actions in modulating Lh and Fsh activities. While loss of function and genome editing studies had the premise to elucidate the exact roles of the multiple Gnrhs in reproduction and other processes, they have instead evoked an ongoing debate about these roles and opened new avenues of research that will no doubt lead to new discoveries regarding the not-yet-fully-understood Gnrh system.

Knockout of the Gnrh genes in zebrafish: effects on reproduction and potential compensation by reproductive and feeding-related neuropeptides

Gonadotropin-releasing hormone (GNRH) is known as a pivotal upstream regulator of reproduction in vertebrates. However, reproduction is not compromised in the hypophysiotropic Gnrh3 knockout line in zebrafish (gnrh3-/-). In order to determine if Gnrh2, the only other Gnrh isoform in zebrafish brains, is compensating for the loss of Gnrh3, we generated a double Gnrh knockout zebrafish line. Surprisingly, the loss of both Gnrh isoforms resulted in no major impact on reproduction, indicating that a compensatory response, outside of the Gnrh system, was evoked. A plethora of factors acting along the reproductive hypothalamus-pituitary axis were evaluated as possible compensators based on neuroanatomical and differential gene expression studies. In addition, we also examined the involvement of feeding factors in the brain as potential compensators for Gnrh2, which has known anorexigenic effects. We found that the double knockout fish exhibited upregulation of several genes in the brain, specifically gonadotropin-inhibitory hormone (gnih), secretogranin 2 (scg2), tachykinin 3a (tac3a), and pituitary adenylate cyclase-activating peptide 1 (pacap1), and downregulation of agouti-related peptide 1 (agrp1), indicating the compensation occurs outside of Gnrh cells and therefore is a noncell autonomous response to the loss of Gnrh. While the differential expression of gnih and agrp1 in the double knockout line was confined to the periventricular nucleus and hypothalamus, respectively, the upregulation of scg2 corresponded with a broader neuronal redistribution in the lateral hypothalamus and hindbrain. In conclusion, our results demonstrate the existence of a redundant reproductive regulatory system that comes into play when Gnrh2 and Gnrh3 are lost.

NKB/NK3 system negatively regulates the reproductive axis in sexually immature goldfish (Carassius auratus)

To ascertain the significance of the Neurokinin B/Tachykinin 3 receptor (NKB/NK3) system in goldfish reproduction, two cDNAs encoding tachykinin 3 receptors, namely tacr3a and tacr3b, were cloned. Subsequent studies revealed that the downstream signalling of both Tac3rs can be activated by different NKB peptides, suggesting that the cloned receptors are biologically functional in goldfish. RT-PCR analysis showed that tacr3s are widely expressed in brain regions. During the gonadal development, tacr3a and tacr3b exhibited different expression patterns in the hypothalamus and pituitary. The actions of NKB peptides on reproductive axis was further investigated in vivo. Intraperitoneal injections of NKB peptides significantly reduced the expression of kiss2 and gonadotropin releasing hormone 3 (gnrh3) in the hypothalamus, and the expression of luteinizing hormone beta subunit (lhb) and follicle stimulating hormone beta subunit (fshb) in the pituitary in sexually immature goldfish. Taken together, our findings revealed that NKB/NK3 system plays a negative role in the reproductive axis of immature goldfish.

IGFs Potentiate TAC3-induced SLα Expression via Upregulation of TACR3 Expression in Grass Carp Pituitary Cells

In mammals, the tachykinin 3 (TAC3)/tachykinin receptor 3 (TACR3) systems have been confirmed to play an important role in the regulation of puberty onset. Using grass carp pituitary cells as the model, our recent study found that the TAC3 gene products could significantly induce somatolactin α (SLα) synthesis and secretion via TACR3 activation. In the present study, we seek to examine if pituitary TACR3 can serve as a regulatory target and contribute to TAC3 interactions with other SLα regulators. Firstly, grass carp TACR3 was cloned and tissue distribution showed that it could be highly detected in grass carp pituitary. Using HEK293 cells as the model, functional expression also revealed that grass carp TACR3 exhibited ligand binding selectivity and post-receptor signaling highly comparable to its mammalian counterpart. Using grass carp pituitary cells as the model, TACR3 mRNA expression could be stimulated by insulin-like growth factor (IGF)-I and -II via the IGF-I receptor coupled to phosphatidylinositol-3-kinase (PI₃K)/protein kinase B (Akt) and mitogen-activated protein kinase (MAPK) pathways. Interestingly, IGF-I/-II cotreatment could also significantly enhance TAC3-induced SLα mRNA expression and the potentiating effect was dependent on TACR3 expression and activation of adenylate cyclase (AC)/cAMP/protein kinase A (PKA), phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP₃)/protein kinase C (PKC), and Ca²⁺/calmodulin (CaM)/calmodulin-dependent protein kinase II (CaMK-II) cascades. Besides, IGF-I-induced Akt phosphorylation but not MEK, extracellular signal-regulated kinase (ERK_1/2), and P₃₈MAPK phosphorylation was notably enhanced by TACR3 activation. These results, as a whole, suggest that the potentiating effect of IGFs on TAC3 gene products-induced SLα mRNA expression was mediated by TACR3 upregulation and functional crosstalk of post-receptor signaling in the pituitary.

Expression of tachykinin3 and related reproductive markers in the brain of the African cichlid fish Astatotilapia burtoni

Neurokinin B, encoded by the tachykinin3 gene, plays a crucial role in regulating reproduction in mammals via KNDy neurons and interaction with GnRH. Previous work in teleost fishes has focused on hypothalamic tac3 expression for its role in reproduction, but detailed studies on extra-hypothalamic tac3 expression are limited. Here, we identified two tac3 genes in the social African cichlid fish Astatotilapia burtoni, only one of which produces a functional protein containing the signature tachykinin motif. In situ hybridization for tac3a mRNA identified cell populations throughout the brain. Numerous tac3a cells lie in several thalamic and hypothalamic nuclei, including periventricular nucleus of posterior tuberculum, lateral tuberal nucleus (NLT), and nucleus of the lateral recess (NRL). Scattered tac3-expressing cells are also present in telencephalic parts, such as ventral (Vv) and supracomissural (Vs) part of ventral telencephalon. In contrast to other teleosts, tac3 expression was absent from the pituitary. Using double-fluorescent staining, we localized tac3a-expressing cells in relation to GnRH and kisspeptin cells. Although no GnRH-tac3a colabeled cells were observed, dense GnRH fibers surround and potentially synapse with tac3a cells in the preoptic area. Only minimal (<5%) colabeling of tac3a was observed in kiss2 cells. Despite tac3a expression in many nodes of the mesolimbic reward system, it was absent from tyrosine hydroxylase (TH)-expressing cells, but tac3a cells were located in areas with dense TH fibers. The presence of tac3a-expressing cells throughout the brain, including in socially relevant brain regions, suggest more diverse functions beyond regulation of reproductive physiology that may be conserved across vertebrates.

TAC3 Gene Products Regulate Brain and Digestive System Gene Expression in the Spotted Sea Bass (Lateolabrax maculatus)

Neurokinin B (NKB) is a member of the tachykinin (tac) family that plays important roles in mammalian growth by modulating prolactin (PRL) synthesis and secretion and causing contraction of the stomach and intestine. However, its potential role in regulating growth of teleosts is less clear. We aimed to explore the role that NKB plays in regulating fish growth using the spotted sea bass (Lateolabrax maculatus) as a model. In the present study, two tac3 and two tacr3 genes were identified in the spotted sea bass. Sequence analysis showed that two tac3 transcripts, tac3a and tac3b, encode four NKBs: NKBa-13, NKBa-10, NKBb-13, and NKBb-10. Expression analysis in different tissues showed that both genes are highly expressed in the brain, stomach and intestine of the spotted sea bass. In situ hybridization indicated that the tac3a and tac3b mRNAs are both localized in several brain regions, such as the telencephalon and hypothalamus, and that tacr3a and tacr3b are localized in the intestinal villus and gastric gland. To investigate the potential role of NKBs in regulating growth, in vitro experiments were performed to detect the effect of NKBs on growth-related gene expression in the brain and brain-gut peptide (BGP)-related genes in the stomach and intestine. NKBb-13 was the most critical ligand in regulating the expression of growth-related genes in the brain and brain-gut peptide (BGP)-related genes in the stomach. The expression of cholecystokinin (cck) was enhanced by NKBa-13, NKBa-10, and NKBb-10 but not NKBb-13 in the intestine. In general, our results showed that NKBs participate in regulating the growth of spotted sea bass.

Deciphering Direct and Indirect Effects of Neurokinin B and GnRH in the Brain-Pituitary Axis of Tilapia

Neurokinin B (NKB) and its cognate receptor (NK3R) are emerging as important components of the neuroendocrine regulation of reproduction. Unlike mammalian tac3, which encodes only one mature peptide (namely NKB), two mature peptides are predicted for each tac3 gene in fish and frogs. Therefore, it was designated as Neurokinin F (NKF). Hormone analogs with high and long-lasting biological activity are important tools for physiological and biological research; however, the availability of piscine-specific analogs is very limited. Therefore, we have developed specific NKB and NKF analogs based on the structure of the mammalian NKB analog-senktide. These analogs, specifically designed for longer half-lives by methylation of proteolysis sites, exhibited activity equal to those of the native NKB and NKF in short-term signal-transduction assays of tilapia NKB receptors. However, the analogs were found to be able to significantly increase the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and growth hormone (GH) in tilapia, as fast as 1 h after intraperitoneal (IP) injection. The impact of the analogs on LH and FSH secretion lasted longer compared to the effect of native peptides and salmon GnRH analog (sGnRHa). In addition, we harvested pituitaries 24 h post injection and measured LH, FSH and GH mRNA synthesis. Both analogs elevated mRNA levels of LH and GH, but only NKB analog increased FSH mRNA levels in the pituitary and all GnRH forms in the brain. NKB receptors were co-localized with all three types the GnRH neurons in tilapia brain in situ. We previously showed a direct effect of NKB at the pituitary level, and these new results suggest that the stronger impact of the NKB analog on GTH release is also due to an indirect effect through the activation of GnRH neurons. These results suggest that novel synthetic NKB analogs may serve as a tool for both research and agricultural purposes. Finally, the biological activity and regulatory role of NKB in tilapia brain and pituitary suggest that the NKB/NKBR system in fish is an important reproductive regulator in a similar way to the kisspeptin system in mammals.

NK3R Mediates the EGF-Induced SLα Secretion and mRNA Expression in Grass Carp Pituitary

Epidermal growth factor (EGF) is a potent regulator of cell function in many cell types. In mammals, the EGF/EGFR system played an important role in both pituitary physiology and pathology. However, it is not clear about the pituitary action of EGF in lower vertebrates. In this study, using grass carp as a model, we found that EGF could stimulate NK3R mRNA and protein expression through pituitary ErbB1 and ErbB2 coupled to MEK/ERK and PI3K/Akt/mTOR pathways. In addition, EGF could also induce pituitary somatolactin α (SLα) secretion and mRNA expression in a dose- and time-dependent manner in vivo and in vitro. The stimulatory actions of EGF on SLα mRNA expression were also mediated by PI3K/Akt/mTOR and MEK/ERK pathways coupled to ErbB1 and ErbB2 activation. Our previous study has reported that neurokinin B (NKB) could also induce SLα secretion and mRNA expression in carp pituitary cells. In the present study, interestingly, we found that EGF could significantly enhance NKB-induced SLα mRNA expression. Further studies found that NK3R antagonist SB222200 could block EGF-induced SLα mRNA expression, indicating an NK3R requirement. Furthermore, cAMP/PKA inhibitors and PLC/PKC inhibitors could both abolish EGF- and EGF+NKB-induced SLα mRNA expression, which further supported that EGF-induced SLα mRNA expression is NK3R dependent.

Characterization of a tachykinin signalling system in the bivalve mollusc Crassostrea gigas

Although tachykinin-like neuropeptides have been identified in molluscs more than two decades ago, knowledge on their function and signalling has so far remained largely elusive. We developed a cell-based assay to address the functionality of the tachykinin G-protein coupled receptor (Cragi-TKR) in the oyster Crassostrea gigas. The oyster tachykinin neuropeptides that are derived from the tachykinin precursor gene Cragi-TK activate the Cragi-TKR in nanomolar concentrations. Receptor activation is sensitive to Ala-substitution of critical Cragi-TK amino acid residues. The Cragi-TKR gene is expressed in a variety of tissues, albeit at higher levels in the visceral ganglia (VG) of the nervous system. Fluctuations of Cragi-TKR expression is in line with a role for TK signalling in C. gigas reproduction. The expression level of the Cragi-TK gene in the VG depends on the nutritional status of the oyster, suggesting a role for TK signalling in the complex regulation of feeding in C. gigas.

Neurokinin B signaling in hermaphroditic species, a study of the orange-spotted grouper (Epinephelus coioides)

Neurokinin B (NKB) plays important roles in the mammalian reproductive axis by modulating the release of gonadotropin-releasing hormone (GnRH) and gonadotropins. In the present study, the tac3 cDNA was cloned from a hermaphroditic species, the orange-spotted grouper. Sequence analysis showed that the grouper Tac3 precursor encoded two tachykinin peptides, NKB and NKB-related peptide (NKBRP). Expression analysis in different tissues revealed that tac3 mRNA was highly expressed in the brain of the orange-spotted grouper. In situ hybridization further revealed that it was localized in some hypothalamic nuclei associated with reproductive regulation. During ovarian development, an increase of tac3 expression in the hypothalamus was observed at vitellogenesis stage. Intraperitoneal administration of NKB could increase the gnrh1 and lhβ mRNA levels, and enhance the serum estrogen levels, but did not significantly influence lhβ expression in cultured pituitary cells, indicating that NKB does not directly exert its actions on the pituitary gland. However, it was found that NKBRP had no effect on the expression of two gnrhs and two gths in vivo and in vitro. Effects of sex steroids on tac3 expression were further investigated. During the 17-methyltestosterone-induced sex change in the orange-spotted grouper, hypothalamic tac3 expression showed no significant change. Interestingly, ovariectomy greatly stimulated tac3 expression, while the 17β-estradiol treatment reversed this effect. In general, our data highly indicated that NKB signaling could activate the reproductive axis in the orange-spotted grouper. Our study is the first description of the NKB signaling in the hermaphroditic species.

Tachykinin-3 Genes and Peptides Characterized in a Basal Teleost, the European Eel: Evolutionary Perspective and Pituitary Role

In mammals, neurokinin B (NKB) is a short peptide encoded by the gene tac3. It is involved in the brain control of reproduction by stimulating gonadotropin-releasing hormone (GnRH) neurons, mainly via kisspeptin. We investigated tac3 genes and peptides in a basal teleost, the European eel, which shows an atypical blockade of the sexual maturation at a prepubertal stage. Two tac3 paralogous genes (tac3a and tac3b) were identified in the eel genome, each encoding two peptides (NKBa or b and NKB-related peptide NKB-RPa or b). Amino acid sequence of eel NKBa is identical to human NKB, and the three others are novel peptide sequences. The four eel peptides present the characteristic C-terminal tachykinin sequence, as well as a similar alpha helix 3D structure. Tac3 genes were identified in silico in 52 species of vertebrates, and a phylogeny analysis was performed on the predicted TAC3 pre-pro-peptide sequences. A synteny analysis was also done to further assess the evolutionary history of tac3 genes. Duplicated tac3 genes in teleosts likely result from the teleost-specific whole genome duplication (3R). Among teleosts, TAC3b precursor sequences are more divergent than TAC3a, and a loss of tac3b gene would have even occurred in some teleost lineages. NKB-RP peptide, encoded beside NKB by tac3 gene in actinopterygians and basal sarcopterygians, would have been lost in ancestral amniotes. Tissue distribution of eel tac3a and tac3b mRNAs showed major expression of both transcripts in the brain especially in the diencephalon, as analyzed by specific qPCRs. Human NKB has been tested in vitro on primary culture of eel pituitary cells. Human NKB dose-dependently inhibited the expression of lhβ, while having no effect on other glycoprotein hormone subunits (fshβ, tshβ, and gpα) nor on gh. Human NKB also dose-dependently inhibited the expression of GnRH receptor (gnrh-r2). The four eel peptides have been synthesized and also tested in vitro. They all inhibited the expression of both lhβ and of gnrh-r2. This reveals a potential dual inhibitory role of the four peptides encoded by the two tac3 genes in eel reproduction, exerted at the pituitary level on both luteinizing hormone and GnRH receptor.

TAC1 Gene Products Regulate Pituitary Hormone Secretion and Gene Expression in Prepubertal Grass Carp Pituitary Cells

Tachykinin-1 (TAC1) is known to have diverse functions in mammals, but similar information is scarce in fish species. Using grass carp as a model, the pituitary actions, receptor specificity and postreceptor signaling of TAC1 gene products, namely substance P (SP) and neurokinin A (NKA), were examined. TAC1 encoding SP and NKA as well as tachykinin receptors NK1R and NK2R were cloned in the carp pituitary. The newly cloned receptors were shown to be functional with properties similar to mammalian counterparts. In carp pituitary cells, SP and NKA could trigger luteinizing hormone (LH), prolactin (PRL), and somatolactin α (SLα) secretion, with parallel rises in PRL and SLα transcripts. Short-term SP treatment (3 hours) induced LH release, whereas prolonged induction (24 hours) could attenuate LHβ messenger RNA (mRNA) expression. At pituitary cell level, LH, PRL, and SLα regulation by TAC1 gene products were mediated by NK1R, NK2R, and NK3R, respectively. Apparently, SP- and NKA-induced LH and SLα secretion and transcript expression were mediated by adenylyl cyclase/cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA), phospholiphase C (PLC)/inositol 1,4,5-triphosphate/protein kinase C (PKC), and Ca2+/calmodulin (CaM)/CaM-dependent protein kinase-II pathways. The signal transduction for PRL responses was similar, except for the absence of a PKC component. Regarding SP inhibition of LHβ mRNA expression, the cAMP/PKA- and PLC/PKC-dependent (but not Ca2+/CaM-dependent) cascades were involved. These results, as a whole, suggest that TAC1 gene products play a role in LH, PRL, and SLα regulation via overlapping postreceptor signaling coupled to different subtypes of tachykinin receptor expressed in the carp pituitary.

Novel Functional Role of NK3R Expression in the Potentiating Effects on Somatolactin α Autoregulation in grass carp pituitary cells

In our previous study, NKB/NK3R system has been shown to act at the pituitary level to up-regulate SLα synthesis and secretion in grass carp. However, whether NK3R expression can serve as a regulatory target at the pituitary level and contribute to NKB interactions with other SLα regulators is still unclear. In current study, using grass carp pituitary cells as a model, we have a novel finding that co-treatment of SLα/SLβ with carp TAC3 gene products, could induce a noticeable enhancement in SLα mRNA expression and these potentiating effects occurred with a parallel rise in NK3R transcript level after SLα/SLβ treatment. Interestingly, the stimulatory effects of SLα/SLβ on NK3R gene expression could be further potentiated by co-treatment with IGF-I/-II and simultaneous exposure of carp pituitary cells to SLα/SLβ and IGF-I/-II in the presence of TAC3 gene products was found to markedly elevated SLα mRNA expression (20 fold increase) and this synergistic stimulation was mediated by cAMP/PKA-, PLC/PKC- and Ca²⁺ -dependent cascades functionally coupled with NK3R activation. These findings suggest that local release of SLα via functional interactions with IGF-I/-II and TAC3/NK3R system may constitute a potent stimulatory signal for SLα gene expression in the carp pituitary via up-regulation of NK3R expression.

Neurokinin B Exerts Direct Effects on the Ovary to Stimulate Estradiol Production

Neurokinin B (NKB) and its receptor, NK3R, play critical roles in reproduction by regulating the secretion of the hypothalamic GnRH. NKB and NK3R genes are also expressed in the ovary; however, their physiological roles within the ovary are unknown. The aim of this study was to determine whether NKB acts directly on the ovary to regulate reproduction. Injection of NKB into zebrafish accelerated follicle development, increased the mRNA levels of cyp11a1 and cyp19a1, and enhanced estradiol production. Similarly, NKB induced cyp11a1 and cyp19a1 expression in primary cultures of zebrafish follicular cells and stimulated estradiol production from cultured follicles. Furthermore, NKB activates cAMP response element-binding protein and ERK, and ERK inhibitors abolished the effect of NKB on cyp11a1, whereas protein kinase A and calmodulin-dependent protein kinase II inhibitors that blocked the activation of cAMP response element-binding protein, attenuated the effect of NKB on cyp19a1 expression. In a human granulosa cell line, COV434, a NKB agonist, senktide, also increased CYP11A1 and CYP19A1 mRNA levels and enhanced aromatase protein levels and activities. Small interfering RNA-mediated knockdown of NK3R reduced senktide-induced CYP11A1 and CYP19A1 mRNA levels. Finally, we found that NK3R mRNA was strongly down-regulated in granulosa cells obtained from polycystic ovary syndrome (PCOS) patients when compared with non-PCOS subjects. Taken together, our findings establish a direct action of NKB to induce ovarian estrogen production and raise the possibility that defective signaling of this pathway may contribute to the development of PCOS.

Identification of prohormones and pituitary neuropeptides in the African cichlid, Astatotilapia burtoni

Background

Cichlid fishes have evolved remarkably diverse reproductive, social, and feeding behaviors. Cell-to-cell signaling molecules, notably neuropeptides and peptide hormones, are known to regulate these behaviors across vertebrates. This class of signaling molecules derives from prohormone genes that have undergone multiple duplications and losses in fishes. Whether and how subfunctionalization, neofunctionalization, or losses of neuropeptides and peptide hormones have contributed to fish behavioral diversity is largely unknown. Information on fish prohormones has been limited and is complicated by the whole genome duplication of the teleost ancestor. We combined bioinformatics, mass spectrometry-enabled peptidomics, and molecular techniques to identify the suite of neuropeptide prohormones and pituitary peptide products in Astatotilapia burtoni, a well-studied member of the diverse African cichlid clade.

Results

Utilizing the A. burtoni genome, we identified 148 prohormone genes, with 21 identified as a single copy and 39 with at least 2 duplicated copies. Retention of prohormone duplicates was therefore 41 %, which is markedly above previous reports for the genome-wide average in teleosts. Beyond the expected whole genome duplication, differences between cichlids and mammals can be attributed to gene loss in tetrapods and additional duplication after divergence. Mass spectrometric analysis of the pituitary identified 620 unique peptide sequences that were matched to 120 unique proteins. Finally, we used in situ hybridization to localize the expression of galanin, a prohormone with exceptional sequence divergence in cichlids, as well as the expression of a proopiomelanocortin, prohormone that has undergone an additional duplication in some bony fish lineages.

Conclusion

We characterized the A. burtoni prohormone complement. Two thirds of prohormone families contain duplications either from the teleost whole genome duplication or a more recent duplication. Our bioinformatic and mass spectrometric findings provide information on a major vertebrate clade that will further our understanding of the functional ramifications of these prohormone losses, duplications, and sequence changes across vertebrate evolution. In the context of the cichlid radiation, these findings will also facilitate the exploration of neuropeptide and peptide hormone function in behavioral diversity both within A. burtoni and across cichlid and other fish species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2914-9) contains supplementary material, which is available to authorized users.

Neurokinin B-related Peptide Suppresses the Expression of GnRH I, Kiss2 and tac3 in the Brain of Mature Female Nile tilapia Oreochromis niloticus

Neurokinin B (NKB) and neurokinin B related peptide (NKBRP) belong to tachykinin peptide family. Theyact as a neurotransmitter and/or neuromodulator. Mutation of NKB and/or its cognate receptor, NK3R resulted in hypogonadotropic hypogonadism in mammals, implying a strong involvement of NKB/NK3R system in controlling mammalian reproduction. Teleosts possess NKBRP as well as NKB, but their roles in fish reproduction need to be clarified. In this study, NKB and NKBRP coding gene (tac3) was cloned from Nile tilapia and sequenced. Based on the sequence, Nile tilapia NKB and NKBRP peptide were synthesized and their biological potencies were tested in vitro pituitary culture. The synthetic NKBRP showed direct inhibitory effect on the expression of GTH subunits at the pituitary level. This inhibitory effect was confirmed in vivo by means of intraperitoneal (ip) injection of synthetic NKB and NKBRP to mature female tilapia (20 pmol/g body weight [BW]). Both NKB and NKBRP had no effect on the plasma level of sex steroids, E2 and 11-KT. However, NKBRP caused declines of expression level of GnRH I, Kiss2 and tac3 mRNAs in the brain while NKB seemed to have no distinct effect. These results indicate some inhibitory roles of NKBRP in reproduction of mature female Nile tilapia, although their exact functions are not clear at the moment.

Goldfish neurokinin B: Cloning, tissue distribution, and potential role in regulating reproduction

Neurokinin B (NKB) is a member of the tackykinin (TAC) family known to play a critical role in the neuroendocrine regulation of reproduction in mammals. However, its biological functions in teleosts are less clear. The aim of this study was to determine the role of NKB in fish reproduction using goldfish as a model. Two transcripts, TAC3a and TAC3b, which encode several NKBs, including NKBa-13, NKBa-10, NKBb-13, and NKBb-11, were cloned. Phylogenetic analysis revealed that NKBa-10 and NKBb-11 are closely related to mammalian NKB, while NKB-13s are more conserved in teleosts. Quantitative real-time PCR analyses in various tissues showed that TAC3a and TAC3b mRNAs were mainly expressed in the brain. In situ hybridization further detected TAC3a and TAC3b mRNAs in several regions of the brain known to be involved in the regulation of reproduction and metabolism, as well as in the neurohypophysis of the pituitary. To investigate the potential role of NKBs in reproduction, goldfish were injected intraperitoneally with synthetic NKBa-13, -10, NKBb-13, or -11 peptides and the mRNA levels of hypothalamic gonadotropin-releasing hormone (GnRH) and pituitary gonadotropin subunits were measured. NKBa-13, -10, or NKBb-13, but not -11, significantly increased hypothalamic salmon GnRH and pituitary FSHβ and LHβ mRNA levels in both female and male goldfish. Finally, ovariectomy increased, while estradiol replacement reduced, TAC3a mRNA levels without affecting TAC3b expression in the hypothalamus. These data suggest that NKBa-13, -10, and NKBb-13 play a role in mediating the estrogen negative feedback regulation of gonadotropins.

Direct regulation of gonadotropin release by neurokinin B in tilapia (Oreochromis niloticus)

Neurokinin B (NKB) was recently identified as a key regulator of reproduction in mammals and fish. Fish were found to possess a specific novel neurokinin termed NKF. To study the role of NKB/NKF in the regulation of fish reproduction and to investigate the role of NKB/NKF and their receptors in the piscine pituitary, we have identified the NKB/tachikinin 3 receptor (tac3r) system in tilapia. Bioinformatics and phylogenetic analyses have demonstrated that the tilapia holds 1 putative tac3 gene and 2 NKB receptor genes (tac3ra and tac3rb) that clustered with other piscine Tac3 and NKB receptor lineages. Furthermore, we found that in African cichlids, NKB peptides differ from other vertebrate NKBs in their C-terminal sequence, possessing isoleucine instead of valine as the X in the NKB FXGLM-NH2-terminal consensus sequence. Signal transduction analysis demonstrated that tilapia NKB (tiNKB), tiNKF, and human NKB activated both CRE-luc and SRE-luc transcriptional activity of both tilapia and human NKB receptors. Two hours after ip injection of tiNKB, the plasma levels of both FSH and LH were increased, whereas tiNKF was more effective in increasing LH levels. However, tiNKB was more effective than tiNKF in increasing both FSH and LH from tilapia pituitary dispersed cells. Using in situ hybridization and fluorescent immunohistochemistry, we have shown that LH cells possess tac3, tac3ra, and tac3rb mRNAs, whereas FSH cells possess mainly tac3rb and tac3ra and tac3 to a much lesser extent. These results suggest that the members of the NKB/tac3r system may serve as paracrine/autocrine regulators of gonadotropin release in fish pituitary.

Neurokinin B and reproductive functions: "KNDy neuron" model in mammals and the emerging story in fish

In mammals, neurokinin B (NKB), the gene product of the tachykinin family member TAC3, is known to be a key regulator for episodic release of luteinizing hormone (LH). Its regulatory actions are mediated by a subpopulation of kisspeptin neurons within the arcuate nucleus with co-expression of NKB and dynorphin A (commonly called the "KNDy neurons"). By forming an "autosynaptic feedback loop" within the hypothalamus, the KNDy neurons can modulate gonadotropin-releasing hormone (GnRH) pulsatility and subsequent LH release in the pituitary. NKB regulation of LH secretion has been recently demonstrated in zebrafish, suggesting that the reproductive functions of NKB may be conserved from fish to mammals. Interestingly, the TAC3 genes in fish not only encode the mature peptide of NKB but also a novel tachykinin-like peptide, namely NKB-related peptide (or neurokinin F). Recent studies in zebrafish also reveal that the neuroanatomy of TAC3/kisspeptin system within the fish brain is quite different from that of mammals. In this article, the current ideas of "KNDy neuron" model for GnRH regulation and steroid feedback, other reproductive functions of NKB including its local actions in the gonad and placenta, the revised model of tachykinin evolution from invertebrates to vertebrates, as well as the emerging story of the two TAC3 gene products in fish, NKB and NKB-related peptide, will be reviewed with stress on the areas with interesting questions for future investigations.

Novel pituitary actions of TAC3 gene products in fish model: receptor specificity and signal transduction for prolactin and somatolactin α regulation by neurokinin B (NKB) and NKB-related peptide in carp pituitary cells

TAC3 is a member of tachykinins, and its gene product neurokinin B (NKB) has recently emerged as a key regulator for LH through modulation of kisspeptin/GnRH system within the hypothalamus. In fish models, TAC3 not only encodes NKB but also a novel tachykinin-like peptide called NKB-related peptide (NKBRP), and the pituitary actions of these TAC3 gene products are still unknown. Using grass carp as a model, the direct effects and postreceptor signaling for the 2 TAC3 products were examined at the pituitary level. Grass carp TAC3 was cloned and confirmed to encode NKB and NKBRP similar to that of other fish species. In carp pituitary cells, NKB and NKBRP treatment did not affect LH release and gene expression but up-regulated prolactin (PRL) and somatolactin (SL)α secretion, protein production, and transcript expression. The stimulation by these 2 TAC3 gene products on PRL and SLα release and mRNA levels were mediated by pituitary NK2 and NK3 receptors, respectively. Apparently, NKB- and NKBRP-induced SLα secretion and transcript expression were caused by adenylate cyclase/cAMP/protein kinase A, phospholipase C/inositol 1,4,5-triphosphate/protein kinase C and Ca(2+)/calmodulin/Ca(2+)/calmodulin-dependent protein kinase II activation. The signal transduction for the corresponding responses on PRL release and mRNA expression were also similar, except that the protein kinase C component was not involved. These findings suggest that the 2 TAC3 gene products do not play a role in LH regulation at the pituitary level in carp species but may serve as novel stimulators for PRL and SLα synthesis and secretion via overlapping postreceptor signaling mechanisms coupled to NK2 and NK3 receptors, respectively.

Does Kisspeptin Belong to the Proposed RF-Amide Peptide Family?

Kisspeptin (KISS) plays a key role in regulating reproduction by binding to its receptor, GPR54. Because of the Arg-Phe (RF) sequence at its carboxyl terminus, KISS has been proposed to be a member of the RF-amide peptide family consisting of neuropeptide FF (NPFF), neuropeptide VF (NPVF), pyroglutamylated RF-amide peptide (QRFP), and prolactin-releasing hormone (PRLH). Evolutionary relationships of protein families can be determined through phylogenetic analysis. However, phylogenetic analysis among related peptide families often fails to provide sufficient information because only short mature peptide sequences from full preprohormone sequences are conserved. Considering the concept of the coevolution of peptide ligands and their cognate receptors, evolutionary relationships among related receptor families provide clues to explore relationships between their peptides. Although receptors for NPFF, NPVF, and QRFP are phylogenetically clustered together, receptors for PRLH and KISS are on different branches of the phylogenetic tree. In particular, KISS has been proposed to be a member of the KISS/galanin/spexin family based on synteny analysis and the phylogenetic relationship between their receptors. This article discusses the evolutionary history of the receptors for the proposed RF-amide peptide family and proposes that, from an evolutionary aspect, KISS has emerged from an ancestor, which is distinct from those of the other RF-amide peptides, and so should be classed separately.

Reproductive neuropeptides: prevalence of GnRH and KNDy neural signalling components in a model avian, gallus gallus

Diverse external and internal environmental factors are integrated in the hypothalamus to regulate the reproductive system. This is mediated through the pulsatile secretion of GnRH into the portal system to stimulate pituitary gonadotrophin secretion, which in turn regulates gonadal function. A single subpopulation of neurones termed 'KNDy neurones' located in the hypothalamic arcuate nucleus co-localise kisspeptin (Kiss), neurokinin B (NKB) and dynorphin (Dyn) and are responsive to negative feedback effects of sex steroids. The co-ordinated secretion from KNDy neurones appears to modulate the pulsatile release of GnRH, acting as a proximate pacemaker. This review briefly describes the neuropeptidergic control of reproduction in the avian class, highlighting the status of reproductive neuropeptide signalling systems homologous to those found in mammalian genomes. Genes encoding the GnRH system are complete in the chicken with similar roles to the mammalian counterparts, whereas genes encoding Kiss signalling components appear missing in the avian lineage, indicating a differing set of hypothalamic signals controlling avian reproduction. Gene sequences encoding both NKB and Dyn signalling components are present in the chicken genome, but expression analysis and functional studies remain to be completed. The focus of this article is to describe the avian complement of neuropeptidergic reproductive hormones and provide insights into the putative mechanisms that regulate reproduction in birds. These postulations highlight differences in reproductive strategies of birds in terms of gonadal steroid feedback systems, integration of metabolic signals and seasonality. Also included are propositions of KNDy neuropeptide gene silencing and plasticity in utilisation of these neuropeptides during avian evolution.

Natalisin, a tachykinin-like signaling system, regulates sexual activity and fecundity in insects

An arthropod-specific peptidergic system, the neuropeptide designated here as natalisin and its receptor, was identified and investigated in three holometabolous insect species: Drosophila melanogaster, Tribolium castaneum, and Bombyx mori. In all three species, natalisin expression was observed in 3-4 pairs of the brain neurons: the anterior dorso-lateral interneurons, inferior contralateral interneurons, and small pars intercerebralis neurons. In B. mori, natalisin also was expressed in two additional pairs of contralateral interneurons in the subesophageal ganglion. Natalisin-RNAi and the activation or silencing of the neural activities in the natalisin-specific cells in D. melanogaster induced significant defects in the mating behaviors of both males and females. Knockdown of natalisin expression in T. castaneum resulted in significant reduction in the fecundity. The similarity of the natalisin C-terminal motifs to those of vertebrate tachykinins and of tachykinin-related peptides in arthropods led us to identify the natalisin receptor. A G protein-coupled receptor, previously known as tachykinin receptor 86C (also known as the neurokinin K receptor of D. melanogaster), now has been recognized as a bona fide natalisin receptor. Taken together, the taxonomic distribution pattern of the natalisin gene and the phylogeny of the receptor suggest that natalisin is an ancestral sibling of tachykinin that evolved only in the arthropod lineage.

Expression of tachykinin receptors (tacr1a and tacr1b) in zebrafish: influence of cocaine and opioid receptors

Opioid and tachykinin receptors (TACRs) are closely related in addiction and pain processes. In zebrafish, opioid receptors have been cloned and characterized both biochemically and pharmacologically. However, the tacr1 gene has not yet been described in zebrafish. The aim of this research was to identify the tacr1 gene, study the effects of cocaine on tacr1, and analyze the interaction between tacr1 and opioid receptors. We have identified a duplicate of tacr1 gene in zebrafish, designated as tacr1a and tacr1b. Phylogenetic analyses revealed an alignment of these receptors in the Tacr1 fish cluster, with a clear distinction from other TACR1s of amphibians, birds, and mammals. Our qPCR results showed that tacr1a and tacr1b mRNAs are expressed during embryonic development. Whole-mount in situ hybridization showed tacr1 expression in the CNS and in the peripheral tissues. Cocaine (1.5 μM) induced an upregulation of tacr1a and tacr1b at 24 and 48 h post-fertilization (hpf; except for tacr1a at 48 hpf, which was downregulated). By contrast, HEK-293 cells transfected with tacr1a and tacr1b and exposed to cocaine showed a downregulation of tacr1s. The knockdown of ZfDOR2 and ZfMOR, opioid receptors, induced a down- and upregulation of tacr1a and tacr1b respectively. In conclusion, tacr1a and tacr1b in zebrafish are widely expressed throughout the CNS and peripherally, suggesting a critical role of these tacr1s during embryogenesis. tacr1a and tacr1b mRNA expression is altered by cocaine exposure and by the knockdown of opioid receptors. Thus, zebrafish can provide clues for a better understanding of the relationship between tachykinin and opioid receptors in pain and addiction during embryonic development.

Substance P mRNA expression during zebrafish development: influence of mu opioid receptor and cocaine

Zebrafish has emerged as an important vertebrate animal model for the study of human diseases and for developmental studies in mammals. Since there are few studies of the tachykinin 1 gene (TAC1), precursor of substance P (SP), in relation to embryonic development, we aimed to study the expression of SP transcript (mRNA) and determine the influence of cocaine and opioid receptors on the expression of this neuropeptide. In order to analyse the spatial and temporal SP mRNA expression in zebrafish, we cloned - based on human TAC1 sequence - the sequence that originates SP. Phylogenetic analyses of the precursor of SP, revealed an alignment in the fish cluster, with a clear distinction from other species (amphibians, birds and mammals). Real time PCR (qPCR) results showed that SP mRNA was expressed in several stages of embryonic development, where it increased progressively from gastrula-8hpf (hour post-fertilisation) to the end of the embryogenesis-72hpf. SP mRNA was expressed mainly in the spinal cord in embryos at 20-30hpf, whereas at 36, 42 and 48hpf embryos SP mRNA was expressed mainly in the CNS telencephalon, diencephalon, hypothalamus, rhombomeres, epiphysis and in peripheral areas (heart and somites). Exposure of embryos to 1.5μM cocaine altered the SP mRNA expression at 24 (increasing) and 48hpf (decreasing). We also report that knockdown of μ-opioid receptor induced an increase of SP mRNA expression while the knockdown of the two delta opioid receptors did not produce changes in SP mRNA expression. In conclusion, SP mRNA in zebrafish is expressed during embryonic development in the CNS and peripherally, suggesting that SP would play a critical role during embryogenesis. Furthermore, cocaine exposure and the knockdown of μ-opioid receptor affect the SP mRNA expression. These observations can be important in the pain and addiction field where SP is involved.