Neuropeptide Y (NPY) is known as a potent orexigenic signal in vertebrates, but its role in Atlantic salmon has not yet been fully established. In this study, we identified three npy paralogs, named npya1, npya2, and npyb, in the Atlantic salmon genome. In silico analysis revealed that these genes are well conserved across the vertebrate’s lineage and the mature peptide sequences shared at least 77% of identity with the human homolog. We analyzed mRNA expression of npy paralogs in eight brain regions of Atlantic salmon post-smolt, and the effect of 4 days of fasting on the npy expression level. Results show that npya1 was the most abundant paralog, and was predominantly expressed in the telencephalon, followed by the midbrain and olfactory bulb. npya2 mRNA was highly abundant in hypothalamus and midbrain, while npyb was found to be highest expressed in the telencephalon, with low mRNA expression levels detected in all the other brain regions. 4 days of fasting resulted in a significant (p < 0.05) decrease of npya1 mRNA expression in the olfactory bulb, increased npya2 mRNA expression in the midbrain and decreased npyb mRNA expression in the pituitary. In the hypothalamus, the vertebrate appetite center, expression of the npy paralogs was not significantly affected by feeding status. However, we observed a trend of increased npya2 mRNA expression (p = 0.099) following 4 days of fasting. Altogether, our findings provide a solid basis for further research on appetite and energy metabolism in Atlantic salmon.
Cocaine- and amphetamine-regulated transcript (CART) has been known to be involved in feeding and energy balance in mammals, acting as an anorexigenic neuropeptide in hypothalamus. In Atlantic salmon, little is known about Cart brain localization and its function. In this study, in silico analysis revealed the existence of 10 cart paralogs, here named cart1a, 1b1, 1b2, 2a, 2b1, 2b2, 3a1, 3a2, 3b, and 4. The Atlantic salmon Cart sequences shared from 19 to 50% of identity with the human homolog and between 25 and 90% of sequence identity among paralogs, except for Cart4 which only shared 18–23% of identity. We further explored cart mRNA expressions in 8 brain regions (Olfactory Bulb-OB, Telencephalon-TEL, Midbrain-MB, Cerebellum-CE, Hypothalamus-HYP, Saccus vasculosus-SV, Pituitary-PT, and Brain Stem-BS) of Atlantic salmon smolt under 4 days of fasting and continuous fed conditions. The cart paralogs analyzed were widely distributed among the brain regions and OB, TEL, HYP, MB, and BS seemed to be the major sites of expression. The expression of cart1a and 1b showed quite similar pattern in MB, HYP, and BS. The expression of cart2a had the highest in MB followed by HYP and TEL. The cart3a transcript was widely distributed in rostrocaudal regions of brain except in OB and SV whereas cart3b was predominantly expressed in BS followed by MB. Expression of cart4 was high in HYP followed by TEL. With regards to effect of feeding status the Atlantic salmon cart2b, which is the most abundant among the paralogs, was upregulated after 4 days of fasting in OB, MB, and HYP compared to fed group. This may suggest an unexpected, but possible orexigenic role of cart2b in Atlantic salmon or a fasting induced stress effect. No other significant effect was observed. Collectively, the differential expressions of the cart paralogs in different brain regions suggest that they may have roles in regional integration of appetite signals and are possibly involved in regulating other brain functions in Atlantic salmon. The fact that salmon has 10 cart paralogs, while mammalians only one, opens interesting perspectives for comparative research on evolutionary adaptations of gene function in the control of appetite and energy homeostasis.
In mammals, the main center for appetite control is the arcuate nucleus (ARC), which contains different neuronal populations. Anorexigenic neurons co-express cocaine- and amphetamine-regulated transcript ( cart) and proopiomelanocortin ( pomc) neuropeptides. The pomc peptide undergoes posttranslational modifications and one of its products, α-MSH, is released together with CART to inhibit appetite. The orexigenic neurons co-express and release neuropeptide Y (NPY) and agouti-related peptide (AgRP) to stimulate appetite. Together, these two ARC neuronal populations comprise the key neuropeptide expression of the melanocortin system (MCS).The MCS is relatively well-conserved among vertebrates, including teleost species. However, in Atlantic salmon ( Salmo salar), the salmonid-specific fourth round whole-genome duplication has resulted in the presence of several paralogous genes, some of which may have resulted in divergent functions. Our in-silico analyses have shown that in Atlantic salmon the key potential neuropeptides of the MCS repertoire comprises three npy, one agrp, ten cart and three pomc genes.Analyses of mRNA distribution of the genes for the MCS neuropeptides across brain regions in salmon reveal significant spatial and quantitative differences in their expression profile between paralogs. In situ hybridization analyses show that agrp1 is only expressed in the nucleus lateralis tuberis (NLT) region of the Atlantic salmon, the putative homolog to the mammalian ARC. In this NLT area, npya, agrp1, cart2b, and pomca are predominantly localized in distinct neurons, however, some neurons co-express cart2b/ pomca. This contrasts with what is known for higher vertebrates and show that co-expression is not required for homeostatic feeding control. Further, studies of fed and fasted fish support that agrp1 acts as an orexigenic signal in Atlantic salmon. In addition, the distinct and well-defined localization of agrp1 in the NLT region suggest that the hypothalamic agrp1 gene may serve as a biomarker for appetite and feed intake in the Atlantic salmon. Research Council of Norway (Projects# 221568; 254894, 267626, 268987, 315106, 317770) This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.
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