New Insights Into the Evolution of Corticotropin-Releasing Hormone Family With a Special Focus on Teleosts

Corticotropin-releasing hormone receptor 1 mediates the enhanced locomotor activity and metabolic demands to an acute thermal stress in adult zebrafish

We recently showed that Crh-Crhr1 signalling is essential for acute stress-related locomotor activity in zebrafish larvae. However, the possibility that Crhr1 activation may also initiate the acute metabolic demands for stress coping was unexplored. Here, we tested the hypothesis that Crhr1 signalling is essential for the thermal stressor-induced increases in the acute metabolic rate, a key response for coping with the enhanced energy demands during stress. We tested this by using a wildtype (WT) and a ubiquitous Crhr1 knockout (crhr1-/-) zebrafish and subjecting them to an acute thermal stressor (TS: +5°C above ambient for 60 min). The TS induced the heat shock proteins response in both genotypes, but the elevated cortisol response observed in the WT was absent in the crhr1-/- mutant. The TS also increased the locomotor activity and the metabolic rate in the WT fish, but this response was inhibited in the crhr1-/- mutants. To test if this was due to a lack of TS-induced cortisol elevation in the crhr1-/- mutant, we mimicked the response in the WT fish by treating them with metyrapone, an 11β-hydroxylase inhibitor. While metyrapone inhibited the TS-induced cortisol elevation in the WT, it did not affect the metabolic rate. The lack of Crhr1 also reduced the swimming performance, and the lower Ucrit in the mutants corresponded with alterations in muscle energy metabolism. Together, our results indicate that Crh-Crhr1 signalling, independent of downstream cortisol action, is essential for the TS-induced acute hyperlocomotor activity and the associated increases in the metabolic demand for stress coping.

Reserpine Causes Neuroendocrine Toxicity, Inducing Impairments in Cognition via Disturbing Hypothalamic-Pituitary-Thyroid Axis in Zebrafish

Reserpine is used as a cheap and effective first-line antihypertensive, and presently, it is applied as treatment for difficult-to-control cases of hypertension. Despite its significance, reserpine's neuroendocrine toxicity remains largely underexplored. Here, we investigated the effects of reserpine on development, locomotion, central nervous system (CNS) neurons, thyroid development, and the expression of genes related to neurodevelopment, endocrine, learning and memory, and depression in zebrafish exposed to different doses of reserpine ranging from 0.5 mg/L to 16 mg/L. The results of our study demonstrated that reserpine exerts dose-dependent toxicity on the neuroendocrine system (NES). An investigation into its underlying mechanism suggests that reserpine disrupted the hypothalamic-pituitary-thyroid (HPT) axis via down-regulating hhex, tg, and tshβ genes, impairing thyroid hormone synthesis and endocrine balance. Meanwhile, it affected neurodevelopment, as evidenced by the reduced expression of gfap, gap43, mbp, syn2a, and tuba1b genes, which compromised neuronal structure and function, while impaired neurotransmitter release and uptake could occur due to the suppression of crhb and mao genes. To conclude, these findings illustrate the interconnected impact of pathways involved in endocrine and neurodevelopment in reserpine-induced toxicity.

Response of CRH system in brain and gill of marine medaka to seawater acidification

Corticotropin-releasing hormone (CRH) is mainly secreted by the hypothalamus to regulate stress when environmental factors change. Gills contact with water directly and may also secrete CRH to maintain local homeostasis. Ocean acidification changes water chemical parameters and is becoming an important environmental stressor for marine fish. The response of brain and gill CRH systems to ocean acidification remains unclear. In this study, marine medaka were exposed to CO2-acidified seawater (440 ppm, 1000 ppm, and 1800 ppm CO2) for 2 h, 4 h, 24 h, and 7 d, respectively. At 2 h and 4 h, the expression of crh mRNA in gills increased with increasing CO2 concentration. Crh protein is expressed mainly in the lamellae cells. crhbp and crhr1 expression also increased significantly. However, at 2 h and 4 h, acidification caused little changes in these genes and Crh protein expression in the brain. At 7 d, Crh-positive cells were detected in the hypothalamus; moreover, Crh protein expression in the whole brain increased. It is suggested that CRH autocrine secretion in gills is responsible for local acid-base regulation rather than systemic mobilization after short-term acidification stress, which may help the rapid regulation of body damage caused by environmental stress.

Exploring transcriptional and post-transcriptional epigenetic regulation of crf and 11βhsd2 in rainbow trout brain during chronic social stress

Using dominance hierarchies in juvenile rainbow trout (Oncorhynchus mykiss) as a model of chronic social stress in fish, we explored whether epigenetic transcriptional and post-transcriptional mechanisms are involved in the gene expression of corticotropin-releasing factor (crf) and 11β-hydroxysteroid dehydrogenase (11βhsd2), key factors involved in the regulation of the endocrine stress axis response. In juvenile rainbow trout pairs, subordinate individuals display sustained elevation of circulating cortisol concentrations. Cortisol production is controlled by the hypothalamic-pituitary-interrenal (HPI) axis in fish and initiated by CRF release from the preoptic area (POA). Given that crf is modulated during chronic social stress, and that such stress has been implicated in the epigenetic regulation of crf in other taxa, we probed a role for epigenetic regulation of crf transcript abundance in chronically stressed rainbow trout. We also investigated the regulation of the cortisol-metabolising enzyme 11βhsd2 in the POA, which is upregulated in subordinates. The potential involvement of DNA methylation and microRNAs (miRNAs) in the regulation of crf transcript abundance was investigated during social stress in the POA of fish, as was the potential involvement of miRNAs in 11βhsd2 regulation. Although transcript abundances of crf were elevated in subordinate fish after 4 days, DNA methylation profiles within putative promoter sequences upstream of the crf gene were not significantly affected by chronic stress. An inverse relationship between crf and its predicted posttranscriptional regulator miR-103a-3p in the POA suggests that miRNAs may be involved in mediating the effects of chronic social stress on key components of the endocrine stress axis.

The Caudal Neurosecretory System: A Still Enigmatic Second Neuroendocrine Complex in Fish

The caudal neurosecretory system (CNSS) is a neuroendocrine complex, whose existence is specific to fishes. In teleosts, it consists of neurosecretory cells (Dahlgren cells) whose fibers are associated with a neurohemal terminal tissue (urophysis). In other actinopterygians as well as in chondrichthyes, the system is devoid of urophysis, so that Dahlgren cells end in a diffuse neurohemal region. Structurally, it has many similarities with the hypothalamic-neurohypophysial system. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most notable ones being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is still hypothetical, and its role is poorly understood. Observations and experimental data gave some evidences of a possible involvement in osmoregulation, stress, and reproduction. But one may question the benefit for fish to possess this second neurosecretory system, while the central hypothalamic-pituitary complex already controls such functions. As an introduction of our review, a brief report on the discovery of the CNSS is given. A description of its organization follows, and our review then focuses on the neuroendocrinology of the CNSS with the different factors it produces and secretes. The current knowledge on the ontogenesis and developmental origin of the CNSS is also reported, as well as its evolution. A special focus is finally given on what is known on its potential physiological roles.