GHRP-6 mimics ghrelin-induced stimulation of food intake and suppression of locomotor activity in goldfish

Feed Supplementation with the GHRP-6 Peptide, a Ghrelin Analog, Improves Feed Intake, Growth Performance and Aerobic Metabolism in the Gilthead Sea Bream Sparus aurata

The aquaculture sector has experienced rapid and important growth with the subsequent increase of feeding and nutritional issues for sustaining this activity, mainly related to the use of high quality, safe and environmentally friendly feed ingredients. The use of additives in aquafeeds has proven to be a suitable option to improve different productive indicators in farmed fish. In the present study, the effect of adding the GHRP-6 peptide, a ghrelin analog, to a commercial diet of gilthead sea bream (Sparus aurata) was studied at two proportions (100 or 500 μg/kg of feed). Both experimental diets show an increase in growth performance, as well as in feed efficiency after 97 days of experiment. The lower inclusion of GHRP-6 (100 μg/kg) results in a better aerobic metabolism, while the higher inclusion significantly increased plasma GH levels in agreement with the GH secretagogue effects of ghrelin. Similar growth outcome and differences between GHRP-6 levels in aerobic metabolism and GH stimulation suggest that improvements in culture performance by this peptide may occur through different mechanisms. Taken together, this compound can be considered as a viable dietary supplement for increasing production efficiency of sea bream aquaculture, although a better understanding of its dose-specific effects is still required.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

Crosstalking between the "gut-brain" hormone ghrelin and the circadian system in the goldfish. Effects on clock gene expression and food anticipatory activity

Ghrelin is a potent orexigenic signal mainly synthesized in the stomach and foregut of vertebrates. Recent studies in rodents point out that ghrelin could also act as an input for the circadian system and/or as an output of peripheral food-entrainable oscillators, being involved in the food anticipatory activity (FAA). In this study we pursue the possible interaction of ghrelin with the circadian system in a teleost, the goldfish (Carassius auratus). First, we analyzed if ghrelin is able to modulate the core clock functioning by regulating clock gene expression in fish under a light/dark cycle 12L:12D and fed at 10 am. As expected the acute intraperitoneal (IP) injection of goldfish ghrelin (gGRL[1-19], 44 pmol/g bw) induced the expression of hypothalamic orexin. Moreover, ghrelin also induced (∼ 2-fold) some Per clock genes in hypothalamus and liver. This effect was partially counteracted in liver by the ghrelin antagonist ([D-Lys(3)]-GHRP-6, 100 pmol/g bw). Second, we investigated if ghrelin is involved in daily FAA rhythms. With this aim locomotor activity was studied in response to IP injections (5-10 days) of gGRL[1-19] and [D-Lys(3)]-GHRP-6 at the doses above indicated. Ghrelin and saline injected fish showed similar 24h activity patterns. However, ghrelin antagonist treatment abolished the FAA in schedule fed fish under 24h light, suggesting the involvement of the endogenous ghrelin system in this pre-feeding activity. Altogether these results suggest that ghrelin could be acting as an input for the entrainment of the food-entrainable oscillators in the circadian organization of goldfish.

Orexin A enhances locomotor activity and induces anxiogenic-like action in the goldfish, Carassius auratus

Orexin acts as an orexigenic factor for the regulation of appetite and rhythmicity in rodents. In goldfish, intracerebroventricular (ICV) administration of orexin A has been shown to affect not only food intake, but also locomotor activity. However, as there is still no information regarding the effect of orexin A on emotional behavior in goldfish, we investigated the effect of orexin A on psychomotor activity in this species. Intracerebroventricular administration of synthetic orexin A at 2 and 4pmol/g body weight (BW) enhanced locomotor activity, and this enhancement by orexin A at 4pmol/g BW was attenuated by treatment with the orexin receptor 1 antagonist, SB334867, at 10pmol/g BW. Since intact goldfish prefer a black to a white background area, or the lower to the upper area of a tank, we used two types of preference tests (black/white and upper/lower tests) for measuring anxiety-like behavior in goldfish. Intracerebroventricular administration of orexin A at 4pmol/g BW shortened the time spent in the white background area, and increased the time taken to move from the lower to the upper area. This action of orexin A mimicked that of the central-type benzodiazepine receptor inverse agonist, FG-7142 (an anxiogenic agent), at 4pmol/g BW. The anxiogenic-like effect of orexin A was abolished by treatment with SB334867 at 10pmol/g BW. These results indicate that orexin A potently affects psychomotor activity in goldfish.

Stimulatory effect of ghrelin on food intake in bullfrog larvae

Ghrelin is a potent orexigenic peptide implicated in appetite regulation in rodents. However, except for teleost fish, the involvement of ghrelin in the regulation of feeding in non-mammalian vertebrates has not been well studied. Anuran amphibian larvae feed and grow during the pre- and prometamorphic stages, but, thereafter they stop feeding as the metamorphic climax approaches. Therefore, orexigenic factors seem to play important roles in growing larvae. In the present study, we examined the effect of intraperitoneal (IP) or intracerebroventricular (ICV) administration of synthetic bullfrog ghrelin (n-octanoylated 28-amino acid form) on food intake in larvae at the prometamorphic stages. Cumulative food intake was significantly increased by IP (8 and 16pmol/g body weight (BW)) or ICV (0.5 and 1pmol/g BW) administration of ghrelin during a 15-min observation period. The orexigenic action of ghrelin at 8pmol/g BW (IP) or at 0.5pmol/g BW (ICV) was blocked by treatment with a growth hormone secretagogue-receptor antagonist, [D-Lys(3)]GHRP-6 at 80pmol/g BW (IP) or at 5pmol/g BW (ICV). We then investigated the effect of feeding status on expression levels of the ghrelin transcript in the hypothalamus and gastrointestinal tract. Ghrelin mRNA levels in both were decreased 15 and 60min after feeding. These results indicate that ghrelin acts as an orexigenic factor in bullfrog larvae.

Ovine corticotropin-releasing hormone (oCRH) exerts an anxiogenic-like action in the goldfish, Carassius auratus

Corticotropin-releasing hormone (CRH) is a member of the hypothalamic neuropeptide family that includes urocortins, urotensin I and sauvagine in vertebrates. CRH and urocortin act as anorexigenic factors for satiety regulation in rodents. In a goldfish model, intracerebroventricular (ICV) administration of ovine CRH (oCRH) affects not only food intake, but also locomotor activity. However, there is no information regarding the psychophysiological roles of CRH in goldfish. Therefore, we investigated the effect of oCRH on psychomotor activity in this species. ICV administration of synthetic oCRH at 20 pmol/g body weight (BW) enhanced locomotor activity. Since intact goldfish prefer the lower to the upper area of a tank, we developed a method for measuring the time taken for fish to move from the lower to the upper area. ICV administration of oCRH at 20 pmol/g BW and the central-type benzodiazepine receptor inverse agonist FG-7142 (an anxiogenic agent) at 1-4 pmol/g BW both increased the time taken to move from the lower to the upper area. This anxiogenic-like effect of oCRH was abolished by the CRH receptor antagonist α-helical CRH(9-41) (100 pmol/g BW). These results indicate that CRH can potently affect locomotor and psychomotor activities in goldfish.

The role of ghrelin in energy balance regulation in fish

Knowledge about the endocrine regulation of energy balance in fish is of interest for basic as well as aquaculture research. Ghrelin is a peptide hormone that was first identified in fish 10 years ago and has important roles in the control of food intake and metabolism. Both ghrelin and its receptor, the growth hormone secretagogue receptor (GHS-R), have been found in numerous fish species. Their tissue distributions support the idea that ghrelin has an integrative role in the regulation of energy balance at both the central nervous system level and systemic level. In tilapia and goldfish, ghrelin treatment appears to increase food intake and to stimulate lipogenesis and tissue fat deposition to promote a more positive energy status. In rainbow trout, on the other hand, ghrelin decreases food intake. Goldfish and rainbow trout are the fish species in which the mode of action of ghrelin on food intake has been most thoroughly investigated. The results from these studies indicate that ghrelin alters food intake by acting on well-known appetite signals, such as CRH, NPY and orexin, in the hypothalamus in a species-specific manner. In goldfish, sensory fibres of the vagus nerve convey the signal from gut-derived ghrelin to modulate appetite. The data also indicate that ghrelin may modulate foraging/swimming activity and the perception of food in fish. Results related to the effects of energy status, temperature, and stressors on plasma ghrelin/tissue ghrelin mRNA levels are occasionally inconsistent between short- and long-term studies, between the protein and mRNA, and between species. Recent data also imply a role of ghrelin in carbohydrate metabolism. More functional studies are required to understand the role of ghrelin and its mechanisms of action in the regulation of energy balance among fish.

Regulation of feeding behavior and psychomotor activity by corticotropin-releasing hormone (CRH) in fish

Corticotropin-releasing hormone (CRH) is a hypothalamic neuropeptide belonging to a family of neuropeptides that includes urocortins, urotensin I, and sauvagine in vertebrates. CRH and urocortin act as anorexigenic factors for satiety regulation in fish. In a goldfish model, intracerebroventricular (ICV) administration of CRH has been shown to affect not only food intake, but also locomotor and psychomotor activities. In particular, CRH elicits anxiety-like behavior as an anxiogenic neuropeptide in goldfish, as is the case in rodents. This paper reviews current knowledge of CRH and its related peptides derived from studies of teleost fish, as representative non-mammals, focusing particularly on the role of the CRH system, and examines its significance from a comparative viewpoint.

Ghrelin receptors in non-Mammalian vertebrates

The growth hormone secretagogue-receptor (GHS-R) was discovered in humans and pigs in 1996. The endogenous ligand, ghrelin, was discovered 3 years later, in 1999, and our understanding of the physiological significance of the ghrelin system in vertebrates has grown steadily since then. Although the ghrelin system in non-mammalian vertebrates is a subject of great interest, protein sequence data for the receptor in non-mammalian vertebrates has been limited until recently, and related biological information has not been well organized. In this review, we summarize current information related to the ghrelin receptor in non-mammalian vertebrates.