Stimulation of feeding behavior and food consumption in the goldfish, Carassius auratus, by orexin-A and orexin-B

Orexin-A is composed of a highly conservedC-terminal and a specific, hydrophilicN-terminal region, revealing the structural basis of specific recognition by the orexin-1 receptor

Orexins-A and B, also called hypocretins-1 and 2, respectively, are neuropeptides that regulate feeding and sleep-wakefulness by binding to two orphan G protein-coupled receptors named orexin-1 (OX(1)R) and orexin-2 (OX(2)R). The sequences and functions of orexins-A and B are similar to each other, but the high sequence homology (68%) is limited in their C-terminal half regions (residues 15-33). The sequence of the N-terminal half region of orexin-A (residues 1-14), containing two disulfide bonds, is very different from that of orexin-B. The structure of orexin-A was determined using two-dimensional homonuclear and (15)N and (13)C natural abundance heteronuclear NMR experiments. Orexin-A had a compact conformation in the N-terminal half region, which contained a short helix (III:Cys6-Gln9) and was fixed by the two disulfide bonds, and a helix-turn-helix conformation (I:Leu16-Ala23 and II:Asn25-Thr32) in the remaining C-terminal half region. The C-terminal half region had both hydrophobic and hydrophilic residues, which existed on separate surfaces to provide an amphipathic character in helices I and II. The nine residues on the hydrophobic surface are also well conserved in orexin-B, and it was reported that the substitution of each of them with alanine resulted in a significant drop in the functional potency at the receptors. Therefore, we suggest that they form the surface responsible for the main hydrophobic interaction with the receptors. On the other hand, the residues on the hydrophilic surface, together with the hydrophilic residues in the N-terminal half region that form a cluster, are known to make only small contributions to the binding to the receptors through similar alanine-scan experiments. However, since our structure of orexin-A showed that large conformational and electrostatical differences between orexins-A and B were rather concentrated in the N-terminal half regions, we suggest that the region of orexin-A is important for the preference for orexin-A of OX(1)R.

Differential distribution of hypocretin (orexin) and melanin-concentrating hormone in the goldfish brain

The orexigenic peptides hypocretin (orexin) and melanin-concentrating hormone (MCH) are involved in the control of food intake and in other homeostatic functions including sleep and arousal. In this article we study the distribution of these peptides in the brain of the goldfish (Carassius auratus), focusing on those regions particularly related to feeding, sleep, and arousal. Although the general distribution of these peptides in goldfish shows many similarities to those described previously in other species, we observed some noteworthy differences. As in other vertebrates, the peptidergic somata lie in the anterolateral hypothalamus. In goldfish, both hypocretin and MCH immunoreactive cell bodies project fibers to the ventral telencephalon, thalamus, and hypothalamus. At mesencephalic levels fibers reach the deep layers of the optic tectum and also course sparsely through the mesencephalic tegmentum. In contrast to the strong innervation of locus coeruleus and raphe in mammal, the MCH and hypocretin systems in goldfish barely innervate these aminergic populations related to the regulation of sleep and arousal. MCH, but not hypocretin, immunoreactive fibers terminate substantially in the sensory layer of the vagal gustatory lobe of goldfish, while both peptidergic systems distribute to the primary visceral sensory areas of the medulla and pons. The strong involvement of these peptidergic systems with the hypothalamus and general visceral nuclei, but not with locus coeruleus or raphe nuclei support the view that these peptides originally played a role in regulation of energy balance and evolved secondarily to influence sleep-wakefulness systems in amniote vertebrates.

Neuropeptides and the control of food intake in fish

The brain, particularly the hypothalamus, integrates input from factors that stimulate (orexigenic) and inhibit (anorexigenic) food intake. In fish, the identification of appetite regulators has been achieved by the use of both peptide injections followed by measurements of food intake, and by molecular cloning combined with gene expression studies. Neuropeptide Y (NPY) is the most potent orexigenic factor in fish. Other orexigenic peptides, orexin A and B and galanin, have been found to interact with NPY in the control of food intake in an interdependent and coordinated manner. On the other hand cholecystokinin (CCK), cocaine and amphetamine-regulated transcript (CART), and corticotropin-releasing factor (CRF) are potent anorexigenic factors in fish, the latter being involved in stress-related anorexia. CCK and CART have synergistic effects on food intake and modulate the actions of NPY and orexins. Although leptin has not yet been identified in fish, administration of mammalian leptin inhibits food intake in goldfish. Moreover, leptin induces CCK gene expression in the hypothalamus and its actions are mediated at least in part by CCK. Other orexigenic factors have been identified in teleost fish, including the agouti-related protein (AgRP) and ghrelin. Additional anorexigenic factors include bombesin (or gastrin-releasing peptide), alpha-melanocyte-stimulating hormone (alpha-MSH), tachykinins, and urotensin I. In goldfish, nutritional status can modify the expression of mRNAs encoding a number of these peptides, which provides further evidence for their roles as appetite regulators: (1) brain mRNA expression of CCK, CART, tachykinins, galanin, ghrelin, and NPY undergo peri-prandial variations; and (2) fasting increases the brain mRNA expression of NPY, AgRP, and ghrelin as well as serum ghrelin levels, and decreases the brain mRNA expression of tachykinins, CART, and CCK. This review will provide an overview of recent findings in this field.

Distribution of orexin/hypocretin immunoreactivity in the nervous system of the green Treefrog, Hyla cinerea

We examined the distribution of orexin/hypocretin immunoreactive neurons and projections throughout the brain of the green treefrog (Hyla cinerea). Orexin A and B neurons were located in a single population centered on the suprachiasmatic nucleus. Orexin A and B fibers were visible across the brain, with the highest density within the preoptic area and hypothalamus. Our data suggest different distributions of orexin neurons but not projections between families of amphibians.

Clinical significance of daytime plasma orexin-A-like immunoreactivity concentrations in patients with obstructive sleep apnea hypopnea syndrome

BACKGROUND

Polysomnography (PSG) tests are very complicated and time consuming, despite their clinical benefits in the diagnosis of patients with obstructive sleep apnea hypopnea syndrome (OSAHS). A plasma marker would be desirable to select patients suspected of OSAHS for further PSG studies. We have recently reported that orexin-A concentrations in plasma collected immediately after waking early in the morning were significantly lower in patients with OSAHS than in controls.

OBJECTIVES

We conducted the present study to assess the clinical usefulness of the measurement of orexin-A concentrations in plasma obtained in the daytime as a diagnostic predictor to screen patients with OSAHS.

METHODS

Blood samples were collected in the daytime from 19 male patients with suspected sleep-disordered breathing. Plasma orexin-A concentrations were measured by radioimmunoassay before performing PSG.

RESULTS

PSG was conducted in all 19 subjects. PSG showed that 14 subjects had OSAHS and 5 subjects did not. Plasma orexin-A concentrations were significantly lower in patients with OSAHS (4.9 +/- 0.8 pmol/l, mean +/- SE, n = 14) than in control subjects (12.3 +/- 1.9 pmol/l, n = 5) (p = 0.0004).

CONCLUSIONS

These findings suggest that the orexin-A concentration in plasma obtained even in the daytime may be a useful plasma marker for screening OSAHS.

Functional opioid pathways are necessary for hypocretin-1 (orexin-A)-induced feeding

We investigated the interaction of the orexigenic neuropeptide, hypocretin-1 (Hcrt-1, also known as orexin-A), with endogenous opioids (also orexigenic neuropeptides). Rats were injected with naltrexone (NTX, nonspecific opioid antagonist) i.p., i.c.v., in the lateral hypothalamus (LH), and in the accumbens shell (AcbSh), and naloxone methiodide (nonspecific opioid antagonist unable to cross the blood brain barrier) was injected i.p. Rats were then injected with Hcrt-1 in the LH. Food intake was measured for up to 4h thereafter. Rats were also pretreated with NTX in the LH, with Hcrt-1 injected in the AcbSh. NTX suppressed Hcrt-1-induced feeding only when injected i.p., i.c.v., and in the AcbSh. These studies reveal the necessity for functional central opioidergic pathways involving the AcbSh, but not the LH in Hcrt-1-induced feeding.

Role of leptin in the control of feeding of goldfish Carassius auratus: interactions with cholecystokinin, neuropeptide Y and orexin A, and modulation by fasting

To assess the role of leptin on food intake regulation in goldfish, we examined the effects of central (intracerebroventricular, ICV) and peripheral (intraperitoneal, IP) injections of recombinant murine leptin on feeding behavior. Centrally (100 ng/g) and peripherally (300 ng/g) injected leptin both caused a significant decrease in food intake, compared to the saline-treated controls. To test the hypothesis that leptin influenced orexigenic neuropeptide systems in goldfish, fish were co-injected with neuropeptide Y (NPY) or orexin A and leptin. Both NPY (5 ng/g) and orexin A (10 ng/g) significantly increased food intake. Fish co-injected ICV with NPY (5 ng/g) or orexin A (10 ng/g) and leptin (1 or 10 ng/g) had a food intake lower than that of fish treated with NPY or orexin A alone. NPY mRNA expression in goldfish brain was reduced 2 and 6 h following central injection of leptin. To test the hypothesis that the cholecystokinin (CCK) mediates the effects of leptin in goldfish, fish were simultaneously injected ICV with an ineffective dose of leptin (10 ng/g) and either ICV or IP with an ineffective doses of CCK (1 ng/g ICV or 25 ng/g IP). These fish had a food intake lower than vehicle-treated fish, suggesting that leptin potentiates the satiety actions of CCK. CCK hypothalamic mRNA expression was increased 2 h following central treatment with leptin. The CCK receptor antagonist proglumide blocked both central and peripheral CCK satiety effects. Blockade of CCK brain receptors by proglumide resulted in an inhibition of the leptin-induced decrease in food intake and an attenuation of the inhibiting action of leptin on both NPY- and orexin A-induced feeding. These data suggests that CCK has a role in mediating the effects of leptin on food intake. Fasting potentiated the actions of leptin and attenuated the effects of CCK. Whereas fasting had no effects on the brain mRNA expression of CCK, it increased the brain mRNA expression of NPY and decreased the expression of CART. These changes in neuropeptide expression were partially reversed when fish were treated ICV with leptin. These results provide strong evidence that, in goldfish, leptin influences food intake, in part by modulating the orexigenic effects of NPY and orexin and that its actions are mediated, at least in part, by CCK.

Orexins and appetite regulation

Initial research on the functional significance of two novel hypothalamic neuropeptides, orexin-A and orexin-B, suggested an important role in appetite regulation. Since then, however, these peptides have also been shown to influence a wide range of other physiological and behavioural processes. In this paper, we review the now quite extensive literature on orexins and appetite control, and consider their additional effects within this context. Although the evidence for orexin (particularly orexin-A and the orexin-1 receptor) involvement in many aspects of ingestive physiology and behaviour is incontrovertible, central administration of orexins is also associated with increased EEG arousal and wakefulness, locomotor activity and grooming, sympathetic and HPA activity, and pain thresholds. Since the orexin system is selectively activated by signals indicating severe nutritional depletion, it would be highly adaptive for a hungry animal not only to seek sustenance but also to remain fully alert to dangers in the environment. Crucial evidence indicates that orexin-A increases food intake by delaying the onset of a behaviourally normal satiety sequence. In contrast, a selective orexin-1 receptor antagonist (SB-334867) suppresses food intake and advances the onset of a normal satiety sequence. These data suggest that orexin-1 receptors mediate the episodic signalling of satiety and appear to bridge the transition from eating to resting in the rats' feeding-sleep cycle. The argument is developed that the diverse physiological and behavioural effects of orexins can best be understood in terms of an integrated set of reactions which function to rectify nutritional status without compromising personal survival. Indeed, many of the non-ingestive effects of orexin administration are identical to the cluster of active defences mediated via the lateral and dorsolateral columns of the midbrain periaqueductal gray matter, i.e., somatomotor activation, vigilance, tachycardia, hypertension and non-opioid analgesia. In our view, therefore, the LH orexin system is very well placed to orchestrate the diverse subsystems involved in foraging under potentially dangerous circumstances, i.e., finding and ingesting food without oneself becoming a meal for someone else.

Molecular cloning of chicken prepro-orexin cDNA and preferential expression in the chicken hypothalamus

Chicken prepro-orexin cDNA has been cloned, sequenced and characterized. The predicted amino acid sequence of chicken prepro-orexin cDNA revealed that orexin-A and -B are highly conserved among vertebrate species. In situ hybridization and immunohistochemistry localized orexin-positive cell bodies in the periventricular hypothalamic nucleus extending into the lateral hypothalamic area. Comparisons of orexin gene expression in the brains of 24-h-fasted and ad libitum-fed chickens were made using semi-quantitative RT-PCR. No significant differences in orexin mRNA expression were observed.

Hypocretins/orexins as integrators of physiological information: lessons from mutant animals

The hypocretins/orexins (hcrts) are two recently described neuropeptides derived from the same precursor and expressed in a few thousand neurons in the perifornical area of the lateral hypothalamus, which project throughout the brain. The hypocretins bind to two G-protein coupled receptors with different selective affinities. Positional cloning of the gene responsible for a canine model of narcolepsy revealed that this disease is caused by mutations in hypocretin receptor type 2. Parallel studies with hypocretin/orexin knockout mice showed behavioral arrests reminiscent of narcolepsy-like attacks. Narcoleptic patients have decreased hypocretin-containing neurons suggesting that narcolepsy in humans is caused by selective neurodegeneration of hypocretinergic neurons. Additional functions for the hypocretins on regulation of energy balance neuroendocrine release and sympathetic outflow have been described. Here we review studies in humans and mutant animals that have provided clues about the functions of the hypocretinergic system, which appear to involve the coherent regulation of networks that dictate the states of arousal.

Orexins and feeding: special occasions or everyday occurrence?

Neurons expressing prepro-orexin, the precursor of orexin-A and -B, are found in the lateral hypothalamic area, a region classically implicated in driving feeding. Orexin-A induces feeding transiently when injected centrally, and food intake can be decreased when orexin action is disrupted by immunoneutralization of orexin-A, or by pharmacological blockade of orexin receptors, or by transgenic knockout of orexin. Here, we argue that orexin neurons may act to stimulate feeding in the short term, and that important regulatory signals may be a fall in plasma glucose (stimulatory), countered by satiety signals generated by eating, such as gastric distention (inhibitory).

Orexins in the brain-gut axis

Orexins (hypocretins) are a novel pair of neuropeptides implicated in the regulation of energy balances and arousal. Previous reports have indicated that orexins are produced only in the lateral hypothalamic area, although orexin-containing nerve fibers were observed throughout the neuroaxis. Recent evidence shows that orexins and functional orexin receptors are found in the periphery. Vagal and spinal primary afferent neurons, enteric neurons, and endocrine cells in both the gut and pancreas display orexin- and orexin receptor-like immunoreactivity. Orexins excite secretomotor neurons in the guinea pig gut and modulate gastric and intestinal motility and secretion. In addition, orexins modulate hormone release from pancreatic endocrine cells. Moreover, fasting up-regulates the phosphorylated form of cAMP response element binding protein in orexin-immunoreactive enteric neurons, indicating a functional response to food status in these cells. The purpose of this article is to summarize evidence for the existence of a brain-gut network of orexin-containing cells that appears to play a role in the acute regulation of energy homeostasis.

Orexin receptor-1 (OX-R1) immunoreactivity in chemically identified neurons of the hypothalamus: focus on orexin targets involved in control of food and water intake

The neuropeptides orexin-A and orexin-B are produced in neurons of the lateral hypothalamic area and have been implicated to be involved in the regulation of food/water intake and sleep-wake control. The orexins act at two different G-protein-coupled orexin receptors (OX-R1 and OX-R2) that are derived from separate genes and expressed differentially throughout the central nervous system. In the present study, we have used a polyclonal antipeptide antiserum to analyse in detail the distribution of OX-R1-immunoreactive neurons in the rat hypothalamus. In order to identify the chemical mediators of orexin action in the hypothalamus, the OX-R1-containing neurons were characterized with regard to the content of peptides shown previously to affect ingestive and drinking behaviour. Neurons containing OX-R1 immunoreactivity were widely distributed in the hypothalamus with cell bodies located in the suprachiasmatic, periventricular, paraventricular (both magno- and parvocellular division), supraoptic, arcuate, ventromedial, dorsomedial and tuberomammillary nuclei and the lateral hypothalamic area. In magnocellular neurons of the paraventricular and supraoptic nuclei, OX-R1 immunoreactivity was seen in both vasopressin- and oxytocin-containing neurons. OX-R1 immunoreactivity was demonstrated in vasopressin and vasoactive intestinal polypeptide (VIP) neurons of the suprachiasmatic nucleus, in somatostatin neurons of the periventricular nucleus and in corticotropin-releasing hormone (CRH) neurons of the parvocellular paraventricular nucleus. In the arcuate nucleus, OX-R1 immunoreactivity was present in neuropeptide Y (NPY) and agouti-related peptide (AGRP) neurons of the ventromedial part as well as in proopiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) neurons of the ventrolateral division. In the lateral hypothalamic area, OX-R1 immunoreactivity was demonstrated in melanin-concentrating hormone (MCH)- and orexin-containing neurons. In the hypothalamic tuberomammillary nucleus, OX-R1-immunoreactivity was shown in many histamine-containing neurons. The results support the idea that orexins have important actions on hypothalamic neurons that control food intake and fluid balance, but also that orexins may regulate other neuroendocrine systems.

Interactions between orexin A, NPY and galanin in the control of food intake of the goldfish, Carassius auratus

The neuropeptides orexin A (OXA), neuropeptide Y (NPY) and galanin (GAL) have been shown to play a role in the regulation of food intake in mammals. They also significantly stimulate feeding in goldfish. In order to assess the interactions between these peptides in the control of feeding in goldfish, we investigated the effects of central injection of specific receptor antagonists for NPY (BIBP 3226) and GAL (M40) on OXA-induced feeding and the effects of desensitization of orexin receptors on NPY- and GAL-induced feeding. We investigated the effects of BIBP 3226 on GAL-induced feeding and the effects of M40 on NPY-induced feeding. We also examined the effects of coinjection of each pair of neuropeptides on feeding behavior. Injections of 10 ng/g OXA, 5 ng/g NPY and 10 ng/g GAL each induced an increase in feeding. Fish treated with 5 ng/g BIBP or 20 ng/g M40 had food consumption similar to saline controls. BIBP at 5 ng/g significantly reduced NPY- and OXA-induced feeding. Injections of 20 ng/g M40 significantly decreased GAL-induced feeding, but had no effect on OXA-induced feeding. Blocking of orexin receptors by treatment with high doses of OXA (100 ng/g) resulted in a decrease in both NPY- and GAL-induced feeding. Coinjection with 0.5 ng/g OXA and either 0.5 ng/g NPY or 0.5 ng/g GAL resulted in a food intake higher than that observed in saline control fish and in fish treated with NPY or GAL alone at 0.5 ng/g. NPY mRNA expression was increased in the telencephalon and in the hypothalamus compared to saline-treated fish, following injection of OXA. These results indicate that both NPY and GAL are at least, in part, dependent on coaction with OXA for the stimulation of food intake and feeding behavior in goldfish. In addition, the effects of OXA are mediated, in part, by the NPY pathway. This suggests a functional interdependence between these three peptidergic systems in the control of energy balance in goldfish.

To eat or to sleep? Orexin in the regulation of feeding and wakefulness

Orexin-A and orexin-B are neuropeptides originally identified as endogenous ligands for two orphan G-protein-coupled receptors. Orexin neuropeptides (also known as hypocretins) are produced by a small group of neurons in the lateral hypothalamic and perifornical areas, a region classically implicated in the control of mammalian feeding behavior. Orexin neurons project throughout the central nervous system (CNS) to nuclei known to be important in the control of feeding, sleep-wakefulness, neuroendocrine homeostasis, and autonomic regulation. orexin mRNA expression is upregulated by fasting and insulin-induced hypoglycemia. C-fos expression in orexin neurons, an indicator of neuronal activation, is positively correlated with wakefulness and negatively correlated with rapid eye movement (REM) and non-REM sleep states. Intracerebroventricular administration of orexins has been shown to significantly increase food consumption, wakefulness, and locomotor activity in rodent models. Conversely, an orexin receptor antagonist inhibits food consumption. Targeted disruption of the orexin gene in mice produces a syndrome remarkably similar to human and canine narcolepsy, a sleep disorder characterized by excessive daytime sleepiness, cataplexy, and other pathological manifestations of the intrusion of REM sleep-related features into wakefulness. Furthermore, orexin knockout mice are hypophagic compared with weight and age-matched littermates, suggesting a role in modulating energy metabolism. These findings suggest that the orexin neuropeptide system plays a significant role in feeding and sleep-wakefulness regulation, possibly by coordinating the complex behavioral and physiologic responses of these complementary homeostatic functions.

Hypocretin/orexin, sleep and narcolepsy

The discovery that hypocretins are involved in narcolepsy, a disorder associated with excessive daytime sleepiness, cataplexy and unusually rapid transitions to rapid-eye-movement sleep, opens a new field of investigation in the area of sleep control physiology. Hypocretin-1 and -2 (also called orexin-A and -B) are newly discovered neuropeptides processed from a common precursor, preprohypocretin. Hypocretin-containing cells are located exclusively in the lateral hypothalamus, with widespread projections to the entire neuroaxis. Two known receptors, Hcrtr1 and Hcrtr2, have been reported. The functional significance of the hypocretin system is rapidly emerging in both animals and humans. Hypocretin abnormalities cause narcolepsy in dogs, human and mice. The role of the hypocretin system in normal sleep regulation is more uncertain. We believe hypocretin cells drive cholinergic and monoaminergic activity across the sleep cycle. Input from the suprachiasmatic nucleus to hypocretin-containing neurons may explain the occurrence of clock-dependent alertness. Other functions are suggested by pharmacological and neurochemical experiments. These include regulation of food intake, neuroendocrine function, autonomic nervous system activity and energy balance.

SB-334867, a selective orexin-1 receptor antagonist, enhances behavioural satiety and blocks the hyperphagic effect of orexin-A in rats

Intracerebroventricular (i.c.v.) administration of the novel hypothalamic neuropeptide orexin-A stimulates food intake in rats, and delays the onset of behavioural satiety (i.e. the natural transition from feeding to resting). Furthermore, preliminary findings with the selective orexin-1 receptor antagonist, SB-334867, suggest that orexin-A regulation of food intake is mediated via the orexin-1 receptor. At present, however, little is known about either the intrinsic effects of SB-334867 on the normal structure of feeding behaviour, or its effects upon orexin-A-induced behavioural change. In the present study, we have employed a continuous monitoring technique to characterize the effects of SB-334867 (3-30 mg/kg, i.p.) on the microstructure of rat behaviour during a 1-h test with palatable wet mash. Administered alone, SB-334867 (30 mg/kg, but not lower doses) significantly reduced food intake and most active behaviours (eating, grooming, sniffing, locomotion and rearing), while increasing resting. Although suggestive of a behaviourally nonselective (i.e. sedative) action, the structure of feeding behaviour was well-preserved at this dose level, with the reduction in behavioural output clearly attributable to an earlier onset of behavioural satiety. As previously reported, orexin-A (10 microg per rat i.c.v.) stimulated food intake, increased grooming and delayed the onset of behavioural satiety. Pretreatment with SB-334867 dose-dependently blocked these effects of orexin-A, with significant antagonism evident at dose levels (3-10 mg/kg) below those required to produce intrinsic behavioural effects under present test conditions. Together, these findings strongly support the view that orexin-A is involved in the regulation of feeding patterns and that this influence is mediated through the orexin-1 receptor.

Regulatory peptides and control of food intake in non-mammalian vertebrates

The current view of the control of food intake involves a central feeding system in the hypothalamus receiving input from peripheral systems. The presence of food in the gut stimulates the release of several regulatory peptides that control gut motility and secretion. Some of these peptides also act as feedback satiety signals, responsible for termination of a meal. Among the regulatory peptides suggested as peripheral satiety signals are cholecystokinin and gastrin releasing peptide. A more long-term peripheral regulation of food intake has also been postulated and leptin has been suggested as a regulator of food intake. Several regulatory peptides mediate orexigenic or anorexigenic effects in the central feeding system. Neuropeptide Y and galanin both act centrally and stimulate the intake of food, while corticotropin releasing factor reduces food intake. At present, most information about the regulation of food intake is gained from mammalian studies and these findings are used as a base for a discussion on the current knowledge of how regulatory peptides control appetite in non-mammalian vertebrates.

Effects of CART peptides on food consumption, feeding and associated behaviors in the goldfish, Carassius auratus: actions on neuropeptide Y- and orexin A-induced feeding

CART (cocaine- and amphetamine-regulated transcript) peptides are novel brain neuropeptides that have been shown to have a role in the control of feeding behavior in mammals. The effects of intracereboventricular (i.c.v.) administration of two CART fragments, CART (62-76) and CART (55-102) on feeding and behavioral activity of goldfish (Carassius auratus) were examined. Both CART peptide fragments inhibited food intake in goldfish. Co-injection of CART peptide and neuropeptide Y (NPY) reduced the increase in feeding caused by injection of NPY alone. CART (55-102) was more potent than CART (62-76). Co-treatment with CART (55-102) and orexin A inhibited the feeding response induced by orexin A alone. Feeding behavior was stimulated relative to saline controls following injection of CART (62-76) alone and co-injection of NPY and CART (62-76), but was not affected by treatment with CART (55-102) alone, NPY and CART (55-102) or orexin A and CART (55-102). Total behavioral activity was increased with NPY, orexin A, and both CART fragments alone as compared to saline controls, as well as in fish co-treated with NPY and CART (62-76) or NPY and CART (55-102) as compared to saline controls and NPY-treated fish. Tremors were seen in fish treated with CART (55-102) alone, and in fish co-treated with NPY and CART (55-102) and orexin A and CART (55-102). Co-treatment of the fish with NPY but not with orexin A significantly lowered the frequency of fish showing tremors as compared to fish treated with CART alone. These results indicate that CART peptides are involved in the regulatory pathways of feeding and behavioral activity in goldfish.

Dose-response effects of orexin-A on food intake and the behavioural satiety sequence in rats

Although intracerebroventricular (i.c.v.) administration of orexin-A has been reported to stimulate food intake and/or feeding behaviour in rats, mice and goldfish, little attention has thus far been paid to its effects on normal patterns of feeding. In the present study, a continuous monitoring technique was used to characterise the effects of this novel neuropeptide on the microstructure of rat behaviour during a 1-h test with palatable wet mash. Particular attention was devoted to the behavioural satiety sequence, in which feeding is followed by grooming and resting. Although results confirmed the hyperphagic effects of orexin-A (3.33-30.0 microg i.c. v.), gross behavioural analysis failed to reveal any reliable effects of peptide treatment on eating, drinking, sniffing, grooming, resting, locomotion or rearing. However, microstructural analysis revealed behavioural effects of orexin-A that are both dose- and time-dependent. At lower doses (3.33-10.0 microg), orexin-A primarily delayed behavioural satiety, i.e. the normal transition from eating to resting. In contrast, the 30 microg dose initially induced a sedative-like effect, significantly suppressing eating and other active behaviours for the first 15-20 min of the test period. This sedative-like effect resulted in a phase-shifting of the entire behavioural sequence with higher than control levels of eating, grooming, locomotion, rearing and sniffing observed over the second half of the test session. Present findings illustrate the advantages of microstructural behavioural analysis and suggest that the hyperphagic response to low doses of orexin-A results largely from a delay in behavioural satiety while that seen in response to high doses may occur in rebound to initial behavioural suppression. Further studies will be required to confirm the identity of the specific orexin receptors (i.e. OX(1) or OX(2)) involved in mediating the dose-dependent behavioural effects reported.

Regulation of food intake by neuropeptide Y in goldfish

In mammals, neuropeptide Y (NPY) is a potent orexigenic factor. In the present study, third brain ventricle (intracerebroventricular) injection of goldfish NPY (gNPY) caused a dose-dependent increase in food intake in goldfish, and intracerebroventricular administration of NPY Y1-receptor antagonist BIBP-3226 decreased food intake; the actions of gNPY were blocked by simultaneous injection of BIBP-3226. Goldfish maintained on a daily scheduled feeding regimen display an increase in NPY mRNA levels in the telencephalon-preoptic area and hypothalamus shortly before feeding; however, a decrease occured in optic tectum-thalamus. In both fed and unfed fish, brain NPY mRNA levels decreased after scheduled feeding. Restriction in daily food ration intake for 1 wk or food deprivation for 72 h resulted in increased brain NPY mRNA levels. Results from these studies demonstrate that NPY is a physiological brain signal involved in feeding behavior in goldfish, mediating its effects, at least in part, through Y1-like receptors in the brain.

Brain regulation of feeding behavior and food intake in fish

In mammals, the orexigenic and anorexigenic neuronal systems are morphologically and functionally connected, forming an interconnected network in the hypothalamus to govern food intake and body weight. However, there are relatively few studies on the brain control of feeding behavior in fish. Recent studies using mammalian neuropeptides or fish homologs of mammalian neuropeptides indicate that brain orexigenic signal molecules include neuropeptide Y, orexins, galanin and beta-endorphin, whereas brain anorexigenic signal molecules include cholecystokinin, bombesin, corticotropin-releasing factor, cocaine- and amphetamine-regulated transcript, and serotonin. Tachykinins may also have an anorectic action in fish. The brain hypothalamic area is associated with regulation of food intake, while sites outside the hypothalamus are also involved in this function. There is correlation between short-term changes in serum growth hormone levels and feeding behavior, although possible mechanisms integrating these functions remain to be defined.

Differential distribution and regulation of OX1 and OX2 orexin/hypocretin receptor messenger RNA in the brain upon fasting

To further understand the functions of the orexin/hypocretin system, we examined the expression and regulation of the orexin/hypocretin receptor (OX1R and OX2R) mRNA in the brain by using quantitative in situ hybridization. Expression of OX1R and OX2R mRNA exhibited distinct distribution patterns. Within the hypothalamus, expression for the OX1R mRNA was largely restricted in the ventromedial (VMH) and dorsomedial hypothalamic nuclei, while high levels of OX2R mRNA were contained in the paraventricular nucleus, VMH, and arcuate nucleus as well as in mammilary nuclei. In the amygdala, OX1R mRNA was expressed throughout the amygdaloid complex with robust labeling in the medial nucleus, while OX2R mRNA was only present in the posterior cortical nucleus of amygdala. High levels of OX2R mRNA were also observed in the ventral tegmental area. Moreover, both OX1R and OX2R mRNA were observed in the hippocampus, some thalamic nuclei, and subthalamic nuclei. Furthermore, we analyzed the effect of fasting on levels of OX1R and OX2R mRNA in the hypothalamic and amygdaloid subregions. After 20 h of fasting, levels of OX1R mRNA were significantly increased in the VMH and the medial division of amygdala. An initial decrease (14 h) and a subsequent increase (20 h) in OX1R mRNA levels after fasting were observed in the dorsomedial hypothalamic nucleus and lateral division of amygdala. Levels of OX2R mRNA were augmented in the arcuate nucleus, but remained unchanged in the dorsomedial hypothalamic nucleus, paraventricular hypothalamic nucleus, and amygdala following fasting. The time-dependent and region-specific regulatory patterns of OX1R and OX2R suggest that they may participate in distinct neural circuits under the condition of food deprivation.