Molecular cloning and expression of cDNA encoding brain preprocholecystokinin in goldfish

Molecular characterization and tissue distribution of cholecystokinin and its receptor in Yangtze sturgeon (Acipenser dabryanus) and their response to different feeding conditions

Yangtze sturgeon (Acipenser dabryanus) is a species endemic to Yangtze River drainage in China and is listed as a critically endangered species on the IUCN Red List. In the present study, cholecystokinin (CCK), one of the most important neuroregulatory digestive genes, and its receptor (CCKr) were identified from the full-length transcriptome analysis of A. dabryanus. The deduced amino acid sequences of CCK and CCKr from A. dabryanus showed structural features common to those in other vertebrates. Gene expression profile analysis showed that CCK and CCKr were universally expressed in different tissues, and both had the highest expression in the brain. Starvation and refeeding significantly regulated the expression levels of CCK and CCKr in the brain, suggesting that CCK and CCKr were involved in feed intake regulation in A. dabryanus as in mammals. In addition, the expression levels of CCK and CCKr under different feeding frequencies were studied. Compared with the control group (fed two times a day), the expression levels of CCK and CCKr in the intestine and brain did not change significantly in the other groups after 8 weeks of rearing, indicating that the feeding frequency might not influence the appetite of A. dabryanus. The present work provides a basis for further investigation into the regulation of feeding in A. dabryanus.

Effect of intracerebroventricular administration of two molecular forms of sulfated CCK octapeptide on anxiety-like behavior in the zebrafish danio rerio

Cholecystokinin octapeptide with sulfate (CCK-8s) regulates feeding behavior and psychomotor activity. In rodents and goldfish, intracerebroventricular (ICV) injection of CCK-8s decreases food intake and also induces anxiety-like behavior. The zebrafish has several merits for investigating the psychophysiological roles of neuropeptides. However, little is known about the brain localization of CCK and the behavioral action of CCK-8s in this species. Here we investigated the brain localization of CCK-like immunoreactivity and found that it was distributed throughout the brain. As CCK-like immunoreactivity was particularly evident in the ventral habenular nucleus, the interpeduncular nucleus and superior raphe, we subsequently examined the effect of zebrafish (zf) CCK-8s on psychomotor control. Since the zebrafish possesses two molecular forms of zfCCK-8s (zfCCKA-8s and zfCCKB-8s), two synthetic peptides were administered intracerebroventricularly at 1, 5 and 10 pmol g^-1 body weight (BW). As the zebrafish shows a greater preference for the lower area of a tank than for to the upper area, we used this preference for assessment of anxiety-like behavior. ICV administration of zfCCKA-8 s or zfCCKB-8s at 10 pmol g^-1 BW significantly shortened the time spent in the upper area. The actions of these peptides mimicked that of the central-type benzodiazepine receptor inverse agonist FG-7142 (an anxiogenic agent) at 10 pmol g^-1 BW. The anxiogenic-like action of the two peptides was attenuated by treatment with the CCK receptor antagonist proglumide at 200 pmol g^-1 BW. These results indicate that zfCCKA-8s and zfCCKB-8s potently induce anxiety-like behavior via the CCK receptor-signaling pathway in the zebrafish brain.

Intracerebroventricular administration of sulphated cholecystokinin octapeptide induces anxiety-like behaviour in goldfish

Sulphated cholecystokinin octapeptide (CCK-8s) is involved in feeding regulation as an anorexigenic neuropeptide in vertebrates. In rodents, i.c.v. administration of CCK-8s has been shown to affect not only feeding behaviour, but also psychomotor activity. However, there is still no information available concerning the psychophysiological effects of CCK-8s in goldfish. Therefore, in the present study, we examined the effect of synthetic goldfish (gf) CCK-8s on psychomotor activity in this species. Intracerebroventricular administration of gfCCK-8s at 0.1, 0.5 and 2.5 pmol g^-1 body weight (BW) did not affect swimming distance (locomotor activity). Because goldfish prefer the lower to the upper area of a tank, we used this as a preference test (upper/lower test) to assess anxiety-like behaviour. Intracerebroventricular administration of gfCCK-8s at 2.5 pmol g^-1 BW shortened the time spent in the upper area. The action of gfCCK-8s mimicked that of FG-7142 (the central-type benzodiazepine receptor inverse agonist, an anxiogenic agent) at 5 and 10 pmol g^-1 BW. The anxiogenic-like effect of gfCCK-8s was abolished by treatment with the CCK receptor antagonist proglumide at 50 pmol g^-1 BW. We also investigated the localisation of CCK/gastrin-like immunoreactivity in the goldfish brain. CCK/gastrin-like immunoreactivity was observed in the anxiety-related regions (the nucleus habenularis and the interpeduncular nucleus). These data indicate that gfCCK-8s potently affects psychomotor activity in goldfish, and exerts an anxiogenic-like effect via the CCK receptor-signalling pathway.

CCK reduces the food intake mainly through CCK1R in Siberian sturgeon (Acipenser baerii Brandt)

To explore the effect of CCK on food intake in Siberian sturgeon, cck cDNA sequence of 1005 bp was obtained, and cck mRNA possessed the highest expression in brain. The expressions of cck were significantly increased after feeding 1 and 3 h, while displaying significant decrease after fasting within 15 days in brain and duodenum. Re-feeding for 3 days induced cck level returned to basic level. Acute i.p. injection experiment showed 100 and 200 ng/g BW CCK8 inhibited the food intake in 0–1 h together with the cumulative food intake within 3 h. 7 days chronic i.p. injection of 100 and 200 ng/g BW CCK8, both daily food intake and cumulative food intake were significantly decreased. In addition, chronic i.p injection of CCK8 induced the expression of feeding related factors changes including cck, ucn3, cart, apelin, pyy and npy in respective organization. Moreover, as revealed by the results, Lorglumide, the CCK1R selective antagonist, effectively reversed the inhibitory effects of CCK8 on food intake and the levels of feeding related factors. On the other hand, LY 225910, the CCK2R selective antagonist, partially reversed these effects. These results indicate CCK is a satiety factor inhibits the feeding of Siberian sturgeon primarily through CCK1R.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.

Appetite regulating factors in pacu (Piaractus mesopotamicus): Tissue distribution and effects of food quantity and quality on gene expression

The pacu Piaractus mesopotamicus is an omnivorous fish considered a promising species for aquaculture. Little is known about the endocrine regulation of feeding in this species. In this study, transcripts for orexin, cocaine and amphetamine regulated transcript (CART), cholecystokinin (CCK) and leptin were isolated in pacu. Orexin, CCK and leptin have widespread mRNA distributions in brain and periphery, CART is limited to the brain. To examine the role of these peptides in the regulation of feeding and energy status, mRNA expression levels were compared between fed and fasted fish and around feeding time. Both orexin and CART brain expressions were affected by fasting and displayed periprandial changes, suggesting a role in both short- and long-term regulation of feeding. CCK intestinal expression decreased in fasted fish and displayed periprandial changes, suggesting CCK acts as a peripheral satiety factor. Leptin was not affected by fasting but displayed periprandial changes, suggesting a role as a short-term regulator. To examine if these peptides are affected by diet, brain and gut expressions were assessed in fish fed with different diets containing soy protein concentrate. Food intake, weight gain and expressions of orexin, CART, CCK and leptin were little affected by replacement of fish protein with soy protein, suggesting that pacu is able to tolerate and grow well with a diet rich in plant material. Overall, our results suggest that orexin, CART, CCK and leptin are involved in the physiology of feeding of pacu and that their expressions are little affected by plant-based diets.

Appetite regulating factors in dourado, Salminus brasiliensis: cDNA cloning and effects of fasting and feeding on gene expression

The dourado, Salminus brasiliensis (Cuvier, 1816) is a freshwater piscivorous Characin native to South American rivers. Owing to the high quality of its flesh and its fast growth, it is the object of both capture fisheries and fish farming. However, very little is known about the endocrine regulation of feeding and metabolism of dourado. In this study, cDNAs for orexin, CART and CCK were isolated in dourado, and their mRNA tissue distributions examined. In order to assess the role of these peptides in the regulation of feeding of dourado, the effects of fasting and feeding on mRNA expression levels of orexin, CART and CCK in the brain as well as CCK in the intestine were assessed. Whereas orexin and CCK have widespread mRNA distributions in the brain and peripheral organs, CART seems to be mostly limited to the brain. Orexin brain expression increased with fasting and displayed periprandial changes, suggesting it is involved in both long- and short-term regulation of feeding and appetite. CART and CCK hypothalamic expressions were not affected by fasting, but displayed periprandial changes with post-feeding decreases, suggesting roles in short-term satiation. CCK expression in the anterior intestine was not affected by fasting and did not display periprandial changes. Overall, our results suggest that orexin, CART and CCK are involved in the physiology of feeding of dourado.

Digestive Physiological Characteristics of the Gobiidae: - Characteristics of CCK-producing Cells and Mucus-secreting Goblet Cells of Stomach Fish and Stomachless Fish

In this study, we investigated the characteristics of CCK-producing cells and mucus-secreting goblet cells with respect to stomach fish and stomachless fish of the Gobiidae in order to provide a basis for understanding the digestive physiology. Hairychin goby (Sagamia geneionema), which is stomachless fish, the numbers of mucus-secreting goblet cells is highest in the posterior intestine portion (P<0.05), while CCK-producing cells are scattered throughout the intestine. Gluttonous goby (Chasmichthys gulosus), which is stomach fish, mucus-secreting goblet cells are most abundant in the mid intestine portion (P<0.05), whereas CCK-producing cells are observed only in the anterior and mid intestine portion. Trident goby (Tridentiger obscurus) which is stomach fish, mucus-secreting goblet cells were most abundant in the mid intestine portion (P<0.05). CCK-producing cells are found in the anterior and mid intestine portion. Giurine goby, Rhinogobius giurinus which is also stomach fish, the largest number of mucus-secreting goblet cells showed in anterior intestine portion except for esophagus (P<0.05). CCK-producing cells are present only in the anterior and mid intestine portion. In S. geneionema, digestive action occurs in the posterior intestine portion to protect and functions to activate digestion. In contrast, in C. gulosus, T. obscurus and R. giurinus, their digestive action occurs in the anterior and mid intestine portion to protect and functions to activate digestion. Further studies of the modes of food ingestion by these fish, the contents of their digestive tracts, and the staining characteristics of the goblet cells need to be carried out.

Appetite regulating peptides in red-bellied piranha, Pygocentrus nattereri: cloning, tissue distribution and effect of fasting on mRNA expression levels

cDNAs encoding the appetite regulating peptides apelin, cocaine and amphetamine regulated transcript (CART), cholecystokinin (CCK), peptide YY (PYY) and orexin were isolated in red-bellied piranha and their mRNA tissue and brain distributions examined. When compared to other fish, the sequences obtained for all peptides were most similar to that of other Characiforme fish, as well as to Cypriniformes. All peptides were widely expressed within the brain and in several peripheral tissues, including gastrointestinal tract. In order to assess the role of these peptides in the regulation of feeding of red-bellied piranha, we compared the brain mRNA expression levels of these peptides, as well as the gut mRNA expression of CCK and PYY, between fed and 7-day fasted fish. Within the brain, fasting induced a significant increase in both apelin and orexin mRNA expressions and a decrease in CART mRNA expression, but there where were no significant differences for either PYY or CCK brain mRNA expressions between fed and fasted fish. Within the intestine, PYY mRNA expression was lower in fasted fish compared to fed fish but there was no significant difference for CCK intestine mRNA expression between fed and fasted fish. Our results suggest that these peptides, perhaps with the exception of CCK, play a major role in the regulation of feeding of red-bellied piranha.

Evolution of the gastrin-cholecystokinin gene family revealed by synteny analysis

Gastrin (GAST) and cholecystokinin (CCK) are two structurally and functionally related peptide hormones that exert many functions, including regulation of gastric and pancreatic secretion, feeding behaviour and energy homeostasis. GAST and CCK genes are assumed to have diverged from a common ancestral gene, over 500 million years ago in the vertebrate lineage. However, although a large number of GAST and CCK-related sequences have been identified both in vertebrate and non-vertebrate species, the evolutionary history of the GAST/CCK family remains little understood. To address this issue, we used extensive genome synteny comparisons of vertebrate chromosomes, in particular to evaluate the impact of whole-genome duplications. In the present study, we confirm that the GAST/CCK family in vertebrates is composed of two paralogous genes, namely GAST and CCK, and even three in teleosts, namely GAST, CCK1 and CCK2. We also show that the GAST and CCK genes arose by duplications of a single ancestral gene through the 2R and that the two copies of the CCK gene found in teleosts have probably been generated through the 3R. Finally, our results suggest that the vertebrate ancestor possessed four members of the GAST/CCK family, of which two have likely been lost during evolution.

Changes in expression of appetite-regulating hormones in the cunner (Tautogolabrus adspersus) during short-term fasting and winter torpor

Feeding in vertebrates is controlled by a number of appetite stimulating (orexigenic, e.g., orexin and neuropeptide Y, NPY) and appetite suppressing (anorexigenic, e.g., cholecystokinin, CCK and cocaine- and amphetamine-regulated transcript, CART) hormones. Cunners (Tautogolabrus adspersus) survive the winter in shallow coastal waters by entering a torpor-like state, during which they forgo feeding. In order to better understand the mechanisms regulating appetite/fasting in these fish, quantitative real-time PCR was used to measure transcript expression levels of four appetite-regulating hormones: NPY, CART, orexin and CCK in the forebrain (hypothalamus and telencephalon) and CCK in the gut of fed, short-term summer fasted, and natural winter torpor cunners. Summer fasting induced a decrease in hypothalamic orexin levels and telencephalon NPY, CART and CCK mRNA levels. All brain hormone mRNA levels decreased during natural torpor as compared to fed summer fish. In the gut, CCK expression levels decreased during summer fasting. These results indicate that, in cunner, orexin, NPY, CART and CCK may play a role in appetite regulation and might mediate different physiological responses to short-term summer fasting and torpor-induced long-term fasting.

Feed intake and brain neuropeptide Y (NPY) and cholecystokinin (CCK) gene expression in juvenile cobia fed plant-based protein diets with different lysine to arginine ratios

Cobia (Rachycentron canadum, Actinopterygii, Perciformes;10.5±0.1g) were fed to satiation with three plant-based protein test diets with different lysine (L) to arginine (A) ratios (LL/A, 0.8; BL/A, 1.1; and HL/A, 1.8), using a commercial diet as control for six weeks. The test diets contained 730 g kg(-1) plant ingredients with 505-529 g protein, 90.2-93.9 g lipid kg(-1) dry matter; control diet contained 550 g protein and 95 g lipid kg(-1) dry matter. Periprandial expression of brain NPY and CCK (npy and cck) was measured twice (weeks 1 and 6). At week one, npy levels were higher in pre-feeding than postfeeding cobia for all diets, except LL/A. At week six, npy levels in pre-feeding were higher than in postfeeding cobia for all diets. cck in pre-feeding cobia did not differ from that in postfeeding for all diets, at either time point. Cobia fed LL/A had lower feed intake (FI) than cobia fed BL/A and control diet, but no clear correlations between dietary L/A ratio and FI, growth and expression of npy and cck were detected. The data suggest that NPY serves as an orexigenic factor, but further studies are necessary to describe links between dietary L/A and regulation of appetite and FI in cobia.

Characterization of cholecystokinin-producing cells and mucus-secreting goblet cells in the blacktip grouper, Epinephelus fasciatus

The characteristics and distributions of cholecystokinin (CCK)-producing cells and mucus-secreting goblet cells were investigated in the digestive tract of the blacktip grouper (Epinephelus fasciatus). CCK-producing cells were scattered throughout the digestive tract. The highest frequency of CCK-producing cells was observed in the anterior intestine portion and pyloric ceca, with a very small number of cells distributed as far as the rectum. Mucus-secreting goblet cells were found to differ remarkably in their regional distributions and relative frequencies. High frequencies of mucus-secreting goblet cells were found in the digestive tract, mainly in the anterior intestine portion and pyloric ceca, but not the esophagus; the frequency decreased slightly toward the rectum. Our result suggests that food digested by gastric acid in the stomach moves on the anterior (including the pyloric ceca) and mid intestine portion, thereby ensuring effective stimulation of the CCK-producing cells. In addition, the distribution pattern of the CCK-producing cells closely resembled that of mucus-secreting goblet cells. In E. fasciatus, CCK-producing cells and mucus-secreting goblet cells seem to be well adapted to promoting optimal control of the digestive process.

Molecular characterization of cholecystokinin in grass carp (Ctenopharyngodon idellus): cloning, localization, developmental profile, and effect of fasting and refeeding on expression in the brain and intestine

Cholecystokinin (CCK) is a multi-functional brain-gut peptide in fish and mammals. To investigate the role of CCK in appetite regulation in fish, a 770-bp full-length cDNA sequence of CCK gene was obtained by RT-PCR and rapid amplification of cDNA ends methods in grass carp Ctenopharyngodon idellus. Homology analysis showed that the CCK cDNA sequence of grass carp had the highest similarity (90 %) to that of goldfish Carassius auratus and a higher similarity (>70 %) to those of other teleosts than to mammals. The PCR amplification using genomic DNA identified that the CCK gene of grass carp was comprised of three exons and two introns. Real-time quantitative PCR was used to detect CCK mRNA expression in adult tissues. High levels of gene expression were found in the hypothalamus and pituitary; moderate levels in the intestine, muscle and white adipose tissue; and low levels in other tissues. During early development (i.e., fertilized eggs to 35-day post-hatching larvae) the levels of CCK mRNA expression were higher during embryonic developmental stages than during post-hatch larval stages. Fasting decreased CCK mRNA expression levels in the brain and intestine, whereas refeeding resulted in an increase of expression. The results suggest that CCK mRNA expression has obvious tissue specificity and may have a role in feed intake regulation in grass carp.

Pre- and postprandial changes in orexigenic and anorexigenic factors in channel catfish (Ictalurus punctatus)

Ghrelin (GRLN), cocaine and amphetamine regulated transcript (CART), neuropeptide Y (NPY), and cholecystokinin (CCK) are neuropeptides involved in the regulation of appetite and feeding in vertebrates. We examined pre- and postprandial changes in the expression of plasma GHRL and mRNAs encoding GRLN, CART, NPY, and CCK in channel catfish. Fish were entrained to eat at 0900 h for 2 weeks. Fish were then sampled at 0700, 0800, and 0900 h. Remaining fish were either offered feed at 0900 h (Fed) or fasted (Unfed). Fish sampling continued at 0.5, 1, 2, and 4 h post feeding. Feeding increased abundance of whole brain CART mRNA out to 4 h with no effect observed in unfed fish. Whole brain NPY expression peaked at 0.5 h in both treatments. NPY expression then declined in fed fish but remained elevated in unfed fish. No differences in plasma or stomach GRLN expression were observed. Two separate cDNAs for CCK were identified. Brain CCKa and CCKb expression increased after feeding. These results suggest CART, NPY, and CCK play roles in the regulation of channel catfish feeding. Taken together, these results provide new insights into the neural and gastroenteric mechanisms regulating appetite in channel catfish.

Cholecystokinin: molecular cloning and immunohistochemical localization in the gastrointestinal tract of larval red drum, Sciaenops ocellatus (L.)

The current study sought to clarify the role of cholecystokinin (CCK) in the digestion of larval red drum (Sciaenops ocellatus) in order to better characterize the processes limiting the utilization of microparticulate diets at first feeding. The red drum CCK cDNA, isolated from adult anterior intestine and pyloric caeca, contains a 414 base pair (bp) open reading frame encoding a deduced amino acid sequence of 138 residues which is highly similar to preprocholecystokinin from other vertebrates. The mature CCK octapeptide has the same amino acid sequence as that found in mammals and in Atlantic herring (Clupea harengus). Tissue distribution analysis of adult and juvenile red drum using primers specific for red drum CCK mRNA revealed bright bands in samples from the brain, pyloric caeca, anterior intestine, and gonad with fainter bands seen in all other tissues. Immunohistochemical analysis of larval red drum showed that CCK-immunoreactive (CCK-IR) cells were present as early as 3 days post hatch (DPH) in some fish and were present in all fish by 6 DPH. CCK-IR cells were found in the anterior midgut in early larvae and had spread to the first bend of the gut by day 6. In older larvae (18+ DPH), CCK-IR cells were found in large numbers in the anterior intestine and in the developing pyloric caeca. The sequence and distribution of CCK mRNA along with the presence of CCK-IR cells in early red drum larvae suggest that CCK is present and may be capable of regulating pancreatic secretion in early red drum larvae.

Cloning, distribution and effects of season and nutritional status on the expression of neuropeptide Y (NPY), cocaine and amphetamine regulated transcript (CART) and cholecystokinin (CCK) in winter flounder (Pseudopleuronectes americanus)

cDNAs encoding for neuropeptide Y (NPY), cocaine and amphetamine regulated transcript (CART) and cholecystokinin (CCK) were cloned in winter flounder, a species that undergoes a period of natural fasting during the winter. Tissue distribution studies show that these peptides are present in several peripheral tissues, including gut and gonads, as well as within the brain. We assessed the effects of season and fasting on the expression of these peptides. Our results show that NPY and CCK, but not CART, show seasonal differences in expression with higher hypothalamic NPY and lower gut CCK expression levels in the winter. In the summer, fasting induced an increase in hypothalamic NPY expression levels and a decrease in gut CCK levels, but did not affect hypothalamic CART expression levels. None of the peptides examined was affected by fasting in the winter. Our results suggest that NPY and CCK, but maybe not CART, might have a major role in the regulation of feeding in winter flounder and might contribute to the seasonal fluctuations in appetite in this species.

Neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART) and cholecystokinin (CCK) in winter skate (Raja ocellata): cDNA cloning, tissue distribution and mRNA expression responses to fasting

cDNAs encoding for neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART) and cholecystokinin (CCK) were cloned in an elasmobranch fish, the winter skate. mRNA tissue distribution was examined for the three peptides as well as the effects of two weeks of fasting on their expression. Skate NPY, CART and CCK sequences display similarities with sequences for teleost fish but in general the degree of identity is relatively low (50%). All three peptides are present in brain and in several peripheral tissues, including gut and gonads. Within the brain, the three peptides are expressed in the hypothalamus, telencephalon, optic tectum and cerebellum. Two weeks of fasting induced an increase in telencephalon NPY and an increase in CCK in the gut but had no effects on hypothalamic NPY, CART and CCK, or on telencephalon CART. Our results provide basis for further investigation into the regulation of feeding in winter skate.

Neuronal interaction between melanin-concentrating hormone- and alpha-melanocyte-stimulating hormone-containing neurons in the goldfish hypothalamus

Intracerebroventricular (ICV) administration of melanin-concentrating hormone (MCH) inhibits food intake in goldfish, unlike in rodents, suggesting that its anorexigenic action is mediated by alpha-melanocyte-stimulating hormone (alpha-MSH) but not corticotropin-releasing hormone. This led us to investigate whether MCH-containing neurons in the goldfish brain have direct inputs to alpha-MSH-containing neurons, using a confocal laser scanning microscope, and to examine whether the anorexigenic action of MCH is also mediated by other anorexigenic neuropeptides, such as cholecystokinin (CCK) and pituitary adenylate cyclase-activating polypeptide (PACAP), using their receptor antagonists. MCH- and alpha-MSH-like immunoreactivities were distributed throughout the brain, especially in the diencephalon. MCH-containing nerve fibers or endings lay in close apposition to alpha-MSH-containing neurons in the hypothalamus in the posterior part of the nucleus lateralis tuberis (NLTp). The inhibitory effect of ICV-injected MCH on food intake was not affected by treatment with a CCK A/CCK B receptor antagonist, proglumide, or a PACAP receptor (PAC(1) receptor) antagonist, PACAP((6-38)). ICV administration of MCH at a dose sufficient to inhibit food consumption also did not influence expression of the mRNAs encoding CCK and PACAP. These results strongly suggest that MCH-containing neurons provide direct input to alpha-MSH-containing neurons in the NLTp of goldfish, and that MCH plays a crucial role in the regulation of feeding behavior as an anorexigenic neuropeptide via the alpha-MSH (melanocortin 4 receptor)-signaling pathway.

Role of cholecystokinin and its antagonist proglumide on macronutrient selection in European sea bass Dicentrarchus labrax, L

Teleost fish are able to adjust their energy intake when fed on pure macronutrient sources, although the exact mechanisms regulating macronutrient selection remain unknown. Since cholecystokinin (CCK) has been reported to modify macronutrient selection patterns in mammals, we explored the effect of CCK administered orally to European sea bass on the selection of separately encapsulated macronutrients. CCK doses of 0.05, 0.15 and 0.25 mg/kg BW administered in gelatine capsules for 5 consecutive days produced a significant inhibition of total food intake (21, 28 and 51%, respectively) at highest doses, evenly reducing the quantity of all the macronutrients ingested and, without affecting their relative proportions in the diet. Oral administration of proglumide, a non-specific CCK receptor antagonist, at doses of 5, 15 and 25 mg/kg BW, induced a quantitative total food intake increase of 2, 18 and 44%, respectively, and an increase of 52% in CH and 43% in P quantity ingested at highest dose. Co-administration of proglumide (25 mg/kg BW) and CCK (0.25 mg/kg BW) in a single preload capsule blocked the effects observed with CCK alone. In conclusion, orally administered CCK induced an anorexigenic effect on both total food and single macronutrient intake, an effect that is counteracted by the CCK antagonist proglumide.

Changes in cholecystokinin and peptide Y gene expression with feeding in yellowtail (Seriola quinqueradiata): relation to pancreatic exocrine regulation

In fish, the regulation of digestive enzyme secretion by hormonal control such as cholecystokinin (CCK) and neuropeptide Y (NPY)-related peptide is not well understood. To investigate the roles of fish CCK and peptide Y (PY) in digestive enzyme secretion, mRNA levels of CCK and PY, pyloric caeca enzyme activities and mRNA levels of pancreatic digestive enzymes (lipase, trypsin and amylase) were measured at pre- and post-prandial stages in yellowtail. Pyloric caeca were sampled at 0, 0.5, 1.5, 3, 6, 12, 24 and 48 h after feeding. The mRNA levels of trypsin and amylase increased after feeding, suggesting that transcription was induced by feed ingestion. Digestive enzyme activities decreased in exocrine pancreas after feeding, suggesting the stored enzyme was secreted from pancreas post-prandially. mRNA levels for CCK displayed a time-dependent increase, peaking between 1.5 and 3 h after-feeding followed by a rapid decrease 3 to 6 h after feeding. The mRNA expression pattern of PY was inverse to the pattern of CCK, decreasing until 1.5 h after feeding and then rising to initial levels by 12 h after feeding. These results suggest that CCK and PY work antagonistically in the exocrine pancreas of yellowtail.

The role of neuropeptide Y, orexins, cocaine and amphetamine-related transcript, cholecystokinin, amylin and leptin in the regulation of feeding in fish

In fish, as in all vertebrates, the brain is the primary center of regulation of food intake. Afferent signals continuously inform the central nervous system about changes in energy homeostasis. The brain interprets and integrates these signals and responds with efferent signals that help maintain a constant energy balance. Neuropeptides that originate from the hypothalamus regulate food intake either by stimulating (orexigenic factors) or inhibiting (anorexigenic factors) appetite. Studies using brain or peripheral peptide injections have shown that neuropeptide Y (NPY) and orexins are potent orexigenic factors in fish, whereas cocaine and amphetamine-related transcript (CART) peptides decrease food intake. Complex interactions exist between these central neuropeptide systems. For example, NPY and orexins have synergistic effects on food intake and they are both modulated by CART peptides. These systems are also influenced by endocrine factors from the periphery, including hormones from the gut, such as cholecystokinin (CCK), the pancreatic hormone amylin and the adipocyte hormone leptin. Fasting or ingestion of a meal induces changes in the mRNA expression of NPY, orexins and CART, suggesting that nutritional status modulates the action of these systems. This brief review will focus on our current knowledge on the structure and role of these six appetite-regulating peptides in fish.

Cholecystokinin and peptide Y in yellowtail (Seriola quinqueradiata): molecular cloning, real-time quantitative RT-PCR, and response to feeding and fasting

In fish, the peptide hormones cholecystokinin (CCK) and peptide Y (PY) may be involved in pancreatic exocrine secretion, as found with mammalian CCK and peptide YY (PYY); CCK stimulates, whereas PYY inhibits, pancreatic exocrine secretion in mammals. However, there is very little information on these hormones in fish; in particular, the function of PY is still unknown. Therefore, as a first step for understanding the role of CCK and PY in regulating pancreatic exocrine in fish, the cDNAs of CCK and PY were cloned from the digestive tract of yellowtail (Seriola quinqueradiata). The peptide sequence of yellowtail CCK-8, DYLGWMDF, is identical to sequences found in several teleosts. The mature form of yellowtail PY consists of 36 amino acids and has high identity to other fish PYs (88.9-97.2%). Real-time quantitative RT-PCR assays were developed to measure yellowtail CCK and PY mRNA levels. CCK mRNA levels were extremely high in the brain and, among the digestive organs, high concentrations were found in the pyloric caeca and anterior intestine. PY mRNA levels were low in the brain and highest in the anterior intestine. In fasting experiments, mRNA levels of CCK and PY in the anterior intestine showed an antagonistic change after fasting; CCK decreased whereas PY increased. These data suggest that CCK and PY in yellowtail may relate to digestion including, enzyme secretion.

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.

Cholecystokinin mRNA in Atlantic herring, Clupea harengus--molecular cloning, characterization, and distribution in the digestive tract during the early life stages

The mRNA of the peptide hormone cholecystokinin (CCK) was isolated from juvenile Atlantic herring, Clupea harengus, by RT-PCR. The open reading frame encodes a 137 amino acid-long precursor protein. The peptide sequence of herring CCK-8, DYMGWMDF, is identical to that of higher vertebrates and elasmobranchs, and contains methionine in the sixth position from the C-terminus, which has not been reported previously in teleosts. Expression analysis by in situ hybridization shows that positive endocrine-like cells were mainly located in the pyloric caeca and to a less extent in the rectum of the juvenile. A few positive cells were also found in the pyloric portion of the stomach and the intestine. CCK cells were present in all the larvae examined from the day of hatching onwards. Although the CCK cells were scattered throughout the whole midgut, no signals were detected in either the foregut or the hindgut. Since herring larvae have a straight gut, the distribution pattern of CCK cells seems to be reflected in the anatomy of the gut.

Effects of cholecystokinin and bombesin on the expression of preprosomatostatin-encoding genes in goldfish forebrain

It was previously demonstrated that both cholecystokinin (CCK) and bombesin (BBS) stimulate growth hormone (GH) secretion in goldfish. Both peptides induce satiety and it was speculated that they integrate satiation and the postprandial increase in GH circulating levels. In the present paper we investigated the effects of CCK and BBS on the forebrain expression of the somatostatin gene family in goldfish to analyze if somatostatin peptides may be part of the effector mechanisms of CCK and BBS. We found that peripherally as well as centrally administered CCK decreases mRNA levels of preprosomatostatin (PSS)-I that encodes for SRIF-14, having no effects on PSS-II and PSS-III, which encode for gSRIF-28 and [Pro2] SRIF-14, respectively. In addition, a direct action on the pituitary to stimulate GH release, this inhibition of PSS-I expression provides a possible mechanism for CCK to increase postprandial GH levels. On the other hand, BBS inhibits the forebrain expression of PSS-I and PSS-II but does not affect PSS-III regardless of the route of administration. We conclude that this could be the most likely mechanism of action of BBS to increase GH secretion, since there are few BBS-immunoreactive (IR) fibers and BBS binding sites in the anterior pituitary of goldfish.

In situ localization of preprogalanin mRNA in the goldfish brain and changes in its expression during feeding and starvation

In this paper, we report (i) the in situ localization, and (ii) meal time related and starvation induced changes in preprogalanin mRNA expression in the goldfish brain. The specific brain nuclei that express galanin mRNA are the area ventralis telencephali pars ventralis, nucleus preopticus periventricularis, nucleus lateralis tuberis, and the nucleus recessus lateralis. No changes in preprandial preprogalanin mRNA expression were found in the brain regions studied. No changes in postprandial preprogalanin mRNA expression were found compared to the preprandial levels. However, in unfed fish, a significant increase in preprogalanin mRNA expression was found in the telencephalon (1 and 3 h) and hypothalamus (3 h) after the scheduled feeding time. The postprandial preprogalanin mRNA expression in the telencephalon and hypothalamus of fed fish at 1 and 3 h were significantly lower than the mRNA expression levels in the unfed fish at the same time. Preprogalanin mRNA expression levels remain unaltered during 7 days of starvation. The presence of preprogalanin mRNA in brain nuclei involved in the regulation of food intake and pituitary hormone secretion suggests important physiological roles for galanin in goldfish.

Endogenous hypothalamic somatostatins differentially regulate growth hormone secretion from goldfish pituitary somatotropes in vitro

Using Southern blot analysis of RT-PCR products, mRNA for three different somatostatin (SS) precursors (PSS-I, -II, and -III), which encode for SS(14), goldfish brain (gb)SS(28), and [Pro(2)]SS(14), respectively, were detected in goldfish hypothalamus. PSS-I and -II mRNA, but not PSS-III mRNA, were also detected in cultured pituitary cells. We subsequently examined the effects of the mature peptides, SS(14), gbSS(28), and [Pro(2)]SS(14), on somatotrope signaling and GH secretion. The gbSS(28) was more potent than either SS(14) or [Pro(2)]SS(14) in reducing basal GH release but was the least effective in reducing basal cellular cAMP. The ability of SS(14), [Pro(2)]SS(14), and gbSS(28) to attenuate GH responses to GnRH were comparable. However, gbSS(28) was less effective than SS(14) and [Pro(2)]SS(14) in diminishing dopamine- and pituitary adenylate cyclase-activating polypeptide-stimulated GH release, as well as GH release resulting from the activation of their underlying signaling cascades. In contrast, the actions of a different 28-amino-acid SS, mammalian SS(28), were more similar to those of SS(14) and [Pro(2)]SS(14). We conclude that, in goldfish, SSs differentially couple to the intracellular cascades regulating GH secretion from pituitary somatotropes. This raises the possibility that such differences may allow for the selective regulation of various aspects of somatotrope function by different SS peptides.

Molecular cloning, pharmacological characterization, and brain mapping of the melanocortin 4 receptor in the goldfish: involvement in the control of food intake

We report cloning, pharmacological characterization, tissue distribution, detailed brain mapping, and role in control of food intake of melanocortin 4 receptor in goldfish (gMC4R). The gMC4R protein has 68% identity with the human ortholog and is conserved in important functional domains. Pharmacological profiling showed similar affinities and potency order to hMC4R for MSH peptides, whereas MTII and HS024 were identified as high-affinity agonist and antagonist analogs, respectively. The gMC4R-mRNA was found in brain and some peripheral tissues including the ovary, gill, and spleen. Detailed MC4R-mRNA mapping showed expression in main neuroendocrine and food intake-controlling areas. High expression levels were found in the telencephalon, preoptic area, ventral thalamus, tuberal hypothalamus, and hypothalamic inferior lobe. By RT-PCR, low levels were also detected in the cerebellum, medulla, and spinal cord. Intracerebroventricular MTII administration inhibited food intake in 24-h fasted animals in a dose-dependent manner, whereas HS024 stimulated food intake in fed animals, suggesting that melanocortins exert a tonic inhibitory effect on food intake, which is mediated through central MC4R signaling. The conserved central expression pattern and physiological role in regulation of food intake for the MC4R suggests that neuronal pathways of the melanocortin system may be important for regulation of energy homeostasis in most vertebrates.

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.

Identification of gastrin and multiple cholecystokinin genes in teleost

To identify the teleost gastrin, CCK/gastrin family genes were isolated from puffer and flounder. The cDNA of puffer gastrin, CCK1 and CCK2 were 678, 752 and 533bp, respectively. Puffer gastrin gene consists of three exons, in contrast, CCKs consist of four exons. Flounder gastrin mRNA (526bp) was expressed in the intestine but not in the brain. It was developed synchronously with the stomach differentiation in the larval stage. The phylogenetic analysis shows that puffer and flounder gastrin classified into the vertebrate gastrin cluster and two types of CCK were probably produced by the genome duplication occurred in teleost phylogeny.

Characterization of two forms of cocaine- and amphetamine-regulated transcript (CART) peptide precursors in goldfish: molecular cloning and distribution, modulation of expression by nutritional status, and interactions with leptin

Complementary DNAs encoding two forms of cocaine- and amphetamine-regulated transcript (CART) peptide precursors were identified from goldfish brain and named CART I and CART II. Each cDNA contains a signal peptide sequence, the putative CART-like peptide, and a carboxy-terminal extension peptide. Form I encodes a 117-amino acid pro-CART, whereas form II encodes a 120-amino acid pro-CART. Both forms resemble mammalian CART peptides. Each goldfish CART precursor is encoded by three exons interrupted by two introns within genomic DNA. RT-PCR, slot blot, and Northern blot analysis showed that the mRNAs for form I and II precursors have a widespread distribution. Form I and II are present in the brain, pituitary, eye, gonads, and kidney. Form I is also present in the gill. In the brain, form I is predominant in the olfactory bulb and hypothalamus, and form II is predominant in the optic tectum. Food deprivation for 96 h induced a decrease in form I mRNA levels in the telencephalon-preoptic region, hypothalamus, and olfactory bulb and in form II mRNA expression in the olfactory bulb. An increase in mRNA levels was observed 2 h following a meal in the olfactory bulbs and hypothalamus for form I whereas no postprandial changes in form II mRNA levels were observed. Intracerebroventricular injections of human CART alone induced a significant decrease in food intake. Injections of leptin reinforced the inhibition of feeding behavior and food intake seen in CART-treated fish. Central injection of leptin induced an increase in CART I mRNA in the optic tectum, hypothalamus, and olfactory bulbs but had no effect on CART II mRNA expression in the brain. These results suggest that CART peptides act as leptin-regulated satiety factors in goldfish and that they might have other physiological roles besides feeding, possibly in sensory information processing.

Identification and distribution of CCK-related peptides and mRNAs in the rainbow trout, Oncorhynchus mykiss

Peptides homologous to mammalian cholecystokinin (CCK), and their corresponding cDNAs, have been isolated and sequenced from the rainbow trout, Oncorhynchus mykiss. Three cDNAs encoding CCK-like preprohormones were identified from the brain. The cDNAs encode three different putative CCK-8 peptides containing Asn, Leu or Thr, in position 6 (counting from the C-terminus). Hence, the trout CCKs are named CCK-N, CCK-L and CCK-T respectively. RT-PCR showed differential expression of the three mRNAs although all were detected in the brain and intestine, similar to the expression pattern of CCK in tetrapods. In situ hybridization on trout brain sections using (35)S-labeled gene-specific antisense oligonucleotides showed that the three mRNAs were present in different parts, suggesting that the three CCK peptides may have different functions in the brain. Purification of CCK-immunoreactive material from the trout brain resulted in two CCK octapeptides: DYNGWMDF(.)NH2 (CCK-N) and DYLGWMDF(.)NH2 (CCK-L) present in equal amounts. In the pyloric caeca, three forms of CCK-L were identified, consisting of 7, 8 and 21 residues, respectively. The last was dominating and had the sequence ASGPGPSHKIKDRDYLGWMDF(.)NH2. All isolated peptides were fully sulfated. The trout is the first species in which three different CCK-like cDNAs have been identified.

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.

Molecular cloning and expression of cDNA encoding a brain bombesin/gastrin-releasing peptide-like peptide in goldfish

A complementary DNA (cDNA) of 928 bp encoding a bombesin (BBS)/gastrin-releasing peptide (GRP) precursor was identified from goldfish brain. Goldfish BBS/GRP messenger RNA (mRNA) encodes a 157 amino acid precursor, which contains a signal peptide sequence, the 22 amino acid putative BBS/GRP-like peptide, and a carboxy-terminal extension peptide. Reverse transcription-polymerase chain reaction (PCR) (RT-PCR) demonstrated that the mRNA for this precursor has a widespread distribution in goldfish brain, and is also present in skin, gastrointestinal tract, gonad, and gill. Phylogenetic analysis of BBS/GRP-like peptide precursors in vertebrates shows that goldfish BBS/GRP is more closely related to the known GRP precursors than to BBS precursors.

Postprandial, seasonal and sexual variations in cholecystokinin gene expression in goldfish brain

Recently, we described the complete nucleotide sequence of cholecystokinin (CCK) mRNA and the deduced amino acid sequence of the precursor on the basis of molecular cloning and sequence analysis of cDNA from goldfish brain. In this study, we investigated the hypothesis that CCK has a role in feeding behavior by examining CCK gene expression in the brain of goldfish using Northern blot. We showed that CCK gene is widely but differentially expressed in broad areas of the goldfish brain, including the olfactory bulbs, telencephalon and preoptic region, hypothalamus, optic tectum-thalamus and posterior brain regions, with highest levels in hypothalamus. We found that CCK mRNA levels in goldfish olfactory bulbs, telencephalon-preoptic region, optic tectum-thalamus, and posterior brain were influenced by sex at least sometime of the seasonal gonadal cycle, with female fish having higher levels than males during at least one of the four seasonal sampling times. We also observed a transient and acute increase in the CCK mRNA levels in the olfactory bulbs, telencephalon-preoptic region, hypothalamus, and posterior brain at 120 min after a meal. These widespread postprandial changes in CCK gene expression in goldfish brain indicate that CCK peptides have multiple roles in regulation of feeding behavior in goldfish. This supports the idea that CCK plays a role as a satiety factor in goldfish.

Differential expression of corticotropin-releasing factor (CRF) and urotensin I precursor genes, and evidence of CRF gene expression regulated by cortisol in goldfish brain

Corticotropin-releasing factor (CRF) and urotensin I (UI) precursor cDNAs were cloned and sequenced from a goldfish brain cDNA library in order to investigate the distribution of CRF and UI mRNAs in goldfish brain and the regulation of CRF and UI gene expression. The CRF (966-bp) and UI (769-bp) cDNAs encode 163- and 146-amino acid precursors, respectively, and consist of a signal peptide sequence, a cryptic region, and a 41-amino acid mature peptide at the carboxy terminal. The deduced amino acid sequences of the CRF and UI peptides exhibit a sequence identity of 54%. Northern blot analysis revealed a single size of CRF (1.3 kb) and UI (2.0 kb) mRNAs, which are expressed in the telencephalon-preoptic, hypothalamic, optic tectum-thalamus, and posterior brain regions, but not in the pituitary. In addition, while the CRF gene is strongly expressed in the olfactory bulbs, the UI gene is not. In brain regions in which both genes are expressed, the mRNA levels of CRF were three- to sevenfold higher that those of UI. While the low expression levels of the UI gene prevented further analysis of its regulation, the regulation of CRF gene expression by cortisol was examined. In response to intraperitoneal implants of cortisol (300 microg/g BW) the level of CRF mRNA in the telencephalon-preoptic region decreased to 69% of control values at 6 and 24 h posttreatment. In sham-treated fish, in parallel with a transient injection stress-elicited increase in plasma cortisol, CRF mRNA levels declined to 72% of control value at 6 h postinjection and recovered after 24 h. Injection of the glucocorticoid antagonist, RU-486 (100 microg/g BW), prevented the reduction in CRF gene expression associated with the injection stress at 6 h and increased CRF mRNA levels to 145% of control value after 24 h. In contrast, the various implants had no effect on CRF mRNA levels in either the hypothalamus or the optic tectum-thalamus region. These results provide evidence of differential expression of the CRF and UI genes in hypothalamic and extrahypothalamic regions of goldfish brain. Furthermore, they demonstrate that stress levels of plasma cortisol can lead to a decrease in CRF gene expression that is mediated by glucocorticoid receptors in the telencephalon-preoptic region and give an indication of the regional specificity of the regulation of CRF gene expression by cortisol.