Cholecystokinin

Despite the profuse development of the enteric nervous system, the main route of communication between the brain and the gut in relation to energy homeostasis is via the circulation. One of the hormones first identified in regulating energy homeostasis was the gastrointestinal hormone cholecystokinin (CCK), which is secreted by I cells in the duodenum and the jejunum into the circulation in response to nutrient ingestion (protein and fatty acids; Larsson & Rehfeld, Reference Larsson and Rehfeld1978; Bray, Reference Bray2000; Halford & Blundell, Reference Halford and Blundell2000a; Badman & Flier, Reference Badman and Flier2005; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005; Perez-Tilve et al. Reference Perez-Tilve, Nogueiras, Mallo, Benoit and Tschoep2006). CKK exists in several molecular forms, the major forms in the plasma being CCK-8, -33 and -39 (Halford & Blundell, Reference Halford and Blundell2000a; Konturek et al. Reference Konturek, Konturek, Pawlik and Brzozowski2004; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005). Once secreted, CCK reduces meal size and duration in both man and animals (Gibbs et al. Reference Gibbs, Young and Smith1973; Kissileff et al. Reference Kissileff, Pi-Sunyer, Thornton and Smith1981; Smith et al. Reference Smith, Gibbs, Jerome, Pi-Sunyer, Kissileff and Thornton1981a; Pi-Sunyer et al. Reference Pi-Sunyer, Kissileff, Thornton and Smith1982; Muurahainen et al. Reference Muurahainen, Kissileff, DeRogatis and Pi-Sunyer1988) and infusion of a CCK antagonist increases energy intake in human subjects (Beglinger et al. Reference Beglinger, Degen, Matzinger, D'Amato and Drewe2001). However, despite its anorectic actions, repeated administration of CKK does not influence body weight because although meal frequency is increased, there is no overall change in feeding (West et al. Reference West, Fey and Woods1984; Wei & Mojsov, Reference Wei and Mojsov1995). Thus, CCK is mostly involved in the short-term control of food intake, together with distension of the upper gastrointestinal tract (Konturek et al. Reference Konturek, Konturek, Pawlik and Brzozowski2004; Badman & Flier, Reference Badman and Flier2005).

CCK signals via two distinct G-protein-coupled receptors termed CCK_A and CCK_B (Wank et al. Reference Wank, Harkins, Jensen, Shapira, de Weerth and Slattery1992a; Halford & Blundell, Reference Halford and Blundell2000a). Both receptors are widely expressed in the CNS and in the periphery (Moran et al. Reference Moran, Robinson, Goldrich and McHugh1986; Moran et al. Reference Moran, Norgren, Crosby and McHugh1990; Wank et al. Reference Wank, Harkins, Jensen, Shapira, de Weerth and Slattery1992a,Reference Wank, Pisegna and de Weerthb). The effect of CCK on food intake is mediated via CCK_A (Asin et al. Reference Asin, Gore, Bednarz, Holladay and Nadzan1992; Halford & Blundell, Reference Halford and Blundell2000a). CKK crosses the brain–blood barrier (BBB; Reidelberger et al. Reference Reidelberger, Hernandez, Fritzsch and Hulce2004) and acts on neuropeptide Y (NPY) neurons in the dorsomedial nucleus of the hypothalamus (DMH), as well as the NST in the brainstem (Moran et al. Reference Moran, Baldessarini, Salorio, Lowery and Schwartz1997; Bi et al. Reference Bi, Ladenheim, Schwartz and Moran2001). The effects of CKK on feeding are also mediated through paracrine and neuroendocrine activation of vagal fibres (Reidelberger & Solomon, Reference Reidelberger and Solomon1986; Schwartz & Moran, Reference Schwartz and Moran1994; Moran et al. Reference Moran, Baldessarini, Salorio, Lowery and Schwartz1997).

Glucagon-like peptide-1 and oxyntomodulin

The preproglucagon gene product yields two important satiety peptides, glucagon-like peptide (GLP)-1 and oxyntomodulin (OXM; Tang-Christensen et al. Reference Tang-Christensen, Vrang and Larsen2001; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005). The preproglucagon gene is widely expressed in the gut, the pancreas and the NTS in the brainstem. Tissue-specific processing of preproglucagon by prohormone convertases 1 and 2 produces different products: glucagon is the main product in the pancreas; GLP-1 and -2 and OXM are the major products in CNS and gut (Tang-Christensen et al. Reference Tang-Christensen, Vrang and Larsen2001; Badman & Flier, Reference Badman and Flier2005; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005).

GLP-1 and OXM are released from L cells in response to NEFA and carbohydrates (Ghatei et al. Reference Ghatei, Uttenthal, Christofides, Bryant and Bloom1983; Le Quellec et al. Reference Le Quellec, Kervran, Blache, Ciurana and Bataille1992; Herrmann et al. Reference Herrmann, Goke, Richter, Fehmann, Arnold and Goke1995; Hirasawa et al. Reference Hirasawa, Tsumaya, Awaji, Katsuma, Adachi, Yamada, Sugimoto, Miyazaki and Tsujimoto2005). Both peptides inhibit feeding when they are centrally or peripherally administrated (Turton et al. Reference Turton, O'Shea, Gunn, Beak, Edwards and Meeran1996; Dakin et al. Reference Dakin, Small, Batterham, Neary, Cohen, Patterson, Ghatei and Bloom2004), and chronic administration of GLP-1 and OXM decreases weight gain and adiposity in rodents (Meeran et al. Reference Meeran, O'Shea, Edwards, Turton, Heath and Gunn1999; Dakin et al. Reference Dakin, Small, Batterham, Neary, Cohen, Patterson, Ghatei and Bloom2004). The actions of both GLP-1 and OXM on feeding may be mediated via the GLP-1 receptor, which is expressed in the hypothalamus, brainstem and periphery (Uttenthal et al. Reference Uttenthal, Toledano and Blazquez1992; Wei & Mojsov, Reference Wei and Mojsov1995; Shughrue et al. Reference Shughrue, Lane and Merchenthaler1996; Bullock et al. Reference Bullock, Heller and Habener1996). The anorectic effect of GLP-1 and OXM is also present in man (Flint et al. Reference Flint, Raben, Astrup and Holst1998, Reference Flint, Raben, Rehfeld, Holst and Astrup2000, Reference Flint, Raben, Ersboll, Holst and Astrup2001; Gutzwiller et al. Reference Gutzwiller, Goke, Drewe, Hildebrand, Ketterer, Handschin, Winterhalder, Conen and Beglinger1999; Naslund et al. Reference Naslund, Barkeling, King, Gutniak, Blundell, Holst, Rossner and Hellstrom1999; Verdich et al. Reference Verdich, Flint, Gutzwiller, Naslund, Beglinger, Hellstrom, Long, Morgan, Holst and Astrup2001; Meier et al. Reference Meier, Gallwitz, Schmidt and Nauck2002). Despite this evidence, it has been reported that some of the anorectic effects of GLP-1 may be related to taste aversion and visceral illness (Shughrue et al. Reference Shughrue, Lane and Merchenthaler1996; Bullock et al. Reference Bullock, Heller and Habener1996; Thiele et al. Reference Thiele, van Dijk, Campfield, Smith, Burn, Woods, Bernstein and Seeley1997; Yamamoto et al. Reference Yamamoto, Lee, Marcus, Williams, Overton, Lopez, Hollenberg, Baggio, Saper, Drucker and Elmquist2002). Regardless of the anorectic actions of GLP-1 and OXM reported in rodents, GLP-1 receptor-knock-out mice have normal feeding behaviour (Scrocchi et al. Reference Scrocchi, Brown, Maclusky, Brubaker, Auerbach, Joyner and Drucker1996, Reference Scrocchi, Brown and Drucker1997, Reference Scrocchi, Hill, Saleh, Perkins and Drucker2000).

Preproglucagon also yields GLP-2. The role of GLP-2 has not been fully established; however, central administration reduces feeding, probably via GLP-1 receptor (Badman & Flier, Reference Badman and Flier2005). No effect of GLP-2 on feeding has been reported in man (Schmidt et al. Reference Schmidt, Naslund, Gryback, Jacobsson, Hartmann, Holst and Hellstrom2003).

Peptide YY

Peptide YY (PYY) is secreted postprandially by the L cells of the gastrointestinal tract, especially in the most distal portions such as the ileum, colon and rectum; PYY secretion is correlated with energy intake (Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005). There are two main forms of PYY in the circulation: PYY_1–36; PYY_3–36 (Grandt et al. Reference Grandt, Schimiczek, Beglinger, Layer, Goebell, Eysselein and Reeve1994; Batterham et al. Reference Batterham, Cowley, Small, Herzog, Cohen and Dakin2002; Wynne et al. Reference Wynne, Stanley and Bloom2004; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005). Peripheral administration of PYY has several actions, including delaying gastric emptying and gastric secretion, and increasing ileum absorption. It has also been reported that peripheral administration of PYY_3–36 inhibits food intake and reduces weight gain in rodents, primates and man (Batterham et al. Reference Batterham, Cowley, Small, Herzog, Cohen and Dakin2002; Challis et al. Reference Challis, Pinnock, Coll, Carter, Dickson and O'Rahilly2003; Moran et al. Reference Moran, Smedh, Kinzig, Scott, Knipp and Ladenheim2005). PYY crosses the BBB and probably exerts its actions via the presynaptic Y₂ receptor of NPY neurons in the ARC, releasing inhibition of pro-opiomelanocortin (POMC) neurons and consequently inhibiting feeding (Broberger et al. Reference Broberger, Landry, Wong, Walsh and Hokfelt1997; Batterham et al. Reference Batterham, Cowley, Small, Herzog, Cohen and Dakin2002; Challis et al. Reference Challis, Pinnock, Coll, Carter, Dickson and O'Rahilly2003).

Despite this evidence, the anorectic effect of PYY_3–36 is controversial and not easily duplicated (Tschop et al. Reference Tschop, Castaneda, Joost, Thone-Reineke, Ortmann and Klaus2004; Coll et al. Reference Coll, Challis and O'Rahilly2004a). Indeed, in contrast to peripheral injection central administration of both PYY_1–36 and PYY_3–36 stimulates feeding in rodents (Stanley et al. Reference Stanley, Daniel, Chin and Leibowitz1985; Clark et al. Reference Clark, Sahu, Kalra, Balasubramaniam and Kalra1987; Hagan et al. Reference Hagan, Castaneda, Sumaya, Fleming, Galloway and Moss1998; Corpa et al. Reference Corpa, McQuade, Krasnicki and Conze2001). It has also been suggested that the anorexic effect of PYY may be partially mediated by an aversive response (Halatchev & Cone, Reference Halatchev and Cone2005).

Bombesin

Bombesin is a peptide that is widely distributed in the mammalian gut. Plasma levels of bombesin increase markedly after food intake (Gibbs et al. Reference Gibbs, Fauser, Rowe, Rolls, Rolls and Maddison1979; Wynne et al. Reference Wynne, Stanley and Bloom2004), and peripheral and central administration of bombesin is anorectic (Gibbs et al. Reference Gibbs, Fauser, Rowe, Rolls, Rolls and Maddison1979; Smith et al. Reference Smith, Jerome and Gibbs1981b). Bombesin is structurally very similar to gastrin-releasing peptide and neuromedin B, and binds to their receptors. Additionally, a bombesin-3 receptor has been cloned (Ladenheim et al. Reference Ladenheim, Moore, Salorio, Mantey, Taylor, Coy, Jensen and Moran1997). Knocking out bombesin-3 receptor induces moderate hyperphagia, obesity and metabolic alterations in mice (Ohki-Hamazaki et al. Reference Ohki-Hamazaki, Watase, Yamamoto, Ogura, Yamano, Yamada, Maeno, Imaki, Kikuyama, Wada and Wada1997).

Gastric inhibitory polypeptide

Gastric inhibitory polypeptide is secreted from the duodenal K cells, predominantly in response to ingested fat. Mice fed a high-fat diet have increased levels of gastric inhibitory polypeptide together with obesity (Wynne et al. Reference Wynne, Stanley and Bloom2004; Badman & Flier, Reference Badman and Flier2005), whereas mice lacking the gastric inhibitory polypeptide receptor are protected against obesity induced by both a high-fat diet and leptin deficiency (ob/ob mice; Miyawaki et al. Reference Miyawaki, Yamada, Ban, Ihara, Tsukiyama and Zhou2002). Thus, gastric inhibitory polypeptide may be involved in the development of obesity in response to high fat intake.

Ghrelin

Ghrelin is a twenty-eight-amino acid acylated hormone mainly synthesised and secreted by the gut in the gastric oxyntic cells (A/X cells) at the fundus of the stomach, as well as the duodenum, ileum, caecum and colon (Kojima et al. Reference Kojima, Hosoda, Date, Nakazato, Matsuo and Kangawa1999; Date et al. Reference Date, Kojima, Hosoda, Sawaguchi, Mondal, Suganuma, Matsukura, Kangawa and Nakazato2000; Gualillo et al. Reference Gualillo, Caminos, Blanco, García-Caballero, Kojima, Kangawa, Diéguez and Casanueva2001; Sakata et al. Reference Sakata, Nakamura, Yamazaki, Matsubara, Hayashi, Kangawa and Sakai2002). Ghrelin expression has also been detected in other tissues, such as the hypothalamus (Kojima et al. Reference Kojima, Hosoda, Date, Nakazato, Matsuo and Kangawa1999; Horvath et al. Reference Horvath, Diano, Sotonyi, Heiman and Tschop2001; Cowley et al. Reference Cowley, Smith, Diano, Tschop, Pronchuk and Grove2003) testis (Barreiro et al. Reference Barreiro, Gaytan, Caminos, Pinilla, Casanueva, Aguilar, Dieguez and Tena-Sempere2002; Tena-Sempere et al. Reference Tena-Sempere, Barreiro, Gonzalez, Gaytan, Zhang, Caminos, Pinilla, Casanueva, Dieguez and Aguilar2002), pituitary (Caminos et al. Reference Caminos, Nogueiras, Blanco, Seoane, Bravo, Alvarez, Garcia-Caballero, Casanueva and Dieguez2003a), ovary (Caminos et al. Reference Caminos, Tena-Sempere, Gaytan, Sanchez-Criado, Barreiro, Nogueiras, Casanueva, Aguilar and Dieguez2003b; Gaytan et al. Reference Gaytan, Barreiro, Chopin, Herington, Morales, Pinilla, Casanueva, Aguilar, Dieguez and Tena-Sempere2003), heart (Iglesias et al. Reference Iglesias, Pineiro, Blanco, Gallego, Dieguez, Gualillo, Gonzalez-Juanatey and Lago2004) and placenta (Gualillo et al. Reference Gualillo, Caminos, Blanco, García-Caballero, Kojima, Kangawa, Diéguez and Casanueva2001).

Ghrelin, which was initially identified as the endogenous ligand of the growth hormone secretagogue receptor (GHS-R), exerts a potent and specific growth hormone-releasing activity in vitro (Kojima et al. Reference Kojima, Hosoda, Date, Nakazato, Matsuo and Kangawa1999) and in vivo (Arvat et al. Reference Arvat, Di Vito, Broglio, Papotti, Muccioli, Dieguez, Casanueva, Deghenghi, Camanni and Ghigo2000; Seoane et al. Reference Seoane, Tovar, Baldelli, Arvat, Ghigo, Casanueva and Diéguez2000), as well as increasing the transcription rate of the Pit-1 gene (García et al. Reference García, Alvarez, Smith and Diéguez2001). Further studies have led to the recognition that ghrelin also plays an important role in energy homeostasis. Ghrelin administration induces positive energy balance in rodents by decreasing fat utilization without markedly changing energy expenditure or locomotor activity (Wren et al. Reference Wren, Small, Ward, Murphy, Dakin, Taheri, Kennedy, Roberts, Morgan, Ghatei and Bloom2000; Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001). Furthermore, peripheral and central administration of ghrelin to rodents increases feeding, as well as fat mass, and reduces fat utilization (Tschop et al. Reference Tschop, Smiley and Heiman2000; Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001; Wren et al. Reference Wren, Small, Abbott, Dhillo, Seal, Cohen, Batterham, Taheri, Stanley, Ghatei and Bloom2001b; Seoane et al. Reference Seoane, López, Tovar, Casanueva, Señarís and Diéguez2003). Plasma levels of ghrelin are regulated by food intake, rising during fasting and immediately before meals, and falling after food intake (Ariyasu et al. Reference Ariyasu, Takaya, Tagami, Ogawa, Hosoda and Akamizu2001; Cummings et al. Reference Cummings, Purnell, Frayo, Schmidova, Wisse and Weigle2001; Tschop et al. Reference Tschop, Wawarta, Riepl, Friedrich, Bidlingmaier, Landgraf and Folwaczny2001a). These changes in ghrelin expression are directly modulated by energy intake and nutritional signals such as blood glucose and ingestion of fat or carbohydrate (Tschop et al. Reference Tschop, Smiley and Heiman2000; Sakata et al. Reference Sakata, Nakamura, Yamazaki, Matsubara, Hayashi, Kangawa and Sakai2002). For this reason a physiological role of ghrelin in meal initiation has been proposed (Cummings et al. Reference Cummings, Purnell, Frayo, Schmidova, Wisse and Weigle2001; Cummings & Shannon, Reference Cummings and Shannon2003). This suggestion is supported by experiments (Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001) that use anti-ghrelin antibodies to block the actions of ghrelin, which results in an attenuation of fasting-induced refeeding.

The effects of ghrelin on feeding and growth hormone secretion are mediated via type 1a GHS-R (Kojima et al. Reference Kojima, Hosoda, Date, Nakazato, Matsuo and Kangawa1999; Tschop et al. Reference Tschop, Smiley and Heiman2000; Chen et al. Reference Chen, Trumbauer, Chen, Weingarth, Adams and Frazier2004; Sun et al. Reference Sun, Wang, Zheng and Smith2004). However, the orexigenic effects of ghrelin are independent of growth hormone-releasing properties (Tschop et al. Reference Tschop, Smiley and Heiman2000; Shintani et al. Reference Shintani, Ogawa, Ebihara, Aizawa-Abe, Miyanaga and Takaya2001; Tamura et al. Reference Tamura, Kamegai, Shimizu, Ishii, Sugihara and Oikawa2002). The expression of GHS-R in the hypothalamus is nutritionally regulated in a nucleus-specific manner, with fasting increasing the mRNA levels of GHS-R in the ARC but not in the ventromedial nucleus of the hypothalamus (VMH). Additionally, the level of GHS-R expression in the ARC, but not in the VMH, is reduced by leptin and increased by ghrelin in a growth hormone-dependent fashion (Fig. 1; Nogueiras et al. Reference Nogueiras, Tovar, Mitchell, Rayner, Archer, Diéguez and Williams2004b).

Fig. 1. Regulation of growth hormone secretogogue receptor (GHS-R) in the rat hypothalamus by leptin and ghrelin. Effects of intracerebroventricular leptin (■; a, b) and ghrelin (; c, d) on GHS-R expression in the arcuate nucleus (a, c) and the ventromedial nucleus of the hypothalamus (b, d) in fed and 48 h-fasted rats. (□), Vehicle. Values are means with their standard errors represented by vertical bars. Mean values were significantly different from the corresponding values for fed rats: *P<0·05. Mean values were significantly different from the corresponding values for vehicle-fed rats: †P<0·05, ††P<0·01, †††P<0·001.

Ghrelin is also important in the regulation of energy homeostasis in man. Intravenous administration of ghrelin to healthy volunteers increases food intake (Wren et al. Reference Wren, Seal, Cohen, Brynes, Frost, Murphy, Dhillo, Ghatei and Bloom2001a). Moreover, the rise in preprandial ghrelin correlates with hunger scores in human subjects eating spontaneously (Cummings et al. Reference Cummings, Frayo, Marmonier, Aubert and Chapelot2004). Interestingly, the levels of ghrelin are correlated with adiposity in man, with an inverse relationship between plasma ghrelin levels and BMI (Tschop et al. Reference Tschop, Weyer, Tataranni, Devanarayan, Ravussin and Heiman2001b). Obese human subjects show reduced levels of plasma ghrelin, which rise to normal after diet-induced weight loss (Hansen et al. Reference Hansen, Dall, Hosoda, Kojima, Kangawa, Christiansen and Jorgensen2002; Cummings et al. Reference Cummings, Weigle, Frayo, Breen, Ma, Dellinger and Purnell2002b). Moreover, in obese individuals the postprandial regulation of ghrelin seems to be altered, which may be related to continuous food intake and/or obesity (English et al. Reference English, Ghatei, Malik, Bloom and Wilding2002). Finally, the severe hyperphagia seen in patients with Prader-Willi syndrome is associated with elevated ghrelin levels, in contrast to other forms of obesity in which ghrelin levels are low (Cummings et al. Reference Cummings, Clement, Purnell, Vaisse, Foster, Frayo, Schwartz, Basdevant and Weigle2002a).

In the CNS the action of ghrelin on feeding is mainly exerted via the ARC. GHS-R mRNA is expressed in neurons in the ARC co-expressing NPY and agoutirelated peptide (AgRP; Guan et al. Reference Guan, Yu, Palyha, McKee, Feighner, Sirinathsinghji, Smith, Van Der Ploeg and Howard1997; Tannenbaum et al. Reference Tannenbaum, Lapointe, Beaudet and Howard1998; Willesen et al. Reference Willesen, Kristensen and Romer1999; Zigman et al. Reference Zigman, Jones, Lee, Saper and Elmquist2006), and the central administration of ghrelin increases the mRNA content of NPY and AgRP in the ARC in fed and fasting conditions (Kamegai et al. Reference Kamegai, Tamura, Shimizu, Ishii, Sugihara and Wakabayashi2001; Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001; Seoane et al. Reference Seoane, López, Tovar, Casanueva, Señarís and Diéguez2003). There is also some evidence that orexin (also termed hypocretin; OX) neurons in the lateral hypothalamic area (LHA; Lawrence et al. Reference Lawrence, Snape, Baudoin and Luckman2002; Toshinai et al. Reference Toshinai, Date, Murakami, Shimada, Mondal and Shimbara2003) and neurons in the NTS and the area postrema in the brainstem may mediate the orexigenic actions of ghrelin (Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001; Lawrence et al. Reference Lawrence, Snape, Baudoin and Luckman2002). Recent data also indicate that ghrelin acts in the hypothalamus by altering fatty acid metabolism and AMP-activated protein kinase. It has been demonstrated that ghrelin increases hypothalamic AMP-activated protein kinase phosphorylation levels, activating it. This action may be associated to specific changes in hypothalamic neuropeptides, although the exact molecular mechanisms and anatomical details of this interaction have not been fully identified (Andersson et al. Reference Andersson, Filipsson, Abbott, Woods, Smith, Bloom, Carling and Small2004; Kola et al. Reference Kola, Hubina, Tucci, Kirkham, Garcia, Mitchell, Williams, Hawley, Hardie, Grossman and Korbonits2005).

Despite ghrelin having a potent action in regulating food intake, both ghrelin-knock-out mice and mice lacking GHS-R type 1a have normal feeding patterns and body composition on a standard diet (Sun et al. Reference Sun, Ahmed and Smith2003). However, on a high-fat diet the absence of ghrelin (Wortley et al. Reference Wortley, Del Rincon, Murray, García, Iida, Thorner and Sleeman2005) or the ghrelin receptor (Zigman et al. Reference Zigman, Nakano, Coppari, Balthasar, Marcus and Lee2005) protects against early-onset obesity; in both cases this reduced weight gain is associated with decreased adiposity and increased energy expenditure and locomotor activity. These data suggest that ghrelin, like leptin, may play an important role in the development of hypothalamic systems regulating energy balance (Grove & Cowley, Reference Grove and Cowley2005). Very interestingly, elimination of ghrelin improves the diabetic phenotype but not obese phenotype of ob/ob mice (Sun et al. Reference Sun, Asnicar, Saha, Chan and Smith2006).

Finally, it has recently been reported that obestatin, a new peptide derived from the ghrelin gene, inhibits food intake by acting through the orphan receptor GPR39 (Nogueiras & Tschop, Reference Nogueiras and Tschop2005; Zhang et al. Reference Zhang, Ren, Avsian-Kretchmer, Luo, Rauch, Klein and Hsueh2005). Despite this evidence there are some discrepancies in relation to the anorectic effect of obestatin (Nogueiras et al. Reference Nogueiras, Pfluger, Tovar, Myrtha, Mitchell and Morris2006) as well as its binding to GPR39 (Holst et al. Reference Holst, Egerod, Schild, Vickers, Cheetham, Gerlach, Storjohann, Stidsen, Jones, Beck-Sickinger and Schwartz2006). If the anorectic effect is confirmed, this finding could provide a new drug target for the treatment of obesity.

Arcuate nucleus

The ARC is considered as the ‘master hypothalamic centre’ for feeding control. It is situated around the base of the third ventricle and lies immediately above the median eminence. The ARC–median eminence is a circumventricular organ in which the BBB is modified, allowing the entry of peptides and proteins from the circulation, such as PYY, GLP-1, leptin and insulin (Banks et al. Reference Banks, Kastin, Huang, Jaspan and Maness1996; Kastin et al. Reference Kastin, Akerstrom and Pan2002; Nonaka et al. Reference Nonaka, Shioda, Niehoff and Banks2003; Woods et al. Reference Woods, Seeley, Baskin and Schwartz2003).

Two distinct neuronal populations in the ARC integrate peripheral nutritional and/or feeding signals. One set of neurons in the ventromedial part of the ARC express the orexigenic neuropeptides NPY and AgRP (Broberger et al. Reference Broberger, Johansen, Johansson, Schalling and Hokfelt1998b; Hahn et al. Reference Hahn, Breininger, Baskin and Schwartz1998). These neurons mostly project to the PVN. In the ventrolateral part of the ARC there is a second population of neurons that express the anorexigenic products of POMC, the precursor of α-MSH, and also CART (Elias et al. Reference Elias, Lee, Kelly, Aschkenasi, Ahima, Couceyro, Kuhar, Saper and Elmquist1998a; Kristensen et al. Reference Kristensen, Judge, Thim, Ribel, Christjansen and Wulff1998). This set of neurons projects more broadly within the CNS to hypothalamic nuclei such as the DMH, the LHA and the perifornical area, as well as the PVN. Thus, AgRP/NPY and CART/POMC neurons act as the primary hypothalamic site of action of peripheral hormones such as insulin and leptin. ARC neurons, in turn, project to secondary hypothalamic nuclei (‘second order neurons’) such as the PVN and the LHA. In these second-order neurons the release of neuropeptides is regulated to modulate energy intake (Schwartz et al. Reference Schwartz, Woods, Porte, Seeley and Baskin2000; Flier, Reference Flier2004).

Paraventricular nucleus

The PVN integrates neuropeptide signals from numerous CNS regions, including the ARC and brainstem (Sawchenko & Swanson, Reference Sawchenko and Swanson1983). Administration into the PVN of almost all of the known orexigenic and anorexigenic signalling molecules alters appetite (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999). Furthermore, CART/POMC neurons originating in the ARC potentiate inhibitory γ-aminobutyric acidergic signalling in the PVN and reduce feeding, while AgRP/NPY neurons inhibit this γ-aminobutyric acidergic signalling and stimulate food intake (Cowley et al. Reference Cowley, Pronchuk, Fan, Dinulescu, Colmers and Cone1999). Despite the large number of neuropeptides acting on the PVN, recent work suggests that they act to regulate feeding through a common mechanism involving AMP-activated protein kinase (Minokoshi et al. Reference Minokoshi, Alquier, Furukawa, Kim, Lee and Xue2004; López et al. Reference López, Lelliott, Tovar, Kimber, Gallego and Virtue2006).

The PVN also plays a major role in the integration of food intake and neuroendocrine function. AgRP/NPY and CART/POMC neurons in the ARC project to thyrotrophin-releasing hormone neurons in the PVN (Legradi & Lechan, Reference Legradi and Lechan1999; Fekete et al. Reference Fekete, Legradi, Mihaly, Huang, Tatro, Rand, Emerson and Lechan2000). AgRP/NPY inhibits pro-thyrotrophin-releasing hormone gene expression (Fekete et al. Reference Fekete, Sarkar, Rand, Harney, Emerson, Bianco and Lechan2002), while α-melanocyte-stimulating hormone stimulates pro-thyrotrophin-releasing hormone expression and inhibits the fasting-induced suppression of thyrotrophin-releasing hormone (Fekete et al. Reference Fekete, Legradi, Mihaly, Huang, Tatro, Rand, Emerson and Lechan2000). The PVH also contains corticotrophin-releasing hormone neurons, which form reciprocal circuits with NPY neurons in the ARC (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999).

Ventromedial nucleus of the hypothalamus

The VMH (as distinct from thalamic ventromedial nucleus) has long been considered as the ‘satiety centre’; since the finding that bilateral lesions in this nucleus induce hyperphagia and obesity (Hecherington & Ranson, Reference Hecherington and Ranson1942; Anand & Brobeck, Reference Anand and Brobeck1951). The VMH mainly receives projections from AgRP/NPY and CART/POMC neurons in the ARC. Additionally, the VMH neurons project their axons to the ARC, DMH and LHA, as well as brainstem regions such as the NTS (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Pinto et al. Reference Pinto, Roseberry, Liu, Diano, Shanabrough, Cai, Friedman and Horvath2004; Sternson et al. Reference Sternson, Shepherd and Friedman2005).

The VMH has been considered as a ‘reception nucleus’ for peripheral signals, as well as central signals. VMH neurons show a high abundance of leptin, ghrelin, oestrogen, thyroid hormone and neuropeptide receptors (Shughrue et al. Reference Shughrue, Lane and Merchenthaler1997; Roselli et al. Reference Roselli, Jorgensen, Doyle and Ronnekleiv1997; Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Nogueiras et al. Reference Nogueiras, Tovar, Mitchell, Rayner, Archer, Diéguez and Williams2004b; King, Reference King2006). However, despite the identification of these receptors, the molecular mechanisms regulating feeding in the VMH have not yet been well established. Some evidence suggests that brain-derived neurotrophic factor and steroidogenic factor-1 may play crucial roles in mediating body weight in this nucleus. Mice with reduced brain-derived neurotrophic factor receptor expression (Xu et al. Reference Xu, Goulding, Zang, Cepoi, Cone, Jones, Tecott and Reichardt2003), and with reduced brain-derived neurotrophic factor signalling (Xu et al. Reference Xu, Goulding, Zang, Cepoi, Cone, Jones, Tecott and Reichardt2003), and also steroidogenic factor-1-knock-out mice (Majdic et al. Reference Majdic, Young, Gomez-Sanchez, Anderson, Szczepaniak, Dobbins, McGarry and Parker2002), have increased body weight. Furthermore, activation of SF-1 neurons by leptin is required for normal body-weight homeostasis (Dhillon et al. Reference Dhillon, Zigman, Ye, Lee, McGovern and Tang2006). Finally, it has recently been reported that fatty acid synthase (FAS) and malonyl-CoA levels in this nucleus may play an important physiological role in the regulation of feeding (López et al. Reference López, Lelliott, Tovar, Kimber, Gallego and Virtue2006).

Dorsomedial nucleus of the hypothalamus

Like the VMH, the DMH has long been considered to be an integrative centre, processing information from other hypothalamic areas (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999). The DMH is located immediately dorsal to the VMH and has extensive direct connections with other hypothalamic nuclei (e.g. the PVN and the LHA), as well as the brainstem (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Bellinger & Bernardis, Reference Bellinger and Bernardis2002). Destruction of the DMH induces hyperphagia and obesity, although less dramatically than VMH lesions (Bellinger & Bernardis, Reference Bellinger and Bernardis2002). Very recent evidence has also demonstrated that the DMH is critical for the expression of food-entrainable circadian rhythms (Gooley et al. Reference Gooley, Schomer and Saper2006).

The DMH contains NPY-expressing cell bodies, which are involved in the hyperphagia observed in pregnant and lactating rats (Li et al. Reference Li, Chen and Smith1998; García et al. Reference García, López, Gualillo, Seoane, Diéguez and Señarís2003). Moreover, CART-expressing neurons are highly abundant in the DMH; the exact function of these cells is unknown, but they are probably involved in fasting-induced responses (Henry et al. Reference Henry, Rao, Ikenasio, Mountjoy, Tilbrook and Clarke2001).

Lateral hypothalamic area

Although the involvement of the LHA, including the perifornical area, in the regulation of feeding has been known for >60 years (Hecherington & Ranson, Reference Hecherington and Ranson1942; Anand & Brobeck, Reference Anand and Brobeck1951), the molecular mechanisms involved had remained unknown until 15 years ago when MCH was identified as the first orexigenic peptide exclusively expressed in this nucleus (Qu et al. Reference Qu, Ludwig, Gammeltoft, Piper, Pelleymounter, Cullen, Mathes, Przypek, Kanarek and Maratos-Flier1996). Other neuropeptides important in the regulation of feeding are also highly expressed in this area, such as galanin (Hakansson et al. Reference Hakansson, Brown, Ghilardi, Skoda and Meister1998), dynorphin (Chou et al. Reference Chou, Lee, Lu, Elmquist, Hara and Willie2001), CART-encoded peptides (Koylu et al. Reference Koylu, Couceyro, Lambert and Kuhar1998) and OX (de Lecea et al. Reference de Lecea, Kilduff, Peyron, Gao, Foye and Danielson1998; Sakurai et al. Reference Sakurai, Amemiya, Ishii, Matsuzaki, Chemelli and Tanaka1998; López et al. Reference López, Tena-Sempere, García-Caballero, Señarís, Diéguez, de Lecea and Sutcliffe2005b).

MCH and prepro-OX are each expressed by a different cell population, both of which receive projections from AgRP/NPY and CART/POMC neurons in the ARC (Broberger et al. Reference Broberger, de Lecea, Sutcliffe and Hokfelt1998a; Elias et al. Reference Elias, Saper, Maratos-Flier, Tritos, Lee and Kelly1998b; Horvath et al. Reference Horvath, Diano and van den Pol1999). Additionally, both sets of neurons express leptin receptors, indicating that their actions may be integrated (Hakansson et al. Reference Hakansson, Brown, Ghilardi, Skoda and Meister1998, Reference Hakansson, de Lecea, Sutcliffe, Yanagisawa and Meister1999). The LHA also contains a large number of glucose-sensing neurons (Bernardis & Bellinger, Reference Bernardis and Bellinger1996). Orexin neurons in the LHA respond to a fall in glucose levels with an increase in activity (Cai et al. Reference Cai, Widdowson, Harrold, Wilson, Buckingham, Arch, Tadayyon, Clapham, Wilding and Williams1999; Moriguchi et al. Reference Moriguchi, Sakurai, Nambu, Yanagisawa and Goto1999). LHA neurons project widely to a large number of extrahypothalamic areas. Major targets of the MCH and OX neurons include the brainstem motor systems, sympathetic and parasympathetic preganglionic nuclei in the medulla and spinal cord, the locus coeruleus, the medial raphe nucleus, the tuberomammillary nucleus and the cerebral cortex. All these areas are fundamental in different aspects of food intake regulation, from feeding-related behaviours to arousal and motor activity (Willie et al. Reference Willie, Chemelli, Sinton and Yanagisawa2001; López et al. Reference López, Diéguez, Señarís and Pandalai2001a, Reference López, Tena-Sempere, García-Caballero, Señarís, Diéguez, de Lecea and Sutcliffe2005b; Saper et al. Reference Saper, Chou and Elmquist2002; Steininger et al. Reference Steininger, Kilduff, Behan, Benca and Landry2004). Thus, these second-order neurons in the LHA play a fundamental role in integrating information from ARC neurons before sending it to other CNS areas involved in feeding control.

Neuropeptide Y

NPY is a thirty-six-amino acid peptide belonging to the PP-fold family of peptides, which also includes PYY and PP (Conlon, Reference Conlon2002). NPY is widely distributed in the CNS and is one of the most potent stimulators of food intake; repeated third ventricle or PVN administration of NPY induces striking hyperphagia and obesity (Stanley et al. Reference Stanley, Kyrkouli, Lampert and Leibowitz1986; Zarjevski et al. Reference Zarjevski, Cusin, Vettor, Rohner-Jeanrenaud and Jeanrenaud1993). Central administration of NPY also reduces brown fat thermogenesis (Billington et al. Reference Billington, Briggs, Grace and Levine1991), suppresses sympathetic nerve activity (Egawa et al. Reference Egawa, Yoshimatsu and Bray1991) and inhibits the thyroid axis (Fekete et al. Reference Fekete, Sarkar, Rand, Harney, Emerson, Bianco and Lechan2002) in order to reduce energy expenditure. Additionally, NPY induces hyperinsulinaemia (Moltz & McDonald, Reference Moltz and McDonald1985; Zarjevski et al. Reference Zarjevski, Cusin, Vettor, Rohner-Jeanrenaud and Jeanrenaud1993), hypercorticosteronaemia (Zarjevski et al. Reference Zarjevski, Cusin, Vettor, Rohner-Jeanrenaud and Jeanrenaud1993) and reduced plasma testosterone levels (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999); effects that are independent of increased food intake. NPY mRNA levels and NPY release in the ARC respond to changes in energy status, being increased after fasting and food restriction and decreased after refeeding (Sanacora et al. Reference Sanacora, Kershaw, Finkelstein and White1990; Kalra et al. Reference Kalra, Dube, Sahu, Phelps and Kalra1991; Swart et al. Reference Swart, Jahng, Overton and Houpt2002).

Regardless of its potent orexigenic effect, NPY-knock-out mice show normal body weight and adiposity (Erickson et al. Reference Erickson, Clegg and Palmiter1996), probably related to a compensatory and redundant mechanism in the orexigenic pathways, particularly in relation to AgRP. However, it has been reported recently that selective ablation of AgRP/NPY neurons in adult mice results in hypophagia and leanness, demonstrating direct evidence for a critical role of these neurons in the regulation of energy homeostasis (Gropp et al. Reference Gropp, Shanabrough, Borok, Xu, Janoschek and Buch2005; Luquet et al. Reference Luquet, Perez, Hnasko and Palmiter2005).

NPY, as part of the PP-fold family of peptides, binds and activates G-protein-coupled receptors termed Y₁–Y₆ (Larhammar, Reference Larhammar1996; Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999). Y₁–Y₅ receptors are present in rat brain, but Y₆ is only active in mice, being absent in rats and inactive in primates (Inui, Reference Inui1999). The orexigenic action of NPY is thought to be mediated by hypothalamic Y₁, Y₂, Y₄ and Y₅ receptors (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Williams G et al. Reference Williams, Kaplan and Grill2000, Reference Williams, Bing, Cai, Harrold, King and Liu2001; Stanley et al. Reference Stanley, Wynne, McGowan and Bloom2005).

Melanocortin system (α-melanocyte-stimulating hormone/pro-opiomelanocortin and agouti-related peptide)

Among the hypothalamic neuropeptide systems that regulate feeding, melanocortins play a prominent role (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Cone, Reference Cone1999, Reference Cone2005; Coll et al. Reference Coll, Farooqi, Challis, Yeo and O'Rahilly2004b). The central melanocortin system modulates energy homeostasis through the anorectic actions of the agonist α-melanocyte-stimulating hormone (a POMC cleavage product) and the endogenous orexigenic antagonist AgRP (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Cone, Reference Cone1999, Reference Cone2005; Coll et al. Reference Coll, Farooqi, Challis, Yeo and O'Rahilly2004b). Five melanocortin receptors (MCnR, n 1–5) have been identified. The feeding-related effects of both α-melanocyte-stimulating hormone and AgRP are mediated via MC3R and MC4R. Both receptors are widely expressed in the hypothalamus and are found in the ARC, VMH and PVN (Kalra et al. Reference Kalra, Dube, Pu, Xu, Horvath and Kalra1999; Cone, Reference Cone1999, Reference Cone2005; Coll et al. Reference Coll, Farooqi, Challis, Yeo and O'Rahilly2004b).

Circulating hormones such as insulin (Kim et al. Reference Kim, Grace, Welch, Billington and Levine1999), leptin (Ahima & Flier, Reference Ahima and Flier2000b), ghrelin (Nakazato et al. Reference Nakazato, Murakami, Date, Kojima, Matsuo, Kangawa and Matsukura2001; Cowley et al. Reference Cowley, Smith, Diano, Tschop, Pronchuk and Grove2003; Seoane et al. Reference Seoane, López, Tovar, Casanueva, Señarís and Diéguez2003), PYY (Batterham et al. Reference Batterham, Cowley, Small, Herzog, Cohen and Dakin2002), glucocorticoids (Savontaus et al. Reference Savontaus, Conwell and Wardlaw2002) and oestrogens (Fodor & Delemarre-van de Waal, Reference Fodor and Delemarre-van de2001; Tritos et al. Reference Tritos, Segal-Lieberman, Vezeridis and Maratos-Flier2004) act on melanocortin AgRP and POMC neurons, providing information on energy status from the periphery. Hypothalamic POMC mRNA expression is regulated by nutritional status, with low levels during fasting that return to normal after leptin treatment or refeeding (Schwartz et al. Reference Schwartz, Seeley, Woods, Weigle, Campfield, Burn and Baskin1997; Swart et al. Reference Swart, Jahng, Overton and Houpt2002). In contrast, AgRP mRNA expression is increased by fasting, but unlike NPY mRNA levels, which are decreased after refeeding, AgRP levels remain elevated (Swart et al. Reference Swart, Jahng, Overton and Houpt2002). Recent evidence also suggests that circulating macronutrients, such as glucose (Sergeyev et al. Reference Sergeyev, Broberger, Gorbatyuk and Hokfelt2000; Fraley et al. Reference Fraley, Dinh and Ritter2002; Ibrahim et al. Reference Ibrahim, Bosch, Smart, Qiu, Rubinstein, Ronnekleiv, Low and Kelly2003) and lipids (Obici et al. Reference Obici, Feng, Morgan, Stein, Karkanias and Rossetti2002b; Morgan et al. Reference Morgan, Obici and Rossetti2004) modulate AgRP and POMC neurons.

The role of melanocortin signalling in body-weight homeostasis is fully supported by the phenotype of transgenic and knock-out mice, as well as identified human mutations. Transgenic mice over-expressing AgRP are obese (Ollmann et al. Reference Ollmann, Wilson, Yang, Kerns, Chen, Gantz and Barsh1997) and reduction of hypothalamic AgRP by RNA interference reduces body weight (Makimura et al. Reference Makimura, Mizuno, Mastaitis, Agami and Mobbs2002). However, AgRP-knock-out mice (as well as the double knock-out AgRP/NPY) show normal body weight and food intake (Qian et al. Reference Qian, Chen, Weingarth, Trumbauer, Novi and Guan2002), while selective ablation of AgRP/NPY neurons in adult mice results in hypophagia and leanness (Gropp et al. Reference Gropp, Shanabrough, Borok, Xu, Janoschek and Buch2005; Luquet et al. Reference Luquet, Perez, Hnasko and Palmiter2005). The role of AgRP in human obesity is not well defined, but a polymorphism in the human AgRP gene in man is associated with reduced body weight and fat mass (Marks et al. Reference Marks, Boucher, Lanouette, Perusse, Brookhart, Comuzzie, Chagnon and Cone2004). POMC-knock-out mice (Yaswen et al. Reference Yaswen, Diehl, Brennan and Hochgeschwender1999; Challis et al. Reference Challis, Coll, Yeo, Pinnock, Dickson and Thresher2004; Coll et al. Reference Coll, Challis, López, Piper, Yeo and O'Rahilly2005) and man (Kru de et al. Reference Krude, Biebermann, Luck, Horn, Brabant and Gruters1998) are hyperphagic and obese and display adrenal insufficiency. MC4R-knock-out mice (Huszar et al. Reference Huszar, Lynch, Fairchild-Huntress, Dunmore, Fang and Berkemeier1997; Fan et al. Reference Fan, Boston, Kesterson, Hruby and Cone1997) and man (Yeo et al. Reference Yeo, Farooqi, Aminian, Halsall, Stanhope and O'Rahilly1998; Farooqi et al. Reference Farooqi, Yeo, Keogh, Aminian, Jebb, Butler, Cheetham and O'Rahilly2000, Reference Farooqi, Yeo and O'Rahilly2003) also show hyperphagia and obesity. Finally, MC3R-knock-out mice display an increase in adiposity (Butler et al. Reference Butler, Kesterson, Khong, Cullen, Pelleymounter, Dekoning, Baetscher and Cone2000).

Melanin-concentrating hormone

MCH is an orexigenic neuropeptide expressed in the LHA–perifornical area. Central administration of MCH increases food intake and adiposity in rats and mice (Qu et al. Reference Qu, Ludwig, Gammeltoft, Piper, Pelleymounter, Cullen, Mathes, Przypek, Kanarek and Maratos-Flier1996; Marsh et al. Reference Marsh, Weingarth, Novi, Chen, Trumbauer and Chen2002). MCH receptor 1 antagonists inhibit food intake and induce weight loss (Borowsky et al. Reference Borowsky, Durkin, Ogozalek, Marzabadi, DeLeon and Lagu2002). MCH expression is regulated by nutritional status; fasting induces MCH mRNA expression and leptin decreases it (Qu et al. Reference Qu, Ludwig, Gammeltoft, Piper, Pelleymounter, Cullen, Mathes, Przypek, Kanarek and Maratos-Flier1996; Tritos et al. Reference Tritos, Mastaitis, Kokkotou and Maratos-Flier2001).

The important role of MCH in appetite regulation is supported by the phenotype of GM models. Transgenic mice over-expressing MCH display hyperphagia and obesity (Ludwig et al. Reference Ludwig, Tritos, Mastaitis, Kulkarni, Kokkotou, Elmquist, Lowell, Flier and Maratos-Flier2001; Marsh et al. Reference Marsh, Weingarth, Novi, Chen, Trumbauer and Chen2002), while MCH-knock-out mice are hypophagic and lean (Shimada et al. Reference Shimada, Tritos, Lowell, Flier and Maratos-Flier1998). Finally, the double knock-outs leptin/MCH have lower weight gain and adiposity compared with leptin-deficient ob/ob mice (Segal-Lieberman et al. Reference Segal-Lieberman, Bradley, Kokkotou, Carlson, Trombly, Wang, Bates, Myers, Flier and Maratos-Flier2003), suggesting that MCH is a downstream mediator of leptin effects on feeding. Finally, MCH receptor1-knock-out mice display a lean phenotype as a result of increased energy expenditure (Marsh et al. Reference Marsh, Weingarth, Novi, Chen, Trumbauer and Chen2002).

Orexins

The OX (OX-A and OX-B), or hypocretins (hypocretins 1 and 2), are neuropeptides derived from the common precursor prepro-OX (prepro-hyprocretin) expressed in the LHA/perifornical area (de Lecea et al. Reference de Lecea, Kilduff, Peyron, Gao, Foye and Danielson1998; Sakurai et al. Reference Sakurai, Amemiya, Ishii, Matsuzaki, Chemelli and Tanaka1998). Two different OX receptors have been cloned, termed OX 1 receptor (or hypocretin receptor 1) OX 2 receptor (or hypocretin receptor 2). Although OX expression in the brain is only located in the LHA–perifornical area (Broberger et al. Reference Broberger, de Lecea, Sutcliffe and Hokfelt1998a; Elias et al. Reference Elias, Saper, Maratos-Flier, Tritos, Lee and Kelly1998b; Horvath et al. Reference Horvath, Diano and van den Pol1999), OX receptors show a widespread distribution in the CNS, with high levels of abundance in some hypothalamic nuclei (ARC, DMH, LHA, PVN and VMH; Marcus et al. Reference Marcus, Aschkenasi, Lee, Chemelli, Saper, Yanagisawa and Elmquist2001; Backberg et al. Reference Backberg, Hervieu, Wilson and Meister2002). OX receptors are also present in the adrenal gland (López et al. Reference López, Senarís, Gallego, García-Caballero, Lago, Seoane, Casanueva and Diéguez1999), pituitary (Blanco et al. Reference Blanco, López, García-Caballero, Gallego, Vázquez-Boquete, Morel, Señaris, Casanueva, Diéguez and Beiras2001) gut (Kirchgessner & Liu, Reference Kirchgessner and Liu1999), testis, kidney, ovary and placenta (Johren et al. Reference Johren, Neidert, Kummer, Dendorfer and Dominiak2001).

OX are important regulators of the sleep–wake cycle and the absence of OX signalling causes narcolepsy (Willie et al. Reference Willie, Chemelli, Sinton and Yanagisawa2001; Taheri et al. Reference Taheri, Zeitzer and Mignot2002; Sutcliffe & de Lecea, Reference Sutcliffe and de Lecea2002). However, evidence also links OX to endocrine function (López et al. Reference López, Senarís, Gallego, García-Caballero, Lago, Seoane, Casanueva and Diéguez1999, Reference López, Seoane, Señarís and Diéguez2001b, Reference López, Seoane, Tovar, Nogueiras, Diéguez and Señarís2004, Reference López, Tena-Sempere, García-Caballero, Señarís, Diéguez, de Lecea and Sutcliffe2005b; Barreiro et al. Reference Barreiro, Pineda, Navarro, López, Suominen, Pinilla, Senarís, Toppari, Aguilar, Diéguez and Tena-Sempere2004; Seoane et al. Reference Seoane, Tovar, Pérez, Mallo, López, Señarís, Casanueva and Diéguez2004) and food intake regulation (Sakurai et al. Reference Sakurai, Amemiya, Ishii, Matsuzaki, Chemelli and Tanaka1998; López et al. Reference López, Diéguez, Señarís and Pandalai2001a, Reference López, Tena-Sempere, García-Caballero, Señarís, Diéguez, de Lecea and Sutcliffe2005b). Central administration of OX to rats stimulates feeding via a NPY-dependent mechanism (Sakurai et al. Reference Sakurai, Amemiya, Ishii, Matsuzaki, Chemelli and Tanaka1998; Dube et al. Reference Dube, Horvath, Kalra and Kalra2000; Ida et al. Reference Ida, Nakahara, Kuroiwa, Fukui, Nakazato, Murakami and Murakami2000; Jain et al. Reference Jain, Horvath, Kalra and Kalra2000; Yamanaka et al. Reference Yamanaka, Kunii, Nambu, Tsujino, Sakai, Matsuzaki, Miwa, Goto and Sakurai2000; López et al. Reference López, Seoane, García, Diéguez and Señarís2002). Other evidence has linked the feeding actions of OX-A to opioids, corticotrophin-releasing hormone (Ida et al. Reference Ida, Nakahara, Kuroiwa, Fukui, Nakazato, Murakami and Murakami2000), urocortin and melanocortins (Wang & Kotz, Reference Wang and Kotz2002). Finally, prepro-OX-knock-out mice (Willie et al. Reference Willie, Chemelli, Sinton and Yanagisawa2001) and the OX/ataxin-3 transgenic mice in which OX-containing neurons are ablated (Hara et al. Reference Hara, Beuckmann, Nambu, Willie, Chemelli, Sinton, Sugiyama, Yagami, Goto, Yanagisawa and Sakurai2001) are hypophagic.

OX neurons are also responsive to peripheral signals regulating food intake. The expression of prepro-OX is increased in fasting and restored to normal by leptin (Sakurai et al. Reference Sakurai, Amemiya, Ishii, Matsuzaki, Chemelli and Tanaka1998; López et al. Reference López, Seoane, García, Lago, Casanueva, Senarís and Diéguez2000; Zhu et al. Reference Zhu, Yamanaka, Kunii, Tsujino, Goto and Sakurai2002; Yamanaka et al. Reference Yamanaka, Beuckmann, Willie, Hara, Tsujino and Mieda2003). OX neurons in the lateral hypothalamus are also sensitive to glucose, being activated during hypoglycaemia (Cai et al. Reference Cai, Widdowson, Harrold, Wilson, Buckingham, Arch, Tadayyon, Clapham, Wilding and Williams1999; Griffond et al. Reference Griffond, Risold, Jacquemard, Colard and Fellmann1999; Moriguchi et al. Reference Moriguchi, Sakurai, Nambu, Yanagisawa and Goto1999). It has been also proposed that visceral feeding-related signals regulate OX actions, which are thought to act via the vagus nerve and the NST. Thus, stimuli acting as ‘terminate-eating’ signals, such as gastric distension and glucose concentrations in the portal vein, appear to be important in the regulation of OX (Cai et al. Reference Cai, Evans, Lister, Leslie, Arch, Wilson and Williams2001, Reference Cai, Liu, Evans, Clapham, Wilson, Arch, Morris and Williams2002).

Despite evidence supporting the orexigenic effect of OX, it has been proposed (Hagan et al. Reference Hagan, Leslie, Patel, Evans, Wattam and Holmes1999) that these effects are secondary, and are related to the state of arousal and vigilance necessary for normal feeding. However, OX-A-induced feeding that is independent of arousal activation has been reported (Kotz et al. Reference Kotz, Teske, Levine and Wang2002).

Cocaine- and amphetamine-regulated transcript

CART is the third-most-abundant transcript in the hypothalamus and is expressed in the ARC, DMH, LHA and PVN (Kristensen et al. Reference Kristensen, Judge, Thim, Ribel, Christjansen and Wulff1998; Elias et al. Reference Elias, Lee, Kelly, Ahima, Kuhar, Saper and Elmquist2001; Hunter et al. Reference Hunter, Philpot, Vicentic, Dominguez, Hubert and Kuhar2004). Food deprivation decreases ARC expression of CART, while peripheral leptin treatment in ob/ob mice increases CART expression (Kristensen et al. Reference Kristensen, Judge, Thim, Ribel, Christjansen and Wulff1998). Central administration of CART-(1–102) and CART-(82–103) inhibits feeding (Kristensen et al. Reference Kristensen, Judge, Thim, Ribel, Christjansen and Wulff1998) and CART-knock-out mice display a predisposition to become obese on a high-fat diet (Asnicar et al. Reference Asnicar, Smith, Yang, Heiman, Fox, Chen, Hsiung and Koster2001), an age-related increase in body weight and impaired glucose metabolism (Wierup et al. Reference Wierup, Richards, Bannon, Kuhar, Ahren and Sundler2005), supporting the role of CART in the hypothalamic mechanism regulating food intake.

Opioids

Opioids play an important role in the regulation of feeding. The anatomical site for opioid action is the nucleus accumbens (Zhang & Kelley Reference Zhang and Kelley2000; Zhang et al. Reference Zhang, Balmadrid and Kelley2003). Mice lacking enkephalin or β-endorphin lose the reinforcing property of food, despite the palatability. This effect is overridden after fasting, indicating that homeostatic mechanisms can overrule the hedonistic pathway (Hayward et al. Reference Hayward, Pintar and Low2002). In man opiate antagonists decrease food palatability without altering subjective hunger (Yeomans et al. Reference Yeomans, Wright, Macleod and Critchley1990; Drewnowski et al. Reference Drewnowski, Krahn, Demitrack, Nairn and Gosnell1992).

Endocannabinoids

The effect of marijuana (Cannabis sativa) to increase appetite has been known for many years (Di Marzo & Matias, Reference Di Marzo and Matias2005; Lichtman & Cravatt, Reference Lichtman and Cravatt2005). The primary constituent of cannabis is Δ⁹tetrahydrocannabinol; this molecule, along with other naturally-occurring and synthetic cannabinoids (CB), binds to two separate G-protein-coupled receptors: the CB1 receptor, which is located in the CNS and periphery; the CB2 receptor, which is primarily found in cells of the immune system (Matsuda et al. Reference Matsuda, Lolait, Brownstein, Young and Bonner1990; Devane et al. Reference Devane, Hanus, Breuer, Pertwee, Stevenson, Griffin, Gibson, Mandelbaum, Etinger and Mechoulam1992; Munro et al. Reference Munro, Thomas and Abu-Shaar1993). These receptors also bind endogenous ligands, the EC, which include the fatty acid amide N-arachidonoyl ethanolamine (anandamide) and the monoacylglycerol 2-arachidonoylglycerol (Di Marzo et al. Reference Di Marzo, Goparaju, Wang, Liu, Batkai, Jarai, Fezza, Miura, Palmiter, Sugiura and Kunos2001; Cota et al. Reference Cota, Marsicano, Lutz, Vicennati, Stalla, Pasquali and Pagotto2003a; Di Marzo & Matias, Reference Di Marzo and Matias2005; Lichtman & Cravatt, Reference Lichtman and Cravatt2005).

Central and peripheral administration of EC stimulates food intake (Williams et al. Reference Williams, Rogers and Kirkham1998; Koch, Reference Koch2001; Cota et al. Reference Cota, Marsicano, Lutz, Vicennati, Stalla, Pasquali and Pagotto2003a). This orexigenic effect is mediated via CB1 receptors in the hypothalamus, which co-localise with CART, corticotrophin-releasing hormone, MCH and OX (Cota et al. Reference Cota, Marsicano, Tschop, Grubler, Flachskamm and Schubert2003b). Additionally, CB1-knock-out mice display hypophagia and leanness (Cota et al. Reference Cota, Marsicano, Tschop, Grubler, Flachskamm and Schubert2003b), and leptin-deficient signalling is associated with high hypothalamic EC levels (Di Marzo et al. Reference Di Marzo, Goparaju, Wang, Liu, Batkai, Jarai, Fezza, Miura, Palmiter, Sugiura and Kunos2001). Recent evidence (Verty et al. Reference Verty, McFarlane, McGregor and Mallet2004) has also shown that the EC receptors are located downstream from the melanocortin system.

Together this evidence supports the important role of the EC system in the regulation of feeding. In fact, there is currently a CB1 selective antagonist, Rimonabant (SR141716), in use in phase III clinical trials that may be a potentially promising anti-obesity drug (Di Marzo & Matias, Reference Di Marzo and Matias2005).

Dopamine

The dopaminergic system is also important in the rewarding circuitry of feeding regulation (Fulton et al. Reference Fulton, Pissios, Manchon, Stiles, Frank, Pothos, Maratos-Flier and Flier2006; Hommel et al. Reference Hommel, Trinko, Sears, Georgescu, Liu, Gao, Thurmon, Marinelli and DiLeone2006). In fact, mice lacking tyrosine hydroxylase, the enzyme synthesising dopamine, are hypophagic (Szczypka et al. Reference Szczypka, Kwok, Brot, Marck, Matsumoto, Donahue and Palmiter2001). These actions are mediated via D₁ and D₂ receptors (Szczypka et al. Reference Szczypka, Kwok, Brot, Marck, Matsumoto, Donahue and Palmiter2001).

Serotonin

Serotonin plays an important role in regulating both rewarding and homeostatic mechanisms (Halford & Blundell, Reference Halford and Blundell2000b). Serotonin actions of food intake are mediated via the melanocortin system (Heisler et al. Reference Heisler, Cowley, Tecott, Fan, Low and Smart2002, Reference Heisler, Cowley, Kishi, Tecott, Fan and Low2003). In fact, the currently-discontinued anorectic agent fenfluramine mediates its actions via serotonin and melanocortins (Heisler et al. Reference Heisler, Cowley, Tecott, Fan, Low and Smart2002).