Molecular Psychiatry (1998) 3, 204–206  1998 Stockton Press All rights reserved 1359–4184/98 $12.00

NEWS & VIEWS Bombesin, obesity, and social behavior

Recent studies on mice with knockouts for bombesin-like receptors demonstrate selective involvement of those receptors in neuroendocrine function, social responses, and locomotor activity.

Bombesin is a tetradecapeptide originally purified from mutant mice at the age when size difference was sig- the skin of the European frog Bombina bombina.1 Two nificant. Both glucose tolerance and tolerance bombesin-like , -releasing peptide tests showed impaired glucose metabolism in mutants. (GRP) and (NMB), have been identified Serum concentrations of thyroid hormones and gluca- in mammals.1 GRP and NMB exert their function by gon were normal. In terms of energy balance, BRS-3 binding to high affinity G-protein coupled receptors on deficient mice exhibited a reduction of oxygen con- the cell surface; those are the GRP-preferring receptor sumption at the resting state even before body size dif- (GRPR) and the NMB-preferring receptor (NMBR).2 In ferences between wild-type and mutant mice became addition, a third subtype of mammalian bombesin significant. Subsequently, mild hyperphagia was receptors (BRS-3) has been cloned: however, high observed when the size difference was evident. Feed- affinity natural ligand(s) specific to BRS-3 have not ing efficiency was increased in mutant mice. Loco- been identified yet.3,4 Mammalian bombesin-like pep- motor activity of mutant mice was similar to that of tides and their receptors are widely distributed in the wild-type mice, and non-shivering brown-fat thermo- central nervous system and peripheral organs. They genesis did not appear to be affected in mutant mice. modulate exocrine and endocrine processes, smooth These surprising results on BRS-3 deficient mice muscle contraction, metabolism, homeostasis, and demonstrate that BRS-3 is required for the regulation behavior.1 However, the molecular basis of the diver- of endocrine function and metabolism. In contrast to sity of biological effects of the bombesin system other obese models such as OB/OB,8 BRS-3 deficient remains elusive. Distinct distributions of bombesin mice exhibited mild obesity and mild hyperphagia. receptor subtypes in various tissues suggest inde- BRS-3 deficient mice could therefore be an important pendent roles of the receptors in such biological new model for human obesity and associated dis- responses.2,5 We have generated both GRPR and BRS- orders. Further studies on physiological and pathophy- 3 deficient mice in an effort to determine in vivo the siological roles of BRS-3 both in vivo and in vitro function of these receptors.6,7 should facilitate the development of new prevention and treatment strategies for obesity. In this regard, identification of specific ligands to BRS-3 represents a BRS-3 deficient mice new opportunity for research in this field. Further- Unexpectedly, we observed mild obesity and metabolic more, molecular genetic analyses of obesity in humans defects in BRS-3 deficient mice.6 This surprising result might test the hypothesis of a possible central role of suggests that BRS-3 plays a crucial role in adiposity BRS-3 in the molecular mechanisms of obesity. In con- and energy balance. Size difference between wild-type trast to GRPR and NMBR, BRS-3 is almost exclusively and mutant mice became evident around 16 weeks of expressed in the brain and spinal cord of mice.5 In age, and got larger with age (Figure 1). At 40 weeks of brain, BRS-3 is dominantly expressed in hypothalamic age, mutant mice gain about 40% more body weight nuclei including the paraventricular, arcuate and dor- compared with wild-type mice. Increased mass of the somedial nuclei.5 These nuclei are known to be adipose tissue was likely responsible for the weight involved in the regulation of the autonomic nervous gain in mutants. Hypertension and impaired glucose system and in endocrine function. Since gross anatom- and lipid metabolism were associated with the obesity ical changes were not observed in the hypothalamic of those animals. The plasma levels of insulin were region of BRS-3 deficient mice, BRS-3 deficiency is high and serum growth hormone levels were low in likely to cause functional but not morphological alter- ations in the hypothalamus. There is increasing evi- dence that the hypothalamus is one of the targets for 9,10 Correspondence: K Wada, MD, PhD, National Institute of Neuro- . Besides leptin, sig- science, 4-1-1 Ogawahigashi, Kodaira, Tokyo, 187-8502, Japan. naling in the hypothalamus has been implicated in E-mail: wadaȰncnaxp.ncnp.go.jp energy balance.11–13 It has also been shown that tar- News & Views 205

Figure 1 Left: Growth curve of wild-type (+/Y) (n = 10) and BRS-3 deficient mice (−/Y) (n = 8). Values represent mean ± SEM. Right: Representative pictures of wild-type (+/Y) and BRS-3 deficient (−/Y) mice. geted mutation of -4 receptor caused obesity in mice.14 Although elevated levels of plasma leptin in BRS-3 deficiency mice seem to reflect the increased mass of adipose tissue, the fact that BRS-3 deficient mice become obese despite the increase of leptin level suggests a reduction of leptin sensitivity in those mice. Further studies should address the func- tional relation between BRS-3 and leptin or melanocor- tin receptors in the hypothalamus.

GRPR deficient mice In contrast to BRS-3 deficient mice, we observed that GRPR deficient mice did not exhibit any metabolic or 7 endocrine effects. Rather, GRPR deficient mice Figure 2 Left: Increased locomotor activity during the active showed behavioral abnormalities which were not (dark) period in GRPR deficient mice. Spontaneous locomotor observed in BRS-3 deficient mice. GRPR deficient mice activity of individually housed mice (4 to 5-month-old, exhibited increased locomotor activity in the dark per- mutant n = 4; wild-type n = 4) was monitored for 2 weeks by iod but not in the light period (Figure 2). Furthermore, a sensor of magnetic field under a 24-h dark cycle. Increased GRPR deficient mice exhibited increased social locomotor activity during the active (dark) period was also responses such as sniffing, mounting, and approaching observed in mutant mice under a 12-h light/12-h dark cycle. behaviors against an intruder. Aggressive behaviors Right: Increased social responses in GRPR deficient mice. = = such as fighting and biting were not altered in those Behavior of male mutant (n 10) and wild-type (n 10) mice mutants. These results demonstrate a possible involve- against an intruder were scored every 10 s for 5 min. The number of events of sniffing, mounting, and approaching the ment of GRPR in behavioral and social responses. We intruder’s head were counted as social responses. The inci- observed the same phenotype in two different mice dence of biting and fighting was scored as aggressive lines derived from two independent ES cells as well as response. Before examination, each mouse (4-month-old) was in all tested generations. Although genetic effects of the housed individually for 4 weeks, and intruders (ICR mice) 129 ES strain linked to the targeted GRPR locus cannot were housed in a group of five mice. Values are mean ± SEM. be ignored, it is likely that GRPR is involved in the Statistics were performed with two-tailed Student’s t-test. regulation of social responses and locomotor activities. *, P Ͻ 0.01; **, P Ͻ 0.002. In the brain, BRS-3-expressing regions are included in the regions where GRPR is expressed. However, pheno- types are quite different between the two mutants, ties in social behavior.15 Recently, targeted mutation of which indicates differential roles of these two recep- dvl1, one of three mammalian homologues of Droso- tors in brain function. In social behavior, it is of parti- phila segment polarity gene Dishevelled, was cular interest that the GRPR mutant mice showed dif- reported.16 In contrast to GRPR deficient mice, dvl1 ferences between social and aggressive responses. deficient mice showed reduced social interaction. In Some mutant mice have been reported to show aggress- humans, disruption of GRPR gene was reported in a ive behavior, whereas few mutants exhibit abnormali- patient with autism and multiple exostoses.17 GRPR News & Views 206 deficient mice may therefore provide a useful model BRS-3: a novel bombesin receptor subtype selectively expressed in for psychiatric disorders. testis and lung carcinoma cells. J Biol Chem 1993; 268: 5979–5984. 4 Gorbulev V, Akhundova A, Buchner H, Fahrenholz F. Molecular cloning of a new bombesin receptor subtype expressed in uterus during pregnancy. Eur J Biochem 1992; 208: 405–410. Conclusions 5 Ohki-Hamazaki H, Wada E, Matsui K, Wada K. Cloning and expression of the neuromedin B receptor and the third subtype of The bombesin system has not been very well charac- bombesin receptor genes in the mouse. Brain Res 1997; 762: terized in the brain. This may be due to a lack of spe- 165–172. cific antagonists or agonists or to the diversity of 6 Ohki-Hamazaki H, Watase K, Yamamoto K, Ogura H, Yamano M, physiological effects of bombesin. Bombesin receptors Yamada K et al. Mice lacking bombesin receptor subtype-3 develop are phylogenetically most related to recep- metabolic defects and obesity. Nature 1997; 390: 165–169. 7 Wada E, Watase K, Yamada K, Ogura H, Yamano M, Inomata Y et tors whose importance is well-known. Our studies on al. Generation and characterization of mice lacking gastrin-releas- bombesin receptor knock-out mice indicate the necess- ing peptide receptor. Biochem Biophys Res Comm 1997; 239: 28– ity and potential fruitfulness of further efforts to exam- 33. ine in detail the role of bombesin in brain function. 8 Leibel RL, Chung WK, Chua SC Jr. The molecular genetics of rodent single gene obesities. J Biol Chem 1997; 272: 31937–31940. 9 Satoh N, Ogawa Y, Katsuura G, Hayase M, Tsuji T, Imagawa K et 1 1 1 1 K Wada , E Wada , K Watase , K Yamada al. The arcuate nucleus as a primary site of satiety effect of leptin and H Ohki-Hamazaki2 in rats. Neurosci Lett 1997; 224: 149–152. 1Department of Degenerative Neurological Diseases 10 Spanswick D, Smith MA, Groppi VE, Logan SD, Ashford MLJ. Lep- tin inhibits hypothalamic neurons by activation of ATP-sensitive National Institute of Neuroscience potassium channels. Nature 1997; 390: 521–525. National Center of Neurology and Psychiatry 11 Fan W, Boston BA, Kesterson RA, Hruby VJ, Cone RD. Role of mel- Kodaira, Tokyo 187-8502, Japan anocortinergic neurons in feeding and the agouti obesity syndrome. Nature 1997; 385: 165–168. 2Department of Neurochemistry 12 Boston BA, Blaydon KM, Varnerin J, Cone RD. Independent and additive effects of central POMC and leptin pathways on murine Tokyo Institute of Psychiatry obesity. Science 1997; 278: 1641–1644. Kamikitazawa, Setagaya-ku 13 Seeley RJ, Yagaloff KA, Fisher SL, Burn P, Thiele TE, van Dijk G et Tokyo 156-8585, Japan al. Melanocortin receptors in leptin effects. Nature 1997; 390: 349. 14 Huszar D, Lynch CA, Fairchild-Huntress V, Dunmore JH, Fang Q, Berkemeier LR et al. Targeted disruption of the melanocortin-4 References receptor results in obesity in mice. Cell 1997; 88: 131–141. 15 Tecott LH, Barondes SH. Behavioral genetics: genes and aggressive- 1 Lebacq-Verheyden AM, Trepel J, Sausville EA, Battey J. Bombesin ness. Curr Biol 1996; 6: 238–240. and gastrin releasing peptide: , secretagogues, and 16 Lijam N, Paylor R, McDonald MP, Crawley JN, Deng CX, Herrup K growth factors. In: Sporn MB, Roberts AB (eds). Handbook of et al. Social interaction and sensorimotor gating abnormalities in Experimental Pharmacology, Vol 95/II, Peptide Growth Factors and mice lacking Dvl1. Cell 1997; 90: 895–905. Their Receptors II. 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