Hypophagic and Lean Gene Information which Controls Appetite Laboratory for Cell Culture Development Fig. 1 We showed that appetite, the most important drive in living organisms, is genetically controlled and is not associated with one's consciousness. The hypothalamus plays important roles in life maintenance such as regulating the body temperature and food intake. It has been recognized that "appetite information" based on the levels of sugar, insulin and leptin, a hormone secreted by adipocytes, in the blood is transmitted from the peripheral tissues to the hypothalamus which in turn transmits "appetite information" through neurotransmitter substances in the brain. We have clarified for the first time that the acetylcholine neurotransmitter acts on this transmission mechanism, using muscarinic receptor. The muscarinic receptor, one of the receptors which transmit the information of this neurotransmitter substance acetylcholine, has five different subtypes (M1-M5), and it is known that each subtype is expressed in the brain. It seemed that the physiological functions of these subtypes had been elucidated completely by extensive pharmacological analysis. However, the specificity of an antagonist in selectively matching to a particular subtype is not absolute. Furthermore, it is disadvantageous that the specificity of an antagonist is lost when present at high concentrations. Therefore, the specific physiological functions of the subtypes remained unclarified. To address this issue, Cell Culture Development team (Masaharu Ogawa team leader) and National Institutes of Health research group (US) collaborated to generate mice that lack the gene for each muscarinic receptor subtype and analyzed the specific physiological functions of these subtypes. In the mice that lack the gene for M3 muscarinic receptor, reduction in body weight (about 23% reduction in the body weight) which was caused by the reduction in food consumption (about 25% reduction) was observed; this characteristic was not observed in other mice lacking other genes for other subtypes of the muscarinic receptor. Moreover, the amount of adipose tissue in the mice decreased markedly compared with that in the other mature mice even though their body lengths were the same (Fig. 1). Blood test results indicate that the amount of leptin in blood, which originated from the adipose tissue, decreased markedly. Although the reduction in the leptin level was information from the peripheral tissue indicating "hunger", paradoxically the M3-lacking mouse exhibit hypophagic. This mouse did not exhibit anything unusual in its physical and metabolic activities, or learning ability which require high-stage functions of the brain. These results indicate that appetite information was blocked in the central nervous system of the M3-lacking mouse. In fact, the M3-lacking mouse did not respond to the administration of a neurotransmitter substance, AGRP, which promotes appetite inside the brain. This suggests that the information was blocked in melanin-concentrating hormone (MCH)-producing neurons on which the AGRP receptor is expressed (Fig. 2). To date, it is difficult to develop a functional inhibitor that acts on each muscarinic subtype, because the receptor proteins of the subtypes of muscarinic receptors are quite similar in structure. By analyzing the function of receptor subtypes at the gene level, we clarified the mechanism through which muscarinic receptor subtypes maintain certain life processes, which was a target of our study. Such a genetic approach is considered to lead to efficient development of novel medicines that are effective against the target receptor subtype, as well as an effective method for exploring new physiological functions of the subtypes. We expect that significance of the genetic approach will increase further in the future. 　 Fig. 2 The route of eating stimulus in the hypothalamus and eating behavior following the intra-brain administration of neurotransmitter substances which stimulate appetite. Leptin is secreted from adipocytes when satisfied with food, enters the blood circulation and binds with the leptin receptor that exists in the arcuate nucleus (ARC). This results in the synthesis and secretion of melanocortyn which competes with (agouti-related peptide) AGRP, a neurotransmitter substance which enhances the appetite. This in turn inhibits synthesis and secretion of MCH resulting in the control of eating behavior. Our functional analysis reveals that the blockade of the neural circuitry occurs in MCH-producing neurons of the M3-lacking mouse. The M3-lacking mouse does not respond to AGRP, but does respond to orexin and MCH, which are involved in downstream of the neural circuitry and took food, similar to the control group. Results of this experiment indicate that the transmission of information regarding "hunger" from the peripheral tissues is interrupted because of the insufficiency in the function of MCH-producing neurons in the lateral hypothalamic area (LHA). magnified scene by clicking image M. Yamada, T. Miyakawa, A. Duttaroy, A. Yamanaka, T. Moriguchi, R. Makita, M. Ogawa, C.J. Chou, B. Xia, J.N. Crawley, C.C. Felder, C.-X Deng., J. Wess Mice lacking the M3 muscarinic acetylcholine receptor are hypophagic and lean. Nature, 410, pp207-212 (2001)