Ken-Ichiro Nakajima

The selection of the appropriate energy substrate under different physiological conditions is a key aspect of the energy metabolism homeostasis. We here show that Nos1 (nitric oxide synthase 1)-expressing cells in the paraventricular hypothalamic nucleus (PVH) serve as a pivotal node for controlling whole-body fat consumption in male mice. Nos1 neurons account for ~30% of PVH neurons that convey signals via polysynaptic pathways to individual peripheral tissues, including skeletal muscle and brown (BAT) and white (WAT) adipose tissues. Activation of these Nos1 neurons in the PVH area induces WAT lipolysis and fat oxidation in other peripheral tissues via the sympathetic nervous system, thereby increasing whole-body fat consumption. Inhibition of these neurons abolishes the increase in fat consumption during the light period, whereas long-term silencing lead to obesity independent of energy intake. These neurons are also necessary for cold-induced thermogenesis in BAT and the rapid increase in fat consumption elicited by food deprivation or other stressors. Nos1 neurons in the PVH area are therefore essential for controlling fat consumption and energy homeostasis.

The gustatory system plays an important role in evaluating food quality in animals and humans. While some tastes are intrinsically appetitive, such as sweet, which is elicited from high‐calorie nutrients, the other tastes, such as sour and bitter, are aversive and elicited by toxic substances. In mice, taste signals are relayed by multiple regions of the brain, including the nucleus of the solitary tract, and the parabrachial nucleus (PBN) of the pons, before reaching the gustatory cortex via the gustatory thalamus. Recent advances in taste research using mice expressing Cre recombinase in specific neuronal populations, together with chemogenetic/optogenetic tools, have enabled us to identify genetically defined neurons involved in taste transduction pathways across several areas of the brain. While gustatory pathways play a fundamental role in taste transduction, taste preferences are not always stable, but rather vary depending on internal states. This review summarizes recent progress in research on neural circuits that modify the taste information depending on internal states in mice.

Feeding is one of the most fundamental activities in the survival and reproduction of animals. During feeding, the gustatory system functions as a gatekeeper to evaluate food quality. Accumulated evidence in the field of taste research has shown that 5 basic tastes (sweet, umami, sour, bitter, and salty) are sensed by the corresponding taste receptors expressed in taste receptor cells on the tongue. In contrast, brain mechanisms that transduce or modify taste information have been less studied. In this review, I introduce our recent findings on the sweet taste transduction in the brainstem of mice and explain the hypothalamic neuronal network regulating hunger-induced taste modification. Finally, future perspectives are discussed.

Feeding is essential for survival and taste greatly influences our feeding behaviors. Palatable tastes such as sweet trigger feeding as a symbol of a calorie-rich diet containing sugar or proteins, while unpalatable tastes such as bitter terminate further consumption as a warning against ingestion of harmful substances. Therefore, taste is considered a criterion to distinguish whether food is edible. However, perception of taste is also modulated by physiological changes associated with internal states such as hunger or satiety. Empirically, during hunger state, humans find ordinary food more attractive and feel less aversion to food they usually dislike. Although functional magnetic resonance imaging studies performed in primates and in humans have indicated that some brain areas show state-dependent response to tastes, the mechanisms of how the brain senses tastes during different internal states are poorly understood. Recently, using newly developed molecular and genetic tools as well as in vivo imaging, researchers have identified many specific neuronal populations or neural circuits regulating feeding behaviors and taste perception process in the central nervous system. These studies could help us understand the interplay between homeostatic regulation of energy and taste perception to guide proper feeding behaviors.

Macronutrient intake is associated with cardiometabolic health, ageing and longevity, but the mechanisms underlying its regulation have remained unclear. Most rodents increase carbohydrate selection under certain physiological and pathological conditions such as fasting. When presented with a choice between a basally preferable high‐fat diet (HFD) and a high‐carbohydrate diet (HCD) such as a high‐sucrose diet, fasted mice first eat the HFD and then switch to the HCD during the first few hours of refeeding and continue to eat the HCD up to 24 h in the two‐diet choice approach. Such consumption of an HCD after fasting reverses the fasting‐induced increase in the plasma concentration of ketone bodies more rapidly than does refeeding with an HFD alone. 5′‐AMP‐activated protein kinase (AMPK)‐regulated neurons in the paraventricular nucleus of the hypothalamus (PVH) that express corticotropin‐releasing hormone (CRH) are necessary and sufficient for the fasting‐induced selection of carbohydrate over an HFD in mice. These neurons appear to contribute to a fasting‐induced increase in the positive valence of carbohydrate without affecting the preference for more palatable and energy‐dense diets such as an HFD. Identification of the neural circuits in which AMPK‐regulated CRH neurons in the PVH of mice are embedded should shed new light on the physiological and molecular mechanisms responsible for macronutrient selection. image

The gustatory system plays a critical role in sensing appetitive and aversive taste stimuli for evaluating food quality. Although taste preference is known to change depending on internal states such as hunger, a mechanistic insight remains unclear. Here, we examine the neuronal mechanisms regulating hunger-induced taste modification. Starved mice exhibit an increased preference for sweetness and tolerance for aversive taste. This hunger-induced taste modification is recapitulated by selective activation of orexigenic Agouti-related peptide (AgRP)-expressing neurons in the hypothalamus projecting to the lateral hypothalamus, but not to other regions. Glutamatergic, but not GABAergic, neurons in the lateral hypothalamus function as downstream neurons of AgRP neurons. Importantly, these neurons play a key role in modulating preferences for both appetitive and aversive tastes by using distinct pathways projecting to the lateral septum or the lateral habenula, respectively. Our results suggest that these hypothalamic circuits would be important for optimizing feeding behavior under fasting.

Agouti-related peptide (AgRP) neurons of the hypothalamus play a key role in regulating food intake and body weight, by releasing three different orexigenic molecules: AgRP; GABA; and neuropeptide Y. AgRP neurons express various G protein-coupled receptors (GPCRs) with different coupling properties, including G_s-linked GPCRs. At present, the potential role of G_s-coupled GPCRs in regulating the activity of AgRP neurons remains unknown. Here we show that the activation of G_s-coupled receptors expressed by AgRP neurons leads to a robust and sustained increase in food intake. We also provide detailed mechanistic data linking the stimulation of this class of receptors to the observed feeding phenotype. Moreover, we show that this pathway is clearly distinct from other GPCR signalling cascades that are operative in AgRP neurons. Our data suggest that drugs able to inhibit this signalling pathway may become useful for the treatment of obesity.

Neoculin (NCL) is a heterodimeric protein isolated from the edible fruit ofCurculigo latifolia. It exerts a taste-modifying activity by converting sourness to sweetness. We previously demonstrated that NCL changes its action on the human sweet receptor hT1R2-hT1R3 from antagonism to agonism as the pH changes from neutral to acidic values and that the histidine residues of NCL molecule play critical roles in this pH-dependent functional change. Here, we comprehensively screened key amino acid residues of NCL using nuclear magnetic resonance (NMR) spectroscopy and alanine scanning mutagenesis. We found that the mutations of Arg48, Tyr65, Val72 and Phe94 of NCL basic subunit increased or decreased both the antagonist and agonist activities. The mutations had only a slight effect on the pH-dependent functional change. These residues should determine the affinity of NCL for the receptor regardless of pH. Their locations were separated from the histidine residues responsible for the pH-dependent functional change in the tertiary structure. From these results, we concluded that NCL interacts with hT1R2-hT1R3 through a pH-independent affinity interface including the four residues and a pH-dependent activation interface including the histidine residues. Thus, the receptor activation is induced by local structural changes in the pH-dependent interface.