Dorsomedial hypothalamic lesions alter intake of an imbalanced amino acid diet in rats

Optogenetic activation of leptin- and glucose-regulated GABAergic neurons in dorsomedial hypothalamus promotes food intake via inhibitory synaptic transmission to paraventricular nucleus of hypothalamus

OBJECTIVE

The dorsomedial hypothalamus (DMH) has been considered an orexigenic nucleus, since the DMH lesion reduced food intake and body weight and induced resistance to diet-induced obesity. The DMH expresses feeding regulatory neuropeptides and receptors including neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), cholecystokinin (CCK), leptin receptor, and melanocortin 3/4 receptors. However, the principal neurons generating the orexigenic function in the DMH remain to be defined. This study aimed to clarify the role of the DMH GABAergic neurons in feeding regulation by using optogenetics and electrophysiological techniques.

METHODS

We generated the mice expressing ChRFR-C167A, a bistable chimeric channelrhodopsin, selectively in GABAergic neurons of DMH via locally injected adeno-associated virus 2. Food intake after optogenetic activation of DMH GABAergic neurons was measured. Electrophysiological properties of DMH GABAergic neurons were measured using slice patch clamp.

RESULTS

Optogenetic activation of DMH GABAergic neurons promoted food intake. Leptin hyperpolarized and lowering glucose depolarized half of DMH GABAergic neurons, suggesting their orexigenic property. Optical activation of axonal terminals of DMH GABAergic neurons at the paraventricular nucleus of hypothalamus (PVN), where anorexigenic neurons are localized, increased inhibitory postsynaptic currents on PVN neurons and promoted food intake.

CONCLUSION

DMH GABAergic neurons are regulated by metabolic signals leptin and glucose and, once activated, promote food intake via inhibitory synaptic transmission to PVN.

Lateral hypothalamus as a sensor-regulator in respiratory and metabolic control

Physiological fluctuations in the levels of hormones, nutrients, and gasses are sensed in parallel by interacting control systems distributed throughout the brain and body. We discuss the logic of this arrangement and the definitions of "sensing"; and then focus on lateral hypothalamic (LH) control of energy balance and respiration. LH neurons control diverse behavioral and autonomic processes by projecting throughout the neuraxis. Three recently characterized types of LH cells are discussed here. LH orexin/hypocretin (ORX) neurons fire predominantly during wakefulness and are thought to promote reward-seeking, arousal, obesity resistance, and adaptive thermogenesis. Bidirectional control of ORX cells by extracellular macronutrients may add a new regulatory loop to these processes. ORX neurons also stimulate breathing and are activated by acid/CO2in vivo and in vitro. LH melanin-concentrating hormone (MCH) neurons fire mostly during sleep, promote physical inactivity, weight gain, and may impair glucose tolerance. Reported stimulation of MCH neurons by glucose may thus modulate energy homeostasis. Leptin receptor (LepR) neurons of the LH are distinct from ORX and MCH neurons, and may suppress feeding and locomotion by signaling to the mesolimbic dopamine system and local ORX neurons. Integration within the ORX-MCH-LepR microcircuit is suggested by anatomical and behavioral data, but requires clarification with direct assays of functional connectivity. Further studies of how LH circuits counteract evolutionarily-relevant environmental fluctuations will provide key information about the logic and fragilities of brain controllers of healthy homeostasis.

Hypothalamic signaling in anorexia induced by indispensable amino acid deficiency

Animals exhibit a rapid and sustained anorexia when fed a diet that is deficient in a single indispensable amino acid (IAA). The chemosensor for IAA deficiency resides within the anterior piriform cortex (APC). Although the cellular and molecular mechanisms by which the APC detects IAA deficiency are well established, the efferent neural pathways that reduce feeding in response to an IAA-deficient diet remain to be fully characterized. In the present work, we investigated whether 1) central melanocortin signaling is involved in IAA deficiency-induced anorexia (IAADA) and 2) IAADA engages other key appetite-regulating neuronal populations in the hypothalamus. Rats and mice that consumed a valine-deficient diet (VDD) for 2-3 wk exhibited marked reductions in food intake, body weight, fat and lean body mass, body temperature, and white adipose tissue leptin gene expression, as well as a paradoxical increase in brown adipose tissue uncoupling protein-1 mRNA. Animals consuming the VDD had altered hypothalamic gene expression, typical of starvation. Pharmacological and genetic blockade of central melanocortin signaling failed to increase long-term food intake in this model. Chronic IAA deficiency was associated with a marked upregulation of corticotropin-releasing hormone expression in the lateral hypothalamus, particularly in the parasubthalamic nucleus, an area heavily innervated by efferent projections from the APC. Our observations indicate that the hypothalamic melanocortin system plays a minor role in acute, but not chronic, IAADA and suggest that the restraint on feeding is analogous to that observed after chronic dehydration.

The brain's response to an essential amino acid-deficient diet and the circuitous route to a better meal

The essential (indispensable) amino acids (IAA) are neither synthesized nor stored in metazoans, yet they are the building blocks of protein. Survival depends on availability of these protein precursors, which must be obtained in the diet; it follows that food selection is critical for IAA homeostasis. If even one of the IAA is depleted, its tRNA becomes quickly deacylated and the levels of charged tRNA fall, leading to disruption of global protein synthesis. As they have priority in the diet, second only to energy, the missing IAA must be restored promptly or protein catabolism ensues. Animals detect and reject an IAA-deficient meal in 20 min, but how? Here, we review the molecular basis for sensing IAA depletion and repletion in the brain's IAA chemosensor, the anterior piriform cortex (APC). As animals stop eating an IAA-deficient meal, they display foraging and altered choice behaviors, to improve their chances of encountering a better food. Within 2 h, sensory cues are associated with IAA depletion or repletion, leading to learned aversions and preferences that support better food selection. We show neural projections from the APC to appetitive and consummatory motor control centers, and to hedonic, motivational brain areas that reinforce these adaptive behaviors.

Stress-induced alterations in catecholamine enzymes gene expression in the hypothalamic dorsomedial nucleus are modulated by caudal brain and not hypothalamic paraventricular nucleus neurons

The hypothalamic dorsomedial nucleus (DMN) represents an important coordinate center for regulation of autonomic and neuroendocrine systems, especially during stress response. The present study was focused on the gene expression of catecholamine-synthesizing enzymes and the protein levels of tyrosine hydroxylase in DMN, both in control and stressed rats. Moreover, pathways modulating the gene expression of tyrosine hydroxylase in DMN during immobilization (IMO) stress were also investigated. Gene expressions of all catecholamine-synthesizing enzymes were detected in DMN samples. While the levels of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA were increased in IMO rats, aromatic L-amino acid decarboxylase and dopamine-beta-hydroxylase mRNA remained unchanged. Tyrosine hydroxylase protein levels were significantly elevated in the DMN only after repeated IMO stress. Postero-lateral deafferentations of the DMN, or transections of the ascending catecholaminergic pathways originating in the lower brainstem abolished the IMO-induced increase of tyrosine hydroxylase gene expression in the DMN. Nevertheless, postero-lateral deafferentations of the hypothalamic paraventricular nucleus (PVN), which separate the DMN from the PVN, had no effect on IMO-induced elevation of tyrosine hydroxylase mRNA in the DMN. The present data indicate that certain DMN neurons synthesize mRNA of catecholamine enzymes. The stress-induced increase of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA in DMN neurons indicates the involvement of these catecholaminergic neurons in stress response. The gene expression of tyrosine hydroxylase in DMN is modulated by lower brainstem and/or spinal cord, but not by PVN afferents.

Mechanisms of Food Intake Repression in Indispensable Amino Acid Deficiency

Animals reject diets that lead to indispensable amino acid (IAA) depletion or deficiency. This behavior is adaptive, as continued IAA depletion is incompatible with maintenance of protein synthesis and survival. Following rejection of the diet, animals begin foraging for a better IAA source and develop conditioned aversions to cues associated with the deficient diet. These responses require a sensory system to detect the IAA depletion and alert the appropriate neural circuitry for the behavior. The chemosensor for IAA deprivation is in the highly excitable anterior piriform cortex (APC) of the brain. Recently, the well-conserved general AA control non-derepressing system of yeast was discovered to be activated by IAA deprivation via uncharged tRNA in mammalian APC. This system provides the sensory limb of the mechanism for recognition of IAA depletion that leads to activation of the APC, diet rejection, and subsequent adaptive strategies.

Co-localization of phosphorylated extracellular signal-regulated protein kinases 1/2 (ERK1/2) and phosphorylated eukaryotic initiation factor 2alpha (eIF2alpha) in response to a threonine-devoid diet

The anterior piriform cortex (APC) has been shown to be an essential brain structure for the detection of dietary indispensable amino acid (IAA) deficiency, but little has been known about possible molecular detection mechanisms. Increased phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2alpha (eIF2alpha) has been directly linked to amino acid deficiency in yeast. Recently, we have shown increased phosphorylation of eIF2alpha (p-eIF2alpha) in the rat APC 20 minutes after ingestion of an IAA-deficient meal. We suggest that if phosphorylation of eIF2alpha is an important mechanism in detection of IAA deficiency, then APC neurons that show p-eIF2alpha should also show molecular evidence of potentiation. The present research demonstrates increased expression and co-localization of p-eIF2alpha and phosphorylated extracellular signal-regulated protein kinase 1/2 (p-ERK1/2) in APC neurons, but not in the primary motor or agranular insular cortices in response to an IAA-deficient diet. ERK1/2 is an element of the mitogen-activated protein kinase cascade, an intraneuronal signaling mechanism associated with neuronal activation. The region of the APC that responds to IAA deficiency with increased p-eIF2alpha and p-ERK1/2 labeling ranges from 3.1 to 2.5 mm rostral of bregma. Within this region, only a few neurons respond to IAA deficiency with co-localization of abundant p-eIF2alpha and p-ERK1/2. These chemosensory neurons probably detect IAA deficiency and generate neuronal signaling to other portions of the brain, changing feeding behavior.

Role of MAP kinase in signaling indispensable amino acid deficiency in the brain

Deficiencies of indispensable amino acids (IAAs) appear to be sensed in the anterior piriform cortex (APC) where neurons are activated and potentiated, however, the mediating intracellular signaling mechanisms are largely unexplored. It is postulated that signaling of amino acid deficiency may share many of the same pathways seen with long-term potentiation (LTP). Phosphorylation of mitogen-activated protein kinase (pMAP kinase) has been shown to be a necessary signaling event for the genesis and maintenance of LTP. Immunoperoxidase immunohistochemistry was used to determine the number of neurons showing activation of the MAP kinase signal transduction system. Relative to rats eating a corrected diet, rats consuming threonine-devoid diet showed significantly greater pMAP kinase labeling in the APC, dorsomedial hypothalamus, and the paraventricular hypothalamic nucleus. These are areas previously associated with control of food intake. However, since the dorsomedial hypothalamus and the paraventricular hypothalamic nucleus have not previously been implicated as chemosensory areas for IAAs, phosphorylated MAP kinase expression in these areas may reflect secondary activation.

The dorsomedial hypothalamic nucleus and its role in ingestive behavior and body weight regulation: lessons learned from lesioning studies

This review article discusses the well-established role of the dorsomedial hypothalamic nucleus (DMN) in feeding, drinking and body weight (BW) regulation. DMN lesions (L) in both weanling and mature rats of both sexes produce hypophagia, hypodipsia and reduced ponderal and linear growth in the presence of normal body composition. The growth reduction is not due to a deficient secretion of growth hormone, insulin-like growth factor-1, thyroxine, triiodothyronine or insulin. DMNL rats actively defend their lower BW (BW settling point) by becoming either hyper- or hypophagic, depending on the experimental manipulation, thereby defending both lean and fat mass. They also regulate their 24-h caloric intake, but they may overeat during the first hour of refeeding following a fast, possibly due to a reduced ability to monitor blood glucose or to respond to cholecystokinin (CCK). 2-Deoxy-D-glucose (2DG) increases c-fos expression in orexin-A neurons in the DMN, and DMNL eliminated the orexigenic effect of 2DG. DMNL rats on high-fat diets do not get as obese as controls, which may be due to a reduction of DMN neuropeptide Y (NPY). Rats lacking DMN CCK-A receptors are obese and have increased expression of NPY in the DMN, supporting earlier data that CCK may act at the DMN to suppress food intake. Excitotoxin studies showed that loss of DMN cell somata, and not fibers of passage, is important in the development of the DMNL syndrome. The DMN is a site where opioids increase food intake and knife-cut studies have shown that fibers traveling to/from the DMN are important in this response. An interaction of glucose and opioids in DMN may also be involved in the control of food intake. DMN knife cuts interrupting fibers in the posterior and ventral directions additively produce the hypophagia and reduced linear and ponderal growth observed after DMNL. Ventral cuts may interrupt important connections with the arcuate nucleus. Lateral and posterior DMN cuts additively produce the hypodipsic effect seen after DMNL, but DMNL rats respond normally to all water-regulatory challenges, i.e., the hypophagia is not due to a primary hypodipsia. The DMN has been shown to be involved in the rat's feeding response to an imbalanced amino acid diet. These data show the DMN has an important role in many processes that control both food intake and BW regulation.

Effects of dorsomedial hypothalamic nuclei lesions on intake of an imbalanced amino acid diet

Within 3 h of ingesting an imbalanced amino acid diet (Imb), rats show attenuated intake, which can be ameliorated by prior administration of the serotonin receptor antagonist tropisetron (Trop). Earlier work in which the dorsomedial hypothalamic nucleus (DMN) was electrolytically lesioned (DMNL) determined that this structure plays a role in the early detection of and subsequent adaptation to Imb. However, that study did not address whether cell bodies in the DMN, fibers of passage, or both were involved in the DMNL response to Imb. In the present investigation in experiment 1, rats were given electrolytic DMNL or a sham operation (Sham). The rats were injected with saline (Sal) or Trop just before introduction of Imb. By 3 h Sal-DMNL rats consumed more Imb than did the Sal-Sham rats; intake was normal by 12 h. Trop enhanced Imb intake, with Trop and DMNL being additive. By day 4 the DMNL rats were eating and gaining weight less than were Sham rats. In experiment 2, DMN cell bodies were destroyed by ibotenic acid (Ibo). Sal-injected Ibo-lesioned and Sham rats showed similar food intake depression on Imb; Trop similarly increased Imb intake in both groups. By day 4 both Ibo-L rats were eating and gaining weight less than were Sham rats. In experiment 3, groups of rats were given knife cuts posterior, lateral, ventral, dorsal, or anterior to the DMN. During the first 3 h of consuming Imb, all cuts except posterior enhanced the intake of Imb. Over the next 24 h the anterior cut group continued to eat more Imb than did the Sham rats. In experiment 4 DMNL rats were given novel diets; the DMNL rats did not display a neophilic response. The data suggest that fiber tracts that pass through the DMN may be involved in the early detection of Imb. DMN cell bodies, or fibers of passage, are not involved in the Trop effect. Finally, DMN cell bodies are necessary for proper long-term adaptation to Imb.

Effect of dorsomedial hypothalamic nuclei knife cuts on ingestive behavior

Previous findings show that rats with electrolytic or excitotoxic lesions in the dorsomedial hypothalamic nucleus (DMN) are hypophagic and hypodipsic and have reduced ponderal and linear growth but normal body composition. DMN-lesioned (DMNL) rats also show altered ingestive responses to naloxone. The present study investigated the intrahypothalamic nerve pathways involved in these DMNL effects and the response of the pathways to deprivation challenges by placing knife cuts posterior (Post), lateral (Lat), ventral (Vent), dorsal, or anterior to the DMN or by administering sham operations. One major finding was that rats with Post or Vent were hypophagic (P < 0. 05) and had reduced body weight but responded normally to deprivation challenges. Post and Lat groups were hypodipsic (P < 0. 05), but plasma Na+, K+, and osmolality and 24-h post-water-deprivation drinking responses were similar in all groups. Naloxone did not suppress the intake of Post rats. It appears that the hypophagia and the reduced body weight after DMNL involve fibers entering or leaving the DMN from ventral and posterior directions, and they may be part of an opioid feeding system.