Pro-opiomelanocortin (POMC)- and agouti-related peptide (AgRP)-expressing neurons of the arcuate nucleus of the hypothalamus (ARC) are oppositely regulated by caloric depletion and coordinately ...stimulate and inhibit homeostatic satiety, respectively. This bimodality is principally underscored by the antagonistic actions of these ligands at downstream melanocortin-4 receptors (MC4R) in the paraventricular nucleus of the hypothalamus (PVH). Although this population is critical to energy balance, the underlying neural circuitry remains unknown. Using mice expressing Cre recombinase in MC4R neurons, we demonstrate bidirectional control of feeding following real-time activation and inhibition of PVH(MC4R) neurons and further identify these cells as a functional exponent of ARC(AgRP) neuron-driven hunger. Moreover, we reveal this function to be mediated by a PVH(MC4R)→lateral parabrachial nucleus (LPBN) pathway. Activation of this circuit encodes positive valence, but only in calorically depleted mice. Thus, the satiating and appetitive nature of PVH(MC4R)→LPBN neurons supports the principles of drive reduction and highlights this circuit as a promising target for antiobesity drug development.
Agouti-related peptide (AGRP)-expressing neurons are activated by fasting-this causes hunger
, an aversive state that motivates the seeking and consumption of food
. Eating returns AGRP neuron ...activity towards baseline on three distinct timescales: rapidly and transiently following sensory detection of food cues
, slowly and longer-lasting in response to nutrients in the gut
, and even more slowly and permanently with restoration of energy balance
. The rapid regulation by food cues is of particular interest as its neurobiological basis and purpose are unknown. Given that AGRP neuron activity is aversive
, the sensory cue-linked reductions in activity could function to guide behaviour. To evaluate this, we first identified the circuit mediating sensory cue inhibition and then selectively perturbed it to determine function. Here, we show that a lateral hypothalamic glutamatergic → dorsomedial hypothalamic GABAergic (γ-aminobutyric acid-producing)
→ AGRP neuron circuit mediates this regulation. Interference with this circuit impairs food cue inhibition of AGRP neurons and, notably, greatly impairs learning of a sensory cue-initiated food-acquisition task. This is specific for food, as learning of an identical water-acquisition task is unaffected. We propose that decreases in aversive AGRP neuron activity
mediated by this food-specific circuit increases the incentive salience
of food cues, and thus facilitates the learning of food-acquisition tasks.
Hypoglycemia engenders an autonomically mediated counterregulatory (CR)-response that stimulates endogenous glucose production to maintain concentrations within an appropriate physiological range. ...Although the involvement of the brain in preserving normoglycemia has been established, the neurocircuitry underlying centrally mediated CR-responses remains unclear. Here we demonstrate that lateral parabrachial nucleus cholecystokinin (CCKLPBN) neurons are a population of glucose-sensing cells (glucose inhibited) with counterregulatory capacity. Furthermore, we reveal that steroidogenic-factor 1 (SF1)-expressing neurons of the ventromedial nucleus of the hypothalamus (SF1VMH) are the specific target of CCKLPBN glucoregulatory neurons. This discrete CCKLPBN→SF1VMH neurocircuit is both necessary and sufficient for the induction of CR-responses. Together, these data identify CCKLPBN neurons, and specifically CCK neuropeptide, as glucoregulatory and provide significant insight into the homeostatic mechanisms controlling CR-responses to hypoglycemia.
Display omitted
•CCKLPBN neurons are glucose inhibited and activated by hypoglycemia•CCKLPBN neurons are necessary and sufficient for counterregulatory (CR)-responses•CCK neuropeptide is the key mediator of CCKLPBN neuron-mediated CR-responses•CCKLPBN neuron-induced CR-responses require downstream SF1VMH neurons
The counterregulatory response (CRR) to hypoglycemia is critical for the maintenance of normoglycemia and governed by the brain. Garfield et al. identify a population of brainstem CCK neurons that directly sense extracellular glucose concentrations and, via their connection to SF1 hypothalamic neurons, promote CRR.
Optimal size at birth dictates perinatal survival and long-term risk of developing common disorders such as obesity, type 2 diabetes and cardiovascular disease. The imprinted Grb10 gene encodes a ...signalling adaptor protein capable of inhibiting receptor tyrosine kinases, including the insulin receptor (Insr) and insulin-like growth factor type 1 receptor (Igf1r). Grb10 restricts fetal growth such that Grb10 knockout (KO) mice are at birth some 25-35% larger than wild type. Using a mouse genetic approach, we test the widely held assumption that Grb10 influences growth through interaction with Igf1r, which has a highly conserved growth promoting role.
Should Grb10 interact with Igf1r to regulate growth Grb10:Igf1r double mutant mice should be indistinguishable from Igf1r KO single mutants, which are around half normal size at birth. Instead, Grb10:Igf1r double mutants were intermediate in size between Grb10 KO and Igf1r KO single mutants, indicating additive effects of the two signalling proteins having opposite actions in separate pathways. Some organs examined followed a similar pattern, though Grb10 KO neonates exhibited sparing of the brain and kidneys, whereas the influence of Igf1r extended to all organs. An interaction between Grb10 and Insr was similarly investigated. While there was no general evidence for a major interaction for fetal growth regulation, the liver was an exception. The liver in Grb10 KO mutants was disproportionately overgrown with evidence of excess lipid storage in hepatocytes, whereas Grb10:Insr double mutants were indistinguishable from Insr single mutants or wild types.
Grb10 acts largely independently of Igf1r or Insr to control fetal growth and has a more variable influence on individual organs. Only the disproportionate overgrowth and excess lipid storage seen in the Grb10 KO neonatal liver can be explained through an interaction between Grb10 and the Insr. Our findings are important for understanding how positive and negative influences on fetal growth dictate size and tissue proportions at birth.
The attenuation of food intake as induced by an increase in serotonergic (5-hydroxytryptamine, 5-HT) efficacy has been a target
of antiobesity pharmacotherapies. However, the induction of tolerance ...and/or side-effects limited the clinical utility of
the earliest serotonin-related medications. With the global prevalence of obesity rising, there has been renewed interest
in the manipulation of the serotonergic system as a point of pharmacological intervention. The serotonin 2C receptor (5-HT 2C R), serotonin 1B (rodent)/serotonin 1Dβ (human) receptor (5-HT 1B/1Dβ R) and serotonin 6 receptor (5-HT 6 R) represent the most promising serotonin receptor therapeutic targets. Canonical serotonin receptor compounds have given
way to a myriad of novel receptor-selective ligands, many of which have observable anorectic effects. Here we review serotonergic
compounds reducing ingestive behaviour and discuss their clinical potential for the treatment of obesity.
Agouti-related peptide (AgRP) neurons of the arcuate nucleus of the hypothalamus (ARC) promote homeostatic feeding at times of caloric insufficiency, yet they are rapidly suppressed by food-related ...sensory cues before ingestion. Here we identify a highly selective inhibitory afferent to AgRP neurons that serves as a neural determinant of this rapid modulation. Specifically, GABAergic projections arising from the ventral compartment of the dorsomedial nucleus of the hypothalamus (vDMH) contribute to the preconsummatory modulation of ARC
neurons. In a manner reciprocal to ARC
neurons, ARC-projecting leptin receptor-expressing GABAergic vDMH neurons exhibit rapid activation upon availability of food that additionally reflects the relative value of the food. Thus, leptin receptor-expressing GABAergic vDMH neurons form part of the sensory network that relays real-time information about the nature and availability of food to dynamically modulate ARC
neuron activity and feeding behavior.
An inverse relationship between brain serotonin and food intake and body weight has been known for more than 30
years. Specifically, augmentation of brain serotonin inhibits food intake, while ...depletion of brain serotonin promotes hyperphagia and weight gain. Through the decades, serotonin receptors have been identified and their function in the serotonergic regulation of food intake clarified. Recent refined genetic studies now indicate that a primary mechanism through which serotonin influences appetite and body weight is via serotonin 2C receptor (5-HT
2CR) and serotonin 1B receptor (5-HT
1BR) influencing the activity of endogenous melanocortin receptor agonists and antagonists at the melanocortin 4 receptor (MC4R). However, other mechanisms are also possible and the challenge of future research is to delineate them in the complete elucidation of the complex neurocircuitry underlying the serotonergic control of appetite and body weight.
► Serotonergic tone is inversely correlated with food intake. ► Distinct serotonin receptors exert effects on appetite. ► Multiple brain effector pathways mediate the effects of serotonin on food intake.
In this work we summarize our understanding of melanocortin 4 receptor (MC4R) pathway activation, aiming to define a safe and effective therapeutic targeting strategy for the MC4R. Delineation of ...cellular MC4R pathways has provided evidence for distinct MC4R signaling events characterized by unique receptor activation kinetics. While these studies remain narrow in scope, and have largely been explored with peptidic agonists, the results provide a possible correlation between distinct ligand groups and differential MC4R activation kinetics. In addition, when a set of small-molecule and peptide MC4R agonists are compared, evidence of biased signaling has been reported. The results of such mechanistic studies are discussed.
The parabrachial nucleus is important for thermoregulation because it relays skin temperature information from the spinal cord to the hypothalamus. Prior work in rats localized thermosensory relay ...neurons to its lateral subdivision (LPB), but the genetic and neurochemical identity of these neurons remains unknown. To determine the identity of LPB thermosensory neurons, we exposed mice to a warm (36°C) or cool (4°C) ambient temperature. Each condition activated neurons in distinct LPB subregions that receive input from the spinal cord. Most c-Fos+ neurons in these LPB subregions expressed the transcription factor marker FoxP2. Consistent with prior evidence that LPB thermosensory relay neurons are glutamatergic, all FoxP2+ neurons in these subregions colocalized with green fluorescent protein (GFP) in reporter mice for Vglut2, but not for Vgat. Prodynorphin (Pdyn)-expressing neurons were identified using a GFP reporter mouse and formed a caudal subset of LPB FoxP2+ neurons, primarily in the dorsal lateral subnucleus (PBdL). Warm exposure activated many FoxP2+ neurons within PBdL. Half of the c-Fos+ neurons in PBdL were Pdyn+, and most of these project into the preoptic area. Cool exposure activated a separate FoxP2+ cluster of neurons in the far-rostral LPB, which we named the rostral-to-external lateral subnucleus (PBreL). These findings improve our understanding of LPB organization and reveal that Pdyn-IRES-Cre mice provide genetic access to warm-activated, FoxP2+ glutamatergic neurons in PBdL, many of which project to the hypothalamus.
Primary mouse embryonic fibroblasts (PMEFs) have a number of properties that make them an attractive cell culture model. Relative to other primary explant cultures they are easy to establish and ...maintain, proliferate rapidly and, as a result, large numbers of cells can be produced from a single embryo within several days following explantation. This allows, for instance, for ready comparison of wild-type and knockout cells derived from the same litter of animals. PMEFs can be expanded through several passages before they reach crisis and can be used to establish cell lines following spontaneous transformation or following derivation from strains carrying mutations, such as in the gene encoding the tumour suppressor Trp53. They have been widely used as feeders to support other cultured cell types, notably embryonic stem cells, as well as for the study of a diverse range of cellular phenomena using microscopic, biochemical and molecular biological techniques. Here, we describe a simple and reliable method for the derivation and maintenance of PMEFs.
PDF
