Transient cAMP production drives rapid and sustained spiking in brainstem parabrachial neurons to suppress feeding

Brief stimuli can trigger longer-lasting brain states. G-protein-coupled receptors (GPCRs) could help sustain such states by coupling slow-timescale molecular signals to neuronal excitability. Brainstem parabrachial nucleus glutamatergic (PBNGlut) neurons regulate sustained brain states such as pain and express Gs-coupled GPCRs that increase cAMP signaling. We asked whether cAMP in PBNGlut neurons directly influences their excitability and effects on behavior. Both brief tail shocks and brief optogenetic stimulation of cAMP production in PBNGlut neurons drove minutes-long suppression of feeding. This suppression matched the duration of prolonged elevations in cAMP, protein kinase A (PKA) activity, and calcium activity in vivo and ex vivo, as well as sustained, PKA-dependent increases in action potential firing ex vivo. Shortening this elevation in cAMP reduced the duration of feeding suppression following tail shocks. Thus, molecular signaling in PBNGlut neurons helps prolong neural activity and behavioral states evoked by brief, salient bodily stimuli.

Neural basis for fasting activation of the hypothalamic-pituitary-adrenal axis

Fasting initiates a multitude of adaptations to allow survival. Activation of the hypothalamic-pituitary-adrenal (HPA) axis and subsequent release of glucocorticoid hormones is a key response that mobilizes fuel stores to meet energy demands1-5. Despite the importance of the HPA axis response, the neural mechanisms that drive its activation during energy deficit are unknown. Here, we show that fasting-activated hypothalamic agouti-related peptide (AgRP)-expressing neurons trigger and are essential for fasting-induced HPA axis activation. AgRP neurons do so through projections to the paraventricular hypothalamus (PVH), where, in a mechanism not previously described for AgRP neurons, they presynaptically inhibit the terminals of tonically active GABAergic afferents from the bed nucleus of the stria terminalis (BNST) that otherwise restrain activity of corticotrophin-releasing hormone (CRH)-expressing neurons. This disinhibition of PVHCrh neurons requires γ-aminobutyric acid (GABA)/GABA-B receptor signalling and potently activates the HPA axis. Notably, stimulation of the HPA axis by AgRP neurons is independent of their induction of hunger, showing that these canonical 'hunger neurons' drive many distinctly different adaptations to the fasted state. Together, our findings identify the neural basis for fasting-induced HPA axis activation and uncover a unique means by which AgRP neurons activate downstream neurons: through presynaptic inhibition of GABAergic afferents. Given the potency of this disinhibition of tonically active BNST afferents, other activators of the HPA axis, such as psychological stress, may also work by reducing BNST inhibitory tone onto PVHCrh neurons.

Competition between stochastic neuropeptide signals calibrates the rate of satiation

We investigated how transmission of hunger- and satiety-promoting neuropeptides, NPY and αMSH, is integrated at the level of intracellular signaling to control feeding. Receptors for these peptides use the second messenger cAMP. How cAMP integrates opposing peptide signals to regulate energy balance, and the in vivo spatiotemporal dynamics of endogenous peptidergic signaling, remain largely unknown. We show that AgRP axon stimulation in the paraventricular hypothalamus evokes probabilistic NPY release that triggers stochastic cAMP decrements in downstream MC4R-expressing neurons (PVHMC4R). Meanwhile, POMC axon stimulation triggers stochastic, αMSH-dependent cAMP increments. Release of either peptide impacts a ~100 μm diameter region, and when these peptide signals overlap, they compete to control cAMP. The competition is reflected by hunger-state-dependent differences in the amplitude and persistence of cAMP transients: hunger peptides are more efficacious in the fasted state, satiety peptides in the fed state. Feeding resolves the competition by simultaneously elevating αMSH release and suppressing NPY release, thereby sustaining elevated cAMP in PVHMC4R neurons. In turn, cAMP potentiates feeding-related excitatory inputs and promotes satiation across minutes. Our findings highlight how biochemical integration of opposing, quantal peptide signals during energy intake orchestrates a gradual transition between stable states of hunger and satiety.

Competition between stochastic neuropeptide signals calibrates the rate of satiation

We investigated how transmission of hunger- and satiety-promoting neuropeptides, NPY and αMSH, is integrated at the level of intracellular signaling to control feeding. Receptors for these peptides use the second messenger cAMP, but the messenger's spatiotemporal dynamics and role in energy balance are controversial. We show that AgRP axon stimulation in the paraventricular hypothalamus evokes probabilistic and spatially restricted NPY release that triggers stochastic cAMP decrements in downstream MC4R-expressing neurons (PVH ^MC4R ). Meanwhile, POMC axon stimulation triggers stochastic, αMSH-dependent cAMP increments. NPY and αMSH competitively control cAMP, as reflected by hunger-state-dependent differences in the amplitude and persistence of cAMP transients evoked by each peptide. During feeding bouts, elevated αMSH release and suppressed NPY release cooperatively sustain elevated cAMP in PVH ^MC4R neurons, thereby potentiating feeding-related excitatory inputs and promoting satiation across minutes. Our findings highlight how state-dependent integration of opposing, quantal peptidergic events by a common biochemical target calibrates energy intake.

A synaptic amplifier of hunger for regaining body weight in the hypothalamus

Restricting caloric intake effectively reduces body weight, but most dieters fail long-term adherence to caloric deficit and eventually regain lost weight. Hypothalamic circuits that control hunger drive critically determine body weight; yet, how weight loss sculpts these circuits to motivate food consumption until lost weight is regained remains unclear. Here, we probe the contribution of synaptic plasticity in discrete excitatory afferents on hunger-promoting AgRP neurons. We reveal a crucial role for activity-dependent, remarkably long-lasting amplification of synaptic activity originating from paraventricular hypothalamus thyrotropin-releasing (PVHTRH) neurons in long-term body weight control. Silencing PVHTRH neurons inhibits the potentiation of excitatory input to AgRP neurons and diminishes concomitant regain of lost weight. Brief stimulation of the pathway is sufficient to enduringly potentiate this glutamatergic hunger synapse and triggers an NMDAR-dependent gaining of body weight that enduringly persists. Identification of this activity-dependent synaptic amplifier provides a previously unrecognized target to combat regain of lost weight.

Transient cAMP production drives rapid and sustained spiking in brainstem parabrachial neurons to suppress feeding

Brief stimuli can trigger longer lasting brain states. G protein-coupled receptors (GPCRs) could help sustain such states by coupling slow-timescale molecular signals to neuronal excitability. Brainstem parabrachial nucleus glutamatergic neurons (PBN Glut ) regulate sustained brain states such as pain, and express G s -coupled GPCRs that increase cAMP signaling. We asked whether cAMP directly influences PBN Glut excitability and behavior. Both brief tail shocks and brief optogenetic stimulation of cAMP production in PBN Glut neurons drove minutes-long suppression of feeding. This suppression matched the duration of prolonged elevations in cAMP, Protein Kinase A (PKA), and calcium activity in vivo and in vitro. Shortening this elevation in cAMP reduced the duration of feeding suppression following tail shocks. cAMP elevations in PBN Glut neurons rapidly lead to sustained increases in action potential firing via PKA-dependent mechanisms. Thus, molecular signaling in PBN Glut neurons helps prolong neural activity and behavioral states evoked by brief, salient bodily stimuli.

Arginine-vasopressin mediates counter-regulatory glucagon release and is diminished in type 1 diabetes

Insulin-induced hypoglycemia is a major treatment barrier in type-1 diabetes (T1D). Accordingly, it is important that we understand the mechanisms regulating the circulating levels of glucagon. Varying glucose over the range of concentrations that occur physiologically between the fed and fuel-deprived states (8 to 4 mM) has no significant effect on glucagon secretion in the perfused mouse pancreas or in isolated mouse islets (in vitro), and yet associates with dramatic increases in plasma glucagon. The identity of the systemic factor(s) that elevates circulating glucagon remains unknown. Here, we show that arginine-vasopressin (AVP), secreted from the posterior pituitary, stimulates glucagon secretion. Alpha-cells express high levels of the vasopressin 1b receptor (V1bR) gene (Avpr1b). Activation of AVP neurons in vivo increased circulating copeptin (the C-terminal segment of the AVP precursor peptide) and increased blood glucose; effects blocked by pharmacological antagonism of either the glucagon receptor or V1bR. AVP also mediates the stimulatory effects of hypoglycemia produced by exogenous insulin and 2-deoxy-D-glucose on glucagon secretion. We show that the A1/C1 neurons of the medulla oblongata drive AVP neuron activation in response to insulin-induced hypoglycemia. AVP injection increased cytoplasmic Ca2+ in alpha-cells (implanted into the anterior chamber of the eye) and glucagon release. Hypoglycemia also increases circulating levels of AVP/copeptin in humans and this hormone stimulates glucagon secretion from human islets. In patients with T1D, hypoglycemia failed to increase both copeptin and glucagon. These findings suggest that AVP is a physiological systemic regulator of glucagon secretion and that this mechanism becomes impaired in T1D.

Neural basis for regulation of vasopressin secretion by anticipated disturbances in osmolality

Water balance, tracked by extracellular osmolality, is regulated by feedback and feedforward mechanisms. Feedback regulation is reactive, occurring as deviations in osmolality are detected. Feedforward or presystemic regulation is proactive, occurring when disturbances in osmolality are anticipated. Vasopressin (AVP) is a key hormone regulating water balance and is released during hyperosmolality to limit renal water excretion. AVP neurons are under feedback and feedforward regulation. Not only do they respond to disturbances in blood osmolality, but they are also rapidly suppressed and stimulated, respectively, by drinking and eating, which will ultimately decrease and increase osmolality. Here, we demonstrate that AVP neuron activity is regulated by multiple anatomically and functionally distinct neural circuits. Notably, presystemic regulation during drinking and eating are mediated by non-overlapping circuits that involve the lamina terminalis and hypothalamic arcuate nucleus, respectively. These findings reveal neural mechanisms that support differential regulation of AVP release by diverse behavioral and physiological stimuli.

Fine-tuning the amount of water present in the body at any given time is a tight balancing act. The hormone vasopressin helps to ensure that organisms do not get too dehydrated by allowing water in the urine to be reabsorbed into the bloodstream. A group of vasopressin neurons in the brain trigger the release of the hormone if water levels get too low (as reflected by an increase in osmolality, the level of substances dissolved in a unit of blood). However, these cells also receive additional information that allows them to predict and respond to upcoming changes in water levels. For example, drinking water while dehydrated ‘switches off’ the neurons, even before osmolality is restored in the blood to normal levels. Eating, on the other hand, rapidly activates vasopressin neurons before the food is digested and blood osmolality increases as a result.

How vasopressin neurons receive this ‘anticipatory’ information remains unclear. Kim et al. explored this question in mice by inhibiting different sets of brain cells one by one, and then examining whether the neurons could still exhibit anticipatory responses. This revealed a remarkable division of labor in the neural circuits that regulate vasopressin neurons: two completely different sets of neurons from distinct areas of the brain are dedicated to relaying anticipatory information about either water or food intake.

These findings help to understand how healthy levels of water can be maintained in the body. Overall, they give a glimpse into the neural mechanisms that underlie anticipatory forms of regulation, which can also take place when hunger or thirst neurons ‘foresee’ that food or water will be consumed.

Estimation of Current and Future Physiological States in Insular Cortex

Interoception, the sense of internal bodily signals, is essential for physiological homeostasis, cognition, and emotions. While human insular cortex (InsCtx) is implicated in interoception, the cellular and circuit mechanisms remain unclear. We imaged mouse InsCtx neurons during two physiological deficiency states: hunger and thirst. InsCtx ongoing activity patterns reliably tracked the gradual return to homeostasis but not changes in behavior. Accordingly, while artificial induction of hunger or thirst in sated mice via activation of specific hypothalamic neurons (AgRP or SFOGLUT) restored cue-evoked food- or water-seeking, InsCtx ongoing activity continued to reflect physiological satiety. During natural hunger or thirst, food or water cues rapidly and transiently shifted InsCtx population activity to the future satiety-related pattern. During artificial hunger or thirst, food or water cues further shifted activity beyond the current satiety-related pattern. Together with circuit-mapping experiments, these findings suggest that InsCtx integrates visceral-sensory signals of current physiological state with hypothalamus-gated amygdala inputs that signal upcoming ingestion of food or water to compute a prediction of future physiological state.

Leptin's hunger-suppressing effects are mediated by the hypothalamic-pituitary-adrenocortical axis in rodents

Leptin informs the brain about sufficiency of fuel stores. When insufficient, leptin levels fall, triggering compensatory increases in appetite. Falling leptin is first sensed by hypothalamic neurons, which then initiate adaptive responses. With regard to hunger, it is thought that leptin-sensing neurons work entirely via circuits within the central nervous system (CNS). Very unexpectedly, however, we now show this is not the case. Instead, stimulation of hunger requires an intervening endocrine step, namely activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Increased corticosterone then activates AgRP neurons to fully increase hunger. Importantly, this is true for 2 forms of low leptin-induced hunger, fasting and poorly controlled type 1 diabetes. Hypoglycemia, which also stimulates hunger by activating CNS neurons, albeit independently of leptin, similarly recruits and requires this pathway by which HPA axis activity stimulates AgRP neurons. Thus, HPA axis regulation of AgRP neurons is a previously underappreciated step in homeostatic regulation of hunger.

Lateral hypothalamic neurotensin neurons promote arousal and hyperthermia

Sleep and wakefulness are greatly influenced by various physiological and psychological factors, but the neuronal elements responsible for organizing sleep-wake behavior in response to these factors are largely unknown. In this study, we report that a subset of neurons in the lateral hypothalamic area (LH) expressing the neuropeptide neurotensin (Nts) is critical for orchestrating sleep-wake responses to acute psychological and physiological challenges or stressors. We show that selective activation of Nts^LH neurons with chemogenetic or optogenetic methods elicits rapid transitions from non-rapid eye movement (NREM) sleep to wakefulness and produces sustained arousal, higher locomotor activity (LMA), and hyperthermia, which are commonly observed after acute stress exposure. On the other hand, selective chemogenetic inhibition of Nts^LH neurons attenuates the arousal, LMA, and body temperature (Tb) responses to a psychological stress (a novel environment) and augments the responses to a physiological stress (fasting).

A neurotensin-producing subset of neurons in the lateral hypothalamus promote arousal and thermogenesis; these neurons are necessary for appropriate sleep-wake and body temperature responses to various stressors.

Adjusting sleep-wake behavior in response to environmental and physiological challenges may not only be of protective value, but can also be vital for the survival of the organism. For example, while it is crucial to increase wake to explore a novel environment to search for potential threats and food sources, it is also necessary to decrease wake and reduce energy expenditure during prolonged absence of food. In this study, we report that a subset of neurons in the lateral hypothalamic area (LH) expressing the neuropeptide neurotensin (Nts) is critical for orchestrating sleep-wake responses to such challenges. We show that brief activation of Nts^LH neurons in mice evokes immediate arousals from sleep, while their sustained activation increases wake, locomotor activity, and body temperature for several hours. In contrast, when Nts^LH neurons are inhibited, mice are neither able to sustain wake in a novel environment nor able to reduce wake during food deprivation. These data suggest that Nts^LH neurons may be necessary for generating appropriate sleep-wake responses to a wide variety of environmental and physiological challenges.

Ventrolateral periaqueductal gray mediates rapid eye movement sleep regulation by melanin-concentrating hormone neurons

Neurons containing melanin-concentrating hormone (MCH) in the lateral hypothalamic area (LH) have been shown to promote rapid eye movement sleep (REMs) in mice. However, the downstream neural pathways through which MCH neurons influence REMs remained unclear. Because MCH neurons are considered to be primarily inhibitory, we hypothesized that these neurons inhibit the midbrain 'REMs-suppressing' region consisting of the ventrolateral periaqueductal gray and the lateral pontine tegmentum (vlPAG/LPT) to promote REMs. To test this hypothesis, we optogenetically inhibited MCH terminals in the vlPAG/LPT under baseline conditions as well as with simultaneous chemogenetic activation of MCH soma. We found that inhibition of MCH terminals in the vlPAG/LPT significantly reduced transitions into REMs during spontaneous sleep-wake cycles and prevented the increase in REMs transitions observed after chemogenetic activation of MCH neurons. These results strongly suggest that the vlPAG/LPT may be an essential relay through which MCH neurons modulate REMs.

The Paraventricular Hypothalamus Regulates Satiety and Prevents Obesity via Two Genetically Distinct Circuits

SIM1-expressing paraventricular hypothalamus (PVH) neurons are key regulators of energy balance. Within the PVH^SIM1 population, melanocortin-4 receptor-expressing (PVH^MC4R) neurons are known to regulate satiety and bodyweight, yet they account for only half of PVH^SIM1 neuron-mediated regulation. Here we report that PVH prodynorphin-expressing (PVH^PDYN) neurons, which notably lack MC4Rs, function independently and additively with PVH^MC4R neurons to account for the totality of PVH^SIM1 neuron-mediated satiety. Moreover, PVH^PDYN neurons are necessary for prevention of obesity in an independent but equipotent manner to PVH^MC4R neurons. While PVH^PDYN and PVH^MC4R neurons both project to the parabrachial complex (PB), they synaptically engage distinct efferent nodes, the pre-locus coeruleus (pLC), and central lateral parabrachial nucleus (cLPBN), respectively. PB-projecting PVH^PDYN neurons, like PVH^MC4R neurons, receive input from interoceptive ARC^AgRP neurons, respond to caloric state, and are sufficient and necessary to control food intake. This expands the CNS satiety circuitry to include two non-overlapping PVH to hindbrain circuits.

PACAP neurons in the ventral premammillary nucleus regulate reproductive function in the female mouse

Pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1) is a neuromodulator implicated in anxiety, metabolism and reproductive behavior. PACAP global knockout mice have decreased fertility and PACAP modulates LH release. However, its source and role at the hypothalamic level remain unknown. We demonstrate that PACAP-expressing neurons of the ventral premamillary nucleus of the hypothalamus (PMV^PACAP) project to, and make direct contact with, kisspeptin neurons in the arcuate and AVPV/PeN nuclei and a subset of these neurons respond to PACAP exposure. Targeted deletion of PACAP from the PMV through stereotaxic virally mediated cre- injection or genetic cross to LepR-i-cre mice with Adcyap1^fl/fl mice led to delayed puberty onset and impaired reproductive function in female, but not male, mice. We propose a new role for PACAP-expressing neurons in the PMV in the relay of nutritional state information to regulate GnRH release by modulating the activity of kisspeptin neurons, thereby regulating reproduction in female mice.

Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling

Sodium deficiency increases angiotensin II (ATII) and aldosterone, which synergistically stimulate sodium retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTS^HSD2 neurons) were shown to drive sodium appetite. Here we investigate the basis for NTS^HSD2 neuron activation, identify the circuit by which NTS^HSD2 neurons drive appetite, and uncover an interaction between the NTS^HSD2 circuit and ATII signaling. NTS^HSD2 neurons respond to sodium deficiency with spontaneous pacemaker-like activity-the consequence of "cardiac" HCN and Na_v1.5 channels. Remarkably, NTS^HSD2 neurons are necessary for sodium appetite, and with concurrent ATII signaling their activity is sufficient to produce rapid consumption. Importantly, NTS^HSD2 neurons stimulate appetite via projections to the vlBNST, which is also the effector site for ATII-responsive SFO neurons. The interaction between angiotensin signaling and NTS^HSD2 neurons provides a neuronal context for the long-standing "synergy hypothesis" of sodium appetite regulation.

Homeostatic circuits selectively gate food cue responses in insular cortex

Physiological needs bias perception and attention to relevant sensory cues. This process is 'hijacked' by drug addiction, causing cue-induced cravings and relapse. Similarly, its dysregulation contributes to failed diets, obesity, and eating disorders. Neuroimaging studies in humans have implicated insular cortex in these phenomena. However, it remains unclear how 'cognitive' cortical representations of motivationally relevant cues are biased by subcortical circuits that drive specific motivational states. Here we develop a microprism-based cellular imaging approach to monitor visual cue responses in the insular cortex of behaving mice across hunger states. Insular cortex neurons demonstrate food-cue-biased responses that are abolished during satiety. Unexpectedly, while multiple satiety-related visceral signals converge in insular cortex, chemogenetic activation of hypothalamic 'hunger neurons' (expressing agouti-related peptide (AgRP)) bypasses these signals to restore hunger-like response patterns in insular cortex. Circuit mapping and pathway-specific manipulations uncover a pathway from AgRP neurons to insular cortex via the paraventricular thalamus and basolateral amygdala. These results reveal a neural basis for state-specific biased processing of motivationally relevant cues.

Melanin-concentrating hormone neurons specifically promote rapid eye movement sleep in mice

Currently available evidence indicates that neurons containing melanin-concentrating hormone (MCH) in the lateral hypothalamus are critical modulators of sleep-wakefulness, but their precise role in this function is not clear. Studies employing optogenetic stimulation of MCH neurons have yielded inconsistent results, presumably due to differences in the optogenetic stimulation protocols, which do not approximate normal patterns of cell firing. In order to resolve this discrepancy, we (1) selectively activated the MCH neurons using a chemogenetic approach (Cre-dependent hM3Dq expression) and (2) selectively destroyed MCH neurons using a genetically targeted diphtheria toxin deletion method, and studied the changes in sleep-wake in mice. Our results indicate that selective activation of MCH neurons causes specific increases in rapid eye movement (REM) sleep without altering wake or non-REM (NREM) sleep. On the other hand, selective deletions of MCH neurons altered the diurnal rhythm of wake and REM sleep without altering their total amounts. These results indicate that activation of MCH neurons primarily drives REM sleep and their presence may be necessary for normal expression of diurnal variation of REM sleep and wake.

Appetite controlled by a cholecystokinin nucleus of the solitary tract to hypothalamus neurocircuit

The nucleus of the solitary tract (NTS) is a key gateway for meal-related signals entering the brain from the periphery. However, the chemical mediators crucial to this process have not been fully elucidated. We reveal that a subset of NTS neurons containing cholecystokinin (CCK^NTS) is responsive to nutritional state and that their activation reduces appetite and body weight in mice. Cell-specific anterograde tracing revealed that CCK^NTS neurons provide a distinctive innervation of the paraventricular nucleus of the hypothalamus (PVH), with fibers and varicosities in close apposition to a subset of melanocortin-4 receptor (MC4R^PVH) cells, which are also responsive to CCK. Optogenetic activation of CCK^NTS axon terminals within the PVH reveal the satiating function of CCK^NTS neurons to be mediated by a CCK^NTS→PVH pathway that also encodes positive valence. These data identify the functional significance of CCK^NTS neurons and reveal a sufficient and discrete NTS to hypothalamus circuit controlling appetite.

DOI:http://dx.doi.org/10.7554/eLife.12225.001

Obesity primarily results from eating more food than the body requires, the energy from which is then stored as fat. In recent years obesity has become increasingly common, with the resulting health problems presenting one of the major healthcare challenges of the twenty-first century. New ways to tackle the obesity epidemic are therefore required to improve human health on a global scale.

To regulate how much food is eaten, the gut sends chemical messengers to the brain about how much food has been consumed. These messengers activate particular cells in the brain that signal to other brain regions to trigger a decision about whether we’ve had enough food to eat. This raises a question: if we can artificially activate these cells, can we ‘trick’ the brain into thinking that food has been consumed?

A brain region called the nucleus of the solitary tract (NTS) is known to play a key role in receiving signals from the gut about meals. By studying mice, D’Agostino et al. found that cells in the NTS that make a brain hormone called cholecystokinin (CCK) are particularly activated by food.

Further experiments then used a technique called optogenetics to activate these cells in mice that had free access to different types of food. This activation significantly reduced how hungry the mice were, causing them to eat less food and lose weight. D’Agostino et al. also showed that CCK cells relay the signal about food intake to a brain region called the hypothalamus.

Overall, D’Agostino et al. have found a way to trick the brain into thinking that food has been eaten when it actually hasn’t, and for this reason mice eat less without feeling hungry and lose weight. The next step is to try and find a way to activate the CCK cells in obese humans who have health complications associated with excess body weight.

DOI:http://dx.doi.org/10.7554/eLife.12225.002

Excitatory transmission onto AgRP neurons is regulated by cJun NH2-terminal kinase 3 in response to metabolic stress

The cJun NH₂-terminal kinase (JNK) signaling pathway is implicated in the response to metabolic stress. Indeed, it is established that the ubiquitously expressed JNK1 and JNK2 isoforms regulate energy expenditure and insulin resistance. However, the role of the neuron-specific isoform JNK3 is unclear. Here we demonstrate that JNK3 deficiency causes hyperphagia selectively in high fat diet (HFD)-fed mice. JNK3 deficiency in neurons that express the leptin receptor LEPRb was sufficient to cause HFD-dependent hyperphagia. Studies of sub-groups of leptin-responsive neurons demonstrated that JNK3 deficiency in AgRP neurons, but not POMC neurons, was sufficient to cause the hyperphagic response. These effects of JNK3 deficiency were associated with enhanced excitatory signaling by AgRP neurons in HFD-fed mice. JNK3 therefore provides a mechanism that contributes to homeostatic regulation of energy balance in response to metabolic stress.

DOI:http://dx.doi.org/10.7554/eLife.10031.001

Consuming the right amount of food is important for health. Eating too little for a long time causes damage to organs, and overeating can cause harm as well, in the form of conditions such as obesity and type 2 diabetes. Several signaling molecules and brain regions are linked to controlling food consumption and ensuring the body receives the correct amount of nutrients to fuel its activities.

Previous studies have found that two proteins called JNK1 and JNK2, which are found in most tissues of the body, can reduce how much energy cells use. This can trigger insulin resistance and fat accumulation, and so suggests that blocking the activity of these proteins may help to treat type 2 diabetes and obesity. However, the role of another JNK protein – JNK3, which is mostly found in the brain – was not known.

Now, Vernia, Morel et al. have investigated the role of JNK3 in metabolism. It was found that JNK3 reduced the amount of food consumed by mice provided with a cafeteria (high fat) diet. Mice that lacked JNK3 ate far more food and gained more weight on a high fat diet than normal mice. However, JNK3 played no role in food consumption when mice were fed a standard chow diet. Treating normal mice with leptin – an appetite-suppressing hormone – caused them to lose weight, but did not affect mice that lacked JNK3.

Examining the brains of the mice revealed that in normal mice, JNK3 in a specific sub-population of neurons decreases the production of proteins that promote eating. However, the proteins continued to be produced in mice that lacked JNK3, encouraging overeating.

Overall, the results suggest that blocking the activity of all the JNK proteins will not help treat obesity and diabetes as shutting down JNK3 could encourage overeating. Therefore, future investigation into treatments for these conditions should focus on drugs that specifically target JNK1 and JNK2, and not JNK3.

DOI:http://dx.doi.org/10.7554/eLife.10031.002

A parabrachial-hypothalamic cholecystokinin neurocircuit controls counterregulatory responses to hypoglycemia

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 (CCK(LPBN)) 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 (SF1(VMH)) are the specific target of CCK(LPBN) glucoregulatory neurons. This discrete CCK(LPBN)→SF1(VMH) neurocircuit is both necessary and sufficient for the induction of CR-responses. Together, these data identify CCK(LPBN) neurons, and specifically CCK neuropeptide, as glucoregulatory and provide significant insight into the homeostatic mechanisms controlling CR-responses to hypoglycemia.

Epigenetic therapy for Friedreich ataxia

OBJECTIVE

To investigate whether a histone deacetylase inhibitor (HDACi) would be effective in an in vitro model for the neurodegenerative disease Friedreich ataxia (FRDA) and to evaluate safety and surrogate markers of efficacy in a phase I clinical trial in patients.

METHODS

We used a human FRDA neuronal cell model, derived from patient induced pluripotent stem cells, to determine the efficacy of a 2-aminobenzamide HDACi (109) as a modulator of FXN gene expression and chromatin histone modifications. FRDA patients were dosed in 4 cohorts, ranging from 30mg/day to 240mg/day of the formulated drug product of HDACi 109, RG2833. Patients were monitored for adverse effects as well as for increases in FXN mRNA, frataxin protein, and chromatin modification in blood cells.

RESULTS

In the neuronal cell model, HDACi 109/RG2833 increases FXN mRNA levels and frataxin protein, with concomitant changes in the epigenetic state of the gene. Chromatin signatures indicate that histone H3 lysine 9 is a key residue for gene silencing through methylation and reactivation through acetylation, mediated by the HDACi. Drug treatment in FRDA patients demonstrated increased FXN mRNA and H3 lysine 9 acetylation in peripheral blood mononuclear cells. No safety issues were encountered.

INTERPRETATION

Drug exposure inducing epigenetic changes in neurons in vitro is comparable to the exposure required in patients to see epigenetic changes in circulating lymphoid cells and increases in gene expression. These findings provide a proof of concept for the development of an epigenetic therapy for this fatal neurological disease.

An excitatory paraventricular nucleus to AgRP neuron circuit that drives hunger

Hunger is a hard-wired motivational state essential for survival. Agouti-related peptide (AgRP)-expressing neurons in the arcuate nucleus (ARC) at the base of the hypothalamus are crucial to the control of hunger. They are activated by caloric deficiency and, when naturally or artificially stimulated, they potently induce intense hunger and subsequent food intake. Consistent with their obligatory role in regulating appetite, genetic ablation or chemogenetic inhibition of AgRP neurons decreases feeding. Excitatory input to AgRP neurons is important in caloric-deficiency-induced activation, and is notable for its remarkable degree of caloric-state-dependent synaptic plasticity. Despite the important role of excitatory input, its source(s) has been unknown. Here, through the use of Cre-recombinase-enabled, cell-specific neuron mapping techniques in mice, we have discovered strong excitatory drive that, unexpectedly, emanates from the hypothalamic paraventricular nucleus, specifically from subsets of neurons expressing thyrotropin-releasing hormone (TRH) and pituitary adenylate cyclase-activating polypeptide (PACAP, also known as ADCYAP1). Chemogenetic stimulation of these afferent neurons in sated mice markedly activates AgRP neurons and induces intense feeding. Conversely, acute inhibition in mice with caloric-deficiency-induced hunger decreases feeding. Discovery of these afferent neurons capable of triggering hunger advances understanding of how this intense motivational state is regulated.

Presynaptic and postsynaptic NMDA receptors mediate distinct effects of brain-derived neurotrophic factor on synaptic transmission

In addition to its effects on neuronal survival and differentiation, brain-derived neurotrophic factor (BDNF) plays an important role in modulating synaptic transmission and plasticity in many brain areas, most notably the neocortex and hippocampus. These effects may underlie a role for BDNF in learning and memory as well as developmental plasticity. Consistent with localization of the tropomyosin-related kinase B receptor to both sides of the synapse, BDNF appears to have pre- and postsynaptic effects, but the underlying cellular mechanisms are unclear and it is not known whether pre- and postsynaptic modulations by BDNF occur simultaneously. To address these issues, we recorded dual-component (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-D-aspartate [NMDA]) miniature excitatory postsynaptic currents (mEPSCs) from cortical and hippocampal pyramidal neurons and dentate gyrus granule cells from acute brain slices. BDNF had no effect on the fast component of mEPSC decay or on the peak amplitude, suggesting that BDNF did not modulate postsynaptic AMPA receptors, although BDNF rapidly modulated NMDA receptors, as seen by an enhancement of the slow component of mEPSC decay that was prevented by blocking postsynaptic NMDA receptors. At the same time, BDNF acted presynaptically to enhance mEPSC frequency. Surprisingly, the effect on frequency was also NMDA receptor dependent, but required activation of presynaptic, not postsynaptic, NMDA receptors. BDNF also enhanced action potential-dependent glutamate release via presynaptic NMDA receptors, an effect that was unmasked when voltage-gated calcium channels were partially inhibited. Our results indicate that BDNF acutely modulates presynaptic release and postsynaptic responsiveness through simultaneous effects on pre- and postsynaptic NMDA receptors.