GABAergic RIP-Cre neurons in the arcuate nucleus selectively regulate energy expenditure.

Neural regulation of energy expenditure is incompletely understood. By genetically disrupting GABAergic transmission in a cell-specific fashion, and by combining this with selective pharmacogenetic activation and optogenetic mapping techniques, we have uncovered an arcuate-based circuit that selectively drives energy expenditure. Specifically, mice lacking synaptic GABA release from RIP-Cre neurons have reduced energy expenditure, become obese and are extremely sensitive to high-fat diet-induced obesity, the latter due to defective diet-induced thermogenesis. Leptin's ability to stimulate thermogenesis, but not to reduce feeding, is markedly attenuated. Acute, selective activation of arcuate GABAergic RIP-Cre neurons, which monosynaptically innervate PVH neurons projecting to the NTS, rapidly stimulates brown fat and increases energy expenditure but does not affect feeding. Importantly, this response is dependent upon GABA release from RIP-Cre neurons. Thus, GABAergic RIP-Cre neurons in the arcuate selectively drive energy expenditure, contribute to leptin's stimulatory effect on thermogenesis, and protect against diet-induced obesity.

MC4R-expressing glutamatergic neurons in the paraventricular hypothalamus regulate feeding and are synaptically connected to the parabrachial nucleus.

Activation of melanocortin-4 receptors (MC4Rs) restrains feeding and prevents obesity; however, the identity, location, and axonal projections of the neurons bearing MC4Rs that control feeding remain unknown. Reexpression of MC4Rs on single-minded 1 (SIM1)(+) neurons in mice otherwise lacking MC4Rs is sufficient to abolish hyperphagia. Thus, MC4Rs on SIM1(+) neurons, possibly in the paraventricular hypothalamus (PVH) and/or amygdala, regulate food intake. It is unknown, however, whether they are also necessary, a distinction required for excluding redundant sites of action. Hence, the location and nature of obesity-preventing MC4R-expressing neurons are unknown. Here, by deleting and reexpressing MC4Rs from cre-expressing neurons, establishing both necessity and sufficiency, we demonstrate that the MC4R-expressing neurons regulating feeding are SIM1(+), located in the PVH, glutamatergic and not GABAergic, and do not express oxytocin, corticotropin-releasing hormone, vasopressin, or prodynorphin. Importantly, these excitatory MC4R-expressing PVH neurons are synaptically connected to neurons in the parabrachial nucleus, which relays visceral information to the forebrain. This suggests a basis for the feeding-regulating effects of MC4Rs.

Synaptic glutamate release by ventromedial hypothalamic neurons is part of the neurocircuitry that prevents hypoglycemia.

The importance of neuropeptides in the hypothalamus has been experimentally established. Due to difficulties in assessing function in vivo, the roles of the fast-acting neurotransmitters glutamate and GABA are largely unknown. Synaptic vesicular transporters (VGLUTs for glutamate and VGAT for GABA) are required for vesicular uptake and, consequently, synaptic release of neurotransmitters. Ventromedial hypothalamic (VMH) neurons are predominantly glutamatergic and express VGLUT2. To evaluate the role of glutamate release from VMH neurons, we generated mice lacking VGLUT2 selectively in SF1 neurons (a major subset of VMH neurons). These mice have hypoglycemia during fasting secondary to impaired fasting-induced increases in the glucose-raising pancreatic hormone glucagon and impaired induction in liver of mRNAs encoding PGC-1alpha and the gluconeogenic enzymes PEPCK and G6Pase. Similarly, these mice have defective counterregulatory responses to insulin-induced hypoglycemia and 2-deoxyglucose (an antimetabolite). Thus, glutamate release from VMH neurons is an important component of the neurocircuitry that functions to prevent hypoglycemia.

Acute effects of leptin require PI3K signaling in hypothalamic proopiomelanocortin neurons in mice.

Normal food intake and body weight homeostasis require the direct action of leptin on hypothalamic proopiomelanocortin (POMC) neurons. It has been proposed that leptin action requires PI3K activity. We therefore assessed the contribution of PI3K signaling to leptin's effects on POMC neurons and organismal energy balance. Leptin caused a rapid depolarization of POMC neurons and an increase in action potential frequency in patch-clamp recordings of hypothalamic slices. Pharmacologic inhibition of PI3K prevented this depolarization and increased POMC firing rate, indicating a PI3K-dependent mechanism of leptin action. Mice with genetically disrupted PI3K signaling in POMC cells failed to undergo POMC depolarization or increased firing frequency in response to leptin. Insulin's ability to hyperpolarize POMC neurons was also abolished in these mice. Moreover, targeted disruption of PI3K blunted the suppression of feeding elicited by central leptin administration. Despite these differences, mice with impaired PI3K signaling in POMC neurons exhibited normal long-term body weight regulation. Collectively, these results suggest that PI3K signaling in POMC neurons is essential for leptin-induced activation and insulin-induced inhibition of POMC cells and for the acute suppression of food intake elicited by leptin, but is not a major contributor to the regulation of long-term organismal energy homeostasis.

Leptin directly activates SF1 neurons in the VMH, and this action by leptin is required for normal body-weight homeostasis.

Leptin, an adipocyte-derived hormone, acts directly on the brain to control food intake and energy expenditure. An important question is the identity of first-order neurons initiating leptin's anti-obesity effects. A widely held view is that most, if not all, of leptin's effects are mediated by neurons located in the arcuate nucleus of the hypothalamus. However, leptin receptors (LEPRs) are expressed in other sites as well, including the ventromedial hypothalamus (VMH). The possible role of leptin acting in "nonarcuate" sites has largely been ignored. In the present study, we show that leptin depolarizes and increases the firing rate of steroidogenic factor-1 (SF1)-positive neurons in the VMH. We also show, by generating mice that lack LEPRs on SF1-positive neurons, that leptin action at this site plays an important role in reducing body weight and, of note, in resisting diet-induced obesity. These results reveal a critical role for leptin action on VMH neurons.

The calcium-sensing receptor (CaR) is involved in strontium ranelate-induced osteoblast proliferation.

Strontium ranelate has several beneficial effects on bone and reduces the risk of vertebral and hip fractures in women with postmenopausal osteoporosis. We investigated whether Sr(2+) acts via a cell surface calcium-sensing receptor (CaR) in HEK293 cells stably transfected with the bovine CaR (HEK-CaR) and rat primary osteoblasts (POBs) expressing the CaR endogenously. Elevating Ca(o)(2+) or Sr(2+) concentration-dependently activated the CaR in HEK-CaR but not in non-transfected cells, but the potency of Sr(2+) varied depending on the biological response tested. Sr(2+) was less potent than Ca(o)(2+) in stimulating inositol phosphate accumulation and in increasing Ca(i)(2+), but was comparable to Ca(o)(2+) in stimulating ERK phosphorylation and a non-selective cation channel, suggesting that Ca(2+) and Sr(2+) have differential effects on specific cellular processes. With physiological concentrations of Ca(o)(2+), Sr(2+)-induced further CaR activation. Neither Sr(2+) nor Ca(o)(2+) affected the four parameters just described in non-transfected cells. In POB, Sr(2+) stimulated cellular proliferation. This effect was CaR-mediated, as transfecting the cells with a dominant negative bovine CaR significantly attenuated Ca(o)(2+)-stimulated POB proliferation. Finally, Sr(2+) significantly increased the mRNA levels of the immediate early genes, c-fos and egr-1, which are involved in POB proliferation, and this effect was attenuated by overexpressing the dominant negative CaR. In conclusion, Sr(2+) is a full CaR agonist in HEK-CaR and POB, and, therefore, the anabolic effect of Sr(2+) on bone in vivo could be mediated, in part, by the CaR.

Amyloid beta-protein induced electrophysiological changes are dependent on aggregation state: N-methyl-D-aspartate (NMDA) versus non-NMDA receptor/channel activation.

Alzheimer's disease (AD) is a progressive neurodegenerative disease, however, the underlying mechanism driving this condition is unknown. Unexplored is the possibility that the time-dependent generation of different Abeta assemblies may underlie the pathogenic cascade with biophysically distinct structures interacting with unique biological targets. Thus, the presence of subtle alterations in synaptic function during the earliest clinical phase of AD may be mediated by diffusible assemblies of the amyloid beta-protein (Abeta). Using primary neocortical cultures, here we compare the synaptic responses induced by two different Abeta assemblies, protofibrils (PFs) and fibrils (FBs), and demonstrate for the first time that neuronal activation was selectively dependent on the assembly state of Abeta. PF-induced activity was specifically attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, D-APV. In contrast, the non-NMDA glutamate receptor antagonist, NBQX, preferentially reduced FB-induced activity. In support, removal of Mg(2+) from the medium, which enhances NMDA channels, increased both PF- or FB-induced activation, but D-APV was more effective in attenuating PF-induced excitatory activity. These findings suggest that PFs may activate neurons differently than fibrils and lend support to the hypothesis that pre-fibrillar assemblies of Abeta may play an important role in the development of AD-type synaptic deficits.

ROCK1 in AgRP neurons regulates energy expenditure and locomotor activity in male mice.

Normal leptin signaling is essential for the maintenance of body weight homeostasis. Proopiomelanocortin- and agouti-related peptide (AgRP)-producing neurons play critical roles in regulating energy metabolism. Our recent work demonstrates that deletion of Rho-kinase 1 (ROCK1) in the AgRP neurons of mice increased body weight and adiposity. Here, we report that selective loss of ROCK1 in AgRP neurons caused a significant decrease in energy expenditure and locomotor activity of mice. These effects were independent of any change in food intake. Furthermore, AgRP neuron-specific ROCK1-deficient mice displayed central leptin resistance, as evidenced by impaired Signal Transducer and Activator of Transcription 3 activation in response to leptin administration. Leptin's ability to hyperpolarize and decrease firing rate of AgRP neurons was also abolished in the absence of ROCK1. Moreover, diet-induced and genetic forms of obesity resulted in reduced ROCK1 activity in murine arcuate nucleus. Of note, high-fat diet also impaired leptin-stimulated ROCK1 activity in arcuate nucleus, suggesting that a defect in hypothalamic ROCK1 activity may contribute to the pathogenesis of central leptin resistance in obesity. Together, these data demonstrate that ROCK1 activation in hypothalamic AgRP neurons is required for the homeostatic regulation of energy expenditure and adiposity. These results further support previous work identifying ROCK1 as a key regulator of energy balance and suggest that targeting ROCK1 in the hypothalamus may lead to development of antiobesity therapeutics.

Fasting activation of AgRP neurons requires NMDA receptors and involves spinogenesis and increased excitatory tone.

AgRP neuron activity drives feeding and weight gain whereas that of nearby POMC neurons does the opposite. However, the role of excitatory glutamatergic input in controlling these neurons is unknown. To address this question, we generated mice lacking NMDA receptors (NMDARs) on either AgRP or POMC neurons. Deletion of NMDARs from AgRP neurons markedly reduced weight, body fat and food intake whereas deletion from POMC neurons had no effect. Activation of AgRP neurons by fasting, as assessed by c-Fos, Agrp and Npy mRNA expression, AMPA receptor-mediated EPSCs, depolarization and firing rates, required NMDARs. Furthermore, AgRP but not POMC neurons have dendritic spines and increased glutamatergic input onto AgRP neurons caused by fasting was paralleled by an increase in spines, suggesting fasting induced synaptogenesis and spinogenesis. Thus glutamatergic synaptic transmission and its modulation by NMDARs play key roles in controlling AgRP neurons and determining the cellular and behavioral response to fasting.

Rho-kinase regulates energy balance by targeting hypothalamic leptin receptor signaling.

Leptin regulates energy balance. However, knowledge of the critical intracellular transducers of leptin signaling remains incomplete. We found that Rho-kinase 1 (ROCK1) regulates leptin action on body weight homeostasis by activating JAK2, an initial trigger of leptin receptor signaling. Leptin promoted the physical interaction of JAK2 and ROCK1, thereby increasing phosphorylation of JAK2 and downstream activation of Stat3 and FOXO1. Mice lacking ROCK1 in either pro-opiomelanocortin (POMC) or agouti-related protein neurons, mediators of leptin action, displayed obesity and impaired leptin sensitivity. In addition, deletion of ROCK1 in the arcuate nucleus markedly enhanced food intake, resulting in severe obesity. Notably, ROCK1 was a specific mediator of leptin, but not insulin, regulation of POMC neuronal activity. Our data identify ROCK1 as a key regulator of leptin action on energy homeostasis.

Leptin action on GABAergic neurons prevents obesity and reduces inhibitory tone to POMC neurons.

Leptin acts in the brain to prevent obesity. The underlying neurocircuitry responsible for this is poorly understood, in part because of incomplete knowledge regarding first-order, leptin-responsive neurons. To address this, we and others have been removing leptin receptors from candidate first-order neurons. While functionally relevant neurons have been identified, the observed effects have been small, suggesting that most first-order neurons remain unidentified. Here we take an alternative approach and test whether first-order neurons are inhibitory (GABAergic, VGAT⁺) or excitatory (glutamatergic, VGLUT2⁺). Remarkably, the vast majority of leptin's antiobesity effects are mediated by GABAergic neurons; glutamatergic neurons play only a minor role. Leptin, working directly on presynaptic GABAergic neurons, many of which appear not to express AgRP, reduces inhibitory tone to postsynaptic POMC neurons. As POMC neurons prevent obesity, their disinhibition by leptin action on presynaptic GABAergic neurons probably mediates, at least in part, leptin's antiobesity effects.

Rapid, reversible activation of AgRP neurons drives feeding behavior in mice.

Several different neuronal populations are involved in regulating energy homeostasis. Among these, agouti-related protein (AgRP) neurons are thought to promote feeding and weight gain; however, the evidence supporting this view is incomplete. Using designer receptors exclusively activated by designer drugs (DREADD) technology to provide specific and reversible regulation of neuronal activity in mice, we have demonstrated that acute activation of AgRP neurons rapidly and dramatically induces feeding, reduces energy expenditure, and ultimately increases fat stores. All these effects returned to baseline after stimulation was withdrawn. In contrast, inhibiting AgRP neuronal activity in hungry mice reduced food intake. Together, these findings demonstrate that AgRP neuron activity is both necessary and sufficient for feeding. Of interest, activating AgRP neurons potently increased motivation for feeding and also drove intense food-seeking behavior, demonstrating that AgRP neurons engage brain sites controlling multiple levels of feeding behavior. Due to its ease of use and suitability for both acute and chronic regulation, DREADD technology is ideally suited for investigating the neural circuits hypothesized to regulate energy balance.

Glucose stimulation of hypothalamic MCH neurons involves K(ATP) channels, is modulated by UCP2, and regulates peripheral glucose homeostasis.

Blood glucose levels are tightly controlled, a process thought to be orchestrated primarily by peripheral mechanisms (insulin secretion by ß cells, and insulin action on muscle, fat, and liver). The brain also plays an important, albeit less well-defined role. Subsets of neurons in the brain are excited by glucose; in many cases this involves ATP-mediated closure of K(ATP) channels. To understand the relevance of this, we are manipulating glucose sensing within glucose-excited neurons. In the present study, we demonstrate that glucose excitation of MCH-expressing neurons in the lateral hypothalamus is mediated by K(ATP) channels and is negatively regulated by UCP2 (a mitochondrial protein that reduces ATP production), and that glucose sensing by MCH neurons plays an important role in regulating glucose homeostasis. Combined, the glucose-excited neurons are likely to play key, previously unexpected roles in regulating blood glucose.

Synaptic release of GABA by AgRP neurons is required for normal regulation of energy balance.

The physiologic importance of GABAergic neurotransmission in hypothalamic neurocircuits is unknown. To examine the importance of GABA release from agouti-related protein (AgRP) neurons (which also release AgRP and neuropeptide Y), we generated mice with an AgRP neuron-specific deletion of vesicular GABA transporter. These mice are lean, resistant to obesity and have an attenuated hyperphagic response to ghrelin. Thus, GABA release from AgRP neurons is important in regulating energy balance.