Jon M. Resch

A molecular census of arcuate hypothalamus and median eminence cell types

The hypothalamic arcuate–median eminence complex (Arc-ME) controls energy balance, fertility and growth through molecularly distinct cell types, many of which remain unknown. To catalog cell types in an unbiased way, we profiled gene expression in 20,921 individual cells in and around the adult mouse Arc-ME using Drop-seq. We identify 50 transcriptionally distinct Arc-ME cell populations, including a rare tanycyte population at the Arc-ME diffusion barrier, a new leptin-sensing neuron population, multiple agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) subtypes, and an orexigenic somatostatin neuron population. We extended Drop-seq to detect dynamic expression changes across relevant physiological perturbations, revealing cell type–specific responses to energy status, including distinct responses in AgRP and POMC neuron subtypes. Finally, integrating our data with human genome-wide association study data implicates two previously unknown neuron populations in the genetic control of obesity. This resource will accelerate biological discovery by providing insights into molecular and cell type diversity from which function can be inferred.

A rapidly acting glutamatergic ARC→PVH satiety circuit postsynaptically regulated by α-MSH.

Arcuate nucleus (ARC) neurons sense the fed or fasted state and regulate hunger. Agouti-related protein (AgRP) neurons in the ARC (ARCAgRP neurons) are stimulated by fasting and, once activated, they rapidly (within minutes) drive hunger. Pro-opiomelanocortin (ARCPOMC) neurons are viewed as the counterpoint to ARCAgRP neurons. They are regulated in an opposite fashion and decrease hunger. However, unlike ARCAgRP neurons, ARCPOMC neurons are extremely slow in affecting hunger (many hours). Thus, a temporally analogous, rapid ARC satiety pathway does not exist or is presently unidentified. Here we show that glutamate-releasing ARC neurons expressing oxytocin receptor, unlike ARCPOMC neurons, rapidly cause satiety when chemo- or optogenetically manipulated. These glutamatergic ARC projections synaptically converge with GABAergic ARCAgRP projections on melanocortin-4 receptor (MC4R)-expressing satiety neurons in the paraventricular hypothalamus (PVHMC4R neurons). Transmission across the ARCGlutamatergic→PVHMC4R synapse is potentiated by the ARCPOMC neuron-derived MC4R agonist, α-melanocyte stimulating hormone (α-MSH). This excitatory ARC→PVH satiety circuit, and its modulation by α-MSH, provides insight into regulation of hunger and satiety.

Activation of the Nrf2-ARE pathway in muscle and spinal cord during ALS-like pathology in mice expressing mutant SOD1

Oxidative stress plays a key role in the neuronal loss exhibited in amyotrophic lateral sclerosis (ALS), an event precipitating irreversible muscle atrophy. By crossing ALS mouse models (SOD(G93A) and SOD(H46RH48Q)) with an antioxidant response element (ARE) reporter mouse, we identified activation characteristics of the ARE system throughout the timecourse of motor neuron disease. Surprisingly, the earliest and most significant activation of this genetic sensor of oxidative stress occurred in the distal muscles of mutant SOD mice. The resultant data supports existing hypotheses that the muscle is somehow implicated during the initial pathology of these mice. Subsequently, Nrf2-ARE activation appears to progress in a retrograde fashion along the motor pathway. These data provide timely information concerning the contributions of the Nrf2-ARE pathway in ALS disease progression.

Stimulation of the hypothalamic ventromedial nuclei by pituitary adenylate cyclase-activating polypeptide induces hypophagia and thermogenesis

Numerous studies have demonstrated that the hypothalamic ventromedial nuclei (VMN) regulate energy homeostasis by integrating and utilizing behavioral and metabolic mechanisms. The VMN heavily express pituitary adenylate cyclase-activating polypeptide (PACAP) type I receptors (PAC1R). Despite the receptor distribution, most PACAP experiments investigating affects on feeding have focused on intracerebroventricular administration or global knockout mice. To identify the specific contribution of PACAP signaling in the VMN, we injected PACAP directly into the VMN and measured feeding behavior and indices of energy expenditure. Following an acute injection of PACAP, nocturnal food intake was significantly reduced for 6 h after injections without evidence of malaise. In addition, PACAP-induced suppression of feeding also occurred following an overnight fast and could be blocked by a specific PAC1R antagonist. Metabolically, VMN-specific injections of PACAP significantly increased both core body temperature and spontaneous locomotor activity with a concurrent increase in brown adipose uncoupling protein 1 mRNA expression. To determine which signaling pathways were responsive to PACAP administration into the VMN, we measured mRNA expression of well-characterized hypothalamic neuropeptide regulators of feeding. One hour after PACAP administration, expression of pro-opiomelanocortin mRNA was significantly increased in the arcuate nuclei (ARC), with no changes in neuropeptide Y and agouti-related polypeptide mRNA levels. This suggests that PAC1R expressing VMN neurons projecting to pro-opiomelanocortin neurons contribute to hypophagia by involving melanocortin signaling. While the VMN also abundantly express PACAP protein, the present study demonstrates that PACAP input to the VMN can influence the control of energy homeostasis.

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.

Intrahypothalamic Pituitary Adenylate Cyclase-activating Polypeptide Regulates Energy Balance via Site-specific Actions on Feeding and Metabolism

Numerous studies have demonstrated that both the hypothalamic paraventricular nuclei (PVN) and ventromedial nuclei (VMN) regulate energy homeostasis through behavioral and metabolic mechanisms. Receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are abundantly expressed in these nuclei, suggesting PACAP may be critical for the regulation of feeding behavior and body weight. To characterize the unique behavioral and physiological responses attributed to select hypothalamic cell groups, PACAP was site-specifically injected into the PVN or VMN. Overall food intake was significantly reduced by PACAP at both sites; however, meal pattern analysis revealed that only injections into the PVN produced significant reductions in meal size, duration, and total time spent eating. PACAP-mediated hypophagia in both the PVN and VMN was abolished by PAC1R antagonism, whereas pretreatment with a VPACR antagonist had no effect. PACAP injections into the VMN produced unique changes in metabolic parameters, including significant increases in core body temperature and spontaneous locomotor activity that was PAC1R dependent whereas, PVN injections of PACAP had no effect. Finally, PACAP-containing afferents were identified using the neuronal tracer cholera toxin subunit B (CTB) injected unilaterally into the PVN or VMN. CTB signal from PVN injections was colocalized with PACAP mRNA in the medial anterior bed nucleus of the stria terminalis, VMN, and lateral parabrachial nucleus (LPB), whereas CTB signal from VMN injections was highly colocalized with PACAP mRNA in the medial amygdala and LPB. These brain regions are known to influence energy homeostasis perhaps, in part, through PACAP projections to the PVN and VMN.

Inhibition of Food Intake by PACAP in the Hypothalamic Ventromedial Nuclei is Mediated by NMDA Receptors

Central injections of pituitary adenylate cyclase-activating polypeptide (PACAP) into the ventromedial nuclei (VMN) of the hypothalamus produce hypophagia that is dependent upon the PAC1 receptor; however, the signaling downstream of this receptor in the VMN is unknown. Though PACAP signaling has many targets, this neuropeptide has been shown to influence glutamate signaling in several brain regions through mechanisms involving NMDA receptor potentiation via activation of the Src family of protein tyrosine kinases. With this in mind, we examined the Src-NMDA receptor signaling pathway as a target for PACAP signaling in the VMN that may mediate its effects on feeding behavior. Under nocturnal feeding conditions, NMDA receptor antagonism prior to PACAP administration into the VMN attenuated PACAP-mediated decreases in feeding suggesting that glutamatergic signaling via NMDA receptors is necessary for PACAP-induced hypophagia. Furthermore, PACAP administration into the VMN resulted in increased tyrosine phosphorylation of the GluN2B subunit of the NMDA receptor, and inhibition of Src kinase activity also blocked the effects of PACAP administration into the VMN on feeding behavior. These results indicate that PACAP neurotransmission in the VMN likely augments glutamate signaling by potentiating NMDA receptors activity through the tyrosine phosphorylation events mediated by the Src kinase family, and modulation of NMDA receptor activity by PACAP in the hypothalamus may be a primary mechanism for its regulation of food intake.

Serotonin mediated changes in corticotropin releasing factor mRNA expression and feeding behavior isolated to the hypothalamic paraventricular nuclei.

Fenfluramine reduces hunger and promotes body weight loss by increasing central serotonin (5-HT) signaling. More recently, neuropeptides have been linked to the regulation of feeding behavior, metabolism and body weight. To examine possible interactions between 5-HT and neuropeptides in appetite control, fenfluramine (200 nmol/0.5 μl/side) was administered directly into the hypothalamic paraventricular nuclei (PVN) of male rats. Bilateral fenfluramine produced significant hypophagia and increased expression of PVN corticotropin releasing factor (CRF) mRNA and neuropeptide Y (NPY) mRNA in the arcuate nucleus within the first hour after drug administration. Fenfluramines effects on feeding behavior and mRNA expression were blocked by PVN injections of a 5-HT(1-2) receptor antagonist, metergoline (15 nmol/0.5 μl/side). These data suggest that 5-HT neurons targeting hypothalamic paraventricular CRF neurons may participate in an appetite control circuit for reducing food intake.

Augmented Cystine–Glutamate Exchange by Pituitary Adenylate Cyclase-activating Polypeptide Signaling via the VPAC1 Receptor

In the central nervous system, cystine import in exchange for glutamate through system xc− is critical for the production of the antioxidant glutathione by astrocytes, as well as the maintenance of extracellular glutamate. Therefore, regulation of system xc− activity affects multiple aspects of cellular physiology and may contribute to disease states. Pituitary adenylate cyclase‐activating polypeptide (PACAP) is a neuronally derived peptide that has already been demonstrated to modulate multiple aspects of glutamate signaling suggesting PACAP may also target activity of cystine–glutamate exchange via system xc− . In this study, 24‐h treatment of primary cortical cultures containing neurons and glia with PACAP concentration‐dependently increased system xc− function as measured by radiolabeled cystine uptake. Furthermore, the increase in cystine uptake was completely abolished by the system xc− inhibitor, (S)‐4‐carboxyphenylglycine (CPG), attributing increases in cystine uptake specifically to system xc− activity. Time course and quantitative PCR results indicate that PACAP signaling may increase cystine–glutamate exchange by increasing expression of xCT, the catalytic subunit of system xc− . Furthermore, the potentiation of system xc− activity by PACAP occurs via a PKA‐dependent pathway that is not mediated by the PAC1R, but rather the shared vasoactive intestinal polypeptide receptor VPAC1R. Finally, assessment of neuronal, astrocytic, and microglial‐enriched cultures demonstrated that only astrocyte‐enriched cultures exhibit enhanced cystine uptake following both PACAP and VIP treatment. These data introduce a novel mechanism by which both PACAP and VIP regulate system xc− activity. Synapse 68:604–612, 2014.

N-acetylcysteine decreases binge eating in a rodent model.

Binge-eating behavior involves rapid consumption of highly palatable foods leading to increased weight gain. Feeding in binge disorders resembles other compulsive behaviors, many of which are responsive to N-acetylcysteine (NAC), which is a cysteine prodrug often used to promote non-vesicular glutamate release by a cystine–glutamate antiporter. To examine the potential for NAC to alter a form of compulsive eating, we examined the impact of NAC on binge eating in a rodent model. Specifically, we monitored consumption of standard chow and a high-fat, high carbohydrate western diet (WD) in a rodent limited-access binge paradigm. Before each session, rats received either a systemic or intraventricular injection of NAC. Both systemic and central administration of NAC resulted in significant reductions of binge eating the WD without decreasing standard chow consumption. The reduction in WD was not attributable to general malaise as NAC did not produce condition taste aversion. These results are consistent with the clinical evidence of NAC to reduce or reverse compulsive behaviors, such as, drug addiction, skin picking and hair pulling.