Dopamine in the nucleus accumbens shell controls systemic glucose metabolism via the lateral hypothalamus and hepatic vagal innervation in rodents.

BACKGROUND: Growing evidence demonstrates the role of the striatal dopamine system in the regulation of glucose metabolism. Treatment with dopamine antagonists is associated with insulin resistance and hyperglycemia, while dopamine agonists are used in treatment of type 2 diabetes. The mechanism underlying striatal dopamine effects in glucose metabolism, however is not fully understood. Here, we provide mechanistic insights into the role of nucleus accumbens shell (sNAc) dopaminergic signaling in systemic glucose metabolism. METHODS: Endogenous glucose production (EGP), blood glucose and mRNA expression in the lateral hypothalamic area (LHA) in male Wistar rats were measured following infusion of vanoxerine (VNX, dopamine reuptake inhibitor) in the sNAc. Thereafter, we analyzed projections from sNAc Drd1-expressing neurons to LHA using D1-Cre male Long-Evans rats, Cre-dependent viral tracers and fluorescence immunohistochemistry. Brain slice electrophysiology in adult mice was used to study spontaneous excitatory postsynaptic currents of sNAc Drd1-expressing neurons following VNX application. Finally, we assessed whether GABAergic LHA activity and hepatic vagal innervation were required for the effect of sNAc-VNX on glucose metabolism by combining infusion of sNAc-VNX with LHA-bicuculline, performing vagal recordings and combining infusion of sNAc-VNX with hepatic vagal denervation. RESULTS: VNX infusion in the sNAc strongly decreased endogenous glucose production, prevented glucose increases over time, reduced Slc17A6 and Hcrt mRNA in LHA, and increased vagal activity. Furthermore, sNAc Drd1-expressing neurons increased spontaneous firing following VNX application, and viral tracing of sNAc Drd1-expressing neurons revealed direct projections to LHA with on average 67 % of orexin cells directly targeted by sNAc Drd1-expressing neurons. Importantly, the sNAc-VNX-induced effect on glucose metabolism was dependent on GABAergic signaling in the LHA and on intact hepatic vagal innervation. CONCLUSIONS: We show that sNAc dopaminergic signaling modulates hepatic glucose metabolism through GABAergic inputs to glutamatergic LHA cells and hepatic vagal innervation. This demonstrates that striatal control of glucose metabolism involves a dopaminergic sNAc-LHA-liver axis and provides a potential explanation for the effects of dopamine agonists and antagonists on glucose metabolism.

Anterior hypothalamic parvalbumin neurons are glutamatergic and promote escape behavior.

The anterior hypothalamic area (AHA) is a critical structure for defensive responding. Here, we identified a cluster of parvalbumin-expressing neurons in the AHA (AHAPV) that are glutamatergic with fast-spiking properties and send axonal projections to the dorsal premammillary nucleus (PMD). Using in vivo functional imaging, optogenetics, and behavioral assays, we determined the role of these AHAPV neurons in regulating behaviors essential for survival. We observed that AHAPV neuronal activity significantly increases when mice are exposed to a predator, and in a real-time place preference assay, we found that AHAPV neuron photoactivation is aversive. Moreover, activation of both AHAPV neurons and the AHAPV → PMD pathway triggers escape responding during a predator-looming test. Furthermore, escape responding is impaired after AHAPV neuron ablation, and anxiety-like behavior as measured by the open field and elevated plus maze assays does not seem to be affected by AHAPV neuron ablation. Finally, whole-brain metabolic mapping using positron emission tomography combined with AHAPV neuron photoactivation revealed discrete activation of downstream areas involved in arousal, affective, and defensive behaviors including the amygdala and the substantia nigra. Our results indicate that AHAPV neurons are a functional glutamatergic circuit element mediating defensive behaviors, thus expanding the identity of genetically defined neurons orchestrating fight-or-flight responses. Together, our work will serve as a foundation for understanding neuropsychiatric disorders triggered by escape such as post-traumatic stress disorder (PTSD).

Acts of appetite: neural circuits governing the appetitive, consummatory, and terminating phases of feeding.

The overconsumption of highly caloric and palatable foods has caused a surge in obesity rates in the past half century, thereby posing a healthcare challenge due to the array of comorbidities linked to heightened body fat accrual. Developing treatments to manage body weight requires a grasp of the neurobiological basis of appetite. In this Review, we discuss advances in neuroscience that have identified brain regions and neural circuits that coordinate distinct phases of eating: food procurement, food consumption, and meal termination. While pioneering work identified several hypothalamic nuclei to be involved in feeding, more recent studies have explored how neuronal populations beyond the hypothalamus, such as the mesolimbic pathway and nodes in the hindbrain, interconnect to modulate appetite. We also examine how long-term exposure to a calorically dense diet rewires feeding circuits and alters the response of motivational systems to food. Understanding how the nervous system regulates eating behaviour will bolster the development of medical strategies that will help individuals to maintain a healthy body weight.

Regulation of body weight and food intake by AGRP neurons during opioid dependence and abstinence in mice.

Dysregulation of body weight maintenance and opioid dependence are often treated as independent disorders. Here, we assessed the effects of both acute and long-term administration of morphine with and without chemogenetic activation of agouti-related peptide (AGRP)-expressing neurons in the arcuate nucleus (ARCAGRP neurons) to elucidate whether morphine and neuronal activation affect feeding behavior and body weight. First, we characterized interactions of opioids and energy deficit in wild-type mice. We observed that opioid administration attenuated both fasting-induced refeeding and ghrelin-stimulated feeding. Moreover, antagonism of opioid receptors blocked fasting-induced refeeding behavior. Next, we interfaced chemogenetics with opioid dependence. For chemogenetic experiments of ARCAGRP neurons, we conducted a priori behavioral qualification and post-mortem FOS immunostaining verification of arcuate activation following ARCAGRP chemogenetic activation. We administered clozapine during short-term and long-term morphine administration paradigms to determine the effects of dependence on food intake and body weight. We found that morphine occluded feeding behavior characteristic of chemogenetic activation of ARCAGRP neurons. Notably, activation of ARCAGRP neurons attenuated opioid-induced weight loss but did not evoke weight gain during opioid dependence. Consistent with these findings, we observed that morphine administration did not block fasting-induced activation of the ARC. Together, these results highlight the strength of opioidergic effects on body weight maintenance and demonstrate the utility of ARCAGRP neuron manipulations as a lever to influence energy balance throughout the development of opioid dependence.

Illuminating the brain: an interview with Karl Deisseroth.

Yeka Aponte, a principal investigator at the NIH and an adjunct professor at Johns Hopkins Neuroscience, interviewed her mentor and colleague, Karl Deisseroth, research scientist and psychiatrist at Stanford School of Medicine, about his pioneering work in optogenetics and ongoing research.

Hypothalamic control of interoceptive hunger.

While energy balance is critical to survival, many factors influence food intake beyond caloric need or "hunger." Despite this, some neurons that drive feeding in mice are routinely referred to as "hunger neurons," whereas others are not. To understand how specific hypothalamic circuits control interoceptive hunger, we trained mice to discriminate fasted from sated periods. We then manipulated three hypothalamic neuronal populations with well-known effects on feeding while mice performed this task. While activation of ARCAGRP neurons in sated mice caused mice to report being food-restricted, LHVGAT neuron activation or LHVGLUT2 neuron inhibition did not. In contrast, LHVGAT neuron inhibition or LHVGLUT2 neuron activation in fasted mice attenuated natural hunger, whereas ARCAGRP neuron inhibition did not. Each neuronal population evoked distinct effects on food consumption and reward. After satiety- or sickness-induced devaluation, ARCAGRP neurons drove calorie-specific feeding, while LHVGAT neurons drove calorie-indiscriminate food intake. Our data support a role for ARCAGRP neurons in homeostatic feeding and implicate them in driving a hunger-like internal state that directs behavior toward caloric food sources. Moreover, manipulations of LH circuits did not evoke hunger-like effects in sated mice, suggesting that they may govern feeding more related to reward, compulsion, or generalized consumption than to energy balance, but also that these LH circuits can be powerful negative appetite modulators in fasted mice. This study highlights the complexity of hypothalamic feeding regulation and can be used as a framework to characterize how other neuronal circuits affect hunger and identify potential therapeutic targets for eating disorders.

Fluorescence microendoscopy for in vivo deep-brain imaging of neuronal circuits.

Imaging neuronal activity in awake, behaving animals has become a groundbreaking method in neuroscience that has rapidly enhanced our understanding of how the brain works. In vivo microendoscopic imaging has enabled researchers to see inside the brains of experimental animals and thus has emerged as a technology fit to answer many experimental questions. By combining microendoscopy with cutting edge targeting strategies and sophisticated analysis tools, neuronal activity patterns that underlie changes in behavior and physiology can be identified. However, new users may find it challenging to understand the techniques and to leverage this technology to best suit their needs. Here we present a background and overview of the necessary components for performing in vivo optical calcium imaging and offer some detailed guidance for current recommended approaches.

Lateral hypothalamic LEPR neurons drive appetitive but not consummatory behaviors.

Assigning behavioral roles to genetically defined neurons within the lateral hypothalamus (LH) is an ongoing challenge. We demonstrate that a subpopulation of LH GABAergic neurons expressing leptin receptors (LHLEPR) specifically drives appetitive behaviors in mice. Ablation of LH GABAergic neurons (LHVGAT) decreases weight gain and food intake, whereas LHLEPR ablation does not. Appetitive learning in a Pavlovian conditioning paradigm is delayed in LHVGAT-ablated mice but prevented entirely in LHLEPR-ablated mice. Both LHVGAT and LHLEPR neurons bidirectionally modulate reward-related behaviors, but only LHVGAT neurons affect feeding. In the Pavlovian paradigm, only LHLEPR activity discriminates between conditioned cues. Optogenetic activation or inhibition of either population in this task disrupts discrimination. However, manipulations of LHLEPR→VTA projections evoke divergent effects on responding. Unlike food-oriented learning, chemogenetic inhibition of LHLEPR neurons does not alter cocaine-conditioned place preference but attenuates cocaine sensitization. Thus, LHLEPR neurons may specifically regulate appetitive behaviors toward non-drug reinforcers.

An excitatory lateral hypothalamic circuit orchestrating pain behaviors in mice.

Understanding how neuronal circuits control nociceptive processing will advance the search for novel analgesics. We use functional imaging to demonstrate that lateral hypothalamic parvalbumin-positive (LHPV) glutamatergic neurons respond to acute thermal stimuli and a persistent inflammatory irritant. Moreover, their chemogenetic modulation alters both pain-related behavioral adaptations and the unpleasantness of a noxious stimulus. In two models of persistent pain, optogenetic activation of LHPV neurons or their ventrolateral periaqueductal gray area (vlPAG) axonal projections attenuates nociception, and neuroanatomical tracing reveals that LHPV neurons preferentially target glutamatergic over GABAergic neurons in the vlPAG. By contrast, LHPV projections to the lateral habenula regulate aversion but not nociception. Finally, we find that LHPV activation evokes additive to synergistic antinociceptive interactions with morphine and restores morphine antinociception following the development of morphine tolerance. Our findings identify LHPV neurons as a lateral hypothalamic cell type involved in nociception and demonstrate their potential as a target for analgesia.

Glutamatergic fast-spiking parvalbumin neurons in the lateral hypothalamus: Electrophysiological properties to behavior.

Throughout the central nervous system, neurons expressing the calcium-binding protein parvalbumin have been typically classified as GABAergic with fast-spiking characteristics. However, new methods that allow systematic characterization of the cytoarchitectural organization, connectivity, activity patterns, neurotransmitter nature, and function of genetically-distinct cell types have revealed populations of parvalbumin-positive neurons that are glutamatergic. Remarkably, such findings challenge longstanding concepts that fast-spiking neurons are exclusively GABAergic, suggesting conservation of the fast-spiking phenotype across at least two neurotransmitter systems. This review focuses on the recent advancements that have begun to reveal the functional roles of lateral hypothalamic parvalbumin-positive neurons in regulating behaviors essential for survival.

Lateral hypothalamic fast-spiking parvalbumin neurons modulate nociception through connections in the periaqueductal gray area.

A pivotal role of the lateral hypothalamus (LH) in regulating appetitive and reward-related behaviors has been evident for decades. However, the contributions of LH circuits to other survival behaviors have been less explored. Here we examine how lateral hypothalamic neurons that express the calcium-binding protein parvalbumin (PVALB; LHPV neurons), a small cluster of neurons within the LH glutamatergic circuitry, modulate nociception in mice. We find that photostimulation of LHPV neurons suppresses nociception to an acute, noxious thermal stimulus, whereas photoinhibition potentiates thermal nociception. Moreover, we demonstrate that LHPV axons form functional excitatory synapses on neurons in the ventrolateral periaqueductal gray (vlPAG), and photostimulation of these axons mediates antinociception to both thermal and chemical visceral noxious stimuli. Interestingly, this antinociceptive effect appears to occur independently of opioidergic mechanisms, as antagonism of µ-opioid receptors with systemically-administered naltrexone does not abolish the antinociception evoked by activation of this LHPV→vlPAG pathway. This study directly implicates LHPV neurons in modulating nociception, thus expanding the repertoire of survival behaviors regulated by LH circuits.

High-throughput synapse-resolving two-photon fluorescence microendoscopy for deep-brain volumetric imaging in vivo.

Optical imaging has become a powerful tool for studying brains in vivo. The opacity of adult brains makes microendoscopy, with an optical probe such as a gradient index (GRIN) lens embedded into brain tissue to provide optical relay, the method of choice for imaging neurons and neural activity in deeply buried brain structures. Incorporating a Bessel focus scanning module into two-photon fluorescence microendoscopy, we extended the excitation focus axially and improved its lateral resolution. Scanning the Bessel focus in 2D, we imaged volumes of neurons at high-throughput while resolving fine structures such as synaptic terminals. We applied this approach to the volumetric anatomical imaging of dendritic spines and axonal boutons in the mouse hippocampus, and functional imaging of GABAergic neurons in the mouse lateral hypothalamus in vivo.

Activation of a lateral hypothalamic-ventral tegmental circuit gates motivation.

Across species, motivated states such as food-seeking and consumption are essential for survival. The lateral hypothalamus (LH) is known to play a fundamental role in regulating feeding and reward-related behaviors. However, the contributions of neuronal subpopulations in the LH have not been thoroughly identified. Here we examine how lateral hypothalamic leptin receptor-expressing (LHLEPR) neurons, a subset of GABAergic cells, regulate motivation in mice. We find that LHLEPR neuronal activation significantly increases progressive ratio (PR) performance, while inhibition decreases responding. Moreover, we mapped LHLEPR axonal projections and demonstrated that they target the ventral tegmental area (VTA), form functional inhibitory synapses with non-dopaminergic VTA neurons, and their activation promotes motivation for food. Finally, we find that LHLEPR neurons also regulate motivation to obtain water, suggesting that they may play a generalized role in motivation. Together, these results identify LHLEPR neurons as modulators within a hypothalamic-ventral tegmental circuit that gates motivation.

Electrophysiological properties and projections of lateral hypothalamic parvalbumin positive neurons.

Cracking the cytoarchitectural organization, activity patterns, and neurotransmitter nature of genetically-distinct cell types in the lateral hypothalamus (LH) is fundamental to develop a mechanistic understanding of how activity dynamics within this brain region are generated and operate together through synaptic connections to regulate circuit function. However, the precise mechanisms through which LH circuits orchestrate such dynamics have remained elusive due to the heterogeneity of the intermingled and functionally distinct cell types in this brain region. Here we reveal that a cell type in the mouse LH identified by the expression of the calcium-binding protein parvalbumin (PVALB; LHPV) is fast-spiking, releases the excitatory neurotransmitter glutamate, and sends long range projections throughout the brain. Thus, our findings challenge long-standing concepts that define neurons with a fast-spiking phenotype as exclusively GABAergic. Furthermore, we provide for the first time a detailed characterization of the electrophysiological properties of these neurons. Our work identifies LHPV neurons as a novel functional component within the LH glutamatergic circuitry.

The nutrient sensor OGT in PVN neurons regulates feeding.

Maintaining energy homeostasis is crucial for the survival and health of organisms. The brain regulates feeding by responding to dietary factors and metabolic signals from peripheral organs. It is unclear how the brain interprets these signals. O-GlcNAc transferase (OGT) catalyzes the posttranslational modification of proteins by O-GlcNAc and is regulated by nutrient access. Here, we show that acute deletion of OGT from αCaMKII-positive neurons in adult mice caused obesity from overeating. The hyperphagia derived from the paraventricular nucleus (PVN) of the hypothalamus, where loss of OGT was associated with impaired satiety. These results identify O-GlcNAcylation in αCaMKII neurons of the PVN as an important molecular mechanism that regulates feeding behavior.

Minimally invasive microendoscopy system for in vivo functional imaging of deep nuclei in the mouse brain.

The ability to image neurons anywhere in the mammalian brain is a major goal of optical microscopy. Here we describe a minimally invasive microendoscopy system for studying the morphology and function of neurons at depth. Utilizing a guide cannula with an ultrathin wall, we demonstrated in vivo two-photon fluorescence imaging of deeply buried nuclei such as the striatum (2.5 mm depth), substantia nigra (4.4 mm depth) and lateral hypothalamus (5.0 mm depth) in mouse brain. We reported, for the first time, the observation of neuronal activity with subcellular resolution in the lateral hypothalamus and substantia nigra of head-fixed awake mice.