Ventral tegmental area neurotensin signaling links the lateral hypothalamus to locomotor activity and striatal dopamine efflux in male mice

Hungry for relief: Potential for neurotensin to address comorbid obesity and pain

Chronic pain and obesity frequently occur together. An ideal therapy would alleviate pain without weight gain, and most optimally, could promote weight loss. The neuropeptide neurotensin (Nts) has been separately implicated in reducing weight and pain but could it be a common actionable target for both pain and obesity? Here we review the current knowledge of Nts signaling via its receptors in modulating body weight and pain processing. Evaluating the mechanism by which Nts impacts ingestive behavior, body weight, and analgesia has potential to identify common physiologic mechanisms underlying weight and pain comorbidities, and whether Nts may be common actionable targets for both.

Neurotensin receptor 1-biased ligand attenuates neurotensin-mediated excitation of ventral tegmental area dopamine neurons and dopamine release in the nucleus accumbens

Strong expression of the G protein-coupled receptor (GPCR) neurotensin receptor 1 (NTR1) in ventral tegmental area (VTA) dopamine (DA) neurons and terminals makes it an attractive target to modulate DA neuron activity and normalize DA-related pathologies. Recent studies have identified a novel class of NTR1 ligand that shows promising effects in preclinical models of addiction. A lead molecule, SBI-0654553 (SBI-553), can act as a positive allosteric modulator of NTR1 β-arrestin recruitment while simultaneously antagonizing NTR1 Gq protein signaling. Using cell-attached recordings from mouse VTA DA neurons we discovered that, unlike neurotensin (NT), SBI-553 did not independently increase spontaneous firing. Instead, SBI-553 blocked the NT-mediated increase in firing. SBI-553 also antagonized the effects of NT on dopamine D2 auto-receptor signaling, potentially through its inhibitory effects on G-protein signaling. We also measured DA release directly, using fast-scan cyclic voltammetry in the nucleus accumbens and observed antagonist effects of SBI-553 on an NT-induced increase in DA release. Further, in vivo administration of SBI-553 did not notably change basal or cocaine-evoked DA release measured in NAc using fiber photometry. Overall, these results indicate that SBI-553 blunts NT's effects on spontaneous DA neuron firing, D2 auto-receptor function, and DA release, without independently affecting these measures. In the presence of NT, SBI-553 has an inhibitory effect on mesolimbic DA activity, which could contribute to its efficacy in animal models of psychostimulant use.

Circuit coordination of opposing neuropeptide and neurotransmitter signals

Fast-acting neurotransmitters and slow, modulatory neuropeptides are co-released from neurons in the central nervous system, albeit from distinct synaptic vesicles1. The mechanisms of how co-released neurotransmitters and neuropeptides that have opposing actions-for example, stimulatory versus inhibitory-work together to exert control of neural circuit output remain unclear. This has been difficult to resolve owing to the inability to selectively isolate these signalling pathways in a cell- and circuit-specific manner. Here we developed a genetic-based anatomical disconnect procedure that utilizes distinct DNA recombinases to independently facilitate CRISPR-Cas9 mutagenesis2 of neurotransmitter- and neuropeptide-related genes in distinct cell types in two different brain regions simultaneously. We demonstrate that neurons within the lateral hypothalamus that produce the stimulatory neuropeptide neurotensin and the inhibitory neurotransmitter GABA (γ-aminobutyric acid) utilize these signals to coordinately activate dopamine-producing neurons of the ventral tegmental area. We show that GABA release from lateral hypothalamus neurotensin neurons inhibits GABA neurons within the ventral tegmental area, disinhibiting dopamine neurons and causing a rapid rise in calcium, whereas neurotensin directly generates a slow inactivating calcium signal in dopamine neurons that is dependent on the expression of neurotensin receptor 1 (Ntsr1). We further show that these two signals work together to regulate dopamine neuron responses to maximize behavioural responding. Thus, a neurotransmitter and a neuropeptide with opposing signals can act on distinct timescales through different cell types to enhance circuit output and optimize behaviour.

Neuropeptides Modulate Feeding via the Dopamine Reward Pathway

Dopamine (DA) is a catecholamine neurotransmitter widely distributed in the central nervous system. It participates in various physiological functions, such as feeding, anxiety, fear, sleeping and arousal. The regulation of feeding is exceptionally complex, involving energy homeostasis and reward motivation. The reward system comprises the ventral tegmental area (VTA), nucleus accumbens (NAc), hypothalamus, and limbic system. This paper illustrates the detailed mechanisms of eight typical orexigenic and anorexic neuropeptides that regulate food intake through the reward system. According to recent literature, neuropeptides released from the hypothalamus and other brain regions regulate reward feeding predominantly through dopaminergic neurons projecting from the VTA to the NAc. In addition, their effect on the dopaminergic system is mediated by the prefrontal cortex, paraventricular thalamus, laterodorsal tegmental area, amygdala, and complex neural circuits. Research on neuropeptides involved in reward feeding can help identify more targets to treat diseases with metabolic disorders, such as obesity.

Neurotensin Release from Dopamine Neurons Drives Long-Term Depression of Substantia Nigra Dopamine Signaling

Midbrain dopamine neurons play central physiological roles in voluntary movement, reward learning, and motivated behavior. Inhibitory signaling at somatodendritic dopamine D2 receptor (D2R) synapses modulates excitability of dopamine neurons. The neuropeptide neurotensin is expressed by many inputs to the midbrain and induces LTD of D2R synaptic currents (LTD_DA); however, the source of neurotensin that is responsible for LTD_DA is not known. Here we show, in brain slices from male and female mice, that LTD_DA is driven by neurotensin released by dopamine neurons themselves. Optogenetic stimulation of dopamine neurons was sufficient to induce LTD_DA in the substantia nigra, but not the VTA, and was dependent on neurotensin receptor signaling, postsynaptic calcium, and vacuolar-type H⁺-ATPase activity in the postsynaptic cell. These findings reveal a novel form of signaling between dopamine neurons involving release of the peptide neurotensin, which may act as a feedforward mechanism to increase dopamine neuron excitability.SIGNIFICANCE STATEMENT Dopamine neurons in the midbrain play a critical role in reward learning and the initiation of movement. Aberrant dopamine neuron function is implicated in a range of diseases and disorders, including Parkinson's disease, schizophrenia, obesity, and substance use disorders. D2 receptor-mediated PSCs are produced by a rare form of dendrodendritic synaptic transmission between dopamine neurons. These D2 receptor-mediated PSCs undergo LTD following application of the neuropeptide neurotensin. Here we show that release of neurotensin by dopamine neurons themselves is sufficient to induce LTD of dopamine transmission in the substantia nigra. Neurotensin signaling therefore mediates a second form of interdopamine neuron communication and may provide a mechanism by which dopamine neurons maintain excitability when nigral dopamine is elevated.

Time to drink: Activating lateral hypothalamic area neurotensin neurons promotes intake of fluid over food in a time-dependent manner

The lateral hypothalamic area (LHA) is essential for ingestive behavior but has primarily been studied in modulating feeding, with comparatively scant attention on drinking. This is partly because most LHA neurons simultaneously promote feeding and drinking, suggesting that ingestive behaviors track together. A notable exception are LHA neurons expressing neurotensin (LHANts neurons): activating these neurons promotes water intake but modestly restrains feeding. Here we investigated the connectivity of LHANts neurons, their necessity and sufficiency for drinking and feeding, and how timing and resource availability influence their modulation of these behaviors. LHANts neurons project broadly throughout the brain, including to the lateral preoptic area (LPO), a brain region implicated in modulating drinking behavior. LHANts neurons also receive inputs from brain regions implicated in sensing hydration and energy status. While activation of LHANts neurons is not required to maintain homeostatic water or food intake, it selectively promotes drinking during the light cycle, when ingestive drive is low. Activating LHANts neurons during this period also increases willingness to work for water or palatable fluids, regardless of their caloric content. By contrast, LHANts neuronal activation during the dark cycle does not promote drinking, but suppresses feeding during this time. Finally, we demonstrate that the activation of the LHANts → LPO projection is sufficient to mediate drinking behavior, but does not suppress feeding as observed after generally activating all LHANts neurons. Overall, our work suggests how and when LHANts neurons oppositely modulate ingestive behaviors.

The physiological control of eating: signals, neurons, and networks

During the past 30 yr, investigating the physiology of eating behaviors has generated a truly vast literature. This is fueled in part by a dramatic increase in obesity and its comorbidities that has coincided with an ever increasing sophistication of genetically based manipulations. These techniques have produced results with a remarkable degree of cell specificity, particularly at the cell signaling level, and have played a lead role in advancing the field. However, putting these findings into a brain-wide context that connects physiological signals and neurons to behavior and somatic physiology requires a thorough consideration of neuronal connections: a field that has also seen an extraordinary technological revolution. Our goal is to present a comprehensive and balanced assessment of how physiological signals associated with energy homeostasis interact at many brain levels to control eating behaviors. A major theme is that these signals engage sets of interacting neural networks throughout the brain that are defined by specific neural connections. We begin by discussing some fundamental concepts, including ones that still engender vigorous debate, that provide the necessary frameworks for understanding how the brain controls meal initiation and termination. These include key word definitions, ATP availability as the pivotal regulated variable in energy homeostasis, neuropeptide signaling, homeostatic and hedonic eating, and meal structure. Within this context, we discuss network models of how key regions in the endbrain (or telencephalon), hypothalamus, hindbrain, medulla, vagus nerve, and spinal cord work together with the gastrointestinal tract to enable the complex motor events that permit animals to eat in diverse situations.

A circuit from lateral septum neurotensin neurons to tuberal nucleus controls hedonic feeding

Feeding behavior is regulated by both the homeostatic needs of the body and hedonic values of the food. Easy access to palatable energy-dense foods and the consequent obesity epidemic stress the urgent need for a better understanding of neural circuits that regulate hedonic feeding. Here, we report that neurotensin-positive neurons in the lateral septum (LS^Nts) play a crucial role in regulating hedonic feeding. Silencing LS^Nts specifically promotes feeding of palatable food, whereas activation of LS^Nts suppresses overall feeding. LS^Nts neurons project to the tuberal nucleus (TU) via GABA signaling to regulate hedonic feeding, while the neurotensin signal from LS^Nts→the supramammillary nucleus (SUM) is sufficient to suppress overall feeding. In vivo calcium imaging and optogenetic manipulation reveal two populations of LS^Nts neurons that are activated and inhibited during feeding, which contribute to food seeking and consumption, respectively. Chronic activation of LS^Nts or LS^Nts→TU is sufficient to reduce high-fat diet-induced obesity. Our findings suggest that LS^Nts→TU is a key pathway in regulating hedonic feeding.

EASI-FISH for thick tissue defines lateral hypothalamus spatio-molecular organization

Determining the spatial organization and morphological characteristics of molecularly defined cell types is a major bottleneck for characterizing the architecture underpinning brain function. We developed Expansion-Assisted Iterative Fluorescence In Situ Hybridization (EASI-FISH) to survey gene expression in brain tissue, as well as a turnkey computational pipeline to rapidly process large EASI-FISH image datasets. EASI-FISH was optimized for thick brain sections (300 μm) to facilitate reconstruction of spatio-molecular domains that generalize across brains. Using the EASI-FISH pipeline, we investigated the spatial distribution of dozens of molecularly defined cell types in the lateral hypothalamic area (LHA), a brain region with poorly defined anatomical organization. Mapping cell types in the LHA revealed nine spatially and molecularly defined subregions. EASI-FISH also facilitates iterative reanalysis of scRNA-seq datasets to determine marker-genes that further dissociated spatial and morphological heterogeneity. The EASI-FISH pipeline democratizes mapping molecularly defined cell types, enabling discoveries about brain organization.

l-Menthol increases extracellular dopamine and c-Fos-like immunoreactivity in the dorsal striatum, and promotes ambulatory activity in mice

Similar to psychostimulants, the peripheral administration of menthol promotes mouse motor activity, and the neurotransmitter dopamine has been suggested to be involved in this effect. The present study aimed to elucidate the effects of l-menthol on parts of the central nervous system that are involved in motor effects. The subcutaneous administration of l-menthol significantly increased the number of c-Fos-like immunoreactive nuclei in the dorsal striatum of the mice, and motor activity was promoted. It also increased the extracellular dopamine level in the dorsal striatum of the mice. These observations indicated that after subcutaneous administration, l-menthol enhances dopamine-mediated neurotransmission, and activates neuronal activity in the dorsal striatum, thereby promoting motor activity in mice.

Activation of ventral tegmental area neurotensin Receptor-1 neurons promotes weight loss

Dopamine (DA) neurons in the ventral tegmental area (VTA) modulate physical activity and feeding behaviors that are disrupted in obesity. Yet, the heterogeneity of VTA DA neurons has hindered determination of which ones might be leveraged to support weight loss. We hypothesized that increased activity in the subset of VTA DA neurons expressing neurotensin receptor-1 (NtsR1) might promote weight loss behaviors. To test this, we used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to activate VTA NtsR1 neurons in normal weight and diet-induced obese mice. Acute activation of VTA NtsR1 neurons (24hr) significantly decreased body weight in normal weight and obese mice by reducing food intake and increasing physical activity. Moreover, daily activation of VTA NtsR1 neurons in obese mice sustained weight loss over 7 days. Activating VTA NtsR1 neurons also suppressed how much mice worked to obtain sucrose rewards, even when there was high motivation to consume. However, VTA NtsR1 neural activation was not reinforcing, nor did it invoke liabilities associated with whole-body NtsR1 agonism such as anxiety, vasodepressor response or hypothermia. Activating VTA NtsR1 neurons therefore promotes dual behaviors that support weight loss without causing adverse effects, and is worth further exploration for managing obesity.

The Role of Central Neurotensin in Regulating Feeding and Body Weight

The small peptide neurotensin (Nts) is implicated in myriad processes including analgesia, thermoregulation, reward, arousal, blood pressure, and modulation of feeding and body weight. Alterations in Nts have recently been described in individuals with obesity or eating disorders, suggesting that disrupted Nts signaling may contribute to body weight disturbance. Curiously, Nts mediates seemingly opposing regulation of body weight via different tissues. Peripherally acting Nts promotes fat absorption and weight gain, whereas central Nts signaling suppresses feeding and weight gain. Thus, because Nts is pleiotropic, a location-based approach must be used to understand its contributions to disordered body weight and whether the Nts system might be leveraged to improve metabolic health. Here we review the role of Nts signaling in the brain to understand the sites, receptors, and mechanisms by which Nts can promote behaviors that modify body weight. New techniques permitting site-specific modulation of Nts and Nts receptor-expressing cells suggest that, even in the brain, not all Nts circuitry exerts the same function. Intriguingly, there may be dedicated brain regions and circuits via which Nts specifically suppresses feeding behavior and weight gain vs other Nts-attributed physiology. Defining the central mechanisms by which Nts signaling modifies body weight may suggest strategies to correct disrupted energy balance, as needed to address overweight, obesity, and eating disorders.

The Role of Mediobasal Hypothalamic PACAP in the Control of Body Weight and Metabolism

Body energy homeostasis results from balancing energy intake and energy expenditure. Central nervous system administration of pituitary adenylate cyclase activating polypeptide (PACAP) dramatically alters metabolic function, but the physiologic mechanism of this neuropeptide remains poorly defined. PACAP is expressed in the mediobasal hypothalamus (MBH), a brain area essential for energy balance. Ventromedial hypothalamic nucleus (VMN) neurons contain, by far, the largest and most dense population of PACAP in the medial hypothalamus. This region is involved in coordinating the sympathetic nervous system in response to metabolic cues in order to re-establish energy homeostasis. Additionally, the metabolic cue of leptin signaling in the VMN regulates PACAP expression. We hypothesized that PACAP may play a role in the various effector systems of energy homeostasis, and tested its role by using VMN-directed, but MBH encompassing, adeno-associated virus (AAVCre) injections to ablate Adcyap1 (gene coding for PACAP) in mice (Adcyap1MBHKO mice). Adcyap1MBHKO mice rapidly gained body weight and adiposity, becoming hyperinsulinemic and hyperglycemic. Adcyap1MBHKO mice exhibited decreased oxygen consumption (VO2), without changes in activity. These effects appear to be due at least in part to brown adipose tissue (BAT) dysfunction, and we show that PACAP-expressing cells in the MBH can stimulate BAT thermogenesis. While we observed disruption of glucose clearance during hyperinsulinemic/euglycemic clamp studies in obese Adcyap1MBHKO mice, these parameters were normal prior to the onset of obesity. Thus, MBH PACAP plays important roles in the regulation of metabolic rate and energy balance through multiple effector systems on multiple time scales, which highlight the diverse set of functions for PACAP in overall energy homeostasis.

LSD1-BDNF activity in lateral hypothalamus-medial forebrain bundle area is essential for reward seeking behavior

Reward induces activity-dependant gene expression and synaptic plasticity-related changes. Lysine-specific histone demethylase 1 (LSD1), a key enzyme driving histone modifications, regulates transcription in neural circuits of memory and emotional behavior. Herein, we focus on the role of LSD1 in modulating the expression of brain derived neurotrophic factor (BDNF), the master regulator of synaptic plasticity, in the lateral hypothalamus-medial forebrain bundle (LH-MFB) circuit during positive reinforcement. Rats, trained for intracranial self-stimulation (ICSS) via an electrode-cannula assembly in the LH-MFB area, were assayed for lever press activity, epigenetic parameters and dendritic sprouting. LSD1 expression and markers of synaptic plasticity like BDNF and dendritic arborization in the LH, showed distinct increase in conditioned animals. H3K4me2 levels at Bdnf IV and Bdnf IX promoters were increased in ICSS-conditioned rats, but H3K9me2 was decreased. While intra LH-MFB treatment with pan Lsd1 siRNA inhibited lever press activity, analyses of LH tissue showed reduction in BDNF expression and levels of H3K4me2 and H3K9me2. However, co-administration of BDNF peptide restored lever press activity mitigated by Lsd1 siRNA. BDNF expression in LH, driven by LSD1 via histone demethylation, may play an important role in reshaping the reward pathway and hold the key to decode the molecular basis of addiction.

Hypothalamic insulin signalling as a nexus regulating mood and metabolism

Insulin has long been known as a metabolic hormone critical in the treatment of diabetes for its peripheral effects on blood glucose. However, in the last 50 years, insulin has entered the realm of neuroendocrinology and many studies have described its function on insulin receptors in the brain in relation to both metabolic and mood disorders. Indeed, rodent models of impaired insulin signalling show signs of dysregulated energy and glucose homeostasis, as well as anxiety-like and depressive behaviours. Importantly, many metabolic diseases such as obesity and diabetes increase the risk of developing mood disorders; however, the brain mechanisms underlying the connection between metabolism and mood remain unresolved. We present the current literature on the importance of the insulin receptor with respect to regulating glucose and energy homeostasis and mood-related behaviours. Specifically, we hypothesise that the insulin receptor in the hypothalamus, classically known as the homeostatic centre of the brain, plays a causal role in linking metabolic and behavioural effects of insulin signalling. In this review, we discuss insulin signalling in the hypothalamus as a critical point of neural integration controlling metabolism and mood.

A microfluidic chip for on-line derivatization and application to in vivo neurochemical monitoring

Microfluidic chips can perform a broad range of automated fluid manipulation operations for chemical analysis including on-line reactions. Derivatization reactions carried out on-chip reduce manual sample preparation and improve experimental throughput. In this work we develop a chip for on-line benzoyl chloride derivatization coupled to microdialysis, an in vivo sampling technique. Benzoyl chloride derivatization is useful for the analysis of small molecule neurochemicals in complex biological matrices using HPLC-MS/MS. The addition of one or more benzoyl groups to small, polar compounds containing amines, phenols, thiols, and certain alcohols improves reversed phase chromatographic retention, electrospray ionization efficiency, and analyte stability. The current derivatization protocol requires a three-step manual sample preparation, which ultimately limits the utility of this method for rapid sample collection and large sample sets. A glass microfluidic chip was developed for derivatizing microdialysis fractions on-line as they exit the probe for collection and off-line analysis with HPLC-MS/MS. Calibration curves for 21 neurochemicals prepared using the on-chip method showed linearity (R² > 0.99), limits of detection (0.1-500 nM), and peak area RSDs (4-14%) comparable to manual derivatization. Method temporal resolution was investigated both in vitro and in vivo showing rapid rise times for all analytes, which was limited by fraction length (3 min) rather than the device. The platform was applied to basal measurements in the striatum of awake rats where 19 of 21 neurochemicals were above the limit of detection. For a typical 2 h study, a minimum of 120 pipetting steps are eliminated per animal. Such a device provides a useful tool for the analysis of small molecules in biological matrices which may extend beyond microdialysis to other sampling techniques.

Chronic high-fat diet consumption induces an alteration in plasma/brain neurotensin signaling, metabolic disturbance, systemic inflammation/oxidative stress, brain apoptosis, and dendritic spine loss

Chronic high-fat diet (HFD) consumption caused not only negative effects on obesity and metabolic disturbance, but also instigated several brain pathologies, including dendritic spine loss. In addition, alterations in plasma/brain neurotensin (NT) levels and NT signaling were observed in obesity. However, the mechanistic link between the NT levels in plasma and brain, NT signaling, and peripheral/brain pathologies following prolonged HFD consumption still needs to be elucidated. We hypothesized that an increase in peripheral/brain NT signaling were associated with peripheral/brain pathologies after prolonged HFD consumption. Male Wistar rats (n = 24) were given either a normal diet (ND) or a HFD for 12 and 40 weeks. At the end of each time course, metabolic parameters and plasma NT levels were measured. Rats were then decapitated and the brains were examined the levels of brain NT, hippocampal reactive oxygen species, the number of Iba-1 positive cells, the dendritic spine densities, and the expression of NT-, mitophagy-, autophagy-, and apoptotic-related proteins. The findings showed an increase in the level of plasma NT with dyslipidemia, metabolic disturbances, systemic inflammation/oxidative stress, and hippocampal pathologies in rats fed HFD for 12 and 40 weeks. The expression of brain NT signaling and brain apoptosis were markedly increased after 40 weeks of HFD feeding. These results indicated that the alteration in the level of circulating/brain NT and its downstream signaling were associated with central and peripheral pathologies after long-term HFD intake. Therefore, these alterations in NT level or its signaling could be considered as a therapeutic target in treating obesity.

Ventromedial hypothalamic nucleus neuronal subset regulates blood glucose independently of insulin

To identify neurons that specifically increase blood glucose from among the diversely functioning cell types in the ventromedial hypothalamic nucleus (VMN), we studied the cholecystokinin receptor B-expressing (CCKBR-expressing) VMN targets of glucose-elevating parabrachial nucleus neurons. Activation of these VMNCCKBR neurons increased blood glucose. Furthermore, although silencing the broader VMN decreased energy expenditure and promoted weight gain without altering blood glucose levels, silencing VMNCCKBR neurons decreased hIepatic glucose production, insulin-independently decreasing blood glucose without altering energy balance. Silencing VMNCCKBR neurons also impaired the counterregulatory response to insulin-induced hypoglycemia and glucoprivation and replicated hypoglycemia-associated autonomic failure. Hence, VMNCCKBR cells represent a specialized subset of VMN cells that function to elevate glucose. These cells not only mediate the allostatic response to hypoglycemia but also modulate the homeostatic setpoint for blood glucose in an insulin-independent manner, consistent with a role for the brain in the insulin-independent control of glucose homeostasis.

Lateral hypothalamic area neuropeptides modulate ventral tegmental area dopamine neurons and feeding

Understanding how the brain coordinates energy status with the motivation to eat is crucial to identify strategies to improve disordered body weight. The ventral tegmental area (VTA), known as the core of the mesolimbic system, is of particular interest in this regard because it controls the motivation to consume palatable, calorie-dense foods and to engage in volitional activity. The VTA is largely composed of dopamine (DA) neurons, but modulating these DA neurons has been alternately linked with promoting and suppressing feeding, suggesting heterogeneity in their function. Subsets of VTA DA neurons have recently been described based on their anatomical distribution, electrophysiological features, connectivity and molecular expression, but to date there are no signatures to categorize how DA neurons control feeding. Assessing the neuropeptide receptors expressed by VTA DA neurons may be useful in this regard, as many neuropeptides mediate anorexic or orexigenic responses. In particular, the lateral hypothalamic area (LHA) releases a wide variety of feeding-modulating neuropeptides to the VTA. Since VTA neurons intercept LHA neuropeptides known to either evoke or suppress feeding, expression of the cognate neuropeptide receptors within the VTA may point to VTA DA neuronal mechanisms to promote or suppress feeding, respectively. Here we review the role of the VTA in energy balance and the LHA neuropeptide signaling systems that act in the VTA, whose receptors might be used to classify how VTA DA neurons contribute to energy balance.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

The neural circuitry supporting successful spatial navigation despite variable movement speeds

Ants who have successfully navigated the long distance between their foraging spot and their nest dozens of times will drastically overshoot their destination if the size of their legs is doubled by the addition of stilts. This observation reflects a navigational strategy called path integration, a strategy also utilized by mammals. Path integration necessitates that animals keep track of their movement speed and use it to precisely and instantly modify where they think they are and where they want to go. Here we review the neural circuitry that has evolved to integrate speed and space. We start with the rate and temporal codes for speed in the hippocampus and work backwards towards the motor and sensory systems. We highlight the need for experiments designed to differentiate the respective contributions of motor efference copy versus sensory inputs. In particular, we discuss the importance of high-resolution tracking of the latency of speed-encoding as a precise way to disentangle the sensory versus motor computations that enable successful spatial navigation at very different speeds.

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Neurotensin (Nts) is a neuropeptide implicated in the regulation of many facets of physiology, including cardiovascular tone, pain processing, ingestive behaviors, locomotor drive, sleep, addiction and social behaviors. Yet, there is incomplete understanding about how the various populations of Nts neurons distributed throughout the brain mediate such physiology. This knowledge gap largely stemmed from the inability to simultaneously identify Nts cell bodies and manipulate them in vivo. One means of overcoming this obstacle is to study Nts^Cre mice crossed onto a Cre-inducible green fluorescent reporter line (Nts^Cre;GFP mice), as these mice permit both visualization and in vivo modulation of specific populations of Nts neurons (using Cre-inducible viral and genetic tools) to reveal their function. Here we provide a comprehensive characterization of the distribution and relative densities of the Nts-GFP populations observed throughout the male Nts^Cre;GFP mouse brain, which will pave the way for future work to define their physiologic roles. We also compared the distribution of Nts-GFP neurons with Nts-In situ Hybridization (Nts-ISH) data from the adult mouse brain. By comparing these data sets we can distinguish Nts-GFP populations that may only transiently express Nts during development but not in the mature brain, and hence which populations may not be amenable to Cre-mediated manipulation in adult Nts^Cre;GFP mice. This atlas of Nts-GFP neurons will facilitate future studies using the Nts^Cre;GFP line to describe the physiological functions of individual Nts populations and how modulating them may be useful to treat disease.

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 (LH^LEPR) neurons, a subset of GABAergic cells, regulate motivation in mice. We find that LH^LEPR neuronal activation significantly increases progressive ratio (PR) performance, while inhibition decreases responding. Moreover, we mapped LH^LEPR 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 LH^LEPR neurons also regulate motivation to obtain water, suggesting that they may play a generalized role in motivation. Together, these results identify LH^LEPR neurons as modulators within a hypothalamic-ventral tegmental circuit that gates motivation.

Effects of GABA and Leptin Receptor-Expressing Neurons in the Lateral Hypothalamus on Feeding, Locomotion, and Thermogenesis

OBJECTIVE

The lateral hypothalamus (LH) is known for its role in feeding, and it also regulates other aspects of energy homeostasis. How genetically defined LH neuronal subpopulations mediate LH effects on energy homeostasis remains poorly understood. The behavioral effects of chemogenetically activating LH gamma-aminobutyric acid (GABA) and the more selective population of LH GABA neurons that coexpress the leptin receptor (LepR) were compared.

METHODS

LepR-cre and VGAT-cre mice were injected with AAV5-hSyn-DIO-hM3DGq-mCherry in the LH. The behavioral effects of LH GABA or LH LepR neuronal activation on feeding, locomotion, thermogenesis, and body weight were assessed.

RESULTS

The activation of LH GABA neurons increased body temperature (P ≤ 0.008) and decreased body weight (P ≤ 0.01) despite decreased locomotor activity (P = 0.03) and transiently increased chow intake (P ≤ 0.009). Also, similar to other studies, this study found that activation of LH GABA neurons induced gnawing on both food and nonfood (P = 0.001) items. Activation of LH LepR neurons decreased body weight (P ≤ 0.01) and chow intake when presented on the cage floor (P ≤ 0.04) but not when presented in the cage top and increased locomotor activity (P = 0.002) and body temperature (P = 0.03).

CONCLUSIONS

LH LepR neurons are a subset of LH GABA neurons, and LH LepR activation more specifically regulates energy homeostasis to promote a negative energy balance.

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.

Modulating Dopamine Signaling and Behavior with Chemogenetics: Concepts, Progress, and Challenges

For more than 60 years, dopamine (DA) has been known as a critical modulatory neurotransmitter regulating locomotion, reward-based motivation, and endocrine functions. Disturbances in DA signaling have been linked to an array of different neurologic and psychiatric disorders, including Parkinson's disease, schizophrenia, and addiction, but the underlying pathologic mechanisms have never been fully elucidated. One major obstacle limiting interpretation of standard pharmacological and transgenic interventions is the complexity of the DA system, which only appears to widen as research progresses. Nonetheless, development of new genetic tools, such as chemogenetics, has led to an entirely new era for functional studies of neuronal signaling. By exploiting receptors that are engineered to respond selectively to an otherwise inert ligand, so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetics enables pharmacological remote control of neuronal activity. Here we review the recent, extensive application of this technique to the DA field and how its use has advanced the study of the DA system and contributed to our general understanding of DA signaling and related behaviors. Moreover, we discuss the challenges and pitfalls associated with the chemogenetic technology, such as the metabolism of the DREADD ligand clozapine N-oxide (CNO) to the D2 receptor antagonist clozapine. We conclude that despite the recent concerns regarding CNO, the chemogenetic toolbox provides an exceptional approach to study neuronal function. The huge potential should promote continued investigations and additional refinements to further expound key mechanisms of DA signaling and circuitries in normal as well as maladaptive behaviors.

Single-cell transcriptomic analysis of the lateral hypothalamic area reveals molecularly distinct populations of inhibitory and excitatory neurons

The lateral hypothalamic area (LHA) coordinates an array of fundamental behaviors, including sleeping, waking, feeding, stress and motivated behavior. The wide spectrum of functions ascribed to the LHA may be explained by a heterogeneous population of neurons, the full diversity of which is poorly understood. We employed a droplet-based single-cell RNA-sequencing approach to develop a comprehensive census of molecularly distinct cell types in the mouse LHA. Neuronal populations were classified based on fast neurotransmitter phenotype and expression of neuropeptides, transcription factors and synaptic proteins, among other gene categories. We define 15 distinct populations of glutamatergic neurons and 15 of GABAergic neurons, including known and novel cell types. We further characterize a novel population of somatostatin-expressing neurons through anatomical and behavioral approaches, identifying a role for these neurons in specific forms of innate locomotor behavior. This study lays the groundwork for better understanding the circuit-level underpinnings of LHA function.

Distinct Subsets of Lateral Hypothalamic Neurotensin Neurons are Activated by Leptin or Dehydration

The lateral hypothalamic area (LHA) is essential for ingestive behavior but it remains unclear how LHA neurons coordinate feeding vs. drinking. Most LHA populations promote food and water consumption but LHA neurotensin (Nts) neurons preferentially induce water intake while suppressing feeding. We identified two molecularly and projection-specified subpopulations of LHA Nts neurons that are positioned to coordinate either feeding or drinking. One subpopulation co-expresses the long form of the leptin receptor (LepRb) and is activated by the anorectic hormone leptin (Nts^LepRb neurons). A separate subpopulation lacks LepRb and is activated by dehydration (Nts^Dehy neurons). These molecularly distinct LHA Nts subpopulations also differ in connectivity: Nts^LepRb neurons project to the ventral tegmental area and substantia nigra compacta but Nts^Dehy neurons do not. Intriguingly, the LHA Nts subpopulations cannot be discriminated via their classical neurotransmitter content, as we found that all LHA Nts neurons are GABAergic. Collectively, our data identify two molecularly- and projection-specified subpopulations of LHA Nts neurons that intercept either leptin or dehydration cues, and which conceivably could regulate feeding vs. drinking behavior. Selective regulation of these LHA Nts subpopulations might be useful to specialize treatment for ingestive disorders such as polydipsia or obesity.

Lateral Hypothalamic Mc3R-Expressing Neurons Modulate Locomotor Activity, Energy Expenditure, and Adiposity in Male Mice

The central melanocortin system plays a crucial role in the control of energy balance. Although the decreased energy expenditure and increased adiposity of melanocortin-3 receptor (Mc3R)-null mice suggest the importance of Mc3R-regulated neurons in energy homeostasis, the roles for specific subsets of Mc3R neurons in energy balance have yet to be determined. Because the lateral hypothalamic area (LHA) contributes to the control of energy expenditure and feeding, we generated Mc3rcre mice to determine the roles of LHA Mc3R (Mc3RLHA) neurons in energy homeostasis. We found that Mc3RLHA neurons overlap extensively with LHA neuron markers that contribute to the control of energy balance (neurotensin, galanin, and leptin receptor) and project to brain areas involved in the control of feeding, locomotion, and energy expenditure, consistent with potential roles for Mc3RLHA neurons in these processes. Indeed, selective chemogenetic activation of Mc3RLHA neurons increased locomotor activity and augmented refeeding after a fast. Although the ablation of Mc3RLHA neurons did not alter food intake, mice lacking Mc3RLHA neurons displayed decreased energy expenditure and locomotor activity, along with increased body mass and adiposity. Thus, Mc3R neurons lie within LHA neurocircuitry that modulates locomotor activity and energy expenditure and contribute to energy balance control.

Potential roles of neurotensin on cognition in conditions of obese-insulin resistance

Neurotensin is an endogenous tridecapeptide that can be found in both central and peripheral nervous systems. Under normal physiological conditions, neurotensin is involved in the regulation of pain, body temperature, physical activity, appetite as well as learning and memory. In addition, it plays an important role in fat metabolism. Previous studies have demonstrated that alterations of neurotensin levels were associated with several neuropathological conditions such as Alzheimer's disease, mood disorders, and obesity associated eating disorders. Obesity has been shown to be associated with low-grade systemic inflammation, brain inflammation, and cognitive decline. Several pieces of evidence suggest that neurotensin might play a role in cognitive decline following obesity. However, the underlying mechanisms of neurotensin on cognition under obese-insulin resistant condition are still unclear. In this review, the current available evidence from in vitro, in vivo and clinical studies regarding the role of neurotensin in the physiological condition and obesity in association with cognition are comprehensively summarized and discussed. The studies which report controversial findings regarding these issues are also presented and discussed.

Activation of lateral hypothalamic area neurotensin-expressing neurons promotes drinking

Animals must ingest water via drinking to maintain fluid homeostasis, yet the neurons that specifically promote drinking behavior are incompletely characterized. The lateral hypothalamic area (LHA) as a whole is essential for drinking behavior but most LHA neurons indiscriminately promote drinking and feeding. By contrast, activating neurotensin (Nts)-expressing LHA neurons (termed LHA Nts neurons) causes mice to immediately drink water with a delayed suppression of feeding. We therefore hypothesized that LHA Nts neurons are sufficient to induce drinking behavior and that these neurons specifically bias for fluid intake over food intake. To test this hypothesis we used designer receptors exclusively activated by designer drugs (DREADDs) to selectively activate LHA Nts neurons and studied the impact on fluid intake, fluid preference and feeding. Activation of LHA Nts neurons stimulated drinking in water-replete and dehydrated mice, indicating that these neurons are sufficient to promote water intake regardless of homeostatic need. Interestingly, mice with activated LHA Nts neurons drank any fluid that was provided regardless of its palatability, but if given a choice they preferred water or palatable solutions over unpalatable (quinine) or dehydrating (hypertonic saline) solutions. Notably, acute activation of LHA Nts neurons robustly promoted fluid but not food intake. Overall, our study confirms that activation of LHA Nts neurons is sufficient to induce drinking behavior and biases for fluid intake. Hence, LHA Nts neurons may be important targets for orchestrating the appropriate ingestive behavior necessary to maintain fluid homeostasis. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

In vivo detection of optically-evoked opioid peptide release

Though the last decade has seen accelerated advances in techniques and technologies to perturb neuronal circuitry in the brain, we are still poorly equipped to adequately dissect endogenous peptide release in vivo. To this end we developed a system that combines in vivo optogenetics with microdialysis and a highly sensitive mass spectrometry-based assay to measure opioid peptide release in freely moving rodents.

Lateral Hypothalamic Area Neurotensin Neurons Are Required for Control of Orexin Neurons and Energy Balance

The lateral hypothalamic area (LHA) is essential for motivated ingestive and locomotor behaviors that impact body weight, yet it remains unclear how the neurochemically defined subpopulations of LHA neurons contribute to energy balance. In particular, the role of the large population of LHA neurotensin (Nts) neurons has remained ambiguous due to the lack of methods to easily visualize and modulate these neurons. Because LHA Nts neurons are activated by leptin and other anorectic cues and they modulate dopamine or local LHA orexin neurons implicated in energy balance, they may have important, unappreciated roles for coordinating behaviors necessary for proper body weight. In this study, we genetically ablated or chemogenetically inhibited LHA Nts neurons in adult mice to determine their necessity for control of motivated behaviors and body weight. Genetic ablation of LHA Nts neurons resulted in profoundly increased adiposity compared with mice with intact LHA Nts neurons, as well as diminished locomotor activity, energy expenditure, and water intake. Complete loss of LHA Nts neurons also led to downregulation of orexin, revealing important cross-talk between the LHA Nts and orexin populations in maintenance of behavior and body weight. In contrast, chemogenetic inhibition of intact LHA Nts neurons did not disrupt orexin expression, but it suppressed locomotor activity and the adaptive response to leptin. Taken together, these data reveal the necessity of LHA Nts neurons and their activation for controlling energy balance, and that LHA Nts neurons influence behavior and body weight via orexin-dependent and orexin-independent mechanisms.

Neurotensin Receptor-1 Identifies a Subset of Ventral Tegmental Dopamine Neurons that Coordinates Energy Balance

Dopamine (DA) neurons in the ventral tegmental area (VTA) are heterogeneous and differentially regulate ingestive and locomotor behaviors that affect energy balance. Identification of which VTA DA neurons mediate behaviors that limit weight gain has been hindered, however, by the lack of molecular markers to distinguish VTA DA populations. Here, we identified a specific subset of VTA DA neurons that express neurotensin receptor-1 (NtsR1) and preferentially comprise mesolimbic, but not mesocortical, DA neurons. Genetically targeted ablation of VTA NtsR1 neurons uncouples motivated feeding and physical activity, biasing behavior toward energy expenditure and protecting mice from age-related and diet-induced weight gain. VTA NtsR1 neurons thus represent a molecularly defined subset of DA neurons that are essential for the coordination of energy balance. Modulation of VTA NtsR1 neurons may therefore be useful to promote behaviors that prevent the development of obesity.

Determination of neurotensin projections to the ventral tegmental area in mice

Pharmacologic treatment with the neuropeptide neurotensin (Nts) modifies motivated behaviors such as feeding, locomotor activity, and reproduction. Dopamine (DA) neurons of the ventral tegmental area (VTA) control these behaviors, and Nts directly modulates the activity of DA neurons via Nts receptor-1. While Nts sources to the VTA have been described in starlings and rats, the endogenous sources of Nts to the VTA of mice remain incompletely understood, impeding determination of which Nts circuits orchestrate specific behaviors in this model. To overcome this obstacle we injected the retrograde tracer Fluoro-Gold into the VTA of mice that express GFP in Nts neurons. Identification of GFP-Nts cells that accumulate Fluoro-Gold revealed the Nts afferents to the VTA in mice. Similar to rats, most Nts afferents to the VTA of mice arise from the medial and lateral preoptic areas (POA) and the lateral hypothalamic area (LHA), brain regions that are critical for coordination of feeding and reproduction. Additionally, the VTA receives dense input from Nts neurons in the nucleus accumbens shell (NAsh) of mice, and minor Nts projections from the amygdala and periaqueductal gray area. Collectively, our data reveal multiple populations of Nts neurons that provide direct afferents to the VTA and which may regulate specific aspects of motivated behavior. This work lays the foundation for understanding endogenous Nts actions in the VTA, and how circuit-specific Nts modulation may be useful to correct motivational and affective deficits in neuropsychiatric disease.

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

Role of central neurotensin in regulating feeding: Implications for the development and treatment of body weight disorders

The peptide neurotensin (Nts) was discovered within the brain over 40years ago and is implicated in regulating analgesia, body temperature, blood pressure, locomotor activity and feeding. Recent evidence suggests, however, that these disparate processes may be controlled via specific populations of Nts neurons and receptors. The neuronal mediators of Nts anorectic action are now beginning to be understood, and, as such, modulating specific Nts pathways might be useful in treating feeding and body weight disorders. This review considers mechanisms through which Nts normally regulates feeding and how disruptions in Nts signaling might contribute to the disordered feeding and body weight of schizophrenia, Parkinson's disease, anorexia nervosa, and obesity. Defining how Nts specifically mediates feeding vs. other aspects of physiology will inform the design of therapeutics that modify body weight without disrupting other important Nts-mediated physiology.

Lateral Hypothalamic Neurotensin Neurons Orchestrate Dual Weight Loss Behaviors via Distinct Mechanisms

The central mechanism by which neurotensin (Nts) potentiates weight loss has remained elusive. We leveraged chemogenetics to reveal that Nts-expressing neurons of the lateral hypothalamic area (LHA) promote weight loss in mice by increasing volitional activity and restraining food intake. Intriguingly, these dual weight loss behaviors are mediated by distinct signaling pathways: Nts action via NtsR1 is essential for the anorectic effect of the LHA Nts circuit, but not for regulation of locomotor or drinking behavior. Furthermore, although LHA Nts neurons cannot reduce intake of freely available obesogenic foods, they effectively restrain motivated feeding in hungry, weight-restricted animals. LHA Nts neurons are thus vital mediators of central Nts action, particularly in the face of negative energy balance. Enhanced action via LHA Nts neurons may, therefore, be useful to suppress the increased appetitive drive that occurs after lifestyle-mediated weight loss and, hence, to prevent weight regain.

A leptin-regulated circuit controls glucose mobilization during noxious stimuli

Adipocytes secrete the hormone leptin to signal the sufficiency of energy stores. Reductions in circulating leptin concentrations reflect a negative energy balance, which augments sympathetic nervous system (SNS) activation in response to metabolically demanding emergencies. This process ensures adequate glucose mobilization despite low energy stores. We report that leptin receptor-expressing neurons (LepRb neurons) in the periaqueductal gray (PAG), the largest population of LepRb neurons in the brain stem, mediate this process. Application of noxious stimuli, which often signal the need to mobilize glucose to support an appropriate response, activated PAG LepRb neurons, which project to and activate parabrachial nucleus (PBN) neurons that control SNS activation and glucose mobilization. Furthermore, activating PAG LepRb neurons increased SNS activity and blood glucose concentrations, while ablating LepRb in PAG neurons augmented glucose mobilization in response to noxious stimuli. Thus, decreased leptin action on PAG LepRb neurons augments the autonomic response to noxious stimuli, ensuring sufficient glucose mobilization during periods of acute demand in the face of diminished energy stores.

Lateral Hypothalamic Control of the Ventral Tegmental Area: Reward Evaluation and the Driving of Motivated Behavior

The lateral hypothalamus (LH) plays an important role in many motivated behaviors, sleep-wake states, food intake, drug-seeking, energy balance, etc. It is also home to a heterogeneous population of neurons that express and co-express multiple neuropeptides including hypocretin (Hcrt), melanin-concentrating hormone (MCH), cocaine- and amphetamine-regulated transcript (CART) and neurotensin (NT). These neurons project widely throughout the brain to areas such as the locus coeruleus, the bed nucleus of the stria terminalis, the amygdala and the ventral tegmental area (VTA). Lateral hypothalamic projections to the VTA are believed to be important for driving behavior due to the involvement of dopaminergic reward circuitry. The purpose of this article is to review current knowledge regarding the lateral hypothalamic connections to the VTA and the role they play in driving these behaviors.

Microdialysis Coupled with LC-MS/MS for In Vivo Neurochemical Monitoring

Microdialysis is a powerful sampling technique used to monitor small molecules in vivo. Despite the many applications of microdialysis sampling, it is limited by the method of analyzing the resulting samples. An emerging technique for analysis of microdialysis samples is liquid chromatography-tandem mass spectrometry (LC-MS/MS). This technique is highly versatile, allowing multiplexed analysis of neurotransmitters, metabolites, and neuropeptides. Using LC-MS/MS for polar neurotransmitters is hampered by weak retention reverse phase LC columns. Several derivatization reagents have been utilized to enhance separation and resolution of neurochemicals in dialysate samples including benzoyl chloride (BzCl), dansyl chloride, formaldehyde, ethylchloroformate, and propionic anhydride. BzCl reacts with amine and phenol groups so that many neurotransmitters can be labeled. Besides improving separation on reverse phase columns, this reagent also increases sensitivity. It is available in a heavy form so that it can be used to make stable-isotope labeled internal standard for improved quantification. Using BzCl with LC-MS/MS has allowed for measuring as many as 70 neurochemicals in a single assay. With slightly different conditions, LC-MS/MS has also been used for monitoring endocannabinoids. LC-MS/MS is also useful for neuropeptide assay because it allows for highly sensitive, sequence specific measurement of most peptides. These advances have allowed for multiplexed neurotransmitter measurements in behavioral, circuit analysis, and drug effect studies.

Cholinergic Mesopontine Signals Govern Locomotion and Reward through Dissociable Midbrain Pathways

The mesopontine tegmentum, including the pedunculopontine and laterodorsal tegmental nuclei (PPN and LDT), provides major cholinergic inputs to midbrain and regulates locomotion and reward. To delineate the underlying projection-specific circuit mechanisms, we employed optogenetics to control mesopontine cholinergic neurons at somata and at divergent projections within distinct midbrain areas. Bidirectional manipulation of PPN cholinergic cell bodies exerted opposing effects on locomotor behavior and reinforcement learning. These motor and reward effects were separable via limiting photostimulation to PPN cholinergic terminals in the ventral substantia nigra pars compacta (vSNc) or to the ventral tegmental area (VTA), respectively. LDT cholinergic neurons also form connections with vSNc and VTA neurons; however, although photo-excitation of LDT cholinergic terminals in the VTA caused positive reinforcement, LDT-to-vSNc modulation did not alter locomotion or reward. Therefore, the selective targeting of projection-specific mesopontine cholinergic pathways may offer increased benefit in treating movement and addiction disorders.

Galanin-Expressing GABA Neurons in the Lateral Hypothalamus Modulate Food Reward and Noncompulsive Locomotion

The lateral hypothalamus (LHA) integrates reward and appetitive behavior and is composed of many overlapping neuronal populations. Recent studies associated LHA GABAergic neurons (LHA ^GABA ), which densely innervate the ventral tegmental area (VTA), with modulation of food reward and consumption; yet, LHA ^GABA projections to the VTA exclusively modulated food consumption, not reward. We identified a subpopulation of LHA ^GABA neurons that coexpress the neuropeptide galanin (LHA ^Gal ). These LHA ^Gal neurons also modulate food reward, but lack direct VTA innervation. We hypothesized that LHA ^Gal neurons may represent a subpopulation of LHA ^GABA neurons that mediates food reward independent of direct VTA innervation. We used chemogenetic activation of LHA ^Gal or LHA ^GABA neurons in mice to compare their role in feeding behavior. We further analyzed locomotor behavior to understand how differential VTA connectivity and transmitter release in these LHA neurons influences this behavior. LHA ^Gal or LHA ^GABA neuronal activation both increased operant food-seeking behavior, but only activation of LHA ^GABA neurons increased overall chow consumption. Additionally, LHA ^Gal or LHA ^GABA neuronal activation similarly induced locomotor activity, but with striking differences in modality. Activation of LHA ^GABA neurons induced compulsive-like locomotor behavior; while LHA ^Gal neurons induced locomotor activity without compulsivity. Thus, LHA ^Gal neurons define a subpopulation of LHA ^GABA neurons without direct VTA innervation that mediate noncompulsive food-seeking behavior. We speculate that the striking difference in compulsive-like locomotor behavior is also based on differential VTA innervation. The downstream neural network responsible for this behavior and a potential role for galanin as neuromodulator remains to be identified.SIGNIFICANCE STATEMENT The lateral hypothalamus (LHA) regulates motivated feeding behavior via GABAergic LHA neurons. The molecular identity of LHA ^GABA neurons is heterogeneous and largely undefined. Here we introduce LHA ^Gal neurons as a subset of LHA ^GABA neurons that lack direct innervation of the ventral tegmental area (VTA). LHA ^Gal neurons are sufficient to drive motivated feeding and locomotor activity similar to LHA ^GABA neurons, but without inducing compulsive-like behaviors, which we propose to require direct VTA innervation. Our study integrates galanin-expressing LHA neurons into our current understanding of the neuronal circuits and molecular mechanisms of the LHA that contribute to motivated feeding behaviors.

Loss of Action via Neurotensin-Leptin Receptor Neurons Disrupts Leptin and Ghrelin-Mediated Control of Energy Balance

The hormones ghrelin and leptin act via the lateral hypothalamic area (LHA) to modify energy balance, but the underlying neural mechanisms remain unclear. We investigated how leptin and ghrelin engage LHA neurons to modify energy balance behaviors and whether there is any crosstalk between leptin and ghrelin-responsive circuits. We demonstrate that ghrelin activates LHA neurons expressing hypocretin/orexin (OX) to increase food intake. Leptin mediates anorectic actions via separate neurons expressing the long form of the leptin receptor (LepRb), many of which coexpress the neuropeptide neurotensin (Nts); we refer to these as NtsLepRb neurons. Because NtsLepRb neurons inhibit OX neurons, we hypothesized that disruption of the NtsLepRb neuronal circuit would impair both NtsLepRb and OX neurons from responding to their respective hormonal cues, thus compromising adaptive energy balance. Indeed, mice with developmental deletion of LepRb specifically from NtsLepRb neurons exhibit blunted adaptive responses to leptin and ghrelin that discoordinate the mesolimbic dopamine system and ingestive and locomotor behaviors, leading to weight gain. Collectively, these data reveal a crucial role for LepRb in the proper formation of LHA circuits, and that NtsLepRb neurons are important neuronal hubs within the LHA for hormone-mediated control of ingestive and locomotor behaviors.

Hormones and Neuropeptide Receptor Heteromers in the Ventral Tegmental Area. Targets for the Treatment of Loss of Control of Food Intake and Substance Use Disorders

Hormones and neuropeptides represent biological correlates of internal homeostatic signals detected and integrated in the hypothalamus, which establishes a robust functional connection with the ventral tegmental area (VTA). The hypothalamus-VTA connection determines the ability of these signals to influence central dopaminergic neurotransmission and, therefore, their ability to increase responsiveness to their reward-associated stimuli and to establish appropriate associative learning. The hypothalamus also provides the main source of the multiple neuropeptides that are released in the VTA. With volume transmission of neuropeptides and hormones, extrasynaptic receptors within the VTA provide a fine-tune mechanism, which depends on the ability of molecularly different G protein-coupled receptors (GPCRs) to form heteromers. GPCR heteromer is defined as a macromolecular complex composed of at least two different receptor units (protomers) with biochemical properties that are demonstrably different from those of its individual components. GPCR heteromers can provide unique allosteric properties to specific ligands, which provides new avenues for drug development. We have identified specific GPCR heteromers in the VTA that integrate orexin and CRF neurotransmission and opioid and galanin neurotransmission, which play a very significant role in the modulation of dopaminergic neuronal activity and which can constitute targets for the treatment of loss of control of food intake and substance use disorders.