Medial prefrontal D1 dopamine neurons control food intake

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu₂ and mGlu₃) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu₃ induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu₃-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu₃-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu₃ activation in vivo prevented the stress-induced maladaptive changes to amydalo-cortical physiology and motivated behavior. These data demonstrate that mGlu₃ mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu₃ modulation as a treatment for stress-related psychiatric disorders.

VIPergic neurons of the infralimbic and prelimbic cortices control palatable food intake through separate cognitive pathways

The prefrontal cortex controls food reward seeking and ingestion, playing important roles in directing attention, regulating motivation towards reward pursuit, and the assignment of reward salience and value. The cell types that mediate these behavioral functions, however, are not well described. We report here that optogenetic activation of vasoactive peptide expressing (VIP) interneurons in both the infralimbic (IL) and prelimbic (PL) divisions of the medial prefrontal cortex in mice is sufficient to reduce acute, binge-like intake of high calorie palatable food in the absence of any effect on low calorie rodent chow intake in the sated animal. In addition, we discovered that the behavioral mechanisms associated with these changes in feeding differed between animals that underwent either IL or PL VIPergic stimulation. While IL VIP neurons showed the ability to reduce palatable food intake, this effect was dependent upon the novelty and relative value of the food source. In addition, IL VIP neuron activation significantly reduced novel object- and novel social investigative behavior. Activation of PL VIP neurons, however, produced a reduction in high calorie palatable food intake that was independent of food novelty. Neither IL nor PL VIP excitation changed motivation to obtain food reward. Our data show how neurochemically-defined populations of cortical interneurons can regulate specific aspects of food reward-driven behavior, resulting in a selective reduction in intake of highly valued food.

Chronic unpredictable stress promotes cell-specific plasticity in prefrontal cortex D1 and D2 pyramidal neurons

Exposure to unpredictable environmental stress is widely recognized as a major determinant for risk and severity in neuropsychiatric disorders such as major depressive disorder, anxiety, schizophrenia, and PTSD. The ability of ostensibly unrelated disorders to give rise to seemingly similar psychiatric phenotypes highlights a need to identify circuit-level concepts that could unify diverse factors under a common pathophysiology. Although difficult to disentangle a causative effect of stress from other factors on medial prefrontal cortex (PFC) dysfunction, a wealth of data from humans and rodents demonstrates that the PFC is a key target of stress. The present study sought to identify a model of chronic unpredictable stress (CUS) which induces affective behaviors in C57BL6J mice and once established, measure stress-related alterations in intrinsic excitability and synaptic regulation of mPFC layer 5/6 pyramidal neurons. Adult male mice received 2 weeks of 'less intense' stress or 2 or 4 weeks of 'more intense' CUS followed by sucrose preference for assessment of anhedonia, elevated plus maze for assessment of anxiety and forced swim test for assessment of depressive-like behaviors. Our findings indicate that more intense CUS exposure results in increased anhedonia, anxiety, and depressive behaviors, while the less intense stress results in no measured behavioral phenotypes. Once a behavioral model was established, mice were euthanized approximately 21 days post-stress for whole-cell patch clamp recordings from layer 5/6 pyramidal neurons in the prelimbic (PrL) and infralimbic (IL) cortices. No significant differences were initially observed in intrinsic cell excitability in either region. However, post-hoc analysis and subsequent confirmation using transgenic mice expressing tdtomato or eGFP under control of dopamine D1-or D2-type receptor showed that D1-expressing pyramidal neurons (D1-PYR) in the PrL exhibit reduced thresholds to fire an action potential (increased excitability) but impaired firing capacity at more depolarized potentials, whereas D2-expressing pyramidal neurons (D2-PYR) showed an overall reduction in excitability and spike firing frequency. Examination of synaptic transmission showed that D1-and D2-PYR exhibit differences in basal excitatory and inhibitory signaling under naïve conditions. In CUS mice, D1-PYR showed increased frequency of both miniature excitatory and inhibitory postsynaptic currents, whereas D2-PYR only showed a reduction in excitatory currents. These findings demonstrate that D1-and D2-PYR subpopulations differentially undergo stress-induced intrinsic and synaptic plasticity that may have functional implications for stress-related pathology, and that these adaptations may reflect unique differences in basal properties regulating output of these cells.

Time-dependent assessment of stimulus-evoked regional dopamine release

To date, the spatiotemporal release of specific neurotransmitters at physiological levels in the human brain cannot be detected. Here, we present a method that relates minute-by-minute fluctuations of the positron emission tomography (PET) radioligand [11C]raclopride directly to subsecond dopamine release events. We show theoretically that synaptic dopamine release induces low frequency temporal variations of extrasynaptic extracellular dopamine levels, at time scales of one minute, that can evoke detectable temporal variations in the [11C]raclopride signal. Hence, dopaminergic activity can be monitored via temporal fluctuations in the [11C]raclopride PET signal. We validate this theory using fast-scan cyclic voltammetry and [11C]raclopride PET in mice during chemogenetic activation of dopaminergic neurons. We then apply the method to data from human subjects given a palatable milkshake and discover immediate and-for the first time-delayed food-induced dopamine release. This method enables time-dependent regional monitoring of stimulus-evoked dopamine release at physiological levels.

Optogenetic stimulation of medial prefrontal cortex Drd1 neurons produces rapid and long-lasting antidepressant effects

Impaired function in the medial prefrontal cortex (mPFC) contributes to depression, and the therapeutic response produced by novel rapid-acting antidepressants such as ketamine are mediated by mPFC activity. The mPFC contains multiple types of pyramidal cells, but it is unclear whether a particular subtype mediates the rapid antidepressant actions of ketamine. Here we tested two major subtypes, Drd1 and Drd2 dopamine receptor expressing pyramidal neurons and found that activating Drd1 expressing pyramidal cells in the mPFC produces rapid and long-lasting antidepressant and anxiolytic responses. In contrast, photostimulation of Drd2 expressing pyramidal cells was ineffective across anxiety-like and depression-like measures. Disruption of Drd1 activity also blocked the rapid antidepressant effects of ketamine. Finally, we demonstrate that stimulation of mPFC Drd1 terminals in the BLA recapitulates the antidepressant effects of somatic stimulation. These findings aid in understanding the cellular target neurons in the mPFC and the downstream circuitry involved in rapid antidepressant responses.

Dopamine tunes prefrontal outputs to orchestrate aversive processing

Decades of research suggest that the mesocortical dopamine system exerts powerful control over mPFC physiology and function. Indeed, dopamine signaling in the medial prefrontal cortex (mPFC) is implicated in a vast array of processes, including working memory, stimulus discrimination, stress responses, and emotional and behavioral control. Consequently, even slight perturbations within this delicate system result in profound disruptions of mPFC-mediated processes. Many neuropsychiatric disorders are associated with dysregulation of mesocortical dopamine, including schizophrenia, depression, attention deficit hyperactivity disorder, post-traumatic stress disorder, among others. Here, we review the anatomy and functions of the mesocortical dopamine system. In contrast to the canonical role of striatal dopamine in reward-related functions, recent work has revealed that mesocortical dopamine fine-tunes distinct efferent projection populations in a manner that biases subsequent behavior towards responding to stimuli associated with potentially aversive outcomes. We propose a framework wherein dopamine can serve as a signal for switching mPFC states by orchestrating how information is routed to the rest of the brain.

Prefrontal-accumbens opioid plasticity: Implications for relapse and dependence

In addiction, an individual's ability to inhibit drug seeking and drug taking is thought to reflect a pathological strengthening of drug-seeking behaviors or impairments in the capacity to control maladaptive behavior. These processes are not mutually exclusive and reflect drug-induced modifications within prefrontal cortical and nucleus accumbens circuits, however unlike psychostimulants such as cocaine, far less is known about the temporal, anatomical, and cellular dynamics of these changes. We discuss what is known regarding opioid-induced adaptations in intrinsic membrane physiology and pre-/postsynaptic neurotransmission in principle pyramidal and medium spiny neurons in the medial prefrontal cortex and nucleus accumbens from electrophysiological studies and explore how circuit specific adaptations may contribute to unique facets of opioid addiction.

Hippocampal deficits in neurodevelopmental disorders

Neurodevelopmental disorders result from impaired development or maturation of the central nervous system. Both genetic and environmental factors can contribute to the pathogenesis of these disorders; however, the exact causes are frequently complex and unclear. Individuals with neurodevelopmental disorders may have deficits with diverse manifestations, including challenges with sensory function, motor function, learning, memory, executive function, emotion, anxiety, and social ability. Although these functions are mediated by multiple brain regions, many of them are dependent on the hippocampus. Extensive research supports important roles of the mammalian hippocampus in learning and cognition. In addition, with its high levels of activity-dependent synaptic plasticity and lifelong neurogenesis, the hippocampus is sensitive to experience and exposure and susceptible to disease and injury. In this review, we first summarize hippocampal deficits seen in several human neurodevelopmental disorders, and then discuss hippocampal impairment including hippocampus-dependent behavioral deficits found in animal models of these neurodevelopmental disorders.

Uneven balance of power between hypothalamic peptidergic neurons in the control of feeding

The interplay between the anorexigenic and orexigenic neurons in the arcuate nucleus that contributes to the control of feeding remains elusive. Using optogenetic stimulation, we show that activation of POMC neurons rapidly inhibits feeding behavior in fasted animals. However, simultaneous stimulation of both POMC neurons and a subset of the orexigenic neurons that express AgRP is sufficient to reverse that inhibition and trigger intense feeding behavior. We used 3D imaging and functional studies to illuminate the anatomical underpinning of both the inhibitory and excitatory events. Our work suggests that translational applications that aim to control appetite need to target the activation rather than the inhibition mechanisms.

Two classes of peptide-producing neurons in the arcuate nucleus (Arc) of the hypothalamus are known to exert opposing actions on feeding: the anorexigenic neurons that express proopiomelanocortin (POMC) and the orexigenic neurons that express agouti-related protein (AgRP) and neuropeptide Y (NPY). These neurons are thought to arise from a common embryonic progenitor, but our anatomical and functional understanding of the interplay of these two peptidergic systems that contribute to the control of feeding remains incomplete. The present study uses a combination of optogenetic stimulation with viral and transgenic approaches, coupled with neural activity mapping and brain transparency visualization to demonstrate the following: (i) selective activation of Arc POMC neurons inhibits food consumption rapidly in unsated animals; (ii) activation of Arc neurons arising from POMC-expressing progenitors, including POMC and a subset of AgRP neurons, triggers robust feeding behavior, even in the face of satiety signals from POMC neurons; (iii) the opposing effects on food intake are associated with distinct neuronal projection and activation patterns of adult hypothalamic POMC neurons versus Arc neurons derived from POMC-expressing lineages; and (iv) the increased food intake following the activation of orexigenic neurons derived from POMC-expressing progenitors engages an extensive neural network that involves the endogenous opioid system. Together, these findings shed further light on the dynamic balance between two peptidergic systems in the moment-to-moment regulation of feeding behavior.

Early High-Fat Diet Exposure Causes Dysregulation of the Orexin and Dopamine Neuronal Populations in Nonhuman Primates

Maternal obesity and consumption of a high-fat diet (HFD) during pregnancy has a negative impact on offspring, including an increased risk for the development of obesity in adolescence. The mechanism for this transferred metabolic risk is unclear, but many studies have focused on the brain due to its important role in appetite and body-weight regulation. Two main pathways regulate appetite in the brain; homeostatic regulation that occurs predominantly in hypothalamic circuits and hedonic regulation of feeding that occurs via dopaminergic pathways. The current proposal examined the impact of early HFD exposure on the dopaminergic control of hedonic feeding pathways in a translational nonhuman primate model. Japanese macaque offspring from mothers consuming a control (CTR) or HFD were weaned onto control or HFD at an average 8 months of age yielding four groups: maternal and post-weaning control diet (mCTRpCTR), maternal control diet and post-weaning HFD (mCTRpHFD), maternal HFD and post-weaning control diet (mHFDpCTR) and maternal and post-weaning HFD (mHFDpHFD). Brains from 13-month-old offspring were evaluated for expression of neuropeptides that regulate dopaminergic pathways including orexin, melanin-concentrating hormone (MCH) in the lateral hypothalamus (LH), and tyrosine hydroxylase expression in the ventral tegmental area (VTA). Orexin cell numbers in the LH were significantly increased in animals exposed to a post-weaning HFD, while no difference was observed for orexin mRNA content or MCH cell numbers. Orexin fiber projections to the rostral VTA were significantly reduced in mCTRpHFD, mHFDpCTR, and mHFDpHFD groups, but these differences were not significant in the caudal VTA. There was no difference in the percentage of dopamine neurons receiving close appositions from orexin fibers in either the rostral or caudal VTA, nor was there any difference between groups in the number of orexin contacts per TH cell. In conclusion, the current study finds that prolonged early exposure to HFD during the in utero and postnatal period causes alterations at several levels in the dopaminergic circuits regulating reward.

Methyleugenol counteracts anorexigenic signals in association with GABAergic inhibition in the central amygdala

Feeding can be inhibited by satiety, sickness, or food unpalatability. The central nucleus of the amygdala (CeA) has been considered the key region for processing multiple anorexigenic signals, although the detailed cellular and molecular mechanisms remain largely unclear. Here we identify that methyleugenol (ME), a novel agonist of A type ionotropic γ-aminobutyric acid receptors (GABA_ARs), significantly counteracts the anorexigenic effects caused by satiety or sickness in association with GABAergic inhibition in the CeA. Electrophysiologically, ME enhanced GABAergic transmission and repressed neuronal excitability of the CeA. Behaviorally, ME increased feeding but not affect locomotor activity and basal anxiety in naïve mice. Notably, both systemic and CeA-specific delivery of ME significantly rescued satiety- or sickness-induced inhibition of feeding. The effects of ME were mainly dependent on the GABA_ARs in the CeA. Indeed, viral-mediated, the CeA region-specific genetic knockdown of the γ2 subunit of GABA_ARs largely abolished the above pharmacological effects, while its re-expression in a subpopulation of GABAergic neurons in the CeA, that produce protein kinase C-δ (PKC-δ), recovered the effects of ME on anorexigenic signals. Taken together, these results reveal a novel molecular mechanism for counter-anorexigenic signals dependent on GABAergic inhibition in the CeA, suggesting the possibility of ME as a leading compound for anorexia treatment.

Modulation of appetitive motivation by prefrontal cortical mu-opioid receptors is dependent upon local dopamine D1 receptor signaling

Opioid neurotransmission has been implicated in psychiatric disorders featuring impaired control over appetitive motivation, such as addiction and binge-eating disorder. We have previously shown that infusions of the μ-opioid receptor (μOR) agonist DAMGO into the ventromedial prefrontal cortex (vmPFC) induced hyperphagia, increased motor activity, and augmented sucrose-reinforced responding in the task progressive ratio (PR) task, which assesses the motivational value of an incentive. These effects were not reproduced by intra-PFC infusion of a variety of dopamine (DA) agonists and antagonists, suggesting that manipulation of intra-PFC DA systems alone is not sufficient to reproduce μOR-like effects. Nevertheless, this does not rule out interactions between PFC DA and μ-opioid systems. Here we used intra-vmPFC drug cocktails containing DAMGO and SCH 23390 (a DA D1 receptor antagonist) to determine whether increases in appetitive motivation and motor activity elicited by intra-vmPFC μOR stimulation require intact signaling through vmPFC D1 receptors. Blockade of D1 receptors with SCH 23390 attenuated the enhancement of PR breakpoint, and increases in exploratory-like behavior and feeding initiation elicited by intra-vmPFC μOR stimulation. These results establish that intra-vmPFC D1 signaling is required for the expression of behavioral effects evoked by μOR stimulation within the PFC, and further suggest that D1 tone plays an enabling or permissive role in the expression of μOR -elicited effects. Simultaneous targeting of both μ-opioid and D1 systems may represent a more efficacious treatment strategy (compared to μOR blockade alone) for psychiatric disorders characterized by dysregulated appetitive motivation.

A hippocampus to prefrontal cortex neural pathway inhibits food motivation through glucagon-like peptide-1 signaling

The hippocampus and the medial prefrontal cortex (mPFC) are traditionally associated with regulating memory and executive function, respectively. The contribution of these brain regions to food intake control, however, is poorly understood. The present study identifies a novel neural pathway through which monosynaptic glutamatergic ventral hippocampal field CA1 (vCA1) to mPFC connectivity inhibits food-motivated behaviors through vCA1 glucagon-like peptide-1 receptor (GLP-1R). Results demonstrate that vCA1-targeted RNA interference-mediated GLP-1R knockdown increases motivated operant responding for palatable food. Chemogenetic disconnection of monosynaptic glutamatergic vCA1 to mPFC projections using designer receptors exclusively activated by designer drugs (DREADDs)-mediated synaptic silencing ablates the food intake and body weight reduction following vCA1 GLP-1R activation. Neuropharmacological experiments further reveal that vCA1 GLP-1R activation reduces food intake and inhibits impulsive operant responding for palatable food via downstream communication to mPFC NMDA receptors. Overall these findings identify a novel neural pathway regulating higher-order cognitive aspects of feeding behavior.

Somatostatin Neurons in the Basal Forebrain Promote High-Calorie Food Intake

Obesity has become a global issue, and the overconsumption of food is thought to be a major contributor. However, the regulatory neural circuits that regulate palatable food consumption remain unclear. Here, we report that somatostatin (SOM) neurons and GABAergic (VGAT) neurons in the basal forebrain (BF) play specific roles in regulating feeding. Optogenetic stimulation of BF SOM neurons increased fat and sucrose intake within minutes and promoted anxiety-like behaviors. Furthermore, optogenetic stimulation of projections from BF SOM neurons to the lateral hypothalamic area (LHA) selectively resulted in fat intake. In addition, activation of BF VGAT neurons rapidly induced general food intake and gnawing behaviors. Whole-brain mapping of inputs and outputs showed that BF SOM neurons form bidirectional connections with several brain areas important in feeding and regulation of emotion. Collectively, these results suggest that BF SOM neurons play a selective role in hedonic feeding.

Cloning and functional characterization of the DA2 receptor gene in Chinese mitten crab (Eriocheir sinensis)

Dopamine (DA) plays a modulatory role in numerous physiological processes such as light adaptation and food intake, and exerts these functions through DA receptors (DARs). This study presents, for the first time, isolation and characterization of the dopamine receptor 2(DA₂ receptor) cDNA from the intestinal tissue of Eriocheir sinensis, an economically important freshwater aquaculture species in China. The DA₂ receptor cDNA sequence, which was obtained by rapid amplification of cDNA ends, is 2369bp long, encode peptide of 589 amino acid, and is highly homologous to related sequences in crustaceans. Analysis of the deduced amino acid sequence and the structure of the DA₂ indicated that this receptor is a member of the family of G protein-coupled receptors (GPCRs), as it contains seven transmembrane domains and other common signatures of GPCRs. RT-PCR showed that the expression of the DA₂ receptor gene was distributed in various tissues, and high expression levels were observed in the cranial ganglia and the thoracic ganglia. Further study of the effect of photoperiod on DA₂ expression showed that constant darkness induced a significant increase in DA₂ expression in the cranial ganglia. Finally, analysis of DA₂ receptor expression under different feeding statuses showed that there was significantly greater expression in the hepatopancreas and intestines after feeding than before feeding, but there were no differences in expression between the before feeding and during feeding periods in either tissue. Our results indicate that the DA₂ receptor structurally belongs to the family of G protein-coupled receptors, and that the cranial ganglia are the main tissues in which the DA₂ receptor participates in light adaptation during dark hours. In addition, the DA₂ receptor in E. sinensis may be involved in the physiological regulation of the hepatopancreas and digestive tract after the ingestion of food. This study provides a foundation for further exploration of the light adaptation and digestive functions of the DA₂ receptor in decapods.

Food Addiction and Binge Eating: Lessons Learned from Animal Models

The feeding process is required for basic life, influenced by environment cues and tightly regulated according to demands of the internal milieu by regulatory brain circuits. Although eating behaviour cannot be considered "addictive" under normal circumstances, people can become "addicted" to this behaviour, similarly to how some people are addicted to drugs. The symptoms, cravings and causes of "eating addiction" are remarkably similar to those experienced by drug addicts, and both drug-seeking behaviour as eating addiction share the same neural pathways. However, while the drug addiction process has been highly characterised, eating addiction is a nascent field. In fact, there is still a great controversy over the concept of "food addiction". This review aims to summarize the most relevant animal models of "eating addictive behaviour", emphasising binge eating disorder, that could help us to understand the neurobiological mechanisms hidden under this behaviour, and to improve the psychotherapy and pharmacological treatment in patients suffering from these pathologies.

Overlapping Brain Circuits for Homeostatic and Hedonic Feeding

Central regulation of food intake is a key mechanism contributing to energy homeostasis. Many neural circuits that are thought to orchestrate feeding behavior overlap with the brain's reward circuitry both anatomically and functionally. Manipulation of numerous neural pathways can simultaneously influence food intake and reward. Two key systems underlying these processes-those controlling homeostatic and hedonic feeding-are often treated as independent. Homeostatic feeding is necessary for basic metabolic processes and survival, while hedonic feeding is driven by sensory perception or pleasure. Despite this distinction, their functional and anatomical overlap implies considerable interaction that is often overlooked. Here, we argue that the neurocircuits controlling homeostatic feeding and hedonic feeding are not completely dissociable given the current data and urge researchers to assess behaviors extending beyond food intake in investigations of the neural control of feeding.

Parallels and Overlap: The Integration of Homeostatic Signals by Mesolimbic Dopamine Neurons

Motivated behaviors are often initiated in response to perturbations of homeostasis. Indeed, animals and humans have fundamental drives to procure (appetitive behaviors) and eventually ingest (consummatory behaviors) substances based on deficits in body fluid (e.g., thirst) and energy balance (e.g., hunger). Consumption, in turn, reinforces motivated behavior and is therefore considered rewarding. Over the years, the constructs of homeostatic (within the purview of the hypothalamus) and reward (within the purview of mesolimbic circuitry) have been used to describe need-based vs. need-free consumption. However, many experiments have demonstrated that mesolimbic circuits and "higher-order" brain regions are also profoundly influenced by changes to physiological state, which in turn generate behaviors that are poised to maintain homeostasis. Mesolimbic pathways, particularly dopamine neurons of the ventral tegmental area (VTA) and their projections to nucleus accumbens (NAc), can be robustly modulated by a variety of energy balance signals, including post-ingestive feedback relaying nutrient content and hormonal signals reflecting hunger and satiety. Moreover, physiological states can also impact VTA-NAc responses to non-nutritive rewards, such as drugs of abuse. Coupled with recent evidence showing hypothalamic structures are modulated in anticipation of replenished need, classic boundaries between circuits that convey perturbations in homeostasis and those that drive motivated behavior are being questioned. In the current review, we examine data that have revealed the importance of mesolimbic dopamine neurons and their downstream pathways as a dynamic neurobiological mechanism that provides an interface between physiological state, perturbations to homeostasis, and reward-seeking behaviors.

Optogenetic and chemogenetic techniques for neurogastroenterology

Optogenetics and chemogenetics comprise a wide variety of applications in which genetically encoded actuators and indicators are used to modulate and monitor activity with high cellular specificity. Over the past 10 years, development of these genetically encoded tools has contributed tremendously to our understanding of integrated physiology. In concert with the continued refinement of probes, strategies to target transgene expression to specific cell types have also made much progress in the past 20 years. In addition, the successful implementation of optogenetic and chemogenetic techniques thrives thanks to ongoing advances in live imaging microscopy and optical technology. Although innovation of optogenetic and chemogenetic methods has been primarily driven by researchers studying the central nervous system, these techniques also hold great promise to boost research in neurogastroenterology. In this Review, we describe the different classes of tools that are currently available and give an overview of the strategies to target them to specific cell types in the gut wall. We discuss the possibilities and limitations of optogenetic and chemogenetic technology in the gut and provide an overview of their current use, with a focus on the enteric nervous system. Furthermore, we suggest some experiments that can advance our understanding of how the intrinsic and extrinsic neural networks of the gut control gastrointestinal function.

Antipsychotic Drugs: From Receptor-binding Profiles to Metabolic Side Effects

BACKGROUND

Antipsychotic-induced metabolic side effects are major concerns in psychopharmacology and clinical psychiatry. Their pathogenetic mechanisms are still not elucidated.

METHODS

Herein, we review the impact of neurotransmitters on metabolic regulation, providing insights into antipsychotic-induced metabolic side effects.

RESULTS

Antipsychotic drugs seem to interfere with feeding behaviors and energy balance, processes that control metabolic regulation. Reward and energy balance centers in central nervous system constitute the central level of metabolic regulation. The peripheral level consists of skeletal muscles, the liver, the pancreas, the adipose tissue and neuroendocrine connections. Neurotransmitter receptors have crucial roles in metabolic regulation and they are also targets of antipsychotic drugs. Interaction of antipsychotics with neurotransmitters could have both protective and harmful effects on metabolism.

CONCLUSION

Emerging evidence suggests that antipsychotics have different liabilities to induce obesity, diabetes and dyslipidemia. However this diversity cannot be explained merely by drugs'pharmacodynamic profiles, highlighting the need for further research.

Appetite suppressive role of medial septal glutamatergic neurons

Feeding behavior is controlled by diverse neurons and neural circuits primarily concentrated in the hypothalamus and hindbrain in mammals. In this study, by using chemo/optogenetic techniques along with feeding assays, we investigate how neurons within the medial septal complex (MSc), a brain area implicated in emotion and cognition, contribute to food intake. We find that chemo/optogenetic activation of MSc glutamatergic neurons profoundly reduces food intake during both light and dark periods of the rodent light cycle. Furthermore, we find that selective activation of MSc glutamatergic projections in paraventricular hypothalamus (PVH) reduces food intake, suggesting that MSc glutamatergic neurons suppress feeding by activating downstream neurons in the PVH. Open-field behavioral assays reveal that these neurons do not overtly affect anxiety levels and locomotion. Collectively, our findings demonstrate that septal glutamatergic neurons exert anorexigenic effects by projecting to the PVH without affecting anxiety and physical activities.

Dopamine terminals from the ventral tegmental area gate intrinsic inhibition in the prefrontal cortex

Spike frequency adaptation (SFA or accommodation) and calcium‐activated potassium channels that underlie after‐hyperpolarization potentials (AHP) regulate repetitive firing of neurons. Precisely how neuromodulators such as dopamine from the ventral tegmental area (VTA) regulate SFA and AHP (together referred to as intrinsic inhibition) in the prefrontal cortex (PFC) remains unclear. Using whole cell electrophysiology, we measured intrinsic inhibition in prelimbic (PL) layer 5 pyramidal cells of male adult rats. Results demonstrate that bath application of dopamine reduced intrinsic inhibition (EC₅₀: 25.0 μmol/L). This dopamine action was facilitated by coapplication of cocaine (1 μmol/L), a blocker of dopamine reuptake. To evaluate VTA dopamine terminals in PFC slices, we transfected VTA dopamine cells of TH::Cre rats in vivo with Cre‐dependent AAVs to express channelrhodopsin‐2 (ChR2) or designer receptors exclusively activated by designer drugs (DREADDS). In PFC slices from these animals, stimulation of VTA terminals with either blue light to activate ChR2 or bath application of clozapine‐N‐oxide (CNO) to activate Gq‐DREADDs produced a similar reduction in intrinsic inhibition in PL neurons. Electrophysiological recordings from cells expressing retrograde fluorescent tracers showed that this plasticity occurs in PL neurons projecting to the accumbens core. Collectively, these data highlight an ability of VTA terminals to gate intrinsic inhibition in the PFC, and under appropriate circumstances, enhance PL neuronal firing. These cellular actions of dopamine may be important for dopamine‐dependent behaviors involving cocaine and cue‐reward associations within cortical–striatal circuits.

Combined Social and Spatial Coding in a Descending Projection from the Prefrontal Cortex

Social behaviors are crucial to all mammals. Although the prelimbic cortex (PL, part of medial prefrontal cortex) has been implicated in social behavior, it is not clear which neurons are relevant or how they contribute. We found that PL contains anatomically and molecularly distinct subpopulations that target three downstream regions that have been implicated in social behavior: the nucleus accumbens (NAc), amygdala, and ventral tegmental area. Activation of NAc-projecting PL neurons (PL-NAc), but not the other subpopulations, decreased the preference for a social target. To determine what information PL-NAc neurons convey, we selectively recorded from them and found that individual neurons were active during social investigation, but only in specific spatial locations. Spatially specific manipulation of these neurons bidirectionally regulated the formation of a social-spatial association. Thus, the unexpected combination of social and spatial information within the PL-NAc may contribute to social behavior by supporting social-spatial learning.

An appetite for life: brain regulation of hunger and satiety

Obesity results from the consumption of food in excess of bodily energy requirements, with the excess energy stored as adipose tissue. Sequelae of obesity, such as heart disease and type 2 diabetes, consistently rank among the top causes of death worldwide. The global prevalence of obesity highlights the urgency of understanding the mechanisms regulating hunger and satiety. Appetite, defined as the motivational drive to obtain food, is regulated by a complex neurocircuitry which integrates a variety of interoceptive signals to gauge nutritional state and guide appropriate levels of food-seeking. Here we review key recent developments in the identification of cell groups, neural circuits, endogenous and exogenous substances, and intracellular signaling pathways which drive hunger and satiety. We also consider particularly promising pharmacological targets for appetite modulation.

Cognitive impairment and gene expression alterations in a rodent model of binge eating disorder

Binge eating disorder (BED) is defined as recurrent, distressing over-consumption of palatable food (PF) in a short time period. Clinical studies suggest that individuals with BED may have impairments in cognitive processes, executive functioning, impulse control, and decision-making, which may play a role in sustaining binge eating behavior. These clinical reports, however, are limited and often conflicting. In this study, we used a limited access rat model of binge-like behavior in order to further explore the effects of binge eating on cognition. In binge eating prone (BEP) rats, we found novel object recognition (NOR) as well as Barnes maze reversal learning (BM-RL) deficits. Aberrant gene expression of brain derived neurotrophic factor (Bdnf) and tropomyosin receptor kinase B (TrkB) in the hippocampus (HPC)-prefrontal cortex (PFC) network was observed in BEP rats. Additionally, the NOR deficits were correlated with reductions in the expression of TrkB and insulin receptor (Ir) in the CA3 region of the hippocampus. Furthermore, up-regulation of serotonin-2C (5-HT_2C) receptors in the orbitoprefrontal cortex (OFC) was associated with BM-RL deficit. Finally, in the nucleus accumbens (NAc), we found decreased dopamine receptor 2 (Drd2) expression among BEP rats. Taken together, these data suggest that binge eating vegetable shortening may induce contextual and reversal learning deficits which may be mediated, at least in part, by the altered expression of genes in the CA3-OFC-NAc neural network.

Optogenetics and pharmacogenetics: principles and applications

Remote and selective spatiotemporal control of the activity of neurons to regulate behavior and physiological functions has been a long-sought goal in system neuroscience. Identification and subsequent bioengineering of light-sensitive ion channels (e.g., channelrhodopsins, halorhodopsin, and archaerhodopsins) from the bacteria have made it possible to use light to artificially modulate neuronal activity, namely optogenetics. Recent advance in genetics has also allowed development of novel pharmacological tools to selectively and remotely control neuronal activity using engineered G protein-coupled receptors, which can be activated by otherwise inert drug-like small molecules such as the designer receptors exclusively activated by designer drug, a form of chemogenetics. The cutting-edge optogenetics and pharmacogenetics are powerful tools in neuroscience that allow selective and bidirectional modulation of the activity of defined populations of neurons with unprecedented specificity. These novel toolboxes are enabling significant advances in deciphering how the nervous system works and its influence on various physiological processes in health and disease. Here, we discuss the fundamental elements of optogenetics and chemogenetics approaches and some of the applications that yielded significant advances in various areas of neuroscience and beyond.

Projection targets of medial frontal D1DR-expressing neurons

Prefrontal neurons expressing D1-type dopamine receptors (D1DRs) have been implicated in a variety of cognitive processes including working memory and timing. Although D1DRs are most strongly expressed on layer V/VI projection neurons, it is unknown which brain areas are specifically targeted by these projections. Here we selectively marked D1DR neurons using cre-loxP techniques with AAV carrying mCherry fluorescent protein, and traced projection targets of D1DR+ neurons in the mouse medial frontal cortex (MFC). We found relatively strong MFC D1DR+ projections to cortical areas as well as projections to basal ganglia and thalamic nuclei. We found relatively weaker MFC D1DR+ projections to the brainstem, hypothalamus, and other subcortical nuclei. These data intimate that MFC D1DR+ projections are well-positioned to powerfully influence cortical processing and have subcortical specificity. Thus MFC D1DR+ projection neurons may play a key role in tuning cortical networks during goal-directed behavior.

Prefrontal Dopamine D1 and D2 Receptors Regulate Dissociable Aspects of Decision Making via Distinct Ventral Striatal and Amygdalar Circuits

Mesocortical dopamine (DA) regulates a variety of cognitive functions via actions on D1 and/or D2 receptors. For example, risk/reward decision making is modulated differentially by these two receptors within the prefrontal cortex (PFC), with D2 receptors enabling flexible decision making and D1 receptors promoting persistence in choice biases. However, it is unclear how DA mediates opposing patterns of behavior by acting on different receptors within the same terminal region. We explored the possibility that DA may act on separate networks of PFC neurons that are modulated by D1 or D2 receptors and in turn interface with divergent downstream structures such as the basolateral amygdala (BLA) or nucleus accumbens (NAc). Decision making was assessed using a probabilistic discounting task in which well trained male rats chose between small/certain or large/risky rewards, with the odds of obtaining the larger reward changing systematically within a session. Selective disruption of D1 or D2 modulation of separate PFC output pathways was achieved using unilateral intra-PFC infusions of DA antagonists combined with contralateral inactivation of the BLA or NAc. Disrupting D2 (but not D1) modulation of PFC→BLA circuitry impaired adjustments in decision biases in response to changes in reward probabilities. In contrast, disrupting D1 modulation of PFC→NAc networks reduced risky choice, attenuating reward sensitivity and increasing sensitivity to reward omissions. These findings reveal that mesocortical DA can facilitate dissociable components of reward seeking and action selection by acting on different functional networks of PFC neurons that can be distinguished by the subcortical projection targets with which they interface.SIGNIFICANCE STATEMENT Prefrontal cortical dopamine regulates a variety of executive functions governed by the frontal lobes via actions on D1 and D2 receptors. These receptors can in some instances mediate different patterns of behavior, but the mechanisms underlying these dissociable actions are unclear. Using a selective disconnection approach, we reveal that D1 and D2 receptors can facilitate diverse aspects of decision making by acting on separate networks of prefrontal neurons that interface with distinct striatal or amygdalar targets. These findings reveal an additional level of complexity in how mesocortical DA regulates different forms of cognition via actions on different receptors, highlighting how it may act upon distinct cortical microcircuits to drive different patterns of behavior.

Neural Circuit Mechanisms Underlying Emotional Regulation of Homeostatic Feeding

The neural circuits controlling feeding and emotional behaviors are intricately and reciprocally connected. Recent technological developments, including cell type-specific optogenetic and chemogenetic approaches, allow functional characterization of genetically defined cell populations and neural circuits in feeding and emotional processes. Here we review recent studies that have utilized circuit-based manipulations to decipher the functional interactions between neural circuits controlling feeding and those controlling emotional processes. Specifically, we highlight newly described neural circuit interactions between classical emotion-related brain regions, such as the hippocampus and amygdala, and homeostatic feeding circuitry in the arcuate nucleus and lateral hypothalamus (LH). Together these circuits will provide a template for future studies to examine functional interactions between feeding and emotion.

D3 Receptors Regulate Excitability in a Unique Class of Prefrontal Pyramidal Cells

The D3 dopamine receptor, a member of the G_i-coupled D2 family of dopamine receptors, is expressed throughout limbic circuits affected in neuropsychiatric disorders, including prefrontal cortex (PFC). These receptors are important for prefrontal executive function because pharmacological and genetic manipulations that affect prefrontal D3 receptors alter anxiety, social interaction, and reversal learning. However, the mechanisms by which D3 receptors regulate prefrontal circuits and whether D3 receptors regulate specific prefrontal subnetworks remains unknown. Here, we combine dopamine receptor reporter lines, anatomical tracing techniques, and electrophysiology to show that D3 receptor expression defines a novel subclass of layer 5 glutamatergic pyramidal cell in mouse PFC (either sex). D3-receptor-expressing pyramidal neurons are electrophysiologically and anatomically separable from neighboring neurons expressing D1 or D2 receptors based on their dendritic morphology and subthreshold and suprathreshold intrinsic excitability. D3-receptor-expressing neurons send axonal projections to intratelencephalic (IT) targets, including contralateral cortex, nucleus accumbens, and basolateral amygdala. Within these neurons, D3 receptor activation was found to regulate low-voltage-activated Ca_V3.2 calcium channels localized to the axon initial segment, which suppressed action potential (AP) excitability, particularly when APs occurred at high frequency. Therefore, these data indicate that D3 receptors regulate the excitability of a unique, IT prefrontal cell population, thereby defining novel circuitry and cellular actions for D3 receptors in PFC.SIGNIFICANCE STATEMENT The D3 dopamine receptor, a member of the G_i-coupled D2 family of dopamine receptors, are expressed throughout limbic circuits, including prefrontal cortex (PFC). They are of broad interest as a site for therapeutic intervention in serious mental illness, yet we know very little about their distribution or function within PFC. Here, we show that D3 receptors define a unique population of glutamatergic principal cells in mouse PFC that largely lack expression of D1 or D2 receptors. Within these cells, we find that D3 receptors regulate the ability to generate high-frequency action potential bursts through mechanisms not supported by other dopamine receptors. These results define unique circuitry and cellular actions for D3 receptors in regulating PFC networks.

Feeding-modulatory effects of mu-opioids in the medial prefrontal cortex: a review of recent findings and comparison to opioid actions in the nucleus accumbens

RATIONALE

Whereas reward-modulatory opioid actions have been intensively studied in subcortical sites such as the nucleus accumbens (Acb), the role of cortical opioid transmission has received comparatively little attention.

OBJECTIVES

The objective of this study is to describe recent findings on the motivational actions of opioids in the prefrontal cortex (PFC), emphasizing studies of food motivation and ingestion. PFC-based opioid effects will be compared/contrasted to those elicited from the Acb, to glean possible common functional principles. Finally, the motivational effects of opioids will be placed within a network context involving the PFC, Acb, and hypothalamus.

RESULTS

Mu-opioid receptor (μ-OR) stimulation in both the Acb and PFC induces eating and enhances food-seeking instrumental behaviors; μ-OR signaling also enhances taste reactivity within a highly circumscribed zone of medial Acb shell. In both the Acb and PFC, opioid-sensitive zones are aligned topographically with the sectors that project to feeding-modulatory zones of the hypothalamus and intact glutamate transmission in the lateral/perifornical (LH-PeF) hypothalamic areas is required for both Acb- and PFC-driven feeding. Conversely, opioid-mediated feeding responses elicited from the PFC are negatively modulated by AMPA signaling in the Acb shell.

CONCLUSIONS

Opioid signaling in the PFC engages functionally opposed PFC➔hypothalamus and PFC➔Acb circuits, which, respectively, drive and limit non-homeostatic feeding, producing a disorganized and "fragmented" pattern of impulsive food-seeking behaviors and hyperactivity. In addition, opioids act directly in the Acb to facilitate food motivation and taste hedonics. Further study of this cortico-striato-hypothalamic circuit, and incorporation of additional opioid-responsive telencephalic structures, could yield insights with translational relevance for eating disorders and obesity.

Hippocampus Contributions to Food Intake Control: Mnemonic, Neuroanatomical, and Endocrine Mechanisms

Food intake is a complex behavior that can occur or cease to occur for a multitude of reasons. Decisions about where, when, what, and how much to eat are not merely reflexive responses to food-relevant stimuli or to changes in energy status. Rather, feeding behavior is modulated by various contextual factors and by previous experiences. The data reviewed here support the perspective that neurons in multiple hippocampal subregions constitute an important neural substrate linking the external context, the internal context, and mnemonic and cognitive information to control both appetitive and ingestive behavior. Feeding behavior is heavily influenced by hippocampal-dependent mnemonic functions, including episodic meal-related memories and conditional learned associations between food-related stimuli and postingestive consequences. These mnemonic processes are undoubtedly influenced by both external and internal factors relating to food availability, location, and physiological energy status. The afferent and efferent neuroanatomical connectivity of the subregions of the hippocampus is reviewed with regard to the integration of visuospatial and olfactory sensory information (the external context) with endocrine and gastrointestinal interoceptive stimuli (the internal context). Also discussed are recent findings demonstrating that peripherally derived endocrine signals act on receptors in hippocampal neurons to reduce (leptin, glucagon-like peptide-1) or increase (ghrelin) food intake and learned food reward-driven responding, thereby highlighting endocrine and neuropeptidergic signaling in hippocampal neurons as a novel substrate of importance in the higher-order regulation of feeding behavior.

Preliminary evidence of sex differences in behavioral and neural responses to palatable food reward in rats

The female bias in eating disorder prevalence is the largest in all of psychiatry. Binge eating on palatable food (PF) is a core, maladaptive symptom that cuts across all major types of eating disorders and can be studied via animal models. Using an individual differences rat model of binge eating that identifies binge eating prone (BEP) and binge eating resistant (BER) phenotypes, we previously showed that, compared with males, females consume more PF and are more likely to be classified as BEP. One potential explanation for this sex difference is that PF is inherently more rewarding to females, leading to higher rates of binge eating. Here we tested the hypothesis that females have more robust behavioral and neural responses to PF reward than males. Adult male (N=18) and female (N=17) Sprague-Dawley rats were exposed to the Conditioned Place Preference paradigm using PF as the unconditioned stimulus. Select males (N=9) and females (N=9) were video-recorded during three of the PF-paired conditioning sessions to score feeding behavior. Following CPP, 13 male and 12 female rats were exposed to PF just prior to sacrifice to induce expression of the neural activation marker Fos, and Fos expression was quantified in mesocorticolimbic, hypothalamic, and amygdalar circuits. In the CPP paradigm, females displayed a more robust shift in preference for the chamber paired with PF compared with males, and behavioral analyses revealed that average duration of individual feeding bouts during pairing sessions was longer in females than in males. Fos expression was significantly higher in females vs. males in select regions of the mesocorticolimbic reward circuit, with no sex differences in hypothalamic or amygdalar regions. These results provide initial evidence that PF may be more rewarding to females than to males, possibly due to heightened responsiveness of neural substrates that mediate the hedonic and motivational responses to PF, which in part, may underlie sex differences in binge eating proneness.

Regulation of Alcohol Extinction and Cue-Induced Reinstatement by Specific Projections among Medial Prefrontal Cortex, Nucleus Accumbens, and Basolateral Amygdala

The ability to inhibit drinking is a significant challenge for recovering alcoholics, especially in the presence of alcohol-associated cues. Previous studies have demonstrated that the regulation of cue-guided alcohol seeking is mediated by the basolateral amygdala (BLA), nucleus accumbens (NAc), and medial prefrontal cortex (mPFC). However, given the high interconnectivity between these structures, it is unclear how mPFC projections to each subcortical structure, as well as projections between BLA and NAc, mediate alcohol-seeking behaviors. Here, we evaluate how cortico-striatal, cortico-amygdalar, and amygdalo-striatal projections control extinction and relapse in a rat model of alcohol seeking. Specifically, we used a combinatorial viral technique to express diphtheria toxin receptors in specific neuron populations based on their projection targets. We then used this strategy to create directionally selective ablations of three distinct pathways after acquisition of ethanol self-administration but before extinction and reinstatement. We demonstrate that ablation of mPFC neurons projecting to NAc, but not BLA, blocks cue-induced reinstatement of alcohol seeking and neither pathway is necessary for extinction of responding. Further, we show that ablating BLA neurons that project to NAc disrupts extinction of alcohol approach behaviors and attenuates reinstatement. Together, these data provide evidence that the mPFC→NAc pathway is necessary for cue-induced reinstatement of alcohol seeking, expand our understanding of how the BLA→NAc pathway regulates alcohol behavior, and introduce a new methodology for the manipulation of target-specific neural projections.SIGNIFICANCE STATEMENT The vast majority of recovering alcoholics will relapse at least once and understanding how the brain regulates relapse will be key to developing more effective behavior and pharmacological therapies for alcoholism. Given the high interconnectivity of cortical, striatal, and limbic structures that regulate alcohol intake, it has been difficult to disentangle how separate projections between them may control different aspects of these complex behaviors. Here, we demonstrate a new approach for noninvasively ablating each of these pathways and testing their necessity for both extinction and relapse. We show that inputs to the nucleus accumbens from medial prefrontal cortex and amygdala regulate alcohol-seeking behaviors differentially, adding to our understanding of the neural control of alcoholism.

Distinct recruitment of basolateral amygdala-medial prefrontal cortex pathways across Pavlovian appetitive conditioning

Associative learning can enable environmental cues to signal food and stimulate feeding, independent of physiological hunger. Two forebrain regions necessary in cue driven feeding, the basolateral area of the amygdala and the medial prefrontal cortex, communicate via extensive, topographically organized connections. The basolateral nucleus (BLA) sends extensive projections to the prelimbic cortex (PL), and our aim here was to determine if this pathway was selectively recruited during cue-food associative learning. The anterior and posterior basolateral nuclei are recruited during different phases of cue-food learning, and thus we examined whether distinct pathways that originate in these nuclei and project to the PL are differently recruited during early and late stages of learning. To accomplish this we used neuroanatomical tract tracing combined with the detection of Fos induction. To identify projecting neurons within the BLA, prior to training, rats received a retrograde tracer, Fluoro-Gold (FG) into the PL. Rats were given either one or ten sessions of tone-food presentations (Paired group) or tone-only presentations (Control group). The Paired group learned the tone-food association quickly and robustly and had greater Fos induction within the anterior and posterior BLA during early and late learning compared to the Control group. Notably, the Paired group had more double-labeled neurons (FG + Fos) during late training compared to the Control group, specifically in the anterior BLA. This demonstrates selective recruitment of the anterior BLA-PL pathway by late cue-food learning. These findings indicate plasticity and specificity in the BLA-PL pathways across cue-food associative learning.

Lip Sync: Gamma Rhythms Orchestrate Top-Down Control of Feeding Circuits

Feeding involves the coordinated action of networks across many brain regions. New work recently published in Nature indicates that the prefrontal cortex, septum, and lateral hypothalamus circuits synchronize at gamma rhythms (30-90 Hz) to regulate feeding-related behaviors (Carus-Cadavieco et al., 2017).

Test-retest measurements of dopamine D1-type receptors using simultaneous PET/MRI imaging

PURPOSE

The role of dopamine D₁-type receptor (D₁R)-expressing neurons in the regulation of motivated behavior and reward prediction has not yet been fully established. As a prerequisite for future research assessing D₁-mediated neuronal network regulation using simultaneous PET/MRI and D₁R-selective [¹¹C]SCH23390, this study investigated the stability of central D₁R measurements between two independent PET/MRI sessions under baseline conditions.

METHODS

Thirteen healthy volunteers (7 female, age 33 ± 13 yrs) underwent 90-min emission scans, each after 90-s bolus injection of 486 ± 16 MBq [¹¹C]SCH23390, on two separate days within 2-4 weeks using a PET/MRI system. Parametric images of D₁R distribution volume ratio (DVR) and binding potential (BP_ND) were generated by a multi-linear reference tissue model with two parameters and the cerebellar cortex as receptor-free reference region. Volume-of-interest (VOI) analysis was performed with manual VOIs drawn on consecutive transverse MRI slices for brain regions with high and low D₁R density.

RESULTS

The DVR varied from 2.5 ± 0.3 to 2.9 ± 0.5 in regions with high D₁R density (e.g. the head of the caudate) and from 1.2 ± 0.1 to 1.6 ± 0.2 in regions with low D₁R density (e.g. the prefrontal cortex). The absolute variability of the DVR ranged from 2.4% ± 1.3% to 5.1% ± 5.3%, while Bland-Altman analyses revealed very low differences in mean DVR (e.g. 0.013 ± 0.17 for the nucleus accumbens). Intraclass correlation (one-way, random) indicated very high agreement (0.93 in average) for both DVR and BP_ND values. Accordingly, the absolute variability of BP_ND ranged from 7.0% ± 4.7% to 12.5% ± 10.6%; however, there were regions with very low D₁R content, such as the occipital cortex, with higher mean variability.

CONCLUSION

The test-retest reliability of D₁R measurements in this study was very high. This was the case not only for D₁R-rich brain areas, but also for regions with low D₁R density. These results will provide a solid base for future joint PET/MRI data analyses in stimulation-dependent mapping of D₁R-containing neurons and their effects on projections in neuronal circuits that determine behavior.

Integrated Control of Predatory Hunting by the Central Nucleus of the Amygdala

Superior predatory skills led to the evolutionary triumph of jawed vertebrates. However, the mechanisms by which the vertebrate brain controls predation remain largely unknown. Here, we reveal a critical role for the central nucleus of the amygdala in predatory hunting. Both optogenetic and chemogenetic stimulation of central amygdala of mice elicited predatory-like attacks upon both insect and artificial prey. Coordinated control of cervical and mandibular musculatures, which is necessary for accurately positioning lethal bites on prey, was mediated by a central amygdala projection to the reticular formation in the brainstem. In contrast, prey pursuit was mediated by projections to the midbrain periaqueductal gray matter. Targeted lesions to these two pathways separately disrupted biting attacks upon prey versus the initiation of prey pursuit. Our findings delineate a neural network that integrates distinct behavioral modules and suggest that central amygdala neurons instruct predatory hunting across jawed vertebrates.

Optogenetic Stimulation of Frontal D1 Neurons Compensates for Impaired Temporal Control of Action in Dopamine-Depleted Mice

Disrupted mesocortical dopamine contributes to cognitive symptoms of Parkinson's disease (PD). Past work has implicated medial frontal neurons expressing D1 dopamine receptors (D1DRs) in temporal processing. Here, we investigated whether these neurons can compensate for behavioral deficits resulting from midbrain dopamine dysfunction. We report three main results. First, both PD patients and mice with ventral tegmental area (VTA) dopamine depletion had attenuated delta activity (1-4 Hz) in the medial frontal cortex (MFC) during interval timing. Second, we found that optogenetically stimulating MFC D1DR neurons could increase ramping activity among MFC neurons. Finally, stimulating MFC D1DR neurons specifically at delta frequencies (2 Hz) compensated for deficits in temporal control of action caused by VTA dopamine depletion. Our results suggest that cortical networks can be targeted by frequency-specific brain stimulation to improve dopamine-dependent cognitive processing.

Reduction of Cocaine-Induced Locomotor Effects by Enriched Environment Is Associated with Cell-Specific Accumulation of ΔFosB in Striatal and Cortical Subregions

BACKGROUND

Early exposure to enriched environments has been shown to decrease the locomotor effects induced by repeated injections of cocaine and modify basal and cocaine-induced total protein levels of the transcription factor ΔFosB in the whole striatum of mice. In this study, we aimed at characterizing whether the profile of ΔFosB accumulation induced by enriched environments and cocaine would be similar or different in terms of brain areas and cell type.

METHODS

We used mice expressing the eGFP protein in D1 receptor positive (D1R(+)) neurons to determine whether Δ FosB induced by enriched environment or cocaine injections (5×15 mg/kg) would occur in selective subpopulations of neurons in several subregions of the striatum and prefrontal cortex.

RESULTS

We found that: (1) exposure to enriched environment reduces cocaine-induced locomotor activation, confirming our previous findings; (2) exposure to enriched environment by itself increases the accumulation of Δ FosB mostly in D1R(-) cells in the shell part of the nucleus accumbens and dorsal striatum, whereas in the nucleus accumbens core, Δ FosB accumulates in both D1R(+) and D1R(-) neurons; (3) in standard environment mice, cocaine induces accumulation of Δ FosB selectively in D1R(+) cells in the nucleus accumbens, dorsal striatum, and infralimbic cortex; and (4) the effects of enriched environments and cocaine on accumulation of Δ FosB were reciprocally blocked by their combination.

CONCLUSIONS

Altogether, these results suggest that the enriched environment-induced reduction in behavioral effects of cocaine might result from 2 distinct effects on ΔFosB in striatal medium-sized spiny neurons belonging to the direct and indirect pathways.

Chemogenetic inhibition of the medial prefrontal cortex reverses the effects of REM sleep loss on sucrose consumption

Rapid eye movement (REM) sleep loss is associated with increased consumption of weight-promoting foods. The prefrontal cortex (PFC) is thought to mediate reward anticipation. However, the precise role of the PFC in mediating reward responses to highly palatable foods (HPF) after REM sleep deprivation is unclear. We selectively reduced REM sleep in mice over a 25–48 hr period and chemogenetically inhibited the medial PFC (mPFC) by using an altered glutamate-gated and ivermectin-gated chloride channel that facilitated neuronal inhibition through hyperpolarizing infected neurons. HPF consumption was measured while the mPFC was inactivated and REM sleep loss was induced. We found that REM sleep loss increased HPF consumption compared to control animals. However, mPFC inactivation reversed the effect of REM sleep loss on sucrose consumption without affecting fat consumption. Our findings provide, for the first time, a causal link between REM sleep, mPFC function and HPF consumption.

DOI:http://dx.doi.org/10.7554/eLife.20269.001

Gamma-Aminobutyric Acidergic Projections From the Dorsal Raphe to the Nucleus Accumbens Are Regulated by Neuromedin U

BACKGROUND

Neuromedin U (NMU) is a neuropeptide enriched in the nucleus accumbens shell (NAcSh), a brain region associated with reward. While NMU and its receptor, NMU receptor 2 (NMUR2), have been studied for the ability to regulate food reward, NMU has not been studied in the context of drugs of abuse (e.g., cocaine). Furthermore, the neuroanatomical pathways that express NMUR2 and its ultrastructural localization are unknown.

METHODS

Immunohistochemistry was used to determine the synaptic localization of NMUR2 in the NAcSh and characterize which neurons express this receptor (n = 17). The functional outcome of NMU on NMUR2 was examined using microdialysis (n = 16). The behavioral effects of NMU microinjection directly to the NAcSh were investigated using cocaine-evoked locomotion (n = 93). The specific effects of NMUR2 knockdown on cocaine-evoked locomotion were evaluated using viral-mediated RNA interference (n = 40).

RESULTS

NMUR2 is localized to presynaptic gamma-aminobutyric acidergic nerve terminals in the NAcSh originating from the dorsal raphe nucleus. Furthermore, NMU microinjection to the NAcSh decreased local gamma-aminobutyric acid concentrations. Next, we evaluated the effects of NMU microinjection on behavioral sensitization to cocaine. When repeatedly administered throughout the sensitization regimen, NMU attenuated cocaine-evoked hyperactivity. Additionally, small hairpin RNA-mediated knockdown of presynaptic NMUR2 in the NAcSh using a retrograde viral vector potentiated cocaine sensitization.

CONCLUSIONS

Together, these data reveal that NMUR2 modulates a novel gamma-aminobutyric acidergic pathway from the dorsal raphe nucleus to the NAcSh to influence behavioral responses to cocaine.

Amygdala Circuits for Fear Memory: A Key Role for Dopamine Regulation

In addition to modulating a number of cognitive functions including reward, punishment, motivation, and salience, dopamine (DA) plays a pivotal role in regulating threat-related emotional memory. Changes in neural circuits of the amygdala nuclei are also critically involved in the acquisition and expression of emotional memory. In this review, we summarize the regulation of amygdala circuits by DA. Specifically, we describe DA signaling in the amygdala, and DA regulation of synaptic transmission and synaptic plasticity of the amygdala neurons. Finally, we discuss a potential contribution of DA-related mechanisms to the pathogenesis of posttraumatic stress disorder.

Effects of D1 receptor knockout on fear and reward learning

Dopamine signaling is involved in a variety of neurobiological processes that contribute to learning and memory. D1-like dopamine receptors (including D1 and D5 receptors) are thought to be involved in memory and reward processes, but pharmacological approaches have been limited in their ability to distinguish between D1 and D5 receptors. Here, we examine the effects of a specific knockout of D1 receptors in associative learning tasks involving aversive (shock) or appetitive (cocaine) unconditioned stimuli. We find that D1 knockout mice show similar levels of cued and contextual fear conditioning to WT controls following conditioning protocols involving one, two, or four shocks. D1 knockout mice show increased generalization of fear conditioning and extinction across contexts, revealed as increased freezing to a novel context following conditioning and decreased freezing to an extinguished cue during a contextual renewal test. Further, D1 knockout mice show mild enhancements in extinction following an injection of SKF81297, a D1/D5 receptor agonist, suggesting a role for D5 receptors in extinction enhancements induced by nonspecific pharmacological agonists. Finally, although D1 knockout mice show decreased locomotion induced by cocaine, they are able to form a cocaine-induced conditioned place preference. We discuss these findings in terms of the role of dopamine D1 receptors in general learning and memory processes.

Innate Fear-Induced Weight Regulation in the C57BL/6J Mouse

Regulation of body weight is an important strategy for small prey animals to avoid capture. Field and laboratory studies have shown that prey animals reduce body size when subjected to long-term predator stimuli. However, the causes of predator-induced weight regulation are highly variable and the underlying mechanisms remain unclear. Understanding this phenomenon is important for gaining a better understanding of how animals regulate body weight under ethologically relevant conditions and has implications for obesity. Here we expose inbred C57BL/6J mice to a fear-inducing odorant (2,4,5-trimethylthiazole; mT) to model predation-induced weight regulation. Eight week-old mice were put on a 45% high fat diet (HFD) or chow diet (5% fat) and exposed daily to mT, an equally aversive dose of butyric acid (BA), or a neutral control scent (almond). mT-exposed mice in both diet groups gained significantly less weight over a 6-week period than BA-exposed mice. This differential weight gain appears unlikely to be due to differences in food intake and activity level, or brown adipose thermogenesis between the mT and BA groups. However, following chronic mT exposure we find increases in ΔFosB protein, a marker for long-term neural plasticity, in the dorsomedial hypothalamus (DMH)-an area previously implicated in chronic stress and defensive responses, as well as weight regulation. This study establishes a simplified and robust laboratory model of predation-mediated weight regulation with inbred lab mice and fear-inducing odor, and suggests a likely, yet undetermined, metabolic adaptation as contributing to this response.

Amphetamine alters neural response to sucrose in healthy women

Amphetamine, likely via action on the brain's dopaminergic systems, induces anorectic eating behavior and blunts dopaminergic midbrain activation to rewards. Past work has hypothesized that this blunted reward responsivity is a result of increasing tonic over phasic DA activity. We sought to extend past findings to sweet taste during fMRI following single-blind administration of dextroamphetamine and placebo in 11 healthy women. We hypothesized that neural response in both limbic and cognitive sweet taste circuits would mirror past work with monetary rewards by effectively blunting sweet taste reward, and 'equalizing' it's rewarding taste with receipt of water. Behavioral results showed that amphetamine reduced self-reported hunger (supporting the existence of amphetamine anorexia) and increased self-report euphoria. In addition, region of Interest analysis revealed significant treatment by taste interactions in the middle insula and dorsal anterior cingulate confirming the 'equalizing' hypothesis in the cingulate, but unlike monetary reinforcers, the insula actually evinced enhanced separation between tastes on the amphetamine day. These results suggest a divergence from prior research using monetary reinforcers when extended to primary reinforcers, and may hint that altering dopaminergic signaling in the insula and anterior cingulate may be a target for pharmacological manipulation of appetite, and the treatment of obesity.