Double deletion of melanocortin 4 receptors and SAPAP3 corrects compulsive behavior and obesity in mice

The midline thalamic nucleus reuniens promotes compulsive-like grooming in rodents

Obsessive-compulsive disorder (OCD), a disabling and notoriously treatment-resistant neuropsychiatric disorder, affects 2-3% of the general population and is characterized by recurring, intrusive thoughts (obsessions) and repetitive, ritualistic behaviors (compulsions). Although long associated with dysfunction within the cortico-striato-thalamic-cortical circuits, the thalamic role in OCD pathogenesis remains highly understudied in the literature. Here, we identified a rat thalamic nucleus - the reuniens (NRe) - that mediates persistent, compulsive self-grooming behavior. Optogenetic activation of this nucleus triggers immediate, excessive grooming with strong irresistibility, increases anxiety, and induces negative affective valence. A thalamic-hypothalamic pathway linking NRe to the dorsal premammillary nucleus (PMd) was discovered to mediate excessive self-grooming behavior and render it a defensive coping response to stress, mirroring the compulsions faced by OCD patients. Given the close resemblance between this self-grooming behavior and the clinical manifestations of OCD, the results from this study highlight the role of NRe in mediating OCD-like compulsive behaviors. This can be attributed to NRe's position at the nexus of an extensive frontal-striatal-thalamic network regulating cognition, emotion, and stress-related behaviors, suggesting NRe as a potential novel target for intervention.

Action inflexibility and compulsive-like behavior accompany neurobiological alterations in the anterior orbitofrontal cortex and associated striatal nuclei

The orbitofrontal cortex (OFC) is a large cortical structure, expansive across anterior-posterior axes. It is essential for flexibly updating learned behaviors, and paradoxically, also implicated in inflexible and compulsive-like behaviors. Here, we investigated mice bred to display inflexible reward-seeking behaviors that are insensitive to action consequences. We found that these mice also demonstrate insensitivity to Pavlovian-to-instrumental transfer, as well as compulsive-like grooming behavior that is ameliorated by fluoxetine and inhibitory, but not excitatory, chemogenetic modulation of excitatory OFC neurons. Thus, these mice offer the opportunity to identify neurobiological factors associated with inflexible and compulsive-like behavior. Experimentally bred mice suffer excitatory dendritic spine attrition, as well as changes in inhibitory synapse-associated proteins, GAD67/GAD1 and SLITRK3, largely in the anterior and not posterior OFC (or medial frontal cortex). They also display higher levels of the excitatory synaptic marker striatin in the nucleus accumbens and lower levels of the excitatory synaptic marker SAPAP3 in the dorsal striatum, striatal nuclei that receive input from the anterior OFC. Together, our findings point to the anterior OFC as a potential locus controlling action flexibility and compulsive-like behavior alike.

New horizons for obsessive-compulsive disorder drug discovery: is targeting glutamate receptors the answer?

INTRODUCTION

Over the past decade, glutamate has emerged as a prominent focus in the field of obsessive-compulsive disorder (OCD) pathophysiology. A convergence of evidence from genetic, preclinical, and clinical studies points to glutamatergic dysfunction as a key feature of this condition. In light of these findings, there has been a growing interest in exploring the potential of glutamatergic agents in the treatment of OCD.

AREAS COVERED

This paper reviews the literature on glutamate transmission in OCD. In addition, the authors examine the results of clinical trials investigating the efficacy of glutamatergic agents in the treatment of OCD patients.

EXPERT OPINION

Along with the recognition of neuroinflammation in the brain in OCD, the evidence of glutamate dysfunction represents one of the most promising recent discoveries for understanding the mechanisms involved in OCD. The importance of this discovery lies primarily in its pharmacological implications and has led to intense research activity in the field of glutamatergic agents. While this research has not yet had a substantial clinical impact, targeting glutamate receptors remains a promising horizon for the successful treatment of OCD patients.

The genetics of trichotillomania and excoriation disorder: A systematic review

BACKGROUND

Trichotillomania (TTM) and excoriation disorder (ED) are impairing obsessive-compulsive related disorders that are common in the general population and for which there are no clear first-line medications, highlighting the need to better understand the underlying biology of these disorders to inform treatments. Given the importance of genetics in obsessive-compulsive disorder (OCD), evaluating genetic factors underlying TTM and ED may advance knowledge about the pathophysiology of these body-focused repetitive behaviors.

AIM

In this systematic review, we summarize the available evidence on the genetics of TTM and ED and highlight gaps in the field warranting further research.

METHOD

We systematically searched Embase, PsycInfo, PubMed, Medline, Scopus, and Web of Science for original studies in genetic epidemiology (family or twin studies) and molecular genetics (candidate gene and genome-wide) published up to June 2023.

RESULTS

Of the 3536 records identified, 109 studies were included in this review. These studies indicated that genetic factors play an important role in the development of TTM and ED, some of which may be shared across the OCD spectrum, but there are no known high-confidence specific genetic risk factors for either TTM or ED.

CONCLUSIONS

Our review underscores the need for additional genome-wide research conducted on the genetics of TTM and ED, for instance, genome-wide association and whole-genome/whole-exome DNA sequencing studies. Recent advances in genomics have led to the discovery of risk genes in several psychiatric disorders, including related conditions such as OCD, but to date, TTM and ED have remained understudied.

A molecularly defined orbitofrontal cortical neuron population controls compulsive-like behavior, but not inflexible choice or habit

Habits are familiar behaviors triggered by cues, not outcome predictability, and are insensitive to changes in the environment. They are adaptive under many circumstances but can be considered antecedent to compulsions and intrusive thoughts that drive persistent, potentially maladaptive behavior. Whether compulsive-like and habit-like behaviors share neural substrates is still being determined. Here, we investigated mice bred to display inflexible reward-seeking behaviors that are insensitive to action consequences. We found that these mice demonstrate habitual response biases and compulsive-like grooming behavior that was reversible by fluoxetine and ketamine. They also suffer dendritic spine attrition on excitatory neurons in the orbitofrontal cortex (OFC). Nevertheless, synaptic melanocortin 4 receptor (MC4R), a factor implicated in compulsive behavior, is preserved, leading to the hypothesis that Mc4r+ OFC neurons may drive aberrant behaviors. Repeated chemogenetic stimulation of Mc4r+ OFC neurons triggered compulsive and not inflexible or habitual response biases in otherwise typical mice. Thus, Mc4r+ neurons within the OFC appear to drive compulsive-like behavior that is dissociable from habitual behavior. Understanding which neuron populations trigger distinct behaviors may advance efforts to mitigate harmful compulsions.

Human microbiota from drug-naive patients with obsessive-compulsive disorder drives behavioral symptoms and neuroinflammation via succinic acid in mice

Emerging evidence suggests that the gut microbiota is closely related to psychiatric disorders. However, little is known about the role of the gut microbiota in the development of obsessive-compulsive disorder (OCD). Here, to investigate the contribution of gut microbiota to the pathogenesis of OCD, we transplanted fecal microbiota from first-episode, drug-naive OCD patients or demographically matched healthy individuals into antibiotic-treated specific pathogen-free (SPF) mice and showed that colonization with OCD microbiota is sufficient to induce core behavioral deficits, including abnormal anxiety-like and compulsive-like behaviors. The fecal microbiota was analyzed using 16 S rRNA full-length sequencing, and the results demonstrated a clear separation of the fecal microbiota of mice colonized with OCD and control microbiota. Notably, microbiota from OCD-colonized mice resulted in injured neuronal morphology and function in the mPFC, with inflammation in the mPFC and colon. Unbiased metabolomic analyses of the serum and mPFC region revealed the accumulation of succinic acid (SA) in OCD-colonized mice. SA impeded neuronal activity and induced an inflammatory response in both the colon and mPFC, impacting intestinal permeability and brain function, which act as vital signal mediators in gut microbiota-brain-immune crosstalk. Manipulations of dimethyl malonate (DM) have been reported to exert neuroprotective effects by suppressing the oxidation of accumulated succinic acid, attenuating the downstream inflammatory response and neuronal damage, and can help to partly improve abnormal behavior and reduce neuroinflammation and intestinal inflammation in OCD-colonized mice. We propose that the gut microbiota likely regulates brain function and behaviors in mice via succinic acid signaling, which contributes to the pathophysiology of OCD through gut-brain crosstalk and may provide new insights into the treatment of this disorder.

Informational Analysis and Prediction of Obsessive-Compulsive Disorder Pathogenesis

OBJECTIVE

We aimed to predict the possible mechanism of obsessive-compulsive disorder (OCD) by integrating and analyzing mRNA sequencing results from two datasets and to provide direction for future studies into the pathogenesis of OCD.

METHODS

Two OCD datasets, GSE78104 and GSE60190, were obtained, and the intersection of the two gene sets with differential expression in OCD samples was selected. Kyoto Encyclopedia of Genes and Genomes (KEGG) signal pathway enrichment and Gene Ontology (GO) analyses were performed using the Database for Annotation, Visualization, and Integrated Discovery (DAVID) online analysis website for the genes at the intersection, and the data were mapped using http://www.bioinformatics.com.cn. After genes with p≤0.05 had been screened out, protein-protein interaction (PPI) interaction analysis was conducted using Metascape to screen the key Molecular Complex Detection (MCODE) genes. MCODE genes were then enriched using the KEGG signaling pathway and GO classification.

RESULTS

A total of 3,449 differentially expressed genes (DEGs) were obtained from the GSE78104 and GSE60190 datasets. KEGG, GO, and Gene Set Enrichment Analysis analyses of DEGs showed that the onset of OCD was related to oxidative phosphorylation and other metabolic processes, which may have a similar pathogenesis to other neurodegenerative diseases. Single-gene PPI analysis of SAPAP3 revealed that the mechanism by which SAPAP3 knockout induces OCD may also be caused by affecting oxidative phosphorylation.

CONCLUSION

The mechanism of SAPAP3 knockout-induced OCD in mice may be due to the oxidative phosphorylation process in the body. Future studies on the neural circuit mechanism of OCD should be conducted.

Lateral septum as a melanocortin downstream site in obesity development

The melanocortin pathway is well established to be critical for body-weight regulation in both rodents and humans. Despite extensive studies focusing on this pathway, the downstream brain sites that mediate its action are not clear. Here, we found that, among the known paraventricular hypothalamic (PVH) neuron groups, those expressing melanocortin receptors 4 (PVH^Mc4R) preferably project to the ventral part of the lateral septum (LSv), a brain region known to be involved in emotional behaviors. Photostimulation of PVH^Mc4R neuron terminals in the LSv reduces feeding and causes aversion, whereas deletion of Mc4Rs or disruption of glutamate release from LSv-projecting PVH neurons causes obesity. In addition, disruption of AMPA receptor function in PVH-projected LSv neurons causes obesity. Importantly, chronic inhibition of PVH- or PVH^Mc4R-projected LSv neurons causes obesity associated with reduced energy expenditure. Thus, the LSv functions as an important node in mediating melanocortin action on body-weight regulation.

Peripheral actions and direct central-local communications of melanocortin 4 receptor signaling

Melanocortin 4 receptor (MC4R), the most important monogenetic cause of human metabolic disorders, has been of great interest to many researchers in the field of energy homeostasis and public health. Because MC4R is a vital pharmaceutical target for maintaining controllable appetite and body weight for professional athletes, previous studies have mainly focused on the central, rather than the peripheral, roles of MC4R. Thus, the local expression of MC4R and its behavioral regulation remain unclear. In an attempt to shed light on different directions for future studies of MC4R signaling, we review a series of recent and important studies exploring the peripheral functions of MC4R and the direct physiological interaction between peripheral organs and central MC4R neurons in this article.

SAPAP Scaffold Proteins: From Synaptic Function to Neuropsychiatric Disorders

Excitatory (glutamatergic) synaptic transmission underlies many aspects of brain activity and the genesis of normal human behavior. The postsynaptic scaffolding proteins SAP90/PSD-95-associated proteins (SAPAPs), which are abundant components of the postsynaptic density (PSD) at excitatory synapses, play critical roles in synaptic structure, formation, development, plasticity, and signaling. The convergence of human genetic data with recent in vitro and in vivo animal model data indicates that mutations in the genes encoding SAPAP1-4 are associated with neurological and psychiatric disorders, and that dysfunction of SAPAP scaffolding proteins may contribute to the pathogenesis of various neuropsychiatric disorders, such as schizophrenia, autism spectrum disorders, obsessive compulsive disorders, Alzheimer's disease, and bipolar disorder. Here, we review recent major genetic, epigenetic, molecular, behavioral, electrophysiological, and circuitry studies that have advanced our knowledge by clarifying the roles of SAPAP proteins at the synapses, providing new insights into the mechanistic links to neurodevelopmental and neuropsychiatric disorders.

The hypothalamus for whole-body physiology: from metabolism to aging

Obesity and aging are two important epidemic factors for metabolic syndrome and many other health issues, which contribute to devastating diseases such as cardiovascular diseases, stroke and cancers. The brain plays a central role in controlling metabolic physiology in that it integrates information from other metabolic organs, sends regulatory projections and orchestrates the whole-body function. Emerging studies suggest that brain dysfunction in sensing various internal cues or processing external cues may have profound effects on metabolic and other physiological functions. This review highlights brain dysfunction linked to genetic mutations, sex, brain inflammation, microbiota, stress as causes for whole-body pathophysiology, arguing brain dysfunction as a root cause for the epidemic of aging and obesity-related disorders. We also speculate key issues that need to be addressed on how to reveal relevant brain dysfunction that underlines the development of these disorders and diseases in order to develop new treatment strategies against these health problems.

Inter-individual variability amplified through breeding reveals control of reward-related action strategies by Melanocortin-4 Receptor in the dorsomedial striatum

In day-to-day life, we often must choose between pursuing familiar behaviors or adjusting behaviors when new strategies might be more fruitful. The dorsomedial striatum (DMS) is indispensable for arbitrating between old and new action strategies. To uncover molecular mechanisms, we trained mice to generate nose poke responses for food, then uncoupled the predictive relationship between one action and its outcome. We then bred the mice that failed to rapidly modify responding. This breeding created offspring with the same tendencies, failing to inhibit behaviors that were not reinforced. These mice had less post-synaptic density protein 95 in the DMS. Also, densities of the melanocortin-4 receptor (MC4R), a high-affinity receptor for α-melanocyte-stimulating hormone, predicted individuals' response strategies. Specifically, high MC4R levels were associated with poor response inhibition. We next found that reducing Mc4r in the DMS in otherwise typical mice expedited response inhibition, allowing mice to modify behavior when rewards were unavailable or lost value. This process required inputs from the orbitofrontal cortex, a brain region canonically associated with response strategy switching. Thus, MC4R in the DMS appears to propel reward-seeking behavior, even when it is not fruitful, while moderating MC4R presence increases the capacity of mice to inhibit such behaviors.

Disruption of prepulse inhibition is associated with compulsive behavior severity and nucleus accumbens dopamine receptor changes in Sapap3 knockout mice

Obsessive compulsive disorder (OCD) is associated with disruption of sensorimotor gating, which may contribute to difficulties inhibiting intrusive thoughts and compulsive rituals. Neural mechanisms underlying these disturbances are unclear; however, striatal dopamine is implicated in regulation of sensorimotor gating and OCD pathophysiology. The goal of this study was to examine the relationships between sensorimotor gating, compulsive behavior, and striatal dopamine receptor levels in Sapap3 knockout mice (KOs), a widely used preclinical model system for OCD research. We found a trend for disruption of sensorimotor gating in Sapap3-KOs using the translational measure prepulse inhibition (PPI); however, there was significant heterogeneity in both PPI and compulsive grooming in KOs. Disruption of PPI was significantly correlated with a more severe compulsive phenotype. In addition, PPI disruption and compulsive grooming severity were associated with reduced dopamine D1 and D2/3 receptor density in the nucleus accumbens core (NAcC). Compulsive grooming progressively worsened in Sapap3-KOs tested longitudinally, but PPI disruption was first detected in high-grooming KOs at 7 months of age. Through detailed characterization of individual differences in OCD-relevant behavioral and neurochemical measures, our findings suggest that NAcC dopamine receptor changes may be involved in disruption of sensorimotor gating and compulsive behavior relevant to OCD.

Alternative Frameworks for Advancing the Study of Eating Disorders

Eating disorders are life-interrupting psychiatric conditions with high morbidity and mortality, yet the basic mechanisms underlying these conditions are understudied compared with other psychiatric disorders. In this opinion, we suggest that recent knowledge gleaned from genomic and neuroimaging investigations of eating disorders in humans presents a rich opportunity to sharpen animal models of eating disorders and to identify neural mechanisms that contribute to the risk and maintenance of these conditions. Our article reflects the state of the science, with a primary focus on anorexia nervosa (AN) and binge-eating behavior, and encourages further study of all conditions categorized under feeding and eating disorders.

Progression of obsessive compulsive disorder-like grooming in Sapap3 knockout mice: A longitudinal [11C]ABP688 PET study

We aimed to evaluate [3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-0-¹¹C-methyloxime] ([¹¹C]ABP688) small animal positron emission tomography (μPET) as a biomarker to visualize possible longitudinal changes in metabotropic glutamate receptor 5 (mGluR5) availability in the brain of SAP90/PSD-95 associated protein 3 (Sapap3) knockout (ko) mice, showing obsessive compulsive disorder (OCD)-like behavior.

METHODS

Alongside the assessment of grooming, we performed [¹¹C]ABP688 μPET/CT imaging in wildtype (wt; n=10) and ko (n=11) mice both at 3 and 9 months. Using the simplified reference tissue method (SRTM), the nondisplaceable binding potential (BP_ND) was calculated representing the in vivo availability of the metabotropic glutamate receptor 5 (mGluR5) in the brain with the cerebellum as a reference region. Longitudinal voxel-based statistical parametric mapping (SPM) was performed on BP_ND images. Results were verified using [¹¹C]ABP688 ex vivo autoradiography, [³H]ABP688 in vitro autoradiography, and mGluR5 immunohistochemistry.

RESULTS

Cross-sectional comparisons revealed significantly increased grooming parameters in ko animals, at both time points. A significant longitudinal increase in % grooming duration (+268.25%; p<0.05) reflected aggravation of this behavior in ko mice. [¹¹C]ABP688 μPET revealed significantly lower mGluR5 availability in the cortex, striatum, hippocampus, and amygdala of ko mice at both ages. A significant longitudinal BP_ND decline was present for ko mice (p<0.01: cortex -17.14%, striatum -19.82%, amygdala -23.57%; p<0.05: hippocampus -15.53%), which was confirmed by SPM (p<0.01).

CONCLUSION

Sapap3 ko mice show a decline in mGluR5 availability in OCD relevant brain regions parallel to the worsening of OCD-like behavior. This demonstrates a potential role for [¹¹C]ABP688 PET as a biomarker to monitor disease progression in vivo.

The Role of Genetic Variation of BMI, Body Composition, and Fat Distribution for Mental Traits and Disorders: A Look-Up and Mendelian Randomization Study

Anthropometric traits and mental disorders or traits are known to be associated clinically and to show genetic overlap. We aimed to identify genetic variants with relevance for mental disorders/traits and either (i) body mass index (or obesity), (ii) body composition, (and/or) (iii) body fat distribution. We performed a look-up analysis of 1,005 genome-wide significant SNPs for BMI, body composition, and body fat distribution in 15 mental disorders/traits. We identified 40 independent loci with one or more SNPs fulfilling our threshold significance criterion (P < 4.98 × 10-5) for the mental phenotypes. The majority of loci was associated with schizophrenia, educational attainment, and/or intelligence. Fewer associations were found for bipolar disorder, neuroticism, attention deficit/hyperactivity disorder, major depressive disorder, depressive symptoms, and well-being. Unique associations with measures of body fat distribution adjusted for BMI were identified at five loci only. To investigate the potential causality between body fat distribution and schizophrenia, we performed two-sample Mendelian randomization analyses. We found no causal effect of body fat distribution on schizophrenia and vice versa. In conclusion, we identified 40 loci which may contribute to genetic overlaps between mental disorders/traits and BMI and/or shape related phenotypes. The majority of loci identified for body composition overlapped with BMI loci, thus suggesting pleiotropic effects.

Instrumental learning in a mouse model for obsessive-compulsive disorder: Impaired habit formation in Sapap3 mutants

It has been hypothesized that maladaptive habit formation contributes to compulsivity in psychiatric disorders such as obsessive-compulsive disorder (OCD). Here, we used an established animal model of OCD, Sapap3 knockout mice (SAPAP3^-/-), to investigate the balance of goal-directed and habitual behavior in compulsive individuals and if altered habit formation is associated with compulsive-like behavior. We subjected 24 SAPAP3^-/- and 24 wildtype littermates (WT) to two different schedules of reinforcement in a within-subjects design: a random-ratio (RR) schedule to promote goal-directedness, and a random-interval (RI) schedule, known to facilitate habitual responding. SAPAP3^-/- acquired responding under both schedules, but showed lower response rates and fewer attempts to collect food pellets than WT, indicative of altered reward processing. As expected, WT were sensitive to sensory-specific satiety (outcome devaluation) following RR training, but not RI training, demonstrating schedule-specific acquisition of goal-directed and habitual responding, respectively. In contrast, SAPAP3^-/- were sensitive to outcome devaluation after both RR and RI training, suggesting decreased engagement of a habitual response strategy. No linear relation was observed between increased grooming and behavior during the outcome devaluation test in SAPAP3^-/-. Together, our findings demonstrate altered reward processing and impaired habit learning in SAPAP3^-/-. We report a diminished propensity to form habits in these mice, which albeit inconsistent with the predominant idea of excessive habit formation in OCD, nonetheless points at dysregulation of behavioral automation in the context of compulsivity. Thus, the habit hypothesis of compulsivity should be updated to state that an imbalance of habitual and goal-directed responding in either direction can contribute to the development of compulsive behavior.

Late onset obesity in mice with targeted deletion of potassium inward rectifier Kir7.1 from cells expressing the melanocortin-4 receptor

Energy stores in fat tissue are determined in part by the activity of hypothalamic neurones expressing the melanocortin-4 receptor (MC4R). Even a partial reduction in MC4R expression levels in mice, rats or humans produces hyperphagia and morbid obesity. Thus, it is of great interest to understand the molecular basis of neuromodulation by the MC4R. The MC4R is a G protein-coupled receptor that signals efficiently through Gα_S , and this signalling pathway is essential for normal MC4R function in vivo. However, previous data from hypothalamic slice preparations indicated that activation of the MC4R depolarised neurones via G protein-independent regulation of the ion channel Kir7.1. In the present study, we show that deletion of Kcnj13 (ie, the gene encoding Kir7.1) specifically from MC4R neurones produced resistance to melanocortin peptide-induced depolarisation of MC4R paraventricular nucleus neurones in brain slices, resistance to the sustained anorexic effect of exogenously administered melanocortin peptides, late onset obesity, increased linear growth and glucose intolerance. Some MC4R-mediated phenotypes appeared intact, including Agouti-related peptide-induced stimulation of food intake and MC4R-mediated induction of peptide YY release from intestinal L cells. Thus, a subset of the consequences of MC4R signalling in vivo appears to be dependent on expression of the Kir7.1 channel in MC4R cells.

A neural basis for antagonistic control of feeding and compulsive behaviors

Abnormal feeding often co-exists with compulsive behaviors, but the underlying neural basis remains unknown. Excessive self-grooming in rodents is associated with compulsivity. Here, we show that optogenetically manipulating the activity of lateral hypothalamus (LH) projections targeting the paraventricular hypothalamus (PVH) differentially promotes either feeding or repetitive self-grooming. Whereas selective activation of GABAergic LH→PVH inputs induces feeding, activation of glutamatergic inputs promotes self-grooming. Strikingly, targeted stimulation of GABAergic LH→PVH leads to rapid and reversible transitions to feeding from induced intense self-grooming, while activating glutamatergic LH→PVH or PVH neurons causes rapid and reversible transitions to self-grooming from voracious feeding induced by fasting. Further, specific inhibition of either LH→PVH GABAergic action or PVH neurons reduces self-grooming induced by stress. Thus, we have uncovered a parallel LH→PVH projection circuit for antagonistic control of feeding and self-grooming through dynamic modulation of PVH neuron activity, revealing a common neural pathway that underlies feeding and compulsive behaviors.

Toll-like receptor 4 deficiency alters nucleus accumbens synaptic physiology and drug reward behavior

Behavioral manifestations of drug-seeking behavior are causally linked to alterations of synaptic strength onto nucleus accumbens (NAc) medium spiny neurons (MSN). Although neuron-driven changes in physiology and behavior are well characterized, there is a lack of knowledge of the role of the immune system in mediating such effects. Toll-like receptor 4 (TLR4) is a pattern recognition molecule of the innate immune system, and evidence suggests that it modulates drug-related behavior. Using TLR4 knockout (TLR4.KO) mice, we show that TLR4 plays a role in NAc synaptic physiology and behavior. In addition to differences in the pharmacological profile of N-methyl-d-aspartate receptors (NMDAR) in the NAc core, TLR4.KO animals exhibit a deficit in low-frequency stimulation-induced NMDAR-dependent long-term depression (LTD). Interestingly, the synaptic difference is region specific as no differences were found in excitatory synaptic properties in the NAc shell. Consistent with altered NAc LTD, TLR4.KO animals exhibit an attenuation in drug reward learning. Finally, we show that TLR4 in the NAc core is primarily expressed on microglia. These results suggest that TLR4 influences NAc MSN synaptic physiology and drug reward learning and behavior.

Automated classification of self-grooming in mice using open-source software

BACKGROUND

Manual analysis of behavior is labor intensive and subject to inter-rater variability. Although considerable progress in automation of analysis has been made, complex behavior such as grooming still lacks satisfactory automated quantification.

NEW METHOD

We trained a freely available, automated classifier, Janelia Automatic Animal Behavior Annotator (JAABA), to quantify self-grooming duration and number of bouts based on video recordings of SAPAP3 knockout mice (a mouse line that self-grooms excessively) and wild-type animals.

RESULTS

We compared the JAABA classifier with human expert observers to test its ability to measure self-grooming in three scenarios: mice in an open field, mice on an elevated plus-maze, and tethered mice in an open field. In each scenario, the classifier identified both grooming and non-grooming with great accuracy and correlated highly with results obtained by human observers. Consistently, the JAABA classifier confirmed previous reports of excessive grooming in SAPAP3 knockout mice.

COMPARISON WITH EXISTING METHODS

Thus far, manual analysis was regarded as the only valid quantification method for self-grooming. We demonstrate that the JAABA classifier is a valid and reliable scoring tool, more cost-efficient than manual scoring, easy to use, requires minimal effort, provides high throughput, and prevents inter-rater variability.

CONCLUSION

We introduce the JAABA classifier as an efficient analysis tool for the assessment of rodent self-grooming with expert quality. In our "how-to" instructions, we provide all information necessary to implement behavioral classification with JAABA.

The Eating-Disorder Associated HDAC4A778T Mutation Alters Feeding Behaviors in Female Mice

BACKGROUND

While eating disorders (EDs) are thought to result from a combination of environmental and psychological stressors superimposed on genetic vulnerability, the neurobiological basis of EDs remains incompletely understood. We recently reported that a rare missense mutation in the gene for the transcriptional repressor histone deacetylase 4 (HDAC4) is associated with the risk of developing an ED in humans.

METHODS

To understand the biological consequences of this missense mutation, we created transgenic mice carrying this mutation by introducing the alanine to threonine mutation at position 778 of mouse Hdac4 (corresponding to position 786 of the human protein). Bioinformatic analysis to identify Hdac4-regulated genes was performed using available databases.

RESULTS

Male mice heterozygous for HDAC4^A778T did not show any metabolic or behavioral differences. In contrast, female mice heterozygous for HDAC4^A778T display several ED-related feeding and behavioral deficits depending on housing condition. Individually housed HDAC4^A778T female mice exhibit reduced effortful responding for high-fat diet and compulsive grooming, whereas group-housed female mice display increased weight gain on high-fat diet, reduced behavioral despair, and increased anxiety-like behaviors. Bioinformatic analysis identifies mitochondrial biogenesis including synthesis of glutamate/gamma-aminobutyric acid as a potential transcriptional target of HDAC4^A778T activity relevant to the behavioral deficits identified in this new mouse model of disordered eating.

CONCLUSIONS

The HDAC4^A778T mouse line is a novel model of ED-related behaviors and identifies mitochondrial biogenesis as a potential molecular pathway contributing to behavioral deficits.

Genetic interaction of DGAT2 and FAAH in the development of human obesity

PURPOSE

DGAT2 is the critical catalyzing enzyme for triglyceride biosynthesis, and excess triglyceride accumulation in fat tissues is a fundamental process for obesity. Mutations in DGAT2 or other genes interacting with DGAT2 associated with adiposity have not been reported in human to date.

METHODS

DGAT2 mutation was identified based on our in-home database-exome sequencing 227 young obese subjects (body-mass index (BMI), 35.1-61.7 kg/m²) and 219 lean controls (BMI, 17.5-23.0 kg/m²), further validated in 1190 lean subjects and the pedigree of the proband. The trios of the proband were further subjected to whole-exome sequencing to explore the candidate genes for obesity. The mutations in DGAT2 and FAAH were functionally evaluated in vitro.

RESULTS

We detected two rare variants in DGAT2 with no significant difference between obese and lean individuals. One novel heterozygous nonsense variant c.382C > T (p.R128*) was identified in one obese subject but not in 219 lean subjects and another 1190 lean subjects. Notably, in vitro study showed that R128* mutation severely damaged the TG-biosynthesis ability of DGAT2, and all other R128* carriers in the pedigree were lean. Thus, we further identified a loss-of-function variant c. 944G > T (p.R315I) in FAAH in the proband inheriting from his obese father. Importantly, FAAH overexpression inhibited DGAT2 expression and TG synthesis, while R315I mutant largely eliminated this inhibitory effect. We first report loss-of-function mutations in DGAT2 and FAAH in one obese subject, which may interact with each other to affect the adiposity penetrance, providing a model of genetic interaction associated with human obesity.

Rodent models of obsessive compulsive disorder: Evaluating validity to interpret emerging neurobiology

Obsessive Compulsive Disorder (OCD) is a common neuropsychiatric disorder with unknown molecular underpinnings. Identification of genetic and non-genetic risk factors has largely been elusive, primarily because of a lack of power. In contrast, neuroimaging has consistently implicated the cortico-striatal-thalamo-cortical circuits in OCD. Pharmacological treatment studies also show specificity, with consistent response of OCD symptoms to chronic treatment with serotonin reuptake inhibitors; although most patients are left with residual impairment. In theory, animal models could provide a bridge from the neuroimaging and pharmacology data to an understanding of pathophysiology at the cellular and molecular level. Several mouse models have been proposed using genetic, immunological, pharmacological, and optogenetic tools. These experimental model systems allow testing of hypotheses about the origins of compulsive behavior. Several models have generated behavior that appears compulsive-like, particularly excessive grooming, and some have demonstrated response to chronic serotonin reuptake inhibitors, establishing both face validity and predictive validity. Construct validity is more difficult to establish in the context of a limited understanding of OCD risk factors. Our current models may help us to dissect the circuits and molecular pathways that can elicit OCD-relevant behavior in rodents. We can hope that this growing understanding, coupled with developing technology, will prepare us when robust OCD risk factors are better understood.

Hunger and Satiety Gauge Reward Sensitivity

Many of the neurocircuits and hormones known to underlie the sensations of hunger and satiety also substantially alter the activity of the dopaminergic reward system. Much interest lies in the ways that hunger, satiety, and reward tie together, as the epidemic of obesity seems tied to the recent development and mass availability of highly palatable foods. In this review, we will first discuss the basic neurocircuitry of the midbrain and basal forebrain reward system. We will elaborate how several important mediators of hunger-the agouti-related protein neurons of the arcuate nucleus, the lateral hypothalamic nucleus, and ghrelin-enhance the sensitivity of the dopaminergic reward system. Then, we will elaborate how mediators of satiety-the nucleus tractus solitarius, pro-opiomelanocortin neurons of the arcuate nucleus, and its peripheral hormonal influences such as leptin-reduce the reward system sensitivity. We hope to provide a template by which future research may identify the ways in which highly rewarding foods bypass this balanced system to produce excessive food consumption.

The Effects of Dietary Fat and Iron Interaction on Brain Regional Iron Contents and Stereotypical Behaviors in Male C57BL/6J Mice

Adequate brain iron levels are essential for enzyme activities, myelination, and neurotransmitter synthesis in the brain. Although systemic iron deficiency has been found in genetically or dietary-induced obese subjects, the effects of obesity-associated iron dysregulation in brain regions have not been examined. The objective of this study was to examine the effect of dietary fat and iron interaction on brain regional iron contents and regional-associated behavior patterns in a mouse model. Thirty C57BL/6J male weanling mice were randomly assigned to six dietary treatment groups (n = 5) with varying fat (control/high) and iron (control/high/low) contents. The stereotypical behaviors were measured during the 24th week. Blood, liver, and brain tissues were collected at the end of the 24th week. Brains were dissected into the hippocampus, midbrain, striatum, and thalamus regions. Iron contents and ferritin heavy chain (FtH) protein and mRNA expressions in these regions were measured. Correlations between stereotypical behaviors and brain regional iron contents were analyzed at the 5% significance level. Results showed that high-fat diet altered the stereotypical behaviors such as inactivity and total distance traveled (P < 0.05). The high-fat diet altered brain iron contents and FtH protein and mRNA expressions in a regional-specific manner: (1) high-fat diet significantly decreased the brain iron content in the striatum (P < 0.05), but not other regions, and (2) thalamus has a more distinct change in FtH mRNA expression compared with other regions. Furthermore, high-fat diet resulted in a significant decreased total distance traveled and a significant correlation between iron content and sleeping in midbrain (P < 0.05). Dietary iron also decreased brain iron content and FtH protein expression in a regionally specific manner. The effect of interaction between dietary fat and iron was observed in brain iron content and behaviors. All these findings will lay foundations to further explore the links among obesity, behaviors, and brain iron alteration.

Escaping the Golden Cage: Animal Models of Eating Disorders in the Post-Diagnostic and Statistical Manual Era

Eating disorders (EDs) are severe, life-threatening mental illnesses characterized by marked disturbances in body image and eating patterns. Attempts to understand the neurobiological basis of EDs have been hindered by the perception that EDs are primarily socially reinforced behaviors and not the result of a pathophysiologic process. This view is reflected by the diagnostic criteria of anorexia nervosa and bulimia nervosa, which emphasize intrapsychic conflicts such as "inability to maintain body weight," "undue influence of body weight or shape on self-evaluation," and "denial of the seriousness of low body weight" over neuropsychological measures. The neuropsychological constructs introduced within the research domain criteria (RDoC) matrix offer new hope for determining the neural substrate underlying the biological predisposition to EDs. We present selected studies demonstrating deficits in patients with EDs within each domain of the RDoC and propose a set of behavioral tasks in model systems that reflect aspects of that deficit. Finally, we propose a battery of tasks to examine comprehensively the function of neural circuits relevant to the development of EDs.

Neurobiology of rodent self-grooming and its value for translational neuroscience

Self-grooming is a complex innate behaviour with an evolutionarily conserved sequencing pattern and is one of the most frequently performed behavioural activities in rodents. In this Review, we discuss the neurobiology of rodent self-grooming, and we highlight studies of rodent models of neuropsychiatric disorders--including models of autism spectrum disorder and obsessive compulsive disorder--that have assessed self-grooming phenotypes. We suggest that rodent self-grooming may be a useful measure of repetitive behaviour in such models, and therefore of value to translational psychiatry. Assessment of rodent self-grooming may also be useful for understanding the neural circuits that are involved in complex sequential patterns of action.

A review of animal models of obsessive-compulsive disorder: a focus on developmental, immune, endocrine and behavioral models

INTRODUCTION

Obsessive-compulsive disorder (OCD) is a neuropsychiatric condition characterized by intrusive thoughts (obsessions) and/or repetitive behaviors (compulsions). Several models of OCD exist, many which employ behaviors such as over-grooming or hoarding as correlates for compulsive behaviors - often using a response to serotonergic agents as evidence for their validity. Recent discoveries in the genetics of OCD and the identification of aberrancies of glutamatergic, hormonal, and immune pathways in the OCD phenotype highlight a need to review existing of animal models of OCD. The focus of attention to these pathways may lead to possible new targets for drug discovery.

AREAS COVERED

In this review, the authors describe frameworks for animal models in OCD conceptualized as either biological (e.g., developmental, genetic, and endocrine pathways), or behavioral (e.g., repetitive grooming, and stereotypies). In addition, the authors give special attention to the emerging role of glutamate in OCD.

EXPERT OPINION

While many animal models for OCD demonstrate pathologic repetitive behavior phenotypes, which are relieved by serotoninergic agents, animal models based on reversal learning, perseverative responding, and neurodevelopmental mechanisms represent robust new paradigms. Glutamatergic influences in these new animal models suggest that drug discovery using neuroprotective approaches may represent a new stage for pharmacologic developments in OCD.

Evidence of long-term gene flow and selection during domestication from analyses of Eurasian wild and domestic pig genomes

Traditionally, the process of domestication is assumed to be initiated by humans, involve few individuals and rely on reproductive isolation between wild and domestic forms. We analyzed pig domestication using over 100 genome sequences and tested whether pig domestication followed a traditional linear model or a more complex, reticulate model. We found that the assumptions of traditional models, such as reproductive isolation and strong domestication bottlenecks, are incompatible with the genetic data. In addition, our results show that, despite gene flow, the genomes of domestic pigs have strong signatures of selection at loci that affect behavior and morphology. We argue that recurrent selection for domestic traits likely counteracted the homogenizing effect of gene flow from wild boars and created 'islands of domestication' in the genome. Our results have major ramifications for the understanding of animal domestication and suggest that future studies should employ models that do not assume reproductive isolation.

Regulation of glucose tolerance and sympathetic activity by MC4R signaling in the lateral hypothalamus

Melanocortin 4 receptor (MC4R) signaling mediates diverse physiological functions, including energy balance, glucose homeostasis, and autonomic activity. Although the lateral hypothalamic area (LHA) is known to express MC4Rs and to receive input from leptin-responsive arcuate proopiomelanocortin neurons, the physiological functions of MC4Rs in the LHA are incompletely understood. We report that MC4R(LHA) signaling regulates glucose tolerance and sympathetic nerve activity. Restoring expression of MC4Rs specifically in the LHA improves glucose intolerance in obese MC4R-null mice without affecting body weight or circulating insulin levels. Fluorodeoxyglucose-mediated tracing of whole-body glucose uptake identifies the interscapular brown adipose tissue (iBAT) as a primary source where glucose uptake is increased in MC4R(LHA) mice. Direct multifiber sympathetic nerve recording further reveals that sympathetic traffic to iBAT is significantly increased in MC4R(LHA) mice, which accompanies a significant elevation of Glut4 expression in iBAT. Finally, bilateral iBAT denervation prevents the glucoregulatory effect of MC4R(LHA) signaling. These results identify a novel role for MC4R(LHA) signaling in the control of sympathetic nerve activity and glucose tolerance independent of energy balance.

Behavioral disturbances in estrogen-related receptor alpha-null mice

Eating disorders, such as anorexia nervosa and bulimia nervosa, are common and severe mental illnesses of unknown etiology. Recently, we identified a rare missense mutation in the transcription factor estrogen-related receptor alpha (ESRRA) that is associated with the development of eating disorders. However, little is known about ESRRA function in the brain. Here, we report that Esrra is expressed in the mouse brain and demonstrate that Esrra levels are regulated by energy reserves. Esrra-null female mice display a reduced operant response to a high-fat diet, compulsivity/behavioral rigidity, and social deficits. Selective Esrra knockdown in the prefrontal and orbitofrontal cortices of adult female mice recapitulates reduced operant response and increased compulsivity, respectively. These results indicate that Esrra deficiency in the mouse brain impairs behavioral responses in multiple functional domains.

Animal models of tic disorders: a translational perspective

Tics are repetitive, sudden movements and/or vocalizations, typically enacted as maladaptive responses to intrusive premonitory urges. The most severe tic disorder, Tourette syndrome (TS), is a childhood-onset condition featuring multiple motor and at least one phonic tic for a duration longer than 1 year. The pharmacological treatment of TS is mainly based on antipsychotic agents; while these drugs are often effective in reducing tic severity and frequency, their therapeutic compliance is limited by serious motor and cognitive side effects. The identification of novel therapeutic targets and development of better treatments for tic disorders is conditional on the development of animal models with high translational validity. In addition, these experimental tools can prove extremely useful to test hypotheses on the etiology and neurobiological bases of TS and related conditions. In recent years, the translational value of these animal models has been enhanced, thanks to a significant re-organization of our conceptual framework of neuropsychiatric disorders, with a greater focus on endophenotypes and quantitative indices, rather than qualitative descriptors. Given the complex and multifactorial nature of TS and other tic disorders, the selection of animal models that can appropriately capture specific symptomatic aspects of these conditions can pose significant theoretical and methodological challenges. In this article, we will review the state of the art on the available animal models of tic disorders, based on genetic mutations, environmental interventions as well as pharmacological manipulations. Furthermore, we will outline emerging lines of translational research showing how some of these experimental preparations have led to significant progress in the identification of novel therapeutic targets for tic disorders.

P7C3 neuroprotective chemicals block axonal degeneration and preserve function after traumatic brain injury

The P7C3 class of neuroprotective aminopropyl carbazoles has been shown to block neuronal cell death in models of neurodegeneration. We now show that P7C3 molecules additionally preserve axonal integrity after injury, before neuronal cell death occurs, in a rodent model of blast-mediated traumatic brain injury (TBI). This protective quality may be linked to the ability of P7C3 molecules to activate nicotinamide phosphoribosyltransferase, the rate-limiting enzyme in nicotinamide adenine dinucleotide salvage. Initiation of daily treatment with our recently reported lead agent, P7C3-S243, 1 day after blast-mediated TBI blocks axonal degeneration and preserves normal synaptic activity, learning and memory, and motor coordination in mice. We additionally report persistent neurologic deficits and acquisition of an anxiety-like phenotype in untreated animals 8 months after blast exposure. Optimized variants of P7C3 thus offer hope for identifying neuroprotective agents for conditions involving axonal damage, neuronal cell death, or both, such as occurs in TBI.

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

Developing 'integrative' zebrafish models of behavioral and metabolic disorders

Recently, the pathophysiological overlap between metabolic and mental disorders has received increased recognition. Zebrafish (Danio rerio) are rapidly becoming a popular model organism for translational biomedical research due to their genetic tractability, low cost, quick reproductive cycle, and ease of behavioral, pharmacological or genetic manipulation. High homology to mammalian physiology and the availability of well-developed assays also make the zebrafish an attractive organism for studying human disorders. Zebrafish neurobehavioral and endocrine phenotypes show promise for the use of zebrafish in studies of stress, obesity and related behavioral and metabolic disorders. Here, we discuss the parallels between zebrafish and other model species in stress and obesity physiology, as well as outline the available zebrafish models of weight gain, metabolic deficits, feeding, stress, anxiety and related behavioral disorders. Overall, zebrafish demonstrate a strong potential for modeling human behavioral and metabolic disorders, and their comorbidity.