Y1 receptors regulate aggressive behavior by modulating serotonin pathways

Connectivity and molecular profiles of Foxp2- and Dbx1-lineage neurons in the accessory olfactory bulb and medial amygdala

In terrestrial vertebrates, the olfactory system is divided into main (MOS) and accessory (AOS) components that process both volatile and nonvolatile cues to generate appropriate behavioral responses. While much is known regarding the molecular diversity of neurons that comprise the MOS, less is known about the AOS. Here, focusing on the vomeronasal organ (VNO), the accessory olfactory bulb (AOB), and the medial amygdala (MeA), we reveal that populations of neurons in the AOS can be molecularly subdivided based on their ongoing or prior expression of the transcription factors Foxp2 or Dbx1, which delineate separate populations of GABAergic output neurons in the MeA. We show that a majority of AOB neurons that project directly to the MeA are of the Foxp2 lineage. Using single-neuron patch-clamp electrophysiology, we further reveal that in addition to sex-specific differences across lineage, the frequency of excitatory input to MeA Dbx1- and Foxp2-lineage neurons differs between sexes. Together, this work uncovers a novel molecular diversity of AOS neurons, and lineage and sex differences in patterns of connectivity.

Lessons from lonely flies: Molecular and neuronal mechanisms underlying social isolation

Animals respond to changes in the environment which affect their internal state by adapting their behaviors. Social isolation is a form of passive environmental stressor that alters behaviors across animal kingdom, including humans, rodents, and fruit flies. Social isolation is known to increase violence, disrupt sleep and increase depression leading to poor mental and physical health. Recent evidences from several model organisms suggest that social isolation leads to remodeling of the transcriptional and epigenetic landscape which alters behavioral outcomes. In this review, we explore how manipulating social experience of fruit fly Drosophila melanogaster can shed light on molecular and neuronal mechanisms underlying isolation driven behaviors. We discuss the recent advances made using the powerful genetic toolkit and behavioral assays in Drosophila to uncover role of neuromodulators, sensory modalities, pheromones, neuronal circuits and molecular mechanisms in mediating social isolation. The insights gained from these studies could be crucial for developing effective therapeutic interventions in future.

The Neuromodulatory Basis of Aggression: Lessons From the Humble Fruit Fly

Aggression is an intrinsic trait that organisms of almost all species, humans included, use to get access to food, shelter, and mating partners. To maximize fitness in the wild, an organism must vary the intensity of aggression toward the same or different stimuli. How much of this variation is genetic and how much is externally induced, is largely unknown but is likely to be a combination of both. Irrespective of the source, one of the principal physiological mechanisms altering the aggression intensity involves neuromodulation. Any change or variation in aggression intensity is most likely governed by a complex interaction of several neuromodulators acting via a meshwork of neural circuits. Resolving aggression-specific neural circuits in a mammalian model has proven challenging due to the highly complex nature of the mammalian brain. In that regard, the fruit fly model Drosophila melanogaster has provided insights into the circuit-driven mechanisms of aggression regulation and its underlying neuromodulatory basis. Despite morphological dissimilarities, the fly brain shares striking similarities with the mammalian brain in genes, neuromodulatory systems, and circuit-organization, making the findings from the fly model extremely valuable for understanding the fundamental circuit logic of human aggression. This review discusses our current understanding of how neuromodulators regulate aggression based on findings from the fruit fly model. We specifically focus on the roles of Serotonin (5-HT), Dopamine (DA), Octopamine (OA), Acetylcholine (ACTH), Sex Peptides (SP), Tachykinin (TK), Neuropeptide F (NPF), and Drosulfakinin (Dsk) in fruit fly male and female aggression.

Mapping the neurocircuitry of impulsive aggression through the pharmacologic review of anti-impulsive aggressive agents

Impulsive aggression, in contradistinction to premeditated aggression in humans or predatory aggression in animals, corresponds to defensive aggression in animal models. At the core of the neurocircuitry of impulsive aggression, from murine to feline to human species, it is the medial amygdala-mediobasal hypothalamus-dorsal periaqueductal gray pathway. Here, we update current knowledge on the neurocircuitry of impulsive aggression by placing the neurocircuitry and its neurophysiological substrates into the top-down/bottom-up hypothesis of impulsive aggression. We then reverse the neurotranslational approach, which applies neuroscience to developing therapeutic drugs, and apply current understanding of potential mechanisms of anti-impulsive aggression agents to further clarify, at least heuristically and hypothetically, the dynamic biochemical components of the neurocircuitry of impulsive aggression. To do this, we searched the medical literature for studies attempting to clarify the neurobiological and neurochemical effects of the five most widely studied anti-impulsive aggressive agents, particularly as they pertain to the top-down/bottom-up hypothesis. Multiple different mechanisms are discussed, all of which fitting in the hypothesis by way of either promoting the "top-down" part (i.e., enhancing inhibitory neurotransmitters), or suppressing the "bottom-up" part (i.e., decreasing excitatory neurotransmitters). The hypothesis appears consistent with the current psychopharmacological understanding of these agents, as well as to account for the likely multifactorial etiology of the condition. Limitations of the hypothesis and future directions are finally discussed.

Neuroendocrine Mechanisms Underlying Non-breeding Aggression: Common Strategies Between Birds and Fish

Aggression is an adaptive behavior that plays an important role in gaining access to limited resources. Aggression may occur uncoupled from reproduction, thus offering a valuable context to further understand its neural and hormonal regulation. This review focuses on the contributions from song sparrows (Melospiza melodia) and the weakly electric banded knifefish (Gymnotus omarorum). Together, these models offer clues about the underlying mechanisms of non-breeding aggression, especially the potential roles of neuropeptide Y (NPY) and brain-derived estrogens. The orexigenic NPY is well-conserved between birds and teleost fish, increases in response to low food intake, and influences sex steroid synthesis. In non-breeding M. melodia, NPY increases in the social behavior network, and NPY-Y1 receptor expression is upregulated in response to a territorial challenge. In G. omarorum, NPY is upregulated in the preoptic area of dominant, but not subordinate, individuals. We hypothesize that NPY may signal a seasonal decrease in food availability and promote non-breeding aggression. In both animal models, non-breeding aggression is estrogen-dependent but gonad-independent. In non-breeding M. melodia, neurosteroid synthesis rapidly increases in response to a territorial challenge. In G. omarorum, brain aromatase is upregulated in dominant but not subordinate fish. In both species, the dramatic decrease in food availability in the non-breeding season may promote non-breeding aggression, via changes in NPY and/or neurosteroid signaling.

Update on the Mechanism and Treatment of Sevoflurane-Induced Postoperative Cognitive Dysfunction

Sevoflurane is one of the most widely used anesthetics for the induction and maintenance of general anesthesia in surgical patients. Sevoflurane treatment may increase the incidence of postoperative cognitive dysfunction (POCD), and patients with POCD exhibit lower cognitive abilities than before the operation. POCD affects the lives of patients and places an additional burden on patients and their families. Understanding the mechanism of sevoflurane-induced POCD may improve prevention and treatment of POCD. In this paper, we review the diagnosis of POCD, introduce animal models of POCD in clinical research, analyze the possible mechanisms of sevoflurane-induced POCD, and summarize advances in treatment for this condition.

Peripheral-specific Y1 receptor antagonism increases thermogenesis and protects against diet-induced obesity

Obesity is caused by an imbalance between food intake and energy expenditure (EE). Here we identify a conserved pathway that links signalling through peripheral Y1 receptors (Y1R) to the control of EE. Selective antagonism of peripheral Y1R, via the non-brain penetrable antagonist BIBO3304, leads to a significant reduction in body weight gain due to enhanced EE thereby reducing fat mass. Specifically thermogenesis in brown adipose tissue (BAT) due to elevated UCP1 is enhanced accompanied by extensive browning of white adipose tissue both in mice and humans. Importantly, selective ablation of Y1R from adipocytes protects against diet-induced obesity. Furthermore, peripheral specific Y1R antagonism also improves glucose homeostasis mainly driven by dynamic changes in Akt activity in BAT. Together, these data suggest that selective peripheral only Y1R antagonism via BIBO3304, or a functional analogue, could be developed as a safer and more effective treatment option to mitigate diet-induced obesity.

Sex differences in behavioral and metabolic effects of gene inactivation: The neuropeptide Y and Y receptors in the brain

Brain and gonadal hormones interplay controls metabolic and behavioral functions in a sex-related manner. However, most translational neuroscience research related to animal models of endocrine and psychiatric disorders are often carried out in male animals only. The Neuropeptide Y (NPY) system shows sex-dependent differences and is sensitive to gonadal steroids. Based on published data from our and other laboratories, in this review we will discuss the sex related differences of NPY action on energy balance, bone homeostasis and behavior in rodents with the genetic manipulation of genes encoding NPY and its Y1, Y2 and Y5 cognate receptors. Comparative analyses of the phenotype of transgenic and knockout NPY and Y receptor rodents unravels sex dependent differences in the functions of this neurotransmission system, potentially helping to develop therapeutics for a variety of sex-related disorders including metabolic syndrome, osteoporosis and ethanol addiction.

NPY/NPF-Related Neuropeptide FLP-34 Signals from Serotonergic Neurons to Modulate Aversive Olfactory Learning in Caenorhabditis elegans

Aversive learning is fundamental for animals to increase chances of survival. In addition to classical neurotransmitters, neuropeptides have emerged to modulate such complex behaviors. Among them, neuropeptide Y (NPY) is well known to promote aversive memory acquisition in mammals. Here we identify an NPY/neuropeptide F (NPF)-related neuropeptide system in Caenorhabditis elegans and show that this FLP-34/NPR-11 system is required for learning negative associations, a process that is reminiscent of NPY signaling in mammals. The Caenorhabditis elegans NPY/NPF ortholog FLP-34 displays conserved structural hallmarks of bilaterian-wide NPY/NPF neuropeptides. We show that it is required for aversive olfactory learning after pairing diacetyl with the absence of food, but not for appetitive olfactory learning in response to butanone. To mediate diacetyl learning and thus integrate the aversive food context with the diacetyl odor, FLP-34 is released from serotonergic neurons and signals through its evolutionarily conserved NPY/NPF GPCR, NPR-11, in downstream AIA interneurons. NPR-11 activation in the AIA integration center results in avoidance of a previously attractive stimulus. This study opens perspectives for a deeper understanding of stress conditions in which aversive learning results in excessive avoidance.SIGNIFICANCE STATEMENT Aversive learning evolved early in evolution to promote avoidance of dangerous and stressful situations. In addition to classical neurotransmitters, neuropeptides are emerging as modulators of complex behaviors, including learning and memory. Here, we identified the evolutionary ortholog of neuropeptide Y/neuropeptide F in the nematode Caenorhabditis elegans, and we discovered that it is required for olfactory aversive learning. In addition, we elucidated the neural circuit underlying this avoidance behavior, and we discovered a novel coordinated action of Caenorhabditis elegans neuropeptide Y/neuropeptide F and serotonin that could aid in our understanding of the molecular mechanisms underlying stress disorders in which excessive avoidance results in maladaptive behaviors.

Ecotype differences in aggression, neural activity and behaviorally relevant gene expression in cichlid fish

In Lake Malawi, two ecologically distinct lineages of cichlid fishes (rock- vs sand-dwelling ecotypes, each comprised of over 200 species) evolved within the last million years. The rock-dwelling species (Mbuna) are aggressively territorial year-round and males court and spawn with females over rocky substrate. In contrast, males of sand-dwelling species are not territorial and instead aggregate on seasonal breeding leks in which males construct courtship "bowers" in the sand. However, little is known about how phenotypic variation in aggression is produced by the genome. In this study, we first quantify and compare behavior in seven cichlid species, demonstrating substantial ecotype and species differences in unconditioned mirror-elicited aggression. Second, we compare neural activity in mirror-elicited aggression in two representative species, Mchenga conophoros (sand-dwelling) and Petrotilapia chitimba (rock-dwelling). Finally, we compare gene expression patterns between these two species, specifically within neurons activated during mirror aggression. We identified a large number of genes showing differential expression in mirror-elicited aggression, as well as many genes that differ between ecotypes. These genes, which may underly species differences in behavior, include several neuropeptides, genes involved in the synthesis of steroid hormones and neurotransmitter activity. This work lays the foundation for future experiments using this emerging genetic model system to investigate the genomic basis of evolved species differences in both brain and behavior.

Neuropeptide Y deficiency induces anxiety-like behaviours in zebrafish (Danio rerio)

Neuropeptide Y (NPY) controls energy homeostasis including orexigenic actions in mammalians and non-mammalians. Recently, NPY has attracted attention as a mediator of emotional behaviour and psychosomatic diseases. However, its functions are not fully understood. We established npy gene-deficient (NPY-KO) zebrafish (Danio rerio) to assess the relationship between NPY and emotional behaviours. The NPY-KO zebrafish exhibited similar growth, but pomc and avp mRNA levels in the brain were higher as compared to wild-type fish. NPY-KO zebrafish exhibited several anxiety-like behaviours, such as a decrease in social interaction in mirror test and decreased locomotion in black-white test. The acute cold stress-treated NPY-KO zebrafish exhibited anxiety-like behaviours such as remaining stationary and swimming along the side of the tank in the mirror test. Moreover, expression levels of anxiety-associated genes (orx and cck) and catecholamine production (gr, mr, th1 and th2) were significantly higher in NPY-KO zebrafish than in wild-type fish. We demonstrated that NPY-KO zebrafish have an anxiety phenotype and a stress-vulnerability like NPY-KO mice, whereby orx and/or catecholamine signalling may be involved in the mechanism actions.

The stalk-eyed fly as a model for aggression - is there a conserved role for 5-HT between vertebrates and invertebrates?

Serotonin (5-HT) has largely been accepted to be inhibitory to vertebrate aggression, whereas an opposing stimulatory role has been proposed for invertebrates. Herein, we argue that critical gaps in our understanding of the nuanced role of 5-HT in invertebrate systems drove this conclusion prematurely, and that emerging data suggest a previously unrecognized level of phylogenetic conservation with respect to neurochemical mechanisms regulating the expression of aggressive behaviors. This is especially apparent when considering the interplay among factors governing 5-HT activity, many of which share functional homology across taxa. We discuss recent findings using insect models, with an emphasis on the stalk-eyed fly, to demonstrate how particular 5-HT receptor subtypes mediate the intensity of aggression with respect to discrete stages of the interaction (initiation, escalation and termination), which mirrors the complex behavioral regulation currently recognized in vertebrates. Further similarities emerge when considering the contribution of neuropeptides, which interact with 5-HT to ultimately determine contest progression and outcome. Relative to knowledge in vertebrates, much less is known about the function of 5-HT receptors and neuropeptides in invertebrate aggression, particularly with respect to sex, species and context, prompting the need for further studies. Our Commentary highlights the need to consider multiple factors when determining potential taxonomic differences, and raises the possibility of more similarities than differences between vertebrates and invertebrates with regard to the modulatory effect of 5-HT on aggression.

Serotonin differentially affects morph-specific behavior in divergent populations of a horned beetle

Associations between animal weapons and corresponding aggressive behaviors are among the most characteristic features of species, yet at the same time their co-expression is itself often strongly dependent on context, such as male condition or population ecology. Yet the mechanisms that modulate associations between aggression, morphology, and biological context remain poorly understood. The biogenic amine serotonin has been shown to regulate a wide range of aggressive and morph-specific behaviors in diverse insect species. However, the extent to which serotonin may coordinate the expression of behavior with morphology across biological contexts remains unclear. In this study, we pharmacologically increased serotonin biosynthesis in males of the polyphenic beetle, Onthophagus taurus, and assessed how this manipulation affects both aggressive and non-aggressive behaviors in alternative fighter and sneaker morphs, as well as in males derived from two rapidly diverging populations characterized by disparate levels of competition for mates. We find (i) that enhancing serotonin biosynthesis increases most measures of aggressive behaviors, but influences only a subset of nonaggressive behaviors, (ii) that similar serotonin-mediated behavioral changes manifest in both morphs within populations more often than just a single morph, and (iii) that males derived from the two focal populations have diverged in their behavioral responsiveness to serotonin up-regulation. Collectively, our study suggests that serotonin signaling plays a critical role in the regulation of male behavior and its evolution, including in the context of rapid, short-term population divergence.

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin's (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2's involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.

Neuropeptide Y impairs the acquisition of conditioned defeat in Syrian hamsters

Recent evidence indicates that Neuropeptide Y (NPY) may function as a potent anxiolytic as well as a resilience factor that can insulate the brain from the effects of stress. However, most of these studies have utilized physical stressors such as shock or restraint. In the present study, we use an ethologically-based model in Syrian hamsters (Mesocricetus auratus) called Conditioned Defeat (CD) to investigate whether NPY can ameliorate the effect of social defeat stress. In the CD model, a male Syrian hamster is socially defeated by a larger, more aggressive conspecific. Subsequently, when paired with a smaller, non-aggressive intruder (NAI) in its own home cage, changes in its behavioral repertoire occur, including a reduction in aggression and chemosensory (social) investigation, and a concomitant increase in submissive behaviors. In Experiment 1, hamsters were infused intracerebroventricularly (icv) with NPY prior to social defeat, and 24-hours later, hamsters were exposed to a NAI. Results indicate that NPY significantly reduced submissive/defensive behaviors in socially defeated hamsters compared to control animals. In Experiment 2, we examined whether this effect was mediated by the NPY Y1 receptor. Subjects were first pre-treated with the Y1 receptor antagonist BIBP 3226 or vehicle, followed by NPY and then socially defeated. Upon testing with a NAI 24-hours later, pretreatment with BIBP 3226 failed to block the NPY effect compared to controls. These results demonstrate that NPY may function as an important resilience factor in socially defeated hamsters, but that these effects are not mediated by the Y1 receptor.

Conditional inactivation of Npy1r gene in mice induces behavioural inflexibility and orbitofrontal cortex hyperactivity that are reversed by escitalopram

Cognitive flexibility is the ability to rapidly adapt established patterns of behaviour in the face of changing circumstance and depends critically on the orbitofrontal cortex (OFC). Impaired flexibility also results from altered serotonin transmission in the OFC. The Y1 (Y1R) and Y5 (Y5R) receptors for neuropeptide Y (NPY) colocalize in several brain regions and have overlapping functions in regulating cognition and emotional behaviour. The targeted disruption of gene encoding Y1R (Npy1r gene) in Y5R containing neurons (Npy1r^Y5R-/- mice) increases anxiety-like behaviour and spatial reference memory. Here we used the same conditional system to analyse whether the coordinated expression of the Y1R and Y5R might be required for behavioural flexibility in reversal learning tasks, OFC serotoninergic tone and OFC neural activity, as detected by immunohistochemical quantification of the immediate-early gene, c-Fos. In addition, we investigated whether the acute treatment of Npy1r^Y5R-/- mice with the selective serotonin reuptake inhibitor escitalopram affected behavioural flexibility and OFC c-Fos expression. Npy1r^Y5R-/- male mice exhibit an impairment in performing the reversal task of the Morris water maze and the water T-maze but normal spatial learning, working memory and sociability, compared to their control siblings. Furthermore, Npy1r^Y5R-/- male mice display decreased 5-hydroxytriptamine (5-HT) positive fibres and increased baseline neural activity in OFC. Importantly, escitalopram normalizes OFC neural activity and restores behavioural flexibility of Npy1r^Y5R-/- male mice. These findings suggest that the inactivation of Y1R in Y5R containing neurons increases pyramidal neuron activity and dysregulates serotoninergic tone in OFC, whereby contributing to reversal learning impairment.

Interoceptive contributions to healthy eating and obesity

Obesity results from persistent failure by the brain to balance food intake with energy needs, resulting in a state of chronic energy surplus. Although there are many factors that predispose individuals to weight gain and obesity, the current review focuses on two ways eating behavior may be influenced by sensitivity to interoceptive signals of hunger, satiety, and metabolic energy reserves. First, obesity may be related to hypersensitivity to interoceptive signals of hunger, leading to positive alliesthesia for food cues that undermine attempts to change unhealthy eating behaviors. Second, overeating and obesity may arise from an inability to accurately detect interoceptive signals of satiety and positive energy balance. The findings reviewed herein demonstrate that obesity may be related to altered interoception, and warrant the continued development of novel obesity interventions aimed at promoting interoceptive awareness.

The central mechanism of risperidone-induced hyperprolactinemia

Risperidone is known to increase prolactin secretion in treating mental illness patients. This side-effect is thought to be mediated via central signaling pathway. However, the exact pathway involved between risperidone and hyperprolactinemia are still unknown. Therefore, we have treated mice with risperidone and investigated the central mechanisms. The present study showed that in risperidone treated group, the level of the serum prolactin significantly increased, which was consistent with increased positive prolactin staining in pituitary gland. Elevated c-fos expression was observed in the arcuate hypothalamic nucleus (Arc) where we found 65% c-fos positive neurons co-localised with neuropeptide Y (NPY) in mice treated with risperidone. In addition, the results from in situ hybridization showed that the NPY mRNA in the Arc was significantly increased, whereas the tyrosine hydroxylase (TH) mRNA dramatically decreased compared with control group in the paraventricular hypothalamic nucleus (PVN). These findings revealed that risperidone may mediate the transcriptional regulation of Arc NPY and TH in the PVN. Furthermore, risperidone induced a decreased dopamine synthesis in the PVN and thus reduced the dopamine-induced inhibition of prolactin release, ultimately lead to hyperprolactinemia. Therefore, insights into these neuronal mechanisms open up potential new ways to treat schizophrenia patients in order to ameliorate hyperprolactinemia.

Neuromodulation and Strategic Action Choice in Drosophila Aggression

In this review, I discuss current knowledge and outstanding questions on the neuromodulators that influence aggressive behavior of the fruit fly Drosophila melanogaster. I first present evidence that Drosophila exchange information during an agonistic interaction and choose appropriate actions based on this information. I then discuss the influence of several biogenic amines and neuropeptides on aggressive behavior. One striking characteristic of neuromodulation is that it can configure a neural circuit dynamically, enabling one circuit to generate multiple outcomes. I suggest a consensus effect of each neuromodulatory molecule on Drosophila aggression, as well as effects of receptor proteins where relevant data are available. Lastly, I consider neuromodulation in the context of strategic action choices during agonistic interactions. Genetic components of neuromodulatory systems are highly conserved across animals, suggesting that molecular and cellular mechanisms controlling Drosophila aggression can shed light on neural principles governing action choice during social interactions.

Migraine: A disorder of metabolism?

The treatment and prevention of migraine within the last decade has become largely pharmacological. While there is little doubt that the advent of drugs (e.g. triptans) has helped many migraine sufferers to lead a normal life, there is still little knowledge with respect to the factors responsible for precipitating a migraine attack. Evidence from biochemical and behavioural studies from a number of disciplines is integrated to put forward the proposal that migraine is part of a cascade of events, which together act to protect the organism when confronted by a metabolic challenge.

Aggression in non-human vertebrates: Genetic mechanisms and molecular pathways

Aggression is an adaptive behavioral trait that is important for the establishment of social hierarchies and competition for mating partners, food, and territories. While a certain level of aggression can be beneficial for the survival of an individual or species, abnormal aggression levels can be detrimental. Abnormal aggression is commonly found in human patients with psychiatric disorders. The predisposition to aggression is influenced by a combination of environmental and genetic factors and a large number of genes have been associated with aggression in both human and animal studies. In this review, we compare and contrast aggression studies in zebrafish and mouse. We present gene ontology and pathway analyses of genes linked to aggression and discuss the molecular pathways that underpin agonistic behavior in these species. © 2015 Wiley Periodicals, Inc.

Genetics of aggressive behavior: An overview

The Research Domain Criteria (RDoC) address three types of aggression: frustrative non-reward, defensive aggression and offensive/proactive aggression. This review sought to present the evidence for genetic underpinnings of aggression and to determine to what degree prior studies have examined phenotypes that fit into the RDoC framework. Although the constructs of defensive and offensive aggression have been widely used in the animal genetics literature, the human literature is mostly agnostic with regard to all the RDoC constructs. We know from twin studies that about half the variance in behavior may be explained by genetic risk factors. This is true for both dimensional, trait-like, measures of aggression and categorical definitions of psychopathology. The non-shared environment seems to have a moderate influence with the effects of shared environment being unclear. Human molecular genetic studies of aggression are in an early stage. The most promising candidates are in the dopaminergic and serotonergic systems along with hormonal regulators. Genome-wide association studies have not yet achieved genome-wide significance, but current samples are too small to detect variants having the small effects one would expect for a complex disorder. The strongest molecular evidence for a genetic basis for aggression comes from animal models comparing aggressive and non-aggressive strains or documenting the effects of gene knockouts. Although we have learned much from these prior studies, future studies should improve the measurement of aggression by using a systematic method of measurement such as that proposed by the RDoC initiative.

Regulation of energy homeostasis by the NPY system

Obesity develops when energy intake exceeds energy expenditure over time. Numerous neurotransmitters, hormones, and factors have been implicated to coordinately control energy homeostasis, centrally and peripherally. However, the neuropeptide Y (NPY) system has emerged as the one with the most critical functions in this process. While NPY centrally promotes feeding and reduces energy expenditure, peptide YY (PYY) and pancreatic polypeptide (PP), the other family members, mediate satiety. Importantly, recent research has uncovered additional functions for these peptides that go beyond the simple feeding/satiety circuits and indicate a more extensive function in controlling energy homeostasis. In this review, we will discuss the actions of the NPY system in the regulation of energy balance, with a particular focus on energy expenditure.

Neuromolecular responses to social challenge: common mechanisms across mouse, stickleback fish, and honey bee

Certain complex phenotypes appear repeatedly across diverse species due to processes of evolutionary conservation and convergence. In some contexts like developmental body patterning, there is increased appreciation that common molecular mechanisms underlie common phenotypes; these molecular mechanisms include highly conserved genes and networks that may be modified by lineage-specific mutations. However, the existence of deeply conserved mechanisms for social behaviors has not yet been demonstrated. We used a comparative genomics approach to determine whether shared neuromolecular mechanisms could underlie behavioral response to territory intrusion across species spanning a broad phylogenetic range: house mouse (Mus musculus), stickleback fish (Gasterosteus aculeatus), and honey bee (Apis mellifera). Territory intrusion modulated similar brain functional processes in each species, including those associated with hormone-mediated signal transduction and neurodevelopment. Changes in chromosome organization and energy metabolism appear to be core, conserved processes involved in the response to territory intrusion. We also found that several homologous transcription factors that are typically associated with neural development were modulated across all three species, suggesting that shared neuronal effects may involve transcriptional cascades of evolutionarily conserved genes. Furthermore, immunohistochemical analyses of a subset of these transcription factors in mouse again implicated modulation of energy metabolism in the behavioral response. These results provide support for conserved genetic "toolkits" that are used in independent evolutions of the response to social challenge in diverse taxa.

Nitric oxide and serotonin interactions in aggression

Violence is a significant public health problem worldwide. Neurobiological research on violence and aggression attempts to elucidate the cellular and molecular pathways that increase the propensity toward this behavior. Research over the past 40 years has implicated several brain regions and neurotransmitters in aggression, mainly using rodent models. Perhaps the strongest association is the link between serotonin and aggression, which has compelling interactions with the nitric oxide system. Recently, new insights into these relationships have been added as modern techniques allow more sophisticated analyses. This chapter will discuss current developments implicating serotonin and nitric oxide in aggressive behavior. Recently developed high-resolution methods for examining the neurobiological basis of aggression will be considered, with emphasis on future directions for the field.

The time course of aggressive behaviour in juvenile matrinxã Brycon amazonicus fed with dietary L-tryptophan supplementation

This study evaluated the influence of dietary L-tryptophan (TRP) supplementation on the time course of aggressive behaviour and on neuroendocrine and hormonal indicators in juvenile matrinxã Brycon amazonicus. Supplementation with TRP promoted a change in the fight pattern at the beginning of an interaction with an intruder, resulting in decreased aggressive behaviours during the first 20 min. The decrease in aggression did not persist throughout the interaction but increased at 3 and 6 h after the beginning of the fight. Monoamine levels in the hypothalamus were not influenced by TRP before or after the fight; however, the hypothalamic serotonin (5-HT) concentration and the 5-hydroxyindole-3-acetic acid (5HIAA):5-HT ratio were significantly correlated with the reduction in aggressive behaviour at the beginning of the fight. Cortisol was not altered by TRP before the fight. After the fight cortisol increased to higher levels in B. amazonicus fed with supplementary TRP. These results indicate that TRP supplementation alters the aggressive behaviour of B. amazonicus and that this effect is limited to the beginning of the fight, suggesting a transient effect of TRP on aggressive behaviour. This is the first study reporting the effects of TRP supplementation on the time course of aggressive interaction in fishes.

Neuropeptide Y and posttraumatic stress disorder

Resiliency to the adverse effects of extraordinary emotional trauma on the brain varies within the human population. Accordingly, some people cope better than others with traumatic stress. Neuropeptide Y (NPY) is a 36-amino-acid peptide transmitter abundantly expressed in forebrain limbic and brain stem areas that regulate stress and emotional behaviors. Studies largely in rodents demonstrate a role for NPY in promoting coping with stress. Moreover, accruing data from the genetic to the physiological implicate NPY as a potential 'resilience-to-stress' factor in humans. Here, we consolidate findings from preclinical and clinical studies of NPY that are of relevance to stress-associated syndromes, most prototypically posttraumatic stress disorder (PTSD). Collectively, these data suggest that reduced central nervous system (CNS) NPY concentrations or function may be associated with PTSD. We also link specific symptoms of human PTSD with extant findings in the NPY field to reveal potential physiological contributions of the neuropeptide to the disorder. In pursuit of understanding the physiological basis and treatment of PTSD, the NPY system is an attractive target.

Ominous odors: olfactory control of instinctive fear and aggression in mice

Aggression and fear are often thought to be distinct behavioral states, yet they share several common output responses. In the mouse, both can be initiated by specialized odor cues. How these cues signal through the olfactory system to promote behavior is largely unknown. Recent experiments have started to uncover the relevant signaling ligands, chemosensory receptors, and responsive sensory neurons that together enable the precise manipulation of behaviorally relevant neural circuits. Moreover, the use of molecular genetics and new experimental strategies has begun to reveal how the central nervous system processes olfactory information to initiate aggression and fear. A sensory-initiated comparative study of these two fundamental threat reactions promises to offer new mechanistic insight.

Cerebrospinal fluid neuropeptide Y-like immunoreactivity correlates with impulsive aggression in human subjects

BACKGROUND

Neurochemical studies have pointed to a modulatory role in human aggression for a number of central neurotransmitters; some (e.g., serotonin) appear to play an inhibitory role, while others (e.g., vasopressin) appear to play a facilitator role in the modulation of aggression. While recent animal studies of neuropeptide Y (NPY) have suggested a facilitator role for central NPY in the modulation of aggression, no human studies of central NPY have yet been reported regarding aggression.

METHODS

Basal lumbar cerebrospinal fluid (CSF) was obtained from 60 physically healthy subjects with personality disorder (PD) (n=40) and from healthy volunteers (n=20). These samples were then assessed for CSF NPY-like immunoreactivity (NPY-LI) and other neurotransmitter-related species in CSF and correlated with measures of aggression and impulsivity.

RESULTS

Cerebrospinal fluid NPY-LI was higher in PD subjects compared with healthy volunteers and in subjects with intermittent explosive disorder compared with those without intermittent explosive disorder. In PD subjects, CSF NPY-LI was directly correlated with composite measures of aggression and impulsivity and a composite measure of impulsive aggression. Group differences in CSF NPY-LI concentration were accounted for by measures of impulsive aggression.

CONCLUSIONS

These data suggest a direct relationship between CSF NPY-immunoreactivity concentration and measures of impulsive aggression in human subjects. This adds to the complex picture of the central neuromodulatory role of impulsive aggression in human subjects.

Brain serotonin receptors and transporters: initiation vs. termination of escalated aggression

RATIONALE

Recent findings have shown a complexly regulated 5-HT system as it is linked to different kinds of aggression.

OBJECTIVE

We focus on (1) phasic and tonic changes of 5-HT and (2) state and trait of aggression, and emphasize the different receptor subtypes, their role in specific brain regions, feed-back regulation and modulation by other amines, acids and peptides.

RESULTS

New pharmacological tools differentiate the first three 5-HT receptor families and their modulation by GABA, glutamate and CRF. Activation of 5-HT(1A), 5-HT(1B) and 5-HT(2A/2C) receptors in mesocorticolimbic areas, reduce species-typical and other aggressive behaviors. In contrast, agonists at 5-HT(1A) and 5-HT(1B) receptors in the medial prefrontal cortex or septal area can increase aggressive behavior under specific conditions. Activation of serotonin transporters reduce mainly pathological aggression. Genetic analyses of aggressive individuals have identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT(1B), 5-HT transporter, Pet1, MAOA) or indirectly (e.g., Neuropeptide Y, αCaMKII, NOS, BDNF). Dysfunction in genes for MAOA escalates pathological aggression in rodents and humans, particularly in interaction with specific experiences.

CONCLUSIONS

Feedback to autoreceptors of the 5-HT(1) family and modulation via heteroreceptors are important in the expression of aggressive behavior. Tonic increase of the 5-HT(2) family expression may cause escalated aggression, whereas the phasic increase of 5-HT(2) receptors inhibits aggressive behaviors. Polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT modulate aggression, often requiring interaction with the rearing environment.

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Delayed stress-induced differences in locomotor and depression-related behaviour in female neuropeptide-Y Y1 receptor knockout mice

Neuropeptide-Y acting through Y1 receptors reduces anxiety and stress sensitivity in rodents. In Y1 receptor knockout (Y1⁻/⁻) mice, however, anxiety-related behaviour is altered only in a context-dependent manner. Here, we investigated whether stress causes a delayed change in the emotional-affective behaviour of female Y1⁻/⁻ mice. Locomotor and anxiety-related behaviour was assessed with the elevated plus-maze (EPM) test and depression-like behaviour with the forced swim test (FST). These behavioural tests were also used as experimental stress paradigms. Locomotion and anxiety-like behaviour did not differ between naïve control and Y1⁻/⁻ mice. One week after the FST, locomotion was reduced in control animals but unchanged in Y1⁻/⁻ mice, whereas anxiety-like behaviour remained unaltered in both genotypes. Depression-like behaviour (immobility) was identical in naïve control and Y1⁻/⁻ mice but, 1 week after the EPM test, was attenuated in Y1⁻/⁻ mice relative to control animals. Our data show that naïve female Y1⁻/⁻ mice do not grossly differ from female control animals in their locomotor and depression-like behaviour. Exposure to the stress associated with behavioural testing, however, leads to delayed genotype-dependent differences in locomotion and depression-like behaviour. These findings attest to a role of Y1 receptor signalling in the control of stress coping and/or adaptation.

Unravelling the neurophysiological basis of aggression in a fish model

Background

Aggression is a near-universal behaviour with substantial influence on and implications for human and animal social systems. The neurophysiological basis of aggression is, however, poorly understood in all species and approaches adopted to study this complex behaviour have often been oversimplified. We applied targeted expression profiling on 40 genes, spanning eight neurological pathways and in four distinct regions of the brain, in combination with behavioural observations and pharmacological manipulations, to screen for regulatory pathways of aggression in the zebrafish (Danio rerio), an animal model in which social rank and aggressiveness tightly correlate.

Results

Substantial differences occurred in gene expression profiles between dominant and subordinate males associated with phenotypic differences in aggressiveness and, for the chosen gene set, they occurred mainly in the hypothalamus and telencephalon. The patterns of differentially-expressed genes implied multifactorial control of aggression in zebrafish, including the hypothalamo-neurohypophysial-system, serotonin, somatostatin, dopamine, hypothalamo-pituitary-interrenal, hypothalamo-pituitary-gonadal and histamine pathways, and the latter is a novel finding outside mammals. Pharmacological manipulations of various nodes within the hypothalamo-neurohypophysial-system and serotonin pathways supported their functional involvement. We also observed differences in expression profiles in the brains of dominant versus subordinate females that suggested sex-conserved control of aggression. For example, in the HNS pathway, the gene encoding arginine vasotocin (AVT), previously believed specific to male behaviours, was amongst those genes most associated with aggression, and AVT inhibited dominant female aggression, as in males. However, sex-specific differences in the expression profiles also occurred, including differences in aggression-associated tryptophan hydroxylases and estrogen receptors.

Conclusions

Thus, through an integrated approach, combining gene expression profiling, behavioural analyses, and pharmacological manipulations, we identified candidate genes and pathways that appear to play significant roles in regulating aggression in fish. Many of these are novel for non-mammalian systems. We further present a validated system for advancing our understanding of the mechanistic underpinnings of complex behaviours using a fish model.

Acoustic startle response and sensorimotor gating in a genetic mouse model for the Y1 receptor

Recent research has highlighted a potential role for neuropeptide Y (NPY) and its Y(1) receptor in the development of schizophrenia. Genetic as well as molecular biological studies have demonstrated reduced levels of NPY in schizophrenia patients. Importantly, Y(1) receptors may mediate some of the potential effects of NPY on schizophrenia, as decreased Y(1) receptor expression has been found in the lymphocytes of schizophrenia patients. To clarify NPY's role in schizophrenia, we investigated a genetic animal model for Y(1) deficiency in regard to (i) acoustic startle response (ASR), (ii) habituation to ASR and (iii) sensorimotor gating [i.e. prepulse inhibition (PPI)] using two different PPI protocols. Mutant and wild type-like mice were screened for baseline behaviours and after pharmacological challenge with the psychotropic drugs dexamphetamine (DEX) and MK-801. Y(1) knockout mice (Y(1)(-/-)) showed a moderate reduction of the ASR and an impaired ASR habituation at baseline and after DEX treatment. The baseline PPI performance of Y(1) mutant mice was unaltered their response to DEX and MK-801 challenge was moderately different compared to control mice, which was dependent on the PPI protocol used. MK-801 challenge had a protocol-dependent differential effect in Y(1)(-/-) mice and DEX a more pronounced impact at the highest prepulse intensities. In conclusion, it appears that the Y(1) receptor influences the acoustic startle response and its habituation but does not play a major role in sensorimotor gating. Further explorations into the effects of Y(1) deficiency seem valid.

Fear of harm, a possible phenotype of pediatric bipolar disorder: a dimensional approach to diagnosis for genotyping psychiatric syndromes

BACKGROUND

In a prior concordance study of affected sibling pairs with a community diagnosis of pediatric bipolar disorder (PBD) a behavioral phenotype termed Fear of Harm (FOH) was found to have one of the strongest concordance coefficients (rho) between probands and siblings, and the widest contrasts between the rho-estimates for the proband/sibling vs. proband/comparison pairs [Papolos, D., Hennen, J., Cockerham, M.S, Lachman, H., 2007]. A strategy for identifying phenotypic subtypes: concordance of symptom dimensions between sibling pairs who met screening criteria for a genetic linkage study of childhood-onset bipolar disorder using the Child Bipolar Questionnaire (CBQ) was employed. J. Affect. Disord. 99, 27-36.]. We used the Child Bipolar Questionnaire (OUT) (CBQ) to further elucidate this behavioral phenotype of PBD. We hypothesized that selective factors including parent reported symptoms of mania and depression, would be distinguishing features of impairment between groups defined by 1) the magnitude of their score on a continuous measure of FOH, and 2) the high FOH group would have significantly greater levels of severity on course of illness variables. These measures included earlier age of onset of first psychiatric symptoms, first hospitalization, and frequency of psychiatric hospitalizations, as well as, degree of social impairment as determined by exposure to the juvenile justice system and school performance problems.

METHODS

The sample was comprised of children with community diagnoses of bipolar disorder or at risk for the illness based on enriched family history with multiple first degree relatives diagnosed with BPD (N=5335). Included were all subjects who had >40 positively endorsed CBQ symptom items at frequencies of very often, almost always, and always. This group was divided randomly into two groups, the exploratory group (N=2668) and the hypothesis testing (study) group (N=2666). The exploratory group was used for the development of hypotheses and the study group was used to test these hypotheses on a new set of data. All results reported here derive from the latter group. In subsequent analyses, we classified each child as having a high degree of FOH, low FOH, or no FOH. We examined a subset of the sample for differences in age of onset of first psychiatric symptoms, course of illness and measures of symptom severity. These groups were compared using the chi-square procedure for categorical data and the Analysis of Variance (ANOVA) with Scheffe pair wise tests for continuous variables. The Child Bipolar Questionnaire V.2.0, the Yale-Brown Obsessive Compulsive Scale (YBOCS) and the Overt Aggression Scale (OAS) were the principal instruments used to obtain diagnostic information for this study.

RESULTS

We found that children representative of the FOH phenotype when compared to children with PBD who lack this trait had higher indices of severity of mania and depression, as well as other indices that reflect severity and course of illness. Trait factors were derived from a factor analysis of CBQ in a large population of children diagnosed with or at risk for PBD, and used to further elucidate trait features of children with FOH. Children with the FOH traits were also more likely to be defined by six CBQ factors; Sleep/Arousal, Harm to Self and Others, Territorial Aggression, Anxiety, Self-esteem, Psychosis/Parasomnias/Sweet Cravings/Obsessions (PPSO).

LIMITATIONS

This data is derived from samples enriched with bipolar disorder cases. Further validation is needed with samples in which childhood-onset BD is rarer and diagnoses more diverse. Clinician diagnosis was not validated via research interview.

CONCLUSIONS

The FOH phenotype, as defined by a metric derived from combining items from the YBOCS/OAS, is a clinically homogeneous behavioral phenotype of PBD with early age of onset, severe manic and depressive symptoms, and significant social impairment that is strongly associated with 6 CBQ factors and can be easily identified using the CBQ. Through the examination of dimensional features of PBD in an enriched sample of large size, we were able to further refine a phenotype and identify clinical dimensions potentially linked to endophenotypic markers that may prove fruitful in differential diagnosis, treatment and etiological studies of PBD. The nature of the sets of specific symptoms that comprise the FOH factors enabled us to propose a biological model for the phenotype (OUT) that involves a complex orexigenic circuit which links hypothalamic, limbic, and other brain nuclei primarily responsible for the regulation of behavioral and proposed physiological features of the FOH phenotype.

Obese reversal by a chronic energy restricted diet leaves an increased Arc NPY/AgRP, but no alteration in POMC/CART, mRNA expression in diet-induced obese mice

Weight regain after weight loss is a major hurdle for combating obesity. The aim of this study is to examine orexigenic and anorectic neuropeptides of the hypothalamic arcuate nucleus (Arc) in response to weight loss after chronic energy intake restriction. Thirty mice were fed with a high-fat diet for 8 weeks and then classified as diet-induced obese (DIO; n=10) or diet-resistant (DR; n=10) mice according to the highest and lowest body weight gainers. Five mice from DIO and DR groups were placed on an energy restricted diet or continued on their high-fat diet ad libitum for 6 weeks. An additional five mice were on a LF diet throughout the course of this study as controls. Results showed that a six-week energy restricted diet completely reversed the increased body weight, fat mass and leptin in the DIO mice to the levels of the LF and DR mice. Arc neuropeptide Y (NPY) and agouti-related protein (AgRP) mRNA expression in DIO mice after obesity reversal were significantly higher than DIO mice without obesity reversal (17%, 47%, both p<0.05), while the Arc pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) mRNA showed no difference. Both NPY and AgRP expression in DIO mice were negatively correlated with plasma leptin (R=-0.78, p<0.05; R=-0.72, p<0.05). In conclusion, while chronic energy restriction will lead to weight loss, it can up-regulate hypothalamic orexigenic peptides, which may be an important contributing factor to weight regain after a weight loss program from an energy restricted diet.

Mutations in many genes affect aggressive behavior in Drosophila melanogaster

Background

Aggressive behavior in animals is important for survival and reproduction. Identifying the underlying genes and environmental contexts that affect aggressive behavior is important for understanding the evolutionary forces that maintain variation for aggressive behavior in natural populations, and to develop therapeutic interventions to modulate extreme levels of aggressive behavior in humans. While the role of neurotransmitters and a few other molecules in mediating and modulating levels of aggression is well established, it is likely that many additional genetic pathways remain undiscovered. Drosophila melanogaster has recently been established as an excellent model organism for studying the genetic basis of aggressive behavior. Here, we present the results of a screen of 170 Drosophila P-element insertional mutations for quantitative differences in aggressive behavior from their co-isogenic control line.

Results

We identified 59 mutations in 57 genes that affect aggressive behavior, none of which had been previously implicated to affect aggression. Thirty-two of these mutants exhibited increased aggression, while 27 lines were less aggressive than the control. Many of the genes affect the development and function of the nervous system, and are thus plausibly relevant to the execution of complex behaviors. Others affect basic cellular and metabolic processes, or are mutations in computationally predicted genes for which aggressive behavior is the first biological annotation. Most of the mutations had pleiotropic effects on other complex traits. We characterized nine of these mutations in greater detail by assessing transcript levels throughout development, morphological changes in the mushroom bodies, and restoration of control levels of aggression in revertant alleles. All of the P-element insertions affected the tagged genes, and had pleiotropic effects on brain morphology.

Conclusion

This study reveals that many more genes than previously suspected affect aggressive behavior, and that these genes have widespread pleiotropic effects. Given the conservation of aggressive behavior among different animal species, these are novel candidate genes for future study in other animals, including humans.

Quantitative trait loci for aggressive behavior in Drosophila melanogaster

Aggressive behavior is observed across animal taxa and is likely to be evolutionarily conserved. Although potentially advantageous, aggression can have social and health consequences in humans, and is a component of a number of psychiatric disorders. As a complex genetic trait, it is modulated by numerous quantitative trait loci (QTL) with allelic effects that can vary in direction and magnitude and that are sensitive to environmental perturbations. Assays to quantify aggressive behavior in Drosophila melanogaster have been developed, making this an ideal model system in which to dissect the genomic architecture underlying manifestation of and variation in aggressive behavior. Here, we map QTL affecting variation in aggression between two wild-type Drosophila strains. We identified a minimum of five QTL in a genomewide scan: two on chromosome 2 and three on chromosome 3. At least three and possibly all five of these QTL interact epistatically. We used deficiency complementation mapping to subdivide two linked, epistatically interacting QTL of large effect on chromosome 3 into at least six QTL, and complementation tests to mutations identified four candidate quantitative trait genes. Extensive epistasis poses a serious challenge for understanding the genetic basis of complex traits.

Peptidergic influences on proliferation, migration, and placement of neural progenitors in the adult mouse forebrain

Neural progenitor proliferation, differentiation, and migration are continually ongoing processes in the subventricular zone (SVZ) and rostral migratory stream (RMS) of the adult brain. There is evidence that peptidergic systems may be involved in the molecular cascades regulating these neurogenic processes, and we examined a possible influence of neuropeptide Y (NPY) and cholecystokinin (CCK) systems in cell proliferation and neuroblast formation in the SVZ and RMS and generation of interneurons in the olfactory bulb (OB). We show that NPY and the Y1 and Y2 receptor (R) proteins are expressed in and surrounding the SVZ and RMS and that Y1R is located on neuroblasts in the anterior RMS. Mice deficient in Y1Rs or Y2Rs have fewer Ki-67-immunoreactive (ir) proliferating precursor cells and doublecortin-ir neuroblasts in the SVZ and RMS than WT mice, and less calbindin-, calretinin-, and tyrosine hydroxylase-ir interneurons in the OB. Mice lacking CCK1Rs have fewer proliferating cells and neuroblasts than normal and a shortage of interneurons in the OB. These findings suggest that both NPY and CCK through their receptors help to regulate the proliferation of precursor cells, the amount of neuroblast cells in the SVZ and RMS, and influence the differentiation of OB interneurons.

Homeobox gene Sax2 deficiency causes an imbalance in energy homeostasis

The brain, in particular the hypothalamus and the brainstem, plays a critical role in the regulation of energy homeostasis by incorporating signals from the periphery and translating them into feeding behavior. Here we show that the homeobox gene Sax2, which is expressed predominantly in the brainstem, in the vicinity of serotonergic neurons, contributes to this physiological balance. Sax2 deficiency results in a decrease of fat and glycogen storage, reduced blood glucose levels, and raised serotonin levels in the hindbrain. Surprisingly, in the brainstem the expression levels of pro-opiomelanocortin and neuropeptide Y were indicative of a fasting condition, opposed to the observed high serotonin levels implying satiation. Furthermore, Sax2-directed lacZ expression reveals a dramatic change of the distribution of Sax2-expressing cells in the null mutant occurring during perinatal development. These data strongly suggest that Sax2 is required for the coordinated crosstalk of factors involved in the maintenance of energy homeostasis.

The neuropeptide Y Y1 receptor subtype is necessary for the anxiolytic-like effects of neuropeptide Y, but not the antidepressant-like effects of fluoxetine, in mice

RATIONALE

Neuropeptide Y (NPY) is implicated in the pathophysiology of affective illness. Multiple receptor subtypes (Y1R, Y2R, and Y5R) have been suggested to contribute to NPY's effects on rodent anxiety and depression-related behaviors.

OBJECTIVES

To further elucidate the role of Y1R in (1) NPY's anxiolytic-like effects and (2) fluoxetine's antidepressant-like and neurogenesis-inducing effects.

METHODS

Mice lacking Y1R were assessed for spontaneous anxiety-like behavior (open field, elevated plus-maze, and light/dark exploration test) and Pavlovian fear conditioning, and for the anxiolytic-like effects of intracerebroventricularly (icv)-administrated NPY (elevated plus-maze). Next, Y1R -/- were assessed for the antidepressant-like effects of acute fluoxetine in the forced swim test and chronic fluoxetine in the novelty-induced hypophagia test, as well as for chronic fluoxetine-induced hippocampal neurogenesis.

RESULTS

Y1R -/- exhibited largely normal baseline behavior as compared to +/+ littermate controls. Intraventricular administration of NPY in Y1R -/- mice failed to produce the normal anxiolytic-like effect in the elevated plus-maze test seen in +/+ mice. Y1R mutant mice showed higher immobility in the forced swim test and longer latencies in the novelty-induced hypophagia test. In addition, Y1R -/- mice responded normally to the acute and chronic effects of fluoxetine treatment in the forced swim test and the novelty-induced hypophagia test, respectively, as well as increased neuronal precursor cell proliferation in the hippocampus.

CONCLUSIONS

These data demonstrate that Y1R is necessary for the anxiolytic-like effects of icv NPY, but not for the antidepressant-like or neurogenesis-inducing effects of fluoxetine. The present study supports targeting Y1R as a novel therapeutic target for anxiety disorders.

Y1 receptors are critical for the proliferation of adult mouse precursor cells in the olfactory neuroepithelium

While the regenerative capacity of the olfactory neuroepithelium has been well studied less is known about the molecular events controlling precursor cell activity. Neuropeptide Y (NPY) is expressed at high levels in the olfactory system, and NPY has been shown to play a role in neuroregeneration of the brain. In this study, we show that the numbers of olfactory neurospheres derived from NPY, NPY/peptide YY, and Y1 receptor knockout mice are decreased compared with wild type (WT) controls. Furthermore, flow cytometric analysis of isolated horizontal basal cells, globose basal cells, and glandular cells showed that only glandular cells derived from WT mice, but not from NPY and Y1 receptor knockout mice, formed secondary neurospheres suggesting a critical role for NPY signaling in this process. Interestingly, olfactory function tests revealed that olfaction in Y1 knockout mice is impaired compared with those of WT mice, probably because of the reduced number of olfactory neurons formed. Together these results indicate that NPY and the Y1 receptor are required for the normal proliferation of adult olfactory precursors and olfactory function.

Tumor-induced anorexia and weight loss are mediated by the TGF-beta superfamily cytokine MIC-1

Anorexia and weight loss are part of the wasting syndrome of late-stage cancer, are a major cause of morbidity and mortality in cancer, and are thought to be cytokine mediated. Macrophage inhibitory cytokine-1 (MIC-1) is produced by many cancers. Examination of sera from individuals with advanced prostate cancer showed a direct relationship between MIC-1 abundance and cancer-associated weight loss. In mice with xenografted prostate tumors, elevated MIC-1 levels were also associated with marked weight, fat and lean tissue loss that was mediated by decreased food intake and was reversed by administration of antibody to MIC-1. Additionally, normal mice given systemic MIC-1 and transgenic mice overexpressing MIC-1 showed hypophagia and reduced body weight. MIC-1 mediates its effects by central mechanisms that implicate the hypothalamic transforming growth factor-beta receptor II, extracellular signal-regulated kinases 1 and 2, signal transducer and activator of transcription-3, neuropeptide Y and pro-opiomelanocortin. Thus, MIC-1 is a newly defined central regulator of appetite and a potential target for the treatment of both cancer anorexia and weight loss, as well as of obesity.