Genes and aggressiveness. Behavioral genetics

Neural circuit mechanisms that govern inter-male attack in mice

Individuals of many species fight with conspecifics to gain access to or defend critical resources essential for survival and reproduction. Such intraspecific fighting is evolutionarily selected for in a species-, sex-, and environment-dependent manner when the value of resources secured exceeds the cost of fighting. One such example is males fighting for chances to mate with females. Recent advances in new tools open up ways to dissect the detailed neural circuit mechanisms that govern intraspecific, particularly inter-male, aggression in the model organism Mus musculus (house mouse). By targeting and functional manipulating genetically defined populations of neurons and their projections, these studies reveal a core neural circuit that controls the display of reactive male-male attacks in mice, from sensory detection to decision making and action selection. Here, we summarize these critical results. We then describe various modulatory inputs that route into the core circuit to afford state-dependent and top-down modulation of inter-male attacks. While reviewing these exciting developments, we note that how the inter-male attack circuit converges or diverges with neural circuits that mediate other forms of social interactions remain not fully understood. Finally, we emphasize the importance of combining circuit, pharmacological, and genetic analysis when studying the neural control of aggression in the future.

The peacefulness gene promotes aggression in Drosophila

Natural aggressiveness is commonly observed in all animal species, and is displayed frequently when animals compete for food, territory and mating. Aggression is an innate behaviour, and is influenced by both environmental and genetic factors. However, the genetics of aggression remains largely unclear. In this study, we identify the peacefulness (pfs) gene as a novel player in the control of male-male aggression in Drosophila. Mutations in pfs decreased intermale aggressiveness, but did not affect locomotor activity, olfactory avoidance response and sexual behaviours. pfs encodes for the evolutionarily conserved molybdenum cofactor (MoCo) synthesis 1 protein (Mocs1), which catalyzes the first step in the MoCo biosynthesis pathway. Neuronal-specific knockdown of pfs decreased aggressiveness. By contrast, overexpression of pfs greatly increased aggressiveness. Knocking down Cinnamon (Cin) catalyzing the final step in the MoCo synthesis pathway, caused a pfs-like aggression phenotype. In humans, inhibition of MoCo-dependent enzymes displays anti-aggressive effects. Thus, the control of aggression by Pfs-dependent MoCo pathways may be conserved throughout evolution.

Characterization of head transcriptome and analysis of gene expression involved in caste differentiation and aggression in Odontotermes formosanus (Shiraki)

BACKGROUND

The subterranean termite Odontotermes formosanus (Shiraki) is a serious insect pest of trees and dams in China. To date, very little is known about genomic or transcriptomic data for caste differentiation and aggression in O. formosanus. Hence, studies on transcriptome and gene expression profiling are helpful to better understand molecular basis underlying caste differentiation and aggressive behavior in O. formosanus.

METHODOLOGY AND PRINCIPAL FINDINGS

Using the Illumina sequencing, we obtained more than 57 million sequencing reads derived from the heads of O. formosanus. These reads were assembled into 116,885 unique sequences (mean size  =  536 bp). Of the unigenes, 30,646 (26.22%) had significant similarity with proteins in the NCBI nonredundant protein database and Swiss-Prot database (E-value<10(-5)). Of these annotated unigenes, 10,409 and 9,009 unigenes were assigned to gene ontology categories and clusters of orthologous groups, respectively. In total, 19,611 (25.52%) unigenes were mapped onto 242 pathways using the Kyoto Encyclopedia of Genes and Genomes Pathway database (KEGG). A total of 11,661 simple sequence repeats (SSRs) were predicted from the current transcriptome database. Moreover, we detected seven putative genes involved in caste differentiation and six putative genes involved in aggression. The qPCR analysis showed that there were significant differences in the expression levels of the three putative genes hexamerin 2, β-glycosidase and bicaudal D involved in caste differentiation and one putative gene Cyp6a20 involved in aggression among workers, soldiers and larvae of O. formosanus.

CONCLUSIONS

To our knowledge, this is the first study to characterize the complete head transcriptome of a higher fungus-cultivating termite using high-throughput sequencing. Our study has provided the comprehensive sequence resources available for elucidating molecular basis underlying caste differentiation and aggressive behavior in O. formosanus.

Gene expression in aminergic and peptidergic cells during aggression and defeat: relevance to violence, depression and drug abuse

In this review, we examine how experiences in social confrontations alter gene expression in mesocorticolimbic cells. The focus is on the target of attack and threat due to the prominent role of social defeat stress in the study of coping mechanisms and victimization. The initial operational definition of the socially defeated mouse by Ginsburg and Allee (1942) enabled the characterization of key endocrine, cardiovascular, and metabolic events during the initial response to an aggressive opponent and during the ensuing adaptations. Brief episodes of social defeat stress induce an augmented response to stimulant challenge as reflected by increased locomotion and increased extracellular dopamine (DA) in the nucleus accumbens (NAC). Cells in the ventral tegmental area (VTA) that project to the NAC were more active as indicated by increased expression of c-fos and Fos-immunoreactivity and BDNF. Intermittent episodes of social defeat stress result in increased mRNA for MOR in brainstem and limbic structures. These behavioral and neurobiological indices of sensitization persist for several months after the stress experience. The episodically defeated rats also self-administered intravenous cocaine during continuous access for 24 h ("binge"). By contrast, continuous social stress, particularly in the form of social subordination stress, leads to reduced appetite, compromised endocrine activities, and cardiovascular and metabolic abnormalities, and prefer sweets less as index of anhedonia. Cocaine challenges in subordinate rats result in a blunted psychomotor stimulant response and a reduced DA release in NAC. Subordinate rats self-administer cocaine less during continuous access conditions. These contrasting patterns of social stress result from continuous vs. intermittent exposure to social stress, suggesting divergent neuroadaptations for increased vulnerability to cocaine self-administration vs. deteriorated reward mechanisms characteristic of depressive-like profiles.

A common genetic target for environmental and heritable influences on aggressiveness in Drosophila

Environmental and genetic factors can modulate aggressiveness, but the biological mechanisms underlying their influence are largely unknown. Social experience with conspecifics suppresses aggressiveness in both vertebrate and invertebrate species, including Drosophila. We searched for genes whose expression levels correlate with the influence of social experience on aggressiveness in Drosophila by performing microarray analysis of head tissue from socially isolated (aggressive) vs. socially experienced (nonaggressive) male flies. Among approximately 200 differentially expressed genes, only one was also present in a gene set previously identified by profiling Drosophila strains subjected to genetic selection for differences in aggressiveness [Dierick HA, Greenspan RJ (2006) Nat Genet 38:1023-1031]. This gene, Cyp6a20, encodes a cytochrome P450. Social experience increased Cyp6a20 expression and decreased aggressiveness in a reversible manner. In Cyp6a20 mutants, aggressiveness was increased in group-housed but not socially isolated flies. These data identify a common genetic target for environmental and heritable influences on aggressiveness. Cyp6a20 is expressed in a subset of nonneuronal support cells associated with pheromone-sensing olfactory sensilla, suggesting that social experience may influence aggressiveness by regulating pheromone sensitivity.

Association of aggressive behavior in Korean male schizophrenic patients with polymorphisms in the serotonin transporter promoter and catecholamine-O-methyltransferase genes

The incidence of aggressive behavior in patients with schizophrenia is higher than in the general population. Among particular gene polymorphisms posited to be involved in psychiatric disorders, the catecholamine-O-methyltransferase (COMT) and serotonin transporter (5-HTTPR) genes have been the focus of recent research on aggression. In this study, we hypothesized that both the COMT and the 5-HTTPR genotypes may be dependent on and related to aggression in Korean patients with schizophrenia. The subjects were 168 unrelated male schizophrenic patients diagnosed according to DSM-IV. Among two psychiatric hospital staff and medical university students, 158 unrelated male subjects with no lifetime history of psychiatric disorders were recruited to establish the COMT and 5-HTTPR genotype distribution in the general population. All episodes of aggression from the last discharge to readmission were rated. The Total Overt Aggression Scale (OAS) score (sum of the scores of all episodes of aggression), highest OAS score (highest individual episode score, 0-16), OAS category, and OAS category score (mean score within each category) were recorded. There were statistically significant effects of COMT genotype on the mean OAS 4 (physical aggression against other people) score and the highest OAS score. The most predictive was the OAS 4 score. There was a statistically significant effect of 5-HTTPR genotype on mean total score. Thus, the COMT gene is associated with the severity of aggression and with physical aggression against other people, whereas the 5-HTTPR gene is associated with the summary score of all episodes of aggression.

Aggression and defeat: persistent effects on cocaine self-administration and gene expression in peptidergic and aminergic mesocorticolimbic circuits

The question of how ostensibly aversive social stress experiences in an aggressive confrontation can persistently increase intense drug taking such as cocaine 'bingeing' needs to be resolved. The biology of social conflict highlights distinctive behavioral, cardiovascular and endocrine profiles of dominant and subordinate animals, as seen also in rodents and primates under laboratory conditions. In contrast to continuous subordination stress that produces chronic pathophysiological consequences and often is fatal, animals adapt to brief episodes of social defeat stress, but show enduring functional activation in mesocorticolimbic microcircuits. Uncontrollable episodes of social defeat stress produce long-lasting tolerance to opiate analgesia and, concurrently, behavioral sensitization to challenges with either amphetamine or cocaine. One week after a single social defeat stress, cross-sensitization to cocaine is evident in terms of enhanced motor activity as well as in terms of increased Fos labeling in the periaqueductal grey area, the locus coeruleus, and the dorsal raphe nuclei. When challenged with a low amphetamine dose, the behavioral and neural effects of repeated brief episodes of social defeat stress persist for months. Previous exposure to social defeat stress can (1). significantly shorten the latency to acquire cocaine self-administration, (2). maintain this behavior at low cocaine unit doses, (3). significantly increase the levels of cocaine taking during a 24 h binge of continuous drug availability, (4). dysregulate the timing of consecutive infusions, and (5). abolish the circadian pattern of self-administration. Amygdaloid modulation, especially originating from central and basolateral nuclei, of dopaminergic pathways via peptidergic and glutamatergic neurons appears to be a key mechanism by which social defeat stress affects cocaine self-administration. Social stress alters the feedback from prefrontal cortex and thereby may contribute to the dysregulation of dopaminergic activity that is necessary for cocaine self-administration.

Anxiety-related traits in mice with modified genes of the serotonergic pathway

The neurobiology of anxiety is complex, reflecting the cumulative physiological effects of multiple genes. These genes are interactive with each other and with the environment in which they are expressed. Variation in genes coding for proteins that control serotonin (5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction and cellular pathways have been implicated in the genetics of anxiety and related disorders. Here, we selected anxiety and avoidance as paradigmatic traits and behavior and cover both traditional studies with inbred murine strains and selected lines which have been modified by gene knockout technologies. The design of a mouse model partially or completely lacking a gene of interest during all stages of development (constitutive knockout) or in a spatio-temporal context (conditional knockout) is among the prime strategies directed at elucidating the role of genetic factors in fear and anxiety. In many cases, knockout mice have been able to confirm what has already been anticipated based on pharmacological studies. In other instances, knockout studies have changed views of the relevance of 5-HT homeostasis in brain development and plasticity as well as processes underlying emotional behavior. In this review, we discuss the pertinent literature regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, monoamine oxidase A and other components of the serotonergic pathway. Finally, we attempt to identify future directions of genetic manipulation in animal models to advance our understanding of brain dysregulation characteristic of anxiety disorders.

Toward a molecular architecture of personality

Epidemiological studies provided a large body of evidence that personality dimensions are influenced by genetic factors and that the genetic component is highly complex, polygenic, and epistatic. However, consistent findings on the genetic basis of personality have yet remained sparse. In recent years, molecular genetics has begun to identify specific genes coding in particular for components of the serotonergic and dopaminergic neurotransmitter systems representing quantitative trait loci (QTLs) for behavioral traits. The QTL concept suggests that complex traits are not attributable to single genes. According to this polygenic model, the genetic basis of personality and behavior and its pathological variations thus results from additive or nonadditive interactions of various genes. As the number of suitable candidate genes constantly increases, the QTL model provides a reasonable explanation for the genetic basis of personality and its disorders. In this review, the current knowledge on the impact of a large number of candidate gene polymorphisms (e.g. variations in serotonin and dopamine receptor and serotonin transporter genes) on personality and temperament is summarized. Additionally, investigations of gene-gene and gene-environment interactions in humans and animals, which currently intensify the identification of genes that underlie behavioral variations, are examined. The findings converge on the notion that a probabilistic rather than deterministic impact of genes on the expression of behavior will contribute to the demystification of behavioral disorders.

Mouse anxiety: the power of knockout

The role of the neurokinine 1 receptors (NK1R) and its endogenous ligand, the neuropeptide substance P (SP) in the pathophysiology and treatment response of anxiety and depression has been controversial. In a new study, however, mice with a targeted inactivation of the NK1R show a phenotype that is associated with anxiety-related behaviors and stress responses. Since this behavioral phenotype was associated with an increase in serotonergic function, NK1R knockout mice represent a systematic model for advanced investigations of SP/NK1R and serotonin system interaction in anxiety responses. The availability of an increasing number of knockout mouse anxiety models as well as integration of emerging tools and technologies for genetic analysis will provide the groundwork for the genetic dissection of anxiety and related disorders.

Variation of serotonergic gene expression: neurodevelopment and the complexity of response to psychopharmacologic drugs

Individual differences in drug effects and treatment response are relatively enduring, continuously distributed, as well as substantially heritable, and are therefore likely to result from an interplay of multiple genomic variations with environmental influences. As the etiology and pathogenesis of behavioral and psychiatric disorders is genetically complex, so is the response to drug treatment. Psychopharmacologic drug response depends on the structure and functional expression of gene products, which may be direct drug targets or may indirectly modify the development and synaptic plasticity of neural networks critically involved in drug response. While formation and integration of these neural networks is dependent on the action of manifold proteins, converging lines of evidence indicate that genetically controlled variability in the expression of genes critical to the development and plasticity of distinct neurocircuits influences a wide spectrum of quantitative traits including treatment response. During brain development, neurotransmitter systems (e.g. serotonergic system), which are frequently targeted by psychotropic drugs, control neuronal specification, differentiation, and phenotype maintenance. The formation and maturation of these neurotransmitter systems, in turn, is directed by an intrinsic genetic program. Based on the notion that complex gene-gene and gene environment interactions in the regulation of brain plasticity are presumed to contribute to interindividual differences in drug response, the concept of developmental psychopharmacogenetics is emerging. This review appraises prototypical genomic variation with impact on gene expression and complementary studies of genetic and environmental effects on brain development and synaptic plasticity in the mouse model. Although special emphasis is given to molecular mechanisms of neurodevelopmental genetics, relevant conceptual and methodological issues pertinent to the dissection of the psychopharmacogenetic-neurodevelopmental interface are also considered.

Effects of group selection for productivity and longevity on blood concentrations of serotonin, catecholamines, and corticosterone of laying hens

Selection of a line of White Leghorn chickens for high group productivity and longevity resulted in reducing cannibalism and flightiness in multiple-hen cages. Improvements in survival might have been due to changes of physiological homeostasis. The objective of the present study was to test the hypothesis that genetic selection for high (HGPS) and low (LGPS) group productivity and survivability also altered regulation of neuroendocrine homeostasis. Hens were randomly assigned to individual cages at 17 wk of age. At 21 wk of age, blood concentrations of dopamine, epinephrine, norepinephrine, and serotonin were measured using HPLC assay. Blood concentrations of corticosterone were measured using radioimmunoassay. The LGPS hens had greater blood concentrations of dopamine and epinephrine than the HGPS hens (P < 0.01). The blood concentration of norepinephrine was not significantly different between the lines, but the ratio of epinephrine to norepinephrine was greater in the LGPS hens (P < 0.01). The blood concentrations of serotonin were also higher in the LGPS hens compared to those in the HGPS hens (P < 0.01). Although the HGPS hens tended to have a higher level of blood corticosterone, the difference was not significant (1.87 +/- 0.19 vs. 1.49 +/- 0.21 ng/mL; P = 0.08). The results suggest that selection for group productivity and survivability alters the chickens' neuroendocrine homeostasis, and these changes may correlate with its line-unique coping ability to domestic environments and survivability.

Molecular manipulations as tools for enhancing our understanding of 5-HT neurotransmission

A developing trend in exploring the sites at which drugs act is to use molecular rather than chemical agents to alter receptors, intracellular signalling mechanisms or gene expression. The 5-HT neurotransmission system is targeted by drugs useful in many behavioural disorders, including anxiety, depression, psychosis and eating disorders. It also regulates many physiological functions and provides some examples of the potential use of these new molecular approaches. This article reviews the progress made in the molecular manipulation of 5-HT receptors and discusses the potential of such tools for the treatment of diseases associated with the 5-HT transmission system.

Y chromosome, urinary chemosignals, and an agonistic behavior (offense) of mice

In mice, offense is one type of agonistic behavior associated with attacks. Offense of male mice was measured in a panel of testers design. The mice were DBA1 (D1) and DBA1.C57BL10-Y (D1.B10-Y). These are congenic for the male-specific, nonrecombining part of the Y chromosome. For the behavioral experiments, urine from D1 or D1.B10-Y mice was daubed on gonadectomized opponents. The opponents were of two genotypes, D1 or D1.B10-Y. The experimental subjects were of the same two genotypes. There were main effects for strain of experimental subject and strain of urine donor as well as interactions for strain of experimental subject x strain of gonadectomized opponent, strain of gonadectomized opponent x strain of urine donor, and strain of experimental subject x strain of gonadectomized opponent x strain of urine donor. These findings are consistent with a model in which this part of the Y chromosome affects testosterone-dependent pheromones and non-testosterone-dependent odor types acting as motivating stimuli, the olfactory perception of motivating stimuli for offense, and the motivational mechanism for offense.

Mean genes and the biology of aggression: a critical review of recent animal and human research

Recent genetic work has suggested that abnormalities in serotonin biochemistry are directly causally linked to aggressive behavior, and there appears to be a consensus in the psychiatric literature that low levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid are specifically associated with impulsive violent behavior. We review the limitations of the genetic studies and conduct a meta-analysis of 39 studies linking 5-HIAA to aggression in humans. No differences in mean 5-HIAA levels were found between groups of violent impulsive psychiatric patients and groups of subjects diagnosed with other psychiatric or medical conditions not considered to involve violence once these levels had been corrected for three nonpsychiatric sources of variation (age, sex and height). However, mean 5-HIAA levels in both of these groups were lower than the mean corrected level in groups of normal healthy volunteers. The results confirm an association between low 5-HIAA levels and psychiatric disorders, but fail to support any specific relationship between low 5-HIAA levels and impulsive aggression or criminality. It is premature and misleading to speak of "mean genes" (Hen 1996) or a specific neurochemistry of aggressive behavior.