The Selective Metabotropic Glutamate 2/3 Receptor Agonist MGS0028 Reverses Isolation Rearing–Induced Abnormal Behaviors in Mice

(R)-ketamine restores anterior insular cortex activity and cognitive deficits in social isolation-reared mice

Chronic social isolation increases the risk of mental health problems, including cognitive impairments and depression. While subanesthetic ketamine is considered effective for cognitive impairments in patients with depression, the neural mechanisms underlying its effects are not well understood. Here we identified unique activation of the anterior insular cortex (aIC) as a characteristic feature in brain-wide regions of mice reared in social isolation and treated with (R)-ketamine, a ketamine enantiomer. Using fiber photometry recording on freely moving mice, we found that social isolation attenuates aIC neuronal activation upon social contact and that (R)-ketamine, but not (S)-ketamine, is able to counteracts this reduction. (R)-ketamine facilitated social cognition in social isolation-reared mice during the social memory test. aIC inactivation offset the effect of (R)-ketamine on social memory. Our results suggest that (R)-ketamine has promising potential as an effective intervention for social cognitive deficits by restoring aIC function.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Broad spectrum efficacy with LY2969822, an oral prodrug of metabotropic glutamate 2/3 receptor agonist LY2934747, in rodent pain models

BACKGROUND AND PURPOSE

A body of evidence suggests activation of metabotropic glutamate 2/3 (mGlu_2/3 ) receptors would be an effective analgesic in chronic pain conditions. Thus, the analgesic properties of a novel mGlu_2/3 receptor agonist prodrug were investigated.

EXPERIMENTAL APPROACH

After oral absorption, the prodrug LY2969822 rapidly converts to the brain penetrant, potent and subtype-selective mGlu_2/3 receptor agonist LY2934747. Behavioural assessments of allodynia, hyperalgesia and nocifensive behaviours were determined in preclinical pain models after administration of LY2969822 0.3-10 mg·kg^-1 . In addition, the ability of i.v. LY2934747 to modulate dorsal horn spinal cord wide dynamic range (WDR) neurons in spinal nerve ligated (SNL) rats was assessed.

KEY RESULTS

Following treatment with LY2934747, the spontaneous activity and electrically-evoked wind-up of WDR neurons in rats that had undergone spinal nerve ligation and developed mechanical allodynia were suppressed. In a model of sensitization, orally administered LY2969822 prevented the nociceptive behaviours induced by an intraplantar injection of formalin. The on-target nature of this effect was confirmed by blockade with an mGlu_2/3 receptor antagonist. LY2969822 prevented capsaicin-induced tactile hypersensitivity, reversed the SNL-induced tactile hypersensitivity and reversed complete Freund's adjuvant - induced mechanical hyperalgesia. The mGlu_2/3 receptor agonist prodrug demonstrated efficacy in visceral pain models, including a colorectal distension model and partially prevented the nocifensive behaviours in the mouse acetic acid writhing model.

CONCLUSIONS AND IMPLICATIONS

Following oral administration of the prodrug LY2969822, the mGlu_2/3 receptor agonist LY2934747 was formed and this attenuated pain behaviours across a broad range of preclinical pain models.

Comparative Effects of LY3020371, a Potent and Selective Metabotropic Glutamate (mGlu) 2/3 Receptor Antagonist, and Ketamine, a Noncompetitive N-Methyl-d-Aspartate Receptor Antagonist in Rodents: Evidence Supporting the Use of mGlu2/3 Antagonists, for the Treatment of Depression

The ability of the N-methyl-d-aspartate receptor antagonist ketamine to alleviate symptoms in patients suffering from treatment-resistant depression (TRD) is well documented. In this paper, we directly compare in vivo biologic responses in rodents elicited by a recently discovered metabotropic glutamate (mGlu) 2/3 receptor antagonist 2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY3020371) with those produced by ketamine. Both LY3020371 and ketamine increased the number of spontaneously active dopamine cells in the ventral tegmental area of anesthetized rats, increased O₂ in the anterior cingulate cortex, promoted wakefulness, enhanced the efflux of biogenic amines in the prefrontal cortex, and produced antidepressant-related behavioral effects in rodent models. The ability of LY3020371 to produce antidepressant-like effects in the forced-swim assay in rats was associated with cerebrospinal fluid (CSF) drug levels that matched concentrations required for functional antagonist activity in native rat brain tissue preparations. Metabolomic pathway analyses from analytes recovered from rat CSF and hippocampus demonstrated that both LY3020371 and ketamine activated common pathways involving GRIA2 and ADORA1. A diester analog of LY3020371 [bis(((isopropoxycarbonyl)oxy)-methyl) (1S,2R,3S,4S,5R,6R)-2-amino-3-(((3,4-difluorophenyl)thio)methyl)-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylate (LY3027788)] was an effective oral prodrug; when given orally, it recapitulated effects of intravenous doses of LY3020371 in the forced-swim and wake-promotion assays, and augmented the antidepressant-like effects of fluoxetine or citalopram without altering plasma or brain levels of these compounds. The broad overlap of biologic responses produced by LY3020371 and ketamine supports the hypothesis that mGlu2/3 receptor blockade might be a novel therapeutic approach for the treatment of TRD patients. LY3020371 and LY3027788 represent molecules that are ready for clinical tests of this hypothesis.

Divergent effects of isolation rearing on prepulse inhibition, activity, anxiety and hippocampal-dependent memory in Roman high- and low-avoidance rats: A putative model of schizophrenia-relevant features

Social isolation of rats induces a constellation of behavioral alterations known as "isolation syndrome" that are consistent with some of the positive and cognitive symptoms observed in schizophrenic patients. In the present study we have assessed whether isolation rearing of inbred Roman high-avoidance (RHA-I) and Roman low-avoidance (RLA-I) strains can lead to the appearance of some of the key features of the "isolation syndrome", such as prepulse inhibition (PPI) deficits, increased anxious behavior, hyperactivity and memory/learning impairments. Compared to RLA-I rats, the results show that isolation rearing (IR) in RHA-I rats has a more profound impact, as they exhibit isolation-induced PPI deficits, increased anxiety, hyperactivity and long-term reference memory deficits, while isolated RLA-I rats only exhibit deficits in a spatial working memory task. These results give further support to the validity of RHA-I rats as a genetically-based model of schizophrenia relevant-symptoms.

The neurobiological basis of human aggression: A review on genetic and epigenetic mechanisms

Aggression is an evolutionary conserved behavior present in most species including humans. Inadequate aggression can lead to long-term detrimental personal and societal effects. Here, we differentiate between proactive and reactive forms of aggression and review the genetic determinants of it. Heritability estimates of aggression in general vary between studies due to differing assessment instruments for aggressive behavior (AB) as well as age and gender of study participants. In addition, especially non-shared environmental factors shape AB. Current hypotheses suggest that environmental effects such as early life stress or chronic psychosocial risk factors (e.g., maltreatment) and variation in genes related to neuroendocrine, dopaminergic as well as serotonergic systems increase the risk to develop AB. In this review, we summarize the current knowledge of the genetics of human aggression based on twin studies, genetic association studies, animal models, and epigenetic analyses with the aim to differentiate between mechanisms associated with proactive or reactive aggression. We hypothesize that from a genetic perspective, the aminergic systems are likely to regulate both reactive and proactive aggression, whereas the endocrine pathways seem to be more involved in regulation of reactive aggression through modulation of impulsivity. Epigenetic studies on aggression have associated non-genetic risk factors with modifications of the stress response and the immune system. Finally, we point to the urgent need for further genome-wide analyses and the integration of genetic and epigenetic information to understand individual differences in reactive and proactive AB. © 2015 Wiley Periodicals, Inc.

Psychopharmacological Studies in Mice

Since 1998, when the laboratory of Medicinal Pharmacology was established in the Graduate School of Pharmaceutical Sciences, Osaka University, I have been interested in psychopharmacological research topics. During this period, we identified a number of novel regulatory mechanisms that control the prefrontal dopamine system through functional interaction between serotonin1A and dopamine D2 receptors or between serotonin1A and σ1 receptors. Our findings suggest that strategies that enhance the prefrontal dopamine system may have therapeutic potential in the treatment of psychiatric disorders. We also found that environmental factors during development strongly impact the psychological state in adulthood. Furthermore, we clarified the pharmacological profiles of the acetylcholinesterase inhibitors donepezil, galantamine, and rivastigmine, providing novel insights into their mechanisms of action. Finally, we developed the female encounter test, a novel method for evaluating motivation in mice. This simple method should help advance future psychopharmacological research. In this review, we summarize the major findings obtained from our recent studies in mice.

Rivastigmine improves isolation rearing-induced prepulse inhibition deficits via muscarinic acetylcholine receptors in mice

RATIONALE

The acetylcholinesterase inhibitors donepezil, galantamine, and rivastigmine are used for the treatment of Alzheimer's disease. We previously demonstrated that donepezil and galantamine differentially affect isolation rearing-induced prepulse inhibition (PPI) deficits and that this might be due to differential effects on brain muscarinic acetylcholine (mACh) receptor function in mice.

OBJECTIVES

We examined the effects of rivastigmine on isolation rearing-induced PPI deficits, brain ACh levels, and mACh receptor function in mice.

METHODS

Acoustic startle responses were measured in a startle chamber. Microdialysis was performed, and the levels of dopamine and ACh in the prefrontal cortex were measured.

RESULTS

Rivastigmine (0.3 mg/kg) improved PPI deficits, and this improvement was antagonized by the mACh receptor antagonist telenzepine but not by the nicotinic ACh receptor antagonist mecamylamine. Rivastigmine increased extracellular ACh levels by approximately 2-3-fold, less than the increase produced by galantamine. Rivastigmine enhanced the effect of the mACh receptor agonist N-desmethylclozapine on prefrontal dopamine release, a marker of mACh receptor function, and this increase was blocked by telenzepine. In contrast, galantamine did not affect N-desmethylclozapine-induced dopamine release. Furthermore, rivastigmine did not affect cortical dopamine release induced by the serotonin1A receptor agonist osemozotan, suggesting that the effect of rivastigmine has specificity for mACh receptors.

CONCLUSIONS

Taken together with our previous finding that marked increases in ACh levels are required for the PPI deficit improvement induced by galantamine, our present results suggest that rivastigmine improves isolation rearing-induced PPI deficits by increasing ACh levels and by concomitantly enhancing mACh receptor function.

Social Isolation Stress Induces Anxious-Depressive-Like Behavior and Alterations of Neuroplasticity-Related Genes in Adult Male Mice

Stress is a major risk factor in the onset of several neuropsychiatric disorders including anxiety and depression. Although several studies have shown that social isolation stress during postweaning period induces behavioral and brain molecular changes, the effects of social isolation on behavior during adulthood have been less characterized. Aim of this work was to investigate the relationship between the behavioral alterations and brain molecular changes induced by chronic social isolation stress in adult male mice. Plasma corticosterone levels and adrenal glands weight were also analyzed. Socially isolated (SI) mice showed higher locomotor activity, spent less time in the open field center, and displayed higher immobility time in the tail suspension test compared to group-housed (GH) mice. SI mice exhibited reduced plasma corticosterone levels and reduced difference between right and left adrenal glands. SI showed lower mRNA levels of the BDNF-7 splice variant, c-Fos, Arc, and Egr-1 in both hippocampus and prefrontal cortex compared to GH mice. Finally, SI mice exhibited selectively reduced mGluR1 and mGluR2 levels in the prefrontal cortex. Altogether, these results suggest that anxious- and depressive-like behavior induced by social isolation stress correlates with reduction of several neuroplasticity-related genes in the hippocampus and prefrontal cortex of adult male mice.

In vitro pharmacological and rat pharmacokinetic characterization of LY3020371, a potent and selective mGlu2/3 receptor antagonist

Metabotropic glutamate _2/3 (mGlu_2/3) receptors are of considerable interest owing to their role in modulating glutamate transmission via presynaptic, postsynaptic and glial mechanisms. As part of our ongoing efforts to identify novel ligands for these receptors, we have discovered (1S,2R,3S,4S,5R,6R)-2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid; (LY3020371), a potent and selective orthosteric mGlu_2/3 receptor antagonist. In this account, we characterize the effects of LY3020371 in membranes and cells expressing human recombinant mGlu receptor subtypes as well as in native rodent and human brain tissue preparations, providing important translational information for this molecule. In membranes from cells expressing recombinant human mGlu₂ and mGlu₃ receptor subtypes, LY3020371.HCl competitively displaced binding of the mGlu_2/3 agonist ligand [³H]-459477 with high affinity (hmGlu₂ K_i = 5.26 nM; hmGlu₃ Ki = 2.50 nM). In cells expressing hmGlu₂ receptors, LY3020371.HCl potently blocked mGlu_2/3 agonist (DCG-IV)-inhibited, forskolin-stimulated cAMP formation (IC₅₀ = 16.2 nM), an effect that was similarly observed in hmGlu₃-expressing cells (IC₅₀ = 6.21 nM). Evaluation of LY3020371 in cells expressing the other human mGlu receptor subtypes revealed high mGlu_2/3 receptor selectivity. In rat native tissue assays, LY3020371 demonstrated effective displacement of [³H]-459477 from frontal cortical membranes (K_i = 33 nM), and functional antagonist activity in cortical synaptosomes measuring both the reversal of agonist-suppressed second messenger production (IC₅₀ = 29 nM) and agonist-inhibited, K⁺-evoked glutamate release (IC₅₀ = 86 nM). Antagonism was fully recapitulated in both primary cultured cortical neurons where LY3020371 blocked agonist-suppressed spontaneous Ca²⁺ oscillations (IC₅₀ = 34 nM) and in an intact hippocampal slice preparation (IC₅₀ = 46 nM). Functional antagonist activity was similarly demonstrated in synaptosomes prepared from epileptic human cortical or hippocampal tissues, suggesting a translation of the mGlu_2/3 antagonist pharmacology from rat to human. Intravenous dosing of LY3020371 in rats led to cerebrospinal fluid drug levels that are expected to effectively block mGlu_2/3 receptors in vivo. Taken together, these results establish LY3020371 as an important new pharmacological tool for studying mGlu_2/3 receptors in vitro and in vivo. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Involvement of prefrontal AMPA receptors in encounter stimulation-induced hyperactivity in isolation-reared mice

We recently showed that social encounter stimulation induces hyperactivity in mice reared in social isolation from early life and this is associated with the transient activation of prefrontal dopaminergic and serotonergic systems. In the present study, we examined the effect of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 2, 3-dioxo-6-nitro-1, 2, 3, 4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) on encounter-induced behavioural and neurochemical changes to study the role of the receptor in abnormal behaviours in isolation-reared mice. The encounter to an intruder mouse induced hyperactivity with transient increases in prefrontal dopamine and serotonin levels in isolation-reared mice. NBQX attenuated the encounter-induced hyperactivity and the associated neurochemical changes in isolation-reared mice. In addition, NBQX reduced aggressive behaviour and cognitive impairment in isolation-reared mice, but did not affect depressive-like behaviour or spontaneous hyper-locomotion in these animals. The AMPA receptor agonist (S)-AMPA increased prefrontal dopamine and serotonin release, and this effect was higher in isolation-reared mice than in the group-reared mice, suggesting higher prefrontal AMPA receptor activity in isolation-reared mice. Furthermore, isolation rearing increased the expression of AMPA receptor subunits (GluR1, GluR2 and GluR3) and GluR1 Ser845 phosphorylation in the prefrontal cortex, but not in the hippocampus or nucleus accumbens. Taken together, these results suggest that an increase in AMPA receptor activity in the prefrontal cortex contributes to some, but not all, abnormal behaviours in isolation-reared mice.

Cerebrospinal fluid glutamate concentration correlates with impulsive aggression in human subjects

Neurochemical studies have pointed to a modulatory role in human aggression for various central neurotransmitters. Some (e.g., serotonin) appear to play an inhibitory role, while others appear to play a facilitator role. While recent animal studies of glutaminergic activity suggest a facilitator role for central glutamate in the modulation of aggression, no human studies of central glutaminergic indices have yet been reported regarding aggression. Basal lumbar cerebrospinal fluid (CSF) was obtained from 38 physically healthy subjects with DSM-IV Personality Disorder (PD: n = 28) and from Healthy Volunteers (HV: n = 10) and assayed for glutamate, and other neurotransmitters, in CSF and correlated with measures of aggression and impulsivity. CSF Glutamate levels did not differ between the PD and HC subjects but did directly correlate with composite measures of both aggression and impulsivity and a composite measure of impulsive aggression in both groups. These data suggest a positive relationship between CSF Glutamate levels and measures of impulsive aggression in human subjects. Thus, glutamate function may contribute to the complex central neuromodulation of impulsive aggression in human subjects.

Role of social encounter-induced activation of prefrontal serotonergic systems in the abnormal behaviors of isolation-reared mice

Isolation-reared male rodents show abnormal behaviors such as hyperlocomotion, aggressive behaviors, deficits of prepulse inhibition, and depression- and anxiety-like behaviors, but the neurochemical mechanism for the effects of psychological stress in these animals is not fully understood. This study examined the effects of social interactions between isolation-reared mice and intruder mice on brain monoaminergic systems. A cage was divided into two compartments by a mesh partition to prevent direct physical interactions. The 20-min encounter with an intruder elicited a restless and hyperexcitable state (hyperactivity) in male, but not in female, isolation-reared mice, whereas encounters with a sleeping intruder or a novel object did not. Although the encounter did not affect prefrontal neuronal-activity-marker c-Fos expression, dopamine (DA) levels, or serotonin (5-HT) levels in male group-reared mice or female isolation-reared mice, it increased prefrontal c-Fos expression, DA levels, and 5-HT levels in male isolation-reared mice. Furthermore, encounter-induced increases in c-Fos expression in the dorsal raphe nucleus and ventral tegmental area, but not in the nucleus accumbens shell, were much greater in isolation-reared than group-reared male mice. A 5-HT1A receptor agonist, a metabotropic glutamate 2/3 receptor agonist, and a gamma-aminobutyric acid A receptor agonist attenuated isolation-induced aggressive behaviors and encounter-induced hyperactivity, c-Fos expression in the prefrontal cortex and dorsal raphe nucleus, and increases in prefrontal 5-HT levels. These findings suggest that the prefrontal DA and 5-HT systems are activated by encounter stimulation in male isolation-reared mice, and the encounter-induced activation of 5-HT system triggers the induction of some abnormal behaviors in male isolation-reared mice. Furthermore, this study implies that the encounter stimulation-induced signal has a pharmacological significance.

Involvement of spinal 5-HT1A receptors in isolation rearing-induced hypoalgesia in mice

RATIONALE

Isolation rearing in rodents causes not only abnormal behaviors which resemble the clinical symptoms of schizophrenia but also hypoalgesia in thermal nociception models. However, the mechanism of the hypoalgesia is not known.

OBJECTIVES

The present study investigated the effect of isolation rearing on acute pain and the descending pain inhibitory pathways in mice.

RESULTS

Rearing in isolation for 6 weeks from post-weaning reduced pain sensitivity in the hot plate test and acetic acid-induced writhing test. Isolation rearing also reduced the intraplantar capsaicin-induced licking behavior. Capsaicin increased c-Fos expression, a neuronal activity marker, in the spinal cord and primary somatosensory cortex both in group- and isolation-reared mice, but this effect did not differ between groups. On the other hand, c-Fos expression in the anterior cingulate cortex, periaqueductal gray matter, and rostral ventromedial medulla, but not in the spinal cord or somatosensory cortex, was enhanced by isolation rearing. Systemic administration of WAY100635 (serotonin (5-HT)1A receptor antagonist), but not of ketanserin (5-HT2 receptor antagonist), prazosin (α1-adrenoceptor antagonist), or yohimbine (α2-adrenoceptor antagonist), attenuated isolation rearing-induced hypoalgesia in capsaicin-induced licking behavior. Attenuation of isolation rearing-induced hypoalgesia was also observed following the intrathecal injection of WAY100635. Naloxone, an opioid receptor antagonist, did not affect the hypoalgesia in isolation-reared mice.

CONCLUSIONS

These findings suggest that isolation rearing causes hypoalgesia in mouse models of acute pain and imply that the spinal 5-HT1A receptor activation probably through descending serotonergic inhibitory pathway is involved in isolation rearing-induced hypoalgesia.

Stimulation of metabotropic glutamate (mGlu) 2 receptor and blockade of mGlu1 receptor improve social memory impairment elicited by MK-801 in rats

Glutamatergic dysfunction has been implicated in psychiatric disorders such as schizophrenia. Both the stimulation of the metabotropic glutamate (mGlu) 2/3 receptor and the blockade of the mGlu1 receptor have been shown to be effective in a number of animal models of schizophrenia. However, the efficacy for social cognition, which is poorly managed by current medication, has not been fully addressed. The present study evaluated the effects of an mGlu2/3-receptor agonist and an mGlu1-receptor antagonist on social memory impairment in rats. Pretreatment with an mGlu2/3-receptor agonist, (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268), or an mGlu1-receptor antagonist, (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone (JNJ16259685), improved social memory impairment induced by 5R,10S-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801) without affecting the social interactions. In addition, the intraperitoneal administration of an mGlu2-receptor potentiator, 3'-[[(2-cyclopentyl-2,3-dihydro-6,7-dimethyl-1-oxo-1H-inden-5-yl)oxy]methyl]-[1,1'-biphenyl]-4-carboxylic acid (BINA), also improved the MK-801-induced impairment of social memory, which was blocked by pretreatment with an mGlu2/3-receptor antagonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495). These findings indicate that both the stimulation of the mGlu2 receptor and the inhibition of an mGlu1 receptor improve social memory impairment elicited by MK-801, and both manipulations could be effective approaches for the treatment of certain cognitive dysfunctions observed in schizophrenic patients.

Neurogenetics of aggressive behavior: studies in rodents

Aggressive behavior is observed in many animal species, such as insects, fish, lizards, frogs, and most mammals including humans. This wide range of conservation underscores the importance of aggressive behavior in the animals' survival and fitness, and the likely heritability of this behavior. Although typical patterns of aggressive behavior differ between species, there are several concordances in the neurobiology of aggression among rodents, primates, and humans. Studies with rodent models may eventually help us to understand the neurogenetic architecture of aggression in humans. However, it is important to recognize the difference between the ecological and ethological significance of aggressive behavior (species-typical aggression) and maladaptive violence (escalated aggression) when applying the findings of aggression research using animal models to human or veterinary medicine. Well-studied rodent models for aggressive behavior in the laboratory setting include the mouse (Mus musculus), rat (Rattus norvegicus), hamster (Mesocricetus auratus), and prairie vole (Microtus ochrogaster). The neural circuits of rodent aggression have been gradually elucidated by several techniques, e.g., immunohistochemistry of immediate-early gene (c-Fos) expression, intracranial drug microinjection, in vivo microdialysis, and optogenetics techniques. Also, evidence accumulated from the analysis of gene-knockout mice shows the involvement of several genes in aggression. Here, we review the brain circuits that have been implicated in aggression, such as the hypothalamus, prefrontal cortex (PFC), dorsal raphe nucleus (DRN), nucleus accumbens (NAc), and olfactory system. We then discuss the roles of glutamate and γ-aminobutyric acid (GABA), excitatory and inhibitory amino acids in the brain, as well as their receptors, in controlling aggressive behavior, focusing mainly on recent findings. At the end of this chapter, we discuss how genes can be identified that underlie individual differences in aggression, using the so-called forward genetics approach.