Involvement of the GABAergic system on shock-induced aggressive behavior in two strains of mice

Letrozole delays acquisition of water maze task in female BALB/c mice: Possible involvement of anxiety

Letrozole, an aromatase inhibitor preventing estrogen synthesis from testosterone, is used as an adjuvant therapy in estrogen receptor-positive breast cancer patients. However, like other aromatase inhibitors, it induces many side effects, including impaired cognition. Despite its negative effect in humans, results from animal models are inconsistent and suggest that letrozole can either impair or improve cognition. Here, we studied the effects of chronic letrozole treatment on cognitive behavior of adult female BALB/c mice, a relevant animal model for breast cancer studies, to develop an appropriate animal model aimed at testing therapies to mitigate side effects of letrozole. In Morris water maze, letrozole 0.1 mg/kg impaired reference learning and memory. Interestingly, most of the letrozole 0.1 mg/kg-treated mice were able to learn the new platform position in reversal training and performed similar to control mice in a reversal probe test. Results of the reversal test suggest that letrozole did not completely disrupt spatial navigation, but rather delayed acquisition of spatial information. The delay might be related to increased anxiety as suggested by increased thigmotactic behavior during the reference memory training. The learning impairment was water maze-specific since we did not observe impairment in other spatial tasks such as in Y-maze or object location test. In contrast, the dose of 0.3 mg/kg did not have effect on water maze learning and facilitated locomotor habituation and recognition in novel object recognition test. The current study shows that letrozole dose-dependently modulates behavioral response and that its effects are task-dependent.

Valproate selectively suppresses adolescent anabolic/androgenic steroid-induced aggressive behavior: implications for a role of hypothalamic γ-aminobutyric acid neural signaling

Pubertal male Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids (AASs) during adolescence (P27-P56) display a highly intense aggressive phenotype that shares many behavioral similarities with pathological aggression in youth. Anticonvulsant drugs like valproate that enhance the activity of the γ-aminobutyric acid (GABA) neural system in the brain have recently gained acceptance as a primary treatment for pathological aggression. This study examined whether valproate would selectively suppress adolescent AAS-induced aggressive behavior and whether GABA neural signaling through GABAA subtype receptors in the latero-anterior hypothalamus (LAH; an area of convergence for developmental and neuroplastic changes that underlie aggression in hamsters) modulate the aggression-suppressing effect of this anticonvulsant medication. Valproate (1.0-10.0 mg/kg, intraperitoneal) selectively suppressed the aggressive phenotype in a dose-dependent fashion, with the effective anti-aggressive effects beginning at 5 mg/kg, intraperitoneally. Microinfusion of the GABAA receptor antagonist bicuculline (7.0-700 ng) into the LAH reversed valproate's suppression of AAS-induced aggression in a dose-dependent fashion. At the 70 ng dose of bicuculline, animals expressed the highly aggressive baseline phenotype normally observed in AAS-treated animals. These studies provide preclinical evidence that the anticonvulsant valproate selectively suppresses adolescent, AAS-induced aggression and that this suppression is modulated, in part, by GABA neural signaling within the LAH.

The effects of Topiramate on isolation-induced aggression: a behavioral and immunohistochemical study in mice

Topiramate, an antiepileptic drug, has been found to be useful for the treatment of aggression in clinical populations. Most preclinical studies related to Topiramate have been focused exclusively on the quantitative aspects of the aggressive behavior between mice. However, there is still limited knowledge regarding the effects of Topiramate on neuronal mechanisms occurring in aggressive mice. The present work aims to understand further the effects of the antiepileptic drug Topiramate on aggressive behaviors, and on the neural correlates underlying such behaviors. To achieve this, we combined the resident-intruder model of isolation-induced aggression in mice with two drug regimens of Topiramate administration (30.0 mg/kg; acute and sub-chronic treatments). Our data showed that both acute and subchronic treatments decreased the intensity of agonistic encounters and reinforced social behavior. By using C-fos immunoreactivity, we investigated the neuronal activation of several brain regions involved in aggressive behavior following subchronic treatment. We found that Topiramate produced activation in several cortical areas and in the lateral septum of resident brain mice compared with their controls. However, Topiramate induced inhibition in the medial nucleus of the amygdala, the dorsomedial nucleus of the periaqueductal gray, and especially in the anterior hypothalamic nucleus. Finally, we performed microinfusion of Topiramate (0.1 and 0.3 mM) into the lateral septum and anterior hypothalamus on offensive behaviors in isolation-induced-aggression paradigm. Interestingly, the microinfusion of Topiramate into the lateral septum has the capacity to alleviate aggressive behavior, without affecting social behavior. However, the microinfusion of Topiramate into the anterior hypothalamus decreased aggressive behavior and slightly reinforced social behavior. Our observations supported that the dose of 0.1 mM of Topiramate appeared more efficacy to treat aggression in adult mice. These pharmacological characteristics may account for Topiramate efficacy on aggressive symptoms in psychiatric patients.

Alcohol and violence: neuropeptidergic modulation of monoamine systems

Neurobiological processes underlying the epidemiologically established link between alcohol and several types of social, aggressive, and violent behavior remain poorly understood. Acute low doses of alcohol, as well as withdrawal from long-term alcohol use, may lead to escalated aggressive behavior in a subset of individuals. An urgent task will be to disentangle the host of interacting genetic and environmental risk factors in individuals who are predisposed to engage in escalated aggressive behavior. The modulation of 5-hydroxytryptamine impulse flow by gamma-aminobutyric acid (GABA) and glutamate, acting via distinct ionotropic and metabotropic receptor subtypes in the dorsal raphe nucleus during alcohol consumption, is of critical significance in the suppression and escalation of aggressive behavior. In anticipation and reaction to aggressive behavior, neuropeptides such as corticotropin-releasing factor, neuropeptide Y, opioid peptides, and vasopressin interact with monoamines, GABA, and glutamate to attenuate and amplify aggressive behavior in alcohol-consuming individuals. These neuromodulators represent novel molecular targets for intervention that await clinical validation. Intermittent episodes of brief social defeat during aggressive confrontations are sufficient to cause long-lasting neuroadaptations that can lead to the escalation of alcohol consumption.

γ-Aminobutyric acid neural signaling in the lateroanterior hypothalamus modulates aggressive behavior in adolescent anabolic/androgenic steroid-treated hamsters

Male Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids (AAS) during adolescence (P27-P56) display highly escalated and mature forms of offensive aggression correlated with increased γ-aminobutyric acid (GABA) afferent development as well as decreased GABAA receptors in the lateroanterior hypothalamus (LAH) - an area of convergence for developmental and neuroplastic changes that underlie offensive aggressive behaviors in hamsters. This study investigated whether microinfusion of a GABAA receptor agonist (muscimol; 0.01-1.0 pmol/l) or antagonist (bicuculline; 0.04-4.0 pmol/l) directly into the LAH modulate adolescent AAS-induced offensive aggression. Activation of LAH GABAA receptors enhanced adolescent AAS-induced offensive aggression, beginning at the 0.1 pmol/l dose, when compared with AAS-treated animals injected with saline into the LAH. Importantly, GABAA receptor agonism within the LAH significantly increased the frequency of belly/rear attacks, while simultaneously decreasing the frequency of frontal attacks. These data identify a neuroanatomical locus where GABAA receptor activation functions to enhance aggression in adolescent AAS-treated animals, while also promoting the display of mature forms of aggression and suppressing juvenile play behaviors.

Chronic tiagabine administration and aggressive responding in individuals with a history of substance abuse and antisocial behavior

Anticonvulsants, notably those which modulate GABA activity, have shown efficacy in reducing aggressive behavior. Previously, we found dose-related decreases in human aggressive responding following acute tiagabine administration. Here, we examined the effects of chronic tiagabine over a 5-week period. Twelve individuals at increased risk for aggressive and violent behavior (currently on parole/probation with personality and/or substance use disorders) were randomly assigned to placebo (n = 6) or an escalating dose sequence of placebo, 4 mg, 8 mg, 12 mg, placebo (n = 6). Data were analyzed using both frequentist and Bayesian mixed models, evaluating aggressive behavior as a function of time, dose condition, and their interaction. For aggressive responding, there was a significant interaction of drug condition and time. Aggression in the tiagabine condition decreased for each additional week in the study, while participants in the placebo condition failed to demonstrate similar change over time. For monetary-reinforced responding, no drug or drug by time interactions were observed, suggesting specificity of drug effects on aggression. The small number of subjects limits the generality of the findings, and previous studies with tiagabine are limited to acute dosing and case report investigations. However, the present data provide an indication that tiagabine merits further examination as an agent for management of impulsive aggression.

Adolescent exposure to anabolic/androgenic steroids and the neurobiology of offensive aggression: a hypothalamic neural model based on findings in pubertal Syrian hamsters

Considerable public attention has been focused on the issue of youth violence, particularly that associated with drug use. It is documented that anabolic steroid use by teenagers is associated with a higher incidence of aggressive behavior and serious violence, yet little is known about how these drugs produce the aggressive phenotype. Here we discuss work from our laboratory on the relationship between the development and activity of select neurotransmitter systems in the anterior hypothalamus and anabolic steroid-induced offensive aggression using pubertal male Syrian hamsters (Mesocricetus auratus) as an adolescent animal model, with the express goal of synthesizing these data into an cogent neural model of the developmental adaptations that may underlie anabolic steroid-induced aggressive behavior. Notably, alterations in each of the neural systems identified as important components of the anabolic steroid-induced aggressive response occurred in a sub-division of the anterior hypothalamic brain region we identified as the hamster equivalent of the latero-anterior hypothalamus, indicating that this sub-region of the hypothalamus is an important site of convergence for anabolic steroid-induced neural adaptations that precipitate offensive aggression. Based on these findings we present in this review a neural model to explain the neurochemical regulation of anabolic steroid-induced offensive aggression showing the hypothetical interaction between the arginine vasopressin, serotonin, dopamine, gamma-aminobutyric acid, and glutamate neural systems in the anterior hypothalamic brain region.

Flunitrazepam in combination with alcohol engenders high levels of aggression in mice and rats

RATIONALE

Higher doses of benzodiazepines and alcohol induce sedation and sleep; however, in low to moderate doses these drugs can increase aggressive behavior.

OBJECTIVES

To assess firstly the effects of ethanol, secondly the effects of flunitrazepam, a so-called club drug, and thirdly the effects of flunitrazepam plus alcohol on aggression in mice and rats.

METHODS

Exhaustive behavioral records of confrontations between a male resident and a male intruder were obtained twice a week, using CF-1 mice and Wistar rats. The salient aggressive and non-aggressive elements in the resident's repertoire were analyzed. Initially, the effects of ethanol (1.0g/kg), and secondly flunitrazepam (0; 0.01; 0.1; and 0.3mg/kg) were determined in all mice and rats; subsequently, flunitrazepam or vehicle, given intraperitoneally (0; 0.01; 0.1; and 0.3mg/kg) was administered plus ethanol 1.0g/kg or vehicle via gavage.

RESULTS

The most significant finding is the escalation of aggression after a moderate dose of ethanol, and a low dose of flunitrazepam. The largest increase in aggressive behavior occurred after combined flunitrazepam plus ethanol treatment in mice and rats.

CONCLUSIONS

Ethanol can heighten aggressive behavior and flunitrazepam further increases this effect in male mice and rats.

Shock-induced aggression in mice is modified by lithium

Aggression is associated with numerous psychiatric disorders. Evidence suggests that lithium decreases aggression in humans and rats. The effects of lithium on aggression related behavior, and in particular shock-induced aggression, has not been as thoroughly explored in mice. Male mice were treated with lithium and tested in the shock-induced aggression and dominance tube tests. Mice treated with lithium were also assessed for thermal pain and shock sensitivity in the hot plate and jump-flinch tests. In the shock-induced aggression paradigm chronic lithium significantly decreased both the frequency and duration of attacks, without affecting social interaction or behavior in the dominance tube. Acute lithium significantly decreased the total duration of attacks and social interaction but did not affect behavior in the dominance tube test. Neither treatment regimen had an effect on temperature sensitivity in the hot plate test or on activity levels in the open field. However, chronic lithium modified the response of mice to shock in the jump-flinch test, but not at the shock level used in the aggression test. The results of this study indicate that lithium decreases shock-induced aggression in mice, but effects on baseline response to shock confound interpretation of this behavioral effect of lithium.

Effects of acute tiagabine administration on aggressive responses of adult male parolees

Experimental and clinical studies have supported a relationship between gamma-aminobutyric acid (GABA) and aggressive behavior in non-humans and humans. Tiagabine is a GABA uptake inhibitor that has been shown to produce acute behavioral effects in animals. In addition, tiagabine has been shown to decrease aggression in agitated patients when administered chronically. The present study was designed to investigate the effects of acute administration of tiagabine on aggressive responding on a laboratory task in adult humans. Ten adult males participated in experimental sessions on the Point Subtraction Aggression Paradigm (PSAP), which provided subjects with aggressive, escape, and monetary-reinforced response options. All subjects received four acute oral doses of Tiagabine (4, 8, 12 and 16 mg) separated by placebo sessions. Tiagabine decreased aggression at doses that either did not affect, or affected to a lesser extent, monetary-reinforced responding. The results are consistent with some prior research using the PSAP showing a possible unique role for GABA in the regulation of human aggression. A possible behavioral mechanism for the rate-decreasing effects on aggressive responding produced in the present study is that tiagabine may modify aggressive responding by suppressing reactions to aversive stimuli.

Strain differences of neurosteroid levels in mouse brain

Neurosteroids, pregnenolone (Preg), dehydroepiandrosterone (DHEA) and their sulfates (PregS and DHEAS) are reported to exert their modulatory effects of neuronal excitability and synaptic plasticity via amino acid receptors, which affect and regulate the learning and memory process, mood, and depression. Although the brain levels of these steroids have been reported in rodents, the strain differences of the levels of these steroids have not been demonstrated. We examined the concentrations of Preg, 17-OH-Preg, DHEA, androstenediol (ADIOL) and their sulfates in whole brains from DBA/2, C57BL/6, BALB/c, ddY and ICR mice, the genetic backgrounds of which are different. No differences in the brain levels of Preg and DHEA were found among the strains. In contrast, PregS levels in DBA/2 were significantly lower than in the others, while DHEAS concentrations in DBA/2 were significantly higher than those in other strains. Strain differences were found in 17-OH-Preg, ADIOL and 17-OH-PregS but not in ADIOLS levels. The ranges of Preg and PregS levels were the highest among the steroids studied. Further, we measured serum these steroid levels. Although strain differences were also found in serum steroids, correlation study between brain and serum levels revealed that brain neurosteroids studied may not come from peripheral circulation. In conclusion, this is the first report of demonstrating mammalian brain levels of 17-OH-Preg, ADIOL, 17-OH-PregS and ADIOLS and the strain differences in neurosteroid levels in mice brains. The differences in levels may involve the strain differences in their behavior, e.g. aggression, adaptation to stress or learning, in mice.

Neurosteroids, GABAA receptors, and escalated aggressive behavior

Aggressive behavior can serve important adaptive functions in social species. However, if it exceeds the species-typical pattern, it may become maladaptive. Very high or escalated levels of aggressive behavior can be induced in laboratory rodents by pharmacological (alcohol-heightened aggression), environmental (social instigation), or behavioral (frustration-induced aggression) means. These various forms of escalated aggressive behavior may be useful in further elucidating the neurochemical control over aggression and violence. One neurochemical system most consistently linked with escalated aggression is the GABAergic system, in conjunction with other amines and peptides. Although direct stimulation of GABA receptors generally suppresses aggression, a number of studies have found that positive allosteric modulators of GABAA receptors can cause increases in aggressive behavior. For example, alcohol, benzodiazepines, and many neurosteroids are all positive modulators of the GABAA receptor and all can cause increased levels of aggressive behavior. These effects are dose-dependent and higher doses of these compounds generally shift from heightening aggressive behavior to being sedative and anti-aggressive. In addition, these modulators interact with each other and can have additive effects on the GABAA receptor and on behavior, including aggression. The GABAA receptor is a heteropentameric protein that can be constituted from various subunits. It has been shown that subunit composition can affect sensitivity of the receptor to some modulators and that subunit composition differentially affects the sedative vs anxiolytic actions of benzodiazepines. Initial studies targeting alpha subunits of the GABAA receptor point to their significant role in the aggression-heightening effects of alcohol, benzodiazepines, and neurosteroids.

Perturbations of plasma cortisol and DHEA-S following discontinuation of cocaine use in cocaine addicts

Changes in plasma levels of cortisol and dehydroepiandrosterone sulfate (DHEA-S) following cocaine discontinuation were assessed in hospitalized chronic cocaine users. Measurements were performed after 6, 9, 18 and 21 days of abstinence. Repeated measures ANOVAs revealed significant time effects for cortisol (P<0.02) and DHEA-S (P<0.001). Changes in the two hormones did not follow the same course. Levels of cortisol were highest on day 6 and then subsequently decreased, whereas DHEA-S levels were low on day 6 and highest on day 18. Repeated measures ANCOVAs were used to test the overall effects of total duration of cocaine use, daily or weekly cocaine amounts consumed, or frequency of use on cortisol secretion. Analyses revealed a significant effect of frequency of use only (P<0.04). More sustained cocaine use was associated with higher cortisol levels and less pronounced cortisol decline after discontinuation of cocaine use, but drug intake variables had no influence on DHEA-S. The effects of presence or absence of life-long histories of aggression were also assessed. Repeated measures ANOVAs revealed a near significant group x time interaction for cortisol, which declined more dramatically in aggressive addicts than in non-aggressive addicts after day 6. DHEA-S was consistently higher in aggressive cocaine addicts, although this effect did not reach statistical significance. There was a noticeable difference in the dynamics of normalization of adrenal hormones between the two groups, with DHEA-S/cortisol ratios rising more dramatically during cocaine abstinence in aggressive than in non-aggressive addicts. In conclusion, lingering neuroendocrine perturbations persist after discontinuation of cocaine use in addicts. Some of these changes could be associated with an increased relapse risk.

Gabapentin for treatment of behavioral and psychological symptoms of dementia

OBJECTIVE

To report the use of gabapentin in the treatment of behavioral and psychological symptoms of dementia (BPSD) and to review the available literature relating to the use of gabapentin in this population.

CASE SUMMARY

A 62-year-old white man was admitted to the hospital due to a worsening state of confusion, anxiety, depressed mood, insomnia, and verbal and physical aggressiveness toward his wife. He had a past medical history significant for vascular dementia. He had been intolerant of or had failed to respond to numerous antidepressants, benzodiazepines, and neuroleptics. The addition of gabapentin to the patient's medication regimen resulted in reduced agitation, sexual inappropriateness, and lability. He was discharged to his home on a dose of gabapentin 300 mg three times daily.

DATA SOURCES

A MEDLINE search (1966-August 2000) was performed to identify case reports and clinical trials discussing the efficacy of gabapentin in the treatment of BPSD.

DISCUSSION

Gabapentin, like other anticonvulsants, has been used with success in several psychiatric illnesses. Available literature indicates that the drug may have some efficacy in the treatment of BPSD. It has a favorable adverse effect profile in the elderly, which makes it an attractive altemative to standard therapies, including benzodiazepines and neuroleptics. Optimal dosing remains unclear.

CONCLUSIONS

This case suggests that gabapentin is a reasonable alternative therapy for patients whose behavioral symptoms do not respond to conventional agents.

Selection, evolution of behavior and animal models in behavioral neuroscience

We investigated whether genetic differences in various forms of intraspecific aggression and anxiety in four different genetic lines of mice (i.e. wild, outbred Swiss-CD1, inbred DBA/2 and inbred C57/BL6N) may reflect modifications in behavioral strategy. Experiments 1 and 2 used ethologically based paradigms to analyze aggressive and anxiety responses both in social (i.e. aggression) and non-social (i.e. novel environment exploration) contexts. In Experiment 3, an anxiolytic drug (chlordiazepoxide (CDP)) was used to examine possible differences in proximal mechanisms underlying anxiety-related behaviors. The data show that intrasexual aggression, infanticide and maternal aggressions are related and covarying. Genetic lines with the highest levels of intermale attack (i.e. Wild and Swiss-CD1) also have highest levels of infanticide, interfemale attack and maternal aggression but, interestingly, the lowest levels of anxiety. In fact, exploratory behavior is lower and risk assessment behavior markedly higher in DBA/2 and C57/BL6N mice (i.e. the less aggressive strains) compared to Swiss and Wild genetic lines. Although reproductive status influences anxiety levels in female mice, our findings show that (contrary to previous studies) lactating mice are more anxious than virgin females in terms of risk assessment activities. These data demonstrate the importance of studying behavior in a more ecologically-relevant context which emphasizes the function of behavior in a specific situation. Moreover, differential strain sensitivity to the behavioral effects of CDP suggests that genetic lines of mice may differ in the underlying mechanisms mediating behavior. It is therefore possible that artificial selection of different genotypes has resulted in differences in proximate mechanisms modulating the levels of aggression and anxiety, thereby leading to modification of social behavior. Overall, the results presented here suggest that subtle genetic alterations in specific underlying neural mechanisms are likely to cause profound effects on behavioral responses and their adaptive significance. Implications for behavioral neuroscience research that seeks to understand both the proximal and ultimate mechanisms of behavior are discussed.

The emerging role of valproate in bipolar disorder and other psychiatric disorders

Although valproate, a simple branched-chain fatty acid, is generally considered to be an antiepileptic agent, a large literature dating back to 1966 describes its use in primary psychiatric disorders. The significant role that gamma-aminobutyric acid plays in mood provided the rationale to examine valproate in this regard. Numerous uncontrolled as well as placebo- and lithium-controlled studies verified the drug's efficacy in the short-term management of bipolar and schizoaffective disorders. The response appears to be independent of response to traditional therapies such as lithium, neuroleptics, and carbamazepine, and may be maintained for extended periods of time. Valproate should be considered not only in patients with mood disorders who are intolerant of or nonresponsive to traditional therapies, but also in those with rapid cycling, electroencephalographic abnormalities, head trauma antedating the onset of psychiatric illness, or any other factor suggesting an organic component. Preliminary uncontrolled studies suggest that the drug may also eventually play a role in the management of panic disorder and behavioral dyscontrol (agitation, aggression, temper outbursts). Its adverse event profile is well known from years of experience in the management of epilepsy and does not appear to be altered in the presence of psychiatric disorders. Similarly, the drug-drug interaction potential of valproate is reasonably well known, although further research into interactions with psychotropic agents is warranted.

Sodium valproate in the treatment of behavioral disturbance in dementia

Management of the behavioral complications of dementia, including agitation and aggression, presents a tremendous challenge to families and caregivers of afflicted patients. Most previous pharmacotherapies have shown minimal efficacy and significant side effects. We report our initial, open-label experience using the anticonvulsant sodium valproate in four dementia patients with severe behavioral disturbance. The drug was well tolerated by all patients, with no side effects or laboratory abnormalities, during 1- to 3-month trials. Two patients showed significant improvement in behavior, and a third had a transient response. Valproate may be a useful agent in the treatment of behavioral disturbance in dementia; controlled trials are needed to document its efficacy.

Strain-dependent effects of post-training GABA receptor agonists and antagonists on memory storage in mice

Post-training administration of the GABA-A and GABA-B receptor agonists muscimol and baclofen dose-dependently impaired retention of an inhibitory avoidance response in C57 mice, while improving memory consolidation in the DBA strain. By contrast, picrotoxin (blocker of GABA-activated ionophores), bicuculline (GABA-A antagonist) and CGP 35348 (GABA-B antagonist) dose-dependently improved retention in C57 mice and impaired it in DBA mice. These effects cannot be ascribed to non-specific actions of the drugs on retention performance, as the latencies during the retention test of those mice that had not received footshock during the training were not lengthened by the post-training drug administration. The effects on retention performance induced by GABA agonists and antagonists are probably due to an effect on memory consolidation, since they are observed when the drugs are given at short, but not at long, intervals after training. These results are discussed in terms of possible interaction of GABA systems with endogenous opioid and dopamine systems, whose activation has been shown to produce strain-dependent effects on memory processes. The possible utilization of these results for a genetic behavioral approach with recombinant inbred (RI) mice is also considered.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

Catecholamines and aggression in animals

This paper assesses the evidence for the role of catecholamines in the aggressive behaviour of animals. The effects of manipulating dopamine and noradrenaline function, either alone or in combination, are considered with respect to two categories of aggression, predatory and affective. Affective aggression is further subdivided into shock-induced defensive fighting, isolation-induced aggression and irritable aggression. The results indicate that catecholamines may not have a specific role in aggressive behaviour. Rather, they may act more to excite or inhibit general behavioural systems, although certain treatments do have a specific influence on aggressive behaviour. The review also highlights certain problems concerning the psychopharmacology of aggression; different species may make varying responses to the same treatment, whilst treatments exerting a similar pharmacological action may result in diverse behavioural effects.

Age-dependent changes of brain GABA levels, turnover rates and shock-induced aggressive behavior in inbred strains of mice

Shock-induced aggressive behavior (SIAB) is absent or very weak in C57BL/6 (C57) mice at the age of 12 weeks while it reaches high levels at the age of 20 weeks. This age-dependent increase of aggressive responses is absent in DBA/2 (DBA) mice. Aggressive C57 mice (20 week old) are characterized by lower GABA levels in amygdala, striatum and substantia nigra than both non-aggressive C57 (12 week old) and DBA mice (12-20 week old). Concerning turnover rate, C57 mice at the age of 20 weeks show lower turnover rate values in cerebellum and raphe and higher values in septum in comparison with 12 week old mice of the same strain. These results are discussed in terms of the role of GABA function in brain areas which are involved in the control of emotionality and aggressive behavior.

Prolyl-leucyl-glycinamide (PLG): inhibition of offensive aggression in mice

The effects on offensive aggression of the endogenous peptide-leucyl-glycinamide (PLG, MIF-1) and the exogenous opiate antagonist, naloxone, were examined in male mice. PLG (0.01-10 mg/Kg) reduced, in a dose-dependent manner, the incidence and intensity of offensive aggression in dominant resident mice. PLG was more potent than naloxone (1.0 mg/Kg). In a number of cases, PLG completely eliminated the display of offensive aggression towards intruder mice. These results raise the possibility that PLG may function as an "anti-aggressive" peptide whose actions may include antagonistic and/or modulatory influences on both opioid and non-opioid systems.

Effects of the potentiation of the GABAergic neurotransmission in the olfactory bulbs on mouse-killing behavior

Intra olfactory bulb administration of three classes of GABA-mimetics (GABAa agonists, inhibitors of reuptake, inhibitors of GABA degradation) clearly inhibit mouse-killing behavior, without sedation. A linear correlation is observed between GABA levels increase in the olfactory bulbs and muricidal inhibition following local injection of valproic acid and gamma-vinyl GABA, two GABA-T inhibitors; the differences observed between these two compounds may be due to the differences in their mechanism of action on GABA-T activity and to the different pool of GABA on which they act. No diffusion to extra bulbar sites were observed after local administration of gamma-vinyl GABA. This evidence suggests an inhibitory role of GABA from olfactory bulbs in the modulation of mouse-killing behavior.

Effects of gender, gonadectomy and social status on attack directed towards female intruders by resident mice

Small unisexed groups (triads) of either male of female Swiss strain mice attack lactating intruders introduced into their home-cages. Female residents are, however, generally much more aggressive towards such intruders than are males. Ovariectomy of female residents (on day 15 or day 50 of life) has little effect on such responses, but castration of male residents significantly increases their attack on lactating intruders to a level resembling that of females. The introduction of the lactating female into the home-cage of sham-operated or gonadectomized male or female residents also enhances intragroup fighting with males (irrespective of their endocrine status) being the more aggressive. Finally, the social status of the male alters the response towards lactating females. Dominants generally show higher levels of attack, an effect that can be augmented by castration.

The effect of age on two kinds of aggressive behavior in inbred strains of mice

Shock-induced aggressive behavior is absent or very weak in C57BL/6 mice at the age of 12 weeks, increases at the age of 16 weeks and reaches the highest levels at the age of 20 weeks. This age-dependent increase of aggressive responses is absent in BALB/c and DBA/2 mice at the three ages examined in our experiments. Social isolation induces a clear age-dependent increase of aggressive behavior in BALB/c and DBA/2 mice (the highest level being evident at 20 weeks of age) but not in C57BL/6 mice which are characterized by the lowest or absence of aggressive responses. These results are discussed in terms of the role of developmental and genetic factors in the expression of shock- and isolation-induced aggressive behavior in the laboratory mouse.

Elicitation of conspecific attack or defense in the male rat by intraventricular injection of a GABA agonist or antagonist

The involvement of central GABAergic mechanisms in the control over offensive and defensive behaviours in the rat was studied using intracerebroventricular injections (5 microliter) of a GABA agonist (THIP) or a GABA antagonist (bicuculline methiodide). Intracerebroventricular injections of THIP (1.25 and 2.5 micrograms) induced attacks and offensive sideways towards an untreated partner, in animals placed in a neutral area where no aggressive reactions occur in controls. Social approach behaviours (partner investigation, allogrooming) were also increased in both attacking and non-attacking animals, whereas individual behaviours (cage exploration, autogrooming, immobile posture) were decreased. Inversely, intracerebroventricular injections of bicuculline methiodide (62.5 and 125 ng) suppressed offensive items (attacks, offensive sideways, upright postures) in resident animals confronted with untreated intruders and increased occurrence of defensive sideways. This treatment also decreased reactions oriented towards the partner (investigation, allogrooming and crawl under/over), while increasing individual behaviours (cage exploration, immobile posture). These data demonstrate that activation of central GABA receptors elicits intraspecific offensive behaviours in the rat. On the contrary, blockage of these receptors induces defensive reactions and suppresses offensive behaviours. The involvement of these receptors in the neural control over aggressive behaviour in the rat is discussed.

Age-dependent changes in brain GABA turnover rates in two inbred strains of mice

gamma-Aminobutyric acid (GABA) steady-state levels and turnover rates have been determined in 15 brain areas of 21-day- and 3-month-old DBA/2J (DBA) and C57B1/6J (C57) mice. These two inbred strains differ by their susceptibility to audiogenic seizures; moreover, the involvement of GABAergic neurotransmission has been suggested in the control of this behavior. Turnover rates are generally higher at 21 days than at 3 months of age. There are few significant differences in the GABA steady-state levels between 21-day-old seizure-prone DBA mice when compared with seizure-resistant C57 mice. In the DBA mice, the steady-state level is higher in the olfactory bulbs and lower in the posterior colliculus and the olfactory tubercles than in the C57 mice. Although there are some significant differences in GABA turnover rates and steady-state levels, intra or inter strains, it is difficult to correlate directly these differences with seizure susceptibility.

Gabaergic modulation of mouse-killing in the rat

When GABA-potentiating compounds were administered IP to rats with prior experience of mouse-killing behaviour, a reduction of killing was observed with gamma-vinyl GABA (200 and 400 mg/kg) and nipecotic acid amide (400 mg/kg), while no significant effect was noted following injection of dipropylacetate or THIP. The inhibitory effects of gamma-vinyl GABA and nipecotic acid amide were not reversed by subsequent injection of picrotoxin and were associated with sedation as observed in open field and actograph tests. When GABA-potentiating compounds were administered to food-deprived rats exposed for the first time to a mouse (initial elicitation), administration of gamma-vinyl GABA, dipropylacetate, nipecotic acid amide or THIP increased the incidence of mouse-killing behaviour. Conversely, the incidence of mouse-killing under the same conditions was reduced following injections of picrotoxin. These results do not support the hypothesis that the general activation of GABAergic mechanisms inhibits mouse-killing behaviour in rats. On the contrary, data obtained in naive animals suggest that potentiation of these mechanisms actually facilitates the initial elicitation of this behaviour.

Disinhibition of muricide and irritability by intraseptal muscimol

In two experiments we have found and replicated the observation that intraseptal muscimol profoundly facilitates muricide. It also increases irritability (response to handling). These effects are specific to aggressive behaviors in that the drug affects neither activity nor chocolate chip acceptance. The effects of the GABA synthesis inhibitor thiosemicarbazide depend upon the site of injection within the septum; in more anterior loci the drug produces the expected increase in muricide latency; in more posterior sites it produces an anomalous facilitation of muricide. The serotonergic agents quipazine and metergoline have no significant effect when injected into any of these sites. These results suggest that the septal neurons mediating the muricide-inhibitory effect of electrical stimulation [29] are subject to local, GABAergic, control. Inhibition of these neurons by muscimol produces a net disinhibition of muricide.

Effects of n-di-propylacetate on aggressive behavior and brain GABA level in isolated mice

n-di-Propylacetate (nDPA, valproate) a GABA-T inhibitor, injected IP at the dose of 300 mg/kg antagonized agonistic behavior of isolated DBA/2 mice in a time-dependent fashion in parallel to an increase of GABA levels in olfactory bulb, striatum, posterior colliculus and septum. After 75 min, aggressive responses were higher than those after 15 to 45 min and significantly lower in comparison with those of saline injected mice. After 120 min aggressive behavior was not different from that of control mice. The concentration of GABA in the striatum and olfactory bulb returned to control value 75 and 120 min after drug administration, respectively. After 120 min GABA levels in posterior colliculus and septum were lower than those after 15 to 75 min, although significantly higher in comparison with those of saline injected mice. The results are discussed in terms of the possible involvement of olfactory bulb and striatum in GABA-mediated control of isolation-induced aggressive behavior in mice.

Induction of mouse-killing in the rat by intraventricular injection of a GABA-agonist

Intracerebroventricular injections of THIP (2.5 and 5.0 micrograms in 5 microliter) facilitated elicitation of mouse-killing in killer rats placed in a non-familiar environment. The same doses induced well organized mouse-killing responses in 60% of non-killer rats. Concomitantly food intake was elicited. Exploratory activity as well as orientation and approach towards any sensory stimulus were also increased. On the contrary, intracerebroventricular injections of bicuculline methiodide (65 and 125 ng in 5 microliters) suppressed aggressive responses in killer rats. The data support the view that GABA receptors are involved in mechanisms which facilitate elicitation of mouse-killing behaviour as well as other positively motivated responses.

GABAergic influences on defensive fighting in rats

The involvement of GABAergic mechanisms in shock-induced defensive fighting in rats was investigated in a series of three experiments. In Experiment 1, sodium n-dipropylacetate (100-200 mg/kg) failed to produce significant behavioural change whilst gamma-vinyl-GABA (100-200 mg/kg) induced a selective and dose-dependent reduction in fighting. In Experiment 2, although inconsistent behavioural effects were obtained with (+)-bicuculline (0.25-4 mg/kg), a biphasic influence on defensive fighting was observed with picrotoxin (0.125-2 mg/kg). The inhibitory effect on fighting, induced by the highest dose of picrotoxin, was related to motor impairment. In Experiment 3, muscimol reduced fighting at doses above 0.25 mg/kg with motor disruption evident only at the highest dose used (1 mg/kg). A dose-dependent inhibition of defensive fighting was observed with 1-baclofen (0.15-1.2 mg/kg) which, at the highest dose tested, also impaired motor coordination. None of the compounds tested significantly altered shock thresholds. Results are discussed in relation to the hypothesized inhibitory role of GABA in the mediation of aggressive behaviours.

gamma-Aminobutyric acid in brain areas of isolated aggressive or non-aggressive inbred strains of mice

In order to investigate the effects of social isolation on aggressive behavior and GABA levels in different brain areas, inbred mice of the C57 Bl/6 and the DBA/2 strains were housed individually over a period of 8 weeks. Social isolation induced a clear increase of aggressive responses only in the DBA/2 strain and a decrease of GABA levels in septum, striatum, olfactory bulb and posterior colliculus in both the C57Bl/6 and in the DBA/2 strains. An increase of neurotransmitter concentration was observed in amygdala of DBA mice. DBA mice when compared to C57 mice showed significantly lower levels of GABA in olfactory bulb and striatum. These results are discussed in light of several previous studies which have pointed out a correlation between a deficiency of GABA mediated inhibition in some brain areas and different kinds of aggressive behavior as well as the possibility of a blockade of aggressive behavior by potentiation of GABAergic mediated inhibition. A possible suggestion emerging from our results is that the aggressive responses exhibited by isolated DBA mice but not by isolated C57 mice may be related to lower levels of the inhibitory neurotransmitter in the olfactory bulb and striatum.