The inhibitory effects of fluprazine on parental aggression in female mice are dependent upon intruder sex

To attack, or not to attack? The role of serotonin transporter genotype in the display of maternal aggression

Aggressive behavior in males has been intensively investigated regarding the influence of the brain serotonergic system. Despite some inconsistencies, a general conclusion is that low levels of serotonin (5-HT) are associated with high levels of male aggression. The role of the serotonergic system for female aggression is less well researched. Female mice rarely show intraspecific aggressive behavior, except during lactation, when they may exhibit intense aggression towards intruders to protect their pups. The aim of the present study was to investigate the impact of 5-HT transporter (5-HTT) inactivation on maternal aggression in mice. Therefore, lactating homozygous and heterozygous 5-HTT knockout as well as wildtype mice were confronted with male intruders in their home cages. Homozygous 5-HTT knockout dams, which exhibit highest levels of extracellular 5-HT in the brain, were significantly less prone to initiate offensive aggression than wildtype controls. Moreover, they showed longer latencies to attack the intruder, attacked less often and displayed an overall lower frequency of offensive aggressive behavior patterns than wildtype dams. Heterozygous 5-HTT knockout mothers generally showed intermediate levels of aggressive behavior. Thus, our data indicate that higher extracellular including synaptic levels of 5-HT are associated with lower intensity of aggressive behavior in lactating mice, adding support to the inhibitory role of 5-HT in aggression also in females.

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.

Mouse females devoid of exposure to males during fetal development exhibit increased maternal behavior

Many sex differences can be found in the expression of aggression and parental nurturing behaviors. It is important to determine if these are modulated by prenatal conditions. Here, using assisted reproduction technologies, we generated females that were (mixed-sex) or were not (same-sex) exposed to males during fetal development, raised them by cross fostering among fosters' own female only pups to control for effects of postnatal environment, and compared their reproductive abilities and behavior. There were no differences between females from the two prenatal conditions in estrus cycle length and length of time spent at individual estrus cycle stages. Both types of females had similar ovulation efficiency and bred equally well yielding comparable litter size and progeny sex ratio. Females from the two prenatal conditions were also indistinguishable in social behavior and exhibited normal social responses towards unfamiliar females in the three-chamber social approach and social proximity tests. When urine was collected from both types of females and used as a point source in a scent-marking paradigm, exposed males showed a similar distribution and extent of urinary scent marking in response to urine from each type of female but tended to engage in higher durations of sniffing the urine from same-sex females. When females were tested in a resident-intruder paradigm 3 days after giving birth, same-sex females exhibited enhancement of pup grooming and an overall decrease of non-pup activity prior to male intruder introduction, and after introduction were more defensive as evidenced by higher rates of burying, open-mouth threat/lunges, and attacks towards the male, and decreased latencies to display these defensive behaviors. Our results suggest that females devoid of male exposure during fetal development have reproductive abilities similar to those of females from mixed-sex pregnancies, and have normal social interactions with other females. However, they exhibit hyper-maternal behavior both in terms of the care and defense of pups in front of a male intruder, and potentially produce a pheromonal milieu that renders them more attractive to males during olfactory investigations.

Parental precaution: neurobiological means and adaptive ends

Humans invest precious reproductive resources in just a few offspring, who remain vulnerable for an extended period of their lifetimes relative to other primates. Therefore, it is likely that humans evolved a rich precautionary psychology that assists in the formidable task of protecting offspring. In this review, we integrate precautionary behaviors during pregnancy and postpartum with the adaptive functions they may serve and what is known of their biological mediators, particularly brain systems motivating security and attachment. We highlight the role of reproductive hormones in (i) priming parental affiliation with young to incentivize offspring protection, (ii) focusing parental attention on cues of potential threat, and (iii) facilitating maternal defense against potentially dangerous conspecifics and predators. Throughout, we center discussion on adaptive responses to threats of disease, accident and assault as common causes of child mortality in the ancestral past.

Disruption of the vasopressin 1b receptor gene impairs the attack component of aggressive behavior in mice

Vasopressin affects behavior via its two brain receptors, the vasopressin 1a and vasopressin 1b receptors (Avpr1b). Recent work from our laboratory has shown that disruption of the Avpr1b gene reduces intermale aggression and reduces social motivation. Here, we further characterized the aggressive phenotype in Avpr1b -/- (knockout) mice. We tested maternal aggression and predatory behavior. We also analyzed the extent to which food deprivation and competition over food increases intermale aggression. We quantified defensive behavior in Avpr1b -/- mice and later tested offensive aggression in these same mice. Our results show that attack behavior toward a conspecific is consistently reduced in Avpr1b -/- mice. Predatory behavior is normal, suggesting that the deficit is not because of a global inability to detect and attack stimuli. Food deprivation, competition for food and previous experience increase aggression in both Avpr1b +/+ and -/- mice. However, in these circumstances, the level of aggression seen in knockout mice is still less than that observed in wild-type mice. Defensive avoidance behaviors, such as boxing and fleeing, are largely intact in knockout mice. Avpr1b -/- mice do not display as many 'retaliatory' attacks as the Avpr1b +/+ mice. Interestingly, when territorial aggression was measured following the defensive behavior testing, Avpr1b -/- mice typically show less initial aggressive behavior than wild-type mice, but do show a significant increase in aggression with repeated testing. These studies confirm that deficits in aggression in Avpr1b -/- mice are limited to aggressive behavior involving the attack of a conspecific. We hypothesize that Avpr1b plays an important role in the central processing that couples the detection and perception of social cues (which appears normal) with the appropriate behavioral response.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.

Problems in the study of rodent aggression

Laboratory research has produced detailed descriptions of aggression and defense patterns in the rat, mouse, and hamster, showing strong similarities, but also some differences, across these species. Research on target sites for attack, in conjunction with analyses of the situational antecedents of attack behaviors and of responsivity of these to conditions that elicit fear, has also provided a strong basis for analysis of offensive and defensive aggression strategies and for identification of combinations of these modalities such as may occur in maternal aggression. These patterns have been empirically differentiated from phenomena such as play fighting or predation and compared for laboratory rodents and their wild ancestors. An array of tasks, suitable for use with pharmacological and experimental manipulations, is available for analysis of both aggression and defense. These developments should produce a firm basis for research using animal models to analyze a broad array of aggression-related phenomena, including systematic approaches to understanding the normal antecedents and consequences of each of several differentiable types of aggressive behavior. Despite this strong empirical and analytic background, laboratory animal aggression research has been in a period of decline, spanning several decades, relative to comparable research focusing on areas such as sexual behavior or stress. Problems that may have contributed to the relative neglect of aggression research include confusion about the interpretation of different tasks for eliciting aggression; difficulties and labor intensiveness of observational measures needed for an adequate differentiation of offensive and defensive behaviors; analytic difficulties stemming from the sensitivity of offensive aggression to the inhibitory effects of fear or defensiveness; lack of a clear relationship between categories of aggressive behavior as defined in animal studies and those used in human aggression research; and the social and political difficulties undermining support for research on a topic that, when applied to humans, provides a stigmatizing label. While all of these provide some rationale for eschewing aggression research, aggression remains a serious social, economic, health, and political problem. The neglect of research in this area contributes to an ongoing failure to understand the degree of similarity across mammalian species in the antecedents, neural systems, behavioral expression, and outcomes of aggression. This failure, in turn, hinders analyses of normal and abnormal forms of aggression and of the appropriate roles of the former in society, reducing the possibility of sensitive and effective approaches to control inappropriate human aggressive behaviors.

Interaction of nitric oxide and serotonin in aggressive behavior

Nitric oxide (NO) modulates many behavioral and neuroendocrine responses. Genetic or pharmacological inhibition of the synthetic enzyme that produces NO in neurons evokes elevated and sustained aggression in male mice. Recently, the excessive aggressive and impulsive traits of neuronal NO synthase knockout (nNOS-/-) mice were shown to be caused by reductions in serotonin (5-HT) turnover and deficient 5-HT1A and 5-HT1B receptor function in brain regions regulating emotion. The consistently high levels of aggression observed in nNOS-/- mice could be reversed by 5-HT precursors and by treatment with specific 5-HT1A and 5-HT1B receptor agonists. The expression of the aggressive phenotype of nNOS-/- knockout mice requires isolated housing prior to testing. The effects of social factors such as housing condition and maternal care can affect 5-HT and aggression, but the interaction among extrinsic factors, 5-HT, NO, and aggression remains unspecified. Taken together, NO appears to play an important role in normal brain 5-HT function and may have significant implications for the treatment of psychiatric disorders characterized by aggressive and impulsive behaviors.

Sensory, hormonal, and neural control of maternal aggression in laboratory rodents

Parental animals of many rodent species display fierce and persistent aggression toward unfamiliar conspecifics that appears to protect their often altricial and defenseless young. We herein review studies of the sensory, hormonal, neuroanatomical, and neurochemical mechanisms underlying maternal aggression in laboratory rodents. The relationship between maternal aggression and fearfulness or anxiety is also discussed.

Sexual and aggressive interactions in a visible burrow system with provisioned burrows

The visible burrow system (VBS) is a habitat providing burrows and an open area for mixed-set rat colonies. Provisioning of food and water in the burrows makes it unnecessary for potentially defensive animals to leave the burrows to eat/drink on the surface, and enables evaluation of new types of agonistic interactions that may emerge when this necessity is removed. In such colonies, subordinate males showed high magnitude tunnel guarding behavior, occupying a tunnel opening onto the surface and confronting the dominant. Dominants, in response, made lunges into the tunnels, but quickly retreated without gaining entry, apparently stopped by contact with the defender's vibrissae. Dominants also made and continued to make lateral attacks to the wall adjacent to the tunnels guarded by subordinates, although these were useless in terms of affording contact with the subordinate. Dominant-female agonistic interactions were more frequent than those of dominants and subordinates. These were largely initiated by the male, and involved female defensive behavior. Nonetheless, females, unlike subordinates, failed to show tunnel guarding and continued to utilize the surface freely. They also spent more time in the vicinity of the dominant over days of colony formation. This apparent paradox may reflect that females were seldom wounded, and that the initial site of male contact with females was the female's anogenital area, findings suggesting that interactions of males and females often reflect male sexual advances, countered by female defenses that effectively protect nonestrus females from mounting and copulation.

An evolutionary approach to behavioral pharmacology: using drugs to understand proximate and ultimate mechanisms of different forms of aggression in mice

Psychoactive drugs (Fluprazine and Chlordiazepoxide--CDP) were used as probes to test both differences or similarities in neurochemical substrates (proximal causations) and adaptive significance (ultimate causations) of different forms of intraspecific aggression in wild mice and laboratory Swiss CD-1 counterparts. Fluprazine (1-5 mg/kg) inhibited maternal attack on female, but not on male intruders. Thus, phenotypically different attack behaviors (offence and defence respectively) which have different functions may be modulated by different neurochemical substrates. Intrasexual attack and infanticide which are phenotypically different, but share similar functions (i.e. competition for mates and resources) were equally inhibited by Fluprazine (2 mg/kg) both in males and females of wild and laboratory mice. This indicates that the neural substrates of these behaviors are related and similarly regulated in the two sexes. Fluprazine was used to test the prediction of the evolutionary model on fighting strategies in male-male asymmetric contests as far as fighting ability and resource value (mating and cohabitation with a female) are concerned. Fluprazine inhibited the intensity of fighting (i.e. more 'defensive' behavioral phenotype of attack) only in animals without previous positive fighting experience, suggesting that different behavioral strategies are based on different neurochemical modulation. Experience of attack also influenced the effects of CDP (2.5-5 mg/kg) in both lactating females and male resident mice. The reported proaggressive effects of benzodiazepines were observed only in animals with prior fighting experience in both cases. Thus the understanding of the effects of drugs on behavior demands consideration of the biological variability (e.g. genetic, previous experience and/or interindividual differences) and the adaptive significance of behavior in the experimental context. On this background ethopharmacology can be defined as an evolutionary approach to the study of a drugs effect on neurochemical mechanisms and functions of behavior.

Comparing different forms of male and female aggression in wild and laboratory mice: an ethopharmacological study

The purpose of this study was to compare the effects of 2 mg/kg fluprazine (a serotonergic psychoactive drug with antiaggressive properties) on intrasexual attack, infanticide, and predation (on an insect larva) in males and females of wild and Swiss mice. The results showed that, in both stocks of mice, fluprazine significantly inhibited intrasexual and infanticidal attack in both sexes, but predatory attack was not altered by the drug treatment. Motivational and neural substrates underlying intrasexual attack and infanticide appear, thus, to be related to each other, and similarly modulated in both males and females. Conversely, predatory attack seems to be under a different neurohumoral control. The similar regulation of proximal mechanisms of aggressive behavior observed in wild and Swiss mice suggests a common neurobiology of aggression. For this reason, the outbred laboratory Swiss mice appear to be a reliable model for studies on causal and functional mechanisms of aggression.

Recent developments in anxiety, stress, and depression

Recent research in the development, analysis, and pharmacology of animal tests of state anxiety is discussed, including the use of responses to predator odours, the role of learning in modifying the anxiety measured in the plus-maze, and the roles of cholinergic, NMDA, and dopaminergic systems. Developmental and genetic factors are considered with particular reference to the development of tests of trait anxiety. The roles of 5-HT1A receptors in anxiety, depression, impulsivitity, and agonistic behaviours are discussed. Recent studies on the impacts of stress on neurotransmitter, endocrine, and immune systems and the interactions between these systems are discussed, with particular emphasis on their contributions to the development of pathologic states relevant to anxiety and depression.

Effects of chlordiazepoxide on maternal aggression in mice depend on experience of resident and sex of intruder

Lactating mice respond differentially to intruders of differing sex, displaying defensive attack against the male and offensive attack against the female. Such a phenotypic dichotomy appears to have adaptive value in that unfamiliar males pose a much greater threat to the offspring than do females. The present study examined the effects of the benzodiazepine anxiolytic chlordiazepoxide (CDP) (2.5-10.0 mg/kg) on this differential response pattern in aggression-naive (nonscreened) (NS) and aggression-experienced (screened) (S) lactating female mice (Mus musculus domesticus) confronting intruders of either sex in a 10-min test. This procedure was used to evaluate the influence of both the type of opponent and previous aggressive experience on basal behavioural profiles and drug action. Results showed that both intruder sex and prior screening for attack modulated the behaviour of lactating females toward intruders. In turn, both variables strongly influenced CDP effects on maternal aggression. In particular, in S dams, CDP dose-dependently increased maternal attack against males but decreased attack against female intruders. Conversely, in NS dams, CDP decreased attack (and fear) against males but did not affect it against females. In both S and NS conditions, CDP modified the attack strategy of lactating females against the male, switching it from a defensive to an offensive pattern. Exploration, social investigation, eating, and immobility were differentially affected by the drug treatment, depending on screening and/or intruder sex condition. These differential effects of CDP between S and NS conditions, toward either male or female intruders, cannot be fully explained by differences in the baseline levels of these behaviours. Alternative hypotheses are discussed. These findings demonstrate that the effects of CDP on maternal attack behaviour depend on not only the drug but also the object of attack, and hence the function of attack and the prior experience of the attacker.

Effects of intracerebroventricular administration of 5-HT receptor agonists on the maternal aggression of rats

This study attempted to analyze the effects of 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), TFMPP (1-(3-trifluoromethyl-phenyl)piperazine hydrochloride), and DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) on maternal aggressive behavior. Female Wistar rats were divided into 4 groups of 12 animals each. They received an intracerebroventricular (i.c.v.) injection of: (1) saline, (2) 8-OH-DPAT (20 micrograms/rat), (3) TFMPP (100 micrograms/rat), and (4) DOI (100 micrograms/rat). 5-HT1A (8-OH-DPAT) and 5-HT2 (DOI) receptor agonists decreased the frequency of attack 15 but not 55 min after i.c.v. injection. The 5-HT1B/D receptor agonist (TFMPP), in the dose studied, showed no significant difference as compared to saline. Pup care and non-aggressive social interaction with the intruder were not affected by any drug. These data suggest that 5-HT1A and 5-HT2 receptor agonists can specifically inhibit maternal aggression without affecting maternal care; however, this effect is of short duration.

5-HT1A receptor influences on rodent social and agonistic behavior: a review and empirical study

Investigations examining the influence of 5-HT1A receptors in murine agonistic and social behavior have reported either specific or nonspecific attenuation of offensive behavior. To clarify this situation, the effects of three 5-HT1A agonists were examined on isolation-induced aggression and social behavior in male mice. 8-OH-DPAT (0.025-1.25 mg/kg) increased social behavior, rearing, and digging. Offensive behavior was reduced, without concomitant sedation. Ipsapirone (0.1-10.0 mg/kg) reduced naso-nasal behavior, whilst enhancing stretched-attend behavior, cage-exploration, and rearing. Offensive and defensive behaviors were attenuated, without reductions in activity. MDL 73005 EF (0.25-8.0 mg/kg) reduced social behaviors, cage-exploration and rearing while maintenance behavior was increased. Offensive and defensive behaviors showed attenuation. Current results corroborate previous findings with respect to 5-HT1A receptor involvement in murine agonistic behavior and anxiety. Data also connote that the behavioral specificity of 5-HT1A ligands should be interpreted in terms of response competition rather than solely concomitant sedation.

Fluprazine inhibits intermale attack and infanticide, but not predation, in male mice

The effects of fluprazine (1, 2, and 5 mg/kg) on intermale attack, infanticide and predation (insect larvae) by male mice were assessed. Fluprazine dose-dependently inhibited attacks by males on conspecific intruders and genetically unrelated mouse pups. However, predatory attack on insect larvae was unaltered by any dose of the compound. Thus the neurohumoral substrates underlying intraspecific attack and pup killing may be similar to each other, but different from those modulating predatory attack and prey killing. These data support the hypothesis that male infanticide is a form of intraspecific aggression and not an expression of intraspecific predation (cannibalism). Drug-induced stimulation of paternal behavior in some previously infanticidal males suggests that serotonergic substrates may also be involved in the natural mechanisms which mediate the inhibition of infanticide and promote parental care.

Serenics fluprazine (DU 27716) and eltoprazine (DU 28853) enhance neophobic and emotional behaviour in mice

Two tests designed to elicit responses to novelty and to aversive stimuli were used to study the effects of the serenics fluprazine and eltoprazine on the behaviour of male Swiss mice: a free exploratory test (fluprazine; 2.5, 5 and 10 mg/kg; eltoprazine: 2.5, 5, 10 and 15 mg/kg) and a two-box choice procedure (fluprazine: 1.25, 2.5, 5 and 7.5 mg/kg; eltoprazine: 2.5, 5, 7.5 and 10 mg/kg). Both drugs increased the neophobic reaction, as well as the avoidance of a brightly illuminated box. These effects closely resemble those of psychostimulant drugs such as methamphetamine and caffeine. It is hypothesized that the behavioural changes induced by these drugs may be due to a nonspecific increase of the emotional reactivity of animals.