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

Effects of early life social stress on maternal behavior and neuroendocrinology

Maternal mood disorders such as depression and chronic anxiety can negatively affect the lives of both mothers and their adult offspring. An active focus of maternal depression and anxiety research has been the role of chronic social stress in the development of these disorders. Chronic exposure to social stress is common in humans, especially in lactating mothers, and postpartum mood disorders have been correlated with high levels of social conflict and low levels of social support. Recent studies have described an effective and ethologically relevant chronic social stress (CSS) based rodent model for postpartum depression and anxiety. Since CSS attenuates maternal behavior and impairs both dam and offspring growth, it was hypothesized that CSS is an ethologically relevant form of early life stress for the developing female offspring and may have effects on subsequent adult maternal behavior and neuroendocrinology. Dams exposed to early life CSS as infants display substantial increases in pup retrieval and nursing behavior that are specifically associated with attenuated oxytocin, prolactin, and vasopressin gene expression in brain nuclei involved in the control of maternal behavior. Since the growth patterns of both groups were similar despite substantial increases in nursing duration, the early life CSS dams exhibited an attenuated nursing efficiency. It is concluded that early life CSS has long term effects on the neuroendocrinology of maternal care (oxytocin and prolactin) which results in decreased nursing efficiency in the adult dams. The data support the use of early life CSS as an effective model for stress-induced impairments in nursing, such as those associated with postpartum depression and anxiety.

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.

Assessing postpartum maternal care, alloparental behavior, and infanticide in mice: with notes on chemosensory influences

Chemosensory signaling influences maternal care and other innate behaviors toward conspecific young animals in rodents. In this chapter, we describe basic protocols for assessment of postpartum maternal behavior and other pup-directed behaviors in laboratory mice. The specific aim of this protocol is to screen out the abnormal phenotypes in parenting of genetic mutant mice under the standard housing condition. The possible underlying mechanisms for a given abnormality in the mother-young interaction are briefly suggested as well.

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.

Effects of cocaine on maternal behavior and neurochemistry

Drug addiction is a chronic relapsing disorder that involves drug seeking and abuse despite the negative social and health consequences. While the potential effects of cocaine on child development have been extensively studied over the last 30 years, few researchers have focused on the effects of cocaine on maternal behavior, which includes offspring care and maternal aggression towards an unfamiliar individual. In humans, maternal cocaine use can lead to child neglect, abuse, and disrupt the mother-child bond. While it has been argued the developmental effects of maternal cocaine use on children were initially overstated, it is clear that disruptions of typical maternal behavior (i.e. postpartum depression, anxiety disorders) are detrimental to the physical and emotional health of offspring. Cocaine use in mothers is commonly associated with psychological disorders, including depression and anxiety, and it is postulated that many of the negative effects of maternal cocaine use on offspring are mediated through changes in maternal behavior. This review will summarize research on cocaine and maternal behavior in animal and human studies, discuss potential mechanisms, and suggest therapeutic strategies for treating cocaine-affected maternal behavior which may improve the physical and behavioral health of both mother and child. The primary objective is to stimulate future communication, cooperation, and collaboration between researchers who use animals and humans to study cocaine and maternal behavior.

Oxytocin increases amygdala reactivity to threatening scenes in females

The neuropeptide oxytocin (OT) is well known for its profound effects on social behavior, which appear to be mediated by an OT-dependent modulation of amygdala activity in the context of social stimuli. In humans, OT decreases amygdala reactivity to threatening faces in males, but enhances amygdala reactivity to similar faces in females, suggesting sex-specific differences in OT-dependent threat-processing. To further explore whether OT generally enhances amygdala-dependent threat-processing in females, we used functional magnetic resonance imaging (fMRI) in a randomized within-subject crossover design to measure amygdala activity in response to threatening and non-threatening scenes in 14 females following intranasal administration of OT or placebo. Participants' eye movements were recorded to investigate whether an OT-dependent modulation of amygdala activity is accompanied by enhanced exploration of salient scene features. Although OT had no effect on participants' gazing behavior, it increased amygdala reactivity to scenes depicting social and non-social threat. In females, OT may, thus, enhance the detection of threatening stimuli in the environment, potentially by interacting with gonadal steroids, such as progesterone and estrogen.

Glutamic acid decarboxylase 65 and 67 expression in the lateral septum is up-regulated in association with the postpartum period in mice

The postpartum period in mammals undergoes a variety of physiological adaptations, including metabolic, behavioral and neuroendocrine alterations. GABA signaling has been strongly linked to various emotional states, stress responses and offspring protection. However, whether GABA signaling may change in the lateral septum (LS), a core brain region for regulating behavioral, emotional and stress responses in postpartum mice has not previously been examined. In this study, we tested whether the expression of two isoforms of glutamic acid decarboxylase (GAD), GAD65 (GAD2) and GAD67 (GAD1), the rate-limiting enzyme for GABA synthesis, exhibits altered expression in postpartum mice relative to nonmaternal, virgin mice. Using microdissected septal tissue from virgin and age-matched postpartum females, quantitative real-time PCR and Western blotting were carried out to assess GAD mRNA and protein expression, respectively. We found both protein and mRNA expression of GAD67 in the whole septum was up-regulated in postpartum mice. By contrast, no significant difference in the whole septum was observed in GAD65 expression. We then conducted a finer level of analysis using smaller microdissections and found GAD67 to be significantly increased in rostral LS, but not in caudal LS or medial septum (MS). Further, GAD65 mRNA expression in rostral LS, but not in caudal LS or MS was also significantly elevated in postpartum mice. These findings suggest that an increased GABA production in rostral LS of the postpartum mice via elevated GAD65 and GAD67 expression may contribute to multiple alterations in behavioral and emotional states, and responses to stress that occur during the postpartum period. Given that rostral LS contains GABA neurons that are projection neurons or local interneurons, it still needs to be determined whether the function of elevated GABA is for local or distant action or both.

Is it all in the family? The effects of early social structure on neural-behavioral systems of prairie voles (Microtus ochrogaster)

The transition to parenthood is generally associated with a reduction in anxiety or anxiety-like behavior across a wide range of species. In some species, juveniles provide supplementary parental care for younger siblings, a behavior known as alloparenting. Although the fitness consequences of alloparenting behavior have been a focus of evolutionary research, less is known about how alloparenting behavior impacts affective states. In the socially monogamous prairie vole (Microtus ochrogaster), most juveniles exhibit alloparenting behavior, making the species an ideal model for examining the effects of alloparenting on future behavioral outcomes. We randomly assigned juvenile voles to alloparenting (AL) or no alloparenting (NoAL) groups and behaviorally phenotyped them for anxiety-like and social behaviors using the elevated plus maze (EPM), open field test (OFT), startle box, social interaction test, juvenile affiliation test, and partner preference test. AL voles displayed more anxiety-like and less exploratory behaviors than NoAL voles, spending significantly less time in the open arms of the EPM and center of an open field. We dissected the CA1 region of the hippocampus and the bed nucleus of the stria terminalis (BNST) from brains of behaviorally phenotyped voles and nontested siblings as well. Decreased brain-derived neurotrophic factor (BDNF) expression in CA1 has generally been associated with increased anxiety-like behavior in other rodents, while an anxiogenic role for BDNF in BNST is less established. Western blot analyses showed that alloparenting experience increased expression of BDNF in the BNST but decreased BDNF expression in the CA1 region of hippocampus (CA1) of nontested voles. There were similar differences in BNST BDNF of behaviorally phenotyped voles, and BDNF levels within this region were negatively correlated with exploratory behavior (i.e. time in center of OFT). Our results suggest that BDNF signaling in BNST and CA1 fluctuate with alloparenting experience, and they contribute to an increasingly complex "BDNF hypothesis" in which behavioral effects of this molecule are region-specific.

Both oxytocin and vasopressin are mediators of maternal care and aggression in rodents: from central release to sites of action

In the mammalian peripartum period, the activity of both the brain oxytocin and vasopressin system is elevated as part of the physiological adaptations occurring in the mother. This is reflected by increased expression and intracerebral release of oxytocin and vasopressin, as well as increased neuropeptide receptor expression and binding. In this review we discuss the functional role of the brain oxytocin and vasopressin system in the context of maternal behavior, specifically maternal care and maternal aggression in rodents. In order to enable the identification of significant and peptide-specific contributions to the display of maternal behavior, various complementary animal models of maternal care and/or maternal aggression were studied, including rats selectively bred for differences in anxiety-related behavior (HAB and LAB dams), monitoring of local neuropeptide release during ongoing maternal behavior, and local pharmacological or genetic manipulations of the neuropeptide systems. The medial preoptic area was identified as a major site for oxytocin- and vasopressin-mediated maternal care. Furthermore, both oxytocin and vasopressin release and receptor activation in the central amygdala and the bed nucleus of the stria terminalis play an important role for maternal aggression. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

The regulation of social recognition, social communication and aggression: vasopressin in the social behavior neural network

Neuropeptides in the arginine vasotocin/arginine vasopressin (AVT/AVP) family play a major role in the regulation of social behavior by their actions in the brain. In mammals, AVP is found within a circuit of recriprocally connected limbic structures that form the social behavior neural network. This review examines the role played by AVP within this network in controlling social processes that are critical for the formation and maintenance of social relationships: social recognition, social communication and aggression. Studies in a number of mammalian species indicate that AVP and AVP V1a receptors are ideally suited to regulate the expression of social processes because of their plasticity in response to factors that influence social behavior. The pattern of AVP innervation and V1a receptors across the social behavior neural network may determine the potential range and intensity of social responses that individuals display in different social situations. Although fundamental information on how social behavior is wired in the brain is still lacking, it is clear that different social behaviors can be influenced by the actions of AVP in the same region of the network and that AVP can act within multiple regions of this network to regulate the expression of individual social behaviors. The existing data suggest that AVP can influence social behavior by modulating the interpretation of sensory information, by influencing decision making and by triggering complex motor outputs. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Prolactin, neurogenesis, and maternal behaviors

Elevated prolactin during pregnancy increases neurogenesis in the subventricular zone of the lateral ventricle (SVZ) of the maternal brain. Evidence from our laboratory has shown that low prolactin in early pregnancy, and the consequent suppression of neurogenesis in the SVZ in the adult brain, is associated with increased postpartum anxiety and markedly impaired maternal behavior. Daughters of low prolactin mothers also display increased anxiety and a significant delay in the onset of puberty, which is associated with epigenetic changes in neuronal development (see Fig. 1). This suggests that, in rodents, low prolactin in early pregnancy exerts long-term effects that influence maternal mood postpartum, and offspring development. This mini-review aims to summarize the evidence showing that the prolactin-induced increase in SVZ neurogenesis during pregnancy underlies normal postpartum maternal interactions with pups.

Behavioral effects of bidirectional selection for behavior towards human in virgin and lactate Norway rats

Although numerous studies have demonstrated strong differences in behavioral, hormonal and neurobiological characteristics between male rats selected for elimination (tame) and enhancement (aggressive) of aggressiveness towards humans, few studies have examined changes in female behavior under this selection. The objective of the current work was to evaluate the effects of bidirectional selection for aggressiveness towards humans on behavioral profiles of virgin and lactating rats compared with the behavior in tame, aggressive and unselected (wild-type) females. The behavior of virgin females was studied using the light-dark box, the startle response test and the modified glove test. Tame females were less anxious and more tolerant towards humans than unselected and aggressive rats. Principal component analysis of all behavioral parameters produced three independent factors, explaining 66.37% of the total variability. The measures of behavior towards humans and the measures of anxiety mainly loaded on PC1 (first principal component) which separated the tame females from the unselected and aggressive ones. These data suggest the genetic correlation between the selected behavior towards humans and anxiety-related behavior in virgin rats. No significant effect of line was found for PC2 scores, associated with risk assessment behavior. Measurements of freezing behavior mainly loaded on PC3, and this component separated rats of different genetic groups from each other. The behavior of lactating rats was studied in maternal defense and pup retrieval tests. Females of selected lines did not significantly differ in behavioral measurements of these tests and were characterized by higher maternal motivation than unselected rats. It is suggested that long-term breeding of tame and aggressive rats in captivity has reduced the threshold for maternal behavior.

Advantage of rare infanticide strategies in an invasion experiment of behavioural polymorphism

Killing conspecific infants (infanticide) is among the most puzzling phenomena in nature. Stable polymorphism in such behaviour could be maintained by negative frequency-dependent selection (benefit of rare types). However, it is currently unknown whether there is genetic polymorphism in infanticidal behaviour or whether infanticide may have any fitness advantages when rare. Here we show genetic polymorphism in non-parental infanticide. Our novel invasion experiment confirms negative frequency-dependent selection in wild bank vole populations, where resource benefits allow an infanticidal strategy to invade a population of non-infanticidal individuals. The results show that infanticidal behaviour is highly heritable with genetic correlation across the sexes. Thus, a positive correlative response in male behaviour is expected when selection operates on females only and vice versa. Our results, on one hand, demonstrate potential benefits of infanticide, and on the other, they open a new perspective of correlative evolution of infanticide in females and males.

Neurotensin induced Egr-1 activity is altered in the postpartum period in mice

Neurotensin (NT) is a 13 amino acid neuropeptide that is identical in mice and humans and is released from and acts upon a number of social brain regions. Recent work indicates NT neurotransmission may be altered in postpartum females and support the onset of some maternal behaviors. In a recent study, we highlighted how virgin and postpartum brains from mice selected for high offspring protection differ in response to injected NT (0.1 μg) relative to vehicle when examining c-Fos profiles across the CNS. In this companion study we use a second marker for brain activity, Egr-1, and evaluate multiple brain regions. Common significant increased Egr-1 responses to NT (relative to vehicle) were found in both female groups only in ventromedial hypothalamus. In lateral periaqueductal gray, virgin mice showed a significant Egr-1 increase with NT (relative to vehicle), but maternal mice did not. When comparing NT injections, virgin (relative to maternal) mice had significantly higher activity in five regions, including anterior hypothalamus, lateral hypothalamus, somatosensory cortex, paraventricular nucleus, and zona incerta; no regions were higher in maternal mice. A Principal Components Analysis was also used for data mining and in virgin mice, greater changes in activity hubs were found with NT (relative to vehicle) than for maternal mice. Overall, a lower sensitivity to NT in terms of Egr-1 reactivity in the maternal state was highlighted and this is consistent with previous c-Fos results. These findings provide additional insight into the mechanisms by which NT functions in the CNS.

Exposure to repeated maternal aggression induces depressive-like behavior and increases startle in adult female rats

The stress response is a multifaceted physiological reaction that engages a wide range of systems. Animal studies examining stress and the stress response employ diverse methods as stressors. While many of these stressors are capable of inducing a stress response in animals, a need exists for an ethologically relevant stressor for female rats. The purpose of the current study was to use an ethologically relevant social stressor to induce behavioral alterations in adult female rats. Adult (postnatal day 90) female Wistar rats were repeatedly exposed to lactating Long Evans female rats to simulate chronic stress. After six days of sessions, intruder females exposed to defeat were tested in the sucrose consumption test, the forced swim test, acoustic startle test, elevated plus maze, and open field test. At the conclusion of behavioral testing, animals were restrained for 30 min and trunk blood was collected for assessment of serum hormones. Female rats exposed to maternal aggression exhibited decreased sucrose consumption, and impaired coping behavior in the forced swim test. Additionally, female rats exposed to repeated maternal aggression exhibited an increased acoustic startle response. No changes were observed in female rats in the elevated plus maze or open field test. Serum hormones were unaltered due to repeated exposure to maternal aggression. These data indicate the importance of the social experience in the development of stress-related behaviors: an acerbic social experience in female rats precipitates the manifestation of depressive-like behaviors and an enhanced startle response.

A bold view of the lactating brain: functional magnetic resonance imaging studies of suckling in awake dams

Functional magnetic resonance imaging (fMRI) has been used to investigate the responsiveness of the maternal rat brain to pup-suckling under various experimental paradigms. Our research employing the lactating rat model has explored the cortical sensory processing of pup stimuli and the effect of suckling on the brain's reward system. Suckling was observed to increase blood-oxygen-level-dependent (BOLD) signal intensity in the midbrain, striatum and prefrontal cortex, which are areas that receive prominent dopaminergic inputs. The BOLD activation of the reward system occurs in parallel with the activation of extensive cortical sensory areas. The observed regions include the olfactory cortex, auditory cortex and gustatory cortex, and could correspond to cortical representations of pup odours, vocalisations and taste that are active during lactation. Activation patterns within reward regions are consistent with past research on maternal motivation and we explore the possibility that exposure to drugs of abuse might be disruptive of maternal neural responses to pups, particularly in the prefrontal cortex. Our ongoing fMRI studies support and extend past research on the maternal rat brain and its functional neurocircuitry.

The levels of analysis revisited

The term levels of analysis has been used in several ways: to distinguish between ultimate and proximate levels, to categorize different kinds of research questions and to differentiate levels of reductionism. Because questions regarding ultimate function and proximate mechanisms are logically distinct, I suggest that distinguishing between these two levels is the best use of the term. Integrating across levels in research has potential risks, but many benefits. Consideration at one level can help generate novel hypotheses at the other, define categories of behaviour and set criteria that must be addressed. Taking an adaptationist stance thus strengthens research on proximate mechanisms. Similarly, it is critical for researchers studying adaptation and function to have detailed knowledge of proximate mechanisms that may constrain or modulate evolutionary processes. Despite the benefits of integrating across ultimate and proximate levels, failure to clearly identify levels of analysis, and whether or not hypotheses are exclusive alternatives, can create false debates. Such non-alternative hypotheses may occur between or within levels, and are not limited to integrative approaches. In this review, I survey different uses of the term levels of analysis and the benefits of integration, and highlight examples of false debate within and between levels. The best integrative biology reciprocally uses ultimate and proximate hypotheses to generate a more complete understanding of behaviour.

Exposure to chronic pregnancy stress reverses peripartum-associated adaptations: implications for postpartum anxiety and mood disorders

Maternal adaptations, such as decreased anxiety and attenuated stress responsiveness, are necessary to enable successful postnatal development of the offspring. However, there is growing evidence that they are also required to protect the mental health of the mother and that exposure to chronic stress during pregnancy may prevent such adaptations. Overcrowding stress (24 h) and restraint stress (2 × 1 h) were employed on alternate days between pregnancy d 4-16 to examine the impact of chronic pregnancy stress on relevant behavioral, neuroendocrine, and neuronal peripartum adaptations. To determine whether the chronic stress-induced alterations were specific to the peripartum period, we included virgins as controls. Validating the stress procedure, we demonstrated decreased body-weight gain and increased adrenal weight in stressed dams, relative to their nonstressed controls. Chronic stress prevented a number of peripartum adaptations, including basal plasma hypercorticosterone levels, increased oxytocin mRNA expression in the hypothalamic paraventricular nucleus, and anxiolysis. However, chronic stress did not prevent the peripartum-associated decrease in CRH mRNA expression or attenuate corticosterone response to an acute stressor, nor did it affect hypothalamic vasopressin mRNA expression. Illustrating the specificity of these stress-induced changes to the peripartum period, none of these parameters were affected in stressed virgins. Although chronic stress did not alter depression-related behavior, it reversed the response to acute imipramine treatment and increased active maternal behavior in lactation. Thus, prevention of the peripartum-associated increases in basal corticosterone and oxytocin system activity by pregnancy stress reveal two alterations that may increase the risk of postpartum psychiatric disorders, particularly anxiety.

Maternal defense is modulated by beta adrenergic receptors in lateral septum in mice

Maternal defense (offspring protection) is a critical and highly conserved component of maternal care in mammalian systems that involves dramatic shifts in a female's behavioral response to social cues. Numerous changes occur in neuronal signaling and connectivity in the postpartum female, including decreases in norepinephrine (NE) signaling in subregions of the CNS. In this study using a strain of mice selected for maternal defense, we examined whether possible changes in NE signaling in the lateral septum (LS) could facilitate expression of maternal aggression. In separate studies that utilized a repeated measures design, mice were tested for maternal defense following intra-LS injections of either the β-adrenergic receptor agonist isoproterenol (10 μg or 30 μg) or vehicle (Experiment 1), the β-adrenergic receptor antagonist propranolol (2 μg) or vehicle (Experiment 2), or the β1-receptor antagonist, atenolol (Experiment 3). Mice were also evaluated for light-dark performance and pup retrieval. Thirty micrograms of the agonist isoproterenol significantly decreased number of attacks and time aggressive relative to vehicle without affecting pup retrieval or light-dark box performance. In contrast, the antagonist propranolol significantly increased maternal aggression (lowered latency to attack and increased total attack time) without altering light-dark box test. The β1-specific antagonist, atenolol, significantly decreased latency to attack (1 μg vs. vehicle) without altering other measures. Although the findings were identified in a unique strain of mice, the results of these studies support the hypothesis that changes in NE signaling in LS during the postpartum period contribute to the expression of offspring protection.

Maternal defense: breast feeding increases aggression by reducing stress

Mothers in numerous species exhibit heightened aggression in defense of their young. This shift typically coincides with the duration of lactation in nonhuman mammals, which suggests that human mothers may display similarly accentuated aggressiveness while breast feeding. Here we report the first behavioral evidence for heightened aggression in lactating humans. Breast-feeding mothers inflicted louder and longer punitive sound bursts on unduly aggressive confederates than did formula-feeding mothers or women who had never been pregnant. Maternal aggression in other mammals is thought to be facilitated by the buffering effect of lactation on stress responses. Consistent with the animal literature, our results showed that while lactating women were aggressing, they exhibited lower systolic blood pressure than did formula-feeding or never-pregnant women while they were aggressing. Mediation analyses indicated that reduced arousal during lactation may disinhibit female aggression. Together, our results highlight the contributions of breast feeding to both protecting infants and buffering maternal stress.

Neuromolecular basis of parental behavior in laboratory mice and rats: with special emphasis on technical issues of using mouse genetics

To support the well-being of the parent-infant relationship, the neuromolecular mechanisms of parental behaviors should be clarified. From neuroanatomical analyses in laboratory rats, the medial preoptic area (MPOA) has been shown to be of critical importance in parental retrieving behavior. More recently, various gene-targeted mouse strains have been found to be defective in different aspects of parental behaviors, contributing to the identification of molecules and signaling pathways required for the behavior. Therefore, the neuromolecular basis of "mother love" is now a fully approachable research field in modern molecular neuroscience. In this review, we will provide a summary of the required brain areas and gene for parental behavior in laboratory mice (Mus musculus) and rats (Rattus norvegicus). Basic protocols and technical considerations on studying the mechanism of parental behavior using genetically-engineered mouse strains will also be presented.

The neuroendocrinology of primate maternal behavior

In nonhuman primates and humans, similar to other mammals, hormones are not strictly necessary for the expression of maternal behavior, but nevertheless influence variation in maternal responsiveness and parental behavior both within and between individuals. A growing number of correlational and experimental studies have indicated that high circulating estrogen concentrations during pregnancy increase maternal motivation and responsiveness to infant stimuli, while effects of prepartum or postpartum estrogens and progestogens on maternal behavior are less clear. Prolactin is thought to play a role in promoting paternal and alloparental care in primates, but little is known about the relationship between this hormone and maternal behavior. High circulating cortisol levels appear to enhance arousal and responsiveness to infant stimuli in young, relatively inexperienced female primates, but interfere with the expression of maternal behavior in older and more experienced mothers. Among neuropeptides and neurotransmitters, preliminary evidence indicates that oxytocin and endogenous opioids affect maternal attachment to infants, including maintenance of contact, grooming, and responses to separation. Brain serotonin affects anxiety and impulsivity, which in turn may affect maternal behaviors such as infant retrieval or rejection of infants' attempts to make contact with the mother. Although our understanding of the neuroendocrine correlates of primate maternal behavior has grown substantially in the last two decades, very little is known about the mechanisms underlying these effects, e.g., the extent to which these mechanisms may involve changes in perception, emotion, or cognition.

Changes in CNS response to neurotensin accompany the postpartum period in mice

Neurotensin (NT) is a highly conserved neuropeptide in mammals. Recent studies suggest that altered NT neurotransmission in postpartum females could promote the emergence of some maternal behaviors, including offspring protection. Here we evaluated how virgin and postpartum brains from mice selected for high maternal defense differ in response to NT. Virgin and postpartum mice were injected with either vehicle or 0.1 μg NT icv and brains were evaluated for c-Fos immunoreactivity, an indirect marker of neuronal activity. Using ANOVA analysis, common significant responses to NT were found in both female groups in four brain regions, including supraoptic nucleus, ventromedial nucleus, bed nucleus of stria terminalis dorsal, and a subregion of lateral septum (LS). For postpartum mice, only one additional region showed a significant response to NT relative to vehicle, whereas for virgin mice seven unique brain regions showed a significant c-Fos response: nucleus accumbens shell, paraventricular nucleus, central amygdala, and substantia nigra. Using a principal components analysis of c-Fos, we identified regions within each group with highly correlated activity. As expected, virgin and postpartum mice (vehicle conditions) showed different activity hubs and in the postpartum group the hubs matched regions linked to maternal care. The response to injected NT was different in the maternal and virgin groups with maternal mice showing a stronger coordinated activity in periaqueductal gray whereas virgin mice showed a stronger septal and amygdala linking of activity. Together, these results indicate neuronal responses of virgin and postpartum mice to NT and highlight pathways by which NT can alter maternal responses.

Autoradiographic analysis of GABAA receptor binding in the neural anxiety network of postpartum and non-postpartum laboratory rats

Postpartum female rats exhibit a suppression of anxiety-related behaviors when compared to diestrous virgin females, pregnant females, and males. This blunted anxiety promotes optimal maternal care and involves elevated GABA neurotransmission, possibly including greater density of GABA(A) and benzodiazepine receptors in the postpartum brain. We here examined autoradiographic binding of [(3)H]muscimol to measure the total population of GABA(A) receptors and [(3)H]flunitrazepam to assess density of benzodiazepine sites in the medial prefrontal cortex, bed nucleus of the stria terminalis, amygdala, hippocampus, and periaqueductal gray of female rats sacrificed on day 7 postpartum, day 10 of pregnancy, or as diestrous virgins. A group of sexually naïve male rats was also included. We found that [(3)H]muscimol binding did not differ among groups in any site but that diestrous virgin females had greater [(3)H]flunitrazepam binding in the CA1 and dentate gyrus of the hippocampus compared to mid-pregnant females and males. Notably, postpartum and diestrous virgin females did not significantly differ in binding of either ligand in any site examined. This is the first study to evaluate the densities of GABA(A) and benzodiazepine binding sites simultaneously across three female reproductive states and sex with a focus on brain sites influencing anxiety-related behaviors. The results suggest that changes in other GABA(A) receptor characteristics such as subunit composition, or increased presynaptic GABA release during interactions with offspring, must instead play a greater role in the postpartum suppression of anxiety in laboratory rats.

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.

Functional identification of an aggression locus in the mouse hypothalamus

Electrical stimulation of certain hypothalamic regions in cats and rodents can elicit attack behavior, but the exact location of relevant cells within these regions, their requirement for naturally occurring aggression and their relationship to mating circuits have not been clear. Genetic methods for neural circuit manipulation in mice provide a potentially powerful approach to this problem, but brain stimulation-evoked aggression has never been demonstrated in this species. Here we show that optogenetic, but not electrical, stimulation of neurons in the ventromedial hypothalamus, ventrolateral subdivision (VMHvl) causes male mice to attack both females and inanimate objects, as well as males. Pharmacogenetic silencing of VMHvl reversibly inhibits inter-male aggression. Immediate early gene analysis and single unit recordings from VMHvl during social interactions reveal overlapping but distinct neuronal subpopulations involved in fighting and mating. Neurons activated during attack are inhibited during mating, suggesting a potential neural substrate for competition between these behaviors.

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

RATIONALE

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

OBJECTIVE

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

RESULTS

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

CONCLUSIONS

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

Social instigation and aggression in postpartum female rats: role of 5-Ht1A and 5-Ht1B receptors in the dorsal raphé nucleus and prefrontal cortex

RATIONALE

5-HT(1A) and 5-HT(1B) receptor agonists effectively reduce aggressive behavior in males that has been escalated by social instigation. Important sites of action for these drugs are the receptors in dorsal raphé nuclei (DRN) and the ventral-orbital prefrontal cortex (VO PFC). DRN and VO PFC areas are particularly relevant in the inhibitory control of escalated aggressive and impulsive behavior.

OBJECTIVES

The objectives of this study are to assess the anti-aggressive effects of 5-HT(1A) (8-OH-DPAT) and 5-HT(1B) (CP-93,129) receptor agonists microinjected into DRN and VO PFC, respectively, and to study the aggressive behavior in postpartum female Wistar rats using the social instigation protocol to increase aggression.

METHODS AND RESULTS

8-OH-DPAT (0.56 μg) in the DRN increased aggressive behavior in postpartum female rats. By contrast, CP-93,129 (1.0 μg) microinjected into VO PFC decreased the number of attack bites and lateral threats. 5-HT(1A) and 5-HT(1B) receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming and the latter increasing these acts. When 8-OH-DPAT was microinjected into DRN and CP-93,129 was microinjected into VO PFC in female rats at the same time, maternal aggression decreased. Specific participation of 5-HT(1B) receptors was verified by reversal of the anti-aggressive effects using the selective antagonist SB-224,289 (1.0 μg).

CONCLUSIONS

The decrease in maternal aggressive behavior after microinjections of 5-HT(1B) receptor agonists into the VO PFC and DRN of female postpartum rats that were instigated socially supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner, due to activation of 5-HT(1B) receptors at the soma and terminals.

Behavioral and pharmacogenetics of aggressive behavior

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

Using transgenic mouse models to study oxytocin's role in the facilitation of species propagation

Oxytocin and its receptor are important for a wide range of effects, from social memory to uterine contractions. It is an evolutionarily well-conserved hormone that is particularly important in social and gregarious animals. Research on small mammals has yielded a rich literature on oxytocin's many functions. Recently a new tool has been created that has furthered our understanding of oxytocin's role in behavior: transgenic mice that lack either the ability to synthesize oxytocin or the oxytocin receptor itself. The study of these lines, while still in its infancy, is already bearing fruit and offers the promise of insight into some human disorders characterized by aberrant social behavior.

Effects of chronic and intermittent cocaine treatment on dominance, aggression, and oxytocin levels in post-lactational rats

RATIONALE

Little is known about mechanisms underlying female rodent aggression during the late postpartum period with no pups present. Studies of aggression, dominance, and oxytocin (OT) response in cocaine-treated females are sparse.

OBJECTIVES

This study was designed to examine dominance (drinking success) and aggression in a limited-access drinking model of water competition. Acute OT level measures were made on postpartum day (PPD) 36 in several brain regions of interest. Chronic and intermittent cocaine- and saline-treated and untreated rats 10 days post-weaning were tested (without pups) over PPDs 31-35 following cessation of cocaine treatment 10-30 days before testing.

METHODS

Subjects were water-deprived overnight, and triads consisting of an untreated control (UN), a chronic continuous saline-treated (CS), and chronic continuous cocaine-treated (CC; 30 mg/kg/day throughout gestation) or a UN, an intermittent saline-treated (IS), and an intermittent cocaine-treated (IC; 30 mg/kg two consecutive days every 4 days throughout gestation until PPD 20) female were tested for aggression and drinking behavior during 5 min sessions on five consecutive days. The amygdala, medial preoptic area (MPOA), and ventral tegmental area were assayed for OT levels.

RESULTS

CC and IC females were more aggressive than controls, but only IC females drank more often than controls. OT levels were lower in the MPOA of IC and CC females than in controls.

CONCLUSIONS

Findings demonstrate that long after cessation of treatment, CC- and IC-treated non-lactating females (no pups present) had higher rates of aggression, altered drinking behavior, and acutely lower MPOA OT levels.

GABAA receptor signaling in caudal periaqueductal gray regulates maternal aggression and maternal care in mice

Maternal aggression (maternal defense) is exhibited by lactating females towards intruders and contributes to the protection of offspring. Enhancement of Gamma-Aminobutyric acid (GABA)(A) receptor signaling by benzodiazepines elevates maternal aggression, and we previously found indirect evidence (via c-Fos immunohistochemistry) that caudal periaqueductal gray (cPAG) and lateral septum (LS) could be sites where benzodiazepines increase aggression. We recently found that GABA(A) receptor signaling in LS modulates maternal aggression, and in this study, we tested the hypothesis that GABA(A) receptor signaling in cPAG also regulates this behavior. Site-directed injections to cPAG were made in lactating mice using the GABA(A) receptor antagonist, bicuculline (3-9 ng) or the GABA(A) receptor positive modulator, chlordiazepoxide (CDP), a benzodiazepine (2.5-20 microg). Maternal aggression, other maternal behaviors, and anxiety-like measures (using the light-dark box) were then examined. GABA(A) receptor positive modulator did not increase aggression, which could have resulted from a ceiling effect. However, 8 ng and 9 ng of bicuculline in cPAG significantly decreased maternal aggression without altering other maternal behaviors or light-dark box performance, suggesting some GABA(A) receptor signaling in cPAG is required for full maternal aggression expression. Additionally, 7 ng of bicuculline significantly increased licking/grooming of pups, and decreased the number of transitions between the light and dark compartments of the light-dark box without affecting aggression. Given these results indicating that antagonizing GABA(A) receptor in cPAG dose-dependently promotes offspring grooming behavior while impairing aggression, it is possible that the cPAG represents a key site for decision making (aggression versus other behaviors) in the lactating female.

Oxytocin and human social behavior

Despite a general consensus that oxytocin (OT) has prosocial effects, there is no clear agreement on how these effects are achieved. Human research on OT is reviewed under three broad research initiatives: attachment and trust, social memory, and fear reduction. As an organizing perspective for scholars' current knowledge, a tentative model of the causes and effects of alterations in OT level is proposed. The model must remain provisional until conceptual and methodological problems are addressed that arise from a failure to distinguish between traits and states, differing research paradigms used in relation to OT as an independent versus dependent variable, and the possibility that OT effects depend on the initial emotional state of the individual. Social and personality psychologists have important roles to play in developing more rigorous and creative research designs.

Progesterone and maternal aggressive behavior in rats

Females usually display low levels of aggressiveness; however, during lactation, the aggressive behavior against intruders to the nest area is an important component of the maternal behavioral repertoire. The present study aimed to analyze the influence of progesterone (P4) on the maternal aggressive behavior in rats. Lactating rat were ovariectomized on the first day after delivery and, on the 6th postpartum day, aggressive behaviors against a male intruder were recorded. Also in the 6th PPD, the effects of a P4 receptor antagonist (RU 486) as well as of finasteride - which inhibits the conversion of P4 to its metabolite allopregnanolone - on the aggressive behavior of non-ovariectomized lactating rats were analyzed. Finally, plasma concentration of prolactin was measured on the 8th PPD. This study shows, for the first time, that ovariectomy just after parturition reduces some aspects of the maternal behavior (frequency of licking) and the aggressive behavior and increased plasma prolactin. On the other hand, the administration of RU486 induced a marked increase in the aggressiveness of lactating females. No changes were detected after finasteride injection. Gonadal hormones after parturition seem necessary for the development of maternal aggressive behavior. Furthermore, our results suggest that the increase in P4 levels throughout the postpartum period could be one of the causes for the natural reduction of the aggressive behavior in lactating rats.

Vasopressin released within the central amygdala promotes maternal aggression

Vasopressin regulates important aspects of social behaviour. Although vasopressin is more prominent in the expression of male social behaviours, we recently demonstrated its role in the fine-tuned maintenance of maternal care in lactating rats. Here, we investigate the involvement of brain vasopressin in the regulation of maternal aggression in lactating Wistar rats selectively bred for either high (HAB) or low (LAB) anxiety-related behaviour. The genetically determined elevation in vasopressin mRNA expression was confirmed within the hypothalamic paraventricular nucleus of virgin and lactating HAB rats and was additionally found in limbic brain areas. Lactating HAB dams are more maternally aggressive as part of their generally higher level of maternal care compared with LAB rats. Using intracerebral microdialysis, we describe increased vasopressin release within the central amygdala, but not the paraventricular nucleus, during maternal aggression only in HAB dams. Moreover, the release of vasopressin within the central amygdala was positively correlated with the display of offensive behaviour. Blockade of local vasopressin actions by bilateral administration of a selective vasopressin V1a receptor antagonist into the central amygdala reduced maternal aggression in HAB dams, whereas synthetic vasopressin increased the low level of aggression in LAB rats. Vasopressin receptor binding within the central amygdala or the paraventricular nucleus was similar in HAB and LAB females. In conclusion, vasopressin is an important neuropeptide regulating maternal aggressive behaviour, thus further extending its involvement in female social behaviour. Differences in intracerebral vasopressin release within the central amygdala rather than local vasopressin receptor binding contribute to the level of maternal aggression.

Effects of photoperiod and experience on aggressive behavior in female California mice

Aggressive behavior among females is observed in many species, but the mechanisms of this behavior have historically been understudied. In many species of rodents, winter-like short day photoperiods induce increased aggression levels compared to summer-like long day photoperiods. Recent reports in hamsters show that short days also increase aggression in females. We examined the effects of photoperiod on aggression in female California mice, and for the first time compare brain activity of aggression-tested female rodents under different photoperiods. We observed that female California mice were more aggressive when housed in short days versus long days. Intriguingly, we also observed that under long days female attack latency decreases with repeated testing in resident-intruder tests. These data suggest that winner effects that have been described in males may also occur in females. We also used the expression of phosphorylated extracellular signal-regulated kinases (pERK) in the brain to estimate brain activity during aggression tests. pERK can alter neuronal activity in the short term and in the long term can act as a transcription factor. Using immunoblot analyses we observed that aggression-induced pERK expression in the female bed nucleus of the stria terminalis and medial amygdala occurs under both long and short days. Thus, the mechanisms controlling increased aggression under short days are still unclear and additional study is needed.

GABA(A) receptor signaling in the lateral septum regulates maternal aggression in mice

Maternal aggression (maternal defense) is a fierce aggression produced by lactating females toward intruders that plays an important role in protection of vulnerable offspring. Enhancement of GABA(A) receptor signaling by benzodiazepines increases maternal aggression, and we recently found indirect evidence that lateral septum (LS) could be a key site where benzodiazepines elevate aggression. In this study, we directly tested the hypothesis that activation of GABA(A) receptors in LS would promote maternal aggression while inhibition of this receptor would decrease aggression. Site-directed injections to LS were made using the GABA(A) receptor antagonist, bicuculline (3-30 ng), or the GABA(A) receptor agonists, chlordiazepoxide, a benzodiazepine (2.5-5 microg), and muscimol (0.05-5 ng). Maternal aggression and other behavioral measures were then evaluated in lactating mice. Neither GABA(A) receptor agonist elevated aggression, which could reflect a ceiling effect. However, 7 ng of the GABA(A) receptor antagonist, bicuculline, in LS significantly decreased maternal aggression without altering other maternal behaviors or light-dark box performance, suggesting some GABA(A) receptor signaling in LS is required for full maternal aggression expression. Together, these results confirm a role for GABA(A) receptor signaling in LS in the regulation of maternal aggression.

Gestational exposure to the organophosphate chlorpyrifos alters social-emotional behaviour and impairs responsiveness to the serotonin transporter inhibitor fluvoxamine in mice

BACKGROUND

The organophosphate chlorpyrifos (CPF) is a pesticide largely used worldwide. Studies from animal models indicate that CPF exposure during development at low doses can target different neurotransmitter systems in the absence of overt cholinergic effects.

METHODS

Late gestational exposure (gestational days 14-17) to CPF at the dose of 6 mg/kg was evaluated in CD-1 mice at adulthood. Neurobehavioural effects likely involving serotonin (5-hydroxytryptamine, 5HT) transmission were assessed both in males and females, through the light-dark exploration test to assess CPF effects on anxiety profiles and the forced swimming test to evaluate the response to the 5HT transporter (5HTT) inhibitor fluvoxamine (30 mg/kg). In females only, we evaluated the effects of gestational exposure to CPF on maternal aggression, under basal condition or after injection of fluvoxamine.

RESULTS

Gestational CPF exposure increased anxiety levels only in female mice, as shown by the augmented thigmotaxis behaviour and the lower latency to enter in the dark compartment. In the forced swimming test, no differences between CPF and control mice were found when assessed under basal condition (saline administration), but both male and female CPF mice missed to show the typical behavioural effects of the 5HTT inhibitor fluvoxamine. During maternal aggression, CPF females showed lower propensity to and intensity of aggressive behaviour, together with mild decreased responsiveness to fluvoxamine administration.

CONCLUSIONS

Overall, the present results confirm a specific and sex-dependent vulnerability of affective/emotional domains to developmental CPF exposure. Furthermore, data provide clear indication on the disrupting effects of prenatal CPF on serotoninergic transmission.

Enhanced maternal aggression and associated changes in neuropeptide gene expression in multiparous rats

Although it has often been speculated that prior reproductive experience improves subsequent maternal care, few studies have examined specific changes in behavior during a 1st versus 2nd lactation. During lactation, mothers display heightened aggression toward male intruders, purportedly to protect vulnerable young. In the current study, maternal aggression was examined in primiparous and age-matched multiparous females on postpartum days 5 (PPD5) and PPD15. Expression of oxytocin, oxytocin receptor, arginine vasopressin, arginine vasopressin V1a receptors, and corticotrophin-releasing hormone mRNA was measured following aggression testing at both time points using real-time quantitative PCR in brain regions previously implicated in the regulation of maternal aggression. Multiparity significantly enhanced maternal aggression on PPD5 but not on PPD15. In addition, this increased aggression was associated with region- and gene-specific changes in mRNA expression. These findings indicate that reproductive experience enhances maternal aggression, an effect that may be mediated by region-specific alterations in neuropeptidergic activity. The adaptations observed in multiparous females provide an innate model for the study of neuroplasticity in the regulation of aggression.

Gonadal hormones modulate the display of conditioned defeat in male Syrian hamsters

It has been widely reported that gonadal hormones influence the display of aggression in Syrian hamsters; conversely, much less is known about whether gonadal hormones modulate submissive/defensive behaviors in these animals. Following social defeat, male hamsters no longer display normal territorial aggression but instead display submissive/defensive behavior in the presence of a smaller opponent, a phenomenon we have termed conditioned defeat (CD). The purpose of the present study was to examine the effect of gonadal hormones on the display of CD in male hamsters. In Experiment 1, males were castrated or sham-operated. The castrated males were significantly more submissive following social defeat relative to their intact counterparts. The increased submissive behavior in the castrated males during CD testing was particularly surprising, given the fact that they were attacked significantly less during CD training. In Experiment 2a, males were castrated and given hormone replacement. Castrated males treated with testosterone or dihydrotestosterone displayed significantly less submissive behavior following social defeat than did those treated with cholesterol or estradiol. Finally, in Experiment 2b, there was no effect of hormone replacement on aggressive behavior in non-defeated hamsters suggesting that the decrease in submissive behavior in males treated with dihydrotestosterone or testosterone is specific to being previously defeated. Taken together the data indicate that the presence of androgens reduces the display of submission in defeated male hamsters. More importantly, these findings suggest that androgens may have a protective effect against the development of depression-like or anxiety-like behaviors following exposure to an ethologically relevant stressor.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

The advantage of social living: brain neuropeptides mediate the beneficial consequences of sex and motherhood

Living in social groups is clearly beneficial for many species, often resulting in increased survival, enhanced fitness of the group, and progression of brain development and cognitive abilities. The development of the social brain has been promoted on the basis (i) of activation of reward centres by social stimuli, (ii) of positive consequences of close social interactions on emotionality (which is reinforcing by itself) and on general fitness, and (iii) of negative health consequences in the absence or as a result of sudden interruption of social interactions. For example, social interactions as seen between mother and child or between mating partners have beneficial effects on the mental and physical health state, in particular on adaptive processes related to emotional and physiological stress coping in both sexes. Here, the neurobiological basis of social behaviour, in particular the involvement of the brain neuropeptides, oxytocin and prolactin, in mediating such positive health effects will be discussed.

Aggression is related to frontal serotonin-1A receptor distribution as revealed by PET in healthy subjects

OBJECTIVES

Various studies indicate that serotonin regulates impulsivity and the inhibitory control of aggression. Aggression is also known to be modified by sex hormones, which exert influence on serotonergic neurotransmission. The present study aimed to elucidate potential interactions between human aggression, the inhibitory serotonergic 5-HT(1A) receptor, and sex hormones.

EXPERIMENTAL DESIGN

Thirty-three healthy volunteers (16 women, aged 26.24 +/- 5.5 yr) completed a validated questionnaire incorporating five dimensions of aggression. Subsequently, all subjects underwent positron emission tomography with the radioligand [carbonyl-(11)C]WAY-100635 to quantify 5-HT(1A) binding potentials (BP(ND)s) in the prefrontal cortex, limbic areas, and midbrain. Also, plasma levels of testosterone, 17beta-estradiol and sex hormone-binding globulin (SHBG) were measured. Relations between aggression scores, regional 5-HT(1A) BP(ND)s, and hormone levels were analyzed using correlations, multivariate analyses of variance, and linear regressions.

PRINCIPAL OBSERVATIONS

Statistical analyses revealed higher 5-HT(1A) receptor BP(ND)s in subjects exhibiting higher aggression scores in prefrontal (all P < 0.041) and anterior cingulate cortices (P = 0.016). More aggressive subjects were also characterized by lower SHBG levels (P = 0.015). Moreover, higher SHBG levels were associated with lower 5-HT(1A) BP(ND)s in frontal (P = 0.048) and cingulate cortices (all P < 0.013) and in the amygdala (P = 0.03).

CONCLUSIONS

The present study provides first-time evidence for a specific interrelation between the 5-HT(1A) receptor distribution, sex hormones, and aggression in humans. Our findings point to a reduced down-stream control due to higher amounts or activities of frontal 5-HT(1A) receptors in more aggressive subjects, which is presumably modulated by sex hormones.

Blood oxygen level-dependent signal responses in corticolimbic 'emotions' circuitry of lactating rats facing intruder threat to pups

Lactating rats must continuously maintain a critical balance between caring for pups and aggressively responding to nest threats. We tested the neural response of lactating females to the presentation of their own pups and novel intruder males using blood oxygen level-dependent functional magnetic resonance imaging at 7 T. Dams were presented with a single sequence of a control stimulus, pups or a male intruder in one imaging session (n = 7-9). To further determine the selectivity of neural processing, dams were imaged for their response to a male intruder in both the absence and presence of their pups (n = 6). Several maternal cortical and limbic brain regions were significantly activated by intruder presentation but not by pups or a control stimulus. These included the nucleus accumbens, periaqueductal gray, anterior cingulate, anterior thalamus, basal nucleus of the amygdala, temporal cortex, prelimbic/orbital area and insula. The nucleus accumbens, periaqueductal gray, temporal cortex and mediodorsal thalamus still showed greater neural activity when compared with intruder presentation in the absence of pups. The present results suggest that the high emotional state generated by a threat to pups involves robust activation of classical limbic regions controlling emotional responses. This pattern of blood oxygen level-dependent activity may precede behavioral states upon which lactating rats initiate attacks against a potential threat to offspring.

Oxytocin: the great facilitator of life

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Maternal profiling of corticotropin-releasing factor receptor 2 deficient mice in association with restraint stress

Mice deficient in corticotropin-releasing factor receptor 2 (CRF2) (C57BL/6J:129Sv background) exhibit impaired maternal defense (protection of offspring) and are more reactive to stressors than wild-type mice. To further understand CRF2's role in maternal behavior, we crossed the knockout mice with a line bred for high maternal defense that also has elevated maternal care relative to inbred lines. Maternal care was normal in knockout mice (relative to wild-type). Maternal defense was impaired as previously observed. Exposure to a mild stressor (15 min restraint) did not trigger deficits in maternal defense in either genotype as determined by a two-way repeated measures ANOVA analysis. However, when examining difference scores between unrestrained and restrained conditions, knockout mice exhibited significant decreases in maternal defense with stress, suggesting knockouts are more susceptible to a mild stressor's effects. To gain possible insights into brain activity differences between WT and KO mice, we examined c-Fos expression in association with stress. Unrestrained KO mice exhibited significantly lower c-Fos levels relative to unrestrained WT mice in 9 regions, including lateral septum and periaqueductal gray. For WT mice, restraint stress triggered c-Fos activity increases in 3 regions while for KO mice, restraint stress triggered c-Fos increases in 16 regions. Taken together, our results suggest both altered behavioral and c-Fos responses to stress in lactating CRF2 KO mice.