-Fos expression in female hamster brain following sexual and aggressive behaviors

Aggression Unleashed: Neural Circuits from Scent to Brain

Aggression is a fundamental behavior with essential roles in dominance assertion, resource acquisition, and self-defense across the animal kingdom. However, dysregulation of the aggression circuitry can have severe consequences in humans, leading to economic, emotional, and societal burdens. Previous inconsistencies in aggression research have been due to limitations in techniques for studying these neurons at a high spatial resolution, resulting in an incomplete understanding of the neural mechanisms underlying aggression. Recent advancements in optogenetics, pharmacogenetics, single-cell RNA sequencing, and in vivo electrophysiology have provided new insights into this complex circuitry. This review aims to explore the aggression-provoking stimuli and their detection in rodents, particularly through the olfactory systems. Additionally, we will examine the core regions associated with aggression, their interactions, and their connection with the prefrontal cortex. We will also discuss the significance of top-down cognitive control systems in regulating atypical expressions of aggressive behavior. While the focus will primarily be on rodent circuitry, we will briefly touch upon the modulation of aggression in humans through the prefrontal cortex and discuss emerging therapeutic interventions that may benefit individuals with aggression disorders. This comprehensive understanding of the neural substrates of aggression will pave the way for the development of novel therapeutic strategies and clinical interventions. This approach contrasts with the broader perspective on neural mechanisms of aggression across species, aiming for a more focused analysis of specific pathways and their implications for therapeutic interventions.

Cupid's quiver: Integrating sensory cues in rodent mating systems

In many animal species, males and females exploit different mating strategies, display sex-typical behaviors, and use distinct systems to recognize ethologically relevant cues. Mate selection thus requires mutual recognition across diverse social interactions based on distinct sensory signals. These sex differences in courtship and mating behaviors correspond to differences in sensory systems and downstream neural substrates engaged to recognize and respond to courtship signals. In many rodents, males tend to rely heavily on volatile olfactory and pheromone cues, while females appear to be guided more by a combination of these chemosensory signals with acoustic cues in the form of ultrasonic vocalizations. The mechanisms by which chemical and acoustic cues are integrated to control behavior are understudied in mating but are known to be important in the control of maternal behaviors. Socially monogamous species constitute a behaviorally distinct group of rodents. In these species, anatomic differences between males and females outside the nervous system are less prominent than in species with non-monogamous mating systems, and both sexes engage in more symmetric social behaviors and form attachments. Nevertheless, despite the apparent similarities in behaviors displayed by monogamous males and females, the circuitry supporting social, mating, and attachment behaviors in these species is increasingly thought to differ between the sexes. Sex differences in sensory modalities most important for mate recognition in across species are of particular interest and present a wealth of questions yet to be answered. Here, we discuss how distinct sensory cues may be integrated to drive social and attachment behaviors in rodents, and the differing roles of specific sensory systems in eliciting displays of behavior by females or males.

Functions of habenula in reproduction and socio-reproductive behaviours

Habenula is an evolutionarily conserved structure in the brain of vertebrates. Recent reports have drawn attention to the habenula as a processing centre for emotional decision-making and its role in psychiatric disorders. Emotional decision-making process is also known to be closely associated with reproductive conditions. The habenula receives innervations from reproductive centres within the brain and signals from key reproductive neuroendocrine regulators such as gonadal sex steroids, gonadotropin-releasing hormone (GnRH), and kisspeptin. In this review, based on morphological, biochemical, physiological, and pharmacological evidence we discuss an emerging role of the habenula in reproduction. Further, we discuss the modulatory role of reproductive endocrine factors in the habenula and their association with socio-reproductive behaviours such as mating, anxiety and aggression.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

One cranium, two brains not yet introduced: Distinct but complementary views of the social brain

Social behavior is pervasive across the animal kingdom, and elucidating how the brain enables animals to respond to social contexts is of great interest and profound importance. Our understanding of 'the social brain' has been fractured as it has matured. Two drastically different conceptualizations of the social brain have emerged with relatively little awareness of each other. In this review, we briefly recount the history behind the two dominant definitions of a social brain. The divide that has emerged between these visions can, in part, be attributed to differential attention to cortical or sub-cortical regions in the brain, and differences in methodology, comparative perspectives, and emphasis on functional specificity or generality. We discuss how these factors contribute to a lack of communication between research efforts, and propose ways in which each version of the social brain can benefit from the perspectives, tools, and approaches of the other. Interface between the two characterizations of social brain networks is sure to provide essential insight into what the social brain encompasses.

The role of central and medial amygdala in normal and abnormal aggression: A review of classical approaches

The involvement of the amygdala in aggression is supported by overwhelming evidence. Frequently, however, the amygdala is studied as a whole, despite its complex internal organization. To reveal the role of various subdivisions, here we review the involvement of the central and medial amygdala in male rivalry aggression, maternal aggression, predatory aggression, and models of abnormal aggression where violent behavior is associated with increased or decreased arousal. We conclude that: (1) rivalry aggression is controlled by the medial amygdala; (2) predatory aggression is controlled by the central amygdala; (3) hypoarousal-associated violent aggression recruits both nuclei, (4) a specific upregulation of the medial amygdala was observed in hyperarousal-driven aggression. These patterns of amygdala activation were used to build four alternative models of the aggression circuitry, each being specific to particular forms of aggression. The separate study of the roles of amygdala subdivisions may not only improve our understanding of aggressive behavior, but also the differential control of aggression and violent behaviors of various types, including those associated with various psychopathologies.

Overcoming neonatal sickness: Sex-specific effects of sickness on physiology and social behavior

Early-life environmental stressors, including sickness, have the potential to disrupt development in ways that could severely impact fitness. Despite what is known about the effects of sickness on reproduction, the precise physiological mechanisms have not yet been determined. The goal of this study was to investigate the effects of a neonatal immune challenge on adult reproductive physiology and opposite-sex social behavior. Male and female Siberian hamster (Phodopus sungorus) pups were administered lipopolysaccharide ([LPS]; a cell wall component of gram-negative bacteria) or saline injections on postnatal days 3 and 5 and body mass, food intake, and measures of reproductive maturity were taken throughout development. In adulthood, hamsters were placed in staged mating pairs with reproductively mature individuals of the opposite sex, during which a series of behaviors were scored. We found that although males and females showed no change in food intake, body mass, or reproductive behaviors, LPS-treated females had abnormal estrous cycles and smaller ovaries. Females also showed increased investigation of and increased aggression towards males in a reproductive context. In contrast, LPS-treated males showed no change in any physiological measures, nor did they show any changes in behavior. The present findings demonstrate that females may be more robustly affected by neonatal sickness than males and that these effects could have potential impacts on reproductive success. Collectively, the results of this study can be used to expand upon what is already known about sickness and reproduction, specifically the importance of social behaviors involved in pre-copulation and information necessary to choose the appropriate mate.

Social instigation and repeated aggressive confrontations in male Swiss mice: analysis of plasma corticosterone, CRF and BDNF levels in limbic brain areas

Introduction:

Agonistic behaviors help to ensure survival, provide advantage in competition, and communicate social status. The resident-intruder paradigm, an animal model based on male intraspecific confrontations, can be an ethologically relevant tool to investigate the neurobiology of aggressive behavior.

Objectives:

To examine behavioral and neurobiological mechanisms of aggressive behavior in male Swiss mice exposed to repeated confrontations in the resident intruder paradigm.

Methods:

Behavioral analysis was performed in association with measurements of plasma corticosterone of mice repeatedly exposed to a potential rival nearby, but inaccessible (social instigation), or to 10 sessions of social instigation followed by direct aggressive encounters. Moreover, corticotropin-releasing factor (CRF) and brain-derived neurotrophic factor (BNDF) were measured in the brain of these animals. Control mice were exposed to neither social instigation nor aggressive confrontations.

Results:

Mice exposed to aggressive confrontations exhibited a similar pattern of species-typical aggressive and non-aggressive behaviors on the first and the last session. Moreover, in contrast to social instigation only, repeated aggressive confrontations promoted an increase in plasma corticosterone. After 10 aggressive confrontation sessions, mice presented a non-significant trend toward reducing hippocampal levels of CRF, which inversely correlated with plasma corticosterone levels. Conversely, repeated sessions of social instigation or aggressive confrontation did not alter BDNF concentrations at the prefrontal cortex and hippocampus.

Conclusion:

Exposure to repeated episodes of aggressive encounters did not promote habituation over time. Additionally, CRF seems to be involved in physiological responses to social stressors.

Measuring virgin female aggression in the female intruder test (FIT): effects of oxytocin, estrous cycle, and anxiety

The costs of violence and aggression in our society have stimulated the scientific search for the predictors and causes of aggression. The majority of studies have focused on males, which are considered to be more aggressive than females in most species. However, rates of offensive behavior in girls and young women are considerable and are currently rising in Western society. The extrapolation of scientific results from males to young, non-maternal females is a priori limited, based on the profound sex differences in brain areas and functioning of neurotransmitters involved in aggression. Therefore, we established a paradigm to assess aggressive behavior in young virgin female rats, i.e. the female intruder test (FIT). We found that approximately 40% of un-manipulated adult (10-11 weeks old) female Wistar rats attack an intruder female during the FIT, independent of their estrous phase or that of their intruder. In addition, adolescent (7-8 weeks old) female rats selected for high anxiety behavior (HABs) displayed significantly more aggression than non-selected (NAB) or low-anxiety (LAB) rats. Intracerebroventricular infusion of oxytocin (OXT, 0.1 µg/5 µl) inhibited aggressive behavior in adult NAB and LAB, but not HAB females. Adolescent NAB rats that had been aggressive towards their intruder showed increased pERK immunoreactivity (IR) in the hypothalamic attack area and reduced pERK-IR in OXT neurons in the paraventricular hypothalamic nucleus compared to non-aggressive NAB rats. Taken together, aggressive behavior in young virgin female rats is partly dependent on trait anxiety, and appears to be under considerable OXT control.

The effects of exogenous melatonin and melatonin receptor blockade on aggression and estrogen-dependent gene expression in male California mice (Peromyscus californicus)

Photoperiodic regulation of aggression has been well established in several vertebrate species, with rodents demonstrating increased aggression in short day photoperiods as compared to long day photoperiods. Previous work suggests that estrogens regulate aggression via rapid nongenomic pathways in short days and act more slowly in long days, most likely via genomic pathways. The current study therefore examines the role of melatonin in mediating aggression and estrogen-dependent gene transcription. In Experiment 1, male California mice were housed under long day photoperiods and were treated with either 0.3 μg/g of melatonin, 40 mg/kg of the melatonin receptor antagonist luzindole, or vehicle for 10 days. We found that melatonin administration significantly increased aggression as compared to mice receiving vehicle, but this phenotype was not completely ameliorated by luzindole. In Experiment 2, male California mice were injected with either 1mg/kg of the aromatase inhibitor letrozole or vehicle, and oxytocin receptor (OTR), estrogen receptor alpha (ERα), and c-fos gene expression was examined in the bed nucleus of the stria terminalis (BNST) and medial preoptic area (MPOA). In the BNST, but not MPOA, OTR mRNA was significantly downregulated following letrozole administration, indicating that OTR is an estrogen-dependent gene in the BNST. In contrast, ERα was not estrogen dependent in either brain region. In the MPOA, OTR mRNA was inhibited by melatonin, and luzindole suppressed this effect. C-fos and ERα did not differ between treatments in any brain region examined. These results suggest that it is unlikely that melatonin facilitates aggression via broad spectrum regulation of estrogen-dependent gene expression. Instead, melatonin may act via regulation of other transcription factors such as extracellular signal regulated kinase.

Gonadotropin-inhibitory hormone reduces sexual motivation but not lordosis behavior in female Syrian hamsters (Mesocricetus auratus)

Reproductive success is maximized when female sexual motivation and behavior coincide with the time of optimal fertility. Both processes depend upon coordinated hormonal events, beginning with signaling by the gonadotropin-releasing hormone (GnRH) neuronal system. Two neuropeptidergic systems that lie upstream of GnRH, gonadotropin-inhibitory hormone (GnIH; also known as RFamide related peptide-3) and kisspeptin, are potent inhibitory and excitatory modulators of GnRH, respectively, that participate in the timing of the preovulatory luteinizing hormone (LH) surge and ovulation. Whether these neuropeptides serve as neuromodulators to coordinate female sexual behavior with the limited window of fertility has not been thoroughly explored. In the present study, either intact or ovariectomized, hormone-treated female hamsters were implanted for fifteen days with chronic release osmotic pumps filled with GnIH or saline. The effect of GnIH on sexual motivation, vaginal scent marking, and lordosis was examined. Following mating, FOS activation was quantified in brain regions implicated in the regulation of female sexual behavior. Intracerebroventricular administration of GnIH reduced sexual motivation and vaginal scent marking, but not lordosis behavior. GnIH administration altered FOS expression in key neural loci implicated in female reproductive behavior, including the medial preoptic area, medial amygdala and bed nucleus of the stria terminalis, independent of changes in circulating gonadal steroids and kisspeptin cell activation. Together, these data point to GnIH as an important modulator of female proceptive sexual behavior and motivation, independent of downstream alterations in sex steroid production.

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.

Dopaminergic modulation of incentive motivation in adolescence: age-related changes in signaling, individual differences, and implications for the development of self-regulation

Behavioral activation that is associated with incentive-reward motivation increases in adolescence relative to childhood and adulthood. This quadratic developmental pattern is generally supported by behavioral and experimental neuroscience findings. It is suggested that a focus on changes in dopamine neurotransmission is informative in understanding the mechanism for this adolescent increase in reward-related behavioral activation and subsequent decline into adulthood. Evidence is presented to indicate that incentive-reward motivation is modulated by mesoaccumbens dopamine, and that it increases in adolescence before declining into adulthood because of normative developmental changes at the molecular level. Potential mechanisms of variation in functional mesoaccumbens dopamine transmission are discussed with a focus on the interplay between tonic and phasic modes of dopamine transmission in modulating both general incentive-motivational biases and the efficacy of reward learning during exposure to novel reward experiences. Interactions between individual difference factors and these age-related trends are discussed.

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

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

Estradiol alters Fos-immunoreactivity in the hippocampus and dorsal striatum during place and response learning in middle-aged but not young adult female rats

Evidence from lesion and inactivation studies suggests that the hippocampus (HPC) and dorsal striatum compete for control over navigation behavior, and there is some evidence in males that the structure with greater relative activation controls behavior. Estradiol has been shown to enhance HPC-dependent place learning and impair dorsal striatum-dependent response learning in female rats, possibly by increasing hippocampal activation and/or decreasing striatal activation. We used Fos-immunoreactivity (Fos-IR) to examine the activation of several subregions of the HPC and striatum in ovariectomized female rats with or without estradiol replacement 30 min after place or response learning. In 4-month-old rats, neither task nor estradiol increased Fos-IR above explore control levels in any subregion analyzed, even though estradiol impaired response learning. In 12-month-old rats, estradiol increased Fos-IR in the dentate gyrus, dorsal medial striatum, and dorsal lateral striatum in place task learners, while the absence of estradiol increased Fos-IR in these regions in response task learners. However, learning rate was not affected by estradiol in either task. We also included a group of long-term ovariectomized 12-month-old rats that displayed impaired place learning and altered Fos-IR in CA1 of the HPC. These results suggest that task-specific effects of estradiol on hippocampal and striatal activation emerge across age but that relative hippocampal and striatal activation are not related to learning rate during spatial navigation learning.

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.

Testosterone programs adult social behavior before and during, but not after, adolescence

Whereas the adolescent brain is a major target for gonadal hormones, our understanding of hormonal influences on adolescent neural and behavioral development remains limited. These experiments investigated how variations in the timing of testosterone (T) exposure, relative to adolescence, alters the strength of steroid-sensitive neural circuits underlying social behavior in male Syrian hamsters. Experiment 1 simulated early, on-time, and late pubertal development by gonadectomizing males on postnatal d 10 and treating with SILASTIC brand T implants for 19 d before, during, or after adolescence. T treatment before or during, but not after, adolescence facilitated mating behavior in adulthood. In addition, preadolescent T treatments most effectively increased mating behavior overall, indicating that the timing of exposure to pubertal hormones contributes to individual differences in adult behavior. Experiment 2 examined the effects of preadolescent T treatment on behavior and brain regional volumes within the mating neural circuit of juvenile males (i.e. still preadolescent). Although preadolescent T treatment did not induce reproductive behavior in juvenile males, it did increase volumes of the bed nucleus of the stria terminalis, sexually dimorphic nucleus, posterodorsal medial amygdala, and posteroventral medial amygdala to adult-typical size. In contrast, juvenile anterodorsal medial amygdala and ventromedial hypothalamus volumes were not changed by preadolescent T treatment yet differed significantly in volume from adult controls, suggesting that further maturation of these brain regions during adolescence is required for the expression of male reproductive behavior. Thus, adolescent maturation of social behavior may involve both steroid-independent and -dependent processes, and adolescence marks the end of a postnatal period of sensitivity to steroid-dependent organization of the brain.

Imaging the neural circuitry and chemical control of aggressive motivation

BACKGROUND

With the advent of functional magnetic resonance imaging (fMRI) in awake animals it is possible to resolve patterns of neuronal activity across the entire brain with high spatial and temporal resolution. Synchronized changes in neuronal activity across multiple brain areas can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories and emotions for particular behaviors. To this end, fMRI in conscious rats combined with 3D computational analysis was used to identifying the putative distributed neural circuit involved in aggressive motivation and how this circuit is affected by drugs that block aggressive behavior.

RESULTS

To trigger aggressive motivation, male rats were presented with their female cage mate plus a novel male intruder in the bore of the magnet during image acquisition. As expected, brain areas previously identified as critical in the organization and expression of aggressive behavior were activated, e.g., lateral hypothalamus, medial basal amygdala. Unexpected was the intense activation of the forebrain cortex and anterior thalamic nuclei. Oral administration of a selective vasopressin V1a receptor antagonist SRX251 or the selective serotonin reuptake inhibitor fluoxetine, drugs that block aggressive behavior, both caused a general suppression of the distributed neural circuit involved in aggressive motivation. However, the effect of SRX251, but not fluoxetine, was specific to aggression as brain activation in response to a novel sexually receptive female was unaffected.

CONCLUSION

The putative neural circuit of aggressive motivation identified with fMRI includes neural substrates contributing to emotional expression (i.e. cortical and medial amygdala, BNST, lateral hypothalamus), emotional experience (i.e. hippocampus, forebrain cortex, anterior cingulate, retrosplenial cortex) and the anterior thalamic nuclei that bridge the motor and cognitive components of aggressive responding. Drugs that block vasopressin neurotransmission or enhance serotonin activity suppress activity in this putative neural circuit of aggressive motivation, particularly the anterior thalamic nuclei.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Sex-role reversal is reflected in the brain of African black coucals (Centropus grillii)

In most bird species males compete over access to females and have elevated circulating androgen levels when they establish and defend a breeding territory or guard a mate. Testosterone is involved in the regulation of territorial aggression and sexual display in males. In few bird species the traditional sex-roles are reversed and females are highly aggressive and compete over access to males. Such species represent excellent models to study the hormonal modulation of aggressive behavior in females. Plasma sex steroid concentrations in sex-role reversed species follow the patterns of birds with "traditional" sex-roles. The neural mechanisms modulating endocrine secretion and hormone-behavior interactions in sex-role reversed birds are currently unknown. We investigated the sex differences in the mRNA expression of androgen receptors, estrogen receptor alpha, and aromatase in two brain nuclei involved in reproductive and aggressive behavior in the black coucal, the nucleus taeniae and the bed nucleus of the stria terminalis. In the bed nucleus there were no sex differences in the receptor or aromatase expression. In the nucleus taeniae, however, we show for the first time, that females have a higher mRNA expression of androgen receptors than males. These results suggest that the expression of agonistic and courtship behavior in females does not depend on elevated blood hormone levels, but may be regulated via increased steroid hormone sensitivity in particular target areas in the brain. Hence, aggression in females and males may indeed be modulated by the same hormones, but regulated at different levels of the neuroendocrine cascade.

Repeated quinpirole treatments produce neurochemical sensitization and associated behavioral changes in female hamsters

RATIONALE

Repeated stimulation of dopaminergic pathways with dopamine receptor agonists can produce both neurochemical and behavioral sensitization.

OBJECTIVES

The present study was designed to examine whether repeated treatment with the D2-like dopamine receptor agonist, quinpirole, would produce neurochemical sensitization of D1 dopamine receptor-mediated processes and associated behavioral changes in female hamsters in a manner analogous to that previously used to sensitize heterologous dopamine signaling pathways in derived cell lines.

MATERIALS AND METHODS

Female hamsters received two injections of quinpirole (1.5 mg/kg) or saline each week for 7 weeks, during which time pouching behavior and body weight were monitored. Over the next 2 weeks, hamsters were tested for differences in prepulse inhibition of the acoustic startle response (PPI) and sexual behavior. Adenylate cyclase activation assays were then performed on dissected tissue from the nucleus accumbens and caudate-putamen.

RESULTS

Repeated treatment with quinpirole increased pouching behavior and body weight and disrupted PPI. No changes in sexual activity in response to repeated quinpirole were found. Prior quinpirole treatment enhanced D1 dopamine receptor-stimulated adenylate cyclase activity in the caudate-putamen that was blocked by co-incubation with the D1 dopamine antagonist, SCH23390.

CONCLUSIONS

These results show that repeated activation of D2-like receptors in vivo can produce changes in feeding behavior and sensory processing that is associated with sensitization of D1 dopamine receptor-mediated signaling in the caudate-putamen.

Individual differences in estrogen receptor alpha in select brain nuclei are associated with individual differences in aggression

Steroid hormones play an important role in modulating social behavior in many species. Estrogens are thought to act on an interconnected network of hypothalamic and limbic brain areas to affect aggressive behavior, although the specific nuclei unknown remain unspecified. We show that individual variation in estrogen receptor alpha (ERalpha) immunoreactivity in the lateral septum (LS), ventral bed nucleus of the stria terminalis (vBNST), and anterior hypothalamus (AHA) of CD-1 mice is positively correlated with aggressive behavior. When males were treated with fadrozole (an aromatase inhibitor), aggressive behavior was reduced, although castration did not reduce aggression. These results suggest that estrogens modulate aggressive behavior by acting on a circuit that includes the LS, vBNST, and AHA and that the source of estrogens is non-gonadal. Fadrozole also decreased c-fos expression in the lateral septum following aggressive encounters. Although the effects of estrogen on aggression appear to involve regulation of neuronal activity in the LS, additional processes are likely involved. These results suggest that estrogen acts in a specific subset of a complex network of nuclei to affect aggressive behavior.

The bed nucleus of the stria terminalis in the Syrian hamster: subnuclei and connections of the posterior division

The bed nucleus of the stria terminalis is a key part of a ring of cells extending between the centromedial amygdala and bed nucleus of the stria terminalis referred to as the extended amygdala. The present study describes the architecture of the bed nucleus of the stria terminalis and the connections of subnuclei in posterior bed nucleus of the stria terminalis. The hamster bed nucleus of the stria terminalis is readily allotted to anterior and posterior divisions separated by the fibers of the body of the anterior commissure. The anterior division has four subnuclei: anteromedial, anterointermediate, anterolateral, and anteroventral. Within the posterior division, there are three distinct regions: posteromedial, posterointermediate, and posterolateral. In hamsters, the posterior bed nucleus of the stria terminalis contributes to male sexual behavior, particularly chemoinvestigation. Moreover, the posterior bed nucleus of the stria terminalis is part of a neural circuit essential for mating, including the medial amygdaloid nucleus and medial preoptic area. The connections of bed nucleus of the stria terminalis, posteromedial part, bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posterolateral part were visualized by co-injection of anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (cholera toxin B) tract tracers. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have dense bidirectional connections with medial amygdaloid nucleus and cortical amygdala via the stria terminalis and ventral amygdalofugal pathway. These subnuclei also maintain bidirectional connections with steroid-concentrating areas including lateral septum, medial preoptic area, hypothalamus, and periaqueductal gray. The bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part receive projections from the subiculum and send projections to deep mesencephalic nuclei. By contrast, the bed nucleus of the stria terminalis, posterolateral part is connected with the central amygdala, lateral hypothalamus, subthalamic nucleus, nucleus accumbens, substantia innominata, substantia nigra and thalamus. Thus, the bed nucleus of the stria terminalis, posterointermediate part and bed nucleus of the stria terminalis, posteromedial part have similar connections with areas involved in social behaviors. The bed nucleus of the stria terminalis, posterolateral part maintains connections with areas involved in motivational circuits. This supports the concept of distinct circuits within the extended amygdala which differentially link the centromedial amygdala and bed nucleus of the stria terminalis.

Call type-specific differences in vocalization-related afferents to the periaqueductal gray of squirrel monkeys (Saimiri sciureus)

In a recent retrograde tracing study in the squirrel monkey, we found that regions in the midbrain periaqueductal gray (PAG) producing different call types when pharmacologically stimulated, receive their input largely from the same structures. The aim of the present study was to find out, whether there are quantitative differences in this input. For this reason, we counted retrogradely labeled neurons in various brain regions after injections of wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) into three different vocalization-eliciting PAG sites: one site producing non-aversive contact calls (clucking); a second site producing slightly aversive social mobbing calls (cackling); and a third site producing highly aversive defensive threat calls (shrieking). Cell counting was carried out by the help of the optical fractionator technique. Six squirrel monkeys were used, two for each call type. In some regions, marked differences in the number of retrogradely labeled cells between the three call type groups occured. Such regions are the nucl. accumbens, preoptic area, posterior hypothalamus, anterior cingulate cortex, subcallosal gyrus and the nucl. striae terminalis. In some of these regions, the number of retrogradely labeled cells correlated positively (posterior hypothalamus) or negatively (preoptic area, nucl. striae terminalis) with the "aversiveness" of the elicited call type. Other regions of interest, e.g., the dorsomedial prefrontal and precallosal cortex, amygdala and hypothalamic regions surrounding the fornix, revealed no clear differences in their afferent projections to the different vocalization-eliciting PAG sites. The results make clear that distinct vocalization-controlling regions in the PAG receive a qualitatively similar but quantitatively differentiated input.

Differential fos activation in virgin and lactating mice in response to an intruder

Lactating (L) mice display fierce aggression towards novel, male mice, while virgin (V) mice do not. This study compares patterns of brain activation in V and L mice in response to a novel intruder using immunohistochemical detection of Fos (Fos-IR). Animals were sampled 120 min after either a sham or real 10 min test with a male intruder. L mice were aggressive towards intruders, but V mice were not. In general, Fos-IR for both groups increased with exposure to an intruder, with L mice showing higher increases in Fos-IR than V mice. In only medial preoptic nucleus and ventral portion of bed nucleus of stria terminalis (BNST) was Fos-IR significantly increased in both groups with testing. In V mice, testing resulted in Fos-IR increases in an additional 10 regions examined that did not reach significance in L mice, including lateral septum, lateral and medial preoptic areas, and anterior hypothalamus. Fos-IR also increased with testing in nine regions unique to L mice, including the mitral and granular layers of accessory olfactory bulb, regions of the amygdala, dorsal BNST, and caudal portions of the hypothalamic attack area. These increases in Fos-IR with testing suggest alterations in the circuitry governing response to pheromonal cues and imply some commonalities between the circuitries governing maternal aggression and intermale aggression. These results support the hypothesis that pregnancy and lactation induce substantial changes in brain circuitry and function; changes that enable maternal defense of offspring by altering the neural response to an intruder male.

Visualizing sexual dimorphism in the brain

Sexually dimorphic behaviors are likely to involve neural pathways that express the androgen receptor (AR). We have genetically modified the AR locus to visualize dimorphisms in neuronal populations that express AR. Analysis of AR-positive neurons reveals both known dimorphisms in the preoptic area of the hypothalamus and the bed nucleus of the stria terminalis as well as novel dimorphic islands in the basal forebrain with a clarity unencumbered by the vast population of AR-negative neurons. This genetic approach allows the visualization of dimorphic subpopulations of AR-positive neurons along with their projections and may ultimately permit an association between neural circuits and specific dimorphic behaviors.

c-fos Changes following an aggressive encounter in female California mice: a synthesis of behavior, hormone changes and neural activity

Although there has been growing interest in the neuroanatomical and physiological mechanisms underlying aggressive behavior, little work has focused on possible mechanisms controlling natural plasticity in aggression. In the current study, we used naturally occurring changes in aggression level displayed by female Peromyscus californicus across the estrous cycle and parallel changes in c-fos expression to examine possible brain regions involved in mediating this plasticity. We found that c-fos expression was increased in females exposed to a conspecific female intruder compared with control females in numerous brain regions thought to be involved in the control of aggression. More importantly, we found that c-fos increased in the bed nucleus of the stria terminalis (BNST) and ventral lateral septum (LSv) only in the more aggressive, diestrous females, and not in the less aggressive, proestrous and estrous females. Conversely, c-fos increased in the medial amygdala (MeA) across all stages of estrus compared with controls, suggesting the MeA is not involved in mediating changes in individual levels of aggression. Moreover, we found correlations between several measures of aggression and c-fos expression in the BNST and LSv but not the MeA, again suggesting a role in mediating aggression plasticity for the former two but not the latter brain region. We further hypothesize that the BNST and the LSv may be involved more generally in mediating natural changes in aggression, such as increases often observed after individuals win aggressive interactions against conspecifics.

Reproductive activation of pine voles (Microtus pinetorum): examination of physiological markers

We tested the hypothesis that the presence of an opposite-sex conspecific will result in time-related changes in measures of reproductive activation. We housed male-female pairs of pine voles together for 0, 2, 6, 12, or 24 h before collecting blood, reproductive organs and brains for immunocytochemical analysis of LHRH and c-fos. Control animals were never exposed to an opposite-sex conspecific. Following exposure to a male, there was a significant increase in uterine weight but not in LH levels. In males, there were no changes in peripheral indices of activation, i.e. LH levels, testes and seminal vesicle weights were not altered. Consistent with no change in circulating levels of LH, there was no change in LHRH immunoreactivity at any time. However, c-fos immunoreactivity was significantly greater in both males and females in the cingulate cortex and rostral bed nucleus of the stria terminalis (BNST) at 2 h, and in the caudal BNST at 2, 6 and 12 h. Similarly, c-fos immunoreactivity was increased in the rostral MPOA in both males and females at 2 and 6 h. However, in the caudal MPOA, there was a significant interaction between sex and time due to increased c-fos immunoreactivity in females only at 6 h. These results indicate that, in both male and female pine voles, exposure to an opposite-sex conspecific is sufficient to produce rapid, neural activation in brain areas known to be involved in reproductive activation and sexual behavior. This early activation did not occur in LHRH neurons. It is not known if this activation, particularly at early times, is due to reproductive activation or to the formation of pair bonds.

Patterns of neural activation associated with exposure to odors from a familiar winner in male golden hamsters

The neural mechanisms underlying recognition of familiar individuals and responses appropriate to them are not well known. Previous studies with male golden hamsters have shown that, after a series of brief aggressive encounters, a loser selectively avoids his own, familiar winner but does not avoid other males. Using this paradigm, we investigated activity in 20 areas of the brain using immunohistochemistry for c-Fos and Egr-1 during exposure to a familiar winner compared to control groups not exposed to another male. Behavioral data showed that 1 day after fights males that lost avoided the familiar winner, suggesting that they recognized this individual. The c-Fos and Egr-1 immunohistochemistry showed that the losers exposed to familiar winners had a greater density of stained cells in the basolateral amygdala, the CA1 region of anterior dorsal hippocampus and the dorsal subiculum than control groups had in these areas. These results suggest that these brain areas may be involved in the memory for other males, the learned fear of familiar winners, or related processes.

Sexual experience alters D1 receptor-mediated cyclic AMP production in the nucleus accumbens of female Syrian hamsters

Drugs of abuse produce long-term changes in dopamine neurotransmission and receptor-effected intracellular signaling. Similar changes in neuronal activity are mediated by motivated behaviors. To explore cellular mechanisms underlying these neuroadaptations following sexual experience, cyclic AMP accumulation following stimulation of D1 dopamine receptors, G-proteins, and adenylate cyclase was compared in the nucleus accumbens and caudate nucleus of sexually naive and experienced female hamsters following sexual behavior. Direct stimulation of adenylate cyclase with forskolin or indirectly by activation of G-proteins with Gpp(NH)p produced dose-dependent increases in the formation of cyclic AMP in the nucleus accumbens and caudate nucleus, with no effects of sexual experience on these measures. Specific D1 receptor stimulation increased Gpp(NH)p-induced adenylate cyclase activity in the nucleus accumbens and caudate nucleus of all animals. Interestingly, this stimulation was further enhanced only in membranes from the nucleus accumbens, but not from the caudate nucleus, of sexually experienced hamsters compared to the response of naive females. These results demonstrate that sexual behavior experience can sensitize mesolimbic dopamine pathways and that this sensitization occurs through an increase in D1 receptor-mediated signaling.

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

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

Representations of motivational drives in mesial cortex, medial thalamus, hypothalamus and midbrain

We propose that neural representations of motivational drives, including sexual desire, hunger, thirst, fear, power-dominance, the motivational aspect of pain, the need for sleep, and nurturance, are represented in four areas in the brain. These are located in the medial hypothalamic/preoptic area, the periaqueductal gray matter (PAG) in the midbrain/pons, the midline and intralaminar thalamic nuclei, and in the anterior part of the mesial cortex, including the medial prefrontal and anterior cingulate areas. We attempt to determine the locations of each of these representations within the hypothalamus/preoptic area, periaqueductal gray and cortex, based on the available literature on activation of brain structures by stimuli that evoke these forms of motivation, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on hodological data. We discuss the hierarchical organization of the representations for a given drive, outputs from these representations to premotor structures in the medulla, caudate-putamen, and cortex, and their contributions to involuntary, learned-sequential (operant) and voluntary behaviors.

Oxytocin inhibits aggression in female Syrian hamsters

Dominant subordinate relationships are formed as the result of social conflict and are maintained at least in part by communication. At this time, little is known about the neural mechanisms that are responsible for coordinating the social behaviours (e.g. aggression) that occur in association with the formation and maintenance of these relationships. The purpose of the present study was to investigate the role of oxytocin (OXT) within the medial preoptic anterior hypothalamic continuum (MPOA-AH) in the control of aggression in female hamsters. OXT injected into the MPOA-AH immediately before testing significantly reduced the duration of aggression in a dose-dependent manner. Injection of an OXT antagonist 30 min before testing significantly increased the duration of aggression. In contrast, the duration of aggression was not altered when hamsters were tested either 30 min after injection of OXT or immediately following injection of an OXT-antagonist. These data support the hypothesis that OXT release within the MPOA-AH regulates social behaviours important in the formation and maintenance of dominant subordinate relationships in female hamsters.

Expression of c-fos gene activation during rough and tumble play in juvenile rats

Rough and tumble (R&T) play is an intrinsic behavior in most mammals. However, unlike sex and aggression, play has not been well characterized in terms of neuronal circuitry. We employed in situ hybridization to explore the differences of c-fos mRNA activation in juvenile rats that had been allowed R&T play for a total of 30 min before sacrifice contrasted to animals with comparable histories that had received no play. Densitometric estimates of c-fos gene activation revealed that the deep and dorsolateral tectum, inferior colliculus, dorsal periaquaductal gray, ventromedial hypothalamus, dorsal and ventral striatum, and somatosensory cortex were significantly more activated in animals that had played than those that had not. Prior play dominance and amount of social experience had no clear effects on the levels of c-fos gene expression. This provides a variety of new hypotheses concerning the role of various brain areas in the elaboration of R&T play behavior, but the important role of other types of motor arousal in the differential effects were not evaluated in this study.

Neural background of glucocorticoid dysfunction-induced abnormal aggression in rats: involvement of fear- and stress-related structures

Glucocorticoid hypofunction is associated with persistent aggression in some psychologically disordered human subjects and, as reported recently, induces abnormal forms of aggression in rats. Here we report on the effects of glucocorticoid hypofunction on aggression-induced neural activation. Rats were adrenalectomized, and implanted with low-release glucocorticoid pellets. After one week recovery, they were challenged by an unfamiliar intruder in their home-cage. Neural activation was studied by c-Fos protein immunocytochemistry. Aggressive encounters in controls induced c-Fos activation in all brain areas relevant for the control of aggression (cortex, amygdala, septum, hypothalamus, periaqueductal grey and the locus coeruleus). Very intense c-Fos activation was observed in the medial amygdala, the hypothalamic attack area and the periaqueductal grey matter which constitute a downward stimulatory stream that activates attack behaviour. The experimentally induced glucocorticoid hypofunction dramatically increased attacks targeted towards vulnerable parts of the opponent's body (mainly the head). This abnormal behaviour was not associated with changes in the activation of brain centres involved in the control of aggression. However, the activation of brain centres involved in both the stress response (the parvocellular part of the hypothalamic paraventricular nucleus) and fear reactions (central amygdala) were markedly increased. An acute glucocorticoid treatment abolished both behavioural and neural consequences of glucocorticoid hypofunction. Our data suggest that glucocorticoid hypofunction-induced abnormal forms of aggressiveness are related to increased sensitivity to stressors and fear-eliciting stimuli. This assumption is supported by the finding that fearful situations induce attack patterns in intact rats that are similar to those induced by glucocorticoid hypofunction.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

Role of the archistriatal nucleus taeniae in the sexual behavior of male Japanese quail (Coturnix japonica): a comparison of function with the medial nucleus of the amygdala in mammals

Nucleus taeniae (Tn) is a prominent cell group within the medial archistriatum of birds. Based upon similarities in sex-steroid binding sites, this nucleus has been hypothesized to be homologous to the medial nucleus of the amygdala (Me) in mammals, which is known to modulate the expression of sexual behavior in rodents. We therefore tested whether or not Tn likewise plays a role in the expression of sexual behavior in male Japanese quail. We found that bilateral damage to Tn produced deficits in several components of male responses toward female stimuli that were indicative of decreased sexual arousal, including goal-oriented responses, vocalizations associated with courtship, and motor reflexes that precede copulation. Our results suggest that Tn influences a wide range of behavioral functions in response to sexual stimuli, and they indicate a function for this nucleus similar to that subserved by the Me in mammals. These results strengthen the argument that these sex-steroid accumulating cell groups are homologous and suggest a conservation of function for them despite the vastly divergent evolutionary histories separating birds and mammals.

Fos expression in female hamsters after various stimuli associated with mating

Detection of the expression of c-fos mRNA or its protein product, Fos, has been used to indicate differences in neuronal response to exogenous stimuli. Factors contributing to differences in Fos expression as a result of various stimuli associated with mating have been extensively studied in the female rat. Less is known about the factors that contribute to Fos expression in female hamsters. Female hamsters differ from female rats in several aspects of sexual behavior; therefore, it seems likely that Fos expression may also differ. The purpose of this study was to determine which factors associated with mating selectively affect Fos expression in the female hamster. Animals were ovariectomized, hormone treated, and then exposed to several behavioral conditions. Fos expression in several brain areas was then assessed via immunocytochemistry (ICC). As has been found by others, mating increases Fos immunoreactivity in a number of brain regions. Specifically, vaginal-cervical stimulation (VCS) was determined to be the salient factor contributing to Fos expression in the preoptic area (POA) and bed nucleus of the stria terminalis (BNST) of ovariectomized hormone primed female hamsters that received a mating interaction.

Mating modifies c-fos expression in the brain of male and female rabbits

Copulation in rabbits provokes behavioral and neuroendocrine changes in both sexes. To investigate if the activity of particular brain regions is modified accordingly we quantified, by the reverse transcription-polymerase chain reaction method, c-fos expression in the preoptic area, hypothalamus, hippocampus, and frontal cortex of male and female rabbits before mating, immediately afterwards, and 1 h later. Mating immediately increased c-fos expression in the hypothalamus of both sexes, the frontal cortex of females, and the preoptic area of males. c-fos expression did not change in the hippocampus after mating in either sex but decreased in the preoptic area of females following mating. Results show that mating provokes changes in brain activity, in a gender- and region-specific manner, which may underlie the behavioral and endocrine consequences of copulation in rabbits.

Effects of sexual interactions with a male on fos-like immunoreactivity in the female quail brain

Sexual interactions can cause changes in plasma hormone levels and activate immediate early genes within the mammalian brain. There are marked anatomical differences between the regions activated that relate directly to the sexual specific behaviour and neuroendocrinology of each sex. The aim of this study was to determine if such a sexual dimorphism exists in birds by examining the brain regions stimulated in adult virgin female Japanese quail (Coturnix japonica) during sexual behaviour and comparing this to previously reported data concerning males. Female quail were allowed to freely interact with adult males and both female and male sexual behaviour was recorded. Contrary to previous findings in male quail, no significant induction of Fos-like immunoreactive (FLI) cells was observed following sexual interactions in the preoptic area of females; this area is fundamentally involved in the control of male-type copulatory behaviour. Sexual interactions significantly induced FLI cells in the hyperstriatum ventrale, the part of the archistriatum just lateral to the anterior commissure, and the nucleus intercollicularis. Moreover, prominent activation was detected throughout most of the ventromedial nucleus of the hypothalamus, a region reported to be rich in oestrogen receptors. FLI induction was not a consequence of sexual behaviour induced changes in luteinizing hormone (LH) as plasma LH levels were unaltered. Instead, brain activation must be a consequence of copulation-associated somatosensory inputs or direct stimuli originating from the male. Male quail, like the majority of other birds, lack an intromittant organ (penis) so that the somatosensory inputs to the female are rather different from those in mammals; the precise nature of these inputs is yet to be determined.

The medial extended amygdala in male reproductive behavior. A node in the mammalian social behavior network

Hormonal and chemosensory signals regulate social behaviors in a wide variety of mammals. In the male Syrian hamster, these signals are integrated in nuclei of the medial extended amygdala, where olfactory and vomeronasal system transmission is modulated by populations of androgen- and estrogen-sensitive neurons. Evidence from behavioral changes following lesions and from immediate early gene expression supports the hypothesis that the medial extended amygdala and medial preoptic area belong to a circuit that functions selectively in male sexual behavior. However, accumulated behavioral, neuroanatomical, and neuroendocrine data in hamsters, other rodents, and other mammals indicate that this circuit is embedded in a larger integrated network that controls not only male mating behavior, but female sexual behavior, parental behavior, and various forms of aggression. In this context, perhaps an individual animal's social responses can be more easily understood as a repertoire of closely interrelated, hormone-regulated behaviors, shaped by development and experience and modulated acutely by the environmental signals and the hormonal milieu of the brain.

Effects of a D1 antagonist and of sexual experience on copulation-induced Fos-like immunoreactivity in the medial preoptic nucleus

The medial preoptic nucleus (MPN) of the medial preoptic area (MPOA) and the medial amygdala are two brain regions in which male rat sexual behavior increased Fos-like immunoreactivity (Fos-Li). Dopamine is released in the MPOA during male rat sexual behavior and facilitates copulation. Psychostimulants, which increase dopamine levels, induce Fos-Li in the striatum through D1 receptors. We examined whether copulation-induced Fos-Li in the MPN was also mediated through D1 receptors. In Experiment 1, sexually inexperienced male rats that received the D1 antagonist Schering 39166 prior to their first sexual experience had fewer Fos-Li cells in the MPN than did those that received vehicle. In Experiment 2, no significant effect of the D1 antagonist was observed on copulation-induced Fos-Li in male rats that had received repeated sexual experiences prior to the drug test day. Sexual experience increases copulatory efficiency; the mechanisms by which this improvement occurs are unclear. In Experiment 3, copulation by highly experienced male rats led to greater Fos-Li in the MPN than did copulation by sexually naive males. Although there were no differences between groups in amygdala Fos-Li in these studies, in several groups Fos-Li in the medial amygdala was positively correlated with the post-ejaculatory interval. These experiments indicate that (1) stimulation of D1 receptors may contribute to the transient copulation-induced increase in Fos-Li in the MPN, and (2) repeated sexual experiences enhanced copulation-induced Fos-Li in the MPN, which may represent a marker of altered responsiveness of neurons in the MPN to sexual or conditioned stimuli.

Protein synthesis in the medial preoptic area is important for the mating-induced decrease in estrus duration in hamsters

Sexual receptivity in female hamsters potentially lasts for about 16 h. However, vaginal cervical stimulation (VCS) from a male during mating eventually reduces receptivity and can shorten the duration of behavioral estrus. The process by which this change in response to the male takes place is unknown. Recently, detection of the Fos protein has indicated that the medial preoptic area (POA) is one of the brain regions particularly responsive to VCS. Additionally, the POA may have an inhibitory effect on sexual receptivity. To determine if protein synthesis in the POA is required to initiate the VCS-induced decrease in estrus duration, a protein synthesis inhibitor (anisomycin, 0.50 microg) or a control substance (cholesterol) was applied bilaterally to the POA of steroid-primed ovariectomized female hamsters. Females were tested with a sexually active male at five time points following the initial test for sexual receptivity (hour 1, 2, 6, 12, and 24). Half of the females tested were allowed to receive VCS from a male, while half were fitted with vaginal masks to prevent penile intromission. Each group receiving VCS showed a significant decrease in lordosis duration evident between hour 2 and hour 6, except the group which received anisomycin in the POA. In this respect the POA anisomycin group was similar to animals which did not receive VCS. Hamsters with vaginal masks and the anisomycin/POA animals allowed to receive VCS exhibited their first decrease in lordosis duration between hour 6 and hour 12. These results indicate that protein synthesis is important for VCS-induced decrease in estrus duration in the POA.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Effect of photoperiod on neural estrogen and progestin receptor immunoreactivity in female Syrian hamsters

This study explored the possibility that reduced behavioral responsiveness to estradiol and progesterone in female Syrian hamsters exposed to a short photoperiod is associated with a reduction in the concentration of neural steroid receptors. The effects of long and short photoperiod (LP; SP) exposure on steroid receptor immunoreactivity were examined in the ventromedial hypothalamus (VMH), medial tuberal region (mTu), medial preoptic area (mPOA), medial nucleus of the amygdala (mAMYG), and the arcuate nucleus (ARC) of ovariectomized hamsters. In Experiment 1, exposure to SP for ten weeks attenuated the lordosis response following sequential treatment with estradiol and progesterone. In a separate group of animals not given hormones, SP decreased the staining intensity of estrogen receptor immunoreactive (ERIR) cells in the mPOA while increasing the number of detectable ERIR cells in part of the mAMYG. In Experiment 2, SP diminished the lordosis response as it did in Experiment 1. One week later, the same females were administered estradiol systemically to induce progestin receptors (PR). Animals housed in SP showed significantly reduced progestin receptor immunoreactivity (PRIR) in the VMH, mTu, mPOA, mAMYG, and ARC. Experiment 3 examined whether the results of Experiment 2 might have been influenced by photoperiodic effects on peripheral metabolism of estradiol. Among hamsters housed in LP or SP, PRs were induced by estradiol implanted unilaterally in the medial basal hypothalamus, thus bypassing possible photoperiodic effects on peripheral estradiol availability. This treatment resulted in significantly fewer cells with detectable PRIR in the VMH and mPOA of SP females, suggesting that the photoperiodic influences on PR induction observed in Experiment 2 do not depend on alterations in the peripheral availability of estradiol.

Social stress in hamsters: defeat activates specific neurocircuits within the brain

During an agonistic encounter, subordinate male hamsters display defensive and submissive postures and show increased secretion of glucocorticoids, whereas dominant males do not. To determine whether specific neuronal pathways are activated during the behavioral and neuroendocrine responses of subordinate males, expression of c-fos mRNA within the brains of subordinate males was compared with the pattern in dominant males after fighting. After 1 week of handling, pairs of hamsters were either swapped between cages (handled control males), or were allowed to interact for 30 min [dominant (DOM) males and subordinate (SUB) males]. A second group of control animals that received no handling or social stimulation (unhandled control males) were also included. After testing, all animals were killed by decapitation, their brains were removed for c-fos in situ hybridization, and trunk blood was collected for analysis of plasma cortisol and corticosterone levels. Exposure of males to their partner's cage for 30 min resulted in increased expression of c-fos mRNA in multiple brain regions. In addition, fighting increased c-fos expression in the medial amygdaloid nucleus of both DOM and SUB males as well as having more selective effects. In DOM males, c-fos expression was elevated within the supraoptic nucleus of the hypothalamus. In SUB males, c-fos expression increased within a multitude of brain areas, including cingulate cortex, lateral septum, bed nucleus of the stria terminalis, medial preoptic area, several hypothalamic nuclei, central amygdaloid nucleus, amygdalohippocampal area, dorsal periaqueductal gray, dorsal raphe, cuneiform nucleus, and locus coeruleus. These findings are discussed in relation to neurocircuits associated with behavioral arousal and stress.

Intromissive stimulation from the male increases extracellular dopamine release from fluoro-gold-identified neurons within the midbrain of female hamsters

Extracellular concentrations of dopamine in the nucleus accumbens were monitored using microdialysis in ovariectomized female Syrian hamsters hormonally primed with estradiol and progesterone or with a similar regimen of oil injections. Some females in each of these groups had their vaginas occluded with tape, whereas the remaining females' vaginas stayed unoccluded. When exposed to a male, both groups of hormonally primed females showed high levels of lordosis. However, only in the hormone-primed, unoccluded females were there significant elevations of dialysate dopamine during the sexual interactions with the male. There were no significant elevations in dopamine levels in the oil-treated females during interactions with the male. These data suggest that nucleus accumbens dopamine is responsive to stimuli associated with the vaginocervical stimulation received by the female during intromissions by the male. Histological analyses were based on Fluoro-Gold efflux through the probes combined with immunocytochemistry for tyrosine hydroxylase. Probe placements in the rostral accumbens, caudal accumbens, or rostral bed nucleus of the stria terminalis were not distinguishable based on analyses of basal dopamine levels, volume of Fluoro-Gold injection sites, or Fluoro-Gold labeling of midbrain, tyrosine hydroxylase-stained neurons. The number of midbrain neurons containing Fluoro-Gold was positively related to basal dopamine levels, indicating that the amount of dopamine recovered from the nucleus accumbens in microdialysis studies is a function of the number of neurons contributing to the terminal field in the region of the probe.

Implications of immediate-early gene induction in the brain following sexual stimulation of female and male rodents

Induction of immediate-early genes (IEGs), such as c-fos, has been widely used to mark the activation of brain regions following different types of sexual stimulation and behavior. A relatively common set of hormone-concentrating basal forebrain and midbrain structures in female and male rodents is activated by copulatory stimulation, in particular, stimulation of sensory nerves that innervate the penis or vagina/cervix, olfactory or pheromonal stimuli, and conditioned sexual incentives. These regions include the preoptic area, lateral septum, bed nucleus of the stria terminalis, paraventricular hypothalamus, ventromedial hypothalamus, medial amygdala, ventral premammillary nuclei, ventral tegmentum, central tegmental field, mesencephalic central gray, and peripeduncular nuclei. Regions that do not contain classic intracellular steroid receptors, such as the ventral and dorsal striatum or cortex, are also activated. IEGs have also been colocalized with cytoplasmic proteins like GnRH and oxytocin, and have been used in conjunction with retrograde tracers to reveal functional pathways associated with different sexual behaviors. Steroid hormones can also alter the ability of sexual stimulation to induce IEGs. Despite the many similarities, some differences in IEG induction between sexes have also been found. We review these findings and raise the question of what IEG induction in the brain actually means for sexual behavior, that is, whether it indicates the perception of sexual stimulation, commands for motor output, or the stimulation of a future behavioral or neuroendocrine event related to the consequences of sexual stimulation. To understand the role of a particular activated region, the behavioral or neuroendocrine effects of lesions, electrical stimulation, drug or hormone infusions, must also be known.