Serotonin type 3 receptors modulate the aggression-stimulating effects of adolescent cocaine exposure in Syrian hamsters (Mesocricetus auratus)

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Contemporary Pharmacotherapeutics and the Management of Aggressive Behavior in an Adolescent Animal Model of Maladaptive Aggression

Objective

Antipsychotic and anticonvulsant medications are increasingly being used as pharmacotherapeutic treatments for maladaptive aggression in youth, yet no information is available regarding whether these drugs exhibit aggression- specific suppression in preclinical studies employing adolescent animal models of maladaptive aggression. This study examined whether the commonly used antipsychotics medications haloperidol and risperidone and the anticonvulsant medication valproate exert selective aggression-suppressing effects using a validated adolescent animal model of maladaptive aggression.

Methods

Twenty-seven-day old Syrian hamsters (Mesocricetus auratus) were administered testosterone for 30 consecutive days during the first 4 weeks of adolescent development. The following day (during late adolescence), experimental animals received a single dose of haloperidol (0.00, 0.025, 0.50, 1.0 mg/kg), risperidone (0.00, 0.01, 0.03, 1.0 mg/kg), or valproate (0.00, 1.0, 5.0, 10.0 mg/kg) and tested for offensive aggression using the Resident/Intruder Paradigm. As a baseline, non-aggressive behavioral control, a separate set of pubertal hamsters was treated with sesame oil vehicle during the first 4 weeks of adolescence and then tested for aggression during late adolescence in parallel with the haloperidol, risperidone or valproate-treated experimental animals.

Results

Risperidone and valproate selectively reduced the highly impulsive and intense maladaptive aggressive phenotype in a dose-dependent fashion. While haloperidol marginally reduced aggressive responding, its effects were non-specific as the decrease in aggression was concurrent with reductions in a several ancillary (non-aggressive) behaviors.

Conclusion

These studies provide pre-clinical evidence that the contemporary pharmacotherapeutics risperidone and valproate exert specific aggression-suppressing effects in an adolescent animal model of maladaptive aggression.

Serotonin type-3 receptors differentially modulate anxiety and aggression during withdrawal from adolescent anabolic steroid exposure

Male Syrian hamsters (Mesocricetus auratus) administered anabolic/androgenic steroids during adolescent development display increased aggression and decreased anxious behavior during the adolescent exposure period. Upon withdrawal from anabolic/androgenic steroids, this neurobehavioral relationship shifts and hamsters exhibit decreased aggression and increased anxious behavior. This study investigated the hypothesis that alterations in anterior hypothalamic signaling through serotonin type-3 receptors modulate the behavioral shift between adolescent anabolic/androgenic steroid-induced aggressive and anxious behaviors during the withdrawal period. To test this, hamsters were administered anabolic/androgenic steroids during adolescence then withdrawn from drug exposure for 21 days and tested for aggressive and anxious behaviors following direct pharmacological manipulation of serotonin type-3 receptor signaling within the latero-anterior hypothalamus. Blockade of latero-anterior hypothalamic serotonin type-3 receptors both increased aggression and decreased anxious behavior in steroid-treated hamsters, effectively reversing the pattern of behavioral responding normally observed during anabolic/androgenic steroid withdrawal. These findings suggest that the state of serotonin neural signaling within the latero-anterior hypothalamus plays an important role in behavioral shifting between aggressive and anxious behaviors following adolescent exposure to anabolic/androgenic steroids.

The neurobiology of aggression and violence

Aggression and violence represent a significant public health concern and a clinical challenge for the mental healthcare provider. A great deal has been revealed regarding the neurobiology of violence and aggression, and an integration of this body of knowledge will ultimately serve to advance clinical diagnostics and therapeutic interventions. We will review here the latest findings regarding the neurobiology of aggression and violence. First, we will introduce the construct of aggression, with a focus on issues related to its heterogeneity, as well as the importance of refining the aggression phenotype in order to reduce pathophysiologic variability. Next we will examine the neuroanatomy of aggression and violence, focusing on regional volumes, functional studies, and interregional connectivity. Significant emphasis will be on the amygdala, as well as amygdala-frontal circuitry. Then we will turn our attention to the neurochemistry and molecular genetics of aggression and violence, examining the extensive findings on the serotonergic system, as well as the growing literature on the dopaminergic and vasopressinergic systems. We will also address the contribution of steroid hormones, namely, cortisol and testosterone. Finally, we will summarize these findings with a focus on reconciling inconsistencies and potential clinical implications; and, then we will suggest areas of focus for future directions in the field.

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

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

Aggression and anxiety in adolescent AAS-treated hamsters: A role for 5HT3 receptors

Previously, we have shown that anabolic androgenic steroid (AAS) exposure throughout adolescence stimulates offensive aggression while also reducing anxious behaviors during the exposure period. Interestingly, AAS exposure through development correlates with alterations to the serotonin system in regions known to contain 5HT3 receptors that influence the control of both aggression and anxiety. Despite these effects, little is known about whether these separate developmental AAS-induced behavioral alterations occur as a function of a common neuroanatomical locus. To begin to address this question, we localized 5HT3 receptors in regions that have been implicated in aggression and anxiety. To examine the impact these receptors may have on AAS alterations to behavior, we microinjected the 5HT3 agonist mCPBG directly into a region know for its influence over aggressive behavior, the lateral division of the anterior hypothalamus, and recorded alterations to anxious behaviors using the elevated plus maze. AAS exposure primarily reduced the presence of 5HT3 receptors in aggression/anxiety regions. Accordingly, mCPBG blocked the anxiolytic effects of adolescent AAS exposure. These data suggest that the 5HT3 receptor plays a critical role in the circuit modulating developmental AAS-induced changes to both aggressive and anxious behaviors, and further implicates the lateral division of the anterior hypothalamus as an important center for the negative behavioral effects of developmental AAS-exposure.

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Aggression-reducing effects of F15599, a novel selective 5-HT1A receptor agonist, after microinjection into the ventral orbital prefrontal cortex, but not in infralimbic cortex in male mice

BACKGROUND

The 5-HT1A receptor subtype has been postulated to modulate aggressive behavior particularly when it is excessive. F15599 is a high affinity and selective 5-HT1A receptor agonist that exhibits biased agonism for postsynaptic receptors that are preferentially coupled to Gαi3 protein subunits, with more potent action in the cortex, and with potential for selectively reducing aggression.

OBJECTIVES AND METHODS

The aims of the current study were to investigate the anti-aggressive effects of the novel 5-HT1A receptor agonist, F15599, microinjected into the ventral orbital prefrontal cortex (VO PFC) and into the infralimbic cortex (ILC) of CF-1 male mice that had been previously socially provoked and to confirm the specific action at this receptor by blocking its effects using the 5-HT1A receptor antagonist, WAY100,635.

RESULTS

Microinjection of the lower doses of F15599 (0.03 and 0.1 μg) into the VO PFC, but not into the ILC, significantly reduced the frequency of attack bites and sideways threats, without affecting other elements of the behavioral repertoire related to aggression such as pursuing and sniffing the intruder and tail rattle. There were also no changes observed in the duration of walking and rearing. Pretreatment with WAY100,635 prevented the anti-aggressive effects of F15599 when microinjected into VO PFC.

CONCLUSIONS

The present results demonstrated that F15599 is effective in reducing the most intense behavioral elements of aggressive behavior in male mice, when microinjected into the VO PFC, but not into the ILC, without affecting nonaggressive behavior, and confirmed the critical role of this cortical region and specifically the 5-HT1A heteroreceptors in the modulation of escalated aggressive behavior.

Adolescent anabolic/androgenic steroids: Aggression and anxiety during exposure predict behavioral responding during withdrawal in Syrian hamsters (Mesocricetus auratus)

In the U.S. and worldwide anabolic/androgenic steroid use remains high in the adolescent population. This is concerning given that anabolic/androgenic steroid use is associated with a higher incidence of aggressive behavior during exposure and anxiety during withdrawal. This study uses pubertal Syrian hamsters (Mesocricetus auratus) to investigate the hypothesis that an inverse behavioral relationship exists between anabolic/androgenic steroid-induced aggression and anxiety across adolescent exposure and withdrawal. In the first experiment, we examined aggression and anxiety during adolescent anabolic/androgenic steroid exposure and withdrawal. Adolescent anabolic/androgenic steroid administration produced significant increases in aggression and decreases in anxiety during the exposure period followed by significant decreases in aggression and increases in anxiety during anabolic/androgenic steroid withdrawal. In a second experiment, anabolic/androgenic steroid exposed animals were separated into groups based on their aggressive response during the exposure period and then tested for anxiety during exposure and then for both aggression and anxiety during withdrawal. Data were analyzed using a within-subjects repeated measures predictive analysis. Linear regression analysis revealed that the difference in aggressive responding between the anabolic/androgenic steroid exposure and withdrawal periods was a significant predictor of differences in anxiety for both days of testing. Moreover, the combined data suggest that the decrease in aggressive behavior from exposure to withdrawal predicts an increase in anxiety-like responding within these same animals during this time span. Together these findings indicate that early anabolic/androgenic steroid exposure has potent aggression- and anxiety-eliciting effects and that these behavioral changes occur alongside a predictive relationship that exists between these two behaviors over time.

The role of the serotonergic system at the interface of aggression and suicide

Alterations in serotonin (5-HT) neurochemistry have been implicated in the aetiology of all major neuropsychiatric disorders, ranging from schizophrenia to mood and anxiety-spectrum disorders. This review will focus on the multifaceted implications of 5-HT-ergic dysfunctions in the pathophysiology of aggressive and suicidal behaviours. After a brief overview of the anatomical distribution of the 5-HT-ergic system in the key brain areas that govern aggression and suicidal behaviours, the implication of 5-HT markers (5-HT receptors, transporter as well as synthetic and metabolic enzymes) in these conditions is discussed. In this regard, particular emphasis is placed on the integration of pharmacological and genetic evidence from animal studies with the findings of human experimental and genetic association studies. Traditional views postulated an inverse relationship between 5-HT and aggression and suicidal behaviours; however, ample evidence has shown that this perspective may be overly simplistic, and that such pathological manifestations may reflect alterations in 5-HT homoeostasis due to the interaction of genetic, environmental and gender-related factors, particularly during early critical developmental stages. The development of animal models that may capture the complexity of such interactions promises to afford a powerful tool to elucidate the pathophysiology of impulsive aggression and suicidability, and identify new effective therapies for these conditions.

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.

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.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Paliperidone suppresses the development of the aggressive phenotype in a developmentally sensitive animal model of escalated aggression

RATIONALE

Atypical antipsychotics are commonly prescribed to clinically referred youngsters for treatment of heightened aggressive behavior associated with various psychiatric disorders. Previously, we demonstrated risperidone's anti-aggressive effects using a well-validated animal model of offensive aggression. Paliperidone, the main active metabolite of risperidone, is a potent serotonin-2A and dopamine-2 receptor antagonist with slightly different pharmacodynamic properties compared to risperidone. Given that much of risperidone's therapeutic efficacy is due to its active metabolite, paliperidone may effectively suppress aggression with fewer adverse side effects.

OBJECTIVES

Investigate whether paliperidone administration would reduce heightened aggressive behavior induced by low-dose cocaine exposure in a developmentally sensitive model of offensive aggression.

MATERIALS AND METHODS

Male Syrian hamsters (n = 12/group) were administered an acute dose of paliperidone (0.05, 0.1, 0.2, and 0.3 mg/kg) and then tested for aggressive behavior using the resident-intruder paradigm. To investigate the effects of chronic paliperidone administration, a separate set of animals (n = 12/group) was exposed to repeated paliperidone administration (0.1 mg kg(-1) day(-1)) during different developmental periods and varying lengths of time (1-4 weeks).

RESULTS

Experiment 1 results revealed a dose-dependent decrease in bite and attack behaviors with an effective dose observed at 0.1 mg/kg. In Experiment 2, the maximal reduction in aggressive behavior in response to chronic paliperidone treatment was observed in animals treated during the third week of adolescence, and this reduction occurred without concomitant alterations in non-aggressive behaviors.

CONCLUSIONS

These results support the specific aggression-suppressing properties of paliperidone and the potential use of this compound in the treatment of maladaptive aggression in clinical settings.

Adolescent anabolic-androgenic steroid exposure alters lateral anterior hypothalamic serotonin-2A receptors in aggressive male hamsters

Chronic anabolic-androgenic steroid (AAS) treatment during adolescence facilitates offensive aggression in male Syrian hamsters (Mesocricetus auratus). Serotonin (5-HT) modulates aggressive behavior and has been shown to be altered after chronic treatment with AAS. Furthermore, 5-HT type 2 receptors have been implicated in the control of aggression. For example, treatment with 5-HT(2A) receptor antagonists suppress the generation of the offensive aggressive phenotype. However, it is unclear whether these receptors are sensitive to adolescent AAS exposure. The current study assessed whether treatment with AAS throughout adolescence influenced the immunohistochemical localization of 5-HT(2A) in areas of the hamster brain implicated in the control of aggression. Hamsters were administered AAS (5.0 mg/kg) each day throughout adolescence, scored for offensive aggression, and then examined for differences in 5-HT(2A)-immunoreactivity (5-HT(2A)-ir). When compared with non-aggressive oil-treated controls, aggressive AAS-treated hamsters showed significant increases in 5-HT(2A)-ir fibers in the lateral portion of the anterior hypothalamus (LAH). Further analysis revealed that AAS treatment also produced a significant increase in the number of cells expressing 5-HT(2A)-ir in the LAH. Together, these results support a role for altered 5-HT(2A) expression and further implicate the LAH as a central brain region important in the control of adolescent AAS-induced offensive aggression.

Repeated risperidone administration during puberty prevents the generation of the aggressive phenotype in a developmentally immature animal model of escalated aggression

Risperidone has been shown to be clinically effective for the treatment of aggressive behavior in children, yet until recently no information was available regarding whether risperidone exhibits aggression-specific suppression in preclinical studies employing validated developmentally immature animal models of escalated aggression. Recently, using a pharmacologic animal model of escalated offensive aggression, we reported that acute risperidone treatment selectively and dose-dependently reduces the expression of the adult aggressive phenotype, with a significant reduction in aggressive responses observed at 0.1 mg/kg, i.e., a dose within the range administered to children and adolescents in the clinical setting. This study examined whether repeated exposure to risperidone during puberty would prevent the generation of the highly escalated aggressive phenotype in this animal model. To test this hypothesis, the aggression-eliciting stimulus (i.e., cocaine hydrochloride, 0.5 mg/kg/dayx28 days) was co-administered with an aggression-suppressing dose of risperidone (i.e., 0.1 mg/kg/day) during different time frames of puberty and for varied lengths of time (i.e., 1-4 weeks), and then animals were scored for targeted measures of offensive aggression during late puberty. Risperidone administration prevented the generation of the adult aggressive phenotype, with a complete blockade of matured offensive responses (i.e., lateral attacks and flank/rump bites) seen only after prolonged periods of exposure to risperidone (i.e., 3-4 weeks). The selective prevention of these aggressive responses, while leaving other measures of aggression intact (e.g., upright offensive postures), suggest that risperidone is acting in a highly discriminatory anti-aggressive fashion, targeting neurobehavioral elements important for the mature aggressive response pattern.

Anabolic androgenic steroids differentially affect social behaviors in adolescent and adult male Syrian hamsters

Anabolic androgenic steroids (AAS) are synthetic derivatives of testosterone used by over half a million adolescents in the United States for their tissue-building potency and performance-enhancing effects. AAS also affect behavior, including reports of heightened aggression and changes in sexual libido. The expression of sexual and aggressive behaviors is a function of complex interactions among hormones, social context, and the brain, which is extensively remodeled during adolescence. Thus, AAS may have different consequences on behavior during adolescence and adulthood. Using a rodent model, these studies directly compared the effects of AAS on the expression of male sexual and aggressive behaviors in adolescents and adults. Male Syrian hamsters were injected daily for 14 days with either vehicle or an AAS cocktail containing testosterone cypionate (2 mg/kg), nandrolone decanoate (2 mg/kg), and boldenone undecylenate (1 mg/kg), either during adolescence (27-41 days of age) or in adulthood (63-77 days of age). The day after the last injection, males were tested for either sexual behavior with a receptive female or agonistic behavior with a male intruder. Adolescent males treated with AAS showed significant increases in sexual and aggressive behaviors relative to vehicle-treated adolescents. In contrast, AAS-treated adults showed significantly lower levels of sexual behavior compared with vehicle-treated adults and did not show heightened aggression. Thus, adolescents, but not adults, displayed significantly higher behavioral responses to AAS, suggesting that the still-developing adolescent brain is more vulnerable than the adult brain to the adverse consequences of AAS on the nervous system and behavior.

Adolescent maturation of cocaine-sensitive neural mechanisms

Both clinical and animal studies have shown that adolescents undergo a late maturation of the central nervous system, which may underlie adolescent typical behaviors. In particular, decreased behavioral response to cocaine has been found in adolescents as compared to adults. In the present study, cocaine was used as a tool to explore adolescent brain maturation. Juvenile (postnatal day (P) 27), adolescent (P37), and adult (P90) male Sprague-Dawley rats were treated acutely with cocaine (750 microg/kg/injection x 2, i.v.), and c-fos mRNA expression, a marker of neuronal activation, was evaluated by in situ hybridization. Cocaine-induced c-fos mRNA was similar across ages in the dorsal caudate putamen (CPu), nucleus accumbens, and lateral bed nucleus of the stria terminalis. In contrast, there was a diminished response in juvenile/adolescent ventral CPu and in juvenile central nucleus of the amygdala, and an increased response in juvenile/adolescent cortex. Further studies evaluated the mechanism of the late maturation of cocaine response in ventral CPu. No significant age differences were observed in regional dopamine (DA) transporter binding. Although striatal DA content was significantly reduced at P27 as compared to adult, there was no difference between dorsal and ventral subregions. In contrast, basal- and cocaine-induced extracellular DA overflow, as measured by in vivo microdialysis, was lower in juvenile ventral CPu than in the adults. This age difference was not observed in dorsal CPu. These findings suggest that impulse activity in DA afferents to ventral CPu is immature in adolescents. In conclusion, the present study showed that cocaine-sensitive neuronal circuits continue to mature during adolescence.

Risperidone exerts potent anti-aggressive effects in a developmentally immature animal model of escalated aggression

BACKGROUND

Risperidone has been shown to be clinically effective for the treatment of aggressive behavior in children, yet no information is available regarding whether risperidone exhibits aggression-specific suppression in preclinical studies that use validated developmentally immature animal models of escalated aggression. Previously, we have shown that exposure to low doses of the psychostimulant cocaine-hydrochloride (.5 mg/kg intraperitoneally) during the majority of pubertal development (postnatal days [P]27-57) generates animals that exhibit a high level of offensive aggression. This study examined whether risperidone exerts selective aggression-suppressing effects by using this pharmacologic animal model of highly escalated offensive aggression.

METHODS

Experimental hamsters were tested for offensive aggression after the acute administration of risperidone (.05-1.0 mg/kg, intraperitoneally).

RESULTS

Risperidone dose-dependently reduced the highly aggressive phenotype, with a significant reduction observed at .1-.2 mg/kg for most aggressive responses measured. Experimental animals treated with higher doses of risperidone (.3-1.0 mg/kg) showed significant reductions in aggression and social interest toward intruders, indicating more general behavioral inhibition.

CONCLUSIONS

These studies provide evidence that risperidone exerts specific aggression-suppressing effects in a developmentally immature animal model of escalated aggression.

Interactions between the neural regulation of stress and aggression

Socially aggressive interaction is stressful. What is more, social aggression is stressful for both dominant and subordinate animals. Much of the neurocircuitry for stress and aggression overlap. The pattern of neurochemical and hormonal events stimulated by social interaction make it clear that subtle differences in this pattern of response distinguish social rank. The neurotransmitter serotonin (5-HT) responds rapidly to stress, and also appears to play the most important role for inhibitory regulation of aggressive interactions. In addition, the adrenocortical/interrenal steroid hormones corticosterone and cortisol are responsive to stress and influence aggression. However, while 5-HT and glucocorticoids can both be inhibitory to aggression, the relationship between 5-HT and glucocorticoids is not straightforward, and much of the distinctions in function depend upon timing. Neither is inhibitory during the early stressful phase of aggression. This transmitter-hormone combination follows and influences a four-stage functional pattern of effect: (1) predisposed (positively or negatively) toward aggression, (2) motivated toward behavior, (3) responsive to stress (including aggression) and passively allowing aggression, and finally (4) chronically applied 5-HT and glucocorticoids inhibit aggression.

Repeated anabolic/androgenic steroid exposure during adolescence alters phosphate-activated glutaminase and glutamate receptor 1 (GluR1) subunit immunoreactivity in Hamster brain: correlation with offensive aggression

Male Syrian hamsters (Mesocricetus auratus) treated with moderately high doses (5.0mg/kg/day) of anabolic/androgenic steroids (AAS) during adolescence (P27-P56) display highly escalated offensive aggression. The current study examined whether adolescent AAS-exposure influenced the immunohistochemical localization of phosphate-activated glutaminase (PAG), the rate-limiting enzyme in the synthesis of glutamate, a fast-acting neurotransmitter implicated in the modulation of aggression in various species and models of aggression, as well as glutamate receptor 1 subunit (GluR1). Hamsters were administered AAS during adolescence, scored for offensive aggression using the resident-intruder paradigm, and then examined for changes in PAG and GluR1 immunoreactivity in areas of the brain implicated in aggression control. When compared with sesame oil-treated control animals, aggressive AAS-treated hamsters displayed a significant increase in the number of PAG- and area density of GluR1-containing neurons in several notable aggression regions, although the differential pattern of expression did not appear to overlap across brain regions. Together, these results suggest that altered glutamate synthesis and GluR1 receptor expression in specific aggression areas may be involved in adolescent AAS-induced offensive aggression.

Lasting changes in neuronal activation patterns in select forebrain regions of aggressive, adolescent anabolic/androgenic steroid-treated hamsters

Repeated exposure to anabolic/androgenic steroids (AAS) during adolescence stimulates high levels of offensive aggression in Syrian hamsters. The current study investigated whether adolescent AAS exposure activated neurons in areas of hamster forebrain implicated in aggressive behavior by examining the expression of FOS, i.e., the protein product of the immediate early gene c-fos shown to be a reliably sensitive marker of neuronal activation. Adolescent AAS-treated hamsters and sesame oil-treated littermates were scored for offensive aggression and then sacrificed 1 day later and examined for the number of FOS immunoreactive (FOS-ir) cells in regions of the hamster forebrain important for aggression control. When compared with non-aggressive, oil-treated controls, aggressive AAS-treated hamsters showed persistent increases in the number of FOS-ir cells in select aggression regions, namely the anterior hypothalamus and lateral septum. However, no differences in FOS-ir cells were found in other areas implicated in aggression such as the ventrolateral hypothalamus, bed nucleus of the stria terminals, central and/or medial amygdala or in non-aggression areas, such as the samatosensory cortex and the suprachiasmatic nucleus. These results suggest that adolescent AAS exposure may constitutively activate neurons in select forebrain areas critical for the regulation of aggression in hamsters. A model for how persistent activation of neurons in one of these brain regions (i.e., the anterior hypothalamus) may facilitate the development of the aggressive phenotype in adolescent-AAS exposed animals is presented.

From genes to aggressive behavior: the role of serotonergic system

Recent investigations in neurogenomics have opened up new lines of research into a crucial genetic problem-the pathway from genes to behavior. This paper concentrates on the involvement of protein elements in the brain neurotransmitter serotonin (5-HT) system in the genetic control of aggressive behavior. Specifically, it describes: (1) the effect of the knockout of MAO A, the principal enzyme in 5-HT degradation, (2) the association of intermale aggression with the polymorphism in the Tph2 gene encoding the key enzyme in 5-HT synthesis in the brain, tryptophan hydroxylase (TPH), and (3) the effect of selective breeding for nonaggressive behavior on 5-HT metabolism, TPH activity and 5-HT(1A) receptors in the brain. The review provides converging lines of evidence that: (1) brain 5-HT contributes to a critical mechanism underlying genetically defined individual differences in aggressiveness, and (2) genes encoding pivotal enzymes in 5-HT metabolism (TPH and MAO A), 5-HT-transporter, 5-HT(1A) and 5-HT(1B) receptors belong to a group of genes that modulate aggressive behavior.

Plasticity in anterior hypothalamic vasopressin correlates with aggression during anabolic-androgenic steroid withdrawal in hamsters

In hamsters, adolescent anabolic-androgenic steroid (AAS) exposure facilitates offensive aggression, in part by altering the development and activity of anterior hypothalamic arginine vasopressin (AH-AVP). This study assessed whether these effects were lasting by examining aggression and AH-AVP during AAS withdrawal. Adolescent hamsters administered AAS were tested as adults for aggression at 1, 4, 11, 18, or 25 days of withdrawal, sacrificed the following day, and examined for AH-AVP afferent innervation using immunohistochemistry. Through Day 12 of withdrawal, aggression and AVP were significantly higher in AAS-treated hamsters than in controls. These differences were no longer observable by Day 19 of withdrawal, at which point the behavior and neurobiology of AAS-treated hamsters reverted to that observed in controls. These data indicate that adolescent AAS exposure has short-term, reversible effects on both aggression and AH-AVP, correlating AH-AVP with the aggressive/nonaggressive behavioral phenotype during AAS withdrawal.

Serotonin-1B receptor activity and expression modulate the aggression-stimulating effects of adolescent anabolic steroid exposure in hamsters

Repeated high dose (5.0 mg/kg) anabolic-androgenic steroid (AAS) exposure during adolescence stimulates offensive aggression in male Syrian hamsters. These studies examined whether AAS-induced aggression was regulated by the activity of serotonin (5HT) type-1B receptors and correlated with altered 5HT1B expression. AAS-treated hamsters were tested for offensive aggression following the administration of the 5HT1B agonist anpirtoline (0.125-0.5 mg/kg). Anpirtoline dose-dependently reduced select components of the AAS-induced aggressive response, with significant reductions observed at 0.25 mg/kg. Aggressive, AAS-treated hamsters showed significant decreases in the area covered by 5HT1B-containing neuronal puncta and increases in the number of 5HT1B-containing neuronal somata in select brain regions implicated in aggression control. Together, these data support a role for site-specific alterations in 5HT1B signaling and expression in adolescent AAS-induced aggression.

Glucocorticoid interaction with aggression in non-mammalian vertebrates: reciprocal action

Socially aggressive interaction is stressful, and as such, glucocorticoids are typically secreted during aggressive interaction in a variety of vertebrates, which may both potentiate and inhibit aggression. The behavioral relationship between corticosterone and/or cortisol in non-mammalian (as well as mammalian) vertebrates is dependent on timing, magnitude, context, and coordination of physiological and behavioral responses. Chronically elevated plasma glucocorticoids reliably inhibit aggressive behavior, consistent with an evolutionarily adaptive behavioral strategy among subordinate and submissive individuals. Acute elevation of plasma glucocorticoids may either promote an actively aggressive response via action in specialized local regions of the brain such as the anterior hypothalamus, or is permissive to escalated aggression and/or activity. Although the permissive effect of glucocorticoids on aggression does not suggest an active role for the hormone, the corticosteroids may be necessary for full expression of aggressive behavior, as in the lizard Anolis carolinensis. These effects suggest that short-term stress may generally be best counteracted by an actively aggressive response, at least for socially dominant proactive individuals. An acute and active response may be evolutionarily maladaptive under chronic, uncontrollable and unpredictable circumstances. It appears that subordinate reactive individuals often produce compulsorily chronic responses that inhibit aggression and promote submissive behavior.

5-HT(3) receptors, alcohol and aggressive behavior in mice

Alcohol is a positive modulator at the 5-HT(3) receptor, which has been implicated in alcohol drinking, anxiety and aggression. The reported experiments explored the role of the 5-HT(3) receptor in aggressive behavior and alcohol-heightened aggression. Male, CFW mice were trained to self-administer 1.0 g/kg of alcohol, after which they confronted an intruder. Half of the CFW mice exhibited consistently increased aggressive behavior after alcohol and were designated as showing alcohol-heightened aggression, the others showed no increase and were designated as showing alcohol non-heightened aggression. The 5-HT(3) antagonist, ondansetron (0.01-1.0 mg/kg), significantly reduced aggression in both groups of CFW mice without affecting non-aggressive behaviors. Zacopride also reduced aggression effectively in both groups of mice, but at high doses began to affect walking. Male B6SJL/F2 transgenic 5-HT(3) over-expressing mice (TG) and wild-type mice (WT) were tested for aggressive behavior in their home cage. In those individuals that fought in tests of resident-intruder aggression, no differences were found in aggression after alcohol intake. In tests of aggression without alcohol intake, zacopride reduced aggression in both TG and WT mice at a dose of 56 mg/kg. Antagonism of 5-HT(3) receptors shows promising anti-aggressive effects, although these effects depend on the genetic background of the mice.

Anterior hypothalamic vasopressin modulates the aggression-stimulating effects of adolescent cocaine exposure in Syrian hamsters

Repeated low-dose cocaine treatment (0.5 mg/kg/day) during adolescence induces offensive aggression in male Syrian hamsters (Mesocricetus auratus). This study examines the hypothesis that adolescent cocaine exposure predisposes hamsters to heightened levels of aggressive behavior by increasing the activity of the anterior hypothalamic-vasopressinergic neural system. In a first experiment, adolescent male hamsters were treated with low-dose cocaine and then scored for offensive aggression in the absence or presence of vasopressin receptor antagonists applied directly to the anterior hypothalamus. Adolescent cocaine-treated hamsters displayed highly escalated offensive aggression that could be reversed by blocking the activity of vasopressin receptors within the anterior hypothalamus. In a second set of experiments, adolescent hamsters were administered low-dose cocaine or vehicle, tested for offensive aggression, and then examined for differences in vasopressin innervation patterns and expression levels in the anterior hypothalamus, as well as the basal- and stimulated-release of vasopressin in this same brain region. Aggressive, adolescent cocaine-treated hamsters showed no differences in vasopressin afferent innervation and/or peptide levels in the anterior hypothalamus compared with non-aggressive, saline-treated littermates. Conversely, significant increases in stimulated, but not basal, vasopressin release were detected from the anterior hypothalamus of aggressive, cocaine-treated animals compared with non-aggressive, saline-treated controls. Together, these data suggest that adolescent cocaine exposure increases aggression by increasing stimulated release of vasopressin in the anterior hypothalamus, providing direct evidence for a causal role of anterior hypothalamic-vasopressin activity in adolescent cocaine-induced offensive aggression. A model for how alterations in anterior hypothalamic-vasopressin neural functioning may facilitate the development of the aggressive phenotype in adolescent-cocaine exposed animals is presented.

Does Serotonin Influence Aggression? Comparing Regional Activity before and during Social Interaction

Serotonin is widely believed to exert inhibitory control over aggressive behavior and intent. In addition, a number of studies of fish, reptiles, and mammals, including the lizard Anolis carolinensis, have demonstrated that serotonergic activity is stimulated by aggressive social interaction in both dominant and subordinate males. As serotonergic activity does not appear to inhibit agonistic behavior during combative social interaction, we investigated the possibility that the negative correlation between serotonergic activity and aggression exists before aggressive behavior begins. To do this, putatively dominant and more aggressive males were determined by their speed overcoming stress (latency to feeding after capture) and their celerity to court females. Serotonergic activities before aggression are differentiated by social rank in a region-specific manner. Among aggressive males baseline serotonergic activity is lower in the septum, nucleus accumbens, striatum, medial amygdala, anterior hypothalamus, raphe, and locus ceruleus but not in the hippocampus, lateral amygdala, preoptic area, substantia nigra, or ventral tegmental area. However, in regions such as the nucleus accumbens, where low serotonergic activity may help promote aggression, agonistic behavior also stimulates the greatest rise in serotonergic activity among the most aggressive males, most likely as a result of the stress associated with social interaction.

Repeated cocaine exposure during adolescence alters glutamic acid decarboxylase-65 (GAD65) immunoreactivity in hamster brain: correlation with offensive aggression

Male Syrian hamsters (Mesocricetus auratus) treated with low-dose (0.5 mg/kg/day) cocaine throughout adolescence (P27-P56) display highly escalated offensive aggression. The current study examined whether adolescent cocaine exposure influenced the immunohistochemical localization of glutamic acid decarboxylase-65 (GAD65), the rate-limiting enzyme in the synthesis of gamma-aminobutyric acid (GABA), a fast-acting neurotransmitter implicated in the modulation of aggression in various species and models of aggression. Hamsters were administered low doses of cocaine throughout adolescence, scored for offensive aggression using the resident-intruder paradigm, and then examined for changes in GAD65 immunoreactivity in areas of the brain implicated in aggression control. When compared with saline-treated control animals, aggressive cocaine-treated hamsters showed significant differences in the area covered by GAD65 puncta in several notable aggression regions, including the anterior hypothalamus, the medial and central amygdaloid nuclei, and the lateral septum. However, no differences in GAD65 puncta were found in other aggression areas, such as the bed nucleus of the stria terminalis, the ventrolateral hypothalamus, and the corticomedial amygdala. Together, these results suggest that altered GABA synthesis and function in specific aggression areas may be involved in adolescent cocaine-facilitated offensive aggression.

Aggression upon adolescent cocaine exposure linked to serotonin anomalies: theoretical comment on Ricci et al. (2004)

Aggressive tendencies may be linked to the psychopharmacology of cocaine, yet few experimental approaches have been brought to bear on understanding the neurobiological implications of cocaine exposure during the developmentally sensitive period of adolescence. In this issue, Melloni and coauthors (L. A. Ricci, J. M. Grimes, & R. H. Melloni, 2004) present convincing evidence that the development of serotonin signal cascades in key brain regions can be disrupted by cocaine administration, resulting in an aggressive response in adolescents. These findings may allow the development of new therapeutic approaches to tailor pharmacotherapy for adolescents experiencing problems with aggressive behavior and/or impulse control associated with illicit drug use.

Persistent activation of select forebrain regions in aggressive, adolescent cocaine-treated hamsters

Hamsters repeatedly exposed to cocaine throughout adolescence display highly escalated offensive aggression compared to saline-treated littermates. The current study investigated whether adolescent cocaine exposure activated neurons in areas of hamster forebrain implicated in aggressive behavior by examining the expression of FOS, i.e., the protein product of the immediate early gene c-fos shown to be a reliably sensitive marker of neuronal activation. Adolescent cocaine-treated hamsters and saline-treated littermates were scored for offensive aggression and then sacrificed 1 day later and examined for the number of FOS immunoreactive (FOS-ir) cells in regions of the hamster forebrain important for aggression control. When compared with non-aggressive, saline-treated controls, aggressive cocaine-treated hamsters showed persistent increases in the number of FOS-ir cells in several aggression regions, including the anterior hypothalamus, nucleus circularis, lateral hypothalamus (i.e., the hypothalamic attack area), lateral septum, and medial and corticomedial amygdaloid nuclei. Conversely, aggressive cocaine-treated hamsters showed a significant decrease in FOS-ir cells in the medial supraoptic nucleus, bed nucleus of the stria terminalis, and central amygdala when compared with controls. However, no differences in FOS-ir cells were found in other areas implicated in aggression such as the paraventricular hypothalamic nucleus, or in a number of non-aggression areas. These results suggest that adolescent cocaine exposure may constitutively activate neurons in select forebrain areas critical for the regulation of aggression in hamsters. A model for how persistent activation of neurons in one of these brain regions (i.e., the hypothalamus) may facilitate the development of the aggressive phenotype in adolescent cocaine-exposed animals is presented.