Effects of acute treatment with 8-OH-DPAT and fluoxetine on aggressive behaviour in male song sparrows (Melospiza melodia morphna)

The serotonin blocker Ketanserin reduces coral reef fish Ctenochaetus striatus aggressive behaviour during between-species social interactions

A multitude of species engages in social interactions not only with their conspecifics but also with other species. Such interspecific interactions can be either positive, like helping, or negative, like aggressive behaviour. However, the physiological mechanisms of these behaviours remain unclear. Here, we manipulated the serotonin system, a well-known neurohormone for regulating intraspecific aggressive behaviour, to investigate its role in interspecific aggression. We tested whether serotonin blockade affects the aggressive behaviour of a coral reef fish species (Ctenochaetus striatus) that engages in mutualistic interactions with another species, the cleaner fish (Labroides dimidiatus). Although this mutualistic cleaning relationship may appear positive, cleaner fish do not always cooperate and remove ectoparasites from the other coral reef fish ("clients") but tend to cheat and bite the client's protective layer of mucus. Client fish thus often apply control mechanisms, like chasing, to deter their cleaner fish partners from cheating. Our findings show that blocking serotonin receptors 5-HT2A and 5-HT2C with ketanserin reduced the client fish's aggressive behaviour towards cleaner fish, but in the context where the latter did not cheat. These results are evidence of the involvement of serotonin in regulating aggressive behaviour at the between-species social interactions level. Yet, the direction of effect we found here is the opposite of previous findings using a similar experimental set-up and ecological context but with a different client fish species (Scolopsis bilineatus). Together, it suggests that serotonin's role in aggressive behaviour is complex, and at least in this mutualistic ecological context, its function is species-dependent. This warrants, to some extent, careful interpretations from single-species studies looking into the physiological mechanisms of social behaviour.

Neuroendocrine mechanisms contributing to the coevolution of sociality and communication

Communication is inherently social, so signaling systems should evolve with social systems. The 'social complexity hypothesis' posits that social complexity necessitates communicative complexity and is generally supported in vocalizing mammals. This hypothesis, however, has seldom been tested outside the acoustic modality, and comparisons across studies are confounded by varying definitions of complexity. Moreover, proximate mechanisms underlying coevolution of sociality and communication remain largely unexamined. In this review, we argue that to uncover how sociality and communication coevolve, we need to examine variation in the neuroendocrine mechanisms that coregulate social behavior and signal production and perception. Specifically, we focus on steroid hormones, monoamines, and nonapeptides, which modulate both social behavior and sensorimotor circuits and are likely targets of selection during social evolution. Lastly, we highlight weakly electric fishes as an ideal system in which to comparatively address the proximate mechanisms underlying relationships between social and signal diversity in a novel modality.

The role of serotonin in modulating common waxbill behaviour

Abstract

Serotonin or 5-hydroxytryptamine (5-HT) is a monoaminergic neurotransmitter that is known to influence behaviour in various animal species. Its actions, however, are complex and not well-understood yet. Here, we tested whether and how two 5-HT receptor agonists and a 5-HT receptor antagonist influence behaviour in common waxbills (Estrilda astrild), focusing on aggression, movement and feeding. We applied acute administration of either 8-OH-DPAT (a 5-HT1A receptor agonist), fluoxetine (a selective serotonin reuptake inhibitor; SSRI) or WAY 100,635 (a 5-HT1A receptor antagonist), and then quantified behaviour in the context of competition for food. Waxbills treated with the SSRI fluoxetine showed an overall decrease of aggressive behaviour, activity and feeding, while we found no significant effects of treatment with the other serotonergic enhancer (8-OH-DPAT) or with the antagonist WAY 100,635. Since both 8-OH-DPAT and WAY 100,635 act mainly on 5-HT1A receptor pathways, while fluoxetine more generally affects 5-HT pathways, our results suggest that receptors other than 5-HT1A are important for serotonergic modulation of waxbill behaviour.

Significance statement

The serotonergic system is of interest for current behavioural research due to its influence on a range of behaviours, including aggression, affiliative behaviour, feeding and locomotion in various species. There are, however, numerous discrepancies regarding the behavioural effects of serotonin across studies. We used acute pharmacological manipulations of the serotonergic system in common waxbills, using two serotonin enhancers (8-OH-DPAT and fluoxetine) and a serotonin blocker (WAY 100,635). Behavioural effects of these pharmacological manipulations on aggressiveness, movement and feeding, during tests of competition over food, indicated an anxiogenic-like effect of fluoxetine, but not of 8-OH-DPAT and WAY 100,635. This suggests a distinct role for different serotonergic pathways on waxbill behaviour.

Effects of the 5-HT1A receptor agonist 8-OH-DPAT on aggressive behavior in juvenile pufferfish, Takifugu rubripes

Severe aggressive behavior of juvenile pufferfish affects economic efficiency and fish welfare in aquaculture. 5-HT plays an important role in regulating the aggressive behavior of fish in aquaculture environment. This study examined the effects of different concentrations (0, 0.25, 0.5, 1 mg/kg) of 8-OH-DPAT, a selective 5-HT_1A receptor agonist, on the aggressive behavior of juvenile pufferfish. Forty-five minutes after drug injection, the aggressive behavior of juvenile fish was recorded for 20 min, including the latency to the first attack and the frequency of aggressive behaviors. The results showed no significant differences in the latency to the first attack of juvenile fish among treatment groups. During the first 10 min of the observation period, there was no significant difference in the total aggressive acts and locomotor activity among treatment groups. Total aggressive acts and locomotor activity were the least in the 1 mg/kg 8-OH-DPAT-treated during the 20 min observation period. Both aggressive behavior and locomotor activity were negatively correlated with 8-OH-DPAT treatment overall, respectively. The above results suggested that the serotonergic system activation had suppressive effects on aggressive behavior and locomotor activity in juvenile pufferfish.

The serotonin 1A receptor modulates the social behaviour within groups of a cooperatively-breeding cichlid

The neurotransmitter serotonin (5-HT) reduces aggressive behaviour in a number of vertebrates, and the 5-HT_1A receptor is known to be involved in this regulation. However, the role of this receptor in the modulation of sociopositive behaviour remains largely unknown. Here we investigated the role of the 5-HT_1A receptor in the regulation of aggressive, submissive and affiliative behaviour in the cooperatively-breeding cichlid Neolamprologus pulcher. In two experiments, we performed intramuscular injections of a 5-HT_1A agonist (8-OH-DPAT) and antagonist (Way-100635) followed by recordings of social behaviour of injected fish within their social groups. We determined the concentrations and post-injection times when the drugs had the greatest effect on social behaviour. We recorded spontaneous social behaviour in both experiments. In the second experiment we also recorded behaviour after social groups received a territorial challenge by live presentations of either conspecifics or egg predators. The 5-HT_1A agonist caused an increase in aggression and a decrease in submission and affiliation, whereas the antagonist had the opposite effects. Thus, the 5-HT_1A receptor plays an important regulatory role not only for aggressive but also sociopositive behaviour.

A mutation in monoamine oxidase (MAO) affects the evolution of stress behavior in the blind cavefish Astyanax mexicanus

The neurotransmitter serotonin controls a variety of physiological and behavioral processes. In humans, mutations affecting monoamine oxidase (MAO), the serotonin-degrading enzyme, are highly deleterious. Yet, blind cavefish of the species Astyanax mexicanus carry a partial loss-of-function mutation in MAO (P106L) and thrive in their subterranean environment. Here, we established four fish lines, corresponding to the blind cave-dwelling and the sighted river-dwelling morphs of this species, with or without the mutation, in order to decipher the exact contribution of mao P106L in the evolution of cavefish neurobehavioral traits. Unexpectedly, although mao P106L appeared to be an excellent candidate for the genetic determinism of the loss of aggressive and schooling behaviors in cavefish, we demonstrated that it was not the case. Similarly, the anatomical variations in monoaminergic systems observed between cavefish and surface fish brains were independent from mao P106L, and rather due to other, morph-dependent developmental processes. However, we found that mao P106L strongly affected anxiety-like behaviors. Cortisol measurements showed lower basal levels and an increased amplitude of stress response after a change of environment in fish carrying the mutation. Finally, we studied the distribution of the P106L mao allele in wild populations of cave and river A. mexicanus, and discovered that the mutant allele was present - and sometimes fixed - in all populations inhabiting caves of the Sierra de El Abra. The possibility that this partial loss-of-function mao allele evolves under a selective or a neutral regime in the particular cave environment is discussed.

Disruption of energy homeostasis by food restriction or high ambient temperature exposure affects gonadal function in male house finches (Haemorhous mexicanus)

Reproductive success requires that individuals acquire sufficient energy resources. Restricting food availability or increasing energy expenditure (e.g., thermoregulation) inhibits reproductive development in multiple avian species, but the nature of the energy-related signal mediating this effect is unclear. To investigate this question, we examined reproductive and metabolic physiology in male house finches that either underwent moderate food restriction (FR) or were exposed to high temperature (HT), in which birds were held at a high, but not locally atypical, ambient temperature cycle (37.8 °C day, 29.4 °C night) compared to a control group (CT; 29.4 °C day, 21.1 °C night). We hypothesized that FR and HT inhibit reproductive development by lowering available metabolic fuel, in particular plasma glucose (GLU) and free fatty acids (FFA). Following FR for 4 weeks, finches lost body mass, had marginally higher plasma FFA, and experienced a 90% reduction in testis mass compared to CT birds. Four weeks of HT exposure resulted in reduced voluntary food consumption and muscle mass, as well as an 80% reduction in testis mass relative to CT birds. Both FR and HT birds expressed less testicular 17β-hydroxysteroid dehydrogenase (17β-HSD) mRNA than controls but the expression of other testicular genes measured was unaffected by either treatment. Neither treatment significantly influenced plasma GLU. This study is among the first to demonstrate a negative effect of HT on reproductive development in a wild bird. Further studies are needed to clarify the role of metabolic mediators and their involvement under various conditions of energy availability and demand.

An Early-Life Stage Alternative Testing Strategy for Assessing the Impacts of Environmental Chemicals in Birds

Early-life stage (ELS) toxicity tests are recognized as an advancement over current testing methodologies in terms of cost, animal use, and biological relevance. However, standardized ELS tests are not presently available for some vertebrate taxa, including birds. The present study describes a Japanese quail (Coturnix japonica) ELS test that is a promising candidate for standardization and applies it to test 8 environmental chemicals (ethinylestradiol, benzo[a]pyrene, chlorpyrifos, fluoxetine, lead(II)nitrate, trenbolone, seleno-L-methionine, hexabromocyclododecane). Individual chemicals were injected into the air cell of unincubated Japanese quail eggs at 3 concentrations, all predicted to cause ≤20% mortality. Survival to embryonic day 16 was consistently high (>90%) among the vehicle-injected controls. All chemicals, except ethinylestradiol, were detected in liver tissue, most at concentrations suggestive of embryonic clearance. Adverse effects were observed for 5 of the 8 chemicals; chlorpyrifos (41.1 µg/g) significantly increased developmental abnormalities and decreased embryo and gallbladder mass. Ethinylestradiol (54.2 µg/g) and hexabromocyclododecane (0.02 µg/g) decreased embryo mass and tarsus length, respectively. Benzo[a]pyrene (0.83 µg/g) and fluoxetine hydrochloride (32.7 µg/g) exceeded the 20% mortality cutoff. No effects were observed following lead(II)nitrate, seleno-L-methionine, or trenbolone exposure up to 10.7, 0.07, and 4.4 µg/g, respectively. Overall, our ELS approach was time- and cost-effective, caused minimal mortality in controls, effectively delivered diverse chemicals to the embryo, and permitted identification of apical outcomes, all of which provide support toward standardization. Environ Toxicol Chem 2019;39:141-154. © 2019 SETAC.

Cannabidiol attenuates aggressive behavior induced by social isolation in mice: Involvement of 5-HT1A and CB1 receptors

Long-term single housing increases aggressive behavior in mice, a condition named isolation-induced aggression or territorial aggression, which can be attenuated by anxiolytic, antidepressant, and antipsychotic drugs. Preclinical and clinical findings indicate that cannabidiol (CBD), a non-psychotomimetic compound from Cannabis sativa, has anxiolytic, antidepressant, and antipsychotic properties. Few studies, however, have investigated the effects of CBD on aggressive behaviors. Here, we investigated whether CBD (5, 15, 30, and 60 mg/kg; i.p.) could attenuate social isolation-induced aggressive behavior in the resident-intruder test. Male Swiss mice (7-8 weeks) were single-housed for 10 days (resident mice) to induce aggressive behaviors, while conspecific mice of same sex and age (intruder mice) were group-housed. During the test, the intruder was placed into the resident's home-cage and aggressive behaviors initiated by the resident, including the latency for the first attack, number of attacks, and total duration of aggressive encounters, were recorded. The involvement of 5-HT1A and CB1 receptors (CB1R) in the effects of CBD was also investigated. All tested CBD doses induced anti-aggressive effects, indicated by a decrease in the number of attacks. CBD, at intermediary doses (15 and 30 mg/kg), also increased latency to attack the intruder and decreased the duration of aggressive encounters. No CBD dose interfered with locomotor behavior. CBD anti-aggressive effects were attenuated by the 5-HT1A receptor antagonist WAY100635 (0.3 mg/kg) and the CB1 antagonist AM251 (1 mg/kg), suggesting that CBD decreases social isolation-induced aggressive behaviors through a mechanism associated with the activation of 5-HT1A and CB1 receptors. Also, CBD decreased c-Fos protein expression, a neuronal activity marker, in the lateral periaqueductal gray (lPAG) in social-isolated mice exposed to the resident-intruder test, indicating a potential involvement of this brain region in the drug effects. Taken together, our findings suggest that CBD may be therapeutically useful to treat aggressive behaviors that are usually associated with psychiatric disorders.

Predictive framework for estimating exposure of birds to pharmaceuticals

We present and evaluate a framework for estimating concentrations of pharmaceuticals over time in wildlife feeding at wastewater treatment plants (WWTPs). The framework is composed of a series of predictive steps involving the estimation of pharmaceutical concentration in wastewater, accumulation into wildlife food items, and uptake by wildlife with subsequent distribution into, and elimination from, tissues. Because many pharmacokinetic parameters for wildlife are unavailable for the majority of drugs in use, a read-across approach was employed using either rodent or human data on absorption, distribution, metabolism, and excretion. Comparison of the different steps in the framework against experimental data for the scenario where birds are feeding on a WWTP contaminated with fluoxetine showed that estimated concentrations in wastewater treatment works were lower than measured concentrations; concentrations in food could be reasonably estimated if experimental bioaccumulation data are available; and read-across from rodent data worked better than human to bird read-across. The framework provides adequate predictions of plasma concentrations and of elimination behavior in birds but yields poor predictions of distribution in tissues. The approach holds promise, but it is important that we improve our understanding of the physiological similarities and differences between wild birds and domesticated laboratory mammals used in pharmaceutical efficacy/safety trials, so that the wealth of data available can be applied more effectively in ecological risk assessments. Environ Toxicol Chem 2017;36:2335-2344. © 2017 SETAC.

Serotonergic activation during courtship and aggression in the brown anole, Anolis sagrei

The role of serotonin (5-hydroxytryptamine, 5-HT) in social behavior regulation is not fully understood. While 5-HT release in nuclei of the social behavior network has generally been associated with inhibition of aggressive behavior across multiple classes of vertebrates, less is known about its effects on sexual, especially non-copulatory courtship display behaviors. Furthermore, most research has examined effects at 5-HT release sites, while studies examining the behavioral relevance of source cell populations have generated contradictory findings. This study utilized immunohistochemistry to examine the colocalization of 5-HT with Fos, an immediate early gene product and marker of neural activity, in the raphe and superior reticular nuclei of male brown anoles (Anolis sagrei) exposed to either aggression, courtship, or control social interactions. Supporting previous research, copulation was associated with a decrease in 5-HT activity, while a novel link between 5-HT activity and latency to non-copulatory courtship was also found. Within the aggression group, intensity and frequency of behavior were both associated with decreased 5-HT activity. An effect of social context was also seen, with anoles exposed to either courtship or aggression encounters showing decreased 5-HT activity in certain raphe and superior reticular nuclei populations compared to controls. Interestingly, context effects and behavioral effects were seen at separate brain nuclei, suggesting the presence of separate systems with distinct functional roles.

Pre-GnRH and GnRH-induced testosterone levels do not vary across behavioral contexts: A role for individual variation

Hormones can facilitate the expression of behavior, but relatively few studies have considered individual variation and repeatability in hormone-behavior relationships. Repeated measures of hormones are valuable because repeatability in hormone levels might be a mechanism that drives repeatability in behavior ("personality"). Testosterone is predicted to promote territorial aggression and suppress parental behaviors. In our population of eastern bluebirds (Sialia sialis), parental care and nest defense aggression toward a heterospecific are repeatable. We tested the hypothesis that repeatability of testosterone levels within individuals underlies repeatable behaviors observed in our population. We measured nestling provisioning and aggressive nest defense against a heterospecific. After behavioral observations we captured either the male or female bluebird, and determined initial testosterone levels and maximum capacity of the gonads to secrete testosterone by injecting gonadotropin-releasing hormone (GnRH). We found among-individual variation in initial testosterone levels for males and females. Individual males were repeatable in both initial and GnRH-induced testosterone levels across behavioral contexts, while individual females were repeatable in GnRH-induced testosterone levels. However, testosterone levels were not significantly related to parental or nest defense behaviors, suggesting that repeatable testosterone levels may not drive repeatable parental and heterospecific nest defense behaviors in this population. The absence of a relationship between testosterone and parental and heterospecific nest defense behaviors might be due to among-individual variation in testosterone levels. Considering the sources of variation in testosterone levels may reveal why some populations exhibit high individual variation in hormone levels.

Dehydroepiandrosterone Heightens Aggression and Increases Androgen Receptor and Aromatase mRNA Expression in the Brain of a Male Songbird

Dehydroepiandrosterone (DHEA) is a testosterone/oestrogen precursor and known modulator of vertebrate aggression. Male song sparrows (Melospiza melodia morphna) show high aggression during breeding and nonbreeding life-history stages when circulating DHEA levels are high, and low aggression during molt when DHEA levels are low. We previously showed that androgen receptor and aromatase mRNA expression are higher during breeding and/or nonbreeding in brain regions associated with reproductive and aggressive behaviour, although the potential role of DHEA in mediating these seasonal changes remained unclear. In the present study, nonbreeding male song sparrows were captured and held in the laboratory under short days (8 : 16 h light/dark cycle) and implanted with s.c. DHEA-filled or empty (control) implants for 14 days. DHEA implants increased aggression in a laboratory-based simulated territorial intrusion. Brains of DHEA-implanted birds showed higher aromatase mRNA expression in the preoptic area (POA) and higher androgen receptor mRNA expression in the periventricular nucleus of the medial striatum (pvMSt) and ventromedial nucleus of the hypothalamus. The DHEA-induced increases in aromatase expression in the POA and androgen receptor expression in the pvMSt are consistent with previously reported seasonal increases in these markers associated with naturally elevated DHEA levels. This suggests that DHEA facilitates seasonal increases in aggression in nonbreeding male song sparrows by up-regulating steroid signalling/synthesis machinery in a brain region-specific fashion.

Behavioural and physiological responses of birds to environmentally relevant concentrations of an antidepressant

Many wildlife species forage on sewage-contaminated food, for example, at wastewater treatment plants and on fields fertilized with sewage sludge. The resultant exposure to human pharmaceuticals remains poorly studied for terrestrial species. On the basis of predicted exposure levels in the wild, we administered the common antidepressant fluoxetine (FLUOX) or control treatment via prey to wild-caught starlings (Sturnus vulgaris) for 22 weeks over winter. To investigate responses to fluoxetine, birds were moved from their group aviaries into individual cages for 2 days. Boldness, exploration and activity levels showed no treatment effects but controls and FLUOX birds habituated differently to isolation in terms of the concentration of corticosterone (CORT) metabolites in faeces. The controls that excreted higher concentrations of CORT metabolites on day 1 lost more body mass by day 2 of isolation than those which excreted lower levels of CORT metabolites. CORT metabolites and mass loss were unrelated in FLUOX birds. When we investigated the movements of birds in their group aviaries, we found the controls made a higher frequency of visits to food trays than FLUOX birds around the important foraging periods of sunrise and sunset, as is optimal for wintering birds. Although individual variability makes interpreting the sub-lethal endpoints measured challenging, our data suggest that fluoxetine at environmentally relevant concentrations can significantly alter behaviour and physiology.

Non-invasive administration of 17β-estradiol rapidly increases aggressive behavior in non-breeding, but not breeding, male song sparrows

17β-Estradiol (E2) acts in the brain via genomic and non-genomic mechanisms to influence physiology and behavior. There is seasonal plasticity in the mechanisms by which E2 activates aggression, and non-genomic mechanisms appear to predominate during the non-breeding season. Male song sparrows (Melospiza melodia) display E2-dependent territorial aggression throughout the year. Field studies show that song sparrow aggression during a territorial intrusion is similar in the non-breeding and breeding seasons, but aggression after an intrusion ends differs seasonally. Non-breeding males stop behaving aggressively within minutes whereas breeding males remain aggressive for hours. We hypothesize that this seasonal plasticity in the persistence of aggression relates to seasonal plasticity in E2 signaling. We used a non-invasive route of E2 administration to compare the non-genomic (within 20min) effects of E2 on aggressive behavior in captive non-breeding and breeding season males. E2 rapidly increased barrier contacts (attacks) during an intrusion by 173% in non-breeding season males only. Given that these effects were observed within 20min of E2 administration, they likely occurred via a non-genomic mechanism of action. The present data, taken together with past work, suggest that environmental cues associated with the non-breeding season influence the molecular mechanisms through which E2 influences behavior. In song sparrows, transient expression of aggressive behavior during the non-breeding season is highly adaptive: it minimizes energy expenditure and maximizes the amount of time available for foraging. In all, these data suggest the intriguing possibility that aggression in the non-breeding season may be activated by a non-genomic E2 mechanism due to the fitness benefits associated with rapid and transient expression of aggression.

Serotonin modulates electrosensory processing and behavior via 5-HT2-like receptors

Efficient sensory processing of the environment is a critical function for any organism to survive and is accomplished by having neurons adapt their responses to stimuli based on behavioral context in part through neuromodulators such as serotonin (5-HT). We have recently shown that one critical function of the serotonergic system in weakly electric fish is to enhance sensory pyramidal neuron responses within the electrosensory lateral line lobe (ELL) to stimuli caused by same sex conspecifics, thereby enhancing their perception. This enhancement is accomplished by making pyramidal neurons more excitable through downregulation of potassium channels. However, the nature of the 5-HT receptors that mediate this effect is not known. Here we show that the 5-HT2 receptor antagonist ketanserin (ket) can effectively block the effects of 5-HT on pyramidal neuron excitability in vitro. Indeed, 5-HT application subsequent to ket application did not cause any significant changes in neuron excitability and responses to current injection. We further show that ket applied in vivo can block the effects of 5-HT on behavioral responses. Thus, our results strongly suggest that the previously observed effects of 5-HT on sensory processing within ELL and their consequences for behavior are mediated by 5-HT2 receptors.

Neuromodulation of the agonistic behavior in two species of weakly electric fish that display different types of aggression

Agonistic behavior has shaped sociality across evolution. Though extremely diverse in types of displays and timing, agonistic encounters always follow the same conserved phases (evaluation, contest and post-resolution) and depend on homologous neural circuits modulated by the same neuroendocrine mediators across vertebrates. Among neuromodulators, serotonin (5-HT) is the main inhibitor of aggression, and arginine vasotocin (AVT) underlies sexual, individual and social context differences in behavior across vertebrate taxa. We aim to demonstrate that a distinct spatio-temporal pattern of activation of the social behavior network characterizes each type of aggression by exploring its modulation by both the 5-HT and AVT systems. We analyze the neuromodulation of aggression between the intermale reproduction-related aggression displayed by the gregarious Brachyhypopomus gauderio and the non-breeding intrasexual and intersexual territorial aggression displayed by the solitary Gymnotus omarorum. Differences in the telencephalic activity of 5-HT between species were paralleled by a differential serotonergic modulation through 1A receptors that inhibited aggression in the territorial aggression of G. omarorum but not in the reproduction-related aggression of B. gauderio. AVT injection increased the motivation towards aggression in the territorial aggression of G. omarorum but not in the reproduction-related aggression of B. gauderio, whereas the electric submission and dominance observed in G. omarorum and B. gauderio, respectively, were both AVT-dependent in a distinctive way. The advantages of our model species allowed us to identify precise target areas and mechanisms of the neuromodulation of two types of aggression that may represent more general and conserved strategies of the control of social behavior among vertebrates.

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.

Differential serotonergic modulation of two types of aggression in weakly electric fish

Agonistic aggression has provided an excellent framework to study how conserved circuits and neurochemical mediators control species-specific and context-dependent behavior. The principal inhibitory control upon aggression is serotonin (5-HT) dependent, and the activation of 5-HT(1A) receptors is involved in its action. To address whether the serotonergic system differentially regulates different types of aggression, we used two species of weakly electric fish: the solitary Gymnotus omarorum and the gregarious Brachyhypopomus gauderio, which display distinctive types of aggression as part of each species' natural behavioral repertoire. We found that in the reproduction-related aggression displayed by B. gauderio after conflict resolution, the serotonergic activity follows the classic pattern in which subordinates exhibit higher 5-HT levels than controls. After the territorial aggression displayed by G. omarorum, however, both dominants and subordinates show lower 5-HT levels than controls, indicating a different response of the serotonergic system. Further, we found interspecific differences in basal serotonin turnover and in the dynamic profile of the changes in 5-HT levels from pre-contest to post-contest. Finally, we found the expected reduction of aggression and outcome shift in the territorial aggression of G. omarorum after 8-OH-DPAT (5-HT(1A) receptor agonist) administration, but no effect in the reproduction-related aggression of B. gauderio. Our results demonstrate the differential participation of the serotonergic system in the modulation of two types of aggression that we speculate may be a general strategy of the neuroendocrine control of aggression across vertebrates.

Seasonal changes in patterns of gene expression in avian song control brain regions

Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log₂ 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.

Non-migratory stonechats show seasonal changes in the hormonal regulation of non-seasonal territorial aggression

In many birds and mammals, male territorial aggression is modulated by elevated circulating concentrations of the steroid hormone testosterone (T) during the breeding season. However, many species are territorial also during the non-breeding season, when plasma T levels are basal. The endocrine control of non-breeding territorial aggression differs considerably between species, and previous studies on wintering birds suggest differences between migratory and resident species. We investigated the endocrine modulation of territorial aggression during the breeding and non-breeding season in a resident population of European stonechats (Saxicola torquata rubicola). We recorded the aggressive response to a simulated territorial intrusion in spring and winter. Then, we compared the territorial aggression between seasons and in an experiment in which we blocked the androgenic and estrogenic action of T. We found no difference in the aggressive response between the breeding and the non-breeding season. However, similarly to what is found in migratory stonechats, the hormonal treatment decreased aggressive behaviors in resident males in the breeding season, whereas no effects were recorded in the non-breeding season. When we compared the aggressive responses of untreated birds with those obtained from migratory populations in a previous study, we found that territorial aggression of resident males was lower than that of migratory males during the breeding season. Our results show that in a resident population of stonechats T and/or its metabolites control territorial aggression in the breeding but not in the non-breeding season. In addition, our study supports the hypothesis that migratory status does modulate the intensity of aggressive behavior.

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.

The role of androgen receptors in regulating territorial aggression in male song sparrows

This paper examines the role that androgen receptors (ARs) play in modulating aggressive behavior in male song sparrows, Melospiza melodia morphna. Song sparrows are seasonally breeding, territorial birds that maintain year-round territories with male-female pair bonds formed during the spring breeding season. Plasma testosterone levels peak as territories are established and mates acquired. In late summer, testosterone levels fall and remain basal during the non-breeding season. We examined the role of ARs in regulating territorial aggression in captive song sparrows under short- and long-day conditions as well as just prior to, and at the start of the breading season in freely living birds using the nonsteroidal antiandrogen flutamide to block AR function. Birds were implanted with either empty or drug filled silastic implants for 18 to 42 days and then challenged with a novel male decoy to assess the individual birds level of male-male aggression. Freely living birds remained on their home territory and underwent a simulated territorial intrusion, whereas laboratory-held birds were assessed using a laboratory simulated territorial intrusion and remained in their home cage. Experimental treatment of male song sparrows decreased aggressive behavior during the pre-breeding life history substage (March-April) in freely living birds as well as in laboratory-held birds under long-day (16L:8D) conditions. During the early breeding substage (April-May) there was no measurable effect of flutamide treatment on aggressive behavior, nor was there a difference in behavior in the (8L:16D) laboratory birds. This demonstrates that ARs are an important component of the neuroendocrine control of aggressive behavior. Given that flutamide only affected aggression during the pre-breeding substage and in LD birds, the results suggest that AR dependent control of aggressive behavior changes as song sparrow life history states change.

The effect of increased serotonergic neurotransmission on aggression: a critical meta-analytical review of preclinical studies

RATIONALE

The role of serotonin (5-HT) on aggression has been extensively studied; nonetheless, the role of this neurotransmitter in aggression is still inconclusive.

OBJECTIVES

The current meta-analytical review investigated the role of increased 5-HT neurotransmission in aggression.

METHODS

Preclinical studies using serotonin reuptake inhibitors, 5-hydroxytryptophan, L-tryptophan, or serotonin (5-HT) to increase 5-HT levels were included in this meta-analysis. An overall effect of serotonin on aggression was calculated, and the role of several moderator variables was analyzed.

RESULTS

A total of 218 effect sizes revealed that increased 5-HT had an overall significant inhibitory effect on aggression (r = 0.3). The results showed that increased 5-HT had the strongest inhibitory effect on aggression when (1) a specific strain or species (e.g., Long Evans) was used; (2) aggression was offensive or predatory and/or induced by administration of 5,7-dihydroxytryptamine or p-chlorophenylalanine; (3) zimelidine, sertraline, L-tryptophan, citalopram, or 5-HT were used to increase 5-HT; (4) treatment was acute; (5) long chronic treatment durations were used; and (6) time between last injection and behavior testing was within 8 h before or after peak plasma concentration of drug. In contrast, the results revealed that increased-5-HT-facilitated aggression could be predicted when (1) Wistar rats, (2) social isolation or stress to induce aggression, and/or (3) animals treated for less than 3 weeks were used.

CONCLUSIONS

Although 5-HT has an overall inhibitory effect on aggression, the animal's genetic background, drug, treatment time, aggression inducing paradigm, and aggression type are critical variables that influence and modify this effect.

Serotonin, social status and sex change in the bluebanded goby Lythrypnus dalli

In a variety of vertebrates, highly aggressive individuals tend to have high social status and low serotonergic function. In the sex changing fish Lythrypnus dalli, serotonin (5-HT) may be involved as a mediator between the social environment and the reproductive system because social status is a critical cue in regulating sex change. Subordination inhibits sex change in L. dalli, and it is associated with higher serotonergic activity in other species. We tested the hypothesis that high serotonergic activity has an inhibitory effect on sex change. In a social situation permissive to sex change, we administered to the dominant female implants containing the serotonin precursor 5-hydroxytryptophan (5-HTP). In a social situation not conducive to sex change, we administered either the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) or the 5-HT(1A) receptor antagonist p-MPPI. After three weeks we used HPLC to measure brain levels of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). We also performed PCPA, p-MPPI and fluoxetine injections in size-matched pairs of females to assess its effect on dominance status. Males and newly sex changed fish showed a trend for higher levels of 5-HIAA and 5-HT/5-HIAA ratio than females. The different implants treatments did not affect the probability of sex change. Interestingly, this species does not seem to fit the pattern seen in other vertebrates where dominant individuals have lower serotonergic activity than subordinates.

Serotonin 5-HT(2) and 5-HT(1A)-like receptors differentially modulate aggressive behaviors in Drosophila melanogaster

Aggressive behavior is widespread throughout the animal kingdom, and is a complex social behavior influenced by both genetics and environment. Animals typically fight over resources that include food, territory, and sexual partners. Of all the neurotransmitters, serotonin (5-HT) has been the most implicated in modulating aggressive behaviors in mammalian systems. In the fruit fly, Drosophila melanogaster, the involvement of 5-HT itself in aggressive behaviors has been recently established, however, the underlying mechanisms have largely remained elusive. Here we describe the influence of different 5-HT receptor subtypes on aggressive behaviors in Drosophila. Drosophila express homologs of three mammalian 5-HT receptors: the 5-HT(1A), 5-HT(2), and 5-HT(7) receptors. Significantly, these receptors mediate important behaviors in mammalian systems ranging from feeding, aggression, and sleep, to cognition. To examine the role of the 5-HT(2)Dro receptor, we utilized the selective 5-HT(2) receptor agonist (R)-1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI), and the 5-HT(2) receptor antagonist, ketanserin. To examine the role of 5-HT(1A)-like receptors we used the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT), and the 5-HT(1A) receptor antagonist WAY100635. We find that activation of 5-HT(2) receptors with (R)-DOI appears to decrease overall aggression, whereas activation of 5-HT(1A)-like receptors with 8-OH-DPAT increases overall aggression. Furthermore, the different 5-HT receptor circuitries appear to mediate different aspects of aggression: 5-HT(2) receptor manipulation primarily alters lunging and boxing, whereas 5-HT(1A)-like receptor manipulation primarily affects wing threats and fencing. Elucidating the effects of serotonergic systems on aggression in the fly is a significant advancement not only in establishing the fly as a system to study aggression, but as a system relevant to elucidating molecular mechanisms underlying aggression in mammals, including humans.

Serotonergic activation of 5HT1A and 5HT2 receptors modulates sexually dimorphic communication signals in the weakly electric fish Apteronotus leptorhynchus

Serotonin modulates agonistic and reproductive behavior across vertebrate species. 5HT(1A) and 5HT(1B) receptors mediate many serotonergic effects on social behavior, but other receptors, including 5HT(2) receptors, may also contribute. We investigated serotonergic regulation of electrocommunication signals in the weakly electric fish Apteronotus leptorhynchus. During social interactions, these fish modulate their electric organ discharges (EODs) to produce signals known as chirps. Males chirp more than females and produce two chirp types. Males produce high-frequency chirps as courtship signals; whereas both sexes produce low-frequency chirps during same-sex interactions. Serotonergic innervation of the prepacemaker nucleus, which controls chirping, is more robust in females than males. Serotonin inhibits chirping and may contribute to sexual dimorphism and individual variation in chirping. We elicited chirps with EOD playbacks and pharmacologically manipulated serotonin receptors to determine which receptors regulated chirping. We also asked whether serotonin receptor activation generally modulated chirping or more specifically targeted particular chirp types. Agonists and antagonists of 5HT(1B/1D) receptors (CP-94253 and GR-125743) did not affect chirping. The 5HT(1A) receptor agonist 8OH-DPAT specifically increased production of high-frequency chirps. The 5HT(2) receptor agonist DOI decreased chirping. Receptor antagonists (WAY-100635 and MDL-11939) opposed the effects of their corresponding agonists. These results suggest that serotonergic inhibition of chirping may be mediated by 5HT(2) receptors, but that serotonergic activation of 5HT(1A) receptors specifically increases the production of high-frequency chirps. The enhancement of chirping by 5HT(1A) receptors may result from interactions with cortisol and/or arginine vasotocin, which similarly enhance chirping and are influenced by 5HT(1A) activity in other systems.

Combined effects of DHEA and fadrozole on aggression and neural VIP immunoreactivity in the non-breeding male song sparrow

The male Song Sparrow, Melospiza melodia morphna, shows high levels of aggression in its non-breeding season, concomitant with basal levels of circulating testosterone (T) and estradiol (E(2)). However, administration of fadrozole, an aromatase inhibitor, decreases non-breeding aggression in the field. Circulating levels of dehydroepiandrosterone (DHEA), an androgen/estrogen precursor, correspond to the seasonal expression of aggression in this species, with high levels in the breeding and non-breeding seasons when aggression is also high, and lower levels during the molt when aggression is low. We test the hypothesis that circulating DHEA up-regulates non-breeding aggression via an aromatase-mediated mechanism. We also hypothesize that this up-regulation of aggression is partially mediated by changes in vasoactive intestinal polypeptide (VIP) in the lateral extent of the bed nucleus of the stria terminalis (BSTl) and lateral septum (LS). Birds were administered either DHEA, fadrozole, or both for 2 weeks and tested for aggression in a lab-based paradigm. As predicted, birds given DHEA were significantly more aggressive. However, fadrozole did not block this effect, and, when administered without DHEA, also led to increased aggression over controls. These results may be explained by the fact that the behaviors measured in field tests, which include more direct attack behaviors, may be under different hormonal regulation than the behaviors measured in the lab paradigm, which represent warning, or threat, behaviors. VIP immunoreactivity (VIP-ir) changed across multiple brain regions with this treatment regimen, most notably in the LSO/VFI subdivision of the lateral septum.

Fish on Prozac: a simple, noninvasive physiology laboratory investigating the mechanisms of aggressive behavior in Betta splendens

The neuromodulator serotonin is an important regulator of aggressive behavior in vertebrates. Experimentally increasing synaptic levels of serotonin with fluoxetine, a selective serotonin reuptake inhibitor, has been shown to reliably decrease the expression of aggressive behavior. Here, we describe a method by which fluoxetine can be noninvasively administered to male Betta splendens (an attractive model for the study of aggressive behavior) and describe a simple laboratory exercise that allows students to experimentally investigate the physiological mechanisms of aggressive behavior. We demonstrate that relatively short-term exposure (3 h) of male bettas to as little as 3 microg/ml of fluoxetine-treated aquarium water is sufficient to reduce the expression of specific aggressive behaviors. We emphasize the physiological concepts that can be addressed with this exercise, including the role of the serotonergic system in regulating aggression, and the interplay of environmental contaminants and physiology in regulating the expression of behavior. We also highlight important aspects of experimental design. This exercise can be flexibly altered to accommodate one or several laboratory periods. It is also low cost, is low impact to the animals, and requires minimal preparation time for instructors.

Serotonin decreases aggression via 5-HT1A receptors in the fighting fish Betta splendens

The role of the monoamine neurotransmitter serotonin (5-HT) in the modulation of conspecific aggression in the fighting fish (Betta splendens) was investigated using pharmacological manipulations. We used a fish's response to its mirror image as our index of aggressive behavior. We also investigated the effects of some manipulations on monoamine levels in the B. splendens brain. Acute treatment with 5-HT and with the 5-HT1A receptor agonist 8-OH-DPAT both decreased aggressive behavior; however, treatment with the 5-HT1A receptor antagonist WAY-100635 did not increase aggression. Chronic treatment with the selective serotonin reuptake inhibitor fluoxetine caused no significant changes in aggressive behavior and a significant decline in 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations. Treatment with the serotonin synthesis inhibitor p-chlorophenylalanine resulted in no change in aggression, yet serotonergic activity decreased significantly. Finally, a diet supplemented with L-tryptophan (Trp), the precursor to 5-HT, showed no consistent effects on aggressive behavior or brain monoamine concentrations. These results suggest a complex role for serotonin in the expression of aggression in teleost fishes, and that B. splendens may be a useful model organism in pharmacological and toxicological studies.

Serotonin in a diencephalic nucleus controlling communication in an electric fish: Sexual dimorphism and relationship to indicators of dominance

Serotonin regulates aggressive behavior. The production or release of serotonin is sexually dimorphic and related to social rank in many species. We examined serotonin expression in the central posterior/prepacemaker nucleus (CP/PPn) of the electric fish Apteronotus leptorhynchus. The CP/PPn is a thalamic nucleus that controls agonistic and reproductive electrocommunication signals known as chirps and gradual frequency rises. In parts of the CP/PPn that control chirping, females had more than twice as many serotonergic fibers and terminals as did males. Serotonin immunoreactivity in chirp-controlling areas of the CP/PPn was also negatively correlated with two indicators of dominance: electric organ discharge (EOD) frequency and body mass. Within sexes, the negative correlation between EOD frequency and serotonergic innervation of the PPn was significant in females, but not in males. Females with higher EOD frequencies had less serotonin in the CP/PPn than did females with lower EOD frequencies. Thus, the CP/PPn contained more serotonin in females than in males, and in particular, more serotonin in females with EOD frequencies typical of social subordinates than in females with EOD frequencies typical of social dominants. These results, combined with previous findings that serotonin inhibits chirping and that females chirp much less than males, suggest that serotonin may link sex, social rank, and the production of agonistic communication signals. The relative simplicity of the neural circuits that control the EOD and chirping make the electromotor system well-suited for studying the cellular, physiological, and behavioral mechanisms by which serotonin modulates agonistic communication.

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.

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

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

The effect of citalopram hydrobromide on 5-HT2A receptors in the impulsive-aggressive dog, as measured with 123I-5-I-R91150 SPECT

PURPOSE

Involvement of the serotonergic system in impulsive aggression has been demonstrated in both human and animal studies. The purpose of the present study was to investigate the effect of citalopram hydrobromide (a selective serotonin re-uptake inhibitor) on the 5-HT(2A) receptor and brain perfusion in impulsive-aggressive dogs by means of single-photon emission computed tomography.

METHODS

The binding index of the radioligand (123)I-5-I-R91150 was measured before and after treatment with citalopram hydrobromide in nine impulsive-aggressive dogs. Regional perfusion was measured with (99m)Tc-ethyl cysteinate dimer (ECD). Behaviour was assessed before treatment and again after 6 weeks of treatment.

RESULTS

A correlation was found between decreased binding and behavioural improvement in eight out of nine dogs. The 5-HT(2A) receptor binding index was significantly reduced after citalopram hydrobromide treatment in all cortical regions but not in the subcortical area. None of the dogs displayed alterations in perfusion on the post-treatment scans.

CONCLUSION

This study supports previous findings regarding the involvement of the serotonergic system in impulsive aggression in dogs in general. More specifically, the effect of treatment on the 5-HT(2A) receptor binding index could be demonstrated and the decreased binding index correlated with behavioural improvement.

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.

Increased sensitivity of the serotonergic system during the breeding season in free-living American tree sparrows

In order to understand the physiological role of serotonin in regulating aggressive behaviour it is important to understand how this neuromodulator acts within the context of a naturally fluctuating social and physical environment. To accomplish this, we examined the effect of the selective serotonin reuptake inhibitor fluoxetine during the breeding season in free-living male American tree sparrows (Spizella arborea) in Northern Alaska. During this time period males are maximally aggressive towards territorial intruders. Male, territorial sparrows were injected with either vehicle or a 10 mg/kg dose of fluoxetine. One hour later, aggression was measured using a simulated territorial intrusion. Depending upon when birds were sampled, the aggression scores for vehicle and fluoxetine treatments were grouped according to the number of days after territorial behaviour was initiated. The three groups were: early, days 1-5; middle, days 6-10; and late, days 11-15. There was a significant overall difference between groups (F(5,36)=5.18, P<0.0015). Post hoc analysis demonstrated that the level of aggression did not differ between the three groups of saline injected birds. However, fluoxetine injected birds showed a time dependent decrease in aggression. When compared to control birds the middle and late fluoxetine groups had significantly less aggression. Furthermore, the late group of fluoxetine treated birds were significantly less aggressive than the early group of fluoxetine birds. This demonstrates that free-living male ATSPs show a rapid change in their behavioural response to fluoxetine across the first 2 weeks of the breeding season. The rapidity of the change in responsiveness suggests a dynamic sensitivity of the serotonergic system.

Plasma corticosterone increases during migratory restlessness in the captive white-crowned sparrow Zonotrichia leucophrys gambelli

Plasma corticosterone increases during the period of spring migration in a variety of bird species. Long-distance migrants show elevations in corticosterone specifically in association with the stage of flight, suggesting that corticosterone may support flight-related processes, for example, locomotor activity and/or energy mobilization. The pattern of corticosterone secretion as it relates to migratory flight has hitherto not been clearly described in migrants that frequently interrupt flight to refuel, for example, the Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). The Gambel's white-crowned sparrow fuels by day and expresses peak migratory activity during the first few hours of night. To determine if plasma corticosterone increases in association with the stage of migratory flight also in this short-bout migrant, we induced captive white-crowned sparrows to enter into the migratory condition by placing photosensitive birds on long days (16L:8D) and then evaluated birds for plasma corticosterone and locomotor activity during four time points of the day. Patterns found in long-day birds were compared to those observed in short-day controls (8L:16D). Differences in energy metabolism as determined from plasma metabolites were also evaluated. We found that locomotor activity and corticosterone were significantly elevated at the onset of the dark period, but only in long-day birds. Plasma beta-hydroxybutyrate (a ketone body) was also elevated. Thus, findings suggest that plasma corticosterone and ketogenesis increase in association with migratory restlessness in a short-bout migrant. In fact, corticosterone may play a regulatory role, because it shows a trend to increase already before night-time activity.

Neural responses to territorial challenge and nonsocial stress in male song sparrows: segregation, integration, and modulation by a vasopressin V1 antagonist

The present experiments were conducted to determine (1) which basal forebrain regions and/or their peptidergic components are responsive to social challenge and nonsocial stress, and (2) the influence of an arginine vasopressin V(1) antagonist (AVPa) on these responses. Experiments were conducted in wild-caught male song sparrows (Melospiza melodia) that were housed on seminatural territories (field-based flight cages). Subjects were each fitted with a chronic guide cannula directed at the lateral ventricle and exposed to one of five conditions before sacrifice and histochemistry: saline + simulated territorial intrusion (STI; consisting of song playback and presentation of a caged conspecific male), AVPa + STI, saline + empty cage, AVPa + empty cage, unhandled. Two tissue series were prepared and immunofluorescently double-labeled for ZENK (egr-1) protein and either arginine vasotocin (AVT; avian homologue of AVP) or corticotropin releasing factor (CRF). The results indicate that the neuronal populations that are sensitive to nonsocial stress (capture, handling and infusion) and STI are at least partially segregated. Increases in ZENK-immunoreactive (-ir) nuclei following handling and infusion were observed in a large number of areas, whereas neural responses that were specific to STI were more limited. However, multiple areas showed responses to both handling and STI. AVPa infusions significantly reduced or eliminated most experimental increases in ZENK-ir, suggesting a broad role for endogenous AVT in the modulation of baseline activity and/or stress responsivity, and a much more limited role in the specific response to social challenge. Particular attention is given to the numerous zones of the lateral septum (LS), which are differentially responsive to handling, STI, and V(1)-like receptor blockade. These data suggest that septal AVT modulates neural responses to general stressors, not social stimuli specifically. Thus, species differences in septal AVT function (as previously described in songbirds) likely reflect differences in the relationship of stress or anxiety to species-specific behaviors, or to behavior in species-typical contexts.

Malnutrition during brain growth spurt alters the effect of fluoxetine on aggressive behavior in adult rats

Malnutrition effect during the suckling period on aggressive behavior was investigated in adult rats treated and not treated with fluoxetine, a selective serotonin reuptake inhibitor. Sixty-four Wistar male rats were allocated in two groups, according to their mothers' diet during lactation. The well-nourished group was fed by mothers receiving a 23% protein diet; the malnourished one by mothers receiving a 8% protein diet. Following weaning, all rats received the 23% protein diet. On the 90th day after birth, each nutritional group was divided into two subgroups, one receiving a single daily injection of fluoxetine (10 mg/kg) and the other of a saline solution (0.9% NaCl) for 14 days. Treatment with Fluoxetine reduced aggressive response in well-nourished but not in malnourished rats. These findings suggest that the serotoninergic system was affected by malnutrition during the critical period of brain development, and persisted even after a long period of nutritional recovery.

The low-affinity glucocorticoid receptor regulates feeding and lipid breakdown in the migratory Gambel's white-crowned sparrow Zonotrichia leucophrys gambelii

Plasma corticosterone increases during spring migration in a variety of bird species, including the Gambel's white-crowned sparrow Zonotrichia leucophrys gambelii. Corticosterone is elevated specifically in association with migratory flight, suggesting that corticosterone may promote processes such as energy mobilization and/or migratory activity. General effects of glucocorticoids support such a prediction. Because glucocorticoids exert permissive effects on food intake, corticosterone may also participate in the regulation of migratory hyperphagia. To examine the role of corticosterone during migration, we induced Gambel's white-crowned sparrows to enter the migratory condition and compared food intake and locomotor activity between controls and birds injected with RU486--an antagonist to the low-affinity glucocorticoid receptor (GR). In addition, we investigated effects of RU486 in birds that were subjected to a short-term fast. Results indicate that RU486 did not affect locomotor activity. However, consistent with its effects in mammals, RU486 suppressed food intake. Thus, hyperphagia and migratory restlessness, the two behaviors that characterize migration, may be regulated by different mechanisms. Lastly, RU486 antagonized fasting-induced lipid mobilization, as evidenced by decreased plasma free fatty acids. Thus, data on spring migrants suggest that endogenous corticosterone levels act through the GR to support hyperphagia and that the GR promotes availability of lipid fuel substrates in association with periods of energetic demand, e.g. during migratory flight.