Mechanisms for quick and variable responses

Postweaning Isolation Alters the Responses of Auditory Neurons to Serotonergic Modulation

Juvenile social experience, such as social isolation, has profound effects on communicative behavior, including signal production and reception. In the current study, we explored responsiveness to the neuromodulator serotonin as a potential mechanistic link between early life social isolation and auditory processing. The serotonergic system is sensitive to social isolation in many brain regions including the inferior colliculus (IC), an auditory midbrain nucleus. We investigated the effects of social experience on serotonergic responsiveness by measuring cFos, an immediate early gene product, in the IC of female mice. Serotonin was manipulated pharmacologically by administering fenfluramine, pCPA, or saline to mice that had undergone an extreme dichotomy in social experience after weaning: being housed in social groups versus individually. These mice were exposed to a 60-min recording of vocalizations from an opposite-sex interaction and perfused. Using immunohistochemistry, we measured the density of cFos-positive (cFos+) nuclei in the major subdivisions of the IC. Housing condition, drug treatment, and IC subregion all had a significant effect on cFos+ density. The central IC showed the highest density of cFos+ cells and also the most pronounced effects of housing condition and drug treatment. In the central IC, cFos+ density was higher following fenfluramine treatment than saline, and lower following pCPA treatment than fenfluramine. Individually housed mice showed a higher cFos+ density than socially housed mice in both of the pharmacological treatment groups, but not in the saline group. Drug treatment but not housing condition had strong effects on the behaviors of grooming, digging, rearing, and movement. Once the effects of drug condition were controlled, there were no across-individual correlations between cFos+ densities and behaviors. These findings suggest that the responses of auditory neurons to neuromodulation by serotonin are influenced by early life experience.

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.

Neural and endocrine responses to social stress differ during actual and virtual aggressive interactions or physiological sign stimuli

Neural and endocrine responses provide quantitative measures that can be used for discriminating behavioral output analyses. Experimental design differences often make it difficult to compare results with respect to the mechanisms producing behavioral actions. We hypothesize that comparisons of distinctive behavioral paradigms or modification of social signals can aid in teasing apart the subtle differences in animal responses to social stress. Eyespots are a unique sympathetically activated sign stimulus of the lizard Anolis carolinensis that influence aggression and social dominance. Eyespot formation along with measurements of central and plasma monoamines enable comparison of paired male aggressive interactions with those provoked by a mirror image. The results suggest that experiments employing artificial application of sign stimuli in dyadic interactions amplify behavioral, neural and endocrine responses, and foreshorten behavioral interactions compared to those that develop among pairs naturally. While the use of mirrors to induce aggressive behavior produces simulated interactions that appear normal, some behavioral, neural, and endocrine responses are amplified in these experiments as well. In contrast, mirror image interactions also limit the level of certain behavioral and neuroendocrine responses. As true social communication does not occur during interaction with mirror images, rank relationships can never be established. Multiple experimental approaches, such as combining naturalistic social interactions with virtual exchanges and/or manipulation of sign stimuli, can often provide added depth to understanding the motivation, context, and mechanisms that produce specific behaviors. The addition of endocrine and neural measurements helps identify the contributions of specific behavioral elements to the social processes proceeding.

Effects of Chattonella antiqua on the swimming behavior and brain monoamine metabolism of juvenile yellowtail (Seriola quinqueradiata)

Being the precursor of serotonin and melatonin, dietary supplementation with tryptophan (TRP) may modulates behavior, stress responses, and antioxidant capacity in fish. In this study, effects of Chattonella exposure on the swimming behavior and brain monoamine metabolism of yellowtail fed a commercial diet (control diet) or that enriched by 1.5% L-TRP (TRP + diet) were investigated. A 7-day dietary TRP supplementation elevated spontaneous swimming speed of yellowtail and mitigated their behavioral response to Chattonella (250 cells/mL) exposure. A 30-day dietary TRP supplementation elevated growth of juvenile yellowtail. Lethal exposure to Chattonella (1000 cells/mL) significantly elevated the turnover rates of serotonin, dopamine, and norepinephrine metabolism in fish fed control diet, but did not alter the serotonin turnover rate in fish fed TRP + diet. Our results suggested that dietary supplementation with TRP had potential to mitigate the stress response in yellowtail to Chattonella, partly via mediating their brain monoamine metabolism.

Arginine Vasotocin, the Social Neuropeptide of Amphibians and Reptiles

Arginine vasotocin (AVT) is the non-mammalian homolog of arginine vasopressin (AVP) and, like vasopressin, serves as an important modulator of social behavior in addition to its peripheral functions related to osmoregulation, reproductive physiology, and stress hormone release. In amphibians and reptiles, the neuroanatomical organization of brain AVT cells and fibers broadly resembles that seen in mammals and other taxa. Both parvocellular and magnocellular AVT-containing neurons are present in multiple populations located mainly in the basal forebrain from the accumbens-amygdala area to the preoptic area and hypothalamus, from which originate widespread fiber connections spanning the brain with a particularly heavy innervation of areas associated with social behavior and decision-making. As for mammalian AVP, AVT is present in greater amounts in males in many brain areas, and its presence varies seasonally, with hormonal state, and in males with differing social status. AVT's social influence is also conserved across herpetological taxa, with significant effects on social signaling and aggression, and, based on the very small number of studies investigating more complex social behaviors in amphibians and reptiles, AVT may also modulate parental care and social bonding when it is present in these vertebrates. Within this conserved pattern, however, both AVT anatomy and social behavior effects vary significantly across species. Accounting for this diversity represents a challenge to understanding the mechanisms by which AVT exerts its behavioral effects, as well are a potential tool for discerning the structure-function relationships underlying AVT's many effects on behavior.

Putting the "Biology" Back into "Neurobiology": The Strength of Diversity in Animal Model Systems for Neuroscience Research

Current trends in neuroscience research have moved toward a reliance on rodent animal models to study most aspects of brain function. Such laboratory-reared animals are highly inbred, have been disengaged from their natural environments for generations and appear to be of limited predictive value for successful clinical outcomes. In this Perspective article, we argue that research on a rich diversity of animal model systems is fundamental to new discoveries in evolutionarily conserved core physiological and molecular mechanisms that are the foundation of human brain function. Analysis of neural circuits across phyla will reveal general computational solutions that form the basis for adaptive behavioral responses. Further, we stress that development of ethoexperimental approaches to improve our understanding of behavioral nuance will help to realign our research strategies with therapeutic goals and improve the translational validity of specific animal models. Finally, we suggest that neuroscience has a role in environmental conservation of habitat and fauna that will preserve and protect the ecological settings that drive species-specific behavioral adaptations. A rich biodiversity will enhance our understanding of human brain function and lead in unpredicted directions for development of therapeutic treatments for neurological disorders.

Changes in regional brain monoaminergic activity and temporary down-regulation in stress response from dietary supplementation with l-tryptophan in Atlantic cod (Gadus morhua)

The brain monoamines serotonin (5-hydroxytryptamine; 5-HT) and dopamine (DA) both play an integrative role in behavioural and neuroendocrine responses to challenges, and comparative models suggest common mechanisms for dietary modulation of transmission by these signal substances in vertebrates. Previous studies in teleosts demonstrate that 7 d of dietary administration with L-tryptophan (Trp), the direct precursor of 5-HT, suppresses the endocrine stress response. The present study investigated how long the suppressive effects of a Trp-enriched feed regimen, at doses corresponding to two, three or four times the Trp levels in commercial feed, last in juvenile Atlantic cod (Gadus morhua) when the fish are reintroduced to a diet with standard amino acid composition. We also wanted to determine whether Trp supplementation induced changes in brain monoaminergic neurochemistry in those forebrain structures innervated by DA and 5-HTergic neurons, by measuring regional activity of DA and 5-HT in the lateral pallial regions (Dl) of the telencephalon and nucleus lateralis tuberis (NLT) of the hypothalamus. Dietary Trp resulted in a dose-dependent suppression in plasma cortisol among fish exposed to confinement stress on the first day following experimental diet; however, such an effect was not observed at 2 or 6 d after Trp treatment. Feeding the fish with moderate Trp doses also evoked a general increase in DA and 5-HT-ergic activity, suggesting that these neural circuits within the NLT and Dl may be indirectly involved in regulating the acute stress response.

Social regulation of serotonin in the auditory midbrain

The neuromodulator serotonin regulates auditory processing and can increase within minutes in response to stimuli like broadband noise as well as nonauditory stressors. Little is known about the serotonergic response in the auditory system to more natural stimuli such as social interactions. Using carbon-fiber voltammetry, we measured extracellular serotonin in the auditory midbrain of resident male mice during encounters with a male intruder. Serotonin increased in the inferior colliculus (IC) over the course of a 15 minute interaction, but not when mice were separated with a perforated barrier. Several behaviors, including the amount of immobility and anogenital investigation performed by the resident, were correlated with the serotonergic response. Multiple intrinsic factors associated with individual mice also correlated with the serotonergic response. One of these was age: older mice had smaller serotonergic responses to the social interaction. In a second interaction, individual identity predicted serotonergic responses that were highly consistent with those in the first interaction, even when mice were paired with different intruders. Serotonin was also significantly elevated in the second social interaction relative to the first, suggesting a role for social experience. These findings show that during social interaction, serotonin in the IC is influenced by extrinsic factors such as the directness of social interaction and intrinsic factors including age, individual identity, and experience.

Social rank modulates brain arginine vasotocin immunoreactivity in false clown anemonefish (Amphiprion ocellaris)

The brain nanopeptide arginine vasotocin (AVT) and its mammalian homolog arginine vasopressin are involved in the regulation of social and reproductive behavior. We investigated the relationship between social rank formation and the brain AVT system in the false clown anemonefish (Amphiprion ocellaris), which forms a social rank that leads to sex differentiation in higher-ranked individuals. Tanks of three sexually immature fish were kept for 90 days and each fish's behavior was observed once a month. The social rank of each individual was distinguishable by behavior, but gonadosomatic index (GSI) did not differ significantly. The number of AVT neurons in the magnocellular layer in the preoptic area (POA) increased in subordinate individuals and declined with increasing hierarchical dominance. These results suggest that social rank formation modulates AVT production in the brain of the clown anemonefish and may influence their later sex differentiation.

Regulation of pseudosexual behavior in the parthenogenetic whiptail lizard, Cnemidophorus uniparens

Neuroendocrine mechanisms underlying complementary behaviors like male-typical mounting and female-typical receptivity are most often studied independently in males and females, respectively. Cnemidophorus uniparens is a unisexual lizard species consisting only of females that alternately express male- and female-like pseudosexual behavior across the ovarian cycle. Intact, postovulatory (PostOv), and ovariectomized (OVX), androgen-implanted animals [OVX plus testosterone (T)] exhibit male-like mounting, but not receptivity, whereas intact, preovulatory (PreOv), and OVX lizards injected with estradiol [OVX plus estrogen (E)] express receptivity, but not mounting. We tested whether the serotonergic system in the preoptic area (POA) and ventromedial nucleus of the hypothalamus (VMN) gates the reciprocal inhibition characterizing this alternating expression of mounting and receptivity. Serotonergic signaling at the POA appears to be key to gating male-like behavior. Postovulatory and OVX plus T animals have lower intracellular serotonin (5-HT) levels, and greater abundance of inhibitory 5-HT1A receptor mRNA in the POA compared with both PreOv and OVX plus E lizards. Moreover, injecting 5-HT into the POA of OVX plus T animals suppresses mounting, whereas injection into VMN of OVX plus E lizards suppresses receptivity. Although 5-HT levels in the VMN do not differ across the ovarian cycle or between hormonally manipulated animals, PreOv and OVX plus E lizards have a lower abundance of 5-HT2A mRNA in the VMN. Stimulating 5-HT1A receptors using systemic drug administration inhibits mounting, whereas activating 5-HT2A receptors facilitates receptivity. This study illuminates how male- and female-typical sexual behaviors share common neural circuits, and that 5-HT regulates these naturally complementary, and mutually exclusive, behaviors.

Memory systems in the chick: regional and temporal control by noradrenaline

Learning starts with the information about a situation or experience delivered to different brain areas in terms of visual, olfactory, auditory and tactile inputs. Memory processing occurs in different brain locations in a well-defined temporal sequence of physiologically based stages and biochemical cascades. Using neuropharmacological techniques in one species and a robust bead discrimination task, we have been able to chart the passage of memory from acquisition to consolidation in the chick and to dissect out the multiple roles for noradrenaline in consolidating this memory. Fortunately only a small fraction of sensory input is remembered and it is clear that modulatory neurotransmitters play a key role in determining what is remembered. We have identified roles for noradrenaline in the mesopallium or 'avian cortex', the hippocampus, medial striatum or basal ganglia and teased out the different effects of noradrenaline in each of these areas based on the receptor subtypes activated by the transmitter and the stages on which they act. Noradrenergic input from the locus coeruleus controls memory processing at two critical times after training-acquisition (0-2.5 min after training) and consolidation (25-30 min after training). We have also elucidated some of the cellular mechanisms whereby noradrenaline achieves memory modulation and finds that it has actions on both neurones and astrocytes with particularly important effects on energy metabolism in astrocytes. The memory system of the chick is very similar to that of mammals in terms of brain regions recruited in memory processing and in the ways memory is modulated by noradrenaline.

Nuclear organization and morphology of serotonergic neurons in the brain of the Nile crocodile, Crocodylus niloticus

The present study describes the location and nuclear organization of the serotonergic system in a representative of the order Crocodylia, the Nile crocodile (Crocodylus niloticus). We found evidence for serotonergic neurons in three regions of the brain, including the diencephalon, rostral and caudal brainstem, as previously reported in several other species of reptile. Within the diencephalon we found neurons in the periventricular organ of the hypothalamus, but not in the infundibular recess as noted in some other reptilian species. In addition we found serotonergic neurons in the pretectal nucleus, this being the first description of these neurons in any species. Within the rostral brainstem we found medial and lateral divisions of the superior raphe nucleus and a widely dispersed group of neurons in the tegmentum, the superior reticular nucleus. In the caudal brainstem we observed the inferior raphe nucleus and the inferior reticular nucleus. While much of the serotonergic system of the Nile crocodile is similar to that seen in other reptiles the entire suite of features appears to distinguish the crocodile studied from the members of the Squamate (lizards and snakes) and Testudine (turtles, tortoises and terrapins) reptiles previously studied. The observations are suggestive of order-specific patterns of nuclear organization of this system in the reptiles, reflecting potential evolutionary constraints in the mutability of the nuclear organization as seen for similar systems in mammals.

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.

Memory of opponents is more potent than visual sign stimuli after social hierarchy has been established

During agonistic interactions between male Anolis carolinensis, perception of a visual sign stimulus (darkened eyespots) not only inhibits aggression and promotes initial attainment of dominant social status, but also evokes distinct neuroendocrine responses in each opponent. This study was designed to examine the effect of eyespot manipulation on behavior and social rank during a second interaction between opponents that had previously established a natural dyadic social hierarchy. Prior to a second interaction, eyespots of familiar size-matched combatants were manipulated to reverse information conveyed by this visual signal. Eyespots on the previously dominant male were masked with green paint to indicate low aggression and social status. Previously subordinate males had their eyespots permanently marked with black paint to convey high aggression and status. Opponents were then re-paired for a second 10 min interaction following either 1 or 3 days of separation. Aggression was generally decreased and social status between pairs remained reasonably consistent. Unlike rapidly activated monoaminergic activity that occurs following the initial pairing, most brain areas sampled were not affected when animals were re-introduced, regardless of visual signal reversal or length of separation between interactions. However in males with "normal" eyespot color, dominant males had reduced serotonergic activity in CA(3) and raphé, while subordinate males exhibited elevated CA(3) dopaminergic activity. Reversing eyespot color also reversed serotonergic activity in raphé and dopaminergic activity in CA(3) after 3 days of separation. The results suggest that males remember previous opponents, and respond appropriately to their previous social rank in spite of eyespot color.

Behavioral and neurophysiological responses of European sea bass groups reared under food constraint

The individual food-demand behavior of juvenile European sea bass (Dicentrarchus labrax, L.) reared in groups under self-feeding conditions was investigated. The triggering activity on self-feeder, i.e. index of the food-demand activity, agonistic interactions and territorial behavior were monitored for periods of 42 to 68 days in six groups of 50 fish. The specific growth rate was calculated and the brain serotonergic activity was used as a stable index of social stress. Inter-individual differences appeared in triggering activity and three groups were distinguished: 3-5 high-triggering fish, 17-30 low-triggering fish and the remaining individuals were null-triggering fish. There were no significant differences in specific growth rates calculated at the end of the experiment (day 42 or day 68) between individuals with high, low, and null food-demand (ANOVA, p>0.05). No territorial or agonistic behaviors were observed, however, there were significant differences in brain serotonergic activity between the three triggering groups (ANOVA, p=0.050 in telencephalon and p=0.004 in cerebellum). Specifically, high-triggering fish had lower serotonergic turnover than low or null-triggering fish. We put forth the hypothesis that fish with low or null-triggering activity could be stressed by the high activity of high-triggering individuals.

Serotonergic modulation of male-like pseudocopulatory behavior in the parthenogenetic whiptail lizard, Cnemidophorus uniparens

Hormone-neurotransmitter interactions form an important link through which hormones influence a variety of behavioral processes. Typically, sexual behavior is dimorphic with males mounting receptive females. In the all-female lizard species Cnemidophorus uniparens, individuals display both male-like pseudocopulation and female-like receptivity. These respective behavioral states are correlated with high circulating concentrations of progesterone following ovulation and of estrogen preceding it. In sexual species, serotonin is involved in male-typical mounting, and, as reported here, in male-like pseudosexual behavior in this unisexual species. In the first study, C. uniparens were ovariectomized and treated systemically with exogenous androgen, a hormonal regimen that results in individuals displaying only male-like pseudosexual behavior. An increase in serotonin levels in the preoptic area coupled with the suppression of male-like pseudocopulation was observed in androgen-treated lizards injected with 5-hydroxytryptophan (the precursor of serotonin) and clorgyline (a monoamine oxidase inhibitor) compared to vehicle-treated controls. Our second experiment involved ovariectomizing lizards and either injecting them with estradiol or implanting them with either an empty (Blank) or a progesterone- or testosterone-containing Silastic capsule. Treatment with para-chlorophenylalanine (an inhibitor of tryptophan hydroxylase) facilitated male-like pseudosexual behavior depending on the circulating hormonal milieu and decreased serotonin levels in the preoptic area. Our data suggest that serotonin is inhibitory to male-like pseudosexual behavior in C. uniparens but more importantly that the hormonal environment modulates the serotonin system at the level of the preoptic area, with the serotonergic system then establishing behavioral thresholds that allow for this behavior to be "gated".

Aggression and vasotocin are associated with dominant–subordinate relationships in zebrafish

Agonistic interactions are present throughout the animal kingdom as well as in humans. In this report, we present a model system to study neurological correlates of dominant-subordinate relationships. Zebrafish, Danio rerio, has been used as a model system for developmental biology for decades. We propose here that it is also an excellent model for studying social behavior. Adult male zebrafish were separated for 5 days and then pairs were formed and allowed to interact for 5 days. Under these conditions, aggression is prevalent and dominant-subordinate relationships are quickly established. Dominant behavior is characterized by a repeated pattern of chasing and biting, whereas subordinates engage in retreats. By day 5, the dominant-subordinate relationship was firmly established and there were differences in behavior over time. Chases, bites and retreats were all less frequent on day 5 of the social interaction than on day 1. Arginine vasotocin is the teleostean homologue of arginine vasopressin, a neuropeptide whose expression has been linked to aggression and social position in mammals. Immunohistochemistry indicated differences in vasotocin staining between dominant and subordinate individuals. Dominant individuals express vasotocin in one to three pairs of large cells in the magnocellular preoptic area whereas subordinate individuals express vasotocin in 7-11 pairs of small cells in the parvocellular preoptic area. These results suggest that the vasotocinergic system may play a role in shaping dominant-subordinate relationships and agonistic behavior in this model organism.

Dopaminergic activity modulation via aggression, status, and a visual social signal

Social interaction may elicit aggression, establish social rank, and be influenced by changes in central dopaminergic activity. In the lizard Anolis carolinensis, a sign stimulus (darkening of postorbital skin or eyespots) inhibits aggressive response from opponents, in part because it forms more rapidly in dominant males. The authors report that artificially hiding or darkening eyespots influences central dopaminergic activity, social status, and aggression during dyadic social interaction. All males that viewed an opponent with eyespots painted black became subordinate and exhibited elevated dopamine in raphe, lateral amygdala, and medial amygdala but decreased dopamine in septum and locus ceruleus. In contrast, males that viewed opponents with hidden eyespots (painted green) became dominant and had increased dopamine in striatum, nucleus accumbens, hypothalamus, and combined substantia nigra/ventral tegmental area.

Stress induces rapid changes in central catecholaminergic activity in Anolis carolinensis: Restraint and forced physical activity

Immobilization stress and physical activity separately influence monoaminergic function. In addition, it appears that stress and locomotion reciprocally modulate neuroendocrine responses, with forced exercise ameliorating stress-induced serotonergic activity in lizards. To investigate the interaction of forced physical activity and restraint stress on central dopamine (DA), norepinephrine (NE), and epinephrine (Epi), we measured these catecholamines and their metabolites in select brain regions of stressed and exercised male Anolis carolinensis lizards. Animals were handled briefly to elicit restraint stress, with some lizards additionally forced to run on a track until exhaustion, or half that time (50% of average time to exhaustion), compared to a control group that experienced no restraint or exercise. Norepinephrine concentrations in the hippocampus and locus ceruleus decreased with restraint stress, but returned to control levels following forced exhaustion. Levels of NE in the raphé nuclei and area postrema, and epinephrine in raphé became elevated following restraint stress, and returned to control levels following forced physical activity to 50% or 100% exhaustion. Striatal DA increased as animals were exercised to 50% of exhaustion, and returned to baseline with exhaustion. At exhaustion, striatal Epi levels were diminished, compared with controls. In the area postrema, exhaustion reversed a decline in epinephrine levels that followed forced physical activity. These results suggest that stress stimulates a rapid influence on central catecholamines. In addition, forced exercise, and even exhaustion, may alleviate the effects of restraint stress on central monoamines.

Modulation of aggressive behaviour by fighting experience: mechanisms and contest outcomes

Experience in aggressive contests often affects behaviour during, and the outcome of, later contests. This review discusses evidence for, variations in, and consequences of such effects. Generally, prior winning experiences increase, and prior losing experiences decrease, the probability of winning in later contests, reflecting modifications of expected fighting ability. We examine differences in the methodologies used to study experience effects, and the relative importance and persistence of winning and losing experiences within and across taxa. We review the voluminous, but somewhat disconnected, literature on the neuroendocrine mechanisms that mediate experience effects. Most studies focus on only one of a number of possible mechanisms without providing a comprehensive view of how these mechanisms are integrated into overt behaviour. More carefully controlled work on the mechanisms underlying experience effects is needed before firm conclusions can be drawn. Behavioural changes during contests that relate to prior experience fall into two general categories. Losing experiences decrease willingness to engage in a contest while winning experiences increase willingness to escalate a contest. As expected from the sequential assessment model of contest behaviour, experiences become less important to outcomes of contests that escalate to physical fighting.A limited number of studies indicate that integration of multiple experiences can influence current contest behaviour. Details of multiple experience integration for any species are virtually unknown. We propose a simple additive model for this integration of multiple experiences into an individual's expected fighting ability. The model accounts for different magnitudes of experience effects and the possible decline in experience effects over time. Predicting contest outcomes based on prior experiences requires an algorithm that translates experience differences into contest outcomes. We propose two general types of model, one based solely on individual differences in integrated multiple experiences and the other based on the probability contests reach the escalated phase. The difference models include four algorithms reflecting possible decision rules that convert the perceived fighting abilities of two rivals into their probabilities of winning. The second type of algorithm focuses on how experience influences the probability that a subsequent contest will escalate and the fact that escalated contests may not be influenced by prior experience. Neither type of algorithm has been systematically investigated.Finally, we review models for the formation of dominance hierarchies that assume that prior experience influences contest outcome. Numerous models have reached varied conclusions depending on which factors examined in this review are included. We know relatively little about the importance of and variation in experience effects in nature and how they influence the dynamics of aggressive interactions in social groups and random assemblages of individuals. Researchers should be very active in this area in the next decade. The role of experience must be integrated with other influences on contest outcome, such as prior residency, to arrive at a more complete picture of variations in contest outcomes. We expect that this integrated view will be important in understanding other types of interactions between individuals, such as mating and predator-prey interactions, that also are affected significantly by prior experiences.

Does Individual Variation in Stress Responses and Agonistic Behavior Reflect Divergent Stress Coping Strategies in Juvenile Rainbow Trout?

Individual rainbow trout were transferred to visual isolation in experimental aquaria. As a measure of the speed of acclimation, individual food intake was quantified during the first 6 d following transfer. Following acclimation, aggression was quantified by subjecting the fish to three resident-intruder tests, with 30 d of recovery between the tests. Moreover, between the resident-intruder tests (i.e., two times) the fish were exposed to an unfamiliar environment and their cortisol response was measured. The results of this study show that individuals of juvenile rainbow trout differ distinctly in their response to changes in their environment, and that this diversity in behavior is reflected by consistent behavioral traits displayed by individual fish. These traits have proven to be consistent not only over time but also across situations, revealing two distinct behavioral profiles, in the same manner as shown in studies on proactive and reactive mammals. Our results also show that the reactivity of the hypothalamic-pituitary-interrenal (HPI) axis, when exposed to a stressor, is a consistent physiological trait in juvenile rainbow trout. We found that difference in HPI axis reactivity is linked to the different behavioral profiles. However, HPI axis reactivity could not be linked directly to the singular behavioral traits measured. In other words, we did not find that the consistent behavioral traits shown by the fish were associated with a difference in HPI axis reactivity in the same manner as the reactivity of the hypothalamic-pituitary-adrenocortical axis does in mammals. Taken together, our results show that stress coping strategies akin to what has been described as reactive and proactive stress coping in mammals appear to exist in juvenile rainbow trout.

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.

Fluoxetine-treated male wrasses exhibit low AVT expression

In many species, increasing serotonergic activity can reduce aggression and reverse dominance relationships. These effects may in part be mediated through interactions with the arginine vasotocin/vasopressin (AVT/AVP) system. We tested this hypothesis in a territorial coral reef fish, the bluehead wrasse (Thalassoma bifasciatum), by experimentally enhancing serotonergic neurotransmission, using the selective serotonin re-uptake inhibitor (SSRI) fluoxetine. Terminal phase (TP) males received 2 weeks of nightly intraperitoneal fluoxetine injections (6 microg/g body weight) and were then tested for their aggressive response to an intruder and killed to examine AVT phenotype in the preoptic area of the hypothalamus (POA), an area important to social behavior in fishes. Our previously published study demonstrated that fluoxetine-treated males are less aggressive [H.A.N. Perreault, K. Semsar, J. Godwin, Fluoxetine treatment decreases territorial aggression in a coral reef fish, Physiol. and Behav. 79 (2003) 719-724.]. Here, further study of these same fluoxetine-treated males shows approximately twofold lower AVT mRNA expression relative to saline-treated controls in all regions of the POA (all p< or =0.05) without any changes in AVT-ir soma size (all p>0.4). This study experimentally supports the hypothesis that behavioral effects of SSRIs may be mediated in part through interactions with the AVT/AVP system. These results parallel findings from rodents and humans and are consistent with an indirect neurosteroidogenic rather than a solely direct serotonergic mechanism for SSRI effects on the AVT/AVP system. Furthermore, they suggest that SSRI effects on neuroendocrine function may be best modeled in animals with sensitive stress responses such as those found in nondomesticated animals.

Dynamics and mechanics of social rank reversal

Stable social relationships are rearranged over time as resources such as favored territorial positions change. We test the hypotheses that social rank relationships are relatively stable, and although social signals influence aggression and rank, they are not as important as memory of an opponent. In addition, we hypothesize that eyespots, aggression and corticosterone influence serotonin and N-methyl-D: -aspartate (NMDA) systems in limbic structures involved in learning and memory. In stable adult dominant-subordinate relationships in the lizard Anolis carolinensis, social rank can be reversed by pharmacological elevation of limbic serotonergic activity. Any pair of specific experiences: behaving aggressively, viewing aggression or perceiving sign stimuli indicative of dominant rank also elevate serotonergic activity. Differences in the extent of serotonergic activation may be a discriminating and consolidating factor in attaining superior rank. For instance, socially aggressive encounters lead to increases in plasma corticosterone that stimulate both serotonergic activity and expression of the NMDA receptor subunit 2B (NR(2B)) within the CA(3) region of the lizard hippocampus. Integration of these systems will regulate opponent recognition and memory, motivation to attack or retreat, and behavioral and physiological reactions to stressful social interactions. Contextually appropriate social responses provide a modifiable basis for coping with the flexibility of social relationships.

Dominance status predicts response to nonsocial forced movement stress in the green anole lizard (Anolis carolinensis)

We used changes in body color and eyespot formation, two somatic indices of stress controlled mainly by catecholamine activity, to compare the reactions of dominant and subordinate male green anole lizards (Anolis carolinensis) to a nonsocial stressor, forced movement. Individual males were pretested by subjecting them to 10 min of forced movement induced by chasing them around their home cage with a slender wooden stick. Stress responses were assayed via changes in body color (progressive darkening from green to brown indicating increasing stress) and expression of a black postorbital eyespot (which appears with increasing catecholaminergic stress responses). Lizards were paired and allowed to form stable dominant/subordinate relationships for 2 weeks. After that period of stable social status, dominants and subordinates were separated and subjected to the same forced-movement stress. There was no difference between experimental groups in the pretest. After assuming positions in the dominance hierarchy, however, dominant males showed reduced somatic indicators of stress and were quicker to recover from the stress. The data suggest that animals that assumed the dominant position decreased their stress response relative to the pretest, while animals that assumed the subordinate position increased their stress response relative to the pretest. The results indicate that dominant social status may have advantages beyond the realm of social interactions by enhancing an individual's ability to tolerate other, nonsocial stressful events.

Serotonergic response to social stress and artificial social sign stimuli during paired interactions between male Anolis carolinensis

Serotonergic activity is influenced by social status and manipulation of social signals. In the lizard Anolis carolinensis, eyespot formation, i.e. darkening of postorbital skin from green to black, appears during stressful and agonistic situations, forming first in males that become dominant. To assess the effect of eyespots on central serotonergic activity during social interaction, males were paired by weight and painted postorbitally with green or black paint. Manipulation of eyespot color influenced social interactions and status. All males that viewed an opponent with black painted eyespots became subordinate. In these subordinate animals, serotonergic activity was elevated in hippocampus, striatum, nucleus accumbens and locus ceruleus. In contrast, males that viewed opponents with hidden eyespots (painted green) and became dominant had increased serotonergic activity in hypothalamus, medial amygdala and raphé. Pre-painted eyespots produced results that distinguish dominant and subordinate relationships based on serotonergic activity not previously seen in unmanipulated pairs. Results from experiments using pairs are similar to those using mirrors for medial amygdala and locus ceruleus, but not hippocampus, nucleus accumbens or raphé. Decreased hypothalamic serotonin was associated with increased aggressive behavior. These results, when compared with previous studies, suggest some flexibility in central serotonergic systems, which may shape dominant and subordinate rank acquisition, and appear to be influenced by the completion of social role formation. Furthermore, social status and central serotonergic activity was influenced by a visual cue, the presence or absence of postorbital eyespots on an opponent.

The green anole (Anolis carolinensis): a reptilian model for laboratory studies of reproductive morphology and behavior

The green anol (Anolis carolinensis) is an excellent reptilian model for studying reproductive behavior and the neural and muscular morphology that supports it. This lizard has been the subject of behavioral and ecological study for more than 100 yr, and a rich literature exists on its natural history. Both courtship and copulatory behaviors reveal sex and seasonal differences, which allow for the study of mechanisms regulation naturally occurring variation in performance at multiple levels within a single animal model. Green anoles are readily obtained due to their abundance in the wild; once in the laboratory, they are easily maintained, bred, and reared. Background on the natural history and husbandry of this lizard is provided, and the authors' research program on the regulation of reproductive anatomy and behavior is reviewed, Discussion includes the similarities and differences in the mechanisms mediating both structure and function compared with more traditional animal models. This type of comparative research will make it possible to identify the fundamental principles governing reproductive biology, thus advancing both basic and applied knowledge.

Social stress and corticosterone regionally upregulate limbic N-methyl-d-aspartatereceptor (NR) subunit type NR2A and NR2B in the lizard anolis carolinensis

Social aggression in the lizard Anolis carolinensis produces dominant and subordinate relationships while elevating corticosterone levels and monoaminergic transmitter activity in hippocampus (medial and mediodorsal cortex). Adaptive social behavior for dominant and subordinate male A. carolinensis is learned during aggressive interaction and therefore was hypothesized to involve hippocampus and regulation of N-methyl-d-aspartate (NMDA) receptors. To test the effects of social stress and corticosterone on NMDA receptor subunits (NR), male lizards were either paired or given two injections of corticosterone 1 day apart. Paired males were allowed to form dominant-subordinate relationships and were killed 1 day later. Groups included isolated controls, dominant males, subordinate males and males injected with corticosterone. Brains were processed for glutamate receptor subunit immunohistochemistry and fluorescence was analyzed by image analysis for NR(2A) and NR(2B) in the small and large cell divisions of the medial and mediodorsal cortex. In the small granule cell division there were no significant differences in NR(2A) or NR(2B) immunoreactivity among all groups. In contrast, there was a significant upregulation of NR(2A) and NR(2B) subunits in the large pyramidal cell division in all three experimental groups as compared with controls. The results revealed significantly increased NR(2A) and NR(2B) subunits in behaving animals, whereas animals simply injected with corticosterone showed less of an effect, although they were significantly increased over control. Upregulation of NR(2) subunits occurs during stressful social interactions and is likely to be regulated in part by glucocorticoids. The data also suggest that learning social roles during stressful aggressive interactions may involve NMDA receptor-mediated mechanisms.

A neural network underlying individual differences in emotion and aggression in male golden hamsters

In rodents, aggressive behavior can be altered by experimental manipulations of emotional responsiveness. The goal of this study was to identify characteristics of emotional reactivity associated with individual differences in aggressive behavior and their integration within a common neural network. Male golden hamsters were first screened for offensive aggression. Then, the animals were trained through immediate reinforcement and tested for their adaptation to a delayed reward. Similar protocols have been used to test behaviors associated with frustration. At first, all hamsters showed increased frequency of bar pressing per reward during delayed reinforcement. However, Low-Aggression animals were able to adapt to the delay and showed a decreased rate of bar pressing per reward within 5 days. In contrast, High-Aggression animals maintained a high rate of bar pressing per reward. In addition, brains were collected after immediate reward training or delayed reward testing, and labeled for pCREB-immunoreactivity as a marker of trans-synaptic activity. In High-Aggression individuals, elevated density of cyclic AMP response element binding protein, phosphorylated (pCREB) immunostaining was found within the anterior hypothalamus, an area critical to the control of aggression. Delayed reinforcement was associated with enhanced pCREB immunostaining within the central amygdala, medial amygdala and preoptic area/hypothalamus continuum. Further analysis of the data also showed a positive correlation in labeling density between the lateral septum and the anterior hypothalamus, specifically in Low-Aggression animals exposed to delayed reward. Therefore, as High-Aggression individuals lack control of their emotional reactivity, they are also characterized by a de-synchronization between the inhibitory output of the septum and the aggression areas of the hypothalamus. Finally, our data also show that frustration is associated with an extensive activation of the preoptic area/hypothalamus continuum and amygdala.

Interaction effects of corticosterone and experience on aggressive behavior in the green anole lizard

Aggressive encounters are accompanied by a release of stress hormone, and this corticosterone (CORT) secretion could influence aggressive behavior in subsequent encounters. We investigated the modulating effects of CORT on aggressive behavior in the context of a 5-day social experience in male green anole lizards. In Experiment 1, we measured plasma CORT levels in animals that were exposed for different times to aggressive males. In Experiment 2, using metyrapone, a CORT synthesis blocker, we tested whether CORT secretion in response to the aggressive stimulus plays a role in experience-dependent facilitation of aggressive behavior. We hypothesized that aggressive encounters would increase plasma CORT levels, and that blocking CORT synthesis with metyrapone treatment during the aggressive encounter would cause an animal to become more aggressive. We also tested whether blocking CORT would interfere with the influence of 5-day social experience on animals' behavior in a subsequent aggressive encounter. Animals that were exposed to another male showed higher plasma CORT levels immediately after the 10 min encounter than animals exposed to the non-social video, and this high level was maintained through day 5. Within the aggressive video groups, in Experiment 2, there was a distinctly different pattern in displays depending on drug condition: vehicle-injected animals showed gradual increases followed by decreases in aggressive behavioral responses to the video as the five days proceeded (habituation), while animals injected with metyrapone started out with high aggressive behavior and did not decrease behavioral responses at later trials (no habituation). Finally, when tested with a novel conspecific on day 6, animals previously injected with metyrapone showed no higher aggression than did animals previously injected with vehicle and exposed to the aggressive video. These results suggest that blocking CORT synthesis during the exposure to the aggressive video induced animals to remain aggressive toward the repetitive stimulus without habituating, while not becoming more aggressive than controls toward a novel challenger.

Sociality, stress, and the corpus striatum of the green anolis lizard

The green anolis lizard, Anolis carolinensis, is a uniquely convenient species with great potential for providing insights about the causes and consequences of social behavior from an evolutionary perspective. In this species, social interactions are mediated by visual displays in which specific units of behavior are combined in various ways to communicate several more-or-less specific messages. Two related research programs that utilize this species converge in provocative ways to provide insight into this phenomenon. The first program is centered on the basal ganglia, now known to be crucial to the expression of aggressive territoriality in this species, and the second research program examines the way the physiological stress response is involved in aggression and its subsequent adaptive outcomes. Both the neural and the neuroendocrine systems affect the progress of social interactions as well as the subsequent social dominance relationships when combatants subsequently live together. Further, because body color depends almost exclusively on the stress response, skin color provides a unique in situ bioassay of otherwise inaccessible information about the animal's internal state. The fullest understanding of the physiological ethology of this model species will depend on an interdisciplinary approach that considers both proximate (physiological) and ultimate (evolutionary) causes of displays. Questions thus arising include how the nervous system controls and assembles the specific units of behavior-motor patterns and autonomic reflexes-into displays that are adaptive in specific contexts.

The anabolic-androgenic steroid nandrolone induces alterations in the density of serotonergic 5HT1B and 5HT2 receptors in the male rat brain

Anabolic-androgenic steroids (AAS) are partly misused by males in order to become brave and intoxicated and these agents are highly associated with psychosis, disinhibition, aggression and acts of violence. Since such behavioral states have been related to an imbalanced serotonergic system and the involvement of the serotonergic 5HT(1B) and the 5HT(2) receptors, it was important to discern the impact of AAS on these receptors. The objective of our study was to investigate the effects of 2 weeks of treatment with the AAS nandrolone decanoate at three different doses (1, 5, 15 mg/kg/day) on the total specific binding of the radioligands [(125)I]-(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (5HT(2) receptors) by autoradiography. All doses caused a significant down-regulation of the 5HT(1B) receptor density in the hippocampal CA(1) and in the medial globus pallidus and a significant up-regulation of the 5HT(2) receptor density in the nucleus accumbens shell. Alterations in receptor density were also observed in the lateral globus pallidus, ventromedial hypothalamus, the amygdala and in the intermediate layers of various cortex regions. In conclusion, serotonergic 5HT(1B) or 5HT(2) receptors are likely to play important roles in mediating observed emotional states and behavioral changes among AAS abusers.

Sex differences and hormone influences on tyrosine hydroxylase immunoreactive cells in the leopard frog

We examined sex differences in tyrosine hydroxylase immunoreactive (TH-ir) cell populations in the preoptic area (POA), suprachiasmatic nucleus (SCN), posterior tuberculum (TP), and caudal hypothalamus (Hy) in the leopard frog (Rana pipiens), in addition to the effects of natural variation in sex steroid hormones on these same populations in both sexes. All four of these populations have been shown to be dopaminergic. Gonadal sex, androgens, and estrogen all influenced TH-ir cell numbers, but in a complicated pattern of interactions. After factoring out the effects of sex steroids by multiple regression, TH-ir cell numbers in all four areas differed between the sexes, with males having a greater number of TH-ir cells. The influence of androgens and estrogen differed by region and sex of the animals. Androgens were the main influence on TH-ir cell numbers in the POA and SCN. Plasma androgen concentrations were positively correlated with TH-ir cell numbers in both areas in males. In females, androgen concentration was negatively correlated with TH-ir cell numbers in the POA; there was no significant relationship in the SCN in females. In the more caudal populations, estrogen (E2) levels were positively correlated with TH-ir cell numbers in the TP of both males and females. In the caudal hypothalamus, E2 levels were positively correlated with TH-ir cell numbers in females, but there was no significant correlation in males. The results indicate that gonadal sex imposes a baseline sex difference in the four TH-ir (dopamine) populations, resulting in a higher number of such cells in males. Individual and sex-linked differences in gonadal steroid hormones lead to variation around this baseline condition, with androgens having a greater influence on rostral populations and estrogen on caudal populations. Last, an individual's gonadal sex determines the effect that androgens and estrogen have on each population.

Temporal patterns of limbic monoamine and plasma corticosterone response during social stress

Dominant and subordinate males respond differently to the stress of social interaction. After an hour of social interaction, subordinate male Anolis carolinensis have elevated serotonergic activity in hippocampus, but dominant males do not. In other species, and using other stressors, the activation of hippocampal serotonergic activity is much more rapid than one hour. To elucidate early stress responsiveness, adult male A. carolinensis were divided into four groups: isolated controls, and pairs of males sampled after 10, 20 or 40 minutes of aggressive interaction. Development of dominant-subordinate relationships was determined by behavior and by the celerity of eyespot darkening. Serotonergic activity in the hippocampus, nucleus accumbens and amygdala was elevated rapidly and equally in both dominant and subordinate males, as were plasma corticosterone concentrations. Serotonergic activity remained elevated through 40 minutes in hippocampus and nucleus accumbens. Only subordinate males had elevated corticosterone levels at 40 minutes. Social status does not impede socially induced stress responses. Rather, rapid regulation of serotonergic stress responses appears to be a mediating factor in determining both behavioral output and social status. Temporal expressions of monoaminergic and endocrine stress responses are distinctive between males of dominant and subordinate social status. Such temporal patterns of transmitter and glucocorticoid activity may reflect neurocircuitry adaptations that result in behavior modified to fit social status.

Serotonin modulates the electric waveform of the gymnotiform electric fish Brachyhypopomus pinnicaudatus

The gymnotiform electric fish Brachyhypopomus pinnicaudatus communicates with a sexually dimorphic electric waveform, the electric organ discharge (EOD). Males display pronounced circadian rhythms in the amplitude and duration of their EODs. Changes in the social environment influence the magnitudes of these circadian rhythms and also produce more transient responses in the EOD waveforms. Here we show that injections of serotonin produce quick, transient, dose-dependent enhancements of the male EOD characters similar to those induced by encounters with another male. The response to serotonin administered peripherally begins 5-10 min post injection and lasts approximately 3 h. The magnitude of the response to serotonin is tightly associated with the magnitude of the day-to-night swing of the circadian rhythm prior to injection. Taken together these findings suggest that the male's social environment influences his response to serotonin by altering the function of some part of the downstream chain between the serotonin receptors and the ion channels involved in control of the EOD waveform. Although chronic activation of serotonin circuitry is widely known to elicit subordinate behavior, we find that 5-HT initially increases a dominance signal in these fish. These findings are consistent with the emerging view that serotonin facilitates different adaptive responses to acute and chronic social challenge and stress.

Rapid glucocorticoid stimulation and GABAergic inhibition of hippocampal serotonergic response: in vivo dialysis in the lizard anolis carolinensis

Central serotonin (5-HT) is activated during stressful situations and aggressive interactions in a number of species. Glucocorticoids secreted peripherally during stressful events feed back on central systems and may affect 5-HT mediation of stress-induced behavioral events. To test the neuromodulatory effect of stress hormone secretion, serotonin overflow was measured from the hippocampus of the lizard Anolis carolinensis. Microdialysis was used to collect repeated samples from anesthetized lizards, with perfusate measured by HPLC with electrochemical analysis. Following initially high levels of 5-HT, concentrations stabilized to basal levels after approximately 2 h. Intracortical infusion of 200 ng/ml corticosterone evoked transient increases in 5-HT release of approximately 400%. The effect of corticosterone on 5-HT overflow appears to be dose dependent as 20 ng/ml stimulated an increase of 200%, whereas 2 ng/ml stimulated a 50% increase. Administration of 0.1 and 1 ng/ml GABA via the dialysis probe significantly inhibited 5-HT overflow by 20 and 40%, respectively. The duration of GABA inhibition is greater than the stimulatory response for glucocorticoids. Short-lived glucocorticoid stimulation of 5-HT release suggests a possible mechanism for endocrine mediation of continuously changing social behavioral events.

Relationships between hormones and aggressive behavior in green anole lizards: an analysis using structural equation modeling

We investigated the relationship between aggressive behavior and circulating androgens in the context of agonistic social interaction and examined the effect of this interaction on the androgen-aggression relationship in response to a subsequent social challenge in male Anolis carolinensis lizards. Individuals comprising an aggressive encounter group were exposed to an aggressive conspecific male for 10 min per day during a 5-day encounter period, while controls were exposed to a neutral stimulus for the same period. On the sixth day, their responses to an intruder test were observed. At intervals, individuals were sacrificed to monitor plasma androgen levels. Structural equation modeling (SEM) was used to test three a priori interaction models of the relationship between social stimulus, aggressive behavior, and androgen. Model 1 posits that exposure to a social stimulus influences androgen and aggressive behavior independently. In Model 2, a social stimulus triggers aggressive behavior, which in turn increases circulating levels of androgen. In Model 3, exposure to a social stimulus influences circulating androgen levels, which in turn triggers aggressive behavior. During the 5 days of the encounter period, circulating testosterone (T) levels of the aggressive encounter group followed the same pattern as their aggressive behavioral responses, while the control group did not show significant changes in their aggressive behavior or T level. Our SEM results supported Model 2. A means analysis showed that during the intruder test, animals with 5 days of aggressive encounters showed more aggressive responses than did control animals, while their circulating androgen levels did not differ. This further supports Model 2, suggesting that an animal's own aggressive behavior may trigger increases in levels of plasma androgen.

The role of monoaminergic nuclei during aggression and sympathetic social signaling

A social sign stimulus that is sympathetically induced affects aggressive approaches and influences serotonergic, dopaminergic and noradrenergic activity in the brainstem nuclei of Anolis carolinensis. Darkening of postorbital skin via sympathetic activation of adrenal catecholamines and beta(2)-adrenergic receptors provides a visual signal that forms more rapidly in dominant than subordinate males during social interactions. This signal limits aggressive interactions. Males were painted postorbitally with green or black paint and then exposed to a mirror. Aggressive approaches to the mirror were inhibited in males viewing a reflection with darkened eyespots, and increased in males viewing a reflection without eyespots (hidden). Noradrenergic turnover in the raphe and locus ceruleus were greatest in test subjects that viewed a reflection with eyespots hidden by green paint. Perception of darkened eyespots stimulated greater serotonergic turnover in raphe, locus ceruleus and substantia nigra/ventral tegmental area (SN/VTA). Dopaminergic turnover was higher in the raphe and SN/VTA of Anolis that viewed a reflection with darkened eyespots. However, these animals had lower dopamine turnover in the locus ceruleus than isolated and hidden eyespot groups. Of the possible roles of perikarya on central function and behavior, our results suggest feedback, cross-nuclear regulation, and some independence of function between nuclei and the forebrain terminal fields. Decreased serotonergic activity corresponds with increased aggression only in the raphe, suggesting that the raphe nuclei might be important for this behavioral trait. Increased serotonergic, noradrenergic and dopaminergic activities in SN/VTA in Anolis that view a reflected opponent with dark eyespots suggests that the SN/VTA might be directly involved in recognition of this social sign stimulus and the resulting inhibition of aggression.