Brain serotonergic neurons: their role in a form of dominance-subordination behavior in rats

The Neural Circuit Architecture of Social Hierarchy in Rodents and Primates

Social status is recognized as a major determinant of social behavior and health among animals; however, the neural circuits supporting the formation and navigation of social hierarchies remain under extensive research. Available evidence suggests the prefrontal cortex is a keystone in this circuit, but upstream and downstream candidates are progressively emerging. In this review, we compare and integrate findings from rodent and primate studies to create a model of the neural and cellular networks supporting social hierarchies, both from a macro (i.e., circuits) to a micro-scale perspective (microcircuits and synapses). We start by summarizing the literature on the prefrontal cortex and other relevant brain regions to expand the current “prefrontal-centric” view of social hierarchy behaviors. Based on connectivity data we also discuss candidate regions that might inspire further investigation, as well as the caveats and strategies that have been used to further our understanding of the biological substrates underpinning social hierarchy and dominance.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Competition for sucrose pellets in tetrads of male Wistar, Fischer or Sprague–Dawley rats: Is intra-group ranking reflected in the level of anxiety?

Competition for palatable food or fluids within groups of rats has been previously used to mirror intra-group ranking. The paradigm of competition for sucrose pellets in non-food-deprived male Wistar rats was here extended from triads to tetrads aiming at evaluating whether the number of poor-performing rats, those animals being likely to model aspects of human psychopathologies (anxiety/depression/social withdrawal), could be increased. To evaluate potential superiority over the previously used Wistar strain, establishment and stability of the ranking was also assessed in tetrads of male Fischer and Sprague-Dawley rats. Clear and stable rank orders were seen in around 60-70% of both triads and tetrads of Wistar rats: a high-performing, a medium-performing and one (in triads) or two (in tetrads) poor-performing rats were identifiable, indicating that the number of poor-performing rats had increased in tetrads. Comparable rank orders were also seen in tetrads of Fischer and Sprague-Dawley rats. At the end of an extended period of repeated testing, tetrads of these two strains, as well as some selected Wistar tetrads, were tested in the elevated zero-maze and plasma corticosterone levels were determined. The differentiation in competition-performance among cage mates was not paralleled by a difference in performance in the elevated zero-maze or in plasma corticosterone levels in any of the three strains. These data indicate that the level of anxiety in a non-social paradigm, the elevated zero-maze, does not reflect the competition-performance within the home cage and thus, the dominant/subordinate status in this food-competition paradigm may not reflect/being caused by different levels of anxiety.

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.

Monoaminergic changes associated with socially induced sex reversal in the saddleback wrasse

The process of sex reversal in fishes is socially mediated and requires a total reorganization of the hypothalamo-pituitary-gonadal axis. When the ratio of males to females in a population of saddleback wrasse (Thalassoma dupperrey) is too low, the largest female becomes male over the course of 6 to 8 weeks. This event requires the conversion of external social cues into internal chemical cues. In an attempt to investigate the role monoamines might play in this process, two females were housed together in floating enclosures in order to induce sex reversal in the larger. Brains were sampled at various time points throughout the process of sex reversal. Monoamines were measured in the amygdala, preoptic area, ventral hypothalamus, locus coeruleus and raphe nucleus. Changes were demonstrated in monoamine metabolism for all brain regions examined. The most important changes in monoamine-system activation were seen during the first week of sex reversal. It is during this time that transitional animals undergo behavioral sex reversal. There is an increase in serotonergic activity in the amygdala which is likely related to territorial acquisition. The absence of male aggression results in a less stressful environment for the female and a reduction in serotonergic activity in the preoptic area allowing for an increase in noradrenergic activity potentially triggering the reorganization of the reproductive axis. In the ventral hypothalamus, there is a decrease in noradrenergic and increase in dopaminergic activity associated with this change from female to male. The locus coeruleus shows an increase in noradrenergic activity later in the process of sex reversal which is probably a response to more circulating androgens. In the raphe nucleus, there is a decrease in serotonergic activity at the time of behavioral sex reversal. This decrease in serotonergic activity is linked to the behavioral component of sex reversal. This study suggests that monoamines play a very important role in both behavioral and gonadal sex reversal in the saddleback wrasse, the former under the control of serotonin in the raphe and the latter mediated via serotonergic effects on norepinephrine in the preoptic area.

Immunological consequences of the reversal of social status in C57BL/6J mice

The present study has shown that the immune response can be influenced by the reversal of animal's social status. For this purpose testing of agonistic interactions under the sensory contact conditions was performed for pairs of either aggressive or submissive C57BL/6J male mice with preliminary experience of 10 or 20 daily confrontations. Tests continued for 10 or 20 days, respectively. The reversal of aggressive behavior into submissive one in C57BL/6J mice resulted in immunosuppression only following 20 daily confrontations compared to the controls as well as to aggressive males which did not change their behavior and showed the increased immune response. Despite the fact that previous experience of defeat was associated with a reduced immune reaction, the occurrence of aggressive behavior in formerly submissive mice during 10 or 20 tests of daily confrontations produced immunostimulation. It is suggested that changes in the neurochemical pattern of the brain can be considered as an underlying basis for the linkage between the immune response and specific behavioral profile.

Serotonin reverses dominant social status

Social stress from aggressive interaction is expressed differently in specific brain regions of dominant and subordinate male Anolis carolinensis. Prior to aggressive behavior, the outcome is predictable via the celerity of postorbital coloration: Dominant males exhibit more rapid eyespot darkening. Serotonergic activation is manifest rapidly (1 h) in hippocampus, nucleus accumbens and brainstem of subordinate males, and is expressed more rapidly in dominant males. Amygdalar serotonergic activation responds rapidly (1 h) in dominant males, but is expressed slowly (1 w) and chronically in subordinate males. We hypothesized that chronic (1 w) serotonin elevation, manipulated by the selective serotonin reuptake inhibitor sertraline, would decrease aggressiveness and result in subordinate status. Dominant status was established in pairs of male A. carolinensis. The pairs were separated and treated with sertraline or vehicle. Sertraline was given in food to either the dominant or the subordinate male, both males or neither male for 1 week. Pairs were reintroduced, and behavior and social status recorded. When both dominant and subordinate males were treated with sertraline (or vehicle), or when subordinate males alone were treated with sertraline, previously established social relationships remained unchanged or became associative. However, when dominant males alone were treated with sertraline, their social status was reversed (43%) or negated (57%). Latency to eyespot darkening was significantly retarded in dominant males treated with sertraline, and aggressive displays and attacks were reduced. Chronic 5-HT elevation is consistent with subordinate status. Social status and aggressive disposition do not appear to be immutable, but may be changed by neuroendocrine mechanisms that mediate adaptation to environmental conditions like stress.

Selective reduction of monoamine oxidase A and B in the frontal cortex of subordinate rats

We have previously shown that subordination causes a reduction in the levels of 5-hydroxytryptamine and dopamine selectively in the frontal cortex [6]. These monoamines are catabolised mainly by the enzyme monoamine oxidase (MAO) which exists in two isoforms; MAO-A and MAO-B. The present study was carried out to determine whether there is any change in the activity of these two iso-enzymes induced by subordination and if any such alteration is confined to the frontal cortex. The animal model of dominance-subordination used was a worker-parasite paradigm in male Wistar rats. The enzyme activities were measured in five brain regions, the frontal cortex, entorhinal cortex, hippocampus, hypothalamus and striatum, using kynuramine as the substrate. Clorgyline and L-deprenyl were used in vitro to block the activities of MAO-A and MAO-B, respectively. There was a significant (P < 0.001) reduction in the activity of MAO-A as well as MAO-B selectively in the frontal cortex of the subordinate animals. This finding may suggest a reduced neurotransmitter turnover in the serotonergic and dopaminergic neurons terminating in the frontal cortex.

Selective effects of 8-OH-DPAT on social competition in the rat

Previous research has demonstrated that dominant-subordinate relationships measured in small groups of rats competing for access to palatable food or fluids can be disrupted by both anxiolytic and anxiogenic drugs, and it has been proposed as a possible animal model of anxiety. The present study investigated the effects of the selective 5-HT1A agonist 8-OH-DPAT on the rank order of triads of rats measured in terms of access to sweetened milk. The effect of 8-OH-DPAT on locomotor activity and intake of sweetened milk was also determined. 8-OH-DPAT (25 and 37.5 micrograms/kg) significantly increased the subordinate animals position in the social hierarchy without effect on the individual intakes of sweetened milk or locomotor activity. The same doses administered to dominant animals had no effect on any of the parameters measured. The 8-OH-DPAT-induced increase in social competition in subordinate rats was dissociable from effects on feeding behavior and locomotor activity. The results from this study provide further evidence that social competition in groups of rats may represent a model that can be used to detect drugs acting via receptor mechanisms believed to be implicated in anxiety.

Decreased dominance in a limited access test but normal maternal behavior in micrencephalic rats

Micrencephalic offspring produced by gestational treatment with the antimitotic compound/methylazoxymethanol acetate (MAM) are remarkable for substantial preservation of function concurrent with severe neural stunting. Altered behaviors in these micrencephalics, including light shyness and response perseveration, are similar to those produced by frontal cortex lesions. Consistent with this, the frontal cortex is one of several regions severely stunted by gestational MAM treatment. Because the frontal cortex has been implicated in rodent social behavior, maternal behavior in females and dominance in both sexes were assessed. Dominance was measured via water competition in 24-h water-deprived dyads (1 control and 1 MAM) matched for sex and body weight. Micrencephalic rats exhibited shorter drinking time than controls (males: 101 vs. 219 s, p < 0.001; females: 114 vs. 176 s, p < 0.03), indicating that micrencephalics were more submissive. For maternal behavior tests, micrencephalic and control females were bred to control males and pup retrieval was measured on postnatal days 3-13. Micrencephalic dams were unimpaired in any aspect of pup retrieval. Of 8 standard behavior measures used here and previously, access time in water competition tests produced the clearest differentiation between control and micrencephalic rats. These studies indicate that at least one aspect of social dominance in both sexes is severely reduced by MAM treatment while maternal behavior remains intact.

Serotonergic control of androgen-induced dominance

The present study investigates the role of serotonergic systems in anabolic steroid-induced aggression. An animal model of aggressive dominance was used to assess the chronic effects of testosterone propionate. When rats that had become dominant following administration of testosterone propionate received serotonergic agonists with selectivity for the 5-HT1A receptor (8-OH-DPAT, buspirone, gepirone), the 5-H1B receptor (eltoprazine, TFMPP), or the 5-HT2A/2C receptor (DOM), a dose-dependent decrease in dominance was demonstrated. Pretreatment with three serotonergic antagonists (pizotyline, pirenpirone, and pindolol) blocked agonist-induced reductions in dominance in varying degrees. Nonserotonergic agonists with CNS depressant effects were also tested in dominant animals. The benzodiazepine, chlordiazepoxide, did not reduce dominance except at doses that interfered with motor behavior. The opioid agonist, morphine, dose dependently decreased dominance, but this effect was reversible with administration of the serotonergic antagonist, pirenpirone, suggesting the antidominant effect of morphine had a serotonergic component. Biochemical experiments demonstrated that following chronic testosterone propionate, there was a decrease in levels of 5-HT and 5-HIAA in the hippocampus but not in the striatum or the frontal cortex. Chronic testosterone propionate also caused an increase in the affinity of [3H]8-OH-DPAT for the 5-HT1A receptor but no corresponding change in the density of 5-HT1A binding sites in the hippocampus. There was also no change in the properties of the 5-HT2 receptor in the frontal cortex following chronic testosterone propionate. These data suggest that serotonergic systems may play an important role in the control of anabolic steroid-induced aggressive dominance.

The Flinders sensitive line rats: a genetic animal model of depression

The Flinders Sensitive Line (FSL) rat, selectively bred for increased responses to the anticholinesterase DFP, was originally proposed as an animal model of depression because, like depressed humans, it is supersensitive to the behavioral and hormonal effects of cholinergic (muscarinic) agonists. The present review critically examines earlier and recent data collected on FSL rats to assess whether the model has good face, construct and/or predictive validity. With respect to face validity, FSL rats resemble depressed humans, at least superficially, in that they demonstrate: (a) reduced locomotor activity, (b) reduced body weight, (c) increased REM sleep, and (d) cognitive (learning) difficulties. So far, studies designed to assess the presence of anhedonia, a cardinal symptom of melancholic depression, have been inconclusive, but there are trends for the FSL rats to be more anhedonic than their control counterparts, the Flinders Resistant Line (FRL) rats, when exposed to chronic mild stress. Thus, FSL rats fulfill the criterion of face validity. Because FSL rats also are more sensitive to cholinergic agonists and have phase advanced circadian rhythms, they meet the criteria for the cholinergic and circadian rhythm models of depression and, therefore, have good construct validity. A key behavioral symptom exhibited by the FSL rat is demonstration of an exaggerated immobility when exposed to stressors such as foot shock and forced swimming. This behavioral abnormality has been normalized by a number of well-recognized antidepressant drugs such as imipramine and desipramine, as well as newer generation antidepressants with promising clinical effects such as sertraline and rolipram. However, several treatments that have not been routinely used to treat depression (lithium, exposure to bright light, the anticholinesterase DFP) have been ineffective in reversing the exaggerated immobility. Thus, the evidence in the present review indicates that the FSL rat model of depression fulfills the criteria of face, construct, and predictive validities.

Reversal of testosterone-induced dominance by the serotonergic agonist quipazine

Anabolic steroids and other androgens, such as testosterone propionate (TP), have a facilitatory role in the expression of aggressive behavior. Based upon literature indicating an inverse relationship between aggression and the central neurotransmitter serotonin (5-HT), the present study was undertaken to investigate the role of 5-HT in androgen-induced aggression. In this study, an animal model of aggression involving competition between male rat pairs for sugar pellets was used to investigate the effects of TP. When TP was administered daily (30 mg/kg) to nondominant rats, these animals became dominant. Dominant behavior was found to be stable throughout the study with continued daily administration of TP. To test the serotonergic component of TP-induced aggression, the serotonergic agonist 2-(1-piperazinyl) quinolone dimaleate (quipazine) was administered acutely to TP-dominant rats. Quipazine dose dependently reduced aggressive dominance in TP-dominant rats, as well as in naturally dominant rats. When the serotonergic antagonists pirenpirone or pizotyline were coadministered with quipazine to either group of dominant rats, they blocked the effect of quipazine in reducing dominance. However, when 1-[1H-Indol-4-yloxy]-3-[isopropylamino]-2-propanol (pindolol), a drug that acts at both beta-adrenergic receptors and at 5-HT1A and 5-HT1B receptors, was coadministered with quipazine there was a reversal of the quipazine effect on aggression only in TP-dominant rats. These results indicate that androgen-induced aggression may involve a complex alteration in serotonergic neurotransmission.

Effect of verapamil on submissive behavior in genetically bred hypercholinergic rats in a water competition test

Male hypercholinergic FSL (Flinders Sensitive Line) and control FRL (Flinders Resistant Line) rats were placed on a water deprivation schedule and tested for dominance behavior with FSL/FRL pairs competing for water. FSL rats spent significantly less time drinking than their FRL partners. Acute injection of 10 mg/kg of verapamil, a calcium channel inhibitor, to FSL rats markedly increased their drinking time without influencing water intake in individually tested rats. This effect of the drug was no longer seen after prolonged 4 day treatment. It is suggested that submissiveness of FSL animals in the water competition test might be due to increased fear which is alleviated by verapamil treatment. Tolerance seems to develop to this effect of the drug.

Competition for sucrose-pellets in triads of male Wistar rats: disinhibitory effect of individual housing in poor-performing rats

Within triads of male Wistar rats, some animals almost completely abstain from competition for palatable sucrose-pellets (so-called poor-performing rats). Individual housing temporarily helped these rats to behave more competitively. Such changes in the poor-performing rats' competition-scores are discussed in relation to previously described, isolation-induced alterations in serotonergic and noradrenergic neurotransmission.

Competition for sucrose-pellets in triads of male Wistar rats: effects of acute and subchronic chlordiazepoxide

Within triads of male Wistar rats, some animals almost completely abstain from competition for palatable sucrose pellets (so-called poor-performing rats), whereas other rats consistently win the competition (so-called high-performing rats). Subchronic (5 mg/kg; 5 consecutive days), but not acute (0.1-20 mg/kg), treatment with chlordiazepoxide temporarily helped poor-performing rats to behave more competitively. This finding, considered together with parallel studies (using high-performing rats), suggested that chlordiazepoxide's beneficial effect was only demonstrable when the poor-performing rats had become tolerant to the drug's initial sedative effect.

Selective increase of offensive behavior in the rat following intrahypothalamic 5,7-DHT-induced serotonin depletion

Cerebral serotonin (5-HT) depletions usually increase aggressive behaviors and more specifically facilitate elicitation of offensive behaviors. In order to localize the brain structures involved in this effect, 5,7-dihydroxytryptamine (5,7-DHT), a neurotoxin of 5-HT neurons, was injected into the ascending serotonergic pathway within the lateral hypothalamus, thus depleting 5-HT only in the forebrain structures. The effects of such treatment on offensive and defensive as well as social and non-social behaviors were studied in resident rats confronted with untreated intruders. Pretreatment with desipramine protected noradrenergic neurons. The content of 5-HT fell to 25% of controls, whereas noradrenaline was maintained at 90% in the forebrain anterior to the injection site. Ethological analysis of both resident's and intruder's behavior showed that offensive items were increased in 5,7-DHT-treated residents, whereas defensive items were increased in their non-treated partners; non-social activities were unchanged. Control of mouse-killing behavior during a 2-h test in the same animals showed a clear increase in elicitation of killing in 5,7-DHT-injected rats. These results confirm that the inhibitory control of serotonin is exerted specifically on offensive aggression. They suggest that forebrain structures are involved in this control.

Inhibition of affective aggression and dominance in rats after thyrotropin-releasing hormone (TRH) microinjection into the nucleus accumbens

The effect of 10 micrograms TRH injected bilaterally into the nucleus accumbens septi on two models of affective aggression and on dominance in a water-competition task was investigated in pairs of male Wistar rats. TRH significantly suppressed affective shock-induced and apomorphine-induced fighting. It also decreased dominance when administered to dominant rats while no effect was noted upon injection into subordinate animals. The peptide influenced neither water consumption in thirsty rats nor the pain threshold in a hot plate test.

Competition for sucrose-pellets in triads of male Wistar rats: effects of three serotonergic drugs

1. Within triads of rats, individuals can be discerned according to their competition for sucrose-pellets as a high-, a medium- and a poor-performing animal. 2. The competition-rates, expressed as scores, are affected by serotonergic drugs: upon inhibiting tryptophan-hydroxylase, the characteristic abstention of the poor-performing rats can temporarily be overcome; quipazine, on the other hand, leads to a dose-dependent decrease in the competition-rate of high-performing rats. 3. Such findings are indicative for a regulatory effect of some serotonergic mechanisms on a competition-behavior evoked within the familiar social context.

Competition for sucrose-pellets in triads of male Wistar rats: the individuals' performances are differing but stable

After having individually been accustomed to consume palatable sucrose-pellets, the individual's competition-rate for sucrose-pellets within its familiar triad has been observed in male Wistar rats. In this fixed-triad, food competition paradigm high-performing rats (high competition-rate, majority of pellets consumed), medium-performing rats (medium competition-rate, consumption of some pellets) and poor-performing rats (almost completely abstaining from competition with only rare incidence of pellet consumption) can be distinguished. Intra-group rank-orders, defined according to the individuals' competition-rates (expressed as a score), were present in many triads and, once established, these were stable over months. No additional differences were observed between high- and poor-performing rats, neither when these were individually tested in an open field or in the elevated plus-maze, nor with regard to the preference for a sucrose solution.

Comparison of desipramine, amitriptyline, zimeldine and alaproclate in six animal models used to investigate antidepressant drugs

In the present paper the acute actions primarily of the tricyclic antidepressants amitriptyline and desipramine, the atypical antidepressant zimeldine and the potential antidepressant alaproclate were evaluated in six models used for studying antidepressant agents. These included the forced swim test, a modified learned helplessness procedure, the clonidine hypothermia test, the social dominance test (using the interaction with clonidine), a differential-reinforcement-of-low-rates (DRL-72s) schedule and conditioned avoidance response. The results showed desipramine to be effective in all the tests employed. Zimeldine was effective in the learned helplessness, DRL-72s and domination tests, but also caused notable deficits in two-way active avoidance response. Alaproclate was effective in all the tests except the domination paradigm. Amitriptyline was effective in all tests employed. The results are discussed in relation to the possible mechanism of action of these compounds in the test models employed.

Motivational versus motor impairment after haloperidol injection or 6-OHDA lesions in the ventral tegmental area or substantia nigra in rats

The effects of blockade of dopaminergic receptors as well as the lesions of dopaminergic neurons on the behaviour of rats in "motivational" and "motor" tests were examined. Both systemic injection of haloperidol and 6-OHDA lesions in the ventral tegmental area (VTA) or substantia nigra, pars compacta (SN/pC) induced a decrease in the number of wins made by thirsty animals fighting for water in a water competition test. The rats with lesions in the VTA became insensitive to motivational thirst stimuli. The reaction of rats lesioned in the SN/pC was still dependent on the level of thirst. Both groups did not differ from controls in the amount of water intake. In the rotating rod test the animals with destroyed nigrostriatal system showed a substantial motor impairment, while destruction of the mesolimbic system had no effect in that test. The role of dopaminergic systems in terms of their involvement in the control of motivational arousal and motor performance has been discussed.

Parachlorophenylalanine-induced serotonin depletion increases offensive but not defensive aggression in male rats

Cerebral 5-HT depletion has been shown to facilitate elicitation of various kinds of aggressive behavior in rats. The question as to whether both offensive and defensive aggressive reactions are affected to the same extent was examined in a resident-intruder paradigm where an ethological analysis of the two animals allows an evaluation of non-social activities as well as agonistic interactions, including both offense and defense. PCPA (375 mg/kg IP) was administered either to the resident or the intruder and the interactions with an untreated conspecific were recorded in the resident's home cage for an 8 min period three days after injection when 5-HT was maximally reduced. PCPA treatment increased the occurrence of social approach and offensive postures in resident rats, whereas their untreated partners displayed more defensive reactions. When intruders were injected, only non-significant increases in approach and offense were observed. In no case did PCPA affect occurrence of defensive postures in the injected animals. These results confirm that serotonin plays a role in controlling offensive aggression but not defensive behavior.