Social hierarchy affects gene expression for catecholamine biosynthetic enzymes in rat adrenal glands

Status-dependent metabolic effects of social interactions in a group-living fish

Social interactions can sometimes be a source of stress, but social companions can also ameliorate and buffer against stress. Stress and metabolism are closely linked, but the degree to which social companions modulate metabolic responses during stressful situations-and whether such effects differ depending on social rank-is poorly understood. To investigate this question, we studied Neolamprologus pulcher, a group-living cichlid fish endemic to Lake Tanganyika and measured the metabolic responses of dominant and subordinate individuals when they were either visible or concealed from one another. When individuals could see each other, subordinates had lower maximum metabolic rates and tended to take longer to recover following an exhaustive chase compared with dominants. In contrast, metabolic responses of dominants and subordinates did not differ when individuals could not see one another. These findings suggest that the presence of a dominant individual has negative metabolic consequences for subordinates, even in stable social groups with strong prosocial relationships.

Endocannabinoid basis of personality-Insights from animal model of social behavior

Rationale: The endocannabinoid system is known to be involved in learning, memory, emotional processing and regulation of personality patterns. Here we assessed the endocannabinoid profile in the brains of mice with strong characteristics of social dominance and submissiveness. Methods: A lipidomics approach was employed to assess the endocannabinoidome in the brains of Dominant (Dom) and Submissive (Sub) mice. The endocannabinoid showing the greatest difference in concentration in the brain between the groups, docosatetraenoyl ethanolamine (DEA), was synthesized, and its effects on the physiological and behavioral responses of Dom and Sub mice were evaluated. mRNA expression of the endocannabinoid receptors and enzymes involved in PUFA biosynthesis was assessed using qRT-PCR. Results: Targeted LC/MS analysis revealed that long-chain polyunsaturated ethanolamides including arachidonoyl ethanolamide (AEA), DEA, docosatrienoyl ethanolamide (DTEA), eicosatrienoyl ethanolamide (ETEA), eicosapentaenoyl ethanolamide (EPEA) and docosahexaenoyl ethanolamide (DHEA) were higher in the Sub compared with the Dom mice. Untargeted LC/MS analysis showed that the parent fatty acids, docosatetraenoic (DA) and eicosapentaenoic (EPA), were higher in Sub vs. Dom. Gene expression analysis revealed increased mRNA expression of genes encoding the desaturase FADS2 and the elongase ELOVL5 in Sub mice compared with Dom mice. Acute DEA administration at the dose of 15 mg/kg produced antinociceptive and locomotion-inducing effects in Sub mice, but not in Dom mice. Subchronic treatment with DEA at the dose of 5 mg/kg augmented dominant behavior in wild-type ICR and Dom mice but not in Sub mice. Conclusion: This study suggests that the endocannabinoid system may play a role in the regulation of dominance and submissiveness, functional elements of social behavior and personality. While currently we have only scratched the surface, understanding the role of the endocannabinoid system in personality may help in revealing the mechanisms underlying the etiopathology of psychiatric disorders.

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.

Neonatal handling (resilience) attenuates water-avoidance stress induced enhancement of chronic mechanical hyperalgesia in the rat

Chronic stress is well known to exacerbate pain. We tested the hypothesis that neonatal handling, which induces resilience to the negative impact of stress by increasing the quality and quantity of maternal care, attenuates the mechanical hyperalgesia produced by water-avoidance stress in the adult rat. Neonatal male rats underwent the handling protocol on postnatal days 2-9, weaned at 21 days and tested for muscle mechanical nociceptive threshold at postnatal days 50-75. Decrease in mechanical nociceptive threshold in skeletal muscle in adult rats, produced by exposure to water-avoidance stress, was significantly attenuated by neonatal handling. Neonatal handling also attenuated the mechanical hyperalgesia produced by intramuscular administration of the pronociceptive inflammatory mediator, prostaglandin E2 in rats exposed as adults to water-avoidance stress. Neonatal handling, which induces a smaller corticosterone response in adult rats exposed to a stressor as well as changes in central nervous system neurotransmitter systems, attenuates mechanical hyperalgesia produced by water-avoidance stress and enhanced prostaglandin hyperalgesia in adult animals.

Antioxidant treatment strategies for hyperphenylalaninemia

Hyperphenylalaninemia (HPA) leads to increased oxidative stress in patients with phenylketonuria (PKU) and in animal models of PKU. Early diagnosis and immediate adherence to a phenylalanine-restricted diet prevents HPA and, consequently, severe brain damage. However, treated adolescent and adult PKU patients have difficulties complying with the diet, leading to an oscillation of phenylalanine levels and associated oxidative stress. The brain is especially susceptible to reactive species, and oxidative stress might add to the impaired cognitive function found in these patients. The restricted PKU diet has a very limited nutrient content from natural foods and almost no animal protein, which reduces the intake of important compounds. These specific compounds can act as scavengers of reactive species and can be co-factors of antioxidant enzymes. Supplementation with nutrients, vitamins, and tetrahydropterin has given quite promising results in patients and animal models. Antioxidant supplementation has been studied in HPA, however there is no consensus about its always beneficial effects. In this way, regular exercise could be a beneficial addition on antioxidant status in PKU patients. A deeper understanding of PKU molecular biochemistry, and genetics, as well as the need for improved targeted treatment options, could lead to the development of new therapeutic strategies.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

Studies of socially housed rodents have provided significant information regarding the consequences of exposure to stressors. Psychosocial stressors are known to alter the ingestion of ethanol and the activity of the dopaminergic neuronal system. Since both stressors and ethanol are known to affect the function of dopaminergic neurons, we employed amphetamine to assess the role of this neural system on the ingestion of ethanol by psychosocially stressed male rats. Male rats housed two per cage were designated as dominant or subdominant rats based on evaluations of agonistic behavior and body weight changes. The dyad-housed rats and a group of single-housed rats were sequentially assessed for ethanol intake after injections of saline or amphetamine (0.3, 0.9 or 2.7 mg/kg i.p.) both prior to dyad housing and subsequently again during dyad-housing. Prior to dyad housing ethanol intake of future subdominant rats was higher than that of future dominant rats. Dyad-housing significantly increased ethanol intake of dominant rats. Pre-dyad the highest dose of amphetamine potently depressed ethanol ingestion. Sensitivity to amphetamine's depressant effect on ethanol intake was higher at the dyad test in all subjects, most prominently in single-housed rats. In contrast to the single-housed rats, the dyad-housed rats displayed saccharin anhedonia. It can be concluded that dopaminergic system modulates, at least partially, the psychosocial stress-induced changes in ethanol intake. Furthermore, the level of ethanol ingestion at the pre-dyad test was predictive of future hierarchical status.

Differential sensitivity to amphetamine's effect on open field behavior of psychosocially stressed male rats

RATIONALE

Studies of socially housed rodents have provided significant information regarding the mechanisms of stress and of stress-related disorders.

OBJECTIVE

Since psychosocial stress is known to alter the functional activity of dopaminergic system, we employed amphetamine (AMP) to evaluate the involvement dopamine in mediating the behavioral consequences of psychosocial stress.

METHODS

Male rats housed two per cage were designated as dominant (DOM) or subdominant (Sdom) based on initial evaluations of agonistic behaviors and body weight changes. Diad-housed rats and a group of single-housed (SiH) rats were tested in an open field after injections of saline or amphetamine (0.9 or 2.7 mg/kg IP) prior to and again while diad-housing.

RESULTS

Compared to future DOM rats, saline-injected future Sdom rats entered the open field center less frequently, spent less time in rearing behavior and groomed less. At the pre-diad test AMP treatment elevated locomotor activity of all rats, while stimulation of center entries was more marked in future DOM rats. At the diad test, AMP's locomotor stimulant effect was evident in all experimental groups with DOM rats showing higher effects compared to Sdom and SiH rats. Amphetamine's stimulation of center entries in DOM rats was similar to the pre-diad test, but it was diminished in Sdom rats, while stimulation of rearing behavior was most evident in diad-housed rats.

CONCLUSION

The dopaminergic system modulates the psychosocial stress-induced differences in explorative and emotional behaviors. Furthermore, behavioral traits like frequency of grooming behavior and of center entries were predictive of future hierarchical status.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Organizational role for pubertal androgens on adult hypothalamic-pituitary-adrenal sensitivity to testosterone in the male rat

The inhibitory effect of androgens on the hypothalamic-pituitary-adrenal (HPA) axis in basal and stress conditions in adult male rats is well documented. Major sex-related neuroendocrine changes take place during puberty. There is a robust rise in production and secretion of gonadal steroids, which is thought to underlie numerous neural and behavioural changes brought on after puberty. The present study investigated the effect of the pubertal rise in gonadal steroid levels on the subsequent adult corticosterone profile, particularly the sensitivity of the adult HPA axis to testosterone. Animals were castrated either prepubertally (28 days) or in adulthood (11 weeks) and adult animals were subsequently treated with subcutaneous implants containing either testosterone or cholesterol. Using an automated blood sampling system, blood was collected from each freely moving, conscious rat every 10 min (i) over a 24 h period; (ii) in response to 10 min of noise stress, and (iii) following an immunological challenge with lipopolysaccharide (LPS). Analysis revealed that testosterone treatment did not significantly affect overall corticosterone release over the 24 h period in adult animals castrated before puberty in contrast to animals castrated in adulthood in which testosterone significantly suppressed corticosterone secretion. Following either a noise stress or LPS injection, testosterone treatment did not affect the hypothalamic or adrenal stress response in animals castrated prepubertally. Testosterone significantly suppressed the corticotrophin-releasing hormone and arginine vasopressin mRNA as well as the corticosterone response to LPS in castrated animals that had had their testes intact over puberty. These data provide evidence that puberty is a critical organizational period during which rising levels of gonadal steroids programme the sensitivity of the adult HPA axis to gonadal steroids in adulthood.

Social housing influences the composition of volatile compounds in the preputial glands of male rats

In rodents the preputial glands are one of the major sources of pheromones. These volatile chemosignaling compounds are known to elicit specific behavioral and physiological effects in their conspecifics. While social stress can alter both the behavior and hormonal status of rodents, little is known about its influence on the volatile constituents of the preputial glands. We have examined the composition of volatile compounds in the preputial glands of gonadally intact male rats housed for 70 days in either unisex triads (three/cage) or singly. The rank status of triad-housed rats was based on quantitative behavioral assessments taken during the initial 30 min of triad housing. Dominant rats had heavier preputial glands compared to subdominant and subordinate rats. Capillary gas chromatography-mass spectrometry identified 56 volatile preputial compounds, of these 17 did not differ between groups while 26 compounds were significantly higher in the single-housed compared to the triad-housed rats. Six additional volatile compounds were higher in the dominant compared to the other 3 groups, while another six compounds were higher in both the dominant and single-housed rats compared to the subdominant and subordinate rats. It can be concluded that both housing condition and social rank status have significant but different effects on the composition of volatile compounds found in preputial glands of male rats. The physiological and behavioral significance of these changes in preputial gland volatile compound composition in rats remain to be investigated.

Differing effects of acute and chronic stressors on plasma osteocalcin and leptin in rats

Stressor activation of the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis can have profound effects on bone and also appetite and metabolism. We tested in rats the response of plasma osteocalcin (pOC, a bone biomarker that is acutely stress responsive), corticosterone, and leptin to (1) ethanol consumption (5% w/v) in a liquid diet (compared with ad libitum and pair-fed rats), (2) acute restraint, and (3) acute (once, 1 h) and (4) chronic (1 h/day for 7 weeks) social aggression. Basal pOC concentration did not differ with ethanol diet or social interaction, but was elevated by both foot restraint immobilization (Imo) and restraint in wire mesh cylinders (WMR). As previously reported for chronic Imo, ingestion of ethanol blunted the pOC response to Imo. Plasma corticosterone concentration was increased by acute WMR and acute social interaction but was unaltered by chronic social interaction. Plasma leptin concentration was markedly increased by Imo in ad libitum fed, but only slightly in ethanol or pair-fed rats. In contrast, the data reflect significant differences between acute and chronic stressor effects since chronic social stress had little effect on pOC or plasma corticosterone, but tended to decrease leptin level in relation to dominance. Lack of significant impact of prolonged ethanol intake or social aggression suggests physiological adaptation.

Psychosocial stress alters ethanol's effect on open field behaviors

Psychosocial stress, including social rank status, has been shown to alter spontaneously occurring behaviors in rodents as well as the behavioral effects of drugs of abuse. In this study, rats were repeatedly evaluated in a modified open field following: their initial exposure, and after intraperitoneal injections of saline and 0.75 g/kg ethanol (EtOH). All subjects were first tested while under single housing conditions, then again following 35 days of differential housing (singly or 3 rats/cage) with social status determined by scoring agonistic behavior at triad formation. The data suggest that (1) future subordinate rats differed with respect to specific aspects of behavior displayed in a 'novel' open field arena, (2) future subordinate rats were more emotional since they showed greater "anxiety-like" behavior and less exploratory behavior, (3) subordinate rats were more impaired by the saline injection stress, (4) subordinate rats were more sensitive to the depressant effects of EtOH, (5) grooming behavior did not show habituation, in contrast to the other behaviors, but showed sensitization on the second test. Overall, subordinate rats may have differed from their cage mates in innate anxiety, and this may underlie their distinct response to both stressors and EtOH. Furthermore, while EtOH had mostly stimulant effects in naive rats, psychosocial stress and/or repeated testing resulted in enhancement of EtOH's depressant effects.

Housing and rank status of male Long-Evans rats modify ethanol's effect on open-field behaviors

RATIONALE

Psychosocial stress is known to alter behavior of rodents. While psychosocial stress may alter the response to some drugs, the response to ethanol (EtOH) has not been evaluated.

OBJECTIVE

To examine open-field behaviors of triad- and singly housed rats treated acutely or voluntarily ingesting EtOH.

METHOD

Triad-housed rats were categorized as dominant, subdominant, or subordinate based on assessments of offensive and defensive behaviors. Open-field behaviors were monitored during a 10-min test in rats voluntarily ingesting a 6% solution of EtOH for 2 weeks (1), and after an i.p. injection of saline, 0.5 or 1.0 g kg(-1) of EtOH (2).

RESULTS

Daily intake of EtOH was highest in subdominant and lowest in dominant rats. Overall, open-field behaviors did not differ between water- and EtOH-consuming triad- or singly housed rats. The 0.5-g kg(-1) dose of EtOH enhanced locomotor activity only in triad-housed rats, center entries primarily in singly housed rats, and head-poke behavior in dominant and singly housed rats. Rearing behavior was not altered by the 0.5-g kg(-1) dose, but in singly housed rats, rearing behavior was depressed by the 1.0-g kg(-1) dose. This larger dose of EtOH had no effect on the other behaviors.

CONCLUSIONS

EtOH's effects on open-field behaviors show behavioral specificity and vary with the subject's housing and/or rank status. EtOH's acute anxiolytic-like effect was primarily evident in singly housed rats.