Coping with social conflict: impact on the activity of tyrosine hydroxylase in the limbic system and in the adrenals

Chronic psychosocial stressors in adulthood: Studies in mice, rats and tree shrews

Human psychological stress is the major environmental risk factor for major depression and certain of the anxiety disorders. Psychological stressors often occur in the context of the adult social environment, and they or the memory formed of them impact on the individual across an extended period, thereby constituting chronic psychosocial stress (CPS). Psychosocial stressors often involve loss to the individual, such as the ending of a social relationship or the onset of interpersonal conflict leading to loss of social control and predictability. Given the difficulty in studying the etio-pathophysiological processes mediating between CPS and brain and behavior pathologies in human, considerable effort has been undertaken to study manipulations of the social environment that constitute adulthood chronic psychosocial stressors in other mammals. The majority of such research has been conducted in rodents; the focus for a considerable time period was on rats and more recently both rats and mice have been investigated, the latter species in particular providing the opportunity for essential gene x chronic psychosocial stressor interaction studies. Key studies in the tree shrew demonstrate that this approach should not be limited to rodents, however. The animal adult CPS paradigms are based on resident-intruder confrontations. These are typified by the intruder-subject's brief proximate interactions with and attacks by, and otherwise continuous distal exposure to, the resident stressor. In contrast to humans where cognitive capacities are such that the stressor pertains in its physical absence, the periods of continuous distal exposure are apparently essential in these species. Whilst the focus of this review is on the stressor rather than the stress response, we also describe some of the depression- and anxiety disorder-relevant effects on behavior, physiology and brain structure-function of chronic psychosocial stressors, as well as evidence for the predictive validity of such models in terms of chronic antidepressant efficacy. Nonetheless, there are limitations in the methods used to date, most importantly the current emphasis on studying CPS in males, despite the much higher disorder prevalence in women compared to men. Future studies will need to address these limitations.

Animal models of major depression and their clinical implications

Major depressive disorder is a common, complex, and potentially life-threatening mental disorder that imposes a severe social and economic burden worldwide. Over the years, numerous animal models have been established to elucidate pathophysiology that underlies depression and to test novel antidepressant treatment strategies. Despite these substantial efforts, the animal models available currently are of limited utility for these purposes, probably because none of the models mimics this complex disorder fully. It is presumable that psychiatric illnesses, such as affective disorders, are related to the complexity of the human brain. Here, we summarize the animal models that are used most commonly for depression, and discuss their advantages and limitations. We discuss genetic models, including the recently developed optogenetic tools and the stress models, such as the social stress, chronic mild stress, learned helplessness, and early-life stress paradigms. Moreover, we summarize briefly the olfactory bulbectomy model, as well as models that are based on pharmacological manipulations and disruption of the circadian rhythm. Finally, we highlight common misinterpretations and often-neglected important issues in this field.

Tree shrews at the German Primate Center

Abstract. For many years, Tupaia (family Tupaiidae), most commonly known as tree shrews, have been studied almost exclusively by zoologists resulting in a controversial debate on their taxonomic status among mammals. Today, tree shrews are placed in the order Scandentia; they are valuable, widely accepted and increasingly used model animals as an alternative to rodents and non-human primates in biomedical research. After a brief description on how tree shrews entered science and their taxonomic odyssey, the present article describes the history of the tree shrew (Tupaia belangeri) colony at the German Primate Center and selected aspects of our work with special emphasis on the psychosocial stress model in these animals.

Social stress, therapeutics and drug abuse: preclinical models of escalated and depressed intake

The impact of ostensibly aversive social stresses on triggering, amplifying and prolonging intensely rewarding drug taking is an apparent contradiction in need of resolution. Social stress encompasses various types of significant life events ranging from maternal separation stress, brief episodes of social confrontations in adolescence and adulthood, to continuous subordination stress, each with its own behavioral and physiological profile. The neural circuit comprising the VTA-accumbens-PFC-amygdala is activated by brief episodes of social stress, which is critical for the DA-mediated behavioral sensitization and increased stimulant consumption. A second neural circuit comprising the raphe-PFC-hippocampus is activated by continuous subordination stress and other types of uncontrollable stress. In terms of the development of therapeutics, brief maternal separation stress has proven useful in characterizing compounds acting on subtypes of GABA, glutamate, serotonin and opioid receptors with anxiolytic potential. While large increases in alcohol and cocaine intake during adulthood have been seen after prolonged maternal separation experiences during the first two weeks of rodent life, these effects may be modulated by additional yet to be identified factors. Brief episodes of defeat stress can engender behavioral sensitization that is relevant to escalated and prolonged self-administration of stimulants and possibly opioids, whereas continuous subordination stress leads to anhedonia-like effects. Understanding the intracellular cascade of events for the transition from episodic to continuous social stress in infancy and adulthood may provide insight into the modulation of basic reward processes that are critical for addictive and affective disorders.

Dietary Tyrosine Protects Striatal Dopamine Receptors from the Adverse Effects of REM Sleep Deprivation

L-Tyrosine is a non-essential amino acid that is produced as an intermediary metabolite in the conversion of phenylalanine to 3,4-dihyroxyphenylalanine (DOPA), and is a precursor of the neurotransmitter dopamine. In previous studies, tyrosine pretreatment was shown to protect against the neurochemical and behavioral deficits of acute stress caused by tail shock or cold exposure in rodents. The present study addressed the hypothesis that tyrosine administration may be an effective counter-measure to dopamine-mediated behaviors induced by rapid eye-movement sleep deprivation (RSD). In order to test the hypothesis, Sprague-Dawley rats were divided into 9 treatment groups: RSD-treated rats on normal-protein diet (20% casein: 1% tyrosine, 1% valine); tank control (TC) rats on a normal diet; cage control (CC) rats on normal diet; RSD-treated rats on 4% tyrosine diet; TC rats on 4% tyrosine diet; CC rats on 4% tyrosine diet; RSD-treated rats on 4% valine diet; TC rats on 4% valine diet; CC rats on 4% valine diet. In the RSD group receiving tyrosine, there was no apparent change in Bmax for binding of the dopamine D2 receptor ligand [(3)H]YM-09151-2 in the striata as compared to the respective TC and CC groups; whereas RSD-treated rats maintained on the normal diet and valine supplementation demonstrated expected increases in Bmax for ligand binding. The TC group on the tyrosine diet showed attenuated catalepsy compared to the corresponding CC group, while the RSD group consuming tyrosine showed a catalepsy that was significantly increased, and similar to that of cage control animais on a control diet. These data suggest that the tyrosine-supplemented diet significantly attenuated RSD-induced changes in striatal dopamine D2 receptors, and the effect appeared sufficient to influence RSD-induced behaviors.

Behavioral and autonomic responses to intermittent social stress: differential protection by clonidine and metoprolol

The present study investigated physiological and pharmacological characteristics of socially "stressed" animals. Specifically, we examined (1) to what degree autonomic and behavioral "stress" reactions during intermittent confrontations between an intruder male adult Long-Evans rat with an aggressive resident undergo habituation, and (2) to what extent the defeat-experienced animal can be protected against these "stress" reactions with clonidine or metoprolol, two adrenergic agents with clinical anxiolytic effects. We developed an acute social stress situation that consisted of initially placing an experimental rat as an intruder into the homecage of a resident while the resident was not present, thereafter permitting brief physical agonistic interactions with the reintroduced resident until the intruder was forced into a submissive supine posture and emitted ultrasonic vocalizations (USV), and eventually exposing the intruder to the resident's threats for one hour, while being shielded from potentially injurious attacks ("threat encounter"). Over the course of the initial 4-weekly threat encounters the acute tachycardia but not the hyperthermic stress responses decreased in magnitude. Following the first three threat encounters core temperature (Tc) was significantly elevated for at least 3 h. The Tc was already elevated when the repeatedly defeated intruder was confronted with the olfactory cues of the resident's cage. This conditioned "anticipatory" hyperthermia developed in the course of the first three confrontations and was paralleled by a decrease in exploratory and motor behavior and by an increase in defensive behaviors and in both types of USV emitted in the "low" (20-30 kHz) and the "high" (31-70 kHz) frequency range. Clonidine (0.01-0.1 mg/kg, IP), an alpha 2-adrenergic agonist and metoprolol, a beta-adrenergic blocker (1.0-10.0 mg/kg, IP), dose-dependently prevented the tachycardic response to stress. Only clonidine, but not metoprolol, also attenuated the rise in Tc during the 1-h agonistic interaction. Clonidine decreased those aspects of motor behavior (e.g. rearing, walking) that are of lesser "cost" for the individual but maintained high levels of defensive reactions and increased the duration of "low" USV. The high doses of clonidine (0.06, 0.1 mg/kg) attenuated the homeostatic regulation and sedated the intruder while exposed to threats during a social confrontation. The absence of attenuation of the high level of defensive behavior and the prolonged "low" USV suggest a stress intensification by the higher doses of clonidine. In conclusion, after the fourth encounter, the autonomic, behavioral and vocal response pattern prior to and during repeated weekly confrontations show no evidence for habituation for the following 6 weeks.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential modulation of dopaminergic systems in the rat brain by dietary protein

Rats that consume a diet 50% rich in protein exhibit hyperactivity and hyperresponsiveness to nociceptive stimuli, in which facilitation of dopaminergic activity has been implicated. We studied the regional changes in the concentrations of dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brains of rats that were maintained on high-protein (HP, 50% casein), normal-protein (NP, 20% casein), and low-protein (LP, 8% casein) diets for 36 weeks. Brain nuclei that represented different DAergic systems were punch-dissected and analyzed using HPLC. In the substantia nigra, the striatum, and the dentate gyrus, DA concentrations decreased and increased, respectively, with a decrease and increase in dietary protein (p < 0.05 compared to the NP diet). Similar trends in the effect of the HP diet were observed in the ventral tegmental area, amygdala, frontal cortex, subiculum, centromedial nucleus (CM) of the thalamus, and inferior colliculi (IC), although the differences in DA concentrations were not statistically significant. These brain areas also showed a pattern of decreased DA concentration in association with the LP diet, and the differences were statistically significant (p < 0.05) in the CM and IC. DA concentrations in most regions of the midbrain and brainstem were not different between the diet groups, nor were consistent trends observed in those regions. Also, there were no consistent relationships between DOPAC/DA and HVA/DA ratios and dietary protein level. These data suggest that only discrete dopaminergic neuronal circuits in the rat forebrain were sensitive to changes in dietary protein level.

Long-term impairment of autonomic circadian rhythms after brief intermittent social stress

This experiment was designed to examine the short- and long-term behavioral, cardiovascular, and thermoregulatory responses to brief intermittent agonistic confrontation in rats. The experimental procedure involves resident-intruder confrontations consisting of a 10-min period during which both animals are separated in the home cage of the resident, followed by a brief physical encounter leading to defeat of the intruder and a 10-min period, when the intruder was alone in the home cage of the resident. These 30-min-long confrontations were repeated on 5 consecutive days. Before the first confrontation with a resident, an intruder rat's telemetered heart rate and core temperature show a stable circadian rhythm that is entrained by the light cycle. Acutely, the confrontations produce immediate and large tachycardia and hyperthermia in intruders. A decrease in amplitude of the circadian rhythms for heart rate and core temperature, as detected by cosinor analysis, persists for at least 10 days after the last of five daily brief confrontations with the resident. The defensive upright posture is nearly absent during the first exploration of the resident's home cage, but is displayed by the intruder for one-third of the available time before and after the fifth defeat. Intermittent brief social stress is sufficient to induce profound changes in defensive behavior and long-lasting depression of circadian rhythmicity that persist for weeks.

Extended attack from a resident conspecific is critical to the development of long-lasting analgesia in male intruder mice

Recent studies have shown that intruder mice display qualitatively different forms of analgesia in response to social encounters with resident conspecifics. An acute (non-opioid) reaction is evident when display of defeat is used as test criterion whereas a much longer-lasting (opioid) response is observed when this criterion is exceeded. Neither reaction can be readily attributed to the effects of physical stimulation per se. Two studies were conducted to determine the critical stimulus leading to a 'switchover' from non-opioid to opioid analgesia in intruder mice. The pattern of results indicates that temporal factors per se are unimportant but that extended exposure to attack is essential to the development of an enduring (opioid-typical) analgesia. It is argued that this finding is consistent with the uncontrollability hypothesis which asserts that opioid analgesia is a consequence of a failure in behavioural coping strategies.

Behavioural, physiological and immunological consequences of social status and aggression in chronically coexisting resident-intruder dyads of male rats

The behavioural and physiological consequences of social status and reciprocal fighting in resident-intruder dyads of Long Evans male rats were evaluated. Before a chronic cohabitation of 10 days, residents and intruders were individually housed for one month to increase their aggressiveness. Control animals included isolates, i.e., animals kept individually housed throughout the experiment and pair-housed rats, i.e., pairs of rats housed together from their rats in the laboratory. In 19 out of 20 dyads, a clear dominance relationship developed with an advantage to the resident in 68% of the cases. Dominants showed more exploratory activity than subordinates in a open-field test at the end of the cohabitation period; subordinates groomed longer than animals from other experimental groups. Dominants had lower pain thresholds than individually and pair-housed animals. Both dominants and subordinates had higher tyrosine hydroxylase enzymatic activities in the left adrenal than isolated and pair-housed rats. Subordinates lost body weight and had higher plasma corticosteroid concentrations than animals from the other experimental groups. In addition, they had smaller thymus glands and reduced spleen lymphocyte responses to mitogenic stimulation in vitro, in comparison to dominant animals. These results show that subordination in the dyadic resident-intruder paradigm leads to a complex syndrome of behavioural and physiological changes, some of which may be modulated by the intensity of aggressive interactions.

Continuous social defeat induces an increase of endogenous opioids in discrete brain areas of the mongolian gerbil

Dyads of a victor and a loser of mongolian gerbils (Meriones unguiculatus) coexisted for seven days; isolated animals served as a further experimental group. beta-Endorphin, Met-enkephalin and dynorphin were measured in several brain areas and in the anterior and neurointermediate pituitary. beta-Endorphin and Met-enkephalin were increased in the amygdala of defeated as compared to victorious animals. Met-enkephalin in the hypothalamus and in the striatum were lower in isolated than in coexisting gerbils. Coexistence decreased beta-endorphin in the amygdala and in the hypothalamus as compared to isolation. The results provide biochemical evidence for the role of central endogenous opioid-peptide systems in the physiology of victory and defeat. Dynorphin showed no variation with social conflict and social status.

Gonadal response to psychosocial stress in male tree shrews (Tupaia belangeri) morphometry of testis, epididymis and prostate

Male tree shrews (46 animals in all) were kept under distinct social conditions. Two kinds of controls were investigated: isolated animals as well as males out of female/male couples. Comparing these two groups only little isolation effect was observed. With the other individuals, initially isolated, dyads were formed which coexisted for 7 or 50 days. In a third group of dyads, after 50 days of coexistence social circumstances were aggravated for 5 more days by providing only half of the cage. All dyads established clear cut dominance relationship. Animals behaving dominantly developed no signs of gonadal regression rather that of stimulation concerning especially with the epididymis. In contrast, animals behaving as subordinates revealed profound regressive changes although differentiated with respect to any parameter, both morphological and conditional. In testis, chronical subordination (50 days, mainly 50 + 5 days) leads to a loss of organ weight, narrowing of seminiferous tubules and to enlarged spermatogonia nuclei. The case is similar in the epididymis: loss of organ weight, lowered epithelium and an enormously increased frequency of immature germ cells within tail segments. Whereas prostate weight drops continuously, striking histological regression occurs already after 7 days (extreme diminution of epithelial height and of epithelial cell nuclei).

The effect of 6-OHDA lesions of the lateral septum on schedule-induced polydipsia

In this study the relationship between 6-hydroxydopamine lesions of dopaminergic innervation of the lateral septum and schedule-induced polydipsia was examined. Animals with lesions were found to have a significant increase in water intake during a test involving an intermittent schedule of food delivery. Additional experiments with these animals showed that lesions had no effect on: (i) spontaneous drinking, the amount of drinking after 24 h food or water deprivation, water intake following a hypertonic saline challenge; (ii) eating behavior with or without food deprivation; and (iii) spontaneous locomotor activity. This increase in adjunctive behavior is discussed in the light of an enhanced frustrative effect induced by the septal lesions.

Social status as determinant of alcohol effects on aggressive behavior in squirrel monkeys (Saimiri sciureus)

We investigated the behavioral effects of alcohol, especially those on aggression, on members of established groups of squirrel monkeys. A continuous measurement of monkey behavior in groups revealed consistent differences between dominant and subordinate animals. Dominant monkeys aggressed more frequently than subordinate monkeys, and spent less time in stationary postures. Alcohol produced dose-related biphasic changes in the number of threats, grasps, and displacements exhibited by dominant but not subordinate monkeys. Low doses of alcohol (0.1, 0.3, 0.6 g/kg) increased the frequency of aggressive behavior, and high doses (1.0 g/kg) decreased these behaviors. The largest change in behavior was evident in the first 20-40 min after injection, with a return to baseline 60-120 min later. The aggression-enhancing effect of alcohol in dominant monkeys required a stable baseline, which was obtained by daily administration of distilled water. Low levels of aggressive and associative behavior in subordinate monkeys were relatively unaffected at any dose of alcohol. They were associated with and aggressed against more often by untreated members of the group. The status-dependent effects of alcohol may be related to the neuroendocrine and neurotransmitter profiles characteristic of dominant and subordinate monkeys.

A10 lesion and passive avoidance latency: correlation with limbic tyrosine-hydroxylase-activity

Experiments were carried out with female tree-shrews (Tupaia belangeri). Following 6-hydroxydopamine injections into the ventral tegmental area, considerable variations in tyrosine-hydroxylase activity in the respective target regions of the A10 area occurred. Depletion of the THY-activity in the septum (SE), the entorhinal cortex (ECO), the frontal cortex and the basal ganglia correlated with an impairment in the performance of a passive avoidance task (indicated by testing 24 hours postshock). Partial correlation analysis revealed that the variations of THY-activity in the SE and ECO are primarily related to this failure in passive avoidance acquisition.