Animal models of social stress: effects on behavior and brain neurochemical systems.

Social interactions serve as an evolutionarily important source of stress, and one that is virtually ubiquitous among mammalian species. Animal models of social stress are varied, ranging from a focus on acute, intermittent, or chronic exposure involving agonistic behavior, to social isolation. The relative stressfulness of these experiences may depend on the species, sex, and age of the subjects, and subject sex also appears to influence the value of hypothalamic--pituitary--adrenal (HPA) axis activity as a general criterion for stress response: higher glucocorticoid levels are typically found in dominant females in some species. Social stress models often produce victorious and defeated, or dominant and subordinate, animals that may be compared to each other or to controls, but the appropriateness of specific types of comparisons and the interpretations of their differences may vary for the different models. Social stress strongly impacts behavior, generally reducing aggression and enhancing defensiveness, both inside and outside the stress situation. Social and sexual behaviors may be reduced in subordinate animals, as is activity and responsivity to normally rewarding events. However, some components of these changes may be dependent on the presence of a dominant, rather than representing a longer-term and general alteration in behavior. Social stress effects on brain neurotransmitter systems have been most extensively investigated, and most often found in serotonin and noradrenergic systems, with changes also reported for other monoamine and for peptidergic systems. Morphological changes and alterations of neogenesis and of cell survival particularly involving the hippocampus and dentate gyrus have been reported with severe social stress, as have longer-term changes in HPA axis functioning. These findings indicate that social stress models can provide high magnitude and appropriate stressors for research, but additionally suggest a need for caution in interpretation of the findings of these models and care in analysis of their underlying mechanisms.

Chronic social stress reduces dendritic arbors in CA3 of hippocampus and decreases binding to serotonin transporter sites.

Male rats housed in mixed-sex groups in a visible burrow system (VBS) form a dominance hierarchy in which subordinate animals show stress-related changes in behavior, endocrine function and neurochemistry. Dominants also appear to be moderately stressed compared to controls, although these animals do not develop the more pronounced behavioral and physiological deficits seen in the subordinates. In the present study, we examined the effects of chronic psychosocial stress on the morphology of Golgi-impregnated CA3 pyramidal neurons. In addition, since serotonin has been implicated in the mechanisms mediating the dendritic remodeling seen with other chronic stress regimens, we used quantitative autoradiography to measure binding to the serotonin transporter (5HTT) in hippocampus and dorsal and median raphe. Chronic social stress led to a decrease in the number of branch points and total dendritic length in the apical dendritic trees of CA3 pyramidal neurons in dominant animals compared to unstressed controls; subordinates also had a decreased number of dendritic branch points. [(3)H]paroxetine binding to the 5HTT was decreased in Ammon's horn in both dominants and subordinates compared to controls, while 5HTT binding remained unchanged in dentate gyrus and raphe. The similarity of the changes in 5HTT binding and dendritic arborization between both groups of VBS animals, despite apparent differences in stressor severity, suggests that these changes may be part of the normal adaptive response to chronic social stress. The mechanisms underlying dendritic remodeling in CA3 pyramidal neurons are likely to involve stress-induced changes in glucocorticoids and in 5HT and other transmitters.

Corticosterone and phenytoin reduce neuronal nitric oxide synthase messenger RNA expression in rat hippocampus.

The production and release of the corticosteroids, namely the glucocorticoids and the mineralocorticoids, are regulated by various stimuli, including stress. Previous studies from our laboratory have shown that chronic exposure to stress or to stress levels of glucocorticoids produces atrophy of the apical dendrites of CA3 pyramidal neurons in the hippocampus. This stress-induced dendritic remodeling is blocked by the anti-epileptic drug phenytoin, which suppresses glutamate release, and also by N-methyl-D-aspartate receptor antagonists. These results suggest an interaction between glucocorticoids and excitatory amino acids in the development of stress-induced atrophy of CA3 pyramidal neurons. Since nitric oxide is proposed to play an important role in mediating both the physiological and pathophysiological actions of excitatory amino acids, we examined the regulation of neuronal nitric oxide synthase messenger RNA expression by corticosterone and phenytoin in the rat hippocampus. The expression of neuronal nitric oxide synthase messenger RNA in hippocampal pyramidal neurons and granule neurons of the dentate gyrus was unaffected by 21-day administration of corticosterone (40 mg/kg), phenytoin (40 mg/kg) or the combination of corticosterone and phenytoin. However, in hippocampal interneurons, corticosterone/ phenytoin co-administration led to a significant reduction in neuronal nitric oxide synthase messenger RNA levels when compared with vehicle controls. These results suggest that, during exposure to stress levels of corticosterone, phenytoin inhibits glucocorticoid-induced atrophy of CA3 pyramidal neurons by reducing neuronal nitric oxide synthase expression in hippocampal interneurons. Moreover, these results may provide another example of synaptic plasticity in the hippocampus mediated by nitric oxide synthase.

Decreased 5-HT1A and increased 5-HT2A receptor binding after chronic corticosterone associated with a behavioural indication of depression but not anxiety.

The effects of chronic corticosterone treatment (100 mg pellet implanted for 1 week) were assessed in animal tests of anxiety, exploration and motor activity, and changes in binding to 5-HT1A and 5-HT2A receptors, and the 5-HT transporter, were measured. At the end of the week's treatment, the corticosterone concentration was significantly elevated and there were significant decreases in adrenal, thymus and body weights. However, there were no changes in the measures of anxiety in the social interaction test or on trials 1 and 2 of the elevated plus-maze. Also supporting a dissociation between anxiety and elevated corticosterone concentrations are previous findings that benzodiazepine withdrawal causes increased anxiety but no change in corticosteroid concentrations. Therefore these two situations provide a double dissociation between anxiety and elevated corticosteroids. Decreased 5-HT1A receptor binding in the dentate gyrus and increased 5-HT2A receptor binding in the parietal cortex was found following chronic corticosterone treatment. This reciprocal relationship between 5-HT1A and 5-HT2A receptors has been proposed to be important in mediating depression. The significant decreases in motor activity observed in all the test situations would be compatible with this proposal. Thus the constellation of behavioural and biochemical changes detected after chronic corticosterone treatment is more pertinent to depression than anxiety. One week after removal of the pellets, the behavioural and neurochemical changes had disappeared and the only differences to remain were decreased adrenal, thymus and body weights in the animals that had been treated chronically with corticosterone.

Chronic social stress alters levels of corticotropin-releasing factor and arginine vasopressin mRNA in rat brain.

In the visible burrow system model of chronic social stress, male rats housed in mixed-sex groups quickly form a dominance hierarchy in which the subordinates appear to be severely stressed. A subgroup of subordinates have an impaired corticosterone response after presentation of a novel restraint stressor, leading to their designation as nonresponsive subordinates. To examine the mechanism underlying the blunted corticosterone response in these animals, in situ hybridization histochemistry was used to quantify corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) mRNA expression in the brain. In two separate visible burrow system experiments, the nonresponsive subordinates expressed a significantly lower average number of CRF mRNA grains per cell in the paraventricular hypothalamic nucleus compared with stress-responsive subordinates, dominants (DOM), or cage-housed control (CON) rats. The number of CRF mRNA labeled cells was also significantly lower in nonresponders than in responsive subordinates or DOM. In the central amygdala, CRF mRNA levels were increased in both groups of subordinates compared with CON rats, whereas responsive subordinates exhibited higher levels than the DOM rats as well. AVP mRNA levels did not vary with behavioral rank in any subdivision of the paraventricular hypothalamic nucleus. In the medial amygdala, the number of cells expressing AVP mRNA was significantly greater in CON rats compared with both groups of subordinates, although the average number of AVP mRNA grains per cell did not vary with rank. In addition, the number of AVP-positive cells significantly correlated with plasma testosterone level.

Tianeptine treatment induces regionally specific changes in monoamines.

Tianeptine is an atypical tricyclic antidepressant that facilitates serotonin (5-HT) reuptake. Tianeptine (10 mg/kg) or saline was administered intraperitoneally to male rats daily for 4 days. Monoamine levels were measured in micropunches of discrete brain nuclei that are implicated in mood and cognition. In addition, the rates of 5-HT and norepinephrine (NE) accumulation were determined by the pargyline method. Few changes were noted in the 5-HT system. 5-HT levels were increased by short-term tianeptine in the CA3 region of hippocampus, and 5-hydroxyindoleacetic acid (5-HIAA) was increased in the ventromedial nucleus of hypothalamus, while 5-HT turnover was decreased in preoptic area (POA). In addition, short-term tianeptine treatment increased NE levels in POA, parietal sensory cortex (SCTX) and dorsal raphe (DR), and decreased NE in dentate gyrus. NE turnover was also decreased in DR, SCTX and parietal motor cortex. These data suggest that the short-term neural and behavioral actions of tianeptine may be attributable, in part, to alterations of the norepinephrine system.

Effects of chronic social stress on tyrosine hydroxylase mRNA and protein levels.

Males housed in mixed sex groups quickly form dominance hierarchies; subordinates can be further subdivided into stress responsive subordinates (SRS) and non-responsive subordinates (NRS) based on corticosterone responses to a novel stressor. Tyrosine hydroxylase (TH) mRNA levels measured with in situ hybridization were elevated in locus coeruleus (LC) of NRS compared to singly or pair-housed controls; NRS also had higher TH levels than dominants. TH protein levels determined by immunoautoradiography were also higher in LC of NRS and SRS versus pair-housed controls.

Serotonin receptor binding in a colony model of chronic social stress.

Male rats housed in mixed-sex groups quickly established dominance hierarchies in which subordinates appeared severely stressed. Subordinate rats had elevated basal corticosterone (CORT) levels relative to dominants and individually housed controls. Several subordinates had blunted CORT responses to a novel stressor, leading to the classification of subordinates as either stress-responsive or nonresponsive. Binding to 5-HT1A receptors was reduced in stress-responsive subordinates compared to controls throughout hippocampus and dentate gyrus. Decreased binding was observed in nonresponsive subordinates only in CA3 of hippocampus. In addition, 5-HT1A binding was decreased in CA1, CA3, and CA4 in dominants compared to controls. Binding to 5-HT2 receptors was increased in parietal cortex in both responsive and nonresponsive subordinates compared to controls. No changes were observed in binding to 5-HT1B receptors. These results are discussed in the context of regulation of the serotonergic system by stress and glucocorticoids and possible relevance to the pathophysiology of depression.

Short-term fluoxetine treatment alters monoamine levels and turnover in discrete brain nuclei.

The effects of short-term fluoxetine administration on monoamine levels and turnover were assessed in discrete brain nuclei. Adult male rats received fluoxetine HCl (10 mg/kg) or saline injections intraperitoneally for 4 days and monoamine levels determined by high performance liquid chromatography. The major metabolite of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), was decreased by fluoxetine treatment in the ventromedial hypothalamic nucleus (VMN), the lateral hypothalamic area and the CA1 region of the hippocampus. Fluoxetine treatment significantly increased serotonin (5-HT) levels in the VMN but did not change 5-HT levels in any other area examined. Norepinephrine (NE) levels were higher in fluoxetine-treated rats in the dorsomedial hypothalamic nucleus, dorsal raphe nucleus and parietal motor cortex (MCTX). 5-HT and NE turnover were also determined by the pargyline method. Fluoxetine treatment decreased 5-HT turnover in the VMN and increased 5-HT turnover in the median raphe. NE turnover was decreased in the preoptic area, the MCTX and parietal sensory cortex by fluoxetine administration. These results demonstrate that brain areas with similar 5-HT innervation respond differently to fluoxetine administration and fluoxetine, which selectively alters 5-HT uptake, also affects NE levels and turnover in several brain nuclei.

Effect of 5,7-dihydroxytryptamine, ovariectomy and gonadal steroids on serotonin receptor binding in rat brain.

Quantitative autoradiography was used to assess alterations in serotonin (5-HT) receptor binding in the hypothalamus and hippocampus following denervation with 5,7-dihydroxytryptamine (5,7-DHT), ovariectomy (OVX) and gonadal steroid manipulation. Seven days after 5,7-DHT injection, 5-HT1a receptor density was significantly increased in the ventromedial hypothalamic nucleus (VMN) of intact but not OVX female rats. Under these conditions 5-HT1b receptor density was unchanged in any brain region examined and 5-HT transporter binding was decreased in all 5,7-DHT injected animals. In addition, there was a significant interaction between OVX and 5,7-DHT for both the 5-HT1a receptor and the 5-HT transporter in the VMN, such that OVX inhibited the 5,7-DHT-induced increase in 5-HT1a receptors and attenuated the 5,7-DHT-induced decrease in 5-HT transporter binding. In a separate experiment the effect of gonadal steroid manipulation on 5-HT receptor binding was assessed. In female OVX rats, 5-HT1a receptor density was unchanged by estrogen or estrogen and progesterone administration. In male rats, castration significantly decreased 5-HT1a receptor density in the medial preoptic area. Estrogen and progesterone administration to female OVX rats increased the density of 5-HT1b receptors in the VMN, as compared to estrogen alone. The relationship of these results to the role of 5-HT in mediating lordosis behavior is discussed.

Autoradiographic analyses of the effects of estradiol benzoate on [3H]paroxetine binding in the cerebral cortex and dorsal hippocampus of gonadectomized male and female rats.

Effects of estradiol benzoate (EB) on [3H]paroxetine binding in dorsal hippocampus and cerebral cortex of gonadectomized male and female rats were evaluated by quantitative autoradiography. EB significantly decreased [3H]paroxetine binding in male and female rats in the oriens layers of CA1-CA4, and in the radiata/lacunosum moleculare layers of CA2 and CA3. Sex differences were also noted, with binding of [3H]paroxetine being significantly lower in female rats in the radiata/lacunosum moleculare layers of CA2 and CA4, and in the suprapyramidal dentate. No significant effects of either EB or sex were noted in the cortex.

Alterations of serotonin receptor binding in the hypothalamus following acute denervation.

Quantitative autoradiography was used to determine the effect of acute serotonergic denervation with 5,7-dihydroxytryptamine (5,7-DHT) or serotonin 5HT1a and 5-HT1b receptors in male rats. Seven days after intrahypothalamic 5,7-DHT injection there was a significant increase in the density of 5HT1a receptors in the ventromedial and dorsomedial hypothalamic nuclei (VMN and DMN) of male rats. In adjacent sections. 5-HT1b receptors were significantly increased only in the VMN. No changes in receptor density were observed in the lateral hypothalamic area or hippocampus even though binding of [3H]paroxetine, which labels the presynaptic transporter site, was significantly decreased in all evaluated brain regions in 5,7-DHT-treated animals. In addition to demonstrating that 5-HT1a and 5-HT1b receptors are differentially regulated in different brain areas, these results show that in the brain regions examined both 5-HT1a and 5-HT1b receptors are primarily post-synaptic.