Accumbal dopamine and serotonin in anticipation of the next aggressive episode in rats

Autonomic and limbic neural activities are linked to aggressive behavior, and it is hypothesized that activities in the cardiovascular and monoaminergic systems play a role in preparing for an aggressive challenge. The objective was to learn about the emergence of monoamine activity in nucleus accumbens before an aggressive confrontation that was omitted at the regular time of occurrence, dissociating the motoric from the aminergic activity. Dopamine, serotonin, heart rate and behavioral activity were monitored before, during and after a single 10-min confrontation in resident male Long-Evans rats fitted with a microdialysis probe in the n. accumbens and with a telemetry sender (experiment 1). DA, but not 5-HT efflux, was confirmed to increase in n. accumbens during and after a single aggressive episode. In aggressive males that confronted an opponent daily for 10 days (experiment 2) heart rate rose 1 h before the regularly scheduled encounter relative to control rats, as measured on day 11 in the absence of any aggression. Concurrently, DA levels increased by 60-70% over baseline levels and 5-HT levels decreased by 30-35% compared to baseline levels. These changes were sustained over 1 h, and contrasted with no significant changes in DA, 5-HT, heart rate or behavioral activity in control rats. The rise in mesolimbic DA appears to be significant in anticipating the physiological and behavioral demands of an aggressive episode, and the fall in 5-HT in its termination, dissociated from the actual execution of the behavior.

Blockade of glucocorticoid receptors with ORG 34116 does not normalize stress-induced symptoms in male tree shrews

Glucocorticoid receptors play an important role in the regulation of the activity of the hypothalamo-pituitary-adrenal axis, and are thought to be involved in the pathophysiology of depressive disorders. The present study investigated the effect of the specific glucocorticoid receptor antagonist ORG 34116 (a substituted 11,21 bisarylsteroid compound) in the tree shrew (Tupaia belangeri) chronic psychosocial stress model, an established animal model for depressive disorders. Animals were stressed for 10 days before treatment with ORG 34116 started (25 mg/kg p.o. for 28 days). Stress induced a decrease in body weight, which just failed significance, whereas ORG 34116 did not affect body weight in stress and control animals. ORG 34116 enhanced the stress-induced increase in the concentration of urinary-free cortisol, although no differences between the different experimental groups existed during the last week of treatment. In stressed animals, ORG 34116 did not affect marking behavior, but decreased locomotor activity. Post mortem analysis of 5-HT(1A) receptors revealed a decreased affinity of 3[H]-8-OH-DPAT (3[H]-8-hydroxy-2-[di-n-propylamino]tetralin) binding sites in the hippocampus of animals treated with the glucocorticoid receptor antagonist. In conclusion, under our experimental conditions, the glucocorticoid receptor antagonist ORG 34116 did not normalize the depressive-like symptoms in the psychosocial stress model of male tree shrews. This finding, however, does not exclude that specific central, neuroendocrine and behavioral features are affected by the compound.

Social stress in tree shrews: effects on physiology, brain function, and behavior of subordinate individuals

Social stress is known to be involved in the etiology of central nervous disorders such as depression. In recent years, animal models have been developed that use chronic stress to induce neuroendocrine and central nervous changes that might be similar to those occurring in the course of the development of depressive disorders. The present review gives a summary of observations made in the tree shrew chronic social stress model. During periods of daily social stress, male tree shrews develop symptoms that are known from many depressed patients such as persistent hyperactivities of both the hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system, disturbances in sleeping patterns, and reduced motor activity. Moreover, various physiological parameters indicate an acceleration of the over all metabolic rate in socially stressed tree shrews. Some of these parameters can be renormalized by antidepressants thus supporting the view of the tree shrew social stress paradigm as model for major depression. In the brains of socially stressed animals, monoamine receptors show dynamic changes that reflect adaptation to the persistent monoaminergic hyperactivity during periods of chronic stress. In addition to the changes in neurotransmitter systems, there are structural changes in neurons, e.g., retraction of the dendrites of hippocampal pyramidal neurons. Together, these processes are suggested as a cause of behavioral alterations that can be counteracted by antidepressants in this naturalistic social stress model.

Stress hormone-related psychopathology: pathophysiological and treatment implications

Stress is commonly associated with a variety of psychiatric conditions, including major depression, and with chronic medical conditions, including diabetes and insulin resistance. Whether stress causes these conditions is uncertain, but plausible mechanisms exist by which such effects might occur. To the extent stress-induced hormonal alterations (e.g., chronically elevated cortisol levels and lowered dehydroepiandrosterone [DHEA] levels) contribute to psychiatric and medical disease states, manipulations that normalize these hormonal aberrations should prove therapeutic. In this review, we discuss mechanisms by which hormonal imbalance (discussed in the frameworks of "allostatic load" and "anabolic balance") might contribute to illness. We then review certain clinical manifestations of such hormonal imbalances and discuss pharmacological and behavioural treatment strategies aimed at normalizing hormonal output and lessening psychiatric and physical pathology.

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 subordination stress in male tree shrews: replacement of testosterone affects behavior and central alpha(2)-adrenoceptors

Subordination stress induced by social defeat in male animals is known to inhibit gonadal functions and it has been discussed whether the resulting deficit in testosterone might play a role in subordination behavior. One of the major transmitter systems involved in regulation of behavior is the noradrenergic system. To analyze whether a testosterone replacement can alter subordination behavior and whether this might be related to changes in the brain noradrenergic system, we quantified alpha(2)-adrenoceptors (alpha(2)-ARs) in the central nervous system of male tree shrews. Animals were submitted to chronic subordination stress and received testosterone at the same time. Behavior was monitored during all phases of the experiment: the control period of 10 days, the period of social stress lasting 10 days when subordinates were confronted daily with a dominant male, and, subsequently, the stress and treatment period of 18 days when in parallel to the stress, animals received either injections of testosterone or vehicle. Brain alpha(2)-ARs were quantified by in vitro receptor autoradiography using the antagonist ligand (3)H-RX821002. Locomotor activity decreased significantly during the stress period and was not re-normalized by testosterone. In contrast, testosterone re-normalized scent marking behavior and autogrooming, parameters that had both been reduced due to the subordination stress. Vehicle injections improved none of these behaviors. In 8 of 10 brain regions that were analyzed, numbers of alpha(2)-adrenergic binding sites were increased in stressed animals that received vehicle injections, but a difference between testosterone and vehicle injected animals was only observed in five regions. These brain regions are all known to be involved in emotional behavior (anterior hypothalamus, medial nucleus of the amygdala, cingulate cortex) or autonomic regulation, respectively (solitary tract nucleus, dorsal motor nucleus of vagus). Therefore, our data show that testosterone influences behavior of male subordinates and modulates alpha(2)-AR expression in their brains. Androgen-mediated alterations in receptors occur in brain regions that are known to be involved in emotionality, e.g., in the anterior hypothalamus which regulates aggressive behavior. One can therefore conclude that alpha(2)-ARs contribute to neuronal functions that are responsible for subordination of stress behavior, and that testosterone-induced receptor changes are related to the partial restoration of normal behavior.

Psychosocial stress, glucocorticoids, and structural alterations in the tree shrew hippocampus

Animal models for chronic stress represent an indispensable preclinical approach to human pathology since clinical data point to a major role of psychological stress experiences, acute and/or chronic, to the development of behavioral and physiological disturbances. Chronic emotional arousal is a consequence of various types of social interactions, and one major neurohumoral accompaniment is the activation of the classic stress circuit, the limbic--hypothalamic--pituitary--adrenocortical (LHPA) axis. The adrenocortical glucocorticoid hormones cortisol and corticosterone are principal effectors within this circuit since they affect neurotransmission and neuroendocrine control, thus having profound effects on mood and behavior. Using the experimental paradigm of chronic psychosocial stress in tree shrews, we investigated the impact of aversive chronic social encounters on hippocampal structure and function. In chronically stressed animals, we observed dendritic atrophy of hippocampal pyramidal neurons and an impairment of neurogenesis in the dentate gyrus. However, a stress-induced loss of hippocampal neurons was not observed in this animal model. This review summarizes our recent results on structural changes occurring during chronic stress in neurons of the hippocampus and their potential influence on learning and memory. We discuss whether these changes are reversible and to what extent glucocorticoids might be responsible for the stress-induced effects.

Corticosteroids in relation to fear, anxiety and psychopathology

Corticosteroids play extremely important roles in fear and anxiety. The mechanisms by which corticosteroids exert their effects on behavior are often indirect, because, although corticosteroids do not regulate behavior, they induce chemical changes in particular sets of neurons making certain behavioral outcomes more likely in certain contexts as a result of the strengthening or weakening of particular neural pathways. The timing of corticosteroid increase (before, during or after exposure to a stressor) determines whether and how behavior is affected. The present review shows that different aspects of fear and anxiety are affected differentially by the occupation of the mineralocorticoid receptor (MR) or glucocorticoid receptor (GR) at different phases of the stress response. Corticosteroids, at low circulating levels, exert a permissive action via brain MRs on the mediation of acute freezing behavior and acute fear-related plus-maze behavior. Corticosteroids, at high circulating levels, enhance acquisition, conditioning and consolidation of an inescapable stressful experience via GR-mechanisms. Brain GR-occupation also promotes processes underlying fear potentiation. Fear potentiation can be seen as an adjustment in anticipation of changing demands. However, such feed-forward regulation may be particularly vulnerable to dysfunction. MR and/or GR mechanisms are involved in fear extinction. Brain MRs may be involved in the extinction of passive avoidance, and GRs may be involved in mediating the extinction of active avoidance. In the developing brain, corticosteroids play a facilitatory role in the ontogeny of freezing behavior, probably via GRs in the dorsal hippocampus, and their influence on the development of the septo-hippocampal cholinergic system. Corticosteroids can exert maladaptive rather than adaptive effects when their actions via MRs and GRs are chronically unbalanced due to chronic stress. Both mental health of humans and animal welfare is likely to be seriously threatened after psychosocial stress, prolonged stress, prenatal stress or postnatal stress, especially when maternal care or social support is absent, because these can chronically dysregulate the central MR/GR balance. In such circumstances the normally adaptive corticosteroid responses can become maladaptive.

Acute stress decreases serotonin transporter mRNA in the raphe pontis but not in other raphe nuclei of the rat

In addition to elevated corticosterone levels, stress produces structural changes and neuronal damage especially in the hippocampus. In this line it has been shown, that in rats single or repeated immobilisation markedly reduces brain-derived neurotrophic factor (BDNF) mRNA levels in the hippocampal formation. Since this neurotrophin also controls the efficacy of serotonergic neurotransmission, the aim of the current study was to investigate the effect of acute immobilization stress on the expression of serotonin transporter (SERT) mRNA in the raphe nuclei as a parameter of serotonergic innervation. We have examined the expression of SERT mRNA and of BDNF mRNA in rats upon acute immobilisation by quantitative in situ hybridisation with a (35)S-labelled oligonucleotide probe. Elevated corticosterone levels in stressed animals confirmed as internal controls the effect of stress under our conditions. Acute stress led to a significant decrease of BDNF mRNA in the hippocampus and of SERT mRNA in the raphe pontis, but not in other raphe nuclei investigated. These data provide evidence for fast interactions between neurotrophins, corticosterone and serotonergic neurotransmission under stress conditions.

Regulation of monoamine receptors in the brain: dynamic changes during stress

Monoamine receptors are membrane-bound receptors that are coupled to G-proteins. Upon stimulation by agonists, they initiate a cascade of intracellular events that guide biochemical reactions of the cell. In the central nervous system, they undergo diverse regulatory processes, among which are receptor desensitization, internalization into the cell, and downregulation. These processes vary among different types of monoamine receptors. alpha 2-Adrenoceptors are often downregulated by agonists, and beta-adrenoceptors are internalized rapidly. Others, such as serotonin1A-receptors, are controlled tightly by steroid hormones. Expression of these receptors is reduced by the "stress hormones" glucocorticoids, whereas gonadal hormones such as testosterone can counterbalance the glucocorticoid effects. Because of this, the pattern of monoamine receptors in certain brain regions undergoes dynamic changes when there are elevated concentrations of agonists or when the hormonal milieu changes. Stress is a physiological situation accompanied by the high activity of brain monoaminergic systems and dramatic changes in peripheral hormones. Resulting alterations in monoamine receptors are considered to be in part responsible for changes in the behavior of an individual.

5HT1A-receptor binding in the brain of cyclic and ovariectomized female rats

Although it has been reported that hypothalamic 5HT1A-receptor functioning is modulated by oestrogen and that this modulation contributes to the regulation of female sexual behaviour, there have been no reports up to now showing changes in numbers of these receptors during the oestrus cycle and after oestrogen treatment. We therefore analysed 5HT1A-receptors in eight brain areas of female rats at different stages of the oestrus cycle, and in ovariectomized (OVX) females without and with oestrogen replacement. In-vitro receptor autoradiography with the agonist 3H-8-OH-DPAT(3H-8-hydroxy-2-[di-n-propylamino]tetralin) was used to determine numbers and affinities of 5HTA1A-receptors. To evaluate the hormonal state of the animals, serum concentrations of oestradiol, progesterone, luteinizing hormone (LH), and prolactin were also measured. Hormone determinations confirmed the expected endocrine states of the animals. In the ventromedial hypothalamic nucleus, the number of 3H-8-OH-DPAT binding sites (Bmax-value) during oestrus was increased compared to dioestrus yielding significant differences when using ANOVA statistics. In OVX females, the number of binding sites was decreased compared to pro-oestrus and oestrus, and after oestrogen replacement, it was as high as during oestrus. All other brain areas analysed (medial preoptic area, bed nucleus of the stria terminalis, lateral septum, cingulate cortex, amygdala, hippocampal region CA1, and layers V and VI of the occipital cortex) showed no significant changes in 3H-8-OH-DPAT binding site numbers. Also the affinity of 3H-8-OH-DPAT binding sites did not change during the oestrus cycle, but in the medial preoptic area, oestradiol-treated OVX animals showed a tendency for increased affinity compared to untreated OVX females. This was indicated by a change in Kd which appeared to be significant when groups were compared with the t-test. We conclude from our data, that in the ventromedial hypothalamic nucleus, which is involved in the regulation of sexual function, 5HT1A-receptors are up-regulated during oestrus, that ovariectomy reduces the receptor numbers, and that oestradiol replacement counteracts the effect of ovariectomy. Since the ventromedial hypothalamic nucleus contains a high number of oestrogen receptive cells, our data indicate that oestrogen up-regulates 5HT1A-receptor expression in this nucleus.