Socially defeated male rats display a blunted adrenocortical response to a low dose of 8-OH-DPAT

Sex Differences in Serotonin 5-HT 1A Receptor Responses to Repeated Restraint Stress in Adult Male and Female Rats

BACKGROUND

Male and female rats were exposed to repeated restraint to determine how changes in serotonin (5-hydroxytryptamine; 5-HT) 1A receptors associate with stress hypothalamic-pituitary-adrenal (HPA) axis habituation.

METHODS

In response to 2-hour episodes of restraint, repeated daily for 5 consecutive days, males and females displayed reliable declines in HPA output, indicated by diminished adrenocorticotropin and corticosterone secretory responses. Using the 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) as a pharmacological challenge for inducing hypothermia and elevations in plasma corticosterone, males displayed sensitized hypothermal responses after repeated restraint, whereas corticosterone responses to 8-OH-DPAT were enhanced in both sexes following single or repeated exposure.

RESULTS

Only males showed elevations in 5-HT 1A receptor G-protein coupling responses in the dorsal raphe after repeated restraint, whereas only females showed an increase in 5-HT 1A receptor responses in the hippocampus following single or repeated exposure. G-protein coupling responses within both regions correlated positively with 5-HT 1A receptor binding capacity. Thus, despite expressing similar capacities for stress HPA axis habituation, males and females emerged from repeated restraint to show region-specific changes in 5-HT 1A receptor function that may be explained, at least in part, by changes in receptor availability.

CONCLUSIONS

Based on the hypothermal and corticosteroid responses to 8-OH-DPAT, the present data suggest that stress habituation is met by an increase in the sensitivity of presynaptic 5-HT 1A receptors in males and by an increase in the sensitivity of a population of postsynaptic receptors in both sexes.

Pathogenesis of depression: Insights from human and rodent studies

Major depressive disorder (MDD) will affect one out of every five people in their lifetime and is the leading cause of disability worldwide. Nevertheless, mechanisms associated with the pathogenesis of MDD have yet to be completely understood and current treatments remain ineffective in a large subset of patients. In this review, we summarize the most recent discoveries and insights for which parallel findings have been obtained in human depressed subjects and rodent models of mood disorders in order to examine the potential etiology of depression. These mechanisms range from synaptic plasticity mechanisms to epigenetics and the immune system where there is strong evidence to support a functional role in the development of specific depression symptomology. Ultimately we conclude by discussing how novel therapeutic strategies targeting central and peripheral processes might ultimately aid in the development of effective new treatments for MDD and related stress disorders.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Chronically restricted sleep leads to depression-like changes in neurotransmitter receptor sensitivity and neuroendocrine stress reactivity in rats

STUDY OBJECTIVES

Frequently disrupted and restricted sleep is a common problem for many people in our Western society. In the long run, insufficient sleep may have repercussions for health and may sensitize individuals to psychiatric diseases. In this context, we applied an animal model of chronic sleep restriction to study effects of sleep loss on neurobiological and neuroendocrine systems that have been implied in the pathophysiology of depression, particularly the serotonergic system and the hypothalamic-pituitary-adrenal (HPA) axis.

DESIGN

Adult rats were exposed to a schedule of chronic partial sleep deprivation allowing them only 4 h of sleep per day. Sleep restriction was achieved by placing the animals in slowly rotating drums. To examine the regulation and reactivity of the HPA axis, blood samples were collected to measure adrenocorticotropin (ACTH) and corticosterone (CORT) responses.

MEASUREMENTS AND RESULTS

While one day of restricted sleep had no significant effect on HPA axis stress reactivity, sleep restriction for a week caused a blunted pituitary ACTH response in a conditioned fear paradigm. Despite this lower ACTH response, adrenal CORT release was normal. The blunted pituitary response may be related to reduced sensitivity of serotonin-1A receptors and/or receptors for corticotropin-releasing hormone (CRH), since sleep restricted rats showed similar reductions in ACTH release to direct pharmacological stimulation with a serotonin-1A agonist or CRH.

CONCLUSIONS

Chronic sleep restriction may lead to changes in neurotransmitter receptor systems and neuroendocrine reactivity in a manner similar to that seen in depression. This experimental study thus supports the hypothesis that disrupted and restricted sleep may contribute to the symptomatology of psychiatric disorders.

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.

Animal models of depression in drug discovery: a historical perspective

Over the course of the last 50 years many models of major depressive disorder have been developed on the basis of theoretical aspects of this disorder. These models and procedures have been crucial in the discovery and development of clinically-effective drugs. Notwithstanding, there is presently great concern about the discrepancy between positive outcomes of new candidate drugs in animal models and apparent lack of efficacy in humans i.e., the predictive validity of animal models. Some reasons for this concern lie in the over-reliance in the face value of behavioural models, design of clinical trials, placebo responses, genetic variations in response to drugs, species differences in bioavailability and toxicology, and not least, disinterest of pharmaceutical sponsors to continue developing certain drugs. Present model development is focusing on endophenotypic aspects of behaviours rather than trying to model whole syndromes. This essay traces the origins and theoretical bases of our animal models of depression or depressed-like behaviours in humans and indicates how they have evolved from behavioural assays used to measure the potency and efficacy of potential candidate drugs to tools by which endophenotypes of depression may be identified and verified pharmacologically. A cautionary note is included though to indicate that the true predictive validity of our models will not be fully assessed until we can determine the attrition rate of molecules discovered from new drug targets translating into clinically-effective drugs.

Dominant-submissive behavior as models of mania and depression

This review examines the ways in which dominant-subordinate behavior in animals, as determined in laboratory studies, can be used to model depression and mania in humans. Affective disorders are mood illnesses with two opposite poles, melancholia (depression) and mania that are expressed to different degrees in affected individuals. Dominance and submissiveness are also two contrasting behavioral poles distributed as a continuum along an axis with less or more dominant or submissive animals. The premise of this article is that important elements of both mania and depression can be modeled in rats and mice based on observation of dominant and submissive behavior exhibited under well defined conditions. Studies from our own research, where dominance and submissiveness are defined in a competition test and measured as the relative success of two food-restricted rats to gain access to a feeder, have yielded a paradigm that we call the Dominant Submissive Relationship (DSR). This paradigm results in two models sensitive to drugs used to treat mood disorders. Specifically, drugs used to treat mania inhibit the dominant behavior of rats gaining access to food at the expense of an opponent (Reduction of Dominant Behavior Model or RDBM), whereas antidepressants counteract the behavior of rats losing such encounters; Reduction of Submissive Behavior Model (RSBM). The validation of these models, as well as their advantages and limitations, are discussed and compared with other animal paradigms that utilize animal social behavior to model human mood disturbances.

Early postnatal stress alters the 5-HTergic modulation to emotional stress at postadolescent periods of rats

Several lines of evidence have shown that traumatic events during the early postnatal stage might precipitate long-lasting alteration in the functional properties underlying emotional expression. The aim of the present study was to examine whether the early postnatal stress alters the 5-HTergic mechanism underlying regulation of emotional stress, focusing on the 5-HT(1A) receptor-mediated synaptic responses in adult rats. Pups were exposed to aversive stimulus, footshock (FS) at the postnatal period of the second (2W) and the third week (3W). At postadolescent period (10-12-week-old), electrophysiological and behavioral studies were performed. The 5-HT(1A) receptor agonist tandospirone (10 mg/kg body wt, i.p.) blocked the long-term potentiation (LTP) in the hippocampal CA1 field of 3W-FS, as well as non-FS control, whereas this inhibition was not observed in 2W-FS group. Fear-related freezing behavior observed during exposure to contextual fear conditioning markedly attenuated in 2W-FS, but not in 3W-FS. These data suggest that aversive stress exposed at 2W is attributable to changes in the 5-HT(1A) receptor-mediated synaptic plasticity, which may be responsible for the attenuation of freezing behavior. Thus, 5-HT(1A) receptors appear to play a key role in the 5-HTergic mechanism underlying regulation of emotional stress on the postnatal development of the brain. In other words, the second postnatal week may be the "critical period" for establishing proper behavioral responses to emotional stress in adult rats. (c) 2005 Wiley-Liss, Inc.

The Darwinian concept of stress: benefits of allostasis and costs of allostatic load and the trade-offs in health and disease

Why do we get the stress-related diseases we do? Why do some people have flare ups of autoimmune disease, whereas others suffer from melancholic depression during a stressful period in their life? In the present review possible explanations will be given by using different levels of analysis. First, we explain in evolutionary terms why different organisms adopt different behavioral strategies to cope with stress. It has become clear that natural selection maintains a balance of different traits preserving genes for high aggression (Hawks) and low aggression (Doves) within a population. The existence of these personality types (Hawks-Doves) is widespread in the animal kingdom, not only between males and females but also within the same gender across species. Second, proximate (causal) explanations are given for the different stress responses and how they work. Hawks and Doves differ in underlying physiology and these differences are associated with their respective behavioral strategies; for example, bold Hawks preferentially adopt the fight-flight response when establishing a new territory or defending an existing territory, while cautious Doves show the freeze-hide response to adapt to threats in their environment. Thus, adaptive processes that actively maintain stability through change (allostasis) depend on the personality type and the associated stress responses. Third, we describe how the expression of the various stress responses can result in specific benefits to the organism. Fourth, we discuss how the benefits of allostasis and the costs of adaptation (allostatic load) lead to different trade-offs in health and disease, thereby reinforcing a Darwinian concept of stress. Collectively, this provides some explanation of why individuals may differ in their vulnerability to different stress-related diseases and how this relates to the range of personality types, especially aggressive Hawks and non-aggressive Doves in a population. A conceptual framework is presented showing that Hawks, due to inefficient management of mediators of allostasis, are more likely to be violent, to develop impulse control disorders, hypertension, cardiac arrhythmias, sudden death, atypical depression, chronic fatigue states and inflammation. In contrast, Doves, due to the greater release of mediators of allostasis (surplus), are more susceptible to anxiety disorders, metabolic syndromes, melancholic depression, psychotic states and infection.

Long-term effects of social stress on brain and behavior: a focus on hippocampal functioning

In order to study mechanisms involved in the etiology of human affective disorders, there is an abundant use of various animal models. Next to genetic factors that predispose for psychopathologies, environmental stress is playing an important role in the etiology of these mental diseases. Since the majority of stress stimuli in humans that lead to psychopathology are of social nature, the study of consequences of social stress in experimental animal models is very valuable. The present review focuses on one of these models that uses the resident-intruder paradigm. In particular the long-lasting effects of social defeat in rats will be evaluated. Data from our laboratory on the consequences of social defeat on emotional behavior, stress responsivity and serotonergic functionality are presented. Furthermore, we will go into detail on hippocampal functioning in socially stressed rats. Very recent results show that there is a differential effect of a brief double social defeat and repetitive social defeat stress on dendritic remodeling in hippocampal CA3 neurons and that this has repercussions on hippocampal LTP and LTD. Both the structural and electrophysiological changes of principal neurons in the hippocampal formation after defeat are discussed as to their relationship with the maintenance in cognitive performance that was observed in socially stressed rats. The results are indicative of a large dynamic range in the adaptive plasticity of the brain, allowing the animals to adapt behaviorally to the previously occurred stressful situation with the progression of time.

Social stress alters the severity of acute Theiler's virus infection

Our laboratory has previously shown that restraint stress resulted in decreased Theiler's virus-induced CNS inflammation, while exacerbating illness behaviors during the acute phase of disease. In contrast, social disruption stress (SDR) applied prior to infection led to the development of glucocorticoid (GC) resistance, and these animals developed more severe disease course, with increased inflammation. However, when SDR was applied concurrent with infection, GC resistance fails to develop, disease course is less severe and inflammation was moderate. These results suggest that the effects of SDR on Theiler's virus infection are dependent upon the timing of SDR application in relation to infection.

Activation of post-synaptic 5-HT(1A) receptors in the dorsal hippocampus prevents learned helplessness development

Activation of post-synaptic 5-HT(1A) receptors in the dorsal hippocampus is proposed to mediate stress adaptation. Chronic social stress and high corticosteroid levels would impair this coping mechanism, predisposing animals to learned helplessness. To test the hypothesis that increasing serotonin levels in the dorsal hippocampus would attenuate the development of learned helplessness, rats received inescapable foot-shock (pre-test session) and were tested in a shuttle box 24-h later. Pre-stressed animals showed impairment of escape responses. This effect was prevented by chronic (21 days) treatment with imipramine (15 mg/kg). Similar results were obtained when the animals received bilateral intra-hippocampal injections, immediately after pre-test, of zimelidine (100 nmol/0.5 microl), a serotonin reuptake blocker, or 8-OH-DPAT (10 nmol), a 5-HT(1A) receptor agonist. The zimelidine effect was prevented by pre-treatment with WAY-100635 (30 nmol), a 5-HT(1A) receptor antagonist. These data suggest that facilitation of serotonergic neurotransmission in the dorsal hippocampus mediates adaptation to severe inescapable stress, probably through the activation of post-synaptic 5-HT(1A) receptors.

Behavioral and physiological responses to stress are affected by high-fat feeding in male rats

Interactions between monoaminergic neurochemistry and macronutrient intake have been frequently shown. Because monoaminergic systems in the brain are also closely involved in behavioral and physiological stress responses it can be hypothesized that differences in the macronutrient composition of diets are reflected in these responses. The present studies, therefore, were designed to assess the consequences of a change in dietary macronutrient composition on a variety of physiological and behavioral responses (both acute and long-term) to a number of stressors. The effect of chronic high-fat (HF; 61% kcal from fat) feeding on the stress responses was compared with controls receiving regular high-carbohydrate (HC; 63% kcal from carbohydrates) laboratory chow. Rats were kept on this diet for at least 2 months before they were exposed to either psychological (social defeat) or physiological (lipopolysaccharide, LPS, administration) stress. At baseline, chronic HF feeding caused a slight, but significantly reduction in body temperature relative to that observed in HC-fed rats. Following social defeat or LPS injection, HF feeding caused a faster recovery of the body temperature increase relative to animals on the HC diet. Stress-induced suppression of home cage locomotor activity and body weight gain were also reduced by HF feeding. The serotonergic 5-HT(1a) receptor hyposensitivity that was observed in HC-fed rats 2 weeks after stress was absent in the HF regimen. Although the present results cannot be readily interpreted as showing purely beneficial effects of high-fat diets on stress responsivity, the findings in the present study do encourage further investigation of possible ameliorating effects of high-fat diets on aspects of the behavioral and physiological response stress.

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.

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.

Central serotonin depletion modulates the behavioural, endocrine and physiological responses to repeated social stress and subsequent c-fos expression in the brains of male rats

Intraspecific confrontation has been used to study effect of depleting central serotonin on the adaptation of male rats to repeated social stress (social defeat). Four groups of adult male rats were used (serotonin depletion/sham: stressed; serotonin depletion/sham: non-stressed). Central serotonin was reduced (by 59-97%) by a single infusion of the neurotoxin 5,7-dihydroxtryptamine (150 microg) into the cerebral ventricles; levels of dopamine and noradrenaline were unaltered (rats received appropriate uptake blockers prior to neurotoxic infusions). Sham-operated animals received solute only. Rats were then either exposed daily for 10 days to a second larger aggressive male in the latter's home cage, or simply transferred to an empty cage (control procedure). Rats with reduced serotonin failed to show the increased freezing behaviour during the pre-defeat phase of the social interaction test characteristic of sham animals. There was no change in the residents' behaviour. Core temperature increased during aggressive interaction in sham rats, and this did not adapt with repeated stress. By contrast, stress-induced hyperthermia was accentuated in serotonin-reduced rats as the number of defeat sessions increased. Basal core temperature was unaffected by serotonin depletion. Heart rate increased during social defeat, but this did not adapt with repeated stress; serotonin depletion had no effect on this cardiovascular response. Basal corticosterone was increased in serotonin-depleted rats, but the progressive reduction in stress response over days was not altered. C-fos expression in the brain was not altered in control (non-stressed) rats by serotonin reduction in the areas examined, but there was increased expression after repeated social stress in the medial amygdala of 5-HT depleted rats. These experiments show that reduction of serotonin alters responses to repeated social stress in male rats, and suggests a role for serotonin in the adaptive process.

Behavioral, neuroendocrine and serotonergic consequences of single social defeat and repeated fluoxetine pretreatment in the Lewis rat strain

We have analysed some behavioral, neuroendocrine and serotonergic consequences of a single (30-min) social defeat followed by 14-18 h of sensory contact with the aggressor, in Lewis rats, an inbred strain highly sensitive to chronic social stressors [Berton O. et al. (1998) Neuroscience 82, 147-159]. In addition, we have investigated how the aforementioned consequences are affected by pretreatment with the selective serotonin reuptake inhibitor, fluoxetine (7.5 mg/kg/day for 21 days). A single social defeat triggered hypophagia and body weight loss, and increased anxiety in the elevated plus-maze. It did not affect baseline plasma adrenocorticotropic hormone levels and renin activity, but decreased plasma corticosterone levels. On the other hand, the responses of the latter variables to subsequent acute forced swim stress were blunted (corticosterone) or amplified (adrenocorticotropic hormone, renin activity) by prior defeat. The density of hippocampal serotonin transporters, but not that of hippocampal serotonin-1A and cortical serotonin-2A receptors, was decreased by a single social defeat; in addition, neither tryptophan availability and serotonin synthesis/metabolism, nor serotonin-1A autoreceptor-mediated functions (inhibition of serotonin synthesis, hyperphagia) were affected. Fluoxetine pretreatment diminished social defeat-induced hypophagia, body weight loss and anxiety without affecting these variables in control animals. This pretreatment increased plasma corticosterone levels in resting and acutely stressed rats, but abolished social defeat-elicited corticosterone hyporesponsiveness to acute forced swim stress. Except for a decrease in midbrain serotonin transporter density, fluoxetine did not affect the other serotonergic indices analysed herein, i.e. serotonin-1A and serotonin-2A receptor densities, serotonin synthesis/metabolism. A single social defeat in Lewis rats produces behavioral and endocrine alterations that may model some aspects of human anxiety disorders. In this paradigm, prior fluoxetine treatment is endowed with adaptive behavioral, and possibly neuroendocrine, effects without affecting the key elements of central serotonergic systems analysed herein.

Long-lasting deficient dexamethasone suppression of hypothalamic-pituitary-adrenocortical activation following peripheral CRF challenge in socially defeated rats

The present study focuses on the long-term changes in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis following two short-lasting episodes of intensive stress in the rat stress model of social defeat and the possible similarities with HPA functioning in human affective disorders. Male Wistar rats experienced social defeats on 2 consecutive days by an aggressive male conspecific. The long-term effect of these defeats on resting and ovine corticotropin-releasing factor (oCRF; intravenous (i.v.) 0. 5 microg/kg) induced levels of plasma ACTH and corticosterone (CORT) were measured 1 and 3 weeks later. In a second experiment the glucocorticoid feedback regulation of HPA function was tested in a combined dexamethasone (DEX)/CRF test (DEX; 25 microg/kg s.c., 90 min before oCRF injection, 0.5 microg/kg). The oCRF challenges were performed between 11.00 and 13.00 h (about three hours after start of the light phase). One week after defeat the ACTH response to CRF was significantly enhanced in defeated rats as compared to controls. Three weeks after defeat the ACTH response was back to control levels. The increased ACTH response 1 week after the stressor was not reflected in higher CORT levels. Neither were baseline ACTH and CORT levels affected by the prior stress exposure. DEX pretreatment inhibited pituitary adrenocortical activity, reflected both in reduced baseline and response values of ACTH and CORT. The ACTH response to CRF following DEX administration was significantly higher in defeated rats as compared to controls both at one and three weeks after defeat. A reduced DEX suppression of baseline secretion of ACTH appeared 3 weeks after defeat. The same tendency was apparent in response and baseline values of CORT. The differences in CORT between socially stressed and control treated rats, however, did not reach significance. The possible role of changes in glucocorticoid-(GR) and mineralocorticoid receptor (MR) binding in the altered regulation of HPA activity following defeat were studied in brain and pituitary of male Wistar rats 1 and 3 weeks after defeat. One week after defeat GR-binding decreased in hippocampus and hypothalamus. No changes were observed in GR-binding in the pituitary nor in MR-binding in any of the regions analysed. Three weeks after defeat GR-binding recovered in hippocampus and hypothalamus but at this time MR-binding in hippocampal tissue was seriously decreased. In a fourth experiment vasopressin (AVP) and CRF stores in the external zone of the median eminence (ZEME) were measured by quantitative immunocytochemistry one and three weeks after defeat and compared with controls. Social defeat failed to induce a change in the immunocytochemical stores of AVP or CRF. The present findings show that in rats short-lasting stressors like defeat induce long-lasting, temporal dynamic changes in the regulation of the HPA axis. Since these changes in time are reflected in GRs and MRs in different brain areas an altered corticosteroid receptor binding might play an important role in the affected HPA activity following defeat.

Blockade of the anxiolytic-like action of ipsapirone and buspirone, but not that of 8-OH-DPAT, by adrenalectomy in male rats

The effect of the 5-HT1A agonists ipsapirone (5 mg/kg), buspirone (5 mg/kg) and 8-OH-DPAT (0.5 mg/kg) on experimental anxiety was examined in sham-operated, adrenalectomized and adrenally demedullated male rats. The animal model of anxiety used was the defensive burying test. At the doses selected, all 5-HT1A compounds produced an anxiolytic-like action by reducing the burying behavior in both sham-operated and demedullated rats. However, in adrenalectomized subjects, while 8-OH-DPAT still reduced burying behavior, ipsapirone and buspirone lost their action. Data suggest that adrenocortical secretions play a role in the anxiolytic-like actions of buspirone and ipsapirone, but not in those of 8-OH-DPAT. Buspirone and ipsapirone also produced a reduction in burying behavior latency in sham-operated animals that was not observed in adrenalectomized or adrenally demedullated rats. These data suggest that adrenaline may be participating in the action of these compounds on the burying behavior latency. Present findings support possible direct relationships between the stimulation of 5-HT1A receptors and adrenal secretions.

The hypothalamus: cross-roads of endocrine and behavioural regulation in grooming and aggression

Anatomical and functional studies show that the hypothalamus is at the junction of mechanisms involved in the exploratory appraisal phase of behaviour and mechanisms involved in the execution of specific consummatory acts. However, the hypothalamus is also a crucial link in endocrine regulation. In natural settings it has been shown that behavioural challenges produce large and fast increases in circulating hormones such as testosterone, prolactin, corticotropin and corticosterone. The behavioural function and neural mechanisms of such fast neuroendocrine changes are not well understood. We suggest that behaviourally specific hypothalamic mechanisms, at the cross-roads of behavioural and endocrine regulation, play a role in such neuroendocrine changes. Mild stimulation of the hypothalamic aggressive area, produces stress levels of circulating prolactin, corticotropin, and corticosterone. Surprisingly luteinizing hormone does not change. This increase in stress hormones is due to the stimulation itself, and not caused by the stress of fighting. Similar increases in corticosterone are observed during electrical stimulation of the hypothalamic self-grooming area. The corticosterone response during self-grooming-evoking stimulation is negatively correlated with the amount of self-grooming observed, suggesting that circulating corticosterone exerts a negative feedback control on grooming. Earlier literature, and preliminary data form our laboratory, show that circulating corticosterone exerts a fast positive feedback control over brain mechanisms involved in aggressive behaviour. Such findings suggest that the hormonal responses caused by the activity of behaviourally specific areas of the hypothalamus may be part of a regulation mechanism involved in facilitating or inhibiting the very behavioural responses that can be evoked from those areas. We suggest that studying such mechanisms may provide a new approach to behavioural dysfunctions associated with endocrine disorders and stress.

The effect of bile duct manipulation on cardiovascular responsiveness: in vivo studies

Cholestasis depresses cardiovascular function and responsiveness. In a previous study, the 3-day bile duct manipulated (BDM) rat was validated as the appropriate control for studying in vitro vascular neuroeffector mechanisms in cholestasis. The present study reports the findings on the effect of BDM on cardiovascular function and responsiveness in conscious rats. Cardiovascular responsiveness was assessed by measuring the in vivo pressor responses to a 90 degrees head-up vertical tilt, a controlled hemorrhage, and to intravenously infused norepinephrine, tyramine, the indirectly acting sympathomimetic drug, isoproterenol, the nonselective beta-adrenoceptor agonist, angiotensin I, and angiotensin II. The concentrations of catecholamines and plasma renin activity measured in plasma samples obtained from conscious chronically arterial catheterized BDM rats were compared to identical data obtained from rats in which the bile duct was not manipulated. There were no differences in cardiovascular responsiveness to any of these procedures or drug infusions between the two groups of rats. Plasma catecholamine concentrations and renin activities in the BDM rats were not significantly different from control rats. Although no differences in cardiovascular responsiveness between BDM and control rats were observed, these data confirm the choice of the BDM as the control group for experiments assessing the effects of cholestasis on cardiovascular responsiveness.

Differential effects of social stress on central serotonergic activity and emotional reactivity in Lewis and spontaneously hypertensive rats

Social stress by repeated defeat has been shown to be endowed with neuroendocrine and behavioural effects that render this stress model useful to identify adaptive mechanisms. Among these mechanisms, those related to central serotonergic systems (e.g., hippocampal 5-HT1A receptors, cortical 5-HT2A receptors) have been particularly underlined. Nonetheless, how (i) the neuroendocrine and behavioural effects of social stress are affected by the genetic status of the animal, and (ii) this status affects the relationships between central serotonergic systems and adaptive processes has not been studied so far. The present study has thus analysed the effects of repeated defeat (once a day for seven days) by Long-Evans resident rats upon the psychoneuroendocrine profile of Lewis rats and spontaneously hypertensive rats previously characterized for their contrasting social and anxiety-related behaviours. Repeated defeat decreased in a time-dependent manner, body weight growth and food intake in both strains, these decreases being, however, more severe and longer lasting in Lewis rats. This strain-dependent difference could not be accounted for by differences in physical contacts with the resident rats as the number of attacks and their latency throughout the stress period were similar between spontaneously hypertensive and Lewis rats. When exposed to an elevated plus-maze test of anxiety, the unstressed Lewis rats entered less the open arms than their spontaneously hypertensive counterparts, thus confirming that Lewis rats are more anxious than spontaneously hypertensive rats. This difference was amplified by stress as the latter increased anxiety-related behaviours in Lewis rats only. These strain- and stress-related differences were associated with differences in locomotor activity, this being increased in unstressed Lewis compared with spontaneously hypertensive rats; moreover, stress triggered hypolocomotion in the former but not the latter strain. Lastly, in the forced swimming test. Lewis rats spent more time immobile than spontaneously hypertensive rats with stress increasing immobility in a strain-independent manner. Beside the aforementioned metabolic changes, the activity of the hypothalamo-pituitary-adrenal axis was slightly stimulated in a strain-independent manner by the stressor, as assessed by increased corticosterone levels and adrenal weights, and decreased thymus weights. In Lewis, but not in spontaneously hypertensive rats, midbrain serotonin metabolism was increased by stress, a difference associated with an increased Bmax value of cortical [3H]ketanserin binding at 5-HT2A receptors. On the other hand, the Bmax value of hippocampal [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding at 5-HT1A receptors was decreased by stress, this reduction being amplified in spontaneously hypertensive compared with Lewis rats. This study shows that the psychoneuroendocrine responses to social stress may have a genetic origin, and that the use of socially stressed Lewis and spontaneously hypertensive rats may provide an important paradigm to study adaptive processes. However, whether the aforementioned strain-dependent differences in central serotonergic systems (partly or totally) underlie the distinct profiles of emotivity measured in spontaneously hypertensive and Lewis rats, is discussed in the context of the relationships between serotonergic systems and behavioural responses to novel environments.

The temporal dynamics of the stress response

This paper summarises the available evidence that failure of defense mechanisms in (semi)-natural social groups of animals may lead to serious forms of stress pathology. Hence the study of social stress may provide animal models with a high face validity. However, most of the animal models of human stress-disorders have concentrated on the consequences of chronic exposure to stressors. The present paper considers recent data, indicating that a single experience with a major stressor in the form of social defeat may have long-term consequences ranging from hours to days and weeks. It seems that the experience of a major stressor sensitizes the animal to subsequent stressors. The consequences of these long-term temporal dynamics of the stress response to the development of stress-related disorders and stress-vulnerability are discussed.

Homeostatic disturbances and human aggression

A new model on the nature of human aggression is presented. It rests on the assumption that a pre-established organismic homeostatic modification, based on a decrease in neuronal membrane electric threshold, causes neural facilitation. In turn, this influences the cut-off phenomenon, in particular, neuronal network and therefore either inherited schemata representation, or acquired engram linkage programs run inadequately. These programs adjust the response to working loads of the eight normal serial stages in the body's operational regime activity. The effect of facilitation on these programs is: (1) loss of discrimination when approaching involuntary multi-stimuli; (2) the corruption of acquired engram linkage portions used in neural networks; (3) significant reduction of the voluntary degrees of freedom of response, thus narrowing the body's operational regime activity. This results in damage to certain cognitive links from some acquired engram linkages, enhancing impulse-like program mismatches and causing a unilateral 'fight' response of an aggressive nature.

Adrenaline release by the 5-HT1A receptor agonist 8-OH-DPAT is partly responsible for pituitary activation

In male Wistar rats the effect of adrenalectomy on pituitary activation by the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), was studied. Rats were injected intravenously with 8-OH-DPAT (0.10 mg/kg) in their home cages. Blood samples were withdrawn from freely moving cannulated rats for determination of plasma adrenaline and plasma adrenocorticotropin hormone (ACTH). Adrenalectomized rats showed almost no measurable amounts of plasma adrenaline, but these animals had elevated baseline plasma (ACTH levels as compared to sham-operated rats. 8-OH-DPAT treatment led to a large plasma adrenaline response in the sham-operated animals, which was abolished after adrenalectomy. The plasma ACTH response to 8-OH-DPAT was significantly diminished in the adrenalectomized rats as compared to sham animals. This blunted ACTH response in adrenalectomized rats, however, was still considerable in magnitude. The present data thus indicate that the plasma ACTH response to 8-OH-DPAT is due to at least two different mechanisms. First, via 5-HT1A receptor-mediated adrenaline release, which may consequently stimulate the pituitary. Second, a direct action of 8-OH-DPAT on hypothalamic 5HT1A receptors is assumed, independent of peripheral adrenaline release.