[18F]MPPF and [18F]FDG μPET imaging in rats: impact of transport and restraint stress

Background

Stress exposure can significantly affect serotonergic signaling with a particular impact on 5-HT_1A receptor expression. Positron emission tomography (PET) provides opportunities for molecular imaging of alterations in 5-HT_1A receptor binding following stress exposure. Considering the possible role of 5-HT_1A receptors in stress coping mechanisms, respective imaging approaches are of particular interest.

Material and methods

For twelve consecutive days, Sprague Dawley rats were exposed to daily transport with a 1 h stay in a laboratory or daily transport plus 1 h restraint in a narrow tube. Following, animals were subjected to μPET imaging with 2′-methoxyphenyl-(N-2′-pyridinyl)-p-[¹⁸F]fluoro-benzamidoethylpiperazine ([¹⁸F]MPPF) and 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). Behavioral and biochemical parameters were analyzed to obtain additional information.

Results

In rats with repeated transport, hippocampal [¹⁸F]MPPF binding exceeded that in the naive group, while no difference in [¹⁸F]FDG uptake was detected between the groups. A transient decline in body weight was observed in rats with transport or combined transport and restraint. Thereby, body weight development correlated with [¹⁸F]MPPF binding.

Conclusions

Mild-to-moderate stress associated with daily transport and exposure to a laboratory environment can trigger significant alterations in hippocampal binding of the 5-HT_1A receptor ligand [¹⁸F]MPPF. This finding indicates that utmost care is necessary to control and report transport and associated handling procedures for animals used in μPET studies analyzing the serotonergic system in order to enhance the robustness of conclusions and allow replicability of findings. In view of earlier studies indicating that an increase in hippocampal 5-HT_1A receptor expression may be associated with a resilience to stress, it would be of interest to further evaluate 5-HT_1A receptor imaging approaches as a candidate biomarker for the vulnerability to stress.

Exploring the impact of trauma type and extent of exposure on posttraumatic alterations in 5-HT1A expression

The long-term behavioral, psychological, and neurobiological effects of exposure to potentially traumatic events vary within the human population. Studies conducted on trauma-exposed human subjects suggest that differences in trauma type and extent of exposure combine to affect development, maintenance, and treatment of a variety of psychiatric syndromes. The serotonin 1-A receptor (5-HT1A) is an inhibitory G protein-coupled serotonin receptor encoded by the HTR1A gene that plays a role in regulating serotonin release, physiological stress responding, and emotional behavior. Studies from the preclinical and human literature suggest that dysfunctional expression of 5-HT1A is associated with a multitude of psychiatric symptoms commonly seen in trauma-exposed individuals. Here, we synthesize the literature, including numerous preclinical studies, examining differences in alterations in 5-HT1A expression following trauma exposure. Collectively, these findings suggest that the impact of trauma exposure on 5-HT1A expression is dependent, in part, on trauma type and extent of exposure. Furthermore, preclinical and human studies suggest that this observation likely applies to additional molecular targets and may help explain variation in trauma-induced changes in behavior and treatment responsivity. In order to understand the neurobiological impact of trauma, including the impact on 5-HT1A expression, it is crucial to consider both trauma type and extent of exposure.

The serotonin system in the hippocampus CA3 involves in effects of CSDS on social recognition in adult female mandarin voles (Microtus mandarinus)

Chronic social defeat stress (CSDS) exacerbated the development of stress-related psychiatric disorders, and the social recognition dysfunction is the core feature of many psychiatric disorders. However, the effects of CSDS on female social recognition and the underlying neural mechanisms remain unclear. Using highly aggressive adult female mandarin voles (Microtus mandarinus) as animal model, the aim of this work is to investigate the effects of CSDS on social recognition in adult female rodents and the neurobiological mechanisms underlying these effects. Our results indicate the CSDS disrupted the normal social recognition in adult female voles. Meanwhile, defeated voles exhibited increased neural activity in the DG, CA1 and CA3 of the hippocampus. Furthermore, CSDS reduced levels of serotonin (5-HT) and serotonin 1A receptors (5-HT_1AR) in the CA3. We also discovered that microinjection of 8-OH-DPAT into the CA3 effectively reversed the social recognition deficits induced by CSDS, and an infusion of WAY-100635 into the CA3 of control female voles impaired social recognition. Moreover, targeted activation of the 5-HT neuron projection from the DRN to CA3 by long-term administration of CNO significantly prevented the CSDS induced social recognition deficits. Taken together, our study demonstrated that CSDS induced social recognition deficits in adult female voles, and these effects were mediated by the action of 5-HT on the 5-HT_1AR in the hippocampus CA3. The projection from the DRN to CA3 may be involved in social recognition deficits induced by CSDS.

Adrenergic and noradrenergic regulation of poultry behavior and production

Norepinephrine and epinephrine (noradrenaline and adrenaline) are integral in maintaining behavioral and physiological homeostasis during both aversive and rewarding events. They regulate the response to stressful stimuli through direct activation of adrenergic receptors in the central and sympathetic nervous systems, hormonal activity and through the interaction of the brain, gut, and microbiome. The multiple functions of these catecholamines work synergistically to prepare an individual for a "fight or flight" response. However, hyper-reactivity of this system can lead to increased fearfulness and aggression, decreased health and productivity, and a reduction in overall well-being. Behaviors, such as aggression and certain fear-related behaviors, are a serious problem in the poultry industry that can lead to injury and cannibalism. For decades, catecholamines have been used as a measure of stress in animals. However, few studies have specifically targeted the adrenergic systems as means to reduce behaviors that are damaging or maladapted to their rearing environments and improve animal well-being. This article attempts to address our current understanding of specific, adrenergic-regulated behaviors that impact chicken well-being and production.

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.

Fighting experience alters brain androgen receptor expression dependent on testosterone status

Contest decisions are influenced by the outcomes of recent fights (winner-loser effects). Steroid hormones and serotonin are closely associated with aggression and therefore probably also play important roles in mediating winner-loser effects. In mangrove rivulus fish, Kryptolebias marmoratus, individuals with higher testosterone (T), 11-ketotestosterone and cortisol levels are more capable of winning, but titres of these hormones do not directly mediate winner-loser effects. In this study, we investigated the effects of winning/losing experiences on brain expression levels of the receptor genes for androgen (AR), oestrogen α/β (ERα/β), glucocorticoid (GR) and serotonin (5-HT1AR). The effect of contest experience on AR gene expression depended on T levels: repeated losses decreased, whereas repeated wins increased AR gene expression in individuals with low T but not in individuals with medium or high T levels. These results lend strong support for AR being involved in mediating winner-loser effects, which, in previous studies, were more detectable in individuals with lower T. Furthermore, the expression levels of ERα/β, 5-HT1AR and GR genes were higher in individuals that initiated contests against larger opponents than in those that did not. Overall, contest experience, underlying endocrine state and hormone and serotonin receptor expression patterns interacted to modulate contest decisions jointly.

Neural stem cells respond to stress hormones: distinguishing beneficial from detrimental stress

Neural stem cells (NSCs), the progenitors of the nervous system, control distinct, position-specific functions and are critically involved in the maintenance of homeostasis in the brain. The responses of these cells to various stressful stimuli are shaped by genetic, epigenetic, and environmental factors via mechanisms that are age and developmental stage-dependent and still remain, to a great extent, elusive. Increasing evidence advocates for the beneficial impact of the stress response in various settings, complementing the extensive number of studies on the detrimental effects of stress, particularly in the developing brain. In this review, we discuss suggested mechanisms mediating both the beneficial and detrimental effects of stressors on NSC activity across the lifespan. We focus on the specific effects of secreted factors and we propose NSCs as a “sensor,” capable of distinguishing among the different stressors and adapting its functions accordingly. All the above suggest the intriguing hypothesis that NSCs are an important part of the adaptive response to stressors via direct and indirect, specific mechanisms.

Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?

Adult neurogenesis, the birth of new neurons in the mature brain, has attracted considerable attention in the last decade. One of the earliest identified and most profound factors that affect adult neurogenesis both positively and negatively is stress. Here, we review the complex interplay between stress and adult neurogenesis. In particular, we review the role of the glucocorticoid receptor, the main mediator of the stress response in the proliferation, differentiation, migration, and functional integration of newborn neurons in the hippocampus. We review a multitude of mechanisms regulating glucocorticoid receptor activity in relationship to adult neurogenesis. We postulate a novel concept in which the level of glucocorticoid receptor expression directly regulates the excitation-inhibition balance, which is key for proper neurogenesis. We furthermore argue that an excitation-inhibition dis-balance may underlie aberrant functional integration of newborn neurons that is associated with psychiatric and paroxysmal brain disorders.

Sex differences in anxiety and depression: role of testosterone

Compelling evidence exists for pervasive sex differences in pathological conditions, including anxiety and depressive disorders, with females more than twice as likely to be afflicted. Gonadal hormones may be a major factor in this disparity, given that women are more likely to experience mood disturbances during times of hormonal flux, and testosterone may have protective benefits against anxiety and depression. In this review we focus on the effects of testosterone in males and females, revealed in both human and animal studies. We also present possible neurobiological mechanisms underlying testosterone's mostly protective benefits, including the brain regions, neural circuits, and cellular and molecular pathways involved. While the precise underlying mechanisms remain unclear, both activational and organizational effects of testosterone appear to contribute to these effects. Future clinical studies are necessary in order to better understand when and how testosterone therapy may be effective in both sexes.

Urine scent marking (USM): a novel test for depressive-like behavior and a predictor of stress resiliency in mice

Decreased interest in pleasurable stimuli including social withdrawal and reduced libido are some of the key symptomatic criteria for major depression, and thus assays that measure social and sexual behavior in rodents may be highly appropriate for modeling depressive states. Here we present a novel approach for validating rodent models of depression by assessing male urine scent marking (USM) made in consequence to a spot of urine from a proestrous female. USM is an ethologically important form of sexual communication expressed by males to attract females. The expression of this behavior is highly sensitive and adaptive to environmental cues and social status. We hypothesized that male USM behavior offers a naturalistic measure of social motivation that can be used to evaluate hedonic behaviors relevant to the study of mood disorders. We demonstrated that 1) adult male mice displayed a strong preference for marking proestrous female urine with a high degree of specificity, 2) exposure to chronic social defeat profoundly decreased USM whereas exposure to environmental enrichment increased USM, 3) the standard antidepressant fluoxetine reversed declines in USM induced by social defeat, 4) USM behavior closely correlated with other hedonic measures, and 5) USM scores in non-stressed mice predicted behavioral outcomes after defeat exposure such that mice displaying high preference for marking female urine prior to social defeat showed behavioral resiliency after social defeat. The findings indicate that the USM test is a sensitive, validated measure of psychosocial stress effects that has high predictive value for examination of stress resiliency and vulnerability and their neurobiological substrates.

Cellular consequences of stress and depression

Stress is known to activate distinct neuronal circuits in the brain and induce multiple changes on the cellular level, including alterations in neuronal structures. On the basis of clinical observations that stress often precipitates a depressive disease, chronic psychosocial stress serves as an experimental model to evaluate the cellular and molecular alterations associated with the consequences of major depression. Antidepressants are presently believed to exert their primary biochemical effects by readjusting aberrant intrasynaptic concentrations of neurotransmitters, such as serotonin or noradrenaline, suggesting that imbalances viihin the monoaminergic systems contribute to the disorder (monoaminergic hypothesis of depression). Here, we reviev the results that comprise our understanding of stressful experience on cellular processes, with particular focus on the monoaminergic systems and structural changes within brain target areas of monoaminergic neurons.

Strain-dependent changes in acoustic startle response and its plasticity across adolescence in mice

Acoustic startle response and its plasticity, e.g., habituation and prepulse inhibition (PPI), have been extensively investigated, being altered in several neuropsychiatric disorders. Yet, little is known about the expression of startle-related behaviors during adolescence, a critical phase in the development of a variety of major neuropsychiatric pathologies. The present study investigated for the first time startle behaviors across adolescence in male mice of the inbred strains C57BL/6J and DBA/2J. Pre-pubertal (4 weeks of age) mice displayed reduced startle reactivity and altered PPI compared to adult animals (8 weeks of age), but these effects were observed only in the C57BL/6J strain. Strain differences were also clearly detected for startle response, habituation, and PPI. All effects were modulated by the intensity of the pulse stimulus and were not confounded by differences in anxiety levels. Our data demonstrate that genetic factors and the early adolescent phase are critically important considerations in the design of mouse models of neuropsychiatric disturbances.

Effects of citalopram on serotonin and CRF systems in the midbrain of primates with differences in stress sensitivity

This chapter reviews the neurobiological effects of stress sensitivity and s-citalpram (CIT) treatment observed in our nonhuman primate model of functional hypothalamic amenorrhea (FHA). This type of infertility, also known as stress-induced amenorrhea, is exhibited by cynomolgus macaques. In small populations, some individuals are stress-sensitive (SS) and others are highly stress-resilient (HSR). The SS macaques have suboptimal secretion of estrogen and progesterone during normal menstrual cycles. SS monkeys also have decreased serotonin gene expression and increased CRF expression compared to HSR monkeys. Recently, we found that CIT treatment improved ovarian steroid secretion in SS monkeys, but had no effect in HSR monkeys. Examination of the serotonin system revealed that SS monkeys had significantly lower Fev (fifth Ewing variant, rodent Pet1), TPH2 (tryptophan hydroxylase 2), 5HT1A autoreceptor and SERT (serotonin reuptake transporter) expression in the dorsal raphe than SR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. In contrast, SS monkeys tended to have a higher density of CRF fiber innervation of the dorsal raphe than HSR monkeys, and CIT significantly decreased the CRF fiber density in SS animals. In addition, CIT increased CRF-R2 gene expression in the dorsal raphe. We speculate that in a 15-week time frame, the therapeutic effect of S-citalopram may be achieved through a mechanism involving extracellular serotonin inhibition of CRF and stimulation of CRF-R2, rather than alteration of serotonin-related gene expression.

Testosterone decrease does not play a major role in the suppression of hippocampal cell proliferation following social defeat stress in rats

Stress of social defeat in rodents is known to have a strong and long-lasting effect on brain, physiology and behavior, which bears similarities with certain human stress related psychopathologies. Previous experiments in this lab showed that social defeat stress suppresses testosterone secretion and causes a lasting desensitization of the serotonergic 5-HT(1A) receptors. Testosterone supplementation in socially stressed tree shrews prevented a decrease in hippocampal 5-HT(1A) receptor binding. These receptors are hypothesized to play an important role in neurogenesis in this brain structure. We designed the present experiment to test if social defeat reduces hippocampal cell proliferation and neurogenesis in rats and if testosterone supplementation can prevent this reduction. The results indicate that repeated social defeat stress on 5 successive days induces a significant drop in plasma testosterone levels in male rats and suppresses hippocampal cell proliferation 24h and 3weeks after the end of the stress period. Testosterone supplementation prevented the social stress induced drop in plasma testosterone levels. The hormone supplementation also reduced the negative effect of stress on hippocampal BrdU labeling at 3weeks post-defeat. This effect was, however, rather weak and was caused by the tendency of the hormone in itself to suppress proliferation and the failure to fully recover the proliferation rate. Survival of dentate gyrus cells that either proliferated prior to the stress period or 24h after the last defeat was not affected by the social defeats. Thus the stress-induced lowering of hippocampal cell proliferation is not likely to be caused by transient inhibition of testosterone secretion during social stress.

Chronic social defeat downregulates the 5-HT1A receptor but not Freud-1 or NUDR in the rat prefrontal cortex

The serotonin 1A receptor (5-HT1A) and its associated transcriptional regulators, five prime repressor element under dual repression (Freud-1) and nuclear-deformed epidermal autoregulatory factor (NUDR/Deaf-1) have been previously found to be the repressors for 5-HT1A in the serotonergic raphe neurons, and are also altered in postmortem brains of individuals with major depressive disorder (MDD) and in rats exposed to chronic restraint stress. We sought to find out if rats exposed to chronic social defeat (CSD) stress also show altered expression of these genes. Adult male Wistar rats were exposed to CSD stress for four consecutive weeks following which they were sacrificed and gene expression assessed in the prefrontal cortex (PFC) by quantitative real-time polymerase chain reaction. While CSD had no significant effects on NUDR and Freud-1 mRNA levels, 5-HT1A mRNA levels were significantly downregulated in defeated animals. The data suggest that regulatory factors other than Freud-1 and NUDR may be involved in the regulation of 5-HT1A expression in PFC during CSD stress. Furthermore, decreased levels of 5-HT1A following social defeat in the PFC are consistent with human postmortem results for this receptor in major depression and demonstrate the possibility that this receptor is involved in the pathophysiology of depression and other stress related disorders.

Prenatal alcohol exposure alters biobehavioral reactivity to pain in newborns

OBJECTIVES

To examine biobehavioral responses to an acute pain event in a Cape Town, South Africa, cohort consisting of 28 Cape Colored (mixed ancestry) newborns (n = 14) heavily exposed to alcohol during pregnancy (exposed), and born to abstainers (n = 14) or light (< or = 0.5 oz absolute alcohol/d) drinkers (controls).

METHODS

Mothers were recruited during the third trimester of pregnancy. Newborn data were collected on postpartum day 3 in the maternity obstetrical unit where the infant had been delivered. Heavy prenatal alcohol exposure was defined as maternal consumption of at least 14 drinks/wk or at least 1 incident of binge drinking/mo. Acute stress-related biobehavioral markers [salivary cortisol, heart rate (HR), respiratory sinus arrhythmia (RSA), spectral measures of heart rate variability (HRV), and videotaped facial actions] were collected thrice during a heel lance blood collection (baseline, lance, and recovery). After a feeding and nap, newborns were administered an abbreviated Brazelton Neonatal Behavioral Assessment Scale.

RESULTS

There were no between-group differences in maternal age, marital status, parity, gravidity, depression, anxiety, pregnancy smoking, maternal education, or infant gestational age at birth (all ps > 0.15). In both groups, HR increased with the heel lance and decreased during the postlance period. The alcohol-exposed group had lower mean HR than controls throughout, and showed no change in RSA over time. Cortisol levels showed no change over time in controls but decreased over time in exposed infants. Although facial action analyses revealed no group differences in response to the heel lance, behavioral responses assessed on the Brazelton Neonatal Scale showed less arousal in the exposed group.

CONCLUSIONS

Both cardiac autonomic and hypothalamic-pituitary-adrenal stress reactivity measures suggest a blunted response to an acute noxious event in alcohol-exposed newborns. This is supported by results on the Brazelton Neonatal Scale indicating reduced behavioral arousal in the exposed group. To our knowledge, these data provide the first biobehavioral examination of early pain reactivity in alcohol-exposed newborns and have important implications for understanding neuro-/biobehavioral effects of prenatal alcohol exposure in the newborn period.

Stress sensitive female macaques have decreased fifth Ewing variant (Fev) and serotonin-related gene expression that is not reversed by citalopram

Female cynomolgus monkeys exhibit different degrees of reproductive dysfunction with moderate metabolic and psychosocial stress. When stressed with a paradigm of relocation and diet for 60 days or two menstrual cycles, highly stress resilient monkeys (HSR) continued to ovulate during the stress cycles whereas stress sensitive monkeys (SS) did not. After cessation of stress, monkeys characterized as HSR or SS were administered placebo (PL) or S-citalopram (CIT) for 15 weeks at doses that normalized ovarian steroid secretion in the SS animals and that maintained blood CIT levels in a therapeutic range. After euthanasia, the brain was perfused with 4% paraformaldehyde. The pontine midbrain was blocked and sectioned at 25 microm. The expression of four genes pivotal to serotonin neural function was assessed in the four groups of monkeys (n=4/group). Fev (fifth Ewing variant) ETS transcription factor, tryptophan hydroxylase 2 (TPH2), the serotonin reuptake transporter (SERT), and the 5HT1A autoreceptor were determined at 7-8 levels of the dorsal raphe nucleus with in situ hybridization (ISH) using radiolabeled- and digoxygenin-incorporated riboprobes. Positive pixel area and cell number were measured with Slidebook 4.2 in the digoxigenin assay for Fev. Optical density (OD) and positive pixel area were measured with NIH Image software in the radiolabeled assays for TPH2, SERT and 5HT1A. All data were analyzed with two-way ANOVA. SS monkeys had significantly fewer Fev-positive cells and lower Fev-positive pixel area in the dorsal raphe than HSR monkeys. SS monkeys also had significantly lower levels of TPH2, SERT and 5HT1A mRNAs in the dorsal raphe nucleus than HSR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. These data suggest that SS monkeys have fewer serotonin (5-HT) neurons than HSR monkeys, and that they have deficient Fev expression, which in turn, leads to deficient TPH2, SERT and 5HT1A expression. In addition, the therapeutic effect of CIT is probably achieved through mechanisms other than alteration of 5-HT-related gene expression.

Imaging phenotypes of major depressive disorder: genetic correlates

Imaging techniques are a potentially powerful method of identifying phenotypes that are associated with, or are indicative of, a vulnerability to developing major depressive disorder (MDD). Here we identify seven promising MDD-associated traits identified by magnetic resonance imaging (MRI) or positron emission tomography (PET). We evaluate whether these traits are state-independent, heritable endophenotypes, or state-dependent phenotypes that may be useful markers of treatment efficacy. In MDD, increased activity of the amygdala in response to negative stimuli appears to be a mood-congruent phenomenon, and is likely moderated by the 5-HT transporter gene (SLC6A4) promoter polymorphism (5-HTTLPR). Hippocampal volume loss is characteristic of elderly or chronically-ill samples and may be impacted by the val66met brain-derived neurotrophic factor (BDNF) gene variant and the 5-HTTLPR SLC6A4 polymorphism. White matter pathology is salient in elderly MDD cohorts but is associated with cerebrovascular disease, and is unlikely to be a useful marker of a latent MDD diathesis. Increased blood flow or metabolism of the subgenual anterior cingulate cortex (sgACC), together with gray matter volume loss in this region, is a well-replicated finding in MDD. An attenuation of the usual pattern of fronto-limbic connectivity, particularly a decreased temporal correlation in amygdala-anterior cingulate cortex (ACC) activity, is another MDD-associated trait. Concerning neuroreceptor PET imaging, decreased 5-HT(1A) binding potential in the raphe, medial temporal lobe, and medial prefrontal cortex (mPFC) has been strongly associated with MDD, and may be impacted by a functional single nucleotide polymorphism in the promoter region of the 5-HT(1A) gene (HTR1A: -1019 C/G; rs6295). Potentially indicative of inter-study variation in MDD etiology or mood state, both increased and decreased binding potential of the 5-HT transporter has been reported. Challenges facing the field include the problem of phenotypic and etiological heterogeneity, technological limitations, the confounding effects of medication, and non-disease related inter-individual variation in brain morphology and function. Further advances are likely as epigenetic, copy-number variant, gene-gene interaction, and genome-wide association (GWA) approaches are brought to bear on imaging data.

5-HT(1A) receptor function in major depressive disorder

Dysfunction of the serotonin 1A receptor (5-HT(1A)) may play a role in the genesis of major depressive disorder (MDD). Here we review the pharmacological, post-mortem, positron emission tomography (PET), and genetic evidence in support of this statement. We also touch briefly on two MDD-associated phenotypes, cognitive impairment and somatic pain. The results of pharmacological challenge studies with 5-HT(1A) receptor agonists are indicative of blunted endocrine responses in depressed patients. Lithium, valproate, selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants (TCAs), and other treatment, such as electroconvulsive shock therapy (ECT), all increase post-synaptic 5-HT(1A) receptor signaling through either direct or indirect effects. Reduced somatodendritic and postsynaptic 5-HT(1A) receptor numbers or affinity have been reported in some post-mortem studies of suicide victims, a result consistent with well-replicated PET analyses demonstrating reduced 5-HT(1A) receptor binding potential in diverse regions such as the dorsal raphe, medial prefrontal cortex (mPFC), amygdala and hippocampus. 5-HT(1A) receptor knockout (KO) mice display increased anxiety-related behavior, which, unlike in their wild-type counterparts, cannot be rescued with antidepressant drug (AD) treatment. In humans, the G allele of a single nucleotide polymorphism (SNP) in the 5-HT(1A) receptor gene (HTR1A; rs6295), which abrogates a transcription factor binding site for deformed epidermal autoregulatory factor-1 (Deaf-1) and Hes5, has been reported to be over-represented in MDD cases. Conversely, the C allele has been associated with better response to AD drugs. We raise the possibility that 5-HT(1A) receptor dysfunction represents one potential mechanism underpinning MDD and other stress-related disorders.

Synergistic effects of dehydroepiandrosterone and fluoxetine on proliferation of progenitor cells in the dentate gyrus of the adult male rat

The 5-HT re-uptake inhibitor (SSRI) fluoxetine and the adrenal hormone dehydroepiandrosterone (DHEA) both increase the proliferation of progenitor cells in the adult hippocampus and also have antidepressant activity. This paper explores the combined ability of fluoxetine and DHEA to affect this process in the dentate gyrus of adult rats. We show that DHEA can render an otherwise ineffective dose of fluoxetine (2.5 mg/kg) able to increase progenitor cell proliferation to the same extent as doses four times higher (10 mg/kg). This synergistic action does not appear to be mediated by alterations in brain-derived neurotrophic factor (BDNF) gene expression; or by TrkB, mineralocorticoid, glucocorticoid, or 5-HT (5HT1A) receptor expression in the dentate gyrus; or by altered levels of plasma corticosterone. In a second experiment, the synergism between DHEA and fluoxetine was replicated. Furthermore, flattening the diurnal rhythm of plasma corticosterone by implanting additional corticosterone pellets s.c. prevented the effect of fluoxetine on progenitor cell division. This was not overcome by simultaneous treatment with DHEA, despite the latter's reported anti-glucocorticoid actions. The cellular mechanism for the potentiating action of DHEA on the pro- proliferative effects of fluoxetine in the adult hippocampus remains to be revealed. Since altered neurogenesis has been linked to the onset or recovery from depression, one consequence of these results is to suggest DHEA as a useful adjunct therapy for depression.

Activation of 5-HT1A autoreceptors in the dorsal raphe nucleus reduces the behavioral consequences of social defeat

In animal models, serotonin (5-HT) activity contributes to stress-induced changes in behavior. Syrian hamsters (Mesocricetus auratus) exhibit a stress-induced change in behavior in which social defeat results in increased submissive and defensive behavior and a complete loss of normal territorial aggression directed toward a novel, non-aggressive opponent. We refer to this defeat-induced change in agonistic behavior as conditioned defeat. In this study we tested the hypothesis that 5-HT activity in the dorsal raphe nucleus (DRN) contributes to the acquisition and expression of conditioned defeat. We investigated whether injection of the selective 5-HT1A agonist flesinoxan (200 ng, 400 ng, or 800 ng in 200 nl saline) into the DRN would reduce the acquisition and expression of conditioned defeat. Additionally, we investigated whether injection of the selective 5-HT1A antagonist WAY 100635 (400 ng in 200 nl saline) into the DRN would enhance the acquisition and expression of conditioned defeat following a sub-optimal social defeat experience. We found that injection of flesinoxan into the DRN before exposure to a 15-min social defeat reduced the amount of submissive and defensive behavior shown at testing. We also found that injection of flesinoxan into the DRN before testing similarly reduced submissive and defensive behavior. In addition, we found that WAY 100635 enhanced conditioned defeat when injected either before social defeat or before testing. These data support the hypothesis that the activity of 5-HT cells in the DRN, as regulated by 5-HT1A autoreceptors, contributes to the formation and display of conditioned defeat. Further, our results suggest that 5-HT release in DRN projection regions augments defeat-induced changes in social behavior.

Effects of prenatal ethanol exposure on regulation of basal hypothalamic-pituitary-adrenal activity and hippocampal 5-HT1A receptor mRNA levels in female rats across the estrous cycle

Prenatal ethanol exposure, like other early adverse experiences, is known to alter hypothalamic-pituitary-adrenal (HPA) activity in adulthood. The present study examined the modulatory effects of the gonadal hormones on basal HPA regulation and serotonin Type 1A receptor (5-HT(1A)) mRNA levels in adult female rats prenatally exposed to ethanol (E) compared to that in females from pair-fed (PF) and ad libitum-fed control (C) conditions. We demonstrate, for the first time, long-lasting consequences of prenatal ethanol exposure for basal corticosterone (CORT) regulation and basal levels of hippocampal mineralocorticoid (MR), glucocorticoid (GR) and serotonin Type 1A (5-HT(1A)) receptor mRNA, as a function of estrous cycle stage: (1) basal CORT levels were higher in E compared to C females in proestrus but lower in E and PF compared to C females in estrus; (2) there were no differences among groups in basal levels of adrenocorticotropin (ACTH), estradiol or progesterone; (3) hippocampal MR mRNA levels were decreased in E compared to PF and C females across the estrus cycle, with the greatest effects in proestrus, whereas E (but not PF or C) females had higher hippocampal GR mRNA levels in proestrus than in estrous and diestrus; (4) 5-HT(1A) mRNA levels were increased in E compared to PF and C females in diestrus. That alterations were revealed as a function of estrous cycle stage suggests a role for the ovarian steroids in mediating the adverse effects of ethanol. Furthermore, it appears that ethanol-induced nutritional effects may play a role in mediating at least some of the effects observed. The resetting of HPA activity by early environmental events could be one mechanism linking early life experiences with long-term health consequences. Thus, changes in basal CORT levels, a shift in the MR/GR balance and alterations in 5-HT(1A) receptor mRNA could have important clinical implications for understanding the secondary disabilities, such as an increased incidence of depression, in children with FASD.

Serotonergic mediation of aggression in high and low aggressive chicken strains

Serotonin (5-HT) regulates aggressive behavior via binding to its receptors, such as 5-HT1A and 1B, in humans and rodents. Here we investigate the heritable components of 5-HT regulation of aggressiveness in chickens, utilizing 3 distinct genetic strains. In this study, we used 2 divergently selected strains (high and low group productivity and survivability, respectively; HGPS and LGPS) and a third strain, Dekalb XL (DXL), an aggressive out-group. Hens were paired within the same strain. At 24 wk of age, the subordinate of each pair received a daily i.p. injection of NAN-190 (0.5 mg/kg, a 5-HT1A antagonist), GR-127935 (0.5 mg/kg, a 5-HT1B antagonist), or saline (control) for 5 consecutive days. Frequency of aggressive behaviors was increased in the hens of DXL and LGPS treated with 5-HT1A antagonist and in the HGPS hens treated with 5-HT1B antagonist. The 5-HT1B antagonist-treated HGPS hens and 5-HT1A antagonist-treated LGPS hens also displayed increased feather pecking, but neither antagonist had an effect on feather pecking of DXL hens. This may suggest that multiple mediating factors alter feather pecking behaviors. Among the controls, LGPS hens have higher epinephrine levels than HGPS or DXL hens, indicative of the inferior stress-coping ability of LGPS hens. Treatment with 5-HT1B antagonist reduced epinephrine in LGPS hens but not in DXL or HGPS hens, suggesting a role of 5-HT1B in stress regulation in LGPS hens. The results provide evidence for different heritable serotonergic mediation of aggressive behaviors and stress coping in chickens.

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

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

Interaction between BDNF and serotonin: role in mood disorders

Brain-derived neurotrophic factor (BDNF) and serotonin (5-hydroxytryptamine, 5-HT) are two seemingly distinct signaling systems that play regulatory roles in many neuronal functions including survival, neurogenesis, and synaptic plasticity. A common feature of the two systems is their ability to regulate the development and plasticity of neural circuits involved in mood disorders such as depression and anxiety. BDNF promotes the survival and differentiation of 5-HT neurons. Conversely, administration of antidepressant selective serotonin reuptake inhibitors (SSRIs) enhances BDNF gene expression. There is also evidence for synergism between the two systems in affective behaviors and genetic epitasis between BDNF and the serotonin transporter genes.

Major depression as a disorder of serotonin resistance: inference from diabetes mellitus type II

The multifactorial nature of depression resembles that of other complex disorders such as diabetes mellitus or coronary artery disease. However, while for the latter disorders predisposing and risk factors have been identified, such knowledge is still scarce in depression. In this review we propose to use diabetes mellitus, for which characteristic milestones have been condensed to obesity-hyperinsulinaemia-insulin resistance-diabetes mellitus, as a conceptual analogical model. Based on this model we hypothesize that depression develops according to a similar pattern: prolonged psychological stress-hyperserotonism-serotonin resistance-major depression. We review extensive supporting evidence from human studies and animal models of depression, including stress involvement in the aetiology of depression, evidence for increased synaptic serotonin and decreased 5-HT1A receptor activity. Conceptualizing the pathogenesis of depression as a multi-step process may inspire new concepts, which will eventually lead to delineation of additional preventive and therapeutic interventions similar to those currently practised in diabetes.

Serotonin, 5-HT1A serotonin receptors and proliferation of lymphocytes in major depression patients

Serotonin receptors are present in lymphocytes and might be related to the functionality of these cells in health and in pathology. The serotonergic system is affected in the brain and in peripheral immune cells of depressed patients. The objectives of this work were to evaluate the basal proliferation of lymphocytes, the response to the mitogen concanavalin A, and the role of serotonin 5-HT(1A) receptors. Twenty-nine patients, 19-52 years old, were diagnosed for a major depression episode with the Statistical and Diagnostic Manual-IV of the American Psychiatric Association, approved by ethic committees and gave written consent. The Hamilton depression score was 30.60 +/- 2.65. An apparently healthy group without a family history of psychiatric illness was included. Blood peripheral lymphocytes were isolated by density gradients with Ficoll/Hypaque and differential adhesion to plastic, cultured in 96-well plaques with RPMI-1640 medium with or without 4 mug/ml of concanavalin A. 8-Hydroxy-2-(di-n-propylamino)tetralin (5-40 nM) and WAY-100,478 (0.1-100 microM), agonist and antagonist of 5-HT(1A) receptors, serotonin (12.5-100 nM) or imipramine (0.1-100 microM) were also added. Proliferation was evaluated at 72 h with 3-[4,5-dimethythiazol-2-yl]-2,5-diphenyltetrazolium bromide, and the optical density was 570 nm. Basal proliferation was three times higher in depressed patients than in controls, whereas no response to mitogen was obtained, and 5-HT(1A) receptors significantly reacted to the agonist, with increases of about 31-54% at 10, 20 and 40 nM of the specific agonist, indicating initial activation probably in relation to autoimmunity and overreactivity of these receptors in depression. The antagonist reduced proliferation in mitogen-stimulated lymphocytes, 50% in controls and 70% in depressed patients, with a differential concentration dependency; probably, these receptors are more sensitive in depression due to increased 5-HT(1A) receptor transduction. The antagonist also reduced the stimulation produced by the 5-HT(1A) agonist. Imipramine caused biphasic effects according to concentrations, showing a possible dual role for serotonin, although all values were significantly higher in depressed subjects. The described alterations might be of relevance in the pathophysiology of depression.

Differential effects of acute and chronic social defeat stress on hypothalamic-pituitary-adrenal axis function and hippocampal serotonin release in mice

Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) stress axis and disturbances in serotonin (5-HT) neurotransmission have been implicated in the pathogenesis of depressive disorder. Repeated social defeat of male NMRI mice has been shown to induce increases in core body temperature and corticosterone, indicative of a state of chronic stress in subordinate animals. The present study further characterised the HPA axis response to social defeat stress, and also examined hippocampal extracellular 5-HT release during the stress. Exposure to an acute social defeat elicits increases in plasma adrenocorticotrophic hormone and corticosterone levels, peaking at 15 and 30 min, respectively, and enhances corticotrophin-releasing factor (CRF) mRNA, but not arginine vasopressin (AVP) mRNA within the medial parvocellular division of the hypothalamic paraventricular nucleus. A concomitant increase in hippocampal corticosterone and 5-HT levels is observed. By contrast, although chronic social defeat is associated with greatly elevated corticosterone levels, the predominant drive appears to be via parvocellular AVP rather than CRF. Furthermore, subordinate animals allowed to recover for 9 days after chronic social defeat display an increase in immobility in the forced swimming model of depression, indicating that animals previously exposed to the homotypic defeat stress are sensitised to the behavioural effects of a novel stressor. These results demonstrate that social defeat induces prolonged activation of the HPA axis and alterations in 5-HT neurotransmission that could be of relevance to some of the pathological abnormalities observed in clinical depression.

Chronic psychosocial stress in tree shrews: effect of the substance P (NK1 receptor) antagonist L-760735 and clomipramine on endocrine and behavioral parameters

RATIONALE

Substance P and its preferred receptor, the neurokinin 1 receptor (NK(1)R), have been proposed as possible targets for new antidepressant therapies, although results of a recently completed phase III trial failed to demonstrate that the NK(1)R antagonist MK-869 is more effective than placebo in the treatment of depression.

METHODS

In the present study, we compared the effects of the NK(1)R antagonist L-760735 with the tricyclic antidepressant clomipramine on endocrine and behavioral parameters in chronically stressed tree shrews. Animals were subjected to a 7-day period of psychosocial stress before receiving daily oral administration of L-760735 (10 mg/kg/day) or clomipramine (50 mg/kg/day). The psychosocial stress continued throughout the treatment period of 21 days. Daily morning urine was collected to measure cortisol and norepinephrine levels. All animals were videotaped daily and three types of behavior were analyzed.

RESULTS

Chronic psychosocial stress resulted in a significant increase of urinary cortisol and norepinephrine concentrations. Moreover, stressed animals displayed decreased marking behavior and locomotor activity, while grooming remained unaffected. Neither treatment with clomipramine nor L-760735 was able to normalize the stress-induced elevation of cortisol or norepinephrine. On the behavioral parameters, L-760735 had a time-dependent restorative influence on marking behavior close to normal levels, without affecting locomotor activity. Grooming behavior was significantly increased by the 3 weeks of drug treatment.

CONCLUSIONS

These results suggest that L-760735 was able to counteract certain stress-induced behavioral alterations in an animal model of depression.

Examining SLV-323, a novel NK1 receptor antagonist, in a chronic psychosocial stress model for depression

RATIONALE

Substance P antagonists have been proposed as candidates for a new class of antidepressant compounds.

OBJECTIVES

We examined the effects of SLV-323, a novel neurokinin 1 receptor (NK1R) antagonist, in the chronic psychosocial stress paradigm of adult male tree shrews.

METHODS

Animals were subjected to a 7 day period of psychosocial stress before being treated daily with SLV-323 (20 mg kg(-1) day(-1)). The psychosocial stress continued throughout the treatment period of 28 days. Brain metabolite concentrations were determined in vivo by proton magnetic resonance spectroscopy. Norepinephrine excretion was monitored from daily urine samples, and serum testosterone concentrations were measured at the end of the experiment. All animals were videotaped daily to analyze scent-marking behavior and locomotor activity. Cell proliferation in the dentate gyrus and hippocampal volume were measured postmortem.

RESULTS

Stress significantly decreased cerebral concentrations of N-acetyl-aspartate, total creatine, and choline-containing compounds in vivo and resulted in an increase of urinary norepinephrine and decrease of serum testosterone concentrations. Moreover, stressed animals displayed decreased scent-marking behavior and locomotor activity. The proliferation rate of the granule precursor cells in the dentate gyrus was reduced, and hippocampal volume was mildly decreased. The stress-induced alterations in the central nervous system were partially prevented by concomitant administration of SLV-323, while drug treatment had only a minor effect on the stress-induced behavioral changes.

CONCLUSIONS

The novel NK1R antagonist SLV-323 has certain antidepressant-like effects in a valid animal model of depression.

5-HT1A receptor expression in pyramidal neurons of cortical and limbic brain regions

We studied expression of the 5-HT(1A) receptor in cortical and limbic areas of the brain of the tree shrew. In situ hybridization with a receptor-specific probe and immunocytochemistry with various antibodies was used to identify distinct neurons expressing the receptor. In vitro receptor autoradiography with (3)H-8-OH-DPAT ((3)H-8-hydroxy-2-[di-n-propylamino]tetralin) was performed to visualize receptor-binding sites. In the prefrontal, insular, and occipital cortex, 5-HT(1A) receptor mRNA was expressed in pyramidal neurons of layer 2, whereas (3)H-8-OH-DPAT labeled layers 1 and 2 generating a columnar-like pattern in the prefrontal and occipital cortex. In the striate and ventral occipital cortex, receptor mRNA was present within layers 5 and 6 in pyramidal neurons and Meynert cells. Pyramid-like neurons in the claustrum and anterior olfactory nucleus also expressed the receptor. Principal neurons in hippocampal region CA1 expressed 5-HT(1A) receptor mRNA, and (3)H-8-OH-DPAT labeled both the stratum oriens and stratum radiatum. CA3 pyramidal neurons displayed low 5-HT(1A) receptor expression, whereas granule neurons in the dentate gyrus revealed moderate expression of this receptor. In the amygdala, large pyramid-like neurons in the basal magnocellular nucleus strongly expressed the receptor. Immunocytochemistry with antibodies against parvalbumin, calbindin, and gamma aminobutyric acid (GABA) provided no evidence for 5-HT(1A) receptor expression in GABAergic neurons in cortical and limbic brain areas. Our data agree with previous findings showing that the 5-HT(1A) receptor mediates the modulation of glutamatergic neurons. Expression in the limbic and cortical areas suggested an involvement of 5-HT(1A) receptors in emotional and cognitive processes.

Oestrogen and testosterone modulate the firing activity of dorsal raphe nucleus serotonergic neurones in both male and female rats

Women are twice as likely to suffer from mood disorders than men. Moreover, a growing body of evidence suggests a reciprocal modulation between sex steroids and the serotonin (5-HT) system. A previous study from our laboratory has shown that the progesterone metabolites 5beta-pregnane-3,20-dione (5beta-DHP) and 5alpha-pregnan-3alpha-ol,20-one (3alpha,5alpha-THP), as well as dehydroepiandrosterone (DHEA), increase the firing activity of dorsal raphe nucleus (DRN) 5-HT neurones in female rats. The present study was undertaken to assess the effects of these steroids in male rats, as well as the effects of testosterone and 17beta-oestradiol (17beta-E) in both sexes, and finally to evaluate gender differences in the modulation of the 5-HT neuronal firing activity by these different neuroactive steroids. Male rats were treated i.c.v., for 7 days, with a dose of 50 microg/kg/day of one of the following steroids: progesterone, 5beta-DHP, 3alpha,5alpha-THP, DHEA, testosterone, 17beta-hydroxy-5alpha-androstan-3-one (5alpha-DHT) and 17beta-E. Some rats also received a 3-day administration of testosterone (50 microg/kg/day, i.c.v). Females were treated in the same fashion with testosterone and 17beta-E. Extracellular unitary recordings of 5-HT neurones, obtained in vivo in the DRN of these rats, revealed that testosterone and 17beta-E increased the firing activity of 5-HT neurones in both males and females. In males, the effect of testosterone could already be seen after 3 days of treatment. Neither castration nor any treatment with other steroids significantly modified the firing rate of male 5-HT neurones. Taken together with previous findings, the results of the present study indicate both similarities and differences between sexes in the modulation of 5-HT neurones by some steroids. This could prove important in understanding gender differences in mood disorders.

Social stress in tree shrews as an animal model of depression: an example of a behavioral model of a CNS disorder

Animal models are invaluable in preclinical research on human psychopathology. Valid animal models to study the pathophysiology of depression and specific biological and behavioral responses to antidepressant drug treatments are of prime interest. In order to improve our knowledge of the causal mechanisms of stress-related disorders such as depression, we need animal models that mirror the situation seen in patients. One promising model is the chronic psychosocial stress paradigm in male tree shrews. Coexistence of two males in visual and olfactory contact leads to a stable dominant/subordinate relationship, with the subordinates showing obvious changes in behavioral, neuroendocrine, and central nervous activity that are similar to the signs and symptoms observed during episodes of depression in patients. To discover whether this model, besides its "face validity" for depression, also has "predictive validity," we treated subordinate animals with the tricyclic antidepressant clomipramine and found a time-dependent recovery of both endocrine function and normal behavior. In contrast, the anxiolytic diazepam was ineffective. Chronic psychosocial stress in male tree shrews significantly decreased hippocampal volume and the proliferation rate of the granule precursor cells in the dentate gyrus. These stress-induced changes can be prevented by treating the animals with clomipramine, tianeptine, or the selective neurokinin receptor antagonist L-760,735. In addition to its apparent face and predictive validity, the tree shrew model also has a "molecular validity" due to the degradation routes of psychotropic compounds and gene sequences of receptors are very similar to those in humans. Although further research is required to validate this model fully, it provides an adequate and interesting non-rodent experimental paradigm for preclinical research on depression.

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.

Effects of repeated aggressive encounters on approach to a female and plasma testosterone in male mice

Effects of repeated experience of aggression accompanied by social victories or social defeat in 10 daily agonistic confrontations on testosterone levels in and the behavioral response of CBA/Lac male mice exposed to a receptive female from behind a perforated transparent partition have been examined. Testosterone levels were not changed significantly in the mice that had consistently been victorious over 10 days (winners) or in the mice that had consistently been defeated over 10 days (losers). Losers and controls (mice that had been caged individually for 5 days) responded with increased levels of behavioral activity near the partition and elevated testosterone. Winners showed a significantly poorer behavioral and hormonal response. It is concluded that the repeated display of aggression by male mice led to a reduction in both their behavioral and neuroendocrine responses to an estrous female.

Social stress and corticosterone regionally upregulate limbic N-methyl-d-aspartatereceptor (NR) subunit type NR2A and NR2B in the lizard anolis carolinensis

Social aggression in the lizard Anolis carolinensis produces dominant and subordinate relationships while elevating corticosterone levels and monoaminergic transmitter activity in hippocampus (medial and mediodorsal cortex). Adaptive social behavior for dominant and subordinate male A. carolinensis is learned during aggressive interaction and therefore was hypothesized to involve hippocampus and regulation of N-methyl-d-aspartate (NMDA) receptors. To test the effects of social stress and corticosterone on NMDA receptor subunits (NR), male lizards were either paired or given two injections of corticosterone 1 day apart. Paired males were allowed to form dominant-subordinate relationships and were killed 1 day later. Groups included isolated controls, dominant males, subordinate males and males injected with corticosterone. Brains were processed for glutamate receptor subunit immunohistochemistry and fluorescence was analyzed by image analysis for NR(2A) and NR(2B) in the small and large cell divisions of the medial and mediodorsal cortex. In the small granule cell division there were no significant differences in NR(2A) or NR(2B) immunoreactivity among all groups. In contrast, there was a significant upregulation of NR(2A) and NR(2B) subunits in the large pyramidal cell division in all three experimental groups as compared with controls. The results revealed significantly increased NR(2A) and NR(2B) subunits in behaving animals, whereas animals simply injected with corticosterone showed less of an effect, although they were significantly increased over control. Upregulation of NR(2) subunits occurs during stressful social interactions and is likely to be regulated in part by glucocorticoids. The data also suggest that learning social roles during stressful aggressive interactions may involve NMDA receptor-mediated mechanisms.

Chronic social stress: effects on limbic brain structures

Different types of stressors are known to activate distinct neuronal circuits in the brain. Acute physiological stimuli that are life threatening and require immediate reactions lead to a rapid stimulation of brainstem and hypothalamus to activate efferent visceral pathways. In contrast, psychological stressors activate higher-order brain structures for further interpretations of the perceived endangerment. Common to the later multimodal stressors is that they need cortical processing and, depending on previous experience or ongoing activation, the information is assembled within limbic circuits connecting, e.g., the hippocampus, amygdala and prefrontal cortex to induce neuroendocrine and behavioral responses. In view of the fact that stressful life events often contribute to the etiology of psychopathologies such as depressive episodes, several animal models have been developed to study central nervous mechanisms that are induced by stress. The present review summarizes observations made in the tree shrew chronic psychosocial stress paradigm with particular focus on neurotransmitter systems and structural changes in limbic brain regions.

Mother's voice "buffers" separation-induced receptor changes in the prefrontal cortex of octodon degus

Although the potential vulnerability of the postnatally developing brain toward adverse environmental influences is generally recognized, relatively little is known about the basic mechanisms involved. The plasticity and adaptability of the postnatally developing brain in response to adverse emotional experiences was analyzed in the South American Octodon degus. Our study revealed that repeated brief separation from the parents and exposure to an unfamiliar environment induces an up-regulation of dopamine (D1) and 5-hydroxytrytamine (5HT1(A))-receptor density in the precentral medial, anterior cingulate, prelimbic and infralimbic cortices in female pups. No significant changes of gamma aminobutyric acid (GABA(A)) receptor density were found in deprived animals of both genders. The acoustic presence of the mother during parental separation suppressed the D1-receptor up-regulation as well as the 5-HT1(A)-receptor up-regulation, again only in the female pups. These results demonstrate that that early adverse emotional experience alters aminergic function within the prefrontal cortex in the female but not the male brain. The mother's voice, a powerful emotional signal, can protect the developing cortex from separation-induced receptor changes.

The tree shrews: adjuncts and alternatives to primates as models for biomedical research

The tree shrews are non-rodent, primate-like, small animals. There is increasing interest in using them to establish animal models for medical and biological research. This review focuses on the use of the tree shrews in in vivo studies on viral hepatitis, hepatocellular carcinoma (HCC), myopia, and psychosocial stress. Because of the susceptibility of the tree shrews (Tupaia belangeri) and their hepatocytes to infection with human hepatitis B virus (HBV) in vivo and in vitro, these animals have been used to establish human hepatitis virus-induced hepatitis and human HBV- and aflatoxin B1-associated HCC models. As these animals are phylogenetically close to primates in evolution and have a well-developed visual system and color vision in some species, they have been utilized to establish myopia models. Because dramatic behavioral, physiological, and neuroendocrine changes in subordinate male tree shrews are similar to those observed in depressed human patients, the tree shrews have been successfully employed to experimentally study psychosocial stress. However, the tree shrews holds significant promise as research models and great use could be made of these animals in biomedical research.

Alpha2A and alpha2C-adrenoceptor regulation in the brain: alpha2A changes persist after chronic stress

Stress-induced activation of the central nervous noradrenergic system has been suspected to induce depressive disorders. As episodes of depression often occur some time after a stress experience we investigated whether stress-induced changes in the alpha2-adrenoceptor (alpha2-AR) system persist throughout a post-stress recovery period. Brains of male tree shrews were analysed after 44 days of chronic psychosocial stress and after a subsequent 10-day recovery period. Expression of RNA for alpha2A and alpha2C-adrenoceptors was quantified by in situ hybridization, and receptor binding was determined by in vitro receptor autoradiography. Activities of the sympathetic nervous system and of the hypothalamo-pituitary-adrenal axis were increased during chronic stress but normalized during recovery. Alpha2A-AR RNA in the glutamatergic neurons of the lateral reticular nucleus was elevated significantly after stress and after recovery (by 29% and 17%). In the dorsal motor nucleus of the vagus, subtype A expression was enhanced after recovery (by 33%). In the locus coeruleus, subtype A autoreceptor expression was not changed significantly. Subtype C expression in the caudate nucleus and putamen was elevated by stress (by 5 and 4%, respectively) but normalized during recovery. Quantification of 3H-RX821002 binding revealed receptor upregulation during stress and/or recovery. Our data therefore show: (i) that chronic psychosocial stress differentially regulates expression of alpha2-adrenoceptor subtypes A and C; (ii) that subtype A heteroreceptor expression is persistently upregulated whereas (iii), subtype C upregulation is only transient. The present findings coincide with post mortem studies in depressed patients revealing upregulation of alpha2A-ARs.

Subordination stress alters alternative splicing of the Slo gene in tree shrew adrenals

It was previously hypothesized that stress hormones regulate the alternative splicing of Slo potassium channels, thereby tuning the intrinsic excitability of adrenal chromaffin cells. Male tree shrews subjected to chronic stress by exposure to a dominant male develop robust symptoms with parallels to human depression. We report here that adrenals from males subjected to 4-6 weeks of subordination have a significantly smaller proportion of Slo transcripts with the optional STREX exon (STRess-axis regulated EXon) than unstressed male adrenals. Female adrenals (unstressed) had even lower levels than stressed males. These data suggest both behavioral regulation and sexual dimorphism in ion channel structure. We hypothesize that chromaffin cell excitability and sympathoadrenal function will be altered, and speculate that this may favor passive coping responses in subordinate males and females.