Do stress hormones connect environmental effects with behavior in the forced swim test?

A repeated measures cognitive affective bias test in rats: comparison with forced swim test

RATIONALE

There is an urgent need to identify behaviours in animals that can provide insight into the aetiology and potential treatment of depression in humans.

OBJECTIVES

This study aimed to validate a repeated measures cognitive affective bias (CAB) test in a rat model of chronic stress and compare CAB with forced swim test (FST) measures.

METHODS

Male and female Sprague Dawley rats were trained to associate large and small rewards with scent, spatial, and tactile cues, and their response to an ambiguous tactile stimulus tested. Rats underwent weekly CAB testing for 4 weeks with no intervention, or for 2 weeks of chronic restraint stress (CRS), followed by 2 weeks of fluoxetine, vehicle, or no treatment. CRS rats also underwent the FST at selected timepoints.

RESULTS

In control rats, CAB was positive and remained stable over the 4-week period. In CRS-fluoxetine and CRS-vehicle groups, CAB was initially positive, became negative during chronic restraint stress, and returned to positive by 2 weeks after treatment. However, in the CRS-no treatment group, CAB was variable at the outset and unstable over time. Behaviour in the FST was not affected by treatment, and there was no correlation between CAB and FST outcomes.

CONCLUSIONS

Instability in the CRS-no treatment group precluded interpretation of the impact of fluoxetine on CAB post-CRS. Our results suggest that behaviour in the FST does not reflect or alter affective state and support the use of CAB tests as part of the behavioural testing repertoire for preclinical animal models of affective disorders.

Evidence-based severity assessment of the forced swim test in the rat

The forced swim test (FST) is a traditional assay, which has been used for more than 40 years to assess antidepressant effects of novel drug candidates. In recent years, a debate about the test has focused on the assumption that the FST is highly aversive and burdening for the animals because of the earlier anthropomorphic interpretation and designation as a "behavioral despair test". The Directive 2010/63/EU and the German Animal Welfare law require a prospective severity classification of the planned experimental procedures. Still, an objective examination of the animals' burden in this test has not been performed yet. To fill this gap, we conducted an evidence-based severity assessment of the forced swim test in rats according to a 'standard protocol' with a water temperature of 25°C. We examined parameters representing the physiological and the affective state, and natural as well as locomotion-associated behaviors in three separate experiments to reflect as many dimensions as possible of the animal's condition in the test. Hypothermia was the only effect observed in all animals exposed to the FST when using this standard protocol. Additional adverse effects on body weight, food consumption, and fecal corticosterone metabolite concentrations occurred in response to administration of the antidepressant imipramine, which is frequently used as positive control when testing for antidepressant effects of new substances. We conclude that this version of the FST itself is less severe for the animals than assumed, and we suggest a severity classification of 'moderate' because of the acute and short-lasting effects of hypothermia. To refine the FST according to the 3Rs, we encourage confirming the predictive validity in warmer water temperatures to allow the rats to maintain physiological body temperature.

A systematic mapping review of the evolution of the rat Forced Swim Test: Protocols and outcome parameters

As depression is projected to become the leading mental disease burden globally by 2030, understanding the underlying pathology, as well as screening potential anti-depressants with a higher efficacy, faster onset of action, and/or fewer side-effects is essential. A commonly used test for screening novel antidepressants and studying depression-linked aspects in rodents is the Porsolt Forced Swim Test. The present systematic mappping review gives a comprehensive overview of the evolution and of the most prevalently used set-ups of this test in rats, including the choice of animals (strain, sex, and age), technical aspects of protocol and environment, as well as reported outcome measures. Additionally, we provide an accessible list of all existing publications, to support informed decision-making for procedural and technical aspects of the test, to thereby enhance reproducibility and comparability. This should further contribute to reducing the number of unnecessarily replicated experiments, and consequently, reduce the number of animals used in future.

Pharmacogenetic excitation of the median raphe region affects social and depressive-like behavior and core body temperature in male mice

AIMS

Median raphe region (MRR) is an important bottom-up regulatory center for various behaviors as well as vegetative functions, but detailed descriptions and links between the two are still largely unexplored.

METHODS

Pharmacogenetics was used to study the role of MRR in social (sociability, social interaction, resident intruder test) and emotional behavior (forced swim test) parallel with some vegetative changes (biotelemetry: core body temperature). Additionally, to validate pharmacogenetics, the effect of clozapine-N-oxide (CNO), the ligand of the artificial receptor, was studied by measuring (i) serum and brainstem concentrations of CNO and clozapine; (ii) MRR stimulation induced neurotransmitter release in hippocampus; (iii) CNO induced changes in body temperature and locomotor activity.

KEY FINDINGS

MRR stimulation decreased locomotion, increased friendly social behavior in the resident intruder test and enhanced depressive-like behavior. The latter was accompanied by diminished decrease in core body temperature. Thirty minutes after CNO injection clozapine was predominant in the brainstem. Nonetheless, peripheral CNO injection was able to induce glutamate release in the hippocampus. CNO had no immediate (<30 min) or chronic (repeated injections) effect on the body temperature or locomotion.

SIGNIFICANCE

We confirmed the role of MRR in locomotion, social and depressive-like behavior. Most interestingly, only depressive-like behavior was accompanied by changed body temperature regulation, which was also observed in human depressive disorders previously. This indicates clinical relevance of our findings. Despite low penetration, CNO acts centrally, but does not influence the examined basic parameters, being suitable for repeated behavioral testing.

The forced swim test: Historical, conceptual and methodological considerations and its relationship with individual behavioral traits

The forced swim test (FST), developed by Porsolt and collaborators in 1977 to evaluate antidepressant (AD) treatments in rodents, has become extensively used for this purpose and to evaluate depression-like states. Despite its popularity, studies have raised important concerns regarding its theoretical and predictive validity. In my view and that of others, the FST mainly evaluates coping strategies in an inescapable situation. Although it is reasonable to assume that ADs act favoring active coping whereas negative affective states would favor passive coping, this does not mean that only ADs should enhance active coping or that a depression state has developed, respectively. Given its simplicity, proper interpretation of the FST behavior is critically dependent on how FST behavior relates to other behavioral traits. Unfortunately, this issue has been poorly discussed previously. Then, the present review, using a historical perspective, offers information needed to better understand the meaning and limitations of the FST, discusses critical methodological aspects and analyzes the relationship of FST behavior with classical behavioral traits in rodents.

Behavioral and neuroendocrine consequences of disrupting a long-term monogamous social bond in aging prairie voles

Social support from a spouse, long-term partner, or someone who provides emotional or instrumental support may protect against consequences of aging, including mediating behavioral stress reactivity and altering neurobiological process that underlie short-term stress responses. Therefore, long-term social bonding may have behavioral and neurobiological benefits. The socially monogamous prairie vole provides a valuable experimental model for investigating the benefits of long-term social bonds on short-term stress reactivity in aging animals, given their unique social structure of forming enduring opposite-sex bonds, living in family groups, and bi-parental rearing strategies. Male-female pairs of long-term, cohabitating prairie voles were investigated for short-term behavioral and neuroendocrine stress reactivity following either long-term social pairing (control), or a period of social isolation. In Experiment 1, social isolation was associated with altered behavioral reactivity to an acute swim stressor, and greater neural activation in the hypothalamic paraventricular nucleus, as well as specifically the parvocellular region, following the swim stressor (vs. control). In Experiment 2, social isolation was associated with greater corticosterone reactivity following an acute restraint stressor (vs. control). No sex differences were observed. Exploratory correlation and subgroup analyses revealed systematic relationships among various demographic variables (such as age of the subjects, amount of time the pair cohabitated together, and number of litters the pair reared together) and the behavioral and neuroendocrine outcome measures. These findings may inform our understanding of the benefits of long-term social bonding on modulating short-term behavioral and neuroendocrine responses to stress.LAY SUMMARYReceiving social support from a long-term spouse or partner, or having a strong support network from friends, may have important health benefits as people age. In aging monogamous prairie voles, social isolation from a long-term social partner disrupted behaviors and short-term stress responses, whereas living with a long-term partner protected against these disruptions. This research is important for our understanding of the benefits of social support on stress responses as we age.

Differential effects of imipramine and CORT118335 (Glucocorticoid receptor modulator/mineralocorticoid receptor antagonist) on brain-endocrine stress responses and depression-like behavior in female rats

Depression is commonly associated with hypothalamic-pituitary adrenal (HPA) axis dysfunction that primarily manifests as aberrant glucocorticoid secretion. Glucocorticoids act on Type I mineralocorticoid (MR) and Type II glucocorticoid receptors (GR) to modulate mood and endocrine responses. Successful antidepressant treatment normalizes HPA axis function, in part due to modulatory effects on MR and GR in cortico-limbic structures. Although women are twice as likely to suffer from depression, little is known about how antidepressants modulate brain, endocrine, and behavioral stress responses in females. Here, we assessed the impact of CORT118335 (GR modulator/MR antagonist) and imipramine (tricyclic antidepressant) on neuroendocrine and behavioral responses to restraint or forced swim stress (FST) in female rats (n=10-12/group). Increased immobility CORT118335 in the FST is purported to reflect passive coping or depression-like behavior. CORT118335 dampened adrenocorticotropic hormone (ACTH) and corticosterone responses to the FST, but did not affect immobility. Imipramine suppressed ACTH, but had minimal effects on corticosterone responses to FST. Despite these marginal effects, imipramine decreased immobility, suggesting antidepressant efficacy. In an effort to link brain-endocrine responses with behavior, c-Fos was assessed in HPA axis and mood modulatory regions in response to the FST. CORT118335 upregulated c-Fos expression in the paraventricular nucleus of the hypothalamus. Imipramine decreased c-Fos in the basolateral amygdala and hippocampus (CA1 and CA3), but increased c-Fos in the central amygdala. These data suggest the antidepressant-like (e.g., active coping) properties of imipramine may be due to widespread effects on cortico-limbic circuits that regulate emotional and cognitive processes.

Social defeat stress causes depression-like behavior with metabolite changes in the prefrontal cortex of rats

Major depressive disorder is a serious mental disorder with high morbidity and mortality. The role of social stress in the development of depression remains unclear. Here, we used the social defeat stress paradigm to induce depression-like behavior in rats, then evaluated the behavior of the rats and measured metabolic changes in the prefrontal cortex using gas chromatography-mass spectrometry. Within the first week after the social defeat procedure, the sucrose preference test (SPT), open field test (OFT), elevated plus maze (EPM) and forced swim test (FST) were conducted to examine the depressive-like and anxiety-like behaviors. For our metabolite analysis, multivariate statistics were applied to observe the distribution of all samples and to differentiate the socially defeated group from the control group. Ingenuity pathway analysis was used to find the potential relationships among the differential metabolites. In the OFT and EPM, there were no significant differences between the two experimental groups. In the SPT and FST, socially defeated rats showed less sucrose intake and longer immobility time compared with control rats. Metabolic profiling identified 25 significant variables with good predictability. Ingenuity pathways analysis revealed that “Hereditary Disorder, Neurological Disease, Lipid Metabolism” was the most significantly altered network. Stress-induced alterations of low molecular weight metabolites were observed in the prefrontal cortex of rats. Particularly, lipid metabolism, amino acid metabolism, and energy metabolism were significantly perturbed. The results of this study suggest that repeated social defeat can lead to metabolic changes and depression-like behavior in rats.

Restoration of peripheral V2 receptor vasopressin signaling fails to correct behavioral changes in Brattleboro rats

Beside its hormonal function in salt and water homeostasis, vasopressin released into distinct brain areas plays a crucial role in stress-related behavior resulting in the enhancement of an anxious/depressive-like state. We aimed to investigate whether correction of the peripheral symptoms of congenital absence of AVP also corrects the behavioral alterations in AVP-deficient Brattleboro rats. Wild type (WT) and vasopressin-deficient (KO) male Brattleboro rats were tested. Half of the KO animals were treated by desmopressin (V2-receptor agonist) via osmotic minipump (subcutaneous) to eliminate the peripheral symptoms of vasopressin-deficiency. Anxiety was studied by elevated plus maze (EPM), defensive withdrawal (DW) and marble burying (MB) tests, while depressive-like changes were monitored in forced swimming (FS) and anhedonia by sucrose preference test. Cell activity was examined in septum and amygdala by c-Fos immunohistochemistry after 10 min FS. KO rats spent more time in the open arm of the EPM, spent less time at the periphery of DW and showed less burying behavior in MB suggesting a reduced anxiety state. KO animals showed less floating behavior during FS revealing a less depressive phenotype. Desmopressin treatment compensated the peripheral effects of vasopressin-deficiency without a significant influence on the behavior. The FS-induced c-Fos immunoreactivity in the medial amygdala was different in WT and KO rats, with almost identical levels in KO and desmopressin treated animals. There were no differences in central and basolateral amygdala as well as in lateral septum. Our data confirmed the role of vasopressin in the development of affective disorders through central mechanisms. The involvement of the medial amygdala in the behavioral alterations of vasopressin deficient animals deserves further attention.

Pregnanolone Glutamate, a Novel Use-Dependent NMDA Receptor Inhibitor, Exerts Antidepressant-Like Properties in Animal Models

A number of studies demonstrated a rapid onset of an antidepressant effect of non-competitive N-methyl-d-aspartic acid receptor (NMDAR) antagonists. Nonetheless, its therapeutic potential is rather limited, due to a high coincidence of negative side-effects. Therefore, the challenge seems to be in the development of NMDAR antagonists displaying antidepressant properties, and at the same time maintaining regular physiological function of the NMDAR. Previous results demonstrated that naturally occurring neurosteroid 3α5β-pregnanolone sulfate shows pronounced inhibitory action by a use-dependent mechanism on the tonically active NMDAR. The aim of the present experiments is to find out whether the treatment with pregnanolone 3αC derivatives affects behavioral response to chronic and acute stress in an animal model of depression. Adult male mice were used throughout the study. Repeated social defeat and forced swimming tests were used as animal models of depression. The effect of the drugs on the locomotor/exploratory activity in the open-field test was also tested together with an effect on anxiety in the elevated plus maze. Results showed that pregnanolone glutamate (PG) did not induce hyperlocomotion, whereas both dizocilpine and ketamine significantly increased spontaneous locomotor activity in the open field. In the elevated plus maze, PG displayed anxiolytic-like properties. In forced swimming, PG prolonged time to the first floating. Acute treatment of PG disinhibited suppressed locomotor activity in the repeatedly defeated group-housed mice. Aggressive behavior of isolated mice was reduced after the chronic 30-day administration of PG. PG showed antidepressant-like and anxiolytic-like properties in the used tests, with minimal side-effects. Since PG combines GABA_A receptor potentiation and use-dependent NMDAR inhibition, synthetic derivatives of neuroactive steroids present a promising strategy for the treatment of mood disorders.

Highlights: -

3α5β-pregnanolone glutamate (PG) is a use-dependent antagonist of NMDA receptors.

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We demonstrated that PG did not induce significant hyperlocomotion.

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We showed that PG displayed anxiolytic-like and antidepressant-like properties.

The Antidepressant-like Effect of Ethanol Extract of Daylily Flowers ( Jīn Zhēn Huā) in Rats

According to the prediction of the 2008 World Health Organization (WHO) report, depression will be the highest burden disease by the year 2030. Daylily flower ( Jīn Zhēn Huā ; the flower of Hemerocallis fulva) is traditionally used for soothing in Chinese dietary therapy. The major flavonoid of daylily flowers, rutin, is also characterized to be an antidepressant. In this study, we investigated the antidepressant effects of ethanol extract of daylily flowers (DFEtoH) and rutin by forced swimming test (FST) and neurotransmitter metabolism of brain regions (frontal cortex, hippocampus, striatum, and amygdala). Results show that either short- or long-term tests, the extract and rutin significantly reduce the immobility time and increased swimming time of FST, which are compared with the vehicle (P < 0.05). The extract and rutin also increase the serotonin, norepinephrine, and dopamine concentration of these brain regions (P < 0.05). In long-term tests, the daylily flowers extract markedly increased serotonin concentration and reduced serotonin turnover rate in these brain regions but not frontal cortex. In conclusion, present data illustrated that DFEtoH does have antidepressant-like effects possibly via the regulation of serotonergic system. Moreover, rutin might be playing a very important role in the antidepressant-like effects of DFEtoH.

The selective glucocorticoid receptor antagonist CORT 108297 decreases neuroendocrine stress responses and immobility in the forced swim test

Pre-clinical and clinical studies have employed treatment with glucocorticoid receptor (GR) antagonists in an attempt to limit the deleterious behavioral and physiological effects of excess glucocorticoids. Here, we examined the effects of GR antagonists on neuroendocrine and behavioral stress responses, using two compounds: mifepristone, a GR antagonist that is also a progesterone receptor antagonist, and CORT 108297, a specific GR antagonist lacking anti-progestin activity. Given its well-documented impact on neuroendocrine and behavioral stress responses, imipramine (tricyclic antidepressant) served as a positive control. Male rats were treated for five days with mifepristone (10mg/kg), CORT 108297 (30mg/kg and 60mg/kg), imipramine (10mg/kg) or vehicle and exposed to forced swim test (FST) or restraint stress. Relative to vehicle, imipramine potently suppressed adrenocorticotropin hormone (ACTH) responses to FST and restraint exposure. Imipramine also decreased immobility in the FST, consistent with antidepressant actions. Both doses of CORT 108297 potently suppressed peak corticosterone responses to FST and restraint stress. However, only the higher dose of CORT 108297 (60mg/kg) significantly decreased immobility in the FST. In contrast, mifepristone induced protracted secretion of corticosterone in response to both stressors, and modestly decreased immobility in the FST. Taken together, the data indicate distinct effects of each compound on neuroendocrine stress responses and also highlight dissociation between corticosterone responses and immobility in the FST. Within the context of the present study, our data suggest that CORT 108297 may be an attractive alternative for mitigating neuroendocrine and behavioral states associated with excess glucocorticoid secretion.

Adaptation of the pituitary-adrenal axis to daily repeated forced swim exposure in rats is dependent on the temperature of water

Comparison of exposure to certain predominantly emotional stressors reveals a qualitatively similar neuroendocrine response profile as well as a reduction of physiological responses after daily repeated exposure (adaptation). However, particular physical components of the stressor may interfere with adaptation. As defective adaptation to stress can enhance the probability to develop pathologies, we studied in adult male rats (n = 10/group) swimming behavior (struggling, immobility and mild swim) and physiological responses (ACTH, corticosterone and rectal temperature) to daily repeated exposure to forced swim (20 min, 13 d) at 25 or 36 °C (swim25 or swim36). Rats were repeatedly blood-sampled by tail-nick and hormones measured by radioimmunoassay. Some differences were observed between the two swim temperature groups after the first exposure to forced swim: (a) active behaviors were greater in swim25 than swim36 groups; (b) swim25 but not swim36 caused hypothermia; and (c) swim36 elicited the same ACTH response as swim25, but plasma corticosterone concentration was lower for swim36 at 30 min post-swim. After daily repeated exposure, adaptation in ACTH secretion was observed with swim36 already on day 4, whereas with swim25 adaptation was not observed until day 13 and was of lower magnitude. Nevertheless, after repeated exposure to swim25 a partial protection from hypothermia was observed and the two swim conditions resulted in progressive reduction of active behaviors. Thus, daily repeated swim at 25 °C impairs adaptation of the hypothalamic-pituitary-adrenal axis as compared to swim at 36 °C, supporting the hypothesis that certain physical components of predominantly emotional stressors can interfere with the process of adaptation.

Prior cold water swim stress alters immobility in the forced swim test and associated activation of serotonergic neurons in the rat dorsal raphe nucleus

Prior adverse experience alters behavioral responses to subsequent stressors. For example, exposure to a brief swim increases immobility in a subsequent swim test 24h later. In order to determine if qualitative differences (e.g. 19°C versus 25°C) in an initial stressor (15-min swim) impact behavioral, physiological, and associated neural responses in a 5-min, 25°C swim test 24h later, rats were surgically implanted with biotelemetry devices 1 week prior to experimentation then randomly assigned to one of six conditions (Day 1 (15 min)/Day 2 (5 min)): (1) home cage (HC)/HC, (2) HC/25°C swim, (3) 19°C swim/HC, (4) 19°C swim/25°C swim, (5) 25°C swim/HC, (6) 25°C swim/25°C swim. Core body temperature (Tb) was measured on Days 1 and 2 using biotelemetry; behavior was measured on Day 2. Rats were transcardially perfused with fixative 2h following the onset of the swim on Day 2 for analysis of c-Fos expression in midbrain serotonergic neurons. Cold water (19°C) swim on Day 1 reduced Tb, compared to both 25°C swim and HC groups on Day 1, and, relative to rats exposed to HC conditions on Day 1, reduced the hypothermic response to the 25°C swim on Day 2. The 19°C swim on Day 1, relative to HC exposure on Day 1, increased immobility during the 5-min swim on Day 2. Also, 19°C swim, relative to HC conditions, on Day 1 reduced swim (25°C)-induced increases in c-Fos expression in serotonergic neurons within the dorsal and interfascicular parts of the dorsal raphe nucleus. These results suggest that exposure to a 5-min 19°C cold water swim, but not exposure to a 5-min 25°C swim alters physiological, behavioral and serotonergic responses to a subsequent stressor.

Oxytocin in Brattleboro rats: increased synthesis is contrasted by blunted intrahypothalamic release from supraoptic nucleus neurones

Adult male Brattleboro rats were used to investigate the impact of the congenital absence of vasopressin on the release pattern of oxytocin (OXT) within the hypothalamic supraoptic nucleus (SON) in response to a 10-min forced swimming session and osmotic stimulation. Both immunohistochemical and in situ hybridisation data suggest that vasopressin-deficient animals have more oxytocin-synthesising neurones in the SON than homozygous wild-type controls. Unexpectedly, both forced swimming and peripheral osmotic stimulation resulted in a blunted release profile of oxytocin within the SON of vasopressin-deficient rats compared to controls. A similar intranuclear OXT response to direct osmotic stimulation of the SON by retrodialysis with hypertonic Ringer's solution in both genotypes confirmed the capability of SON neurones to locally release oxytocin in vasopressin-deficient rats, indicating an altered processing of information originating from multisynaptic inputs rather than a deficit in release capacity. Taken together with data obtained in previous studies, the present findings provide evidence suggesting that autocrine and paracrine signalling of magnocellular neurones differs within the paraventricular nucleus and the SON. Thus, significant alterations in intra-SON oxytocin mRNA levels cannot easily be extrapolated to intranuclear release profiles and the local signal intensity of this neuropeptide after physiological stimulation.

Behavioural and neurochemical changes induced by stress-related conditions are counteracted by the neurokinin-2 receptor antagonist saredutant

These experiments were undertaken to assess the mechanisms underlying the antidepressant-like effects of the neurokinin-2 (NK(2)) receptor antagonist saredutant (SR48968) in rats tested in the forced swim test (FST), by analysing hippocampal brain-derived neurotrophic factor (BDNF) and plasma corticosterone [as index of hypothalamic-pituitary-adrenal (HPA) axis activity]. Male Wistar rats received three intraperitoneal injections over 24 h of vehicle, saredutant (5 mg/kg), citalopram (15 mg/kg), clomipramine (50 mg/kg). Rats were subjected to restraint stress (4 h) 24 h prior to the FST procedure. This stress procedure increased immobility and decreased swimming behaviour in the FST; furthermore, it lowered hippocampal BDNF protein expression and increased plasma corticosterone levels. Saredutant and clomipramine or citalopram, used here as positive controls, reduced the immobility time in the FST both under basal conditions and after stress exposure. This effect was not attributable to changes in locomotion, because locomotor activity was unchanged when assessed in the open field test. Pretreatment with para-cholorophenylalanine (150 mg/kg, 72 h and 48 h prior to FST) abolished the effect of citalopram and saredutant on immobility time. At neurochemical level, saredutant attenuated activation of HPA axis in stressed animals more than clomipramine or citalopram. The behavioural effects of saredutant support the hypothesis that NK(2) receptor activity is involved in stress-related disorders. These effects of saredutant may be related to normalization of the HPA axis. Moreover, saredutant increases BDNF expression in the hippocampus, confirming the role of NK(2) receptor blockade in BDNF activation following stressor application.

Maternal neglect with reduced depressive-like behavior and blunted c-fos activation in Brattleboro mothers, the role of central vasopressin

Early mother-infant relationships exert important long-term effects in offspring and are disturbed by factors such as postpartum depression. We aimed to clarify if lack of vasopressin influences maternal behavior paralleled by the development of a depressive-like phenotype. We compared vasopressin-deficient Brattleboro mothers with heterozygous and homozygous normal ones. The following parameters were measured: maternal behavior (undisturbed and separation-induced); anxiety by the elevated plus maze; sucrose and saccharin preference and forced swim behavior. Underlying brain areas were examined by c-fos immunocytochemistry among rest and after swim-stress. In another group of rats, vasopressin 2 receptor agonist was used peripherally to exclude secondary changes due to diabetes insipidus. Results showed that vasopressin-deficient rats spend less time licking-grooming their pups through a centrally driven mechanism. There was no difference between genotypes during the pup retrieval test. Vasopressin-deficient mothers tended to explore more the open arms of the plus maze, showed more preference for sucrose and saccharin and struggled more in the forced swim test, suggesting that they act as less depressive. Under basal conditions, vasopressin-deficient mothers had more c-fos expression in the medial preoptic area, shell of nucleus accumbens, paraventricular nucleus of the hypothalamus and amygdala, but not in other structures. In these areas the swim-stress-induced activation was smaller. In conclusion, vasopressin-deficiency resulted in maternal neglect due to a central effect and was protective against depressive-like behavior probably as a consequence of reduced activation of some stress-related brain structures. The conflicting behavioral data underscores the need for more sex specific studies.

The integration of depressive behaviors and cardiac dysfunction during an operational measure of depression: investigating the role of negative social experiences in an animal model

OBJECTIVE

There is a bidirectional association between depression and cardiovascular disease. The neurobiological mechanisms underlying this association may involve an inability to cope with disrupted social bonds. This study investigated in an animal model the integration of depressive behaviors and cardiac dysfunction after a disrupted social bond and during an operational measure of depression, relative to the protective effects of intact social bonds.

METHODS

Depressive behaviors in the forced swim test and continuous electrocardiographic parameters were measured in 14 adult, female socially monogamous prairie voles (rodents), after 4 weeks of social pairing or isolation.

RESULTS

After social isolation, animals exhibited (all values are mean ± standard error of the mean; isolated versus paired, respectively) increased heart rate (416 ± 14 versus 370 ± 14 bpm, p < .05) and reduced heart rate variability (3.3 ± 0.2 versus 3.9 ± 0.2 ln(ms(2))). During the forced swim test, isolated animals exhibited greater helpless behavior (immobility = 106 ± 11 versus 63 ± 11 seconds, p < .05), increased heart rate (530 ± 22 versus 447 ± 15 bpm, p < .05), reduced heart rate variability (1.8 ± 0.4 versus 2.7 ± 0.2 ln(ms(2)), p < .05), and increased arrhythmias (arrhythmic burden score = 181 ± 46 versus 28 ± 12, p < .05).

CONCLUSIONS

The display of depressive behaviors during an operational measure of depression is coupled with increased heart rate, reduced heart rate variability, and increased arrhythmias, indicative of dysfunctional behavioral and physiological stress coping abilities as a function of social isolation. In contrast, social pairing with a sibling is behaviorally protective and cardioprotective. The present results can provide insight into a possible social mechanism underlying the association between depression and cardiovascular disease in humans.

Swim stress activates serotonergic and nonserotonergic neurons in specific subdivisions of the rat dorsal raphe nucleus in a temperature-dependent manner

Physical (exteroceptive) stimuli and emotional (interoceptive) stimuli are thought to influence stress-related physiologic and behavioral responses through different neural mechanisms. Previous studies have demonstrated that stress-induced activation of brainstem serotonergic systems is influenced by environmental factors such as temperature. In order to further investigate the effects of environmental influences on stress-induced activation of serotonergic systems, we exposed adult male Wistar rats to either home cage control conditions or a 15-min swim in water maintained at 19 °C, 25 °C, or 35 °C and conducted dual immunohistochemical staining for c-Fos, a marker of immediate-early nuclear activation, and tryptophan hydroxylase (TPH), a marker of serotonergic neurons. Changes in core body temperature were documented using biotelemetry. As expected, exposure to cold (19 °C) swim, relative to warm (35 °C) swim, increased c-Fos expression in the external lateral part of the parabrachial nucleus (LPBel), an important part of the spinoparabrachial pathway involved in sensation of cold, cutaneous stimuli, and in serotonergic neurons in the raphe pallidus nucleus (RPa), an important part of the efferent mechanisms controlling thermoregulatory warming responses. In addition, exposure to cold (19 °C) swim, relative to 35 °C swim, increased c-Fos expression in the dorsal raphe nucleus, ventrolateral part/periaqueductal gray (DRVL/VLPAG) and dorsal raphe nucleus, interfascicular part (DRI). Both of these subregions of the dorsal raphe nucleus (DR) have previously been implicated in thermoregulatory responses. Altogether, the data are consistent with the hypothesis that midbrain serotonergic neurons, possibly via activation of afferents to the DR by thermosensitive spinoparabrachial pathways, play a role in integration of physiologic and behavioral responses to interoceptive stress-related cues involved in forced swimming and exteroceptive cues related to cold ambient temperature.