Maria Razzoli

Different susceptibility to social defeat stress of BalbC and C57BL6/J mice

Social stress may precipitate psychopathological disorders in susceptible individuals. The present experiments were focused on the biology beyond the differential susceptibility to social stress. Social defeat, an ethologically relevant stressor known to elicit different coping strategies, was used in two mouse strains differing for baseline emotionality, such as C57BL6/J and BalbC. In separate experiments, in both strains a single social defeat decreased home-cage activity without altering social aversion; it diminished body weight only in defeated BalbC mice. In longitudinal experiments, mice experienced repeated social defeats that induced multiple long-term consequences. Defeated C57BL6/J increased their body weight and food intake; defeated BalbC mice diminished their metabolic efficiency. Only defeated BalbC subjects exhibited increased social avoidance levels; no differences from controls were seen on forced swim test response in defeated mice of either strain. No long-term effects of social defeat were detected in peripheral biomarkers of stress, metabolic, and immune responses, although the analysis of selected internal organs revealed decreases in abdominal fat and gonadal organs in all defeated subjects. These results demonstrated a strain-distinctive profile in the susceptibility to social defeat stress, either acutely or chronically, with metabolic consequences more consistently found in C57BL6/J while social aversion induced predominantly in BalbC subjects.

Early-life stress and antidepressants modulate peripheral biomarkers in a gene-environment rat model of depression.

BACKGROUND Availability of peripheral biomarkers for depression could aid diagnosis and help to predict treatment response. The objective of this work was to analyse the peripheral biomarker response in a gene-environment interaction model of depression. Genetically selected Flinders Sensitive Line (FSL) rats were subjected to maternal separation (MS), since early-life trauma is an important antecedent of depression. An open-ended approach based on a proteomic analysis of serum was combined with the evaluation of depression-associated proteins. METHODS Rats experienced MS and chronically received escitalopram (ESC) or nortryptiline (NOR). Serum proteins were compared by two-dimensional gel electrophoresis. Corticosterone, cytokines, BDNF and C-reactive protein (CRP) were measured by immunoassays. RESULTS Comparing FSL with the control Flinders Resistant Line (FRL), Apo-AI and Apo-AIV, alpha1-macroglobulin, glutathione peroxidase and complement-C3 were significantly modulated. Significant increases were detected in leptin, interleukin (IL) 1alpha and BDNF. CRP levels were significantly reduced. The impact of early-life stress was assessed by comparing FSL+MS versus FSL. Apo-E, alpha1-macroglobulin, complement-C3, transferrin and hemopexin were significantly modulated. The effect of stress in antidepressant response was then evaluated. In the comparison FSL+ESC+MS versus FSL+ESC, albumin, alpha1-macroglobulin, glutathione peroxidase and complement-C3 were modulated and significant reductions were detected in IL4, IL6, IL10, CRP and BDNF. By comparing FSL+NOR+MS versus FSL+NOR proteins like Apo-AIV, pyruvate dehydrogenase, alpha1-macroglobulin, transferrin and complement-C3 showed different levels. CONCLUSIONS Lipid metabolism and immunity proteins were differently expressed in FSL in comparison with FRL. Exposure to MS induced changes in inflammation and transport proteins which became apparent in response to antidepressant treatments. Modulated proteins could suggest biomarker studies in humans.

Repeated long separations from pups produce depression-like behavior in rat mothers

Long maternal (LMS) versus brief maternal (BMS) daily separations of rat pups from their mothers have contrasting effects on their adult stress responses and maternal behavior by, respectively, decreasing and increasing licking received from their mothers. We hypothesized that LMS decreases pup-licking in mothers by inducing learned helplessness, creating a depression-like state. We subjected postpartum rats to LMS (3 h), BMS (15 min) or no separation (NMS) on postpartum days 2-14. After weaning, mothers were given a forced swim test (FST). LMS mothers exhibited more immobility and fewer escape attempts than BMS or NMS mothers. These results suggest that LMS induces a depression-like state, which may account for the reductions in maternal behavior seen in LMS mothers. Immobility in the FST is recognized as an animal model of depression. Therefore, LMS may be a model of maternal depression.

Intraspecific variation in estrogen receptor alpha and the expression of male sociosexual behavior in two populations of prairie voles.

Estrogen (E) regulates a variety of male sociosexual behaviors. We hypothesize that there is a relationship between the distribution of estrogen receptor alpha (ERalpha) and the degree of male social behavior. To test this hypothesis, ERalpha immunoreactivity (IR) was compared in prairie voles (Microtus ochrogaster) from Illinois (IL), which are highly social, and Kansas (KN), which are less social. The expression of androgen receptors (AR) in males also was compared between populations. The expression of ERalpha and AR were compared in brains from KN and IL males and females using immunocytochemistry (ICC). There were significant intrapopulational differences, with males expressing less ERalpha-IR than females in the medial preoptic area, ventromedial nucleus, ventrolateral portion of the hypothalamus, and bed nucleus of the stria terminalis (BST). IL males also displayed less ERalpha-IR in the medial amygdala (MeA) than IL females. While IL males expressed significantly less ERalpha-IR in the BST and MeA than KN males, there was no difference in AR-IR. Differences in the pattern of ERalpha-IR between KN and IL males were behaviorally relevant, as low levels of testosterone (T) were more effective in restoring sexual activity in castrated KN males than IL males. The lack of difference in AR combined with lower expression of ERalpha-IR in IL males suggests that behavioral differences in response to T are associated with aromatization of T to E and that reduced sensitivity to E may facilitate prosocial behavior in males.

Increased phasic activity of VTA dopamine neurons in mice 3 weeks after repeated social defeat.

Social defeat is an ethologically relevant stress inducing neuroadaptive changes in the mesocorticolimbic dopaminergic system. Three weeks after 10 days of daily defeat salient behaviors and in vivo dopamine (DA) neuron firing were evaluated in mice. Prior defeat induced social avoidance and hyperphagia and increased ventral tegmental area (VTA) DA neuron bursting activity. These data extend previous studies and suggest that increased phasic DA neuron firing in the VTA could be considered amongst the features defining the lasting imprint of social defeat stress.

Expression Profiling of a Genetic Animal Model of Depression Reveals Novel Molecular Pathways Underlying Depressive-Like Behaviours

Background The Flinders model is a validated genetic rat model of depression that exhibits a number of behavioural, neurochemical and pharmacological features consistent with those observed in human depression. Principal Findings In this study we have used genome-wide microarray expression profiling of the hippocampus and prefrontal/frontal cortex of Flinders Depression Sensitive (FSL) and control Flinders Depression Resistant (FRL) lines to understand molecular basis for the differences between the two lines. We profiled two independent cohorts of Flinders animals derived from the same colony six months apart, each cohort statistically powered to allow independent as well as combined analysis. Using this approach, we were able to validate using real-time-PCR a core set of gene expression differences that showed statistical significance in each of the temporally distinct cohorts, representing consistently maintained features of the model. Small but statistically significant increases were confirmed for cholinergic (chrm2, chrna7) and serotonergic receptors (Htr1a, Htr2a) in FSL rats consistent with known neurochemical changes in the model. Much larger gene changes were validated in a number of novel genes as exemplified by TMEM176A, which showed 35-fold enrichment in the cortex and 30-fold enrichment in hippocampus of FRL animals relative to FSL. Conclusions These data provide significant insights into the molecular differences underlying the Flinders model, and have potential relevance to broader depression research.

A role for BDNF/TrkB signaling in behavioral and physiological consequences of social defeat stress.

Accumulating evidences underlie the importance of the interplay between environmental and genetic factors in contributing to the risk to develop mental illness. Brain‐derived neurotrophic factor (BDNF) and its Tyrosine receptor kinase B (TrkB) receptor play a fundamental contribution to brain development and plastic adaptations to life events. In the present study, the potential for the BDNF/TrkB contribution in increasing vulnerability to negative social experiences was assessed by subjecting TrkB.T1 overexpressing mice to a chronic social defeat model. TrkB.T1 mice overexpress the dominant‐negative truncated splice variant of TrkB receptor leading to decreased BDNF signaling. After repeated social defeat, mice were assessed in a longitudinal study for behavioral, physiological, endocrine and immune responses potentially related to psychiatric endophenotypes. TrkB.T1 overexpression corresponded to smaller changes in metabolic parameters such as body weight, food intake, feed efficiency and peripheral ghrelin levels compared with wild‐type (wt) littermates following social defeat. Interestingly, 4 weeks after the last defeat, TrkB.T1 overexpressing mice exhibited more consistent social avoidance effects than what observed in wt subjects. Finally, previously unreported effects of TrkB mutations could be observed on lymphoid organ weight and on peripheral immune biomarker levels, such as interleukin‐1α and regulated on activation, normal, T‐cell expressed, and secreted (RANTES), thus suggesting a systemic role of BDNF signaling in immune function. In conclusion, the present data support a contribution of TrkB to stress vulnerability that, given the established role of TrkB in the response to antidepressant treatment, calls for further studies addressing the link between stress susceptibility and variability in drug efficacy.

Conditioning properties of social subordination in rats: behavioral and biochemical correlates of anxiety.

To develop a socially based model of anxiety, the contextual fear conditioning properties of social defeat were examined in rats. Social threat consisted of exposing intruders to aggressive residents in resident home cage, separated by a partition. During 3 daily encounters, intruders were either defeated or threatened by residents, providing the defeated-threatened (DT) and threatened-threatened (TT) groups respectively. On Day 4, both DT and TT animals were subjected to a social threat only. Additional animals received a 4-day exposure to a novel empty cage (EC group). Further DT, TT, and EC rats were confronted to a different context on Day 4. DT rats exhibited a robust and context-specific anxiety-like response, characterized by significant behavioral and biochemical alterations. DT rats showed increased risk assessment and decreased exploration compared to TT and EC rats that in turn were not different towards each other. DT and TT rats exhibited increased ACTH levels, while only DT rats showed enhanced corticosterone and decreased testosterone levels compared to EC. These differences were context-specific since they were absent confronting animals to a different context and since they were not long lasting. Overall, these data demonstrate the induction of an anxiety-like state in rats through a context conditioning process based upon social factors. The social basis of this paradigm offers good face validity with anxiety disorders, which in humans are mainly related to social factors and associated with HPA axis deregulations. The present procedure may provide a useful experimental model to further investigate the neurobiological mechanisms underlying anxiety-related disorders.

Hormonal regulation of agonistic and affiliative behavior in female mongolian gerbils (Meriones unguiculatus).

Ovarian steroids and oxytocin (OT) have been implicated in the regulation of social behaviors. The purpose of the present study was to examine hormonal substrates of aggression and affiliation in the female Mongolian gerbil (Meriones unguiculatus), a highly social, monogamous rodent. Sexually naive adult females were paired with sexually experienced males for 48 h and their interactions videotaped. Females were gonadally intact and tested during vaginal estrus (INT) or ovariectomized and observed after the following treatments, administered by means of sc injections: EBEB (7 days of estradiol-benzoate); EBP (2 days of EB followed by progesterone), SALEB (saline, days 1-5 then 2 days of EB), OTEB (OT for days 1-5 then 2 days of EB); OTOIL (OT for days 1-5 then 2 days of OIL); or SALOIL (saline days 1-5 then 2 days of OIL). During the first hour of pairing INT females displayed higher levels of affiliation and lower levels of sniffing and agonistic behavior than SALOIL females. All hormonal treatments reduced agonistic behaviors when compared to SALOIL, although none of the hormonal treatments restored affiliation to INT levels. During the 48-h test overt aggression varied by treatment with INT, EBEB, EBP, and OTEB females displaying lower levels than SALOIL, while all groups displayed similar levels of affiliation. The results indicate that OT and E play a significant role in regulating male-directed aggressive behavior in females and that the presence of ovarian hormones as well as OT can increase affiliation during initial contact. Over a sustained period of cohabitation social cues appear to be more important in regulating affiliation than gonadal hormones.

Social defeat and isolation induce clear signs of a depression-like state, but modest cardiac alterations in wild-type rats.

Adverse social environments play a relevant role in the onset and progression of mood disorders. On the other hand, depression is an independent risk factor for cardiovascular morbidity. This study was aimed at (i) corroborating the validity of a rat model of depression based on a negative social episode followed by social isolation and (ii) verifying its impact on cardiac function and structure. Pair housed, wild-type Groningen rats (Rattus norvegicus) were implanted with radiotransmitters for ECG, temperature and activity recordings. They were either exposed to a social defeat episode followed by 4-week isolation or left undisturbed with their female partners. The social challenge induced a series of biological changes that are commonly taken as markers of depression in rats, including decreased body weight gain and reduced preference for sucrose consumption, functional and structural changes of the hypothalamic-pituitary-adrenocortical axis, increased anxiety in the elevated plus maze test. The cardiovascular alterations consisted in (i) transitory heart rate circadian rhythm alterations, (ii) lack of habituation of cardiac autonomic responsivity (tachycardia and vagal withdrawal) to an acute stressor, and (iii) moderate hypertrophy affecting the right ventricle of the heart. These results indicate that a depression-like state induced via this model of social challenge was associated with a few modest cardiovascular changes. Further studies are required to confirm the validity of this rat model of depression as a valid preclinical approach to the comprehension of the biological substrates underlying depression-cardiovascular comorbidity.