Social crowding in the night-time reduces an anxiety-like behavior and increases social interaction in adolescent mice

Social environment enrichment alleviates anxiety-like behavior in mice: Involvement of the dopamine system

Rearing environment plays a vital role in maintaining physical and mental health of both animals and humans. Plenty of studies have proved that physical environment enrichment in adolescence has protective effects on emotion, social behavior, learning and memory deficits. However, the following effects of social environment enrichment in adolescence remain largely elusive. Using the paradigm of companion rotation (CR), the present study found that social environment enrichment reduced anxiety-like behaviors of early adult male C57BL/6J mice. CR group also showed significantly higher expression of tyrosine hydroxylase in the ventral tegmental area and dopamine 1 receptor mRNA in the nucleus accumbens shell than control group. Taken together, these findings demonstrate that CR from adolescence to early adulthood can suppress the level of anxiety and upregulate dopaminergic neuron activity in early adult male C57BL/6J mice.

Acute and long-term effects of adolescence stress exposure on rodent adult hippocampal neurogenesis, cognition, and behaviour

Adolescence represents a critical period for brain and behavioural health and characterised by the onset of mood, psychotic and anxiety disorders. In rodents, neurogenesis is very active during adolescence, when is particularly vulnerable to stress. Whether stress-related neurogenesis changes influence adolescence onset of psychiatric symptoms remains largely unknown. A systematic review was conducted on studies investigating changes in hippocampal neurogenesis and neuroplasticity, hippocampal-dependent cognitive functions, and behaviour, occurring after adolescence stress exposure in mice both acutely (at post-natal days 21-65) and in adulthood. A total of 37 studies were identified in the literature. Seven studies showed reduced hippocampal cell proliferation, and out of those two reported increased depressive-like behaviours, in adolescent rodents exposed to stress. Three studies reported a reduction in the number of new-born neurons, which however were not associated with changes in cognition or behaviour. Sixteen studies showed acutely reduced hippocampal neuroplasticity, including pre- and post-synaptic plasticity markers, dendritic spine length and density, and long-term potentiation after stress exposure. Cognitive impairments and depressive-like behaviours were reported by 11 of the 16 studies. Among studies who looked at adolescence stress exposure effects into adulthood, seven showed that the negative effects of stress observed during adolescence on either cell proliferation or hippocampal neuroplasticity, cognitive deficits and depressive-like behaviour, had variable impact in adulthood. Treating adolescent mice with antidepressants, glutamate receptor inhibitors, glucocorticoid antagonists, or healthy diet enriched in omega-3 fatty acids and vitamin A, prevented or reversed those detrimental changes. Future research should investigate the translational value of these preclinical findings. Developing novel tools for measuring hippocampal neurogenesis in live humans, would allow assessing neurogenic changes following stress exposure, investigating relationships with psychiatric symptom onset, and identifying effects of therapeutic interventions.

Stress in adolescence as a first hit in stress-related disease development: Timing and context are crucial

The two-hit stress model predicts that exposure to stress at two different time-points in life may increase or decrease the risk of developing stress-related disorders later in life. Most studies based on the two-hit stress model have investigated early postnatal stress as the first hit with adult stress as the second hit. Adolescence, however, represents another highly sensitive developmental window during which exposure to stressful events may affect programming outcomes following exposure to stress in adulthood. Here, we discuss the programming effects of different types of stressors (social and nonsocial) occurring during adolescence (first hit) and how such stressors affect the responsiveness toward an additional stressor occurring during adulthood (second hit) in rodents. We then provide a comprehensive overview of the potential mechanisms underlying interindividual and sex differences in the resilience/susceptibility to developing stress-related disorders later in life when stress is experienced in two different life stages.

In the pursuit of new social neurons. Neurogenesis and social behavior in mice: A systematic review

Social behaviors have become more relevant to our understanding of the human nervous system because relationships with our peers may require and modulate adult neurogenesis. Here, we review the pieces of evidence we have to date for the divergence of social behaviors in mice by modulation of adult neurogenesis or if social behaviors and the social environment can drive a change in neurogenic processes. Social recognition and memory are deeply affected by antimitotic drugs and irradiation, while NSC transgenic mice may run with lower levels of social discrimination. Interestingly, social living conditions can create a big impact on neurogenesis. Social isolation and social defeat reduce the number of new neurons, while social dominance and enrichment of the social environment increase their number. These new “social neurons” trigger functional modifications with amazing transgenerational effects. All of these suggest that we are facing two bidirectional intertwined variables, and the great challenge now is to understand the cellular and genetic mechanisms that allow this relationship to be used therapeutically.

Influence of diurnal phase on behavioral tests of sensorimotor performance, anxiety, learning and memory in mice

Behavioral measurements in mice are critical tools used to evaluate the effects of interventions. Whilst mice are nocturnal animals, many studies conduct behavioral tests during the day. To better understand the effects of diurnal rhythm on mouse behaviors, we compared the results from behavioral tests conducted in the active and inactive phases. C57BL/6 mice were used in this study; we focus on sensorimotor performance, anxiety, learning and memory. Overall, our results show mice exhibit slightly higher cutaneous sensitivity, better long-term contextual memory, and a greater active avoidance escape response during the active phase. We did not observe significant differences in motor coordination, anxiety, or spatial learning and memory. Furthermore, apart from the elevated-O-maze, there was no remarkable sex effect among these tests. This study provides information on the effects of different diurnal phases on types of behavior and demonstrates the importance of the circadian cycle on learning and memory. Although we did not detect differences in anxiety and spatial learning/memory, diurnal rhythm may interact with other factors to influence these behaviors.

Stress Varies Along the Social Density Continuum

Social stress is ubiquitous in the lives of social animals. While significant research has aimed to understand the specific forms of stress imparted by particular social interactions, less attention has been paid to understanding the behavioral effects and neural underpinnings of stress produced by the presence and magnitude of social interactions. However, in humans and rodents alike, chronically low and chronically high rates of social interaction are associated with a suite of mental health issues, suggesting the need for further research. Here, we review literature examining the behavioral and neurobiological findings associated with changing social density, focusing on research on chronic social isolation and chronic social crowding in rodent models, and synthesize findings in the context of the continuum of social density that can be experienced by social animals. Through this synthesis, we aim to both summarize the state of the field and describe promising avenues for future research that would more clearly define the broad effects of social interaction on the brain and behavior in mammals.

Neurobehavioral effects of alcohol in overcrowded male adolescent rats

Being a critical neurodevelopmental stage that is affected by social conditions, the period of adolescence was chosen as the age of examining possible modification of alcohol neurobehavioral effects by overcrowding. Adolescent male rats (postnatal day 35±1) were subjected to overcrowding and/or injected with ethanol, 2 g/kg, 20% w/v, (i.p.) for one week. 24 h after the last dose, motor, exploratory behavior, sociability and fear responses were assessed using open field, social interaction and defensive probe burying tests, respectively. Wet brain tissue nitric oxide and reduced glutathione contents as well as monoamine levels, namely dopamine, norepinephrine and serotonin, in addition to 5-HIAA were estimated. Overcrowding increased social play and freezing time. Alcohol administration under overcrowding condition impaired sociability and interfered with active fear response. Alcohol in normal or in under overcrowding condition, impaired motor and exploratory behavior and increased anxiety. These results indicate that concomitant exposure of male adolescent rats to overcrowding and alcohol induced adverse behavioral changes.

Post-weaning folate deficiency induces a depression-like state via neuronal immaturity of the dentate gyrus in mice

Folate deficiency has been suggested as a risk factor for depression in preclinical and clinical studies. Several hypotheses of mechanisms underlying folate deficiency-induced depressive symptoms have been proposed, but the detailed mechanisms are still unclear. In this study, we assessed whether post-weaning folate deficiency affect neurological and psychological function. The low folate diet-fed mice showed depression-like behavior in the forced swim test. In contrast, spontaneous locomotor activity, social behavior, coordinated motor skills, anxiety-like behavior and spatial memory did not differ between control and low folate diet-fed mice. In the dentate gyrus (DG) of the hippocampus, decreased number of newborn mature neurons and increased number of immature neurons were observed in low folate diet-fed mice. Staining with Golgi-Cox method revealed that dendritic complexity, spine density and the number of mature spines of neurons were markedly reduced in the DG of low folate diet-fed mice. Stress response of neurons indicated as c-Fos expression was also reduced in the DG of low folate diet-fed mice. These results suggest that reduction in the degree of maturation of newborn hippocampal neurons underlies folate deficiency-induced depressive symptoms.

Antidepressants with different mechanisms of action show different chronopharmacological profiles in the tail suspension test in mice

The circadian system regulates sleep/wake cycles, metabolism, mood, and other functions. It also influences medication efficacy. In this study, we studied the chronopharmacological profiles of antidepressants with various modes of action. We also investigated the effects of dosing time on the pharmacological activity of several antidepressants acting on serotonergic, noradrenergic, and/or dopaminergic neurons. C57BL/6 mice were intraperitoneally administered fluoxetine, imipramine, venlafaxine, or bupropion at 08:00 h (morning), 14:00 h (mid-day), 20:00 h (evening), or 02:00 h (mid-night). Antidepressant activity was evaluated by the tail suspension test. All antidepressants reduced immobility, and their activities varied according to the dosing time. Fluoxetine and imipramine induced relatively strong rhythms with high amplitudes. Their maximal effects were observed in the morning and evening, respectively. Venlafaxine and bupropion induced weak rhythms with maximal effects in the evening and dawn, respectively. These results suggest that the antidepressant activity is associated with circadian fluctuation, and antidepressants with different modes of action have different chronopharmacological profiles. They affect locomotor activity in animals placed in novel (unfamiliar) environments. Fluoxetine, imipramine, and venlafaxine reduced locomotor activity, whereas bupropion increased it. The effects on locomotor activity also vary with circadian rhythm, and the tested drugs showed a maximal effect during the light phase. The peak time was different from that in TST. Plasma and brain levels of all drugs were slightly higher in the morning than in the evening. The dosing time dependency of the antidepressant activity did not correlate with the sedative/stimulatory activity or tissue drug level. Therefore, these latter two factors may have only a small impact on circadian antidepressant activity fluctuations. The relative activity of the serotonergic, noradrenergic, and dopaminergic systems may determine the chronopharmacological profiles of each drug. These results suggest the possibility that drug therapy be optimized by considering the dosing time when the antidepressant activity is high and other pharmacological activities leading to adverse effects are low. Further studies using animal models of depression and in clinical settings are necessary to confirm the effects of dosing time on depressed subjects.

Effects of light and dark phase testing on the investigation of behavioural paradigms in mice: Relevance for behavioural neuroscience

Different timing and light phases are critical factors in behavioural neuroscience, which can greatly affect the experimental outcomes of the performed tests. Despite the fact that time of testing is one of the most common factors that varies across behavioural laboratories, knowledge about the consequences of testing time on behavioural readouts is limited. Thus, in this study we systematically assessed the effect of this factor on the readout of a variety of elementary and recurrent behavioural paradigms in C57Bl/6 mice. Furthermore, we investigated potential neuronal correlates of this phenomenon by analysing how testing time influences the expression pattern of genes relevant for neuronal activation functions and the control of brain circadian rhythms. We show that animals tested in the light phase display reduced social approach behaviour and sensorimotor gating and increased locomotor activity, whereas anxiety-related behaviour and working memory are not affected. In addition, animals tested in the light phase also exhibit increased locomotor response to systemic amphetamine treatment, which is paralleled by alterations in the expression patterns of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the Nucleus Accumbens (NAc) and/or Midbrain (Mid). Lastly, we observed that neuronal activation, indexed by the gene expression levels of cFos, was increased in the NAc and Mid of animals tested during the light phase. Our data thus suggest that daily alterations in gene expression in mesolimbic brain structures might be involved in the different behavioural responses of mice tested in the light- versus the dark-phase. At the same time, our study adds further weight to the notion that the specific timing of testing can indeed strongly affect the readout of a given test. As comparison and reproducibility of findings is pivotal in science, experimental protocols should be clarified in detail to allow appropriate data comparison across different laboratories.

Group housing during adolescence has long-term effects on the adult stress response in female, but not male, zebra finches (Taeniopygia guttata)

Adolescent social interactions can have long-term effects on physiological responses to stressors in later-life. A larger adolescent group size can result in higher stressor-induced secretion of glucocorticoids in adulthood. The effect may be due to a socially-mediated modulation of gonadal hormones, e.g. testosterone. However, group size (number of animals) has been conflated with social density (space per animal). Therefore it is hard to determine the mechanisms through which adolescent group size can affect the stress response. The current study aimed to tease apart the effects of group size and social density during adolescence on the physiological stress response and gonadal hormone levels in adulthood. Adolescent zebra finches were housed in groups varying in size (2 vs. 5 birds per cage) and density (0.03m³ vs. 0.06m³ per bird) during early adolescence (day 40-60). Density was only manipulated in birds raised in groups of five. Glucocorticoid concentration secreted in response to a standard capture and restraint stressor was quantified in adolescence (day 55±1) and adulthood (day 100+). Basal gonadal hormone concentrations (male testosterone, female estradiol) were also quantified in adulthood. Female birds housed in larger groups, independent of social density, secreted a higher glucocorticoid concentration 45min into restraint regardless of age, and had higher peak glucocorticoid concentration in adulthood. Adult gonadal hormone concentrations were not affected by group size or density. Our results suggest that group size, not density, is a social condition that influences the development of the endocrine response to stressors in female zebra finches, and that these effects persist into adulthood. The findings have clear relevance to the social housing conditions necessary for optimal welfare in captive animals, but also elucidate the role of social rearing conditions in the emergence of responses to stressors that may persist across the lifespan and affect fitness of animals in wild populations.

Anti-anhedonic effect of selective serotonin reuptake inhibitors with affinity for sigma-1 receptors in picrotoxin-treated mice

BACKGROUND AND PURPOSE

Prefrontal dopamine release by the combined activation of 5-HT1A and sigma-1 (σ1 ) receptors is enhanced by the GABAA receptor antagonist picrotoxin in mice. Here, we examined whether this neurochemical event was accompanied by behavioural changes.

EXPERIMENTAL APPROACH

Male mice were treated with picrotoxin to decrease GABAA receptor function. Their anhedonic behaviour was measured using the female encounter test. The expression of c-Fos was determined immunohistochemically.

KEY RESULTS

Picrotoxin caused an anxiogenic effect on three behavioural tests, but it did not affect the immobility time in the forced swim test. Picrotoxin decreased female preference in the female encounter test and attenuated the female encounter-induced increase in c-Fos expression in the nucleus accumbens. Picrotoxin-induced anhedonia was ameliorated by fluvoxamine and S-(+)-fluoxetine, selective serotonin reuptake inhibitors with high affinity for the σ1 receptor. The effect of fluvoxamine was blocked by a 5-HT1A or a σ1 receptor antagonist, and co-administration of the σ1 receptor agonist (+)-SKF-10047 and the 5-HT1A receptor agonist osemozotan mimicked the effect of fluvoxamine. By contrast, desipramine, duloxetine and paroxetine, which have little affinity for the σ1 receptor, did not affect picrotoxin-induced anhedonia. The effect of fluvoxamine was blocked by a dopamine D2/3 receptor antagonist. Methylphenidate, an activator of the prefrontal dopamine system, ameliorated picrotoxin-induced anhedonia.

CONCLUSION AND IMPLICATIONS

Picrotoxin-treated mice show anhedonic behaviour that is ameliorated by simultaneous activation of 5-HT1A and σ1 receptors. These findings suggest that the increased prefrontal dopamine release is associated with the anti-anhedonic effect observed in picrotoxin-treated mice.

Repeated daily restraint stress induces adaptive behavioural changes in both adult and juvenile mice

Chronic stress is known to be a risk factor for the development of depression and anxiety, disorders which often begin during adolescence. Restraint stress is a commonly used stressor in adult rodents, however the effects of repeated restraint stress in juvenile mice have not been well characterised. Here we have shown for the first time the behavioural and hormonal effects of repeated restraint stress in both adult and juvenile BALB/c and C57BL/6 mice. Repeated daily restraint stress (2h/day for 3, 7 or 14days) provoked a robust physiological response evident as increased corticosterone levels and decreased body weight after 14days. However, habituation of the stress-response was evident during repeated exposure to the stressor in both adult and juvenile mice. The behavioural changes seen in response to repeated restraint stress were complex. In juvenile mice, repeated restraint stress evoked an increase in exploratory behaviours in the elevated plus maze, a decrease in time spent immobile in the forced swim test and a decrease in sucrose preference. In adult mice fewer behavioural changes were seen. Interestingly BALB/c and C57BL/6 mice showed qualitatively similar response to 3days repeated restraint stress. The behavioural changes we observed, as a result of prior stress exposure, may represent an adaptive stress-coping response or resilience. Both the hormonal and behavioural effects of stress were more pronounced in juvenile mice than in adults. This wider range of behavioural responses seen in juvenile mice might reflect a greater ability to engage in adaptive stress-coping strategies that likely have beneficial effects evident in future stress challenges.

Psychopharmacological Studies in Mice

Since 1998, when the laboratory of Medicinal Pharmacology was established in the Graduate School of Pharmaceutical Sciences, Osaka University, I have been interested in psychopharmacological research topics. During this period, we identified a number of novel regulatory mechanisms that control the prefrontal dopamine system through functional interaction between serotonin1A and dopamine D2 receptors or between serotonin1A and σ1 receptors. Our findings suggest that strategies that enhance the prefrontal dopamine system may have therapeutic potential in the treatment of psychiatric disorders. We also found that environmental factors during development strongly impact the psychological state in adulthood. Furthermore, we clarified the pharmacological profiles of the acetylcholinesterase inhibitors donepezil, galantamine, and rivastigmine, providing novel insights into their mechanisms of action. Finally, we developed the female encounter test, a novel method for evaluating motivation in mice. This simple method should help advance future psychopharmacological research. In this review, we summarize the major findings obtained from our recent studies in mice.

Social regulation of adult neurogenesis: A comparative approach

The social environment sculpts the mammalian brain throughout life. Adult neurogenesis, the birth of new neurons in the mature brain, can be up- or down-regulated by various social manipulations. These include social isolation, social conflict, social status, socio-sexual interactions, and parent/offspring interactions. However, socially-mediated changes in neuron production are often species-, sex-, and/or region-specific. In order to reconcile the variability of social effects on neurogenesis, we need to consider species-specific social adaptations and other contextual variables (e.g. age, social status, reproductive status, etc.) that shift the valence of social stimuli. Using a comparative approach to understand how adult-generated neurons in turn influence social behaviors will shed light on how adult neurogenesis contributes to survival and reproduction in diverse species.

Behavioral and physiological response to onset and termination of social instability in female mice

Chronic stress has been associated with several negative health outcomes and psychopathological conditions. One source of chronic stress might be from ones social environment (e.g., being excluded from a group, losing a loved one, etc.). Specifically, social instability, or frequent changes in the social environment, can result in both physiological and behavioral stress responses. Corticosterone is the primary stress-responsive biomarker in rodents, and it reflects the activity of the hypothalamic-pituitary-adrenal (HPA) axis. Historically, research on the effects of stress has relied on collection of blood, saliva, or other bodily fluids that yield information about moment-to-moment changes in stress physiology. Recently, new sampling techniques involving quantification of glucocorticoids in hair has allowed researchers to view patterns of stress physiology over extended periods of time. This study assessed the effects of chronic social instability on corticosterone levels in female mice. Mice that were subjected to social instability showed elevated hair corticosterone compared to baseline levels and as compared to controls. Additionally, when mice were returned to stable social environments, corticosterone levels returned to levels comparable to baseline and to control animals. This suggests that the corticosterone in hair from female mice can serve as a useful biomarker of chronic stress, and that social instability is a sufficient stressor to elicit an extended HPA response.