Isolation rearing in the rat disrupts the hippocampal response to stress

Sumatriptan attenuates fear-learning despair induced by social isolation stress in mice: Mediating role of hypothalamic-pituitary-adrenal axis

OBJECTIVES

Research has demonstrated that chronic stress experienced early in life can lead to impairments in memory and learning. These deficits are attributed to an imbalance in the interaction between glucocorticoids, the end product of the hypothalamic-pituitary-adrenal (HPA) axis, and glucocorticoid receptors in brain regions responsible for mediating memory, such as the hippocampus. This imbalance can result in detrimental conditions like neuroinflammation. The aim of this study was to assess the impact of sumatriptan, a selective agonist for 5-HT 1B/1D receptors, on fear learning capabilities in a chronic social isolation stress model in mice, with a particular focus on the role of the HPA axis.

METHODS

Mice were assigned to two opposing conditions, including social condition (SC) and isolated condition (IC) for a duration of five weeks. All mice underwent passive avoidance test, with their subsequent freezing behavior serving as an indicator of fear retrieval. Mice in the IC group were administered either a vehicle, sumatriptan, GR-127935 (a selective antagonist for 5-HT 1B/1D receptors), or a combination of sumatriptan and GR-127935 during the testing sessions. At the end, all mice were sacrificed and samples of their serum and hippocampus were collected for further analysis.

RESULTS

Isolation was found to significantly reduce freezing behavior (p<0.001). An increase in the freezing response among IC mice was observed following the administration of varying doses of sumatriptan, as indicated by a one-way ANOVA analysis (p<0.001). However, the mitigating effects of sumatriptan were reversed upon the administration of GR-127935. An ELISA assay conducted before and after the passive avoidance test revealed no significant change in serum corticosterone levels among SC mice. In contrast, a significant increase was observed among IC mice, suggesting hyper-responsiveness of the HPA axis in isolated animals. This hyper-responsiveness was ameliorated following the administration of sumatriptan. Furthermore, both the sumatriptan and SC groups exhibited a similar trend, showing a significant increase in the expression of hippocampal glucocorticoid receptors following the stress of the passive avoidance test. Lastly, the elevated production of inflammatory cytokines (TNF-α, IL-1β) observed following social isolation was attenuated in the sumatriptan group.

CONCLUSION

Sumatriptan improved fear learning probably through modulation of HPA axis and hippocampus neuroinflammation.

Epigenetic Mechanism of Early Life Stress-Induced Depression: Focus on the Neurotransmitter Systems

Depression has an alarmingly high prevalence worldwide. A growing body of evidence indicates that environmental factors significantly affect the neural development and function of the central nervous system and then induce psychiatric disorders. Early life stress (ELS) affects brain development and has been identified as a major cause of depression. It could promote susceptibility to stress in adulthood. Recent studies have found that ELS induces epigenetic changes that subsequently affect transcriptional rates of differentially expressed genes. The epigenetic modifications involved in ELS include histone modifications, DNA methylation, and non-coding RNA. Understanding of these genetic modifications may identify mechanisms that may lead to new interventions for the treatment of depression. Many reports indicate that different types of ELS induce epigenetic modifications of genes involved in the neurotransmitter systems, such as the dopaminergic system, the serotonergic system, the gamma-aminobutyric acid (GABA)-ergic system, and the glutamatergic system, which further regulate gene expression and ultimately induce depression-like behaviors. In this article, we review the effects of epigenetic modifications on the neurotransmitter systems in depression-like outcomes produced by different types of ELS in recent years, aiming to provide new therapeutic targets for patients who suffer from depression.

Effects of age and social isolation on murine hippocampal biochemistry and behavior

Social isolation (SI) is a major health risk in older people leading to cognitive decline. This study examined how SI and age influence performance in the novel object recognition (NOR) and elevated plus maze (EPM) tasks in C57BL/6 mice aged 3 or 24 months. Mice were group-housed (groups of 2-3) or isolated for 2 weeks prior to experimentation. Following NOR and EPM testing hippocampal norepinephrine (NE), 5, hydroxytryptamine (5-HT), 5, hydroxyindole acetic acid (5-HIAA), corticosterone (CORT) and interleukin-6 (IL-6) were determined and serum collected for basal CORT analysis. A separate set of mice were exposed to the forced swim test (FST), sacrificed immediately and serum CORT determined. SI impaired performance in the NOR and the FST, reduced hippocampal 5-HT, increased hippocampal IL-6 and increased serum CORT post-FST in young mice. Aged mice either failed to respond significantly to SI (NOR, FST, hippocampal 5-HT, serum CORT post FST) or SI had synergistic effects with age (hippocampal NE, 5-HIAA:5-HT). In conclusion, the lack of response to SI in the aged mice may affect health by preventing them adapting to new stressors, while the synergistic effects of SI with age would increase allostatic load and enhance the deleterious effects of the ageing process.

Silence, Solitude, and Serotonin: Neural Mechanisms Linking Hearing Loss and Social Isolation

For social animals that communicate acoustically, hearing loss and social isolation are factors that independently influence social behavior. In human subjects, hearing loss may also contribute to objective and subjective measures of social isolation. Although the behavioral relationship between hearing loss and social isolation is evident, there is little understanding of their interdependence at the level of neural systems. Separate lines of research have shown that social isolation and hearing loss independently target the serotonergic system in the rodent brain. These two factors affect both presynaptic and postsynaptic measures of serotonergic anatomy and function, highlighting the sensitivity of serotonergic pathways to both types of insult. The effects of deficits in both acoustic and social inputs are seen not only within the auditory system, but also in other brain regions, suggesting relatively extensive effects of these deficits on serotonergic regulatory systems. Serotonin plays a much-studied role in depression and anxiety, and may also influence several aspects of auditory cognition, including auditory attention and understanding speech in challenging listening conditions. These commonalities suggest that serotonergic pathways are worthy of further exploration as potential intervening mechanisms between the related conditions of hearing loss and social isolation, and the affective and cognitive dysfunctions that follow.

Calbindin Deficits May Underlie Dissociable Effects of 5-HT6 and mGlu7 Antagonists on Glutamate and Cognition in a Dual-Hit Neurodevelopmental Model for Schizophrenia

Despite several compounds entering clinical trials for the negative and cognitive symptoms of schizophrenia, few have progressed beyond phase III. This is partly attributed to a need for improved preclinical models, to understand disease and enable predictive evaluation of novel therapeutics. To this end, one recent approach incorporates "dual-hit" neurodevelopmental insults like neonatal phencyclidine plus isolation rearing (PCP-Iso). Glutamatergic dysfunction contributes to schizophrenia pathophysiology and may represent a treatment target, so we used enzyme-based microsensors to evaluate basal- and drug-evoked glutamate release in hippocampal slices from rats that received neonatal PCP and/or isolation rearing. 5-HT₆ antagonist-evoked glutamate release (thought to be mediated indirectly via GABAergic disinhibition) was reduced in PCP-Iso, as were cognitive effects of a 5-HT₆ antagonist in a hippocampal glutamate-dependent novel object discrimination task. Yet mGlu₇ antagonist-evoked glutamatergic and cognitive responses were spared. Immunohistochemical analyses suggest these findings (which mirror the apparent lack of clinical response to 5-HT₆ antagonists in schizophrenia) are not due to reduced hippocampal 5-HT input in PCP-Iso, but may be explained by reduced calbindin expression. This calcium-binding protein is present in a subset of GABAergic interneurons receiving preferential 5-HT innervation and expressing 5-HT₆ receptors. Its loss (in schizophrenia and PCP-Iso) would be expected to reduce interneuron firing and potentially prevent further 5-HT₆ antagonist-mediated disinhibition, without impacting on responses of VIP-expressing interneurons to mGlu₇ antagonism. This research highlights the importance of improved understanding for selection of appropriate preclinical models, especially where disease neurobiology impacts on cells mediating the effects of potential therapeutics.

Environmental Enrichment and Physical Exercise Attenuate the Depressive-Like Effects Induced by Social Isolation Stress in Rats

We assessed the antidepressant-like effects of environmental enrichment (EE) and physical exercise (PE) compared with the selective serotonin reuptake inhibitor fluoxetine against the depression-related neurobehavioral alterations induced by postweaning social isolation (SI) in rats. After 1 month of SI, rats were submitted to PE (treadmill), EE, or fluoxetine (10 mg/kg), which were compared with naïve SI and group-housed rats. After 1 month, behavior was analyzed in the open field (OFT), the sucrose preference (SPT), and the forced swimming (FST) tests. Afterward, the hippocampal serotonin contents, its metabolite, and turnover were measured. SI induced a depression-related phenotype characterized by a marginal bodyweight gain, anxiety, anhedonia, behavioral despair, and alterations of serotonin metabolism. EE produced the widest and largest antidepressive-like effect, followed by PE and fluoxetine, which were almost equivalent. The treatments, however, affected differentially the neurobehavioral domains investigated. EE exerted its largest effect on anhedonia and was the only treatment inducing anxiolytic-like effects. Fluoxetine, in contrast, produced its largest effect on serotonin metabolism, followed by its anti-behavioral despair action. PE was a middle-ground treatment with broader behavioral outcomes than fluoxetine, but ineffective to reverse the serotonergic alterations induced by SI. The most responsive test to the treatments was the FST, followed closely by the SPT. Although OFT locomotion and body weight varied considerably between groups, they were barely responsive to PE and fluoxetine. From a translational standpoint, our data suggest that exercise and recreational activities may have broader health benefits than antidepressants to overcome confinement and the consequences of chronic stress.

Long-Term Behavioral Effects of Post-weaning Social Isolation in Males and Females

Adolescence is a developmental period associated with vast neural and behavioral changes which are accompanied by altered sensitivity to stimuli, both stressful and rewarding. Perturbations, especially stressful stimuli, during this period have been shown to alter behavior in adulthood. Social isolation rearing is one such perturbation. This review highlights the long-term behavioral consequences of adolescent social isolation rearing in rodents with a specific focus on anxiety- and addiction-related behaviors. Sex-specific effects are discussed where data are available. We then consider changes in monoaminergic neurotransmission as one possible mechanism for the behavioral effects described. This research on both normative and perturbed adolescent development is crucial to understanding and treating the increased vulnerability to psychiatric disorders seen in humans during this life stage.

Social Isolation Rearing Induces Neuropsychiatric Diseases: Updated Overview

Neuropsychiatric and neurologic diseases cause a great burden for individuals, families, and societies. Social isolation rearing can trigger a variety of psychiatric diseases. New advances suggest that epigenetic factors along with other neurochemical changes can be an important topic in neuropsychiatric diseases. It is thought that the prevention of social isolation rearing that occurs around birth can reduce the occurrence of neuropsychiatric diseases. It has been suggested that the environment can induce epigenetic alternation. So, for the diagnosis of a proportion of neuropsychiatric diseases, assessing epigenetic factors may be helpful. Also, apart from epigenetic factors, new advances have been made about new mechanisms of and treatments for such a disorder.

Transcriptomic analysis reveals oxidative phosphorylation activation in an adolescent social isolation rat model

Complex interactions between genetic and environmental factors exert a sustained influence on the pathogenesis of schizophrenia (SCZ). Adolescent social isolation is regarded as a typical paradigm for SCZ. However, the underlying pathological mechanisms are not fully understood. In this study, adolescent Sprague-Dawley (SD) rats were placed in isolation rearing (IR) or social rearing (SR) conditions from postnatal day (PND) 21 to 34 to establish a SCZ disease model and a control model, respectively. Prepulse inhibition (PPI) assays and elevated plus maze tests were performed on PND 56. Next, prefrontal cortex (PFC) tissues were isolated for transcriptomic sequencing and RT-qPCR analyses. The results indicated that adolescent social isolation induced anxious behaviors and disrupted PPIs as well as specific PFC gene expression patterns in adult SD rats. A total of 196 genes were identified as upregulated, and 748 genes were identified as down-regulated in the IR group compared with those in the SR group. Differentially expressed genes (DEGs) were highly enriched in the KEGG pathways associated with the comorbidity of neurological disorder and oxidative phosphorylation (OXPHOS); 26 out of 27 comorbid neurological disorder-associated DEGs overlapped with 31 OXPHOS-associated DEGs. Those 26 overlapping DEGs were all upregulated in the IR group and could easily distinguish the IR group from the SR group; 6 of these DEGs (COX7C, NDUFB11, NDUFA2, NDUFC2, ATP5C1, and COX6A1) were verified by RT-qPCR. Here, we provide a systematic overview of gene expression alterations in adolescent-social-isolation-induced SCZ (ASI-SCZ), which suggests that genes that are associated with the comorbidity of neurological disorders, especially OXPHOS-related genes, contribute to the pathogenesis of ASI-SCZ.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Post-weaning social isolation of rats leads to long-term disruption of the gut microbiota-immune-brain axis

Early-life stress is an established risk for the development of psychiatric disorders. Post-weaning isolation rearing of rats produces lasting developmental changes in behavior and brain function that may have translational pathophysiological relevance to alterations seen in schizophrenia, but the underlying mechanisms are unclear. Accumulating evidence supports the premise that gut microbiota influence brain development and function by affecting inflammatory mediators, the hypothalamic-pituitaryadrenal axis and neurotransmission, but there is little knowledge of whether the microbiota-gut-brain axis might contribute to the development of schizophrenia-related behaviors. To this end the effects of social isolation (SI; a well-validated animal model for schizophrenia)-induced changes in rat behavior were correlated with alterations in gut microbiota, hippocampal neurogenesis and brain cytokine levels. Twenty-four male Lister hooded rats were housed in social groups (group-housed, GH, 3 littermates per cage) or alone (SI) from weaning (post-natal day 24) for four weeks before recording open field exploration, locomotor activity/novel object discrimination (NOD), elevated plus maze, conditioned freezing response (CFR) and restraint stress at one week intervals. Post-mortem caecal microbiota composition, cortical and hippocampal cytokines and neurogenesis were correlated to indices of behavioral changes. SI rats were hyperactive in the open field and locomotor activity chambers traveling further than GH controls in the less aversive peripheral zone. While SI rats showed few alterations in plus maze or NOD they froze for significantly less time than GH following conditioning in the CFR paradigm, consistent with impaired associative learning and memory. SI rats had significantly fewer BrdU/NeuN positive cells in the dentate gyrus than GH controls. SI rats had altered microbiota composition with increases in Actinobacteria and decreases in the class Clostridia compared to GH controls. Differences were also noted at genus level. Positive correlations were seen between microbiota, hippocampal IL-6 and IL-10, conditioned freezing and open field exploration. Adverse early-life stress resulting from continuous SI increased several indices of 'anxiety-like' behavior and impaired associative learning and memory accompanied by changes to gut microbiota, reduced hippocampal IL-6, IL-10 and neurogenesis. This study suggests that early-life stress may produce long-lasting changes in gut microbiota contributing to development of abnormal neuronal and endocrine function and behavior which could play a pivotal role in the aetiology of psychiatric illness.

Social experience alters socially induced serotonergic fluctuations in the inferior colliculus

Past social experience and current social context shape the responses of animals to social signals. The serotonergic system is one potential mechanism by which both experiential and contextual factors could be conveyed to sensory systems, such as the auditory system, for multiple reasons. 1) Many features of the serotonergic system are sensitive to social experience. 2) Elevations in serotonergic activity are triggered by social partners, and variations in socially triggered serotonergic responses reflect behavioral differences among social encounters. 3) Serotonin is an auditory neuromodulator, altering how auditory neurons respond to sounds including conspecific vocalizations. In this study, we tested how social experience influences the socially triggered serotonergic response in the inferior colliculus, an auditory midbrain region with an important role in vocalization processing. We used carbon fiber voltammetry to measure serotonin during social interactions of male mice ( Mus musculus) from different social backgrounds: 4 weeks of grouped or individual housing. When paired with an unfamiliar male, both group-housed and individually housed males demonstrated elevations in serotonin; however, individually housed males exhibited socially triggered serotonergic responses with delayed time courses compared with the group-housed males. Furthermore, group-housed males displayed previously described correlations between the socially triggered serotonergic response and behaviors such as social investigation. In contrast, individually housed males did not show these serotonin-behavior relationships. These results suggest that social experience gained via social housing may shape the ability of the central serotonergic system to encode social context in sensory regions.

NEW & NOTEWORTHY We demonstrate that past social experience influences the fidelity with which the serotonergic system represents social context in an auditory region. Social experience altered the time course of socially triggered serotonergic responses and changed how the serotonergic system reflects behavioral variations among social encounters of the same context. These findings are significant to the study of communication, suggesting that centralized neuromodulatory systems potentially convey integrated information regarding past experience and current context to primary sensory regions.

Early-Life Social Isolation-Induced Depressive-Like Behavior in Rats Results in Microglial Activation and Neuronal Histone Methylation that Are Mitigated by Minocycline

Early-life stress is a potent risk factor for development of psychiatric conditions such as depression. The underlying mechanisms remain poorly understood. Here, we used the early-life social isolation (ESI) model of early-life stress in rats to characterize development of depressive-like behavior, the role of microglia, levels of histone methylation, as well as expression of glutamate receptor subunits in the hippocampus. We found that depressive-like behavior was induced after ESI as determined by sucrose preference and forced swimming tests. Increased expression of microglial activation marker, Iba1, was observed in the hippocampus of the ESI group, while expression of the microglial CD200 receptor, which promotes microglial quiescence, significantly decreased. In addition, increased levels of proinflammatory cytokines, interleukin 1β (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor-alpha (TNF-α) were observed in the hippocampus of the ESI group. Moreover, ESI increased levels of neuronal H3K9me2 (a repressive marker of transcription) and its associated "writer" enzymes, G9a and G9a-like protein, in the hippocampus. ESI also decreased expression of hippocampal NMDA receptor subunits, NR1, and AMPA receptor subunits, GluR1 and GluR2, which are involved in synaptic plasticity, but it did not affect expression of PSD95 and NR2B. Interestingly, treatment with minocycline to block microglial activation induced by ESI inhibited increases in hippocampal microglia and prevented ESI-induced depressive-like behavior as well as increases in IL-1β, IL-6, and TNF-α. Notably, minocycline also triggered downregulation of H3K9me2 expression and restored expression of NR1, GluR1, and GluR2. These results suggest that ESI induces depressive-like behavior, which may be mediated by microglial signaling.

Social Isolation Modulates CLOCK Protein and Beta-Catenin Expression Pattern in Gonadotropin-Inhibitory Hormone Neurons in Male Rats

Postweaning social isolation reduces the amplitude of the daily variation of CLOCK protein in the brain and induces lower reproductive activity. Gonadotropin-inhibitory hormone (GnIH) acts as an inhibitor in the reproductive system and has been linked to stress. Social isolation has been shown to lower neuronal activity of GnIH-expressing neurons in the dorsomedial hypothalamus (DMH). The exact mechanism by which social isolation may affect GnIH is still unclear. We investigated the impact of social isolation on regulatory cellular mechanisms in GnIH neurons. We examined via immunohistochemistry the expression of CLOCK protein at four different times throughout the day in GnIH cells tagged with enhanced fluorescent green protein (EGFP-GnIH) in 9-week-old adult male rats that have been raised for 6 weeks under postweaning social isolation and compared them with group-raised control rats of the same age. We also studied the expression of β-catenin-which has been shown to be affected by circadian proteins such as Bmal1-in EGFP-GnIH neurons to determine whether it could play a role in linking CLOCK in GnIH neurons. We found that social isolation modifies the pattern of CLOCK expression in GnIH neurons in the DMH. Socially isolated rats displayed greater CLOCK expression in the dark phase, while control rats displayed increased CLOCK expression in the light phase. Furthermore, β-catenin expression pattern in GnIH cells was disrupted by social isolation. This suggests that social isolation triggers changes in CLOCK and GnIH expression, which may be associated with an increase in nuclear β-catenin during the dark phase.

Chronic social isolation reduces 5-HT neuronal activity via upregulated SK3 calcium-activated potassium channels

The activity of serotonin (5-HT) neurons is critical for mood regulation. In a mouse model of chronic social isolation, a known risk factor for depressive illness, we show that 5-HT neurons in the dorsal raphe nucleus are less responsive to stimulation. Probing the responsible cellular mechanisms pinpoints a disturbance in the expression and function of small-conductance Ca²⁺-activated K⁺ (SK) channels and reveals an important role for both SK2 and SK3 channels in normal regulation of 5-HT neuronal excitability. Chronic social isolation renders 5-HT neurons insensitive to SK2 blockade, however inhibition of the upregulated SK3 channels restores normal excitability. In vivo, we demonstrate that inhibiting SK channels normalizes chronic social isolation-induced anxiety/depressive-like behaviors. Our experiments reveal a causal link for the first time between SK channel dysregulation and 5-HT neuron activity in a lifelong stress paradigm, suggesting these channels as targets for the development of novel therapies for mood disorders.

Disruptions of sensorimotor gating, cytokines, glycemia, monoamines, and genes in both sexes of rats reared in social isolation can be ameliorated by oral chronic quetiapine administration

The pathogenesis of schizophrenia in patients with metabolic abnormalities remains unclear. Our previous study demonstrated that isolation rearing (IR) induced longitudinal concomitant changes of pro-inflammatory cytokine (pro-CK) levels and metabolic abnormalities with a developmental origin. However, the general consensus, believes that these abnormalities are caused by antipsychotic treatment in schizophrenic patients. The IR paradigm presents with face, construct, and predictive validity for schizophrenia. Therefore, we employed IR rats of both sexes to examine whether chronic quetiapine (QTP, a second-generation antipsychotic medication) treatment induces disruptions of metabolism (body weight, blood pressure, and the glycemic and lipid profiles) or cytokines [interleukin (IL)-1 beta, IL-6, IL-10, interferon-gamma, and tumor necrosis factor (TNF)-alpha], and further, whether it reverses deficits of behaviors [locomotor activity and prepulse inhibition (PPI)] and the expression of monoamines (dopamine and serotonin) and related genes (Htr1a, Htr2a, Htr3a, Drd1a, and Gabbr2). IR induced higher levels of pro-CK, dysglycemia, blood pressure, locomotor activity, and impaired PPI, simultaneously destabilizing cortico-striatal monoamines and relevant genes in both sexes, while QTP demonstrated dose-dependent reversal of these changes, suggesting that QTP might reduce the pro-CKs to regulate these abnormalities. Our data implied that antipsychotics may not be the solitary factor causing metabolic problems in schizophrenia and suggested that inflammatory changes may play a vital role in the developmental pathophysiology of schizophrenia and related metabolic abnormalities.

Individual and combined effects of physical exercise and methylphenidate on orienting behavior and social interaction in spontaneously hypertensive rats

This study determined the duration of exercise and amount of methylphenidate that is needed to affect attentional function and social behavior in spontaneously hypertensive rats (SHR), a commonly used animal model of Attention-Deficit/Hyperactivity Disorder (ADHD). Attention was assessed by measuring the orienting response to repeated presentations of a nonreinforced visual cue. Social behavior was examined by allowing rats to freely explore a large arena containing an unfamiliar conspecific rat. Consistent with their hyper-responsive phenotype, nonexercising SHRs exhibited a high level of orienting behavior and little habituation, as well as hyper-social behavior compared with normo-active rats. Exercise or methylphenidate decreased orienting behavior and social behavior in a dose-dependent fashion. In addition, we found an additive effect of combining doses of exercise and methylphenidate that alone were ineffective in altering behavior. These data indicate that physical exercise and methylphenidate can reduce hyper-responsiveness to irrelevant stimuli and reduce hyper-social behavior in SHR. Moreover, subthreshold doses of methylphenidate can be used in combination with moderate amounts of exercise to reduce distractibility, supporting the notion that exercise may be useful as an adjunctive or replacement therapy in ADHD.

Effects of Sini San used alone and in combination with fluoxetine on central and peripheral 5-HT levels in a rat model of depression

OBJECTIVE

To investigate the effects of Sini San and fluoxetine on the levels of central and peripheral 5-HT in a rat model of depression, and provide new insight into the treatment of depression with integrated Chinese-Western Medicine.

METHODS

A rat model of depression was established by chronic mild stress (CMS). Model rats received either Sini San, fluoxetine, a combination of the two drugs, or no drug treatment. Healthy naive rats were used as controls. Open field and sucrose preference tests were used to assess depression-like behavior. ELISA and immunohistochemistry were used to determine central and peripheral levels of 5-HT.

RESULTS

In the group with no drug treatment, central 5-HT expression decreased while peripheral 5-HT concentrations increased as CMS continued. Four weeks after CMS, Sini San alone was less effective in reducing depression-like behavior than fluoxetine alone or in combination with Sini San, but combined use was more effective than fluoxetine alone. Eight weeks after CMS, Sini San alone or in combination with fluoxetine was more effective in reducing depression-like behavior than fluoxetine alone. Furthermore Sini San and fluoxetine used alone or in combination notably increased central 5-HT expression and decreased peripheral 5-HT levels in the rat model.

CONCLUSION

The results of the present study indicate that there is a synergistic action between the two medicines in the treatment of depression. Sini San exhibited a relatively long lag before its effects were observed; however, by eight weeks the Traditional Chinese Medicine appeared at least as effective as fluoxetine. We suggest that Sini San can replace fluoxetine in the later stages of depression treatment to minimize side effects observed with long-term fluoxetine administration.

Animal Models of Psychosis: Current State and Future Directions

Psychosis is an abnormal mental state characterized by disorganization, delusions and hallucinations. Animal models have become an increasingly important research tool in the effort to understand both the underlying pathophysiology and treatment of psychosis. There are multiple animal models for psychosis, with each formed by the coupling of a manipulation and a measurement. In this manuscript we do not address the diseases of which psychosis is a prominent comorbidity. Instead, we summarize the current state of affairs and future directions for animal models of psychosis. To accomplish this, our manuscript will first discuss relevant behavioral and electrophysiological measurements. We then provide an overview of the different manipulations that are combined with these measurements to produce animal models. The strengths and limitations of each model will be addressed in order to evaluate its cross-species comparability.

Environment and Brain Plasticity: Towards an Endogenous Pharmacotherapy

Brain plasticity refers to the remarkable property of cerebral neurons to change their structure and function in response to experience, a fundamental theoretical theme in the field of basic research and a major focus for neural rehabilitation following brain disease. While much of the early work on this topic was based on deprivation approaches relying on sensory experience reduction procedures, major advances have been recently obtained using the conceptually opposite paradigm of environmental enrichment, whereby an enhanced stimulation is provided at multiple cognitive, sensory, social, and motor levels. In this survey, we aim to review past and recent work concerning the influence exerted by the environment on brain plasticity processes, with special emphasis on the underlying cellular and molecular mechanisms and starting from experimental work on animal models to move to highly relevant work performed in humans. We will initiate introducing the concept of brain plasticity and describing classic paradigmatic examples to illustrate how changes at the level of neuronal properties can ultimately affect and direct key perceptual and behavioral outputs. Then, we describe the remarkable effects elicited by early stressful conditions, maternal care, and preweaning enrichment on central nervous system development, with a separate section focusing on neurodevelopmental disorders. A specific section is dedicated to the striking ability of environmental enrichment and physical exercise to empower adult brain plasticity. Finally, we analyze in the last section the ever-increasing available knowledge on the effects elicited by enriched living conditions on physiological and pathological aging brain processes.

Involvement of spinal 5-HT1A receptors in isolation rearing-induced hypoalgesia in mice

RATIONALE

Isolation rearing in rodents causes not only abnormal behaviors which resemble the clinical symptoms of schizophrenia but also hypoalgesia in thermal nociception models. However, the mechanism of the hypoalgesia is not known.

OBJECTIVES

The present study investigated the effect of isolation rearing on acute pain and the descending pain inhibitory pathways in mice.

RESULTS

Rearing in isolation for 6 weeks from post-weaning reduced pain sensitivity in the hot plate test and acetic acid-induced writhing test. Isolation rearing also reduced the intraplantar capsaicin-induced licking behavior. Capsaicin increased c-Fos expression, a neuronal activity marker, in the spinal cord and primary somatosensory cortex both in group- and isolation-reared mice, but this effect did not differ between groups. On the other hand, c-Fos expression in the anterior cingulate cortex, periaqueductal gray matter, and rostral ventromedial medulla, but not in the spinal cord or somatosensory cortex, was enhanced by isolation rearing. Systemic administration of WAY100635 (serotonin (5-HT)1A receptor antagonist), but not of ketanserin (5-HT2 receptor antagonist), prazosin (α1-adrenoceptor antagonist), or yohimbine (α2-adrenoceptor antagonist), attenuated isolation rearing-induced hypoalgesia in capsaicin-induced licking behavior. Attenuation of isolation rearing-induced hypoalgesia was also observed following the intrathecal injection of WAY100635. Naloxone, an opioid receptor antagonist, did not affect the hypoalgesia in isolation-reared mice.

CONCLUSIONS

These findings suggest that isolation rearing causes hypoalgesia in mouse models of acute pain and imply that the spinal 5-HT1A receptor activation probably through descending serotonergic inhibitory pathway is involved in isolation rearing-induced hypoalgesia.

Have studies of the developmental regulation of behavioral phenotypes revealed the mechanisms of gene-environment interactions?

This review addresses the recent convergence of our long-standing knowledge of the regulation of behavioral phenotypes by developmental experience with recent advances in our understanding of mechanisms regulating gene expression. This review supports a particular perspective on the developmental regulation of behavioral phenotypes: That the role of common developmental experiences (e.g. maternal interactions, peer interactions, exposure to a complex environment, etc.) is to fit individuals to the circumstances of their lives within bounds determined by long-standing (evolutionary) mechanisms that have shaped responses to critical and fundamental types of experience via those aspects of gene structure that regulate gene expression. The phenotype of a given species is not absolute for a given genotype but rather variable within bounds that is determined by mechanisms regulated by experience (e.g. epigenetic mechanisms). This phenotypic variation is not necessarily random, or evenly distributed along a continuum of description or measurement, but often highly disjointed, producing distinct, even opposing, phenotypes. The potentiality for these varying phenotypes is itself the product of evolution, the potential for alternative phenotypes itself conveying evolutionary advantage. Examples of such phenotypic variation, resulting from environmental or experiential influences, have a long history of study in neurobiology, and a number of these will be discussed in this review: neurodevelopmental experiences that produce phenotypic variation in visual perception, cognitive function, and emotional behavior. Although other examples will be discussed, particular emphasis will be made on the role of social behavior on neurodevelopment and phenotypic determination. It will be argued that an important purpose of some aspects of social behavior is regulation of neurobehavioral phenotypes by experience via genetic regulatory mechanisms.

Influence of social isolation in the rat on serotonergic function and memory--relevance to models of schizophrenia and the role of 5-HT₆ receptors

There is increasing awareness of the importance that early environmental factors have on brain development and their role in the neurobiology of neurodevelopmental disorders including schizophrenia. The isolation reared rat attempts to model adverse effects that human social isolation (absence of social contact) can have on normal brain development. The isolation reared rat also models aspects of schizophrenia including the development of persistent learning and memory deficits. This short review concentrates on the effects of isolation rearing on cognition, including deficits in novel object discrimination, and the neural mechanisms that may underlie this impairment. There is evidence that a key effect of social isolation may be loss of neuronal plasticity combined with change in the functional state of various cortical and hippocampal neurotransmitters, including glutamate and serotonin. Reduced glutamate function may underlie the deficits in novel object discrimination, which can be reversed by administration of a 5-HT(6) receptor antagonist. This suggests that the 5-HT(6) antagonists may act by reducing 5-HT(6) receptor mediated activation of GABA, resulting in glutamate disinhibition. Thus drugs acting at 5-HT(6) receptors may offer a novel approach to treat neurodevelopmental cognitive symptoms, including those seen in schizophrenia.

Effects of physical exercise on ADHD-like behavior in male and female adolescent spontaneously hypertensive rats

The present study examined the effects of exercising (voluntary wheel running) during adolescence on attentional function in male and female spontaneously hypertensive rats (SHRs), a commonly used animal model of attention-deficit/hyperactivity disorder (ADHD). Once rats reached adulthood, they received one session in which a light was presented 12 times but not reinforced, followed by training sessions in which the light was paired with a food reward. Male and female SHRs that had access to running wheels exhibited levels of unconditioned orienting behavior that were similar to Wistar-Kyoto rats (normo-active control) while SHRs that did not have access to running wheels exhibited higher levels of unconditioned orienting behavior. When the light was later paired with food there were no differences between the groups of male rats, but exercising female SHRs exhibited a decrease in conditioned food cup behavior. Consistent with their established phenotype, SHR rats exhibited more locomotor activity during an open field exploration session than WKY rats, but there was no relationship between orienting behavior and locomotor activity. Together these data suggest that physical exercise during adolescence can benefit attentional capabilities.

The serotonergic projection from the median raphe nucleus to the ventral hippocampus is involved in the retrieval of fear memory through the corticotropin-releasing factor type 2 receptor

Several different studies have separately established that serotonin, corticotropin-releasing factor (CRF) receptors, and the hippocampus are involved in fear memory retrieval. The main aim of this study is to connect these separate studies. To assess the levels of anxiety/fear, we used the contextual fear-conditioning test and the elevated plus maze test as memory-dependent and memory-independent tasks, respectively. We injected CRF receptor antagonists or vehicle into the median raphe nucleus (MRN) 10 min before behavioral tests. As a result, 1000 ng of astressin 2B (CRF(2) receptor antagonist), but not 250 ng of antalarmin (CRF(1) receptor antagonist), significantly suppressed the expression rate of freezing behavior in the contextual fear-conditioning test. However, in the elevated plus maze test, there was no difference between astressin 2B-injected rats and saline-injected rats in the time spent in open arms. Neither the amount of exploratory behavior nor the moving distance in the EPM of astressin 2B-injected rats differed from that of vehicle-injected rats. Moreover, when we assessed the extracellular serotonin release in the ventral hippocampus in freely moving rats through in vivo microdialysis, it was shown that the blockade of the CRF(2) receptor in the MRN suppressed serotonin release in the ventral hippocampus during fear memory retrieval. These results indicated that endogenous CRF and/or related ligands that were released in the MRN could activate the CRF(2) receptor and stimulate serotonin release in the ventral hippocampus, thereby inducing fear memory retrieval.

Effects of fluoxetine and PCPA on isolation-induced morphine self-administration and startle reactivity

The present study investigated the effects of the SSRI fluoxetine and the serotonin synthesis blocker--parachlorophenylalanine (PCPA) on morphine self-administration and startle reactivity in rats subjected to social isolation during adulthood. Adult Wistar rats were housed individually or in pairs for 21days. They were treated with fluoxetine, PCPA, or vehicle and tested for their startle response and intake of a morphine solution (0.5mg/ml). Socially restricted rats consumed significantly more morphine solution (but not water) than rats living in pairs, in both one-bottle and in two-bottle tests. They also showed significantly higher startle response amplitude. Daily fluoxetine treatment (5mg/kg i.p.) counteracted these behavioral alterations induced by isolation housing while PCPA treatment (200mg/kg for 3 consecutive days) further exacerbated it. Social isolation may increase morphine self-administration and emotional reactivity in the startle box by affecting serotonin. Antidepressants (such as fluoxetine) may normalize or stabilize serotonin function and restore the behavioral changes produced by isolation.

Organic cation transporter inhibition increases medial hypothalamic serotonin under basal conditions and during mild restraint

The dorsomedial hypothalamus (DMH) has been implicated in the coordination of stress responses. Restraint stress or systemic corticosterone (CORT) treatment induces a rapid increase in tissue concentrations of serotonin (5-hydroxytryptamine; 5-HT) in the DMH. Although the mechanism for rapid changes in 5-HT concentrations in the DMH is not clear, earlier results suggest that stress-induced increases in CORT may inhibit 5-HT transport from the extracellular fluid by acting on corticosterone-sensitive organic cation transporters (OCTs). We tested the hypothesis that perfusion of the medial hypothalamus (MH), which includes the DMH, with the OCT blocker decynium 22 (D-22) would potentiate the effects of mild restraint on extracellular 5-HT. Male Sprague-Dawley rats, implanted with a microdialysis probe into the MH, were treated with reverse-dialysis of D-22 (20 microM; 40 min) or vehicle and subjected to either 40 min mild restraint or undisturbed control conditions. Perfusates collected from a separate group of rats were evaluated for the effect of restraint on extracellular CORT concentrations in the MH. Reverse-dialysis of D-22 induced an increase (200%) in extracellular 5-HT concentrations in the MH in undisturbed control rats. Restraint in the absence of D-22 did not significantly affect MH CORT or 5-HT concentrations. However, perfusion of the MH with D-22 during restraint led to an increased magnitude and duration of extracellular 5-HT concentrations, relative to D-22 by itself. These results are consistent with the hypothesis that OCTs in the DMH contribute to the clearance of 5-HT from the extracellular fluid under both baseline conditions and mild restraint.

Sex-dependent alterations in response to maternal deprivation in rats

We review here our latest results regarding short- and long-term effects of a neonatal maternal deprivation (MD) stress [24h at postnatal day (PND) 9] on diverse psychoneuroimmunoendocrine parameters, pointing out the existence of numerous sexual dimorphisms. Behavioral changes observed in MD animals might be at least in part attributable to neurodevelopmental effects of MD-induced elevated corticosterone levels. Our findings of short-term effects of MD on hippocampal and cerebellar neurons and glial cells appear to support this hypothesis. However, it is important to note that these cellular effects were more marked in males than in females. Moreover, in analyzing the effects of this neonatal stress on the endocannabinoid system (hippocampal endocannabinoid levels and CB1 receptors) we have also found that males were more affected by MD. Since all these sexual dimorphisms were found at an early neonatal age (PND 13), they are attributable to organizational effects of gonadal steroids. We discuss the potential implications of the elevated corticosterone and decreased leptin levels shown by MD animals in their diverse functional alterations, including the above mentioned neural effects as well as the intriguing persistent deficit in their immunological system. We also emphasize the necessity of analyzing the important influence of sex as regards the specific consequences of early life stress.

Chronic fluoxetine differentially modulates the hippocampal microtubular and serotonergic system in grouped and isolation reared rats

Social isolation from weaning in rats produces behavioural and hippocampal structural changes at adulthood. Here, rats were group or isolation reared for eight-weeks. Following the initial four-week period of rearing, fluoxetine (10 mg/kg i.p.) was administered for 28 days. Changes in recognition memory, hippocampal monoamines, and cytoskeletal microtubules were investigated. Isolation-rearing for four- or eight-weeks produced recognition memory deficits that were not reversed by fluoxetine. Eight-weeks of isolation decreased alpha-tubulin acetylation (Acet-Tub) and the tyrosinated/detyrosinated alpha-tubulin ratio (Tyr/Glu-Tub), suggesting major alterations in microtubule dynamics and neuronal plasticity. In grouped rats, fluoxetine decreased Acet-Tub without changes in Tyr/Glu-Tub. In isolates, fluoxetine did not affect Acet-Tub but increased Tyr/Glu-Tub. Finally, fluoxetine altered serotonin metabolism in grouped, but not in isolated animals. Therefore, isolation-rearing changes the hippocampal responses of the serotonergic and microtubular system to fluoxetine. These findings show that early-life experience induces behavioural changes paralleled by alterations in cytoskeletal and neurochemical functions.

Early life stress, the development of aggression and neuroendocrine and neurobiological correlates: what can we learn from animal models?

Early life stress (child and adolescent abuse, neglect and trauma) induces robust alterations in emotional and social functioning resulting in enhanced risk for the development of psychopathologies such as mood and aggressive disorders. Here, an overview is given on recent findings in primate and rodent models of early life stress, demonstrating that chronic deprivation of early maternal care as well as chronic deprivation of early physical interactions with peers are profound risk factors for the development of inappropriate aggressive behaviors. Alterations in the hypothalamic-pituitary-adrenocortical (HPA), vasopressin and serotonin systems and their relevance for the regulation of aggression are discussed. Data suggest that social deprivation-induced inappropriate forms of aggression are associated with high or low HPA axis (re)activity and a generally lower functioning of the serotonin system in adulthood. Moreover, genetic and epigenetic modifications in HPA and serotonin systems influence the outcome of early life stress and may even moderate adverse effects of early social deprivation on aggression. A more comprehensive study of aggression, neuroendocrine, neurobiological and (epi)genetic correlates of early life stress using animal models is necessary to provide a better understanding of the invasive aggressive deficits observed in humans exposed to child maltreatment.

Early maternal deprivation induces gender-dependent changes on the expression of hippocampal CB(1) and CB(2) cannabinoid receptors of neonatal rats

Early maternal deprivation (MD) in rats (24 h, postnatal day 9-10) is a model for neurodevelopmental stress. There are some data proving that MD affects the endocannabinoid system (ECS) in a gender-dependent manner, and that these changes may account for the proposed schizophrenia-like phenotype of MD rats. The impact of MD on cannabinoid receptor distribution in the hippocampus is unknown. The aim of this study is to evaluate the expression of CB(1) and CB(2) receptors in diverse relevant subregions (DG, CA1, and CA3) of the hippocampus in 13-day-old rats by immunohistochemistry and densitometry. MD induced a significant decrease in CB(1) immunoreactivity (more marked in males than in females), which was mainly associated with fibers in the strata pyramidale and radiatum of CA1 and in the strata oriens, pyramidale, and radiatum of CA3. In contrast, MD males and females showed a significant increase in CB(2) immunoreactivity in the three hippocampal areas analyzed that was detected in neuropil and puncta in the stratum oriens of CA1 and CA3, and in the polymorphic cell layer of the dentate gyrus. A marked sex dimorphism was observed in CA3, with females exhibiting higher CB(1) immunoreactivity than males, and in dentate gyrus, with females exhibiting lower CB(2) immunoreactivity than males. These results point to a clear association between developmental stress and dysregulation of the ECS. The present MD procedure may provide an interesting experimental model to further address the role of the ECS in neurodevelopmental mental illnesses such as schizophrenia.

Consequences of post-weaning social isolation on anxiety behavior and related neural circuits in rodents

Exposure to adverse experiences in early-life is implicated in the later vulnerability to development of psychiatric disorders, including anxiety and affective disorders in humans. Adverse early-life experiences likely impart their long-term consequences on mental health by disrupting the normal development of neural systems involved in stress responses, emotional behavior and emotional states. Neural systems utilizing the neurotransmitters serotonin, dopamine and the neuropeptide corticotropin-releasing factor (CRF) are implicated in mediating emotive behaviors, and dysfunction of these neurochemical systems is associated with mood/anxiety disorders. These neural systems continue maturing until early or mid-adolescence in humans, thus alterations to their development are likely to contribute to the long-term consequences of adverse early-life experiences. A large body of literature suggests that post-weaning isolation rearing of rodents models the behavioral consequences of adverse early-life experiences in humans. Overall, the majority findings suggest that post-weaning social isolation that encompasses pre-adolescence produces long-lasting alterations to anxiety behavior, while measures of monoaminergic activity in various limbic regions during social isolation suggest alterations to dopamine and serotonin systems. The goal of this review is to evaluate and integrate findings from post-weaning social isolation studies specifically related to altered fear and anxiety behaviors and associated changes in neuroendocrine function and the activity of monoaminergic systems.

Neonatal maternal separation affects endocrine and metabolic stress responses to ether exposure but not to restraint exposure in adult rats

We investigated prolactin secretion and metabolic changes in stress response in adult male rats submitted to periodic maternal separation (MS; 180 min/day) at 2 weeks of life. Restraint and ether exposure were randomly performed when the animals were 10-12 weeks of age. Restraint exposure: the animals were placed into plastic tubes (21 cm long, 4.5 cm diameter) for 20 min. Ether exposure: the rats were exposed to ether for 10 min. Atrial cannulation for blood sampling was performed through the jugular vein 5 days before the experiments. In both protocols, blood samples were taken immediately before (0), and 5, 15 and 20 min after the beginning of stress exposure. Ours results showed attenuated endocrine and metabolic responses to ether exposure in the maternal separation (MS) group compared to the control group. The measured metabolic parameters, plasma glucose, prolactin, lactate, and insulin secretion, were 32%, 55%, 41%, 73% lower (P < 0.01), respectively, in MS than in control animals. On the other hand, the endocrine and metabolic stress responses to restraint exposure were not affected by maternal separation. There was no difference between the MS and the control groups in any of the parameters studied. Our data demonstrated that early life experiences affect the hormonal systems beyond the hypothalamic-pituitary-adrenal axis, such as the central neuronal pathways, and their activities related to hormonal and metabolic responses to stress in adulthood. More importantly, these modifications were specific, but dependent on stress situation affecting mainly the circuitry related to the stress response to ether exposure.

Effect of social isolation on CB1 and D2 receptor and fatty acid amide hydrolase expression in rats

Rearing rats in isolation has been shown to produce behavioral and neurochemical alterations similar to those observed in psychoses such as schizophrenia. Also, a dysregulation in both the endocannabinoid and dopaminergic systems has been implicated in schizophrenia. The aim of this study was to determine if there are differences in CB1 receptor and fatty acid amide hydrolase (FAAH) protein expression, as well as D2 dopamine receptor expression in different brain regions in rats reared in different environmental conditions. Twenty-one-day-old male Sprague-Dawley rats were either reared in individual cages (isolated rats) or in group cages of six per cage (group-housed rats) for 8 weeks. Quantitative fluorescence immunohistochemistry was performed on brain slices using antibodies specific to the CB1 or D2 receptor, or the enzyme FAAH. Raising rats in isolation led to a significant decrease in CB1 receptor expression in the caudate putamen and the amygdala, a significant increase in FAAH expression in the caudate putamen and the nucleus accumbens core and shell, and no significant change in D2 receptor expression in any region studied. These results indicate that the endocannabinoid system is altered in an animal model of aspects of psychosis. This implies that rearing rats under different housing conditions may provide new insight into the role of the endocannabinoid system in the development of psychoses.

The postweaning social isolation in C57BL/6 mice: preferential vulnerability in the male sex

INTRODUCTION

Social deprivation during early life can severely affect mental health later in adulthood, leading to the development of behavioural traits associated with several major psychiatric disorders including schizophrenia. This has led to the application of social isolation in laboratory animals to model the impact of environmental factors on the aetiopathology of schizophrenia. However, controversy exists over the precise behavioural profile and the robustness of some of the reported effects of social isolation rearing.

MATERIALS AND METHODS

Here, we evaluated the efficacy of postweaning social isolation to induce schizophrenia-related behavioural deficits in C57BL/6 mice of both sexes.

RESULTS

The effects of social isolation clearly differed between sexes: isolated male but not female mice exhibited multiple habituation deficits and enhanced locomotor reaction to amphetamine.

DISCUSSION

The preferential vulnerability in the male sex corresponds well with the earlier disease onset and poorer prognosis in male relative to female schizophrenic patients. In contrast, we observed no evidence for a disruption of sensorimotor gating in the prepulse inhibition paradigm despite the efficacy of social isolation to alter startle reactivity. With both success and failure in the induction of schizophrenia-related endophenotypes, the present study thus provides important characterizations and qualifications to the application of the social isolation model in mice.

CONCLUSIONS

We conclude that social isolation in mice represents a valuable tool for the examination of candidate genes within the context of the "two-hit" hypothesis of the aetiological processes in schizophrenia.

Behavioural and neurochemical effects of post-weaning social isolation in rodents-relevance to developmental neuropsychiatric disorders

Exposing mammals to early-life adverse events, including maternal separation or social isolation, profoundly affects brain development and adult behaviour and may contribute to the occurrence of psychiatric disorders, such as depression and schizophrenia in genetically predisposed humans. The molecular mechanisms underlying these environmentally induced developmental adaptations are unclear and best evaluated in animal paradigms with translational salience. Rearing rat pups from weaning in isolation, to prevent social contact with conspecifics, produces reproducible, long-term changes including; neophobia, impaired sensorimotor gating, aggression, cognitive rigidity, reduced prefrontal cortical volume and decreased cortical and hippocampal synaptic plasticity. These alterations are associated with hyperfunction of mesolimbic dopaminergic systems, enhanced presynaptic dopamine (DA) and serotonergic (5-HT) function in the nucleus accumbens (NAcc), hypofunction of mesocortical DA and attenuated 5-HT function in the prefrontal cortex and hippocampus. These behavioural, morphological and neurochemical abnormalities, as reviewed herein, strongly resemble core features of schizophrenia. Therefore unravelling the mechanisms that trigger these sequelae will improve our knowledge of the aetiology of neurodevelopmental psychiatric disorders, enable identification of longitudinal biomarkers of dysfunction and permit predictive screening for novel compounds with potential antipsychotic efficacy.

N,N-dimethyl-thioamphetamine and methyl-thioamphetamine, two non-neurotoxic substrates of 5-HT transporters, have scant in vitro efficacy for the induction of transporter-mediated 5-HT release and currents

We studied two non-neurotoxic amphetamine derivatives (methyl-thioamphetamine, MTA and N,N-dimethylMTA, DMMTA) interacting with serotonin (5-HT) transporters (SERTs) with affinities comparable to that of p-Cl-amphetamine (pCA). The rank order for their maximal effects in inducing both [(3)H]5-HT release from rat brain synaptosomes or hSERT-expressing HEK-293 cells, and currents in hSERT-expressing oocytes, was pCA >> MTA > or = DMMTA. A correlation between drug-induced release and currents is also strengthened by the similar bell shape of the dose-response curves. Release experiments indicated that MTA and DMMTA are SERT substrates although MTA is taken up by HEK-293 cells with a V(max) 40% lower than pCA. The weak effects of MTA and DMMTA in vitro might therefore be due to their properties as 'partial substrates' on the mechanisms, other than translocation, responsible for currents and/or release. After either local or systemic in vivo administration, MTA and DMMTA release 5-HT in a manner comparable to pCA. These findings confirm that the neurotoxic properties of some amphetamine derivatives are independent of their 5-HT-releasing activity in vivo. It is worth noting that only those amphetamine derivatives with high efficiency in inducing 5-HT release and currents in vitro have neurotoxic properties.

Social isolation rearing-induced impairment of the hippocampal neurogenesis is associated with deficits in spatial memory and emotion-related behaviors in juvenile mice

Experiences during brain development may influence the pathogenesis of developmental disorders. Thus, social isolation (SI) rearing after weaning is a useful animal model for studying the pathological mechanisms of such psychiatric diseases. In this study, we examined the effect of SI on neurogenesis in the hippocampal dentate gyrus (DG) relating to memory and emotion-related behaviors. When newly divided cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) before SI, the number of BrdU-positive cells and the rate of differentiation into neurons were significantly decreased after 4-week SI compared with those in group-housed mice. Repeated treatment of fluoxetine prevented the SI-induced impairment of survival of newly divided cells and ameliorated spatial memory impairment and part of aggression in SI mice. Furthermore, we investigated the changes in gene expression in the DG of SI mice by using DNA microarray and real-time PCR. We finally found that SI reduced the expression of development-related genes Nurr1 and Npas4. These findings suggest that communication in juvenile is important in the survival and differentiation of newly divided cells, which may be associated with memory and aggression, and raise the possibility that the reduced expression of Nurr1 and/or Npas4 may contribute to the impairment of neurogenesis and memory and aggression induced by SI.

An intact dopaminergic system is required for context-conditioned release of 5-HT in the nucleus accumbens of postweaning isolation-reared rats

We investigated the effect of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT) on extracellular dopamine and 5-HT levels in the nucleus accumbens of group- and isolation-reared rats. Microdialysis with high-performance liquid chromatography-electrochemical detection was used to quantify dopamine and 5-HT efflux in the nucleus accumbens following foot shock and in association with a conditioned emotional response (CER). Isolation- and group-reared rats received i.p. injections of either saline (0.9%) or AMPT (200 mg/kg) 15 h and 2 h prior to sampling. There was no significant difference between saline-treated isolation- or group-reared rats for basal efflux of dopamine or 5-HT, however as expected, AMPT-treatment significantly reduced dopamine efflux in both groups to an equivalent level (50-55% saline-treated controls). Exposure to mild foot shock stimulated basal dopamine efflux in saline-treated groups only, although the effect was significantly greater in isolation-reared rats. In AMPT-treated rats, foot shock did not affect basal dopamine efflux in either rearing group. Foot shock evoked a prolonged increase in 5-HT efflux in both isolation- and group-reared saline-treated rats but had no effect on 5-HT efflux in AMPT-treated rats. In response to CER, isolation-rearing was associated with significantly greater efflux of both dopamine and 5-HT in saline-treated rats, compared to saline-treated, group-reared controls. However in AMPT-treated rats, efflux of dopamine or 5-HT did not change in response to CER. These data suggest that unconditioned or conditioned stress-induced changes in 5-HT release of the nucleus accumbens are dependent upon intact catecholaminergic neurotransmission. Furthermore, as the contribution of noradrenaline to catecholamine efflux in the nucleus accumbens is relatively minor compared to dopamine, our findings suggest that dopamine efflux in the nucleus accumbens is important for the local regulation of 5-HT release in this region. Finally, these findings implicate the isolation-enhanced presynaptic dopamine function in the accumbens with the augmented ventral striatal 5-HT neurotransmission characterized by isolation-reared rats.

Acquisition of a conditioned avoidance reflex and morphometric characteristics of the sensorimotor cortex in rats subjected to social deprivation in early ontogenesis

Male Wistar rats were subjected to social deprivation from day 22 to day 70 of postnatal development to form a group of isolants. Measures of learning of a passive avoidance and the results of training to active avoidance of painful electrical stimulation in these rats were significantly lower at age 110-120 days than in control rats. The characteristics of movement activity in isolants during training and testing, as well as in the open field test, probably resulted from their high levels of anxiety. Morphometric measurements of the numbers of neurons, satellite glial cells, and free glial cells in the sensorimotor area of the neocortex showed that isolants had a significantly lower density of neural elements per unit area, which resulted from a decrease in the afferent flow due to social isolation.