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

Social isolation and post-weaning environmental enrichment effects on rat emotional behavior and serotonergic system

Social isolation (SI) is related to adverse neurobehavioral effects and neurochemical changes when it occurs early in development. On the other hand, environmental enrichment (EE) is associated with a reduction in anxiety-like and depression-like behavior, as well as an increase in serotonin (5-HT) levels in the prefrontal cortex and hippocampus in rodents. This study systematically reviewed the effects of SI and EE on emotional behavior and serotonergic system components in rats after weaning. Primary experimental studies that used subgroups of rats subjected to SI, EE, and normal social conditions after weaning were considered eligible. Studies that used transgenic rodents, ex vivo studies, in vitro studies, human research, or in silico studies were ineligible. Two authors completed searches in Medline/PubMed, LILACS, Scopus, Web of Science, EMBASE, and Open Gray. The Kappa index was calculated to assess agreement between reviewers and assess study quality. The results showed that the animals exposed to EE showed better adaptation to a new environment. Furthermore, EE increased 5-HT levels in the hippocampus and prefrontal cortex of rodents. Thus, it appears that an EE during the critical period of development may reduce anxiety/depression-like behaviors, as well as increase long-term neurotransmitter response.

Investigating the Robustness of a Rodent "Double Hit" (Post-Weaning Social Isolation and NMDA Receptor Antagonist) Model as an Animal Model for Schizophrenia: A Systematic Review

Schizophrenia is a debilitating psychiatric disorder comprising positive, negative, and cognitive impairments. Most of the animal models developed to understand the neurobiology and mechanism of schizophrenia do not produce all the symptoms of the disease. Therefore, researchers need to develop new animal models with greater translational reliability, and the ability to produce most if not all symptoms of schizophrenia. This review aimed to evaluate the effectiveness of the rodent "double hit" (post-weaning social isolation and N-methyl-D-aspartate (NMDA) receptor antagonist) model to produce symptoms of schizophrenia. This systematic review was developed according to the 2020 PRISMA guidelines and checklist. The MEDLINE (PubMed) and Ebscohost databases were used to search for studies. The systematic review is based on quantitative animal studies. Studies in languages other than English that could be translated sufficiently using Google translate were also included. Data extraction was performed individually by two independent reviewers and discrepancies between them were resolved by a third reviewer. SYRCLE's risk-of-bias tool was used to test the quality and biases of included studies. Our primary search yielded a total of 47 articles, through different study selection processes. Seventeen articles met the inclusion criteria for this systematic review. Ten of the seventeen studies found that the "double hit" model was more effective in developing various symptoms of schizophrenia. Most studies showed that the "double hit" model is robust and capable of inducing cognitive impairments and positive symptoms of schizophrenia.

Involvement of purinergic P2Y1R in antidepressant-like effects of electroacupuncture treatment on social isolation stress mice

Depression is a common neuropsychiatric disorder with high incidence and disability. Electroacupuncture (EA) is effective in the treatment of depression. However, the underlying mechanisms are not fully understood. Social isolation stress during post-weaning period can impair purinergic signaling in the brain of rodents and has emerged as a major risk factor for depression. The purpose of this study was to investigate the involvement of P2Y1 receptor (P2Y1R) in the antidepressant-like effects of EA. In this study, C57BL/6 mice were randomly assigned to group-housed (GH) or social isolated (SI) groups at post-natal day 21. After 6 weeks of social isolation, EA was performed on acupoints "Bai-hui" (GV20) and "Yin-tang" (GV29), or non-acupoints for 4 weeks. The SI mice received either intracerebroventricular injection of a selective P2Y1R agonist, MRS2365 (1 nmol); or a selective P2Y1R antagonist, MRS2179 (2 μmol), before and after EA. We found that SI mice exhibited depression-like behaviors accompanied with anxiety-like behaviors. The expressions of P2Y1R were well co-localized with GFAP-positive astrocytes and increased in the prefrontal cortex and hippocampus of SI mice. After treated with MRS2179, the depression-like behaviors of SI mice were attenuated, but not with MRS2365. Meanwhile, we found that EA could attenuate social isolation caused depression- and anxiety-like behaviors, and inhibited the up-regulation of P2Y1R in the prefrontal cortex and hippocampus of SI mice. Notably, the positive effects of EA on depression-like behaviors of SI mice could be reversed by MRS2365, while MRS2365 had no effect on the anxiolytic-like effects of EA. Therefore, we provide new evidence that EA could ameliorate depression- and anxiety-like behaviors in social isolation stress mice, and P2Y1R was involved in the antidepressant-like effects of EA.

Depresssion, anxiety and other cognitive consequences of social isolation: Drug and non-drug treatments

OBJECTIVE

During the COVID-19 pandemic, quarantine and staying at home is advised. The social relationship between people has become deficient, and human social isolation (SI) has become the consequence of this situation. It was shown that SI has made changes in hippocampal neuroplasticity, which will lead to poor cognitive function and behavioural abnormalities. There is a connection between SI, learning, and memory impairments. In addition, anxiety-like behaviour and increased aggressive mood in long-term isolation have been revealed during the COVID-19 outbreak.

METHODS

Term searches was done in Google Scholar, Scopus, ScienceDirect, Web of Science and PubMed databases as well as hand searching in key resource journals from 1979 to 2020.

RESULTS

Studies have shown that some drug administrations may positively affect or even prevent social isolation consequences in animal models. These drug treatments have included opioid drugs, anti-depressants, Antioxidants, and herbal medications. In addition to drug interventions, there are non-drug treatments that include an enriched environment, regular exercise, and music.

CONCLUSION

This manuscript aims to review improved cognitive impairments induced by SI during COVID-19.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

The Role of Zebrafish and Laboratory Rodents in Schizophrenia Research

Schizophrenia is a severe disorder characterized by positive, negative and cognitive symptoms, which are still not fully understood. The development of efficient antipsychotics requires animal models of a strong validity, therefore the aims of the article were to summarize the construct, face and predictive validity of schizophrenia models based on rodents and zebrafish, to compare the advantages and disadvantages of these models, and to propose future directions in schizophrenia modeling and indicate when it is reasonable to combine these models. The advantages of rodent models stem primarily from the high homology between rodent and human physiology, neurochemistry, brain morphology and circuitry. The advantages of zebrafish models stem in the high fecundity, fast development and transparency of the embryo. Disadvantages of both models originate in behavioral repertoires not allowing specific symptoms to be modeled, even when the models are combined. Especially modeling the verbal component of certain positive, negative and cognitive symptoms is currently impossible.

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.

Dissociation between schizophrenia-relevant behavioral profiles and volumetric brain measures after long-lasting social isolation in Roman rats

Social isolation rearing of rodents is an environmental manipulation known to induce or potentiate psychotic-like symptoms and attentional and cognitive impairments relevant for schizophrenia. When subjected to a 28-week isolation rearing treatment, the Roman high-avoidance (RHA-I) rats display the common behavioral social isolation syndrome, with prepulse inhibition (PPI) deficits, hyperactivity, increased anxiety responses and learning/memory impairments when compared to their low-avoidance (RLA-I) counterparts. These results add face validity to the RHA-I rats as an animal model for schizophrenia-relevant behavioral and cognitive profiles and confirm previous results. The aim here was to further investigate the neuroanatomical effects of the isolation rearing, estimated through volume differences in medial prefrontal cortex (mPFC), dorsal striatum (dSt) and hippocampus (HPC). Results showed a global increase in volume in the mPFC in the isolated rats of both strains, as well as strain effects (RLA > RHA) in the three brain regions. These unexpected but robust results, might have unveiled some kind of compensatory mechanisms due to the particularly long-lasting isolation rearing period, much longer than those commonly used in the literature (which usually range from 4 to 12 weeks).

Enhancement of Aggression Induced by Isolation Rearing is Associated with a Lack of Central Serotonin

Isolation rearing (IR) enhances aggressive behavior, and the central serotonin (5-hydroxytryptamine, 5-HT) system has been linked to IR-induced aggression. However, whether the alteration of central serotonin is the cause or consequence of enhanced aggression is still unknown. In the present study, using mice deficient in central serotonin Tph2^-/- and Lmx1b^-/-, we examined the association between central serotonin and aggression with or without social isolation. We demonstrated that central serotonergic neurons are critical for the enhanced aggression after IR. 5-HT depletion in wild-type mice increased aggression. On the other hand, application of 5-HT in Lmx1b^-/- mice inhibited the enhancement of aggression under social isolation conditions. Dopamine was downregulated in Lmx1b^-/- mice. Similar to 5-HT, L-DOPA decreased aggression in Lmx1b^-/- mice. Our results link the serotoninergic system directly to aggression and this may have clinical implications for aggression-related human conditions.

Urocortins and their unfolding role in mammalian social behavior

The corticotropin-releasing factor (CRF) system is well known for its major role in coordinating the endocrine, autonomic and behavioral responses to stress. These functions have been shown to be mediated mainly by the binding of the CRF neuropeptide to its specific receptor CRFR1. Yet, the CRF system comprises several more neuropeptides, including the three urocortins, UCN1, UCN2 and UCN3, of which the latter two bind specifically to a distinct receptor-CRFR2. Unlike the brain-wide abundant expression of CRF and CRFR1, the brain expression of the urocortins and CRFR2 is rather restricted and seems to be focused in limbic areas associated with social behavior. Here, we will review accumulating evidence from recent studies that unfold the role of UCN2 and UCN3 in regulating mammalian social behavior, via activation of CRFR2.

Involvement of opioid system in behavioral despair induced by social isolation stress in mice

Social isolation stress (SIS) as a type of chronic stress could induce depressive- and anxiety-like behaviors. Our study evaluates the role of opioid system on negative behavioral impacts of SIS in male NMRI mice. We investigated effects of morphine, a nonselective opioid receptor (OR) agonist, naltrexone (NLX), an OR antagonist, naltrindole (NLT), a delta opioid receptor (DOR) antagonist, SNC80, a DOR agonist, U-69593, a kappa opioid receptor (KOR) agonist, nor-Binaltorphimine, a selective KOR antagonist and cyprodime hydrochloride a selective mu opioid receptor (MOR) antagonist on depressive- and anxiety-like behaviors. Using RT-PCR we evaluated ORs gene expression in mice brain. Our findings showed that SIS induced anxiety- and depressive-like behavior in the forced swimming test, open field test, splash test and hole-board test. Moreover, administration of SNC-80 significantly mitigated anxiety- and depressive-like behaviors. NLT decreased grooming-activity in the splash test. Excitingly, administration of agents affecting KOR failed to alter the negative effects of SIS. RT-PCR demonstrated that MOR and KOR gene expression decreased in socially isolated mice; however, SIS did not affect DORs expression. Our findings suggest that SIS at least in part, probably via altering endogenous opioids particularly MORs and KORs but not DORs mediated negative impacts on behavior; also, it could be concluded that DORs might be considered as a novel target for studying depression and anxiety.

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.

Characterization of Behavioral, Signaling and Cytokine Alterations in a Rat Neurodevelopmental Model for Schizophrenia, and Their Reversal by the 5-HT6 Receptor Antagonist SB-399885

Post-weaning social isolation of rats produces neuroanatomical, neurochemical and behavioral alterations resembling some core features of schizophrenia. This study examined the ability of the 5-HT₆ receptor antagonist SB-399885 to reverse isolation-induced cognitive deficits, then investigated alterations in hippocampal cell proliferation and hippocampal and frontal cortical expression of selected intracellular signaling molecules and cytokines. Male Lister hooded rats (weaned on post-natal days 21-24 and housed individually or in groups of 3-4) received six i.p. injections of vehicle (1% Tween 80, 1 mL/kg) or SB-399885 (5 or 10 mg/kg) over a 2-week period starting 40 days post-weaning, on the days that locomotor activity, novel object discrimination (NOD), pre-pulse inhibition of acoustic startle and acquisition, retention and extinction of a conditioned freezing response (CFR) were assessed. Tissue was collected 24 h after the final injection for immunohistochemistry, reverse-phase protein microarray and western blotting. Isolation rearing impaired NOD and cue-mediated CFR, decreased cell proliferation within the dentate gyrus, and elevated hippocampal TNFα levels and Cdc42 expression. SB-399885 reversed the NOD deficit and partially normalized CFR and cell proliferation. These effects were accompanied by altered expression of several members of the c-Jun N-terminal Kinase (JNK) and p38 MAPK signaling pathways (including TAK1, MKK4 and STAT3). Although JNK and p38 themselves were unaltered at this time point hippocampal TAK1 expression and phosphorylation correlated with visual recognition memory in the NOD task. Continued use of this neurodevelopmental model could further elucidate the neurobiology of schizophrenia and aid assessment of novel therapies for drug-resistant cognitive symptoms.

Impaired glucocorticoid-mediated HPA axis negative feedback induced by juvenile social isolation in male rats

We previously demonstrated that socially isolated rats at weaning showed a significant decrease in corticosterone and adrenocorticotropic hormone (ACTH) levels, associated with an enhanced response to acute stressful stimuli. Here we shown that social isolation decreased levels of total corticosterone and of its carrier corticosteroid-binding globulin, but did not influence the availability of the free active fraction of corticosterone, both under basal conditions and after acute stress exposure. Under basal conditions, social isolation increased the abundance of glucocorticoid receptors, while it decreased that of mineralocorticoid receptors. After acute stress exposure, socially isolated rats showed long-lasting corticosterone, ACTH and corticotrophin releasing hormone responses. Moreover, while in the hippocampus and hypothalamus of group-housed rats glucocorticoid receptors expression increased with time and reached a peak when corticosterone levels returned to basal values, in socially isolated rats expression of glucocorticoid receptors did not change. Finally, social isolation also affected the hypothalamic endocannabinoid system: compared to group-housed rats, basal levels of anandamide and cannabinoid receptor type 1 were increased, while basal levels of 2-arachidonoylglycerol were decreased in socially isolated rats and did not change after acute stress exposure. The present results show that social isolation in male rats alters basal HPA axis activity and impairs glucocorticoid-mediated negative feedback after acute stress. Given that social isolation is considered an animal model of several neuropsychiatric disorders, such as generalized anxiety disorder, depression, post-traumatic stress disorder and schizophrenia, these data could contribute to better understand the alterations in HPA axis activity observed in these disorders.

Phencyclidine increased while isolation rearing did not affect progressive ratio responding in rats: Investigating potential models of amotivation in schizophrenia

BACKGROUND

Schizophrenia is a debilitating neurodevelopmental disorder affecting 1% of the global population with heterogeneous symptoms including positive, negative, and cognitive. While treatment for positive symptoms exists, none have been developed to treat negative symptoms. Animal models of schizophrenia are required to test targeted treatments and since patients exhibit reduced effort (breakpoints) for reward in a progressive ratio (PR) task, we examined the PR breakpoints of rats treated with the NMDA receptor antagonist phencyclidine or those reared in isolation - two common manipulations used to induce schizophrenia-relevant behaviors in rodents.

METHODS

In two cohorts, the PR breakpoint for a palatable food reward was examined in Long Evans rats after: 1) a repeated phencyclidine regimen; 2) A subchronic phencyclidine regimen followed by drug washout; and 3) post-weaning social isolation.

RESULTS

Rats treated with repeated phencyclidine and those following washout from phencyclidine exhibited higher PR breakpoints than vehicle-treated rats. The breakpoint of isolation reared rats did not differ from those socially reared, despite abnormalities of these rats in other schizophrenia-relevant behaviors.

CONCLUSION

Despite their common use for modeling other schizophrenia-relevant behaviors neither phencyclidine treatment nor isolation rearing recreated the motivational deficits observed in patients with schizophrenia, as measured by PR breakpoint. Other manipulations, and negative symptom-relevant behaviors, require investigation prior to testing putative therapeutics.

Interaction between harmane, a class of β-carboline alkaloids, and the CA1 serotonergic system in modulation of memory acquisition

This study set to assess the involvement of dorsal hippocampus (CA1) serotonergic system on harmane induced memory acquisition deficit. We used one trial step-down inhibitory avoidancetask to evaluate memory retention and then, open field test to evaluate locomotor activity in adult male NMRI mice. The results showed that pre-training intra-peritoneal (i.p.) administration of harmane (12mg/kg) induced impairment of memory acquisition. Pre-training intra-CA1 administration of 5-HT1B/1D receptor agonist (CP94253; 0.5 and 5ng/mouse) and 5-HT2A/2B/2C receptor agonist (α-methyl 5-HT; 50ng/mouse) impaired memory acquisition. Furthermore, intra-CA1 administration of 5-HT1B/1D receptor antagonist (GR127935; 0.5ng/mouse) and 5-HT2 receptor antagonist (cinancerine; 5ng/mouse) improved memory acquisition. In addition, pre-training intra-CA1 injection of sub-threshold dose of CP94253 (0.05ng/mouse) and α-methyl 5-HT (5ng/mouse) potentiated impairment of memory acquisition induced by harmane (12mg/kg, i.p.). On the other hand, pre-training intra-CA1 infusion of sub-threshold dose of GR127935 (0.05ng/mouse) and cinancerine (0.5ng/mouse) with the administration of harmane (12mg/kg, i.p.) weakened impairment of memory acquisition. Moreover, all above doses of drugs did not change locomotor activity. The present findings suggest that there is an interaction between harmane and the CA1 serotonergic system in modulation of memory acquisition.

Early-Life Social Isolation Influences Mouse Ultrasonic Vocalizations during Male-Male Social Encounters

Early-life social isolation has profound effects on adult social competence. This is often expressed as increased aggression or inappropriate displays of courtship-related behaviors. The social incompetence exhibited by isolated animals could be in part due to an altered ability to participate in communicatory exchanges. House mice (Mus musculus) present an excellent model for exploring this idea, because social isolation has a well-established influence on their social behavior, and mice engage in communication via multiple sensory modalities. Here, we tested the prediction that social isolation during early life would influence ultrasonic vocalizations (USVs) emitted by adult male mice during same-sex social encounters. Starting at three weeks of age, male mice were housed individually or in social groups of four males for five weeks, after which they were placed in one of three types of paired social encounters. Pair types consisted of: two individually housed males, two socially housed males, or an individually housed and a socially housed male (“mixed” pairs). Vocal behavior (USVs) and non-vocal behaviors were recorded from these 15-minute social interactions. Pairs of mice consisting of at least one individually housed male emitted more and longer USVs, with a greater proportional use of USVs containing frequency jumps and 50-kHz components. Individually housed males in the mixed social pairs exhibited increased levels of mounting behavior towards the socially housed males. Mounting in these pairs was positively correlated with increased number and duration of USVs as well as increased proportional use of spectrally more complex USVs. These findings demonstrate that USVs are part of the suite of social behaviors influenced by early-life social isolation, and suggest that altered vocal communication following isolation reflects reduced social competence.

Neurobiology of comorbid post-traumatic stress disorder and alcohol-use disorder

Post-traumatic stress disorder (PTSD) and alcohol-use disorder (AUD) are highly comorbid in humans. Although we have some understanding of the structural and functional brain changes that define each of these disorders, and how those changes contribute to the behavioral symptoms that define them, little is known about the neurobiology of comorbid PTSD and AUD, which may be due in part to a scarcity of adequate animal models for examining this research question. The goal of this review is to summarize the current state-of-the-science on comorbid PTSD and AUD. We summarize epidemiological data documenting the prevalence of this comorbidity, review what is known about the potential neurobiological basis for the frequent co-occurrence of PTSD and AUD and discuss successes and failures of past and current treatment strategies. We also review animal models that aim to examine comorbid PTSD and AUD, highlighting where the models parallel the human condition, and we discuss the strengths and weaknesses of each model. We conclude by discussing key gaps in our knowledge and strategies for addressing them: in particular, we (1) highlight the need for better animal models of the comorbid condition and better clinical trial design, (2) emphasize the need for examination of subpopulation effects and individual differences and (3) urge cross-talk between basic and clinical researchers that is reflected in collaborative work with forward and reverse translational impact.

Neurobehavioural sequelae of social deprivation in rodents revisited: Modelling social adversity for developmental neuropsychiatric disorders

The significance of investigating effects of deprivation of social experience in rodents is reviewed in the context of the review by Robbins et al. (1996) in the Journal of Psychopharmacology (10: 39-47). The early development of the paradigm by which rats were reared post-weaning in social isolation is described and compared with other early experience manipulations. The specification of the neural and behavioural phenotype of the isolate is brought up-to-date, focusing on changes in motivation and cognitive function, as well as on contrasting changes in the dopamine and serotonin systems, and in cortical (including hippocampal) structure and function. The relevance of the isolate for animal models of psychiatric disorders such as attention deficit hyperactivity disorder and schizophrenia is reviewed, and it is considered that the paradigm best exemplifies a manipulation that can be applied to test effects of certain forms of social adversity during adolescence on brain development and behaviour.

Physical interaction between neurofibromin and serotonin 5-HT6 receptor promotes receptor constitutive activity

Active G protein-coupled receptor (GPCR) conformations not only are promoted by agonists but also occur in their absence, leading to constitutive activity. Association of GPCRs with intracellular protein partners might be one of the mechanisms underlying GPCR constitutive activity. Here, we show that serotonin 5 hydroxytryptamine 6 (5-HT₆) receptor constitutively activates the Gs/adenylyl cyclase pathway in various cell types, including neurons. Constitutive activity is strongly reduced by silencing expression of the Ras-GTPase activating protein (Ras-GAP) neurofibromin, a 5-HT₆ receptor partner. Neurofibromin is a multidomain protein encoded by the NF1 gene, the mutation of which causes Neurofibromatosis type 1 (NF1), a genetic disorder characterized by multiple benign and malignant nervous system tumors and cognitive deficits. Disrupting association of 5-HT₆ receptor with neurofibromin Pleckstrin Homology (PH) domain also inhibits receptor constitutive activity, and PH domain expression rescues 5-HT₆ receptor-operated cAMP signaling in neurofibromin-deficient cells. Furthermore, PH domains carrying mutations identified in NF1 patients that prevent interaction with the 5-HT₆ receptor fail to rescue receptor constitutive activity in neurofibromin-depleted cells. Further supporting a role of neurofibromin in agonist-independent Gs signaling elicited by native receptors, the phosphorylation of cAMP-responsive element-binding protein (CREB) is strongly decreased in prefrontal cortex of Nf1^+/- mice compared with WT mice. Moreover, systemic administration of a 5-HT₆ receptor inverse agonist reduces CREB phosphorylation in prefrontal cortex of WT mice but not Nf1^+/- mice. Collectively, these findings suggest that disrupting 5-HT₆ receptor-neurofibromin interaction prevents agonist-independent 5-HT₆ receptor-operated cAMP signaling in prefrontal cortex, an effect that might underlie neuronal abnormalities in NF1 patients.

Divergent effects of isolation rearing on prepulse inhibition, activity, anxiety and hippocampal-dependent memory in Roman high- and low-avoidance rats: A putative model of schizophrenia-relevant features

Social isolation of rats induces a constellation of behavioral alterations known as "isolation syndrome" that are consistent with some of the positive and cognitive symptoms observed in schizophrenic patients. In the present study we have assessed whether isolation rearing of inbred Roman high-avoidance (RHA-I) and Roman low-avoidance (RLA-I) strains can lead to the appearance of some of the key features of the "isolation syndrome", such as prepulse inhibition (PPI) deficits, increased anxious behavior, hyperactivity and memory/learning impairments. Compared to RLA-I rats, the results show that isolation rearing (IR) in RHA-I rats has a more profound impact, as they exhibit isolation-induced PPI deficits, increased anxiety, hyperactivity and long-term reference memory deficits, while isolated RLA-I rats only exhibit deficits in a spatial working memory task. These results give further support to the validity of RHA-I rats as a genetically-based model of schizophrenia relevant-symptoms.

Adolescent Social Isolation as a Model of Heightened Vulnerability to Comorbid Alcoholism and Anxiety Disorders

Individuals diagnosed with anxiety-related illnesses are at increased risk of developing alcoholism, exhibit a telescoped progression of this disease and fare worse in recovery, relative to alcoholics that do not suffer from a comorbid anxiety disorder. Similarly, preclinical evidence supports the notion that stress and anxiety represent major risk factors for the development of alcohol use disorder (AUD). Despite the importance of understanding the link between anxiety and alcoholism, much remains unknown about the neurobiological substrates underlying this relationship. One stumbling block has been the lack of animal models that reliably reproduce the spectrum of behaviors associated with increased vulnerability to these diseases. Here, we review the literature that has examined the behavioral and neurobiological outcomes of a simple rodent adolescent social isolation procedure and discuss its validity as a model of vulnerability to comorbid anxiety disorders and alcoholism. Recent studies have provided strong evidence that adolescent social isolation of male rats leads to the expression of a variety of behaviors linked with increased vulnerability to anxiety and/or AUD, including deficits in sensory gating and fear extinction, and increases in anxiety measures and ethanol drinking. Neurobiological studies are beginning to identify mesolimbic adaptations that may contribute to the behavioral phenotype engendered by this model. Some of these changes include increased excitability of ventral tegmental area dopamine neurons and pyramidal cells in the basolateral amygdala and significant alterations in baseline and stimulated catecholamine signaling. A growing body of evidence suggests that adolescent social isolation may represent a reliable rodent model of heightened vulnerability to anxiety disorders and alcoholism in male rats. These studies provide initial support for the face, construct, and predictive validity of this model and highlight its utility in identifying neurobiological adaptations associated with increased risk of developing these disorders.

The dopamine D₃-preferring D₂/D₃ dopamine receptor partial agonist, cariprazine, reverses behavioural changes in a rat neurodevelopmental model for schizophrenia

Current antipsychotic medication is largely ineffective against the negative and cognitive symptoms of schizophrenia. One promising therapeutic development is to design new molecules that balance actions on dopamine D2 and D3 receptors to maximise benefits and limit adverse effects. This study used two rodent paradigms to investigate the action of the dopamine D3-preferring D3/D2 receptor partial agonist cariprazine. In adult male rats, cariprazine (0.03-0.3 mg/kg i.p.), and the atypical antipsychotic aripiprazole (1-3 mg/kg i.p.) caused dose-dependent reversal of a delay-induced impairment in novel object recognition (NOR). Treating neonatal rat pups with phencyclidine (PCP) and subsequent social isolation produced a syndrome of behavioural alterations in adulthood including hyperactivity in a novel arena, deficits in NOR and fear motivated learning and memory, and a reduction and change in pattern of social interaction accompanied by increased ultrasonic vocalisations (USVs). Acute administration of cariprazine (0.1 and 0.3 mg/kg) and aripiprazole (3 mg/kg) to resultant adult rats reduced neonatal PCP-social isolation induced locomotor hyperactivity and reversed NOR deficits. Cariprazine (0.3 mg/kg) caused a limited reversal of the social interaction deficit but neither drug affected the change in USVs or the deficit in fear motivated learning and memory. Results suggest that in the behavioural tests investigated cariprazine is at least as effective as aripiprazole and in some paradigms it showed additional beneficial features further supporting the advantage of combined dopamine D3/D2 receptor targeting. These findings support recent clinical studies demonstrating the efficacy of cariprazine in treatment of negative symptoms and functional impairment in schizophrenia patients.

Role of CB2 receptors in social and aggressive behavior in male mice

RATIONALE

Male CB1KO mice exhibit stronger aggressive responses than wild-type mice.

OBJECTIVE

This study was designed to examine the role of cannabinoid CB2r in social and aggressive behavior.

METHODS

The social interaction test and resident-intruder paradigm were performed in mice lacking CB2r (CB2KO) and in wild-type (WT) littermates. The effects of the CB2r selective agonist JWH133 (1 and 2 mg/kg) on aggression were also evaluated in Oncins France 1 (OF1) mice. Gene expression analyses of monoamine oxidase-A (MAO-A), catechol-o-methyltransferase (COMT), 5-hydroxytryptamine transporter (5-HTT), and 5-HT1B receptor (5HT1Br) in the dorsal raphe nuclei (DR) and the amygdala (AMY) were carried out using real-time PCR.

RESULTS

Group-housed CB2KO mice exhibited higher levels of aggression in the social interaction test and displayed more aggression than resident WT mice. Isolation increased aggressive behavior in WT mice but did not affect CB2KO animals; however, the latter mice exhibited higher levels of social interaction with their WT counterparts. MAO-A and 5-HTT gene expression was significantly higher in grouped CB2KO mice. The expression of 5HT1Br, COMT, and MAO-A in the AMY was more pronounced in CB2KO mice than in WT counterparts. Acute administration of the CB2 agonist JWH133 significantly reduced the level of aggression in aggressive isolated OF1 mice, an effect that decreased after pretreatment with the CB2 receptor antagonist AM630.

CONCLUSION

Our results suggest that CB2r is implicated in social interaction and aggressive behavior and deserves further consideration as a potential new target for the management of aggression.

5-HT6 Receptor: A New Player Controlling the Development of Neural Circuits

5-HT6 receptor (5-HT6R) is a G protein-coupled receptor that has recently emerged as a new regulator of neural development. In addition to the canonical Gs adenylyl cyclase pathway, recent proteomics approaches reveal that 5-HT6R is able to engage key developmental signaling pathways controlling neuronal circuit formation, neuronal connectivity, and psychiatric-relevant behaviors. For example, at early stages of neuronal development, expression of 5-HT6R constitutively regulates the activity of the cyclin-dependent kinase (Cdk)5 and, through this mechanism, controls cellular processes involved in circuit formation, including neuronal migration and neurite outgrowth. In addition to the Cdk5 pathway, 5-HT6R modulates a variety of key developmental targets such as Fyn, Jab1, and mammalian target of rapamycin (mTOR). Engagement of developmental pathways through 5-HT6R pharmacological manipulation has led to interesting new therapeutic perspectives in the field of psychiatric-related disorders. Indeed, 5-HT6R blockade can rescue a pathological overactivation of the mTOR pathway induced by early life insults in rodents and normalizes the associated social and episodic memory deficits. Here, we review recent evidence supporting the notion that 5-HT6R is at the interface of key developmental signaling pathways and a novel actor in the orchestration of neural circuit formation.

Neurodevelopmental Animal Models Reveal the Convergent Role of Neurotransmitter Systems, Inflammation, and Oxidative Stress as Biomarkers of Schizophrenia: Implications for Novel Drug Development

Schizophrenia is a life altering disease with a complex etiology and pathophysiology, and although antipsychotics are valuable in treating the disorder, certain symptoms and/or sufferers remain resistant to treatment. Our poor understanding of the underlying neuropathological mechanisms of schizophrenia hinders the discovery and development of improved pharmacological treatment, so that filling these gaps is of utmost importance for an improved outcome. A vast amount of clinical data has strongly implicated the role of inflammation and oxidative insults in the pathophysiology of schizophrenia. Preclinical studies using animal models are fundamental in our understanding of disease development and pathology as well as the discovery and development of novel treatment options. In particular, social isolation rearing (SIR) and pre- or postnatal inflammation (PPNI) have shown great promise in mimicking the biobehavioral manifestations of schizophrenia. Furthermore, the "dual-hit" hypothesis of schizophrenia states that a first adverse event such as genetic predisposition or a prenatal insult renders an individual susceptible to develop the disease, while a second insult (e.g., postnatal inflammation, environmental adversity, or drug abuse) may be necessary to precipitate the full-blown syndrome. Animal models that emphasize the "dual-hit" hypothesis therefore provide valuable insight into understanding disease progression. In this Review, we will discuss SIR, PPNI, as well as possible "dual-hit" animal models within the context of the redox-immune-inflammatory hypothesis of schizophrenia, correlating such changes with the recognized monoamine and behavioral alterations of schizophrenia. Finally, based on these models, we will review new therapeutic options, especially those targeting immune-inflammatory and redox pathways.

The poly(I:C)-induced maternal immune activation model in preclinical neuropsychiatric drug discovery

Increasing epidemiological and experimental evidence implicates gestational infections as one important factor involved in the pathogenesis of several neuropsychiatric disorders. Corresponding preclinical model systems based upon maternal immune activation (MIA) by treatment of the pregnant female have been developed. These MIA animal model systems have been successfully used in basic and translational research approaches, contributing to the investigation of the underlying pathophysiological mechanisms at the molecular, cellular and behavioral levels. The present article focuses on the application of a specific MIA rodent paradigm, based upon treatment of the gestating dam with the viral mimic polyinosinic-polycytidilic acid (Poly(I:C)), a synthetic analog of double-stranded RNA (dsRNA) which activates the Toll-like receptor 3 (TLR3) pathway. Important advantages and constraints of this animal model will be discussed, specifically in light of gestational infection as one vulnerability factor contributing to the complex etiology of mood and psychotic disorders, which are likely the result of intricate multi-level gene×environment interactions. Improving our currently incomplete understanding of the molecular pathomechanistic principles underlying these disorders is a prerequisite for the development of alternative therapeutic approaches which are critically needed in light of the important drawbacks and limitations of currently available pharmacological treatment options regarding efficacy and side effects. The particular relevance of the Poly(I:C) MIA model for the discovery of novel drug targets for symptomatic and preventive therapeutic strategies in mood and psychotic disorders is highlighted in this review article.

Role of social factors on cell death, cerebral plasticity and recovery after stroke

Stroke is a serious global health care problem. It is now is the fourth leading cause of death and the primary cause of adult disability in the United States. Substantial evidence from both experimental and clinical studies has demonstrated that social isolation (SI) can increase stroke incidence and impair recovery. Epidemiological studies demonstrate that an increasing number of patients are living alone, and as the aging population increases, loneliness will only increase in prevalence. SI is increasingly identified as an independent risk factor for all-cause mortality. In contrast, individuals with high levels of social support exhibit more rapid and extensive functional and cognitive recovery after a wide variety of pathological insults, including stroke. Clinical data suggests that SI is an important risk factor for increased mortality and delayed functional recovery following ischemic stroke. Attesting to the importance of mortality and behavioral factors in stroke outcome is that these same effects can be reproduced in animal models of experimental stroke. This has allowed researchers to identify several mechanistic changes that occur with affiliative interactions. These include decreased systemic inflammation, elaboration of growth factors including brain derived neurotropic factor (BDNF), enhanced neurogenesis, and improved neuroimmune responsiveness in group housed animals. These may mediate the beneficial effects of social interaction on improving stroke recovery and reducing neuronal death. In this review we provide an overview of the effects of SI on ischemic injury and recovery and discuss their clinical and therapeutic implications.

Ethological concepts enhance the translational value of animal models

The translational value of animal models is an issue of ongoing discussion. We argue that 'Refinement' of animal experiments is needed and this can be achieved by exploiting an ethological approach when setting up and conducting experiments. Ethology aims to assess the functional meaning of behavioral changes, due to experimental manipulation or treatment, in animal models. Although the use of ethological concepts is particularly important for studies involving the measurement of animal behavior (as is the case for most studies on neuro-psychiatric conditions), it will also substantially benefit other disciplines, such as those investigating the immune system or inflammatory response. Using an ethological approach also involves using more optimal testing conditions are employed that have a biological relevance to the animal. Moreover, using a more biological relevant analysis of the data will help to clarify the functional meaning of the modeled readout (e.g. whether it is psychopathological or adaptive in nature). We advocate for instance that more behavioral studies should use animals in group-housed conditions, including the recording of their ultrasonic vocalizations, because (1) social behavior is an essential feature of animal models for human 'social' psychopathologies, such as autism and schizophrenia, and (2) social conditions are indispensable conditions for appropriate behavioral studies in social species, such as the rat. Only when taking these elements into account, the validity of animal experiments and, thus, the translation value of animal models can be enhanced.

Involvement of the nitrergic system in the proconvulsant effect of social isolation stress in male mice

Social isolation stress (SIS) in adolescence is accompanied by neurobehavioral disturbances and pathophysiological changes in certain regions of the CNS such as the hippocampus. In this study, we tested whether SIS impacts seizure susceptibility in postnatal male mice due to a role of hippocampal nitric oxide (NO). To do this, we used the pentylenetetrazole (PTZ) model of clonic seizures, open-field test, hole-board test, forced swimming test, and plasma corticosterone assay. We aimed to evaluate if 4 weeks of SIS is capable of decreasing seizure threshold along with altering affective and neuroendocrine responses in isolated conditioned (IC) animals in comparison with socially conditioned (SC) animals. In addition, we applied subeffective doses of NO precursor L-arginine (25, 50, and 100mg/kg) and NOS inhibitors 7-NI (15 and 40 mg/kg), aminoguanidine (50 and 100mg/kg), and L-NAME (10 and 15 mg/kg) to both IC and SC groups prior to the determination of seizure threshold. Injection of a single dose of all mentioned drugs did not induce changes in seizure threshold of SC mice. On the other hand, L-NAME and 7-NI, but not aminoguanidine, modulated the proconvulsant effect of SIS, while L-arginine augmented the latter effect. We also measured the hippocampal nitrite levels after the administration of the aforementioned drugs. Social isolation stress increased the nitrite levels in comparison with those in SC mice, whereas 7-NI and L-NAME, unlike aminoguanidine, mitigated the effect of SIS. Additionally, L-arginine boosted the effects of SIS on nitrite production. In summary, we showed that SIS enhanced seizure susceptibility in the PTZ model of clonic seizures through the activation of the nitrergic system in the hippocampus. Also, we proved that nNOS, but not iNOS, accounts for these changes following SIS.

Schizophrenia: an integrated sociodevelopmental-cognitive model

Schizophrenia remains a major burden on patients and society. The dopamine hypothesis attempts to explain the pathogenic mechanisms of the disorder, and the neurodevelopmental hypothesis the origins. In the past 10 years an alternative, the cognitive model, has gained popularity. However, the first two theories have not been satisfactorily integrated, and the most influential iteration of the cognitive model makes no mention of dopamine, neurodevelopment, or indeed the brain. In this Review we show that developmental alterations secondary to variant genes, early hazards to the brain, and childhood adversity sensitise the dopamine system, and result in excessive presynaptic dopamine synthesis and release. Social adversity biases the cognitive schema that the individual uses to interpret experiences towards paranoid interpretations. Subsequent stress results in dysregulated dopamine release, causing the misattribution of salience to stimuli, which are then misinterpreted by the biased cognitive processes. The resulting paranoia and hallucinations in turn cause further stress, and eventually repeated dopamine dysregulation hardwires the psychotic beliefs. Finally, we consider the implications of this model for understanding and treatment of schizophrenia.

Evaluating negative-symptom-like behavioural changes in developmental models of schizophrenia

Many lines of evidence suggest that schizophrenia has a major developmental component and that environmental factors that disrupt key stages of development, such as maternal stress during pregnancy as a result of infection or malnutrition, can increase the risk of developing schizophrenia in later life. This review examines how non-clinical neurodevelopmental models pertinent to schizophrenia have been evaluated for their ability to reproduce behavioural deficits related to the negative symptoms of schizophrenia. The more frequently used are the prenatal application of the mitotoxic agent methylazoxymethanol, prenatal immune challenge and the neonatal ventral hippocampus lesion model. In general they have been extensively evaluated in models considered relevant to positive symptoms of schizophrenia. In contrast, very few studies have examined tests related to negative symptoms and, when they have, it has almost exclusively been a social interaction model. Other aspects related to negative symptoms such as anhedonia, affective flattening and avolition have almost never been studied. Further studies examining other components of negative symptomatology are needed to more clearly associate these deficits with a schizophrenia-like profile as social withdrawal is a hallmark of many disorders. Although there are no truly effective treatments for negative symptoms, better characterisation with a broader range of drugs used in schizophrenia will be necessary to better evaluate the utility of these models. In summary, developmental models of schizophrenia have been extensively studied as models of positive symptoms but, given the unmet need in the clinic, the same effort now needs to be made with regard to negative symptoms.

In vivo neurometabolic profiling to characterize the effects of social isolation and ketamine-induced NMDA antagonism: a rodent study at 7.0 T

Continued efforts are undertaken to develop animal models of schizophrenia with translational value in the quest for much needed novel drugs. Existing models mimic specific neurobiological aspects of schizophrenia, but not its full complexity. Here, we used proton magnetic resonance spectroscopy ((1)H-MRS) to assess the metabolic profile in the prefrontal cortex (PFC) of two established models, rearing in social isolation and acute N-methyl-D-aspartate receptor (NMDA-R) antagonism and their combination. Rats reared in social isolation or group housed underwent (1)H-MRS at baseline and dynamically after ketamine challenge (25mg/kg, intraperitoneal) under isoflurane anesthesia. A 7 T animal scanner was used to perform spectra acquisition from the anterior cingulate/medial PFC. LCModel was used for metabolite quantification and effects of rearing and ketamine injection were analyzed. Social isolation did not lead to significant differences in the metabolic profile of the PFC at baseline. Ketamine induced a significant increase in glutamine in both groups with significance specifically reached by the group-housed animals alone. Only rats reared in social isolation showed a significant 11% γ-aminobutyric acid (GABA) decrease. This study provides preliminary evidence that social interactions in early life predict the glutamatergic and GABAergic response to acute NMDA-R blockade. The similarity between the prefrontal GABA reduction in patients with schizophrenia and in rats reared as social isolates after challenge with ketamine suggests good potential translational value of this combined animal model for drug development.

Social isolation disrupts hippocampal neurogenesis in young non-human primates

Social relationships are crucial for the development and maintenance of normal behavior in non-human primates. Animals that are raised in isolation develop abnormal patterns of behavior that persist even when they are later reunited with their parents. In rodents, social isolation is a stressful event and is associated with a decrease in hippocampal neurogenesis but considerably less is known about the effects of social isolation in non-human primates during the transition from adolescence to adulthood. To investigate how social isolation affects young marmosets, these were isolated from other members of the colony for 1 or 3 weeks and evaluated for alterations in their behavior and hippocampal cell proliferation. We found that anxiety-related behaviors like scent-marking and locomotor activity increased after social isolation when compared to baseline levels. In agreement, grooming—an indicative of attenuation of tension—was reduced among isolated marmosets. These results were consistent with increased cortisol levels after 1 and 3 weeks of isolation. After social isolation (1 or 3 weeks), reduced proliferation of neural cells in the subgranular zone of dentate granule cell layer was identified and a smaller proportion of BrdU-positive cells underwent neuronal fate (doublecortin labeling). Our data is consistent with the notion that social deprivation during the transition from adolescence to adulthood leads to stress and produces anxiety-like behaviors that in turn might affect neurogenesis and contribute to the deleterious consequences of prolonged stressful conditions.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

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.

Synthesis of substituted diphenyl sulfones and their structure-activity relationship with the antagonism of 5-НТ6 receptors

Substituted diphenyl sulfones (10a-n) were synthesised, and the structures were confirmed by NMR, LC-MS and X-ray crystallography. Their antagonistic activities towards 5-HT₆ receptor were assessed in a cell-based functional assay. Diphenyl sulfone 10a, in spite of being the smallest and simplest known sulfonyl-containing 5-HT₆R antagonist, showed a strong potency (Ki=1.6 μM). Its derivative with a methylamine substituent, 10g (N-methyl-2-(phenylsulfonyl)aniline), was ∼66-times as active as diphenyl sulfone (Ki=24.3 nM). Addition of a piperazinyl moiety in the para-position relative to the sulfonyl group in compound 10m (N-methyl-2-(phenylsulfonyl)-5-piperazin-1-ylaniline) led to a further 150-fold increase in potency (Ki=0.16 nM) to block the serotonin-induced response of HEK-293 cells that were stably transfected with the human recombinant 5-HT₆ receptor.

Serotonin homeostasis and serotonin receptors as actors of cortical construction: special attention to the 5-HT3A and 5-HT6 receptor subtypes

Cortical circuits control higher-order cognitive processes and their function is highly dependent on their structure that emerges during development. The construction of cortical circuits involves the coordinated interplay between different types of cellular processes such as proliferation, migration, and differentiation of neural and glial cell subtypes. Among the multiple factors that regulate the assembly of cortical circuits, 5-HT is an important developmental signal that impacts on a broad diversity of cellular processes. 5-HT is detected at the onset of embryonic telencephalic formation and a variety of serotonergic receptors are dynamically expressed in the embryonic developing cortex in a region and cell-type specific manner. Among these receptors, the ionotropic 5-HT3A receptor and the metabotropic 5-HT6 receptor have recently been identified as novel serotonergic targets regulating different aspects of cortical construction including neuronal migration and dendritic differentiation. In this review, we focus on the developmental impact of serotonergic systems on the construction of cortical circuits and discuss their potential role in programming risk for human psychiatric disorders.

Severe life stress and oxidative stress in the brain: from animal models to human pathology

SIGNIFICANCE

Severe life stress (SLS), as opposed to trivial everyday stress, is defined as a serious psychosocial event with the potential of causing an impacting psychological traumatism.

RECENT ADVANCES

Numerous studies have attempted to understand how the central nervous system (CNS) responds to SLS. This response includes a variety of morphological and neurochemical modifications; among them, oxidative stress is almost invariably observed. Oxidative stress is defined as disequilibrium between oxidant generation and the antioxidant response.

CRITICAL ISSUES

In this review, we discuss how SLS leads to oxidative stress in the CNS, and how the latter impacts pathophysiological outcomes. We also critically discuss experimental methods that measure oxidative stress in the CNS. The review covers animal models and human observations. Animal models of SLS include sleep deprivation, maternal separation, and social isolation in rodents, and the establishment of hierarchy in non-human primates. In humans, SLS, which is caused by traumatic events such as child abuse, war, and divorce, is also accompanied by oxidative stress in the CNS.

FUTURE DIRECTIONS

The outcome of SLS in humans ranges from resilience, over post-traumatic stress disorder, to development of chronic mental disorders. Defining the sources of oxidative stress in SLS might in the long run provide new therapeutic avenues.