Adolescent Social Isolation Affects Schizophrenia-Like Behavior in the MAM-E17 Model of Schizophrenia

Juvenile social isolation in Sprague Dawley rats does not have a lasting impact on social behavior in adulthood

Adolescent social interactions are essential for shaping adult behavior in humans. While rodent studies have highlighted the impact of social isolation on behavior, many extend isolation into adulthood, making it challenging to pinpoint the long-term consequences of juvenile isolation. To address these challenges, we examined the effects of social isolation using two independent protocols with male and female Sprague Dawley rats. In both prfotocols, rats were isolated during the juvenile stage; however, in one protocol, rats were re-socialized following isolation and tested in adulthood, while in the other, rats were tested immediately after isolation. This approach allowed us to determine whether social deficits emerged following adolescent isolation and if they could be reversed by re-socialization before adulthood. We found that juvenile isolation had no lasting effects but increased motivation for social interaction immediately after isolation. These findings underscore the need to account for housing conditions and isolation protocols when assessing the effects of social isolation.

Sex differences in loneliness, social isolation, and their impact on psychiatric symptoms and cognitive functioning in schizophrenia

BACKGROUND

Social isolation and loneliness, objective and subjective features of dysfunctional social relationships, are more prevalent in patients with schizophrenia (SCZ) than in the general population. This study aimed to explore sex differences in loneliness and social isolation among Chinese chronic SCZ patients, and to investigate their relationships with psychiatric symptoms and cognitive functioning.

METHODS

A total of 323 SCZ patients, comprising 136 males and 187 females, were recruited. Psychopathology, cognitive functioning, loneliness, social isolation were assessed using the Positive and Negative Syndrome Scale (PANSS), the Repeated Battery for Assessment of Neuropsychological Status (RBANS), the UCLA (University of California, Los Angeles) Loneliness Scale (Version 3) and the Social Isolation Index (ISI). Multiple linear regression models were conducted to test the independent, relative, and synergistic efects of loneliness and social isolation on psychiatric symptoms and cognitive performance for male and female patients separately.

RESULTS

Male patients exhibited higher UCLA loneliness scale scores and social isolation scores compared to female patients (ps < 0.05). In male patients, both loneliness and social isolation significantly predicted PANSS total scores (ps< 0.01), negative subscale scores (ps < 0.05) and general psychopathology subscale scores (ps < 0.05). For female patients, loneliness (not social isolation) significantly predicted immediate memory (p < 0.001), language (p = 0.013), delayed memory (p = 0.017), and RBANS total scores (p = 0.002). Further examination of loneliness components in female patients revealed that personal feelings of isolation were negatively associated with language (r = -0.21, p = 0.001) and a negative correlation exists between lack of collective connectedness and delayed memory (r = -0.19, p = 0.048).

CONCLUSION

Loneliness and social isolation are more pronounced in male SCZ patients than in female patients. Both loneliness and social isolation are positively related to psychiatric symptoms in male patients, while loneliness is negatively associated with cognitive functioning in female patients.

Abnormalities in behavior relevant to schizophrenia in embryonic day 17 MAM-exposed rodent models: A systematic review and meta-analysis

BACKGROUND

Neurodevelopmental disorders, notably schizophrenia, present ongoing challenges in mental health. Methylazoxymethanol (MAM), a potent neurodevelopmental disruptor, is implicated in inducing schizophrenia-like structural and functional alterations in rodent models. This study conducts a systematic review and meta-analysis to assess comprehensively the behavioral consequences of embryonic MAM exposure in rodents, focusing on diverse paradigms reflective of schizophrenia-related phenotypes.

METHODS

Employing a meticulous search strategy across PubMed, Embase, Cochrane Library, Sino Med, CNKI, Weip Database, Wan Fang, and Web of Science, this study adheres to PRISMA guidelines. The analysis includes studies examining the impact of embryonic MAM exposure on behavioral outcomes, such as Prepulse Inhibition (PPI), social interaction (SI), novel object recognition (NOR), elevated plus maze (EPM) performance, and open field test (OFT) results. The study protocol is registered with PROSPERO, number 42024521442 CRD.

RESULTS

Involving 19 studies, the meta-analysis reveals nuanced behavioral alterations. MAM-exposed male rats in the EPM group exhibit a Mean Difference of -0.27 (95 % CI: [-1.02, 0.49]) during puberty, with a broader Mean Difference of -0.50 (95 % CI: [-1.97, 0.96]) in adulthood. Combining both stages yields an overall Mean Difference of -0.31 (95 % CI: [-1.01, 0.38]), indicating potential EPM performance differences. Subgroup analysis by MAM dosage levels reveals a Mean Difference of -0.90 (95 % CI: [-1.86, 0.05]) for moderate-dose MAM and 0.65 (95 % CI: [0.29, 1.02]) for high-dose MAM. In the OFT group, adulthood shows a Mean Difference of -1.22 (95 % CI: [-2.14, -0.29]), emphasizing altered exploratory behavior. The NOR group indicates significant Mean Differences of -6.18 (95 % CI: [-8.41, -3.94]) in adulthood, signifying recognition memory deficits. SI assessments show consistent negative Mean Differences during puberty and adulthood for male rats (-1.88 and - 1.87, respectively) and female rats in preestrus and estrus (-1.09).

CONCLUSIONS

This systematic review and meta-analysis offer a comprehensive overview of behavioral consequences linked to embryonic MAM exposure in rodents. Findings underscore intricate relationships between MAM and various behavioral domains relevant to schizophrenia. Dose-dependent effects, developmental stage considerations, and potential sex-specific influences contribute to the complexity of MAM-induced alterations, advancing our understanding of neurodevelopmental disruptions and suggesting avenues for future research and therapeutic interventions targeting the developmental origins of psychiatric disorders.

Isolated during adolescence: long-term impact on social behavior, pain sensitivity, and the oxytocin system in male and female rats

BACKGROUND

Adolescent social isolation (ASI) has profound long-term effects on behavioral and neural development. Despite this, the specific long-term impact of ASI during different adolescent stages and across sexes remain underexplored.

METHODS

Our study addresses this gap by examining the effects of early- and late- adolescent social isolation on both male and female rats. Rats were either isolated (or group-housed) starting from PD 21 (early) or PD 42 (late) for three weeks and then rehoused into groups. In adulthood (PD 90), rats underwent a battery of tests: elevated plus-maze, open field, novel object recognition, social interaction and social recognition memory and hotplate tests. Finally, we analyzed oxytocin receptor binding in several regions in the brains of a second cohort of rats.

RESULTS

Both, male and female rats from the late adolescent social isolation (LASI) groups spent significantly less time interacting in the social interaction test. Additionally, we observed a general decrease in social recognition memory regardless of sex. Both male ASI groups demonstrated heightened thermal pain sensitivity, while the opposite was observed in early adolescent social isolation (EASI) female rats. In the brain, we observed changes in oxytocin receptor (OTR) binding in the paraventricular nucleus of the hypothalamus (PVN) and paraventricular nucleus of the thalamus (PVT) and central amygdala (CeA) with the largest changes in EASI and LASI female rats.

CONCLUSION

Our model demonstrates long-lasting alterations on behavior and oxytocin receptor binding levels following ASI providing insights into the long-term effects of ASI in a time- and sex-specific manner.

The excitatory-inhibitory balance as a target for the development of novel drugs to treat schizophrenia

The intricate balance between excitation and inhibition (E/I) in the brain plays a crucial role in normative information processing. Dysfunctions in the E/I balance have been implicated in various psychiatric disorders, including schizophrenia (SCZ). In particular, abnormalities in GABAergic signaling, specifically in parvalbumin (PV)-containing interneurons, have been consistently observed in SCZ pathophysiology. PV interneuron function is vital for maintaining an ideal E/I balance, and alterations in PV interneuron-mediated inhibition contribute to circuit deficits observed in SCZ, including hippocampus hyperactivity and midbrain dopamine system overdrive. While current antipsychotic medications primarily target D2 dopamine receptors and are effective primarily in treating positive symptoms, novel therapeutic strategies aiming to restore the E/I balance could potentially mitigate not only positive symptoms but also negative symptoms and cognitive deficits. This could involve, for instance, increasing the inhibitory drive onto excitatory neurons or decreasing the putative enhanced pyramidal neuron activity due to functional loss of PV interneurons. Compounds targeting the glycine site at glutamate NMDA receptors and muscarinic acetylcholine receptors on PV interneurons that can increase PV interneuron drive, as well as drugs that increase the postsynaptic action of GABA, such as positive allosteric modulators of α5-GABA-A receptors, and decrease glutamatergic output, such as mGluR2/3 agonists, represent promising approaches. Preventive strategies aiming at E/I balance also represent a path to reduce the risk of transitioning to SCZ in high-risk individuals. Therefore, compounds with novel mechanisms targeting E/I balance provide optimism for more effective and tailored interventions in the management of SCZ.

Sex-dependent emergence of prepubertal social dysfunction and augmented dopamine activity in a neurodevelopmental rodent model relevant for schizophrenia

Schizophrenia is a neurodevelopmental psychiatric disorder that often emerges in adolescence, is characterized by social dysfunction, and has an earlier onset in men. These features have been replicated in rats exposed to the mitotoxin methylazoxymethanol acetate (MAM) on gestational day (GD) 17, which as adults exhibit behavioral impairments and dopamine (DA) system changes consistent with a schizophrenia-relevant rodent model. In humans, social withdrawal is a negative symptom that often precedes disease onset and DA system dysfunction and is more pronounced in men. Children and adolescents at high-risk for schizophrenia exhibit social deficits prior to psychotic symptoms (i.e., prodromal phase), which can be used as a predictive marker for future psychopathology. Adult MAM rats also exhibit deficient social interaction, but less is known regarding the emergence of social dysfunction in this model, whether it varies by sex, and whether it is linked to disrupted DA function. To this end, we characterized the ontogeny of social and DA dysfunction in male and female MAM rats during the prepubertal period (postnatal days 33-43) and found sex-specific changes in motivated social behaviors (play, approach) and DA function. Male MAM rats exhibited reduced social approach and increased VTA DA neuron activity compared to saline-treated (SAL) males, whereas female MAM rats exhibited enhanced play behaviors compared to SAL females but no changes in social approach or VTA population activity during this period. These findings demonstrate sex differences in the emergence of social and DA deficits in the MAM model, in which females exhibit delayed emergence.

Acute stress modulates noradrenergic signaling in the ventral tegmental area-amygdalar circuit

Noradrenergic neurotransmission is a critical mediator of stress responses. In turn, exposure to stress induces noradrenergic system adaptations, some of which are implicated in the etiology of stress-related disorders. Adrenergic receptors (ARs) in the ventral tegmental area (VTA) have been demonstrated to regulate phasic dopamine (DA) release in the forebrain, necessary for behavioral responses to conditional cues. However, the impact of stress on noradrenergic modulation of the VTA has not been previously explored. We demonstrate that ARs in the VTA regulate dopaminergic activity in the VTA-BLA (basolateral amygdala) circuit, a key system for processing stress-related stimuli; and that such control is altered by acute stress. We utilized fast-scan cyclic voltammetry to assess the effects of intra-VTA microinfusion of α₁ -AR and α₂ -AR antagonists (terazosin and RX-821002, respectively), on electrically evoked phasic DA release in the BLA in stress-naïve and stressed (unavoidable electric shocks - UES) anesthetized male Sprague-Dawley rats. In addition, we used western blotting to explore UES-induced alterations in AR protein level in the VTA. Intra-VTA terazosin or RX-821002 dose-dependently attenuated DA release in the BLA. Interestingly, UES decreased the effects of intra-VTA α₂ -AR blockade on DA release (24 h but not 7 days after stress), while the effects of terazosin were unchanged. Despite changes in α₂ -AR physiological function in the VTA, UES did not alter α₂ -AR protein levels in either intracellular or membrane fractions. These findings demonstrate that NA-ergic modulation of the VTA-BLA circuit undergoes significant alterations in response to acute stress, with α₂ -AR signaling indicated as a key target.

Alpha-2A but not 2B/C noradrenergic receptors in ventral tegmental area regulate phasic dopamine release in nucleus accumbens core

Adrenergic receptors (AR) in the ventral tegmental area (VTA) modulate local neuronal activity and, as a consequence, dopamine (DA) release in the mesolimbic forebrain. Such modulation has functional significance: intra-VTA blockade of α₁-AR attenuates behavioral responses to salient environmental stimuli in rat models of drug seeking and conditioned fear as well as phasic DA release in the nucleus accumbens (NAc). In contrast, α₂-AR in the VTA has been suggested to act primarily as autoreceptors, limiting local noradrenergic input. The regulation of noradrenaline efflux by α₂-AR could be of clinical interest, as α₂-AR agonists are proposed as promising pharmacological tools in the treatment of PTSD and substance use disorder. Thus, the aim of our study was to determine the subtype-specificity of α₂-ARs in the VTA capable of modulating phasic DA release. We used fast scan cyclic voltammetry (FSCV) in anaesthetized male rats to measure DA release in the NAc after combined electrical stimulation and infusion of selected α₂-AR antagonists into the VTA. Intra-VTA microinfusion of idazoxan - a non-subtype-specific α₂-AR antagonist, as well as BRL-44408 - a selective α_2A-AR antagonist, attenuated electrically-evoked DA in the NAc. In contrast, local administration of JP-1302 or imiloxan (α_2B- and α_2C-AR antagonists, respectively) had no effect. The effect of BRL-44408 on DA release was attenuated by intra-VTA DA D₂ antagonist (raclopride) pre-administration. Finally, we confirmed the presence of α_2A-AR protein in the VTA using western blotting. In conclusion, these data specify α_2A-, but not α_2B- or α_2C-AR as the receptor subtype controlling NA release in the VTA.

Persistent behavioral and neurobiological consequences of social isolation during adolescence

Meaningful social interactions are a fundamental human need, the lack of which can pose serious risks to an individual's physical and mental health. Across species, peer-oriented social behaviors are dramatically reshaped during adolescence, a developmental period characterized by dynamic changes in brain structure and function as individuals transition into adulthood. Thus, the experience of social isolation during this critical developmental stage may be especially pernicious, as it could permanently derail typical neurobiological processes that are necessary for establishing adaptive adult behaviors. The purpose of this review is to summarize investigations in which rodents were isolated during adolescence, then re-housed in typical social groups prior to testing, thus allowing the investigators to resolve the long-term consequences of social adversity experienced during adolescent sensitive periods, despite subsequent normalization of the social environment. Here, we discuss alterations in social, anxiety-like, cognitive, and decision-making behaviors in previously isolated adult rodents. We then explore corresponding neurobiological findings, focusing on the prefrontal cortex, including changes in synaptic densities and protein levels, white matter and oligodendrocyte function, and neuronal physiology. Made more urgent by the recent wave of social deprivation resulting from the COVID-19 pandemic, especially amongst school-aged adolescents, understanding the mechanisms by which even transient social adversity can negatively impact brain function across the lifespan is of paramount importance.

Inhibition of BET Proteins during Adolescence Affects Prefrontal Cortical Development: Relevance to Schizophrenia

Background: The present study investigated the role of proteins from the bromodomain and extra-terminal (BET) family in schizophrenia-like abnormalities in a neurodevelopmental model of schizophrenia induced by prenatal methylazoxymethanol (MAM) administration (MAM-E17). Methods: An inhibitor of BET proteins, JQ1, was administered during adolescence on postnatal days (P) 23–P29, and behavioural responses (sensorimotor gating, recognition memory) and prefrontal cortical (mPFC) function (long-term potentiation (LTP), molecular and proteomic analyses) studies were performed in adult males and females. Results: Deficits in sensorimotor gating and recognition memory were observed only in MAM-treated males. However, adolescent JQ1 treatment affected animals of both sexes in the control but not MAM-treated groups and reduced behavioural responses in both sexes. An electrophysiological study showed LTP impairments only in male MAM-treated animals, and JQ1 did not affect LTP in the mPFC. In contrast, MAM did not affect activity-dependent gene expression, but JQ1 altered gene expression in both sexes. A proteomic study revealed alterations in MAM-treated groups mainly in males, while JQ1 affected both sexes. Conclusions: MAM-induced schizophrenia-like abnormalities were observed only in males, while adolescent JQ1 treatment affected memory recognition and altered the molecular and proteomic landscape in the mPFC of both sexes. Thus, transient adolescent inhibition of the BET family might prompt permanent alterations in the mPFC.

Prenatal MAM treatment altered fear conditioning following social isolation: Relevance to schizophrenia

Adolescent social isolation (SI) might change the trajectory of brain development. In the present study, we investigated the effect of short-term adolescent SI on fear memory, anxiety and protein levels in the adult medial prefrontal cortex of rats prenatally treated with methylazoxymethanol, MAM-E17 model of schizophrenia. The animals were maintained in standard housing (SH) or social isolation (P30-P40, SI) conditions. Behavioural tests (trace or delay fear conditioning, light/dark box) were performed in late adolescence and early adulthood. The results showed that MAM treatment did not alter fear memory, which was investigated with the use of either trace or delay fear conditioning, at any age, and SI decreased the fear response in adult control animals only under trace conditioning. Neither MAM nor SI influenced anxiety-related behaviour measured in the light/dark box. A proteomics study showed that both MAM and SI changed the protein levels related to synapse maturation and cytoskeletal organization, energy transfer and metabolic processes. Prenatal or adolescent environmental factors are able to change the expression of proteins that are correlated with behavioural impairments. Moreover, SI reversed some alterations in proteins induced by MAM. Thus, normally developing brains showed different responses to adolescent SI than those with altering courses of MAM administration.

Adolescent social isolation affects parvalbumin expression in the medial prefrontal cortex in the MAM-E17 model of schizophrenia

Altered parvalbumin (PV) expression is observed in the prefrontal cortex of subjects with schizophrenia. Environmental context, particularly during adolescence, might regulate PV expression. In the present study, we investigated the effect of adolescent social isolation (SI) on PV expression in the medial prefrontal cortex in a neurodevelopmental model (MAM-E17) of schizophrenia. SI exposure occurred from postnatal day 30 to 40, followed by resocialization until late adolescence or early adulthood. PV mRNA and protein levels, as well as the number of PV cells, were analysed at these ages. Moreover, epigenetic regulation of PV expression by histone methylation was examined by measuring the total and PV gene-bound H3K4me3 levels. MAM only decreased levels of the PV mRNA and protein in adulthood. Decreases in total H3K4me3 levels and its level at the PV gene were also observed at this age. In contrast, in late adolescence, SI induced a decrease in the expression of the PV mRNA in the MAM group that was related to the reduction in total and PV gene-bound H3K4me3 levels. However, at this age, SI increased the levels of the PV protein in both the control and MAM groups. In adulthood, SI did not affect PV mRNA or H3K4me3 levels but decreased levels of the PV protein in both groups. Both MAM and SI failed to change the number of PV cells at any age. The results indicate that adolescent SI accelerated epigenetic impairments of PV expression in MAM-E17 rats; however, subsequent resocialization abolished this dysfunction, but failed to prevent alterations in PV protein.