Effects of diazepam on hippocampal blood flow in people at clinical high risk for psychosis

Elevated hippocampal perfusion has been observed in people at clinical high risk for psychosis (CHR-P). Preclinical evidence suggests that hippocampal hyperactivity is central to the pathophysiology of psychosis, and that peripubertal treatment with diazepam can prevent the development of psychosis-relevant phenotypes. The present experimental medicine study examined whether diazepam can normalize hippocampal perfusion in CHR-P individuals. Using a randomized, double-blind, placebo-controlled, crossover design, 24 CHR-P individuals were assessed with magnetic resonance imaging (MRI) on two occasions, once following a single oral dose of diazepam (5 mg) and once following placebo. Regional cerebral blood flow (rCBF) was measured using 3D pseudo-continuous arterial spin labeling and sampled in native space using participant-specific hippocampus and subfield masks (CA1, subiculum, CA4/dentate gyrus). Twenty-two healthy controls (HC) were scanned using the same MRI acquisition sequence, but without administration of diazepam or placebo. Mixed-design ANCOVAs and linear mixed-effects models were used to examine the effects of group (CHR-P placebo/diazepam vs. HC) and condition (CHR-P diazepam vs. placebo) on rCBF in the hippocampus as a whole and by subfield. Under the placebo condition, CHR-P individuals (mean [±SD] age: 24.1 [±4.8] years, 15 F) showed significantly elevated rCBF compared to HC (mean [±SD] age: 26.5 [±5.1] years, 11 F) in the hippocampus (F(1,41) = 24.7, pFDR < 0.001) and across its subfields (all pFDR < 0.001). Following diazepam, rCBF in the hippocampus (and subfields, all pFDR < 0.001) was significantly reduced (t(69) = -5.1, pFDR < 0.001) and normalized to HC levels (F(1,41) = 0.4, pFDR = 0.204). In conclusion, diazepam normalized hippocampal hyperperfusion in CHR-P individuals, consistent with evidence implicating medial temporal GABAergic dysfunction in increased vulnerability for psychosis.

Effects of Benzodiazepine Exposure on Real-World Clinical Outcomes in Individuals at Clinical High Risk for Psychosis

BACKGROUND AND HYPOTHESIS

Animal models indicate GABAergic dysfunction in the development of psychosis, and that benzodiazepine (BDZ) exposure can prevent the emergence of psychosis-relevant phenotypes. However, whether BDZ exposure influences real-world clinical outcomes in individuals at clinical high risk for psychosis (CHR-P) is unknown.

STUDY DESIGN

This observational cohort study used electronic health record data from CHR-P individuals to investigate whether BDZ exposure (including hypnotics, eg, zopiclone) reduces the risk of developing psychosis and adverse clinical outcomes. Cox proportional-hazards models were employed in both the whole-unmatched sample, and a propensity score matched (PSM) subsample.

STUDY RESULTS

567 CHR-P individuals (306 male, mean[±SD] age = 22.3[±4.9] years) were included after data cleaning. The BDZ-exposed (n = 105) and BDZ-unexposed (n = 462) groups differed on several demographic and clinical characteristics, including psychotic symptom severity. In the whole-unmatched sample, BDZ exposure was associated with increased risk of transition to psychosis (HR = 1.61; 95% CI: 1.03-2.52; P = .037), psychiatric hospital admission (HR = 1.93; 95% CI: 1.13-3.29; P = .017), home visit (HR = 1.64; 95% CI: 1.18-2.28; P = .004), and Accident and Emergency department attendance (HR = 1.88; 95% CI: 1.31-2.72; P < .001). However, after controlling for confounding-by-indication through PSM, BDZ exposure did not modulate the risk of any outcomes (all P > .05). In an analysis restricted to antipsychotic-naïve individuals, BDZ exposure reduced the risk of transition to psychosis numerically, although this was not statistically significant (HR = 0.59; 95% CI: 0.32-1.08; P = .089).

CONCLUSIONS

BDZ exposure in CHR-P individuals was not associated with a reduction in the risk of psychosis transition or adverse clinical outcomes. Results in the whole-unmatched sample suggest BDZ prescription may be more likely in CHR-P individuals with higher symptom severity.

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.

Effects of Benzodiazepine Exposure on Real-World Clinical Outcomes in Individuals at Clinical High-Risk for Psychosis

Background

Animal models indicate GABAergic dysfunction in the development of psychosis, and that benzodiazepine (BDZ) exposure can prevent the emergence of psychosis-relevant phenotypes. However, whether BDZ exposure influences the risk of psychosis in humans is unknown.

Methods

This observational-cohort study used electronic health record data from 818 individuals at clinical high-risk for psychosis (CHR-P) to investigate whether BDZ exposure (including hypnotics e.g., zopiclone) reduces the risk of developing psychosis and adverse clinical outcomes. Cox proportional-hazards models were employed in both the whole-unmatched sample, and a propensity score matched (PSM) subsample.

Results

567 CHR-P individuals were included after data cleaning (105 BDZ-exposed, 462 BDZ-unexposed). 306 (54%) individuals were male, and the mean age was 22.3 years (SD 4.9). The BDZ-exposed and BDZ-unexposed groups differed on several demographic and clinical characteristics, including psychotic symptom severity. In the whole-unmatched sample, BDZ exposure was associated with increased risk of transition to psychosis (HR=1.61; 95%CI:1.03-2.52; P=0.037), psychiatric hospital admission (HR=1.93; 95%CI:1.13-3.29; P=0.017), home visit (HR=1.64; 95%CI:1.18-2.28; P=0.004), and A&E attendance (HR=1.88; 95%CI:1.31-2.72; P<0.001). However, after controlling for confounding-by-indication through PSM, BDZ exposure did not modulate the risk of any outcomes (all P>0.05). In analysis restricted to antipsychotic-naïve individuals, BDZ exposure reduced the risk of transition to psychosis at trend-level (HR=0.59; 95%CI:0.32-1.08; P=0.089).

Conclusions

BDZ exposure in CHR-P individuals was not associated with a reduction in the risk of psychosis transition or other adverse clinical outcomes. Results in the whole-unmatched sample suggest BDZ prescription may be more likely in CHR-P individuals with higher symptom severity.

Insights into the Mechanism of Action of Antipsychotic Drugs Derived from Animal Models: Standard of Care versus Novel Targets

Therapeutic intervention for schizophrenia relies on blockade of dopamine D2 receptors in the associative striatum; however, there is little evidence for baseline overdrive of the dopamine system. Instead, the dopamine system is in a hyper-responsive state due to excessive drive by the hippocampus. This causes more dopamine neurons to be in a spontaneously active, hyper-responsive state. Antipsychotic drugs alleviate this by causing depolarization block, or excessive depolarization-induced dopamine neuron inactivation. Indeed, both first- and second-generation antipsychotic drugs cause depolarization block in the ventral tegmentum to relieve positive symptoms, whereas first-generation drugs also cause depolarization in the nigrostriatal dopamine system to lead to extrapyramidal side effects. However, by blocking dopamine receptors, these drugs are activating multiple synapses downstream from the proposed site of pathology: the loss of inhibitory influence over the hippocampus. An overactive hippocampus not only drives the dopamine-dependent positive symptoms, but via its projections to the amygdala and the neocortex can also drive negative and cognitive symptoms, respectively. On this basis, a novel class of drugs that can reverse schizophrenia at the site of pathology, i.e., the hippocampal overdrive, could be effective in alleviating all three classes of symptoms of schizophrenia while also being better tolerated.

Medial prefrontal cortex to medial septum pathway activation improves cognitive flexibility in rats

BACKGROUND

The medial prefrontal cortex (mPFC) is necessary for cognitive flexibility and projects to medial septum (MS). MS activation improves strategy switching, a common measure of cognitive flexibility, via its ability to regulate midbrain dopamine (DA) neuron population activity. We hypothesized that the mPFC to MS pathway may be the mechanism by which the MS regulates strategy switching and DA neuron population activity.

METHODS

Male and female rats learned a complex discrimination strategy across two different training time points: a constant length (10 days) and a variable length that coincided with each rat meeting an acquisition-level performance threshold (males-5.3 + 0.3 days, females-3.8 + 0.3 days). We then chemogenetically activated or inhibited the mPFC-MS pathway and measured each rat's ability to inhibit the prior learned discrimination strategy and switch to a prior ignored discrimination strategy (strategy switching).

RESULTS

Activation of the mPFC-MS pathway improved strategy switching after 10 days of training in both sexes, while inhibition of the pathway had little to no effect on strategy switching. Neither activation nor inhibition of the mPFC-MS pathway affected strategy switching following the acquisition-level performance threshold. Activation, but not inhibiton, of the mPFC-MS pathway bidirectionally regulated DA activity in ventral tegmental area and substantia nigra pars compacta, similar to MS activation.

CONCLUSIONS

This study presents a top-down circuit from the prefrontal cortex to the midbrain, with the MS being a critical contributor, by which DA activity can be manipulated to promote flexible behavior.

Update on current animal models for schizophrenia: are they still useful?

PURPOSE OF REVIEW

Schizophrenia is a psychiatric disorder that has a significant socioeconomic impact worldwide. Antipsychotic drugs targeting dopamine transmission alleviate psychotic symptoms but with limited efficacy and tolerability. Animal models have long proven useful for drug discovery. The continued need for new treatment highlights the importance of animal models to study schizophrenia. The lack of new therapeutic compounds combined with the shortcomings of clinical design studies potentially decreased the enthusiasm for animal model use.

RECENT FINDINGS

In the current review, we discuss the central role of animal models for schizophrenia in providing new insights into neurobiological features and therapeutic development. The US National Institute of Mental Health released the Research Domain Criteria to guide preclinical model studies. Here, we point out the advances of this approach and debate its potential limitations when using animal models to study schizophrenia from the drug discovery perspective.

SUMMARY

Cross-validated animal models for schizophrenia are crucial to comprehend the cause, pathophysiology, and behavioral and biological features of the disease, to advance prevention and treatment, and the need to carefully evaluate and select appropriate paradigms when investigating novel therapeutic targets.

Sex- and exposure age-dependent effects of adolescent stress on ventral tegmental area dopamine system and its afferent regulators

Adolescent stress is a risk factor for schizophrenia. Emerging evidence suggests that age-dependent sensitive windows for childhood trauma are associated more strongly with adult psychosis, but the neurobiological basis and potential sex differences are unknown.Using in vivo electrophysiology and immunohistology in rats, we systematically compared the effects of two age-defined adolescent stress paradigms, prepubertal (postnatal day [PD] 21-30; PreP-S) and postpubertal (PD41-50; PostP-S) foot-shock and restraint combined stress, on ventral tegmental area (VTA) dopaminergic activity, pyramidal neuron activity in the ventral hippocampus (vHipp) and the basolateral amygdala (BLA), corticoamygdalar functional inhibitory control, and vHipp and BLA parvalbumin interneuron (PVI) impairments. These endpoints were selected based on their well-documented roles in the pathophysiology of psychosis.Overall, we found distinct sex- and exposure age-dependent stress vulnerability. Specifically, while males were selectively vulnerable to PreP-S-induced adult VTA dopamine neuron and vHipp hyperactivities, females were selectively vulnerable to PostP-S. These male selective PreP-S effects were correlated with stress-induced aberrant persistent BLA hyperactivity, dysfunctional prefrontal inhibitory control of BLA neurons, and vHipp/BLA PVI impairments. In contrast, female PostP-S only produced vHipp PVI impairments in adults, with the BLA structure and functions largely unaffected.Our results indicated distinct adolescent-sensitive periods during which stress can sex-dependently confer maximal risks to corticolimbic systems to drive dopamine hyperactivity, which provide critical insights into the neurobiological basis for sex-biased stress-related psychopathologies emphasizing but not limited to schizophrenia. Furthermore, our work also provides a framework for future translational research on age-sensitive targeted interventions.

Levetiracetam Attenuates Adolescent Stress-induced Behavioral and Electrophysiological Changes Associated With Schizophrenia in Adult Rats

BACKGROUND AND HYPOTHESIS

Stress during adolescence is a major risk factor for schizophrenia. We have found previously in rats that adolescent stress caused, in adulthood, behavioral changes and enhanced ventral tegmental area (VTA) dopamine system activity, which were associated with dysregulation of the excitatory-inhibitory (E/I) balance in the ventral hippocampus (vHip). Levetiracetam, an anticonvulsant drug, regulates the release of neurotransmitters, including glutamate, via SV2A inhibition. It also modulates parvalbumin interneuron activity via Kv3.1 channels. Therefore, levetiracetam could ameliorate deficits in the E/I balance. We tested whether levetiracetam attenuate the adolescent stress-induced behavioral changes, vHip hyperactivity, and enhanced VTA dopamine system activity in adult rats.

STUDY DESIGN

Male Sprague-Dawley rats were subjected to a combination of daily footshock (postnatal day [PD] 31-40), and three 1 h-restraint stress sessions (at PD31, 32, and 40). In adulthood (PD62), animals were tested for anxiety responses (elevated plus-maze and light-dark box), social interaction, and cognitive function (novel object recognition test). The activity of vHip pyramidal neurons and VTA dopamine neurons was also recorded.

STUDY RESULTS

Adolescent stress produced anxiety-like responses and impaired sociability and cognitive function. Levetiracetam (10 mg/kg) reversed these changes. Levetiracetam also reversed the increased VTA dopamine neuron population activity and the enhanced firing rate of vHip pyramidal neurons induced by adolescent stress.

CONCLUSIONS

These findings suggest that levetiracetam attenuates the adverse outcomes associated with schizophrenia caused by stress during adolescence.

Dopamine downregulation in novel rodent models useful for the study of postpartum depression

Postpartum depression (PPD) is the most common psychiatric disorder following childbirth and is characterized by maternal mood disturbances, impaired maternal responses, and disrupted caregiving- all of which negatively impact offspring development. Since PPD has detrimental consequences for both mother and child, clinical and preclinical research has focused on identifying brain changes associated with this disorder. In humans, PPD is linked to dysregulated mesolimbic dopamine (DA) system function and altered neural responses (i.e., decreased reward-related activity) to infant-related cues, which are considered hallmark features of PPD. In accordance, rodent models employing translational risk factors useful for the study of PPD have demonstrated alterations in mesolimbic DA system structure and function, and these changes are reviewed here. We also present two novel rodent models based on postpartum adversity exposure (i.e., pup removal, scarcity-adversity) which result in PPD-relevant behavioral changes (e.g., disrupted mother-infant interactions, deficits in maternal behavior, depressive-like phenotypes) and attenuated ventral tegmental area (VTA) DA neuron activity consistent with a hypodopaminergic state. Furthermore, we highlight open questions and future directions for these rodent models. In sum, human and rodent studies converge in showing blunted mesolimbic DA function (i.e., DA downregulation) in PPD. We propose that reduced activity of VTA DA neurons, resulting in downregulation of the mesolimbic DA system, interferes with reward-related processes necessary for maternal motivation and responsiveness. Thus, the mesolimbic DA system may constitute a therapeutic target for ameliorating reward-related deficits in PPD.

Medial septum activation improves strategy switching once strategies are well-learned via bidirectional regulation of dopamine neuron population activity

Strategy switching is a form of cognitive flexibility that requires inhibiting a previously successful strategy and switching to a new strategy of a different categorical modality. It is dependent on dopamine (DA) receptor activation and release in ventral striatum and prefrontal cortex, two primary targets of ventral tegmental area (VTA) DA projections. Although the circuitry that underlies strategy switching early in learning has been studied, few studies have examined it after extended discrimination training. This may be important as DA activity and release patterns change across learning, with several studies demonstrating a critical role for substantia nigra pars compacta (SNc) DA activity and release once behaviors are well-learned. We have demonstrated that medial septum (MS) activation simultaneously increased VTA and decreased SNc DA population activity, as well as improved reversal learning via these actions on DA activity. We hypothesized that MS activation would improve strategy switching both early in learning and after extended training through its ability to increase VTA DA population activity and decrease SNc DA population activity, respectively. We chemogenetically activated the MS of male and female rats and measured their performance on an operant-based strategy switching task following 1, 10, or 15 days of discrimination training. Contrary to our hypothesis, MS activation did not affect strategy switching after 1 day of discrimination training. MS activation improved strategy switching after 10 days of training, but only in females. MS activation improved strategy switching in both sexes after 15 days of training. Infusion of bicuculline into the ventral subiculum (vSub) inhibited the MS-mediated decrease in SNc DA population activity and attenuated the improvement in strategy switching. Intra-vSub infusion of scopolamine inhibited the MS-mediated increase in VTA DA population activity but did not affect the improvement in strategy switching. Intra-vSub infusion of both bicuculline and scopolamine inhibited the MS-mediated effects on DA population activity in both the SNc and VTA and completely prevented the improvement in strategy switching. These data indicate that MS activation improves strategy switching once the original strategy has been sufficiently well-learned, and that this may occur via the MS's regulation of DA neuron responsivity.

Contingent Amygdala Inputs Trigger Heterosynaptic LTP at Hippocampus-To-Accumbens Synapses

The nucleus accumbens shell (NAcSh) is a key brain region where environmental cues acquire incentive salience to reinforce motivated behaviors. Principal medium spiny neurons (MSNs) in the NAcSh receive extensive glutamatergic projections from limbic regions, among which, the ventral hippocampus (vH) transmits information enriched in contextual cues, and the basolateral amygdala (BLA) encodes real-time arousing states. The vH and BLA project convergently to NAcSh MSNs, both activated in a time-locked manner on a cue-conditioned motivational action. In brain slices prepared from male and female mice, we show that co-activation of the two projections induces long-term potentiation (LTP) at vH-to-NAcSh synapses without affecting BLA-to-NAcSh synapses, revealing a heterosynaptic mechanism through which BLA signals persistently increase the temporally contingent vH-to-NAcSh transmission. Furthermore, this LTP is more prominent in dopamine D1 receptor-expressing (D1) MSNs than D2 MSNs and can be prevented by inhibition of either D1 receptors or dopaminergic terminals in NAcSh. This heterosynaptic LTP may provide a dopamine-guided mechanism through which vH-encoded cue inputs that are contingent to BLA activation acquire increased circuit representation to reinforce behavior.SIGNIFICANCE STATEMENT In motivated behaviors, environmental cues associated with arousing stimuli acquire increased incentive salience, processes mediated in part by the nucleus accumbens (NAc). NAc principal neurons receive glutamatergic projections from the ventral hippocampus (vH) and basolateral amygdala (BLA), which transmit information encoding contextual cues and affective states, respectively. Our results show that co-activation of the two projections induces long-term potentiation (LTP) at vH-to-NAc synapses without affecting BLA-to-NAc synapses, revealing a heterosynaptic mechanism through which BLA signals potentiate the temporally contingent vH-to-NAc transmission. Furthermore, this LTP is prevented by inhibition of either D1 receptors or dopaminergic axons. This heterosynaptic LTP may provide a dopamine-guided mechanism through which vH-encoded cue inputs that are contingent to BLA activation acquire increased circuit representation to reinforce behavior.

Using animal models for the studies of schizophrenia and depression: The value of translational models for treatment and prevention

Animal models of psychiatric disorders have been highly effective in advancing the field, identifying circuits related to pathophysiology, and identifying novel therapeutic targets. In this review, we show how animal models, particularly those based on development, have provided essential information regarding circuits involved in disorders, disease progression, and novel targets for intervention and potentially prevention. Nonetheless, in recent years there has been a pushback, largely driven by the US National Institute of Mental Health (NIMH), to shift away from animal models and instead focus on circuits in normal subjects. This has been driven primarily from a lack of discovery of new effective therapeutic targets, and the failure of targets based on preclinical research to show efficacy. We discuss why animal models of complex disorders, when strongly cross-validated by clinical research, are essential to understand disease etiology as well as pathophysiology, and direct new drug discovery. Issues related to shortcomings in clinical trial design that confound translation from animal models as well as the failure to take patient pharmacological history into account are proposed to be a source of the failure of what are likely effective compounds from showing promise in clinical trials.

Monoamine oxidase inhibition in cigarette smokers: From preclinical studies to tobacco product regulation

Monoamine oxidase (MAO) activity is reduced in cigarette smokers and this may promote the reinforcing actions of nicotine, thereby enhancing the addictive properties of cigarettes. At present, it is unclear how cigarette smoking leads to MAO inhibition, but preclinical studies in rodents show that MAO inhibition increases nicotine self-administration, especially at low doses of nicotine. This effect of MAO inhibition develops slowly, likely due to plasticity of brain monoamine systems; studies relying on acute MAO inhibition are unlikely to replicate what happens with smoking. Given that MAO inhibition may reduce the threshold level at which nicotine becomes reinforcing, it is important to consider this in the context of very low nicotine content (VLNC) cigarettes and potential tobacco product regulation. It is also important to consider how this interaction between MAO inhibition and the reinforcing actions of nicotine may be modified in populations that are particularly vulnerable to nicotine dependence. In the context of these issues, we show that the MAO-inhibiting action of cigarette smoke extract (CSE) is similar in VLNC cigarettes and cigarettes with a standard nicotine content. In addition, we present evidence that in a rodent model of schizophrenia the effect of MAO inhibition to enhance nicotine self-administration is absent, and speculate how this may relate to brain serotonin systems. These issues are relevant to the MAO-inhibiting effect of cigarette smoking and its implications to tobacco product regulation.

Nucleus reuniens inactivation reverses stress-induced hypodopaminergic state and altered hippocampal-accumbens synaptic plasticity

The nucleus reuniens of the thalamus (RE) is a pivotal area responsible for the connectivity of the prefrontal-hippocampus pathway that regulates cognitive, executive, and fear learning processes. Recently, it was proposed that the RE participates in the pathophysiological states related to affective dysregulation. We investigated the role of RE in motivational behavioral and electrophysiological dysregulation induced by stress. Adult Sprague-Dawley rats were exposed to a combination of stressors (restraint stress+footshock) for 10 days and tested one to two weeks later in the forced swim test (FST), ventral tegmental area (VTA)dopamine (DA) neuron electrophysiological activity, and hippocampal-nucleus accumbens plasticity. The RE was inactivated by injecting TTX prior to the procedures. The stress exposure increased the immobility in the FST and decreased VTA DA neuron population activity. Whereas an early long-term potentiation (e-LTP) in the ventral hippocampus-nucleus accumbens pathway was found after fimbria high-frequency stimulation in naïve animals, stressed animals showed an early long-term depression (e-LTD). Inactivation of the RE reversed the stress-induced changes in the FST and restored dopaminergic activity. RE inactivation partially recovered the stress-induced abnormal hippocampal-accumbens plasticity observed in controls. Our findings support the role of the RE in regulating affective dysregulation and blunted VTA DA system function induced by stress. Also, it points to the hippocampal-accumbens pathway as a potential neural circuit through which RE could modulate activity. Therefore, RE may represent a key brain region involved in the neurobiology of amotivational states and may provide insights into circuit dysfunction and markers of the maladaptive stress response.

Dopaminergic dysfunction and excitatory/inhibitory imbalance in treatment-resistant schizophrenia and novel neuromodulatory treatment

Antipsychotic drugs are the mainstay in the treatment of schizophrenia. However, one-third of patients do not show adequate improvement in positive symptoms with non-clozapine antipsychotics. Additionally, approximately half of them show poor response to clozapine, electroconvulsive therapy, or other augmentation strategies. However, the development of novel treatment for these conditions is difficult due to the complex and heterogenous pathophysiology of treatment-resistant schizophrenia (TRS). Therefore, this review provides key findings, potential treatments, and a roadmap for future research in this area. First, we review the neurobiological pathophysiology of TRS, particularly the dopaminergic, glutamatergic, and GABAergic pathways. Next, the limitations of existing and promising treatments are presented. Specifically, this article focuses on the therapeutic potential of neuromodulation, including electroconvulsive therapy, repetitive transcranial magnetic stimulation, transcranial direct current stimulation, and deep brain stimulation. Finally, we propose multivariate analyses that integrate various perspectives of the pathogenesis, such as dopaminergic dysfunction and excitatory/inhibitory imbalance, thereby elucidating the heterogeneity of TRS that could not be obtained by conventional statistics. These analyses can in turn lead to a precision medicine approach with closed-loop neuromodulation targeting the detected pathophysiology of TRS.

The D2-like Dopamine Receptor Agonist Quinpirole Microinjected Into the Ventral Pallidum Dose-Dependently Inhibits the VTA and Induces Place Aversion

BACKGROUND

The ventral pallidum (VP) is a dopaminoceptive forebrain structure regulating the ventral tegmental area (VTA) dopaminergic population activity. We have recently demonstrated that in the VP, the D2-like dopamine (DA) receptor agonist quinpirole dose dependently facilitates memory consolidation in inhibitory avoidance and spatial learning. According to our hypothesis, quinpirole microinjected into the VP can modulate the VTA DAergic activity and influence motivation and learning processes of rats.

METHODS

Quinpirole was microinjected at 3 different doses into the VP of male rats, and controls received vehicle. Single unit recordings were employed to assess VTA DAergic activity. To investigate the possible reinforcing or aversive effect of quinpirole in the VP, the conditioned place preference paradigm was used.

RESULTS

Our results showed that intra-VP quinpirole microinjection regulates VTA DAergic neurons according to an inverted U-shaped dose-response curve. The largest dose of quinpirole decreased the population activity and strongly reduced burst activity of the DAergic neurons in the first hour after its application. In contrast, the 2 smaller doses increased DA population activity, but their effect started with a delay 1 hour after their microinjection. The CPP experiments revealed that the largest dose of quinpirole in the VP induced place aversion in the rats. Furthermore, the largest dose of quinpirole induced an acute locomotor activity reduction, while the medium dose led to a long-duration increase in locomotion.

CONCLUSIONS

In summary, quinpirole dose dependently regulates VTA DAergic activity as well as the motivation and motor behavior of the rats at the level of the VP.

Use of prepubertal environment enrichment to prevent dopamine dysregulation in a neurodevelopmental rat model of schizophrenia risk

Here, we present a protocol for delivering environmental enrichment (EE) in discrete postnatal windows to prevent long-term dopamine neuron dysfunctions in a neurodevelopmental rat model of schizophrenia risk. We describe generation of the schizophrenia model through prenatal treatment of rats with methylazoxymethanol acetate (the MAM model) and the saline-treated controls. We then detail the 10-day or 20-day EE paradigms applied on male rats at different ages. This protocol also includes preparation of control groups in regular environment (RE) cages for comparison.

For complete details on the use and execution of this protocol, please refer to Zhu and Grace (2021).

•

Protocol to deliver EE in postnatal windows of a rat model of schizophrenia risk

•

Detailed description of supplies and procedures to construct EE boxes

•

Details of animal loading/offloading, cage maintenance, and design of control groups

•

Expected electrophysiological outcomes and troubleshooting of common pitfalls

GABAA and NMDA receptor density alterations and their behavioral correlates in the gestational methylazoxymethanol acetate model for schizophrenia

Hippocampal hyperactivity driven by GABAergic interneuron deficits and NMDA receptor hypofunction is associated with the hyperdopaminergic state often observed in schizophrenia. Furthermore, previous research in the methylazoxymethanol acetate (MAM) rat model has demonstrated that repeated peripubertal diazepam administration can prevent the emergence of adult hippocampal hyperactivity, dopamine-system hyperactivity, and associated psychosis-relevant behaviors. Here, we sought to characterize hippocampal GABAA and NMDA receptors in MAM-treated rats and to elucidate the receptor mechanisms underlying the promising effects of peripubertal diazepam exposure. Quantitative receptor autoradiography was used to measure receptor density in the dorsal hippocampus CA1, ventral hippocampus CA1, and ventral subiculum. Specifically, [3H]-Ro15-4513 was used to quantify the density of α5GABAA receptors (α5GABAAR), [3H]-flumazenil to quantify α1-3;5GABAAR, and [3H]-MK801 to quantify NMDA receptors. MAM rats exhibited anxiety and schizophrenia-relevant behaviors as measured by elevated plus maze and amphetamine-induced hyperlocomotion (AIH), although diazepam only partially rescued these behaviors. α5GABAAR density was reduced in MAM-treated rats in all hippocampal sub-regions, and negatively correlated with AIH. Ventral hippocampus CA1 α5GABAAR density was positively correlated with anxiety-like behavior. Dorsal hippocampus CA1 NMDA receptor density was increased in MAM-treated rats, and positively correlated with AIH. [3H]-flumazenil revealed no significant effects. Finally, we found no significant effect of diazepam treatment on receptor densities, potentially related to the only partial rescue of schizophrenia-relevant phenotypes. Overall, our findings provide first evidence of α5GABAAR and NMDA receptor abnormalities in the MAM model, suggesting that more selective pharmacological agents may become a novel therapeutic mechanism in schizophrenia.

Adverse clinical outcomes in people at clinical high-risk for psychosis related to altered interactions between hippocampal activity and glutamatergic function

Preclinical rodent models suggest that psychosis involves alterations in the activity and glutamatergic function in the hippocampus, driving dopamine activity through projections to the striatum. The extent to which this model applies to the onset of psychosis in clinical subjects is unclear. We assessed whether interactions between hippocampal glutamatergic function and activity/striatal connectivity are associated with adverse clinical outcomes in people at clinical high-risk (CHR) for psychosis. We measured functional Magnetic Resonance Imaging of hippocampal activation/connectivity, and 1H-Magnetic Resonance Spectroscopy of hippocampal glutamatergic metabolites in 75 CHR participants and 31 healthy volunteers. At follow-up, 12 CHR participants had transitioned to psychosis and 63 had not. Within the clinical high-risk cohort, at follow-up, 35 and 17 participants had a poor or a good functional outcome, respectively. The onset of psychosis (ppeakFWE = 0.003, t = 4.4, z = 4.19) and a poor functional outcome (ppeakFWE < 0.001, t = 5.52, z = 4.81 and ppeakFWE < 0.001, t = 5.25, z = 4.62) were associated with a negative correlation between the hippocampal activation and hippocampal Glx concentration at baseline. In addition, there was a negative association between hippocampal Glx concentration and hippocampo-striatal connectivity (ppeakFWE = 0.016, t = 3.73, z = 3.39, ppeakFWE = 0.014, t = 3.78, z = 3.42, ppeakFWE = 0.011, t = 4.45, z = 3.91, ppeakFWE = 0.003, t = 4.92, z = 4.23) in the total CHR sample, not seen in healthy volunteers. As predicted by preclinical models, adverse clinical outcomes in people at risk for psychosis are associated with altered interactions between hippocampal activity and glutamatergic function.

Nicotine Administration Normalizes Behavioral and Neurophysiological Perturbations in the MAM Rodent Model of Schizophrenia

BACKGROUND

The present study utilized the methylazoxymethanol (MAM) neurodevelopmental rodent model of schizophrenia (SCZ) to evaluate the hypothesis that individuals with SCZ smoke in an attempt to "self-medicate" their symptoms through nicotine (NIC) intake.

METHODS

To explore this question, we examined the effects of acute and chronic administration of NIC in 2 established behavioral tests known to be disrupted in the MAM model: prepulse inhibition of startle and novel object recognition. Additionally, we assessed the effects of acute and chronic NIC on 2 indices of the pathophysiology of SCZ modeled by MAM, elevated dopamine neuron population activity in the ventral tegmental area and neuronal activity in the ventral hippocampus, using in vivo electrophysiological recordings.

RESULTS

Our findings demonstrated that both acute and chronic administration of NIC significantly improved deficits in prepulse inhibition of startle and novel object recognition among MAM rats and normalized elevated ventral tegmental area and ventral hippocampal neuronal activity in these animals.

CONCLUSION

Together, these findings of NIC-induced improvement of deficits lend support for a "self-medication" hypothesis behind increased cigarette smoking in SCZ and illustrate the potential utility of nicotinic modulation in future pharmacotherapies for certain SCZ symptoms.

Early Pup Removal Leads to Social Dysfunction and Dopamine Deficit in Late Postpartum Rats: Prevention by Social Support

Offspring interaction is among the most highly motivated behaviors in maternal mammals and is mediated by mesolimbic dopamine (DA) system activation. Disruption or loss of significant social relationships is among the strongest individual predictors of affective dysregulation and depression onset in humans. However, little is known regarding the effects of disrupted mother–infant attachment (pup removal) in rat dams. Here, we tested the effects of permanent pup removal in rat dams, which were assigned to one of three groups on postpartum day (PD) 1: pups; pups removed, single-housed; or pups removed, co-housed with another dam who also had pups removed; and underwent a behavioral test battery during PD 21–23. In vivo electrophysiological recordings of ventral tegmental area (VTA) DA neurons were performed on PD 22 and 23 in a subset of animals. Pup removal did not impact sucrose consumption or anxiety-like behavior, but increased passive forced swim test (FST) coping responses. Pup-removal effects on social behavior and VTA activity were sensitive to social buffering: only single-housed dams exhibited reduced social motivation and decreased numbers of active DA neurons. Dams that had pups removed and were co-housed did not exhibit changes in social behavior or VTA function. Moreover, no changes in social behavior, FST coping, or VTA activity were found in socially isolated adult virgin females, indicating that effects observed in dams are specific to pup loss. This study showed that deprivation of species-expected social relationships (pups) during the postpartum precipitates an enduring negative affect state (enhanced passive coping, blunted social motivation) and attenuated VTA DA function in the dam, and that a subset of these effects is partially ameliorated through social buffering.

Interactions between hippocampal activity and striatal dopamine in people at clinical high risk for psychosis: relationship to adverse outcomes

Preclinical models propose that increased hippocampal activity drives subcortical dopaminergic dysfunction and leads to psychosis-like symptoms and behaviors. Here, we used multimodal neuroimaging to examine the relationship between hippocampal regional cerebral blood flow (rCBF) and striatal dopamine synthesis capacity in people at clinical high risk (CHR) for psychosis and investigated its association with subsequent clinical and functional outcomes. Ninety-five participants (67 CHR and 28 healthy controls) underwent arterial spin labeling MRI and ¹⁸F-DOPA PET imaging at baseline. CHR participants were followed up for a median of 15 months to determine functional outcomes with the global assessment of function (GAF) scale and clinical outcomes using the comprehensive assessment of at-risk mental states (CAARMS). CHR participants with poor functional outcomes (follow-up GAF < 65, n = 25) showed higher rCBF in the right hippocampus compared to CHRs with good functional outcomes (GAF ≥ 65, n = 25) (p_fwe = 0.026). The relationship between rCBF in this right hippocampal region and striatal dopamine synthesis capacity was also significantly different between groups (p_fwe = 0.035); the association was negative in CHR with poor outcomes (p_fwe = 0.012), but non-significant in CHR with good outcomes. Furthermore, the correlation between right hippocampal rCBF and striatal dopamine function predicted a longitudinal increase in the severity of positive psychotic symptoms within the total CHR group (p = 0.041). There were no differences in rCBF, dopamine, or their associations in the total CHR group relative to controls. These findings indicate that altered interactions between the hippocampus and the subcortical dopamine system are implicated in the pathophysiology of adverse outcomes in the CHR state.

Peripubertal mGluR2/3 Agonist Treatment Prevents Hippocampal Dysfunction and Dopamine System Hyperactivity in Adulthood in MAM Model of Schizophrenia

Pomaglumetad methionil (POM), a group 2 metabotropic glutamate receptor (mGluR2/3) agonist, showed promise as a novel antipsychotic in preclinical research but failed to show efficacy in clinical trials, though it has been suggested that it may be effective in certain patient populations, including early in disease patients. We used the methyazoxymethanol acetate (MAM) rat model of schizophrenia to determine whether POM may prevent the development of dopamine (DA) system dysfunction in a model representative of the hyperdopaminergic state thought to underlie psychosis, compared to control (SAL) rats. MAM and SAL rats were administered either POM (3 mg/kg, i.p.), vehicle (1 ml/kg), or no injection during postnatal day (PD) 31-40. In either late adolescence (PD 47-56) or adulthood (PD 83-96), novel object recognition (NOR) was tested, followed by anesthetized in vivo electrophysiological recordings of VTA DA neuron activity or ventral hippocampal (vHPC) pyramidal neuron activity. MAM rats treated with POM demonstrated increased NOR in adulthood compared to no injection MAM rats, but not compared to vehicle-treated MAM rats. POM-treated MAM rats demonstrated normalized DA neuron population activity and vHPC pyramidal neuron activity compared to vehicle and no injection MAM rats in both late adolescence and adulthood. No significant differences were observed across treatment groups in SAL rats. These results suggest that peripubertal mGluR2/3 agonist administration can prevent the emergence of vHPC pyramidal neuron hyperactivity and increased DA neuron population activity in adult MAM rats.

Beyond Dopamine Receptor Antagonism: New Targets for Schizophrenia Treatment and Prevention

Treatment of schizophrenia (SCZ) historically relies on the use of antipsychotic drugs to treat psychosis, with all of the currently available antipsychotics acting through the antagonism of dopamine D2 receptors. Although antipsychotics reduce psychotic symptoms in many patients, they induce numerous undesirable effects and are not effective against negative and cognitive symptoms. These highlight the need to develop new drugs to treat SCZ. An advanced understanding of the circuitry of SCZ has pointed to pathological origins in the excitation/inhibition balance in regions such as the hippocampus, and restoring function in this region, particularly as a means to compensate for parvalbumin (PV) interneuron loss and resultant hippocampal hyperactivity, may be a more efficacious approach to relieve a broad range of SCZ symptoms. Other targets, such as cholinergic receptors and the trace amine-associated receptor 1 (TAAR1), have also shown some promise for the treatment of SCZ. Importantly, assessing efficacy of novel compounds must take into consideration treatment history of the patient, as preclinical studies suggest prior antipsychotic treatment may interfere with the efficacy of these novel agents. However, while novel therapeutic targets may be more effective in treating SCZ, a more effective approach would be to prevent the transition to SCZ in susceptible individuals. A focus on stress, which has been shown to be a predisposing factor in risk for SCZ, is a possible avenue that has shown promise in preclinical studies. Therefore, therapeutic approaches based on our current understanding of the circuitry of SCZ and its etiology are likely to enable development of more effective therapeutic interventions for this complex disorder.

Stress impacts corticoamygdalar connectivity in an age-dependent manner

Stress is a socio-environmental risk factor for the development of psychiatric disorders, with the age of exposure potentially determining the outcome. Several brain regions mediate stress responsivity, with a prominent role of the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) and their reciprocal inhibitory connectivity. Here we investigated the impact of stress exposure during adolescence and adulthood on the activity of putative pyramidal neurons in the BLA and corticoamygdalar plasticity using in vivo electrophysiology. 155 male Sprague-Dawley rats were subjected to a combination of footshock/restraint stress in either adolescence (postnatal day 31-40) or adulthood (postnatal day 65-74). Both adolescent and adult stress increased the number of spontaneously active putative BLA pyramidal neurons 1-2 weeks, but not 5-6 weeks post stress. High-frequency stimulation (HFS) of BLA and mPFC depressed evoked spike probability in the mPFC and BLA, respectively, in adult but not adolescent rats. In contrast, an adult-like BLA HFS-induced decrease in spike probability of mPFC neurons was found 1-2 weeks post-adolescent stress. Changes in mPFC and BLA neuron discharge were found 1-2 weeks post-adult stress after BLA and mPFC HFS, respectively. All these changes were transient since they were not found 5-6 weeks post adolescent or adult stress. Our findings indicate that stress during adolescence may accelerate the development of BLA-PFC plasticity, probably due to BLA hyperactivity, which can also disrupt the reciprocal communication of BLA-mPFC after adult stress. Therefore, precocious BLA-mPFC connectivity alterations may represent an early adaptive stress response that ultimately may contribute to vulnerability to adult psychiatric disorders.

Prepubertal Environmental Enrichment Prevents Dopamine Dysregulation and Hippocampal Hyperactivity in MAM Schizophrenia Model Rats

BACKGROUND

Schizophrenia (SCZ) is a neurodevelopmental disorder with a progressive, prolonged course. Early prevention for SCZ is promising but overall lacks support from preclinical evidence. Previous studies have tested environmental enrichment (EE) in certain models of SCZ and discovered a broadly beneficial effect in preventing behavioral abnormalities relevant, yet not specific, to the disorder. Nonetheless, whether EE can prevent dopamine (DA) dysregulation, a hallmark of psychosis and SCZ, had not been tested.

METHODS

Using the MAM (methylazoxymethanol acetate) rat model of schizophrenia and saline-treated control animals, we investigated the long-term electrophysiological effects of prepubertal (postnatal day 21-40) EE on DA neurons, pyramidal neurons in the ventral hippocampus, and projection neurons in the basolateral amygdala. Anxiety-related behaviors in the elevated plus maze and locomotor responses to amphetamine were also analyzed.

RESULTS

Prepubertal EE prevented the increased population activity of DA neurons and the associated increase in locomotor response to amphetamine. Prepubertal EE also prevented hyperactivity in the ventral hippocampus but did not prevent hyperactivity in the basolateral amygdala. Anxiety-like behaviors in MAM rats were not ameliorated by prepubertal exposure to EE.

CONCLUSIONS

Twenty-day prepubertal EE is sufficient to prevent DA hyperresponsivity in the MAM model, measured by electrophysiological recordings and locomotor response to amphetamine. This effect is potentially mediated by normalizing excessive firing in the ventral hippocampus without affecting anxiety-like behaviors and basolateral amygdala firing. This study identified EE as a useful preventative approach that may protect against the pathophysiological development of SCZ.

Integrated metastate functional connectivity networks predict change in symptom severity in clinical high risk for psychosis

The ability to identify biomarkers of psychosis risk is essential in defining effective preventive measures to potentially circumvent the transition to psychosis. Using samples of people at clinical high risk for psychosis (CHR) and Healthy controls (HC) who were administered a task fMRI paradigm, we used a framework for labelling time windows of fMRI scans as 'integrated' FC networks to provide a granular representation of functional connectivity (FC). Periods of integration were defined using the 'cartographic profile' of time windows and k-means clustering, and sub-network discovery was carried out using Network Based Statistics (NBS). There were no network differences between CHR and HC groups. Within the CHR group, using integrated FC networks, we identified a sub-network negatively associated with longitudinal changes in the severity of psychotic symptoms. This sub-network comprised brain areas implicated in bottom-up sensory processing and in integration with motor control, suggesting it may be related to the demands of the fMRI task. These data suggest that extracting integrated FC networks may be useful in the investigation of biomarkers of psychosis risk.

Reduced motor cortex GABABR function following chronic alcohol exposure

The GABA_B receptor (GABA_BR) agonist baclofen has been used to treat alcohol and several other substance use disorders (AUD/SUD), yet its underlying neural mechanism remains unclear. The present study aimed to investigate cortical GABA_BR dynamics following chronic alcohol exposure. Ex vivo brain slice recordings from mice chronically exposed to alcohol revealed a reduction in GABA_BR-mediated currents, as well as a decrease of GABA_B1/2R and G-protein-coupled inwardly rectifying potassium channel 2 (GIRK2) activities in the motor cortex. Moreover, our data indicated that these alterations could be attributed to dephosphorylation at the site of serine 783 (ser-783) in GABA_B2 subunit, which regulates the surface expression of GABA_BR. Furthermore, a human study using paired-pulse-transcranial magnetic stimulation (TMS) analysis further demonstrated a reduced cortical inhibition mediated by GABA_BR in patients with AUD. Our findings provide the first evidence that chronic alcohol exposure is associated with significantly impaired cortical GABA_BR function. The ability to promote GABA_BR signaling may account for the therapeutic efficacy of baclofen in AUD.

Emerging therapeutic targets for schizophrenia: a framework for novel treatment strategies for psychosis

Introduction: Antipsychotic drugs are central to the treatment of schizophrenia, but their limitations necessitate improved treatment strategies. Multiple lines of research have implicated glutamatergic dysfunction in the hippocampus as an early source of pathophysiology in schizophrenia. Novel compounds have been designed to treat glutamatergic dysfunction, but they have produced inconsistent results in clinical trials. Areas covered: This review discusses how the hippocampus is thought to drive psychotic symptoms through its influence on the dopamine system. It offers the reader an evaluation of proposed treatment strategies including direct modulation of GABA or glutamate neurotransmission or reducing the deleterious impact of stress on circuit development. Finally, we offer a perspective on aspects of future research that will advance our knowledge and may create new therapeutic opportunities. PubMed was searched for relevant literature between 2010 and 2020 and related studies. Expert opinion: Targeting aberrant excitatory-inhibitory neurotransmission in the hippocampus and its related circuits has the potential to alleviate symptoms and reduce the risk of transition to psychosis if implemented as an early intervention. Longitudinal multimodal brain imaging combined with mechanistic theories generated from animal models can be used to better understand the progression of hippocampal-dopamine circuit dysfunction and heterogeneity in treatment response.

The pathophysiological impact of stress on the dopamine system is dependent on the state of the critical period of vulnerability

Unregulated stress during critical periods of development is proposed to drive deficits consistent with schizophrenia in adults. If accurate, reopening the critical period could make the adult susceptible to pathology. We evaluated the impact of early adolescent and adult stress exposure (combination of daily footshock for 10 days and 3 restraint sessions) on (1) midbrain dopamine (DA) neuron activity, (2) ventral hippocampal (vHipp) pyramidal neuron activity, and (3) the number of parvalbumin (PV) interneurons in the vHipp and their associated perineuronal nets (PNNs). Ventral tegmental area (VTA) DA neuron population activity and vHipp activity was increased 1-2 and 5-6 weeks post-adolescent stress, along with a decrease in the number of PV+, PNN+, PV + /PNN + cells in the vHipp, which are consistent with the MAM model of schizophrenia. In contrast, adult stress decreased VTA DA neuron population activity only at 1-2 weeks post stress, which is consistent with what has been observed in animal models of depression, without impacting vHipp activity and PV/PNN expression. Administration of valproate (VPA), which can re-instate the critical period of plasticity via histone deacetylase (HDAC) inhibition, caused adult stress to produce changes similar to those induced by adolescent stress, presumably by increasing stress vulnerability to early adolescent levels. Our findings indicate that timing of stress is a critical determinant of the pathology produced in the adult: adolescent stress led to circuit deficits that recapitulates schizophrenia, whereas adult stress induced a depression-like hypodopaminergic state. Reopening the critical period in the adult restores vulnerability to stress-induced pathology resembling schizophrenia.

Glutamate in schizophrenia: Neurodevelopmental perspectives and drug development

Research into the neurobiological processes that may lead to the onset of schizophrenia places growing emphasis on the glutamatergic system and brain development. Preclinical studies have shown that neurodevelopmental, genetic, and environmental factors contribute to glutamatergic dysfunction and schizophrenia-related phenotypes. Clinical research has suggested that altered brain glutamate levels may be present before the onset of psychosis and relate to outcome in those at clinical high risk. After psychosis onset, glutamate dysfunction may also relate to the degree of antipsychotic response and clinical outcome. These findings support ongoing efforts to develop pharmacological interventions that target the glutamate system and could suggest that glutamatergic compounds may be more effective in specific patient subgroups or illness stages. In this review, we consider the updated glutamate hypothesis of schizophrenia, from a neurodevelopmental perspective, by reviewing recent preclinical and clinical evidence, and discuss the potential implications for novel therapeutics.

Adolescent Exposure to WIN 55212-2 Render the Nigrostriatal Dopaminergic Pathway Activated During Adulthood

BACKGROUND

During adolescence, neuronal circuits exhibit plasticity in response to physiological changes and to adapt to environmental events. Nigrostriatal dopaminergic pathways are in constant flux during development. Evidence suggests a relationship between early use of cannabinoids and psychiatric disorders characterized by altered dopaminergic systems, such as schizophrenia and addiction. However, the impact of adolescent exposure to cannabinoids on nigrostriatal dopaminergic pathways in adulthood remains unclear. The aim of this research was to determine the effects of repeated activation of cannabinoid receptors during adolescence on dopaminergic activity of nigrostriatal pathways and the mechanisms underlying this impact during adulthood.

METHODS

Male Sprague-Dawley rats were treated with 1.2 mg/kg WIN 55212-2 daily from postnatal day 40 to 65. Then no-net flux microdialysis of dopamine in the dorsolateral striatum, electrophysiological recording of dopaminergic neuronal activity, and microdialysis measures of gamma-aminobutyric acid (GABA) and glutamate in substantia nigra par compacta were carried out during adulthood (postnatal days 72-78).

RESULTS

Repeated activation of cannabinoid receptors during adolescence increased the release of dopamine in dorsolateral striatum accompanied by increased population activity of dopamine neurons and decreased extracellular GABA levels in substantia nigra par compacta in adulthood. Furthermore, perfusion of bicuculline, a GABAa antagonist, into the ventral pallidum reversed the increased dopamine neuron population activity in substantia nigra par compacta induced by adolescent cannabinoid exposure.

CONCLUSIONS

These results suggest that adolescent exposure to cannabinoid agonists produces disinhibition of nigrostriatal dopamine transmission during adulthood mediated by decreased GABAergic input from the ventral pallidum.

Dysregulation of Midbrain Dopamine System and the Pathophysiology of Schizophrenia

Dysregulation of the dopamine system is central to many models of the pathophysiology of psychosis in schizophrenia. However, emerging evidence suggests that this dysregulation is driven by the disruption of upstream circuits that provide afferent control of midbrain dopamine neurons. Furthermore, stress can profoundly disrupt this regulatory circuit, particularly when it is presented at critical vulnerable prepubertal time points. This review will discuss the dopamine system and the circuits that regulate it, focusing on the hippocampus, medial prefrontal cortex, thalamic nuclei, and medial septum, and the impact of stress. A greater understanding of the regulation of the dopamine system and its disruption in schizophrenia may provide a more complete neurobiological framework to interpret clinical findings and develop novel treatments.

Prelimbic medial prefrontal cortex disruption during adolescence increases susceptibility to helpless behavior in adult rats

Major depressive disorder (MDD) is a disabling mental disorder worldwide. Several animal models have been used to study the neurobiology of this disorder, including the learned helplessness (LH) paradigm, in which susceptible animals show helpless behavior indicated by fails to escape a controllable footshock. This behavior has been associated with a downregulation of ventral tegmental area (VTA) dopamine (DA) system activity. The prelimbic portion of the prefrontal cortex (plPFC) plays an important role in the modulation of helpless behavior, but so far there is no evidence indicating that its developmental disruption alters susceptibility to helpless behavior. We investigated the impact of plPFC lesion performed at adolescence (postnatal day 31-33) or adulthood (postnatal day 70-72) on anxiety responses (elevated plus-maze), susceptibility to helpless behavior, and the VTA DA system activity in adult Sprague-Dawley rats. Whereas adult plPFC lesions induced neither anxiety responses nor increased susceptibility to helpless behavior (plPFC lesion: 33.3% of helplessness; controls: 30.8% of helplessness rats), adolescent plPFC lesions induced anxiety responses and increased the proportion of rats showing helpless at adulthood (plPFC lesion: 92.3% helplessness; controls: 42.1% helplessness rats). Moreover, only helpless rats in the groups showed a decreased VTA DA system population activity that was confined to the medial portion of the VTA. These findings suggest that the impairment of plPFC activity during adolescence occurs during a critical window for the development of helpless behavior in adult rats, indicating that predisposition or early life adverse events that impair plPFC activity may enhance susceptibility to depression in adulthood.

Functional differentiation in the human ventromedial frontal lobe: A data-driven parcellation

Ventromedial regions of the frontal lobe (vmFL) are thought to play a key role in decision-making and emotional regulation. However, aspects of this area's functional organization, including the presence of a multiple subregions, their functional and anatomical connectivity, and the cross-species homologies of these subregions with those of other species, remain poorly understood. To address this uncertainty, we employed a two-stage parcellation of the region to identify six distinct structures within the region on the basis of data-driven classification of functional connectivity patterns obtained using the meta-analytic connectivity modeling (MACM) approach. From anterior to posterior, the derived subregions included two lateralized posterior regions, an intermediate posterior region, a dorsal and ventral central region, and a single anterior region. The regions were characterized further by functional connectivity derived using resting-state fMRI and functional decoding using the Brain Map database. In general, the regions could be differentiated on the basis of different patterns of functional connectivity with canonical "default mode network" regions and/or subcortical regions such as the striatum. Together, the findings suggest the presence of functionally distinct neural structures within vmFL, consistent with data from experimental animals as well prior demonstrations of anatomical differences within the region. Detailed correspondence with the anterior cingulate, medial orbitofrontal cortex, and rostroventral prefrontal cortex, as well as specific animal homologs are discussed. The findings may suggest future directions for resolving potential functional and structural correspondence of subregions within the frontal lobe across behavioral contexts, and across mammalian species.

Transcranial photoacoustic imaging of NMDA-evoked focal circuit dynamics in the rat hippocampus

OBJECTIVE

We report the transcranial functional photoacoustic (fPA) neuroimaging of N-methyl-D-aspartate (NMDA) evoked neural activity in the rat hippocampus. Concurrent quantitative electroencephalography (qEEG) and microdialysis were used to record real-time circuit dynamics and excitatory neurotransmitter concentrations, respectively.

APPROACH

We hypothesized that location-specific fPA voltage-sensitive dye (VSD) contrast would identify neural activity changes in the hippocampus which correlate with NMDA-evoked excitatory neurotransmission.

MAIN RESULTS

Transcranial fPA VSD imaging at the contralateral side of the microdialysis probe provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration changes. qEEG validated a wide range of glutamatergic excitation, which culminated in focal seizure activity after a high NMDA dose. We conclude that transcranial fPA VSD imaging can distinguish focal glutamate loads in the rat hippocampus, based on the VSD redistribution mechanism which is sensitive to the electrophysiologic membrane potential.

SIGNIFICANCE

Our results suggest the future utility of this emerging technology in both laboratory and clinical sciences as an innovative functional neuroimaging modality.

Neural Circuitry of Novelty Salience Processing in Psychosis Risk: Association With Clinical Outcome

Psychosis has been proposed to develop from dysfunction in a hippocampal-striatal-midbrain circuit, leading to aberrant salience processing. Here, we used functional magnetic resonance imaging (fMRI) during novelty salience processing to investigate this model in people at clinical high risk (CHR) for psychosis according to their subsequent clinical outcomes. Seventy-six CHR participants as defined using the Comprehensive Assessment of At-Risk Mental States (CAARMS) and 31 healthy controls (HC) were studied while performing a novelty salience fMRI task that engaged an a priori hippocampal-striatal-midbrain circuit of interest. The CHR sample was then followed clinically for a mean of 59.7 months (~5 y), when clinical outcomes were assessed in terms of transition (CHR-T) or non-transition (CHR-NT) to psychosis (CAARMS criteria): during this period, 13 individuals (17%) developed a psychotic disorder (CHR-T) and 63 did not. Functional activation and effective connectivity within a hippocampal-striatal-midbrain circuit were compared between groups. In CHR individuals compared to HC, hippocampal response to novel stimuli was significantly attenuated (P = .041 family-wise error corrected). Dynamic Causal Modelling revealed that stimulus novelty modulated effective connectivity from the hippocampus to the striatum, and from the midbrain to the hippocampus, significantly more in CHR participants than in HC. Conversely, stimulus novelty modulated connectivity from the midbrain to the striatum significantly less in CHR participants than in HC, and less in CHR participants who subsequently developed psychosis than in CHR individuals who did not become psychotic. Our findings are consistent with preclinical evidence implicating hippocampal-striatal-midbrain circuit dysfunction in altered salience processing and the onset of psychosis.

Adaptations in reward-related behaviors and mesolimbic dopamine function during motherhood and the postpartum period

Initiation and maintenance of maternal behavior is driven by a complex interaction between the physiology of parturition and offspring stimulation, causing functional changes in maternal brain and behavior. Maternal behaviors are among the most robust and rewarding motivated behaviors. Mesolimbic dopamine (DA) system alterations during pregnancy and the postpartum enable enhanced reward-related responses to offspring stimuli. Here, we review behavioral evidence demonstrating postpartum rodents exhibit a bias towards pups and pup-related stimuli in reward-related tasks. Next, we provide an overview of normative adaptations in the mesolimbic DA system induced by parturition and the postpartum, which likely mediate shifts in offspring valence. We also discuss a causal link between dopaminergic dysfunction and disrupted maternal behaviors, which are recapitulated in postpartum depression (PPD) and relevant rodent models. In sum, mesolimbic DA system activation drives infant-seeking behavior and strengthens the mother-infant bond, potentially representing a therapeutic target for reward-related deficits in PPD.

Noninvasive ultrasound deep brain stimulation of nucleus accumbens induces behavioral avoidance

Ultrasound stimulation is an emerging noninvasive option in treating neuropsychiatric disorders. The present study investigates the behavioral alterations resulting from ultrasound stimulation on the nucleus accumbens (NAc) in freely moving mice. Our results show that an acute ultrasound stimulation on the NAc, rather than the visual cortex or auditory cortex, led to a pronounced avoidance behavior, while repeated NAc ultrasound stimulation resulted in an obvious conditioned place aversion with changes in synaptic protein (GluA1/2 subunit) expression. Notably, NAc ultrasound stimulation suppressed the morphine-induced conditioned place preference. The results provide evidence that NAc ultrasound stimulation can be applied as a potential noninvasive therapeutic option in treating psychiatric disorders.

Postpartum changes in affect-related behavior and VTA dopamine neuron activity in rats

The onset of motherhood is accompanied by alterations in emotional and affective behaviors. Many new mothers experience transient and mild depressive symptoms that typically resolve spontaneously (i.e. postpartum blues) but increase the risk for postpartum depression (PPD). There is little data regarding the neural adaptations occurring in response to parturition and shortly after birth that may be associated with these affective changes. Although the dopamine (DA) system is involved in affect, maternal motivation and PPD, little is known about postpartum DA function. We compared affective behavior in virgin and postpartum adult female rats at early and late time points. In vivo extracellular recordings of VTA DA neurons were performed to evaluate 3 parameters: number of active DA neurons (i.e. population activity), firing rate, and firing pattern. Compared with virgins, postpartum rats exhibited increased anxiety-like behavior in the elevated plus maze at 1-day postpartum; reduced social motivation at 1- and 3-days postpartum, reduced anxiety-like behavior in the novelty suppressed feeding test throughout the first week postpartum and increased forced swim test immobility at 1-day postpartum. 1- and 3-day postpartum females exhibited attenuated VTA population activity without changes in firing rate or pattern. None of these effects were observed in late postpartum females when compared with virgins. These data suggest that parturition induces time-dependent changes in a subset of affect-related behaviors and DA function during the postpartum period in rodents, with early postpartum females exhibiting depression-related phenotypes (i.e. low social motivation, higher immobility, blunted DA activity).

Nicotine Self-administration Is Not Increased in the Methylazoxymethanol Acetate Rodent Model of Schizophrenia

INTRODUCTION

Patients with schizophrenia (SCZ) smoke at a rate of 4-5 times higher than the general population, contributing to negative health consequences in this group. One possible explanation for this increased smoking is that individuals with SCZ find nicotine (NIC) more reinforcing. However, data supporting this possibility are limited.

METHODS

The present experiments examined self-administration of NIC, alone or in combination with other reinforcers, across a range of doses in the methylazoxymethanol acetate (MAM) rodent model of SCZ.

RESULTS

MAM and control animals did not differ in NIC self-administration across a range of doses and schedules of reinforcement, in both standard 1-hour self-administration sessions and 23-hour extended access sessions. However, MAM animals responded less for sucrose or reinforcing visual stimuli alone or when paired with NIC.

CONCLUSIONS

To the extent that MAM-treated rats are a valid model of SCZ, these results suggest that increased NIC reinforcement does not account for increased smoking in SCZ patients.

IMPLICATIONS

This study is the first to utilize nicotine self-administration, the gold standard for studying nicotine reinforcement, in the methylazoxymethanol acetate model of schizophrenia, which is arguably the most comprehensive animal model of the disease currently available. Our assessment found no evidence of increased nicotine reinforcement in methylazoxymethanol acetate animals, suggesting that increased reinforcement may not perpetuate increased smoking in schizophrenia patients.

Stress during critical periods of development and risk for schizophrenia

Schizophrenia is a neurodevelopmental disorder with genetic predisposition, and stress has long been linked to its etiology. While stress affects all stages of the illness, increasing evidence suggests that stress during critical periods of development may be particularly detrimental, increasing individual's vulnerability to psychosis. To thoroughly understand the potential causative role of stress, our group has been focusing on the prenatal methylazoxymethanol acetate (MAM) rodent model, and discovered that MAM offspring display abnormal stress reactivity and heightened anxiety prepubertally, prior to the manifestation of a hyperdopaminergic state. Furthermore, pharmacologically treating anxiety during prepuberty prevented the emergence of the dopamine dysfunction in adulthood. Interestingly, sufficiently strong stressors applied to normal rats selectively during early development can recapitulate multiple schizophrenia-related phenotypes of MAM rats, whereas the same stress paradigm during adulthood only produced short-term depression-related deficits. Altogether, the evidence is thus converging: developmental disruption (genetic or environmental) might render animals more susceptible to the deleterious effects of stress during critical time windows, during which unregulated stress can lead to the emergence of psychosis later in life. As an important region regulating the midbrain dopamine system, the ventral hippocampus is particularly vulnerable to stress, and the distinct maturational profile of its fast-spiking parvalbumin interneurons may largely underlie such vulnerability. In this review, by discussing emerging evidence spanning clinical and basic science studies, we propose developmental stress vulnerability as a novel link between early predispositions and environmental triggering events in the pathophysiology of schizophrenia. This promising line of research can potentially provide not only insights into the etiology, but also a "roadmap" for disease prevention.

Female rats are resistant to the long-lasting neurobehavioral changes induced by adolescent stress exposure

Stress during adolescence is a risk factor for neuropsychiatric diseases, including schizophrenia. We recently observed that peripubertal male rats exposed to a combination of daily footshock plus restraint stress exhibited schizophrenia-like changes. However, numerous studies have shown sex differences in neuropsychiatric diseases as well as on the impact of coping with stress. Thus, we decided to evaluate, in adolescent female rats, the impact of different stressors (restraint stress [RS], footshock [FS], or the combination of FS and RS [FS+RS]) on social interaction (3-chamber social approach test/SAT), anxiety responses (elevated plus-maze/EPM), cognitive function (novel object recognition test/NOR), and dopamine (DA) system responsivity by evaluating locomotor response to amphetamine and in vivo extracellular single unit recordings of DA neurons in the ventral tegmental area (VTA) in adulthood. The impact of FS+RS during early adulthood was also investigated. Adolescent stress had no impact on social behavior, anxiety, cognition and locomotor response to amphetamine. In addition, adolescent stress did not induce long-lasting changes in VTA DA system activity. However, a decrease in the firing rate of VTA DA neurons was found 1-2 weeks post-adolescent stress. Similar to adolescent stress, adult stress did not induce long-lasting behavioral deficits and changes in VTA DA system activity, but FS+RS decreased VTA DA neuron population activity 1-2 weeks post-adult stress. Our results are consistent with previous studies showing that female rodents appear to be more resilient to developmental stress-induced persistent changes than males and may contribute to the delayed onset and lesser severity of schizophrenia in females.

Towards a Unifying Cognitive, Neurophysiological, and Computational Neuroscience Account of Schizophrenia

Psychotic experiences may be understood as altered information processing due to aberrant neural computations. A prominent example of such neural computations is the computation of prediction errors (PEs), which signal the difference between expected and experienced events. Among other areas showing PE coding, hippocampal-prefrontal-striatal neurocircuits play a prominent role in information processing. Dysregulation of dopaminergic signaling, often secondary to psychosocial stress, is thought to interfere with the processing of biologically important events (such as reward prediction errors) and result in the aberrant attribution of salience to irrelevant sensory stimuli and internal representations. Bayesian hierarchical predictive coding offers a promising framework for the identification of dysfunctional neurocomputational processes and the development of a mechanistic understanding of psychotic experience. According to this framework, mismatches between prior beliefs encoded at higher levels of the cortical hierarchy and lower-level (sensory) information can also be thought of as PEs, with important consequences for belief updating. Low levels of precision in the representation of prior beliefs relative to sensory data, as well as dysfunctional interactions between prior beliefs and sensory data in an ever-changing environment, have been suggested as a general mechanism underlying psychotic experiences. Translating the promise of the Bayesian hierarchical predictive coding into patient benefit will come from integrating this framework with existing knowledge of the etiology and pathophysiology of psychosis, especially regarding hippocampal-prefrontal-striatal network function and neural mechanisms of information processing and belief updating.

State-dependent effects of the D2 partial agonist aripiprazole on dopamine neuron activity in the MAM neurodevelopmental model of schizophrenia

Aripiprazole is an antipsychotic drug characterized by partial agonist activity at D₂ receptors to normalize both hyperdopaminergic and hypodopaminergic states. Traditional D₂ antagonist antipsychotic drugs have been shown previously to reduce dopamine neuron activity through action on D₂ autoreceptors to produce an overexcitation-induced cessation of cell firing, referred to as depolarization block. It is unclear whether aripiprazole reduces dopamine neuron activity via inhibition or, as seen following D₂ antagonist administration, depolarization block. The impact of acute and repeated aripiprazole treatment was examined in the methylazoxymethanol acetate (MAM) rodent model to observe its effects on a hyperdopaminergic system, compared to normal rats. We found that administration of aripiprazole acutely or after 1 or 7 days of withdrawal from 21-day repeated treatment led to a decrease in the number of spontaneously active dopamine neurons in MAM rats but not in controls. This reduction was not reversed by apomorphine (100-200 µg/kg i.p. or 20 µg/kg i.v.) administration, suggesting that it was not due to depolarization block. In contrast, 1 h after induction of depolarization block of dopamine neurons by acute haloperidol treatment (0.6 mg/kg i.p.), aripiprazole (1 mg/kg, i.p.) reversed the depolarization block state. Therefore, aripiprazole rapidly reduced the hyperdopaminergic activity selectively in MAM rats. The reduction is unlikely due to depolarization block and persists following 7-day withdrawal from repeated treatment. Aripiprazole also removes haloperidol-induced depolarization block in MAM rats, which may underlie the acute psychotic state often observed with switching to this treatment.

The Circuitry of Dopamine System Regulation and its Disruption in Schizophrenia: Insights Into Treatment and Prevention

Despite evidence for a role of the dopamine system in the pathophysiology of schizophrenia, there has not been substantial evidence that this disorder originates from a pathological change within the dopamine system itself. Current data from human imaging studies and preclinical investigations instead point to a disruption in afferent regulation of the dopamine system, with a focus on the hippocampus. We found that the hippocampus in the methylazoxymethanol acetate (MAM) rodent developmental disruption model of schizophrenia is hyperactive and dysrhythmic, possibly due to loss of parvalbumin interneurons, leading to a hyperresponsive dopamine system. Whereas current therapeutic approaches target dopamine receptor blockade, treatment at the site of pathology may be a more effective therapeutic avenue. This model also provided insights into potential means for prevention of schizophrenia. Specifically, given that stress is a risk factor in schizophrenia, and that stress can damage hippocampal parvalbumin interneurons, we tested whether alleviating stress early in life can effectively circumvent transition to schizophrenia-like states. Administering diazepam prepubertally at an antianxiety dose in MAM rats was effective at preventing the emergence of the hyperdopaminergic state in the adult. Moreover, multiple stressors applied to normal rats at the same time point resulted in pathology similar to the MAM rat. These data suggest that a genetic predisposition leading to stress hyper-responsivity, or exposure to substantial stressors, could be a primary factor leading to the emergence of schizophrenia later in life, and furthermore treating stress at a critical period may be effective in circumventing this transition.

Correction: Prefrontal GABA levels, hippocampal resting perfusion and the risk of psychosis

This article was originally published under NPG's License to Publish, but has now been made available under a [CC BY 4.0] license. The PDF and HTML versions of the paper have been modified accordingly.

Prefrontal GABA levels, hippocampal resting perfusion and the risk of psychosis

Preclinical models propose that the onset of psychosis is associated with hippocampal hyperactivity, thought to be driven by cortical GABAergic interneuron dysfunction and disinhibition of pyramidal neurons. Recent neuroimaging studies suggest that resting hippocampal perfusion is increased in subjects at ultra-high risk (UHR) for psychosis, but how this may be related to GABA concentrations is unknown. The present study used a multimodal neuroimaging approach to address this issue in UHR subjects. Proton magnetic resonance spectroscopy and pulsed-continuous arterial spin labeling imaging were acquired to investigate the relationship between medial prefrontal (MPFC) GABA+ levels (including some contribution from macromolecules) and hippocampal regional cerebral blood flow (rCBF) in 36 individuals at UHR of psychosis, based on preclinical evidence that MPFC dysfunction is involved in hippocampal hyperactivity. The subjects were then clinically monitored for 2 years: during this period, 7 developed a psychotic disorder and 29 did not. At baseline, MPFC GABA+ levels were positively correlated with rCBF in the left hippocampus (region of interest analysis, p = 0.044 family-wise error corrected, FWE). This correlation in the left hippocampus was significantly different in UHR subjects who went on to develop psychosis relative to those who did not (p = 0.022 FWE), suggesting the absence of a correlation in the latter subgroup. These findings provide the first human evidence that MPFC GABA+ concentrations are related to resting hippocampal perfusion in the UHR state, and offer some support for a link between GABA levels and hippocampal function in the development of psychosis.