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

Inflammation and metabolic dysfunction underly anhedonia-like behavior in antidepressant resistant male rats

Inflammation and metabolic dysfunction impair dopamine neurotransmission, which is thought to serve as a critical mechanism underpinning motivational deficits such as anhedonia across a range of psychiatric and neurological disorders. This difficult-to-treat transdiagnostic symptom has important implications for treatment resistant depression (TRD), and may warrant more targeted therapeutic approaches that address the underlying pathophysiological mechanisms. Using the adrenocorticotrophic hormone (ACTH) model of antidepressant treatment resistance we characterized the relationship between antidepressant-like and anhedonia-like behavioral responses to bupropion, mesocortical tyrosine hydroxylase (TH) expression, chronic low-grade inflammation, and metabolic changes in male rats. We demonstrate that chronic ACTH elicited both an antidepressant resistant- and anhedonia-like phenotype in forced swim and effort-related choice behavioral tasks, respectively. This was associated with decreased TH expression in the brain, increased central and peripheral markers of inflammation, and peripheral metabolic disturbances, including impairment of immune cell insulin action. Multivariate analysis revealed that peripheral interleukin-6 (IL-6) levels, immune cell glucose uptake and disturbance of nucleotide metabolism were strongly associated with anhedonia-like behavior. Post-hoc analyses further confirmed strong correlations between TH expression, inflammation and behavioral performance. These data suggest that stress hormone-induced upregulation of inflammation concurrent with the impairment of insulin-mediated glucose uptake into immune cells is associated with disruption of nucleotide metabolism, and potential impaired central dopamine synthesis contributing to the behavioral expression of anhedonia. These results suggest that immunometabolic perturbations concomitant with impaired insulin action at the level of the immune cell result in a metabolically deficient state that directly impacts nucleotide precursors essential for dopamine synthesis and effortful behavior. These results highlight the potential for immune and metabolic markers for individualized treatment of refractory depression and anhedonia.

Oxidative and Excitatory Neurotoxic Stresses in CRISPR/Cas9-Induced Kynurenine Aminotransferase Knockout Mice: A Novel Model for Despair-Based Depression and Post-Traumatic Stress Disorder

BACKGROUNDS

Memory and emotion are especially vulnerable to psychiatric disorders such as post-traumatic stress disorder (PTSD), which is linked to disruptions in serotonin (5-HT) metabolism. Over 90% of the 5-HT precursor tryptophan (Trp) is metabolized via the Trp-kynurenine (KYN) metabolic pathway, which generates a variety of bioactive molecules. Dysregulation of KYN metabolism, particularly low levels of kynurenic acid (KYNA), appears to be linked to neuropsychiatric disorders. The majority of KYNA is produced by the aadat (kat2) gene-encoded mitochondrial kynurenine aminotransferase (KAT) isotype 2. Little is known about the consequences of deleting the KYN enzyme gene.

METHODS

In CRISPR/Cas9-induced aadat knockout (kat2-/-) mice, we examined the effects on emotion, memory, motor function, Trp and its metabolite levels, enzyme activities in the plasma and urine of 8-week-old males compared to wild-type mice.

RESULTS

Transgenic mice showed more depressive-like behaviors in the forced swim test, but not in the tail suspension, anxiety, or memory tests. They also had fewer center field and corner entries, shorter walking distances, and fewer jumping counts in the open field test. Plasma metabolite levels are generally consistent with those of urine: antioxidant KYNs, 5-hydroxyindoleacetic acid, and indole-3-acetic acid levels were lower; enzyme activities in KATs, kynureninase, and monoamine oxidase/aldehyde dehydrogenase were lower, but kynurenine 3-monooxygenase was higher; and oxidative stress and excitotoxicity indices were higher. Transgenic mice displayed depression-like behavior in a learned helplessness model, emotional indifference, and motor deficits, coupled with a decrease in KYNA, a shift of Trp metabolism toward the KYN-3-hydroxykynurenine pathway, and a partial decrease in the gut microbial Trp-indole pathway metabolite.

CONCLUSIONS

This is the first evidence that deleting the aadat gene induces depression-like behaviors uniquely linked to experiences of despair, which appear to be associated with excitatory neurotoxic and oxidative stresses. This may lead to the development of a double-hit preclinical model in despair-based depression, a better understanding of these complex conditions, and more effective therapeutic strategies by elucidating the relationship between Trp metabolism and PTSD pathogenesis.

Pioglitazone attenuates behavioral and electrophysiological dysfunctions induced by two-hit model of schizophrenia in adult rodent offspring

Maternal infection and stress exposure, especially during childhood and adolescence, have been implicated as risk factors for schizophrenia. Both insults induce an exacerbated inflammatory response, which could mediate disturbance of neurodevelopmental processes and, ultimately, malfunctioning of neural systems observed in this disorder. Thus, anti-inflammatory drugs, such as PPARγ agonists, may potentially be used to prevent the development of schizophrenia. Microglia culture was prepared from the offspring of saline or poly(I:C)-injected mice. The cells were pretreated with pioglitazone and then, stimulated by LPS. Proinflammatory mediators and phagocytic activity were measured. Also, pregnant rats were injected with saline or poly(I:C) on GD17. The offspring were subjected to footshock during adolescence and subsequently injected with pioglitazone or vehicle. At adulthood, behavior and dopaminergic activity were evaluated. Pioglitazone reduced proinflammatory mediators induced by poly(I:C) microglia stimulated by LPS without affecting their decreased phagocytic activity. The PPARγ agonist also prevented the emergence of social and cognitive impairments, as well as attenuated the increased number of spontaneously active dopamine neurons in the VTA, observed in both males and females from poly(I:C) and stress group. Therefore, pioglitazone could potentially prevent the emergence of the schizophrenia-like alterations induced by the two-hit model via reduction of microglial activation.

How life events may confer vulnerability to addiction: the role of epigenetics

Substance use disorder (SUD) represents a large and growing global health problem. Despite the strong addictive potency of drugs of abuse, only a minority of those exposed develop SUDs. While certain life experiences (e.g., childhood trauma) may increase subsequent vulnerability to SUDs, mechanisms underlying these effects are not yet well understood. Given the chronic and relapsing nature of SUDs, and the length of time that can elapse between prior life events and subsequent drug exposure, changes in SUD vulnerability almost certainly involve long-term epigenetic dysregulation. To validate this idea, functional effects of specific epigenetic modifications in brain regions mediating reinforcement learning (e.g., nucleus accumbens, prefrontal cortex) have been investigated in a variety of animal models of SUDs. In addition, the effects of epigenetic modifications produced by prior life experiences on subsequent SUD vulnerability have been studied, but mostly in a correlational manner. Here, we review how epigenetic mechanisms impact SUD-related behavior in animal models and summarize our understanding of the relationships among life experiences, epigenetic regulation, and future vulnerability to SUDs. Despite variations in study design, epigenetic modifications that most consistently affect SUD-related behavior are those that produce predominantly unidirectional effects on gene regulation, such as DNA methylation and histone phosphorylation. Evidence explicitly linking environmentally induced epigenetic modifications to subsequent SUD-related behavior is surprisingly sparse. We conclude by offering several directions for future research to begin to address this critical research gap.

A Computational Model for the Simulation of Prepulse Inhibition and Its Modulation by Cortical and Subcortical Units

The sensorimotor gating is a nervous system function that modulates the acoustic startle response (ASR). Prepulse inhibition (PPI) phenomenon is an operational measure of sensorimotor gating, defined as the reduction of ASR when a high intensity sound (pulse) is preceded in milliseconds by a weaker stimulus (prepulse). Brainstem nuclei are associated with the mediation of ASR and PPI, whereas cortical and subcortical regions are associated with their modulation. However, it is still unclear how the modulatory units can influence PPI. In the present work, we developed a computational model of a neural circuit involved in the mediation (brainstem units) and modulation (cortical and subcortical units) of ASR and PPI. The activities of all units were modeled by the leaky-integrator formalism for neural population. The model reproduces basic features of PPI observed in experiments, such as the effects of changes in interstimulus interval, prepulse intensity, and habituation of ASR. The simulation of GABAergic and dopaminergic drugs impaired PPI by their effects over subcortical units activity. The results show that subcortical units constitute a central hub for PPI modulation. The presented computational model offers a valuable tool to investigate the neurobiology associated with disorder-related impairments in PPI.

The neutrophil-lymphocyte ratio in first-episode medication-naïve patients with schizophrenia: A 12-week longitudinal follow-up study

Inflammation has been related to schizophrenia (SZ). The neutrophil-to-lymphocyte ratio (NLR) is an inexpensive inflammatory marker, however, its potential predictive value in patients with SZ has not been extensively investigated. This study aimed to examine whether NLR could predict the clinical response to antipsychotics in this population. One hundred and ninety-five medication-naïve first-episode schizophrenia (MNFES) patients were recruited and received treatment with risperidone for 12 weeks in the present study. Clinical symptoms were evaluated at week 0 and the end of 12 weeks of treatment using the PANSS scales. Complete blood counts were determined at baseline. We found that baseline NEU counts and NLR were positively associated with improvements in clinical symptoms in patients. In addition, MNFES patients with higher baseline NLR values showed a better treatment response to antipsychotics. Linear regression analysis revealed a predictive role of baseline NLR for the improvements of clinical symptoms in SZ patients. Our findings demonstrate that higher NLR was related to better improvements in symptoms after 12 weeks of treatment with antipsychotics, which renders it a promising biomarker of the response to antipsychotics in clinical practice.

Differential Effects of Neonatal Ventral Hippocampus Lesion on Behavior and Corticolimbic Plasticity in Wistar-Kyoto and Spontaneously Hypertensive Rats

Dysfunction of the corticolimbic system, particularly at the dendritic spine level, is a recognized core mechanism in neurodevelopmental disorders such as schizophrenia. Neonatal ventral hippocampus lesion (NVHL) in Sprague-Dawley rats induces both a schizophrenia-related behavioral phenotype and dendritic spine pathology (reduced total number and mature spines) in corticolimbic areas, which is mitigated by antipsychotics. However, there is limited information on the impact of rat strain on NVHL outcomes and antipsychotic effects. We compared the behavioral performance in the open field, novel object recognition (NORT), and social interaction tests, as well as structural neuroplasticity with the Golgi-Cox stain in Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) male rats with and without NVHL. Additionally, we explored the effect of the atypical antipsychotic risperidone (RISP). WKY rats with NVHL displayed motor hyperactivity without impairments in memory and social behavior, accompanied by dendritic spine pathology in the neurons of the prefrontal cortex (PFC) layer 3 and basolateral amygdala. RISP treatment reduced motor activity and had subtle and selective effects on the neuroplasticity alterations. In SH rats, NVHL increased the time spent in the border area during the open field test, impaired the short-term performance in NORT, and reduced social interaction time, deficits that were corrected after RISP administration. The NVHL caused dendritic spine pathology in the PFC layers 3 and 5 of SH rats, which RISP treatment ameliorated. Our results support the utility of the NVHL model for exploring neuroplasticity mechanisms in schizophrenia and understanding pharmacotherapy.

Olanzapine, but not haloperidol, exerts pronounced acute metabolic effects in the methylazoxymethanol rat model

AIM

Widely used second-generation antipsychotics are associated with adverse metabolic effects, contributing to increased cardiovascular mortality. To develop strategies to prevent or treat adverse metabolic effects, preclinical models have a clear role in uncovering underlying molecular mechanisms. However, with few exceptions, preclinical studies have been performed in healthy animals, neglecting the contribution of dysmetabolic features inherent to psychotic disorders.

METHODS

In this study, methylazoxymethanol acetate (MAM) was prenatally administered to pregnant Sprague-Dawley rats at gestational day 17 to induce a well-validated neurodevelopmental model of schizophrenia mimicking its assumed pathogenesis with persistent phenotype. Against this background, the dysmetabolic effects of acute treatment with olanzapine and haloperidol were examined in female rats.

RESULTS

Prenatally MAM-exposed animals exhibited several metabolic features, including lipid disturbances. Half of the MAM rats exposed to olanzapine had pronounced serum lipid profile alteration compared to non-MAM controls, interpreted as a reflection of a delicate MAM-induced metabolic balance disrupted by olanzapine. In accordance with the drugs' clinical metabolic profiles, olanzapine-associated dysmetabolic effects were more pronounced than haloperidol-associated dysmetabolic effects in non-MAM rats and rats exposed to MAM.

CONCLUSION

Our results demonstrate metabolic vulnerability in female prenatally MAM-exposed rats, indicating that findings from healthy animals likely provide an underestimated impression of metabolic dysfunction associated with antipsychotics. In the context of metabolic disturbances, neurodevelopmental models possess a relevant background, and the search for adequate animal models should receive more attention within the field of experimental psychopharmacology.

Prefrontal and Hippocampal Parvalbumin Interneurons in Animal Models for Schizophrenia: A Systematic Review and Meta-analysis

BACKGROUND

Consistent with postmortem findings in patients, most animal models for schizophrenia (SCZ) present abnormal levels of parvalbumin (PV), a marker of fast-spiking GABAergic interneurons, in the prefrontal cortex (PFC) and hippocampus (HIP). However, there are discrepancies in the literature. PV reductions lead to a functional loss of PV interneurons, which is proposed to underly SCZ symptoms. Given its complex etiology, different categories of animal models have been developed to study SCZ, which may distinctly impact PV levels in rodent brain areas.

STUDY DESIGN

We performed a quantitative meta-analysis on PV-positive cell number/density and expression levels in the PFC and HIP of animal models for SCZ based on pharmacological, neurodevelopmental, and genetic manipulations.

RESULTS

Our results confirmed that PV levels are significantly reduced in the PFC and HIP regardless of the animal model. By categorizing into subgroups, we found that all pharmacological models based on NMDA receptor antagonism decreased PV-positive cell number/density or PV expression levels in both brain areas examined. In neurodevelopmental models, abnormal PV levels were confirmed in both brain areas in maternal immune activation models and HIP of the methylazoxymethanol acetate model. In genetic models, negative effects were found in neuregulin 1 and ERBB4 mutant mice in both brain regions and the PFC of dysbindin mutant mice. Regarding sex differences, male rodents exhibited PV reductions in both brain regions only in pharmacological models, while few studies have been conducted in females.

CONCLUSION

Overall, our findings support deficits in prefrontal and hippocampal PV interneurons in animal models for SCZ.

Microglial contribution to the pathology of neurodevelopmental disorders in humans

Microglia are the brain's resident macrophages, which guide various developmental processes crucial for brain maturation, activity, and plasticity. Microglial progenitors enter the telencephalic wall by the 4th postconceptional week and colonise the fetal brain in a manner that spatiotemporally tracks key neurodevelopmental processes in humans. However, much of what we know about how microglia shape neurodevelopment comes from rodent studies. Multiple differences exist between human and rodent microglia warranting further focus on the human condition, particularly as microglia are emerging as critically involved in the pathological signature of various cognitive and neurodevelopmental disorders. In this article, we review the evidence supporting microglial involvement in basic neurodevelopmental processes by focusing on the human species. We next concur on the neuropathological evidence demonstrating whether and how microglia contribute to the aetiology of two neurodevelopmental disorders: autism spectrum conditions and schizophrenia. Next, we highlight how recent technologies have revolutionised our understanding of microglial biology with a focus on how these tools can help us elucidate at unprecedented resolution the links between microglia and neurodevelopmental disorders. We conclude by reviewing which current treatment approaches have shown most promise towards targeting microglia in neurodevelopmental disorders and suggest novel avenues for future consideration.

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.