An investigation into "two hit" effects of BDNF deficiency and young-adult cannabinoid receptor stimulation on prepulse inhibition regulation and memory in mice

The dual hit hypothesis of schizophrenia: Evidence from animal models

Schizophrenia is a heterogeneous psychiatric disorder, which can severely impact social and professional functioning. Epidemiological and clinical studies show that schizophrenia has a multifactorial aetiology comprising genetic and environmental risk factors. Although several risk factors have been identified, it is still not clear how they result in schizophrenia. This knowledge gap, however, can be investigated in animal studies. In this review, we summarise animal studies regarding molecular and cellular mechanisms through which genetic and environmental factors may affect brain development, ultimately causing schizophrenia. Preclinical studies suggest that early environmental risk factors can affect the immune, GABAergic, glutamatergic, or dopaminergic system and thus increase the susceptibility to another risk factor later in life. A second insult, like social isolation, stress, or drug abuse, can further disrupt these systems and the interactions between them, leading to behavioural abnormalities. Surprisingly, first insults like maternal infection and early maternal separation can also have protective effects. Single gene mutations associated with schizophrenia did not have a major impact on the susceptibility to subsequent environmental hits.

BDNF haploinsufficiency induces behavioral endophenotypes of schizophrenia in male mice that are rescued by enriched environment

Brain-derived neurotrophic factor (BDNF) is implicated in a number of processes that are crucial for healthy functioning of the brain. Schizophrenia is associated with low BDNF levels in the brain and blood, however, not much is known about BDNF's role in the different symptoms of schizophrenia. Here, we used BDNF-haploinsufficient (BDNF^+/-) mice to investigate the role of BDNF in different mouse behavioral endophenotypes of schizophrenia. Furthermore, we assessed if an enriched environment can prevent the observed changes. In this study, male mature adult wild-type and BDNF^+/- mice were tested in mouse paradigms for cognitive flexibility (attentional set shifting), sensorimotor gating (prepulse inhibition), and associative emotional learning (safety and fear conditioning). Before these tests, half of the mice had a 2-month exposure to an enriched environment, including running wheels. After the tests, BDNF brain levels were quantified. BDNF^+/- mice had general deficits in the attentional set-shifting task, increased startle magnitudes, and prepulse inhibition deficits. Contextual fear learning was not affected but safety learning was absent. Enriched environment housing completely prevented the observed behavioral deficits in BDNF^+/- mice. Notably, the behavioral performance of the mice was negatively correlated with BDNF protein levels. These novel findings strongly suggest that decreased BDNF levels are associated with several behavioral endophenotypes of schizophrenia. Furthermore, an enriched environment increases BDNF protein to wild-type levels and is thereby able to rescue these behavioral endophenotypes.

Neuron-specific deletion of VEGF or its receptor Flk-1 impairs recognition memory

Vascular endothelial growth factor (VEGF, also known as VEGF-A) is a pleiotropic factor which is expressed by neurons, astrocytes and perivascular macrophages, as well as endothelial cells, in the brain. Recently, VEGF signaling has been implicated in learning and memory, and several clinical and preclinical studies demonstrate that VEGF inhibitors induce cognitive impairment. However, the role of endogenous neuronal VEGF signaling in recognition memory remains unclear. Recently, we have developed mice with forebrain excitatory neuron-specific deletion of VEGF or its receptor, fetal liver kinase 1 (Flk-1) by crossing Camk2a-Cre mice with Vegfa^flox/flox and Flk-1^flox/flox mice, respectively. Using these conditional knockout mice, the present study addressed the influence of forebrain excitatory neuron-specific deletion of VEGF or Flk-1 on recognition memory in the novel object recognition test. The results show that both short-term (2 h) and long-term (24 h) recognition memory are impaired by neuron-specific deletion of either Flk-1 or VEGF. These findings indicate the physiological importance of endogenous neuronal VEGF-Flk-1 signaling in recognition memory. In addition, the current results also suggest that the impairment of neuronal VEGF-Flk-1 signaling can be a cause of anti-VEGF chemotherapy-induced cognitive impairment.

Interaction of reelin and stress on immobility in the forced swim test but not dopamine-mediated locomotor hyperactivity or prepulse inhibition disruption: Relevance to psychotic and mood disorders

RATIONALE

Psychotic disorders, such as schizophrenia, as well as some mood disorders, such as bipolar disorder, have been suggested to share common biological risk factors. One such factor is reelin, a large extracellular matrix glycoprotein that regulates neuronal migration during development as well as numerous activity-dependent processes in the adult brain. The current study sought to evaluate whether a history of stress exposure interacts with endogenous reelin levels to modify behavioural endophenotypes of relevance to psychotic and mood disorders.

METHODS

Heterozygous Reeler Mice (HRM) and wildtype (WT) controls were treated with 50mg/L of corticosterone (CORT) in their drinking water from 6 to 9weeks of age, before undergoing behavioural testing in adulthood. We assessed methamphetamine-induced locomotor hyperactivity, prepulse inhibition (PPI) of acoustic startle, short-term spatial memory in the Y-maze, and depression-like behaviour in the Forced-Swim Test (FST).

RESULTS

HRM genotype or CORT treatment did not affect methamphetamine-induced locomotor hyperactivity, a model of psychosis-like behaviour. At baseline, HRM showed decreased PPI at the commonly used 100msec interstimulus interval (ISI), but not at the 30msec ISI or following challenge with apomorphine. A history of CORT exposure potentiated immobility in the FST amongst HRM, but not WT mice. In the Y-maze, chronic CORT treatment decreased novel arm preference amongst HRM, reflecting reduced short-term spatial memory.

CONCLUSION

These data confirm a significant role of endogenous reelin levels on stress-related behaviour, supporting a possible role in both bipolar disorder and schizophrenia. However, an interaction of reelin deficiency with dopaminergic regulation of psychosis-like behaviour remains unclear.

Effect of adolescent androgen manipulation on psychosis-like behaviour in adulthood in BDNF heterozygous and control mice

RATIONALE

Males are more prone to psychosis, schizophrenia and substance abuse and addiction in adolescence and early adulthood than females. However, the role of androgens during this developmental period is poorly understood.

OBJECTIVES

This study aimed to examine how androgens in adolescence influence psychosis-like behaviour in adulthood and whether brain-derived neurotrophic factor (BDNF) is a mediator of these developmental effects.

METHODS

Wild-type and BDNF heterozygous male mice were castrated at pre-pubescence and implanted with testosterone or dihydrotestosterone (DHT). In adulthood, we assessed amphetamine- and MK-801-induced hyperlocomotion as a model of psychosis-like behaviour. Western blot analysis was used to quantify levels of the dopamine transporter (DAT) and N-methyl-d-aspartate (NMDA) receptor subunits.

RESULTS

While castration itself had little effect on behaviour, adolescent testosterone, but not DHT, significantly reduced amphetamine-induced hyperlocomotion, whereas both testosterone and DHT reduced the effect of MK-801. These effects were similar in mice of either genotype. In wildtype mice, both testosterone and DHT treatment reduced DAT expression in the medial prefrontal cortex (mPFC) but these effects were absent in BDNF heterozygous mice. There were no effects on NMDA receptor subunit levels.

CONCLUSIONS

The differential effect of adolescent testosterone and DHT on amphetamine-induced hyperlocomotion in adulthood suggests involvement of conversion of testosterone to estrogen and subsequent modulation of dopaminergic signalling. In contrast, the similar effect of testosterone and DHT treatment on NMDA receptor-mediated hyperlocomotion indicates it is mediated by androgen receptors. The involvement of BDNF in these hormone effects remains to be elucidated. These results demonstrate that, during adolescence, androgens significantly influence key pathways related to various mental illnesses prevalent in adolescence.

Marijuana use among adolescents is associated with deleterious alterations in mature BDNF

Background

With increases in marijuana use and legalization efforts, it is imperative to establish its impact on the developing brain. Therefore, we investigated whether exposure to marijuana alters brain derived neurotropic-factor (BDNF), given its critical role in brain development and plasticity. We then examined whether onset age of cannabis use was associated with more severe changes. A single site, cohort study following 500 urban healthy American adolescents. Changes in plasma m-BDNF levels were longitudinally assessed, and a multi-method approach was implemented to ascertain marijuana use. Multivariate and general linear model (GLM) regression modeling were utilized to test the main hypothesis, controlling for confounders.

Results

Group-based trajectory modeling identified four distinct groups, characterized by naive (60% control), starters (14%), chronic users (20%), and experimenting/quitters (6%). Compared to controls, those initiating marijuana use had similar pre-existent m-BDNF (1939.2 ± 221 vs. 2640.7 ± 1309 ng/ml, p=0.4) After adjusting for confounding factors, GLM analyses revealed that, compared to controls, younger adolescents increased BDNF levels when experimenting and during moderate marijuana use. Older adolescents had a steeper increase in endogenous BDNF levels, particularly when escalating use. Multivariate analyses confirmed marijuana use as a predictor of m-BDNF (p = 0.001).

Conclusions

This is the first study demonstrating BDNF alterations were not a precondition. Rather, BDNF alteration was secondary to marijuana use, serving as cautionary evidence of marijuana's deleterious effects. Findings suggest that when marijuana use escalates, the BDNF pathway becomes more deregulated. Analyses confirm that age of marijuana use onset influences the magnitude of these changes.

Brain-Derived Neurotrophic Factor (BDNF) Role in Cannabinoid-Mediated Neurogenesis

The adult mammalian brain can produce new neurons in a process called adult neurogenesis, which occurs mainly in the subventricular zone (SVZ) and in the hippocampal dentate gyrus (DG). Brain-derived neurotrophic factor (BDNF) signaling and cannabinoid type 1 and 2 receptors (CB1R and CB2R) have been shown to independently modulate neurogenesis, but how they may interact is unknown. We now used SVZ and DG neurosphere cultures from early (P1-3) postnatal rats to study the CB1R and CB2R crosstalk with BDNF in modulating neurogenesis. BDNF promoted an increase in SVZ and DG stemness and cell proliferation, an effect blocked by a CB2R selective antagonist. CB2R selective activation promoted an increase in DG multipotency, which was inhibited by the presence of a BDNF scavenger. CB1R activation induced an increase in SVZ and DG cell proliferation, being both effects dependent on BDNF. Furthermore, SVZ and DG neuronal differentiation was facilitated by CB1R and/or CB2R activation and this effect was blocked by sequestering endogenous BDNF. Conversely, BDNF promoted neuronal differentiation, an effect abrogated in SVZ cells by CB1R or CB2R blockade while in DG cells was inhibited by CB2R blockade. We conclude that endogenous BDNF is crucial for the cannabinoid-mediated effects on SVZ and DG neurogenesis. On the other hand, cannabinoid receptor signaling is also determinant for BDNF actions upon neurogenesis. These findings provide support for an interaction between BDNF and endocannabinoid signaling to control neurogenesis at distinct levels, further contributing to highlight novel mechanisms in the emerging field of brain repair.

Sensorimotor gating deficits in "two-hit" models of schizophrenia risk factors

Genetic and environmental models of neuropsychiatric disease have grown exponentially over the last 20years. One measure that is often used to evaluate the translational relevance of these models to human neuropsychiatric disease is prepulse inhibition of startle (PPI), an operational measure of sensorimotor gating. Deficient PPI characterizes several neuropsychiatric disorders but has been most extensively studied in schizophrenia. It has become a useful tool in translational neuropharmacological and molecular genetics studies because it can be measured across species using almost the same experimental parameters. Although initial studies of PPI in rodents were pharmacological because of the robust predictive validity of PPI for antipsychotic efficacy, more recently, PPI has become standard common behavioral measures used in genetic and neurodevelopmental models of schizophrenia. Here we review "two hit" models of schizophrenia and discuss the utility of PPI as a tool in phenotyping these models of relevant risk factors. In the review, we consider approaches to rodent models of genetic and neurodevelopmental risk factors and selectively review "two hit" models of gene×environment and environment×environment interactions in which PPI has been measured.

Does genetic BDNF deficiency in rats interact with neurotransmitter control of prepulse inhibition? Implications for schizophrenia

Several studies have suggested a role of BDNF in the development of schizophrenia. For example, post-mortem studies have shown significantly reduced levels of BDNF protein expression in the brain of schizophrenia patients. We investigated the relationship between reduced levels of BDNF in the brain and the regulation of prepulse inhibition (PPI), a behavioral endophenotype of schizophrenia. We used BDNF heterozygous mutant rats which display a 50% decrease of mature BDNF protein levels. Previously, we observed normal baseline PPI and responses to the dopamine D1/D2 receptor agonist, apomorphine, in these rats. Here, we focused on the effects of the NMDA receptor antagonist, MK-801, its interaction with mGluR2/3 and mGluR5 receptors, and the PPI response to serotonergic drugs. MK-801 administration caused a dose-dependent reduction of PPI and increase of startle amplitudes. Baseline PPI and the effect of 0.02-0.1mg/kg of MK-801 were not significantly altered in male or female BDNF heterozygous rats, although the MK-801-induced increase in startle levels was reduced. Co-treatment with the mGluR2/3 agonist, LY379,268, or the mGluR5 antagonist, MPEP, did not alter the effect of MK-801 on PPI in controls or BDNF mutant rats. Treatment with the serotonin-1A receptor agonist, 8-OH-DPAT, the serotonin-2A receptor agonist, DOI, or the serotonin releaser, fenfluramine, induced differential effects on PPI and startle but these effects were not different between the genotypes. These results show that a significant decrease of BDNF protein expression does not lead to reduced PPI at baseline or changes in the regulation of PPI via NMDA receptors or serotonergic mechanisms. These findings in a genetic rat model of BDNF deficiency do not support a role for similar reductions of BDNF levels in schizophrenia in the disruption of PPI, widely reported as an endophenotype of the illness. The potential implications of these results for our understanding of changes in PPI and BDNF expression in schizophrenia are discussed.

The tyrosine kinase inhibitor, sunitinib malate, induces cognitive impairment in vivo via dysregulating VEGFR signaling, apoptotic and autophagic machineries

Chemobrain refers to a cluster of cognitive deficits which affects almost 4-75% of chemotherapy-treated cancer patients. Sunitinib, an FDA-approved multityrosine kinase inhibitor, is currently used in treating different types of tumors. Despite being regarded as targeted therapy which blunts sustained angiogenesis in cancer milieu through inhibiting vascular endothelial growth factor receptor 2 (VEGFR2) signaling, the latter has a cardinal role in cognition. Recent clinical reports warned that sunitinib adversely affected memory processing in cancer patients. Nevertheless, the underlying mechanisms have not been investigated yet. Hence, we explored the impact of a clinically relevant dose of sunitinib on memory processing in vivo and questioned the implication of VEGFR2 signaling, autophagy and apoptosis. Strikingly, sunitinib preferentially impaired spatial cognition as evidenced in Morris water maze, T-maze and passive avoidance task. Consistently, sunitinib degenerated cortical and hippocampal neurons as assessed by histopathological examination and toluidine blue staining. Ultrastructural examination also depicted chromatin condensation, mitochondrial damage and accumulated autophagosomes. Digging deeper, central VEGF/VEGFR2/mTOR signaling was robustly suppressed. Besides, sunitinib boosted cortical and hippocampal p53 and executioner caspase-3 and decreased nuclear factor kappa B and Bcl-2 levels promoting apoptotic cell death. It also profoundly impeded neuronal autophagic flux as shown by decreased beclin-1 and Atg5 and increased p62/SQTSM1 levels. To our knowledge, this is the first study to provide molecular insights into sunitinib-induced chemofog where impeded VEGFR2 signaling and autophagic and hyperactivated apoptotic machineries act in neurodegenerative concert. Importantly, our findings shed light on potential therapeutic strategies to be exploited in the management of sunitinib-induced chemobrain.

The Impact of Exposure to Cannabinoids in Adolescence: Insights From Animal Models

The regular use of cannabis during adolescence is of particular concern because use by this age group seems to be associated with an increased likelihood of deleterious consequences, as reported by several epidemiologic studies. However, despite their unquestionable value, epidemiologic data are inconclusive. Modeling the adolescent phase in animals appears to be a useful approach to investigate the impact of cannabis use on the adolescent brain. In these models, adolescent cannabinoid exposure has been reported to cause long-term impairment in specific components of learning and memory and to have differential effects on anxiety, social behavior, and depressive-like signs. These findings suggest that it may represent, per se or in association with other hits, a risk factor for developing psychotic-like symptoms in adulthood. The neurobiological bases of this association include the induction of alterations in the maturational events of the endocannabinoid system occurring in the adolescent brain. Alterations in the endocannabinoid system may profoundly dysregulate developmental processes in some neurotransmitter systems, such as gamma-aminobutyric acid and glutamate, mainly in the cortex. The resulting picture strongly resembles the one present in schizophrenic patients, highlighting the translational value of this experimental approach.

Schizophrenia and Depression Co-Morbidity: What We have Learned from Animal Models

Patients with schizophrenia are at an increased risk for the development of depression. Overlap in the symptoms and genetic risk factors between the two disorders suggests a common etiological mechanism may underlie the presentation of comorbid depression in schizophrenia. Understanding these shared mechanisms will be important in informing the development of new treatments. Rodent models are powerful tools for understanding gene function as it relates to behavior. Examining rodent models relevant to both schizophrenia and depression reveals a number of common mechanisms. Current models which demonstrate endophenotypes of both schizophrenia and depression are reviewed here, including models of CUB and SUSHI multiple domains 1, PDZ and LIM domain 5, glutamate Delta 1 receptor, diabetic db/db mice, neuropeptide Y, disrupted in schizophrenia 1, and its interacting partners, reelin, maternal immune activation, and social isolation. Neurotransmission, brain connectivity, the immune system, the environment, and metabolism emerge as potential common mechanisms linking these models and potentially explaining comorbid depression in schizophrenia.

Analyzing the influence of BDNF heterozygosity on spatial memory response to 17β-estradiol

The recent use of estrogen-based therapies as adjunctive treatments for the cognitive impairments of schizophrenia has produced promising results; however the mechanism behind estrogen-based cognitive enhancement is relatively unknown. Brain-derived neurotrophic factor (BDNF) regulates learning and memory and its expression is highly responsive to estradiol. We recently found that estradiol modulates the expression of hippocampal parvalbumin-positive GABAergic interneurons, known to regulate neuronal synchrony and cognitive function. What is unknown is whether disruptions to the aforementioned estradiol-parvalbumin pathway alter learning and memory, and whether BDNF may mediate these events. Wild-type (WT) and BDNF heterozygous (+/-) mice were ovariectomized (OVX) at 5 weeks of age and simultaneously received empty, estradiol- or progesterone-filled implants for 7 weeks. At young adulthood, mice were tested for spatial and recognition memory in the Y-maze and novel-object recognition test, respectively. Hippocampal protein expression of BDNF and GABAergic interneuron markers, including parvalbumin, were assessed. WT OVX mice show impaired performance on Y-maze and novel-object recognition test. Estradiol replacement in OVX mice prevented the Y-maze impairment, a Behavioral abnormality of dorsal hippocampal origin. BDNF and parvalbumin protein expression in the dorsal hippocampus and parvalbumin-positive cell number in the dorsal CA1 were significantly reduced by OVX in WT mice, while E2 replacement prevented these deficits. In contrast, BDNF(+/-) mice showed either no response or an opposite response to hormone manipulation in both behavioral and molecular indices. Our data suggest that BDNF status is an important biomarker for predicting responsiveness to estrogenic compounds which have emerged as promising adjunctive therapeutics for schizophrenia patients.

Developmentally vitamin D-deficient rats show enhanced prepulse inhibition after acute Δ9-tetrahydrocannabinol

Developmental vitamin D (DVD) deficiency has been proposed as a risk factor for schizophrenia. DVD-deficient rats show selective cognitive deficits and novelty-induced hyperlocomotion and enhanced locomotor responses from acute treatment with psychomimetic drugs, such as amphetamine and MK-801. Here we aimed to examine the effect of a drug from a different class of psychomimetic/psychoactive compounds, Δ9-tetrahydrocannabinol (THC), on tasks of relevance to the cognitive and positive symptoms of schizophrenia. The aim of this study was to investigate whether DVD deficiency modulates the behavioural effects of THC on tests of delay-dependent memory, sensorimotor gating and locomotion. Adult control and DVD-deficient rats were injected with THC (0, 0.3, 0.6, 1.25, 2.5 mg/kg) 15 min before a delay match to sample (DMTS) task using variable delays (0-24 s). A separate group of rats was injected with either 2.5 mg/kg THC or vehicle before tests of either prepulse inhibition (PPI) of the acoustic startle response or in the open field. Control and DVD-deficient rats showed a similar dose-dependent impairment in performance on the DMTS. The greatest impairment was observed at 2.5 mg/kg for all delays (0-24 s). DVD-deficient rats showed THC-induced enhancement of PPI, which was not observed in control rats. There was no effect of maternal diet on acoustic startle response or locomotor responses in the open field. This study reports the novel findings that DVD-deficient rats were more sensitive to the acute effects of THC on PPI. It appears that prenatal vitamin D deficiency has long-term effects on sensitivity to the behavioural effects of cannabinoids.

Cannabis and adolescent brain development

Heavy cannabis use has been frequently associated with increased rates of mental illness and cognitive impairment, particularly amongst adolescent users. However, the neurobiological processes that underlie these associations are still not well understood. In this review, we discuss the findings of studies examining the acute and chronic effects of cannabis use on the brain, with a particular focus on the impact of commencing use during adolescence. Accumulating evidence from both animal and human studies suggests that regular heavy use during this period is associated with more severe and persistent negative outcomes than use during adulthood, suggesting that the adolescent brain may be particularly vulnerable to the effects of cannabis exposure. As the endocannabinoid system plays an important role in brain development, it is plausible that prolonged use during adolescence results in a disruption in the normative neuromaturational processes that occur during this period. We identify synaptic pruning and white matter development as two processes that may be adversely impacted by cannabis exposure during adolescence. Potentially, alterations in these processes may underlie the cognitive and emotional deficits that have been associated with regular use commencing during adolescence.