Long-term effects of young-adult methamphetamine on dorsal raphe serotonin systems in mice: Role of brain-derived neurotrophic factor

To assess the long-term effects of chronic adolescent methamphetamine (METH) treatment on the serotonin system in the brain, we used serotonin-1A receptor (5-HT_1A) and serotonin transporter (SERT) autoradiography, and quantitative tryptophan-hydroxylase 2 (TPH2) immunohistochemistry in the raphe nuclei of mice. Because of the modulatory role of brain-derived neurotrophic factor (BDNF) on the serotonin system and the effects of METH, we included both BDNF heterozygous (HET) mice and wildtype (WT) controls. Male and female mice of both genotypes were treated with an escalating METH dose regimen from the age of 6-9 weeks. At least two weeks later, acute locomotor hyperactivity induced by a 5 mg/kg D-amphetamine challenge was significantly enhanced in METH-pretreated mice, showing long-term sensitisation. METH pretreatment caused a small, but significant decrease of 5-HT_1A receptor binding in the dorsal raphe nucleus (DRN) of males independent of genotype, but there were no changes in the median raphe nucleus (MRN) or in SERT binding density. METH treatment reduced the number of TPH2 positive cells in ventral subregions of the rostral and medial DRN independent of genotype. METH treatment selectively reduced DRN cell counts in BDNF HET mice compared to wildtype mice in medial and caudal ventrolateral subregions previously associated with panic-like behaviour. The data increase our understanding of the long-term and selective effects of METH on brain serotonin systems. These findings could be relevant for some of the psychosis-like symptoms associated with long-term METH use.

Maternal immune activation targeted to a window of parvalbumin interneuron development improves spatial working memory: Implications for autism

Maternal immune activation (MIA) increases risk for neuropsychiatric disorders such as autism spectrum disorder (ASD) in offspring later in life through unknown causal mechanisms. Growing evidence implicates parvalbumin-containing GABAergic interneurons as a key target in rodent MIA models. We targeted a specific neurodevelopmental window of parvalbumin interneurons in a mouse MIA model to examine effects on spatial working memory, a key domain in ASD that can manifest as either impairments or improvements both clinically and in animal models. Pregnant dams received three consecutive intraperitoneal injections of Polyinosinic:polycytidylic acid (poly(I:C), 5 mg/kg) at gestational days 13, 14 and 15. Spatial working memory was assessed in young adult offspring using touchscreen operant chambers and the Trial-Unique Non-matching to Location (TUNL) task. Anxiety, novelty seeking and short-term memory were assessed using Elevated Plus Maze (EPM) and Y-maze novelty preference tasks. Fluorescent immunohistochemistry was used to assess hippocampal parvalbumin cell density, intensity and co-expression with perineuronal nets. qPCR was used to assess the expression of putatively implicated gene pathways. MIA targeting a window of parvalbumin interneuron development increased spatial working memory performance on the TUNL touchscreen task which was not influenced by anxiety or novelty seeking behaviour. The model reduced fetal mRNA levels of Gad1 and adult hippocampal mRNA levels of Pvalb and the distribution of low intensity parvalbumin interneurons was altered. We speculate a specific timing window for parvalbumin interneuron development underpins the apparently paradoxical improved spatial working memory phenotype found both across several rodent models of autism and clinically in ASD.

Brain-derived Neurotropic Factor val66met is a Strong Predictor of Decision Making and Attention Performance on the CONVIRT Virtual Reality Cognitive Battery

The val66met polymorphism of the brain-derived neurotrophic factor gene has been associated with changes in components of executive functioning such as decision making; however, this relationship remains unclear. Val66met-related changes in attention and visual processing speed may explain potential changes in decision making. Furthermore, chronic stress disrupts executive functions and alters autonomic activity. Because the relationship between val66met and cognition has not been investigated in the context of chronic stress or stress-related autonomic changes, in this study 55 healthy university students completed self-report measures of chronic stress and mental health. Participants then completed a virtual reality cognitive test battery (CONVIRT) measuring decision making, attention, and visual processing reaction times. To measure autonomic activity, saliva alpha amylase and heart rate variability (HRV) were assessed at baseline and after CONVIRT testing. Saliva samples were used to identify val66met genotype. Regression analyses demonstrated that val66met was the strongest predictor of decision making and attention, but not visual processing, where valine/methionine (Val/met) participants had faster reaction times than Val/val participants. Val/met participants also had higher perceived chronic stress and heightened increases in sympathetic activity, but not parasympathetic activity. Neither stress nor autonomic activity moderated the effect of val66met on decision making or attention. This study is the first to investigate the role of val66met in decision making, attention, and visual processing while taking into account chronic stress and autonomic activity. This multifactorial approach revealed that carriers of the Val/met genotype may have better decision making and attention than Val/val carriers.

Behavioural phenotyping of thunder mice with a hypomorphic mutation of heterogeneous nuclear ribonuclear protein L-like (hnRNPLL) and reduced T cell function

Heterogeneous nuclear ribonuclear protein l-like (hnRNPLL) is an RNA binding protein that regulates alternative splicing of mRNA and is abundantly expressed in memory T lymphocytes of the immune system and in the brain. A hypomorphic allele of the gene encoding hnRNPLL (Hnrpll^thunder) selectively reduces T cell accumulation in lymphoid tissues, but little is known about its effects in the brain. Therefore, we exposed Hnrpll^thunder mice to a test battery with relevance for a range of psychiatric illnesses. Thunder mice showed enhanced immobility in the tail-suspension test for depression-related behaviours, impaired short-term spatial memory in the Y-maze and reduced avoidance learning in the active avoidance test. Thus, in addition to its reported effects on immune function, the hnRNPLL mutation in thunder mice selectively affected aspects of behaviour.

Neurobiology of BDNF in fear memory, sensitivity to stress, and stress-related disorders

Brain-derived neurotrophic factor (BDNF) is widely accepted for its involvement in resilience and antidepressant drug action, is a common genetic locus of risk for mental illnesses, and remains one of the most prominently studied molecules within psychiatry. Stress, which arguably remains the "lowest common denominator" risk factor for several mental illnesses, targets BDNF in disease-implicated brain regions and circuits. Altered stress-related responses have also been observed in animal models of BDNF deficiency in vivo, and BDNF is a common downstream intermediary for environmental factors that potentiate anxiety- and depressive-like behavior. However, BDNF's broad functionality has manifested a heterogeneous literature; likely reflecting that BDNF plays a hitherto under-recognized multifactorial role as both a regulator and target of stress hormone signaling within the brain. The role of BDNF in vulnerability to stress and stress-related disorders, such as posttraumatic stress disorder (PTSD), is a prominent example where inconsistent effects have emerged across numerous models, labs, and disciplines. In the current review we provide a contemporary update on the neurobiology of BDNF including new data from the behavioral neuroscience and neuropsychiatry literature on fear memory consolidation and extinction, stress, and PTSD. First we present an overview of recent advances in knowledge on the role of BDNF within the fear circuitry, as well as address mounting evidence whereby stress hormones interact with endogenous BDNF-TrkB signaling to alter brain homeostasis. Glucocorticoid signaling also acutely recruits BDNF to enhance the expression of fear memory. We then include observations that the functional common BDNF Val66Met polymorphism modulates stress susceptibility as well as stress-related and stress-inducible neuropsychiatric endophenotypes in both man and mouse. We conclude by proposing a BDNF stress-sensitivity hypothesis, which posits that disruption of endogenous BDNF activity by common factors (such as the BDNF Val66Met variant) potentiates sensitivity to stress and, by extension, vulnerability to stress-inducible illnesses. Thus, BDNF may induce plasticity to deleteriously promote the encoding of fear and trauma but, conversely, also enable adaptive plasticity during extinction learning to suppress PTSD-like fear responses. Ergo regulators of BDNF availability, such as the Val66Met polymorphism, may orchestrate sensitivity to stress, trauma, and risk of stress-induced disorders such as PTSD. Given an increasing interest in personalized psychiatry and clinically complex cases, this model provides a framework from which to experimentally disentangle the causal actions of BDNF in stress responses, which likely interact to potentiate, produce, and impair treatment of, stress-related psychiatric disorders.

Brain-Derived Neurotrophic Factor Val66Met polymorphism interacts with adolescent stress to alter hippocampal interneuron density and dendritic morphology in mice

Brain-derived neurotrophic factor (BDNF) plays essential roles in GABAergic interneuron development. The common BDNF val66met polymorphism, leads to decreased activity-dependent release of BDNF. The current study used a humanized mouse model of the BDNF val66met polymorphism to determine how reduced activity-dependent release of BDNF, both on its own, and in combination with chronic adolescent stress hormone, impact hippocampal GABAergic interneuron cell density and dendrite morphology. Male and female Val/Val and Met/Met mice were exposed to corticosterone (CORT) or placebo in their drinking water from weeks 6-8, before brains were perfuse-fixed at 15 weeks. Cell density and dendrite morphology of immunofluorescent labelled inhibitory interneurons; somatostatin, parvalbumin and calretinin in the CA1, and 3 and dentate gyrus (DG) across the dorsal (DHP) and ventral hippocampus (VHP) were assessed by confocal z-stack imaging, and IMARIS dendritic mapping software. Mice with the Met/Met genotype showed significantly lower somatostatin cell density compared to Val/Val controls in the DHP, and altered somatostatin interneuron dendrite morphology including branch depth, and spine density. Parvalbumin-positive interneurons were unchanged between genotype groups, however BDNF val66met genotype influenced the dendritic volume, branch level and spine density of parvalbumin interneurons differentially across hippocampal subregions. Contrary to this, no such effects were observed for calretinin-positive interneurons. Adolescent exposure to CORT treatment also significantly altered somatostatin and parvalbumin dendrite branch level and the combined effect of Met/Met genotype and CORT treatment significantly reduced somatostatin and parvalbumin dendrite spine density. In sum, the BDNF^Val66Met polymorphism significantly alters somatostatin and parvalbumin-positive interneuron cell development and dendrite morphology. Additionally, we also report a compounding effect of the Met/Met genotype and chronic adolescent CORT treatment on dendrite spine density, indicating that adolescence is a sensitive period of risk for Val66Met polymorphism carriers.

The effect of 17β-estradiol on maternal immune activation-induced changes in prepulse inhibition and dopamine receptor and transporter binding in female rats

Maternal immune activation (MIA) during pregnancy is associated with an increased risk of development of schizophrenia in later life. 17β-estradiol treatment may improve schizophrenia symptoms, but little is known about its efficacy on MIA-induced psychosis-like behavioural deficits in animals. Therefore, in this study we used the poly(I:C) neurodevelopmental model of schizophrenia to examine whether MIA-induced psychosis-like behavioural and neurochemical changes can be attenuated by chronic treatment (2-6 weeks) with 17β-estradiol. Pregnant rats were treated with saline or the viral mimetic, poly(I:C), on gestational day 15 and adult female offspring were tested for changes in prepulse inhibition (PPI) and density of dopamine D1 and D2 receptors and dopamine transporters in the forebrain compared to control offspring. Poly(I:C)-treated offspring exhibited significantly disrupted PPI, an effect which was reversed by chronic treatment with 17β-estradiol. In control offspring, but not poly(I:C) offspring, PPI was significantly reduced by acute treatment with either the dopamine D1/D2 receptor agonist, apomorphine, or dopamine releaser, methamphetamine. 17β-estradiol restored the effect of apomorphine, but not methamphetamine, on PPI in poly(I:C) offspring. There was a strong trend for a dopamine D2 receptor binding density increase in the nucleus accumbens core region in poly(I:C) offspring, and this was reversed by chronic 17β-estradiol treatment. No changes were found in the nucleus accumbens shell, caudate putamen or frontal cortex or in the density of dopamine D1 receptors or transporters. These findings suggest that 17β-estradiol may improve some symptoms of schizophrenia, an effect that may be mediated by selective changes in dopamine D2 receptor density.

Acute NMDA receptor antagonism impairs working memory performance but not attention in rats-Implications for the NMDAr hypofunction theory of schizophrenia

Cognitive deficits in schizophrenia, which include impairments in working memory and attention, represent some of the most disabling symptoms of this complex psychiatric condition, and lack effective treatments. NMDA receptor (NMDAr) hypofunction is a strong candidate mechanism underlying schizophrenia pathophysiology, and has been modeled preclinically using acute administration of NMDAr antagonists to rodents to investigate biological mechanisms underpinning cognitive dysfunction. However, whether and how NMDAr hypofunction specifically influences all affected cognitive domains is unclear. Here we studied the effects of the NMDAr antagonist MK-801 (dizocilpine) on tasks of attention and working memory in rats using automated touchscreen chambers. Adult male Wistar rats were trained to perform the trial-unique nonmatching to location (TUNL) task of spatial working memory, or the 5-choice serial reaction time task (5CSRTT) of attention. Once trained, rats received injection of vehicle (saline) or low-dose MK-801 (0.06 mg/kg sc) 10 min prior to commencing test sessions. MK-801 significantly impaired working memory, as evidenced by reduced performance accuracy on the TUNL task (p < .0001), compared with vehicle. However, we found no significant effects on attentional processing or perseveration on the 5CSRTT. Additional measures indicated that MK-801 impaired behavioral flexibility in the TUNL task, and decreased response inhibition in both tasks. Using the automated touchscreen system to measure different cognitive functions under the same testing environment, we demonstrate that spatial working memory, response inhibition, and behavioral flexibility are more vulnerable to NMDAr hypofunction than attentional processing. This may have implications for the NMDAr hypofunction hypothesis of schizophrenia. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Effects of beta-hydroxybutyrate administration on MK-801-induced schizophrenia-like behaviour in mice

RATIONALE

Impaired cerebral glucose metabolism is a core pathological feature of schizophrenia. We recently demonstrated that a ketogenic diet, causing a shift from glycolysis to ketosis, normalized schizophrenia-like behaviours in an acute N-methyl-D-aspartate (NMDA) receptor antagonist model of the illness. Ketogenic diet produces the ketone body, β-hydroxybutyrate (BHB), which may serve as an alternative fuel source in its own right without a strict dietary regime.

OBJECTIVE

We hypothesized that chronic administration of BHB replicates the therapeutic effects of ketogenic diet in an acute NMDA receptor hypofunction model of schizophrenia in mice.

METHODS

C57Bl/6 mice were either treated with acute doses of 2 mmol/kg, 10 mmol/kg, or 20 mmol/kg BHB or received daily intraperitoneal injections of 2 mmol/kg BHB or saline for 3 weeks. Behavioural testing assessed the effect of acute challenge with 0.2 mg/kg MK-801 or saline on open field behaviour, social interaction, and prepulse inhibition of startle (PPI).

RESULTS

Acute BHB administration dose-dependently increased BHB plasma levels, whereas the 2 mmol/kg dose increased plasma glucose levels. The highest acute dose of BHB supressed spontaneous locomotor activity, MK-801-induced locomotor hyperactivity and MK-801-induced disruption of PPI. Chronic BHB treatment normalized MK-801-induced hyperlocomotion, reduction of sociability, and disruption of PPI.

CONCLUSION

In conclusion, BHB may present a novel treatment option for patients with schizophrenia by providing an alternative fuel source to normalize impaired glucose metabolism in the brain.

7,8-Dihydroxyflavone Enhances Cue-Conditioned Alcohol Reinstatement in Rats

Alcohol use disorder (AUD) is a detrimental disease that develops through chronic ethanol exposure. Reduced brain-derived neurotrophic factor (BDNF) expression has been associated with AUD and alcohol addiction, however the effects of activation of BDNF signalling in the brain on voluntary alcohol intake reinstatement and relapse are unknown. We therefore trained male and female Sprague Dawley rats in operant chambers to self-administer a 10% ethanol solution. Following baseline acquisition and progressive ratio (PR) analysis, rats were split into drug and vehicle groups during alcohol lever extinction. The animals received two weeks of daily IP injection of either the BDNF receptor, TrkB, agonist, 7,8-dihydroxyflavone (7,8-DHF), or vehicle. During acquisition of alcohol self-administration, males had significantly higher absolute numbers of alcohol-paired lever presses and a higher PR breakpoint. However, after adjusting for body weight, the amount of ethanol was not different between the sexes and the PR breakpoint was higher in females than males. Following extinction, alcohol-primed reinstatement in male rats was not altered by pretreatment with 7,8-DHF when adjusted for body weight. In contrast, in female rats, the weight-adjusted potential amount of ethanol, but not absolute numbers of active lever presses, was significantly enhanced by 7,8-DHF treatment during reinstatement. Analysis of spontaneous locomotor activity in automated photocell cages suggested that the effect of 7,8-DHF was not associated with hyperactivity. These results suggest that stimulation of the TrkB receptor may contribute to reward craving and relapse in AUD, particularly in females.

Sex differences in the effect of maternal immune activation on cognitive and psychosis-like behaviour in Long Evans rats

Maternal immune activation during pregnancy is associated with increased risk of development of schizophrenia in later life. There are sex differences in schizophrenia, particularly in terms of age of onset, course of illness and severity of symptoms. However, there is limited and inconsistent literature on sex differences in the effects of maternal immune activation on behaviour with relevance to schizophrenia. The aim of this study was therefore to investigate sex differences in the effects of maternal immune activation by treating Long Evans rats with poly(I:C) on gestational day 15. We compared adult male and female offspring on spatial working memory in the touchscreen trial-unique nonmatching-to-location task, pairwise discrimination and reversal learning, as well as on prepulse inhibition and psychotropic drug-induced locomotor hyperactivity. Male, but not female poly(I:C) offspring displayed a deficit in spatial working memory, particularly at the longer delay. Neither pairwise discrimination nor reversal learning showed an effect of poly(I:C), but female controls outperformed male controls in the reversal learning task. Significant reduction of prepulse inhibition and enhancement of acute methamphetamine-induced locomotor hyperactivity was found similarly in male and female poly(I:C) offspring. These results show that maternal immune activation induces a range of behavioural effects in the offspring, with sex specificity in the effects of maternal immune activation on some aspects of cognition, but not psychosis-like behaviour.

Pharmacological Mechanisms Involved in Sensory Gating Disruption Induced by (±)-3,4-Methylene- Dioxymethamphetamine (MDMA): Relevance to Schizophrenia

Sensory gating deficits have been demonstrated in schizophrenia, but the mechanisms involved remain unclear. In the present study, we used disruption of paired-pulse gating of evoked potentials in rats by the administration of (±)-3,4-methylene-dioxymethamphetamine (MDMA) to study serotonergic and dopaminergic mechanisms involved in auditory sensory gating deficits. Male Sprague-Dawley rats were instrumented with cortical surface electrodes to record evoked potential changes in response to pairs of 85dB tones (S1 and S2), 500msec apart. Administration of MDMA eliminated the normal reduction in the amplitude of S2 compared to S1, representing disruption of auditory sensory gating. Pretreatment of the animals with the dopamine D1 receptor antagonist, SCH23390, the dopamine D2 receptor antagonist, haloperidol, the serotonin (5-HT)1A receptor antagonist, WAY100635, or the 5-HT2A receptor antagonist, ketanserin, all blocked the effect of MDMA, although the drugs differentially affected the individual S1 and S2 amplitudes. These data show involvement of both dopaminergic and serotonergic mechanisms in disruption of auditory sensory gating by MDMA. These and previous results suggest that MDMA targets serotonergic pathways, involving both 5-HT1A and 5-HT2A receptors, leading to dopaminergic activation, involving both D1 and D2 receptors, and ultimately sensory gating deficits. It is speculated that similar interactive mechanisms are affected in schizophrenia.

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.

Ketogenic diet and olanzapine treatment alone and in combination reduce a pharmacologically-induced prepulse inhibition deficit in female mice

We used the acute NMDA receptor hypoactivity model of schizophrenia in mice to compare the efficacy of a long-term ketogenic diet and a commonly used antipsychotic, olanzapine, and to explore the interaction between these treatments. We found that a ketogenic diet in female mice was as effective as olanzapine to diminish MK-801-induced disruption of prepulse inhibition (PPI). Furthermore, combination of the diet with olanzapine treatment resulted in a similar effect compared to either treatment alone. These results suggest that ketogenic diet can be used effectively together with antipsychotics drugs over an extended period.

GAL3 receptor knockout mice exhibit an alcohol-preferring phenotype

Galanin is a neuropeptide which mediates its effects via three G-protein coupled receptors (GAL_1-3 ). Administration of a GAL₃ antagonist reduces alcohol self-administration in animal models while allelic variation in the GAL₃ gene has been associated with an increased risk of alcohol use disorders in diverse human populations. Based on the association of GAL₃ with alcoholism, we sought to characterize drug-seeking behavior in GAL₃ -deficient mice for the first time. In the two-bottle free choice paradigm, GAL₃ -KO mice consistently showed a significantly increased preference for ethanol over water when compared to wildtype littermates. Furthermore, male GAL₃ -KO mice displayed significantly increased responding for ethanol under operant conditions. These differences in alcohol seeking behavior in GAL₃ -KO mice did not result from altered ethanol metabolism. In contrast to ethanol, GAL₃ -KO mice exhibited similar preference for saccharin and sucrose over water, and a similar preference for a high fat diet over a low fat diet as wildtype littermates. No differences in cognitive and locomotor behaviors were observed in GAL₃ -KO mice to account for increased alcohol seeking behavior. Overall, these findings suggest genetic ablation of GAL₃ in mice increases alcohol consumption.

Ketogenic diet prevents impaired prepulse inhibition of startle in an acute NMDA receptor hypofunction model of schizophrenia

Recent transcriptomic, proteomic and metabolomics studies have highlighted an abnormal cerebral glucose and energy metabolism as one of the potential pathophysiological mechanisms of schizophrenia. This raises the possibility that a metabolically-based intervention might have therapeutic value in the management of schizophrenia, a notion supported by our recent results that a low carbohydrate/high-fat therapeutic ketogenic diet (KD) prevented a variety of behavioural abnormalities induced by pharmacological inhibition of NMDA glutamate receptors. Here we asked if the beneficial effects of KD can be generalised to impaired prepulse inhibition of startle (PPI), a translationally validated endophenotype of schizophrenia, in a pharmacological model in mice. Furthermore, we addressed the issue of whether the effect of KD is linked to the calorie-restricted state typical of the initial phase of KD. We fed male C57BL/6 mice a KD for 7 weeks and tested PPI at 3 and 7 weeks, in the presence and absence of a significant digestible energy deficit, respectively. We used an NMDA receptor hypo-function model of schizophrenia induced by acute injection of dizocilpine (MK-801). We found that KD effectively prevented MK-801-induced PPI impairments at both 3 and 7 weeks, irrespective of the presence or absence of digestible energy deficit. Furthermore, there was a lack of correlation between PPI and body weight changes. These results support the efficacy of the therapeutic KD in a translational model of schizophrenia and furthermore provide evidence against the role of calorie restriction in its mechanism of action.

Short-Term Environmental Stimulation Spatiotemporally Modulates Specific Serotonin Receptor Gene Expression and Behavioral Pharmacology in a Sexually Dimorphic Manner in Huntington's Disease Transgenic Mice

Huntington’s disease (HD) is a neurodegenerative disorder caused by a tandem repeat mutation encoding an expanded polyglutamine tract in the huntingtin protein, which leads to cognitive, psychiatric and motor dysfunction. Exposure to environmental enrichment (EE), which enhances levels of cognitive stimulation and physical activity, has therapeutic effects on cognitive, affective and motor function of transgenic HD mice. The present study investigated gene expression changes and behavioral pharmacology in male and female R6/1 transgenic HD mice at an early time-point in HD progression associated with onset of cognitive and affective abnormalities, following EE and exercise (wheel running) interventions. We have demonstrated changes in expression levels of the serotonin (5-HT) receptor Htr1a, Htr1b, Htr2a and Htr2c genes (encoding the 5-HT_1A, 5-HT_1B, 5-HT_2A and 5-HT_2C receptors, respectively) in HD brains at 8 weeks of age, using quantitative real-time PCR. In contrast, expression of the serotonin transporter (SerT, also known as 5-HTT or Slc6a4) was not altered in these brains. Furthermore, we identified region-specific, sex-specific and environmentally regulated (comparing EE, exercise and standard housing conditions) impacts on gene expression of particular 5-HT receptors, as well as SerT. For example, SerT gene expression was upregulated by exercise (wheel running from 6 to 8 weeks of age) in the hippocampus. Interestingly, when EE was introduced from 6 to 8 weeks of age, Htr2a gene expression was upregulated in the cortex, striatum and hippocampus of male mice. EE also rescued the functional activity of 5-HT₂ receptors as observed in the head-twitch test, reflecting sexually dimorphic effects of environmental stimulation. These findings demonstrate that disruption of the serotonergic system occurs early in HD pathogenesis and, together with previous findings, show that the timing and duration of environmental interventions are critical in terms of their ability to modify gene expression. This study is the first to show that EE is able to selectively enhance both gene expression of a neurotransmitter receptor and the functional consequences on behavioral pharmacology, and links this molecular modulation to the therapeutic effects of environmental stimulation in this neurodegenerative disease.

Brain-Derived Neurotrophic Factor (BDNF): Novel Insights into Regulation and Genetic Variation

Since its discovery, brain-derived neurotrophic factor (BDNF) has spawned a literature that now spans 35 years of research. While all neurotrophins share considerable overlap in sequence homology and their processing, BDNF has become the most widely studied neurotrophin because of its broad roles in brain homeostasis, health, and disease. Although research on BDNF has produced thousands of articles, there remain numerous long-standing questions on aspects of BDNF molecular biology and signaling. Here we provide a comprehensive review, including both a historical narrative and a forward-looking perspective on advances in the actions of BDNF within the brain. We specifically review BDNF’s gene structure, peptide composition (including domains, posttranslational modifications and putative motif sites), mechanisms of transport, signaling pathway recruitment, and other recent developments including the functional effects of genetic variation and the discovery of a new BDNF prodomain ligand. This body of knowledge illustrates a highly conserved and complex role for BDNF within the brain, that promotes the idea that the neurotrophin biology of BDNF is diverse and that any disease involvement is likely to be equally multifarious.

Sex-Dependent Effects of Environmental Enrichment on Spatial Memory and Brain-Derived Neurotrophic Factor (BDNF) Signaling in a Developmental "Two-Hit" Mouse Model Combining BDNF Haploinsufficiency and Chronic Glucocorticoid Stimulation

Neurodevelopmental disorders are thought to be caused by a combination of adverse genetic and environmental insults. The "two-hit" hypothesis suggests that an early first "hit" primes the developing brain to be vulnerable to a second "hit" during adolescence which triggers behavioral dysfunction. We have previously modeled this scenario in mice and found that the combined effect of a genetic hapolinsuffuciency in the brain-derived neurotrophic factor (BDNF) gene (1st hit) and chronic corticosterone (CORT) treatment during adolescence (2nd hit), caused spatial memory impairments in adulthood. Environmental enrichment (EE) protocols are designed to stimulate experience-dependent plasticity and have shown therapeutic actions. This study investigated whether EE can reverse these spatial memory impairments. Wild-type (WT) and BDNF heterozygous (HET) mice were treated with corticosterone (CORT) in their drinking water (50 mg/L) from weeks 6 to 8 and exposed to EE from 7 to 9 weeks. Enriched housing included open top cages with additional toys, tunnels, housing, and platforms. Y-maze novel preference testing, to assess short-term spatial memory, was performed at 10 weeks of age. At week 16 dorsal hippocampus tissue was obtained for Western blot analysis of expression levels of BDNF, the BDNF receptor TrkB, and NMDA receptor subunits, GluNR1, 2A and 2B. As in our previous studies, spatial memory was impaired in our two-hit (BDNF HET + CORT) mice. Simultaneous EE prevented these impairments. However, EE appeared to worsen spatial memory performance in WT mice, particularly those exposed to CORT. While BDNF levels were lower in BDNF HET mice as expected, there were no further effects of CORT or EE in males but a close to significant female CORT × EE × genotype interaction which qualitatively corresponded with Y-maze performance. However, EE caused both sex- and genotype-specific effects on phosphorylated TrkB residues and GluNR expression within the dorsal hippocampus, with GluNR2B levels in males changing in parallel with spatial memory performance. In conclusion, beneficial effects of EE on spatial memory emerge only following two developmental disruptions. The mechanisms by which EE exerts its effects are likely via regulation of multiple activity-dependent pathways, including TrkB and NMDA receptor signaling.

Reelin Haploinsufficiency and Late-Adolescent Corticosterone Treatment Induce Long-Lasting and Female-Specific Molecular Changes in the Dorsal Hippocampus

Reelin depletion and stress seem to affect similar pathways including GABAergic and glutamatergic signaling and both are implicated in psychiatric disorders in late adolescence/early adulthood. The interaction between reelin depletion and stress, however, remains unclear. To investigate this, male and female heterozygous reelin mice (HRM) and wildtype (WT) controls were treated with the stress hormone, corticosterone (CORT), during late adolescence to simulate chronic stress. Glucocorticoid receptors (GR), N-methyl-d-aspartate receptor (NMDAr) subunits, glutamic acid decarboxylase (GAD₆₇) and parvalbumin (PV) were measured in the hippocampus and the prefrontal cortex (PFC) in adulthood. While no changes were seen in male mice, female HRM showed a significant reduction in GR expression in the dorsal hippocampus. In addition, CORT reduced GR levels as well as GluN2B and GluN2C subunits of NMDAr in the dorsal hippocampus in female mice only. CORT furthermore reduced GluN1 levels in the PFC of female mice. The combined effect of HRM and CORT treatment appeared to be additive in terms of GR expression in the dorsal hippocampus. Female-specific CORT-induced changes were associated with overall higher circulating CORT levels in female compared to male mice. This study shows differential effects of reelin depletion and CORT treatment on GR and NMDAr protein expression in male and female mice, suggesting that females are more susceptible to reelin haploinsufficiency as well as late-adolescent stress. These findings shed more light on female-specific vulnerability to stress and have implications for stress-associated mental illnesses with a female bias including anxiety and major depression.

BDNF haploinsufficiency exerts a transient and regionally different influence upon oligodendroglial lineage cells during postnatal development

Brain-Derived Neurotrophic Factor (BDNF) plays important roles in promoting myelination in the developing central nervous system (CNS), however the influence it exerts on oligodendrocyte development in vivo remains unclear. As BDNF knockout mice die in the perinatal period, we undertook a systematic developmental analysis of oligodendroglial lineage cells within multiple CNS regions of BDNF heterozygous (HET) mice. Our data identify that BDNF heterozygosity results in transient reductions in oligodendroglial lineage cell density and progression that are largely restricted to the optic nerve, whereas the corpus callosum, cerebral cortex, basal forebrain and spinal cord white matter tracts are unaffected. In the first two postnatal weeks, BDNF HET mice exhibit reductions in the density of oligodendroglial lineage cells, oligodendrocyte precursor cells (OPCs) and postmitotic oligodendrocytes selectively in the optic nerve, but not in the brain or spinal cord white matter tracts. However, this normalizes later in development. The overall proportion of OPCs and mature oligodendrocytes remains unchanged from P9 to P30 in all CNS regions. This study identifies that BDNF exerts transient effects on oligodendroglial lineage cells selectively in the optic nerve during postnatal development. Taken together, this provides compelling evidence that BDNF haploinsufficiency exerts modest effects upon oligodendroglial cell density and lineage progression in vivo, suggesting its major role is restricted to promoting oligodendrocyte myelination.

The effect of estrogenic compounds on psychosis-like behaviour in female rats

17β-estradiol treatment has shown benefit against schizophrenia symptoms, however long-term use may be associated with negative side-effects. Selective estrogen receptor modulators, such as raloxifene and tamoxifen, have been proposed as suitable alternatives to 17β-estradiol. An isomer of 17β-estradiol, 17α-estradiol, is considered less carcinogenic, and non-feminising in males, however little is known about its potential as a treatment for schizophrenia. Moreover, the mechanism underlying the therapeutic action of estrogens remains unclear. We aimed to investigate the ability of these estrogenic compounds to attenuate psychosis-like behaviour in rats. We used two acute pharmacologically-induced assays of psychosis-like behaviour: psychotomimetic drug-induced hyperlocomotion and disruption of prepulse inhibition (PPI). Female Long Evans rats were either intact, ovariectomised (OVX), or OVX and chronically treated with 17β-estradiol, 17α-estradiol, raloxifene or tamoxifen. Only 17β-estradiol treatment attenuated locomotor hyperactivity induced by the indirect dopamine receptor agonist, methamphetamine. 17β-estradiol- and tamoxifen-treated rats showed attenuated methamphetamine- and apomorphine (dopamine D1/D2 receptor agonist)-induced disruption of PPI. Raloxifene-treated rats showed attenuated apomorphine-induced PPI disruption only. Baseline PPI was significantly reduced following OVX, and this deficit was reversed by all estrogenic compounds. Further, PPI in OVX rats was increased following administration of apomorphine. This study confirms a protective effect of 17β-estradiol in two established animal models of psychosis, while tamoxifen showed beneficial effects against PPI disruption. In contrast, 17α-estradiol and raloxifene showed little effect on dopamine receptor-mediated psychosis-like behaviours. This study highlights the utility of some estrogenic compounds to attenuate psychosis-like behaviour in rats, supporting the notion that estrogens have therapeutic potential for psychotic disorders.

Selective enhancement of NMDA receptor-mediated locomotor hyperactivity by male sex hormones in mice

RATIONALE

Altered glutamate NMDA receptor function is implicated in schizophrenia, and gender differences have been demonstrated in this illness.

OBJECTIVES

This study aimed to investigate the interaction of gonadal hormones with NMDA receptor-mediated locomotor hyperactivity and PPI disruption in mice.

RESULTS

The effect of 0.25 mg/kg of MK-801 on locomotor activity was greater in male mice than in female mice. Gonadectomy (by surgical castration) significantly reduced MK-801-induced hyperlocomotion in male mice, but no effect of gonadectomy was seen in female mice or on amphetamine-induced locomotor hyperactivity. The effect of MK-801 on prepulse inhibition of startle (PPI) was similar in intact and castrated male mice and in ovariectomized (OVX) female mice. In contrast, there was no effect of MK-801 on PPI in intact female mice. Forebrain NMDA receptor density, as measured with [³H]MK-801 autoradiography, was significantly higher in male than in female mice but was not significantly altered by either castration or OVX.

CONCLUSIONS

These results suggest that male sex hormones enhance the effect of NMDA receptor blockade on psychosis-like behaviour. This interaction was not seen in female mice and was independent of NMDA receptor density in the forebrain. Male sex hormones may be involved in psychosis by an interaction with NMDA receptor hypofunction.

Spatial working memory in the touchscreen operant platform is disrupted in female rats by ovariectomy but not estrous cycle

Learning and memory deficits have been described in rats and mice after ovariectomy (OVX) and across the estrous cycle. Preclinical researchers therefore often avoid using female animals and, consequently, a large male bias exists in the preclinical cognitive literature. In the present study we examined the role of sex hormones in the touchscreen operant platform using the spatial working memory trial unique nonmatching-to-location (TUNL) task. Twenty-nine Long Evans rats were trained to acquire the TUNL task including three incremental spatial separations (S0, S1, S2). Following 20 consecutive days of training, subjects in experiment 1 (n=15) remained intact and immediately progressed to TUNL testing, while subjects in experiment 2 were OVX (n=6) or sham-operated (n=8) prior to testing. Subjects were tested on 4 spatial separations (S0-3) with a 1s or 6s delay between the sample and nonmatching stimuli. The estrous cycle of intact rats was monitored during the 4weeks of testing. The estrous cycle phase did not significantly affect performance. In contrast, compared to intact rats, OVX impaired performance at larger spatial separations (S2-3) during the 1s delay condition. Further, during the 6s delay, OVX impaired S2 performance, however not S3. Our results suggest a probable shift in cognitive strategy following OVX, when tested with a large and novel spatial separation. Our findings suggest that ovarian hormone deprivation following OVX, but not estrous cycle, impairs spatial working memory as measured by the TUNL task. This research is relevant for future studies utilising the touchscreen TUNL task and for cognitive testing of female rats.