Conditioned morphine tolerance promotes neurogenesis, dendritic remodelling and pro-plasticity molecules in the adult rat hippocampus.

Structural neuroplasticity of the hippocampus in the form of neurogenesis and dendritic remodelling underlying morphine tolerance is still less known. Therefore, in this study, we aimed to assess whether unconditioned- and conditioned-morphine tolerance can trigger structural neuroplasticity in the dorsal and ventral parts of the adult male rat hippocampus. Evaluation of the levels of neurogenesis markers (Ki67 and DCX) by immunohistochemistry shows that conditioned morphine tolerance is sufficient to increase the baseline topographic level of hippocampal neurogenesis in adult rats. Dendritic spine visualization by Golgi staining shows that the behavioural testing paradigms themselves are sufficient to trigger the hippocampus subregion-specific changes in the dendritic remodelling along the apical dendrites of hippocampal CA1 pyramidal neurons and dentate granule cells in adult rats. Quantitative reverse transcription polymerase chain reaction of Bdnf, Trkb, Rac-1 and RhoA mRNA levels as pro-plasticity molecules, shows that the conditioned morphine tolerance is effective in changing Bdnf and RhoA mRNA levels in the ventral hippocampus of adult rats. In summary, we demonstrate that the acquisition of morphine tolerance promotes adult neurogenesis, dendritic remodelling and pro-plasticity molecules such as Bdnf/Trkb in the rat hippocampus. Indeed, the structural neuroplasticity of the hippocampus may underlie the newly formed aberrant memory and could provide the initial basis for understanding the neurobiological mechanisms of morphine-tolerance plasticity in the hippocampus.

Individuals being high in their sensitivity to the environment: Are sensitive period changes in play?

All individuals on planet earth are sensitive to the environment, but some more than others. These individual differences in sensitivity to environments are seen across many animal species including humans, and can influence personalities as well as vulnerability and resilience to mental disorders. Yet, little is known about the underlying brain mechanisms. Key genes that contribute to individual differences in environmental sensitivity are the serotonin transporter, dopamine D4 receptor and brain-derived neurotrophic factor genes. By synthesizing neurodevelopmental findings of these genetic factors, and discussing them through the lens of mechanisms related to sensitive periods, which are phases of heightened neuronal plasticity during which a certain network is being finetuned by experiences, we propose that these genetic factors delay but extend postnatal sensitive periods. This may explain why sensitive individuals show behavioral features that are characteristic of a young brain state at the level of sensory information processing, such as reduced filtering or blockade of irrelevant information, resulting in a sensory processing system that 'keeps all options open'.

Individual differences in GHB consumption in a new voluntary GHB self-administration model in outbred rats.

BACKGROUND AND PURPOSE: The use of the recreational drug gamma-hydroxybutyric acid (GHB) has increased over the past decade, concomitantly leading to a higher incidence of GHB use disorder. Evidence-based treatment interventions are hardly available and cognitive effects of long-term GHB use remain elusive. In order to study the development of GUD and the causal effects of chronic GHB consumption, a GHB self-administration model is required. EXPERIMENTAL APPROACH: Long Evans rats had access to GHB in their home cage according to a two-bottle choice procedure for 3 months. Intoxication and withdrawal symptoms were assessed using an automated sensor-based setup for longitudinal behavioral monitoring. Rats were trained in an operant environment according to a fixed ratio (FR) 1, 2, and 4 schedule of reinforcement. Addiction-like behaviors were assessed through progressive ratio-, non-reinforced-, and quinine-adulterated operant tests. In addition, the novel object recognition test and elevated plus maze test were performed before and after GHB self-administration to assess memory performance and anxiety-like behavior, respectively. KEY RESULTS: All rats consumed pharmacologically relevant levels of GHB in their home cage, and their intake remained stable over a period of 3 months. No clear withdrawal symptoms were observed following abstinence. Responding under operant conditions was characterized by strong inter-individual differences, where only a subset of rats showed high motivation for GHB, habitual GHB-seeking, and/or continued responding for GHB despite an aversive taste. Male rats showed a reduction in long-term memory performance 3 months after home-cage GHB self-administration. Anxiety-like behavior was not affected by GHB self-administration. CONCLUSION AND IMPLICATIONS: The GHB self-administration model was able to reflect individual susceptibility for addiction-like behavior. The reduction in long-term memory performance upon GHB self-administration calls for further research into the cognitive effects of chronic GHB use in humans.

Hippocampal Trauma Memory Processing Conveying Susceptibility to Traumatic Stress.

While the majority of the population is ever exposed to a traumatic event during their lifetime, only a fraction develops posttraumatic stress disorder (PTSD). Disrupted trauma memory processing has been proposed as a core factor underlying PTSD symptomatology. We used transgenic Targeted-Recombination-in-Active-Populations (TRAP) mice to investigate potential alterations in trauma-related hippocampal memory engrams associated with the development of PTSD-like symptomatology. Mice were exposed to a stress-enhanced fear learning paradigm, in which prior exposure to a stressor affects the learning of a subsequent fearful event (contextual fear conditioning using foot shocks), during which neuronal activity was labeled. One week later, mice were behaviorally phenotyped to identify mice resilient and susceptible to developing PTSD-like symptomatology. Three weeks post-learning, mice were re-exposed to the conditioning context to induce remote fear memory recall, and associated hippocampal neuronal activity was assessed. While no differences in the size of the hippocampal neuronal ensemble activated during fear learning were observed between groups, susceptible mice displayed a smaller ensemble activated upon remote fear memory recall in the ventral CA1, higher regional hippocampal parvalbuminneuronal density and a relatively lower activity of parvalbumininterneurons upon recall. Investigation of potential epigenetic regulators of the engram revealed rather generic (rather than engram-specific) differences between groups, with susceptible mice displaying lower hippocampal histone deacetylase 2 expression, and higher methylation and hydroxymethylation levels. These finding implicate variation in epigenetic regulation within the hippocampus, as well as reduced regional hippocampal activity during remote fear memory recall in interindividual differences in susceptibility to traumatic stress.

Offspring's own serotonin transporter genotype, independently from the maternal one, increases anxiety- and depression-like behavior and alters neuroplasticity markers in rats.

INTRODUCTION: Developmental changes due to early life variations in the serotonin system affect stress-related behavior and neuroplasticity in adulthood. These outcomes can be caused both by offspring's own and maternal serotonergic genotype. We aimed to dissociate the contribution of the own genotype from the influences of mother genotype. METHODS: Sixty-six male homozygous (5-HTT-/-) and heterozygous (5-HTT+/-) serotonin transporter knockout and wild-type rats from constant 5-HTT genotype mothers crossed with varying 5-HTT genotype fathers were subjected to tests assessing anxiety- and depression-like behaviors. Additionally, we measured plasma corticosterone levels and mRNA levels of BDNF, GABA system and HPA-axis components in the prelimbic and infralimbic cortex. Finally, we assessed the effect of paternal 5-HTT genotype on these measurements in 5-HTT+/- offspring receiving their knockout allele from their mother or father. RESULTS: 5-HTT-/- offspring exhibited increased anxiety- and depression-like behavior in the elevated plus maze and sucrose preference test. Furthermore, Bdnf isoform VI expression was reduced in the prelimbic cortex. Bdnf isoform IV and GABA related gene expression was also altered but did not survive false discovery rate (FDR) correction. Finally, 5-HTT+/- offspring from 5-HTT-/- fathers displayed higher levels of anxiety- and depression-like behavior and changes in GABA, BDNF and HPA-axis related gene expression not surviving FDR correction. LIMITATIONS: Only male offspring was tested. CONCLUSIONS: Offspring's own 5-HTT genotype influences stress-related behaviors and Bdnf isoform VI expression, independently of maternal 5-HTT genotype. Paternal 5-HTT genotype separately influenced these outcomes. These findings advance our understanding of the 5-HTT genotype dependent susceptibility to stress-related disorders.

Chronic exposure to imipramine induces a switch from depression-like to mania-like behavior in female serotonin transporter knockout rats: Role of BDNF signaling in the infralimbic cortex.

BACKGROUND: Bipolar disorder (BD) is a highly burdensome psychiatric disorder characterized by alternating states of mania and depression. A major challenge in the clinic is the switch from depression to mania, which is often observed in female BD patients during antidepressant treatment such as imipramine. However, the underlying neural basis is unclear. METHODS: To investigate the potential neuronal pathways, serotonin transporter knockout (SERT KO) rats, an experimental model of female BD patients, were subjected to a battery of behavioral tests under chronic treatment of the antidepressant imipramine. In addition, the expression of brain-derived neurotrophic factor (BDNF) and its downstream signaling was examined in the prefrontal cortex. RESULTS: Chronic exposure to imipramine reduced anxiety and sociability and problem-solving capacity, and increased thigmotaxis and day/night activity in all animals, but specifically in female SERT KO rats, compared to female wild-type (WT) rats. Further, we found an activation of BDNF-TrkB-Akt pathway signaling in the infralimbic, but not prelimbic, cortex after chronic imipramine treatment in SERT KO, but not WT, rats. LIMITATIONS: Repeated testing behaviors could potentially affect the results. Additionally, the imipramine induced changes in behavior and in the BDNF system were measured in separate animals. CONCLUSIONS: Our study indicates that female SERT KO rats, which mirror the female BD patients with the 5-HTTLPR s-allele, are at higher risk of a switch to mania-like behaviors under imipramine treatment. Activation of the BDNF-TrkB-Akt pathway in the infralimbic cortex might contribute to this phenotype, but causal evidence remains to be provided.

The Natural Protoalkaloid Methyl-2-Amino-3-Methoxybenzoate (MAM) Alleviates Positive as well as Cognitive Symptoms in Rat and Mouse Schizophrenia Models.

The development of new antipsychotics with pro-cognitive properties and less side effects represents a priority in schizophrenia drug research. In this study, we present for the first time a preclinical exploration of the effects of the promising natural atypical antipsychotic Methyl-2-Amino-3- Methoxybenzoate (MAM), a brain-penetrable protoalkaloid from the seed of the plant Nigella damascena. Using animal models related to hyperdopaminergic activity, namely the pharmacogenetic apomorphine (D2/D1 receptor agonist)-susceptible (APO-SUS) rat model and pharmacologically induced mouse and rat models of schizophrenia, we found that MAM reduced gnawing stereotypy and climbing behaviours induced by dopaminergic agents. This predicts antipsychotic activity. In line, MAM antagonized apomorphine-induced c-Fos and NPAS4 mRNA levels in post-mortem brain nucleus accumbens and dorsolateral striatum of APO-SUS rats. Furthermore, phencyclidine (PCP, an NMDA receptor antagonist) and 2,5-Dimethoxy-4-iodoamphetamine (DOI, a 5HT2A/2C receptor agonist) induced prepulse inhibition deficits, reflecting the positive symptoms of schizophrenia, which were rescued by treatment with MAM and atypical antipsychotics alike. Post-mortem brain immunostaining revealed that MAM blocked the strong activation of both PCP- and DOI-induced c-Fos immunoreactivity in a number of cortical areas. Finally, during a 28-day subchronic treatment regime, MAM did not induce weight gain, hyperglycemia, hyperlipidemia or hepato- and nephrotoxic effects, side effects known to be induced by atypical antipsychotics. MAM also did not show any cataleptic effects. In conclusion, its brain penetrability, the apparent absence of preclinical side effects, and its ability to antagonize positive and cognitive symptoms associated with schizophrenia make MAM an exciting new antipsychotic drug that deserves clinical testing.

Towards improved screening of toxins for Parkinson's risk.

Parkinson's disease (PD) is a chronic, progressive and disabling neurodegenerative disorder. The prevalence of PD has risen considerably over the past decades. A growing body of evidence suggest that exposure to environmental toxins, including pesticides, solvents and heavy metals (collectively called toxins), is at least in part responsible for this rapid growth. It is worrying that the current screening procedures being applied internationally to test for possible neurotoxicity of specific compounds offer inadequate insights into the risk of developing PD in humans. Improved screening procedures are therefore urgently needed. Our review first substantiates current evidence on the relation between exposure to environmental toxins and the risk of developing PD. We subsequently propose to replace the current standard toxin screening by a well-controlled multi-tier toxin screening involving the following steps: in silico studies (tier 1) followed by in vitro tests (tier 2), aiming to prioritize agents with human relevant routes of exposure. More in depth studies can be undertaken in tier 3, with whole-organism (in)vertebrate models. Tier 4 has a dedicated focus on cell loss in the substantia nigra and on the presumed mechanisms of neurotoxicity in rodent models, which are required to confirm or refute the possible neurotoxicity of any individual compound. This improved screening procedure should not only evaluate new pesticides that seek access to the market, but also critically assess all pesticides that are being used today, acknowledging that none of these has ever been proven to be safe from a perspective of PD. Importantly, the improved screening procedures should not just assess the neurotoxic risk of isolated compounds, but should also specifically look at the cumulative risk conveyed by exposure to commonly used combinations of pesticides (cocktails). The worldwide implementation of such an improved screening procedure, would be an essential step for policy makers and governments to recognize PD-related environmental risk factors.

Cognitive Performance during the Development of Diabetes in the Zucker Diabetic Fatty Rat.

Increased insulin levels may support the development of neural circuits involved in cognition, while chronic mild inflammation may also result in cognitive impairment. This study aimed to gain more insight into whether cognition is already impacted during adolescence in a genetic rat model for obesity and type 2 diabetes. Visual discrimination learning throughout adolescence and the level of motivation during early adulthood were investigated in Zucker Diabetic Fatty (ZDF) obese and ZDF lean rats using operant touchscreens. Blood glucose, insulin, and lipids were longitudinally analyzed. Histological analyses were performed in the liver, white adipose tissues, and the prefrontal cortex. Prior to the experiments with the genetic ZDF research model, all experimental assays were performed in two groups of outbred Long Evans rats to investigate the effect of different feeding circumstances. Adolescent ZDF obese rats outperformed ZDF lean rats on visual discrimination performance. During the longitudinal cognitive testing period, insulin levels sharply increased over weeks in ZDF obese rats and were significantly enhanced from 6 weeks of age onwards. Early signs of liver steatosis and enlarged adipocytes in white adipose tissue were observed in early adult ZDF obese rats. Histological analyses in early adulthood showed no group differences in the number of prefrontal cortex neurons and microglia, nor PSD95 and SIRT1 mRNA expression levels. Together, our data show that adolescent ZDF obese rats even display enhanced cognition despite their early diabetic profile.

Chronic Lithium Treatment Alters NMDA and AMPA Receptor Synaptic Availability and Dendritic Spine Organization in the Rat Hippocampus.

BACKGROUND: The mechanisms underlying the action of lithium (LiCl) in bipolar disorder(BD) are still far from being completely understood. Previous evidence has revealed that BD is characterized by glutamate hyperexcitability, suggesting that LiCl may act, at least partially, by toning down glutamatergic signaling abnormalities. OBJECTIVE: In this study, taking advantage of western blot and confocal microscopy, we used a combination of integrative molecular and morphological approaches in rats exposed to repeated administration of LiCl at a therapeutic dose (between 0.6 and 1.2 mmol/l) and sacrificed at two different time points, i.e., 24 hours and 7 days after the last exposure. RESULTS: We report that repeated LiCl treatment activates multiple, parallel, but also converging forms of compensatory neuroplasticity related to glutamatergic signaling. More specifically, LiCl promoted a wave of neuroplasticity in the hippocampus, involving the synaptic recruitment of GluN2A-containing NMDA receptors, GluA1-containing AMPA receptors, and the neurotrophin BDNF that are indicative of a more plastic spine. The latter is evidenced by morphological analyses showing changes in dendritic spine morphology, such as increased length and head diameter of such spines. These changes may counteract the potentially negative extra-synaptic movements of GluN2B-containing NMDA receptors as well as the increase in the formation of GluA2-lacking Ca2+-permeable AMPA receptors. CONCLUSION: Our findings highlight a previously unknown cohesive picture of the glutamatergic implications of LiCl action that persist long after the end of its administration, revealing for the first time a profound and persistent reorganization of the glutamatergic postsynaptic density receptor composition and structure.

Bridging the translational gap: what can synaptopathies tell us about autism?

Multiple molecular pathways and cellular processes have been implicated in the neurobiology of autism and other neurodevelopmental conditions. There is a current focus on synaptic gene conditions, or synaptopathies, which refer to clinical conditions associated with rare genetic variants disrupting genes involved in synaptic biology. Synaptopathies are commonly associated with autism and developmental delay and may be associated with a range of other neuropsychiatric outcomes. Altered synaptic biology is suggested by both preclinical and clinical studies in autism based on evidence of differences in early brain structural development and altered glutamatergic and GABAergic neurotransmission potentially perturbing excitatory and inhibitory balance. This review focusses on the NRXN-NLGN-SHANK pathway, which is implicated in the synaptic assembly, trans-synaptic signalling, and synaptic functioning. We provide an overview of the insights from preclinical molecular studies of the pathway. Concentrating on NRXN1 deletion and SHANK3 mutations, we discuss emerging understanding of cellular processes and electrophysiology from induced pluripotent stem cells (iPSC) models derived from individuals with synaptopathies, neuroimaging and behavioural findings in animal models of Nrxn1 and Shank3 synaptic gene conditions, and key findings regarding autism features, brain and behavioural phenotypes from human clinical studies of synaptopathies. The identification of molecular-based biomarkers from preclinical models aims to advance the development of targeted therapeutic treatments. However, it remains challenging to translate preclinical animal models and iPSC studies to interpret human brain development and autism features. We discuss the existing challenges in preclinical and clinical synaptopathy research, and potential solutions to align methodologies across preclinical and clinical research. Bridging the translational gap between preclinical and clinical studies will be necessary to understand biological mechanisms, to identify targeted therapies, and ultimately to progress towards personalised approaches for complex neurodevelopmental conditions such as autism.

Altered responsiveness to glutamatergic modulation by MK-801 and to repeated stress of immune challenge in female dopamine transporter knockout rats.

Chronic stress is a key factor in psychiatric and neurological disorders often worsening disease symptoms. In this study, a unique animal model, the dopamine transporter knockout (DAT-KO) rat exhibiting behavioral signs resembling those occurring in mania, schizophrenia, attention deficit hyperactivity disorder, and obsessive-compulsive disorder was used. We have tested the hypothesis that the hyperdopaminergic state in DAT-KO rats (i) modulates behavioral response to the NMDA antagonist MK-801 (dizocilpine) and (ii) leads to abnormal endocrine and immune activation under subchronic stress induced by an immune challenge. Glutamatergic modulation with MK-801 induced a different behavioral pattern. While the WT rats responded to MK-801 injection with a robust rise in their locomotor activity, the hyperactive DAT-KO rats exhibited reduced locomotion. Signs of chronic stress including increased basal corticosterone and aldosterone but blunted anxiety were demonstrated in rats lacking the DAT. Repeated injections of increasing doses of lipopolysaccharide (LPS, 5 days) did not modify plasma prolactin concentrations which were however significantly lower in DAT-KO than in WT rats. Concentrations of plasma high mobility group box 1 (HMGB1) protein were significantly higher in LPS-treated DAT-KO than in WT rats. The gene expression of interleukin-6 in the anterior pituitary increased under the stress induced by the immune challenge in the WT but not the DAT-KO rats. The most evident differences between the genotypes were revealed in the spleen. The splenic gene expression of interleukin-1ß, interleukin-6, and HMGB1 was lower and that of ferritin was higher in DAT-KO compared to WT rats. Obtained results emphasize the functional interaction of the endocrine and immune systems with monoamine and glutamatergic neurotransmission in the mechanisms leading to behavioral alterations and psychiatric disorders associated with dopamine dysfunction.

An Overview of the Putative Structural and Functional Properties of the GHBh1 Receptor through a Bioinformatics Approach.

The neurotransmitter γ-hydroxybutyric acid (GHB) is suggested to be involved in neuronal energy homeostasis processes, but the substance is also used as a recreational drug and as a prescription medication for narcolepsy. GHB has several high-affinity targets in the brain, commonly generalized as the GHB receptor. However, little is known about the structural and functional properties of GHB receptor subtypes. This opinion article discusses the literature on the putative structural and functional properties of the GHBh1 receptor subtype. GHBh1 contains 11 transmembrane helices and at least one intracellular intrinsically disordered region (IDR). Additionally, GHBh1 shows a 100% overlap in amino acid sequence with the Riboflavin (vitamin B2) transporter, which opens the possibility of a possible dual-function (transceptor) structure. Riboflavin and GHB also share specific neuroprotective properties. Further research into the GHBh1 receptor subtype may pave the way for future therapeutic possibilities for GHB.

Aberrant ventral dentate gyrus structure and function in trauma susceptible mice.

Post-traumatic stress disorder (PTSD) is a psychiatric disorder vulnerable individuals can develop following a traumatic event, whereas others are resilient. Enhanced insight into the mechanistic underpinnings contributing to these inter-individual differences in trauma susceptibility is key to improved treatment and prevention. Aberrant function of the hippocampal dentate gyrus (DG) may contribute to its psychopathology, with the dorsal DG potentially encoding trauma memory generalization and the ventral DG anxiety. Using a mouse model, we hypothesized that susceptibility to develop PTSD-like symptoms following trauma will be underpinned by aberrant DG structure and function. Mice were exposed to a traumatic event (unpredictable, inescapable foot shocks) and tested for PTSD-like symptomatology following recovery. In four independent experiments, DG neuronal morphology, synaptic protein gene and protein expression, and neuronal activity during trauma encoding and recall were assessed. Behaviorally, trauma-susceptible animals displayed increased anxiety-like behavior already prior to trauma, increased novelty-induced freezing, but no clear differences in remote trauma memory recall. Comparison of the ventral DG of trauma susceptible vs resilient mice revealed lower spine density, reduced expression of the postsynaptic protein homer1b/c gene and protein, a larger population of neurons active during trauma encoding, and a greater presence of somatostatin neurons. In contrast, the dorsal DG of trauma-susceptible animals did not differ in terms of spine density or gene expression but displayed more active neurons during trauma encoding and a lower amount of somatostatin neurons. Collectively, we here report on specific structural and functional changes in the ventral DG in trauma susceptible male mice.

Alterations of mitochondrial dynamics in serotonin transporter knockout rats: A possible role in the fear extinction recall mechanisms.

Stress-related mental disorders encompass a plethora of pathologies that share the exposure to a negative environment as trigger for their development. The vulnerability to the effects of a negative environment is not equal to all but differs between individuals based on the genetic background makeup. Here, to study the molecular mechanisms potentially underlying increased threat anticipation, we employed an animal model showing this symptom (5-HTT knockout rats) which we exposed to Pavlovian fear conditioning (FC). We investigated the role of mitochondria, taking advantage of the recent evidence showing that the dynamic of these organelles is dysregulated after stress exposure. Behavioral experiments revealed that, during the second day of extinction of the FC paradigm, 5-HTT knockout (5-HTT-/-) animals showed a lack of fear extinction recall. From a mechanistic standpoint, we carried out our molecular analyses on the amygdala and prefrontal cortex, given their role in the management of the fear response due to their tight connection. We demonstrated that mitochondrial dynamics are impaired in the amygdala and prefrontal cortex of 5-HTT-/- rats. The dissection of the potential contributing factors revealed a critical role in the mechanisms regulating fission and fusion that are dysregulated in transgenic animals. Furthermore, mitochondrial oxidative phosphorylation, mitochondrial biogenesis, and the production of antioxidant enzymes were altered in these brain regions in 5-HTT-/- rats. In summary, our data suggest that increased extracellular 5-HT levels cause an unbalance of mitochondrial functionality that could contribute to the reduced extinction recall of 5-HTT-/- rats, pointing out the role of mitochondrial dynamics in the etiology of psychiatric disorders. Our findings, also, provide some interesting insights into the targeted development of drugs to treat such disorders.

Enduring effects of early-life adversity on reward processes: A systematic review and meta-analysis of animal studies.

Two-thirds of individuals experience adversity during childhood such as neglect, abuse or highly-stressful events. Early-life adversity (ELA) increases the life-long risk of developing mood and substance use disorders. Reward-related deficits has emerged as a key endophenotype of such psychiatric disorders. Animal models are invaluable for studying how ELA leads to reward deficits. However, the existing literature is heterogenous with difficult to reconcile findings. To create an overview, we conducted a systematic review containing multiple meta-analyses regarding the effects of ELA on reward processes overall and on specific aspects of reward processing in animal models. A comprehensive search identified 120 studies. Most studies omitted key details resulting in unclear risk of bias. Overall meta-analysis showed that ELA significantly reduced reward behaviors (SMD: -0.42 [-0.60; -0.24]). The magnitude of ELA effects significantly increased with longer exposure. When reward domains were analyzed separately, ELA only significantly dampened reward responsiveness (SMD: -0.525[-0.786; -0.264]) and social reward processing (SMD: -0.374 [-0.663; -0.084]), suggesting that ELA might lead to deficits in specific reward domains.

Longitudinal assessment of amygdala activity in mice susceptible to trauma.

Resilience to consequences of trauma exposure contains relevant information about the processes that contribute to the maintenance of mental health in the face of adversity; information that is essential to improve treatment success of stress-related mental diseases. Prior literature has implicated aberrant amygdala (re)activity as potential factor contributing to trauma susceptibility. However, it remains to be resolved which amygdalar subregions and neuronal subclasses are involved, and when - i.e., pre-, peri- or post-trauma exposure - and under what conditions changes in amygdala (re)activity manifest themselves. Here, we implemented a preclinical rodent model for PTSD that entailed exposure to a traumatic event (severe, unpredictable foot shock) followed by a trigger (mild, predictable foot shock). Using behavioral phenotyping, trauma susceptible vs. resilient mice were identified and pre-, peri- or post-trauma amygdala activity was compared. Neuronal activity was tagged in living mice by the use of the ArcTRAP transgenic mouse line, labeling all activated (i.e., Arc-expressing) neurons by a systemic injection of tamoxifen. Furthermore, we assessed amygdala responses during fear memory recall, induced by either (re-)exposure to the trauma, trigger, or a novel, yet similar context, and analyzed behavioral fear responses under these conditions, as well as basal anxiety in the mice. Results revealed no major differences dissociating susceptible vs. resilient mice prior to trauma exposure, but exaggerated activity in specifically the basolateral amygdala (BLA) peri-trauma that predicted susceptibility to later PTSD-like symptoms. Post-trauma, susceptible mice did not display altered basal amygdala activity, but BLA hyperreactivity in response to trigger context re-exposure, and BLA hyporesponsivity in response to the trauma context. Exposure to the novel, similar context evoked a differential temporal pattern of freezing behavior in susceptible mice and an increased activity of amygdalar somatostatin-expressing neurons specifically. As such, these results for the first time show that deviant BLA activity during fear learning predicts susceptibility to its long-term consequences and that aberrant subsequent BLA responses to stressful contexts depend on the exact context.

Corrigendum: Measuring Locomotor Activity and Behavioral Aspects of Rodents Living in the Home-Cage.

[This corrects the article DOI: 10.3389/fnbeh.2022.877323.].

Dopamine Transporter Knockout Rats Show Impaired Wellbeing in a Multimodal Severity Assessment Approach.

In preclinical psychiatry research, animals are central to modeling and understanding biological mechanisms of behavior and psychiatric disorders. We here present the first multimodal severity assessment of a genetically modified rat strain used in psychiatric research, lacking the dopamine transporter (DAT) gene and showing endophenotypes of several dopamine-associated disorders. Absence of the DAT leads to high extracellular dopamine (DA) levels and has been associated with locomotor hyperactivity, compulsive behaviors and stereotypies in the past. The German Animal Welfare Law, which is based on the EU Directive (2010/63/EU), requires a prospective severity assessment for every animal experiment, depending on the extent of the expected degree of pain, suffering, distress or lasting harm that the animals will experience. This should consider all procedures but also the impact of the genotype on the phenotype. Therefore, we examined multiple parameters indicating animal welfare, like burrowing behavior, social interaction, saccharin preference, baseline stress hormone levels and nesting behavior. Additionally, a footprint analysis was performed and home cage activity was analyzed for a more detailed characterization of locomotion. DAT KO rats demonstrated reduced burrowing, social interaction and saccharin preference. We also found pronounced stereotypies and alterations in the gait analysis in DAT KO rats. Moreover, we confirmed the hyperactivity and the impaired sensorimotor gating mechanisms to assure that our rats are exhibiting the correct phenotype. In conclusion, we provide evidence that DAT KO rats show alterations in natural behavior patterns and deduce that the marked stereotypies are a sign for coping difficulties, both indicating a negative influence of the genotype on wellbeing. We suggest to assess further rat models in an objectified severity assessment as previously done in mice to create a relative severity assessment based on scientific evidence. Until then, we propose the classification of homozygous DAT KO rats as "moderate" in accordance with the criteria of the EU directive 2010/63.

Repeated testing modulates chronic unpredictable mild stress effects in male rats.

Depression is a highly prevalent, debilitating mental disorder. Chronic unpredictable mild stress (CUMS) is the most widely applied model to study this affliction in rodents. While studies incorporating CUMS prior to an intervention often require long-lasting stress effects that persist after exposure is ceased, the longevity of these effects is rarely studied. Additionally, it is unclear whether behavioural assessments can be performed before and after interventions without repeated testing effects. In rats, we investigated CUMS effects on components of depressive-like behaviour both acutely after stress cessation and after a recovery period, as well as effects of repeated testing. We observed acute disruptions of the circadian locomotor rhythm and a reduced sucrose preference immediately after CUMS exposure. While circadian locomotor rhythm effects persisted up until four weeks after stress cessation, independently of repeated testing, sucrose preference effects did not. Interestingly, CUMS animals tested once after a recovery period of four weeks showed reduced anxiety-like behaviour in the open field and elevated plus maze compared to their control group and repeatedly-tested CUMS animals. These findings suggest that distinct CUMS-induced components of depressive-like behaviour are affected differentially by recovery time and repeated testing; these aspects should be considered carefully in future study designs.