Effective Use of Animal Models for Therapeutic Development in Psychiatric and Substance Use Disorders

Post-traumatic stress disorder: clinical and translational neuroscience from cells to circuits

Post-traumatic stress disorder (PTSD) is a maladaptive and debilitating psychiatric disorder, characterized by re-experiencing, avoidance, negative emotions and thoughts, and hyperarousal in the months and years following exposure to severe trauma. PTSD has a prevalence of approximately 6-8% in the general population, although this can increase to 25% among groups who have experienced severe psychological trauma, such as combat veterans, refugees and victims of assault. The risk of developing PTSD in the aftermath of severe trauma is determined by multiple factors, including genetics - at least 30-40% of the risk of PTSD is heritable - and past history, for example, prior adult and childhood trauma. Many of the primary symptoms of PTSD, including hyperarousal and sleep dysregulation, are increasingly understood through translational neuroscience. In addition, a large amount of evidence suggests that PTSD can be viewed, at least in part, as a disorder that involves dysregulation of normal fear processes. The neural circuitry underlying fear and threat-related behaviour and learning in mammals, including the amygdala-hippocampus-medial prefrontal cortex circuit, is among the most well-understood in behavioural neuroscience. Furthermore, the study of threat-responding and its underlying circuitry has led to rapid progress in understanding learning and memory processes. By combining molecular-genetic approaches with a translational, mechanistic knowledge of fear circuitry, transformational advances in the conceptual framework, diagnosis and treatment of PTSD are possible. In this Review, we describe the clinical features and current treatments for PTSD, examine the neurobiology of symptom domains, highlight genomic advances and discuss translational approaches to understanding mechanisms and identifying new treatments and interventions for this devastating syndrome.

The Effects of Drug Treatments for ADHD in Measures of Cognitive Performance

Based on core symptoms of inattention and deficient impulse control, and the identification of effective pharmacotherapies such as amphetamine (AMP; Adderall^®), methylphenidate (MPH; Ritalin^®), and atomoxetine (ATX; Strattera^®), ADHD is a clinical condition which provides opportunity for translational research. Neuropsychological tests such as the 5-Choice and Continuous Performance Tasks, which measure aspects of attention and impulse control in animals and humans, provide scope for both forward (animal to human) and reverse (human to animal) translation. Rodent studies support pro-attentive effects of AMP and MPH and effectiveness in controlling some forms of impulsive behavior. In contrast, any pro-attentive effects of ATX appear to be less consistent, the most reliable effects of ATX are recorded in tests of impulsivity. These differences may account for AMP and MPH being recognized as first-line treatments for ADHD with a higher efficacy relative to ATX. DSM-5 classifies three "presentations" of ADHD: predominantly inattentive type (ADHD-I), predominantly hyperactive/impulsive type (ADHD-HI), or combined (ADHD-C). Presently, it is unclear whether AMP, MPH, or ATX has differential levels of efficacy across these presentation types. Nonetheless, these studies encourage confidence for the forward translation of NCEs in efforts to identify newer pharmacotherapies for ADHD.

Study on the Safety of Human Oligodendrocyte Precursor Cell Transplantation in Young Animals and Its Efficacy on Myelination

Oligodendrocyte precursor cells (OPCs) can differentiate into myelinating oligodendrocytes during embryonic development, thereby representing an important potential source for myelin repair or regeneration. To the best of our knowledge, there are very few OPCs from human sources (human-derived OPCs [hOPCs]). In this study, we aimed to evaluate the safety and remyelination capacity of hOPCs developed in our laboratory, transplanted into the lateral ventricles of young animals. Several acute and chronic toxicity experiments were conducted in which different doses of hOPCs were transplanted into the lateral ventricles of Sprague–Dawley rats of different ages. The toxicity, biodistribution, and tumor formation ability of the injected hOPCs were examined by evaluating the rats' vital signs, developmental indicators, neural reflexes, as well as by hematology, immunology, and pathology. In addition, the hOPCs were transplanted into the corpus callosum of the shiverer mouse to verify cell myelination efficacy. Overall, our results show that transplanted hOPCs into young mice are nontoxic to their organ function or immune system. The transplanted cells engrafted in the brain and did not appear in other organs, nor did they cause tissue proliferation or tumor formation. In terms of efficacy, the transplanted hOPCs were able to form myelin in the corpus callosum, alleviate the trembling phenotype of shiverer mice, and promote normal development. The transplantation of hOPCs is safe; they can effectively form myelin in the brain, thereby providing a theoretical basis for the future clinical transplantation of hOPCs.

Innovative screening models for the discovery of new schizophrenia drug therapies: an integrated approach

Introduction: Schizophrenia is a severe psychiatric disorder affecting millions worldwide. However, available treatment options do not fully address the disease. Whereas current antipsychotics may control psychotic symptoms, they seem notoriously ineffective in improving negative and cognitive symptoms or in preventing functional decline. As the etiology of schizophrenia eludes us, the development of valid animal models for screening new drug targets appears to be a strenuous task.Areas covered: In this review, the authors present the key concepts that validate animal models of schizophrenia, as well as the different screening approaches for novel schizophrenia treatments. The models covered are either based on major neurotransmitter systems or neurodevelopmental, immune, and genetic approaches.Expert opinion: Sadly, due to inertia, research focuses on developing 'anti-psychotics', instead of 'anti-schizophrenia' drugs that would tackle the entire syndrome of schizophrenia. Whereas no perfect model may ever exist, combining different experimental designs may enhance validity, as the over-reliance on a single model is inappropriate. Multi-model approaches incorporating vulnerability, the 'two-hit' hypothesis, and endophenotypes offer a promise for developing new strategies for schizophrenia treatment. Forward and reverse translation between preclinical and clinical research will increase the probability of success and limit failures in drug development.

Short-active photoperiod gestation induces psychiatry-relevant behavior in healthy mice but a resiliency to such effects are seen in mice with reduced dopamine transporter expression

A higher incidence of multiple psychiatric disorders occurs in people born in late winter/early spring. Reduced light exposure/activity level impacts adult rodent behavior and neural mechanisms, yet few studies have investigated such light exposure on gestating fetuses. A dysfunctional dopamine system is implicated in most psychiatric disorders, and genetic polymorphisms reducing expression of the dopamine transporter (DAT) are associated with some conditions. Furthermore, adult mice with reduced DAT expression (DAT-HT) were hypersensitive to short active (SA; 19:5 L:D) photoperiod exposure versus their wildtype (WT) littermates. Effects of SA photoperiod exposure during gestation in these mice have not been examined. We confirmed adult females exhibit a heightened corticosterone response when in SA photoperiod. We then tested DAT-HT mice and WT littermates in psychiatry-relevant behavioral tests after SA or normal active (NA; 12:12 L:D) photoperiod exposure during gestation and early life. SA-born WT mice exhibited sensorimotor gating deficits (males), increased reward preference, less immobility, open arm avoidance (females), less motivation to obtain a reward, and reversal learning deficits, vs. NA-born WT mice. DAT-HT mice were largely resilient to these effects, however. Future studies will determine the mechanism(s) by which SA photoperiod exposure influences brain development to predispose toward emergence of psychiatry-relevant behaviors.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Characterization of Amphetamine, Methylphenidate, Nicotine, and Atomoxetine on Measures of Attention, Impulsive Action, and Motivation in the Rat: Implications for Translational Research

Amphetamine (AMP), methylphenidate (MPH), and atomoxetine (ATX) are approved treatments for ADHD, and together with nicotine (NIC), represent pharmacological agents widely studied on cognitive domains including attention and impulsive action in humans. These agents thus represent opportunities for clinical observation to be reinvestigated in the preclinical setting, i.e., reverse translation. The present study investigated each drug in male, Long Evans rats trained to perform either (1) the five-choice serial reaction time task (5-CSRTT), (2) Go/NoGo task, or (3) a progressive ratio (PR) task, for the purpose of studying each drug on attention, impulsive action and motivation. Specific challenges were adopted in the 5-CSRTT designed to tax attention and impulsivity, i.e., high frequency of stimulus presentation (sITI), variable reduction in stimulus duration (sSD), and extended delay to stimulus presentation (10-s ITI). Initially, performance of a large (> 80) cohort of rats in each task variant was conducted to examine performance stability over repeated challenge sessions, and to identify subgroups of "high" and "low" attentive rats (sITI and sSD schedules), and "high" and "low" impulsives (10-s ITI). Using an adaptive sequential study design, the effects of AMP, MPH, ATX, and NIC were examined and contrasting profiles noted across the tests. Both AMP (0.03-0.3 mg/kg) and MPH (1-6 mg/kg) improved attentional performance in the sITI but not sSD or 10-s ITI condition, NIC (0.05-0.2 mg/kg) improved accuracy across all conditions. ATX (0.1-1 mg/kg) detrimentally affected performance in the sITI and sSD condition, notably in "high" performers. In tests of impulsive action, ATX reduced premature responses notably in the 10-s ITI condition, and also reduced false alarms in Go/NoGo. Both AMP and NIC increased premature responses in all task variants, although AMP reduced false alarms highlighting differences between these two measures of impulsive action. The effect of MPH was mixed and appeared baseline dependent. ATX reduced break point for food reinforcement suggesting a detrimental effect on motivation for primary reward. Taken together these studies highlight differences between AMP, MPH, and ATX which may translate to their clinical profiles. NIC had the most reliable effect on attentional accuracy, whereas ATX was reliably effective against all tests of impulsive action.

Effects of 5-HT2C receptor modulation and the NA reuptake inhibitor atomoxetine in tests of compulsive and impulsive behaviour

Drug repositioning has gained strategic value as a reaction to high attrition rates of new drugs as they pass through the clinical development process. The 5-HT2C receptor agonist lorcaserin (Belviq®), and the selective NA reuptake inhibitor atomoxetine (Strattera®) represent two drugs FDA approved for obesity and ADHD respectively. Although both drugs are of differing pharmacological class, each share a property of regulating impulsive behaviours in preclinical studies, and thus represent candidates for consideration in clinical conditions labelled as 'impulsive-compulsive disorders'. The present studies investigated both drugs, as well as the highly selective 5-HT2C agonist CP-809101 in two tests of compulsive action: schedule-induced polydipsia (SIP) and increased perseverative [PSV] (and premature [PREM]) responses emitted during an extended ITI 5-choice task. While lorcaserin (0.06-0.6 mg/kg), CP-809101 (0.1-1 mg/kg) and atomoxetine (0.1-1 mg/kg) each reduced both PREM and PSV measures in the 5-choice task, at equivalent doses only lorcaserin and CP-809101 affected excessive water intake in the SIP task, atomoxetine (0.1-2 mg/kg) was essentially ineffective. Further evidence supporting a role of the 5-HT2C receptor as an important regulator of impulsive-compulsive behaviours, the selective antagonist SB-242084 produced the opposing effects to lorcaserin, i.e promoting both impulsive and compulsive behaviours. The profile of atomoxetine may suggest differences in the nature of compulsive action measured either as non-regulatory drinking in the SIP task, and PSV responses made in a 5-choice task. These studies support the consideration of 5-HT2C receptor agonists, typified by lorcaserin, and atomoxetine as potential treatments for clinical conditions categorised as 'impulsive-compulsive disorders'. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Tetrahydroprotoberberines: A Novel Source of Pharmacotherapies for Substance Use Disorders?

Tetrahydroprotoberberines (THPBs) are a class of compounds that target both dopamine D1 and D2 families of receptors, making them attractive candidates for treating substance use disorder (SUD). The binding of some THPBs to serotonin and adrenergic receptors, in addition to dopamine receptors, gives rise to complex pharmacological profiles. Significant progress has been made over the last decade in examining these compounds for their therapeutic potential. Here, we evaluate recent discoveries relating to the neural mechanism and therapeutic effects of THPBs, focusing on compounds that have shown promise in animal models of SUD and preliminary clinical studies. Advancements in structure-activity relationship studies and in silico modeling of THPB binding to dopamine receptors have facilitated the synthesis of novel THPBs with enhanced therapeutic properties and provide insights regarding use of the THPB scaffold to serve as a template for innovative drug designs.