Amphetamine psychosis and schizophrenia: a dual model

NMDAR dysfunction and the regulation of dopaminergic transmission in schizophrenia

A substantial body of evidence implicates dysfunction in N-methyl-d-aspartate receptors (NMDARs) in the pathophysiology of schizophrenia. This article illustrates how NMDAR dysfunction may give rise to many of the neurobiological phenomena frequently associated with schizophrenia with a particular focus on how NMDAR dysfunction affects the thalamic reticular nucleus (nRT) and pedunculopontine tegmental nucleus (PPTg). Furthermore, this article presents a model for schizophrenia illustrating how dysfunction in the nRT may interrupt prefrontal regulation of midbrain dopaminergic neurons, and how dysfunction in the PPTg may drive increased, irregular burst firing.

The 3-hit animal models of schizophrenia: Improving strategy to decipher and treat the disease?

Schizophrenia is a complex disease related to combination and interactions between genetic and environmental factors, with an epigenetic influence. After the development of the first mono-factorial animal models of schizophrenia (1-hit), that reproduced patterns of either positive, negative and/or cognitive symptoms, more complex models combining two factors (2-hit) have been developed to better fit with the multifactorial etiology of the disease. In the two past decades, a new way to design animal models of schizophrenia have emerged by adding a third hit (3-hit). This review aims to discuss the relevance of the risk factors chosen for the tuning of the 3-hit animal models, as well as the validities measurements and their contribution to schizophrenia understanding. We intended to establish a comprehensive overview to help in the choice of factors for the design of multiple-hit animal models of schizophrenia.

Insights into the Mechanism of Action of Antipsychotic Drugs Derived from Animal Models: Standard of Care versus Novel Targets

Therapeutic intervention for schizophrenia relies on blockade of dopamine D2 receptors in the associative striatum; however, there is little evidence for baseline overdrive of the dopamine system. Instead, the dopamine system is in a hyper-responsive state due to excessive drive by the hippocampus. This causes more dopamine neurons to be in a spontaneously active, hyper-responsive state. Antipsychotic drugs alleviate this by causing depolarization block, or excessive depolarization-induced dopamine neuron inactivation. Indeed, both first- and second-generation antipsychotic drugs cause depolarization block in the ventral tegmentum to relieve positive symptoms, whereas first-generation drugs also cause depolarization in the nigrostriatal dopamine system to lead to extrapyramidal side effects. However, by blocking dopamine receptors, these drugs are activating multiple synapses downstream from the proposed site of pathology: the loss of inhibitory influence over the hippocampus. An overactive hippocampus not only drives the dopamine-dependent positive symptoms, but via its projections to the amygdala and the neocortex can also drive negative and cognitive symptoms, respectively. On this basis, a novel class of drugs that can reverse schizophrenia at the site of pathology, i.e., the hippocampal overdrive, could be effective in alleviating all three classes of symptoms of schizophrenia while also being better tolerated.

Abnormal neuronal activity in Tourette syndrome and its modulation using deep brain stimulation

Tourette syndrome (TS) is a common childhood-onset disorder characterized by motor and vocal tics that are typically accompanied by a multitude of comorbid symptoms. Pharmacological treatment options are limited, which has led to the exploration of deep brain stimulation (DBS) as a possible treatment for severe cases. Multiple lines of evidence have linked TS with abnormalities in the motor and limbic cortico-basal ganglia (CBG) pathways. Neurophysiological data have only recently started to slowly accumulate from multiple sources: noninvasive imaging and electrophysiological techniques, invasive electrophysiological recordings in TS patients undergoing DBS implantation surgery, and animal models of the disorder. These converging sources point to system-level physiological changes throughout the CBG pathway, including both general altered baseline neuronal activity patterns and specific tic-related activity. DBS has been applied to different regions along the motor and limbic pathways, primarily to the globus pallidus internus, thalamic nuclei, and nucleus accumbens. In line with the findings that also draw on the more abundant application of DBS to Parkinson's disease, this stimulation is assumed to result in changes in the neuronal firing patterns and the passage of information through the stimulated nuclei. We present an overview of recent experimental findings on abnormal neuronal activity associated with TS and the changes in this activity following DBS. These findings are then discussed in the context of current models of CBG function in the normal state, during TS, and finally in the wider context of DBS in CBG-related disorders.

Lithium normalizes amphetamine-induced changes in striatal FoxO1 phosphorylation and behaviors in rats

Administration of the psychostimulant drug amphetamine (AMPH) to animals causes hyperactivity and deficit in prepulse inhibition (PPI) of startle, behaviors that are often observed in neuropsychiatric disorders such as schizophrenia and bipolar disorder. Enhanced central dopamine (DA) transmission is believed to mediate AMPH-induced behavioral alterations. Lithium, a drug used primarily in the treatment of bipolar disorder, is reported to interact with the DA system and antagonize some DA-related behaviors. Here, we provide evidence that AMPH and lithium reciprocally regulate the activity of the transcription factor forkhead box, class O1 (FoxO1), a downstream target of Akt. Administration of d-AMPH (3 mg/kg, intraperitoneally) to Sprague-Dawley rats resulted in a concomitant decrease in levels of phosphorylated (p) Akt as well as p-FoxO1 in the striatum, whereas lithium chloride (LiCl,100 mg/kg, intraperitoneally) exerted the opposite effect, that is, it increased levels of p-Akt and p-FoxO1. Pretreatment of animals with lithium prevented an AMPH-induced decrease in striatal p-Akt and p-FoxO1 levels. Pretreatment of animals with lithium also attenuated AMPH-induced locomotor activity and decreased prepulse inhibition. These in-vivo data suggest that the Akt-FoxO1 pathway may be a common target for the action of dopaminergic and antidopaminergic drugs, and its modulation may be relevant to the treatment of neuropsychiatric disorders.

Behavioral effects of bidirectional modulators of brain monoamines reserpine and d-amphetamine in zebrafish

Brain monoamines play a key role in the regulation of behavior. Reserpine depletes monoamines, and causes depression and hypoactivity in humans and rodents. In contrast, d-amphetamine increases brain monoamines' levels, and evokes hyperactivity and anxiety. However, the effects of these agents on behavior and in relation to monoamine levels remain poorly understood, necessitating further experimental studies to understand their psychotropic action. Zebrafish (Danio rerio) are rapidly emerging as a promising model organism for drug screening and translational neuroscience research. Here, we have examined the acute and long-term effects of reserpine and d-amphetamine on zebrafish behavior in the novel tank test. Overall, d-amphetamine (5 and 10mg/L) evokes anxiogenic-like effects in zebrafish acutely, but not 7 days later. In contrast, reserpine (20 and 40 mg/L) did not evoke overt acute behavioral effects, but markedly reduced activity 7 days later, resembling motor retardation observed in depression and/or Parkinson's disease. Three-dimensional 'temporal' (X, Y, time) reconstructions of zebrafish locomotion further supports these findings, confirming the utility of 3D-based video-tracking analyses in zebrafish models of drug action. Our results show that zebrafish are highly sensitive to drugs bi-directionally modulating brain monoamines, generally paralleling rodent and clinical findings. Collectively, this emphasizes the potential of zebrafish tests to model complex brain disorders associated with monoamine dysregulation.

Redox dysregulation in the pathophysiology of schizophrenia and bipolar disorder: insights from animal models

SIGNIFICANCE

Schizophrenia (SZ) and bipolar disorder (BD) are classified as two distinct diseases. However, accumulating evidence shows that both disorders share genetic, pathological, and epidemiological characteristics. Based on genetic and functional findings, redox dysregulation due to an imbalance between pro-oxidants and antioxidant defense mechanisms has been proposed as a risk factor contributing to their pathophysiology.

RECENT ADVANCES

Altered antioxidant systems and signs of increased oxidative stress are observed in peripheral tissues and brains of SZ and BD patients, including abnormal prefrontal levels of glutathione (GSH), the major cellular redox regulator and antioxidant. Here we review experimental data from rodent models demonstrating that permanent as well as transient GSH deficit results in behavioral, morphological, electrophysiological, and neurochemical alterations analogous to pathologies observed in patients. Mice with GSH deficit display increased stress reactivity, altered social behavior, impaired prepulse inhibition, and exaggerated locomotor responses to psychostimulant injection. These behavioral changes are accompanied by N-methyl-D-aspartate receptor hypofunction, elevated glutamate levels, impairment of parvalbumin GABA interneurons, abnormal neuronal synchronization, altered dopamine neurotransmission, and deficient myelination.

CRITICAL ISSUES

Treatment with the GSH precursor and antioxidant N-acetylcysteine normalizes some of those deficits in mice, but also improves SZ and BD symptoms when given as adjunct to antipsychotic medication.

FUTURE DIRECTIONS

These data demonstrate the usefulness of GSH-deficient rodent models to identify the mechanisms by which a redox imbalance could contribute to the development of SZ and BD pathophysiologies, and to develop novel therapeutic approaches based on antioxidant and redox regulator compounds.

Cannabinoid receptors mediate methamphetamine induction of high frequency gamma oscillations in the nucleus accumbens

Patients suffering from amphetamine-induced psychosis display repetitive behaviors, partially alleviated by antipsychotics, which are reminiscent of rodent stereotypies. Due to recent evidence implicating endocannabinoid involvement in brain disorders, including psychosis, we studied the effects of endocannabinoid signaling on neuronal oscillations of rats exhibiting methamphetamine stereotypy. Neuronal network oscillations were recorded with multiple single electrode arrays aimed at the nucleus accumbens of freely-moving rats. During the experiments, animals were dosed intravenously with the CB1 receptor antagonist rimonabant (0.3 mg/kg) or vehicle followed by an ascending dose regimen of methamphetamine (0.01, 0.1, 1, and 3 mg/kg; cumulative dosing). The effects of drug administration on stereotypy and local gamma oscillations were evaluated. Methamphetamine treatment significantly increased high frequency gamma oscillations (∼80 Hz). Entrainment of a subpopulation of nucleus accumbens neurons to high frequency gamma was associated with stereotypy encoding in putative fast-spiking interneurons, but not in putative medium spiny neurons. The observed ability of methamphetamine to induce both stereotypy and high frequency gamma power was potently disrupted following CB1 receptor blockade. The present data suggest that CB1 receptor-dependent mechanisms are recruited by methamphetamine to modify striatal interneuron oscillations that accompany changes in psychomotor state, further supporting the link between endocannabinoids and schizophrenia spectrum disorders.

Behavioral animal models of antipsychotic drug actions

Basic research in animals represents a fruitful approach to study the neurobiological basis of brain and behavioral disturbances relevant to neuropsychiatric disease and to establish and evaluate novel pharmacological therapies for their treatment. In the context of schizophrenia, there are models employing specific experimental manipulations developed according to specific pathophysiological or etiological hypotheses. The use of selective lesions in adult animals and the acute administration of psychotomimetic agents are indispensable tools in the elucidation of the contribution of specific brain regions or neurotransmitters to the genesis of a specific symptom or collection of symptoms and enjoy some degrees of predictive validity. However, they may be inaccurate, if not inadequate, in capturing the etiological mechanisms or ontology of the disease needed for a complete understanding of the disease and may be limited in the discovery of novel compounds for the treatment of negative and cognitive symptoms of schizophrenia. Under the prevailing consensus of schizophrenia as a disease of neurodevelopmental origin, we have seen the establishment of neurodevelopmental animal models which aim to identify the etiological processes whereby the brain, following specific triggering events, develops into a "schizophrenia-like brain" over time. Many neurodevelopmental models such as the neonatal ventral hippocampus (vHPC) lesion, methylazoxymethanol (MAM), and prenatal immune activation models can mimic a broad spectrum of behavioral, cognitive, and pharmacological abnormalities directly implicated in schizophrenic disease. These models allow pharmacological screens against multiple and coexisting schizophrenia-related dysfunctions while incorporating the disease-relevant concept of abnormal brain development. The multiplicity of existing models is testimonial to the multifactorial nature of schizophrenia, and there are ample opportunities for their integration. Indeed, one ultimate goal must be to incorporate the successes of distinct models into one unitary account of the complex disorder of schizophrenia and to use such unitary approaches in the further development and evaluation of novel antipsychotic treatment strategies.

The neuropsychology of schizophrenia

A model is proposed for integrating the neural and cognitive aspects of the positive symptoms of acute schizophrenia, using evidence from postmortem neuropathology and neurochemistry, clinical and preclinical studies of dopaminergic neurotransmission, anatomical connections between the limbic system and basal ganglia, attentional and other cognitive abnormalities underlying the positive symptoms of schizophrenia, specific animal models of some of these abnormalities, and previous attempts to model the cognitive functions of the septohippocampal system and the motor functions of the basal ganglia. Anatomically, the model emphasises the projections from the septohippocampal system, via the subiculum, and the amygdala to nucleus accumbens, and their interaction with the ascending dopaminergic projection to the accumbens. Psychologically, the model emphasises a failure in acute schizophrenia to integrate stored memories of past regularities of perceptual input with ongoing motor programs in the control of current perception. A number of recent experiments that offer support for the model are briefly described, including anatomical studies of limbic-striatal connections, studies in the rat of the effects of damage to these connections, and of the effects of amphetamine and neuroleptics, on the partial reinforcement extinction effect, latent inhibition and the Kamin blocking effect; and studies of the latter two phenomena in acute and chronic schizophrenics.

Ventral striatal noradrenergic mechanisms contribute to sensorimotor gating deficits induced by amphetamine

The psychotomimetic drug D-amphetamine (AMPH), disrupts prepulse inhibition (PPI) of the startle response, an operational measure of sensorimotor gating that is deficient in schizophrenia patients. Historically, this effect has been attributed to dopaminergic substrates; however, AMPH also increases norepinephrine (NE) levels, and enhancement of central NE transmission has been shown recently to disrupt PPI. This study examined the extent to which NE might participate in AMPH-induced disruptions of PPI and increases in locomotor activity, another classic behavioral effect of AMPH, by determining whether antagonism of postsynaptic NE receptors blocked these effects. Separate groups of male Sprague-Dawley rats received either the α1 receptor antagonist, prazosin (0, 0.3, 1 mg/kg), or the β receptor antagonist timolol (0, 3, 10 mg/kg) before administration of AMPH (0 or 1 mg/kg) before testing for PPI or locomotor activity. As an initial exploration of the anatomical substrates underlying possible α1 receptor-mediated effects on AMPH-induced PPI deficits, the α1 receptor antagonist terazosin (0 or 40 μg/0.5 μl) was microinfused into the nucleus accumbens shell (NAccSh) in conjunction with systemic AMPH administration before startle testing in a separate experiment. Prazosin, but not timolol, blocked AMPH-induced hyperactivity; both drugs reversed AMPH-induced PPI deficits without altering baseline startle responses. Interestingly, AMPH-induced PPI deficits also were partially blocked by terazosin in NAccSh. Thus, behavioral sequelae of AMPH (PPI disruption and hyperactivity) may be mediated in part by NE receptors, with α1 receptors in NAccSh possibly having an important role in the sensorimotor gating deficits induced by this psychotomimetic drug.

A neurocognitive animal model dissociating between acute illness and remission periods of schizophrenia

RATIONALE

The development and validation of animal models of the cognitive impairments of schizophrenia have remained challenging subjects.

OBJECTIVE

We review evidence from a series of experiments concerning an animal model that dissociates between the disruption of attentional capacities during acute illness periods and the cognitive load-dependent impairments that characterize periods of remission. The model focuses on the long-term attentional consequences of an escalating-dosing pretreatment regimen with amphetamine (AMPH).

RESULTS

Acute illness periods are modeled by the administration of AMPH challenges. Such challenges result in extensive impairments in attentional performance and the "freezing" of performance-associated cortical acetylcholine (ACh) release at pretask levels. During periods of remission (in the absence of AMPH challenges), AMPH-pretreated animals' attentional performance is associated with abnormally high levels of performance-associated cortical ACh release, indicative of the elevated attentional effort required to maintain performance. Furthermore, and corresponding with clinical evidence, attentional performance during remission periods is exquisitely vulnerable to distractors, reflecting impaired top-down control and abnormalities in fronto-mesolimbic-basal forebrain circuitry. Finally, this animal model detects the moderately beneficial cognitive effects of low-dose treatment with haloperidol and clozapine that were observed in clinical studies.

CONCLUSIONS

The usefulness and limitations of this model for research on the neuronal mechanisms underlying the cognitive impairments in schizophrenia and for drug-finding efforts are discussed.

Toward a neuro-cognitive animal model of the cognitive symptoms of schizophrenia: disruption of cortical cholinergic neurotransmission following repeated amphetamine exposure in attentional task-performing, but not non-performing, rats

Impairments in attentional functions and capacities represent core aspects of the cognitive symptoms of schizophrenia. Attentional performance has been demonstrated to depend on the integrity and activity of cortical cholinergic inputs. The neurobiological, behavioral, and cognitive effects of repeated exposure to psychostimulants model important aspects of schizophrenia. In the present experiment, prefrontal acetylcholine (ACh) release was measured in attentional task-performing and non-performing rats pretreated with an escalating dosing regimen of amphetamine (AMPH) and following challenges with AMPH. In non-performing rats, pretreatment with AMPH did not affect the increases in ACh release produced by AMPH-challenges. In contrast, attentional task performance-associated increases in ACh release were attenuated in AMPH-pretreated and AMPH-challenged rats. This effect of repeated AMPH exposure on ACh release was already present before task-onset, suggesting that the loss of cognitive control that characterized these animals' performance was a result of cholinergic dysregulation. The findings indicate that the demonstration of repeated AMPH-induced dysregulation of the prefrontal cholinergic input system depends on interactions between the effects of repeated AMPH exposure and cognitive performance-associated recruitment of this neuronal system. Repeated AMPH-induced disruption of prefrontal cholinergic activity and attentional performance represents a useful model to investigate the cholinergic mechanisms contributing to the cognitive impairments of schizophrenia.

The behavioral actions of lithium in rodent models: leads to develop novel therapeutics

For nearly as long as lithium has been in clinical use for the treatment of bipolar disorder, depression, and other conditions, investigators have attempted to characterize its effects on behaviors in rodents. Lithium consistently decreases exploratory activity, rearing, aggression, and amphetamine-induced hyperlocomotion; and it increases the sensitivity to pilocarpine-induced seizures, decreases immobility time in the forced swim test, and attenuates reserpine-induced hypolocomotion. Lithium also predictably induces conditioned taste aversion and alterations in circadian rhythms. The modulation of stereotypy, sensitization, and reward behavior are less consistent actions of the drug. These behavioral models may be relevant to human symptoms and to clinical endophenotypes. It is likely that the actions of lithium in a subset of these animal models are related to the therapeutic efficacy, as well the side effects, of the drug. We conclude with a brief discussion of various molecular mechanisms by which these lithium-sensitive behaviors may be mediated, and comment on the ways in which rat and mouse models can be used more effectively in the future to address persistent questions about the therapeutically relevant molecular actions of lithium.

Compulsive-like effects of repeated administration of quinpirole on drinking behavior in rats

We have previously reported that repeated administrations of quinpirole, a D2/D3 dopamine receptor agonist, facilitate instrumental behavior in rats given the choice between operant and free access to water (contrafreeloading: CFL). The goal of the present study was to investigate the effects of repeated daily administrations of quinpirole (0.5 mg/kg i.p.) on the appetitive versus the consummatory component of water-reinforced behavior, under two experimental conditions. Under one condition, the rats were given access to tap water according to an FR3 schedule of reinforcement. Under the second condition, the rats were given the choice between operant and free access to water. Five major findings were obtained. First, acutely quinpirole suppressed operant behavior and, therefore, water intake for at least 1h. Second, upon repeated administrations tolerance developed to the suppressant effect of quinpirole on instrumental behavior but only to a lesser extent to the antidipsic effect, dissociating the appetitive from the consummatory components of water-reinforced behavior. Third, in CFL conditions quinpirole induced a progressively larger preference for the operant access. Fourth, even when the rats were given the choice between free access to highly palatable saccharine (0.05 or 0.01%) solutions and operant access to tap water, quinpirole shifted the animals towards the operant access. Fifth, repeated quinpirole produced lasting consequences on drinking behavior, since after rehydration and under drug-free conditions quinpirole-pretreated rats ingested larger amounts of water than control rats. In conclusion, the repeated activation of D2/D3 receptors appears to induce the rats to perseverate in performing needless instrumental behavior.

(+)-MK-801 induced social withdrawal in rats; a model for negative symptoms of schizophrenia

Dopaminergic agonists and NMDA-receptor antagonists form the basis for the dopamine and glutamate models of schizophrenia, respectively. In human subjects dopaminergic agonists evoke a psychosis resembling positive symptoms of schizophrenia, while NMDA-receptor antagonists produce both positive and negative symptoms. Consequently, the glutamate model may be considered the most complete of the two models. Alterations in animal behaviour, in response to amphetamine or NMDA-receptor antagonists, are widely used to model schizophrenia. NMDA-receptor antagonist induced social withdrawal in rat is an established model for negative symptoms of schizophrenia. In this study we have set up an automated method, based on video tracking, to assess social behaviour, motor activity and movement pattern in rats. This method was then used to evaluate the effects of amphetamine and the NMDA-receptor antagonist (+)-MK-801, administered as single intraperitoneal injections, on rat behaviour. Amphetamine caused significantly increased motor activity and a tendency towards stimulation of social interactions. (+)-MK-801 also stimulated motor activity, but induced a significant inhibition of social interactions. These results indicate that a single injection of (+)-MK-801 to rats models both positive and negative symptoms of schizophrenia. Amphetamine, in contrast, reflects only the positive symptoms of schizophrenia in this model.

Monoaminergic dysregulation in glutathione-deficient mice: Possible relevance to schizophrenia?

Several lines of research have implicated glutathione (GSH) in schizophrenia. For instance, GSH deficiency has been reported in the prefrontal cortex of schizophrenics in vivo. Further, in rats postnatal GSH-deficiency combined with hyperdopaminergia led to cognitive impairments in the adult. In the present report we studied the effects of 2-day GSH-deficiency with L-buthionine-(S,R)-sulfoximine on monoaminergic function in mice. The effect of GSH-deficiency per se and when combined with the amphetamine and phencyclidine (PCP) models of schizophrenia was investigated. GSH-deficiency significantly altered tissue levels of dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective metabolites homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in a region-specific fashion. The effects of GSH-deficiency on tissue monoamines were distinct from and, generally, did not interact with the effects of amphetamine (5 mg/kg; i.p.) on tissue monoamines. Microdialysis studies showed that extracellular DA-release after amphetamine (5 mg/kg, i.p.) was two-fold increased in the nucleus accumbens of GSH-deficient mice as compared with control mice. Basal DA was unaltered. Further, extracellular levels of HVA in the frontal cortex and hippocampus and 5-HIAA in the nucleus accumbens were elevated by GSH-deficiency per se. Spontaneous locomotor activity in the open field was unchanged in GSH-deficient mice. In contrast, GSH-deficiency modulated the locomotor responses to mid-range doses of amphetamine (1.5 and 5 mg/kg, i.p.). Further, GSH-deficient mice displayed an increased locomotor response to low (2 and 3 mg/kg, i.p.) doses of phencyclidine (PCP). In conclusion, the data presented here show that even short-term GSH-deficiency has consequences for DA and 5-HT function. This was confirmed on both neurochemical and behavioral levels. How GSH and the monoamines interact needs further scrutiny. Moreover, the open field findings suggest reduced or altered N-methyl-d-aspartate (NMDA) receptor function in GSH-deficient mice. Thus, GSH-deficiency can lead to disturbances in DA, 5-HT and NMDA function, a finding that may have relevance for schizophrenia.

Repeated administration of phencyclidine, amphetamine and MK-801 selectively impairs spatial learning in mice: a possible model of psychotomimetic drug-induced cognitive deficits

Cognitive deficits are a key feature of schizophrenia. N-Methyl-D-aspartate (NMDA) receptor antagonists and amphetamine are known to induce psychotic behaviors and cognitive deficits in animals and humans, often affecting visuo-spatial abilities. Phencyclidine (PCP), MK-801 and amphetamine (AMPH) have been used in pharmacological animal models of schizophrenia, but none of these models has focused so far on spatial learning after repeated administration of the drugs. The objective of this study was to test whether repeated administration of PCP, AMPH or MK-801 influenced the performance of mice in a non-associative spatial learning test. CD-1 male mice were given i.p. daily injections of either saline, PCP (5.0, 10.0 mg/kg), AMPH (2.5, 5 mg/kg) or MK-801 (0.3, 0.6 mg/kg), for 5 days. On day 6 all mice were tested in an open field containing five different objects. After three sessions of habituation, each animal's reactivity to object displacement and object substitution was assessed. No significant differences among treatment groups were observed in object exploration or locomotion during the habituation phase. Five days of repeated PCP, AMPH or MK-801 administration selectively and differentially impaired the ability of mice to discriminate a spatial change, while leaving intact the ability to react to a non-spatial change. These data suggest that neurobiological adaptations to drug regimens known to induce psychotic behaviors and alterations in locomotor activity or stereotypies can also alter spatial learning, as assessed in this test, thus indicating that these regimens could also mimic some of the cognitive deficits observed in schizophrenia.

The atypical antipsychotic ziprasidone, but not haloperidol, improves phencyclidine-induced cognitive deficits in a reversal learning task in the rat

The glutamate/N-methyl-D-aspartate receptor antagonist phencyclidine (PCP) has been shown to induce both positive and negative symptoms of schizophrenia, as well as cognitive deficits, thus providing a relatively valid model of psychosis. Isolation rearing from weaning in the rat has been proposed as a non-pharmacological model of psychosis. The aim of the present study was to explore the validity of a combination of these techniques to model cognitive dysfunction associated with schizophrenia. The present study evaluates the effects of the novel antipsychotic ziprasidone and the typical antipsychotic haloperidol in their ability to reverse the cognitive deficit induced by PCP in isolation reared rats and social controls. Rats housed in social isolation (n = 25) or in groups of five (n = 25) from weaning were food deprived and trained to respond for food in an operant reversal learning paradigm. PCP at 1.0 and 1.5 mg/kg (intraperitoneally, i.p.) significantly and selectively impaired reversal task performance in both groups of rats. This impairment was not significantly improved following the coadministration of haloperidol (0.05 mg/kg, i.p.). Higher haloperidol doses (0.1 and 0.25 mg/kg, i.p.) were found to impair task performance, with the social animals being more sensitive than isolation-reared animals. In contrast, ziprasidone (2.5 mg/kg, i.p.) reversed the impairment caused by PCP. This was significant in social animals, while in isolates there was a non-significant enhancement in performance of the reversal task with ziprasidone compared to PCP alone. Thus, PCP produced a selective reversal learning deficit in rats, which was ameliorated following treatment with ziprasidone but not haloperidol. Rearing conditions did not influence performance of the test or the deficit produced by PCP.

Genetic animal models: focus on schizophrenia

The neurobiology of schizophrenia remains poorly understood. Symptoms of schizophrenia are classically thought to be associated with an imbalance of the dopaminergic system. However, the contribution of other neurotransmitters, in particular glutamate, has been increasingly appreciated. The role of individual components of neurotransmitter systems in aberrant behaviors can be experimentally tested in transgenic animals. Dopamine transporter knockout mice display persistently elevated dopaminergic tone and therefore might be appropriate substrates to evaluate the dopamine hypothesis. Similarly, NMDA receptor-deficient mice can be used to evaluate the glutamate hypothesis of schizophrenia. In this review we discuss how such animal models might be relevant for understanding the neurochemical underpinnings of certain manifestations of schizophrenia.

Effects of d-amphetamine on the performance of rats in an animal analogue of the A-X continuous performance test

Schizophrenia patients subjected to the A-X Continuous Performance Test (A-X CPT) show cognitive deficits that are thought to reflect impaired representation and maintenance of context information. An issue deserving attention is to what extent the acute amphetamine model of schizophrenia also models these cognitive deficits. The present experiment examined the effect of acute d-amphetamine (AMP) on the performance of rats in an animal analogue of the A-X CPT. Subjects first learned to solve an A --> X+, B --> X-, A --> Y- discrimination task, with A and B representing visual features; X and Y designating auditory target stimuli; --> signifying a serial presentation; and + and - referring to food reinforcement and non-reinforcement, respectively. Frequency of food-magazine visits was the dependent measure. After mastering the discrimination, rats received test trials under either saline or 0.5 mg/kg AMP (s.c.). At test, the interval between feature and target presentation was varied; reinforcement contingencies were maintained. AMP significantly impaired performance on the A --> X+/B --> X- discrimination by increasing the response level on B --> X- trials. AMP did not significantly affect performance on the A --> X+/A --> Y- discrimination. However, AMP also increased magazine responding in the absence of the presentation of features and targets. A parsimonious conclusion based on these preliminary results is that acute AMP does not affect processing of context information provided by the visual features in this procedure. It rather has a more non-specific response-enhancing effect, especially with respect to stimuli associated with the delivery of food.