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

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

Understanding translational research in schizophrenia: A novel insight into animal models

Schizophrenia affects millions of people worldwide and is a major challenge for the scientific community. Like most psychotic diseases, it is also considered a complicated mental disorder caused by an imbalance in neurotransmitters. Due to the complexity of neuropathology, it is always a complicated disorder. The lack of proper understanding of the pathophysiology makes the disorder unmanageable in clinical settings. However, due to recent advances in animal models, we hope we can have better therapeutic approaches with more success in clinical settings. Dopamine, glutamate, GABA, and serotonin are the neurotransmitters involved in the pathophysiology of schizophrenia. Various animal models have been put forward based on these neurotransmitters, including pharmacological, neurodevelopmental, and genetic models. Polymorphism of genes such as dysbindin, DICS1, and NRG1 has also been reported in schizophrenia. Hypothesis based on dopamine, glutamate, and serotonin are considered successful models of schizophrenia on which drug therapies have been designed to date. New targets like the orexin system, muscarinic and nicotinic receptors, and cannabinoid receptors have been approached to alleviate the negative and cognitive symptoms. The non-pharmacological models like the post-weaning social isolation model (maternal deprivation), the isolation rearing model etc. have been also developed to mimic the symptoms of schizophrenia and to create and test new approaches of drug therapy which is a breakthrough at present in psychiatric disorders. Different behavioral tests have been evaluated in these specific models. This review will highlight the currently available animal models and behavioral tests in psychic disorders concerning schizophrenia.

Anticholinergic Burden and Cognitive Performance in Patients With Schizophrenia: A Systematic Literature Review

Objective: Cognitive impairment in schizophrenia forms the key cause of the disease's disability, leading to serious functional, and socioeconomic implications. Dopaminergic-cholinergic balance is considered essential to cognitive performance in schizophrenia and patients are often treated with many drugs with anticholinergic properties. This study aims to examine the cognitive impact of anticholinergic burden in patients with schizophrenia. Methods: A systematic literature review was performed on English-language studies published on PubMed, Embase, and Web of Science, from inception to June 2021, to identify research studies that examined the effect of anticholinergic load on cognition in clinically stable patients with schizophrenia. No restrictions on study design, age of participants, or geographical distribution were applied. Two researchers performed independently the screening and shortlisting of the eligible articles. A narrative synthesis of the main characteristics and findings of studies included was reported. Results: In total, 17 articles of varying methodological design met the inclusion criteria. Three of them found statistically significant improvement in cognition after anticholinergic tapering without adverse effects. Thirteen studies found a statistically significant association between high anticholinergic burden and cognitive impairment (neurocognitive composite scores and individual cognitive domains such as learning and memory, executive function, processing speed), apart from a study, related to the specific characteristics of clozapine. Conclusions: Medication with increased anticholinergic load has been found in most of the studies to negatively affect neurocognitive performance of patients with schizophrenia. However, the clinical and methodological heterogeneity of studies included limit our interpretation and conclusions.

Acetylcholine Mediates Dynamic Switching Between Information Coding Schemes in Neuronal Networks

Rate coding and phase coding are the two major coding modes seen in the brain. For these two modes, network dynamics must either have a wide distribution of frequencies for rate coding, or a narrow one to achieve stability in phase dynamics for phase coding. Acetylcholine (ACh) is a potent regulator of neural excitability. Acting through the muscarinic receptor, ACh reduces the magnitude of the potassium M-current, a hyperpolarizing current that builds up as neurons fire. The M-current contributes to several excitability features of neurons, becoming a major player in facilitating the transition between Type 1 (integrator) and Type 2 (resonator) excitability. In this paper we argue that this transition enables a dynamic switch between rate coding and phase coding as levels of ACh release change. When a network is in a high ACh state variations in synaptic inputs will lead to a wider distribution of firing rates across the network and this distribution will reflect the network structure or pattern of external input to the network. When ACh is low, network frequencies become narrowly distributed and the structure of a network or pattern of external inputs will be represented through phase relationships between firing neurons. This work provides insights into how modulation of neuronal features influences network dynamics and information processing across brain states.

Sustaining attention to simple visual tasks: a central deficit in schizophrenia? A systematic review

Impairments in sustained attention, that is, the ability to achieve and maintain the focus of cognitive activity on a given stimulation source or task, have been described as central to schizophrenia. Today, sustained attention deficit is still considered as a hallmark of schizophrenia. Nevertheless, current findings on this topic are not consistent. To clarify these findings, we attempt to put these results into perspective according to the type of assessment (i.e., overall and over time assessment), the participants' characteristics (i.e., clinical and demographic characteristics), and the paradigms (i.e., traditionally formatted tasks, go/no-go tasks, and the sustained attention task) and measures used. Two types of assessment lead to opposite findings; they do not evaluate sustained attention the same way. Studies using overall assessments of sustained attention ability tend to reveal a deficit, whereas studies using over time assessments do not. Therefore, further research is needed to investigate the underlying cognitive control mechanisms of changes in sustained attention in schizophrenia.

Thalamic cholinergic innervation makes a specific bottom-up contribution to signal detection: Evidence from Parkinson's disease patients with defined cholinergic losses

Successful behavior depends on the ability to detect and respond to relevant cues, especially under challenging conditions. This essential component of attention has been hypothesized to be mediated by multiple neuromodulator systems, but the contributions of individual systems (e.g., cholinergic, dopaminergic) have remained unclear. The present study addresses this issue by leveraging individual variation in regionally-specific cholinergic denervation in Parkinson's disease (PD) patients, while controlling for variation in dopaminergic denervation. Patients whose dopaminergic and cholinergic nerve terminal integrity had been previously assessed using Positron Emission Tomography (Bohnen et al., 2012) and controls were tested in a signal detection task that manipulates attentional-perceptual challenge and has been used extensively in both rodents and humans to investigate the cholinergic system's role in responding to such challenges (Demeter et al., 2008; McGaughy and Sarter, 1995; see Hasselmo and Sarter 2011 for review). In simple correlation analyses, measures of midbrain dopaminergic, and both cortical and thalamic cholinergic innervation all predicted preserved signal detection under challenge. However, regression analyses also controlling for age, disease severity, and other variables showed that the only significant independent neurotransmitter-related predictor over and above the other variables in the model was thalamic cholinergic integrity. Furthermore, thalamic cholinergic innervation exclusively predicted hits, not correct rejections, indicating a specific contribution to bottom-up salience processing. These results help define regionally-specific contributions of cholinergic function to different aspects of attention and behavior.

Hyperfocusing of attention on goal-related information in schizophrenia: Evidence from electrophysiology

Schizophrenia clearly involves impairments of attention, but the precise nature of these impairments has been difficult to determine. One possibility is that the deficit in attention is a secondary consequence of a deficit in goal maintenance. However, recent research suggests that people with schizophrenia (PSZ) actually focus attention more strongly on objects containing goal-relevant features. To test these competing hypotheses, we recorded event-related potentials (ERPs) from PSZ (N = 20) and healthy control subjects (HCS; N = 20) while they looked for a particular target color at fixation and tried to ignore lateral distractors that sometimes matched the target color (target-color distractors). Goal maintenance was made trivially easy by the continual presentation of a goal reminder. We found that HCS were able to successfully suppress target-color distractors (leading to a distractor positivity ERP component), whereas PSZ focused attention on these items (leading to an N2-posterior-contralateral ERP component). This suggests that, when maintaining a task set, PSZ engage in aberrant focusing of attention, or hyperfocusing, on goal-relevant features. (PsycINFO Database Record

MAM (E17) rodent developmental model of neuropsychiatric disease: disruptions in learning and dysregulation of nucleus accumbens dopamine release, but spared executive function

RATIONALE

Gestational day 17 methylazoxymethanol (MAM) treatment has been shown to reproduce, in rodents, some of the alterations in cortical and mesolimbic circuitries thought to contribute to schizophrenia.

OBJECTIVE

We characterized the behavior of MAM animals in tasks dependent on these circuitries to see what behavioral aspects of schizophrenia the model captures. We then characterized the integrity of mesolimbic dopamine neurotransmission in a subset of animals used in the behavioral experiments.

METHODS

MAM animals' capacity for working memory, attention, and resilience to distraction was tested with two different paradigms. Cue-reward learning and motivation were assayed with Pavlovian conditioned approach. Measurements of electrically stimulated phasic and tonic DA release in the nucleus accumbens with fast-scan cyclic voltammetry were obtained from the same animals used in the Pavlovian task.

RESULTS

MAM animals' basic attentional capacities were intact. MAM animals took longer to acquire the working memory task, but once learned, performed at the same level as shams. MAM animals were also slower to develop a Pavlovian conditioned response, but otherwise no different from controls. These same animals showed alterations in terminal DA release that were unmasked by an amphetamine challenge.

CONCLUSIONS

The predominant behavioral-cognitive feature of the MAM model is a learning impairment that is evident in acquisition of executive function tasks as well as basic Pavlovian associations. MAM animals also have dysregulated terminal DA release, and this may contribute to observed behavioral differences. The MAM model captures some functional impairments of schizophrenia, particularly those related to acquisition of goal-directed behavior.

Amphetamine sensitization in mice is sufficient to produce both manic- and depressive-related behaviors as well as changes in the functional connectivity of corticolimbic structures

It has been suggested that amphetamine abuse and withdrawal mimics the diverse nature of bipolar disorder symptomatology in humans. Here, we determined if a single paradigm of amphetamine sensitization would be sufficient to produce both manic- and depressive-related behaviors in mice. CD-1 mice were subcutaneously dosed for 5 days with 1.8 mg/kg d-amphetamine or vehicle. On days 6-31 of withdrawal, amphetamine-sensitized (AS) mice were compared to vehicle-treated (VT) mice on a range of behavioral and biochemical endpoints. AS mice demonstrated reliable mania- and depression-related behaviors from day 7 to day 28 of withdrawal. Relative to VT mice, AS mice exhibited long-lasting mania-like hyperactivity following either an acute 30-min restraint stress or a low-dose 1 mg/kg d-amphetamine challenge, which was attenuated by the mood-stabilizers lithium and quetiapine. In absence of any challenge, AS mice showed anhedonia-like decreases in sucrose preference and depression-like impairments in the off-line consolidation of motor memory, as reflected by the lack of spontaneous improvement across days of training on the rotarod. AS mice also demonstrated a functional impairment in nest building, an ethologically-relevant activity of daily living. Western blot analyses revealed a significant increase in methylation of histone 3 at lysine 9 (H3K9), but not lysine 4 (H3K4), in hippocampus of AS mice relative to VT mice. In situ hybridization for the immediate-early gene activity-regulated cytoskeleton-associated protein (Arc) further revealed heightened activation of corticolimbic structures, decreased functional connectivity between frontal cortex and striatum, and increased functional connectivity between the amygdala and hippocampus of AS mice. The effects of amphetamine sensitization were blunted in C57BL/6J mice relative to CD-1 mice. These results show that a single amphetamine sensitization protocol is sufficient to produce behavioral, functional, and biochemical phenotypes in mice that are relevant to bipolar disorder.

Attention and the Cholinergic System: Relevance to Schizophrenia

Traditional methods of drug discovery often rely on a unidirectional, "bottom-up" approach: A search for molecular compounds that target a particular neurobiological substrate (e.g., a receptor type), the refinement of those compounds, testing in animal models using high-throughput behavioral screening methods, and then human testing for safety and effectiveness. Many attempts have found the "effectiveness" criterion to be a major stumbling block, and we and others have suggested that success may be improved by an alternative approach that considers the neural circuits mediating the effects of genetic and molecular manipulations on behavior and cognition. We describe our efforts to understand the cholinergic system's role in attention using parallel approaches to test main hypotheses in both rodents and humans as well as generating converging evidence using methods and levels of analysis tailored to each species. The close back-and-forth between these methods has enhanced our understanding of the cholinergic system's role in attention both "bottom-up" and "top-down"-that is, the basic neuroscience identifies potential neuronal circuit-based mechanisms of clinical symptoms, and the patient and genetic populations serve as natural experiments to test and refine hypotheses about its contribution to specific processes. Together, these studies have identified (at least) two major and potentially independent contributions of the cholinergic system to attention: a neuromodulatory component that influences cognitive control in response to challenges from distractors that either make detection more difficult or draw attention away from the distractor, and a phasic or transient cholinergic signal that instigates a shift from ongoing behavior and the activation of cue-associated response. Right prefrontal cortex appears to play a particularly important role in the neuromodulatory component integrating motivational and cognitive influences for top-down control across populations, whereas the transient cholinergic signal involves orbitofrontal regions associated with shifts between internal and external attention. Understanding how these two modes of cholinergic function interact and are perturbed in schizophrenia will be an important prerequisite for developing effective treatments.

Monitoring cholinergic activity during attentional performance in mice heterozygous for the choline transporter: a model of cholinergic capacity limits

Reductions in the capacity of the human choline transporter (SLC5A7, CHT) have been hypothesized to diminish cortical cholinergic neurotransmission, leading to risk for cognitive and mood disorders. To determine the acetylcholine (ACh) release capacity of cortical cholinergic projections in a mouse model of cholinergic hypofunction, the CHT+/- mouse, we assessed extracellular ACh levels while mice performed an operant sustained attention task (SAT). We found that whereas SAT-performance-associated increases in extracellular ACh levels of CHT+/- mice were significantly attenuated relative to wildtype littermates, performance on the SAT was normal. Tetrodotoxin-induced blockade of neuronal excitability reduced both dialysate ACh levels and SAT performance similarly in both genotypes. Likewise, lesions of cholinergic neurons abolished SAT performance in both genotypes. However, cholinergic activation remained more vulnerable to the reverse-dialyzed muscarinic antagonist atropine in CHT+/- mice. Additionally, CHT+/- mice displayed greater SAT-disrupting effects of reverse dialysis of the nAChR antagonist mecamylamine. Receptor binding assays revealed a higher density of α4β2* nAChRs in the cortex of CHT+/- mice compared to controls. These findings reveal compensatory mechanisms that, in the context of moderate cognitive challenges, can overcome the performance deficits expected from the significantly reduced ACh capacity of CHT+/- cholinergic terminals. Further analyses of molecular and functional compensations in the CHT+/- model may provide insights into both risk and resiliency factors involved in cognitive and mood disorders.

Cholinergic control over attention in rats prone to attribute incentive salience to reward cues

Some rats [sign-trackers (STs)] are especially prone to attribute incentive salience to reward cues, relative to others [goal-trackers (GTs)]. Thus, reward cues are more likely to promote maladaptive reward-seeking behavior in STs than GTs. Here, we asked whether STs and GTs differ on another trait that can contribute to poor restraint over behavior evoked by reward cues. We report that, relative to GTs, STs have poor control over attentional performance, due in part to insufficient cholinergic stimulation of cortical circuitry. We found that, relative to GTs, STs showed poor performance on a sustained attention task (SAT). Furthermore, their performance fluctuated rapidly between periods of good to near-chance performance. This finding was reproduced using a separate cohort of rats. As demonstrated earlier, performance on the SAT was associated with increases in extracellular levels of cortical acetylcholine (ACh); however, SAT performance-associated increases in ACh levels were significantly attenuated in STs relative to GTs. Consistent with the view that the modulatory effects of ACh involve stimulation of α4β2* nicotinic ACh receptors (nAChRs), systemic administration of the partial nAChR agonist ABT-089 improved SAT performance in STs and abolished the difference between SAT-associated ACh levels in STs and GTs. Neither the nonselective nAChR agonist nicotine nor the psychostimulant amphetamine improved SAT performance. These findings suggest that individuals who have a propensity to attribute high-incentive salience to reward cues also exhibit relatively poor attentional control. A combination of these traits may render individuals especially vulnerable to disorders, such as obesity and addiction.

Nicotinic acetylcholine receptors controlling attention: behavior, circuits and sensitivity to disruption by nicotine

Attention is a central cognitive function that enables long-term engagement in a task and suppression of irrelevant information to obtain future goals. The prefrontal cortex (PFC) is the main link in integrating emotional and motivational state of an animal to regulate top-down attentional processes. Acetylcholine modulates PFC neuronal networks by activating nicotinic acetylcholine receptors (nAChRs) to support attention. However, how neuronal activity changes in the PFC during attention and which nAChR subtypes mediate this is only rudimentarily understood, but progress is being made. Recently, exciting new insights were obtained in the dynamics of cholinergic signaling in the PFC and modes of acetylcholine transmission via nAChRs in the cortex. In addition, mechanisms are uncovered on how the PFC circuitry is regulated by nAChRs. Novel studies show that endogenous activation of nAChRs in the PFC plays a central role in controlling attention. Here, we review current insights into how different subtypes of nAChRs expressed by distinct types of neurons in the PFC circuitry shape attention. In addition we discuss the impact of nicotine on the cholinergic system and prefrontal cortical circuits. Low concentrations of nicotine, as experienced by smokers, interfere with cholinergic signaling. In the long-term exposure to nicotine during adolescence leads to maladaptive adaptations of the PFC circuitry, which ultimately leads to a decrement in attention performance, again emphasizing the importance of nAChRs in attention.

The presynaptic choline transporter imposes limits on sustained cortical acetylcholine release and attention

Functional variation in the gene encoding the presynaptic choline transporter (CHT) has been linked to attention-deficit/hyperactivity disorder. Here, we report that a heterozygous deletion in the CHT gene in mice (CHT(+/-)) limits the capacity of cholinergic neurons to sustain acetylcholine (ACh) release and attentional performance. Cortical microdialysis and amperometric methods revealed that, whereas wild-type and CHT(+/-) animals support equivalent basal ACh release and choline clearance, CHT(+/-) animals exhibit a significant inability to elevate extracellular ACh following basal forebrain stimulation, in parallel with a diminished choline clearance capacity following cessation of stimulation. Consistent with these findings, the density of CHTs in cortical synaptosomal plasma membrane-enriched fractions from unstimulated CHT(+/-) animals matched those observed in wild-type animals despite reductions in CHT levels in total extracts, achieved via a redistribution of CHT from vesicle pools. As a consequence, in CHT(+/-) animals, basal forebrain stimulation was unable to mobilize wild-type quantities of CHT to the plasma membrane. In behavioral studies, CHT(+/-) mice were impaired in performing a sustained attention task known to depend on cortical cholinergic activity. In wild-type mice, but not CHT(+/-) mice, attentional performance increased the density of CHTs in the synaptosomal membrane in the right frontal cortex. Basal CHT levels in vesicle-enriched membranes predicted the degree of CHT mobilization as well as individual variations in performance on the sustained attention task. Our findings demonstrate biochemical and physiological alterations that underlie cognitive impairments associated with genetically imposed reductions in choline uptake capacity.

Increased distractor vulnerability but preserved vigilance in patients with schizophrenia: evidence from a translational Sustained Attention Task

OBJECTIVE

Attentional deficits represent a core cognitive impairment in schizophrenia. The distractor condition Sustained Attention Task (dSAT) has been identified by the Cognitive Neuroscience Treatment to Improve Cognition in Schizophrenia (CNTRICS) initiative as a promising translational task for assessing schizophrenia-related deficits in attentional selection-control, identifying neuroimaging biomarkers of such deficits, and for preclinical animal research on potential pro-cognitive treatments. Here, we examined whether patients would show specific difficulties in selection-control and in avoiding distraction in the dSAT.

METHOD

Selection-control deficits are measured by comparing attentional performance in the Sustained Attention Task (SAT) without distraction to performance on the task when distraction is present (dSAT). The baseline SAT condition can also be used to assess time-on-task or vigilance effects. Patients with schizophrenia, age- and gender-matched healthy controls and, as an additional control, school-aged children were tested on both the SAT and dSAT.

RESULTS

Compared to healthy controls, patients had reduced performance overall and were differentially vulnerable to distraction. In contrast, patients but not children had preserved vigilance over time.

CONCLUSION

These results demonstrate specific input-selection control impairments in schizophrenia and suggest that patients' distraction-related impairments can be distinguished from general performance impairments and from deficits in other attentional processes (e.g., sustaining attention) evident in other groups.

Leveraging the cortical cholinergic system to enhance attention

Attentional impairments are found in a range of neurodegenerative and neuropsychiatric disorders. However, the development of procognitive enhancers to alleviate these impairments has been hindered by a lack of comprehensive hypotheses regarding the circuitry mediating the targeted attentional functions. Here we discuss the role of the cortical cholinergic system in mediating cue detection and attentional control and propose two target mechanisms for cognition enhancers: stimulation of prefrontal α4β2* nicotinic acetylcholine receptors (nAChR) for the enhancement of cue detection and augmentation of tonic acetylcholine levels for the enhancement of attentional control. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Diminished trkA receptor signaling reveals cholinergic-attentional vulnerability of aging

The cellular mechanisms underlying the exceptional vulnerability of the basal forebrain (BF) cholinergic neurons during pathological aging have remained elusive. Here we employed an adeno-associated viral vector-based RNA interference (AAV-RNAi) strategy to suppress the expression of tropomyosin-related kinase A (trkA) receptors by cholinergic neurons in the nucleus basalis of Meynert/substantia innominata (nMB/SI) of adult and aged rats. Suppression of trkA receptor expression impaired attentional performance selectively in aged rats. Performance correlated with trkA levels in the nMB/SI. trkA knockdown neither affected nMB/SI cholinergic cell counts nor the decrease in cholinergic cell size observed in aged rats. However, trkA suppression augmented an age-related decrease in the density of cortical cholinergic processes and attenuated the capacity of cholinergic neurons to release acetylcholine (ACh). The capacity of cortical synapses to release ACh in vivo was also lower in aged/trkA-AAV-infused rats than in aged or young controls, and it correlated with their attentional performance. Furthermore, age-related increases in cortical proNGF and p75 receptor levels interacted with the vector-induced loss of trkA receptors to shift NGF signaling toward p75-mediated suppression of the cholinergic phenotype, thereby attenuating cholinergic function and impairing attentional performance. These effects model the abnormal trophic regulation of cholinergic neurons and cognitive impairments in patients with early Alzheimer's disease. This rat model is useful for identifying the mechanisms rendering aging cholinergic neurons vulnerable as well as for studying the neuropathological mechanisms that are triggered by disrupted trophic signaling.

Time to pay attention: attentional performance time-stamped prefrontal cholinergic activation, diurnality, and performance

Although the impairments in cognitive performance that result from shifting or disrupting daily rhythms have been demonstrated, the neuronal mechanisms that optimize fixed-time daily performance are poorly understood. We previously demonstrated that daily practice of a sustained attention task (SAT) evokes a diurnal activity pattern in rats. Here, we report that SAT practice at a fixed time produced practice time-stamped increases in prefrontal cholinergic neurotransmission that persisted after SAT practice was terminated and in a different environment. SAT time-stamped cholinergic activation occurred regardless of whether the SAT was practiced during the light or dark phase or in constant-light conditions. In contrast, prior daily practice of an operant schedule of reinforcement, albeit generating more rewards and lever presses per session than the SAT, neither activated the cholinergic system nor affected the animals' nocturnal activity pattern. Likewise, food-restricted animals exhibited strong food anticipatory activity (FAA) and attenuated activity during the dark phase but FAA was not associated with increases in prefrontal cholinergic activity. Removal of cholinergic neurons impaired SAT performance and facilitated the reemergence of nocturnality. Shifting SAT practice away from a fixed time resulted in significantly lower performance. In conclusion, these experiments demonstrated that fixed-time, daily practice of a task assessing attention generates a precisely practice time-stamped activation of the cortical cholinergic input system. Time-stamped cholinergic activation benefits fixed-time performance and, if practiced during the light phase, contributes to a diurnal activity pattern.

CNTRICS final biomarker selection: Control of attention

Attention is widely believed to be dysfunctional in schizophrenia. The Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS) group previously concluded that the processes involved in the top-down control of attention are particularly impaired in schizophrenia and should be the focus of future research. These processes determine which sources of input should be attended, linking goal representations in prefrontal cortex with more posterior regions that implement the actual selection of attended information. A more recent meeting of the CNTRICS group assessed several paradigms that might be useful for identifying biomarkers of attentional control and that could be used for treatment development and assessment. Two types of paradigms were identified as being particularly promising. In one approach, neural activity is measured (using electroencephalography or functional magnetic resonance imaging) during the period between an attention-directing cue and a target. In a second approach, neural activity is measured under low- and high-distraction conditions. These approaches make it possible to identify the goal representations that guide attention and the interactions between these goal representations and the implementation of selection. Although more basic science research with healthy volunteers and preclinical research with schizophrenia patients is needed before these paradigms will be ready to provide clinically useful biomarkers, they hold substantial promise for aiding in the development and assessment of new treatments.

Deficits in attentional control: cholinergic mechanisms and circuitry-based treatment approaches

The cognitive control of attention involves maintaining task rules in working memory (or "online"), monitoring reward and error rates, filtering distractors, and suppressing prepotent, and competitive responses. Weak attentional control increases distractibility and causes attentional lapses, impulsivity, and attentional fatigue. Levels of tonic cholinergic activity (changes over tens of seconds or minutes) modulate cortical circuitry as a function of the demands on cognitive control. Increased cholinergic modulation enhances the representation of cues, by augmenting cue-evoked activity in thalamic glutamatergic afferents, thereby increasing the rate of detection. Such cholinergic modulation is mediated primarily via α4β2* nicotinic acetylcholine receptors. Animal experiments and clinical trials in adult patients with ADHD indicate that attentional symptoms and disorders may benefit from drugs that stimulate this receptor. Tonic cholinergic modulation of cue-evoked glutamatergic transients in prefrontal regions is an essential component of the brain's executive circuitry. This circuitry model guides the development of treatments of deficits in attentional control.

Sustained attention in mice: expanding the translational utility of the SAT by incorporating the Michigan Controlled Access Response Port (MICARP)

Advances in mouse genetic technology have spurred increasing interest in the development of cognitive tasks for mice. Here, we describe and discuss the modifications necessary to adapt a task for the assessment of sustained attention performance for use in mice, including for taxing the top-down control of such performance. The validity of the Sustained Attention Task (SAT), including the distractor version (dSAT), has previously been demonstrated in rats and humans. This task requires moveable or retractable operanda; insertion of operanda into the operant chambers cues animals to respond to a prior signal or non-signal event, reporting either a hit or a miss, or a correct rejection or false alarm, respectively. Retractable levers did not support sufficiently high and stable levels of performance in mice. Given the widespread use of static nose-poke devices for testing operant performance in mice, we therefore designed and fabricated a retractable nose-poke device. As this device extends into chambers, a hole for nose-poking is slowly opened and closed again as the device retracts (termed the "Michigan Controlled Access Response Port", MICARP). Results describe the effects of variation of signal duration and event rate, trial outcome and trial type probability, effects of mice deprivation levels, and the reliability of SAT and dSAT performance. Mice perform the SAT and dSAT at levels comparable to those observed in rats. This task will be of assistance in expanding the translational usefulness of the SAT and dSAT.

Galantamine potentiates the protective effect of rofecoxib and caffeic acid against intrahippocampal Kainic acid-induced cognitive dysfunction in rat

Role of neuroinflammatory mediators particularly cyclooxygenase (COX), lipoxygenase (LOX), have been well suggested in the pathophysiology of neurodegenerative disorders. Rofecoxib is a selective cyclooxygenase 2 enzymes belongs to non-steroidal anti-inflammatory drug, commonly called as coxibs. Whereas, caffeic acid (3,4-dihydroxycinnamic acid) is one of the natural phenolic compounds and reported to inhibit 5-lipoxygenase (5-LOX) activity as one of mechanisms. Present study has been designed to investigate the effects of rofecoxib, caffeic acid and its potentiation by galantamine against intrahippocampal kainic acid-induced cognitive impairment, oxidative damage and mitochondrial respiratory enzyme alterations in rats. Kainic acid (KA) was administrated in the hippocampus region of rat brain. Various behavioral (locomotor activity and memory performances were assessed by using actophotometer and Morris water maze respectively) followed by oxidative stress, mitochondrial enzyme complex were assessed. Intrahippocampal administration of KA significantly impaired locomotor activity, memory performance, mitochondrial enzyme complexes and caused oxidative stress as compared to sham treatment. Rofecoxib (5 and 10mg/kg), caffeic acid (5 and 10mg/kg), Gal (2.5 and 5mg/kg) treatment for 14 days significantly improved locomotor activity, memory retention and oxidative defense (as evidenced by decrease lipid peroxidation, nitrite, increased superoxide dismutase activity and redox ratio) in hippocampus. Besides, alterations in the levels of mitochondrial enzymes and acetylcholine esterase enzyme were significantly restored by rofecoxib and caffeic acid as compared to control. Further, combination of rofecoxib (5mg/kg) with caffeic acid (5mg/kg) and lower dose of gal (2.5mg/kg) with rofecoxib (5mg/kg) treatments significantly potentiated their protective effect which was significant as compared to their effect per se. The results of the present study suggest that galantamine potentiates the protective effect of rofecoxib and caffeic acid against kainic acid induced cognitive impairment and associated oxidative damage.

Challenges to attention: a continuous arterial spin labeling (ASL) study of the effects of distraction on sustained attention

Maintaining attention and performance over time is an essential part of many activities, and effortful cognitive control is required to avoid vigilance decrements and interference from distraction. Regions at or near right middle frontal gyrus (Brodmann's area (BA) 9), as well as in other prefrontal and parietal areas, are often activated in studies of sustained attention (e.g., Cabeza and Nyberg, 2000; Kim et al., 2006; Lim et al., 2010). This activation has often been interpreted as representing the engagement of cognitive control processes. However, such studies are typically implemented at one level of task difficulty, without an experimental manipulation of control demands. The present study used the distractor condition sustained attention task (dSAT), which has been used extensively in animals to determine the role of neuromodulator systems in attentional performance, to test the hypotheses that BA 9 is sensitive to changes in the demand for cognitive control and that this sensitivity reflects an increased engagement of attentional effort. Continuous arterial spin labeling (ASL) was used to measure neural activity in sixteen healthy, young adults performing a sustained attention task under standard conditions and under a distraction condition that provided an experimental manipulation of demands on cognitive control. The distractor impaired behavioral performance and increased activation in right middle frontal gyrus. Larger increases in right middle frontal gyrus activity were associated with greater behavioral vulnerability to the distractor. These findings indicate that while right middle frontal gyrus regions are sensitive to demands for attentional effort and control, they may not be sufficient to maintain performance under challenge. In addition, they demonstrate the sensitivity of ASL methods to variations in task demands, and suggest that the dSAT may be a useful tool for translational cross-species and clinical research.

Modes and models of forebrain cholinergic neuromodulation of cognition

As indicated by the profound cognitive impairments caused by cholinergic receptor antagonists, cholinergic neurotransmission has a vital role in cognitive function, specifically attention and memory encoding. Abnormally regulated cholinergic neurotransmission has been hypothesized to contribute to the cognitive symptoms of neuropsychiatric disorders. Loss of cholinergic neurons enhances the severity of the symptoms of dementia. Cholinergic receptor agonists and acetylcholinesterase inhibitors have been investigated for the treatment of cognitive dysfunction. Evidence from experiments using new techniques for measuring rapid changes in cholinergic neurotransmission provides a novel perspective on the cholinergic regulation of cognitive processes. This evidence indicates that changes in cholinergic modulation on a timescale of seconds is triggered by sensory input cues and serves to facilitate cue detection and attentional performance. Furthermore, the evidence indicates cholinergic induction of evoked intrinsic, persistent spiking mechanisms for active maintenance of sensory input, and planned responses. Models have been developed to describe the neuronal mechanisms underlying the transient modulation of cortical target circuits by cholinergic activity. These models postulate specific locations and roles of nicotinic and muscarinic acetylcholine receptors and that cholinergic neurotransmission is controlled in part by (cortical) target circuits. The available evidence and these models point to new principles governing the development of the next generation of cholinergic treatments for cognitive disorders.

Cholinergic contributions to the cognitive symptoms of schizophrenia and the viability of cholinergic treatments

Effective treatment of the cognitive symptoms of schizophrenia has remained an elusive goal. Despite the intense focus on treatments acting at or via cholinergic mechanisms, little remains known about the dynamic cholinergic abnormalities that contribute to the manifestation of the cognitive symptoms in patients. Evidence from basic neuroscientific and psychopharmacological investigations assists in proposing detailed cholinergic mechanisms and treatment targets for enhancement of attentional performance. Dynamic, cognitive performance-dependent abnormalities in cholinergic activity have been observed in animal models of the disorder and serve to further refine such proposals. Finally, the potential usefulness of individual groups of cholinergic drugs and important issues concerning the interactions between pro-cholinergic and antipsychotic treatments are addressed. The limited evidence available from patient studies and animal models indicates pressing research needs in order to guide the development of cholinergic treatments of the cognitive symptoms of schizophrenia.

Confocal Analysis of Cholinergic and Dopaminergic Inputs onto Pyramidal Cells in the Prefrontal Cortex of Rodents

Cholinergic and dopaminergic projections to the rat medial prefrontal cortex (mPFC) are both involved in cognitive functions including attention. These neuronal systems modulate mPFC neuronal activity mainly through diffuse transmission. In order to better understand the anatomical level of influence of these systems, confocal microscopy with triple-fluorescent immunolabeling was used in three subregions of the mPFC of rats and Drd1a-tdTomato/Drd2-EGFP transgenic mice. The zone of interaction was defined as a reciprocal microproximity between dopaminergic and cholinergic axonal segments as well as pyramidal neurons. The density of varicosities, along these segments was considered as a possible activity-dependant morphological feature. The percentage of cholinergic and dopaminergic fibers in microproximity ranged from 12 to 40% depending on the layer and mPFC subregion. The cholinergic system appeared to have more influence on dopaminergic fibers since a larger proportion of the dopaminergic fibers were within microproximity to cholinergic fibers. The density of both cholinergic and dopaminergic varicosities was significantly elevated within microproximities. The main results indicate that the cholinergic and dopaminergic systems converge on pyramidal cells in mPFC particularly in the layer V. In transgenic mice 93% of the pyramidal cells expressed the transgenic marker for Drd2 expression, but only 22% expressed the maker for Drd1ar expression. Data presented here suggest that the modulation of mPFC by dopaminergic fibers would be mostly inhibitory and localized at the output level whereas the cholinergic modulation would be exerted at the input and output level both through direct interaction with pyramidal cells and dopaminergic fibers.

Enhancement of attentional performance by selective stimulation of alpha4beta2(*) nAChRs: underlying cholinergic mechanisms

Impairments in attention are a major component of the cognitive symptoms of neuropsychiatric and neurodegenerative disorders. Using an operant sustained attention task (SAT), including a distractor condition (dSAT), we assessed the putative pro-attentional effects of the selective alpha4beta2(*) nicotinic acetylcholine receptor (nAChR) agonist S 38232 in comparison with the non-selective agonist nicotine. Neither drug benefited SAT performance. However, in interaction with the increased task demands implemented by distractor presentation, the selective agonist, but not nicotine, enhanced the detection of signals during the post-distractor recovery period. This effect is consistent with the hypothesis that second-long increases in cholinergic activity ('transients') mediate the detection of cues and that nAChR agonists augment such transients. Electrochemical recordings of prefrontal cholinergic transients evoked by S 38232 and nicotine indicated that the alpha4beta2(*) nAChR agonist evoked cholinergic transients that were characterized by a faster rise time and more rapid decay than those evoked by nicotine. Blockade of the alpha7 nAChR 'sharpens' nicotine-evoked transients; therefore, we determined the effects of co-administration of nicotine and the alpha7 nAChR antagonist methyllycaconitine on dSAT performance. Compared with vehicle and nicotine alone, this combined treatment significantly enhanced the detection of signals. These results indicate that compared with nicotine, alpha4beta2(*) nAChR agonists significantly enhance attentional performance and that the dSAT represents a useful behavioral screening tool. The combined behavioral and electrochemical evidence supports the hypothesis that nAChR agonist-evoked cholinergic transients, which are characterized by rapid rise time and fast decay, predict robust drug-induced enhancement of attentional performance.

Discovery of 4-(5-methyloxazolo[4,5-b]pyridin-2-yl)-1,4-diazabicyclo[3.2.2]nonane (CP-810,123), a novel alpha 7 nicotinic acetylcholine receptor agonist for the treatment of cognitive disorders in schizophrenia: synthesis, SAR development, and in vivo efficacy in cognition models

A novel alpha 7 nAChR agonist, 4-(5-methyloxazolo[4,5-b]pyridin-2-yl)-1,4-diazabicyclo[3.2.2]nonane (24, CP-810,123), has been identified as a potential treatment for cognitive deficits associated with psychiatric or neurological conditions including schizophrenia and Alzheimer's disease. Compound 24 is a potent and selective compound with excellent pharmaceutical properties. In rodent, the compound displays high oral bioavailability and excellent brain penetration affording high levels of receptor occupancy and in vivo efficacy in auditory sensory gating and novel object recognition. The structural diversity of this compound and its preclinical in vitro and in vivo package support the hypothesis that alpha 7 nAChR agonists may have potential as a pharmacotherapy for the treatment of cognitive deficits in schizophrenia.

Prefrontal beta2 subunit-containing and alpha7 nicotinic acetylcholine receptors differentially control glutamatergic and cholinergic signaling

One-second-long increases in prefrontal cholinergic activity ("transients") were demonstrated previously to be necessary for the incorporation of cues into ongoing cognitive processes ("cue detection"). Nicotine and, more robustly, selective agonists at alpha4beta2* nicotinic acetylcholine receptors (nAChRs) enhance cue detection and attentional performance by augmenting prefrontal cholinergic activity. The present experiments determined the role of beta2-containing and alpha7 nAChRs in the generation of prefrontal cholinergic and glutamatergic transients in vivo. Transients were evoked by nicotine, the alpha4beta2* nAChR agonist ABT-089 [2-methyl-3-(2-(S)-pyrrolindinylmethoxy) pyridine dihydrochloride], or the alpha7 nAChR agonist A-582941 [2-methyl-5-(6-phenyl-pyridazin-3-yl)-octahydro-pyrrolo[3,4-c]pyrrole]. Transients were recorded in mice lacking beta2 or alpha7 nAChRs and in rats after removal of thalamic glutamatergic or midbrain dopaminergic inputs to prefrontal cortex. The main results indicate that stimulation of alpha4beta2* nAChRs evokes glutamate release and that the presence of thalamic afferents is necessary for the generation of cholinergic transients. ABT-089-evoked transients were completely abolished in mice lacking beta2* nAChRs. The amplitude, but not the decay rate, of nicotine-evoked transients was reduced by beta2* knock-out. Conversely, in mice lacking the alpha7 nAChR, the decay rate, but not the amplitude, of nicotine-evoked cholinergic and glutamatergic transients was attenuated. Substantiating the role of alpha7 nAChR in controlling the duration of release events, stimulation of alpha7 nAChR produced cholinergic transients that lasted 10- to 15-fold longer than those evoked by nicotine. alpha7 nAChR-evoked cholinergic transients are mediated in part by dopaminergic activity. Prefrontal alpha4beta2* nAChRs play a key role in evoking and facilitating the transient glutamatergic-cholinergic interactions that are necessary for cue detection and attentional performance.

Second-by-second analysis of alpha 7 nicotine receptor regulation of glutamate release in the prefrontal cortex of awake rats

These experiments utilized an enzyme-based microelectrode selective for the second-by-second detection of extracellular glutamate to reveal the alpha 7-based nicotinic modulation of glutamate release in the prefrontal cortex (PFC) of freely moving rats. Rats received intracortical infusions of the nonselective nicotinic agonist nicotine (12.0 mM, 1.0 microg/0.4 microl) or the selective alpha 7 agonist choline (2.0 mM/0.4 microl). The selectivity of drug-induced glutamate release was assessed in subgroups of animals pretreated with the alpha 7 antagonist, alpha-bungarotoxin (alpha-BGT, 10 microM), or kynurenine (10 microM) the precursor of the astrocyte-derived, negative allosteric alpha 7 modulator kynurenic acid. Local administration of nicotine increased glutamate signals (maximum amplitude = 4.3 +/- 0.6 microM) that were cleared to baseline levels in 493 +/- 80 seconds. Pretreatment with alpha-BGT or kynurenine attenuated nicotine-induced glutamate by 61% and 60%, respectively. Local administration of choline also increased glutamate signals (maximum amplitude = 6.3 +/- 0.9 microM). In contrast to nicotine-evoked glutamate release, choline-evoked signals were cleared more quickly (28 +/- 6 seconds) and pretreatment with alpha-BGT or kynurenine completely blocked the stimulated glutamate release. Using a method that reveals the temporal dynamics of in vivo glutamate release and clearance, these data indicate a nicotinic modulation of cortical glutamate release that is both alpha 7- and non-alpha 7-mediated. Furthermore, these data may also provide a mechanism underlying the recent focus on alpha 7 full and partial agonists as therapeutic agents in the treatment of cortically mediated cognitive deficits in schizophrenia.

Cognitive enhancement as a pharmacotherapy target for stimulant addiction

BACKGROUND

No medications have been proven to be effective for cocaine and methamphetamine addiction. Attenuation of drug reward has been the main strategy for medications development, but this approach has not led to effective treatments. Thus, there is a need to identify novel treatment targets in addition to the brain reward system.

AIM

To propose a novel treatment strategy for stimulant addiction that will focus on medications enhancing cognitive function and attenuating drug reward.

METHODS

Pre-clinical and clinical literature on potential use of cognitive enhancers for stimulant addiction pharmacotherapy was reviewed.

RESULTS AND CONCLUSIONS

Cocaine and methamphetamine users show significant cognitive impairments, especially in attention, working memory and response inhibition functions. The cognitive impairments seem to be predictive of poor treatment retention and outcome. Medications targeting acetylcholine and norepinephrine are particularly well suited for enhancing cognitive function in stimulant users. Many cholinergic and noradrenergic medications are on the market and have a good safety profile and low abuse potential. These include galantamine, donepezil and rivastigmine (cholinesterase inhibitors), varenicline (partial nicotine agonist), guanfacine (alpha(2)-adrenergic agonist) and atomoxetine (norepinephrine transporter inhibitor). Future clinical studies designed optimally to measure cognitive function as well as drug use behavior would be needed to test the efficacy of these cognitive enhancers for stimulant addiction.

Disruption of mesolimbic regulation of prefrontal cholinergic transmission in an animal model of schizophrenia and normalization by chronic clozapine treatment

Abnormal mesolimbic control of cortical cholinergic activity has been hypothesized to contribute to the cognitive symptoms of schizophrenia. Stimulation of NMDA receptors in nucleus accumbens (NAC) increases acetylcholine (ACh) release in prefrontal cortex (PFC), an activation thought to contribute to attentional processing. Thus, the effects of intra-NAC perfusion of NMDA (250-400 microM) on ACh release in PFC were determined in rats receiving lesions of the ventral hippocampus (VH) as neonates (nVHLX), a neurodevelopmental model of schizophrenia, or as adults (aVHLX). NMDA elevated ACh release (100-150% above baseline) in adults sham-lesioned as neonates or in aVHLX rats. Adult nVHLX were unresponsive to NAC NMDA receptor stimulation. The inability of nVHLX to respond to NMDA emerged over development as a separate experiment demonstrated that evoked ACh release was normal before puberty (100-150% increase) yet, in these same nVHLX animals, absent after puberty. Amphetamine-evoked ACh release was assessed in nVHLX animals to exclude potential limitations in release capacity. Amphetamine produced greater increases in ACh release than in shams, indicating that nVHLX does not impair the capacity of cholinergic neurons to release ACh. Finally, the ability of 13 days of pretreatment with clozapine (1.25 mg/kg/day) to reinstate NMDA-evoked cortical ACh efflux was determined. Clozapine treatment normalized NMDA-evoked ACh release in nVHLX animals. These experiments show that mesolimbic regulation of cortical ACh release is disrupted in postpubertal nVHLX rats and normalized by low-dose treatment of clozapine; supporting the usefulness of nVHLX animals for research on the neuronal mechanisms underlying the cognitive symptoms of schizophrenia.

Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the alpha7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S-ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor L-kynurenine. At 5 mm, S-ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 nm), which by itself tended to reduce basal ACh levels, blocked the ability of S-ESBA to raise extracellular ACh levels. KYNA perfusion (100 nm) also prevented the evoked ACh release caused by d-amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

nAChR agonist-induced cognition enhancement: integration of cognitive and neuronal mechanisms

The identification and characterization of drugs for the treatment of cognitive disorders has been hampered by the absence of comprehensive hypotheses. Such hypotheses consist of (a) a precisely defined cognitive operation that fundamentally underlies a range of cognitive abilities and capacities and, if impaired, contributes to the manifestation of diverse cognitive symptoms; (b) defined neuronal mechanisms proposed to mediate the cognitive operation of interest; (c) evidence indicating that the putative cognition enhancer facilitates these neuronal mechanisms; (d) and evidence indicating that the cognition enhancer facilitates cognitive performance by modulating these underlying neuronal mechanisms. The evidence on the neuronal and attentional effects of nAChR agonists, specifically agonists selective for alpha4beta2* nAChRs, has begun to support such a hypothesis. nAChR agonists facilitate the detection of signals by augmenting the transient increases in prefrontal cholinergic activity that are necessary for a signal to gain control over behavior in attentional contexts. The prefrontal microcircuitry mediating these effects include alpha4beta2* nAChRs situated on the terminals of thalamic inputs and the glutamatergic stimulation of cholinergic terminals via ionotropic glutamate receptors. Collectively, this evidence forms the basis for hypothesis-guided development and characterization of cognition enhancers.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Towards an animal model of an antipsychotic drug-resistant cognitive impairment in schizophrenia: scopolamine induces abnormally persistent latent inhibition, which can be reversed by cognitive enhancers but not by antipsychotic drugs

Schizophrenia symptoms segregate into positive, negative and cognitive, which exhibit differential sensitivity to drugs. Recent efforts to identify treatments targeting cognitive impairments in schizophrenia have directed attention to the cholinergic system for its well documented role in cognition. Relatedly, muscarinic antagonists (e.g. scopolamine) produce an 'antimuscarinic syndrome', characterized by psychosis and cognitive impairments. Latent inhibition (LI) is the poorer conditioning to a stimulus resulting from its non-reinforced pre-exposure. LI indexes the ability to ignore irrelevant stimuli and aberrations of this capacity produced by pro-psychotic agents (e.g. amphetamine, MK-801) are used extensively to model attentional impairments in schizophrenia. We recently showed that LI was disrupted by scopolamine at low doses, and this was reversed by typical and atypical antipsychotic drugs (APDs) and the acetylcholinesterase inhibitor physostigmine. Here, at a higher dose (1.5 mg/kg), scopolamine produced an opposite pole of attentional impairment, namely, attentional perseveration, whereby scopolamine-treated rats persisted in expressing LI under strong conditioning that prevented LI expression in controls. Scopolamine-induced persistent LI was reversed by cholinergic and glycinergic cognitive enhancers (physostigmine and glycine) but was resistant to both typical and atypical APDs (haloperidol and clozapine). The latter sets scopolamine-induced persistent LI apart from scopolamine- and amphetamine-induced disrupted LI, which are reversed by both typical and atypical APDs, as well as from other cases of abnormally persistent LI including MK-801-induced persistent LI, which is reversed by atypical APDs. Thus, scopolamine-induced persistent LI may provide a pharmacological LI model for screening cognitive enhancers that are efficient for the treatment of APD-resistant cognitive impairments in schizophrenia.

CNTRICS final task selection: control of attention

The construct of attention has many facets that have been examined in human and animal research and in healthy and psychiatrically disordered conditions. The Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia (CNTRICS) group concluded that control of attention-the processes that guide selection of task-relevant inputs-is particularly impaired in schizophrenia and could profit from further work with refined measurement tools. Thus, nominations for cognitive tasks that provide discrete measures of control of attention were sought and were then evaluated at the third CNTRICS meeting for their promise for future use in treatment development. This article describes the 5 nominated measures and their strengths and weaknesses for cognitive neuroscience work relevant to treatment development. Two paradigms, Guided Search and the Distractor Condition Sustained Attention Task, were viewed as having the greatest immediate promise for development into tools for treatment research in schizophrenia and are described in more detail by their nominators.

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.

Persistent alterations in cognitive function and prefrontal dopamine D2 receptors following extended, but not limited, access to self-administered cocaine

Drug addicts have deficits in frontocortical function and cognition even long after the discontinuation of drug use. It is not clear, however, whether the cognitive deficits are a consequence of drug use, or are present prior to drug use, and thus are a potential predisposing factor for addiction. To determine if self-administration of cocaine is capable of producing long-lasting alterations in cognition, rats were allowed access to cocaine for either 1 h/day (short access, ShA) or 6 h/day (long access, LgA) for 3 weeks. Between 1 and 30 days after the last self-administration session, we examined performance on a cognitively demanding test of sustained attention that requires an intact medial prefrontal cortex. The expression levels of dopamine D1 and D2 receptor mRNA and D2 protein in the prefrontal cortex were also examined. Early after discontinuation of drug use, LgA (but not ShA) animals were markedly impaired on the sustained attention task. Although the LgA animals improved over time, they continued to show a persistent pattern of performance deficits indicative of a disruption of cognitive flexibility up to 30 days after the discontinuation of drug use. This was accompanied by a significant decrease in DA D2 (but not D1) mRNA in the medial and orbital prefrontal cortex, and D2 receptor protein in the medial prefrontal cortex of LgA (but not ShA) animals. These findings establish that repeated cocaine use is capable of producing persistent alterations in the prefrontal cortex and in cognitive function, and illustrate the usefulness of extended access self-administration procedures for studying the neurobiology of addiction.

The biochemical womb of schizophrenia: A review

The conclusive identification of specific etiological factors or pathogenic processes in the illness of schizophrenia has remained elusive despite great technological progress. The convergence of state-of-art scientific studies in molecular genetics, molecular neuropathophysiology, in vivo brain imaging and psychopharmacology, however, indicates that we may be coming much closer to understanding the genesis of schizophrenia. In near future, the diagnosis and assessment of schizophrenia using biochemical markers may become a "dream come true" for the medical community as well as for the general population. An understanding of the biochemistry/ visa vis pathophysiology of schizophrenia is essential to the discovery of preventive measures and therapeutic intervention.

Detection of the moderately beneficial cognitive effects of low-dose treatment with haloperidol or clozapine in an animal model of the attentional impairments of schizophrenia

The absence of effective cognition enhancers for the treatment of patients with schizophrenia limits the validation of animal models and behavioral tests used for drug finding and characterization. However, low doses of haloperidol and clozapine were documented to produce moderately beneficial effects in patients. Therefore, this experiment was designed to determine the attentional effects of such treatments in a repeated-amphetamine (AMPH) animal model. Animals were trained in an operant-sustained attention task and underwent a 40-day pretreatment period with saline or increasing doses (1-10 mg per kg) of AMPH. After regaining baseline performance following 10 days of saline treatment, animals were treated with haloperidol (0.025 mg per kg), clozapine (2.5 mg per kg), or vehicle for 10 days. Furthermore, the effects of AMPH challenges (1.0 mg per kg) were assessed. In AMPH-pretreated animals, the administration of AMPH challenges resulted in the disruption of attentional performance. Treatment with haloperidol and clozapine attenuated the detrimental performance effects of these challenges, with clozapine exhibiting more robust attenuation. Furthermore, clozapine, but not haloperidol, impaired the performance of control animals. In contrast, the performance of AMPH-pretreated animals remained unaffected by clozapine. As this animal model detects the moderately beneficial cognitive effects of haloperidol and clozapine, it may be useful for preclinical research designed to detect and characterize treatments for the cognitive symptoms of schizophrenia.

Cerebrospinal fluid homovanillic acid is correlated to psychotic features in neurological patients with delirium

OBJECTIVE

The aim of this study was to determine if cerebrospinal fluid (CSF) levels of homovanillic acid (HVA) are related to the clinical features of delirium in a group of patients with acute onset neurological illness.

METHODS

Fifty-one patients with probable acute brain infection were classified as delirious and nondelirious according to Diagnostic and Statistical Manual for Mental Disorders, Fourth Edition (DSM-IV) and Delirium Rating Scale (DRS). CSF HVA concentration was analyzed by high-performance liquid chromatography.

RESULTS

Delirium was present in 60.8% of the total sample. HVA levels were not significantly different between delirious and nondelirious patients. Remarkably, patients with psychotic symptoms shown higher levels of CSF HVA as compared to nonpsychotic patient values. In addition, HVA levels were positively correlated to specific items of DRS such as delusions (r=0.463, P=.001), hallucinations (r=0.438, P=.001), cognitive dysfunction (r=0.286, P=.042) and fluctuation of symptoms (r=0.280, P=.046) in the total sample. Subanalyses excluding patients taking antipsychotic drugs revealed that HVA CSF levels were higher in those patients with delusions, and furthermore, the dopamine metabolite remained positively correlated to delusion subscale of DRS.

CONCLUSIONS

Our results suggest that psychotic symptoms in delirious patients may be related to increased dopamine neurotransmission, as reflected by increased CSF HVA concentration, providing direct evidence to support the dopaminergic theory of psychosis.

Abnormal neurotransmitter release underlying behavioral and cognitive disorders: toward concepts of dynamic and function-specific dysregulation

Abnormalities in the regulation of neurotransmitter release and/or abnormal levels of extracellular neurotransmitter concentrations have remained core components of hypotheses on the neuronal foundations of behavioral and cognitive disorders and the symptoms of neuropsychiatric and neurodegenerative disorders. Furthermore, therapeutic drugs for the treatment of these disorders have been developed and categorized largely on the basis of their effects on neurotransmitter release and resulting receptor stimulation. This perspective stresses the theoretical and practical implications of hypotheses that address the dynamic nature of neurotransmitter dysregulation, including the multiple feedback mechanisms regulating synaptic processes, phasic and tonic components of neurotransmission, compartmentalized release, differentiation between dysregulation of basal vs activated release, and abnormal release from neuronal systems recruited by behavioral and cognitive activity. Several examples illustrate that the nature of the neurotransmitter dysregulation in animal models, including the direction of drug effects on neurotransmitter release, depends fundamentally on the state of activity of the neurotransmitter system of interest and on the behavioral and cognitive functions recruiting these systems. Evidence from evolving techniques for the measurement of neurotransmitter release at high spatial and temporal resolution is likely to advance hypotheses describing the pivotal role of neurotransmitter dysfunction in the development of essential symptoms of major neuropsychiatric disorders, and also to refine neuropharmacological mechanisms to serve as targets for new treatment approaches. The significance and usefulness of hypotheses concerning the abnormal regulation of the release of extracellular concentrations of primary messengers depend on the effective integration of emerging concepts describing the dynamic, compartmentalized, and activity-dependent characteristics of dysregulated neurotransmitter systems.