Dopamine release in the nucleus accumbens: the perspective from aberrations of consciousness in schizophrenia

Linking Anxiolytic Action to Hippocampal "Theta"-A Personal History

This paper provides a personal history of work starting with the discovery that anxiolytic drugs reduce hippocampal theta frequency. It includes parallel work on septal elicitation of theta carried out in Jeffrey Gray's laboratory in Oxford; a statement of my original scientific perspective on the work; and a description of later work in my laboratory in New Zealand confirming the function of theta rhythmicity per se and its mediation of the effects of anxiolytic drugs on behavior. I finish with comments on risk management with such experiments and their use in larger scale theory development.

GABAergic Neurons in the Nucleus Accumbens are Involved in the General Anesthesia Effect of Propofol

The mechanism underlying the hypnosis effect of propofol is still not fully understood. In essence, the nucleus accumbens (NAc) is crucial for regulating wakefulness and may be directly engaged in the principle of general anesthesia. However, the role of NAc in the process of propofol-induced anesthesia is still unknown. We used immunofluorescence, western blotting, and patch-clamp to access the activities of NAc GABAergic neurons during propofol anesthesia, and then we utilized chemogenetic and optogenetic methods to explore the role of NAc GABAergic neurons in regulating propofol-induced general anesthesia states. Moreover, we also conducted behavioral tests to analyze anesthetic induction and emergence. We found out that c-Fos expression was considerably dropped in NAc GABAergic neurons after propofol injection. Meanwhile, patch-clamp recording of brain slices showed that firing frequency induced by step currents in NAc GABAergic neurons significantly decreased after propofol perfusion. Notably, chemically selective stimulation of NAc GABAergic neurons during propofol anesthesia lowered propofol sensitivity, prolonged the induction of propofol anesthesia, and facilitated recovery; the inhibition of NAc GABAergic neurons exerted opposite effects. Furthermore, optogenetic activation of NAc GABAergic neurons promoted emergence whereas the result of optogenetic inhibition was the opposite. Our results demonstrate that NAc GABAergic neurons modulate propofol anesthesia induction and emergence.

A potential link between gambling addiction severity and central dopamine levels: Evidence from spontaneous eye blink rates

Accumulating evidence points at similarities between substance use disorders (SUD) and gambling disorder on the behavioral and neural level. In SUD, attenuation of striatal D2/3-receptor availability is a consistent finding, at least for stimulating substances. For gambling disorder, no clear association with striatal D2/3-receptor availability has been unveiled so far. With its presumably negligible dopaminergic toxicity, possible differences in receptor availability in gambling disorder might constitute a vulnerability marker. Spontaneous eye blink rate (sEBR) is discussed as a potential proxy measure for striatal dopamine D2/3-receptor availability. Here we examined sEBR in 21 male problem gamblers and 20 healthy control participants. In addition, participants completed a screening questionnaire for overall psychopathology and self-reported measures of alcohol and nicotine consumption. We found no significant difference in sEBR between gamblers and controls. However, in gamblers, sEBR was negatively associated with gambling severity and positively associated with psychopathology. A final exploratory analysis revealed that healthy controls with low sEBR displayed higher alcohol and nicotine consumption than healthy participants with high sEBR. Although the exact association between dopamine transmission and sEBR is still debated, our findings reveal that sEBR is sensitive to inter-individual differences in gambling disorder severity in problem gamblers.

Immersion in altered experience: An investigation of the relationship between absorption and psychopathology

Understanding alterations in perceptual experiences as a component of the basic symptom structure of psychosis may improve early detection and the identification of subtle shifts that can precede symptom onset or exacerbation. We explored the phenomenological construct of absorption and psychotic experiences in both clinical (bipolar psychosis and schizophrenia spectrum) and non-clinical participants. Participants with psychosis endorsed significantly higher absorption compared to the non-clinical group. Absorption was positively correlated with all types of hallucinations and multiple types of delusions. The analysis yielded two distinct cluster groups that demarcated a distinction along the continuum of self-disturbance: on characterized by attenuated ego boundaries and the other stable ego boundaries. The study suggests that absorption is a potentially important but under-researched component of psychosis that overlaps with, but is not identical to the more heavily theorized constructs of aberrant salience and hyperreflexivity.

Conflict adaptation in schizophrenia: reviewing past and previewing future efforts

INTRODUCTION

Cognitive control impairments have been suggested to be a critical component in the overall cognitive deficits observed in patients diagnosed with schizophrenia. Here, we zoom in on a specific function of cognitive control, conflict adaptation. Abnormal neural activity patterns have been observed for patients diagnosed with schizophrenia in core conflict adaptation areas such as anterior cingulate cortex and prefrontal cortex. On the one hand, this strongly indicates that conflict adaptation is affected. On the other hand, however, outcomes at the behavioural level are needed to create a window into a precise interpretation of this abnormal neural activity.

METHODS

We present a narrative review of behavioural work within the context of conflict adaptation in schizophrenia, focusing on various major conflict adaptation markers: congruency sequence effects, proportion congruency effects, and post-error and post-conflict slowing. The review emphasises both methodological and theoretical aspects that are relevant to the understanding of conflict adaptation in schizophrenia.

RESULTS

Based on the currently available set of behavioural studies on conflict adaptation, no clear-cut answer can be provided as to the precise conflict adaptation processes that are impaired (and to what extent) in schizophrenia populations.

CONCLUSIONS

Future work is needed in state-of-the-art designs in order to reach better insight into the specifics of conflict adaptation impairments associated with schizophrenia.

Fluoxetine Treatment Rescues Energy Metabolism Pathway Alterations in a Posttraumatic Stress Disorder Mouse Model

Posttraumatic stress disorder (PTSD) is a prevalent psychiatric disorder. Several studies have attempted to characterize molecular alterations associated with PTSD, but most findings were limited to the investigation of specific cellular markers in the periphery or defined brain regions. In the current study, we aimed to unravel affected molecular pathways/mechanisms in the fear circuitry associated with PTSD. We interrogated a foot shock-induced PTSD mouse model by integrating proteomics and metabolomics profiling data. Alterations at the proteome level were analyzed using in vivo (15)N metabolic labeling combined with mass spectrometry in the prelimbic cortex (PrL), anterior cingulate cortex (ACC), basolateral amygdala, central nucleus of the amygdala and CA1 of the hippocampus between shocked and nonshocked (control) mice, with and without fluoxetine treatment. In silico pathway analyses revealed an upregulation of the citric acid cycle pathway in PrL, and downregulation in ACC and nucleus accumbens (NAc). Chronic fluoxetine treatment prevented decreased citric acid cycle activity in NAc and ACC and ameliorated conditioned fear response in shocked mice. Our results shed light on the role of energy metabolism in PTSD pathogenesis and suggest potential therapy through mitochondrial targeting.

You looking at me?: Interpreting social cues in schizophrenia

BACKGROUND

Deficits in the perception of social cues are common in schizophrenia and predict functional outcome. While effective communication depends on deciphering both verbal and non-verbal features, work on non-verbal communication in the disorder is scarce.

METHOD

This behavioural study of 29 individuals with schizophrenia and 25 demographically matched controls used silent video-clips to examine gestural identification, its contextual modulation and related metacognitive representations.

RESULTS

In accord with our principal hypothesis, we observed that individuals with schizophrenia exhibited a preserved ability to identify archetypal gestures and did not differentially infer communicative intent from incidental movements. However, patients were more likely than controls to perceive gestures as self-referential when confirmatory evidence was ambiguous. Furthermore, the severity of their current hallucinatory experience inversely predicted their confidence ratings associated with these self-referential judgements.

CONCLUSIONS

These findings suggest a deficit in the contextual refinement of social-cue processing in schizophrenia that is potentially attributable to impaired monitoring of a mirror mechanism underlying intentional judgements, or to an incomplete semantic representation of gestural actions. Non-verbal communication may be improved in patients through psychotherapeutic interventions that include performance and perception of gestures in group interactions.

A Biologically Inspired Computational Model of Basal Ganglia in Action Selection

The basal ganglia (BG) are a subcortical structure implicated in action selection. The aim of this work is to present a new cognitive neuroscience model of the BG, which aspires to represent a parsimonious balance between simplicity and completeness. The model includes the 3 main pathways operating in the BG circuitry, that is, the direct (Go), indirect (NoGo), and hyperdirect pathways. The main original aspects, compared with previous models, are the use of a two-term Hebb rule to train synapses in the striatum, based exclusively on neuronal activity changes caused by dopamine peaks or dips, and the role of the cholinergic interneurons (affected by dopamine themselves) during learning. Some examples are displayed, concerning a few paradigmatic cases: action selection in basal conditions, action selection in the presence of a strong conflict (where the role of the hyperdirect pathway emerges), synapse changes induced by phasic dopamine, and learning new actions based on a previous history of rewards and punishments. Finally, some simulations show model working in conditions of altered dopamine levels, to illustrate pathological cases (dopamine depletion in parkinsonian subjects or dopamine hypermedication). Due to its parsimonious approach, the model may represent a straightforward tool to analyze BG functionality in behavioral experiments.

The dopaminergic response to acute stress in health and psychopathology: A systematic review

Previous work in animals has shown that dopamine (DA) in cortex and striatum plays an essential role in stress processing. For the first time, we systematically reviewed the in vivo evidence for DAergic stress processing in health and psychopathology in humans. All studies included (n studies=25, n observations=324) utilized DA D2/3 positron emission tomography and measured DAergic activity during an acute stress challenge. The evidence in healthy volunteers (HV) suggests that physiological, but not psychological, stress consistently increases striatal DA release. Instead, increased medial prefrontal cortex (mPFC) DAergic activity in HV was observed during psychological stress. Across brain regions, stress-related DAergic activity was correlated with the physiological and psychological intensity of the stressor. The magnitude of stress-induced DA release was dependent on rearing conditions, personality traits and genetic variations in several SNPs. In psychopathology, preliminary evidence was found for stress-related dorsal striatal DAergic hyperactivity in psychosis spectrum and a blunted response in chronic cannabis use and pain-related disorders, but results were inconsistent. Physiological stress-induced DAergic activity in striatum in HV may reflect somatosensory properties of the stressor and readiness for active fight-or-flight behavior. DAergic activity in HV in the ventral striatum and mPFC may be more related to expectations about the stressor and threat evaluation, respectively. Future studies with increased sample size in HV and psychopathology assessing the functional relevance of stress-induced DAergic activity, the association between cortical and subcortical DAergic activity and the direct comparison of different stressors are necessary to conclusively elucidate the role of the DA system in the stress response.

Noradrenaline-induced release of newly-synthesized accumbal dopamine: differential role of alpha- and beta-adrenoceptors

Previous studies have shown that intra-accumbens infusion of isoproterenol (ISO), a beta-adrenoceptor-agonist, and phenylephrine (PE), an alpha-adrenoceptor-agonist, increase the release of accumbal dopamine (DA). In the present study we analyzed whether the ISO-induced release of DA is sensitive to pretreatment with the DA synthesis inhibitor alpha-methyl-para-tyrosine (AMPT). Earlier studies have shown that the PE-induced release of DA is derived from DA pools that are resistant to AMPT. In addition to PE, the alpha-adrenoceptor-antagonist phentolamine (PA) was also found to increase accumbal DA release. Therefore, we investigated whether similar to the DA-increasing effect of PE, the DA increase induced by PA is resistant to AMPT. Pretreatment with AMPT prevented the ISO-induced increase of accumbal DA. The accumbal DA increase after PA was not reduced by the DA synthesis inhibitor, independently of the amount of DA released. These results show that mesolimbic beta-, but not alpha-adrenoceptors, control the release of accumbal newly-synthesized DA pools. The DA-increasing effects of PE have previously been ascribed to stimulation of presynaptic receptors located on noradrenergic terminals, whereas the DA-increasing effects of PA and ISO have been ascribed to an action of these drugs at postsynaptic receptors on dopaminergic terminals. The fact that AMPT did not affect the accumbal DA response to PE and PA, whereas it did prevent the accumbal DA increase to ISO, supports our previously reported hypothesis that the noradrenergic neurons of the nucleus accumbens containing presynaptic alpha-adrenoceptors impinge upon the dopaminergic terminals in the nucleus accumbens containing postsynaptic adrenoceptors of the alpha but not of the beta type. The putative therapeutic effects of noradrenergic agents in the treatment of DA-related disorders are shortly discussed.

Abnormal neural processing during emotional salience attribution of affective asymmetry in patients with schizophrenia

Aberrant emotional salience attribution has been reported to be an important clinical feature in patients with schizophrenia. Real life stimuli that incorporate both positive and negative emotional traits lead to affective asymmetry such as negativity bias and positivity offset. In this study, we investigated the neural correlates of emotional salience attribution in patients with schizophrenia when affective asymmetry was processed. Fifteen patients with schizophrenia and 14 healthy controls were scanned using functional magnetic resonance imaging (fMRI) while performing an emotion judgment task in which two pictures were juxtaposed. The task consisted of responding to affective asymmetry condition (ambivalent and neutral) and affective symmetry conditions (positive and negative), and group comparisons were performed for each condition. Significantly higher activity in the medial prefrontal cortex and inferior frontal gyrus was observed for the ambivalent condition than for the other conditions in controls, but not in patients. Compared with controls, patients showed decreased activities in the dorsolateral prefrontal cortex, dorsal anterior cingulate cortex, insula, and putamen for the ambivalent condition, but no changes were observed for the neutral condition. Multiple prefrontal hypoactivities during salience attribution of negativity bias in schizophrenia may underlie deficits in the integrative processing of emotional information. Regional abnormalities in the salience network may be the basis of defective emotional salience attribution in schizophrenia, which is likely involved in symptom formation and social dysfunction.

Postnatal choline levels mediate cognitive deficits in a rat model of schizophrenia

In the present study, we investigated whether the essential nutrient choline may protect against schizophrenic-like cognitive deficits in a rat model. Theories regarding the etiology of schizophrenia suggest that early life events render an individual more vulnerable to adult challenges, and the combination may precipitate disease onset. To model this, the adult male offspring of dams who either experienced stress during late gestation or did not were given a 5 mg/kg dose of the NMDA antagonist,MK-801. The presence of both the prenatal challenge of stress and the adult challenge of MK-801 was expected to impair memory in these offspring. Memory was not expected to be impaired in rats that did not experience prenatal stress, but did receive MK-801 as adults. To study whether choline levels altered outcomes in these groups, rats were fed a choline-supplemented, -deficient, or standard diet during the period between the two challenges: beginning at weaning and continuing for 25 days. All rats consumed regular rat chow thereafter. The efficacy of the model was confirmed in the standard fed rats in that only those that were prenatally stressed and received MK-801 as adults displayed impaired memory on a novelty preference test of object recognition. Contrary to this finding and consistent with our hypothesis, choline-supplemented rats that were also both prenatally stressed and given MK-801 as adults showed intact memory. Choline deficiency impaired memory in rats that were just prenatally stressed, just given MK-801 as adults, and subjected to both. Thus, a choline deficient diet may render rats vulnerable to either challenge. Taken together, we offer evidence that developmental choline levels modulate the effects of prenatal stress and/or MK-801 and thereby alter the cognitive outcome in a rat model of schizophrenia.

Cognition-emotion dysinteraction in schizophrenia

Evolving theories of schizophrenia emphasize a "disconnection" in distributed fronto-striatal-limbic neural systems, which may give rise to breakdowns in cognition and emotional function. We discuss these diverse domains of function from the perspective of disrupted neural circuits involved in "cold" cognitive vs. "hot" affective operations and the interplay between these processes. We focus on three research areas that highlight cognition-emotion dysinteractions in schizophrenia: First, we discuss the role of cognitive deficits in the "maintenance" of emotional information. We review recent evidence suggesting that motivational abnormalities in schizophrenia may in part arise due to a disrupted ability to "maintain" affective information over time. Here, dysfunction in a prototypical "cold" cognitive operation may result in "affective" deficits in schizophrenia. Second, we discuss abnormalities in the detection and ascription of salience, manifest as excessive processing of non-emotional stimuli and inappropriate distractibility. We review emerging evidence suggesting deficits in some, but not other, specific emotional processes in schizophrenia - namely an intact ability to perceive emotion "in-the-moment" but poor prospective valuation of stimuli and heightened reactivity to stimuli that ought to be filtered. Third, we discuss abnormalities in learning mechanisms that may give rise to delusions, the fixed, false, and often emotionally charged beliefs that accompany psychosis. We highlight the role of affect in aberrant belief formation, mostly ignored by current theoretical models. Together, we attempt to provide a consilient overview for how breakdowns in neural systems underlying affect and cognition in psychosis interact across symptom domains. We conclude with a brief treatment of the neurobiology of schizophrenia and the need to close our explanatory gap between cellular-level hypotheses and complex behavioral symptoms observed in this illness.

Cognition-emotion dysinteraction in schizophrenia

Evolving theories of schizophrenia emphasize a "disconnection" in distributed fronto-striatal-limbic neural systems, which may give rise to breakdowns in cognition and emotional function. We discuss these diverse domains of function from the perspective of disrupted neural circuits involved in "cold" cognitive vs. "hot" affective operations and the interplay between these processes. We focus on three research areas that highlight cognition-emotion dysinteractions in schizophrenia: First, we discuss the role of cognitive deficits in the "maintenance" of emotional information. We review recent evidence suggesting that motivational abnormalities in schizophrenia may in part arise due to a disrupted ability to "maintain" affective information over time. Here, dysfunction in a prototypical "cold" cognitive operation may result in "affective" deficits in schizophrenia. Second, we discuss abnormalities in the detection and ascription of salience, manifest as excessive processing of non-emotional stimuli and inappropriate distractibility. We review emerging evidence suggesting deficits in some, but not other, specific emotional processes in schizophrenia - namely an intact ability to perceive emotion "in-the-moment" but poor prospective valuation of stimuli and heightened reactivity to stimuli that ought to be filtered. Third, we discuss abnormalities in learning mechanisms that may give rise to delusions, the fixed, false, and often emotionally charged beliefs that accompany psychosis. We highlight the role of affect in aberrant belief formation, mostly ignored by current theoretical models. Together, we attempt to provide a consilient overview for how breakdowns in neural systems underlying affect and cognition in psychosis interact across symptom domains. We conclude with a brief treatment of the neurobiology of schizophrenia and the need to close our explanatory gap between cellular-level hypotheses and complex behavioral symptoms observed in this illness.

Performance on a probabilistic inference task in healthy subjects receiving ketamine compared with patients with schizophrenia

Evidence suggests that some aspects of schizophrenia can be induced in healthy volunteers through acute administration of the non-competitive NMDA-receptor antagonist, ketamine. In probabilistic inference tasks, patients with schizophrenia have been shown to 'jump to conclusions' (JTC) when asked to make a decision. We aimed to test whether healthy participants receiving ketamine would adopt a JTC response pattern resembling that of patients. The paradigmatic task used to investigate JTC has been the 'urn' task, where participants are shown a sequence of beads drawn from one of two 'urns', each containing coloured beads in different proportions. Participants make a decision when they think they know the urn from which beads are being drawn. We compared performance on the urn task between controls receiving acute ketamine or placebo with that of patients with schizophrenia and another group of controls matched to the patient group. Patients were shown to exhibit a JTC response pattern relative to their matched controls, whereas JTC was not evident in controls receiving ketamine relative to placebo. Ketamine does not appear to promote JTC in healthy controls, suggesting that ketamine does not affect probabilistic inferences.

Increased orienting to unexpected action outcomes in schizophrenia

Although some recent research has indicated reduced performance monitoring in patients with schizophrenia, the literature on this topic shows some remarkable inconsistencies. While most studies suggest diminished error signals following error responses, some studies reported normal post-error slowing, while others reported reduced post-error slowing. Here we review these studies and highlight the most important discrepancies. Furthermore, we argue that overall error rates are a mostly neglected issue that can at least partly explain these discrepancies. It has been reported previously that post-error slowing depends on the error rates. Participants or patients that make more errors are likely to show decreased post-error slowing. Therefore, when a group of patients is compared to a group of controls, it is extremely important to match error rates. For this purpose, we developed a procedure where we matched individuals' error rates. In a task where subjects had to press a response key corresponding to one of four colors we manipulated the difficulty on an individual basis by varying the discriminability between the colors. Schizophrenic patients and a group of controls were tested with this procedure showing that differences in accuracy disappear. Interestingly, we can see that in patients, the color values that were needed to reach similar levels of accuracy correlate with the Positive and Negative Syndrome Scale (PANSS) scale, with higher PANSS requiring more color. Most important, we found that schizophrenic patients have increased rather than decreased post-error slowing when the inter-trial interval (ITI) is short. This result can be interpreted within the framework of the orienting account, as it has been demonstrated previously that schizophrenic patients show increased distractibility.

Single tap identification for fast BCI control

One of the major aims of BCI research is devoted to achieving faster and more efficient control of external devices. The identification of individual tap events in a motor imagery BCI is therefore a desirable goal. EEG is recorded from subjects performing and imagining finger taps with their left and right hands. A Differential Evolution based feature selection wrapper is used in order to identify optimal features in the spatial and frequency domains for tap identification. Channel-frequency band combinations are found which allow differentiation of tap vs. no-tap control conditions for executed and imagined taps. Left vs. right hand taps may also be differentiated with features found in this manner. A sliding time window is then used to accurately identify individual taps in the executed tap and imagined tap conditions. Highly statistically significant classification accuracies are achieved with time windows of 0.5 s and more allowing taps to be identified on a single trial basis.

Prefrontal cortical D1 dopamine receptors modulate subcortical D2 dopamine receptor-mediated stress responsiveness

Increased responsiveness to stress plays an important role in the manifestation of schizophrenia symptoms. Evidence indicates that the prefrontal cortex (PFC), and dopamine neurotransmission in the PFC in particular, is involved in the modulation of stress responsiveness. Decreased dopaminergic activity and loss of dopamine fibres have been reported in PFC in schizophrenia patients. Consequently, it was hypothesized that depletion of dopamine in PFC may facilitate increased stress responsiveness. Adult Sprague-Dawley rats received injections of 6-hydroxydopamine or saline bilaterally into the medial PFC (mPFC) following desipramine pretreatment to selectively deplete dopaminergic fibres. Following a 3-wk recovery period, the lesioned and control rats received injections of a D1 or D2 dopamine receptor agonist or vehicle into the mPFC and were immediately subjected to forced swimming as a stressor. Results showed that frequency of locomotion and rearing, behavioural measures indicative of increased dopaminergic activity in the nucleus accumbens (NAc), were significantly increased following stress in prefrontal cortical dopamine-depleted rats. This effect was significantly ameliorated by infusions of a D1 dopamine receptor agonist directly into the mPFC in a dose-dependent manner but not by infusion of a D2 dopamine receptor agonist. In addition, stress-induced behavioural changes in prefrontal cortical dopamine-depleted rats were significantly reduced following selective discrete infusions of a D2 dopamine receptor antagonist into the NAc shell. The results suggest that dopaminergic transmission via D1 receptors in the mPFC modulates D2 dopamine receptor-mediated stress responsiveness in the NAc, a feature that may be disrupted in schizophrenia patients.

The microinjection of AMPA receptor antagonist into the accumbens shell failed to change food intake, but reduced fear-motivated behaviour in free-feeding female rats

This study investigated the effect of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2.5 and 5.0 nmol/side) microinjected into the core and shell sub-regions of the accumbens (Acb) nucleus, on food intake and the level of anxiety in female rats. Bilateral microinjections of CNQX (5.0 nmol/side) into the Acb shell (AP, +1.08 to +2.04), but not into the Acb core, induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since the animals exhibited low level of SAP in the feeding test. The anxiolytic-like effect induced by 5.0 nmol CNQX microinjection into the Acb shell may not be ascribed to changes in the motor activity of the animals, because the frequency of locomotion, rearing and grooming remained unchanged after the drug microinjection. However, neither Acb shell nor Acb core CNQX microinjections were able to change the animals food intake along 1h feeding behaviour evaluation. Food intake remained unchanged 24h after the drug microinjections either into the Acb shell or into the Acb core. The data suggest that AMPA receptor blockade in the Acb nucleus may differentially change the ingestive and defensive behaviours in female rats.

Amphetamine effects in appetitive acquisition depend on the modality of the stimulus rather than its relative validity

Amphetamine has been shown previously to increase the apportioning of associative strength to weak predictors in appetitive Pavlovian conditioning procedures such as latent inhibition and overshadowing. Manipulating the likelihood with which different conditioned stimuli (CSs) predict subsequent delivery of an unconditioned stimulus (UCS) is an alternative method by which the associability of CSs can be influenced. The present experiment tested effects of D-amphetamine (0.5 mg/kg or 1.5 mg/kg administered 15 min prior to conditioning) in appetitive acquisition under partial versus continuous reinforcement of alternative CSs with sucrose pellet UCS delivery. Specifically, male Wistar rats were conditioned to light and tone CSs that were followed by the UCS on 100% or 50% of trials in a cross-over design. It was predicted that amphetamine would disrupt rats' ability to select appropriately the most valid CSs for learning which would be expressed as increased conditioning to weaker, 50% valid CSs. Contrary to prediction, differential responding based on relative validity was preserved under amphetamine, for both light and tone stimuli. Instead, the results showed that responding to light CSs was generally reduced under amphetamine. Conditioning to tone CSs was higher and unaffected by amphetamine. Thus, results demonstrate that amphetamine effects are determined by the properties of the CS used for learning.

Auditory Oddball fMRI in Schizophrenia: Association of Negative Symptoms with Regional Hypoactivation to Novel Distractors

Schizophrenia is associated with abnormal processing of salient stimuli, which may contribute to clinical symptoms. We used fMRI and a standard auditory 3-stimulus task to examine attention processing. Target stimuli and novel distractors were presented to 17 patients and 21 healthy controls and activation was correlated with negative and positive symptoms. To targets, patients overactivated multiple regions including premotor cortex, anterior cingulate, temporal cortex, insula, and hippocampus, and also showed attenuated deactivation within occipital cortex. To distractors, patients overactivated left ventrolateral prefrontal cortex. This overactivation may reflect hypersensitivity to salient stimuli in schizophrenia. Patients also exhibited an inverse correlation between negative symptom severity and activation to novel distractors in the dorsolateral prefrontal cortex, premotor area, and ventral striatum. Novelty-induced activity within prefrontal cortex and ventral striatum may represent a useful intermediate phenotype for studies of negative symptoms.

Appetitive overshadowing is disrupted by systemic amphetamine but not by electrolytic lesions to the nucleus accumbens shell

There is evidence that the indirect dopamine (DA) agonist amphetamine (AMP) can disrupt selective learning in an aversive overshadowing task, consistent with a role for the DA system in this form of salience manipulation. In the following experiments we assessed in the male Wistar rat: (1) whether amphetamine disruption of overshadowing extends to an appetitively motivated overshadowing task; and (2) whether selective electrolytic lesions to the n.acc (shell versus core subfields) disrupt appetitively motivated overshadowing. The experiments used sucrose reward pellets as the unconditioned stimulus (UCS). In each case, a conditioned stimulus (CS, light) was either conditioned alone or in compound together with a more intense CS (noise or tone). The presence of overshadowing was demonstrated as reduced conditioning to the light when it had been previously conditioned in compound compared to when it had been conditioned alone. It was predicted that AMP and lesions to the n.acc shell would disrupt overshadowing. AMP was found to abolish overshadowing at 0.5 mg/kg, but not at 1 mg/kg. Contrary to prediction, the shell lesioned animals did not differ from shams. The results of Experiment 1 add to the evidence that the DA system can moderate salience processing of weaker predictors, also in cases where CS salience is manipulated directly via the physical intensities of the stimuli, as here. However, in terms of the brain structures involved, Experiment 2 suggests that, overshadowing is moderated by projections of the DA system without n.acc.

Behavioral functions of the mesolimbic dopaminergic system: an affective neuroethological perspective

The mesolimbic dopaminergic (ML-DA) system has been recognized for its central role in motivated behaviors, various types of reward, and, more recently, in cognitive processes. Functional theories have emphasized DA's involvement in the orchestration of goal-directed behaviors and in the promotion and reinforcement of learning. The affective neuroethological perspective presented here views the ML-DA system in terms of its ability to activate an instinctual emotional appetitive state (SEEKING) evolved to induce organisms to search for all varieties of life-supporting stimuli and to avoid harms. A description of the anatomical framework in which the ML system is embedded is followed by the argument that the SEEKING disposition emerges through functional integration of ventral basal ganglia (BG) into thalamocortical activities. Filtering cortical and limbic input that spreads into BG, DA transmission promotes the "release" of neural activity patterns that induce active SEEKING behaviors when expressed at the motor level. Reverberation of these patterns constitutes a neurodynamic process for the inclusion of cognitive and perceptual representations within the extended networks of the SEEKING urge. In this way, the SEEKING disposition influences attention, incentive salience, associative learning, and anticipatory predictions. In our view, the rewarding properties of drugs of abuse are, in part, caused by the activation of the SEEKING disposition, ranging from appetitive drive to persistent craving depending on the intensity of the affect. The implications of such a view for understanding addiction are considered, with particular emphasis on factors predisposing individuals to develop compulsive drug seeking behaviors.

The microinjection of AMPA receptor antagonist into the accumbens shell, but not into the accumbens core, induces anxiolysis in an animal model of anxiety

This study investigated the effect of the AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) microinjected into the core and shell sub-regions of the accumbens nucleus (Acb), on the level of fear/anxiety and emotional learning, in female rats submitted to the elevated plus-maze (EPM), an animal model of anxiety. Bilateral microinjections of DNQX (330 and 660 ng) into the Acb shell (AP, +1.08 to +2.16) induced an anxiolytic-like effect in relation to rats microinjected with vehicle, since there was an increased percentage of entries in the open arms of the maze. The 660 ng DNQX microinjection into the Acb shell also increased the percentage of entries into the open arms in relation to 660 ng DNQX microinjection into the Acb core. Prior DNQX microinjections in both core and shell sub-regions of the Acb failed to impair the emotional learning, since the animals exhibited an increase of the open arm avoidance on EPM Trial 2 in relation to EPM trial 1. DNQX microinjections into both sub-regions of the Acb did not change the number of entries into the enclosed arms, either in the EPM Trial 1 or in the EPM Trial 2, which indicates an absence of drug-induced locomotor impairment. Similarly, DNQX microinjections into both sub-regions of the Acb failed to alter the total arm entries, rearing, grooming and head-dipping frequency. The anxiolytic-like effect induced by DNQX suggests that the AMPA receptor in the Acb shell, but not in the Acb core, may underlie anxiety regulation in the EPM.

GABAA and GABAB agonist microinjections into medial accumbens shell increase feeding and induce anxiolysis in an animal model of anxiety

This study investigated the effect of GABAA (muscimol, MUSC) and GABAB (baclofen, BACL) agonist receptors microinjected into medial accumbens shell on feeding and the level of fear in free-feeding rats submitted to the elevated plus-maze (EPM), an animal model of anxiety. Bilateral microinjections of either MUSC (128 pmol/0.2 microl/side) or BACL (128 and 256 pmol/0.2 microl/side) induced an anxiolytic-like effect since they decreased the occurrence of risk assessment, a defensive behaviour positively correlated with the animal anxiety level. Bilateral BACL microinjection (128 pmol), but not MUSC, also increased the head-dipping frequency over the open arms of the EPM, another representative behaviour of anxiety, but negatively correlated with it. In addition to anxiolysis, the present study also showed that the microinjection of MUSC and BACL agonists into rostral sites of the medial Acb shell (AP, +1.2 to +1.6) increased food intake significantly whereas drinking behaviour kept unchanged. Both doses of MUSC and BACL also decreased feeding latency. BACL but not MUSC dose-dependently increased food length. The data indicated a putative role of GABA receptors (especially GABAB) in the medial Acb shell for anxiety modulation in rats.

From prediction error to psychosis: ketamine as a pharmacological model of delusions

Recent cognitive neuropsychiatric models of psychosis emphasize the role of attentional disturbances and inappropriate incentive learning in the development of delusions. These models highlight a pre-psychotic period in which the patient experiences perceptual and attentional disruptions. Irrelevant details and numerous associations between stimuli, thoughts and percepts are imbued with inappropriate significance and the attempt to rationalize and account for these bizarre experiences results in the formation of delusions. The present paper discusses delusion formation in terms of basic associative learning processes. Such processes are driven by prediction error signals. Prediction error refers to mismatches between an organism's expectation in a given environment and what actually happens and it is signalled by both dopaminergic and glutamatergic mechanisms. Disruption of these neurobiological systems may underlie delusion formation. We review similarities between acute psychosis and the psychotic state induced by the NMDA receptor antagonist drug ketamine, which impacts upon both dopaminergic and glutamatergic function. We conclude by suggesting that ketamine may provide an appropriate model to investigate the formative stages of symptom evolution in schizophrenia, and thereby provide a window into the earliest and otherwise inaccessible aspects of the disease process.

Use of magnetic resonance imaging in tracking the course and treatment of schizophrenia

Confirming the early conceptualization of Bleuler (1911) and Kraepelin (1919), magnetic resonance imaging (MRI) studies have demonstrated structural and functional brain abnormalities, predominantly involving the frontal and temporal lobes, in schizophrenia. Most of the abnormalities are already present at illness onset. However, there is, growing evidence for treatment-related neural changes in schizophrenia, such as enlargement of the caudate nucleus (neurotoxic effect) with the use of typical antipsychotics and increases in cortical volumes and improved functional responses (neurotrophic effect) with the use of atypical antipsychotics. More recently, brain changes during the prodrome and transition-to-illness stages of schizophrenia have begun to be characterized. Another area of importance is the use of MRI, as a biological marker, to monitor and define partial or full resistance to medication. Understanding the trait- and state-related influences of brain abnormalities during the course of the illness is critical for developing effective treatment and possibly prevention strategies in schizophrenia.

Cognitive effects of dopaminergic and glutamatergic blockade in nucleus accumbens in pigeons

In earlier studies it was found that glutamatergic transmission within the nucleus accumbens septi is involved in the performance of a learned visual shape discrimination in pigeons. This study examines what effects several kinds of glutamate and dopamine antagonists have on the same task. Pigeons were trained with the relevant discrimination, bilaterally implanted with cannulas into the nucleus accumbens and tested after various transmission blockers had been administered intracerebrally. SCH-23390, a D1 dopamine antagonist, at the dose used, had no effect, and Spiperone, a D2-dopamine and 5HT2a-serotonine antagonist, significantly decreased the error repeat trials. CNQX, a non-NMDA glutamate receptor antagonist, and Cycloleucine, an antagonist of the glycine allosteric site of NMDA receptors, had no effect. CGS-19755, a selective competitive NMDA antagonist, significantly impaired performance by significantly decreasing the percent correct trials and increasing the error repeat trials. CPPG, a II/III metabotropic glutamate antagonist, remarkably improved performance. MMPG, a III/II metabotropic glutamate antagonist, at the dose used, did not have any significant effect. The preparation employed may be a useful animal model of perceptual disturbances in schizophrenia.

The visual search analogue of latent inhibition: implications for theories of irrelevant stimulus processing in normal and schizophrenic groups

Latent inhibition (LI) is a robust phenomenon that is demonstrated when a previously inconsequential stimulus is less effective in a new learning situation than a novel stimulus. Despite LI's simplicity, there is considerable disagreement as to its theoretical basis. Attentional theories claim that unattended stimulus preexposures reduce stimulus associability. Alternatively, it has been asserted that associability is unaffected and that LI is a result of competition/retrieval processes. The present article reviews a series of visual search studies, some with normal subjects, both undifferentiated and divided into low and high schizotypals, and others with pathologies that entail dysfunctional attention, such as schizophrenia, Parkinson's disease, and anxiety. The visual search conditions were designed to model those of traditional LI experiments, while tapping attentional processes independently of the learning scores that index LI. A variety of evidence from these and other studies is used to support the involvement of attentional and retrieval processes in LI. A model of the mechanism of action of these processes in LI is presented, together with its application to schizophrenia.

The effect of Delta9-tetrahydrocannabinol on sensorimotor gating in socially isolated rats

Rearing rats in isolation produces behavioural and neurochemical alterations similar to those observed in schizophrenia. Cannabinoids have also been implicated in inducing psychotic symptoms. In this study, we investigate the effect of the major psychoactive constituent of cannabis and partial cannabinoid CB(1) receptor agonist Delta(9)-tetrahydrocannabinol (THC) on prepulse inhibition (%PPI) of the acoustic startle reflex and on habituation in socially isolated and grouped rats. Deficits in %PPI are reminiscent of sensorimotor gating deficits observed in psychoses. Male Sprague-Dawley rat pups (21 days old) were housed in either single cages (isolated) or in group cages of six per cage (grouped). Eight weeks later the effect of vehicle, THC and the CB(1) receptor antagonist SR 141716 on %PPI was tested. Vehicle treated isolated rats exhibited significantly reduced PPI compared with grouped rats. Isolated rats treated with THC had significantly lower %PPI than vehicle treated groups. This further decrease of %PPI by THC was reversed by pre-treatment with SR 141716, indicating that this effect was mediated by CB(1) receptors. THC had no significant effect on %PPI in grouped rats. SR 141716 had no significant effect on %PPI in either grouped or isolated rats. Habituation did not significantly alter in any treatment group in any treatment group. These results suggest that THC produces significant decreases in sensorimotor gating in rats with already dysfunctional sensorimotor gating processes, but not in normal rats. The lack of effect of SR 141716 in either grouped or isolated rats suggests that normal endocannabinoid function is not critical in sensorimotor gating processes.

The effect of SR 141716 and apomorphine on sensorimotor gating in Swiss mice

The aim of the present study was to investigate in Swiss mice the acute effects of the CB(1) receptor antagonist N-piperidino-5-(4-chlorphenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR 141716) alone and in combination with apomorphine, a D(1)/D(2) receptor agonist, on prepulse inhibition (PPI) of the acoustic startle response, an operational measure of sensorimotor gating. SR 141716 (1 and 3 mg/kg i.p.) had no significant effect on PPI. Apomorphine (3 mg/kg i.p.) significantly disrupted PPI. The PPI of mice injected with SR 141716 (1 mg/kg i.p.) plus apomorphine (3 mg/kg i.p.) was not significantly different to that of vehicle plus apomorphine (3 mg/kg i.p.)-treated mice. However, the higher dose of SR 141716 used (3 mg/kg i.p.) significantly inhibited the disruption of PPI produced by apomorphine. These results suggest that antagonism of CB(1) receptors with SR 141716 has no significant effect on sensorimotor gating in Swiss mice. However, CB(1) receptors appear to be important in the effect of apomorphine on sensorimotor gating, as antagonism of CB(1) receptors with SR 141716 inhibits apomorphine-induced disruptions.

Behavioural consequences of nucleus accumbens dopaminergic stimulation and glutamatergic blocking in pigeons

Upon systemic administration of apomorphine, a potent dopamine agonist, pigeons show a bout of pecking behaviour. When the drug is repeatedly administered a sensitization takes place that is associated with pronounced discrimination learning. Here we show that intra-cerebral injections of apomorphine in the periphery of the nucleus accumbens of pigeons also elicit pecking. We additionally show that injections of 5-amino-phosphonohepatnoic acid, a NMDA-glutamate receptor blocker, into the Acc impairs the performance of a learned visual discrimination incorporating pecking as a choice response. We conclude that, as it is the case in mammals, the control mechanisms of learned sensory-motor behaviour in birds involves dopaminergic and glutamatergic synaptic transmission within the nucleus accumbens area.

Effects of fencamfamine on latent inhibition

The effects of fencamfamine (FCF), an indirect dopamine (DA) agent, were investigated using the latent inhibition (LI) model of schizophrenia. In the LI procedure, rats preexposed (PE) to an unreinforced stimulus show difficulty in subsequent learning of an association in which that stimulus is predictive of an unconditioned stimulus (US). FCF (1.75, 3.5 and 7.0 mg/kg i.p.) yielded an inverse dose-response relationship regarding LI. At 3.5 mg/kg, LI was abolished and no effect was observed at 1.75 and 7.0 mg/kg. The effect of FCF (3.5 mg/kg) on LI was blocked by the antipsychotic risperidone (RIS; 4.0 mg/kg), a D2/5HT2 antagonist. These results confirm the similarity of the behavioral profile of FCF and amphetamine (AMPH). In addition, they provide a further validation of the LI model for psychosis, since RIS was shown to prevent a psychostimulant-induced disruption of LI.

Fear and feeding in the nucleus accumbens shell: rostrocaudal segregation of GABA-elicited defensive behavior versus eating behavior

This study examined localization of positive versus negative motivational functions mediated by GABA circuits within the accumbens shell. Microinjections of a GABA(A) agonist (0, 25, 75, and 225 ng/0.5 microl muscimol) in rostral shell sites elicited appetitive increases in eating behavior. In contrast, microinjections in caudal shell sites elicited defensive burying or paw-treading behavior. Rats whose microinjections landed bilaterally outside of the accumbens shell did not display either behavior. Defensive treading elicited by caudal shell muscimol microinjection appeared to be a negative motivated response to threat (similar in parameters and orientation to normal defensive burying of a threatening electrified shock prod). The nucleus accumbens shell thus appears functionally heterogeneous in coding motivational valence. The demonstration that muscimol elicits positive eating behavior from rostral shell versus negative defensive behavior from caudal shell suggests in particular that GABAergic substrates of positive and negative types of motivated behavior in the nucleus accumbens shell are segregated along a rostrocaudal gradient.

Evaluation of the supervisory system in elderly subjects with and without disinhibition

Disinhibition and irritability, defined as loss of behavioral and emotional control, are frequent in the elderly. The working hypothesis for this study was that these disorders are associated with a cognitive alteration of control processes that manifests as non-routine behavior because of the dysfunction of a general executive component known as the supervisory attentional system (SAS).

METHODS

A total of 28 elderly subjects with mild cognitive impairment were recruited and divided into two groups using the Neuropsychiatric Inventory. Fourteen subjects were allocated to the disinhibited group and 14 subjects matched for age, sex and educational level formed a disinhibition-free control group. The neuropsychological battery included the following tests: Mini Mental Score Evaluation, Boston Naming test, Token test, Trail Making and Verbal Fluency. Two tasks were specifically designed to stress the SAS: 1) A specific verbal sentence arrangement task in which subjects had to use sequential reasoning with verbal material. Each test sequence consisted of a series of words shown in jumbled order. The construction of some sequences had to be done by using familiar routine associations (valid conditions). In contrast, other sequences required the overriding selection of familiar routine associations, which were inappropriate within the general context of the task (invalid conditions). 2) Using the Continuous Performance Test, four aspects were evaluated: sustained, selective, preparation and suppressive attention.

RESULTS

The only group differences in neuropsychological test results were the following: 1) the sentence arrangement task. In comparison with the control group, the disinhibited group was impaired in invalid conditions and the calculated difference between the number of correct responses in invalid conditions minus that in valid conditions was significantly higher; and 2) the CPT. Disinhibited subjects had a significantly lower number of hits, exclusively in the 'suppressive attention' paradigm. These results suggest that subjects with disinhibition have impaired supervisory system function.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

The regulation of forebrain dopamine transmission: relevance to the pathophysiology and psychopathology of schizophrenia

Since the discovery that the therapeutic efficacy of antipsychotic drugs was significantly correlated to their ability to block dopamine D2 receptors, abnormal dopamine transmission in the forebrain has been postulated to underlie psychosis in schizophrenia. In the past 15 years, an impressive amount of clinical and basic research aimed at the study of schizophrenia has indicated that prefrontal and temporal cortical abnormalities may be more important in the etiology of many of the symptoms of schizophrenia, including psychosis. However, the cortical systems that appear to have structural and/or metabolic abnormalities in schizophrenia patients potently regulate forebrain dopamine transmission through a number of mechanisms. In turn, dopamine modulates excitatory transmission mediated by frontal and temporal cortical projections to the basal ganglia and other regions. The present review summarizes the multiple interactions between forebrain DA systems and frontal and temporal corticostriatal transmission. It then examines the role of these interactions in normal behaviors and the psychopathology of schizophrenia.

Abnormal regulation of corticopetal cholinergic neurons and impaired information processing in neuropsychiatric disorders

Cholinergic neurons originating in the basal forebrain innervate all cortical areas and participate in the gating of cortical information processing. Aberrations in the excitability of cortical cholinergic inputs fundamentally alter the processing of sensory stimuli and higher processes, thereby advancing the development of major neuropsychiatric disorders. Cortical cholinergic deafferentation has been considered to be a major neuropathological variable that contributes to the development of age- and dementia-associated impairments in cognition. Conversely, it has been suggested that increases in the excitability of cortical cholinergic inputs mediate the abnormal cognitive processes that escalate into psychotic symptoms and contribute to addictive-drug-seeking behavior, anxiety and phobia. Abnormal regulation of the excitability of cortical cholinergic afferents represents a 'final common pathway' that mediates the manifestation of major neuropsychiatric disorders.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Increased extracellular dopamine in the nucleus accumbens of the rat during associative learning of neutral stimuli

Brain microdialysis was used to study changes in dopamine in the nucleus accumbens and the dorsal striatum during associative learning between two neutral stimuli, flashing light and tone, presented on a paired schedule during stage 1 of a sensory preconditioning paradigm. The tone was subsequently paired with mild footshock using standard aversive conditioning procedures and the formation of a conditioned association between the flashing light and the tone in stage 1 was assessed by measuring the ability of the flashing light to elicit the same conditioned response as the tone when presented at test. The first experiment used behavioural monitoring only, to establish stimulus parameters for subsequent microdialysis experiments. Animals receiving paired presentation of the light and tone in stage 1 showed a conditioned suppression of licking to the light as well as to the tone, indicating that associative learning between the flashing light and the tone had occurred during stage 1, whilst in a separate group of animals given the same stimuli over the same time period but on an explicitly non-paired schedule, the conditioned emotional response was seen to the tone, but not to the light, showing that no association had been formed between the two stimuli during stage 1. In dialysis experiments using the same procedure, we measured a two-fold rise in dopamine in the nucleus accumbens during paired presentation of flashing light and tone, but not during non-paired presentation of the two stimuli. On subsequent test presentation of the two stimuli, we saw increases in accumbal dopamine on presentation of the tone in both groups, reflecting the formation of an association with the footshock in both. However the flashing light elicited an increase in dopamine only in the group which had received paired presentation at stage 1. Thus accumbal dopamine release at test is correlated to the ability of the stimulus to evoke a conditioned response measured behaviourally. Hypotheses of the behavioural function of the mesolimbic dopamine system centre on its role in mediating the effects of biological reinforcers, both rewarding and aversive, conditioned and unconditioned. The present results, showing increases in extracellular dopamine in the nucleus accumbens when an association is formed between two stimuli of which neither is a biological reinforcer nor, prior to formation of the association, affects dopamine levels, suggest a role for accumbal dopamine in the modulation of associative learning in general, not only that involving reinforcement.

Thalamic paraventricular nucleus neurons collateralize to innervate the prefrontal cortex and nucleus accumbens

The prefrontal cortex and nucleus accumbens are primary recipients of medial thalamic inputs, prominently including projections from the thalamic paraventricular nucleus. It is not known if paraventricular neurons collateralize to innervate both the prefrontal cortex and nucleus accumbens. We used dual retrograde tract tracing methods to examine this question. A small population of paraventricular neurons was found to innervate the prefrontal cortex and medial nucleus accumbens. These data suggest that the thalamic paraventricular nucleus may coordinately influence activity in the prefrontal cortex and ventral striatum.

Visual discrimination in pigeons impaired by glutamatergic blockade of nucleus accumbens

The nucleus accumbens septi (Acc) is thought to be involved in the control of cognitive processes and to be implicated in the pathophysiology of schizophrenia. Because perceptual-cognitive distortions are a core symptom in schizophrenia, any evidence that the Acc intervenes in a sensory recognition task in an animal species would be of interest. Pigeons were instrumentally trained to discriminate visual shapes. The acute effects of drug microinjections into the Acc on the discrimination of the training shapes, on the correction responding after errors, and on the generalisation to different shapes were examined. The effects of conduction blockade with lidocaine, glutamatergic blockade with 7-aminophosphonoheptanoic acid, and dopaminergic stimulation with apomorphine on behavioural performance were tested. No effects were observed with lidocaine and apomorphine. A significant and reversible performance disruption to near chance levels was obtained after aminophosphonoheptanoic acid injections into the Acc. It appears that a glutamatergic blockade of the Acc interferes with the visual discrimination processes of pigeons.

Electrolytic lesions of the medial prefrontal cortex in rats disrupt performance on an analog of the Wisconsin Card Sorting Test, but do not disrupt latent inhibition: implications for animal models of schizophrenia

The effects of electrolytic lesions of the medial prefrontal cortex (mPFC) or its subregions were investigated on two cognitive tests that may have relevance to the behavioral impairments of patients with schizophrenia. One task consisted of a delayed non-match-to-sample and reversal of the non-match-to-sample rule, in a Skinner box. The reversal component simulated the essential feature of rule shifting of the Wisconsin Card Sorting Test (WCST), which is a commonly used test for assessing 'frontal-like' deficits in schizophrenia. The second was latent inhibition, in which repeated pre-exposure to a stimulus without consequence retards subsequent associations with that stimulus. Latent inhibition is impaired in acute schizophrenic patients, and its disruption in the rat has been suggested to constitute an animal model of schizophrenia. Expts. 1 and 2 tested the effects of lesions of the dorsal anterior cingulate cortex (dACA) and mPFC, respectively, on the WCST analog. Expt. 3 tested the effects of lesions of the dACA or infralimbic cortex, and Expt. 4 tested the effects of mPFC lesion, on latent inhibition. Lesions of mPFC subregions had no effect. mPFC lesion produced transient deficits in the performance of the DNMS task and impaired the reversal from the non-match-to-sample to the match-to-sample rule, but left the latent inhibition effect intact. Possible relevance of this behavioral profile of mPFC lesion to the 'frontal syndrome' is discussed.

The role of mesolimbic dopaminergic and retrohippocampal afferents to the nucleus accumbens in latent inhibition: implications for schizophrenia

Latent inhibition (LI) consists in a retardation of conditioning seen when the to-be-conditioned stimulus is first presented a number of times without other consequence. Disruption of LI has been proposed as a possible model of the cognitive abnormality that underlies the positive psychotic symptoms of acute schizophrenia. We review here evidence in support of the model, including experiments tending to show that: (1) disruption of LI is characteristic of acute, positively-symptomatic schizophrenia; (2) LI depends upon dopaminergic activity; (3) LI depends specifically upon dopamine release in n. accumbens; (4) LI depends upon the integrity of the hippocampal formation and the retrohippocampal region reciprocally connected to the hippocampal formation; (5) the roles of n. accumbens and the hippocampal system in LI are interconnected.