Associative deficit accounts of disrupted latent inhibition and blocking in schizophrenia

The systemic effects of the enriched environment on the conditioned fear reaction

In this review, a hypothesis is proposed to explain the beneficial effect of an enriched environment (EE) on the conditioned fear reaction (CFR) from the perspective of a functional system of behavioral control. According to the hypothesis, the EE affects all behavioral act components, including the processing of sensory information, memory, motivational and reinforcing systems, and motor activities, which weakens the CFR. Animals raised in the EE have effects that are comparable to those of context (CTX) and CS pre-exposures at latent inhibition. An abundance of stimuli in the EE and constant contact with them provide the formation of CS-noUS and CTX-noUS connections that later, during CFR learning, slow down and diminish fear. The EE also contributes to faster processing of information and habituation to it. As a result, many stimuli in the context lose their significance, and subjects simply ignore them. And finally, the EE affects the motivational and reinforcing brain mechanisms, induces an impairment of search activity, and worsens memory consolidation, which leads to a reduction of CFR.

Aberrant Learned Irrelevance in Patients with First-Episode Schizophrenia-Spectrum Disorder

Emerging evidence has indicated disrupted learned irrelevance (LIrr), a form of selective attention deficit that may contribute to psychotic symptom formation, in schizophrenia. However, previous research mostly focused on chronic patients. There is a paucity of studies on LIrr in first-episode schizophrenia-spectrum disorder (i.e., schizophrenia and schizophreniform disorder; FES), which were limited by small sample size and have produced mixed results. The current study examined a LIrr effect and its relationship with positive symptom severity in 40 briefly-medicated FES patients and 42 demographically-matched healthy controls using a well-validated computerized LIrr paradigm which has been applied in chronic schizophrenia sample. Positive symptoms were assessed by Positive and Negative Syndrome Scale (PANSS) and Psychotic Symptom Rating Scales (PSYRATS). Our results showed that controls demonstrated intact LIrr, with significantly faster learning about previously predictive (relevant) than previously non-predictive (irrelevant) cues. Lack of such normal attention bias towards predictive over non-predictive cues was observed in FES patients, indicating their failure to distinguish between relevant and irrelevant stimuli. Nonetheless, we failed to reveal any significant correlations between learning scores, in particular learning scores for non-predictive cues, and positive symptom measures in FES patients. Learning scores were also not associated with other symptom dimensions, cognitive functions and antipsychotic dose. In conclusion, our findings indicate aberrant LIrr with impaired allocation of attention to relevant versus irrelevant stimuli in briefly-medicated FES patients. Further prospective research is warranted to clarify the longitudinal trajectory of such selective attention deficit and its association with positive symptoms and treatment response in the early course of illness.

BDNF haploinsufficiency induces behavioral endophenotypes of schizophrenia in male mice that are rescued by enriched environment

Brain-derived neurotrophic factor (BDNF) is implicated in a number of processes that are crucial for healthy functioning of the brain. Schizophrenia is associated with low BDNF levels in the brain and blood, however, not much is known about BDNF's role in the different symptoms of schizophrenia. Here, we used BDNF-haploinsufficient (BDNF^+/-) mice to investigate the role of BDNF in different mouse behavioral endophenotypes of schizophrenia. Furthermore, we assessed if an enriched environment can prevent the observed changes. In this study, male mature adult wild-type and BDNF^+/- mice were tested in mouse paradigms for cognitive flexibility (attentional set shifting), sensorimotor gating (prepulse inhibition), and associative emotional learning (safety and fear conditioning). Before these tests, half of the mice had a 2-month exposure to an enriched environment, including running wheels. After the tests, BDNF brain levels were quantified. BDNF^+/- mice had general deficits in the attentional set-shifting task, increased startle magnitudes, and prepulse inhibition deficits. Contextual fear learning was not affected but safety learning was absent. Enriched environment housing completely prevented the observed behavioral deficits in BDNF^+/- mice. Notably, the behavioral performance of the mice was negatively correlated with BDNF protein levels. These novel findings strongly suggest that decreased BDNF levels are associated with several behavioral endophenotypes of schizophrenia. Furthermore, an enriched environment increases BDNF protein to wild-type levels and is thereby able to rescue these behavioral endophenotypes.

Sex Differences in Cue Competition Effects With a Conditioned Taste Aversion Preparation

This study aimed to test whether male and female rats might show differences in cue competition effects in a conditioned taste aversion (CTA) model. Experiment 1 tested for sex differences in overshadowing. After conditioning of a flavored compound AB or only one simple flavor A (being A and B a solution of sugar 10% and salt 1%, counterbalanced), consumption of the A solution at test was larger in the former than in the latter case only in males. Thus, the usual effect of overshadowing was observed in males but not in females. Experiment 2 examined sex differences in blocking with the same stimuli used in Experiment 1. After conditioning of AB, the consumption of B was larger for the animals that previously received a single conditioning trial with A than for those that received unpaired presentations of A and the illness. As observed in Experiment 1, the typical blocking effect appeared only in males but not in females. The present findings thus support the hypothesis that sex dimorphism might be expressed in classical conditioning, or at least, in cue competition effects such as overshadowing and blocking with a taste aversion model.

Neonatal 6-OHDA Lesion Model in Mouse Induces Cognitive Dysfunctions of Attention-Deficit/Hyperactivity Disorder (ADHD) During Young Age

Attention-deficit/hyperactivity disorder (ADHD) is a syndrome characterized by impaired attention, impulsivity and hyperactivity in children. These symptoms are often maintained in adults. During adolescence, prefrontal cortex develops connectivity with other brain regions to engage executive functions such as, latent inhibition, attention and inhibitory control. In our previous work, we demonstrated the validity of the neonatal 6-Hydroxydopamine (6-OHDA) mouse model, a classical neurodevelopmental model mimicking major symptoms of the human ADHD pathology. In order to evaluate pathological forms of executive functions and impulsive behavior in 6-OHDA mice during young age, we first tested latent inhibition (LI) after weaning, and then we evaluated the impulsive behavior using a cliff avoidance reaction test. Our results demonstrated that 6-OHDA mice showed disruption in latent inhibition, suggesting a deficit in selective attention, and displayed repetitive peering-down behavior, indicating a maladaptive impulsive behavior. Subsequently, to assess impulsivity and attention in young mice, we performed a modified 5-choice serial reaction time task test (5-CSRTT), optimizing the degree of food restriction for young animals and shortening the training duration. This test allowed us to demonstrate a deficit in inhibitory control and a loss of accuracy of 6-OHDA mice in the 5-CSRTT. In conclusion, we demonstrated that the 6-OHDA mouse model reproduces human symptoms of ADHD in childhood and early adulthood periods, as seen in human. Taken together, the 6-OHDA mouse model will be useful alongside other animal models to understand the neurobiological mechanisms underlying complex, heterogeneous neurological disorders.

Human latent inhibition: Problems with the stimulus exposure effect

Latent inhibition (LI) is a startlingly simple effect in which preexposure of a stimulus without consequence retards subsequent responding to a stimulus-consequence relation. The effect was first demonstrated with Pavlovian conditioning in animals and was later suggested to be a marker of human psychopathology such as schizophrenia. Individual differences in LI has supported the continued use of animal models to understand human mental health. In this review, we ask whether there is sufficient evidence to support the continued application of LI from animal models to human psychopathology because of the weak evidence for LI in humans. There is considerable variability in the methods used to assess LI, sustaining different theoretical accounts of the effects observed, which differ from the accepted accounts of LI as demonstrated in animals. The review shows that although there have been many experiments testing human LI, none provide the necessary experimental controls to support the conclusion that retarded responding is caused simply by preexposure to a stimulus, as has been demonstrated with animal models. Establishing this conflict, we set out a framework for future research.

Relationship between behavioral measures of anxiety and latent inhibition in mature rats

This study adopted a novel approach to relating nonhuman and human studies of anxiety and latent inhibition, by exploring the degree to which rats’ “temperaments” in relation to anxiety predicted the development of latent inhibition. It investigated whether anxiety levels in one situation (i.e., an elevated-plus maze) involving 38 intact, mature rats, could predict performance on a latent inhibition task (i.e., an animal model of attention), and, thus, reproduce findings from human studies. Rats were subjected to two tasks: a novel within-subject, appetitive stimulus pre-exposure procedure, and an elevated-plus maze task. In the stimulus pre-exposure task, non-reinforced exposure to a light led to facilitation of conditioning (perceptual learning) during the first 3 days, and to retardation of conditioning (latent inhibition) during the last 5 days. In the elevated-plus maze task, moderate levels of anxiety were observed. Regression analyses revealed that anxiety levels (plus maze) were a significant predictor of latent inhibition (stimulus pre-exposure). Measures of locomotor activity did not predict performance on the latent inhibition task. Rats with moderate levels of anxiety had better performance in the late inhibition task than animals with low levels of anxiety. These data and the methodology have implications for understanding nonhuman models of schizophrenia, and for the design of studies investigating these issues with nonhumans.

Abdominal Vagal Afferents Modulate the Brain Transcriptome and Behaviors Relevant to Schizophrenia

Reduced activity of vagal efferents has long been implicated in schizophrenia and appears to be responsible for diminished parasympathetic activity and associated peripheral symptoms such as low heart rate variability and cardiovascular complications in affected individuals. In contrast, only little attention has been paid to the possibility that impaired afferent vagal signaling may be relevant for the disorder's pathophysiology as well. The present study explored this hypothesis using a model of subdiaphragmatic vagal deafferentation (SDA) in male rats. SDA represents the most complete and selective vagal deafferentation method existing to date as it leads to complete disconnection of all abdominal vagal afferents while sparing half of the abdominal vagal efferents. Using next-generation mRNA sequencing, we show that SDA leads to brain transcriptional changes in functional networks annotating with schizophrenia. We further demonstrate that SDA induces a hyperdopaminergic state, which manifests itself as increased sensitivity to acute amphetamine treatment and elevated accumbal levels of dopamine and its major metabolite, 3,4-dihydroxyphenylacetic acid. Our study also shows that SDA impairs sensorimotor gating and the attentional control of associative learning, which were assessed using the paradigms of prepulse inhibition and latent inhibition, respectively. These data provide converging evidence suggesting that the brain transcriptome, dopamine neurochemistry, and behavioral functions implicated in schizophrenia are subject to visceral modulation through abdominal vagal afferents. Our findings may encourage the further establishment and use of therapies for schizophrenia that are based on vagal interventions.SIGNIFICANCE STATEMENT The present work provides a better understanding of how disrupted vagal afferent signaling can contribute to schizophrenia-related brain and behavioral abnormalities. More specifically, it shows that subdiaphragmatic vagal deafferentation (SDA) in rats leads to (1) brain transcriptional changes in functional networks related to schizophrenia, (2) increased sensitivity to dopamine-stimulating drugs and elevated dopamine levels in the nucleus accumbens, and (3) impairments in sensorimotor gating and the attentional control of associative learning. These findings may encourage the further establishment of novel therapies for schizophrenia that are based on vagal interventions.

An examination of the roles of glutamate and sex in latent inhibition: Relevance to the glutamate hypothesis of schizophrenia?

The present study examined the effects of the glutamate receptor antagonist MK-801, the glutamate receptor agonist N-methyl-D-aspartate (NMDA), and sexual dimorphism on latent inhibition to elucidate the glutamate hypothesis of schizophrenia. During the pre-exposure phase, 56 male and 65 female Wistar rats were intracerebroventricularly administered normal saline, MK-801 or NMDA, in the left ventricle and then exposed to a passive avoidance box (or a different context) in three trials over 3 days. Then, all of the rats were placed in the light compartment of the passive avoidance box and were allowed to enter the dark compartment, where they each received a footshock (1mA, 2s) in five trials over 5 days. Injections of the glutamate drugs NMDA and MK-801 did not affect latent inhibition. Sexual dimorphism did not occur in latent inhibition. The present data on the male rats indicated that the glutamate system did not affect latent inhibition, indicating that the glutamate system was not like the dopamine system in terms of mediating the positive symptoms of schizophrenia. The glutamate system might be involved in the negative and cognitive symptoms of schizophrenia. The results may provide information for novel treatments of the negative and cognitive symptoms of schizophrenia.

Ovarian hormones, sleep and cognition across the adult female lifespan: An integrated perspective

Loss of ovarian function in women is associated with sleep disturbances and cognitive decline, which suggest a key role for estrogens and/or progestins in modulating these symptoms. The effects of ovarian hormones on sleep and cognitive processes have been studied in separate research fields that seldom intersect. However, sleep has a considerable impact on cognitive function. Given the tight connections between sleep and cognition, ovarian hormones may influence selective aspects of cognition indirectly, via the modulation of sleep. In support of this hypothesis, a growing body of evidence indicates that the development of sleep disorders following menopause contributes to accelerated cognitive decline and dementia in older women. This paper draws from both the animal and human literature to present an integrated view of the effects of ovarian hormones on sleep and cognition across the adult female lifespan.

Involvement of D1 and D2 dopamine receptor in the retrieval processes in latent inhibition

RATIONALE

Contemporary theories propose that latent inhibition (LI) is due to a process of interference with the context playing a key role as recovery cue. Physiological studies have demonstrated that LI is a process dependent on striatal dopamine. D2 dopamine receptors have been specifically associated with its expression, while D1 receptor has shown a limited function. However, to evaluate the role of dopamine receptors in LI, it is necessary to analyse their activity during recovery phase, where the mechanisms involved in interference processes are performed.

OBJECTIVE

The experiments studied the involvement of the dopaminergic system in the retrieval process of LI. We analysed the effect of the systemic administration of dopaminergic D1 (SCH-23390) and D2 (sulpiride) antagonist during the test phase on LI and on its contextual specificity.

METHODS

Animals were pre-exposed to saccharin solution and conditioned with a LiCl administration in conditioning phase. Dopaminergic antagonist drugs were administered during the test phase. Experiment 2 used the same context in all the phases. Experiment 3 used a new context during conditioning and test phase.

RESULTS

The D2 antagonist increased the LI effect and, in turn, diminished the normally suppressant effect of the context shift on LI. The opposite effect was observed under the D1 antagonist administration. This drug disrupted LI and enhanced the effect that the context shift had on this cognitive process.

CONCLUSIONS

D2 receptor had a relevant role on retrieval processes of pre-exposure learning, while D1 receptor was related with the contextual control of conditioning.

Ketamine administration during the second postnatal week induces enduring schizophrenia-like behavioral symptoms and reduces parvalbumin expression in the medial prefrontal cortex of adult mice

Dysfunctions in the GABAergic system are considered a core feature of schizophrenia. Pharmacological blockade of NMDA receptors (NMDAR), or their genetic ablation in parvalbumin (PV)-expressing GABAergic interneurons can induce schizophrenia-like behavior in animals. NMDAR-mediated currents shape the maturation of GABAergic interneurons during a critical period of development, making transient blockade of NMDARs during this period an attractive model for the developmental changes that occur in the course of schizophrenia's pathophysiology. Here, we examined whether developmental administration of the non-competitive NMDAR antagonist ketamine results in persistent deficits in PFC-dependent behaviors in adult animals. Mice received injections of ketamine (30mg/kg) on postnatal days (PND) 7, 9 and 11, and then tested on a battery of behavioral experiments aimed to mimic major symptoms of schizophrenia in adulthood (between PND 90 and 120). Ketamine treatment reduced the number of cells that expressed PV in the PFC by ∼60% as previously described. Ketamine affected performance in an attentional set-shifting task, impairing the ability of the animals to perform an extradimensional shift to acquire a new strategy. Ketamine-treated animals showed deficits in latent inhibition, novel-object recognition and social novelty detection compared to their SAL-treated littermates. These deficits were not a result of generalized anxiety, as both groups performed comparably on an elevated plus maze. Ketamine treatment did not cause changes in amphetamine-induced hyperlocomotion that are often taken as measures for the positive-like symptoms of the disorder. Thus, ketamine administration during development appears to be a useful model for inducing cognitive and negative symptoms of schizophrenia.

Context-dependent latent inhibition in preweanling rats

Preexposure to a conditioned stimulus (CS) usually weakens conditioning, an effect known as latent inhibition. Similar to other learning interference effects, latent inhibition has been characterized as context-dependent, which means that the magnitude of this effect can be attenuated by changing the context between the different phases of the procedure (e.g., preexposure and conditioning). Latent inhibition has been found with a variety of procedures in infant rats, but the few studies that examined the context-dependency of this phenomenon during this ontogenetic period found no context-change effect. The present study explored the context-dependency of latent inhibition during infancy using a conditioned taste aversion preparation and employing contexts enriched with distinctive odors to increase the possible efficacy of the context manipulation. Experiment 1 showed that three preexposures to the CS (saccharin) were sufficient to retard conditioning to the same CS, although this effect was also observed in a control group preexposed to an alternative taste stimulus (saline), in comparison with a non-preexposed control group. In Experiment 2a, the CS-preexposure effect was found to be specific to the preexposed CS when the number of preexposures was increased. This effect was revealed as context-dependent in Experiment 2b, since it was attenuated by changing the context between preexposure and conditioning. The present result is consistent with recent studies showing the context-dependency of extinction in preweanling rats, thus demonstrating these animals' capacity to learn about context early on in their development.

Deficits in latent inhibition induced by estradiol replacement are ameliorated by haloperidol treatment

There are sex differences in the symptomatology of schizophrenia, and in the response to antipsychotic treatments. One hallmark symptom of schizophrenia is a deficit in selective attention. Selective attention can be measured using a latent inhibition (LI) paradigm in humans; LI can be measured in rodents, and is used as an animal model of the selective attention deficits observed in schizophrenia. In the current experiments LI was used to clarify whether selective attention differs between male rats and ovariectomized (OVX) female rats receiving different estradiol (E2) replacement regimens. An additional aim was to determine whether haloperidol’s (HAL) facilitation of LI is enhanced by E2. Males and OVX female rats were trained in a conditioned emotional response LI paradigm. Females received no E2 replacement, a chronic low dose of E2 via silastic capsule, or a high phasic dose of E2 via silastic capsule accompanied by E2 (10 µg/kg subcutaneous (SC)) injections every 4th day. Actual plasma levels of E2 were determined using an enzyme linked immunosorbent assay. Rats were also administered a vehicle treatment, a 0.05 mg/kg, or a 0.1 mg/kg IP injection of HAL. Males and OVX females that did not receive E2 replacement both exhibited LI, but LI was not observed in the low and high E2 replacement groups. HAL restored LI at a lower dose in the females receiving high E2 replacement compared to females receiving low E2 replacement, indicating that E2 replacement facilitates HAL in restoring LI.

Gender differences in the effects of presynaptic and postsynaptic dopamine agonists on latent inhibition in rats

The present study investigated gender differences in the effects of presynaptic and postsynaptic DA agonists on latent inhibition in the passive avoidance paradigm. During the preexposure phase, 32 male and 32 female Wistar rats were exposed to a passive avoidance box (or a different context) and received drug injections in three trials: the control group received an injection of 10% ascorbic acid in a different context. The experimental groups received injections of 10% ascorbic acid (latent inhibition [LI] group), 1mg/kg of the postsynaptic DA D(1)/D(2) agonist apomorphine (APO group), and 1.5mg/kg of the presynaptic DA agonist methamphetamine (METH group) in a passive avoidance box. All experimental groups were placed in the light compartment of the passive avoidance box and were allowed to enter into the dark compartment to receive a footshock (1mA, 2s) in five trials over 5 days. The latency to enter into the dark compartment was recorded in these five trials. The latent inhibition occurred in the female LI group but not in the male LI group. Regardless of gender, the APO group exhibited an increase in latent inhibition. Male rats in the METH group exhibited a decrease in latent inhibition, but female rats in the METH group exhibited an increase in latent inhibition, indicating that the METH group exhibited sexual dimorphism. The gender factor interacted only with the METH group and not the LI or APO group. The present paper discusses whether gender, the postsynaptic DA D(1)/D(2) agonist APO, and presynaptic DA agonist METH may be related to schizophrenia.

Analysis of sensory, motor and cognitive functions of the coloboma (C3Sn.Cg-Cm/J) mutant mouse

The coloboma mutant mouse (C3Sn.Cg-Cm/J) has been proposed as an animal model of attention-deficit hyperactivity disorder (ADHD) because of excessive locomotion in the open field, yet few studies have looked at other behavioral measures in these mice. We analyzed activity levels of male and female Cm mice and their littermate controls (C3H) in two different types of open field, as well as their hearing (acoustic startle) and sensorimotor gating (prepulse inhibition), pain responsiveness (tail flick and hot plate), motor control (balance beam), motor learning (Rotarod), hippocampal working memory (spontaneous alternation in a Y-maze) and olfactory learning and memory (conditioned odor preference). We found hyperactivity and a lack of habituation in the small and large open fields and a deficit in prepulse inhibition in these mice, as well as a learning deficit in male Cm mice in conditioned odor preference but no deficits in pain perception or spontaneous alternation. Results from the rotarod and balance beam tasks indicate that Cm mice have severe motor co-ordination and balance problems compared to their C3H littermates, suggesting that Cm mice may be a more suitable model of ataxia than ADHD.

Latent inhibition is affected by phase of estrous cycle in female rats

Estrogen has been shown to have a strong modulatory influence on several types of cognition in both women and female rodents. Latent inhibition is a task in which pre-exposure to a neutral stimulus, such as a tone, later impedes the association of that stimulus with a particular consequence, such as a shock. Previous work from our lab demonstrates that high levels of estradiol (E2) administered to ovariectomized (OVX) female rats abolishes latent inhibition when compared to female rats with low levels of E2 or male rats. To determine if this E2-induced impairment also occurs with the natural variations of ovarian hormones during the estrous cycle, this behavior was investigated in cycling female rats. In addition, pre-pubertal male and female rats were also tested in this paradigm to determine if the previously described sex differences are activational or organizational in nature. In a latent inhibition paradigm using a tone and a shock, adult rats were conditioned during different points of the estrous cycle. Rats conditioned during proestrus, a period of high E2 levels, exhibited attenuated latent inhibition when compared to rats conditioned during estrus or metestrus, periods associated with low levels of E2. Moreover, this effect is not seen until puberty indicating it is dependent on the surge of hormones at puberty. This study confirms recent findings that high E2 interferes with latent inhibition and is the first to show this is based in the activational actions of hormones.

Variations in selective and nonselective prediction error with the negative dimension of schizotypy

Two human associative-learning experiments investigated the relationship between the negative dimension of schizotypy and selective and nonselective prediction-error learning. Experiment 1 demonstrates that individuals low, but not high, on the introvertive anhedonia dimension of schizotypy demonstrate Kamin blocking, which has been taken to reflect the operation of selective learning (Rescorla & Wagner, 1972). In complement, Experiment 2 demonstrates that individuals high, but not low, on the same dimension demonstrate asymmetrical learning about the components of a compound stimulus that differ in their associative history, which has been suggested to reflect the operation of nonselective learning (Rescorla, 2000). The implications of this double dissociation for understanding the nature of the cognitive deficit in schizophrenia and for theories of learning are considered.

Altered awareness of action in schizophrenia: a specific deficit in predicting action consequences

Patients suffering from schizophrenia may report unusual experiences of their own actions. They may either feel that external forces are controlling their actions or even their thoughts, or they may feel in control of events that in fact are not caused by their actions. Most theories link these disturbances in the sense of agency to deficits in motor prediction, resulting in a mismatch between predicted and actual sensory feedback at a central comparator mechanism. Such theories therefore can account for situations in which the sense of agency is reduced. However, other experiments as well as clinical observations show an enhanced rather than reduced sense of agency in schizophrenic patients. Here, we distinguish between a predictive and a retrospective mechanism where both contribute to the experience of agency, and show that schizophrenia is associated with a specific impairment to the predictive component. We measured subjective time estimates of self-initiated voluntary action (a key press) that were followed by a sensory effect (a tone). When the voluntary actions had a high probability of causing tones, healthy volunteers showed a predictive shift of the perceptual estimate of the action towards the tone, even on occasional trials where the tone was omitted. No such shift occurred in the absence of the tone on blocks when tones were less frequent. The predictive component of action awareness was calculated as the difference between time estimates on 'action only' trials from blocks with lower and higher tone probabilities. Schizophrenic patients lacked this predictive component of action awareness, showing a shift on 'action only' trials, regardless of the probability of the tone. Importantly, the schizophrenic deficit in predicting the relation between action and effect was strongly correlated with severity of positive psychotic symptoms, specifically delusions and hallucinations. Furthermore, the patients showed an exaggerated retrospective binding between action and tone, shifting the perceived time of action whenever the tone occurred, relative to when it did not occur. Our quantitative, implicit measures show how basic sensory and motor experience may be altered in acute psychosis. The enhanced sense of agency in schizophrenia reflects reliance on retrospection, rather than prediction, to associate actions with external events. The failure to predict the effects of one's own actions may underlie the blurring and confusion in the relationship between the self and the world that characterizes acute psychosis.

Taste memory trace disruption by AP5 administration in basolateral amygdala

N-methyl-D-aspartate receptor has been related to learning and memory processes. Its characteristics make it a key candidate in the modulation of associative processes at physiological level. Traditionally, the main efforts have been directed to show its role in excitatory conditioning. Nevertheless, the studies that have analyzed its implication in inhibitory learning are scarce. We present an experiment where a preexposure effect on the conditioning (latent inhibition) is disrupted by 2-amino-5-phosphonopentanoic acid administered in basolateral amygdala. This data shows interference on taste memory trace, and attenuation of the inhibition effect.

Prefrontal cortex and reversion of atropine-induced disruption of the degraded contingency effect by antipsychotic agents and N-desmethylclozapine in rats

Interactive context processing is a cognitive ability that is altered in psychotic states, including schizophrenia. This deficit has been linked to prefrontal cortical dysfunction in humans. The degraded contingency effect (DCE) is a simple form of interactive context processing by which contextual information interferes with a target conditioned stimulus for control over conditioned responding. We have previously shown that the DCE was disrupted by the muscarinic receptor antagonist atropine and that this disruption was specifically restored by cholinergic drugs displaying an antipsychotic-like profile, such as physostigmine or xanomeline. The DCE was selectively associated with an increase in Fos immunoreactivity in the medial prefrontal cortex (mPFC), an increase that was not observed in the presence of atropine. Here, we set out to test the actions of typical, atypical and potential antipsychotics on atropine-induced disruption of the DCE and the related mPFC Fos-immunoreactivity profile. Low doses of haloperidol, olanzapine, clozapine and N-desmethylclozapine reversed atropine-induced disruption of the DCE, but with different dose-dependent curves (linear shapes for haloperidol and N-desmethylclozapine, inverted U shapes for olanzapine and clozapine). The level of Fos within the mPFC paralleled the pharmacological profile of the different drugs. Compared to contingent control groups, an increased level of Fos immunoreactivity within the mPFC was observed only with doses that reversed atropine-induced disruption of the DCE. These results suggest that the deficit of interactive context processing, which is a hallmark of psychotic states, might originate from a mere deficit of fundamental associative processes. This deficit might result from a cholinergic blockade of the PFC.

Entorhinal cortex and cognition

Understanding the function of the entorhinal cortex (EC) has been an important subject over the years, not least because of its cortical intermediary to and from the hippocampus proper, and because of electrophysiological advances which have started to reveal the physiology in behaving animals. Clearly, a lot more needs to be done but is clear to date that EC is not merely a throughput station providing all hippocampal subfields with sensory information, but that processing within EC contributes significantly to attention, conditioning, event and spatial cognition possibly by compressing representations that overlap in time. These are transmitted to the hippocampus, where they are differentiated again and returned to EC. Preliminary evidence for such a role, but also their possible pitfalls are summarised.

Differential influence of the ventral subiculum on dopaminergic responses observed in core and dorsomedial shell subregions of the nucleus accumbens in latent inhibition

It has previously been reported that dopamine (DA) responses observed in the core and dorsomedial shell parts of the nucleus accumbens (Nacc) in latent inhibition (LI) are dependent on the left entorhinal cortex (ENT). The present study was designed to investigate the influence of the left ventral subiculum (SUB) closely linked to the ENT on the DA responses obtained in the Nacc during LI, using an aversive conditioned olfactory paradigm and in vivo voltammetry in freely moving rats. In the first (pre-exposure) session, functional blockade of the left SUB was achieved by local microinjection of tetrodotoxin (TTX). In the second session, rats were aversively conditioned to banana odor, the conditional stimulus (CS). In the retention (test) session the results were as follows: (1) pre-exposed (PE) conditioned animals microinjected with TTX, displayed aversion toward the CS; (2) in the core part of the Nacc, for PE-TTX-conditioned rats as for non-pre-exposed (NPE) conditioned animals, DA levels remained close to the baseline whereas DA variations in both groups were significantly different from the DA increases observed in PE-conditioned rats microinjected with the solvent (phosphate-buffered saline (PBS)); (3) in the shell part of the Nacc, for PE-TTX-conditioned rats, DA variations were close to or above the baseline. They were situated between the rapid DA increases observed in NPE-conditioned animals and the transient DA decreases obtained in PE-PBS-conditioned animals. These findings suggest that, in parallel to the left ENT, the left SUB controls DA LI-related responses in the Nacc. The present data may also offer new insight into the pathophysiology of schizophrenia.

Determinants of cue interactions

In a Pavlovian conditioning situation, there are many training variables that may affect responding to a conditioned stimulus (CS), such as contiguity, contingency, and the presence of other CSs. This review describes recent experiments that show that some manipulations that usually decrease responding to a CS may have the opposite effect when they are combined with other normally pernicious manipulations. A theoretical framework that explains these so-called counteraction effects is provided. The apparent boundary conditions on the effects and limitations of the theory are discussed.

The amphetamine-induced sensitized state as a model of schizophrenia

Schizophrenia is a serious psychiatric disorder which impacts a broad range of cognitive, behavioural and emotional domains. In animals, exposure to an intermittent, escalating dose regimen of amphetamine induces a sensitized state that appears to share a number of behavioural and neurochemical similarities with schizophrenia. In humans repeated exposure to amphetamine, or other psychomotor stimulants, can induce sensitization as well as psychosis. The following paper evaluates the evidence for the amphetamine-induced sensitized state as an animal model of schizophrenia, focussing separately on the positive, cognitive and negative symptoms associated with this disease. Current evidence supports the use of amphetamine sensitization as a model of the positive symptoms observed in schizophrenia. Additionally, there is increasing evidence for long-lasting cognitive deficits in sensitized animals, especially in the area of attention and/or cognitive flexibility. Other areas of cognition, such as long-term memory, appear to be unaltered in sensitized animals. Finally, little evidence currently exists to either support or refute the use of amphetamine sensitization as a model of negative symptoms. It is concluded that amphetamine sensitization likely impacts behaviour by altering the functioning of mesolimbic dopamine systems and prefrontal cortical function and can serve as a model of certain domains of schizophrenia.

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.

Associations between schizotypal personality traits and the facilitation and inhibition of the speed of contextually cued responses

This study measured schizotypal personality traits in a sample of 33 healthy participants using the Oxford-Liverpool Inventory of Feelings and Experiences. These traits were correlated with measures of inhibition and facilitation of response times (RTs) within a cued letter-comparison task. It was expected that scores on a measure of positive schizotypy, reflecting unrealistically distorted perceptions and beliefs, would be negatively associated with inhibition of RTs to targets that were unexpected in the context of the preceding letter cue. No specific predictions were made for the facilitation of RTs to targets expected in the context of the cues. The predicted negative association for positive schizotypy and inhibition of RTs was confirmed. There was no significant association between positive schizotypy and facilitation of RTs; however, there was an unpredicted finding that facilitation of RTs was increased in individuals with higher levels of disorganized schizotypy. The findings for positive schizotypy were consistent with Hemsley's model, in that high positive schizotypy results from a weakening of contextually elicited inhibitory processes, and is associated with altered functioning of a monitoring system. The normal functioning of the monitoring system is to generate mismatch signals that interrupt information processing when a contextually unexpected stimulus occurs.

Abnormal latent inhibition and impulsivity in coloboma mice, a model of ADHD

Attention deficit hyperactivity disorder (ADHD) is characterized by hyperactivity, inattention, and impulsivity. The coloboma mouse model of ADHD exhibits profound hyperactivity. To determine whether coloboma mice exhibit other signs of ADHD, we assessed latent inhibition as a test of attention, and impulsivity in a delayed reinforcement paradigm. Latent inhibition was present in control mice but was disrupted in coloboma mice. Coloboma mice also exhibited impaired performance on the delayed reinforcement task and were not able to wait as long as control mice to obtain the greater reinforcer. Because norepinephrine mediates hyperactivity in coloboma mice, we examined the role of norepinephrine in disrupted latent inhibition and impulsivity. Reduction of norepinephrine with DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride) restored latent inhibition but did not ameliorate impulsivity. In summary, coloboma mice exhibit hyperactivity, inattention as determined by latent inhibition, and impulsivity, and norepinephrine mediates hyperactivity and inattention but not impulsivity in these mice.

Modeling Madness in Mice: One Piece at a Time

Mouse models that recapitulate the full phenotypic spectrum of a psychiatric disorder, such as schizophrenia, are impossible. However, a more piecemeal recreation of phenotypic components is feasible and promises to harness the power of animal models using approaches that are either off limits or confounded by drug treatment in humans. In that context, animal models will have a central and indispensable role in the process of discovering the causes of psychiatric disorders and generating novel, mechanism-based treatments. Here, we discuss current approaches used to generate animal models of psychiatric disorders, address the different components of these disorders that can be modeled in animals, and describe currently available analytical tools. We also discuss accumulating empirical data and take an in-depth look at what we believe to be the future of animal models made possible by recent advances in psychiatric genetics.

Performance of F2 B6x129 hybrid mice in the Morris water maze, latent inhibition and prepulse inhibition paradigms: Comparison with C57Bl/6J and 129sv inbred mice

Assessment of cognition and information processing in mice is an important tool in preclinical research that focuses on the development of cognitive enhancing drugs. Analysis of transgenic (TG) and knockout (KO) mice is usually performed on a F2 B6x 129 background. In the present study, we have compared performance of F2 B6x 129 hybrid mice (F2 mice) with that of the two parental inbred strains (C57Bl/6J and 129sv mice), and a wild-type (WT) strain (with a combined B6x 129 background) in three cognitive/information processing paradigms. It was found that the F2 mice outperformed either of the parental strains and provide a control sample with good baseline performance in the Morris water maze (MWM). Reliable deficits could be obtained in learning and memory in this paradigm following injections with scopolamine (0.16 mg/kg) in the F2 mice, which can potentially be used to test effects of reference and novel compounds in order to develop cognitive enhancing drugs. Furthermore, it was shown that the four genotypes showed normal latent inhibition (LI) using the conditioned taste aversion (CTA) paradigm and exhibited no differences in prepulse inhibition (PPI) levels. Following the setup of these procedures in mice, we are now able to compare the effects of gene knockout/mutations used for target validation with results in the present study as a frame of reference.

Immunological stress at the maternal-foetal interface: a link between neurodevelopment and adult psychopathology

Maternal infection during pregnancy is associated with a higher incidence of mental disorders, including schizophrenia, in the offspring in later life. Our recent attempt to study this link between prenatal immunological challenge and subsequent psychopathology has led to the establishment of a mouse model demonstrating the emergence of multiple psychotic-like phenotypes following immunological challenge on gestation day (GD) 9. However, little is known about the impact of similar in utero challenge at different times of pregnancy. Here, we compare the efficacy of identical maternal immune stimulation induced by the exposure to polyriboinosinic-polyribocytidilic acid (Poly(I:C)) at a dose of 5mg/kg (i.v.) on distinct days of gestation (GD 6, 9, 13 or 17). The offspring derived were then compared to those collected from vehicle- and non-treated dams in two paradigms of selective associative learning: latent inhibition (LI) and the US-pre-exposure effect (USPEE). LI deficiency was observed in animals born to dams treated with Poly(I:C) on GD 6, 9 or 13, but not in those on GD17. In contrast, a loss of the USPEE was equivalently seen in all Poly(I:C) treatment groups, regardless of treatment times. Evaluation of the acute cytokine response in a separate cohort of pregnant dams receiving Poly(I:C) challenge on either GD9 or GD17 revealed that the ratio of interleukin-10/tumor necrosis factor-alpha was elevated in the GD17 relative to the GD9 group. The present report thus provides evidence that the acute cytokine reaction as well as the long-term pattern of behavioural sequelae of maternal immune challenge can be affected by its precise timing during pregnancy. The present study provides further support to the use of the prenatal Poly(I:C) model in the elucidation of mechanisms involved in the aetiology and disease process of immuno-precipitated neurodevelopmental mental diseases, including but not limited to, schizophrenia.

There is a time and a place for everything: bidirectional modulations of latent inhibition by time-induced context differentiation

Latent inhibition (LI) is defined as poorer evidence of learning with a stimulus that previously was presented without consequence, as compared with a novel or previously attended stimulus. The present article reviews the evidence, mostly from three-stage conditioned taste aversion studies (preexposure, conditioning, and test), that LI can be either attenuated or enhanced depending on the length of the retention interval between conditioning and test and where that interval was spent. Time-induced reduction in LI is observed when the interval context is the same as that of the preexposure, conditioning, and test stages. Super-LI is obtained when a long retention interval is spent in a context that is different from that of the other stages. The differential modulations of LI appear to be the result of the strengthening of primacy effects (i.e., first training disproportionately stronger than subsequent training) by long-interval different contexts, thereby producing super-LI, and the reversal of this effect by long-interval same contexts, thereby producing attenuated LI. The bidirectional effects of time/ context modulations on LI, unaccounted for by current learning theories, are explained, in part, by a time-induced context differentiation process. Implications for theories of LI, learning, and, memory are discussed.

Dopamine manipulations limited to preexposure are sufficient to modulate latent inhibition

Four experiments are reported that demonstrated that dopamine (DA) transmission is involved in the acquisition of latent inhibition (LI) of a conditioned taste aversion. LI refers to weaker conditioning as a consequence of nonreinforced preexposure (PE) of the future conditioned stimulus. Although it is known to depend on DA transmission during the conditioning phase, it is usually thought that the cognitive processes involved in the establishment of LI (during the PE phase) are DA independent. Either amphetamine (AMPH; 0.5 or 1.0 mg/kg) or haloperidol (HAL; 0.1 mg/kg) were injected before 1 or all of the 3 PE sessions. AMPH blocked the acquisition of LI if it was injected before each or before only the last PE session and HAL potentiated LI.

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.

Sensitization to amphetamine, but not phencyclidine, disrupts prepulse inhibition and latent inhibition

RATIONALE

Schizophrenia has been linked to dysregulation of dopamine and glutamate transmitter systems. Attempts to model aspects of schizophrenia in animals have made use of treatments that primarily affect dopaminergic (e.g., amphetamine, Amp) and glutamatergic (e.g., phencyclidine, PCP) function. In addition to exerting short-term acute effects, these agents also induce long-term effects, as seen, for example, in neurochemical and behavioural sensitization.

OBJECTIVES

The goal of this work was to compare Amp- and PCP-sensitized states on two measures of information processing that are impaired in schizophrenia, prepulse inhibition (PPI) of the acoustic startle reflex and latent inhibition (LI).

METHODS

Rats received injections of Amp, PCP or saline 3 days per week for 3 weeks. The Amp dose increased from 1 to 3 mg/kg, at the rate of 1 mg/kg each week. The PCP dose was 3 mg/kg throughout. After various periods of withdrawal rats were tested for PPI and LI.

RESULTS

Repeated intermittent treatment with Amp or PCP resulted in augmented locomotor responses to challenge with each drug, providing an operational index that sensitization had occurred. Rats sensitized to Amp showed disrupted PPI when tested drug free at 3, 21 and 60 days of withdrawal. Amp-sensitized rats also showed abolition of the LI effect. Rats sensitized to PCP did not show deficits in any of these behaviours when tested drug free.

CONCLUSIONS

Because disrupted PPI and LI have both been reported in schizophrenic patients, these results suggest that the Amp-sensitized state may represent a useful model for investigating the neural bases of information processing deficits in schizophrenia.

Antagonism of the nitric oxide synthase inhibitor, L-NAME, of the effects of phencyclidine on latent inhibition in taste aversion conditioning

Latent inhibition (LI) is a behavioural procedure used to evaluate the potential propsychotic and antipsychotic properties of psychoactive drugs. In the present study, a conditioned taste aversion (CTA) procedure was used to investigate the effects of the nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), and the psychotomimetic drugs, phencyclidine (PCP) and d-amphetamine (d-AMP) on LI. PCP (2 mg/kg) and d-AMP (0.5 mg/kg) were both found to enhance LI in this procedure. The effect of d-AMP on LI was less pronounced and this drug also caused a weak disruption of taste aversion conditioning. Pretreatment with L-NAME (10 mg/kg) blocked the LI enhancing effect of PCP on LI but not that of d-AMP. L-NAME by itself caused an attenuation of LI. L-NAME has been shown to block also other behavioural and biochemical effects of PCP in previous studies and these results and the present findings suggest that at least some of the effects PCP are dependent on NO and possibly also that some NOS inhibitors may exert antipsychotic properties.

Haloperidol can increase responding to both discrete and contextual cues in trace conditioned rats

Haloperidol has been shown to enhance attentional selectivity in conditioning procedures. For example, in latent inhibition (LI) it improves animals' ability to treat as irrelevant, stimuli that have previously been presented without consequence. The present study tested whether this finding would generalize to other procedures that present animals with weak predictors. We therefore used a trace conditioning procedure to present rats with a conditioned stimulus (CS) weakened through temporal discontiguity (rather than preexposure in LI) and a flashing light background provided an alternative experimental stimulus. In Experiment 1, a noise CS was paired contiguously (at '0 s') with food or at a 10 s trace interval. In Experiment 2, the trace interval was lengthened to 20 s. In both experiments, haloperidol treatment generally reduced responding in 0 s contiguous groups. By contrast, 0.03 mg/kg haloperidol enhanced conditioning, selectively, to the weakly predictive trace CS, though it was without effect on responding within the trace interval. In addition, again at 0.03 mg/kg, haloperidol significantly increased excitatory conditioning to contextual stimuli in trace groups relative to contiguous groups. At the shorter (10 s) Experiment 1 trace, this result was shown in the extinction test of conditioning to the background stimulus. At the longer (20 s) Experiment 2 trace, this result was shown in the acquisition of responding to the box context in the inter-trial-interval. The demonstration that low dose haloperidol can increase conditioning is novel. This increase was seen selectively with stimuli (both trace-conditioned and contextual) that should have been treated as weak predictors so these results are contrary to what was expected on the basis of haloperidol effects on stimuli weakened through pre-exposure. The possibility that increased contextual conditioning could be relevant to the interpretation of haloperidol-induced enhancement of LI is discounted. However, it is suggested that this result could nonetheless reflect cognitive enhancement.

Systemic or intra-accumbens injection of D-amphetamine delays habituation to a tone stimulus in rats

Dopamine release within the nucleus accumbens shell is suggested to control the salience of environmental stimuli, and previous research has shown that the indirect dopamine agonist D-amphetamine can alter the salience of both aversive and neutral stimuli. In experiment 1, the effect of systemic injection of D-amphetamine (0.5, 1 mg/kg) on fear conditioning to a tone was assessed in an 'off-baseline' conditioned suppression procedure using several footshock intensities. Although the effects of amphetamine on conditioning were unclear, the results indicated a deficit of simple tone habituation in amphetamine-treated rats. In experiment 2, habituation of the orienting reaction to a tone was assessed by the progressive reduction of lick suppression upon repeated presentation of the auditory stimulus. D-Amphetamine delayed tone habituation, whether administered systemically (0.5, 1 mg/kg) or into the nucleus accumbens shell (3, 10 microg/0.5 microl). These data are consistent with electrophysiological and neurochemical data demonstrating the role of nucleus accumbens dopamine in novelty processing. The relevance of the data to latent inhibition is discussed.

Like extinction, latent inhibition of conditioned fear in mice is blocked by systemic inhibition of L-type voltage-gated calcium channels

Having recently shown that extinction of conditioned fear depends on L-type voltage-gated calcium channels (LVGCCs), we have been seeking other protocols that require this unusual induction mechanism. We tested latent inhibition (LI) of fear, because LI resembles extinction except that cue exposures precede, rather than follow, cue-shock pairing. Systemic injections of two LVGCC inhibitors, nifedipine and diltiazem, before pre-exposure blocked LI completely with no evidence of state-dependent learning. The results indicate that extinction and LI share a common molecular requirement and may support the notion that LI, like extinction, is a form of inhibitory learning.

The relevance of irrelevance to schizophrenia

Jeffrey Gray's neuropsychological theory of the positive symptoms of schizophrenia has been highly influential by enabling a strong link between animal and human research. Central to the development and testing of this theory has been the phenomenon and paradigm of latent inhibition (LI-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of the first stimulus). We review findings relating to its alteration in patients with schizophrenia (acute and chronic), people high on dimensions of schizotypy and the effects of amphetamine and anti-psychotic medication in humans. We suggest that many human-LI paradigms still suffer from theoretical and practical limitations, but that recent developments are beginning to address these. Finally we explore the idea that the paradigm of Learned Irrelevance (LIRR-the retardation of learning that one stimulus predicts the occurrence of another due to pre-exposure of both stimuli but in an unrelated manner) might be used to complement studies on LI in exploring the cognitive distortions suffered by patients with schizophrenia.

Towards an immuno-precipitated neurodevelopmental animal model of schizophrenia

Epidemiological studies have indicated an association between maternal bacterial and viral infections during pregnancy and the higher incidence of schizophrenia in the resultant offspring post-puberty. One hypothesis asserts that the reported epidemiological link is mediated by prenatal activation of the foetal immune system in response to the elevation of maternal cytokine level due to infection. Here, we report that pregnant mouse dams receiving a single exposure to the cytokine-releasing agent, polyriboinosinic-polyribocytidilic acid (PolyI:C; at 2.5, 5.0, or 10.0 mg/kg) on gestation day 9 produced offspring that subsequently exhibited multiple schizophrenia-related behavioural deficits in adulthood, in comparison to offspring from vehicle injected or non-injected control dams. The efficacy of the PolyI:C challenge to induce cytokine responses in naïve non-pregnant adult female mice and in foetal brain tissue when injected to pregnant mice were further ascertained in separate subjects: (i) a dose-dependent elevation of interleukin-10 was detected in the adult female mice at 1 and 6h post-injection, (ii) 12 h following prenatal PolyI:C challenge, the foetal levels of interleukin-1beta were elevated. The spectrum of abnormalities included impairments in exploratory behaviour, prepulse inhibition, latent inhibition, the US-pre-exposure effect, spatial working memory; and enhancement in the locomotor response to systemic amphetamine (2.5 mg/kg, i.p.) as well as in discrimination reversal learning. The neuropsychological parallels between prenatal PolyI:C treatment in mice and psychosis in humans, demonstrated here, leads us to conclude that prenatal PolyI:C treatment represents one of the most powerful environmental-developmental models of schizophrenia to date. The uniqueness of this model lies in its epidemiological and immunological relevance. It is, sui generis, ideally suited for the investigation of the neuropsychoimmunological mechanisms implicated in the developmental aetiology and disease processes of schizophrenia.

Intact visual latent inhibition in schizophrenia patients in a within-subject paradigm

People are normally slower to learn a CS-UCS association if they first experience the CS without the UCS. This normal slowing, termed "latent inhibition" (LI), is reported by some to be absent in schizophrenia patients. Our previous studies detected generalized learning deficits but not LI deficits in schizophrenia patients, using between-subject auditory and visual LI paradigms. To understand our divergent results, we developed a within-subject visual LI paradigm that detects LI in normal male subjects that we previously reported to be disrupted by acute treatment with dopamine agonists. In the present study, we verified the ability of this dopamine-sensitive within-subject LI paradigm to detect LI among both male and female normal control subjects, and then used this paradigm to assess LI in schizophrenia patients. Among normals, LI exhibited no sex differences or menstrual cyclicity. Compared to normals, schizophrenia patients exhibited learning deficits with both preexposed (PE) and non-preexposed (NPE) stimuli. Despite these generalized deficits, both acutely hospitalized patients and stable outpatients with schizophrenia exhibited robust LI, as evidenced by significantly faster learning with NPE than PE stimuli. LI deficits in schizophrenia may be paradigm-specific and are not detected by a paradigm that we previously reported to be sensitive to disruption by dopamine agonists.

Expression of the CS- and US-pre-exposure effects in the conditioned taste aversion paradigm and their abolition following systemic amphetamine treatment in C57BL6/J mice

In classical conditioning, pre-exposures to either the to-be-conditioned stimulus (CS) or unconditioned stimulus (US) can retard subsequent conditioning between the CS and US. The present experiment evaluated the expression of these two pre-exposure effects in mice of the C57BL6/J strain, one of the most common background strains for genetically altered mice. We tested whether their expression would be disrupted by amphetamine treatment (2.5 mg/kg, i.p.) in a conditioned taste aversion paradigm with sucrose as the CS and lithium chloride-induced gastric malaise as the US. We found that one pre-exposure (PE) to either the CS or the US reduced aversion to sucrose solution in the controls following conditioning, but no such tendency was evident in the amphetamine-treated mice. The present study represents the first report of amphetamine-induced disruption of the CS-PE effect (ie latent inhibition) in mice, and the first attempt to compare it directly with the US-PE effect in any species. It extended previous reports in rats and humans, suggesting that the sensitivity of latent inhibition to amphetamine is largely comparable across species, thereby lending credence to the use of the latent inhibition effect as a behavioral assay for psychotic-like phenotype in transgenic mice. The parallel observation in the US-PE effect further indicates that its expression, at least in the present conditioned taste aversion paradigm, may also be under similar influence of the dopaminergic system.

Transient disruption of attentional performance following escalating amphetamine administration in rats

RATIONALE

Attentional deficits are thought to be critically involved in the development of positive symptoms in schizophrenia. The present experiment tests the general hypothesis that sensitization of the mesolimbic dopaminergic system contributes to the attentional deficits in schizophrenia.

OBJECTIVES

The present study assessed attentional performance following administration of an escalating amphetamine regimen and subsequent "challenge" amphetamine administration in rats.

METHODS

Rats were trained to perform a two-lever sustained attention task that involved discrimination of visual signals and no signal presentation. After reaching criterion, subjects were assigned to receive escalating amphetamine or saline. Attentional performance was assessed immediately following escalating amphetamine, following "challenge" amphetamine administration (1.0 mg/kg) to amphetamine-pretreated rats, and for 3 days after the challenge session. At the end of this experiment, a dose-response study was conducted with saline-pretreated rats to confirm the appropriateness of the challenge dose.

RESULTS

Amphetamine-pretreated animals demonstrated a transient increase in errors on nonsignal trials following escalating amphetamine administration. The latency to press a lever was decreased during and after challenge amphetamine administration. Administration of 1.0 mg/kg amphetamine did not alter accuracy of amphetamine-pretreated animals or of saline-pretreated animals in the dose-response experiment.

CONCLUSIONS

Prior escalating amphetamine administration transiently disrupted attention, increasing incorrect "claims" for a signal on trials when no signal was presented. The present data support the existing literature that escalating amphetamine regimens may be useful to model the attentional deficits that contribute to the psychotic symptoms in schizophrenia.

Influence of the entorhinal cortex on accumbal and striatal dopaminergic responses in a latent inhibition paradigm

The use of latent inhibition paradigms is one means of investigating the involvement of mesencephalic dopaminergic (DA) neurons in cognitive processes. We have shown recently that DA neurons reaching the core and the dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum are differentially involved in latent inhibition. In other respects, theoretical, behavioral and anatomo-functional data suggest that the entorhinal cortex (ENT) may control latent inhibition expression. In this study, using in vivo voltammetry in freely moving rats, we investigated the influence of the ENT on the DA responses obtained in the core and dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum. For this purpose a reversible inactivation of the left ENT was achieved by the local microinjection of tetrodotoxin, 3 h before pre-exposure to the conditional stimulus (banana odour). During the second session, animals were aversively conditioned to banana odour. Results obtained during the third session (test session), in animals submitted to the reversible blockade of the ENT before the first session were as follows: (1) pre-exposed conditioned animals displayed behavioral aversive responses; (2) where core DA responses were concerned, responses were situated between those observed in pre-exposed and non-pre-exposed conditioned animals; (3) by contrast, where the dorsomedial shell part of the nucleus accumbens and the anterior striatum were concerned, DA variations were not statistically different in pre-exposed and non-pre-exposed conditioned rats. These data suggest that the left ENT exerts a crucial influence over the latent-inhibition-related DA responses in the left dorsomedial shell part of the nucleus accumbens and the left anterior part of the striatum, whereas one or more other brain regions control DA variations in the left core part of the nucleus accumbens. These data may help us to understand the pathophysiology of schizophrenic psychoses.

Latent inhibition is spared by n-methyl-d-aspartate (nmda)-induced ventral hippocampal lesions, but is attenuated following local activation of the ventral hippocampus by intracerebral nmda infusion

Repeated non-reinforced exposures of a neutral stimulus retard the development of a conditioned response to that stimulus when it is subsequently paired with a significant event. This stimulus pre-exposure effect is known as latent inhibition (LI). Early lesion studies have initially suggested an important role for the hippocampus in the normal development and expression of LI. This view has since been modified with the emergence of data derived from selective cell body lesions of the hippocampus and of the entorhinal cortex, with an abolition of LI only seen after lesions of the latter. This suggests that the significance of the hippocampus might have been overestimated in the past, possibly due to interruption of fibres en passage. However, intact behavioural expression of LI following hippocampal damage does not preclude the suggestion that the hippocampus participates in the control and regulation of LI expression in intact animals. The present study demonstrated that whilst cell body lesions of the ventral hippocampus spared LI (as expected), chemical activation of the ventral hippocampus by local N-methyl-D-aspartate infusion disrupted LI. These results parallel our earlier observations on prepulse inhibition (PPI) with similar manipulations [Neuroreport 10 (1999) 2533]. Thus, although the ventral hippocampus is itself not responsible for the behavioural manifestation of LI and PPI, it exerts at least a modulatory control over the form and/or magnitude of their expression. Our results should prompt a re-evaluation of the relative roles of the hippocampus and retrohippocampus in the development and expression of LI.