Latent inhibition in drug naive schizophrenics: relationship to duration of illness and dopamine D2 binding using SPET

A neural plasticity perspective on the schizophrenic condition

Imbalanced plasticity of neural networks in the brain is proposed to underlie deficits in the integration of efferent and afferent processes in schizophrenia. These deficits affect the priming of the behavior implementing systems by prior knowledge, and thus impair both controlled regulation and automatic activation of mental and motor processes. The sense of self as a distinct entity can consequently be undermined. In predominantly reality-distorting patients, hypo-plasticity of neural connectivity may cause the emergence of highly focused but inflexible patterns of activation in their representation and response systems. This may lead to dominance of prepotent patterns of activity in these systems and a relative inability of higher control systems to bias lower level activity towards congruence with the ongoing cognitive and motor context. By contrast, predominantly disorganized patients are characterized by hyper-plastic connectivity. This leads to a weakening of prepotent response tendencies but also, as in reality-distorting patients, to less effective top-down contextual constraining.

Scopolamine induces disruption of latent inhibition which is prevented by antipsychotic drugs and an acetylcholinesterase inhibitor

The fact that muscarinic antagonists may evoke a psychotic state ('antimuscarinic psychosis'), along with findings of cholinergic alterations in schizophrenia, have kindled an interest in the involvement of the cholinergic system in this disorder. Latent inhibition (LI) is a cross-species phenomenon manifested as a poorer conditioning of a stimulus seen when the stage of conditioning is preceded by a stage of repeated nonreinforced pre-exposure to that stimulus, and is considered to index the capacity to ignore irrelevant stimuli. Amphetamine-induced LI disruption and its reversal by antipsychotic drugs (APDs) is a well-established model of positive symptoms of schizophrenia. Here, we tested whether the muscarinic antagonist scopolamine would disrupt LI and whether such disruption would be reversed by APDs and by the acetylcholinesterase inhibitor physostigmine. The results showed that scopolamine at doses of 0.15 and 0.5 mg/kg disrupted LI, and that this effect was due to the action of the drug in the pre-exposure stage, suggesting a role of muscarinic transmission in attentional processes underlying LI. Both the typical and the atypical APDs, haloperidol and clozapine, reversed scopolamine-induced LI disruption when given in conditioning or in both stages, but not in pre-exposure, indicating that the mechanism of antipsychotic action in this model is independent of the mechanism of action of the propsychotic drug. Scopolamine-induced LI disruption was reversed by physostigmine (0.05 and 0.15 mg/kg), which was ineffective in reversing amphetamine-induced LI disruption, pointing to distinct mechanisms underlying LI disruption by these two propsychotic drugs. The latter was further supported by the finding that unlike amphetamine, the LI-disrupting doses of scopolamine did not affect activity levels. We propose scopolamine-induced LI disruption as a model of cholinergic-related positive symptoms in schizophrenia.

A new continuous within-participants latent inhibition task: examining associations with schizotypy dimensions, smoking status and gender

A within-participants latent inhibition task (LI: the retardation in learning that occurs if a stimulus has first been presented without consequence) was developed that produced robust LI, using both reaction time and number of correct responses as the dependent measures. Furthermore, associations were found between the preexposed stimulus and the schizotypy dimension of unusual experiences and smoking status. There were no significant relationships with the non-preexposed stimulus. This pattern of results was found using both reaction time and number of correct responses as the dependent measures which indicates that individuals who smoke and those high in unusual experiences show reduced LI. These findings indicate that the task may have sufficient sensitivity to be used in patients with schizophrenia.

Obsessive–compulsive disorder patients display enhanced latent inhibition on a visual search task

Latent inhibition (LI) is a phenomenon that reflects the ability to ignore irrelevant stimuli. LI is attenuated in some schizophrenic patient groups and in high schizotypal normal participants. One study has found enhanced LI in patients with obsessive-compulsive disorder (OCD [Swerdlow, N. R., Hartston, H. J., & Hartman, P. L., 1999. Enhanced visual latent inhibition in obsessive-compulsive disorder. Biological Psychiatry, 45, 482-488]). The present experiment replicated this finding using a within-subject visual search LI task, with OCD patients displaying more LI than healthy controls. The contrasting LI effects in schizophrenia and OCD are discussed in terms of how these groups differentially process relevant and irrelevant stimuli, and how that outcome affects subsequent behavior.

Withdrawal from continuous amphetamine administration abolishes latent inhibition but leaves prepulse inhibition intact

RATIONALE

Schizophrenia has been associated with dysregulation of dopamine (DA) transmission and impairment in a number of experimental tasks, including sensorimotor gating assessed using prepulse inhibition (PPI) and selective attention assessed using latent inhibition (LI). We have demonstrated in previous studies that after withdrawal from escalating (ESC) dosages of amphetamine (AMPH), animals exhibited disruption of LI but no alteration of PPI. Moreover, these animals always showed behavioural sensitization to an AMPH challenge.

OBJECTIVE

In this study, we were interested in testing whether a different administration schedule would elicit disruption of both LI and PPI.

METHODS

Animals were treated with continuous AMPH release (via osmotic mini-pumps at a dosage of 10 mg kg(-1) day(-1) for 7 days) and tested for their performance in L and PPI during withdrawal in a drug free state. Rats received AMPH treatment during the induction phase in their home cages or in the activity chambers. Following withdrawal, the expression of behavioural sensitization to an AMPH challenge was tested in both cases in the activity chambers.

RESULTS

Animals pretreated with AMPH from both groups did not exhibit behavioural sensitization. Withdrawal from continuous administration induced LI attenuation with no effect on PPI.

CONCLUSIONS

These findings are similar to what was previously found with respect to an ESC AMPH regime. The only difference between the schedules was that the ESC AMPH schedule led to behavioural sensitization whereas the continuous AMPH did not. It is suggested that the expression of sensitization may not be a prerequisite for observed LI disruption.

Latent inhibition and asymmetrical visual-spatial attention in children with ADHD

The research was designed to determine whether the purported hemispheric asymmetries that are associated with attention deficit/hyperactivity disorder (ADHD) affect performance on a selective attention visual search task, and whether any obtained asymmetry will be modulated by methylphenidate. Two groups of children (8-15 years) with ADHD, one with methylphenidate treatment (ADHD+) and one without (ADHD+), were compared to matched controls on a two-stage visual search task. The task assessed right-left visual field asymmetries and the effects of changing a previous distractor into a target. Such a procedure, related to latent inhibition (LI; poorer performance to a previously irrelevant stimulus than to a novel one), can provide evidence for dysfunctional processing of irrelevant stimuli. All three groups exhibited the LI effect. The ADHD group, however, exhibited less LI for left- than right-side targets, an effect absent in the control and ADHD+ groups, suggesting a lateralized attentional deficit for ADHD+ that was normalized by methylphenidate.

Withdrawal from repeated amphetamine administration leads to disruption of prepulse inhibition but not to disruption of latent inhibition

The present study represents a continuous effort to develop an animal model of schizophrenia based on the "endogenous dopamine sensitization" hypothesis. To achieve this goal, withdrawal from an escalating amphetamine (AMPH) regime administration [three injections per day over a period of 4 days and increasing doses from 1 to 10 mg/kg of AMPH or an equivalent volume of saline (SAL)] was employed. Animals exposed to this treatment were evaluated on their performance in attentional (Latent inhibition, LI) and sensorimotor gating (Prepulse inhibition, PPI) tasks in a drug free state and tested for locomotor sensitization following a low dose of AMPH challenge administration.LI using active avoidance, tested on withdrawal day 4, was unaffected. PPI of the acoustic startle response, measured on withdrawal days 6 and 70, was disrupted. On the 76th day of withdrawal, a low challenge dose of AMPH (1 mg/kg) led to a clear locomotor sensitization effect.

Striatal dopaminergic responses observed in latent inhibition are dependent on the hippocampal ventral subicular region

We showed recently that behavioural and striatal dopaminergic (DA) responses obtained in latent inhibition are crucially dependent on the parahippocampal region, the entorhinal cortex. In the present study, we investigated the influence exerted by the hippocampal ventral subicular region (SUB) on the DA responses in the anterior part of the dorsal striatum using in vivo voltammetry in freely moving rats and the same latent inhibition paradigm. To that end, the left SUB was temporarily blocked with tetrodotoxin (TTX) during pre-exposure to a new olfactory stimulus (banana odour). During the second session the animals were aversively conditioned to banana odour. With respect to the results obtained during the test session (third presentation of banana odour), similar changes in behaviour and DA levels were obtained in control and conditioned rats microinjected with the solvent, phosphate-buffered saline (PBS), in the SUB, consistently with a latent inhibition phenomenon. In contrast, after reversible inactivation of the SUB during the pre-exposure session, TTX-pre-exposed conditioned animals displayed aversive behaviour in the test session, and anterior striatal DA variations in these animals differed significantly from those obtained in pre-exposed rats injected locally with PBS. Striatal DA variations obtained in conditioned animals microinjected with TTX were also significantly different from those observed in conditioned non-pre-exposed animals. The present data suggest that, in parallel to the entorhinal cortex, the SUB regulates the latent inhibition-related behavioural and DA responses in the anterior part of the dorsal striatum. These data may provide new insight into the pathophysiology of schizophrenic psychoses.

Sensitization to amphetamine, but not PCP, impairs attentional set shifting: reversal by a D1 receptor agonist injected into the medial prefrontal cortex

RATIONALE

Repeated exposure to psychomotor stimulants can lead to sensitization to their effects, and sensitization has been implicated in the pathophysiology of schizophrenia and drug abuse. These disorders are characterized by cognitive deficits, particularly in prefrontally mediated executive function.

OBJECTIVE

The present experiments were conducted to investigate the effects of sensitizing regimens of amphetamine and phencyclidine (PCP) on attentional set shifting.

METHODS

Rats received injections of amphetamine, PCP or saline three times per week for 5 weeks. Four weeks later, rats were trained to dig for food in one of two bowls, each bowl having an odour and a texture. Only one dimension (odour or texture) correctly predicted which bowl was baited. Rats were then tested on a series of discriminations including those requiring an intra-dimensional shift (IDS), an extra-dimensional shift (EDS) or a reversal of previously relevant and irrelevant stimuli.

RESULTS

Rats sensitized to amphetamine performed normally on the IDS, but were impaired on the EDS, as well as on reversal discriminations. PCP-sensitized rats were unaffected on any of the discriminations. In amphetamine-sensitized rats the deficit at the EDS stage was reversed by infusion of the D(1) receptor agonist SKF38393 into the medial prefrontal cortex (mPFC).

CONCLUSIONS

Results show that the amphetamine-sensitized state impairs prefrontally mediated attentional set shifting. This is consistent with cognitive deficits in schizophrenia and addiction, and with the evidence that amphetamine sensitization is accompanied by functional changes in the mPFC. These results further add to a growing literature showing that activating D(1) receptors in the mPFC improves aspects of cognition.

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.

Midbrain muscarinic receptor mechanisms underlying regulation of mesoaccumbens and nigrostriatal dopaminergic transmission in the rat

Laterodorsal (LDT) and pedunculopontine (PPT) tegmental nuclei in the mesopontine project cholinergic inputs to the midbrain ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), respectively, to directly and indirectly influence the activity of dopamine neuronal cells via actions on muscarinic and nicotinic receptors. The present study investigated the role of midbrain muscarinic receptors in the functional modulation of VTA and SNc dopamine cell activity as reflected by alterations in, respectively, nucleus accumbens (NAc) and striataldopamine efflux. In vivo chronoamperometry was used to measure changes in basal dopamine efflux via stearate-graphite paste electrodes implanted unilaterally in the NAc or striatum of urethane-anaesthetized rats, following blockade or activation of, respectively, VTA or SNc muscarinic receptors. Intra-VTA or -SNc infusion of the muscarinic antagonist scopolamine (200 microg/microL) reduced, respectively, NAc and striatal dopamine efflux while infusion of the muscarinic and nicotinic agonist carbachol (0.5 microg/microL) or the prototypical muscarinic agonist muscarine (0.5 microg/microL) increased NAc and striatal dopamine efflux. Transient decreases in dopamine efflux preceded these increases selectively in the striatum, suggesting a reduction in excitatory or increase in inhibitory drive to the SNc by preferential activation of M3 muscarinic receptors on GABA interneurons and glutamatergic inputs. This was confirmed by showing that selective blockade of M3 receptors with p-F-HHSiD (0.5 microg/microL) increased striatal, but not NAc, dopamine efflux. Together, these findings suggest that midbrain muscarinic receptors, probably M5 subtypes on VTA and SNc dopamine neurons, contribute to the tonic excitatory regulation of forebrain basal dopamine transmission whereas presynaptic M3 receptors serve to counter excessive excitation of nigral dopamine cell activity.

Disruption of learned irrelevance in acute schizophrenia in a novel continuous within-subject paradigm suitable for fMRI

Learned irrelevance (LIrr) is closely related to latent inhibition (LI). In LI a to-be-conditioned stimulus (CS) is prexposed alone prior to the opportunity to learn an association between the CS and an unconditioned stimulus (UCS). In LIrr preexposure consists of intermixed presentations of both CS and UCS in a random relationship to each other. In both paradigms preexposure leads in normal subjects to reduced or retarded learning of the CS-UCS association. Acute schizophrenics fail to show LI. LI is usually demonstrated as a one-off, between-groups difference in trials to learning, so posing problems for neuroimaging. We have developed a novel, continuous, within-subject paradigm in which normal subjects show robust and repeated LIrr. We show that this paradigm is suitable for functional magnetic resonance imaging (fMRI) and gives rise, in normal subjects, to activation in the hippocampal formation, consistent with data from animal experiments on LI. We also report, consistent with previous studies of LI, loss (indeed, significant reversal) of LIrr in acute (first 2 weeks of current psychotic episode) schizophrenics. Chronic schizophrenics failed to demonstrate learning, precluding measurement in this group of LIrr. These findings establish the likely value of the new paradigm for neuroimaging studies of attentional dysfunction in acute schizophrenia.

Learned irrelevance is disrupted in first-episode but not chronic schizophrenia patients

Learned irrelevance (LIrr) is a pre-exposure effect in which uncorrelated presentations of a conditioned stimulus (CS) and an unconditioned stimulus (US) retard subsequent CS-US association. LIrr is closely related to the phenomenon of latent inhibition (LI). LI refers to the retarding effects of inconsequential stimulus pre-exposure on subsequent conditioning to that stimulus, and is considered to reflect the organism's capacity to ignore irrelevant stimuli. LI is disrupted in schizophrenia patients, due to faster learning of the association between the preexposed CS and the US. A new within-subject target-recognition LIrr procedure was applied. The target was either cued by a priming signal or appeared at random, and priming signals were novel or preexposed cues. Schizophrenia patients were compared to age- and sex-matched control subjects. Normal subjects (n = 24) have shown robust LIrr, namely, faster cue-target associations of novel compared to preexposed cues. Schizophrenia patients at the early stages of their first episode (n = 7) showed LIrr disruption, namely, cue-target associations to preexposed cues were as fast as for novel cues. Chronic patients during an acute phase (n = 18) did not show LIrr as they failed to learn the cue-target association. In addition to the LIrr paradigm the same subjects were tested in a covert-orientation task. No differences were observed between the groups on this task. The possible advantages of the new LIrr paradigm are discussed.

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.

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.

Decreased amphetamine-induced locomotion and improved latent inhibition in mice mutant for the M5 muscarinic receptor gene found in the human 15q schizophrenia region

M5 muscarinic receptors are coexpressed with D2 dopamine receptors in the ventral tegmentum and striatum, and are important for reward in rodents. Previously, we reported that disruption of the M5 receptor gene in mice reduced dopamine release in the nucleus accumbens. In this study, we established a polymerase chain reaction (PCR) genotyping method for M5 mutant mice, and, using RT-PCR, found that M5 mRNA expression was highest in the ventral tegmentum, striatum, and thalamus in wild-type mice. In the M5 mutant mice, D2 mRNA expression was increased in several brain structures, including the striatum. Genome mapping studies showed the M5 gene is localized to chromosome 2E4 in mice, and to 15q13 in humans in the region that has been linked to schizophrenia. Amphetamine-induced locomotion, but not baseline locomotion or motor functions, decreased in M5 mutant mice, consistent with lower accumbal dopamine release. Previous reports found latent inhibition improvement in rats following nucleus accumbens lesions, or blockade of dopamine D2 receptors with neuroleptic drugs. Here, latent inhibition was significantly increased in M5 mutant mice as compared with controls, consistent with reduced dopamine function in the nucleus accumbens. In summary, our results showed that M5 gene disruption in mice decreased amphetamine-induced locomotion and increased latent inhibition, suggesting that increased M5 mesolimbic function may be relevant to schizophrenia.

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.

The effect of sulpiride on amphetamine-induced disruption of overshadowing in the rat

Previous studies have demonstrated dopaminergic (DA) modulation of stimulus selection performance in the overshadowing and Kamin blocking (KB) tasks. Systemic administration of the DA agonist D-amphetamine selectively disrupted overshadowing in the rat. In the present study, we examined the ability of the selective DA D2 receptor antagonist sulpiride to reverse amphetamine-induced disruption of overshadowing, in the conditioned emotional response (CER) procedure. In the overshadowing task, two stimuli (light and tone) are presented simultaneously alongside an aversive unconditioned stimulus (CS) (mild footshock); overshadowing consists in the decrease in learning to the less salient stimulus, as compared to when it is conditioned alone. Systemic D-amphetamine (1 mg/kg, i.p.) prevented overshadowing, while having no effect on control learning. Pre-administration of sulpiride (50 and 100 mg/kg, i.p.) did not prevent the ability of D-amphetamine to impair overshadowing. In addition, sulpiride had no significant effect on overshadowing when administered alone. The results are in agreement with previous data indicating that DA modulation of stimulus selection performance is mediated by activation of the DA D1 rather than DA D2 receptor subtype.

Effects of amisulpride, risperidone and chlorpromazine on auditory and visual latent inhibition, prepulse inhibition, executive function and eye movements in healthy volunteers

In view of the evidence that cognitive deficits in schizophrenia are critically important for long-term outcome, it is essential to establish the effects that the various antipsychotic compounds have on cognition, particularly second-generation drugs. This parallel group, placebo-controlled study aimed to compare the effects in healthy volunteers (n = 128) of acute doses of the atypical antipsychotics amisulpride (300 mg) and risperidone (3 mg) to those of chlorpromazine (100 mg) on tests thought relevant to the schizophrenic process: auditory and visual latent inhibition, prepulse inhibition of the acoustic startle response, executive function and eye movements. The drugs tested were not found to affect auditory latent inhibition, prepulse inhibition or executive functioning as measured by the Cambridge Neuropsychological Test Battery and the FAS test of verbal fluency. However, risperidone disrupted and amisulpride showed a trend to disrupt visual latent inhibition. Although amisulpride did not affect eye movements, both risperidone and chlorpromazine decreased peak saccadic velocity and increased antisaccade error rates, which, in the risperidone group, correlated with drug-induced akathisia. It was concluded that single doses of these drugs appear to have little effect on cognition, but may affect eye movement parameters in accordance with the amount of sedation and akathisia they produce. The effect risperidone had on latent inhibition is likely to relate to its serotonergic properties. Furthermore, as the trend for disrupted visual latent inhibition following amisulpride was similar in nature to that which would be expected with amphetamine, it was concluded that its behaviour in this model is consistent with its preferential presynaptic dopamine antagonistic activity in low dose and its efficacy in the negative symptoms of schizophrenia.

The effects of early maternal deprivation on auditory information processing in adult Wistar rats

BACKGROUND

There is now ample evidence that schizophrenia is due to an interaction between genetic and (early) environmental factors which disturbs normal development of the central nervous system and ultimately leads to the development of clinical symptoms. Recently, we showed that a single 24-hour period of maternal deprivation of rat pups at postnatal day 9 leads to a disturbance in prepulse inhibition, similar to what is seen in schizophrenia. The present set of experiments was designed to further characterize the information processing deficits of maternally deprived Wistar rats.

METHODS

Wistar rats were deprived from their mother for 24 hours on postnatal day 9. At adult age, rats were tested in the acoustic startle paradigm for prepulse inhibition and startle habituation. Rats were also tested in the evoked potentials paradigm for auditory sensory gating.

RESULTS

The results show that maternal deprivation led to a reduction in acoustic startle habituation and auditory sensory gating in adult rats. Moreover, maternal deprivation disrupted prepulse inhibition but only when the prepulses were given shortly (50-100 milliseconds) before the startle stimulus. At longer intervals (250-1000 milliseconds), no effect was seen.

CONCLUSIONS

The implications for the model and the development of disturbances in information processes are discussed.

The relation between latent inhibition and symptom-types in young schizophrenics

Latent inhibition (LI), retarded conditioning to a stimulus that has been previously repeatedly presented without reinforcement, was examined in young schizophrenics and normal controls using a within-subject visual search task. Healthy controls exhibited the usual LI effect. LI was potentiated in schizophrenics who simultaneously exhibited high levels of negative symptoms and low levels of positive symptoms. Schizophrenic groups with other combinations of positive and negative symptoms did not differ from controls. The pattern of data suggests that past inconsistencies in the LI-schizophrenia literature may be the result of opposing processes that are associated with positive and negative symptoms.

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.

Amphetamine-sensitized animals show a sensorimotor gating and neurochemical abnormality similar to that of schizophrenia

The aim of these studies was to examine whether amphetamine-induced sensitization in rats could be used as an animal model to study the basis of certain abnormalities seen in schizophrenia. Specifically, these experiments examined whether rats subjected to a sensitizing regimen of amphetamine would show the sensorimotor gating and greater amphetamine-induced displacement of radio-raclopride binding deficit that is observed in schizophrenia. In the first experiment, animals were divided into two groups with each rat receiving an intraperitoneal injection of amphetamine (AMPH) or saline (SAL) (1 ml/kg) three times per week for 3 weeks for a total of nine injections. AMPH dose was increased weekly from 1 mg/kg in the first week to 3 mg/kg in the third. Twenty-two days after the last injection, prepulse inhibition (PPI) of the acoustic startle response was tested. In addition, rats were tested for the effects of a challenge dose of 0.5 mg/kg AMPH on locomotor activity and [3H]raclopride (RAC) binding potential (BP) in the striatum. The tests for PPI confirmed that sensorimotor gating was disrupted in the AMPH-induced sensitized-state rats at baseline. The AMPH-sensitized rats also exhibited higher locomotor response to AMPH and a lower binding of striatal [3H]raclopride when challenged with the drug. The results were replicated and even more pronounced in rats that were treated with AMPH for 5 weeks, with doses ranging from 1mg/kg in the first week to 5 mg/kg in the fifth. These sensorimotor gating deficits and neurochemical (greater AMPH-induced displacement of radio-raclopride binding) abnormalities show similarities with the pathophysiology of schizophrenia and suggest that the AMPH-sensitized-state rats could be used to model certain aspects of schizophrenia.

Haloperidol and clozapine antagonise amphetamine-induced disruption of latent inhibition of conditioned taste aversion

RATIONALE

Latent inhibition (LI) describes a process by which repeated pre-exposure of a stimulus without any consequence retards the learning of subsequent conditioned associations with that stimulus. It is well established that LI is impaired in rats and in humans by injections of the indirect dopamine agonist amphetamine (AMPH), and that this disruption can be prevented by co-administration of either the typical neuroleptic haloperidol (HAL) or the atypical neuroleptic clozapine (CLZ).

OBJECTIVES

Most of what is known of the pharmacology of LI is derived from studies using either the conditioned emotional response or the conditioned active avoidance paradigm. The goal of the present study was to determine whether these results would generalize to the conditioned taste aversion assay.

METHODS

We tested whether AMPH (0.5 mg/kg) pretreatment would disrupt LI of a conditioned aversion to sucrose, and if so, which stage of the procedure is critical for mediating the disruption; in addition, we tested whether HAL (0.2 mg/kg) or CLZ (5.0 mg/kg) could restore such an expected LI disruption.

RESULTS

We determined that AMPH disrupted LI when it was injected before pre-exposure and prior to conditioning, but not if the rats were injected before either stage alone. When HAL or CLZ was given 40 min before AMPH (before both pre-exposure and conditioning), it blocked LI disruption.

CONCLUSION

These results are in line with the pharmacology of LI as derived from other conditioning paradigms. We conclude that the pharmacological regulation of LI in the CTA paradigm is similar to what has been observed previously in the conditioned emotional response and the conditioned active avoidance paradigms.

Animal behavior models of the mechanisms underlying antipsychotic atypicality

This review describes the animal behavior models that provide insight into the mechanisms underlying the critical differences between the actions of typical vs. atypical antipsychotic drugs. Although many of these models are capable of differentiating between antipsychotic and other psychotropic drugs, only a few seem to be able to differentiate between typical and atypical antipsychotics, such as the paw test and the phencyclidine (PCP)-induced disruption of prepulse inhibition (PPI) of startle in rats. Moreover, there is an urgent need for animal models focusing more on the negative and the cognitive symptoms. Hence, improved animal models are crucial for developing better treatments for schizophrenia.

Immune activation during pregnancy in rats leads to a postpubertal emergence of disrupted latent inhibition, dopaminergic hyperfunction, and altered limbic morphology in the offspring: a novel neurodevelopmental model of schizophrenia

Prenatal exposure to infection is associated with increased liability to schizophrenia, and it is believed that such an association is mediated by the maternal immune response, in particular, the proinflammatory cytokines released by the maternal immune system, which may disrupt fetal brain development. Impaired capacity to ignore irrelevant stimuli is one of the central deficits in schizophrenia, and is manifested, among others, in loss of latent inhibition (LI), a phenomenon whereby repeated inconsequential pre-exposure to a stimulus impairs its subsequent capacity to signal significant consequences. We tested the effects of prenatal immune activation induced by peripheral administration of the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I : C) to pregnant dams, on LI in juvenile and adult offspring. Consistent with the characteristic maturational delay of schizophrenia, prenatal immune activation did not affect LI in the juvenile offspring, but led to LI disruption in adulthood. Both haloperidol (0.1 mg/kg) and clozapine (5 mg/kg) reinstated LI in the adult offspring. In addition, prenatal immune activation led to a postpubertal emergence of increased sensitivity to the locomotor-stimulating effects of amphetamine and increased in vitro striatal dopamine release, as well as to morphological alterations in the hippocampus and the entorhinal cortex in the adult offspring, consistent with the well-documented mesolimbic dopaminergic and temporolimbic pathology in schizophrenia. These results suggest that prenatal poly I : C administration may provide a neurodevelopmental model of schizophrenia that reproduces a putative inducing factor; mimics the temporal course as well as some central abnormalities of the disorder; and predicts responsiveness to antipsychotic drugs. Neuropsychopharmacology (2003) 28, 1778-1789. advance online publication, 16 July 2003; doi:10.1038/sj.npp.1300248

Prepulse inhibition during withdrawal from an escalating dosage schedule of amphetamine

RATIONALE

Psychomotor stimulants can induce psychotic states in humans that closely resemble those observed in patients with idiopathic schizophrenia. Attentional and sensorimotor gating impairments are observed in schizophrenic patients using the latent inhibition (LI) and prepulse inhibition (PPI) behavioral assays, respectively. Our previous studies demonstrated that after 4 days of withdrawal from a period of amphetamine (AMPH) administration, animals exhibited disrupted LI but normal PPI.

OBJECTIVE

The aim of the present study was to test PPI in AMPH-withdrawn rats under experimental conditions similar to those used to best demonstrate locomotor sensitization following AMPH withdrawal.

METHODS

We examined the effects on PPI of (1) pairing drug injections with PPI test-associated cues, (2) administration of a low-dose dopamine agonist challenge and (3) testing following longer withdrawal periods (23, 30, 60 days).

RESULTS

Although none of these conditions revealed a disruption of PPI in AMPH-withdrawn rats, we did observe that the acoustic startle response was reduced during a restricted time period following AMPH withdrawal. Similar to our previous findings, AMPH-withdrawn animals showed disrupted LI on day 16 of withdrawal and locomotor sensitization to a challenge injection of AMPH after 62 days of withdrawal.

CONCLUSION

We conclude that the effects of repeated AMPH on PPI are not modulated by the same experimental parameters known to be important for eliciting locomotor sensitization and that withdrawal from the schedule of AMPH administration used in this study models only specific cognitive dysfunctions linked to schizophrenic symptoms, since LI was disrupted but PPI was not affected.

Post-pubertal emergence of disrupted latent inhibition following prenatal immune activation

RATIONALE

There is evidence pointing to an association between prenatal exposure to infection and increased liability to schizophrenia, and it has been suggested that the maternal immune response, in particular, the release of pro-inflammatory cytokines, may interfere with normal fetal brain development. Impaired capacity to ignore irrelevant stimuli is considered one of the central deficits in schizophrenia, and is manifested, among others, in disrupted latent inhibition (LI).

OBJECTIVES

To test the effects of prenatal immune activation on LI in juvenile and adult offspring.

METHODS

Pregnant rats were injected with the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I:C, 4 mg/kg) on gestational day 15. LI was assessed in 35-day and 3-month-old offspring using a thirst motivated conditioned emotional response procedure.

RESULTS

Consistent with the characteristic maturational delay of schizophrenia, prenatal immune activation did not affect LI in the juvenile offspring but led to a post-pubertal emergence of LI disruption. In addition, pronounced alterations in hippocampal morphology resembling those found in schizophrenia, were evident in the adult offspring.

CONCLUSIONS

These results support the hypothesis that immune activation during pregnancy may lead to long-term abnormalities mimicking those observed in schizophrenia.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Differential involvement of dopamine in the anterior and posterior parts of the dorsal striatum in latent inhibition

The involvement of mesostriatal dopaminergic neurons in cognitive operations is not well understood, and needs to be further clarified. The use of latent inhibition paradigms is a means of investigating cognitive processes. In this study, we investigated the involvement in latent inhibition of dopaminergic inputs in the anterior part and posterior part of the dorsal striatum. The latent inhibition phenomenon was observed in a conditioned olfactory aversion paradigm. Changes in extracellular dopamine levels induced by the conditioned olfactory stimulus (banana odor) were monitored in the two parts of the dorsal striatum in the left hemisphere after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. During the conditioning session animals received either an i.p. injection of NaCl (0.9%) (control groups) or an i.p. injection of LiCl (0.15 M) (conditioned groups). Dopamine variations and place preference or aversion toward the stimulus were analyzed simultaneously in pre-exposed and non-pre-exposed animals. Data collected during the retention (test) session were as follows. Where the anterior part of the striatum was concerned, similar enhancements in dopamine levels (+100%) were obtained in pre-exposed and non-pre-exposed control animals, as well as in the pre-exposed experimental animals. In contrast, dopamine levels in the non-pre-exposed experimental group (conditioned animals) remained fairly consistently close to the baseline after the presentation of the olfactory stimulus. Where the posterior part of the striatum was concerned, increases in extracellular dopamine levels were similar (+50%) for the different groups. The present results suggested that dopaminergic neurons reaching the anterior part of the dorsal striatum are implicated in the latent inhibition phenomenon and affective perception, whereas dopaminergic terminals in the posterior part of the dorsal striatum appeared to be involved neither in latent inhibition nor in affective perception of the stimulus, seeming only to be affected by the intrinsic properties of the stimulus. Cognitive as well as affective deficits have been reported in patients with schizophrenia. Thus the present data may be considered in the context of the pathophysiology of schizophrenic psychoses.

Disruption and potentiation of latent inhibition by risperidone: the latent inhibition model of atypical antipsychotic action

Latent inhibition (LI), that is, retarded conditioning to a stimulus following its nonreinforced pre-exposure, is impaired in some subsets of schizophrenia patients and in amphetamine-treated rats. Potentiation of LI by antipsychotic drugs (APDs) given in conditioning, under conditions that do not lead to LI in controls, is a well-established index of antipsychotic activity. Recently, we have shown that the atypical APD, clozapine, in addition disrupts LI if administered in pre-exposure, under conditions that lead to LI in controls. This study demonstrates the same behavioral profile for the atypical APD risperidone. LI was measured in a thirst-motivated conditioned emotional response procedure by comparing suppression of drinking in response to a tone previously paired with a foot shock in rats that received nonreinforced exposure to the tone prior to conditioning (pre-exposed (PE)) and rats for whom the tone was novel (non-pre-exposed (NPE)). We show that under conditions that did not yield LI in vehicle controls (40 pre-exposures and five conditioning trials), risperidone (0.25, 0.5, and 1.2 mg/kg) led to LI when administered in conditioning. Under conditions that led to LI in vehicle controls (40 pre-exposures and two conditioning trials), risperidone (0.25, 0.5, and 2.5 mg/kg) abolished LI when administered in pre-exposure; the latter effect was not evident with haloperidol. In addition, the effects of risperidone administered in both the pre-exposure and conditioning stages were dose-dependent so that the pre-exposure-based action was manifested at lower but not at higher doses. It is concluded that atypical APDs exert in the LI model a dual pattern of effects, which enables detection of their 'typical' action (conditioning-based LI potentiation) as well as a dissociation from typical APDs by their 'atypical' action (pre-exposure-based LI disruption). It is suggested that the former and latter effects are subserved by D2 and 5HT2A antagonism, respectively.

A demonstration of within-subjects latent inhibition in the human: limitations and advantages

The magnitude of latent inhibition (LI) (a retardation of associative learning due to prior exposure to the conditioning stimulus) was measured in healthy volunteers using both a within- and a between-subjects version of the task. Reliable LI was demonstrated for the within-subjects paradigm (using a design that fully counter-balanced stimulus of pre-exposure) but the magnitude of the effect was smaller than for the between-subjects version. Measures of schizotypal personality were found to be associated with reduced LI for the between-subjects task, but not for the within-subjects task. We hypothesised that for the within-subjects task learning about the first stimulus-consequence association (usually that for the not pre-exposed (NPE) stimulus) primes learning about the second stimulus, thus reducing the effect of pre-exposure and restricting the range of LI scores. In turn, this restricted range of LI scores does not allow subtle differences on schizotypal personality dimensions to reveal their effect using this within-subjects paradigm. In conclusion, a within-subjects LI task has been developed which is not open to explanation in terms of differences in stimulus salience. However, the limited range of pre-exposure scores in the current within-subject paradigm may severely limit it is use as an indicator of subtle performance changes.

Effects of typical and atypical antipsychotics on prepulse inhibition and latent inhibition in chronic schizophrenia

BACKGROUND

Prepulse inhibition and latent inhibition are the two animal paradigms currently dominating neuropharmacological research on attentional deficits in schizophrenia. Both paradigms have been shown to have a reasonable amount of face, predictive, and construct validity, but responsiveness to typical and atypical antipsychotics differs between the two, as indicated by animal and human studies. The relationship between the paradigms in schizophrenic patients is still unclear.

METHODS

We tested prepulse inhibition and auditory latent inhibition in a sample of 33 chronic schizophrenic patients medicated either with atypical (n = 17) or typical (n = 16) antipsychotics.

RESULTS

Latent inhibition was found to be intact in both patient groups. Prepulse inhibition was intact in the group receiving atypicals, but deficient in the group receiving typicals (at 60 msec lead interval condition).

CONCLUSIONS

The direct comparison supports the hypothesis that atypical and typical antipsychotics have different effects on prepulse inhibition than on latent inhibition in schizophrenic patients; however, the results may also be explained by a greater sensitivity of the prepulse inhibition method. Because it is crucial to understand why there are considerable differences between the two paradigms and between human and animal studies, research should focus more strongly on comparative approaches.

Dissociation in the involvement of dopaminergic neurons innervating the core and shell subregions of the nucleus accumbens in latent inhibition and affective perception

Mesencephalic dopaminergic neurons have been found to be involved in affective processes. Their implication in cognitive processes appears less well understood. The use of latent inhibition paradigms is a means of studying these kinds of processes. In this study, we investigated the involvement of dopaminergic projections in the core, the dorsomedial shell and the ventromedial shell of the nucleus accumbens, in latent inhibition in olfactory aversive learning. Variations in extracellular dopamine levels induced by an aversively conditioned olfactory stimulus were monitored in the three parts of the nucleus accumbens in the left hemisphere, after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. The parallel between dopamine changes and place preference or aversion toward the stimulus were analyzed in pre-exposed and non-pre-exposed animals. Results showed that dopaminergic neurons innervating the nucleus accumbens are differentially involved in the latent inhibition phenomenon. Dopaminergic neurons innervating the core and the dorsomedial shell subregions of the nucleus accumbens appeared to be involved in latent inhibition processes, unlike those reaching the ventromedial shell. Nonetheless dopamine in the ventromedial shell was found to be involved in affective perception of the stimulus.The present data suggest that dopaminergic neurons innervating the three nucleus accumbens subregions are functionally related to networks involved in parallel processing of the cognitive and affective values of environmental information, and that interaction between these systems, at some levels, may lead to a given behavioral output. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

The partial reinforcement extinction effect in humans: effects of schizophrenia, schizotypy and low doses of amphetamine

The partial reinforcement extinction effect (PREE) was studied in human subjects. It has been suggested that the PREE depends on neural mechanisms critical to the cognitive dysfunction which underlines acute schizophrenia. We therefore predicted that the PREE should be reduced, through decreased resistance to extinction in the partial reinforcement (PR) condition, in various types of individual: (a) healthy volunteers given low doses of oral amphetamine; (b) those in the acute (but not chronic) phase of a schizophrenic illness and; (c) healthy volunteers with high scores on personality measures of schizotypy. Despite obtaining robust demonstrations of PREE in all experiments, none of these predictions were confirmed. A single, low dose, of amphetamine had no effect on either continuous reinforcement (CR) or partial reinforcement (PR). Acute and chronic schizophrenic patients showed a reduced PREE compared to controls. However this was due to increased resistance to extinction in the CR groups. Finally, high schizotypy scores were associated with greater PREE, attributable to both decreased extinction in the CR condition and increased extinction in the PR condition. The results of these experiments on human PREE provide no support that PREE is a valid paradigm with which to explore the cognitive dysfunction underlying schizophrenia.

Latent inhibition and schizophrenia: Pavlovian conditioning of autonomic responses

Latent inhibition (LI) is an important model for understanding cognitive deficits in schizophrenia. Disruption of LI is thought to result from an inability to ignore irrelevant stimuli. The study investigated LI in schizophrenic patients by using Pavlovian conditioning of electrodermal responses in a complete within-subject design. Thirty-two schizophrenic patients (16 acute, unmedicated and 16 medicated patients) and 16 healthy control subjects (matched with respect to age and gender) participated in the study. The experiment consisted of two stages: preexposure and conditioning. During preexposure two visual stimuli were presented. one of which served as the to-be-conditioned stimulus (CSp + ) and the other one was the not-to-be-conditioned stimulus (CSp - ) during the following conditioning ( = acquisition). During acquisition, two novel visual stimuli(CSn + and CSn - ) were introduced. A reaction time task was used as the unconditioned stimulus (US). LI was defined as the difference in response differentiation observed between preexposed and non-preexposed sets of CS + and CS - . During preexposure, the schizophrenic patients did not differ in electrodermal responding from the control subjects, neither concerning the extent of orienting nor the course of habituation. The exposure to novel stimuli at the beginning of the acquisition elicited reduced orienting responses in unmedicated patients compared to medicated patients and control subjects. LI was observed in medicated schizophrenic patients and healthy controls, but not in acute unmedicated patients. Furthermore LI was found to be correlated with the duration of illness: it was attenuated in patients who had suffered their first psychotic episode.

Associative deficit accounts of disrupted latent inhibition and blocking in schizophrenia

Latent inhibition (attenuated responding to a signal due to signal-alone presentations preceding the signal-outcome pairings) and blocking (attenuated responding to Signal B due to Signal A being paired with the outcome prior to pairings of an AB compound with the outcome) are reportedly absent in acute schizophrenics. The common assumption that these phenomena reflect the normal functioning of attention and the observation that rats administered low doses of amphetamine show a similar disruption has resulted in the development of an animal model of attentional dysfunction in acute schizophrenia. Here, we selectively review the experimental and clinical literature concerning latent inhibition and blocking, their disruption in acute schizophrenia, and the current status of this model. We conclude that the construct validity of the model is compromised if latent inhibition and blocking are viewed in attentional terms because experimental data indicate both phenomena can be better understood in associative terms. We favor a framework in which disruption of latent inhibition and blocking in acute schizophrenics is viewed as an inability to compare and express stored representations (i.e. associative performance deficit). This change of perspective does not undermine the potential value of the model, but rather suggests that the nature of its validity should be reconsidered.

Latent inhibition as a function of schizotypality and gender: implications for schizophrenia

In three within-subject experiments, we demonstrated that preexposure to an irrelevant stimulus interfered with performance when that stimulus subsequently predicted the correct location of a target stimulus. This latent inhibition-like effect (LI) was manifest in response time measures, but not errors. As with other related paradigms, LI was a function of an interaction between schizotypy-level and gender. Low schizotypal females and high schizotypal males exhibited significant LI, while high schizotypal females and low schizotypal males failed to produce LI effects. The results, similar to findings with schizophrenic patients, suggest a sexual dimorphism of cognitive dysfunction in schizophrenia, particularly in regard to the processing of irrelevant stimuli.

Latent inhibition deficits in high-schizotypal normals: symptom-specific or anxiety-related?

Latent inhibition (LI) is the phenomenon in which subjects who have repeatedly experienced an irrelevant stimulus perform more poorly on a new learning task with that stimulus than with a novel stimulus, presumably because of a decline in stimulus-specific attention. The present article reviews the literature on LI deficits in high-schizotypal normal subjects and schizophrenic patients. Although LI-deficits have been thought to be specific to these groups, evidence is presented that the effects may be related to the anxiety components of high-schizotypality and related pathologies.

The changing roles and targets for animal models of schizophrenia

Unlike disorders of other fields of medicine (eg., diabetes, heart disease), schizophrenia has been only marginally impacted by the study of animal models. This gap reflects the incomplete understanding of the causes and mechanisms of schizophrenia and the resulting lack of defined targets for model development. However, prior attempts at modeling in animals the complex symptoms of schizophrenia have given way to more promising component models. This review will address the evolving field of animal models of schizophrenia with a focus on models of errors in neurotransmission, and of psychophysiological deficits, with a concluding discussion of the present and future promise of genetic-based models. Evolving models based on the long-held conceptualization of schizophrenia as being based on errors in neurotransmission are discussed as regards the integration of newer findings implicating alterations in dopamine, glutamate and neurotensin function in the pathophysiology and pharmacotherapy of schizophrenia. The case for the more recent conceptualization of schizophrenia as a core deficit in information processing and stimulus filtering is discussed. Animal behavioral paradigms that model psychophysiologic constructs of stimulus processing deficits related to schizophrenia include prepulse inhibition (PPI), a model of sensorimotor gating, or latent inhibition (LI), a model of salience learning. These models represent both better supported associations with schizophrenia and more productive targets and are providing important new information regarding the psychopharmacology of schizophrenia. Genetic models of schizophrenia are based on the demonstrated heritability of the disorder and more recent pharmacogenetic findings for antipsychotic medications. Genetic-based animal models use behavioral or molecular genetic techniques to manipulate behaviors related to schizophrenia by altering the frequencies of related genes. The future development of increasingly informative animal models of schizophrenia will be dependent on a more complete understanding of schizophrenia, an integration of findings across animal models and refinements in the criteria used to assess model "validity" that better reflect the changing nature and roles of animal models of schizophrenia.

Performance on the visual search analog of latent inhibition is modulated by an interaction between schizotypy and gender

Two experiments examined the visual search analog of latent inhibition (LI) and the novel popout (NPO) effect in healthy humans. In Experiments 1 (n=48) and 2 (n=180), subjects judged the positions (left or right side of a computer screen) of a unique target amongst a field of homogeneous distractors. In both experiments, there was a strong LI effect, as indicated by longer response times (RT) to those displays in which the target was previously a distractor and the distractors were previously the target, as compared with displays in which the target was novel and the distractors were previously the target. NPO, faster RT to a display in which the target was novel on a background of familiar distractors than to a display in which both target and distractors were novel, was not obtained. In Experiment 1, LI magnitude was not affected by gender. In Experiment 2, LI magnitude was larger for low schizotypal females than for high schizotypal females, a result not obtained for males. This pattern is similar to one reported for medicated schizophrenic out-patients (Lubow, R.E., Kaplan, O., Abramovich, P., Rudnick, R., Laor, N., 2000. Visual search in schizophrenics: latent inhibition and novel popout effects. Schizophr. Res., in press). Together, these data suggest that the LI deficits found in high schizotypal healthy subjects and in schizophrenic patients represent a dysfunction that is characterized by an inability to reduce attention allocated to irrelevant stimuli, and that this may serve as a trait marker for some subtypes of schizophrenia, particularly those associated with female gender.

The role of neurotensin in the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs

It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather pathologic alterations of several interacting systems. Targeting of neuropeptide neuromodulator systems, capable of concomitantly regulating several transmitter systems, represents a promising approach for the development of increasingly effective and side effect-free antipsychotic drugs. Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia that specifically modulates neurotransmitter systems previously demonstrated to be dysregulated in this disorder. Clinical studies in which cerebrospinal fluid (CSF) NT concentrations have been measured revealed a subset of schizophrenic patients with decreased CSF NT concentrations that are restored by effective antipsychotic drug treatment. Considerable evidence also exists concordant with the involvement of NT systems in the mechanism of action of antipsychotic drugs. The behavioral and biochemical effects of centrally administered NT remarkably resemble those of systemically administered antipsychotic drugs, and antipsychotic drugs increase NT neurotransmission. This concatenation of findings led to the hypothesis that NT functions as an endogenous antipsychotic. Moreover, typical and atypical antipsychotic drugs differentially alter NT neurotransmission in nigrostriatal and mesolimbic dopamine (DA) terminal regions, and these effects are predictive of side effect liability and efficacy, respectively. This review summarizes the evidence in support of a role for the NT system in both the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs.

Latent inhibition, but not prepulse inhibition, is reduced during withdrawal from an escalating dosage schedule of amphetamine

The enhanced locomotor and stereotypic responses of the rat to repeated amphetamine (AMPH) administration are considered to be an animal model of positive schizophrenic symptoms. In contrast, behaviors observed during withdrawal from repeated AMPH are believed to model depression or anxiety. In the present study, the authors tested whether AMPH withdrawal might also elicit behaviors consistent with animal models of schizophrenia, specifically, disruptions in latent inhibition (LI) of 2-way active avoidance and prepulse inhibition (PPI) of startle. Rats treated with escalating doses of AMPH (6 days, 1-5 mg/kg ip) or saline were tested for LI and PPI during withdrawal. LI was eliminated by prior AMPH treatment in rats tested at 4, 13, and 28 days of withdrawal. In contrast, PPI did not differ between AMPH and control groups. These results support an interrelationship between repeated-AMPH and LI-disruption, but not PPI-disruption, models of schizophrenia.