Haloperidol reinstates latent inhibition impaired by hippocampal lesions: data and theory

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

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

Cognitive function in schizophrenia: conflicting findings and future directions

INTRODUCTION

Schizophrenia is a severe mental disorder with multiple psychopathological domains being affected. Several lines of evidence indicate that cognitive impairment serves as the key component of schizophrenia psychopathology. Although there have been a multitude of cognitive studies in schizophrenia, there are many conflicting results. We reasoned that this could be due to individual differences among the patients (i.e. variation in the severity of positive vs. negative symptoms), different task designs, and/or the administration of different antipsychotics.

METHODS

We thus review existing data concentrating on these dimensions, specifically in relation to dopamine function. We focus on most commonly used cognitive domains: learning, working memory, and attention.

RESULTS

We found that the type of cognitive domain under investigation, medication state and type, and severity of positive and negative symptoms can explain the conflicting results in the literature.

CONCLUSIONS

This review points to future studies investigating individual differences among schizophrenia patients in order to reveal the exact relationship between cognitive function, clinical features, and antipsychotic treatment.

Haloperidol and risperidone have specific effects on altered pain sensitivity in the ketamine model of schizophrenia

RATIONALE

The ketamine (ket) model reflects features of schizophrenia as well as secondary symptoms such as altered pain sensitivity.

OBJECTIVES

In the present study, we investigated the effect of subchronic oral treatment with haloperidol (hal, 0.075 mg/kg) and risperidone (ris, 0.2 mg/kg) on altered pain perception and locomotor activity in this model.

RESULTS

In reaction to 5 mg/kg morphine, ket pretreated animals showed a diminished analgesic response. Hal had no analgesic effect per se, but the compound normalised the analgesic reaction to morphine in the ket pretreated animals. The effect of ris was complex. First, there was no analgesic effect per se, and control animals showed a dose-dependent increase in the analgesic index after morphine injection. In the ket group treated with ris, the analgesic response to 5 mg/kg morphine was attenuated and in response to 10 mg/kg analgesia was comparable with that measured in controls. The reduced analgesic effect was not due to pharmacokinetic differences in morphine metabolism. After administration via drinking water in saline-injected control animals, the hal blood serum concentration was 2.6 +/- 0.45 ng/ml. In ket-injected animals, the mean serum concentration of hal amounted to 1.2 +/- 0.44 ng/ml. In the experiment using ris, animals in the control group had higher ris serum concentrations compared with ket-injected animals. In control animals, morphine dose dependently decreased locomotor activity. This effect was significantly stronger in the ket pretreated groups.

CONCLUSIONS

Hal and ris had different effects on altered pain sensitivity. It was hypothesised that these results are connected with alterations in dopamine D2 and mu opioid receptor binding.

Disruption of the US pre-exposure effect and latent inhibition in two-way active avoidance by systemic amphetamine in C57BL/6 mice

RATIONALE

Pre-exposure to either one of the two to-be-associated stimuli alone is known to reduce the efficiency of the learning of their association when they are subsequently paired explicitly. In classical conditioning, pre-exposure to the conditioned stimulus (CS) gives rise to latent inhibition (LI); and pre-exposure to the unconditioned stimulus (US) results in the US pre-exposure effect (USPEE). Considerable evidence supports an important role of central dopamine in the regulation and modulation of LI; it has been suggested that the USPEE may be similarly controlled by dopamine, but this parallelism has only been directly demonstrated in the conditioned taste aversion paradigm.

OBJECTIVE

The present study tested this hypothesis by comparing the efficacy of systemic amphetamine treatment to affect the expression of LI and the USPEE in a two-way active avoidance paradigm.

METHODS

C57BL/6 male mice were tested in active avoidance using a tone CS and a foot-shock US. Twenty-four hours before, they were pre-exposed to 100 presentations of the CS or the US, or to the test apparatus only. Amphetamine (2.5 mg/kg) or saline was administered before stimulus pre-exposure and conditioned avoidance test, in which the mice learned to avoid the shock by shuttling in response to the tone.

RESULTS

Amphetamine disrupted both stimulus pre-exposure effects, thus, lending further support to the hypothesis that the USPEE is similar to LI in its sensitivity to dopamine receptor agonist. Hence, the USPEE paradigm may represent a valuable addition to the study of dopamine-sensitive processes of selective learning currently implicated in LI and Kamin blocking.

Prenatal and postnatal maternal contributions in the infection model of schizophrenia

Epidemiological studies have indicated that the risk of schizophrenia is enhanced by prenatal maternal infection with viral or bacterial pathogens. Recent experimentation in rodents has yielded additional support for a causal relationship between prenatal immune challenge and the emergence of psychosis-related abnormalities in brain and behaviour in later life. However, little is known about the putative roles of maternal postnatal factors in triggering and modulating the emergence of psychopathology following prenatal immunological stimulation. Here, we aimed to dissect the relative contributions of prenatal inflammatory events and postnatal maternal factors in precipitating juvenile and adult psychopathology in the resulting offspring with a cross-fostering design. Pregnant mice were exposed to the viral mimic, polyriboinosinic-polyribocytidilic acid (PolyI:C; at 5 mg/kg, intravenously), or vehicle treatment on gestation day 9, and offspring born to PolyI:C- and vehicle-treated dams were then simultaneously cross-fostered to surrogate rearing mothers, which had either experienced inflammatory or vehicle treatment during pregnancy. Prenatal PolyI:C administration did not affect the expression of latent inhibition (LI) at a juvenile stage of development, but led to the post-pubertal emergence of LI disruption in both aversive classical and instrumental conditioning regardless of the postnatal rearing condition. In addition, deficits in conditioning as such led to a pre- and post-pubertal loss of LI in prenatal control animals that were adopted by PolyI:C-treated surrogate mothers. Our findings thus indicate that the adoption of prenatally immune-challenged neonates by control surrogate mothers does not possess any protective effects against the subsequent emergence of psychopathology in adulthood. At the same time, however, the present study highlights for the first time that the adoption of prenatal control animals by immune-challenged rearing mothers is sufficient to precipitate learning disabilities in the juvenile and adult offspring.

The dual origin hypothesis: An evolutionary brain-behavior framework for analyzing psychiatric disorders

According to the dual origin hypothesis, the cerebral cortex of higher mammals evolved from two primordial brain structures, the amygdala and hippocampal formation. This developmental process defines the orderly principles of cortical connectivity and gives rise to functionally distinct ventral and dorsal systems within the cerebrum. This paper reviews the basic features of the dual origin theory. This model is then applied to understanding symptom production in a number of psychiatric illnesses, with particular reference to recent structural and functional imaging studies. In this paper I propose that psychiatric symptoms can be conceptualized as arising from abnormal processing within dorsal (time-space-motility) or ventral (meaning-motivation) systems, or from a disturbance in the functional interaction/balance between them. Within this framework, one can identify symptom-specific correlations that cross-traditional diagnostic boundaries, as well as potential mechanisms that may explain biologically valid diagnostic entities. Integrating evolutionary, connectional and functional bases across multiple species, the dual origin hypothesis offers a powerful neural systems model to help organize our understanding of psychiatric illness, therein suggesting novel approaches to diagnosis, prevention and treatment.

Latent inhibition of conditioned taste aversion is not disrupted, but can be enhanced, by selective nucleus accumbens shell lesions in rats

Latent inhibition is a form of negative priming in which repeated non-reinforced pre-exposures to a stimulus retard subsequent learning about the predictive significance of that stimulus. The nucleus accumbens shell and the anatomical projection it receives from the hippocampal formation have been attributed a pivotal role in the control or regulation of latent inhibition expression. A number of studies in rats have demonstrated the efficacy of selective shell lesions to disrupt latent inhibition in different associative learning paradigms, including conditioned active avoidance and conditioned emotional response. Here, we extended the test to the conditioned taste aversion paradigm, in which the effect of direct hippocampal damage on latent inhibition remains controversial. We demonstrated the expected effect of selective shell lesions on latent inhibition of conditioned emotional response and of conditioned active avoidance, before evaluating in a separate cohort of rats the effect of comparable selective lesions on latent inhibition of conditioned taste aversion: a null effect of the lesions was first obtained using parameters known to be sensitive to amphetamine treatment, then an enhancement of latent inhibition was revealed with a modified conditioned taste aversion procedure. Our results show that depending on the associative learning paradigm chosen, shell lesions can disrupt or enhance the expression of latent inhibition; and the pattern is reminiscent of that seen following hippocampal damage.

Brain-behaviour relationships in latent inhibition: a computational model

In a series of studies, we applied a neural network to study the neural bases of latent inhibition. We first designed a model able to handle behavioral data and then we investigated whether structures and neural elements in the brain were able to carry out the operations described by network. We demonstrated that the network was able to describe many of the behavioral properties of LI, and elucidate the effects of several manipulations of the dopaminergic system, the hippocampus, and the nucleus accumbens on LI, as well as some of the positive symptoms of schizophrenia. The results support the idea that a 'conceptual nervous system' can be successfully used to relate brain and behavior.

Coantagonism of glutamate receptors and nicotinic acetylcholinergic receptors disrupts fear conditioning and latent inhibition of fear conditioning

The present study investigated the hypothesis that both nicotinic acetylcholinergic receptors (nAChRs) and glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) and N-methyl-d-aspartate glutamate receptors (NMDARs)) are involved in fear conditioning, and may modulate similar processes. The effects of the nAChR antagonist mecamylamine administered alone, the AMPAR antagonist NBQX administered alone, and the NMDAR antagonist MK-801 administered alone on cued fear conditioning, contextual fear conditioning, and latent inhibition of cued fear conditioning were examined. In addition, the effects of coadministration of either mecamylamine and NBQX or mecamylamine and MK-801 on these behaviors were examined. Consistent with previous studies, neither mecamylamine nor NBQX administered alone disrupted any of the tasks. However, coadministration of mecamylamine and NBQX disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. In addition, coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting either task disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. Coadministration of mecamylamine and NBQX, and coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting fear conditioning had little effect on cued fear conditioning. These results suggest that nAChRs and glutamate receptors may support similar processes mediating acquisition of contextual fear conditioning and latent inhibition of fear conditioning.

Rolipram attenuates MK-801-induced deficits in latent inhibition

Latent inhibition is used to examine attention and study cognitive deficits associated with schizophrenia. Research using MK-801, an N-methyl-D-aspartate (NMDA) open channel blocker, implicates glutamate receptors in acquisition of latent inhibition of cued fear conditioning. Evidence suggests an important relationship between NMDA-induced increases in cyclic adenosine monophosphate (cAMP) and learning and memory. The authors examine whether amplification of the cAMP signaling pathway by rolipram, a selective Type 4 cAMP phosphodiesterase inhibitor, reverses MK-801-induced impairments in latent inhibition. One day before training, mice were injected with MK-801, rolipram, MK-801 and rolipram, or vehicle and received 20 preexposures or no preexposures to an auditory conditioned stimulus (CS). Training consisted of 2 CS-footshock unconditioned stimulus pairings. Rolipram attenuated the disruptive effect of MK-801 on latent inhibition, which suggests a role for the cAMP signaling pathway in the task and implicates phosphodiesterase inhibition as a target for treating cognitive impairments associated with schizophrenia.

Simulated apoptosis/neurogenesis regulates learning and memory capabilities of adaptive neural networks

Characterization of neuronal death and neurogenesis in the adult brain of birds, humans, and other mammals raises the possibility that neuronal turnover represents a special form of neuroplasticity associated with stress responses, cognition, and the pathophysiology and treatment of psychiatric disorders. Multilayer neural network models capable of learning alphabetic character representations via incremental synaptic connection strength changes were used to assess additional learning and memory effects incurred by simulation of coordinated apoptotic and neurogenic events in the middle layer. Using a consistent incremental learning capability across all neurons and experimental conditions, increasing the number of middle layer neurons undergoing turnover increased network learning capacity for new information, and increased forgetting of old information. Simulations also showed that specific patterns of neural turnover based on individual neuronal connection characteristics, or the temporal-spatial pattern of neurons chosen for turnover during new learning impacts new learning performance. These simulations predict that apoptotic and neurogenic events could act together to produce specific learning and memory effects beyond those provided by ongoing mechanisms of connection plasticity in neuronal populations. Regulation of rates as well as patterns of neuronal turnover may serve an important function in tuning the informatic properties of plastic networks according to novel informational demands. Analogous regulation in the hippocampus may provide for adaptive cognitive and emotional responses to novel and stressful contexts, or operate suboptimally as a basis for psychiatric disorders. The implications of these elementary simulations for future biological and neural modeling research on apoptosis and neurogenesis are discussed.

Developmental neurocircuitry of motivation in adolescence: a critical period of addiction vulnerability

OBJECTIVE

Epidemiological studies indicate that experimentation with addictive drugs and onset of addictive disorders is primarily concentrated in adolescence and young adulthood. The authors describe basic and clinical data supporting adolescent neurodevelopment as a biologically critical period of greater vulnerability for experimentation with substances and acquisition of substance use disorders.

METHOD

The authors reviewed recent literature regarding neurocircuitry underlying motivation, impulsivity, and addiction, with a focus on studies investigating adolescent neurodevelopment.

RESULTS

Adolescent neurodevelopment occurs in brain regions associated with motivation, impulsivity, and addiction. Adolescent impulsivity and/or novelty seeking as a transitional trait behavior can be explained in part by maturational changes in frontal cortical and subcortical monoaminergic systems. These developmental processes may advantageously promote learning drives for adaptation to adult roles but may also confer greater vulnerability to the addictive actions of drugs.

CONCLUSIONS

An exploration of developmental changes in neurocircuitry involved in impulse control has significant implications for understanding adolescent behavior, addiction vulnerability, and the prevention of addiction in adolescence and adulthood.

Neurodevelopment, impulsivity, and adolescent gambling

The prevalence of problem and pathological gambling in adolescence and young adulthood has been found to be two- to fourfold higher than in adulthood. Given that these high rates might predict future increases across all age groups, it is important to explore the causes of the elevated rates of problem and pathological gambling among youths. This article reviews evidence for a neurobiological basis for adolescent vulnerability to problem and pathological gambling behaviors. We propose that a common trait motif of impulsivity might underlie phenomenology of pathological gambling, commonly comorbid psychiatric disorders, and related aspects of adolescent behavior. Recent advances in understanding the brain mechanisms involved in motivation, reward, and decision-making allow a discussion of neural circuitry underlying impulsivity. Emerging data indicate that important neurodevelopmental events during adolescence occur in brain regions associated with motivation and impulsive behavior. We hypothesize that immaturity of frontal cortical and subcortical monoaminergic systems during normal neurodevelopment underlies adolescent impulsivity as a transitional trait-behavior. While these neurodevelopmental processes may confer advantage by promoting a learning drive for optimal adaptation to adult roles, they may also confer an increased vulnerability to addictive behaviors such as problem and pathological gambling. An exploration of the developmental changes in neural circuitry involved in impulse control has significant implications for understanding adolescent behaviors and treating problem and pathological gambling among youths.

Single-cue delay and trace classical conditioning in schizophrenia

BACKGROUND

Classical conditioning provides a means of addressing mechanisms of learning and can therefore help understand the pathophysiology of memory alteration in schizophrenia.

METHODS

Single cue delay and trace eyeblink conditioning were used in patients with schizophrenia and matched normal control subjects to explore, respectively, cerebellar and hippocampal integrity during learning. We measured percent of conditioned (CRs) and unconditioned responses (URs), their amplitude, and onset and peak latencies. We also accounted for spontaneous blink rates and stimulus-induced responses before learning.

RESULTS

During delay conditioning, patients showed CRs with longer onset and peak latencies and improved efficiency compared to normal volunteers without there being differences between patients and normal control subjects in the percentage of CRs. During trace conditioning, neither group showed an increase in CRs as a function of conditioned stimulus-unconditioned stimulus pairings, in part because the level of spontaneous blink rates exceeded the level of CRs; however, patients with schizophrenia showed increased responding 150-400 msec after the conditioned stimulus and in the last 100-150 msec before the unconditioned stimulus, whereas normal control subjects showed only the latter type of responses. The former type of response was more frequent in patients with schizophrenia even before either trace or delay conditioning.

CONCLUSIONS

These results suggest integrity of cerebellar mechanisms underlying conditioning, although the altered timing of CRs in patients may indicate differences in the modulation of such responses. Both the greater CR onset latency during delay and the presence of early nonadaptive responses during trace are compatible with the pattern of responding seen in animals with hippocampal damage.

Hippocampal dysfunction in schizophrenia

Although not necessarily primary to the disease, hippocampal dysfunction in schizophrenia is suggested by morphological changes in the hippocampal formation reported in schizophrenic patients. This notion receives additional support from studies showing that 1) similar behavioral deficits are exhibited by both schizophrenics and animals with hippocampal lesions, and 2) some of these behavioral deficits are reversed by neuroleptics in both schizophrenics and lesioned animals. A brain-mapped neural network model is used to explain how some impairments in attention can be caused by hippocampal dysfunction and ameliorated by dopaminergic blockers.

Medial dorsal thalamic lesions impair blocking and latent inhibition of the conditioned eyeblink response in rats

The effects of lesions of the medial dorsal thalamic nucleus (MD) on blocking and latent inhibition (LI) of the rat eyeblink response were examined in the present study. Previous work has demonstrated that the cingulate cortex and related thalamic areas are involved in processing conditioning stimuli throughout training. The experiments in the present study tested the hypothesis that disruption of cingulothalamic stimulus processing produced by lesions of the MD would impair 2 types of associative learning that involve decremental changes in attention. In Experiment 1, MD lesions severely impaired blocking. In Experiment 2, MD lesions severely impaired LI. The results indicate that lesions of the MD impair incremental, decremental, or both types of changes in stimulus processing during learning.

The entorhinal cortex-nucleus accumbens pathway and latent inhibition: a behavioral and neurochemical study in rats

Latent inhibition (LI) refers to the decrease in conditioned response produced by the repeated nonrein-forced preexposure to the to-be-conditioned stimulus. Experiment I investigated the effects of electrolytic lesions of the entorhinal cortex on LI in a conditioned emotional response procedure. Entorhinal cortex lesions attenuated LI. Experiments 2 and 3 investigated whether this attenuation of LI could result from a modification in nucleus accumbens (NAcc) dopamine (DA) release. Rats with entorhinal cortex lesions displayed normal spontaneous and amphetamine-induced locomotor activity, as well as normal basal and amphetamine-induced release of DA within the NAcc (assessed by microdialysis). Taken together, these results show that entorhinal cortex lesions disrupt LI in a way that is unlikely to be due to an alteration of DA release within the NAcc.

A neurobiological basis for substance abuse comorbidity in schizophrenia

It is commonly held that substance use comorbidity in schizophrenia represents self-medication, an attempt by patients to alleviate adverse positive and negative symptoms, cognitive impairment, or medication side effects. However, recent advances suggest that increased vulnerability to addictive behavior may reflect the impact of the neuropathology of schizophrenia on the neural circuitry mediating drug reward and reinforcement. We hypothesize that abnormalities in the hippocampal formation and frontal cortex facilitate the positive reinforcing effects of drug reward and reduce inhibitory control over drug-seeking behavior. In this model, disturbances in drug reward are mediated, in part, by dysregulated neural integration of dopamine and glutamate signaling in the nucleus accumbens resulting form frontal cortical and hippocampal dysfunction. Altered integration of these signals would produce neural and motivational changes similar to long-term substance abuse but without the necessity of prior drug exposure. Thus, schizophrenic patients may have a predilection for addictive behavior as a primary disease symptom in parallel to, and in many, cases independent from, their other symptoms.

Nucleus accumbens, entorhinal cortex and latent inhibition: a neural network model

A neural network model of classical conditioning (Schmajuk, Lam, and Gray, J. Exp. Psychol.: Anim. Behav. Process, 22, 1996, 321-349) is applied to the description of the neural substrates of latent inhibition. Experimental data suggest that latent inhibition might be controlled by a circuit that involves the hippocampus, the entorhinal cortex, the nucleus accumbens, and the mesolimbic dopaminergic projection from the ventral tegmental area to the accumbens. By mapping different nodes and connections in the model onto this brain circuit, computer simulations demonstrate that, in most cases, the model provides a good quantitative description of: (1) the impairment of latent inhibition by lesions of the shell of the nucleus accumbens; (2) the restoration of latent inhibition by haloperidol following lesions of the shell; (3) the preservation of latent inhibition by lesions of the core of the nucleus accumbens; (4) the facilitation of latent inhibition by combined shell core lesions and by core lesions with extended conditioning; (5) the impairment of latent inhibition following lesions of the entorhinal cortex or the hippocampus; and (6) the restoration of latent inhibition by haloperidol following lesions of the entorhinal cortex and ventral subiculum. In addition, the model is able to describe neural activity in the nucleus accumbens.