Dissociative effects of apomorphine infusions into the medial prefrontal cortex of rats on latent inhibition, prepulse inhibition and amphetamine-induced locomotion

Adolescent stress during, but not after, pubertal onset impairs indices of prepulse inhibition in adult rats

Exposure to stress during adolescence is a risk factor for developing several psychiatric disorders, many of which involve prefrontal cortex (PFC) dysfunction. The human PFC and analogous rodent medial prefrontal cortex (mPFC) continue to mature functionally and anatomically during adolescence, and some of these maturational events coincide with pubertal onset. As developing brain regions are more susceptible to the negative effects of stress, this may make puberty especially vulnerable. To test this, we exposed male and female rats to isolation and restraint stress during the onset of puberty or during the post-pubertal period of adolescence. In young adulthood, both stressed groups and an unstressed control group underwent testing on a battery of tasks to assess emotional and cognitive behaviors, and the volume of the mPFC was quantified postmortem. Factor analysis revealed only subjects stressed peri-pubertally showed a long-term deficiency compared to controls in prepulse inhibition. Additionally, both sexes showed volumetric mPFC decreases following adolescent stress, and these losses were most pronounced in females. Our findings suggest that pubertal onset may be a vulnerable window wherein adolescents are most susceptible to the negative consequences of stress exposure. Furthermore, it highlights the importance of accounting for pubertal status when studying adolescents.

Pharmaco-electroencephalographic responses in the rat differ between active and inactive locomotor states

Quantitative electroencephalography from freely moving rats is commonly used as a translational tool for predicting drug‐effects in humans. We hypothesized that drug‐effects may be expressed differently depending on whether the rat is in active locomotion or sitting still during recording sessions, and proposed automatic state‐detection as a viable tool for estimating drug‐effects free of hypo‐/hyperlocomotion‐induced effects. We aimed at developing a fully automatic and validated method for detecting two behavioural states: active and inactive, in one‐second intervals and to use the method for evaluating ketamine, DOI, d‐cycloserine, d‐amphetamine, and diazepam effects specifically within each state. The developed state‐detector attained high precision with more than 90% of the detected time correctly classified, and multiple differences between the two detected states were discovered. Ketamine‐induced delta activity was found specifically related to locomotion. Ketamine and DOI suppressed theta and beta oscillations exclusively during inactivity. Characteristic gamma and high‐frequency oscillations (HFO) enhancements of the NMDAR and 5HT_2A modulators, speculated associated with locomotion, were profound and often largest during the inactive state. State‐specific analyses, theoretically eliminating biases from altered occurrence of locomotion, revealed only few effects of d‐amphetamine and diazepam. Overall, drug‐effects were most abundant in the inactive state. In conclusion, this new validated and automatic locomotion state‐detection method enables fast and reliable state‐specific analysis facilitating discovery of state‐dependent drug‐effects and control for altered occurrence of locomotion. This may ultimately lead to better cross‐species translation of electrophysiological effects of pharmacological modulations.

miR-124 dosage regulates prefrontal cortex function by dopaminergic modulation

MicroRNA-124 (miR-124) is evolutionarily highly conserved among species and one of the most abundantly expressed miRNAs in the developing and mature central nervous system (CNS). Previous studies reported that miR-124 plays a role in CNS development, such as neuronal differentiation, maturation, and survival. However, the role of miR-124 in normal brain function has not yet been revealed. Here, we subjected miR-124-1^+/− mice, to a comprehensive behavioral battery. We found that miR-124-1^+/− mice showed impaired prepulse inhibition (PPI), methamphetamine-induced hyperactivity, and social deficits. Whole cell recordings using prefrontal cortex (PFC) slices showed enhanced synaptic transmission in layer 5 pyramidal cells in the miR-124-1^+/− PFC. Based on the results of behavioral and electrophysiological analysis, we focused on genes involved in the dopaminergic system and identified a significant increase of Drd2 expression level in the miR-124-1^+/− PFC. Overexpression or knockdown of Drd2 in the control or miR-124-1^+/− PFC demonstrates that aberrant Drd2 signaling leads to impaired PPI. Furthermore, we identified that expression of glucocorticoid receptor gene Nr3c1, which enhances Drd2 expression, increased in the miR-124-1^+/− PFC. Taken together, the current study suggests that miR-124 dosage modulates PFC function through repressing the Drd2 pathway, suggesting a critical role of miR-124 in normal PFC function.

Adolescent isolation rearing produces a prepulse inhibition deficit correlated with expression of the NMDA GluN1 subunit in the nucleus accumbens

Adolescence is a transition period during which social interaction is necessary for normal brain and behavior development. Severely abnormal social interactions during adolescence can increase the incidence of lifelong psychiatric disease. Decreased prepulse inhibition (PPI) is a quantifiable hallmark of some psychiatric illnesses in humans and can be elicited in rodents by isolation rearing throughout the adolescent transition period. PPI is a measure of sensorimotor gating in which the nucleus accumbens (Acb) is crucially involved. The Acb is comprised of core and shell subregions, which receive convergent dopaminergic and glutamatergic inputs. To gain insight into the neurobiological correlates of adolescent adversity, we conducted electron microscopic immunolabeling of dopamine D1 receptors (D1Rs) and the GluN1 subunit of glutamate NMDA receptors in the Acb of isolation-reared (IR) adult male rats. In all animals, GluN1 was primarily located in dendritic profiles, many of which also contained D1Rs. GluN1 was also observed in perisynaptic glia and axon terminals. In IR rats compared with group-reared controls, GluN1 density was selectively decreased in D1R-containing dendrites of the Acb core. Across all animals, dendritic GluN1 density correlated with average percent PPI, implicating endogenous expression of NMDA receptors of the Acb as a possible substrate of the PPI response. These results suggest that adolescent isolation dampens NMDA-mediated excitation in direct (D1R-containing) output neurons of the Acb, and that these changes influence the operational measure of PPI.

Consequences at adulthood of transient inactivation of the parahippocampal and prefrontal regions during early development: new insights from a disconnection animal model for schizophrenia

The psychic disintegration characteristic of schizophrenia is thought to result from a defective connectivity, of neurodevelopmental origin, between several integrative brain regions. The parahippocampal region and the prefrontal cortex are described as the main regions affected in schizophrenia. Interestingly, latent inhibition (LI) has been found to be reduced in patients with schizophrenia, and the existence of a dopaminergic dysfunction is also generally well accepted in this disorder. In the present review, we have integrated behavioral and neurochemical data obtained in a LI protocol involving adult rats subjected to neonatal functional inactivation of the entorhinal cortex, the ventral subiculum or the prefrontal cortex. The data discussed suggest a subtle and transient functional blockade during early development of the aforementioned brain regions is sufficient to induce schizophrenia-related behavioral and dopaminergic abnormalities in adulthood. In summary, these results support the view that our conceptual and methodological approach, based on functional disconnections, is valid for modeling some aspects of the pathophysiology of schizophrenia from a neurodevelopmental perspective.

Critical role of nitric oxide in the modulation of prepulse inhibition in Swiss mice

RATIONALE

Nitric oxide (NO) modulates the dopamine uptake and release processes and appears to be implicated in dopamine-related pathologies, such as schizophrenia. However, it is unclear whether there is excess or deficient NO synthesis in schizophrenia pathophysiology. Analyses of the intracellular pathways downstream of NO system activation have identified the cyclic nucleotide cyclic guanosine monophosphate (cGMP) as a possible target for drug development. Defects in the sensorimotor gating of the neural mechanism underlying the integration and processing of sensory information have been detected across species through prepulse inhibition (PPI).

OBJECTIVES

The aim of this study was to investigate the effects of NO/cGMP increase on sensorimotor gating modulation during dopamine hyperfunction.

METHODS

Mice were treated with NO donors and subjected to the PPI test. Treatment with the NO donor sodium nitroprusside was preceded by pretreatment with a soluble guanylate cyclase (sGC) inhibitor. Additionally, the mice were treated with NO donors and phosphodiesterases inhibitors prior to amphetamine treatment.

RESULTS

Pretreatment with the NO donors enhanced the PPI response and attenuated the amphetamine-disruptive effects on the PPI. The sGC inhibitor did not modify the sodium nitroprusside effects. Additionally, the cGMP increase induced by a specific phosphodiesterase inhibitor did not modify the amphetamine-disruptive effect.

CONCLUSIONS

This study provides the first demonstration that an increase in NO can improve the PPI response and block the amphetamine-disruptive effects on the PPI response. Our data are consistent with recent clinical results. However, these effects do not appear to be related to an increase in cGMP levels, and further investigation is thus required.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

D-amphetamine and antipsychotic drug effects on latent inhibition in mice lacking dopamine D2 receptors

Drugs that induce psychosis, such as D-amphetamine (AMP), and those that alleviate it, such as antipsychotics, are suggested to exert behavioral effects via dopamine receptor D2 (D2). All antipsychotic drugs are D2 antagonists, but D2 antagonism underlies the severe and debilitating side effects of these drugs; it is therefore important to know whether D2 is necessary for their behavioral effects. Using D2-null mice (Drd2-/-), we first investigated whether D2 is required for AMP disruption of latent inhibition (LI). LI is a process of learning to ignore irrelevant stimuli. Disruption of LI by AMP models impaired attention and abnormal salience allocation consequent to dysregulated dopamine relevant to schizophrenia. AMP disruption of LI was seen in both wild-type (WT) and Drd2-/-. This was in contrast to AMP-induced locomotor hyperactivity, which was reduced in Drd2-/-. AMP disruption of LI was attenuated in mice lacking dopamine receptor D1 (Drd1-/-), suggesting that D1 may play a role in AMP disruption of LI. Further supporting this possibility, we found that D1 antagonist SKF83566 attenuated AMP disruption of LI in WT. Remarkably, both haloperidol and clozapine attenuated AMP disruption of LI in Drd2-/-. This demonstrates that antipsychotic drugs can attenuate AMP disruption of learning to ignore irrelevant stimuli in the absence of D2 receptors. Data suggest that D2 is not essential either for AMP to disrupt or for antipsychotic drugs to reverse AMP disruption of learning to ignore irrelevant stimuli and further that D1 merits investigation in the mediation of AMP disruption of these processes.

Cognitive, emotional and neurochemical effects of repeated maternal separation in adolescent rats

As an adverse early life experience, maternal separation (MS) induces profound neurochemical, cognitive and emotional dysfunction. Previous studies have reported that MS affected prepulse inhibition (PPI), anxiety-related behaviors, dopaminergic and serotonergic activity in adult rats, and in the present study, we investigated the effects of repeated (4h/day) maternal separation during postnatal days 1-21 on PPI and anxiety-related behaviors in an elevated plus maze, as well as dopamine D2 receptor (DRD2) and 5-HT1A receptor expression in the medial prefrontal cortex (mPFC), nucleus accumbens (NAc) and hippocampus in adolescent rats. Our findings show that repeated MS results in reduced PPI, increased anxiety-related behaviors, decreased DRD2 protein expression in the NAc and hippocampus, and decreased 5-HT1A protein expression in the mPFC and hippocampus in adolescent rats. These data further demonstrate that MS can be used as an animal model of neuropsychiatric disease.

Lesion of medial prefrontal dopamine terminals abolishes habituation of accumbens shell dopamine responsiveness to taste stimuli

Taste stimuli increase extracellular dopamine (DA) in the nucleus accumbens (NAc) and in the medial prefrontal cortex (mPFC). This effect shows single-trial habituation in NAc shell but not in core or in mPFC. Morphine sensitization abolishes habituation of DA responsiveness in NAc shell but induces it in mPFC. These observations support the hypothesis of an inhibitory influence of mPFC DA on NAc DA. To test this hypothesis, we used in vivo microdialysis to investigate the effect of mPFC 6-hydroxy-dopamine (6-OHDA) lesions on the NAc DA responsiveness to taste stimuli. 6-OHDA was infused bilaterally in the mPFC of rats implanted with guide cannulae. After 1 week, rats were implanted with an intraoral catheter, microdialysis probes were inserted into the guide cannulae, and dialysate DA was monitored in NAc shell/core after intraoral chocolate. 6-OHDA infusion reduced tissue DA in the mPFC by 75%. Tyrosine hydroxylase immunohistochemistry showed that lesions were confined to the mPFC. mPFC 6-OHDA lesion did not affect the NAc shell DA responsiveness to chocolate in naive rats but abolished habituation in rats pre-exposed to the taste. In the NAc core, mPFC lesion potentiated, delayed and prolonged the stimulatory DA response to taste but failed to affect DA in pre-exposed rats. Behavioural taste reactions and motor activity were not affected. The results indicate a top-down control of NAc DA by mPFC and a reciprocal relationship between DA transmission in these two areas. Moreover, habituation of DA responsiveness in the NAc shell is dependent upon an intact DA input to the mPFC.

Startle reactivity and prepulse inhibition of the acoustic startle response are modulated by catechol-O-methyl-transferase Val(158) Met polymorphism in adults with 22q11 deletion syndrome

22q11 deletion syndrome (22q11DS) is a genetic disorder caused by a microdeletion on chromosome 22, which includes the gene coding for catechol-O-methyl-transferase (COMT). High dopamine (DA) levels due to COMT haplo-insufficiency may be associated with the increased risk of developing schizophrenia in adults with 22q11DS. Reduced prepulse inhibition (PPI) of the acoustic startle response has been associated with schizophrenia and with disrupted DAergic transmission in the prefrontal cortex (PFC). COMT Val(158)Met polymorphism has been shown to influence PPI. We report the first study in adults with 22q11DS to examine PPI of the acoustic startle response and its modulation by COMT Val(158)Met polymorphism. Startle reactivity (SR) and PPI of the acoustic startle response were measured in 23 adults with 22q11DS and 21 healthy controls. 22q11DS subjects were genotyped for the functional COMT Val(158)Met polymorphism. 22q11DS Met hemizygotes showed reduced SR and PPI compared with 22q11DS Val hemizygotes. The effect of COMT Val(158)Met polymorphism on PPI was no longer significant when controlling for baseline SR. Met hemizygosity in 22q11DS is associated with reduced SR and influences PPI indirectly. Decreased PFC functioning following excessive PFC DA levels may be one of the mechanisms by which the Met genotype in 22q11DS disrupts SR.

Impairment of cognitive performance after reelin knockdown in the medial prefrontal cortex of pubertal or adult rats

The glycoprotein reelin is important for embryonic neuronal migration. During adulthood reelin possibly acts as a modulator of synaptic plasticity. Several studies link reduced levels of reelin messenger RNA and protein to the pathophysiology of certain neuropsychiatric disorders. However, little is known about reelin's role for behavioral and cognitive functions in vivo. Therefore, the effect of a reelin knockdown in the medial prefrontal cortex (mPFC) of Wistar rats was examined in behavioral tasks related to neuropsychiatric disorders, such as schizophrenia. Rats treated with reelin antisense phosphothioate oligonucleotides in the mPFC during puberty or adulthood were tested for prepulse inhibition (PPI) of the acoustic startle reflex, spatial working memory, object recognition, and locomotor activity. Reelin quantification in the mPFC was assessed by Western blotting. Local reelin knockdown during puberty or adulthood induced (1) a PPI deficit as well as (2) an impairment of spatial working memory and object recognition following pubertal injections. Western blot analyses showed a distinct and highly selective reelin knockdown in the rats' mPFC. These results indicate that mPFC reelin signaling plays an important role in behavioral tasks with relevance to e.g. schizophrenia. Understanding reelin's function as a neurotrophic modulator of the extracellular matrix may help to achieve new insights into the etiology of certain neuropsychiatric diseases and foster prospective treatment strategies.

The influence of schizophrenia-related neuregulin-1 polymorphisms on sensorimotor gating in healthy males

BACKGROUND

Neuregulin-1 (NRG1) variations have been shown to modulate schizophrenia candidate endophenotypes related to brain structure and function. The objective of this cross-sectional genetic association study was to determine the relationship of six core single-nucleotide polymorphisms within the NRG1 gene identified as promising schizophrenia risk genes (rs6994992, SNP8NRG221132, SNP8NRG241930, rs3924999, rs2439272 and rs10503929) to prepulse inhibition (PPI) of the acoustic startle reflex, a well validated schizophrenia endophenotype.

METHODS

PPI was tested in a highly homogeneous study entry cohort (n = 445) of carefully screened healthy, young male army conscripts originating from the Greek LOGOS project (Learning on Genetics of Schizophrenia Spectrum). The QTPHASE from the UNPHASED package was used for the association analysis of each single-nucleotide polymorphisms or haplotype data.

RESULTS

Reduced PPI, particularly at 75-dB_120-msec and 85-dB_60-msec trials, was related to the SNP8NRG241930 G allele and especially the rs6994992 T allele and rs2439272 C allele. Haplotype analysis followed up by risk versus no-risk groups Analysis of variance confirmed that the rs10503929 and rs3924999 SNPs were also associated with PPI reductions, when combined with rs2439272.

CONCLUSIONS

We provide solid evidence for a role of NRG1 risk genotype variations in PPI reductions in a large and demographically and genetically highly homogeneous cohort of healthy young males. These results further validate NRG1 as a candidate gene for the schizophrenia and spectrum disorders and improve our understanding of its functional mechanisms within the human brain because they suggest an influence of the gene in the neural substrate mediating sensorimotor gating.

Transient exposure of neonatal mice to neuregulin-1 results in hyperdopaminergic states in adulthood: implication in neurodevelopmental hypothesis for schizophrenia

Neuregulin-1 (NRG1) is implicated in the etiology or pathology of schizophrenia, although its biological roles in this illness are not fully understood. Human midbrain dopaminergic neurons highly express NRG1 receptors (ErbB4). To test its neuropathological role in the neurodevelopmental hypothesis of schizophrenia, we administered type-1 NRG1 protein to neonatal mice and evaluated the immediate and subsequent effects on dopaminergic neurons and their associated behaviors. Peripheral NRG1 administration activated midbrain ErbB4 and elevated the expression, phosphorylation and enzyme activity of tyrosine hydroxylase (TH), which ultimately increased dopamine levels. The hyperdopaminergic state was sustained in the medial prefrontal cortex after puberty. There were marked increases in dopaminergic terminals and TH levels. In agreement, higher amounts of dopamine were released from this brain region of NRG1-treated mice following high potassium stimulation. Furthermore, NRG1-treated mice exhibited behavioral impairments in prepulse inhibition, latent inhibition, social behaviors and hypersensitivity to methamphetamine. However, there were no gross abnormalities in brain structures or other phenotypic features of neurons and glial cells. Collectively, our findings provide novel insights into neurotrophic contribution of NRG1 to dopaminergic maldevelopment and schizophrenia pathogenesis.

Lesions to the ventral, but not the dorsal, medial prefrontal cortex enhance latent inhibition

The acquisition of a conditioned response to a stimulus when it is paired with a reinforcer is retarded if the stimulus has previously been repeatedly pre-exposed in the absence of the reinforcer. This effect, called latent inhibition, has previously been found to be insensitive to lesions of the medial prefrontal cortex (mPFC) in rats. Using an on-baseline conditioned emotional response procedure, which is especially sensitive to small variations in the absolute magnitude of latent inhibition, we found increased latent inhibition following excitotoxic lesions of the mPFC (Experiment 1) or the ventral mPFC alone (Experiment 2) as compared with sham-operated control rats. Lesions restricted to the dorsal mPFC, however, were without effect (Experiment 2). These results are consistent with those of experiments employing another type of interference procedure, extinction. Together, these findings suggest that when different contingencies between a stimulus and a reinforcer are established in separate learning phases, lesions to the ventral mPFC result in increased interference between first-learned and second-learned contingencies. As a consequence, retrieval of the second-learned contingency is impaired, and performance is dominated by the first-learned contingency. These findings are discussed in light of the use of latent inhibition to model cognitive deficits in schizophrenia.

Constitutive genetic deletion of the growth regulator Nogo-A induces schizophrenia-related endophenotypes

The membrane protein Nogo-A, which is predominantly expressed by oligodendrocytes in the adult CNS and by neurons mainly during development, is well known for limiting neurite outgrowth and regeneration in the injured mammalian CNS. In addition, it has recently been proposed that abnormal Nogo-A expression or Nogo receptor (NgR) mutations may confer genetic risks for neuropsychiatric disorders of presumed neurodevelopmental origin, such as schizophrenia. We therefore evaluated whether Nogo-A deletion may lead to schizophrenia-like abnormalities in a mouse model of genetic Nogo-A deficiency. Here, we show that systemic, lifelong knock-out of the Nogo-A gene can lead to specific behavioral abnormalities resembling schizophrenia-related endophenotypes: deficient sensorimotor gating, disrupted latent inhibition, perseverative behavior, and increased sensitivity to the locomotor stimulating effects of amphetamine. These behavioral phenotypes were accompanied by altered monoaminergic transmitter levels in specific striatal and limbic structures, as well as changes in dopamine D2 receptor expression in the same brain regions. Nogo-A deletion was further associated with elevated expression of growth-related markers. In contrast, acute antibody-mediated Nogo-A neutralization in adult wild-type mice failed to produce such phenotypes, suggesting that the phenotypes observed in the knock-out mice might be of developmental origin, and that Nogo-A normally subserves critical functions in neurodevelopment. This study provides the first experimental demonstration that Nogo-A bears neuropsychiatric relevance, and alterations in its expression may be one etiological factor in schizophrenia and related disorders.

Effect of apomorphine on cognitive performance and sensorimotor gating in humans

INTRODUCTION

Dysfunction of brain dopamine systems is involved in various neuropsychiatric disorders. Challenge studies with dopamine receptor agonists have been performed to assess dopamine receptor functioning, classically using the release of growth hormone (GH) from the hindbrain as primary outcome measure. The objective of the current study was to assess dopamine receptor functioning at the forebrain level.

METHODS

Fifteen healthy male volunteers received apomorphine sublingually (2 mg), subcutaneously (0.005 mg/kg), and placebo in a balanced, double-blind, cross-over design. Outcome measures were plasma GH levels, performance on an AX continuous performance test, and prepulse inhibition of the acoustic startle. The relation between central outcome measures and apomorphine levels observed in plasma and calculated in the brain was modeled using a two-compartmental pharmacokinetic-pharmacodynamic analysis.

RESULTS

After administration of apomorphine, plasma GH increased and performance on the AX continuous performance test deteriorated, particularly in participants with low baseline performance. Apomorphine disrupted prepulse inhibition (PPI) on high-intensity (85 dB) prepulse trials and improved PPI on low intensity (75 dB) prepulse trials, particularly in participants with low baseline PPI. High cognitive performance at baseline was associated with reduced baseline sensorimotor gating. Neurophysiological measures correlated best with calculated brain apomorphine levels after subcutaneous administration.

CONCLUSION

The apomorphine challenge test appears a useful tool to assess dopamine receptor functioning at the forebrain level. Modulation of the effect of apomorphine by baseline performance levels may be explained by an inverted U-shape relation between prefrontal dopamine functioning and cognitive performance, and mesolimbic dopamine functioning and sensorimotor gating. Future apomorphine challenge tests preferentially use multiple outcome measures, after subcutaneous administration of apomorphine.

Prenatal exposure to infection: a primary mechanism for abnormal dopaminergic development in schizophrenia

RATIONALE

Prenatal exposure to infection is a notable environmental risk factor in the development of schizophrenia. One prevalent hypothesis suggests that infection-induced disruption of early prenatal brain development predisposes the organism to long-lasting structural and functional brain abnormalities. Many of the prenatal infection-induced functional brain abnormalities appear to be closely associated with imbalances in the mesocorticolimbic dopamine system in adult life, suggesting that disruption of functional and structural dopaminergic development may be at the core of the developmental neuropathology associated with psychosis-related abnormalities induced by prenatal exposure to infection.

OBJECTIVES

In this review, we integrate recent findings derived from experimental models in animals with parallel research in humans which supports this hypothesis. We thereby highlight the developmental perspective of abnormal DA functions following in-utero exposure to infection in relation to the developmental and maturational mechanisms potentially involved in schizophrenia.

RESULTS

Experimental investigations show that early prenatal immune challenge can lead to the emergence of early structural and functional alterations in the mesocorticolimbic DA system, long before the onset of the full spectrum of psychosis-associated behavioral and cognitive abnormalities in adulthood.

CONCLUSIONS

Dopaminergic mal-development in general, and following prenatal immune activation in particular, may represent a primary etiopathological mechanism in the development of schizophrenia and related disorders. This hypothesis differs from the view that dopaminergic abnormalities in schizophrenia may be secondary to abnormalities in other brain structures and/or neurotransmitter systems. The existence of primary dopaminergic mechanisms may have important implications for the identification and early treatment of individuals prodromally symptomatic for schizophrenia.

Sensorimotor gating depends on polymorphisms of the serotonin-2A receptor and catechol-O-methyltransferase, but not on neuregulin-1 Arg38Gln genotype: a replication study

BACKGROUND

Prepulse inhibition (PPI) of the acoustic startle response (ASR) is an operational measure of sensorimotor gating and a promising endophenotype of schizophrenia. We have recently shown that the linked serotonin-2A receptor (5-HT(2A)R) A-1438 G and T102C polymorphisms modulate PPI in schizophrenia patients. Moreover, it was shown that genetic variation in the catechol-O-methyltransferase (COMT) and the neuregulin-1 (NRG-1) proteins influences PPI in schizophrenia patients and healthy volunteers. Therefore, we aimed to replicate these results and investigated the impact of the related polymorphisms on PPI in healthy human volunteers.

METHODS

We analyzed the 5-HT(2A)R A-1438 G/T102C (rs6311/rs6313), the COMT Val158Met (rs4680), and the NRG-1 Arg38Gln (rs3924999) polymorphisms, assessing startle reactivity, habituation, and PPI of ASR in 107 healthy Caucasian volunteers.

RESULTS

Subjects homozygous for the 5-HT(2A)R T102C-T/A-1438 G-A allele showed increased PPI levels. In particular, male subjects with the COMT Met158Met-genotype also showed elevated PPI. The NRG-1 Arg38Gln genotype did not have a significant impact on PPI. Startle reactivity was not affected by any of the investigated polymorphisms.

CONCLUSIONS

We confirmed in an independent sample of healthy volunteers that PPI is influenced by genetic variation in the 5-HT(2A)R gene. The influence of the COMT Val158Met genotype on PPI appears to be sex-specific. These results underscore the significance of the serotonin and dopamine systems in the modulation of sensorimotor gating.

Prenatal immune activation leads to multiple changes in basal neurotransmitter levels in the adult brain: implications for brain disorders of neurodevelopmental origin such as schizophrenia

Maternal infection during pregnancy enhances the offspring's risk for severe neuropsychiatric disorders in later life, including schizophrenia. Recent attempts to model this association in animals provided further experimental evidence for a causal relationship between in-utero immune challenge and the postnatal emergence of a wide spectrum of behavioural, pharmacological and neuroanatomical dysfunctions implicated in schizophrenia. However, it still remains unknown whether the prenatal infection-induced changes in brain and behavioural functions may be associated with multiple changes at the neurochemical level. Here, we tested this hypothesis in a recently established mouse model of viral-like infection. Pregnant dams on gestation day 9 were exposed to viral mimetic polyriboinosinic-polyribocytidilic acid (PolyI:C, 5 mg/kg i.v.) or vehicle treatment, and basal neurotransmitter levels were then compared in the adult brains of animals born to PolyI:C- or vehicle-treated mothers by high-performance liquid chromatography on post-mortem tissue. We found that prenatal immune activation significantly increased the levels of dopamine and its major metabolites in the lateral globus pallidus and prefrontal cortex, whilst at the same time it decreased serotonin and its metabolite in the hippocampus, nucleus accumbens and lateral globus pallidus. In addition, a specific reduction of the inhibitory amino acid taurine in the hippocampus was noted in prenatally PolyI:C-exposed offspring relative to controls, whereas central glutamate and gamma-aminobutyric acid (GABA) content was largely unaffected by prenatal immune activation. Our results thus confirm that maternal immunological stimulation during early/middle pregnancy is sufficient to induce long-term changes in multiple neurotransmitter levels in the brains of adult offspring. This further supports the possibility that infection-mediated interference with early fetal brain development may predispose the developing organism to the emergence of neurochemical imbalances in adulthood, which may be critically involved in the precipitation of adult behavioural and pharmacological abnormalities after prenatal immune challenge.

Improvement of prepulse inhibition and executive function by the COMT inhibitor tolcapone depends on COMT Val158Met polymorphism

Recent evidence suggests that prepulse inhibition (PPI) levels relate to executive function possibly by a prefrontal cortex (PFC) dopamine (DA) link. We explored the effects of enhanced PFC DA signaling by the nonstimulant catechol-O-methyltransferase (COMT) inhibitor tolcapone, on PPI and working memory of subjects homozygous for the Val (low PFC DA) and the Met (high PFC DA) alleles of the COMT Val158Met polymorphism. Twelve Val/Val and eleven Met/Met healthy male subjects entered the study. Tolcapone 200 mg was administered in two weekly sessions, according to a balanced, crossover, double-blind, placebo-controlled design. PPI was assessed with 5 dB and 15 dB above background prepulses, at 30-, 60-, and 120 ms prepulse-pulse intervals. Subjects also underwent the n-back and the letter-number sequencing (LNS) tasks. PPI was lower in the Val/Val compared to the Met/Met group in the placebo condition. Tolcapone increased PPI significantly in the Val/Val group and tended to have the opposite effect in the Met/Met group. Baseline startle was not affected by tolcapone in the Val/Val group but it was slightly increased in the Met/Met group. Tolcapone improved performance in the n-back and LNS tasks only in the Val/Val group. Enhancement of PFC DA signaling with tolcapone improves both PPI and working memory in a COMT Val158Met genotype-specific manner. These results suggest that early information processing and working memory may both depend on PFC DA signaling, and that they may both relate to PFC DA levels according to an inverted U-shaped curve function.

Prepulse inhibition of the startle reflex depends on the catechol O-methyltransferase Val158Met gene polymorphism

BACKGROUND

Recent evidence suggests that dopamine (DA) agonist-induced disruption of prepulse inhibition (PPI) depends on basal PPI values, in a manner that suggests an inverted U-shaped relationship between PPI and prefrontal DA levels. This is the first study to examine possible genetic determinants of PPI and the catechol O-methyltransferase (COMT) Val158Met polymorphism, the main catabolic pathway of released DA in the prefrontal cortex (PFC).

METHOD

PPI was measured in 93 healthy males presented with 75-dB and 85-dB prepulses at 60-ms and 120-ms prepulse-pulse intervals. Subjects were grouped according to their COMT status into a Val/Val, a Val/Met and a Met/Met group.

RESULTS

ANOVAs showed that at all prepulse and interval conditions, Val/Val individuals had the lowest PPI, Met/Met the highest, and Val/Met were intermediate.

CONCLUSIONS

These results suggest that PPI is regulated by DA neurotransmission in the PFC and its levels depend on the COMT Val158Met gene polymorphism. These findings enhance the value of the PPI paradigm in examining individual variability of early information processing in healthy subjects and psychiatric disorders associated with changes in PFC DA activity and attentional deficits such as schizophrenia.

The dopamine D(3) receptor Ser9Gly polymorphism modulates prepulse inhibition of the acoustic startle reflex

BACKGROUND

The dopamine D(3) receptor (DRD(3)) is suspected to modulate prepulse inhibition (PPI) in animals and humans, but definite conclusions cannot be drawn due to lack of selective DRD(3) ligands. The Ser9Gly polymorphism is a common variant of the DRD(3) gene and determines the gain of function of the D(3) receptor. This is the first study to examine the influence of the DRD(3) Ser9Gly polymorphism on human PPI.

METHODS

Prepulse inhibition was measured in 101 healthy male subjects presented with 75-dB and 85-dB prepulses at 30-, 60-, and 120-msec prepulse-pulse intervals. Subjects were grouped according to their DRD(3) status into a Gly/Gly, a Ser/Gly, and a Ser/Ser group.

RESULTS

Analyses of variance showed that at all prepulse and interval conditions, Gly/Gly individuals had the lowest PPI and the greatest onset latency facilitation and Ser/Ser individuals had the highest PPI and the lowest onset latency facilitation, while Ser/Gly individuals were intermediate.

CONCLUSIONS

These results suggest that PPI is modulated by the D(3) receptor and its levels depend on the Ser9Gly polymorphism.

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

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

On again, off again effects of gonadectomy on the acoustic startle reflex in adult male rats

Numerous studies have shown sex and/or estrous cycle differences in the acoustic startle reflex (ASR) and its prepulse inhibition (PPI) in humans and animals. However, few have examined the effects of hormone manipulations on these behaviors. This study paired gonadectomy (GDX) in adult male rats with testing for ASR and PPI at 2, 4, 9, 16, 23, 30 and 37 days after surgery. Initial studies of control, GDX and GDX rats given testosterone propionate revealed no group differences in PPI, but did reveal phasic facilitation of the ASR in GDX rats that was greatest on the first and final testing sessions and that was attenuated by testosterone. A second study addressing roles for estrogen and androgen signaling tested new control and GDX rats along with GDX rats given estradiol or the non-aromatizable androgen, 5-alpha-dihydrotestosterone and revealed no group differences in PPI, and increases in ASR in GDX rats that were largest during the first and final testing sessions and that were attenuated by both hormone replacements. However, while responses in GDX rats given testosterone were similar to those of controls, ASR in estradiol- and to a lesser extent in dihydrotestosterone-treated GDX rats were typically lower than in controls. This may suggest that hormone modulation of the ASR requires synergistic estrogen and androgen actions. In the male brain where this can be achieved by local steroid metabolism, the enzymes responsible, e.g., aromatase, could help identify loci in the startle circuitry that may be especially relevant for the hormone modulation observed.

Stimulation of D2 receptors in the prefrontal cortex reduces PCP-induced hyperactivity, acetylcholine release and dopamine metabolism in the nucleus accumbens

The aim of the present study was to investigate the effects of stimulation of D2 receptors in the prefrontal cortex (PFC) on spontaneous motor activity and the hyperactivity induced by the psychomimetic phencyclidine (PCP). In addition, the effects of prefrontal D2 stimulation under PCP treatment on dialysate concentrations of acetylcholine, choline, dopamine, DOPAC and HVA in the nucleus accumbens were also investigated. Sprague-Dawley male rats were implanted with guide cannulae to perform bilateral injections into the medial PFC of the D2 agonist quinpirole (1.5 and 5 microg/side). Horizontal and vertical spontaneous motor activity and the motor activity induced by systemic injections of the PCP (5 mg/kg i.p.) were monitored in the open field. PFC injections of quinpirole (1.5 and 5 microg/side) significantly decreased horizontal and vertical spontaneous motor activity in a dose-related manner. These effects were blocked by the D2 antagonist raclopride (5 microg/side). Microinjections of quinpirole (1.5 and 5 microg/side) into the PFC also significantly attenuated the hyperactivity produced by PCP (5 mg/kg i.p.). PCP also increased dialysate concentrations of acetylcholine, and dopamine metabolites in the nucleus accumbens. These increases were also reduced by injections of quinpirole (5 microg/side) into the PFC. These results suggest that the stimulation of prefrontal D2 receptors plays an inhibitory role in regulating spontaneous and PCP-induced motor activity and also in the neurochemical changes produced by PCP in the nucleus accumbens.

Neurotrophin levels and behaviour in BALB/c mice: impact of intermittent exposure to individual housing and wheel running

This study assessed the effects of intermittent individual housing on behaviour and brain neurotrophins, and whether physical exercise could influence alternate individual-housing-induced effects. Five-week-old BALB/c mice were either housed in enhanced social (E) or standard social (S) housing conditions for 2 weeks. Thereafter they were divided into six groups and for 6 weeks remained in the following experimental conditions: Control groups remained in their respective housing conditions (E-control, S-control); enhanced individual (E-individual) and standard individual (S-individual) groups were exposed every other day to individual cages without running-wheels; enhanced running-wheel (E-wheel) and standard running-wheel (S-wheel) groups were put on alternate days in individual running-wheel cages. Animals were assessed for activity in an automated individual cage system (LABORAS) and brain neurotrophins analysed. Intermittent individual housing increased behavioural activity and reduced nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) levels in frontal cortex; while it increased BDNF level in the amygdala and BDNF protein and mRNA in hippocampus. Besides normalizing motor activity and regulating BDNF and NGF levels in hippocampus, amygdala and cerebellum, physical exercise did not attenuate reduction of cortical NGF and BDNF induced by intermittent individual housing. This study demonstrates that alternate individual housing has significant impact on behaviour and brain neurotrophin levels in mice, which can be partially altered by voluntary physical exercise. Our results also suggest that some changes in neurotrophin levels induced by intermittent individual housing are not similar to those caused by continuous individual housing.

The role of dopamine in conditioning and latent inhibition: what, when, where and how?

It is well established that dopamine is released in the nucleus accumbens (NAC) in animals in rewarding or reinforcing situations, and widely believed that this release is the substrate of, or at least closely related to, the experience of reward. The demonstration of conditioned release of dopamine by stimuli conditioned to primary rewards has reinforced this view. However, a number of observations do not sit comfortably with this interpretation, most notably that dopamine is released equally effectively in NAC by aversive stimuli, and stimuli conditioned to them. Furthermore, additional release of dopamine is seen during conditioning, even if motivational stimuli of either type are not involved. It is suggested here that one important action of NAC dopamine release is to restore the salience of potential conditioned stimuli, when this has been reduced by prior un-reinforced experience. The paradigm of latent inhibition (LI) demonstrates a behavioural effect of this type, and extensive studies on the role of dopamine in LI have been undertaken by us and others. Those studies are reviewed here, together with some previously unpublished data, to demonstrate that (1) amphetamine disruption of LI is indeed a function of calcium-dependant dopamine release in the NAC at the time of conditioning; (2) other drugs acting on LI via changes in dopamine transmission act at the same locus; (3) the disruptive effect of indirect dopamine agonists on LI can be prevented by either D-1 selective receptor antagonists, or D-2 selective receptor antagonists. It is concluded that dopamine release in these very varied behavioural contexts (reward, punishment, conditioning, modulation of salience) must be differentiated in some way, and that this should be investigated. An alternative explanation, if they are not differentiated, would be that the release in fact does have the same functional significance in each case. We suggest that this common significance might be the broadening of attention to take in potentially conditionable stimuli, which have previously been devalued.

The effects of dopamine agonists on prepulse inhibition in healthy men depend on baseline PPI values

RATIONALE AND OBJECTIVES

Dopamine (DA) agonists reliably disrupt prepulse inhibition (PPI) of the startle reflex in animals but less so in humans despite cross-species similarities in the neural regulation of PPI. This study examines whether individual variation in baseline PPI may account for the inconsistencies in DA agonist-induced PPI disruption in humans.

METHODS

Baseline PPI measures were obtained from 32 healthy adult men. Subjects were subsequently tested in three sessions after ingestion of placebo or active drug in a balanced double-blind design. Seventeen subjects were given 0.05 and 0.1 mg of pergolide (a direct DA agonist) and 15 subjects were given 100 and 200 mg of amantadine (an indirect DA agonist). In each treatment group, subjects were assigned to "high" and "low" PPI subgroups based on the median split of their baseline PPI.

RESULTS

Amantadine and pergolide disrupted PPI in high- but not in low-PPI subjects. In contrast, low-PPI subjects showed a trend towards PPI facilitation especially with pergolide.

CONCLUSIONS

Our results suggest that baseline PPI is an important determinant of the effect of DA agonists on PPI.

Early amygdala damage disrupts performance on medial prefrontal cortex-related tasks but spares spatial learning and memory in the rat

Recent studies have demonstrated that the postnatal development of connections between the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) mature around postnatal days 13-15 (pd13-15), whereas these between the BLA and other structures such as the nucleus accumbens and the mediodorsal thalamus are completed by pd7. Accordingly, it is hypothesized that mPFC cytoarchitecture and hence its function may be specifically affected by neonatal (i.e. on pd7) but not later induced (i.e. on pd21) damage to the BLA. To test this hypothesis, rats received excitotoxic lesions to the BLA on either pd7 or pd21 and were subjected to two tests putatively sensitive to mPFC dysfunction, namely food hoarding and spontaneous alternation. In addition, rats were tested for spatial learning and memory, to determine any possible effects on hippocampal function. Consistent with the documented effects of mPFC lesions, pd7 damage to the BLA impaired spontaneous alternation and food hoarding performance, an effect that was not found in rats with BLA lesions induced on pd21. Spatial learning and memory, however, were not affected by the (neonatal) lesion procedure. Together, these results indicate that neonatal BLA damage affects species-specific sequential behavior and flexibility, which may be attributed to abnormal functioning of the mPFC.

Lesions to the basolateral amygdala and the orbitofrontal cortex but not to the medial prefrontal cortex produce an abnormally persistent latent inhibition in rats

Repeated nonreinforced preexposure to a stimulus interferes with the establishment of conditioned responding to this stimulus when it is subsequently paired with reinforcement. This stimulus-preexposure effect is known as latent inhibition (LI). Rather remarkably, LI appears to be resistant to the effects of numerous lesions, including the prefrontal cortex (PFC) and the basolateral amygdala (BLA). However, intact behavioral expression of LI following damage to given brain regions does not preclude the possibility that such regions participate in the regulation of LI expression in the intact brain. The present study showed that lesions of the BLA and the orbitofrontal cortex (OFC) but not of the medial PFC (mPFC) led to an abnormally persistent LI which emerged under conditions that disrupted LI in control rats. LI was measured in a thirst motivated conditioned emotional response procedure by comparing suppression of drinking in response to a tone in rats which received 0 (nonpreexposed) or 40 tone presentations (preexposed) followed by either two or five tone-shock pairings. Control rats showed LI with 40 preexposures and two conditioning trials, but raising the number of conditioning trials to five disrupted LI. OFC- and BLA-lesioned rats showed LI under the former condition but in addition persisted in exhibiting LI under the latter condition. Rats with lesion of the mPFC did not show persistent LI. Thus, although LI does not depend on the integrity of BLA and OFC (because it is present in BLA- and OFC- lesioned rats even under conditions disrupting the phenomenon in normal rats), these regions play an important role in the modulation of its expression, more specifically, in the control of the non-expression of LI when the impact of conditioning increases beyond a certain level.

Neonatal Amygdala Lesions and Juvenile Isolation in the Rat: Differential Effects on Locomotor and Social Behavior Later in Life

Pervasive developmental disorders such as autism are characterized by deficits in social interaction and communication. Disturbed development of limbic structures such as the amygdala might underlie these deficits. The authors examined the effects of amygdala lesions on Postnatal Day 7 and juvenile isolation (2 weeks of individual housing during Weeks 4 and 5 of life) on rat locomotor and social activity later in life. Before puberty, but more pronounced after puberty, lesioned rats displayed enhanced locomotor activity. Adult social behavior was selectively disturbed by the lesion and the isolation procedure. In particular, the combination of neonatal lesions and juvenile isolation severely disrupted social interaction. These results suggest that a combination of neonatal amygdala damage and juvenile isolation may serve as an animal model of certain psychopathological neurodevelopmental disorders, such as autism.

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.

Early environment contributes to developmental disruption of MPFC after neonatal ventral hippocampal lesions in rats

Using a putative animal model of schizophrenia, neonatal rat ventral hippocampal (VH) lesions, combined with cross-fostering Lewis and Fisher rats, we previously demonstrated that the postpubertal expression of amphetamine-induced hyperlocomotion after lesioning depends on the early environment of the pups. However, an important question that emerged from our studies was whether the early environment leads to sparing of function within the VH or to the disruption of another structure, such as the medial prefrontal cortex (MPFC). To answer this question, we took advantage of the natural variation in maternal care of Sprague-Dawley rat dams and separated them into high and low arched back nursing (ABN) groups. Then, on postnatal day 7 (PD7) the pups from the two groups of dams were lesioned in the VH. As a measure of VH function, the rats were tested in a reference memory paradigm, which demonstrated that nVH-lesioned rats raised by high or low ABN dams had pronounced deficits, suggesting that VH functions are not fully spared. Next, the integrity of the MPFC was tested in a number of paradigms in which MPFC function has been implicated. In all three paradigms a similar result was found, that only lesioned rats raised by high ABN dams displayed deficits, such as a lack of MPFC control of amphetamine-induced locomotion, decreased working memory, and decreased anxiety. These results suggest that the early environment does not affect the recovery of the VH to nVH lesion. Rather, the early environment interacts with nVH lesions in such a way that disrupts the development and function of MPFC.

Effects of apomorphine and clozapine on conditioned freezing and latent inhibition

Consistent evidence has shown that learning may be produced in paradigms using electrical stimulation of the inferior colliculus as unconditioned stimulus (US). Much evidence has also been provided for the involvement of dopamine in the setting up of adaptive responses to aversive states generated at this mesencephalic level. The aim of the present study was to determine whether dopaminergic mechanisms are involved in the conditioned freezing behavior and latent inhibition (LI) using either foot shocks (Experiment 1) or inferior colliculus stimulation (Experiment 2) as US and light as conditioned stimulus (CS). To this end, the authors examined the effects of the dopaminergic agonist apomorphine (AP; 0, 0.5, 1.0, and 2.0 mg/kg) and the atypical antipsychotic clozapine (CLZ 0, 1.0, 2.5, and 5.0 mg/kg) on the acquisition of conditioned freezing and LI. Rats were either simply placed in the experimental chamber non-preexposed (NPE) or preexposed (PE) to 50 light presentations. Next, they were submitted to 10 light plus foot shock (or inferior colliculus stimulation) pairings. In the testing session, conditioned freezing under the CS was measured. In both experiments, conditioned freezing was highly significant in the NPE group while previous exposure to nonreinforced light (PE) weakened the strength of the conditioning. Although weaker than that caused by foot shock/light pairings, the occurrence of freezing and LI with the use of inferior colliculus stimulation as US gives additional support to the participation of this structure in the filtering processes of relevant information to higher brain regions. The drug effects depended on whether foot shock or inferior colliculus stimulation was used as US. In Experiment 1, AP produced a dose dependent increase in conditioned freezing without changing LI. On the other hand, CLZ did not change freezing and LI. In Experiment 2 while AP did not change freezing and LI, CLZ clearly increased freezing without changing LI with the use of inferior colliculus as US. The selective effect of AP on conditioned freezing due to light/foot shocks association is consistent with the heightened attentional and cognitive functions of dopaminergic mechanisms of the mesocorticolimbic systems in the setting up of adaptive responses aimed at coping with or signaling the presence of stimuli of aversive nature. The selective effect of CLZ on conditioned freezing is due to light/IC stimulation and may also be due to the known antagonism of 5-HT(2) receptors produced by low doses of this atypical antipsychotic.

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.

Infralimbic D2 receptor influences on anxiety-like behavior and active memory/attention in CD-1 mice

Ventromedial prefrontal cortical (vmPFC) dopamine (DA) influences attentional aspects of cognition and anxiety-like behavioral responding in rodents. The present study investigated the role of D2 receptors on spontaneous alternation in the Y-maze and anxiety-like behavior in a two-trial elevated plus-maze (EPM) procedure in CD-1 mice following vmPFC infusions of the D2 antagonist, sulpiride, and the D2 agonist, quinpirole. Pretrial 1 quinpirole infusions did not influence any anxiety measure (with the exception that the lowest dose increased protected stretch attends), but reduced protected exploration activity (closed-arm entry/time ratios and wall rearing). In Trial 2 24 h later (no injection), quinpirole exerted an anxiolytic behavioral profile relative to Trial 2 control mice (enhanced open-arm entry/time ratios, unprotected head dips), with no effects on protected exploration or risk assessment activity. Pretrial 1 sulpiride infusions enhanced unprotected exploration (open-arm entry/time ratios, unprotected stretch attend, and head dips), but did not influence protected exploration or risk assessment in the EPM. In Trial 2, 24 h later (no injection), sulpiride extended this anxiolytic profile to reduced protected exploration and risk assessment activity (closed-time ratio, protected stretch attend, and head dips). In the Y-maze, whereas quinpirole disrupted alternation performance (5- and 10-nmol dose) concomitant with marked repetitive same-arm returns (SAR) at the highest dose, sulpiride disrupted alternation performance concomitant with marked repetitive SAR behavior at the lowest dose only. These data indicate that although infralimbic (IL) quinpirole and sulpiride infusions similarly disrupted alternation performance in the Y-maze and reduced Trial 2 anxiety-like responding in the EPM, these drugs differentially produced these effects.

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.

Behavioral effects of excitotoxic lesions of ventral medial prefrontal cortex in the rat are hemisphere-dependent

The ventral region of the medial prefrontal cortex (mPFC) is highly sensitive to stressful inputs and implicated in a variety of behaviors. Studies have also demonstrated numerous functional hemispheric asymmetries within this brain area of the rat. The present study examines the effects of ibotenic acid or sham lesions targeting the left, right or bilateral infralimbic cortex, on a variety of behaviors. Lesions (which destroyed infralimbic and ventral prelimbic cortex) were without effect on acquisition or reversal of a spatial learning task in the Morris water maze. Similarly unaffected were spontaneous and amphetamine-induced locomotor activity and sensitization, and prepulse inhibition of the acoustic startle response. In contrast, lesions significantly affected behavior in the elevated plus maze, as right-lesioned animals spent more time exploring the open arms of the maze than shams or left-lesioned rats, while not differing in closed arm entries. As well, in a simple taste aversion paradigm, right-lesioned rats drank significantly more of a sweetened milk/quinine solution than shams and left-lesioned rats, despite not differing in consumption of sweetened milk alone. The anxiolytic effects of right, but not left lesions of ventral mPFC, parallel the asymmetrical suppression of physiological stress responses previously reported for similar lesions. It is suggested that the right ventral mPFC plays a primary role in optimizing cautious and adaptive behavior in potentially threatening situations.

Effects of hippocampal N-methyl-D-aspartate infusion on locomotor activity and prepulse inhibition: differences between the dorsal and ventral hippocampus

Prepulse inhibition (PPI) of the acoustic startle response and open-field locomotor activity were measured after bilateral infusion of N-methyl-D-aspartate into the ventral (0.10, 0.25, 0.50 microg/side) and dorsal (0.10, 0.25, 0.50, 0.70 microg/side) hippocampus of Wistar rats. Dose-dependent hyperactivity and disruption of PPI--behavioral effects related to psychotic symptoms--were observed after ventral infusions but were virtually absent after dorsal infusions. This functional dorsal-ventral difference might be related to the different connections of the dorsal and ventral hippocampus with the amygdala, nucleus accumbens, and prefrontal cortex, which have been implicated in the regulation of locomotor activity and PPI. Hippocampal overactivity has been associated with schizophrenia. The findings suggest that overstimulation of the ventral hippocampal projections may contribute to behavioral outcomes related to psychotic symptoms.

The hippocampal formation--orbitomedial prefrontal cortex circuit in the attentional control of active memory

The long held view that the hippocampal formation is not only essential, but also solely responsible for declarative memory in humans (and by analogy non-human primates) has come into question. Based on extensive reciprocal connection patterns between the hippocampal formation and the orbitoventromedial prefrontal cortex in primates and rats, a central role for the hippocampal formation in the attentional control of behavior is emerging. In this paper, evidence is reviewed showing that the hippocampal-orbitomedial prefrontal cortex circuit may be involved in attentional monitoring of the internal sensorium. This attentional monitoring system, in a sense, is the working memory of viscero-emotional processing. The hippocampal formation can thus be viewed as a discrepancy detector with respect to the relative activational status of cognitive/emotional set in the orbitomedial prefrontal cortex. Discrepancies between the current representation of the internal milieu and the "just-prior" representation held "on-line" in orbitomedial prefrontal cortex associative working memory, are signaled from the hippocampus to the prefrontal cortex prospective attentional systems to activate, process, and reconcile internal (past) with external (present) environments, and finally to effectively alter active working emotional "sets" to exert cognitive-emotional control of behavior.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

The effects of ibotenic acid lesions of the medial and lateral prefrontal cortex on latent inhibition, prepulse inhibition and amphetamine-induced hyperlocomotion

Hypofunction of prefrontal cortical regions, such as dorsolateral and orbital regions, has been suggested to contribute to the symptomatology of schizophrenia. In the rat, the medial and the lateral prefrontal cortices are considered as homologs of the primate dorsolateral and orbital prefrontal cortices, respectively. The present study investigated in rats the effects of lesions of the medial and lateral prefrontal cortices on latent inhibition, prepulse inhibition and amphetamine-induced activity. These paradigms are known to be modulated by the mesolimbic dopaminergic system, a system that has been suggested to be involved in the symptomatology of schizophrenia. Latent inhibition and prepulse inhibition are disrupted in schizophrenic patients as well as in rats treated with amphetamine. Amphetamine-induced activity was tested under dim light (low stress) and bright light (high stress) because stressful situations selectively increase mesocortical dopamine activity. Lateral prefrontal cortex lesioned animals did not differ in their behavior from control animals in any of the paradigms used in this study. Medial prefrontal cortex lesions did not affect latent inhibition but increased prepulse inhibition. In the amphetamine-induced activity experiment, prior to drug administration, open field locomotion was reduced under bright illumination for all lesion groups. After amphetamine administration, medial prefrontal cortex lesions attenuated the hyperlocomotor effect of the drug under the dim light condition and potentiated it under the bright light condition. The results indicate that medial and lateral prefrontal cortex can be functionally differentiated by their involvement in the modulation of behavior requiring mesocorticolimbic dopamine activation. The results in amphetamine induced activity suggest that the behavioral outcomes associated with medial prefrontal cortex depend on the background (stress) against which the evaluation is made. The results also support the notion that prepulse inhibition may be a better model than latent inhibition of the symptoms of schizophrenia associated with dysfunctional prefrontal activity.

Effects of local infusions of dopaminergic drugs into the medial prefrontal cortex of rats on latent inhibition, prepulse inhibition and amphetamine induced activity

Impaired ability to 'gate out' sensory and cognitive information is considered to be a central feature of schizophrenia and is manifested, among others, in disrupted prepulse inhibition (PPI) and latent inhibition (LI). The present study investigated in rats the effects of increasing or decreasing dopamine (DA) receptor activation within the medial prefrontal cortex (mPFC) by local administration of the indirect DA receptor agonist amphetamine (AMPH; 10.0 microg/side) or the DA antagonist cis-flupenthixol (FLU; 12.0 microg/side) on PPI and LI as well as on systemic AMPH-induced activity. The effects of intra-mPFC apomorphine (APO; 10.0 microg/side) on PPI were also tested. AMPH infusions decreased systemic AMPH-induced increase in locomotor activity in the open field, whereas FLU infusion was ineffective. Both infusions had no effect on LI and PPI. However, APO infusions induced a disruption of PPI. These results provide additional evidence that the mPFC is a component of the neural circuitry mediating PPI but plays no role in LI. In addition, they show that the behavioral outcomes produced by DA receptor activation/blockade in the mPFC of the rat cannot be explained by postulating a simple reciprocal relationship between the cortical and subcortical DA systems.