A computer controlled analysis of freezing behaviour

Line- and sex-dependent effects of juvenile stress on contextual fear- and anxiety-related behavior in high- and low-alcohol-preferring mouse lines

Juvenile stress (JS) is a known risk factor for the development of alcohol use disorder (AUD) and post-traumatic stress disorder (PTSD), both of which are frequently co-morbid. Data suggest there may be common, genetically-influenced biological responses to stress that contribute to the development of both AUD and PTSD. The present study investigated the impact of JS on contextual fear learning and extinction, as well as corticosterone (CORT) responses before and after JS, before and after contextual fear conditioning (CFC), and after fear extinction in male and female high-alcohol-preferring (HAP2) and low-alcohol-preferring (LAP2) mouse lines. We also measured unconditioned anxiety-related behavior in the light-dark-transition test before CFC. HAP2 and LAP2 mice did not differ in fear acquisition, but HAP2 mice showed faster fear extinction compared to LAP2 mice. No effects of JS were seen in HAP2 mice, whereas in LAP2 mice, JS reduced fear acquisition in males and facilitated fear extinction in females. Females showed greater fear-related behavior relative to males, regardless of subgroup. HAP2 males demonstrated more anxiolytic-like responses than LAP2 males and LAP2 females demonstrated more anxiolytic-like responses than LAP2 males in the light-dark transition test. HAP2 and LAP2 mice did not differ in CORT during the juvenile stage; however, adult LAP2 mice showed greater CORT levels than HAP2 mice at baseline and after CFC and extinction testing. These findings build upon prior work in these unique mouse lines that differ in genetic propensity toward alcohol preference and provide new information regarding contextual fear learning and extinction mechanisms theorized to contribute to co-morbid AUD and PTSD.

A Systematic Review on the Effect of Transcranial Direct Current and Magnetic Stimulation on Fear Memory and Extinction

Anxiety disorders are among the most prevalent mental disorders. Present treatments such as cognitive behavior therapy and pharmacological treatments show only moderate success, which emphasizes the importance for the development of new treatment protocols. Non-invasive brain stimulation methods such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) have been probed as therapeutic option for anxiety disorders in recent years. Mechanistic information about their mode of action, and most efficient protocols is however limited. Here the fear extinction model can serve as a model of exposure therapies for studying therapeutic mechanisms, and development of appropriate intervention protocols. We systematically reviewed 30 research articles that investigated the impact of rTMS and tDCS on fear memory and extinction in animal models and humans, in clinical and healthy populations. The results of these studies suggest that tDCS and rTMS can be efficient methods to modulate fear memory and extinction. Furthermore, excitability-enhancing stimulation applied over the vmPFC showed the strongest potential to enhance fear extinction. We further discuss factors that determine the efficacy of rTMS and tDCS in the context of the fear extinction model and provide future directions to optimize parameters and protocols of stimulation for research and treatment.

A Freely Available, Self-Calibrating Software for Automatic Measurement of Freezing Behavior

Freezing behavior is commonly used as a measure of associative fear memory. It can be measured by a trained observer, but this task is time-consuming and subject to variation. Commercially available software packages can also be used to quantify freezing; however, they can be expensive and usually require various parameters to be adjusted by the researcher, leading to additional work and variability in results. With this in mind, we developed Phobos, a freely available, self-calibrating software that measures freezing in a set of videos using a brief manual quantification performed by the user to automatically adjust parameters. To optimize the software, we used four different video sets with different features in order to determine the most relevant parameters, the amount of videos needed for calibration and the minimum criteria to consider it reliable. The results of four different users were compared in order to test intra- and interobserver variability in manual and automated freezing scores. Our results suggest that Phobos can be an inexpensive, simple and reliable tool for measurement of fear-related behavior, with intra- and interuser variability similar to that obtained with manual scoring.

Enhanced discriminative aversive learning and amygdala responsivity in 5-HT transporter mutant mice

Genetic variation in the human serotonin transporter (5-HTT) has been linked to altered fear learning but the data are inconsistent and the mechanism is unclear. The present study investigated conditioned aversive learning in 5-HTT knockout (KO) mice while simultaneously recording neural network activity (theta oscillations) and hemodynamic responses (tissue oxygen delivery) from the amygdala, a brain region necessary for forming fearful memories. Conditioned aversive learning was measured using a discrimination learning task in which one auditory cue was paired with foot-shock, whereas a second auditory cue was not. Compared with wild-type mice, 5-HTTKO mice exhibited faster discrimination learning. This effect was associated with stronger theta frequency oscillations and greater hemodynamic changes in the amygdala in response to both the emotionally relevant cues and the unconditioned foot-shock stimulus. Furthermore, hemodynamic responses to the unconditioned stimulus predicted behavioral discrimination performance the following day. Acute pharmacological 5-HTT blockade in wild-type mice produced a similar effect, to the extent that administration of citalopram during the fear conditioning sessions enhanced fear memory recall. Collectively, our data argue that loss of 5-HTT function enhances amygdala responsivity to aversive events and facilitates learning for emotionally relevant cues.

Coordinated Acetylcholine Release in Prefrontal Cortex and Hippocampus Is Associated with Arousal and Reward on Distinct Timescales

Cholinergic neurotransmission throughout the neocortex and hippocampus regulates arousal, learning, and attention. However, owing to the poorly characterized timing and location of acetylcholine release, its detailed behavioral functions remain unclear. Using electrochemical biosensors chronically implanted in mice, we made continuous measurements of the spatiotemporal dynamics of acetylcholine release across multiple behavioral states. We found that tonic levels of acetylcholine release were coordinated between the prefrontal cortex and hippocampus and maximal during training on a rewarded working memory task. Tonic release also increased during REM sleep but was contingent on subsequent wakefulness. In contrast, coordinated phasic acetylcholine release occurred only during the memory task and was strongly localized to reward delivery areas without being contingent on trial outcome. These results show that coordinated acetylcholine release between the prefrontal cortex and hippocampus is associated with reward and arousal on distinct timescales, providing dual mechanisms to support learned behavior acquisition during cognitive task performance.

Vitamin D treatment during pregnancy prevents autism-related phenotypes in a mouse model of maternal immune activation

BACKGROUND

Prenatal exposure to infection is a recognized environmental risk factor for neuropsychiatric disorders of developmental origins such as autism or schizophrenia. Experimental work in animals indicates that this link is mediated by maternal immune activation (MIA) involving interactions between cytokine-associated inflammatory events, oxidative stress, and other pathophysiological processes such as hypoferremia and zinc deficiency. Maternal administration of the viral mimic polyriboinosinic-polyribocytidylic acid (poly(I:C)) in mice produces several behavioral phenotypes in adult offspring of relevance to autism spectrum disorder (ASD) and other neurodevelopmental disorders.

METHODS

Here, we investigated whether some of these phenotypes might also present in juveniles. In addition, given the known immunomodulatory and neuroprotective effects of vitamin D, we also investigated whether the co-administration of vitamin D could block MIA-induced ASD-related behaviors. We co-administered the hormonally active form of vitamin D, 1α,25 dihydroxy vitamin D3 (1,25OHD), simultaneously with poly(I:C) and examined (i) social interaction, stereotyped behavior, emotional learning and memory, and innate anxiety-like behavior in juveniles and (ii) the levels of the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α in maternal plasma and fetal brains.

RESULTS

We show that like adult offspring that were exposed to MIA, juveniles display similar deficits in social approach behavior. Juvenile MIA offspring also show abnormal stereotyped digging and impaired acquisition and expression of tone-cued fear conditioning. Importantly, our study reveals that prenatal administration of 1,25OHD abolishes all these behavioral deficits in poly(I:C)-treated juveniles. However, prenatal administration of vitamin D had no effect on pro-inflammatory cytokine levels in dams or in fetal brains suggesting the anti-inflammatory actions of vitamin D are not the critical mechanism for its preventive actions in this ASD animal model.

CONCLUSIONS

This work raises the possibility that early dietary supplementation with vitamin D may open new avenues for a successful attenuation or even prevention of neurodevelopmental disorders following maternal inflammation during pregnancy.

Increased Serotonin Transporter Expression Reduces Fear and Recruitment of Parvalbumin Interneurons of the Amygdala

Genetic association studies suggest that variations in the 5-hydroxytryptamine (5-HT; serotonin) transporter (5-HTT) gene are associated with susceptibility to psychiatric disorders such as anxiety or posttraumatic stress disorder. Individuals carrying high 5-HTT-expressing gene variants display low amygdala reactivity to fearful stimuli. Mice overexpressing the 5-HTT (5-HTTOE), an animal model of this human variation, show impaired fear, together with reduced fear-evoked theta oscillations in the basolateral amygdala (BLA). However, it is unclear how variation in 5-HTT gene expression impacts on the microcircuitry of the BLA to change behavior. We addressed this issue by investigating the activity of parvalbumin (PV)-expressing interneurons (PVINs), the biggest IN population in the basal amygdala (BA). We found that increased 5-HTT expression impairs the recruitment of PVINs (measured by their c-Fos immunoreactivity) during fear. Ex vivo patch-clamp recordings demonstrated that the depolarizing effect of 5-HT on PVINs was mediated by 5-HT2A receptor. In 5-HTTOE mice, 5-HT-evoked depolarization of PVINs and synaptic inhibition of principal cells, which provide the major output of the BA, were impaired. This deficit was because of reduced 5-HT2A function and not because of increased 5-HT uptake. Collectively, these findings provide novel cellular mechanisms that are likely to contribute to differences in emotional behaviors linked with genetic variations of the 5-HTT.

SERT and uncertainty: serotonin transporter expression influences information processing biases for ambiguous aversive cues in mice

The long allele variant of the serotonin transporter (SERT, 5-HTT) gene-linked polymorphic region (5-HTTLPR) is associated with higher levels of 5-HTT expression and reduced risk of developing affective disorders. However, little is known about the mechanisms underlying this protective effect. One hypothesis is that 5-HTT expression influences aversive information processing, with reduced negative cognitive bias present in those with higher 5-HTT expression. Here we investigated this hypothesis using genetically-modified mice and a novel aversive learning paradigm. Mice with high levels of 5-HTT expression (5-HTT over-expressing, 5-HTTOE mice) and wild-type mice were trained to discriminate between three distinct auditory cues: one cue predicted footshock on all trials (CS+); a second cue predicted the absence of footshock (CS−); and a third cue predicted footshock on 20% of trials (CS20%), and was therefore ambiguous. Wild-type mice exhibited equivalently high levels of fear to the CS+ and CS20% and minimal fear to the CS−. In contrast, 5-HTTOE mice exhibited high levels of fear to the CS+ but minimal fear to the CS− and the CS20%. This selective reduction in fear to ambiguous aversive cues suggests that increased 5-HTT expression reduces negative cognitive bias for stimuli with uncertain outcomes.

Deletion of forebrain glycine transporter 1 enhances conditioned freezing to a reliable, but not an ambiguous, cue for threat in a conditioned freezing paradigm

Enhanced expression of Pavlovian aversive conditioning but not appetitive conditioning may indicate a bias in the processing of threatening or fearful events. Mice with disruption of glycine transporter 1 (GlyT1) in forebrain neurons exhibit such a bias, but they are at the same time highly sensitive to manipulations that hinder the development of the conditioned response (CR) suggesting that the mutation may modify higher cognitive processes that extract predictive information between environmental cues. Here, we further investigated the development of fear conditioning in forebrain neuronal GlyT1 knockout mice when the predictiveness of a tone stimulus for foot-shock was rendered ambiguous by interspersing [tone→no shock] trials in-between [tone→shock] trials during acquisition. The CR to the ambiguous tone CS (conditioned stimulus) was compared with that generated by an unambiguous CS that was always followed by the shock US (unconditioned stimulus) during acquisition. We showed that rendering the CS ambiguous as described significantly attenuated the CR in the mutants, but it was not sufficient to modify the CR in the control mice. It is concluded that disruption of GlyT1 in forebrain neurons does not increase the risk of forming spurious and potentially maladaptive fear associations.

Aversive prediction error signals in the amygdala

Prediction error signals are fundamental to learning. Here, in mice, we show that aversive prediction signals are found in the hemodynamic responses and theta oscillations recorded from the basolateral amygdala. During fear conditioning, amygdala responses evoked by footshock progressively decreased, whereas responses evoked by the auditory cue that predicted footshock concomitantly increased. Unexpected footshock evoked larger amygdala responses than expected footshock. The magnitude of the amygdala response to the footshock predicted behavioral responses the following day. The omission of expected footshock led to a decrease below baseline in the amygdala response suggesting a negative aversive prediction error signal. Thus, in mice, amygdala activity conforms to temporal difference models of aversive learning.

Variation in serotonin transporter expression modulates fear-evoked hemodynamic responses and theta-frequency neuronal oscillations in the amygdala

BACKGROUND

Gene association studies detect an influence of natural variation in the 5-hydroxytryptamine transporter (5-HTT) gene on multiple aspects of individuality in brain function, ranging from personality traits through to susceptibility to psychiatric disorders such as anxiety and depression. The neural substrates of these associations are unknown. Human neuroimaging studies suggest modulation of the amygdala by 5-HTT variation, but this hypothesis is controversial and unresolved, and difficult to investigate further in humans.

METHODS

We used a mouse model in which the 5-HTT is overexpressed throughout the brain and recorded hemodynamic responses (using a novel in vivo voltammetric monitoring method, analogous to blood oxygen level-dependent functional magnetic resonance imaging) and local field potentials during Pavlovian fear conditioning.

RESULTS

Increased 5-HTT expression impaired, but did not prevent, fear learning and significantly reduced amygdala hemodynamic responses to aversive cues. Increased 5-HTT expression was also associated with reduced theta oscillations, which were a feature of aversive cue presentation in controls. Moreover, in control mice, but not those with high 5-HTT expression, there was a strong correlation between theta power and the amplitude of the hemodynamic response.

CONCLUSIONS

Direct experimental manipulation of 5-HTT expression levels throughout the brain markedly altered fear learning, amygdala hemodynamic responses, and neuronal oscillations.

Dysregulation of brain adenosine is detrimental to the expression of conditioned freezing but not general Pavlovian learning

Glutamatergic and dopaminergic neurotransmission is modulated by adenosine, whose ambient level in the brain is in turn regulated by the metabolic enzyme, adenosine kinase (ADK). Brain adenosinergic tone can therefore be effectively reduced and increased by up- and down-regulation of ADK expression, respectively. Although changes in brain ADK levels can yield multiple behavioral effects, the precise functional significance of telencephalon (neocortical and limbic structures) adenosine remains ill-defined. Among the phenotypes identified in transgenic mice with brain-wide ADK overexpression (ADK(TG) mice) and reduced adenosinergic tone, working memory deficiency and potentiated response to systemic N-methyl-d-aspartate receptor blockade were exacerbated by the introduction of local ADK disruption (elevated adenosinergic tone) restricted to the telencephalon (ADK(TG):ADK(Tel-def) mice). These two phenotypes, which are central to schizophrenia cognitive/negative symptoms, appear to be regulated by adenosinergic activities within and outside the telencephalon in a complementary manner. Here, we extended this unique comparison between ADK(TG) mice ADK(TG):ADK(Tel-def) mice to another prominent phenotype previously documented in ADK(TG) mice - namely, impaired Pavlovian conditioned freezing. We found that ADK(TG):ADK(Tel-def) mice again were associated with a more severe phenotype while sharing a similar phenotype profile. Furthermore, we qualified that this Pavlovian phenotype did not translate into a general deficiency in associative learning, since no such deficit was evident in three other (aversive and appetitive) Pavlovian learning paradigms. The present study has thus identified a hitherto unknown function of brain adenosine: the execution of conditioned freezing behavior, which is dependent on the balance of adenosinergic changes between the telencephalon and the rest of the brain.

Comparison of the long-term consequences of withdrawal from repeated amphetamine exposure in adolescence and adulthood on information processing and locomotor sensitization in mice

Repeated administration of the indirect dopamine receptor agonist amphetamine (AMPH) produces robust locomotor sensitization and additional behavioral abnormalities. Accumulating evidence suggests that the developmental timing of drug exposure can critically influence this effect. The present study compared the consequences of withdrawal from repeated AMPH exposure in adolescence and adulthood on information processing and locomotor sensitization in C57BL/6 mice. Animals were injected daily with AMPH (1 or 2.5 mg/kg) or vehicle on 7 consecutive days starting either from postnatal day 35 to 42, or from postnatal day 70 to 77, following which they were given a 4 week withdrawal period before behavioral and pharmacological testing commenced. We found that withdrawal from the higher dose of AMPH (2.5 mg/kg/day) given either in adolescence or adulthood similarly disrupted selective associative learning as measured by the latent inhibition paradigm. None of the AMPH withdrawal groups displayed alterations in sensorimotor gating in the form of prepulse inhibition. Withdrawal from adult AMPH exposure at both doses induced marked locomotor sensitization, whereas adolescent pre-treatment with the higher (2.5 mg/kg/day) but not lower (1 mg/kg/day) dose of AMPH potentiated the locomotor-enhancing effects of acute AMPH re-challenge. Our study suggests that withdrawal from repeated AMPH exposure in adolescence and adulthood has similar consequences on selective associative learning, but the two manipulations differ with respect to their efficacy to induce long-term locomotor sensitization to the drug. The latter finding supports the hypothesis that the precise developmental timing determines, at least in part, the impact on long-term dopamine-associated sensitization processes.

Hemodynamic responses in amygdala and hippocampus distinguish between aversive and neutral cues during Pavlovian fear conditioning in behaving rats

Lesion and electrophysiological studies in rodents have identified the amygdala and hippocampus (HPC) as key structures for Pavlovian fear conditioning, but human functional neuroimaging studies have not consistently found activation of these structures. This could be because hemodynamic responses cannot detect the sparse neuronal activity proposed to underlie conditioned fear. Alternatively, differences in experimental design or fear levels could account for the discrepant findings between rodents and humans. To help distinguish between these alternatives, we used tissue oxygen amperometry to record hemodynamic responses from the basolateral amygdala (BLA), dorsal HPC (dHPC) and ventral HPC (vHPC) in freely-moving rats during the acquisition and extinction of conditioned fear. To enable specific comparison with human studies we used a discriminative paradigm, with one auditory cue [conditioned stimulus (CS)+] that was always followed by footshock, and another auditory cue (CS-) that was never followed by footshock. BLA tissue oxygen signals were significantly higher during CS+ than CS- trials during training and early extinction. In contrast, they were lower during CS+ than CS- trials by the end of extinction. dHPC and vHPC tissue oxygen signals were significantly lower during CS+ than CS- trials throughout extinction. Thus, hemodynamic signals in the amygdala and HPC can detect the different patterns of neuronal activity evoked by threatening vs. neutral stimuli during fear conditioning. Discrepant neuroimaging findings may be due to differences in experimental design and/or fear levels evoked in participants. Our methodology offers a way to improve translation between rodent models and human neuroimaging.

Loss of EphA4 impairs short-term spatial recognition memory performance and locomotor habituation

EphA4 receptor (EphA4) tyrosine kinase is an important regulator of central nervous system development and synaptic plasticity in the mature brain, but its relevance to the control of normal behavior remains largely unexplored. This study is the first attempt to obtain a behavioral profile of constitutive homozygous and heterozygous EphA4 knockout mice. A deficit in locomotor habituation in the open field, impairment in spatial recognition in the Y-maze and reduced probability of spatial spontaneous alternation in the T-maze were identified in homozygous EphA4(-/-) mice, while heterozygo us EphA4(+/-) mice appeared normal on these tests in comparison with wild-type (WT) controls. The multiple phenotypes observed in EphA4(-/-) mice might stem from an underlying deficit in habituation learning, reflecting an elementary form of nonassociative learning that is in contrast to Pavlovian associative learning, which appeared unaffected by EphA4 disruption. A deficit in motor coordination on the accelerating rotarod was also demonstrated only in EphA4(-/-) mice--a finding in keeping with the presence of abnormal gait in EphA4(-/-) mice--although they were able to improve performance over training. There was no evidence for substantial changes in major neurochemical markers in various brain regions rich in EphA4 as shown by post-mortem analysis. This excludes the possibility of major neurochemical compensation in the brain of EphA4(-/-) mice. In summary, we have demonstrated for the first time the behavioral significance of EphA4 disruption, supporting further investigation of EphA4 as a possible target for behavioral interventions where habituation deficits are prominent.

Maternal high-fat diet in mice programs emotional behavior in adulthood

The maternal environment has a significant role in the normal development of the fetus and may have long-term impact on brain development including critical central pathways such as the gamma-aminobutyric acid (GABA), serotonergic and the neurotrophin systems. For example, maternal malnutrition plays an important role in programming many aspects of physiology and behavior including predisposition to mental-health related disorders such as anxiety. Here we investigated the effects of maternal high-fat diet or control diet for nine weeks (prior to gestation, gestation and lactation) on the adult offspring with respect to anxiety related behaviors as well as exploration and conditioned fear response. We found that offspring born to high-fat diet mothers showed increased anxiety-like behaviors, but intact conditioned fear response and exploratory behavior. In addition, brain-derived neurotrophic factor (BDNF) was significantly increased in the dorsal hippocampus, while GABA(A) alpha2 receptor subunit and 5-hydroxytryptamine 1A (5-HT1A) receptor showed increased levels in the ventral hippocampus. In summary, these findings suggest that maternal high-fat diet consumption during critical periods in the development of the fetus, might increase the risk of abnormal behaviors in adulthood related to anxiety.

Examining the sex- and circadian dependency of a learning phenotype in mice with glycine transporter 1 deletion in two Pavlovian conditioning paradigms

Behavioural characterisation of transgenic mice has been instrumental in search of therapeutic targets for the modulation of cognitive function. However, little effort has been devoted to phenotypic characterisation across environmental conditions and genomic differences such as sex and strain, which is essential to translational research. The present study is an effort in this direction. It scrutinised the stability and robustness of the phenotype of enhanced Pavlovian conditioning reported in mice with forebrain neuronal deletion of glycine transporter 1 by evaluating the possible presence of sex and circadian dependency, and its consistency across aversive and appetitive conditioning paradigms. The Pavlovian phenotype was essentially unaffected by the time of testing between the two circadian phases, but it was modified by sex in both conditioning paradigms. We observed that the effect size of the phenotype was strongest in female mice tested during the dark phase in the aversive paradigm. Critically, the presence of the phenotype in female mutants was accompanied by an increase in resistance to extinction. Similarly, enhanced conditioned responding once again emerged solely in female mutants in the appetitive conditioning experiment, which was again associated with an increased resistance to extinction across days, but male mutants exhibited an opposite trend towards facilitation of extinction. The present study has thus added hitherto unknown qualifications and specifications of a previously reported memory enhancing phenotype in this mouse line by identifying the determinants of the magnitude and direction of the expressed phenotype. This in-depth comparative approach is of value to the interpretation of behavioural findings in general.

Impacts of forebrain neuronal glycine transporter 1 disruption in the senescent brain: evidence for age-dependent phenotypes in Pavlovian learning

Genetic deletion of glycine transporter 1 (GlyT1) in forebrain neurons gives rise to multiple-procognitive phenotypes, presumably due to enhanced N-methyl-d-aspartate receptor (NMDAR) functions. However, concerns over possible harmful excitotoxic effects under lifelong elevation of synaptic glycine have been raised. Such effects might accelerate the aging process, weakening or even reversing the procognitive phenotypes identified in adulthood. Here, we examined if one of the most robust phenotypes in the mutant mouse line (CamKIIαCre;GlyT1tm1.2fl/fI), namely, enhanced aversive Pavlovian conditioning, might be modified by age. Comparison between 3-month-old (adult) and 22-month-old (aged) mutants confirmed the presence of this phenotype at both ages. However, the temporal expression of the Pavlovian phenotype was modified in senescence; while adult mutants showed a pronounced within-session extinction, aged mutants did not. Expression of NR2B subunits of NMDAR and neural proliferation were examined in the same animals by immunohistochemistry. These were reduced in the aged mice as expected, but not exacerbated by the mutation. Thus, our results do not substantiate the concerns of neurotoxic effects through lifelong GlyT1 disruption in forebrain neurons, but provide evidence for a modification of phenotypic expression as a function of age. The latter points to the need to further investigate other procognitive phenotypes identified at adulthood in this mutant line. In addition, we revealed here for the first time a clear increase in the number of immature neurons in the hippocampus of the mutants, although the behavioral significance of this phenotype remains to be determined.

Differential effects of post-weaning juvenile stress on the behaviour of C57BL/6 mice in adolescence and adulthood

RATIONALE

There is evidence that events early in post-weaning life influence brain development and subsequent adult behaviour and therefore play an important role in the causation of certain psychiatric disorders in later life. Exposing rodents to stressors during the juvenile period has been suggested as a model of induced predisposition for these disorders.

OBJECTIVE

This is the first study to examine behavioural and pharmacological changes in adolescence and adulthood following juvenile stress in mice.

MATERIALS AND METHODS

Two cohorts of mice were simultaneously exposed to a stress protocol during postnatal days (PND) 25-30. Behavioural assessments reflecting emotional functions, cognitive functions, and psychostimulant sensitivity were then carried out at two time points: one cohort was tested during adolescence (PND 39-54; adolescent group), and the second cohort was tested during adulthood (PND 81-138; adult group).

RESULTS

In the adolescent mice, juvenile stress significantly attenuated conditioned freezing and led to decreased anxiety-like behaviour in the elevated plus maze, whereas no effect was observed on these tests in the adult mice. In contrast, adult mice exhibited poor avoidance learning following juvenile stress. When tested during adulthood, the mice stressed during the juvenile period showed a sensitised response to amphetamine compared to controls, whereas the response during adolescence was similar in stressed and control animals.

CONCLUSIONS

Our results suggest that exposure to stressors during the juvenile period can exert long-term effects on the brain and behaviour and that these effects differ depending on whether the animals are tested during adolescence or adulthood.

Automated assessment of pavlovian conditioned freezing and shock reactivity in mice using the video freeze system

The Pavlovian conditioned freezing paradigm has become a prominent mouse and rat model of learning and memory, as well as of pathological fear. Due to its efficiency, reproducibility and well-defined neurobiology, the paradigm has become widely adopted in large-scale genetic and pharmacological screens. However, one major shortcoming of the use of freezing behavior has been that it has required the use of tedious hand scoring, or a variety of proprietary automated methods that are often poorly validated or difficult to obtain and implement. Here we report an extensive validation of the Video Freeze system in mice, a “turn-key” all-inclusive system for fear conditioning in small animals. Using digital video and near-infrared lighting, the system achieved outstanding performance in scoring both freezing and movement. Given the large-scale adoption of the conditioned freezing paradigm, we encourage similar validation of other automated systems for scoring freezing, or other behaviors.

The amphetamine sensitization model of schizophrenia: relevance beyond psychotic symptoms?

RATIONALE

A sensitized dopamine system may be linked to the genesis of psychotic symptoms in schizophrenia. Following withdrawal from amphetamine exposures, psychotic-like traits have been robustly demonstrated, but the presence of cognitive/mnemonic deficits remains uncertain.

METHODS

Adult male Lewis and Fischer rats, differing in cognitive performance, were exposed intermittently to escalating doses of amphetamine over 5 weeks. This was effective in producing behavioral sensitization to a subsequent amphetamine challenge. Following 27 days of drug withdrawal, the animals were assessed in Pavlovian conditioning, object recognition, and spatial working memory. In addition, prepulse inhibition (PPI), spontaneous motor activity, and anxiety-like behavior were measured.

RESULTS

Amphetamine pretreatment induced behavioral sensitization in both rat strains similarly. Working memory was enhanced in Fischer but not Lewis rats following withdrawal. Spontaneous novel object preference was enhanced in sensitized Fischer rats, but was impaired in sensitized Lewis rats, thus effectively reversing the strain difference in non-sensitized controls. In contrast, Pavlovian fear conditioning remained unaffected and so were anxiety-like behavior, open field activity, and PPI.

CONCLUSION

The face validity of the amphetamine withdrawal model for cognitive deficits was limited to the object recognition memory impairment observed in sensitized Lewis rats. Yet, the possibility that enhancing dopaminergic neurotransmission may facilitate object recognition and spatial working memory performance was demonstrated in sensitized Fischer rats. Identification of the mechanisms underlying such strain-dependent effects would be instrumental in the further specifications of the construct validity, and therefore the limitations and potential of the amphetamine sensitization model of schizophrenia.

Altered mnemonic functions and resistance to N-METHYL-d-Aspartate receptor antagonism by forebrain conditional knockout of glycine transporter 1

Converging evidence from pharmacological and molecular studies has led to the suggestion that inhibition of glycine transporter 1 (GlyT1) constitutes an effective means to boost N-methyl-d-aspartate receptor (NMDAR) activity by increasing the extra-cellular concentration of glycine in the vicinity of glutamatergic synapses. However, the precise extent and limitation of this approach to alter cognitive function, and therefore its potential as a treatment strategy against psychiatric conditions marked by cognitive impairments, remain to be fully examined. Here, we generated mutant mice lacking GlyT1 in the entire forebrain including neurons and glia. This conditional knockout system allows a more precise examination of GlyT1 downregulation in the brain on behavior and cognition. The mutation was highly effective in attenuating the motor-stimulating effect of acute NMDAR blockade by phencyclidine, although no appreciable elevation in NMDAR-mediated excitatory postsynaptic currents (EPSC) was observed in the hippocampus. Enhanced cognitive performance was observed in spatial working memory and object recognition memory while spatial reference memory and associative learning remained unaltered. These findings provide further credence for the potential cognitive enhancing effects of brain GlyT1 inhibition. At the same time, they indicated potential phenotypic differences when compared with other constitutive and conditional GlyT1 knockout lines, and highlighted the possibility of a functional divergence between the neuronal and glia subpopulations of GlyT1 in the regulation of learning and memory processes. The relevance of this distinction to the design of future GlyT1 blockers as therapeutic tools in the treatment of cognitive disorders remains to be further investigated.

Affective and cognitive effects of global deletion of alpha3-containing gamma-aminobutyric acid-A receptors

Gamma-aminobutyric acid (GABA)A receptors characterized by the presence of the alpha3 subunit are the major GABAA receptor subtype expressed in brain stem monoaminergic nuclei. These alpha3-GABAA receptors are therefore in a unique position to regulate monoaminergic functions. To characterize the functional properties of alpha3-GABAA receptors, we present a preliminary assessment of the expression of affective and cognitive behaviour in male mice with a targeted deletion of the Gabra3 gene encoding the alpha3 subunit [alpha3 knockout (KO) mice] on a C57BL/6Jx129X1/SvJ F1 hybrid genetic background. The alpha3 KO mice did not exhibit any gross change of anxiety-like behaviour or spontaneous locomotor behaviour. In the Porsolt forced swim test for potential antidepressant activity, alpha3 KO mice exhibited reduced floating and enhanced swimming behaviour relative to wild-type controls. Performance on a two-choice sucrose preference test, however, revealed no evidence for an increase in sucrose preference in the alpha3 KO mice that would have substantiated a potential phenotype for depression-related behaviour. In contrast, a suggestion of an enhanced negative contrast effect was revealed in a one-bottle sucrose consumption test across different sucrose concentrations. These affective phenotypes were accompanied by alterations in the balance between conditioned responding to the discrete conditioned stimulus and to the context, and a suggestion of faster extinction, in the Pavlovian conditioned freezing paradigm. Spatial learning in the water maze reference memory test, however, was largely unchanged in the alpha3 KO mice, except for a trend of preservation during reversal learning. The novel phenotypes following global deletion of the GABAA receptor alpha3 subunit identified here provided relevant insights, in addition to our earlier study, into the potential behavioural relevance of this specific receptor subtypes in the modulation of both affective and cognitive functions.

Behavioral characterization of mice lacking the neurite outgrowth inhibitor Nogo-A

The membrane protein Nogo-A inhibits neurite outgrowth and regeneration in the injured central nervous system, primarily because of its expression in oligodendrocytes. Hence, deletion of Nogo-A enhances regeneration following spinal cord injury. Yet, the effects of Nogo-A deletion on general behavior and cognition have not been explored. The possibility of potential novel functions of Nogo-A beyond growth inhibition is strongly suggested by the presence of subpopulations of neurons also expressing Nogo-A - not only during development but also in adulthood. We evaluated here Nogo-A(-/-) mice in a series of general basic behavioral assays as well as functional analyses related to brain regions with notable expression levels of Nogo-A. The SHIRPA protocol did not show any major basic behavioral changes in Nogo-A(-/-) mice. Anxiety-related behavior, pain sensitivity, startle reactivity, spatial learning, and associative learning also appeared indistinguishable between Nogo-A(-/-) and control Nogo-A(+/+) mice. However, motor co-ordination and balance were enhanced in Nogo-A(-/-) mice. Spontaneous locomotor activity was also elevated in Nogo-A(-/-) mice, but this was specifically observed in the dark (active) phase of the circadian cycle. Enhanced locomotor reaction to systemic amphetamine in Nogo-A(-/-) mice further pointed to an altered dopaminergic tone in these mice. The present study is the first behavioral characterization of mice lacking Nogo-A and provides significant insights into the potential behavioral relevance of Nogo-A in the modulation of dopaminergic and motor functions.

Deficient maternal care resulting from immunological stress during pregnancy is associated with a sex-dependent enhancement of conditioned fear in the offspring

Activation of maternal stress response systems during pregnancy has been associated with altered postpartum maternal care and subsequent abnormalities in the offspring’s brain and behavioral development. It remains unknown, however, whether similar effects may be induced by exposure to immunological stress during pregnancy. The present study was designed to address this issue in a mouse model of prenatal immune activation by the viral mimic polyriboinosinic–polyribocytidilic acid (PolyI:C). Pregnant mice were exposed to PolyI:C-induced immune challenge or sham treatment, and offspring born to PolyI:C- and sham-treated dams were simultaneously cross-fostered to surrogate rearing mothers, which had either experienced inflammatory or vehicle treatment during pregnancy. We evaluated the effects of the maternal immunological manipulation on postpartum maternal behavior, and we assessed the prenatal and postnatal maternal influences on anxiety- and fear-related behavior in the offspring at the peri-adolescent and adult stage of development. We found that PolyI:C treatment during pregnancy led to changes in postpartum maternal behavior in the form of reduced pup licking/grooming and increased nest building activity. Furthermore, the adoption of neonates by surrogate rearing mothers, which had experienced PolyI:C-induced immunological stress during pregnancy, led to enhanced conditioned fear in the peri-adolescent and adult offspring, an effect that was exclusively seen in female but not male subjects. Unconditioned (innate) anxiety-related behavior as assessed in the elevated plus maze and open field explorations tests were not affected by the prenatal and postnatal manipulations. Our results thus highlight that being raised by gestationally immune-challenged surrogate mothers increases the vulnerability for specific forms of fear-related behavioral pathology in later life, and that this association may be mediated by deficits in postpartum maternal care. This may have important implications for the identification and characterization of early-life risk factors involved in the developmental etiology of fear-related neuropsychiatric disorders.

Behavioral characterization of dysbindin-1 deficient sandy mice

Dysbindin-1 (dystrobrevin binding protein-1) has been reported as a candidate gene associated with schizophrenia. Dysbindin-1 mRNA and protein levels are significantly reduced in the prefrontal cortex and hippocampus of schizophrenia subjects. To understand the in-vivo functions of dysbindin-1, we studied schizophrenia relevant behaviors in adult male Sandy homozygous (sdy/sdy) and heterozygous (sdy/+) mice that have a natural mutation in dysbindin-1 gene (on a DBA/2J background) resulting in loss of protein expression. Spontaneous locomotor activity of sdy/sdy and sdy/+ mice in novel environment was not significantly different from DBA/2J controls. However, on repeated testing in the same environment for 7 days, sdy/sdy mice, in contrast to DBA/2J controls showed a lack of locomotor habituation. Locomotor activating effect of a low dose of d-amphetamine (2.5 mg/kg i.p.), a behavioral measure of mesolimbic dopamine activity, was significantly reduced in the mutant mice. Interestingly, sdy/sdy mice showed enhanced locomotor sensitization to repeated five daily injection of amphetamine. Possible cognitive impairment in Sandy mutants was revealed in novel object recognition test as sdy/sdy and sdy/+ mice spent significantly less time exploring novel objects compared to DBA/2J. Sdy/sdy mice also showed deficits in emotionally motivated learning and memory showing greater freezing response to auditory conditioned stimulus (CS) in fear conditioning paradigm. In thermal nociceptive test, the latency of paw withdrawal in sdy/sdy and sdy/+ animals was significantly higher compared to DBA/2J indicating hypoalgesia in the mutants. Taken together, these data suggest that dysbindin-1 gene deficiency leads to significant changes in cognition and altered responses to psychostimulants.

Effects of withdrawal from an escalating dose of amphetamine on conditioned fear and dopamine response in the medial prefrontal cortex

Neurochemical studies have shown that mesocortical dopamine projections are particularly responsive in aversive situations such as fear conditioning. The present study assessed behavioural and medial prefrontal cortex (mPFC) dopamine responses utilizing in vivo microdialysis during acquisition and expression of a conditioned fear response. In two independent experiments, rats were presented with either two or nine tone-shock pairings during formation of a conditioned fear response. In the second experiment, rats were pre-treated with repeated injections of either amphetamine or saline over a 6-day period and tested during withdrawal. Amphetamine pre-treatment as well as the conditioning procedure itself potentiated an increase in dopamine levels during formation, but not expression of a conditioned fear response. Locomotor activity induced by an amphetamine challenge (1mg/kg) was also enhanced in pre-treated amphetamine compared to saline pre-treated animals (experiment two). However, mPFC dopamine response to amphetamine challenge did not differ between treatment groups. We conclude that while the exact role of mPFC dopamine in behavioural sensitization is yet to be determined, mPFC dopamine release may underlie the increased fear response during acquisition but not expression of fear response.

Automated evaluation of sensitivity to foot shock in mice: inbred strain differences and pharmacological validation

Assessing foot shock sensitivity in rodents can be useful in identifying analgesic or hyperalgesic drugs, and phenotyping inbred or genetically altered mice. Furthermore, as foot shock is an integral part of several rodent behavioral models, sensitivity should also be assessed to accurately interpret behavioral measures from these models. To eliminate variability and increase the efficiency of manually scored shock sensitivity paradigms, we utilized a startle reflex system to automatically quantify responses to varying levels of foot shock. Eight inbred mouse strains were tested for reactivity to foot shock in this system, as well as inherent startle response activity to loud noise bursts. Strain rank order for shock reactivity differed from that for acoustic startle, suggesting that pathways activated in response to each differed. Analgesic doses of morphine and acetaminophen specifically reduced foot shock responses without affecting motor reflexive responses to loud noises in each strain tested. We also tested diazepam and scopolamine, which are often used to disrupt behavior in shock-related paradigms to illustrate the usefulness of this assay. Overall, these results demonstrate that our automated method is a quick and simple way to accurately assess potential foot shock sensitivity differences owing to strain, genotype or drug treatments.

Early deprivation leads to altered behavioural, autonomic and endocrine responses to environmental challenge in adult Fischer rats

Depression is diagnosed on the basis of abnormal positive affects (anhedonia) and negative affects (low mood, helplessness, coping deficit, fatigue), and associated physiological abnormalities include hyperactivity of the HPA endocrine system and autonomic nervous system. Adverse early life environments, including parent-offspring emotional and physical neglect, are associated with traits of altered physiological and neurobiological function and long-term predisposition to depression. Animal studies based on early life adversity can potentially yield environmental models of the developmental behavioural neurobiology of depression. In Wistar rats, we demonstrated that isolation of pups from dam and littermates at room temperature for 4 h per day on P1-14 (early deprivation, ED) led to adulthood anhedonia-like traits of reduced motivation to obtain gustatory reward and reduced social motivation, relative to subjects left undisturbed during infancy (non-handling, NH). We hypothesized that the depression-like effects of ED would be even more pronounced and multiple in the stress hyper-responsive Fischer rat strain. The effects of ED were studied relative to NH and 15 min of daily isolation (early handling, EH). Relative to NH and EH, which exhibited remarkably similar phenotypes, ED led, principally in males, to chronic traits of: reduced motivation for and consumption of gustatory reward; increased activity in the pre-test and test phases of the forced swim test; reduced coping behaviour in an aversive environment; attenuated plasma corticosterone stress response to a normal plasma ACTH stress response; increased hypertensive response to a novel environment; and increased prefrontal cortical serotonin. High sensitivity to an aversive early environment in male Fischer rats therefore constitutes an important model for the study of affective development and its neurobiology.

Levels of neurotrophic factors in the hippocampus and amygdala correlate with anxiety- and fear-related behaviour in C57BL6 mice

The present study tested whether individual differences in anxiety- and fear-related behaviour are associated with between-subjects variation in postmortem brain levels of selected neurotrophic factors. Naïve C57BL6/J mice of both sexes were subjected either to an elevated plus maze test or to a Pavlovian fear conditioning paradigm. Two days after behavioural assays, the mice were sacrificed for postmortem quantification of the protein levels of brain derived neurotrophic factors (BDNF), nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the hippocampus and amygdala. Significant correlations between behavioural measures and postmortem regional neurotrophic factor contents were revealed. The magnitude of anxiety-like behaviour in the elevated plus maze was positively related to dorsal hippocampal BDNF levels, but negatively related to NGF levels in dorsal hippocampus and in the amygdala. On the other hand, the expression of conditioned fear is positively related to amygdala BDNF and NGF levels, and to dorsal hippocampal NGF levels. Our results add to existing reports in human as well as in animals of correlation between anxiety trait and gross measures of hippocampal volume or activation levels. Moreover, a distinction between spontaneous and learned (or conditioned) anxiety/fear would be relevant to the identification of neurotrophin signalling mechanisms in the hippocampus and amygdala implicated in anxiety and related psychopathology.

Effects of dorsal and ventral hippocampal NMDA stimulation on nucleus accumbens core and shell dopamine release

This study has analysed the effects of infusing N-methyl-D-aspartate (NMDA) into either the ventral or dorsal hippocampus on dopamine (DA) transmission in the nucleus accumbens (NAC) core or shell for the first time. Dopamine was measured using in vivo microdialysis with high performance liquid chromatography with electrochemical detection (HPLC-EC). Unilateral NMDA infusion (0.5 microg) into the ventral hippocampus (VH) increased extracellular DA levels in NAC shell during the first 30 min following infusion compared to saline (SAL) infused animals. In contrast, NAC core DA levels were unaffected. NMDA infusion into the dorsal hippocampus (DH) led to a decrease in NAC core DA levels; this effect was not observed in the SAL-infused group. DA levels in NAC shell remained unaltered. At the end of the experiments, we examined the response to a systemic amphetamine (AMPH) injection of 1mg/kg on extracellular DA levels of the NAC core and shell. Interestingly, on2ly animals previously infused with NMDA into the VH exhibited a sensitized DA response in the NAC shell in response to the AMPH injection. We can conclude that VH activation has an acute stimulatory effect on DA release in the shell and that DH activation has a suppressive effect on extracellular DA levels in the core.

Amphetamine withdrawal leads to behavioral sensitization and reduced HPA axis response following amphetamine challenge

Withdrawal from repeated amphetamine (AMPH) administration leads to behavioral sensitization following a drug or a stress challenge and is commonly used to model anhedonia in rats, a core symptom of depression in humans. It is proposed that corticosteroids are involved in the mediation of sensitization and depression. The aim of the present study was to investigate stress and AMPH- induced release of adrenocorticotropic hormone (ACTH) and corticosterone (CORT) during withdrawal from an escalating dosage schedule of AMPH known to produce depression-like effects in rats. Wistar rats were given 3 injections (i.p.) per day over 3 days, escalating from 1 mg/kg to 9 mg/kg and a final injection of 10 mg/kg AMPH or saline on day 4. On day 2 of withdrawal, the animals were tested in the Porsolt swim test. HPA axis activity in response to restraint stress was tested on withdrawal day 14 and in response to AMPH challenge on withdrawal day 30. We found no effect of AMPH withdrawal in the Porsolt swim test and on the ACTH or CORT response following restraint stress. AMPH withdrawn animals expressed behavioral sensitization in terms of locomotion and reduced ACTH and CORT plasma levels following a 1 mg/kg AMPH challenge in comparison to the controls. We conclude that there is no critical involvement of a sensitized HPA axis stress response in the long-term expression of behavioral sensitization.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Disruption of glycine transporter 1 restricted to forebrain neurons is associated with a procognitive and antipsychotic phenotypic profile

The NMDA receptor is thought to play a central role in some forms of neuronal plasticity, including the induction of long-term potentiation. NMDA receptor hypofunction can result in mnemonic impairment and has been implicated in the cognitive symptoms of schizophrenia. The activity of NMDA receptors is controlled by its endogenous coagonist glycine, and a local elevation of glycine levels is expected to enhance NMDA receptor function. Here, we achieved this by the generation of a novel mouse line (CamKIIalphaCre;Glyt1tm1.2fl/fl) with a neuron and forebrain selective disruption of glycine transporter 1 (GlyT1). The mutation led to a significant reduction of GlyT1 and a corresponding reduction of glycine reuptake in forebrain samples, without affecting NMDA receptor expression. NMDA (but not AMPA) receptor-evoked EPSCs recorded in hippocampal slices of mutant mice were 2.5 times of those recorded in littermate controls, suggesting that neuronal GlyT1 normally assumes a specific role in the regulation of NMDA receptor responses. Concomitantly, the mutants were less responsive to phencyclidine than controls. The mutation enhanced aversive Pavlovian conditioning without affecting spontaneous anxiety-like behavior in the elevated plus maze and augmented a form of attentional learning called latent inhibition in three different experimental paradigms: conditioned freezing, conditioned active avoidance, conditioned taste aversion. The CamKIIalphaCre;Glyt1tm1.2fl/fl mouse model thus suggests that augmentation of forebrain neuronal glycine transmission is promnesic and may also offer an effective therapeutic intervention against the cognitive and attentional impairments characteristic of schizophrenia.

The role of voluntary exercise in enriched rearing: A behavioral analysis

The effects of postweaning enriched rearing and home cage voluntary wheel-running exercise in adulthood were contrasted on a comprehensive battery of tests designed to assess mnemonic, attentional, emotional, and motor functions. In a 2 x 2 factorial design, female C57BL/6 mice were housed in groups in either standard or enriched cages, which were equipped with either a running or a locked wheel. They were maintained in the corresponding housing conditions for 2 months postweaning prior to, and throughout, testing. Enriched rearing was associated with anxiogenesis, hypolocomotor activity, enhanced motor skills, retarded extinction of conditioned responding, and improved water maze performance. Exercise as such enhanced motor coordination and facilitated extinction of contextual conditioning. Evidence for an interaction between enrichment and exercise was apparent in the open field test, conditioned freezing to a tone stimulus, prepulse inhibition, and acquisition of water maze reference memory. Hence, care should be taken to control for the unique contribution of wheel-running exercise when it is included as an integral component of the enrichment procedure.

Activation of dopaminergic neurotransmission in the medial prefrontal cortex by N-methyl-d-aspartate stimulation of the ventral hippocampus in rats

Many behavioral functions-including sensorimotor, attentional, memory, and emotional processes-have been associated with hippocampal processes and with dopamine transmission in the medial prefrontal cortex (mPFC). This suggests a functional interaction between hippocampus and prefrontal dopamine. The anatomical substrate for such an interaction is the intimate interconnection between the ventral hippocampus and the dopamine innervation of the mPFC. The present study yielded direct neurochemical evidence for an interaction between ventral hippocampus and prefrontal dopamine transmission in rats by demonstrating that subconvulsive stimulation of the ventral hippocampus with N-methyl-d-aspartate (NMDA; 0.5 mug/side) activates dopamine transmission in the mPFC. Postmortem measurements revealed that bilateral NMDA stimulation of the ventral hippocampus, resulting in locomotor hyperactivity, increased the homovanillic acid/dopamine ratio, an index of dopamine transmission, in the mPFC; indices of dopamine transmission in any of five additionally examined forebrain regions (amygdala, nucleus accumbens shell/core, lateral prefrontal cortex, caudate putamen) were unaltered. In vivo microdialysis measurements in freely moving rats corroborated the suggested activation of prefrontal dopamine transmission by demonstrating that unilateral NMDA stimulation of the ventral hippocampus increased extracellular dopamine in the ipsilateral mPFC. The suggested influence of the ventral hippocampus on prefrontal dopamine may be an important mechanism for hippocampo-prefrontal interactions in normal behavioral processes. Moreover, it indicates that aberrant hippocampal activity, as found in neuropsychiatric diseases, such as schizophrenia and mood disorders, may contribute to disruption of certain cognitive and emotional functions which are extremely sensitive to imbalanced prefrontal dopamine transmission.

GABAA receptors containing the alpha5 subunit mediate the trace effect in aversive and appetitive conditioning and extinction of conditioned fear

A reduction in alpha5 subunit-containing gamma-aminobutyric acid (GABA)A receptors has been reported to enhance some forms of learning in mutant mouse models. This effect has been attributed to impaired alpha5 GABAA receptor-mediated inhibitory modulation in the hippocampus. The introduction of a point mutation (H105R) in the alpha5 subunit is associated with a specific reduction of alpha5 subunit-containing GABAA receptors in the hippocampus. The present study examined the modulation of associative learning and the extinction of conditioned response in these animals. The strength of classical conditioning can be weakened when a trace interval is interposed between the conditioned stimulus and unconditioned stimulus. Here we report that this 'trace effect' in classical conditioning was absent in the mutant mice--they were insensitive to the imposition of a 20-s trace interval. This effect of the mutation was most clearly in the female mice using an aversive conditioning paradigm, and in the male mice using an appetitive conditioning paradigm. These gender-specific phenotypes were accompanied by a resistance to extinction of conditioned fear response in the mutant mice that was apparent in both genders. Our results identify neuronal inhibition in the hippocampus mediated via alpha5 GABAA receptors as a critical control element in the regulation of the acquisition and expression of associative memory.

Effect of social isolation on stress-related behavioural and neuroendocrine state in the rat

The present study investigated the effects of post-weaning social isolation (SI) on behavioural and neuroendocrine reactivity to stress of male and female rats. Innate aspects of fear and anxiety were assessed in the open field and elevated plus maze tests. Spontaneous startle reflex and conditioned fear response were further investigated. The neuroendocrine response of isolates was examined by measuring basal and stress release of ACTH and corticosterone and by evaluating the mRNA expression of mineralocorticoid (MR) and glucocorticoid (GR) receptors using in situ hybridization. Locomotor activity in the open field was not modified by chronic SI. In males, but not females, SI produced an anxiogenic profile in the elevated plus maze. Male isolates showed a trend towards increased startle reflex amplitude relative to socially-reared controls. Moreover, SI in males produced alterations of the HPA axis functioning as reflected by higher basal levels of ACTH, and enhanced release of ACTH and corticosterone following stress. In contrast, startle response or HPA axis functioning were not altered in female isolates. Social isolates from both genders showed reduced contextual fear-conditioning. Finally, the mRNA expression of MR and GR was not modified by SI. The results of the present study suggest that chronic SI increases emotional reactivity to stress and produces a hyperfunction of the HPA axis in adult rats, particularly in males.

Behavioural and cardiovascular responses during latent inhibition of conditioned fear: measurement by telemetry and conditioned freezing

This study assessed freezing behaviour and cardiovascular responses during the expression of latent inhibition of conditioned fear. Animals that were either repeatedly preexposed (PE) to a tone conditioned stimulus (CS) or naive to the tone (non-preexposed; NPE) subsequently experienced three presentations of the tone paired with footshock. Animals were tested 24 h later in the context of the footshock chamber, and on the following day, in the presence of the tone CS. Changes in heart rate and blood pressure were recorded by radio-telemetry. The PE rats spent more time freezing to the conditioned contextual cues and exhibited higher blood pressures during the last half of the context test session than did the NPE animals. During the tone test, the PE rats exhibited less conditioned freezing to the tone CS compared with the NPE animals, i.e. expression of the latent inhibition. This behavioural effect was associated with a significant increase in heart rate, but not blood pressure, in the PE but not the NPE animals. Our results suggest that the increased blood pressures of the PE rats during the context test directly reflect their greater fear of the conditioning context. In contrast, the increased heart rate response but decreased freezing shown by PE rats in response to the tone CS may be due to the fact that lower stress levels (e.g. PE condition) elicit sympathetically-mediated increases in heart rate, whereas higher stress levels (e.g. NPE condition) activate both sympathetic and parasympathetic systems, thus eliminating any CS-induced increase in heart rate in the NPE rats.

The prenatal methylazoxymethanol acetate treatment: a neurodevelopmental animal model for schizophrenia?

The prenatal methylazoxymethanol acetate (MAM) treatment has been proposed as a suitable model for the neurodevelopmental aspects of schizophrenia since the morphological abnormalities it induces in the brain are subtle and in line with most reports of neuropathology in schizophrenic brains. However, the functional aspects of this treatment have not been investigated with behavioural paradigms that are relevant for the psychopathology of the symptoms of schizophrenia. In the present study, we investigated the validity of the prenatal MAM treatment as a developmental model for schizophrenia with a prepulse inhibition of the acoustic startle reflex, latent inhibition, locomotor activity, and cognition and emotionality with freezing in fear conditioning paradigms. We have conducted two studies: in Study I, MAM was injected from E09 to E12, and in Study II MAM was administered at later stages in the embryonic development, from E12 to E15. Morphologically, the prenatal MAM treatment induced mild to severe reduction in brain weights and in the entorhinal cortex, prefrontal cortex and striatum volumes, the severity of the effects depending on the timing of administration. However, despite the morphological abnormalities induced by the MAM treatments, no behavioural deficits were observed in the MAM-treated animals when compared to Controls in prepulse inhibition, latent inhibition with the two-way active avoidance, and in the freezing paradigms. Therefore, due to the consistent lack of treatment effect observed in the present investigation, we conclude that the prenatal MAM treatment has no validity as a behavioural model for schizophrenia.

Long-term social isolation and medial prefrontal cortex: dopaminergic and cholinergic neurotransmission

Rearing rats in social isolation has been suggested as an animal model of schizophrenia, based mainly on the similarity between the attenuation of prepulse inhibition (PPI) in isolated rats and in schizophrenic patients. The medial prefrontal cortex (mPFC) plays a major role in the pathophysiology of schizophrenia. Thus, a postmortem micropunch analysis measuring dopamine (DA), DOPAC (3,4-dihydroxyphenylacetic acid) and homovanillic acid (HVA) in the dorsal and ventral subregion of the mPFC, the caudate putamen (CPu) and nucleus accumbens (NAc) was carried out on socially isolated or group-housed male Sprague-Dawley (SD) rats. Additionally, in vivo microdialysis with D-amphetamine (1 mg/kg ip) stimulation was performed in isolated animals and their controls, examining the ventral mPFC for acetylcholine (ACh), DOPAC and HVA levels. Simultaneously, recording of motor activity was performed. In the neurochemical postmortem tissue analysis we found no difference in any of the brain regions tested between isolated and group-reared animals. Amphetamine increased ACh levels in the mPFC, induced a decrease in DOPAC and HVA levels, and increased motor activity. A close to significant Drug x Housing interaction reflected the fact that the amphetamine-induced decrease of DOPAC was confined to the group-housed animals. In conclusion, social isolation leads only to moderate changes in the dopaminergic system in the mPFC, whereas the cholinergic system remains unaffected.

Rotenone increases glutamate-induced dopamine release but does not affect hydroxyl-free radical formation in rat striatum

Impairment of the mitochondrial complex I has been found in Parkinson's disease and recently long-term treatment with the complex I inhibitor rotenone led to neurodegeneration and Lewy body-like inclusions in rats. To investigate the relationship of free radical formation, complex I inhibition, and dopamine release, rotenone (15 mg/kg s.c.) was injected in male Sprague Dawley rats. Complex I inhibition was measured in the striatum and substantia nigra using the lactate accumulation assay. Dopamine release and free radical formation was determined using striatal microdialysis in combination with the salicylate hydroxylation assay. In a second experiment, glutamate (10 mM) stimulation via the microdialysis probe was used to provoke hydroxyl radical formation and dopamine release 60 min after rotenone or vehicle pretreatment. Rotenone significantly increased striatal and nigral lactate levels. However, rotenone did not produce a significant increase in hydroxyl radical formation and dopamine release, but led to a pronounced hypokinesia. In contrast, rotenone in comparison to vehicle pretreatment produced a significant augmentation of glutamate-induced dopamine release (67-fold and 31-fold increase, respectively) and did not affect the glutamate-induced hydroxyl free radical formation (23-fold and 21-fold increase, respectively). The present study demonstrates that a single systemic rotenone administration does not lead to neurotoxicity, but rather to enhanced glutamate-induced dopamine release with no further increase of hydroxyl free radical formation. Thus, acute complex I inhibition in the presence or absence of high extracellular dopamine and glutamate levels is not critically involved in the formation of hydroxyl free radicals.