Involvement of mesocorticolimbic dopaminergic systems in emotional states

Differential neuronal changes in medial prefrontal cortex, basolateral amygdala and nucleus accumbens after postweaning social isolation

The mesocorticolimbic system contains dopamine (DA)-producing neurons in the ventral tegmental area (VTA) and their projection targets, including the medial prefrontal cortex (mPFC), amygdala (AMY) and nucleus accumbens (NAc). Disruption of this system might attribute to mental illnesses. In the present study, we adopted the postweaning social isolation paradigm to model neuropsychiatric disorders and studied the functional and structural changes of the mesocorticolimbic system. After 8-9 weeks of isolation, rats exhibited hyperlocomotor activity and impaired sensorimotor gating compared to group-reared controls. However, the number of tyrosine hydroxylase-positive VTA neurons and the volume of VTA were not affected. Comparing with group-reared controls, the DA levels in the isolation-reared were not altered in the VTA, mPFC and NAc but decreased in the AMY. In the structural aspect, dendritic features of layer II/III pyramidal mPFC neurons; pyramidal neurons in the basolateral nucleus of amygdala (BLA) and medium spiny neurons in the core region of the NAc (NAcc) were examined. Interestingly, the neuronal changes were region-specific. The mPFC neurons had reduced dendritic complexity, spine density and elongated terminal branches. The BLA neurons had extensive dendritic arbors with short branches but unchanged spine density. The NAcc neurons had reduced total dendritic length but the segment length and spine density remained the same. Together, the results demonstrated the structural and functional changes in the mesocorticolimbic DA system of socially isolated rats. These changes may account for the behavioral impairments in these rats and attribute to the susceptibility to mental disorders related to schizophrenia and depression.

Neurodevelopmental biology associated with childhood sexual abuse

Child maltreatment appears to be the single most preventable cause of mental illness and behavioral dysfunction in the United States. Few published studies examine the developmental and the psychobiological consequences of sexual abuse. There are multiple mechanisms through which sexual abuse can cause post-traumatic stress disorder, activate biological stress response systems, and contribute to adverse brain development. This article will critically review the psychiatric problems associated with maltreatment and the emerging biologic stress system research with a special emphasis on what is known about victimization by sexual abuse.

Extracellular signal-regulated kinase-2 within the ventral tegmental area regulates responses to stress

Neurotrophic factors and their signaling pathways have been implicated in the neurobiological adaptations in response to stress and the regulation of mood-related behaviors. A candidate signaling molecule implicated in mediating these cellular responses is the extracellular signal-regulated kinase (ERK1/2), although its functional role in mood regulation remains to be fully elucidated. Here we show that acute (1 d) or chronic (4 weeks) exposure to unpredictable stress increases phosphorylation of ERK1/2 and of two downstream targets (ribosomal S6 kinase and mitogen- and stress-activated protein kinase 1) within the ventral tegmental area (VTA), an important substrate for motivated behavior and mood regulation. Using herpes simplex virus-mediated gene transfer to assess the functional significance of this ERK induction, we show that overexpressing ERK2 within the VTA increases susceptibility to stress as measured in the forced swim test, responses to unconditioned nociceptive stimuli, and elevated plus maze in Sprague Dawley male rats, and in the tail suspension test and chronic social defeat stress procedure in C57BL/6 male mice. In contrast, blocking ERK2 activity in the VTA produces stress-resistant behavioral responses in these same assays and also blocks a chronic stress-induced reduction in sucrose preference. The effects induced by ERK2 blockade were accompanied by decreases in the firing frequency of VTA dopamine neurons, an important electrophysiological hallmark of resilient-like behavior. Together, these results strongly implicate a role for ERK2 signaling in the VTA as a key modulator of responsiveness to stress and mood-related behaviors.

Influence of methylphenidate on brain development--an update of recent animal experiments

Methylphenidate (MPH) is the most commonly used drug to treat attention deficit/hyperactivity disorder (ADHD) in children effectively and safely. In spite of its widespread application throughout one of the most plastic and sensitive phases of brain development, very little is known to date about its long-term effects on brain structure and function. Hence, this short review updates the influence of MPH on brain development, since recent human and animal studies suggest that MPH alters the dopaminergic system with long-term effects beyond the termination of treatment.

Animal studies imply that the effects of MPH may depend on the neural responder system: Whereas structural and functional parameters are improved by MPH in animals with psychomotor impairments, they remain unaltered or get worse in healthy controls. While recent behavioural studies do not fully support such a differential effect of MPH in ADHD, the animal studies certainly prompt for further investigation of this issue. Furthermore, the abuse of MPH, when (rarely) intravenously applied, may even impair the maturation of dopaminergic fibres in subcortical brain areas. This argues for careful clinical assessment and diagnostics of ADHD symptomatology not only in conjunction with the prescription of MPH. Hence, one should be assured that MPH is only given to children with clear ADHD symptomatology leading to psychosocial impairment. The animal data suggest that under these conditions MPH is supportive for brain development and the related behaviour in children with ADHD.

The psychobiology of neglect

Child neglect, the most prevalent form of child maltreatment, is associated with adverse psychological and educational outcomes. It is hypothesized that these outcomes may be caused by adverse brain development. However, there are very few published cross-sectional studies and no prospective studies that examine the neurodevelopmental consequences of neglect. In this article, the author comprehensively outlines the issues involved in the psychobiological research of child neglect. Pre-clinical and clinical studies will be reviewed. Throughout the article, suggestions for future research opportunities and novel ways to address methodological difficulties inherent in this field of study are offered. The results of recent neuroimaging studies of maltreated children may provide a basis for understanding the early effects of neglect on childhood brain development. The author is comprehensively examining these issues as part of the Federal Child Neglect Consortium.

Noradrenaline and dopamine efflux in the prefrontal cortex in relation to appetitive classical conditioning

We trained rats to learn that an auditory stimulus predicted delivery of reward pellets in the Skinner box. After 2 d of training, we measured changes in efflux of noradrenaline (NA) and dopamine (DA) in the medial prefrontal cortex using microdialysis on the third day. Animals were subjected to a normal rewarded session and an extinction session, in which the auditory stimulus was presented alone. In the rewarded session, both NA and DA efflux were increased, but in extinction, only NA was activated. The data suggest that NA has a role in the reaction to reward-predicting stimuli, which complements that of DA.

Differential distribution of MAP1A isoforms in the adult mouse barrel cortex and comparison with the serotonin 5-HT2A receptor

Microtubule-associated protein 1A (MAP1A) is essential during the late differentiation phase of neuronal development. Here, we demonstrated the presence of two MAP1A isoforms with a differential spatial distribution in the adult mouse barrel cortex. Antibody A stained MAP1A in pyramidal and stellate cells, including dendrites that crossed layer IV in the septa between barrels. The other antibody, BW6 recognized a MAP1A isoform that was mainly confined to the barrel hollow and identified smaller caliber dendrites. Previously, an interaction of MAP1A and the serotonin 5-hydroxytryptamine 2A (5-HT(2A)) receptor was shown in the rat cortex. Here, we identified, by double-immunofluorescent labeling, MAP1A isoform and serotonin 5-HT(2A) receptor distribution. MAP1A co-localized mainly with 5-HT(2A) receptor in larger apical dendrites situated in septa. This differential staining of MAP1A and a serotonin receptor in defined barrel compartments may be due to changes in the expression or processing of MAP1A during dendritic transport as a consequence of functional differences in processing of whisker-related sensory input.

Dissociation between mesocortical dopamine release and fear-related behaviours in two psychogenetically selected lines of rats that differ in coping strategies to aversive conditions

The mesocortical and mesolimbic dopaminergic (DAergic) pathways are activated by either aversive or rewarding stimuli. The functional tone of these DAergic neurons also increases during the execution of cognitive tasks. The present study was designed to examine the relationship between mesocortical and mesolimbic DAergic function and the expression of fear-related behaviours as compared with attention- and cognition-related mechanisms (e.g. coping strategies), in response to aversive conditions. To this aim, we used two psychogenetically selected rat lines, Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh), which display drastically different emotion- and coping-related behaviours in response to stressors: RLA/Verh rats are 'reactive copers' and more fearful than RHA/Verh rats, which are 'proactive copers'. Brain dialysis experiments demonstrated that tail-pinch (TP) and the anxiogenic compounds pentylenetetrazol (PTZ) and ZK 93426 increased DA output in the medial prefrontal cortex (PFCX) of RHA/Verh but not RLA/Verh, rats. In contrast, in the shell compartment of the nucleus accumbens (NAC shell), TP caused a small increase in DA output only in RLA/Verh rats, whereas PTZ and ZK 93426 had no significant effect on either line. RHA/Verh rats displayed more robust and longer lasting coping activity and less frequent freezing and self-grooming episodes than did RLA/Verh rats after TP, PTZ or ZK 93426. This dissociation between fear-related behaviour and cortical DAergic activation argues against the view that the latter may be involved in the control of fear-like responses. We therefore propose that the activation of mesocortical DAergic projections by aversive stimuli underlies the cognitive mechanisms that are triggered in an attempt to gain control over the stressor.

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.

Stimulation of 5-hydroxytryptamine (5-HT(2C) ) receptors in the ventrotegmental area inhibits stress-induced but not basal dopamine release in the rat prefrontal cortex

The present study investigated whether 5-HT(2C) receptors in the ventrotegmental area and prefrontal cortex regulate basal and stimulus-evoked dopamine release in the prefrontal cortex. Using the in vivo microdialysis technique in conscious rats, we studied the effect of a selective 5-HT(2C) receptor agonist, Ro60-0175, on basal and immobilization stress-induced dopamine release in the prefrontal cortex. Ro60-0175 intraperitoneally (2.5 mg/kg) and into the ventrotegmental area (10 microg/0.5 microL) completely antagonized the effect of stress on extracellular dopamine without altering basal levels. Infusion of 10 microm Ro60-0175 through the cortical probe had no significant effect on basal and stress-induced dopamine release. SB242084 (10 mg/kg), a selective antagonist of 5-HT(2C) receptors, significantly increased basal extracellular dopamine and completely prevented the effect of intraperitoneal and intraventrotegmental Ro60-0175 on the stress-induced rise of extracellular dopamine, but had no effect itself in stressed rats. The results show that Ro60-0175 suppresses cortical dopamine release induced by immobilization stress through the stimulation of 5-HT(2C) receptors in the ventrotegmental area. While confirming that endogenous 5-HT acting on 5-HT(2C) receptors tonically inhibit basal dopamine release in the prefrontal cortex, the present findings suggest that the stimulation of 5-HT(2C) receptors with an exogenous agonist preferentially inhibit stimulated release.

Primate anterior cingulate cortex: where motor control, drive and cognition interface

Controversy surrounds the function of the anterior cingulate cortex. Recent discussions about its role in behavioural control have centred on three main issues: its involvement in motor control, its proposed role in cognition and its relationship with the arousal/drive state of the organism. I argue that the overlap of these three domains is key to distinguishing the anterior cingulate cortex from other frontal regions, placing it in a unique position to translate intentions to actions.

Differential environment alters ontogeny of dopamine innervation of the orbital prefrontal cortex in gerbils

In the present study, the influence of postnatal environmental conditions on the structural ontogeny of the orbital prefrontal cortex of adult gerbils (Meriones unguiculatus) was examined. The animals were bred and reared either isolated in standard laboratory cages or grouped in an object-filled environment. At the age of postnatal day 90, dopamine fibers were stained immunocytochemically and innervation density was determined in the orbital prefrontal cortex. By comparison, restricted rearing produced a restraint of the subsequent maturation of orbital prefrontal dopamine innervation, leading to adult fiber densities that were approximately 38% below those in seminaturally reared gerbils. Results are discussed in terms of activity-dependent postnatal maturation of the cortex and adaptive neuroplasticity with regard to previously published data concerning diminished dopamine innervation in the medial prefrontal cortex (Winterfeld et al. [1998]

The prisoners of despair: right hemisphere deficiency and suicide

This paper presents an integrative approach to understanding of the inner experience of suicidal persons in terms of hemispheric asymmetry. The right hemisphere is involved in formation of polysemantic context. Polysemantic context is determined by multiple interconnections among its elements, while each concrete element bears the stamp of the whole context. Left hemisphere functioning leads to formation of monosemantic context. It is suggested that due to functional insufficiency of the right hemisphere the suicidal person demonstrates a compensatory shift to left hemisphere functioning. This shift manifests itself in reversed asymmetry of neurotransmitters, tendency to dissociation, alienated and negative perception of the body, lower sensitivity to pain, disintegration of self-representation, cognitive constriction, overly general nature of personal memories, difficulties in affect regulation as well as such personality traits as low openness to experience and personal constriction. This hypothesis raises a number of suggestions for future research.

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.

Behavioural disturbances associated with hyperdopaminergia in dopamine-transporter knockout mice

Mice lacking the dopamine transporter (DAT-/-) are characterized by high extracellular dopamine levels and spontaneous hyperlocomotion. We performed a detailed analysis of the behavioural phenotype of DAT-/- mice in order to identify other behavioural impairments associated with the hyperdopaminergic tone of these mutant mice. In particular, we investigated locomotor activity, exploration, and social and maternal behaviours, which are known to be regulated by dopamine. DAT-/- mice were easily aroused by novelty and always responded with hyperlocomotion, which interfered with habituation to the testing environment, exploratory behaviour in an open field and the coping response to forced swimming stress. Social behaviours such as interaction with an unknown congener or aggressiveness were not modified in DAT-/- mice compared with DAT+/- and DAT+/+ mice, although the maternal behaviour of mutant females was severely disturbed. Haloperidol and clozapine reversed the hyperactivity in DAT-/- mice, with a rightward shift of the dose-response curve compared with control animals, suggesting a dopamine-mediated effect. These results emphasize the role of dopamine regulation in locomotion, exploration and maternal behaviours and suggest that mice with a genetic deletion of DAT may represent a useful model to elucidate the altered behavioural processes accompanying pathological conditions associated with hyperdopaminergic function.

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

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 and latent inhibition. The present study investigated, in rats, the effects of increasing dopamine receptor activation within the medial prefrontal cortex by local administration of the dopamine receptor agonist apomorphine (9 microg/side) on prepulse inhibition and latent inhibition, as well as on spontaneous and amphetamine-induced activity. Apomorphine infusions decreased spontaneous locomotor activity and blocked amphetamine-induced increase in locomotor activity in the open field, which is in line with the suggestion that dopamine receptor activation in the medial prefrontal cortex inhibits mesolimbic dopamine activity. However, apomorphine infusions induced a disruption of prepulse inhibition, an effect associated with increased dopaminergic activity in the nucleus accumbens, and left the latent inhibition effect intact. While these results support previous evidence that the medial prefrontal cortex is a component of the neural circuitry mediating prepulse inhibition but plays no role in latent inhibition, they show that dopamine receptor activation in the medial prefrontal cortex of the rat produces behavioural outcomes that cannot be explained by postulating a simple reciprocal relationship between the mesocortical and mesolimbic dopamine systems.

The dopaminergic innervation of the avian telencephalon

The present review provides an overview of the distribution of dopaminergic fibers and dopaminoceptive elements within the avian telencephalon, the possible interactions of dopamine (DA) with other biochemically identified systems as revealed by immunocytochemistry, and the involvement of DA in behavioral processes in birds. Primary sensory structures are largely devoid of dopaminergic fibers, DA receptors and the D1-related phosphoprotein DARPP-32, while all these dopaminergic markers gradually increase in density from the secondary sensory to the multimodal association and the limbic and motor output areas. Structures of the avian basal ganglia are most densely innervated but, in contrast to mammals, show a higher D2 than D1 receptor density. In most of the remaining telencephalon D1 receptors clearly outnumber D2 receptors. Dopaminergic fibers in the avian telencephalon often show a peculiar arrangement where fibers coil around the somata and proximal dendrites of neurons like baskets, probably providing them with a massive dopaminergic input. Basket-like innervation of DARPP-32-positive neurons seems to be most prominent in the multimodal association areas. Taken together, these anatomical findings indicate a specific role of DA in higher order learning and sensory-motor processes, while primary sensory processes are less affected. This conclusion is supported by behavioral findings which show that in birds, as in mammals, DA is specifically involved in sensory-motor integration, attention and arousal, learning and working memory. Thus, despite considerable differences in the anatomical organization of the avian and mammalian forebrain, the organization of the dopaminergic system and its behavioral functions are very similar in birds and mammals.

Cholinergic activation in frontal cortex and nucleus accumbens related to basic behavioral manipulations: handling, and the role of post-handling experience

The present experiment is part of a series of studies designed to investigate cerebral cholinergic activity during basic behavioral testing procedures. Using in vivo microdialysis, we monitored extracellular acetylcholine levels in rats which were picked up manually (termed handling) and exposed to an open field, or animals which were picked up and returned to their home cage. These procedures were repeated on two consecutive days. In the lateral precentral area of the frontal cortex, both procedures increased cholinergic activity. However, on the 1 st day of testing, the degree of cholinergic activation was of even greater magnitude in animals which were returned to the home cage after handling than in animals which were exposed to a novel open field. This neurochemical pattern was dissociated from behavioral indices of activation, since rearing and locomotor activity were more pronounced in the open field than in the home cage. In the nucleus accumbens core and shell, where extracellular acetylcholine is provided by cholinergic interneurons, we also found cholinergic activation on both days of testing. However, unlike the frontal cortex, there were no substantial neurochemical differences between animals which were exposed to the open field after handling vs. those which were returned to their home cage. Together, our data suggest that a simple interaction like handling provides a significant stimulus for the animal to which cholinergic activity responds in several forebrain areas. Here, frontal cortical acetylcholine appears to be especially sensitive, with a pattern of activation which is dependent on post-handling experience. These results are discussed with respect to their possible functional implications, and the role of handling as an experimental factor. Since handling is part of many neurobehavioral procedures, handling-induced changes can interact with the imposed independent variables under investigation, such as post-trial pharmacological manipulations, requiring consideration in the interpretation of any experiment employing handling of the subjects.

Dopamine-glutamate interactions in the prefrontal cortex of the conscious rat: studies on ageing

The effects of apomorphine, a D1-D2 dopamine receptor agonist, on the extracellular concentration of glutamate were investigated in the medial prefrontal cortex of young, middle-aged and aged rats. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannula system. Glutamate concentration in the samples were determined by HPLC with fluorometric detection. Apomorphine produced an increase in extracellular concentration of glutamate in medial prefrontal cortex of young rats (178% of baseline) only at 10 microM, but not at 5 and 20 microM. This increase in glutamate concentration induced by apomorphine was significantly attenuated by blockade of D1-D2 dopamine receptors with haloperidol. Apomorphine, at 10 microM, failed to induce an increase in extracellular concentration of glutamate in the prefrontal cortex of middle-aged and aged rats. However, at 20 microM, apomorphine induced an increase in glutamate concentration in the prefrontal cortex of middle-aged rats, but not in aged rats. These data indicate that an interaction between dopamine and glutamate exists in the medial prefrontal cortex and that this interaction deteriorates with age.

Transient increases in catecholaminergic activity in medial prefrontal cortex and nucleus accumbens shell during novelty

Voltammetric recordings with electrochemically modified carbon-fiber electrodes were obtained from specific regions of the forebrain in rats given free-choice access to a novel environment. Entry into novelty increased the catechol signal in the medial prefrontal cortex and shell of the nucleus accumbens by more than 100%, but had no consistent effect in either the neostriatum or accumbal core. In both the medial prefrontal cortex and accumbal shell, moreover, the novelty-induced increase in catecholaminergic activity was detectable only during the initial entry into the novel compartment and did not reappear when animals returned to the familiar environment. These results support increasing evidence for a functional distinction between the accumbal core and shell, with the latter having been linked to brain reward mechanisms. The results also indicate that novelty activates, albeit very transiently, some of the same neurochemical systems believed to play a critical role in the reinforcing effects of certain drugs of abuse.

Rapid sampling of extracellular dopamine in the rat prefrontal cortex during food consumption, handling and exposure to novelty

We report the effects of physiological stimuli on extracellular dopamine (DA) in the medial prefrontal cortex (PFC) of the rat determined on-line in dialysates obtained every 5.5 min. The detection limit for DA was 0.03-0.1 pg/5 microl injection using a conventional HPLC set-up. Basal levels in PFC were at the detection limit, therefore 3 microM nomifensine was included in the Ringer perfusion fluid, producing readily detectable DA levels of 0.9 pg/injection. Perfusion with 3 microM TTX for 30 min decreased DA within 11 min to 10% of control. The routine use of rapid sampling of extracellular DA was applied to study cortical DA release in relation to behaviour. Exposure to a novel environment for 5.5 min led to an increase to 135%. Presentation of a food pellet to food-deprived rats resulted in a rapid increase to 150% within 5.5 min, which lasted 30-40 min, which is 10-20 min more than the time spent eating. Handling the rat for 5.5 min increased DA in PFC within 5.5 min to 160% and in 11 min to 190% of control followed by a 25-min period of a 50% increase, probably reflecting increased arousal. The results suggest that emotional arousal is a common denominator of increased cortical DA release and that responses are graded depending on the intensity of the stimulus.

Dopaminergic modulation of semantic network activation

In order to examine the effect of dopamine on semantic processing, we performed a double-blind, placebo-controlled study. Healthy volunteers (n = 31) were tested in a lexical decision paradigm after ingestion of either L-dopa 100 mg with benserazide 25 mg or placebo. While direct semantic priming was influenced only marginally by L-dopa, the indirect priming effects was reduced significantly. These data support the hypothesis that dopamine increases the signal-to-noise ratio in semantic networks by reducing the spread of semantic processing, thereby leading to a focussing of activation.

Ontogeny of PFC-related behaviours is sensitive to a single non-invasive dose of methamphetamine in neonatal gerbils (Meriones unguiculatus)

A single dose of methamphetamine (50 mg/kg; i.p.) was administered to neonatal male gerbils (Meriones unguiculatus) aged 14 days, and adult prefrontal cortex (PFC)-related behaviours were analysed and compared with saline-treated controls at the age of postnatal day 90. For that purpose, animals were tested for open-field activities and y-maze delayed alternation. This solitary and non-invasive drug challenge, which has recently been found to initiate serious restraint in maturation of the mesoprefrontal dopamine (DA)-system (Dawirs et al., 1994), induces a significant delayed alternation impairment as well as significant increases in open-field motor activity and emotionality. Since an undisturbed development of the prefrontal DA-innervation seems to be a precondition for the maturation of normal PFC-related behaviours, a single early methamphetamine impact may be a suitable animal model for further investigation of structural and functional aspects of non-invasively induced behavioural deficits in rodents. The present results are discussed with regard to the assumption that hypofunctional mesoprefrontal DA-systems might be basic to schizophrenic behaviours in man.

Individual behavioral differences and ethanol consumption in Wistar rats

The aim of this study was to determine if inherent differences in the behavioral measures between individual Wistar male rats influence the level of ethanol consumption. We addressed this question by searching for differences in ethanol intake that an individual animal develops. Measures of ethanol intake were then correlated with the outcome of various behavioral tests. Our results indicate that individual variations of behavioral reactivity may be associated with the predisposition to develop high ethanol intake. Animals with delayed habituation to a novel environment, enhanced locomotor activity during testing in the open field after initial habituation, and excessive locomotor inhibition after administration of 0.07 mg/kg dose of apomorphine, are predisposed to develop high ethanol intake in a free-choice condition. Measures of activity in the forced swim test, apomorphine-induced stereotypy, hypothermia, and locomotor stimulation could not predict a propensity to acquire high ethanol preference. It is suggested that individual differences in the acquisition of high ethanol intake may be influenced, at least partially, by the differences in the sensitivity of brain dopaminergic neurotransmission.

What can genetic models tell us about behavioral plasticity?

Biological diversity and learning have played an essential interactive role in the evolution of species, as intra-specific individual differences have exerted a buffering effect towards environmental changes, and learning ability per se has allowed their maintenance. By exploiting biological diversity individuals with defective learning and memory have been produced that allow the study of the neural substrates of encoding mechanisms, as has been done in studies from Drosophila to rodents. Various aspects of this neurogenetic approach are reviewed and pitfalls are indicated. It is clear that genetic models need to be implemented by an integrated multidisciplinary top-down approach based on behavioral, electrophysiological, histochemical, immunocytochemical and neurochemical techniques. Examples are presented from some animal models that illustrate how a systems level analysis of the neural substrates of information processing can be carried out using such an integrated scheme.

Circadian activity, nociceptive thresholds, nigrostriatal and mesolimbic dopaminergic activity in the Naples High- and Low-Excitability rat lines

These experiments were designed to further characterize the differential phenotypic constellation of the Naples High- (NHE) and Naples Low-Excitability (NLE) lines. In order to determine possible differences between NHE and NLE rats in activity and circadian rhythms, besides reactivity to novelty (selection trait), adult male rats of both strains were tested during two 10-min exposures to a Làt-maze. They were then kept in activity cages continuously for 3 days. Moreover, nociceptive thresholds were measured with the hot-plate and the tail-flick test, to probe the possibility that these rats could be differentially sensitive to nociceptive stimuli. Further, the integrity of the nigro-striatal and mesolimbic system was investigated by measuring tyrosine-hydroxylase activity in the striatum and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the striatum as well as in the nucleus accumbens. In addition, TH activity was measured in the adrenals to probe the sympathetic section of the neurovegetative system. The results indicate that NHE and NLE rats differ by a factor of two in their phasic activity in a Làt-maze. In contrast, no differences in 24-h activity during the dark or light phase could be observed in the activity cages. However, NHE rats anticipated the light-on stimulus in the morning by reducing their activity 1 h earlier than NLE rats. Further, no difference could be found with the hot-plate and the tail-flick test. Finally, biochemical analyses revealed no difference in the NHE and NLE rats in the main terminal zone of mesolimbic system (n. accumbens) nor of nigrostriatal system (striatum) nor in the adrenal glands. In conclusion, since the only consistent difference between NHE and NLE rats appears to be reactivity to spatial novelty, an hippocampus-dependent behavioral trait (selection trait), independent of altered activity in the sympathetic system or dopaminergic activity in the major dopaminergic brain systems, the usefulness of these strains as genetic model to test current hypotheses of spatial processor device(s) in the mammalian brain is supported.