Distinct kinds of novelty processing differentially increase extracellular dopamine in different brain regions

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

Time Perception in Adult ADHD: Findings from a Decade-A Review

Time perception is impaired in adult ADHD. Since the term time perception subsumes different constructs, including time estimation, time reproduction, time production, and duration discrimination, it remains open whether certain domains are more affected than other domains in adult ADHD. The aim of this explorative review is to present the current state of research on time perception in adult ADHD by analysing studies from the past 10 years. A review of the literature addressing adult ADHD time perception, time estimation, and time reproduction was performed. The search strategy was conducted by using the databases "PubMed", "Medline", and "PSYNDEX". The results of the present review indicate that the number of studies on time perception in adult ADHD is very scarce. Moreover, the main investigated domains of time perception in the past decade were time estimation, time reproduction and time management. Whereas some of the found studies were able to demonstrate a distinct deficit in time estimation, time reproduction and time management other studies were unable to demonstrate a clear association between ADHD and time estimation and time reproduction deficits. However, the diagnostic protocols, study design, and methodology varied between studies. Further studies on time estimation and time reproduction need to be carried out.

Lesions of nucleus accumbens shell abolish socially transmitted food preferences

Rats adapt their food choices to conform to their conspecifics' dietary preferences. The nucleus accumbens shell is a relevant brain region to process reward-related and motivated behaviours and social information. Here, we hypothesize that the integrity of the nucleus accumbens shell is necessary to show socially transmitted food preferences. We made excitotoxic and sham lesions of nucleus accumbens shell in male Long-Evans rats who performed a social transmission of food preference task. In this task, observer rats revealed their original preference for one out of two food options. Afterward, they were exposed to a demonstrator rat who was fed with the observer's originally non-preferred food, and the observer's food choices were sampled again. Sham lesioned observer rats changed their food preferences following interaction with the demonstrator, specifically by increasing the intake of their originally non-preferred food type. This interaction-related change in preference was not found after nucleus accumbens shell lesions. The lesion effects on choice were not the consequence of impaired social or non-social motivation, anxiety or sensory or motor function, suggesting that they reflected a genuine deficit in social reward revaluation. These results highlight the role of nucleus accumbens shell in revaluating food rewards to match a conspecific's preferences.

Dopamine activity on the perceptual salience for recognition memory

To survive, animals must recognize relevant stimuli and distinguish them from inconspicuous information. Usually, the properties of the stimuli, such as intensity, duration, frequency, and novelty, among others, determine the salience of the stimulus. However, previously learned experiences also facilitate the perception and processing of information to establish their salience. Here, we propose “perceptual salience” to define how memory mediates the integration of inconspicuous stimuli into a relevant memory trace without apparently altering the recognition of the physical attributes or valence, enabling the detection of stimuli changes in future encounters. The sense of familiarity is essential for successful recognition memory; in general, familiarization allows the transition of labeling a stimulus from the novel (salient) to the familiar (non-salient). The novel object recognition (NOR) and object location recognition (OLRM) memory paradigms represent experimental models of recognition memory that allow us to study the neurobiological mechanisms involved in episodic memory. The catecholaminergic system has been of vital interest due to its role in several aspects of recognition memory. This review will discuss the evidence that indicates changes in dopaminergic activity during exposure to novel objects or places, promoting the consolidation and persistence of memory. We will discuss the relationship between dopaminergic activity and perceptual salience of stimuli enabling learning and consolidation processes necessary for the novel-familiar transition. Finally, we will describe the effect of dopaminergic deregulation observed in some pathologies and its impact on recognition memory.

Disrupted-in-schizophrenia 1 Protein Misassembly Impairs Cognitive Flexibility and Social Behaviors in a Transgenic Rat Model

Alterations in cognitive functions, social behaviors and stress reactions are commonly diagnosed in chronic mental illnesses (CMI). Animal models expressing mutant genes associated to CMI represent either rare mutations or those contributing only minimally to genetic risk. Non-genetic causes of CMI can be modeled by disturbing downstream signaling pathways, for example through inducing protein misassembly or aggregation. The Disrupted-in-Schizophrenia 1 (DISC1) gene was identified to be disrupted and thereby haploinsufficient in a large pedigree where it associated to CMI. The DISC1 protein misassembles to an insoluble protein in a subset of CMI patients and this has been modeled in a rat (tgDISC1 rat) where the full-length, non mutant human transgene was overexpressed and cognitive impairments were observed. Here, we investigated the scope of effects of DISC1 protein misassembly by investigating spatial memory, social behavior and stress resilience. In water maze tasks, the tgDISC1 rats showed intact spatial learning and memory, but were deficient in flexible adaptation to spatial reversal learning compared to littermate controls. They also displayed less social interaction. Additionally, there was a trend towards increased corticosterone levels after restraint stress in the tgDISC1 rats. Our findings suggest that DISC1 protein misassembly leads to disturbances of cognitive flexibility and social behaviors, and might also be involved in stress sensitization. Since the observed behavioral features resemble symptoms of CMI, the tgDISC1 rat may be a valuable model for the investigation of cognitive, social and - possibly - also stress-related symptoms of major mental illnesses.

Altered heparan sulfate metabolism during development triggers dopamine-dependent autistic-behaviours in models of lysosomal storage disorders

Lysosomal storage disorders characterized by altered metabolism of heparan sulfate, including Mucopolysaccharidosis (MPS) III and MPS-II, exhibit lysosomal dysfunctions leading to neurodegeneration and dementia in children. In lysosomal storage disorders, dementia is preceded by severe and therapy-resistant autistic-like symptoms of unknown cause. Using mouse and cellular models of MPS-IIIA, we discovered that autistic-like behaviours are due to increased proliferation of mesencephalic dopamine neurons originating during embryogenesis, which is not due to lysosomal dysfunction, but to altered HS function. Hyperdopaminergia and autistic-like behaviours are corrected by the dopamine D1-like receptor antagonist SCH-23390, providing a potential alternative strategy to the D2-like antagonist haloperidol that has only minimal therapeutic effects in MPS-IIIA. These findings identify embryonic dopaminergic neurodevelopmental defects due to altered function of HS leading to autistic-like behaviours in MPS-II and MPS-IIIA and support evidence showing that altered HS-related gene function is causative of autism.

Behavioural tagging: Effect of novelty exploration on plasticity related molecular signatures

Learning and memory are one of those frontier areas of neurobiology which attract us to investigate the intricacy of this process. Here, we aimed to investigate the general mechanism of "Behavioural Tagging and Capture" in long term memory (LTM) formation and to find the key factors playing role in consolidation of LTM. In this study, we've shown that not only plasticity related proteins (PRPs) but neurotransmitters and immediate early genes (IEGs) also play an important role in memory formation process. It's very well evident that memory traces can last longer if close in time novelty is introduced around memory encoding. Here our results point out that this novelty exploration acts as a modulator in memory consolidation by providing PRPs such as brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), enhancing neurotransmitters (Dopamine), IEGs (cFos) and some enzymes such as acetylcholinesterase (AChE), monoamine oxidase (MAO), sodium-potassium ATPase (Na⁺K⁺-ATPase). Therefore, by using a Novel Object Recognition task (NOR) in combination with novel task exposure, we evaluated the role of molecular markers in memory consolidation employing a behavioural tagging model. The purpose of the current study was first to evaluate the effect of novelty exposure around a single trail of NOR task in a critical time window on memory consolidation in rats after 24 h and second to determine the expression of BDNF, CREB, c-fos, AChE, MAO, Na⁺K⁺-ATPase as potential markers in the medial prefrontal cortex (mPFC) during memory formation. In the present study, to identify and validate the role of these molecular signatures in memory consolidation, infusion of the protein synthesis inhibitor Anisomycin (Ani) was done around the training session that causes a deficit in the formation of LTM when tested 24 h after weak encoding. Altogether, here we are providing the first comprehensive set of evidences indicating that BDNF, CREB, dopamine, some enzymes and c-fos role in modulating LTM by employing behavioural tagging model.

Experimental Dopamine Reuptake Inhibitors in Parkinson's Disease: A Review of the Evidence

Parkinson’s disease (PD) is the second most chronic neurodegenerative disorder worldwide. Deficit of monoamines, particularly dopamine, causes an individually varying compilation of motor and non-motor features. Constraint of presynaptic uptake extends monoamine stay in the synaptic cleft. This review discusses possible benefits of dopamine reuptake inhibition for the treatment of PD. Translation of this pharmacologic principle into positive clinical study results failed to date. Past clinical trial designs did not consider a mandatory, concomitant stable inhibition of glial monoamine turnover, i.e. with monoamine oxidase B inhibitors. These studies focused on improvement of motor behavior and levodopa associated motor complications, which are fluctuations of motor and non-motor behavior. Future clinical investigations in early, levodopa- and dopamine agonist naïve patients shall also aim on alleviation of non-motor symptoms, like fatigue, apathy or cognitive slowing. Oral levodopa/dopa decarboxylase inhibitor application is inevitably necessary with advance of PD. Monoamine reuptake (MRT) inhibition improves the efficacy of levodopa, the blood brain barrier crossing metabolic precursor of dopamine. The pulsatile brain delivery pattern of orally administered levodopa containing formulations results in synaptic dopamine variability. Ups and downs of dopamine counteract the physiologic principle of continuous neurotransmission, particularly in nigrostriatal, respectively mesocorticolimbic pathways, both of which regulate motor respectively non-motor behavior. Thus synaptic dopamine pulsatility overwhelms the existing buffering capacity. Onset of motor and non-motor complications occurs. Future MRT inhibitor studies shall focus on a stabilizing and preventive effect on levodopa related fluctuations of motor and non-motor behavior. Their long-term study designs in advanced levodopa treated patients shall allow a cautious adaptation of oral l-dopa therapy combined with a mandatory inhibition of glial monoamine turnover. Then the evidence for a preventive and beneficial, symptomatic effect of MRT inhibition on motor and non-motor complications will become more likely.

Individual variation in the attribution of incentive salience to social cues

Research on the attribution of incentive salience to drug cues has furthered our understanding of drug self-administration in animals and addiction in humans. The influence of social cues on drug-seeking behavior has garnered attention recently, but few studies have investigated how social cues gain incentive-motivational value. In the present study, a Pavlovian conditioned approach (PCA) procedure was used to identify rats that are more (sign-trackers; STs) or less (goal-trackers; GTs) prone to attribute incentive salience to food reward cues. In Experiment 1, a novel procedure employed social ‘peers’ to compare the tendency of STs and GTs to attribute incentive salience to social reward cues as well as form a social-conditioned place preference. In Experiment 2, social behavior of STs and GTs was compared using social interaction and choice tests. Finally, in Experiment 3, levels of plasma oxytocin were measured in STs and GTs seven days after the last PCA training session, because oxytocin is known to modulate the mesolimbic reward system and social behavior. Compared to GTs, STs attributed more incentive salience to social-related cues and exhibited prosocial behaviors (e.g., social-conditioned place preference, increased social interaction, and social novelty-seeking). No group differences were observed in plasma oxytocin levels. Taken together, these experiments demonstrate individual variation in the attribution of incentive salience to both food- and social-related cues, which has important implications for the pathophysiology of addiction.

c-Fos immunoreactivity in the hypothalamus and reward system of young rats after social novelty exposure

Socialization is an adaptive behavior during the early stages of life because it helps young animals become independent and determines healthy adult social behavior. Therefore, it is probable that the brain areas involved in the processing of social stimuli are more sensitive to social novelty during early life stages. To test this hypothesis, four groups of young male rats were exposed to different socioenvironmental stimuli; nonsocial physical novelty, social familiarity, social novelty, and a control group which received no stimulation. After stimuli exposure, brains were fixed and cut in coronal sections for c-Fos immunohistochemistry. The number of c-Fos-immunoreactive (c-Fos-ir) neurons in the paraventricular nucleus and supraoptic nucleus, the main producers of oxytocin and vasopressin, was compared, as well as in the nucleus accumbens and ventral pallidum, the main areas involved in reinforced behavior. A significantly higher number of c-Fos-ir neurons were found in animals exposed to social novelty in all areas, except in the supraoptic nucleus. In particular, the increase in c-Fos-ir in the paraventricular nucleus seems to be selective in response to social novelty, while the increase of c-Fos-ir in the nucleus accumbens and ventral pallidum suggests that social novelty during youth is a highly rewarding stimulus compared with social familiarity and nonsocial physical novelty.

Activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) regulates anxiety- and novelty-related behaviors

The activity-regulated cytoskeleton-associated protein (Arc, also known as Arg3.1) regulates glutamatergic synapse plasticity and has been linked to neuropsychiatric illness; however, its role in behaviors associated with mood and anxiety disorders remains unclear. We find that stress upregulates Arc expression in the adult mouse nucleus accumbens (NAc)-a brain region implicated in mood and anxiety behaviors. Global Arc knockout mice have altered AMPAR-subunit surface levels in the adult NAc, and the Arc-deficient mice show reductions in anxiety-like behavior, deficits in social novelty preference, and antidepressive-like behavior. Viral-mediated expression of Arc in the adult NAc of male, global Arc KO mice restores normal levels of anxiety-like behavior in the elevated plus maze (EPM). Consistent with this finding, viral-mediated reduction of Arc in the adult NAc reduces anxiety-like behavior in male, but not female, mice in the EPM. NAc-specific reduction of Arc also produced significant deficits in both object and social novelty preference tasks. Together our findings indicate that Arc is essential for regulating normal mood- and anxiety-related behaviors and novelty discrimination, and that Arc's function within the adult NAc contributes to these behavioral effects.

Behavioral characterization of DAT-KO rats and evidence of asocial-like phenotypes in DAT-HET rats: The potential involvement of norepinephrine system

Dopamine (DA) is a key neurotransmitter of the central nervous system, whose availability is regulated by the dopamine transporter (DAT). Deletion of DAT gene leading to hyperdopaminergia was previously performed on mouse models. This enabled recapitulation of the core symptoms of Attention-Deficit / Hyper-activity Disorder (ADHD), which include hyperactivity, inattention and cognitive impairment. We used recently developed DAT knockout (DAT-KO) rats to carry out further behavioral profiling on this novel model of hyperdopaminergia. DAT-KO rats display elevated locomotor activity and restless environmental exploration, associated with a transient anxiety profile. Furthermore, these rats show pronounced stereotypy and compulsive-like behavior at the Marble-Burying test. Homozygous DAT-KO rats mantain intact social interaction when tested in a social-preference task, while heterozygous (HET) rats show high inactivity associated with close proximity to the social stimulus. Ex-vivo evaluation of brain catecholamines highlighted increased levels of norepinephrine in the hippocampus and hypothalamus exclusively of heterozygous rats. Taken together, our data present evidence of unexpected asocial tendencies in heterozygous (DAT-HET) rats associated with neurochemical alterations in norepinephrine neurotransmission. We shed light on the behavioral and neurochemical consequences of altered DAT function in a higher, more complex model of hyperdopaminergia. Unraveling the role of DA neurotransmission in DAT-KO rats has very important implications in the understanding of many psychiatric illnesses, including ADHD, where alterations in DA system have been demonstrated.

The KCNH3 inhibitor ASP2905 shows potential in the treatment of attention deficit/hyperactivity disorder

N-(4-fluorophenyl)-N'-phenyl-N"-(pyrimidin-2-ylmethyl)-1,3,5-triazine-2,4,6-triamine [ASP2905] is a potent and selective inhibitor of the potassium voltage-gated channel subfamily H member 3 (KCNH3) that was originally identified in our laboratory. KCNH3 is concentrated in the forebrain, and its overexpression in mice leads to cognitive deficits. In contrast, Kcnh3 knockout mice exhibit enhanced performance in cognitive tasks such as attention. These data suggest that KCNH3 plays important roles in cognition. Here we investigated the neurochemical and neurophysiological profiles of ASP2905 as well as its effects on cognitive function, focusing on attention. ASP2905 (0.0313 and 0.0625 mg/kg, po) improved the latent learning ability of mice, which reflects attention. Microdialysis assays in rats revealed that ASP2905 increased the efflux of dopamine and acetylcholine in the medial prefrontal cortex (0.03, 0.1 mg/kg, po; 0.1, 1 mg/kg, po, respectively). The activities of these neurotransmitters are closely associated with attention. We used a multiple-trial passive avoidance task to investigate the effects of ASP2905 on inattention and impulsivity in juvenile stroke-prone spontaneously hypertensive rats. ASP2905 (0.1 and 0.3 mg/kg, po) significantly prolonged cumulative latency as effectively as methylphenidate (0.1 and 0.3 mg/kg, sc), which is the gold standard for treating ADHD. Further, ASP2905, amphetamine, and methylphenidate significantly increased the alpha-band power of rats, suggesting that ASP2905 increases arousal, which is a pharmacologically important activity for treating ADHD. In contrast, atomoxetine and guanfacine did not significantly affect power. Together, these findings suggest that ASP2905, which acts through a novel mechanism, is as effective for treating ADHD as currently available drugs such as methylphenidate.

Dopamine receptor activity participates in hippocampal synaptic plasticity associated with novel object recognition

Physiological and behavioral evidence supports that dopamine (DA) receptor signaling influences hippocampal function. While several recent studies examined how DA influences CA1 plasticity and learning, there are fewer studies investigating the influence of DA signaling to the dentate gyrus. The dentate gyrus receives convergent cortical input through the perforant path fiber tracts and has been conceptualized to detect novelty in spatial memory tasks. To test whether DA-receptor activity influences novelty-detection, we used a novel object recognition (NOR) task where mice remember previously presented objects as an indication of learning. Although DA innervation arises from other sources and the main DA signaling may be from those sources, our molecular approaches verified that midbrain dopaminergic fibers also sparsely innervate the dentate gyrus. During the NOR task, wild-type mice spent significantly more time investigating novel objects rather than previously observed objects. Dentate granule cells in slices cut from those mice showed an increased AMPA/NMDA-receptor current ratio indicative of potentiated synaptic transmission. Post-training injection of a D1-like receptor antagonist not only effectively blocked the preference for the novel objects, but also prevented the increased AMPA/NMDA ratio. Consistent with that finding, neither NOR learning nor the increase in the AMPA/NMDA ratio were observed in DA-receptor KO mice under the same experimental conditions. The results indicate that DA-receptor signaling contributes to the successful completion of the NOR task and to the associated synaptic plasticity of the dentate gyrus that likely contributes to the learning.

Different dosing regimens of repeated ketamine administration have opposite effects on novelty processing in rats

Repeated exposure to sub-anesthetic doses of ketamine in rats has been shown to induce cognitive deficits, as well as behavioral changes akin to the negative symptoms of schizophrenia, giving much face validity to the use of ketamine administration as a pharmacological model of schizophrenia. This study sought to further characterize the behavioral effects of two different ketamine pre-treatment regimens, focusing primarily on the effects of repeated ketamine administration on novelty processing, a capacity that is disrupted in schizophrenia. Rats received 5 or 14 intra-peritoneal injections of 30mg/kg ketamine or saline across 5 or 7days, respectively. They were then tested in an associative mismatch detection task to examine their ability to detect novel configurations of familiar audio-visual sequences. Furthermore, rats underwent a sequential novel object and novel object location exploration task. Subsequently, rats were also tested on the delayed matching to place T-maze task, sucrose preference task and locomotor tests involving administering a challenge dose of amphetamine (AMPH). The high-dose ketamine pre-treatment regimen elicited impairments in mismatch detection and working memory. In contrast, the low-dose ketamine pre-treatment regimen improved performance of novelty detection. In addition, low-dose ketamine pre-treated rats showed locomotor sensitization following an AMPH challenge, while the high-dose ketamine pre-treated rats showed an attenuated locomotor response to AMPH, compared to control rats. These findings demonstrate that different regimens of repeated ketamine administration induce alterations in novelty processing in opposite directions, and that differential neural adaptations occurring in the mesolimbic dopamine system may underlie these effects.

Differences in IV alcohol-induced dopamine release in the ventral striatum of social drinkers and nontreatment-seeking alcoholics

BACKGROUND

Striatal dopamine (DA) has been implicated in alcohol use disorders, but it is still unclear whether or not alcohol can induce dopamine release in social drinkers. Furthermore, no data exist on dopamine responses to alcohol in dependent drinkers. We sought to characterize the DA responses to alcohol intoxication in moderately large samples of social drinkers (SD) and nontreatment-seeking alcoholics (NTS).

METHODS

Twenty-four SD and twenty-one NTS received two [(11)C]raclopride (RAC) PET scans; one at rest, and one during an intravenous alcohol infusion, with a prescribed ascent to a target breath alcohol concentration (BrAC), at which it was then "clamped." The alcohol clamp was started 5min after scan start, with a linear increase in BrAC over 15min to the target of 80mg%, the legal threshold for intoxication. Target BrAC was maintained for 30min. Voxel-wise binding potential (BPND) was estimated with MRTM2.

RESULTS

IV EtOH induced significant increases in DA in the right ventral striatum in NTS, but not SD. No decreases in DA were observed in either group.

CONCLUSIONS

Alcohol intoxication results in distinct anatomic profiles of DA responses in SD and NTS, suggesting that in NTS, the striatal DA system may process effects of alcohol intoxication differently than in SD.

Increased mesocorticolimbic dopamine during acute and repeated social defeat stress: modulation by corticotropin releasing factor receptors in the ventral tegmental area

RATIONALE

Stress activates a subset of dopamine neurons in the ventral tegmental area (VTA), increasing extracellular dopamine in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAcSh). The stress neuropeptide corticotropin releasing factor (CRF) and its receptors (CRF-R1 and CRF-R2) are located within the VTA and directly and indirectly influence dopaminergic activity. However, it has yet to be shown in vivo whether VTA CRF receptor activation is necessary for acute and repeated stress-induced dopamine efflux.

OBJECTIVE

With intra-VTA CRF-R1 and CRF-R2 antagonism during social defeat, we assessed whether blockade of these receptors could prevent stress-induced dopamine increases in the mPFC and NAcSh using in vivo microdialysis.

METHODS

Rats were microinjected with a CRF-R1 or CRF-R2 antagonist into the VTA prior to social defeat stress on days 1, 4, 7, and 10. In vivo microdialysis for dopamine in the mPFC and NAcSh was performed during stress on days 1 and 10.

RESULTS

During the first social defeat, extracellular dopamine was significantly elevated in both the mPFC and NAcSh, and this increase in the NAcSh was blocked by intra-VTA CRF-R2, but not CRF-R1, antagonism. During the final social defeat, the dopaminergic increase was neither sensitized nor habituated in the mPFC and NAcSh, and intra-VTA CRF-R2, but not CRF-R1, antagonism prevented the dopamine increase in both brain regions.

CONCLUSION

These findings show that CRF-R2 in the VTA is necessary for acute and repeated stress-induced dopamine efflux in the NAcSh, but is only recruited into mPFC-projecting dopamine neurons with repeated stress exposure.

Accumbal α-adrenoceptors, but not β-adrenoceptors, regulate behaviour that is mediated by reserpine-sensitive storage vesicles

It has previously been demonstrated that mesolimbic α-adrenoceptors, but not β-adrenoceptors, control the release of dopamine that is derived from reserpine-sensitive storage vesicles. The aim of the present study was to investigate whether these storage vesicles also regulate α-adrenoceptor-mediated or β-adrenoceptor-mediated changes in behaviour. Accordingly, rats were pretreated with reserpine before the α-adrenoceptor antagonist phentolamine or the β-adrenoceptor agonist isoproterenol was locally applied to the nucleus accumbens. Both phentolamine and isoproterenol increased the duration of walking, rearing and grooming and decreased the duration of sitting. Reserpine counteracted the behavioural response elicited by phentolamine but not by isoproterenol. The results of the present study demonstrate that mesolimbic α-adrenoceptors, but not β-adrenoceptors, regulate behaviour that is mediated by reserpine-sensitive storage pools. It is hypothesized that the observed α-adrenoceptor-mediated increase in locomotor activity is due to the α-adrenoceptor-mediated increase in the release of accumbal intravesicular dopamine. Our finding that α-adrenoceptors inhibit, whereas β-adrenoceptors stimulate, locomotor activity may help explain why noradrenaline or environmental stressors have previously been found to have opposing effects on the regulation of behaviour.

Modulating the map: dopaminergic tuning of hippocampal spatial coding and interactions

Salient events activate the midbrain dopaminergic system and have important impacts on various aspects of mnemonic function, including the stability of hippocampus-dependent memories. Dopamine is also central to modulation of neocortical memory processing, particularly during prefrontal cortex-dependent working memory. Here, we review the current state of the circuitry and physiology underlying dopamine's actions, suggesting that--alongside local effects within hippocampus and prefrontal cortex--dopamine released from the midbrain ventral tegmental area is well positioned to dynamically tune interactions between limbic-cortical circuits through modulation of rhythmic network activity.

Regionally selective requirement for D1/D5 dopaminergic neurotransmission in the medial prefrontal cortex in object-in-place associative recognition memory

Object-in-place (OiP) memory is critical for remembering the location in which an object was last encountered and depends conjointly on the medial prefrontal cortex, perirhinal cortex, and hippocampus. Here we examined the role of dopamine D₁/D₅ receptor neurotransmission within these brain regions for OiP memory. Bilateral infusion of D₁/D₅ receptor antagonists SCH23390 or SKF83566 into the medial prefrontal cortex, prior to memory acquisition, impaired OiP performance following a 5 min or 1 h delay. Retrieval was unaffected. Intraperirhinal or intrahippocampal infusions of SCH23390 had no effect. These results reveal a selective role for D₁/D₅ receptors in the mPFC during OiP memory encoding.

Noradrenaline-induced release of newly-synthesized accumbal dopamine: differential role of alpha- and beta-adrenoceptors

Previous studies have shown that intra-accumbens infusion of isoproterenol (ISO), a beta-adrenoceptor-agonist, and phenylephrine (PE), an alpha-adrenoceptor-agonist, increase the release of accumbal dopamine (DA). In the present study we analyzed whether the ISO-induced release of DA is sensitive to pretreatment with the DA synthesis inhibitor alpha-methyl-para-tyrosine (AMPT). Earlier studies have shown that the PE-induced release of DA is derived from DA pools that are resistant to AMPT. In addition to PE, the alpha-adrenoceptor-antagonist phentolamine (PA) was also found to increase accumbal DA release. Therefore, we investigated whether similar to the DA-increasing effect of PE, the DA increase induced by PA is resistant to AMPT. Pretreatment with AMPT prevented the ISO-induced increase of accumbal DA. The accumbal DA increase after PA was not reduced by the DA synthesis inhibitor, independently of the amount of DA released. These results show that mesolimbic beta-, but not alpha-adrenoceptors, control the release of accumbal newly-synthesized DA pools. The DA-increasing effects of PE have previously been ascribed to stimulation of presynaptic receptors located on noradrenergic terminals, whereas the DA-increasing effects of PA and ISO have been ascribed to an action of these drugs at postsynaptic receptors on dopaminergic terminals. The fact that AMPT did not affect the accumbal DA response to PE and PA, whereas it did prevent the accumbal DA increase to ISO, supports our previously reported hypothesis that the noradrenergic neurons of the nucleus accumbens containing presynaptic alpha-adrenoceptors impinge upon the dopaminergic terminals in the nucleus accumbens containing postsynaptic adrenoceptors of the alpha but not of the beta type. The putative therapeutic effects of noradrenergic agents in the treatment of DA-related disorders are shortly discussed.

Changes in nucleus accumbens and neostriatal c-Fos and DARPP-32 immunoreactivity during different stages of food-reinforced instrumental training

Nucleus accumbens is involved in several aspects of instrumental behavior, motivation and learning. Recent studies showed that dopamine (DA) release in the accumbens shell was significantly increased on the first day of training on a fixed ratio (FR) 5 schedule (i.e. the transition from FR1 to FR5) compared with those rats that continued FR1 training, even though the rats on their first day of FR5 training received less food reinforcement than rats continuing on the FR1 schedule. Additionally, the second day of FR5 responding was marked by a significant increase in DA release in accumbens core. The present studies employed immunohistochemical methods to characterize the changes in cellular markers of accumbens and neostriatal neural activity that occur during various stages of food-reinforced FR5 training. c-Fos and DARPP-32 immunoreactivity in accumbens shell was significantly increased on the first day of FR5 training, while core c-Fos and DARPP-32 expression showed large increases on the second day of FR5 training. Additional studies showed that c-Fos and DARPP-32 expression in neostriatum increased after more extensive training. Double-labeling studies with immunofluorescence methods indicated that increases in accumbens c-Fos and DARPP-32 expression were primarily seen in substance-P-positive neurons. These increases in accumbens c-Fos and DARPP-32 immunoreactivity seen during the initial phases of FR training may reflect several factors, including novelty, learning, stress or the presentation of a work-related challenge to the organism. Moreover, it appears that the separate subregions of the striatal complex are differentially activated at distinct phases of instrumental training.

Post-learning molecular reactivation underlies taste memory consolidation

It is considered that memory consolidation is a progressive process that requires post-trial stabilization of the information. In this regard, it has been speculated that waves of receptors activation, expression of immediate early genes, and replenishment of receptor subunit pools occur to induce functional or morphological changes to maintain the information for longer periods. In this paper, we will review data related to neuronal changes in the post-acquisition stage of taste aversion learning that could be involved in further stabilization of the memory trace. In order to achieve such stabilization, evidence suggests that the functional integrity of the insular cortex (IC) and the amygdala (AMY) is required. Particularly the increase of extracellular levels of glutamate and activation of N-methyl-d-aspartate (NMDA) receptors within the IC shows a main role in the consolidation process. Additionally the modulatory actions of the dopaminergic system in the IC appear to be involved in the mechanisms that lead to taste aversion memory consolidation through the activation of pathways related to enhancement of protein synthesis such as the Protein Kinase A pathway. In summary, we suggest that post-acquisition molecular and neuronal changes underlying memory consolidation are dependent on the interactions between the AMY and the IC.

Neural systems analysis of decision making during goal-directed navigation

The ability to make adaptive decisions during goal-directed navigation is a fundamental and highly evolved behavior that requires continual coordination of perceptions, learning and memory processes, and the planning of behaviors. Here, a neurobiological account for such coordination is provided by integrating current literatures on spatial context analysis and decision-making. This integration includes discussions of our current understanding of the role of the hippocampal system in experience-dependent navigation, how hippocampal information comes to impact midbrain and striatal decision making systems, and finally the role of the striatum in the implementation of behaviors based on recent decisions. These discussions extend across cellular to neural systems levels of analysis. Not only are key findings described, but also fundamental organizing principles within and across neural systems, as well as between neural systems functions and behavior, are emphasized. It is suggested that studying decision making during goal-directed navigation is a powerful model for studying interactive brain systems and their mediation of complex behaviors.

Slow phasic changes in nucleus accumbens dopamine release during fixed ratio acquisition: a microdialysis study

Nucleus accumbens dopamine (DA) is a critical component of the brain circuitry regulating behavioral output during reinforcement-seeking behavior. Several studies have investigated the characteristics of accumbens DA release during the performance of well-learned operant behaviors, but relatively few have focused on the initial acquisition of particular instrumental behaviors or operant schedules. The present experiments focused on the initial acquisition of operant performance on a reinforcement schedule by studying the transition from a fixed ratio 1 (FR1) schedule to another operant schedule with a higher ratio requirement (i.e. fixed ratio 5 [FR5]). Microdialysis sessions were conducted in different groups of rats that were tested on either the FR1 schedule; the first, second, or third day of FR5 training; or after weeks of FR5 training. Consistent with previous studies, well-trained rats performing on the FR5 schedule after weeks of training showed significant increases in extracellular DA in both core and shell subregions of nucleus accumbens during the behavioral session. On the first day of FR5 training, there was a substantial increase in DA release in nucleus accumbens shell (i.e. approximately 300% of baseline). In contrast, accumbens core DA release was greatest on the second day of FR5 training. In parallel experiments, DA release in core and shell subregions did not significantly increase during free consumption of the same high carbohydrate food pellets that were used in the operant experiments, despite the very high levels of food intake in experienced rats. However, in rats exposed to the high-carbohydrate food for the first time, there was a tendency for extracellular DA to show a small increase. These results demonstrate that transient increases in accumbens DA release occur during the initial acquisition of ratio performance, and suggest that core and shell subregions show different temporal patterns during acquisition of instrumental behavior.

A neoHebbian framework for episodic memory; role of dopamine-dependent late LTP

According to the Hebb rule, the change in the strength of a synapse depends only on the local interaction of presynaptic and postsynaptic events. Studies at many types of synapses indicate that the early phase of long-term potentiation (LTP) has Hebbian properties. However, it is now clear that the Hebb rule does not account for late LTP; this requires an additional signal that is non-local. For novel information and motivational events such as rewards this signal at hippocampal CA1 synapses is mediated by the neuromodulator, dopamine. In this Review we discuss recent experimental findings that support the view that this 'neoHebbian' framework can account for memory behavior in a variety of learning situations.

Dissociable roles of dopamine within the core and medial shell of the nucleus accumbens in memory for objects and place

There is increasing focus on the role of the nucleus accumbens (NAc) in learning and memory, but there is little consensus as to how the core and medial shell subregions of the NAc contribute to these processes. In the current experiments, we used spontaneous object recognition to test rats with 6-hydroxydopamine lesions targeted at the core or medial shell of the NAc on a familiarity discrimination task and a location discrimination task. In the object recognition variant, control animals were able to discriminate the novel object at both 24-hr and 5-min delay. However, in the lesion groups, performance was systematically related to dopamine (DA) levels in the core but not the shell. In the location recognition task, sham-operated animals readily detected the object displacement at test. In the lesion groups, performance impairment was systematically related to DA levels in the shell but not the core. These results suggest that dopamine function within distinct subregions of the NAc plays dissociable roles in the modulation of memory for objects and place.

Distinct patterns of Fos immunoreactivity in striatum and hippocampus induced by different kinds of novelty in mice

In this study the immediate-early gene Fos was used to investigate the response to different novel stimuli in a wide array of brain regions including the hippocampus, the rhinal cortex, the frontal cortex and different components of the striatal complex. Independent groups of CD-1 mice were exposed to three different novelty conditions: (1) novel environment (empty open field); (2) complex novel environment (i.e. open field containing objects); and (3) identity-based detection of novel objects. We observed that a complex novel environment and a knowledge-based novelty modulated Fos levels in both the dorsal and the ventral components of the striatum, while Fos immunoreactivity in the medial temporal lobe was only increased after exposure to novel environments, regardless of their complexity. Finally, we observed a strong increase of Fos levels in the prefrontal cortex in all the three novel conditions examined, indicating a major involvement of this structure in novelty assessment. Overall the present study demonstrates that distinct brain regions are recruited in different kinds of novelty and emphasizes the role of the striatal complex in processing complex novel information.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

Good vibrations: cross-frequency coupling in the human nucleus accumbens during reward processing

The nucleus accumbens is critical for reward-guided learning and decision-making. It is thought to "gate" the flow of a diverse range of information (e.g., rewarding, aversive, and novel events) from limbic afferents to basal ganglia outputs. Gating and information encoding may be achieved via cross-frequency coupling, in which bursts of high-frequency activity occur preferentially during specific phases of slower oscillations. We examined whether the human nucleus accumbens engages such a mechanism by recording electrophysiological activity directly from the accumbens of human patients undergoing deep brain stimulation surgery. Oscillatory activity in the gamma (40-80 Hz) frequency range was synchronized with the phase of simultaneous alpha (8-12 Hz) waves. Further, losing and winning small amounts of money elicited relatively increased gamma oscillation power prior to and following alpha troughs, respectively. Gamma-alpha synchronization may reflect an electrophysiological gating mechanism in the human nucleus accumbens, and the phase differences in gamma-alpha coupling may reflect a reward information coding scheme similar to phase coding.

Mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles

Mesolimbic beta-, but not alpha-adrenoceptors control the accumbal release of dopamine that is derived from alpha-methyl-para-tyrosine-sensitive pools of newly synthesized neurotransmitter. The aim of this study was to investigate which of these adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive pools of previously stored neurotransmitter. Rats, that were divided in low-responders and high-responders to novelty, were pretreated with 1 mg/kg of reserpine before the alpha-adrenergic-agent phentolamine or the beta-adrenergic-agent isoproterenol was locally applied into the nucleus accumbens. The original finding that phentolamine and isoproterenol increased accumbal dopamine levels in low-responders and high-responders was replicated. Reserpine reduced the phentolamine-induced increase of accumbal dopamine in both types of rat. However, phentolamine could still increase accumbal dopamine levels in reserpine-treated high-responders, but not anymore in reserpine-treated low-responders. Reserpine did not reduce the isoproterenol-induced increase of accumbal dopamine in any type of rat. This study demonstrates that mesolimbic alpha-, but not beta-adrenoceptors control the accumbal release of dopamine that is derived from reserpine-sensitive storage vesicles. In addition, these data confirm our previous finding that dopamine can still be released from storage vesicles of reserpinized high-responders, but not of reserpinized low-responders. The collected data underline our notion that alpha- and beta-adrenergic drugs may have therapeutic effects in patients suffering from diseases in which accumbal dopamine is involved.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

Social memory in mice: disruption with an NMDA antagonist and attenuation with antipsychotic drugs

Social recognition reflects the ability of one animal to learn and remember the identity of another. Animal models of social learning and memory are pertinent to several different CNS diseases involving disruptions in cognition. Moreover, the increased understanding of the basic biology of memory increases the likelihood of discovery of memory-enhancing treatments in these human diseases. In the present study, we investigated the effects of the non-competitive NMDA antagonist ketamine on social recognition in mice across a broad dose range (5-30 mg/kg) and time-course (60 min-7 days). We also tested the ability of two antipsychotic drugs, haloperidol and olanzapine, to block the ketamine effect. Our results show that mice demonstrate social recognition over a several day period, with loss of recognition between 3-7 days. Ketamine disrupts social memory at doses which do not affect task performance. Chronic oral administration of haloperidol or olanzapine attenuates these ketamine-induced effects on social recognition, tending to normalize the memory behavior. The neural mechanisms of these actions are not known, although medial temporal lobe memory systems have been implicated.

Attenuation of auditory startle and prepulse inhibition by unexpected changes in ambient illumination through dopaminergic mechanisms

We investigated the role of dopaminergic mechanisms in the attenuation of the acoustic startle response and prepulse inhibition (PPI) in rats by the introduction of unexpected changes in environment illumination. Experiment 1 showed that Dark-to-Light transitions robustly reduce startle responses and PPI. Experiment 2 showed that this phenomenon habituates across repeated testing sessions and reappears after an interval without testing. Experiment 3 demonstrated that haloperidol blocks the startle and PPI-reducing effect of the Dark-to-Light transition. We show how a computational model of acoustic startle response and prepulse inhibition can be extended to incorporate the empirical effects demonstrated in this study. We conclude that sensory gating as measured by prepulse inhibition is markedly attenuated in situations where novel stimuli are introduced during a test session and that dopaminergic systems may be involved in the dynamic changes evoked by the onset of illumination.

The stop null mice model for schizophrenia displays [corrected] cognitive and social deficits partly alleviated by neuroleptics

Recently evidence has accumulated that schizophrenia can arise from primary synaptic defects involving structural proteins particularly, microtubule associated proteins. Previous experiments have demonstrated that a STOP (stable tubule only peptide) gene deletion in mice leads to a phenotype mimicking some aspects of positive symptoms classically observed in schizophrenic patients. In the current study, we determined if STOP null mice demonstrate behavioral abnormalities related to the social and cognitive impairments of schizophrenia. Compared with wild-type mice, STOP null mice exhibited deficits in the non-aggressive component of social recognition, short term working memory and social and spatial learning. As described in humans, learning deficits in STOP null mice were poorly sensitive to long term treatment with typical neuroleptics. Since social and cognitive dysfunction have consistently been considered as central features of schizophrenia, we propose that STOP null mice may provide a useful model to understand the neurobiological correlates of social and cognitive defects in schizophrenia and to develop treatments that better target these symptoms.

Discrimination: from behaviour to brain

Discrimination is a skill needed by many organisms for survival: decisions about food, shelter, and mate selection all require the ability to distinguish among stimuli. This article reviews the how and why of discrimination and how researchers may exploit this natural skill in the laboratory to learn more about what features of stimuli animals use to discriminate. The paper then discusses the possible neurophysiological basis of discrimination and proposes a model, based on one of stimulus-association put forth by Beninger and Gerdjikov (2004) [Beninger, R.J., Gerdjikov, T.V., 2004. The role of signaling molecules in reward-related incentive learning. Neurotox. Res., 6, 91-104], to account for the role of dopamine in how an animal learns to discriminate rewarded from non-rewarded stimuli.

Spatial deficits in a mouse model of Parkinson disease

RATIONALE

Accumulating evidence in humans demonstrated that visuo-spatial deficits are the most consistently reported cognitive abnormalities in Parkinson disease (PD). These deficits have been generally attributed to cortical dopamine degeneration. However, more recent evidence suggests that dopamine loss in the striatum is responsible for the visuo-spatial abnormalities in PD. Studies based on animal models of PD did not specifically address this question.

OBJECTIVES

Thus, the first goal of this study was to analyze the role of dopamine within the dorsal striatum in spatial memory. We tested bilateral 6-OHDA striatal lesioned CD1 mice in an object-place association spatial task. Furthermore, to see whether the effects were selective for spatial information, we measured how the 6-OHDA-lesioned animals responded to a non-spatial change and learned in the one-trial inhibitory avoidance task.

RESULTS

The results demonstrated that bilateral (approximately 75%) dopamine depletion of the striatum impaired spatial change discrimination. On the contrary, no effect of the lesion was observed on non-spatial novelty detection or on passive avoidance learning.

CONCLUSIONS

These results confirm that dopamine depletion is accompanied by cognitive deficits and demonstrate that striatal dopamine dysfunction is sufficient to induce spatial information processing deficits.

Wireless voltammetry recording in unanesthetised behaving rats

In vivo voltammetry is a valuable technique for rapid measurement of dopamine in the brain of freely behaving rats. Using a conventional voltammetry system, however, behavioural freedom is restricted by cables connecting the head assembly to the measurement system. To overcome these difficulties, we developed a wireless voltammetry system utilizing radio waves. This system consisted of a potentiostat and transmitter system that was mounted on the back of the rat, and a receiver and analysis system. A single-step pulse (100-250 mV) was applied at 4 Hz after an activation pulse to a carbon fibre recording electrode (diameter: 7 microm). Measurement of dopamine (detection limit: 2.7 x 10(-7)M) was demonstrated in vitro. In vivo experiment was performed at least 1 week after the recording electrode was implanted in the rat striatum. Administration of 2-phenylethylamine to rats increased dopamine signal current, which was consistent with the result in the microdialysis measurement. During a resident-intruder test, dopamine signal current in a resident rat increased upon introduction of an intruder rat. These results show that the present wireless system is useful for a long-term measurement of dopamine in behaving rats.

Phenotypic characterization of spatial cognition and social behavior in mice with 'knockout' of the schizophrenia risk gene neuregulin 1

Neuregulin-1 (NRG1) has been identified as a candidate susceptibility gene for schizophrenia. In the present study the functional role of the NRG1 gene, as it relates to cognitive and social processes known to be disrupted in schizophrenia, was assessed in mice with heterozygous deletion of transmembrane (TM)-domain NRG1 in comparison with wildtypes (WT). Social affiliative behavior was assessed using the sociability and preference for social novelty paradigm, in terms of time spent in: (i) a chamber containing an unfamiliar conspecific vs. an empty chamber (sociability), or (ii) a chamber containing an unfamiliar conspecific vs. a chamber containing a familiar conspecific (preference for social novelty). Social dominance and aggressive behavior were examined in the resident-intruder paradigm. Spatial learning and memory were assessed using the Barnes maze paradigm, while spatial working memory was measured using the continuous variant of the spontaneous alternation task. Barnes maze data revealed intact spatial learning in NRG1 mutants, with elevated baseline latency to enter the escape hole in male NRG1 mutants reflecting an increase in activity level. Similarly, although a greater number of overall arm entries were found, spontaneous alternation was unaffected in NRG1 mice. Social affiliation data revealed NRG1 mutants to evidence a specific loss of WT preference for spending time with an unfamiliar as opposed to a familiar conspecific. This suggests that NRG1 mutants show a selective impairment in response to social novelty. While spatial learning and working memory processes appear intact, heterozygous deletion of TM-domain NRG1 was associated with disruption to social novelty behavior. These data inform at a novel phenotypic level on the functional role of this gene in the context of its association with risk for schizophrenia.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.