The effects of dopamine D(1) receptor blockade in the prelimbic-infralimbic areas on behavioral flexibility

Brain network function during shifts in learning strategies in portal hypertension animals

Patients with minimal hepatic encephalopathy exhibit early impairments in their ability to shift attentional set. We employed a task-switching protocol to evaluate brain network changes. Strategy switching requires the modification of both the relevant stimulus dimension and the required memory system. Rats were trained in an allocentric (A) and a cue-guided (C) task using a four-arm maze. To examine priming, we changed the order in which the tasks were presented. Five groups of animals were used: a SHAM (sham-operated) A-C group (n=10), a SHAM C-A group (n=8), a PH (portal hypertension) A-C group (n=8), PH C-A group (n=8), and a naïve group (n=10). The triple portal vein ligation method was used to create an animal model of the early evolutive phase of PH. The animals were tested in the four-arm radial water maze in a single 10-trial session each day for six days (three days for the allocentric task and three days for the cue-guided task). The metabolic activities of the brains were studied with cytochrome oxidase histochemistry, and brain network changes were assessed with principal component analysis. The behavioural results revealed significant increases in the numbers of correct choices across training days in all groups studied, and facilitation of the acquisition of the second task was present in the C-A groups. Moreover, different brain network activities were found; in the experimental groups, the performance of A-C switch involved the prefrontal cortex, and the key structures involved in the C-A switch in the other groups were the dentate gyrus of the dorsal hippocampus and the basolateral and central amygdala. These networks have a common nucleus of structures (i.e., the parietal cortex and the dorsal and ventral striatum), whereas other structures were specifically involved in each type of strategy, suggesting that these regions are part of both circuits and may interact with one another during learning.

Prefrontal cortical GABAergic dysfunction contributes to age-related working memory impairment

Working memory functions supported by the prefrontal cortex decline in normal aging. Disruption of corticolimbic GABAergic inhibitory circuits can impair working memory in young subjects; however, relatively little is known regarding how aging impacts prefrontal cortical GABAergic signaling and whether such changes contribute to cognitive deficits. The current study used a rat model to evaluate the effects of aging on expression of prefrontal GABAergic synaptic proteins in relation to working memory decline, and to test whether pharmacological manipulations of prefrontal GABAergic signaling can improve working memory abilities in aged subjects. Results indicate that in aged medial prefrontal cortex (mPFC), expression of the vesicular GABA transporter VGAT was unchanged; however, there was a significant increase in expression of the GABA synthesizing enzyme GAD67, and a significant decrease in the primary neuronal GABA transporter GAT-1 and in both subunits of the GABA(B) receptor (GABA(B)R). Expression of VGAT, GAD67, and GAT-1 was not associated with working memory ability. In contrast, among aged rats, GABA(B)R expression was significantly and negatively associated with working memory performance, such that lower GABA(B)R expression predicted better working memory. Subsequent experiments showed that systemic administration of a GABA(B)R antagonist, CGP55845, dose-dependently enhanced working memory in aged rats. This enhancing effect of systemic CGP55845 was reproduced by direct intra-mPFC administration. Together, these data suggest that age-related dysregulation of GABAergic signaling in prefrontal cortex may play a causal role in impaired working memory and that targeting GABA(B)Rs may provide therapeutic benefit for age-related impairments in executive functions.

Contributions of signaling by dopamine neurons in dorsal striatum to cognitive behaviors corresponding to those observed in Parkinson's disease

Although the cardinal features of Parkinson's disease (PD) are motor symptoms, PD also causes cognitive deficits including cognitive flexibility and working memory, which are strongly associated with prefrontal cortex (PFC) functions. Yet, early stage PD is not characterized by pathology in the PFC but by a loss of dopaminergic (DA) projections from the substantia nigra to the dorsal striatum. Moreover, the degree to which PD symptoms can be ascribed to the loss of DA alone or to the loss of DA neurons is unknown. We addressed these issues by comparing mouse models of either chronic DA depletion or loss of DA projections to the dorsal striatum. We achieved equal levels of striatal DA reduction in both models which ranged from mild (~25%) to moderate (~60%). Both models displayed DA concentration-dependent reductions of motor function as well as mild deficits of cognitive flexibility and working memory. Interestingly, whereas both motor function and cognitive flexibility were more severely impaired after mild ablation of DA neurons as compared to mild loss of DA alone, both models had equal deficits after moderate loss of DA. Our results confirm contributions of nigro-striatal dopamine signaling to cognitive behaviors that are affected in early stage PD. Furthermore, our findings suggest that the phenotype after ablation of DA neurons accrues from factors beyond the mere loss of DA.

The Role of Dopamine in Motor Flexibility

Humans carry out many daily tasks in a seemingly automatic fashion. However, when unexpected changes in the environment occur, we have the capacity to inhibit prepotent behavior and replace it with an alternative one. Such behavioral flexibility is a hallmark of executive functions. The neurotransmitter dopamine is known to be crucial for fast, efficient, and accurate cognitive flexibility. Despite the perceived similarities between cognitive and motor flexibility, less is known regarding the role of dopamine within the motor domain. Therefore, the aim of this study was to determine the role of dopamine in motor flexibility. In a double-blind, five-session, within-subject pharmacological experiment, human participants performed an RT task within a probabilistic context that was either predictable or unpredictable. The probabilistic nature of the predictable context resulted in prediction errors. This required participants to replace the prepotent or prepared action with an unprepared action (motor flexibility). The task was overlearned, and changes in context were explicitly instructed, thus controlling for contributions from other dopamine-related processes such as probabilistic or reversal learning and interactions with other types of uncertainty. We found that dopamine receptor blockade by high-dose haloperidol (D1/D2 dopamine receptors) impaired participants' ability to react to unexpected events occurring in a predictable context, which elicit large prediction errors and necessitate motor flexibility. This effect was not observed with selective D2 receptor blockade (sulpiride), with a general increase in tonic dopamine levels (levodopa), or during an unpredictable context, which evoked minimal prediction error. We propose that dopamine is vital in responding to low-level prediction errors about stimulus outcome that requires motor flexibility.

The prelimbic cortex and subthalamic nucleus contribute to cue-guided behavioral switching

Frontal cortex-basal ganglia circuitry supports behavioral switching when a change in outcome information is used to adapt response patterns. Less is known about whether specific frontal cortex-basal ganglia circuitry supports behavioral switching when cues signal that a change in response patterns should occur. The present experiments investigated whether the prelimbic cortex and subthalamic nucleus in male Long-Evans rats supports cue-guided switching in a conditional discrimination test. Rats learned in a cross-maze that a start arm cue (black or white) signaled which of two maze arms to enter for a food reward. The cue was switched every 3-6 trials. Baclofen and muscimol infused into the prelimbic cortex significantly impaired performance by increasing switch trial errors, as well as trials immediately following a switch trial (perseveration) and after initially making a correct switch (maintenance error). NMDA receptor blockade in the subthalamic nucleus significantly impaired performance by increasing switch errors and perseveration. Contralateral disconnection of these areas significantly reduced conditional discrimination performance by increasing switch and perseverative errors. These findings suggest that the prelimbic area and subthalamic nucleus support the use of cue information to facilitate an initial switch away from a previously relevant response pattern.

Persistent cocaine-induced reversal learning deficits are associated with altered limbic cortico-striatal local field potential synchronization

Repeated exposure to cocaine is known to produce persistent deficits in behavioral flexibility. Evidence suggests that these deficits are mediated in part by a circuit involving the medial prefrontal and orbitofrontal cortices (PFC and OFC), nucleus accumbens (NAC), and basolateral amygdala (BLA). To assess the effects of cocaine on this circuit, we treated rats with cocaine daily for 14 d, followed by 4 weeks of abstinence. Animals were then tested on a cross-maze-based reversal learning and set-shifting task, after which they were anesthetized to allow for recording of spontaneous local field potential (LFP) activity simultaneously from all four regions, in addition to activity evoked from acute BLA stimulation. Cocaine-treated (COC) animals showed specific deficits in reversal learning; furthermore, spontaneous LFP oscillation power was reduced and BLA-induced oscillation power was increased in all regions compared with saline-treated (SAL) rats. Theta-burst stimulation of BLA potentiated BLA-evoked responses in all regions and cocaine challenge reduced spontaneous oscillation power and evoked response amplitude, with no COC/SAL group differences. Notably, cocaine challenge produced differential changes in coherence between OFC-BLA, BLA-NAC, and OFC-NAC in COC and SAL groups. These data indicate that repeated exposure to cocaine can produce changes in oscillatory LFP synchronization along limbic cortico-striatal circuits that persist long into abstinence. Furthermore, the regional specificity of these changes strongly correlates with the observed behavioral deficits. Aberrant synchronization within and between regions and consequent dysregulation of the neurocircuitry involved in executive control may contribute to the long-lasting maladaptive decision making seen in cocaine abusers.

Dopamine and extinction: a convergence of theory with fear and reward circuitry

Research on dopamine lies at the intersection of sophisticated theoretical and neurobiological approaches to learning and memory. Dopamine has been shown to be critical for many processes that drive learning and memory, including motivation, prediction error, incentive salience, memory consolidation, and response output. Theories of dopamine's function in these processes have, for the most part, been developed from behavioral approaches that examine learning mechanisms in reward-related tasks. A parallel and growing literature indicates that dopamine is involved in fear conditioning and extinction. These studies are consistent with long-standing ideas about appetitive-aversive interactions in learning theory and they speak to the general nature of cellular and molecular processes that underlie behavior. We review the behavioral and neurobiological literature showing a role for dopamine in fear conditioning and extinction. At a cellular level, we review dopamine signaling and receptor pharmacology, cellular and molecular events that follow dopamine receptor activation, and brain systems in which dopamine functions. At a behavioral level, we describe theories of learning and dopamine function that could describe the fundamental rules underlying how dopamine modulates different aspects of learning and memory processes.

Dopaminergic control of cognitive flexibility in humans and animals

Striatal dopamine (DA) is thought to code for learned associations between cues and reinforcers and to mediate approach behavior toward a reward. Less is known about the contribution of DA to cognitive flexibility—the ability to adapt behavior in response to changes in the environment. Altered reward processing and impairments in cognitive flexibility are observed in psychiatric disorders such as obsessive compulsive disorder (OCD). Patients with this disorder show a disruption of functioning in the frontostriatal circuit and alterations in DA signaling. In this review we summarize findings from animal and human studies that have investigated the involvement of striatal DA in cognitive flexibility. These findings may provide a better understanding of the role of dopaminergic dysfunction in cognitive inflexibility in psychiatric disorders, such as OCD.

Acute stress impairs set-shifting but not reversal learning

The ability to update and modify previously learned behavioral responses in a changing environment is essential for successful utilization of promising opportunities and for coping with adverse events. Valid models of cognitive flexibility that contribute to behavioral flexibility include set-shifting and reversal learning. One immediate effect of acute stress is the selective impairment of performance on higher-order cognitive control tasks mediated by the medial prefrontal cortex (mPFC) but not the hippocampus. Previous studies show that the mPFC is required for set-shifting but not for reversal learning, therefore the aim of the present experiment is to assess whether exposure to acute stress (15 min of mild tail-pinch stress) given immediately before testing on either a set-shifting or reversal learning tasks would impair performance selectively on the set-shifting task. An automated operant chamber-based task, confirmed that exposure to acute stress significantly disrupts set-shifting but has no effect on reversal learning. Rats exposed to an acute stressor require significantly more trials to reach criterion and make significantly more perseverative errors. Thus, these data reveal that an immediate effect of acute stress is to impair mPFC-dependent cognition selectively by disrupting the ability to inhibit the use of a previously relevant cognitive strategy.

Neural structures underlying set-shifting: roles of medial prefrontal cortex and anterior cingulate cortex

Impaired attentional set-shifting and inflexible decision-making are problems frequently observed during normal aging and in several psychiatric disorders. To understand the neuropathophysiology of underlying inflexible behavior, animal models of attentional set-shifting have been developed to mimic tasks such as the Wisconsin Card Sorting Task (WCST), which tap into a number of cognitive functions including stimulus-response encoding, working memory, attention, error detection, and conflict resolution. Here, we review many of these tasks in several different species and speculate on how prefrontal cortex and anterior cingulate cortex might contribute to normal performance during set-shifting.

Prefrontal dopamine and behavioral flexibility: shifting from an "inverted-U" toward a family of functions

Studies on prefrontal cortex (PFC) dopamine (DA) function have revealed its essential role in mediating a variety of cognitive and executive functions. A general principle that has emerged (primarily from studies on working memory) is that PFC DA, acting on D₁ receptors, regulates cognition in accordance to an “inverted-U” shaped function, so that too little or too much activity has detrimental effects on performance. However, contemporary studies have indicated that the receptor mechanisms through which mesocortical DA regulates different aspects of behavioral flexibility can vary considerably across different DA receptors and cognitive operations. This article will review psychopharmacological and neurochemical data comparing and contrasting the cognitive effects of antagonism and stimulation of different DA receptors in the medial PFC. Thus, set-shifting is dependent on a co-operative interaction between PFC D₁ and D₂ receptors, yet, supranormal stimulation of these receptors does not appear to have detrimental effects on this function. On the other hand, modification of cost/benefit decision biases in situations involving reward uncertainty is regulated in complex and sometimes opposing ways by PFC D₁ vs. D₂ receptors. When viewed collectively, these findings suggest that the “inverted-U” shaped dose-response curve underlying D₁ receptor modulation of working memory is not a one-size-fits-all function. Rather, it appears that mesocortical DA exerts its effects via a family of functions, wherein reduced or excessive DA activity can have a variety of effects across different cognitive domains.

Dopamine D1-like and D2-like receptors in the dorsal striatum control different aspects of attentional performance in the five-choice serial reaction time task under a condition of increased activity of corticostriatal inputs

We investigated the interaction between the corticostriatal glutamatergic afferents and dopamine D1-like and D2-like receptors in the dorsomedial striatum (dm-STR) in attention and executive response control in the five-choice serial reaction time (5-CSRT) task. The competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) injected in the mPFC impaired accuracy and increased premature and perseverative responding, raising GLU, DA, and GABA release in the dm-STR. The D1-like antagonist SCH23390 injected in the dm-STR reversed the CPP-induced accuracy deficit but did not affect the increase in perseverative responding. In contrast, the D2-like antagonist haloperidol injected in the dm-STR reduced the CPP-induced increase in perseverative responding but not the accuracy deficit. The different roles of dorsal striatal D1-like and D2-like receptor were further supported by the finding that activation of D1-like receptor in the dm-STR by SKF38393 impaired accuracy but not perseverative responding while the D2-like agonist quinpirole injected in the dm-STR increased perseverative responding but did not affect accuracy. These findings suggest that integration of cortical information by D1-like receptors in the dm-STR is a key mechanism of the input selection process of attention while the integration of corticostriatal signals by D2-like receptors preserves the ability to switch from one act/response to the next in a complex motor sequence, thus providing for behavioral flexibility.

Comparison of single-dose and extended methamphetamine administration on reversal learning in rats

RATIONALE

Protracted use of methamphetamine (mAMPH) can result in long-term impairments in cognitive function in humans. A previous study reported reversal-specific learning impairments in rats after a binge administration of mAMPH. Several studies show that extended exposure to mAMPH may confer protection against cognitive impairments and the insult to monoamine systems typically observed after larger binge doses.

OBJECTIVES

To explore this issue, we compared the effects of escalating and single doses of mAMPH (and saline, SAL) on retention, reversal learning, and post-mortem analysis of dopamine and serotonin transporters, DAT and SERT.

METHODS

Rats learned to discriminate equiluminant stimuli and then were treated with either: (1) 4 weeks of mAMPH increasing by 0.3 mg/kg, culminating in 6 mg/kg (mAMPH(escal)); (2) 4 weeks of SAL with a single dose of 6 mg/kg on the last day of treatment (mAMPH(single)); or (3) 4 weeks of SAL. Following treatment, rats were tested on retention and reversal learning, with subsequent analysis of DAT and SERT binding across subregions of the striatum and frontoparietal cortex, respectively.

RESULTS

Retention of the pretreatment discrimination was not significantly impaired in either mAMPH treatment group. A significant decrease in ventrolateral striatal DAT binding was observed only in the mAMPH(single) group and frontoparietal SERT was unaffected by either mAMPH treatment. Both treatment groups demonstrated attenuated reversal learning, particularly on measures of accuracy and effort.

CONCLUSIONS

These results show that extended and single-dose pretreatment with mAMPH similarly and selectively affect reversal learning, even in the absence of significant DAT or SERT changes.

The role of prefrontal dopamine D1 receptors in the neural mechanisms of associative learning

Dopamine is thought to play a major role in learning. However, while dopamine D1 receptors (D1Rs) in the prefrontal cortex (PFC) have been shown to modulate working memory-related neural activity, their role in the cellular basis of learning is unknown. We recorded activity from multiple electrodes while injecting the D1R antagonist SCH23390 in the lateral PFC as monkeys learned visuomotor associations. Blocking D1Rs impaired learning of novel associations and decreased cognitive flexibility but spared performance of already familiar associations. This suggests a greater role for prefrontal D1Rs in learning new, rather than performing familiar, associations. There was a corresponding greater decrease in neural selectivity and increase in alpha and beta oscillations in local field potentials for novel than for familiar associations. Our results suggest that weak stimulation of D1Rs observed in aging and psychiatric disorders may impair learning and PFC function by reducing neural selectivity and exacerbating neural oscillations associated with inattention and cognitive deficits.

The emerging role of norepinephrine in cognitive dysfunctions of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1% of the population over age 60. In those patients cognitive dysfunction is a persistent issue that impairs quality of life and productivity. Neuropathological studies demonstrate significant damage in brain regions outside the nigral dopamine (DA) system, including early degeneration of locus coeruleus norepinephrine (LC-NE) neurons, yet discussion of PD and treatment focus has remained dopaminergic-based. Motor symptoms benefit from DA replacement for many years, but other symptoms including several cognitive deficits continue unabated. Recent interest in non-DA substrates of PD highlights early involvement of LC-NE neurons and provides evidence for a prodromal phase, with cognitive disturbance, even in sporadic PD. We outline insights from basic research in LC-NE function to clinical and pathological evidence highlighting a role for NE in PD cognitive dysfunction. We propose that loss of LC-NE regulation, particularly in higher cortical regions, critically underlies certain cognitive dysfunctions in early PD. As a major unmet need for patients, research and use of NE drugs in PD may provide significant benefits for cognitive processing.

The effects of PRX-07034, a novel 5-HT6 antagonist, on cognitive flexibility and working memory in rats

RATIONALE

Accumulating evidence indicates that schizophrenia and autism spectrum disorder patients are marked by cognitive deficits in working memory and strategy switching. There is accumulating evidence that 5-hydroxytryptamine (5-HT)(6) receptors may serve as a useful target to improve cognitive functioning.

OBJECTIVES

In the present experiments, the novel 5-HT(6) antagonist, PRX-07034, was examined for its selectivity of the 5-HT(6) receptor, as well as its effect on delayed spontaneous alternation and strategy switching.

METHODS

The binding affinity of PRX-07034 to the 5-HT(6) receptor, other 5-HT receptors, as well as other G-protein coupled receptors, ion channels, and transporters was evaluated. Cyclic AMP production was measured from transfected HEK-293 cells. In separate behavioral experiments, rats received different doses of PRX-07034 (0.1, 1, or 3 mg/kg, i.p.) 30 min prior to delayed spontaneous alternation testing or prior to the acquisition and switch phases in a place-response switch test.

RESULTS

The results indicated that PRX-07034 is both a potent (Ki = 4-8 nM) and highly selective 5-HT(6) receptor antagonist (≥100-fold selectivity for the 5-HT(6) receptor compared to 68 other GPCRs, ion channels, and transporters, except D(3) (Ki = 71 nM) and 5-HT(1B) (Ki = 260 nM) receptors. For cyclic AMP quantification, PRX-07034 demonstrated antagonist activity (IC(50) = 19 nM) without an effect on basal levels and did not show any agonist activity up to 10 μM. PRX-07034 at 1 and 3 mg/kg (but not 0.1 mg/kg) significantly enhanced delayed spontaneous alternation. The drug at 1 and 3 mg/kg also enhanced switching between a place and response strategy, but did not affect initial learning of either a place or response discrimination.

CONCLUSIONS

These findings demonstrate that PRX-07034 is a selective 5-HT(6) receptor antagonist that may represent a novel treatment for enhancing working memory and cognitive flexibility.

Set shifting in a rodent model of attention-deficit/hyperactivity disorder

Two experiments compared spontaneously hypertensive rats (SHRs; a rodent model of attention-deficit/hyperactivity disorder) and Wistar rats (a normoactive control strain), on the acquisition of a set-shifting strategy. In Experiment 1, SHRs and Wistar rats were equivalent in trials to criterion to learn a brightness or a texture discrimination but SHRs were faster than Wistar rats in shifting to the opposite discrimination when there was 1 or 2 days between the initial discrimination and the shift. In Experiment 2, SHRs and Wistar rats were equivalent in shifting when the shift between discriminations occurred immediately after a criterion had been met in the first discrimination. The results are discussed in terms of a failure of SHRs to store or retrieve an initial discrimination and/or latent inhibition over a delay, leading to faster acquisition of a set-shift. This failure in storage or retrieval may be the result of a hypoactive dopamine system in the prefrontal cortex and nucleus accumbens shell as well as abnormalities in entorhinal cortex in SHRs.

Blockade of NMDA GluN2B receptors selectively impairs behavioral flexibility but not initial discrimination learning

RATIONALE

Behavioral flexibility is the ability to adjust behavior when environmental contingencies change and is compromised in disease states such as schizophrenia, attention deficit hyperactivity disorder, and following damage to the prefrontal cortex.

METHODS

The present study investigated the contribution of N-methyl-D-aspartate GluN2B receptor subunits in the initial learning and in shifting between or within discrimination strategies (i.e., strategy set-shifting and reversal learning), using tasks conducted in operant chambers. Strategy set-shifting required rats initially to learn a visual-cue discrimination (day 1) and on day 2, shift to using an egocentric spatial response strategy to obtain reward. For reversal learning, rats were trained on a response discrimination on day 1 and then required to select the opposite lever on day 2.

RESULTS

Blockade of GluN2B receptors with systemic administration of Ro25-6981 on day 1 did not impair initial acquisition of either a response or visual-cue discrimination nor did these treatments affect performance of strategy or reversal shifts on day 2. However, administration of Ro25-6981 prior to a set-shift or reversal on day 2 significantly impaired performance on both tasks, inducing a selective increase in perseverative errors, indicative of a disruption the ability to suppress a previously acquired prepotent response.

CONCLUSIONS

These data suggest that systemic blockade of GluN2B receptors Ro25-6981 does not appear to interfere with the initial acquisition of simple visual or response discriminations. However, these receptors do appear to play a central and selective role in the modification of previously acquired strategies or stimulus-reward associations, facilitating behavioral inhibition so that novel patterns of behavior may emerge.

Effects of sensitization on the detection of an instrumental contingency

While prior exposure to drugs of abuse permanently changes many behaviors, the underlying psychological mechanisms are relatively obscure. Here, the effects of sensitization on the detection of an action-outcome relationship were assessed, using a particularly stringent contingency degradation procedure. Rats were trained to leverpress until the probability of reinforcement for a response on one lever, or alternative reinforcement for a response on a second lever was reduced to 0.05 per second. Sensitization was then carried out (1mg/kg d-amphetamine/day for 7 days). Then, one reinforcer was also made available for a lack of response on either lever (p=0.05/s), maintaining its contiguity with the original response but eliminating its contingent relationship. Sensitized animals were more active, particularly early in the contingency degradation phase, but reduced responding directed at the degraded action-outcome contingency at a similar rate as controls. However, controls also reduced responding directed at the nondegraded contingency until very late in training, while sensitized animals maintained nondegraded responding at baseline levels. It was suggested that the relatively specific response shown by sensitized animals may reflect either improved action-outcome utilization or discrimination of relevant task features.

Dissociable contributions by prefrontal D1 and D2 receptors to risk-based decision making

Choices between certain and uncertain rewards of different magnitudes have been proposed to be mediated by both the frontal lobes and the mesocorticolimbic dopamine (DA) system. In rats, systemic manipulations of DA activity or inactivation of the medial prefrontal cortex (PFC) disrupt decision making about risks and rewards. However, it is unclear how PFC DA transmission contributes to these processes. We addressed this issue by examining the effects of pharmacological manipulations of D(1) and D(2) receptors in the medial (prelimbic) PFC on choice between small, certain and large, yet probabilistic rewards. Rats were trained on a probabilistic discounting task where one lever delivered one pellet with 100% probability, and the other delivered four pellets, but the probability of receiving reward decreased across blocks of trials (100, 50, 25, 12.5%). D(1) blockade (SCH23390) in the medial PFC decreased preference for the large/risky option. In contrast, D(2) blockade (eticlopride) reduced probabilistic discounting and increased risky choice. The D(1) agonist SKF81297 caused a slight, nonsignificant increase in preference for the large/risky lever. However, D(2) receptor stimulation (quinpirole) induced a true impairment in decision making, flattening the discounting curve and biasing choice away from or toward the risky option when it was more or less advantageous, respectively. These findings suggest that PFC D(1) and D(2) receptors make dissociable, yet complementary, contributions to risk/reward judgments. By striking a fine balance between D(1)/D(2) receptor activity, DA may help refine these judgments, promoting either exploitation of current favorable circumstances or exploration of more profitable ones when conditions change.

Prenatal exposure to a viral mimetic alters behavioural flexibility in male, but not female, rats

Current understanding of the etiology of neurodevelopmental disorders is limited; however, recent epidemiological studies demonstrate a strong correlation between prenatal infection during pregnancy and the development of schizophrenia in adult offspring. In particular, schizophrenia patients subjected to prenatal infection exhibit impairments in executive functions greater than schizophrenia patients not exposed to an infection while in utero. Acute prenatal treatment of rodents with the viral mimetic polyinosinic-polycytidylic acid (PolyI:C) induces behavioural and neuropathological alterations in the adult offspring similar to schizophrenia. However, impairments on tasks of executive function that involve the prefrontal cortex (PFC) have been rarely examined for the prenatal infection model. Hence, we investigated the effects of acute prenatal injection of PolyI:C (4.0 mg/kg, i.v., gestational day 15) on strategy set-shifting and reversal learning in an operant-based task. Our results show male, but not female, PolyI:C-treated adult offspring require more trials to reach criterion and perseverate during set-shifting. An opposite pattern was seen on the reversal day where the PolyI:C-treated male rats made fewer regressive errors. Females took more pre-training days and were slower to respond during the trials when compared to males regardless of prenatal treatment. The present findings validate the utility of the prenatal infection model for examining alterations of executive function, one of the most prominent cognitive symptoms of schizophrenia.

Pharmacological enhancement of memory and executive functioning in laboratory animals

Investigating how different pharmacological compounds may enhance learning, memory, and higher-order cognitive functions in laboratory animals is the first critical step toward the development of cognitive enhancers that may be used to ameliorate impairments in these functions in patients suffering from neuropsychiatric disorders. Rather than focus on one aspect of cognition, or class of drug, in this review we provide a broad overview of how distinct classes of pharmacological compounds may enhance different types of memory and executive functioning, particularly those mediated by the prefrontal cortex. These include recognition memory, attention, working memory, and different components of behavioral flexibility. A key emphasis is placed on comparing and contrasting the effects of certain drugs on different cognitive and mnemonic functions, highlighting methodological issues associated with this type of research, tasks used to investigate these functions, and avenues for future research. Viewed collectively, studies of the neuropharmacological basis of cognition in rodents and non-human primates have identified targets that will hopefully open new avenues for the treatment of cognitive disabilities in persons affected by mental disorders.

Dopamine-mediated MK-801-induced elevation in food-based extinction responding in rats and associated changes in region-specific phosphorylated ERK

RATIONALE

The current study examined the effect of the noncompetitive N-methyl-D: -aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) on the extinction of food-based operant responding.

OBJECTIVES

Experiments were carried out to determine (1) the nature of the elevated responding when rats were under the influence of MK-801, (2) the effect of combined and separate D1- and D2-like dopaminergic antagonists (SCH 23390 and haloperidol) on this MK-801-induced behavioral effect and (3) the neural correlates on and off MK-801 using immunohistochemical localization of the phosphorylated forms of the extracellular signal-regulated kinase-1 and -2.

METHODS

Male Long Evans rats were trained using operant conditioning procedures then treated with the various compounds, and resultant extinction pressing was measured.

RESULTS

A moderate dose (0.1 mg/kg) of MK-801 produced a persistent, elevated lever pressing throughout the entire 30-min extinction session. When animals were pretreated with combined or separate dopamine (DA) D1- or D2-like receptor antagonists, extinction responding under MK-801 was significantly reduced. Examination of pERK1/2 labeling in MK-801-treated animals showed reduced staining in the infralimbic and piriform cortices and elevated staining in the nucleus accumbens compared with controls.

CONCLUSIONS

These data show that MK-801 elevates food-based extinction behavior that can be reduced by DA receptor antagonists. The disrupted extinction behavior may be mediated, in part, by disinhibition of corticostriatal circuits.

Beneficial effects of desipramine on cognitive function of chronically stressed rats are mediated by alpha1-adrenergic receptors in medial prefrontal cortex

Chronic stress is a risk factor for many psychopathological conditions, including depression and anxiety disorders. Cognitive impairments associated with prefrontal cortical dysfunction are a major component of such illnesses. Using an attentional set-shifting test (AST), we have previously shown that elevating noradrenergic activity in rat medial prefrontal cortex (mPFC) can facilitate cognitive set-shifting, and that chronic unpredictable stress (CUS) caused set-shifting deficits. It is not known, however, if noradrenergic modulatory function is compromised by chronic stress, perhaps contributing to the stress-induced cognitive deficit. Thus, the first study investigated whether acutely elevating noradrenergic activity in mPFC still enhances cognitive function after chronic stress. As previously demonstrated, CUS impaired cognitive set-shifting on the AST. This deficit was abolished by acute systemic administration of the alpha(2)-adrenergic autoreceptor antagonist, atipamezole. Microdialysis revealed no differences in extracellular norepinephrine (NE) levels in mPFC of CUS-exposed and unstressed control rats at baseline or during behavioral testing, and comparable increases after atipamezole. In the second experiment, rats were treated chronically with the selective NE reuptake blocker, desipramine, during the CUS treatment through behavioral testing. Again, CUS impaired cognitive set-shifting in vehicle-treated rats, and chronic desipramine treatment prevented such deficits. Acute blockade of post-synaptic alpha(1)-adrenergic receptors in mPFC prior to testing blocked the beneficial effect of desipramine on cognitive set-shifting. These results suggest that desipramine restores cognitive set-shifting capability that has been compromised by chronic stress by activating alpha(1)-adrenergic receptors in the mPFC. Thus, noradrenergic modulatory capability in mPFC remains intact after CUS, and this represents one possible substrate by which antidepressants may exert their beneficial effects in the treatment of depression.

Catecholaminergic depletion within the prelimbic medial prefrontal cortex enhances latent inhibition

Latent inhibition (LI) refers to the reduction in conditioning to a stimulus that has received repeated non-reinforced pre-exposure. Investigations into the neural substrates of LI have focused on the nucleus accumbens (NAc) and its inputs from the hippocampal formation and adjacent cortical areas. Previous work has suggested that lesions to the medial prefrontal cortex (mPFC), another major source of input to the NAc, do not disrupt LI. However, a failure to observe disrupted LI does not preclude the possibility that a particular brain region is involved in the expression of LI. Moreover, the mPFC is a heterogeneous structure and there has been no investigation of a possible role of different regions within the mPFC in regulating LI under conditions that prevent LI in controls. Here, we tested whether 6-hydroxydopamine (6-OHDA)-induced lesions of dopamine (DA) terminals within the prelimbic (PL) and infralimbic (IL) mPFC would lead to the emergence of LI under conditions that do produce LI in controls (weak pre-exposure). LI was measured in a thirst motivated conditioned emotional response procedure with 10 pre-exposures to a noise conditioned stimulus (CS) and two conditioning trials. Sham-operated and IL-lesioned animals did not show LI and conditioned to the pre-exposed CS at comparable levels to the non-pre-exposed controls. 6-OHDA lesions to the PL, however, produced potentiation of LI. These results provide the first demonstration that the PL mPFC is a component of the neural circuitry underpinning LI.

Ventral striatal dopamine modulation of different forms of behavioral flexibility

Different forms of behavioral flexibility are facilitated by interactions between separate regions of the prefrontal cortex and their striatal outputs. However, the contribution of ventral striatal dopamine (DA) to these functions is unclear. The present study assessed the involvement of DA receptors in the nucleus accumbens (NAc) core on either between- or within-strategy shifts using operant chamber-based tasks. Strategy set-shifting required rats initially to learn a visual-cue discrimination and, on the following day, shift to using an egocentric spatial response strategy to obtain reward. For reversal learning, rats were initially trained on a response discrimination and then required to select the opposite lever to receive food reward. Intra-NAc microinfusions of D(1) (SCH23390) but not D(2) (eticlopride) receptor antagonists impaired set-shifting, disrupting the maintenance of a new strategy. Conversely, supranormal activation of D(2) (quinpirole) but not D(1) (SKF81297) receptors also impaired set-shifting, inducing perseverative deficits. However, only infusions of the D(2) agonist impaired reversal learning, but did so without disrupting initial response learning. Thus, mesoaccumbens DA, acting on D(1) receptors, selectively facilitates complex forms of flexibility requiring shifts between different strategies, but does not appear to contribute to simpler forms of flexibility entailing shifts of specific stimulus-reward associations. In contrast, abnormal increases in D(2) receptor activity cause a more general impairment in behavioral flexibility. These findings suggest that deficits in these forms of executive functioning observed in disorders linked to dysfunction of the DA system may be attributable in part to aberrant increases or decreases in mesoaccumbens DA activity.

Dopamine D2/D3 receptor agonist quinpirole impairs spatial reversal learning in rats: investigation of D3 receptor involvement in persistent behavior

RATIONALE

Dopamine is strongly implicated in the ability to shift behavior in response to changing stimulus-reward contingencies.

OBJECTIVES

We investigated the effects of systemic administration of the D2/D3 receptor agonist quinpirole (0.1, 0.3 mg/kg), the D2/D3 receptor antagonist raclopride (0.1, 0.3 mg/kg), the selective D3 antagonist nafadotride (0.3, 1.0 mg/kg), and combined administration of raclopride (0.1 mg/kg) or nafadotride (1.0 mg/kg) with quinpirole (0.3 mg/kg) on spatial discrimination and reversal learning.

MATERIALS AND METHODS

Rats were trained on an instrumental two-lever spatial discrimination and reversal learning task. Both levers were presented, only one of which was reinforced. The rat was required to respond on the reinforced lever under a fixed ratio 3 schedule of reinforcement. Following attainment of criterion, a reversal was introduced.

RESULTS

None of the drugs altered performance during retention of the previously reinforced contingencies. Quinpirole (0.3 mg/kg) significantly impaired reversal learning by increasing both trials and incorrect responses to criterion in reversal phase, a pattern of behavior manifested as increased perseverative responding on the previously reinforced lever. In contrast, neither raclopride nor nafadotride when administered alone altered reversal performance. However, raclopride blocked the quinpirole-induced reversal deficit, whereas combined administration of nafadotride and quinpirole affected not only performance during the reversal but also the retention phase. The reversal impairment resulting from co-administration of nafadotride and quinpirole was associated with both perseverative and learning errors.

CONCLUSIONS

Our data indicate distinct roles for D2 and D3 receptors in the capacity to modify behavior flexibly in the face of environmental change.

Inhibition of the norepinephrine transporter improves behavioral flexibility in rats and monkeys

RATIONALE

Poor cognitive control, including reversal learning deficits, has been reported in children with attention deficit hyperactivity disorder, in stimulant-dependent humans, and in animal models of these disorders; these conditions have each been associated with abnormal catecholaminergic function within the prefrontal cortex.

OBJECTIVES

In the current studies, we sought to explore how elevations in extracellular catecholamine levels, produced by pharmacological inhibition of catecholamine reuptake proteins, affect behavioral flexibility in rats and monkeys.

MATERIALS AND METHODS

Adult male Long-Evans rats and vervet monkeys were trained, respectively, on a four-position discrimination task or a three-choice visual discrimination task. Following systemic administration of pharmacological inhibitors of the dopamine and/or norepinephrine membrane transporters, rats and monkeys were exposed to retention or reversal of acquired discriminations.

RESULTS

In accordance with our a priori hypothesis, we found that drugs that inhibit norepinephrine transporters, such as methylphenidate, atomoxetine, and desipramine, improved reversal performance in rats and monkeys; this was mainly due to a decrease in the number of perseverative errors. Interestingly, the mixed dopamine and norepinephrine transporters inhibitor methylphenidate, if anything, impaired performance during retention in both rats and monkeys, while administration of the selective dopamine transporter inhibitor GBR-12909 increased premature responses but did not alter reversal learning performance.

CONCLUSIONS

Our results suggest that pharmacological inhibition of the membrane norepinephrine, but not membrane dopamine, transporter is associated with enhanced behavioral flexibility. These data, combined with earlier reports, may indicate that enhanced extracellular catecholamine levels in cortical regions, secondary to norepinephrine reuptake inhibition, improves multiple aspects of inhibitory control over responding in rats and monkeys.

Chronic intermittent cold stress and serotonin depletion induce deficits of reversal learning in an attentional set-shifting test in rats

RATIONALE

Chronic stress perturbs modulatory brain neurotransmitter systems, including serotonin (5-HT), and is a risk factor for psychiatric disorders such as depression. Deficits in cognitive flexibility, reflecting prefrontal cortical dysfunction, are prominent in such disorders. Orbitofrontal cortex (OFC) has been implicated specifically in reversal learning, a form of cognitive flexibility modulated by 5-HT.

OBJECTIVES

The objectives of the study were (1) to assess the effects of chronic intermittent cold (CIC) stress, a potent metabolic stressor, on performance of rats in an attentional set-shifting test (AST), and (2) to assess a possible role for serotonin in CIC-induced deficits and test the effects of acute serotonin reuptake blockade.

MATERIALS AND METHODS

Male Sprague-Dawley rats were exposed to CIC stress (14 days x 6 h/day at 4 degrees C) before testing on the AST. In subsequent experiments, brain 5-HT was depleted in naïve rats with para-chlorophenylalanine or 5-HT release was increased acutely in CIC-stressed rats with citalopram (5 mg/kg, s.c.) given 30 min prior to the first reversal task. Microdialysis was used to assess CIC-induced changes in 5-HT release in OFC during testing.

RESULTS

CIC-stressed rats exhibited a selective impairment on the first reversal task in the AST. 5-HT depletion induced a similarly selective deficit in reversal learning. The CIC-induced impairment in reversal learning was attenuated by acute 5-HT reuptake blockade. 5-HT release was reduced in OFC of CIC-stressed rats during behavioral testing.

CONCLUSIONS

The CIC stress-induced impairment of cognitive flexibility may involve dysregulation of 5-HT modulatory function in OFC. Such deficits may thus model relevant symptoms of neuropsychiatric disorders that respond positively to SSRI treatment.

Guidance of instrumental behavior under reversal conditions requires dopamine D1 and D2 receptor activation in the orbitofrontal cortex

The orbitofrontal cortex (OFC) plays a critical role in learning a reversal of stimulus-reward contingencies. Dopamine (DA) neurons probably support reversal learning by emitting prediction error signals that indicate the discrepancy between the actually received reward and its prediction. However, the role of DA receptor-mediated signaling in the OFC to adapt behavior to changing stimulus-reward contingencies is largely unknown. Here we examined the effects of a selective D1 or D2 receptor blockade in the OFC on learning a reversal of previously acquired stimulus-reward magnitude contingencies. Rats were trained on a reaction time (RT) task demanding conditioned lever release with discriminative visual stimuli signaling in advance the upcoming reward magnitude (one or five food pellets). After acquisition, RTs were guided by stimulus-associated reward magnitudes, i.e. RTs of responses were significantly shorter for expected high versus low reward. Thereafter, stimulus-reward magnitude contingencies were reversed and learning was tested under reversal conditions for three blocks after pre-trial infusions of the selective D1 or D2 receptor antagonists R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepinhydrochloride (SCH23390), eticlopride, or vehicle. For comparisons, we included intra-OFC infusions of the selective N-methyl-D-aspartate receptor antagonist AP5. Results revealed that in animals subjected to intra-OFC infusions of SCH23390 or eticlopride learning a reversal of previously acquired stimulus reward-magnitude contingencies was impaired. Thus, in a visual discrimination task as used here, D1 and D2 receptor-mediated signaling in the OFC seems to be necessary to update the reward-predictive significance of stimuli.

Oculomotor and neuropsychological effects of antipsychotic treatment for schizophrenia

Cognitive enhancement has become an important target for drug therapies in schizophrenia. Treatment development in this area requires assessment approaches that are sensitive to procognitive effects of antipsychotic and adjunctive treatments. Ideally, new treatments will have translational characteristics for parallel human and animal research. Previous studies of antipsychotic effects on cognition have relied primarily on paper-and-pencil neuropsychological testing. No study has directly compared neurophysiological biomarkers and neuropsychological testing as strategies for assessing cognitive effects of antipsychotic treatment early in the course of schizophrenia. Antipsychotic-naive patients with schizophrenia were tested before treatment with risperidone and again 6 weeks later. Matched healthy participants were tested over a similar time period. Test-retest reliability, effect sizes of within-subject change, and multivariate/univariate analysis of variance were used to compare 3 neurophysiological tests (visually guided saccade, memory-guided saccade, and antisaccade) with neuropsychological tests covering 4 cognitive domains (executive function, attention, memory, and manual motor function). While both measurement approaches showed robust neurocognitive impairments in patients prior to risperidone treatment, oculomotor biomarkers were more sensitive to treatment-related effects on neurocognitive function than traditional neuropsychological measures. Further, unlike the pattern of modest generalized cognitive improvement suggested by neuropsychological measures, the oculomotor findings revealed a mixed pattern of beneficial and adverse treatment-related effects. These findings warrant further investigation regarding the utility of neurophysiological biomarkers for assessing cognitive outcomes of antipsychotic treatment in clinical trials and in early-phase drug development.

Cannabinoid modulation of executive functions

Executive functions are higher-order cognitive processes such as attention, behavioural flexibility, decision-making, inhibitory control, planning, time estimation and working memory that exert top-down control over behaviour. In addition to the role of cannabinoid signaling in other cognitive functions such as mnemonic processes, interest in its involvement in executive functions has arisen more recently. Here, we will briefly review some of the recent findings indicating a modulatory role of cannabinoid action on executive functioning. In addition, a growing body of evidence suggests that in particular adolescents are more vulnerable for the deleterious effects of drugs of abuse such as cannabis on cognitive functioning. Therefore, in this paper we will also briefly discuss some recent developments in this research field.

Inactivation of the medial prefrontal cortex of the rat impairs strategy set-shifting, but not reversal learning, using a novel, automated procedure

The medial prefrontal cortex (mPFC) of the rat plays an essential role in behavioral flexibility, as lesions or inactivations of this region impair shifting between strategies or attentional sets using a variety of different behavioral tests. In the present study, we assessed the effects of inactivation of the mPFC on strategy set-shifting and reversal learning, using a novel, automated procedure conducted in an operant chamber. In Experiment 1, inactivation of the mPFC with bupivacaine did not impair the initial learning of a visual-cue (i.e.; always press the lever with a cue light illuminated above it) or a response (i.e.; always press the left lever) discrimination. Control rats required greater number of trials to shift from using a visual-cue to a response strategy than the opposite shift. mPFC inactivation impaired performance of a visual-cue-response set-shift, but not the easier response-visual-cue shift. In Experiment 2, pre-exposure to the visual-cue stimulus lights increased the difficulty of the response-visual-cue shift, reflected by a greater number of trials required by control rats to achieve criterion relative to those in Experiment 1. Under these conditions, inactivation of the mPFC did impair performance of this set-shift. In contrast, mPFC inactivation did not affect reversal learning of a response discrimination. These findings highlight the utility of this automated procedure for assessing set-shifting mediated by the mPFC. Furthermore, they reveal that the relative difficulty of the type of shift rats are required to perform has a direct impact on whether or not the mPFC contributes to this form of behavioral flexibility.

A sensitizing regimen of amphetamine that disrupts attentional set-shifting does not disrupt working or long-term memory

Exposure to an intermittent, escalating dose of amphetamine induces a sensitized state that, both behaviourally and neurochemically, mirrors several features linked to the positive symptoms of schizophrenia. Increasingly it is being realized that cognitive deficits are a core component of schizophrenia; therefore we sought to assess the effects of inducing an amphetamine-sensitized state on memory (working and long-term) and cognitive flexibility, two cognitive domains impaired in schizophrenia. Rats were exposed to a sensitizing regimen of amphetamine (1-5 mg/kg; three times per week for 5 weeks; escalating at 1mg/kg per week) or saline. In experiment 1, animals were tested on an operant delayed non-match to position task (working memory). Experiment 2 used a standard fixed-platform location water maze task (long-term memory), while experiment 3 used a variable-platform location water maze task (long-term memory and working memory). Amphetamine-sensitized animals were not impaired on any of these tasks. In experiment 4, animals were assessed on a strategy selection task in which they were first required to learn to locate a food reward using a particular learning strategy (place or response) then to learn to shift to an alternate learning strategy (response or place). Amphetamine-sensitized animals were not impaired on this task. In the final experiment animals were found to be impaired in performance of the extra-dimensional shift component of an attentional set-shifting task. These results suggest that while amphetamine sensitization does not produce memory impairments similar to those seen in schizophrenia, it does produce strong impairments in set-shifting, suggesting changes in prefrontal function similar to those seen in schizophrenia.

Dopamine and corticotropin-releasing factor synergistically alter basolateral amygdala-to-medial prefrontal cortex synaptic transmission: functional switch after chronic cocaine administration

Basolateral amygdala (BLA) neurons provide a major excitatory input to medial prefrontal cortex (mPFC)-layer V pyramidal neurons. Under stressful conditions, commonly associated with chronic cocaine abuse, altered BLA-to-mPFC synaptic transmission could lead to defective emotional information processing and decision making within the mPFC and result in misguided and inappropriate behaviors. We examined the effects of cocaine administered chronically in vivo on EPSCs recorded from a putative BLA-mPFC pathway in vitro and their modulation by dopamine (DA), corticotropin-releasing factor (CRF), and their combination (DA plus CRF). In saline-treated animals, activation of D(1/5) receptors depressed BLA-mPFC EPSCs, whereas CRF1 receptor activation alone had no effect on EPSCs. Activating D(1/5) and CRF1 receptors in combination, however, worked synergistically through presynaptic and postsynaptic mechanisms to depress EPSCs to levels greater than D(1/5) receptor activation alone. After chronic cocaine administration, the function of DA(1/5) and CRF receptors switched from inhibitory to excitatory. In slices from cocaine-treated animals, putative BLA-mPFC EPSCs were depressed through a presynaptic mechanism. Now, activation of either D(1/5) or CRF2 receptors increased the cocaine-induced, depressed EPSCs. Additionally, simultaneous activation of presynaptic D(1/5) and CRF2 receptors led to further enhancement of EPSCs. These data indicate that CRF acting synergistically with DA normally potentiates D(1/5)-induced synaptic depression. However, after chronic cocaine, the combined synergistic actions of DA and CRF switched polarity to enhance facilitation of BLA-mPFC glutamatergic transmission. Also unmasked after acute withdrawal from chronic cocaine are endogenous, tonic-inhibitory D2-like and tonic-facilitatory CRF2 receptor actions. These multiple functional and receptor changes may underlie the altered, possibly aberrant, decision-making process after chronic cocaine.

Procedural learning and cognitive flexibility in a mouse model of restricted, repetitive behaviour

Restricted, repetitive behaviours (e.g., stereotypies, compulsions, rituals) in neurodevelopmental disorders have been linked to alterations in cortico-basal ganglia circuitry. Cognitive processes mediated by this circuitry (e.g., procedural learning, executive function) are likely to be impaired in individuals exhibiting high rates of repetitive behaviour. To test this hypothesis, we assessed both procedural learning and cognitive flexibility (reversal learning) using a T-maze task in deer mice (Peromyscus maniculatus) exhibiting various rates of repetitive behaviour (vertical jumping and backward somersaulting). These mice exhibited high rates of stereotypy when reared in standard rodent cages, and such behaviour was significantly attenuated by housing them in larger more complex environments. Mice reared in complex environments exhibited significantly better procedural and reversal learning than standard caged mice. Thus, early experience associated with the prevention and attenuation of stereotypy was associated with better striatally mediated learning and cognitive flexibility. Stereotypy score was significantly correlated with the number of errors made in reversal learning, and interacted with housing condition to affect overall cognitive performance. Our findings support the applicability of the deer mouse model of spontaneous stereotypy to a wider range of restricted, repetitive behaviour (e.g., insistence on sameness) typical of neurodevelopmental disorders.

Chronic unpredictable stress induces a cognitive deficit and anxiety-like behavior in rats that is prevented by chronic antidepressant drug treatment

Chronic stress is a risk factor for the development of many psychopathological conditions in humans, including major depression and anxiety disorders. There is a high degree of comorbidity of depression and anxiety. Moreover, cognitive impairments associated with frontal lobe dysfunction, including deficits in cognitive set-shifting and behavioral flexibility, are increasingly recognized as major components of depression, anxiety disorders, and other stress-related psychiatric illnesses. To begin to understand the neurobiological mechanisms underlying the cognitive and emotional consequences of chronic stress, it is necessary to employ an animal model that exhibits similar effects. In the present study, a rat model of chronic unpredictable stress (CUS) consistently induced a cognitive impairment in extradimensional set shifting capability in an attentional set shifting test, suggesting an alteration in function of the medial prefrontal cortex. CUS also increased anxiety-like behavior on the elevated plus-maze. Further, chronic treatment both with the selective norepinephrine reuptake blocker, desipramine (7.5 mg/kg/day), and the selective serotonin reuptake blocker, escitalopram (10 mg/kg/day), beginning 1 week before CUS treatment and continuing through the behavioral testing period, prevented the CUS-induced deficit in extradimensional set-shifting. Chronic desipramine treatment also prevented the CUS-induced increase in anxiety-like behavioral reactivity on the plus-maze, but escitalopram was less effective on this measure. Thus, CUS induced both cognitive and emotional disturbances that are similar to components of major depression and anxiety disorders. These effects were prevented by chronic treatment with antidepressant drugs, consistent also with clinical evidence that relapse of depressive episodes can be prevented by antidepressant drug treatment.

Differential effects of inactivation of the orbitofrontal cortex on strategy set-shifting and reversal learning

Different subregions of the rodent prefrontal cortex (PFC) mediate dissociable types of behavioral flexibility. For example, lesions of the medial or orbitofrontal (OFC) regions of the PFC impair extradimensional shifts and reversal learning, respectively, when novel stimuli are used during different phases of the task. In the present study, we assessed the effects of inactivation of the OFC on strategy set-shifting and reversal learning, using a maze based set-shifting task mediated by the medial PFC. Long-Evans rats were trained initially on a visual-cue discrimination to obtain food. On the subsequent day, rats had to shift to using a response strategy (e.g., always turn left). On Day 3 (reversal), rats were required to reverse the direction of their turn (e.g., always turn right). Infusions of the local anesthetic bupivacaine into the OFC did not impair initial visual discrimination learning, nor did it impair performance on the set-shift. In contrast, inactivation of the OFC did impair reversal learning; yet, these rats ceased using the previously acquired response rule as readily as controls. Instead, rats receiving OFC inactivations made a disproportionate number of erroneous arm entries towards the visual-cue, suggested that these animals reverted back to using the original visual-cue based strategy. These findings, in addition to previous data, further support the notion that the OFC and medial PFC play dissociable roles in reversal learning and set-shifting. Furthermore, the lack of effect of OFC inactivations on the set-shift indicates that this type of behavioral flexibility does not require cognitive operations related to reversal learning.

The amphetamine-induced sensitized state as a model of schizophrenia

Schizophrenia is a serious psychiatric disorder which impacts a broad range of cognitive, behavioural and emotional domains. In animals, exposure to an intermittent, escalating dose regimen of amphetamine induces a sensitized state that appears to share a number of behavioural and neurochemical similarities with schizophrenia. In humans repeated exposure to amphetamine, or other psychomotor stimulants, can induce sensitization as well as psychosis. The following paper evaluates the evidence for the amphetamine-induced sensitized state as an animal model of schizophrenia, focussing separately on the positive, cognitive and negative symptoms associated with this disease. Current evidence supports the use of amphetamine sensitization as a model of the positive symptoms observed in schizophrenia. Additionally, there is increasing evidence for long-lasting cognitive deficits in sensitized animals, especially in the area of attention and/or cognitive flexibility. Other areas of cognition, such as long-term memory, appear to be unaltered in sensitized animals. Finally, little evidence currently exists to either support or refute the use of amphetamine sensitization as a model of negative symptoms. It is concluded that amphetamine sensitization likely impacts behaviour by altering the functioning of mesolimbic dopamine systems and prefrontal cortical function and can serve as a model of certain domains of schizophrenia.

Dopamine D2/D3 receptors play a specific role in the reversal of a learned visual discrimination in monkeys

Converging evidence supports a role for mesocorticolimbic dopaminergic systems in a subject's ability to shift behavior in response to changing stimulus-reward contingencies. To characterize the dopaminergic mechanisms involved in this function, we quantified the effects of subtype-specific dopamine (DA) receptor antagonists on acquisition, retention, and reversal of a visual discrimination task in non-human primates (Chlorocebus aethiops sabaeus). We used a modified Wisconsin General Test Apparatus that was equipped with three food boxes, each fitted with a lid bearing a unique visual cue; one of the cues concealed a food reward, whereas the other two concealed an empty box. The monkeys were trained first to acquire a novel discrimination (eg A(+), B(-), C(-)) in a single session, before experiencing either a reversal of the discrimination (eg A(-), B(+), C(-)) or the acquisition of a completely new discrimination (eg D(+), E(-), F(-)), on the following day. Systemic administration of the D(2)/D(3) receptor antagonist raclopride (0.001-0.03 mg/kg) failed to significantly affect the performance of reversal learning when reversal sessions were run without a retention session. But, raclopride (0.03 mg/kg) significantly impaired performance under the reversal condition when reversal sessions were run right after a retention session; however, it did not affect acquisition of a novel visual discrimination. Specifically, raclopride significantly increased the number of reversal errors made before reaching the performance criterion in the reversal, but not in new learning sessions. In contrast, the D(1)/D(5) receptor antagonist SCH 23390 did not significantly modulate acquisition of a novel discrimination or reversal learning at doses (0.001-0.03 mg/kg, i.m.) that did not suppress behavior generally. In addition, none of the drug treatments affected retention of a previously learned discrimination. The results strongly suggest that D(2)/D(3) receptors, but not D(1)/D(5) receptors, selectively mediate reversal learning, without affecting the capacity to learn a new stimulus-reward association. These data support the hypothesis that phasic DA release, acting through D(2)-like receptors, mediates behavioral flexibility.

Differential Regulation of Fronto-Executive Function by the Monoamines and Acetylcholine

The prefrontal cortex (PFC) is innervated by the monoamines, dopamine (DA), noradrenaline (NA), and serotonin, as well as acetylcholine, and the marked influence of these neurochemical systems on prefrontal working memory processes has been widely described. However, their potentially, differential contribution to prefrontal functioning is less well understood. This paper reviews evidence to support the hypothesis that these neurochemical systems recruit distinct fronto-executive operations. Direct comparison of the effects of manipulations of these neuromodulators within PFC on performance of an attentional set-shifting paradigm reveals their differential contribution to distinct task stages. Depletion of prefrontal serotonin selectively disrupts reversal learning but not attentional set formation or set shifting. In contrast, depletion of prefrontal DA disrupts set formation but not reversal learning. NA depletion on the other hand specifically impairs set-shifting, whereas its effects on reversal learning remain unclear. Finally, depletion of prefrontal acetylcholine has no effect on either set formation or set shifting but impairs serial reversal learning. Because these neurochemical systems are known to represent distinct states of stress, arousal, attention, and affect, it is postulated that they augment the different types of executive operation that are recruited and performed within these states via a synergistic interaction with the PFC.

Lesions of the dorsal noradrenergic bundle impair attentional set-shifting in the rat

Rats with medial prefrontal cortex (mPFC) lesions are impaired in attentional set-shifting, when it is required to shift to a previously irrelevant perceptual dimension. The main source of noradrenergic input to the mPFC is from the locus coeruleus via the dorsal noradrenergic ascending bundle (DNAB). This study examined the effects of selective cortical noradrenaline depletion following 6-hydroxydopamine-induced lesions of the DNAB on attentional set-shifting and other aspects of discrimination learning and performance. Rats learned to dig in baited bowls, and then acquired discriminations based on one of two aspects of a bowl--odour or digging medium. The task tested acquisition of novel discriminations (both intra- and extra-dimensional) and reversal learning when contingencies were reversed with the same stimuli. At the conclusion of testing, the DNAB-lesioned rats were shown to have a selective depletion of noradrenaline of approximately 70% within the mPFC (cingulate and prelimbic cortex subregions), with no other significant changes in dopamine or 5-hydroxytryptamine. Rats required more trials to learn new discriminations when attentional shifting was required [extra-dimensional (ED)-shift]. Rats with dorsal noradrenergic ascending bundle (DNAB) lesions were impaired in novel acquisitions when an ED-shift was required, but were unimpaired in reversal learning and other aspects of discrimination learning, relative to controls. These data are consistent with other evidence implicating noradrenaline (NA) in attentional set-shifting, and contrast with effects of manipulations of 5-hydroxytryptamine (5-HT) and acetylcholine within the medial prefrontal cortex (mPFC). The findings are also relevant to recent theorizing about the functions of the coeruleo-cortical noradrenergic system.