The involvement of the nucleus accumbens in the ability of rats to switch to cue-directed behaviours

Nucleus accumbens core and pathogenesis of compulsive checking

To investigate the role of the nucleus accumbens core (NAc) in the development of quinpirole-induced compulsive checking, rats received an excitotoxic lesion of NAc or sham lesion and were injected with quinpirole (0.5 mg/kg) or saline; development of checking behavior was monitored for 10 biweekly tests. The results showed that even after the NAc lesion, quinpirole still induced compulsive checking, suggesting that the pathogenic effects produced by quinpirole lie outside the NAc. Although the NAc lesion did not prevent the induction of compulsive checking, it altered how quickly it develops, suggesting that the NAc normally contributes toward the induction of compulsive checking. Saline-treated rats with an NAc lesion were hyperactive, but did not develop compulsive checking, indicating that hyperactivity by itself is not sufficient for the pathogenesis of compulsive checking. It is proposed that compulsive checking is the exaggerated output of a security motivation system and that the NAc serves as a neural hub for coordinating the orderly activity of neural modules of this motivational system. Evidence is considered suggesting that the neurobiological condition for the pathogenesis of compulsive checking is two-fold: activation of dopamine D2/D3 receptors without concurrent stimulation of D1-like receptors and long-term plastic changes related to quinpirole-induced sensitization.

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.

Abnormally rapid reversal learning and reduced response to antipsychotic drugs following ovariectomy in female rats

Epidemiological and clinical life cycle studies indicate that favorable illness course and better response to antipsychotic drugs (APDs) in women with schizophrenia are positively correlated with estrogen levels. Accordingly, the estrogen hypothesis of schizophrenia proposes a neuroprotective role of estrogen in women vulnerable to schizophrenia. Previously we demonstrated in the rat that low levels of estrogen induced by ovariectomy led to disruption of latent inhibition (LI) reflecting impairment of selective attention, a core deficit of schizophrenia. LI disruption was reversed by 17β-estradiol and the atypical APD clozapine, whereas the typical APD haloperidol was ineffective unless co-administered with 17β-estradiol. Here we aimed to extend these findings by testing ovariectomized rats in another selective attention task, discrimination reversal. Ovariectomy led to a loss of selective attention as manifested in abnormally rapid reversal. The latter was normalized by high dose of 17β-estradiol (150 μg/kg) and clozapine (2.5mg/kg), but not by haloperidol (0.1mg/kg) or lower doses of 17β-estradiol (10 and 50 μg/kg). However, co-administration of haloperidol with 17β-estradiol (50 μg/kg) was effective. In sham rats low 17β-estradiol (10 μg/kg) produced rapid reversal, while high 17β-estradiol (150 μg/kg), haloperidol alone, or haloperidol-17β-estradiol combination reduced reversal speed. Clozapine did not affect reversal speed in sham rats. These results strengthen our previous results in suggesting that schizophrenia-like attentional abnormalities as well as reduced response to APDs in female rats are associated with low level of gonadal hormones. In addition, they support the possibility that estrogen may have an antipsychotic-like action in animal models.

The neuropsychology of schizophrenia

A model is proposed for integrating the neural and cognitive aspects of the positive symptoms of acute schizophrenia, using evidence from postmortem neuropathology and neurochemistry, clinical and preclinical studies of dopaminergic neurotransmission, anatomical connections between the limbic system and basal ganglia, attentional and other cognitive abnormalities underlying the positive symptoms of schizophrenia, specific animal models of some of these abnormalities, and previous attempts to model the cognitive functions of the septohippocampal system and the motor functions of the basal ganglia. Anatomically, the model emphasises the projections from the septohippocampal system, via the subiculum, and the amygdala to nucleus accumbens, and their interaction with the ascending dopaminergic projection to the accumbens. Psychologically, the model emphasises a failure in acute schizophrenia to integrate stored memories of past regularities of perceptual input with ongoing motor programs in the control of current perception. A number of recent experiments that offer support for the model are briefly described, including anatomical studies of limbic-striatal connections, studies in the rat of the effects of damage to these connections, and of the effects of amphetamine and neuroleptics, on the partial reinforcement extinction effect, latent inhibition and the Kamin blocking effect; and studies of the latter two phenomena in acute and chronic schizophrenics.

Dissociable effects of selective 5-HT2A and 5-HT2C receptor antagonists on serial spatial reversal learning in rats

Serotonin (5-hydroxytryptamine, or 5-HT) is strongly implicated in the ability to shift behavior in response to changing stimulus-reward contingencies. However, there is little information on the contribution of different 5-HT receptors in reversal learning. Thus, we investigated the effects of systemic administration of the 5-HT(2A) antagonist M100907 (0, 0.01, 0.03, and 0.1 mg/kg, i.p.) and the 5-HT(2C) antagonist SB 242084 (0, 0.1, 0.3, and 1.0 mg/kg, i.p.) on the performance of an instrumental two-lever spatial discrimination and serial spatial reversal learning task, where both levers were presented and only one 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 series of within-session reversals was presented. Neither M100907 nor SB 242084 altered performance during spatial discrimination and retention of the previously reinforced contingencies. M100907 significantly impaired reversal learning by increasing both trials to criterion (only at the highest dose) and incorrect responses to criterion in Reversal 1, a pattern of behavior manifested as increased perseverative responding on the previously reinforced lever. In contrast, SB 242084 improved reversal learning by decreasing trials and incorrect responses to criterion in Reversal 1, with significantly fewer perseverative responses. These data support the view that 5-HT(2A) and 5-HT(2C) receptors have distinct roles in cognitive flexibility and response inhibition. The improved performance in reversal learning observed following 5-HT(2C) receptor antagonism suggests these receptors may offer the potential for therapeutic advances in a number of neuropsychiatric disorders where cognitive deficits are a feature, including obsessive-compulsive disorder.

Discrete neurochemical coding of distinguishable motivational processes: insights from nucleus accumbens control of feeding

BACKGROUND AND OBJECTIVES

The idea that nucleus accumbens (Acb) dopamine transmission contributes to the neural mediation of reward, at least in a general sense, has achieved wide acceptance. Nevertheless, debate remains over the precise nature of dopamine's role in reward and even over the nature of reward itself. In the present article, evidence is reviewed from studies of food intake, feeding microstructure, instrumental responding for food reinforcement, and dopamine efflux associated with feeding, which suggests that reward processing in the Acb is best understood as an interaction among distinct processes coded by discrete neurotransmitter systems.

RESULTS

In agreement with several theories of Acb dopamine function, it is proposed here that allocation of motor effort in seeking food or food-associated conditioned stimuli can be dissociated from computations relevant to the hedonic evaluation of food during the consummatory act. The former appears to depend upon Acb dopamine transmission and the latter upon striatal opioid peptide release. Moreover, dopamine transmission may play a role in 'stamping in' associations between motor acts and goal attainment and perhaps also neural representations corresponding to rewarding outcomes. Finally, evidence is reviewed that amino acid transmission specifically in the Acb shell acts as a central 'circuit breaker' to flexibly enable or terminate the consummatory act, via descending connections to hypothalamic feeding control systems.

CONCLUSIONS

The heuristic framework outlined above may help explain why dopamine-compromising manipulations that strongly diminish instrumental goal-seeking behaviors leave consummatory activity relatively unaffected.

Latent inhibition is disrupted by nucleus accumbens shell lesion but is abnormally persistent following entire nucleus accumbens lesion: The neural site controlling the expression and disruption of the stimulus preexposure effect

Latent inhibition (LI) is the proactive interference of repeated nonreinforced preexposure to a stimulus with subsequent performance on a learning task involving that stimulus. The present experiments investigated the role of the nucleus accumbens (NAC) in LI. LI was measured in a thirst motivated conditioned emotional response procedure with low or high number of conditioning trials, and in two-way active avoidance procedure with the stages of preexposure and conditioning taking place in the same or different contexts. Sham-lesioned rats showed LI with low but not high number of conditioning trials and if preexposure and conditioning took place in the same context but not if the context was changed between the stages. Lesion to the shell subregion of the NAC disrupted LI but LI was preserved in rats with a combined lesion to the NAC shell and core subregions. Moreover, rats with a combined shell-core lesion persisted in showing LI in spite of high number of conditioning trials and in spite of context change. These results show that the NAC is not essential for the acquisition of LI but rather plays a key role in regulating the expression of LI. Moreover, they suggest that the two subregions of the NAC contribute competitively and cooperatively to this process, selecting the response appropriate to the stimulus-no event or the stimulus-reinforcement association in conditioning.

Maternal immune activation leads to behavioral and pharmacological changes in the adult offspring

Maternal exposure to viral infection has been associated with an increased risk of schizophrenia in the offspring, and it has been suggested that the maternal immune response may interfere with normal fetal brain development. Although studies in rodents have shown that perinatal viral infections can lead to neuropathological and behavioral abnormalities considered relevant to schizophrenia, it is not clear whether these consequences are due to the infection itself or to the maternal immune response to infection. We show that an induction of maternal immune stimulation without exposure to a virus by injecting pregnant dams with the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I:C) leads to abnormal behavioral and pharmacological responses in the adult offspring. As in schizophrenia, these offspring displayed excessive behavioral switching, manifested in the loss of latent inhibition and in rapid reversal learning. Consistent with the clinical pharmacology of schizophrenia, both deficits were alleviated by antipsychotic treatment. In addition, these offspring displayed increased sensitivity to the locomotor-stimulating effects of MK-801, pointing to developmental alterations of the dopaminergic and/or glutamatergic systems. Prenatal poly I:C administration did not produce learning deficits in classical fear conditioning, active avoidance, discrimination learning and water maze. These results show that the maternal immune response is sufficient to cause behavioral and pharmacological alterations relevant to schizophrenia in the adult offspring.

Investigation of the effects of lamotrigine and clozapine in improving reversal-learning impairments induced by acute phencyclidine and D-amphetamine in the rat

RATIONALE

Phencyclidine (PCP), a glutamate/N-methyl-D-aspartate (NMDA) receptor antagonist, has been shown to induce a range of symptoms similar to those of patients with schizophrenia, while D-amphetamine induces predominantly positive symptoms. Previous studies in our laboratory have shown that PCP can selectively impair the performance of an operant reversal-learning task in the rat. Furthermore, we found that the novel antipsychotic ziprasidone, but not the classical antipsychotic haloperidol, could prevent the PCP-induced deficit.

OBJECTIVES

The aim of the present study was to validate the model further using the atypical antipsychotic clozapine and then to investigate the effects of lamotrigine, a broad-spectrum anticonvulsant that is known to reduce glutamate release in vitro and is able to prevent ketamine-induced psychotic symptoms in healthy human volunteers. A further aim was to compare effects of PCP and D-amphetamine in the test and investigate the effects of the typical antipsychotic haloperidol against the latter.

METHODS

Female hooded-Lister rats were food deprived and trained to respond for food in a reversal-learning paradigm.

RESULTS

PCP at 1.5 mg/kg and 2.0 mg/kg and D-amphetamine at 0.5 mg/kg significantly and selectively impaired performance in the reversal phase of the task. The cognitive deficit induced by 1.5 mg/kg PCP was attenuated by prior administration of lamotrigine (20 mg/kg and 30 mg/kg) or clozapine (5 mg/kg), but not haloperidol (0.05 mg/kg). In direct contrast, haloperidol (0.05 mg/kg), but not lamotrigine (25 mg/kg) or clozapine (5 mg/kg), prevented a similar cognitive impairment produced by D-amphetamine (0.5 mg/kg).

CONCLUSIONS

Our findings provide further data to support the use of PCP-induced disruption of reversal learning in rodents to investigate novel antipsychotic drugs. The results also provide evidence for different mechanisms of PCP and D-amphetamine-induced disruption of performance in the test, and their different sensitivities to typical and atypical antipsychotic drugs.

Contrasting effects of dopamine and glutamate receptor antagonist injection in the nucleus accumbens suggest a neural mechanism underlying cue-evoked goal-directed behavior

Discriminative stimuli (DSs) inform animals that reward can be obtained contingent on the performance of a specific behavior. Such stimuli reinstate drug-seeking behavior, evoke dopamine release in the nucleus accumbens (NAc) and excite and inhibit specific subpopulations of NAc neurons. Here we show in rats that DSs can reinstate food-seeking behavior. In addition, we compare the effects of injecting dopamine receptor antagonists into the NAc with those of general NAc inactivation on the performance of a DS task. Selective antagonism of D1 receptors reduced responding to the DS and increased the latency to respond, whereas general inactivation of NAc neuronal activity increased the latency to respond to the DS and increased behaviors extraneous to the task, such as responding in the absence of cues and responding on the inactive lever. Based on these results and our previous findings that NAc neuronal responses to DSs are dependent on the ventral tegmental area, we propose a model for the functional role of NAc neurons in controlling behavioral responses to reward-predictive stimuli.

On the relationship between anticipatory behaviour in a Pavlovian paradigm and Pavlovian-to-Instrumental Transfer in rats (Rattus norvegicus)

The present rat study assessed the relationship between, and the sensitivity of, two different tests for appetitive conditioned responding to differences in the contingency between a conditioned stimulus (CS) and an unconditioned stimulus (US), and to differences in US magnitude. The first test used a Pavlovian-to-Instrumental Transfer (PIT) paradigm, assessing the capacity of the CS to enhance instrumental responding for food. The second test employed a Pavlovian conditioning paradigm with an extended CS-US interval, and total number of behavioural elements in this interval as a dependent measure. The PIT test proved to be sensitive to contingency but not reward magnitude differences, whereas the reverse was true for the Pavlovian test. Although there was a significant correlation between tests in the magnitude of the CS-induced increase of food-magazine entries, the main dependent measure from PIT (number of lever presses) and that from the Pavlovian test (total number of behavioural elements) did not correlate. It is suggested that in the PIT procedure, the CS induces a chain of behavioural responses of which lever pressing is just a single element and that the Pavlovian test, in principle, is more sensitive.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Switching to cue-directed behavior: specific for ventral striatal dopamine but not ventral pallidum/substantia innominata gaba as revealed by a swimming-test procedure in rats

In this study it was investigated whether ventral striatal dopamine-induced changes in switching to cue-directed behavioral patterns were funnelled via the rostral areas of the ventral pallidum/substantia innominata (VP/SI) complex and, if so, whether changes in switching to cue-directed behavioral patterns could be elicited in the VP/SI complex by manipulating GABAergic activity. To this end rats were bilaterally equipped with cannulae directed at the ventral striatum and/or rostral VP/SI complex and subjected to a swimming-test procedure for 6 min. Injections of the dopamine-releasing agent d-amphetamine (10 microg/0.5 microl per side) enhanced the number of different cue-directed behavioral patterns while they had no effect upon the number of different non-cue-directed behavioral patterns in line with previous studies (Life Sci - 1989 1697). This increase was attenuated by a low dose of the GABAa agonist muscimol (1 ng/0.5 microl) into the rostral VP/SI complex. This dose of muscimol when injected alone into the rostral VP/SI complex had no effect upon the number of different cue-directed behavioral patterns. Only the lowest dose of the GABAa antagonist bicuculline (10-25 ng/0.5 microl per side) into the rostral VP/SI complex slightly, and in a non-d-amphetamine-like manner, increased the number of different cue-directed behavioral patterns while none of the doses had an effect on the number of different non-cue-directed behavioral patterns. Both injections of d-amphetamine into the ventral striatum and injections of bicuculline into the rostral VP/SI complex strongly increased motor activity in the 10-min period preceding the swimming test. We conclude from the data that switching to cue-directed behavioral patterns is sensitive to manipulations with the dopaminergic activity in the ventral striatum but not with the GABAergic activity in the VP/SI complex although the VP/SI transmits it to other brain structures. In contrast motor activity is sensitive to manipulations with both ventral striatal dopamine and rostral VP/SI complex GABA.

Actor-critic models of the basal ganglia: new anatomical and computational perspectives

A large number of computational models of information processing in the basal ganglia have been developed in recent years. Prominent in these are actor-critic models of basal ganglia functioning, which build on the strong resemblance between dopamine neuron activity and the temporal difference prediction error signal in the critic, and between dopamine-dependent long-term synaptic plasticity in the striatum and learning guided by a prediction error signal in the actor. We selectively review several actor-critic models of the basal ganglia with an emphasis on two important aspects: the way in which models of the critic reproduce the temporal dynamics of dopamine firing, and the extent to which models of the actor take into account known basal ganglia anatomy and physiology. To complement the efforts to relate basal ganglia mechanisms to reinforcement learning (RL), we introduce an alternative approach to modeling a critic network, which uses Evolutionary Computation techniques to 'evolve' an optimal RL mechanism, and relate the evolved mechanism to the basic model of the critic. We conclude our discussion of models of the critic by a critical discussion of the anatomical plausibility of implementations of a critic in basal ganglia circuitry, and conclude that such implementations build on assumptions that are inconsistent with the known anatomy of the basal ganglia. We return to the actor component of the actor-critic model, which is usually modeled at the striatal level with very little detail. We describe an alternative model of the basal ganglia which takes into account several important, and previously neglected, anatomical and physiological characteristics of basal ganglia-thalamocortical connectivity and suggests that the basal ganglia performs reinforcement-biased dimensionality reduction of cortical inputs. We further suggest that since such selective encoding may bias the representation at the level of the frontal cortex towards the selection of rewarded plans and actions, the reinforcement-driven dimensionality reduction framework may serve as a basis for basal ganglia actor models. We conclude with a short discussion of the dual role of the dopamine signal in RL and in behavioral switching.

Differential effects of a small, unilateral, 6-hydroxydopamine-induced nigral lesion on behavior in high and low responders to novelty

The goal of this study was to develop an animal model that evaluates striatal-specific behavior after partial, unilateral destruction of nigrostriatal neurons. 6-OHDA (1 microg) was injected intranigrally (day 0) to reduce dopaminergic innervation of the dorsal striatum (DS); 6-OHDA (5 microg) was injected to reduce innervation of DS and nucleus accumbens (ACC). We analyzed changes in (a) behavior regulated by dopamine (DA) release in the DS (hindpaw preference from day 5 to day 19, every other day) and the ACC (novelty-induced locomotion on day 16) and (b) apomorphine-induced rotation (on day 21). We used two types of rat that show differences in structure and function of the dopaminergic neurons, namely high (HR) and low (LR) responders to novelty. 6-OHDA (1 microg) significantly decreased TH immunoreactivity (TH-ir) in the DS and increased preference for the hindpaw controlled by the nonlesioned side in HRs and LRs in time. Only in LRs was the significant increase of novelty-induced locomotion accompanied by a significant increase in TH-ir density in the ACC: this suggests a lesion-induced shift in nigrostriatal/mesolimbic balance toward a dominance of the mesolimbic system. The higher 6-OHDA dose significantly decreased TH-ir in the DS and the ACC and increased preference for the hindpaw controlled by the nonlesioned side in HRs and LRs in time. However, this increase occurred significantly earlier in LRs than in HRs. Apomorphine elicited contralateral rotations solely in LRs, and not in HRs, indicating development of supersensitive dopamine receptors in the DS of LRs, but not HRs. The data show that LRs are more susceptible to 6-OHDA than HRs. The relevance of the present data for Parkinson's disease is discussed.

Tonic opioid inhibition of the subiculo-accumbens pathway

There is evidence to suggest that medium spiny neurons (MSNs) in the nucleus accumbens (NAS) should be sensitive to opiate compounds. However, neuronal responses in the NAS evoked by fimbria stimulation (F-D) are insensitive to systemically or iontophoretically administered morphine. The hypothesis of this study was that fimbria-evoked NAS responses may fail to demonstrate sensitivity to morphine because they are under tonic opioid inhibition and can't be further inhibited by opiates. If correct, then pharmacological inhibition of opioid actions on these NAS neuronal responses should result in an increase of response to fimbria stimulation. The effects of systemic and iontophoretic administrations of naloxone on NAS responses evoked by fimbria stimulation were observed. Systemically and locally administered naloxone selectively increased the excitability of accumbens single-unit responses to fimbria stimulation. Conversely, systemic or iontophoretic administration of morphine was without effect on the same types of NAS responses. These observations are consistent with the hypothesis that a tonic opioid inhibition may regulate this pathway. In contrast, naloxone and morphine effect other NAS circuit responses differently than F-D NAS responses. In some cases naloxone and morphine tests have been conducted on different evoked responses from the same neuron. Those results have shown that different responses from the same cell may be differentially affected. Consequently, opioid modulation of activity in the NAS is probably pathway-specific rather than neuron-specific.

Perseveration in schizophrenic patients: a neuropsychological approach for research

Perseveration is a core symptom of schizophrenia, the cause, however, is unknown. It has been shown that for people with frontal lobe lesions, perseveration can be explained with a set-maintenance problem. Perseveration in Parkinson's disease can be explained with problems shifting from one set to another without cues (set-shifting). These disorders can be distinguished using a two-choice task and the Wisconsin Card Sorting Test, that is analysed in phases. Analogs of these tests can be used in animal research. By adding an animal part to the human research, more insight can be gained into the role of specific brain areas in set-maintenance and set-shifting.

Set-maintenance and set-shifting problems in schizophrenic subtypes: relationship to dysfunctions of the fronto-striatal loops

Research with patients suffering from Parkinson's disease and frontal lobe lesions has shown that disturbances in the fronto-striatal loops in the brain can cause perseveration. Perseveration is a core symptom of schizophrenia, yet the cause is not known. For schizophrenic patients disorders of many parts of the fronto-striatal loops are found, for example disturbances of the prefrontal cortex and the striatum. Perseveration in schizophrenia can be explained with set-maintenance problems, related to dysfunction of the prefrontal cortex, or with set-shifting problems that are related to disorders in the striatum. These set-maintenance and set-shifting problems can be distinguished with neuropsychological tests. Regarding the bloodflow patterns for the different subtypes of schizophrenia three problems are expected as explanations for perseveration: set-maintenance problems concerning abstract information, set-maintenance problems shifting between stimuli and enhanced set-shifting with cues.

The role of nucleus accumbens dopamine in motivated behavior: a unifying interpretation with special reference to reward-seeking

Studies addressing behavioral functions of dopamine (DA) in the nucleus accumbens septi (NAS) are reviewed. A role of NAS DA in reward has long been suggested. However, some investigators have questioned the role of NAS DA in rewarding effects because of its role in aversive contexts. As findings supporting the role of NAS DA in mediating aversively motivated behaviors accumulate, it is necessary to accommodate such data for understanding the role of NAS DA in behavior. The aim of the present paper is to provide a unifying interpretation that can account for the functions of NAS DA in a variety of behavioral contexts: (1) its role in appetitive behavioral arousal, (2) its role as a facilitator as well as an inducer of reward processes, and (3) its presently undefined role in aversive contexts. The present analysis suggests that NAS DA plays an important role in sensorimotor integrations that facilitate flexible approach responses. Flexible approach responses are contrasted with fixed instrumental approach responses (habits), which may involve the nigro-striatal DA system more than the meso-accumbens DA system. Functional properties of NAS DA transmission are considered in two stages: unconditioned behavioral invigoration effects and incentive learning effects. (1) When organisms are presented with salient stimuli (e.g., novel stimuli and incentive stimuli), NAS DA is released and invigorates flexible approach responses (invigoration effects). (2) When proximal exteroceptive receptors are stimulated by unconditioned stimuli, NAS DA is released and enables stimulus representations to acquire incentive properties within specific environmental context. It is important to make a distinction that NAS DA is a critical component for the conditional formation of incentive representations but not the retrieval of incentive stimuli or behavioral expressions based on over-learned incentive responses (i.e., habits). Nor is NAS DA essential for the cognitive perception of environmental stimuli. Therefore, even without normal NAS DA transmission, the habit response system still allows animals to perform instrumental responses given that the tasks take place in fixed environment. Such a role of NAS DA as an incentive-property constructor is not limited to appetitive contexts but also aversive contexts. This dual action of NAS DA in invigoration and incentive learning may explain the rewarding effects of NAS DA as well as other effects of NAS DA in a variety of contexts including avoidance and unconditioned/conditioned increases in open-field locomotor activity. Particularly, the present hypothesis offers the following interpretation for the finding that both conditioned and unconditioned aversive stimuli stimulate DA release in the NAS: NAS DA invigorates approach responses toward 'safety'. Moreover, NAS DA modulates incentive properties of the environment so that organisms emit approach responses toward 'safety' (i.e., avoidance responses) when animals later encounter similar environmental contexts. There may be no obligatory relationship between NAS DA release and positive subjective effects, even though these systems probably interact with other brain systems which can mediate such effects. The present conceptual framework may be valuable in understanding the dynamic interplay of NAS DA neurochemistry and behavior, both normal and pathophysiological.

The effects of microinjection of d-amphetamine into the n. accumbens during the late maintenance phase of an ethanol consumption bout

The microinjection of d-amphetamine into the n. accumbens of rats, prior to the start of an operant ethanol self-administration session, increases operant behavior and the amounts of ethanol presented as the reinforcer. Although this effect could result by blocking termination processes regulating a consummatory bout, it could also be a result of enhancing the stimulus control regulating the maintenance of a drinking bout. To explore this issue, rats were trained to self-administer 10% ethanol in an operant situation. Following establishment of stable behavior, they were surgically instrumented so that the n. accumbens could be microinjected with d-amphetamine during a drinking bout, without having to handle the animal. The microinjection of d-amphetamine in the rats self-administering ethanol at the late phase of the drinking bout resulted in a prolonged bout and increased self-administration. During extinction testing, a reinstatement of responding was found following the amphetamine microinjection. The data suggest the most likely action of the amphetamine microinjection was to alter stimulus control factors, which normally regulate the maintenance of drinking, thereby prolonging the bout and increasing intake.

Motor activation by amphetamine infusion into nucleus accumbens core and shell subregions of rats differentially sensitive to dopaminergic drugs

Selective breeding based on activity in a swim test has been used to produce lines of rats that show a high level of activity in the swim test (Swim High-active (SwHi) rats) and a low level of activity in the swim test (Swim Low-active (SwLo) rats). Previous studies have indicated that dopamine (DA) function is enhanced in SwHi rats and reduced in SwLo rats; a principal finding was that SwLo rats showed much smaller increases in ambulatory activity after systemic administration of amphetamine than did SwHi or non-selected rats. In light of the importance of the nucleus accumbens (NAC) in amphetamine-induced activity, the present study investigated whether DA function in NAC differs in SwHi and SwLo rats. Amphetamine was infused bilaterally into either the core or shell subregion of NAC, and ambulation or swim test activity was then measured. In SwLo rats, infusion of amphetamine (0.2-2.0 microg) into either NAC core or shell produced moderate increases in ambulation. In SwHi rats, infusion of amphetamine into NAC shell produced similar moderate increases in ambulation, but infusion into the core produced markedly larger dose-related increases in ambulation. In the swim test, infusion of amphetamine (1.0 microg) increased activity by affecting the dominant behavior of each line; i.e. struggling increased in SwHi rats and floating decreased in SwLo rats, with large effects seen in both lines with infusion into either NAC core or shell. These results support the idea that the distinct behavioral characteristics of SwHi and SwLo rats are mediated in part by differences in NAC-DA function.

Intranucleus accumbens amphetamine infusions enhance responding maintained by a stimulus complex paired with oral ethanol self-administration

Six male Long-Evans rats were trained to self-administer 10% ethanol (v/v) during 30 min operant sessions. A licking response on an empty drinking tube resulted in the presentation of reinforcement from an automatic dipper. During the initiation of ethanol self-administration, a tone-light stimulus complex was paired with all ethanol presentations. When 10% ethanol maintained responding, guide cannulae aimed at the nucleus accumbens (NAcc) were implanted into the brain. The ability of the paired stimulus complex to reinforce a new operant response (i.e., a lever press) was then examined. To test for the development of the new response, responding on one lever resulted in presentation of only the paired tone-light stimulus complex (contingency-associated lever) while responding on an alternate lever had no programmed consequences (no contingency-associated lever). Prior to some new response sessions, amphetamine (5-20 microg/microl) was infused into the NAcc to examine the influence of dopamine on responding maintained by the stimulus complex. Ethanol intake during the sessions prior to new response testing averaged 0.49 +/- 0.07 g/g. During new response sessions no significant differences in lever pressure during no-drug conditions (control, sham, injection or vehicle injection) were observed between the contingency-associated and no contingency-associated levers. Intra-NAcc infusion of amphetamine (5-20 microg/microl) resulted in significant increases in lever pressing only on the contingency-associated lever. These data suggest that increasing NAcc dopamine levels with amphetamine enhanced the ability of the stimulus complex to function as a reinforcer. Further studies examining the ability of potentially more salient stimuli (i.e., taste of ethanol) to function as conditioned reinforcers associated with ethanol self-administration are warranted due to the apparent inability of the paired tone-light stimulus complex to function as a reinforcer without amphetamine-induced activation of the NAcc.

Stimulus dimension shifts in patients with schizophrenia, with and without paranoid hallucinatory symptoms, or obsessive compulsive disorder: strategies, blocking and monoamine status

Reversal, and intra-dimensional (ID) and extra-dimensional (ED) nonreversal discrimination shifts were studied to see if learned inattention to the irrelevant dimension differentially influenced the efficacy of learning and stimulus choice strategy. Performance was compared with conditioned blocking (CB) and monoamine metabolic status between healthy controls, patients with obsessive compulsive disorder (OCD) or schizophrenia with (PH) or without (NP) active paranoid hallucinatory symptoms. PH and NP patients improved learning with practice, but showed an impaired shift on each task. OCD patients were impaired only on the ED-shift. The NP patient's impairment was nonspecific and, unlike PH and controls, it related to reversal performance. All subjects acquired an attentional set for colour reflected in the length of stimulus-response sequences. Analysis of paired-stimulus choice-strategies showed that while all patients showed fewer correct win-stay choices, only PH patients perseverated with lose-stay choices. Learning about the added stimulus in the CB task related to ID-shift efficiency in NP patients. Increases of dopamine activity related to delayed learning but more switches of stimulus choice in the shift-tasks. Increases of serotonin activity correlated with faster learning in controls, OCD and PH patients. In NP patients the opposite held for dopamine and serotonin activity. Thus the two learned inattention tasks have different if related requirements and correlates: the data are consistent with the use of automatic exogenous attention strategies by NP patients, of inefficient controlled attention by PH patients and the automatization of endogenous processes in controls.