Limbic-cortical-ventral striatal activation during retrieval of a discrete cocaine-associated stimulus: a cellular imaging study with gamma protein kinase C expression

We investigated the neuronal activation associated with reexposure to a discrete cocaine-associated stimulus using in situ hybridization to quantify the expression of the plasticity-regulated gene, gamma protein kinase C (gamma PKC), in the limbic-cortical-ventral striatal system. Groups of rats were trained to self-administer cocaine paired with a light stimulus (Paired) or paired with an auditory stimulus but also receiving light presentations yoked to those in the Paired group (Unpaired). Additional groups received noncontingent cocaine-light pairings (Pavlovian) or saline-light pairings (Saline) that were yoked to the Paired group. After acquisition of self-administration by the Paired and Unpaired groups, all groups had a 3 d drug- and training-free period before being reexposed to noncontingent presentations of the light conditioning stimulus during a 5 min test session in the training context. There were four major patterns of results for regional gamma PKC expression 2 hr later. (1) Changes occurred only in groups in which the light was predictive of cocaine. (2) Increases were seen in the amygdala, but decreases were seen in the medial prefrontal cortex. (3) No changes were seen in the hippocampus. (4) Although changes were observed in the basal and central nuclei of the amygdala and the prelimbic cortex in both the Paired and Pavlovian groups, additional changes were observed in the nucleus accumbens core, lateral amygdala, and anterior cingulate cortex in the Pavlovian group. These results suggest not only that regionally selective alterations in gamma PKC expression are an index of the retrieval of Pavlovian associations formed between a drug and a discrete stimulus, but also that a distinct neural circuitry may underlie Pavlovian stimulus-reward associations in cocaine-experienced rats.

Fear memory retrieval induces CREB phosphorylation and Fos expression within the amygdala

Fear memory retrieval has been shown to induce a protein-synthesis dependent re-consolidation of memories within the amygdala. Here, using immunocytochemistry, we investigated the molecular basis of this process in the rat and show that retrieval of a cued fear memory induces the activation, by phosphorylation, of the transcription factor CREB within the basal and lateral nuclei of the amygdala, as well as expression of the CREB-regulated immediate-early gene, c-fos, in the basal amygdala. We also show an increase in CREB phosphorylation within the central nucleus of the amygdala following behavioural testing, with an accompanying increase in Fos-immunoreactive nuclei in animals retrieving the cued association. There were no changes in either phosphorylated CREB or Fos in the hippocampus following exposure to discrete fear stimuli. These results show that activation of CREB, which has been shown to be involved in the formation of long-term fear memories, also accompanies memory retrieval, and also suggest a role for CREB phosphorylation in memory re-consolidation following retrieval.

Cocaine seeking by rats is a goal-directed action

In two experiments rats were trained to self-administer intravenous cocaine on chained schedules using different responses in the initial (drug-seeking) and terminal (drug-taking) links. In both between- (Experiment 1) and within-subject designs (Experiment 2), the drug-taking response was then either extinguished or reinforced in the absence of the opportunity to perform the seeking response. In a subsequent extinction test with the seeking manipulanda alone, the rate of drug seeking was reduced after the prior extinction of the associated taking response. An additional group trained with a sucrose reinforcer showed a comparable devaluation effect. These findings demonstrate that seeking responses for cocaine and food rewards are mediated by a representation of the contingency between seeking responses and the opportunity to take the reward.

Cellular imaging of zif268 expression in the hippocampus and amygdala during contextual and cued fear memory retrieval: selective activation of hippocampal CA1 neurons during the recall of contextual memories

The neuroanatomical and molecular basis of fear memory retrieval was studied by analyzing the expression of the plasticity-associated immediate early gene zif268. Cellular quantitative in situ hybridization revealed that zif268 is expressed within specific regions of the hippocampus and amygdala during fear memory retrieval. Within the hippocampus, increased expression of zif268 was observed within CA1 neurons, but not dentate gyrus neurons, during the retrieval of contextual, but not cued, fear associations. In contrast, zif268 expression was increased within neurons of the amygdala (lateral, basal, and central nuclei) during the retrieval of both contextual and cued fear memories. These results demonstrate activation of hippocampal CA1 neurons in contextual fear memory retrieval that was not merely a correlate of the behavioral expression of fear itself, because it was limited to the retrieval of contextual, and not cued, fear memories. Further studies revealed that the selective increase in hippocampal CA1 zif268 expression seen after contextual fear memory retrieval was limited to the retrieval of recent (24 hr) but not older (28 d) memories. These experiments represent the first demonstration that zif268 expression in specific neuronal populations is associated with memory retrieval and suggest that this gene may contribute to plasticity and reconsolidation accompanying the retrieval process.

The effects of nucleus accumbens core and shell lesions on intravenous heroin self-administration and the acquisition of drug-seeking behaviour under a second-order schedule of heroin reinforcement

RATIONALE

Evidence has implicated the nucleus accumbens (NAcc) in drug-seeking and -taking behaviour. However, the importance of the "core" and "shell" subdivisions of the NAcc in heroin-seeking and -taking behaviour remains unclear.

OBJECTIVES

To investigate the function of the NAcc core and shell in heroin self-administration and heroin-seeking behaviour.

METHODS

Male rats were trained to self-administer heroin (0.12 mg/kg per infusion) under a continuous reinforcement (CRF) schedule. After responding stabilised, rats were given excitotoxic (or sham) lesions of either the NAcc core or shell and after recovery were assessed for their retention of heroin self-administration under CRF. At this point a second-order schedule of reinforcement was introduced, commencing at FR10 (FR1:S) and terminating at FR10 (FR10:S), in which ten lever presses resulted in presentation of the heroin-associated CS+, and completion of ten such units resulted in drug infusion.

RESULTS

Within 7 days, all groups re-acquired responding for heroin under CRF at rates similar to their pre-lesion performance. However, rats with lesions of the NAcc core, but not shell, were severely impaired in the acquisition of heroin-seeking behaviour.

CONCLUSIONS

These results indicate an important role for the core of the NAcc in the acquisition of heroin-seeking behaviour under the control of drug-associated stimuli.

The effects of excitotoxic lesions of the nucleus accumbens core or shell regions on intravenous heroin self-administration in rats

RATIONALE

It has been suggested that the nucleus accumbens (NAcc) may be involved in heroin reward, and the core and shell regions respond differently following administration of a number of drugs of abuse.

OBJECTIVE

The possible role of the NAcc core and shell subregions in the acquisition of heroin self-administration behaviour was investigated.

METHODS

Rats were given selective excitotoxic lesions of either the nucleus accumbens core or shell before the acquisition of responding for i.v. heroin (0.04 mg/infusion) under a continuous reinforcement schedule in daily 3 h sessions. After sham-lesioned rats reached a stable baseline, a between-sessions heroin dose-response function was established.

RESULTS

Rats with lesions of the NAcc shell did not differ significantly from sham controls in either the acquisition of heroin self-administration or in their heroin dose-response function. The NAcc core lesion group showed reduced levels of responding during the acquisition of heroin self-administration and a reduction in responding during the heroin dose-response function, although this behaviour was sensitive to changes in the dose of heroin.

CONCLUSIONS

The NAcc shell does not appear to be critical for heroin self-administration, whereas the NAcc core, although apparently not essential in mediating the rewarding effect of i.v. heroin, may mediate processes that are of special importance during the acquisition of instrumental behaviour.

Role of corticostriatal and nigrostriatal inputs in malonate-induced striatal toxicity

The striatal neuronal loss evident following cellular metabolic compromise may be dependent upon the presence of glutamate and dopamine within the striatum. In order to investigate the relative roles of corticostriatal and nigrostriatal projections in malonate-induced neuronal loss, the extent of toxicity was quantified in animals with cortical lesions to deplete the striatum of glutamate, nigrostriatal lesions to deplete the striatum of dopamine, or both. We found that malonate-induced striatal toxicity was significantly reduced following lesions of either the glutamatergic or dopaminergic afferents to the striatum. The extent of attenuation following the loss of both inputs within the same animal was similar to that seen following lesions of either alone. These data suggest that malonate-induced toxicity in the striatum depends upon the integrity of interactive influences from both glutamatergic and dopaminergic afferents.

The effects of excitotoxic lesions of the basolateral amygdala on the acquisition of heroin-seeking behaviour in rats

RATIONALE

Second-order schedules of drug-self-administration provide a method of examining drug-seeking behaviour, which is maintained in part by the presentation of a discrete, drug-associated light CS. Previous results have found that lesions of the basolateral amygdala (BLA) impair the acquisition of i.v. cocaine self-administration under this type of schedule.

OBJECTIVES

The present experiments examined the effects of excitotoxic lesions of the BLA on the acquisition of i.v. heroin self-administration under both continuous reinforcement and second-order schedules, in order to investigate possible commonalties in the neural basis of heroin- and cocaine-seeking behaviour.

METHODS

Rats received quinolinic acid or sham vehicle lesions of the BLA prior to i.v. self-administration training. Initially, heroin self-administration under a continuous reinforcement schedule was acquired. Each active lever-press resulted in a 0.04 mg i.v. heroin infusion, paired with presentation of a 20-s light conditioned stimulus. Following acquisition of responding under this schedule, the response requirement was gradually increased to a second-order schedule of FI15(FR5:S).

RESULTS

There was no effect of lesions of the BLA on the acquisition of heroin self-administration under a continuous reinforcement schedule. The acquisition of heroin-seeking behaviour under a second-order schedule of self-administration was not affected by lesions of the BLA, but lesioned rats showed a significantly higher baseline level of responding.

CONCLUSIONS

These results indicate that the rewarding effects of heroin do not depend on the integrity of the BLA. The BLA is also not critically involved in mediating heroin-seeking behaviour under a second-order schedule of reinforcement, and this stands in marked contrast to the effects of BLA lesions on the acquisition of cocaine-seeking behaviour. These findings suggest that discrete heroin cues were not critical in maintaining heroin-seeking behaviour under the second-order schedule used here and that other learning systems are engaged in the control of this behaviour.

Second-order schedules of drug reinforcement in rats and monkeys: measurement of reinforcing efficacy and drug-seeking behaviour

RATIONALE AND OBJECTIVES

To review the literature on the use of second-order schedules of drug reinforcement in the context of experimental investigations of the neural and pharmacological mechanisms underlying addictive behaviour in general and drug-seeking behaviour in particular.

METHODS

Second-order schedules of drug reinforcement are described in which responding is maintained not only by the self-administered drug, but also by contingent presentation of drug-paired stimuli that serve as conditioned reinforcers of instrumental behaviour.

RESULTS

The behaviour of rats and monkeys responding under second-order schedules is discussed in relation to self-administered drug dose and the importance of drug-associated cues in maintaining responding for cocaine, morphine or heroin. Drug-seeking behaviour during the period before drug is self-administered is described and compared with drug-seeking behaviour derived from other procedures. In addition, results are summarised that demonstrate the differential involvement of the amygdala and prefrontal cortex in the acquisition of cue-controlled cocaine- and heroin-seeking behaviour, as well as the effects of drugs interacting with D3 dopamine, NMDA and AMPA receptors on drug-seeking behaviour and dopaminergic correlates of drug-paired stimuli presented non-contingently and during responding for cocaine under a second-order schedule.

CONCLUSIONS

We argue that the first, drug-free interval (or other period) of responding under a second-order schedule of reinforcement has particular utility in that it provides a measure of drug-seeking behaviour and reinforcing efficacy that are not affected by the pharmacological effects of recently administered drug. It also provides a means of investigating the role of drug-paired stimuli in drug-seeking behaviour, including its behavioural, neural and neurochemical basis.

Heroin self-administration under a second-order schedule of reinforcement: acquisition and maintenance of heroin-seeking behaviour in rats

RATIONALE

Second-order schedules of heroin self-administration provide a method of measuring heroin-seeking behaviour independently of the effects of the drug on motor behaviour and of investigating the role of heroin-associated stimuli in such heroin-seeking behaviour.

OBJECTIVES

These experiments aimed to establish a second-order schedule of heroin self-administration in rats, similar to that already established in this laboratory for cocaine self-administration and to investigate the role of discrete heroin-associated stimuli in the maintenance of heroin-seeking behaviour under a second-order schedule of reinforcement.

METHODS

Heroin i.v. self-administration (0.04 mg/infusion) was initially contingent upon a lever press, and each infusion was paired with presentation of a 20-s light-conditioned stimulus (CS). Following acquisition of heroin self-administration, the response requirement was progressively increased so that, ultimately, responding was maintained under a fixed interval (FI) 15 min [fixed ratio (FR)5:S] second-order schedule. The effects of varying the dose of heroin (0.01 mg and 0.08 mg/infusion) and pre-treatment with the mu-opiate receptor antagonist, naloxone, on responding under a FI15(FR5:S) schedule were investigated. In addition, the role of the heroin-associated CS on responding was assessed by measuring the effects of omitting the CS during heroin-seeking behaviour and during extinction of responding, as well as the effect of CS presentation on the reinstatement of heroin-seeking behaviour following extinction.

RESULTS

A second-order schedule of heroin self-administration was established. There were no clear effects on heroin-seeking behaviour of increasing or decreasing the dose of heroin. Although no effect of naloxone pre-treatment was seen on heroin-seeking behaviour during the first, drug-free interval of responding, an extinction-like pattern of responding was seen in that interval during subsequent sessions. Omission of the light CS resulted in a reduction in levels of responding for i.v. heroin, indicating its role in maintaining heroin-seeking behaviour. However, under extinction conditions, response-contingent CS presentations did not affect the rate of extinction, nor did non-contingent presentations of the CS following extinction reinstate heroin-seeking behaviour.

CONCLUSIONS

These experiments have established a method of measuring heroin-seeking behaviour in rats by adopting a second-order schedule of i.v. heroin self-administration. The results indicate a relatively weak impact of discrete, heroin-associated cues on heroin-seeking behaviour relative to cocaine-seeking behaviour studied under similar conditions.

Cocaine self-administration in rats differentially alters mRNA levels of the monoamine transporters and striatal neuropeptides

The potential neuroadaptations to cocaine self-administration (SA) were evaluated using quantitative in situ hybridisation histochemistry. Levels of mRNAs of the monoamine transporters, i.e. the primary molecular targets of cocaine, and the striatal neuropeptides substance P and enkephalin, which predominantly exist in different populations of dopaminoceptive striatal neurons, were quantified in rats which had reached different stages of acquisition of cocaine SA. Thus, animals were killed 1 h after completing a self-administration session (i) early in or after acquisition of cocaine SA (ii) after various regimes of chronic cocaine SA, and (iii) a 10-day period of withdrawal from chronic cocaine intake. Control mRNA levels of all molecules under study were those quantified in animals receiving i.v. saline yoked to rats self-administering cocaine (1.5 or 0.75 mg/kg per infusion, depending on the experiment). Monoamine transporter expression was differentially altered by cocaine; dopamine transporter mRNA levels in the ventral tegmental area, but not in the substantia nigra, were increased following withdrawal from cocaine, suggesting a role for the upregulated mesolimbic dopamine transporter in the mechanisms underlying relapse to cocaine taking. By contrast, serotonin transporter mRNA in the dorsal raphé and noradrenaline transporter mRNA in the locus coeruleus remained unaltered under all experimental conditions. In addition, the expression of the striatal neuropeptides was also differentially altered; substance P mRNA levels were transiently increased in the shell of the nucleus accumbens by prolonged cocaine self-administration, but enkephalin mRNA levels in the dorsal and ventral striatum remained unaltered under all conditions.

The role of cortical cholinergic afferent projections in cognition: impact of new selective immunotoxins

Previous investigations aimed at determining the role of corticopetal cholinergic afferents in cognition have relied upon human psychopharmacological studies, neuropsychological analyses of Alzheimer's patients, or psychopharmacological manipulations and excitotoxic lesions in animals. Unfortunately, each approach has its limitations. The interpretation of neuropsychological data relies upon correlations of post-mortem assessments of cholinergic degeneration that may be quite temporally distant from the time of cognitive assessment. In contrast, the use of animals allows direct manipulations of the cholinergic system and the establishment of causal relationships between acetylcholine and cognitive function but is limited by the selectivity of the toxins and drugs available to manipulate the system. The recent introduction of immunotoxins to lesion cortical cholinergic pathways with greater selectivity has allowed the effective testing of these hypotheses of cholinergic functions in cognition. Previous neuropsychological, psychopharmacological and excitotoxic lesion data are reviewed and compared to results produced using the more selective immunotoxins to provide an update to the current hypotheses of the role of corticopetal cholinergic afferents in cognitive function. Additionally, the conceptual and methodological cost and benefits of the methods of infusion used to produce lesions with these immunotoxins is assessed.

The effects of d-amphetamine, chlordiazepoxide, alpha-flupenthixol and behavioural manipulations on choice of signalled and unsignalled delayed reinforcement in rats

RATIONALE

Inability to tolerate delays to reward is an important component of impulsive behaviour, and has been suggested to reflect dysfunction of dopamine systems.

OBJECTIVES

The present experiments examined the effects of signalling a delayed, large reward on rats' ability to choose it over a small, immediate reward, and on the response to amphetamine, a dopamine receptor antagonist, and a benzodiazepine.

METHODS

Three groups of Lister hooded rats were tested on a two-lever discrete-trial delayed reinforcement task in which they chose one pellet delivered immediately or four pellets delivered after a delay. This delay increased from 0 to 60 s during each session. Trials began with illumination of a houselight: in the Houselight group, this remained on during the delay and feeding period. In the No Cue group, the houselight was extinguished at the moment of choice. In the Cue group, a stimulus light was illuminated during the delay. Once trained, the rats were challenged with d-amphetamine (0.3, 1.0, 1.6 mg/kg), chlordiazepoxide (1.0, 3.2, 5.6, 10 mg/kg), alpha-flupenthixol (0.125, 0.25, 0.5 mg/kg), and various behavioural manipulations.

RESULTS

Subjects' choice became and remained sensitive to the delay; the cue speeded learning. Amphetamine decreased choice of the large reinforcer in the No Cue group and increased it in the Cue group. alpha-Flupenthixol and chlordiazepoxide generally decreased preference for the delayed reinforcer; flupenthixol reduced the cue's effects, but chlordiazepoxide did not interact with the cue condition.

CONCLUSIONS

Signals present during a delay can enhance the ability of amphetamine to promote choice of delayed rewards.

Cocaine-seeking by rats: regulation, reinforcement and activation

RATIONALE

In animal models of drug self-administration, response rates often decrease with dose suggesting that a regulative process may mask the reinforcing effects of the drug.

OBJECTIVE

The purpose of the present experiments was to dissociate the role of regulative and reinforcement processes in intravenous cocaine self-administration by rats using a paradigm that explicitly distinguishes between drug-seeking and drug-taking.

METHODS

Rats were trained to respond for intravenous cocaine (0.25 mg/infusion) under a heterogeneous chain (tandem FR1 RI 30 s) FR1 schedule of reinforcement using different levers in the first (seeking) and second (taking) links of the chain. After 10 days of training, rats were switched to one of three doses of cocaine (0.08, 0.25, or 0.5 mg/infusion) and self-administration patterns were recorded for a further ten sessions in experiment 1. In experiment 2, a time-out (TO) period (0, 4, or 12 min) was imposed between successive cycles of the chain schedule. Finally, the effect of allowing animals to perform a drug-taking response on subsequent drug-seeking was assessed in experiment 3.

RESULTS

Having verified that seeking responses for a conventional reinforcer (sucrose) were sensitive to changes in reward magnitude, experiment 1 demonstrated that the number of self-administered infusions was inversely related to dose whereas the latency to initiate drug-seeking increased with dose. Variations in the cocaine dose had no reliable effect on the number of drug seeking response per cycle of the chain schedule. The effect of dose on the latency to initiate drug-seeking was reversed in experiment 2 with increasing TO periods. Moreover, at the longest TO period, drug-seeking responses per cycle increased and the latency to initiate drug seeking decreased with dose. Experiment 3 showed that the latency to drug-seek for the low dose was reduced dramatically when the first drug-seeking response was preceded by a drug-taking response, even when this response did not produce a drug infusion.

CONCLUSIONS

The overall pattern of results suggests that drug-seeking and drug-taking are controlled by three interacting processes: a regulative process depresses drug-seeking in the short-term; behavioral activation enhances drug-seeking and is sustained over longer intervals by higher drug doses; the reinforcing effect of cocaine increases with dose once the satiety producing effects of the drug dissipate.

Dissociation in conditioned dopamine release in the nucleus accumbens core and shell in response to cocaine cues and during cocaine-seeking behavior in rats

The dopaminergic innervation of the nucleus accumbens is generally agreed to mediate the primary reinforcing and locomotor effects of psychostimulants, but there is less consensus on conditioned dopamine (DA) release during drug-seeking behavior. We investigated the neurochemical correlates of drug-seeking behavior under the control of a drug-associated cue [a light conditioned stimulus (CS+)] and to noncontingent presentations of the CS+ in the core and shell subregions of the nucleus accumbens. Rats self-administered cocaine under a continuous reinforcement schedule in which a response on one of two identical levers led to an intravenous cocaine infusion (0.25 mg/infusion) and a 20 sec light CS+. Response requirements for cocaine and the CS+ were then progressively increased until stable responding was established under a second-order schedule of reinforcement. During microdialysis, rats were presented noncontingently with a set of 10 sec CS+ and neutral tone stimuli (CS-) before and after a 90 min period during which they responded for cocaine under a second-order schedule. Results showed the following: (1) nucleus accumbens DA increased in both the core and shell during intravenous cocaine self-administration; (2) noncontingent presentations of a cocaine-associated CS+ led to increased DA release selectively in the nucleus accumbens core; and (3) extracellular DA levels were unaltered in both core and shell during a protracted period of drug-seeking behavior under the control of the same cocaine-associated cue. These results indicate that the mesolimbic dopamine system is activated after exposure to drug-associated stimuli under specific conditions.

Age-dependence of malonate-induced striatal toxicity

Malonate is an inhibitor of cellular metabolism, which, following intrastriatal injection, induces a striatal pathology similar to that seen in Huntington's disease. In two parallel studies, we have investigated the suggested relationship between the neuronal vulnerability to metabolic toxicity and the decline in metabolic function with increasing age. The first experiment investigated malonate-induced neuronal loss in animals aged from 6 weeks up to 27 months, and the second assessed the activities of two mitochondrial enzymes, succinate dehydrogenase and cytochrome oxidase (CYTOX) in animals aged 6 weeks, 3, 8 and 18 months. In the first study, male Lister-Hooded rats received intrastriatal stereotaxic injections of malonate (0.5 or 1.0 M). Animals were killed 10 days after surgery, and the brains were stained with cresyl violet and processed for NADPH-diaphorase activity and glial fibrillary-acidic-protein (GFAP) immunohistochemistry. Animals aged 6 months and older exhibited over 60% striatal neuronal loss. However, the degree of neuronal loss did not show any age-related increase in rats between 6 and 27 months of age, indicating that the extent of malonate-induced toxicity does not increase with age in animals older than 6 months. Infusion of 0.5 M malonate produced smaller lesions, which also demonstrated a consistent extent of neuronal loss from 6 months onwards. Metabolic enzyme activities were decreased in the striatum with increasing age, although this effect was only significant for CYTOX activity. Thus, the pattern of malonate-induced neuronal loss in aged animals partially reflects the changes in metabolic activity during ageing.

Increased acetylcholine release in the rat medial prefrontal cortex during performance of a visual attentional task

Recent studies have suggested a functional link between cortical cholinergic output and attentional task demands, whereby acetylcholine (ACh) release is regulated according to the outcome of ongoing behaviour. To explore this hypothesis we measured ACh efflux in the rat medial prefrontal cortex (mPFC) during between-session manipulations of the cognitive demands of an attentional task. Rats were trained to detect visual stimuli in a five-choice serial reaction time task (5-CSRTT) which involves sustained and divided attention. Following habituation to tethering and implantation with a microdialysis probe in the mPFC, rats were tested in the 5-CSRTT for three consecutive days, with different lengths of stimulus duration. During performance of the 5-CSRTT we measured robust, reproducible, task-related increases in ACh release in the mPFC across all sessions. Variations of the stimulus duration from the standard 0.5 s resulted in the predicted behavioural effects (reductions and increases in choice accuracy with 0.25 s and 5 s, respectively), but there was no evidence of either greater changes in ACh release in the more demanding condition or smaller changes in the less demanding condition. By contrast, in the session with 5-s stimulus duration there was a positive correlation between prefrontal cortical ACh efflux and the total number of trials completed. In summary, the present study shows that ACh efflux in the rat mPFC is increased during performance of a 5-CSRTT, but has found no evidence to support a specific relationship between cholinergic cortical output and attentional performance.

Profiles of cognitive dysfunction in chronic amphetamine and heroin abusers

Groups of subjects whose primary drug of abuse was amphetamine or heroin were compared, together with age- and IQ-matched control subjects. The study consisted of a neuropsychological test battery which included both conventional tests and also computerised tests of recognition memory, spatial working memory, planning, sequence generation, visual discrimination learning, and attentional set-shifting. Many of these tests have previously been shown to be sensitive to cortical damage (including selective lesions of the temporal or frontal lobes) and to cognitive deficits in dementia, basal ganglia disease, and neuropsychiatric disorder. Qualitative differences, as well as some commonalities, were found in the profile of cognitive impairment between the two groups. The chronic amphetamine abusers were significantly impaired in performance on the extra-dimensional shift task (a core component of the Wisconsin Card Sort Test) whereas in contrast, the heroin abusers were impaired in learning the normally easier intra-dimensional shift component. Both groups were impaired in some of tests of spatial working memory. However, the amphetamine group, unlike the heroin group, were not deficient in an index of strategic performance on this test. The heroin group failed to show significant improvement between two blocks of a sequence generation task after training and additionally exhibited more perseverative behavior on this task. The two groups were profoundly, but equivalently impaired on a test of pattern recognition memory sensitive to temporal lobe dysfunction. These results indicate that chronic drug use may lead to distinct patterns of cognitive impairment that may be associated with dysfunction of different components of cortico-striatal circuitry.

Leftward shift in the acquisition of cocaine self-administration in isolation-reared rats: relationship to extracellular levels of dopamine, serotonin and glutamate in the nucleus accumbens and amygdala-striatal FOS expression

RATIONALE

Dopamine dysfunction in the nucleus accumbens is thought to underlie the altered propensity of isolation-reared rats to self-administer psychomotor stimulants.

OBJECTIVE

To identify specific changes in monoamine and glutamate function in the nucleus accumbens and c-fos induction in the amygdala and striatum which may be correlated with altered cocaine self-administration in isolates.

METHODS

In three separate studies, group-reared and isolation-reared rats were trained to self-administer cocaine (0.083. 0.25 or 1.5 mg/kg per IV infusion; FR1), intracerebral microdialysis was used to measure cocaine-induced changes in extracellular levels of dopamine, serotonin and glutamate in the nucleus accumbens and the expression of the immediate-early gene c-fos was quantified using quantitative immunocytochemistry of its protein product Fos in several amygdala and striatal brain regions following cocaine administration.

RESULTS

Isolation-reared rats showed an enhanced sensitivity to self-administer the lowest dose of cocaine but showed retarded acquisition at the highest dose. Isolation rearing produced no effect on basal levels of dopamine, serotonin or glutamate in the nucleus accumbens but potentiated the increase in dopamine efflux, though not serotonin efflux, induced by cocaine. Cocaine increased FOS expression in most amygdala and striatal brain regions examined that were relatively greater in isolation-reared rats in core and shell regions of the nucleus accumbens, medial and lateral regions of the dorsal striatum as well as the central nucleus of the amygdala.

CONCLUSION

These data are consistent with the hypothesis that isolation rearing produces enduring changes in the sensitivity of dopamine-mediated functions in amygdala-striatal circuitry that may be directly related to the altered reinforcing properties of cocaine and other psychomotor stimulants.

Rapid and selective induction of BDNF expression in the hippocampus during contextual learning

The hippocampus is required for many forms of long-term memory in both humans and animals, and formation of long-lasting memories requires the synthesis of new proteins. Furthermore, the long-term potentiation (LTP) of hippocampal synapses, a widely studied model of memory, also depends on both de novo gene transcription and protein synthesis and results in the activation of transcription from promotors containing the cAMP response element (CRE). Expression of several genes is induced during the establishment of LTP; these include the immediate-early genes (IEGs) BDNF (brain-derived neurotrophic factor), zif268 and C/EBPbeta (CCAAT-enhancer binding protein beta), all of which contain CRE sites within their promotor regions. However, these genes induced by LTP are not known to be rapidly induced following learning in a natural setting. Here we demonstrate rapid and selective induction of BDNF expression during hippocampus-dependent contextual learning.

The effect of dopamine depletion from the caudate nucleus of the common marmoset (Callithrix jacchus) on tests of prefrontal cognitive function

This study examined the effects of depletion of dopamine from the caudate nucleus of the common marmoset (Callithrix jacchus), on tasks sensitive to prefrontal damage (attentional set-shifting and spatial delayed response). There was a marked impairment in performance on the spatial delayed response task, but performance on the attentional set-shifting task was relatively preserved except for an impairment in re-engagement of a previously relevant perceptual dimension. This pattern of impairment is distinct from that seen after excitotoxic lesions of the prefrontal cortex and in patients with Parkinson's disease. Though it is not possible to identify specific cognitive functions that are independent of dopaminergic modulation of the caudate nucleus, due to the partial nature of the lesion, the results do provide insight into those cognitive processes that appear most dependent on caudate dopamine.

Disconnection of the anterior cingulate cortex and nucleus accumbens core impairs Pavlovian approach behavior: further evidence for limbic cortical-ventral striatopallidal systems

The nucleus accumbens (NAcc) has been implicated in a variety of forms of reward-related learning, reflecting its anatomical connections with limbic cortical structures. After confirming that excitotoxic lesions of the anterior cingulate cortex (Ant Cing) impaired the acquisition of appetitive Pavlovian conditioning in an autoshaping procedure, the effects of excitotoxic lesions to the NAcc core or shell on autoshaping were also assessed. Only selective core lesions impaired Pavlovian approach. A subsequent experiment studied the effects of a disconnection of the Ant Cing and NAcc core, using an asymmetric lesion procedure, to determine whether these structures interact sequentially as part of a limbic corticostriatal system. Such lesioned rats were also significantly impaired relative to controls at autoshaping. These results demonstrate that the NAcc core and Ant Cing are "nodes" of a corticostriatal circuit involved in stimulus-reward learning.

Enhanced and impaired attentional performance after infusion of D1 dopaminergic receptor agents into rat prefrontal cortex

The role in spatial divided and sustained attention of D1 and D2-like dopamine (DA) receptors in the rat prelimbic medial prefrontal cortex (mPFC) was investigated in a five-choice serial reaction time task. Rats were trained to detect brief flashes of light (0.5-0.25 sec) presented randomly in a spatial array of five apertures. When performance stabilized, animals received bilateral microinfusions of either the D1 DA receptor antagonist SCH 23390, the D1 DA receptor agonist SKF 38393, or the D2 DA antagonist sulpiride into the mPFC. Rats were divided into two groups, with low (<75% correct) and high (>75%) baseline levels of accuracy. Infusions of the D2 receptor antagonist sulpiride had no significant effect on any task variable. SCH 23390 (0.3 microg) selectively impaired the accuracy of attentional performance in rats in the high baseline condition. By contrast, SKF 38393 (0.06 microg) enhanced the accuracy of attentional performance in the low baseline condition, a lower dose (0.03 microg) also increasing the speed of making correct responses. Finally, the beneficial effects of SKF-383893 on choice accuracy were antagonized by SCH 23390 (1.0 microg). The results provide apparently the first demonstration of enhanced cognitive function after local administration of a D1 receptor agonist to the mPFC and suggest dissociable roles of D1 and D2 DA receptors of the mPFC in modulating attentional function.