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.

Dissociable roles of the central and basolateral amygdala in appetitive emotional learning

The amygdala is considered to be a core component of the brain's fear system. Data from neuroimaging studies of normal volunteers and brain-damaged patients perceiving emotional facial expressions, and studies of conditioned freezing in rats, all suggest a specific role for the amygdala in aversive motivation. However, the amygdala may also be critical for emotional processing in positive or appetitive settings. Using an appetitive Pavlovian approach procedure we show a theoretically important dissociation in the effects of excitotoxic lesions of the central nucleus and basolateral area of the amygdala, in the rat. Whilst central nucleus lesions impair appetitive Pavlovian conditioning, basolateral lesions do not. Together with other data, these results not only support the hypothesis that the amygdala is critical for appetitive as well as aversive learning, but are also consistent with amygdala subsystems subserving distinct aspects of emotional learning. Lesions of the dorsal or ventral subiculum were without effect on autoshaping, indicating the lack of involvement of hippocampal processing in this form of emotional behaviour and emphasizing further the neural specificity of the effects seen following central amygdala lesions.

Effects of systemic 3-nitropropionic acid-induced lesions of the dorsal striatum on cannabinoid and mu-opioid receptor binding in the basal ganglia

Systemic administration of 3-nitropropionic acid (3NPA) in experimental animals produces bilateral striatal lesions similar to those seen in Huntington's disease (HD) caudate and putamen. 3H[-CP55,940 binding to cannabinoid receptors in human basal ganglia nuclei has been shown to be highly susceptible to the earliest pathological changes in the HD brain. In this study, to assess further the suitability of 3NPA-induced striatal lesions as a model for HD neuropathology, we examined the effects of striatal lesions induced by the systemic administration of 3NPA on the binding of 3H[-CP55,940 to pre- and postsynaptic cannabinoid receptors in striatum, globus pallidus, entopeduncular nucleus and substantia nigra pars reticulata and also the effect of 3NPA-induced striatal lesions on the binding of 3H[-DAMGO to mu-opioid receptors in striatal striosomes. Systemic administration of 3NPA induced bilateral and symmetrical lesions in dorsolateral striatum. Within the lesion core, 3H[-CP55,940 and 3H[-DAMGO binding density was reduced to background levels. Beyond the immediate borders of the central core of the 3NPA-induced lesion, striatal binding density was not significantly different from that measured in unlesioned rats. 3H[-CP55,940 binding in globus pallidus, entopeduncular nucleus and substantia nigra in 3NPA-lesioned rats was significantly reduced compared to controls, and the individual decreases were similar for each site. However, these reductions were statistically marginal. These data suggest that, while producing striatal lesions which bear some similarity to those seen in HD, the consequences of 3NPA for striatopallidal and striatonigral efferent projections do not reflect the reported neurodegenerative changes seen in the HD brain.

Lesions of the pedunculopontine tegmental nucleus increase sucrose consumption but do not affect discrimination or contrast effects

Pedunculopontine tegmental nucleus (PPTg) lesions block place preferences to drugs or food only when animals are nondeprived. PPTg lesions also disrupt operant responding, but lesioned rats cannot discriminate active from inactive levers. It is not clear, therefore, whether PPTg lesions block reward or disrupt the ability to differentiate changes in reward magnitude. These hypotheses were tested by measuring sucrose consumption, choice, and contrast effects after PPTg lesions. Both sham and lesioned rats consumed greater amounts of a sucrose solution as the concentration and level of deprivation were increased. Given a choice between 2 solutions, all rats consumed more of the higher concentration. Both groups exhibited contrast effects when the concentration was shifted from 32% to 4% within a session. Somewhat surprisingly, lesions increased sucrose intake when rats were food-restricted. These results suggest that PPTg lesions do not disrupt primary motivation or the ability to evaluate and respond to changes in reward strength.

Selective inhibition of cocaine-seeking behaviour by a partial dopamine D3 receptor agonist

Environmental stimuli that are reliably associated with the effects of many abused drugs, especially stimulants such as cocaine, can produce craving and relapse in abstinent human substance abusers. In animals, such cues can induce and maintain drug-seeking behaviour and also reinstate drug-seeking after extinction. Reducing the motivational effects of drug-related cues might therefore be useful in the treatment of addiction. Converging pharmacological, human post-mortem and genetic studies implicate the dopamine D3 receptor in drug addiction. Here we have designed BP 897, the first D3-receptor-selective agonist, as assessed in vitro with recombinant receptors and in vivo with mice bearing disrupted D3-receptor genes. BP 897 is a partial agonist in vitro and acts in vivo as either an agonist or an antagonist. We show that BP 897 inhibits cocaine-seeking behaviour that depends upon the presentation of drug-associated cues, without having any intrinsic, primary rewarding effects. Our data indicate that compounds like BP 897 could be used for reducing the drug craving and vulnerability to relapse that are elicited by drug-associated environmental stimuli.

Associative processes in addiction and reward. The role of amygdala-ventral striatal subsystems

Only recently have the functional implications of the organization of the ventral striatum, amygdala, and related limbic-cortical structures, and their neuroanatomical interactions begun to be clarified. Processes of activation and reward have long been associated with the NAcc and its dopamine innervation, but the precise relationships between these constructs have remained elusive. We have sought to enrich our understanding of the special role of the ventral striatum in coordinating the contribution of different functional subsystems to confer flexibility, as well as coherence and vigor, to goal-directed behavior, through different forms of associative learning. Such appetitive behavior comprises many subcomponents, some of which we have isolated in these experiments to reveal that, not surprisingly, the mechanisms by which an animal sequences responding to reach a goal are complex. The data reveal how the different components, pavlovian approach (or sign-tracking), conditioned reinforcement (whereby pavlovian stimuli control goal-directed action), and also more general response-invigorating processes (often called "activation," "stress," or "drive") may be integrated within the ventral striatum through convergent interactions of the amygdala, other limbic cortical structures, and the mesolimbic dopamine system to produce coherent behavior. The position is probably not far different when considering aversively motivated behavior. Although it may be necessary to employ simplified, even abstract, paradigms for isolating these mechanisms, their concerted action can readily be appreciated in an adaptive, functional setting, such as the responding by rats for intravenous cocaine under a second-order schedule of reinforcement. Here, the interactions of primary reinforcement, psychomotor activation, pavlovian conditioning, and the control that drug cues exert over the integrated drug-seeking response can be seen to operate both serially and concurrently. The power of our analytic techniques for understanding complex motivated behavior has been evident for some time. However, the crucial point is that we are now able to map these components with increasing certainty onto discrete amygdaloid, and other limbic cortical-ventral striatal subsystems. The neural dissection of these mechanisms also serves an important theoretical purpose in helping to validate the various hypothetical constructs and further developing theory. Major challenges remain, not the least of which is an understanding of the operation of the ventral striatum together with its dopaminergic innervation and its interactions with the basolateral amygdala, hippocampal formation, and prefrontal cortex at a more mechanistic, neuronal level.

Effects of contingent and non-contingent cocaine on drug-seeking behavior measured using a second-order schedule of cocaine reinforcement in rats

Rats were trained to respond with intravenous cocaine as the reinforcer under a fixed interval 15-min schedule, during which conditioned stimuli paired with cocaine were presented contingent on completion of a fixed ratio of 10 responses (i.e., second-order schedule of reinforcement). The effects of contingent and noncontingent cocaine were investigated. The results show that pretreatment with noncontingent (i.e., experimenter-administered) cocaine led to a satiation-like effect that was reflected in decreased numbers of responses and a tendency for an increased latency to initiate responding when the doses of cocaine administered were similar to or higher than the training/maintenance dose of cocaine. By contrast, noncontingent administration of cocaine doses lower than the training/maintenance dose, and response-contingent cocaine administration, led to increased drug-seeking behavior, as reflected in increased numbers of responses. The present data indicate that at least two factors determine whether administration of cocaine would lead to drug-seeking behavior: whether the cocaine administration is contingent or noncontingent, and the relative magnitude of the cocaine dose administered in relation to the training/maintenance dose of cocaine.

Effects of excitotoxic lesions of the rat prefrontal cortex on CREB regulation and presynaptic markers of dopamine and amino acid function in the nucleus accumbens

The present study investigated the effects of excitotoxic lesions of the prefrontal cortex (PFC) on dopamine (DA) and excitatory amino acid (EAA) function in the nucleus accumbens core using in vivo microdialysis in freely moving rats. As a postsynaptic marker of neuronal function, the nuclear levels of the transcriptional factor CREB and its active phosphorylated form, CREB-P, were measured in the ventral tegmental area (VTA), and in the core and shell subregions of the nucleus accumbens of sham and lesioned animals. PFC-lesioned animals exhibited a greater locomotor response to novelty and amphetamine administration (125-500 microg/kg i.v.). No change was observed in extracellular levels of glutamate or saturable d-aspartate binding (a marker for the high-affinity EAA transporter) in the nucleus accumbens of PFC-lesioned animals. Extracellular levels of DA were comparable in sham and lesioned animals under tonic conditions, however, following amphetamine administration, DA efflux was significantly attenuated in lesioned animals. No correlation was observed between microdialysate levels of amino acids and the attenuated dopaminergic response to amphetamine in lesioned animals. Further, no effect of the lesion was found on nuclear CREB protein in saline- and amphetamine-treated rats. The density of CREB-P immunoreactive nuclei, while remaining unchanged in the VTA, increased in the nucleus accumbens shell following amphetamine treatment in lesioned animals. The results show that an important modulatory role of the PFC on the behavioural response to novelty and amphetamine is associated with the level of immediate-early gene regulation rather than levels of extracellular DA and amino acids in the ventral striatum.

Central serotonin depletion impairs both the acquisition and performance of a symmetrically reinforced go/no-go conditional visual discrimination

The involvement of central serotonin systems in behavioural disinhibition in the rat was assessed using a symmetrically reinforced go/no-go conditional visual discrimination task. Selective central 5-HT depletion (generally averaging more than 90% in neocortex, hippocampus and striatum) was induced by intracerebroventricular administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) following pretreatment with both a noradrenergic and dopaminergic re-uptake inhibitor. The lesioned animals failed to acquire the conditional visual discrimination. This deficit was due to an inability to withhold responding and thus correctly complete the no-go trials. The lesioned animals responded faster both correctly, during go trials, and incorrectly during no-go trials. Impulsive early responding during the initial 1.2 s of the stimulus presentation was also increased by 5-HT depletion. Subjects that were lesioned after stable performance of the task had been acquired showed a similar, but smaller effect. These animals displayed more accurate performance of the go trials, but poorer performance of the no-go trials. Once again, go trial response latencies were faster and early responses during the no-go trials were increased by the lesion. The results suggest that previous accounts of impulsive responding induced by 5-HT depletion fail to recognise the pervasive nature of this effect, which affects multiple behavioural indices of response disinhibition and can impede the acquisition and performance of discrimination tasks depending on their precise response requirements.

Dissociable deficits in the decision-making cognition of chronic amphetamine abusers, opiate abusers, patients with focal damage to prefrontal cortex, and tryptophan-depleted normal volunteers: evidence for monoaminergic mechanisms

We used a novel computerized decision-making task to compare the decision-making behavior of chronic amphetamine abusers, chronic opiate abusers, and patients with focal lesions of orbital prefrontal cortex (PFC) or dorsolateral/medial PFC. We also assessed the effects of reducing central 5-hydroxytryptamine (5-HT) activity using a tryptophan-depleting amino acid drink in normal volunteers. Chronic amphetamine abusers showed suboptimal decisions (correlated with years of abuse), and deliberated for significantly longer before making their choices. The opiate abusers exhibited only the second of these behavioral changes. Importantly, both sub-optimal choices and increased deliberation times were evident in the patients with damage to orbitofrontal PFC but not other sectors of PFC. Qualitatively, the performance of the subjects with lowered plasma tryptophan was similar to that associated with amphetamine abuse, consistent with recent reports of depleted 5-HT in the orbital regions of PFC of methamphetamine abusers. Overall, these data suggest that chronic amphetamine abusers show similar decision-making deficits to those seen after focal damage to orbitofrontal PFC. These deficits may reflect altered neuromodulation of the orbitofrontal PFC and interconnected limbic-striatal systems by both the ascending 5-HT and mesocortical dopamine (DA) projections.

Forebrain serotonin depletion facilitates the acquisition and performance of a conditional visual discrimination task in rats

Three experiments examined the effects of depleting forebrain 5-hydroxytryptamine (5-HT) on the acquisition and performance of an operant conditional discrimination in the visual modality. In the first experiment, rats with 5-HT lesions induced by infusing the neurotoxin 5,7-dihydroxytryptamine intracerebroventricularly acquired the conditional visual discrimination more rapidly than the sham-operated controls. Following acquisition, a series of manipulations of the task parameters tested the effects of the lesion on cognitive, sensory and motivational aspects of performance. In experiment two, the performance of rats that had acquired the task to asymptote before receiving lesions was assessed. The performance of this second group of serotonin-lesioned rats was similar to that of the pre-acquisition lesioned group following all but one manipulation of the task parameters. When the rate of stimulus presentations was increased, rats with forebrain 5-HT depletions were protected from the disruptive effects on performance seen in the sham-operated controls. This latter finding was also observed in a third experiment, in which the infusion of the 5-HT1A receptor partial agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), directly into the dorsal raphe nucleus improved the performance of unlesioned rats following an increase in the rate of stimulus presentations. The results are discussed in terms of the behavioural, neurochemical and neuroanatomical specificity of serotonin function in appetitive learning and the implications for general theories of the function of serotoninergic processes in cognition.

Effects of excitotoxic lesions of the basolateral amygdala on conditional discrimination learning with primary and conditioned reinforcement

Rats with excitotoxic lesions of the basolateral amygdala (BLA) were not impaired in the acquisition of an appetitive visuospatial conditional discrimination between stimuli varying in temporal frequency that has previously been shown to be sensitive to the effects of lesions of the striatum and cingulate cortex. After asymptotic performance was attained, discrimination was reinforced according to a fixed ratio (FR) schedule under which n presentations of sucrose were provided following n correct responses; each correct response also being reinforced immediately by a light acting as a conditioned reinforcer. Under these conditions of reinforcement when FRn=5, BLA-lesioned rats initially showed transient impairments in several aspects of performance, but rapidly attained control levels over subsequent test sessions. No further impairments occurred when FRn=10/20. However, in various conditions of extinction, further differences in performance were revealed between the BLA-lesioned and control groups, notably a significantly enhanced resistance to extinction when both sucrose and conditioned reinforcement were omitted. The results are discussed in terms of limbic-striatal mechanisms in the control of discrimination learning and the possible role of the amygdala in the mediation of different aspects of conditioned reinforcement.

Dissociation in effects of lesions of the nucleus accumbens core and shell on appetitive pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-amphetamine

Dopamine release within the nucleus accumbens (NAcc) has been associated with both the rewarding and locomotor-stimulant effects of abused drugs. The functions of the NAcc core and shell were investigated in mediating amphetamine-potentiated conditioned reinforcement and locomotion. Rats were initially trained to associate a neutral stimulus (Pavlovian CS) with food reinforcement (US). After excitotoxic lesions that selectively destroyed either the NAcc core or shell, animals underwent additional CS-US training sessions and then were tested for the acquisition of a new instrumental response that produced the CS acting as a conditioned reinforcer (CR). Animals were infused intra-NAcc with D-amphetamine (0, 1, 3, 10, or 20 microg) before each session. Shell lesions affected neither Pavlovian nor instrumental conditioning but completely abolished the potentiative effect of intra-NAcc amphetamine on responding with CR. Core-lesioned animals were impaired during the Pavlovian retraining sessions but showed no deficit in the acquisition of responding with CR. However, the selectivity in stimulant-induced potentiation of the CR lever was reduced, as intra-NAcc amphetamine infusions dose-dependently increased responding on both the CR lever and a nonreinforced (control) lever. Shell lesions produced hypoactivity and attenuated amphetamine-induced activity. In contrast, core lesions resulted in hyperactivity and enhanced the locomotor-stimulating effect of amphetamine. These results indicate a functional dissociation of subregions of the NAcc; the shell is a critical site for stimulant effects underlying the enhancement of responding with CR and locomotion after intra-NAcc injections of amphetamine, whereas the core is implicated in mechanisms underlying the expression of CS-US associations.

Repeated neonatal maternal separation alters intravenous cocaine self-administration in adult rats

Behavioural responses to psychostimulant drugs can be profoundly affected by early environmental influences. The aim of this study was to describe the effects of repeated brief separations of rat pups from their dams during the early neonatal period on cocaine self-administration behaviour as adults. Lister hooded rats exposed to a repeated maternal separation procedure (REMS) showed altered acquisition and maintenance of cocaine self-administration as adults, the effects being dose and gender-dependent. Overall, the patterns of acquisition of self-administration across three doses of cocaine (0.05, 0.08 and 0.5 mg/injection) suggested a rightward shift in the acquisition dose-effect functions for the REMS animals relative to control animals. At 0.05 mg/injection, there was a retarded acquisition of cocaine self-administration in male and female neonatally separated rats. At 0.08 mg/injection there was a facilitated acquisition in female neonatally separated subjects. After establishment of stable self-administration of the training dose, in the same cohort of subjects, rightward and downward shifts in the cocaine self-administration dose-effect functions were determined for female and male REMS subjects, respectively, relative to their controls. The dose-effect function for both female groups was shifted to the left of that of the respective male groups, although the lighter body weights of the females meant that they administered a higher unit dose per unit body weight than the males. Whereas male REMS subjects tended to self-administer less cocaine than the controls at the dose eliciting maximal responding (0.03 mg/injection) and to make fewer lever responses overall at each dose tested, female REMS subjects self-administered significantly more cocaine than their respective controls at a dose of 0.03 mg/injection. There was no differential sensitivity to the rate-altering effects of the selective dopamine D2 receptor antagonist, eticlopride, or to the selective dopamine D1 receptor antagonist, SCH 23390. These data provide further evidence that altered early environment affects drug-taking behaviour in a developmentally specific and gender-specific manner, with the effects of neonatal separation contrasting with previously published data on the effects of post-weaning isolation rearing.

Acquisition, maintenance and reinstatement of intravenous cocaine self-administration under a second-order schedule of reinforcement in rats: effects of conditioned cues and continuous access to cocaine

Second order schedules of IV cocaine reinforcement in rats provide a reliable method for evaluating the effects of conditioned stimuli on cocaine-seeking behaviour, and for measuring the motivational aspects of cocaine reinforcement. In the procedure established here, each infusion of cocaine (0.25 mg/infusion) was initially made contingent on a lever press and was paired with a 20-s light conditioned stimulus (CS). When rats acquired stable rates of cocaine self-administration, the response requirement for cocaine was increased progressively to a second-order schedule of the type FI15 min(FR10:S), whereby the IV cocaine infusion was self-administered following the completion of the first FR10 responses (and CS presentation) after a 15-min fixed interval (FI) had elapsed. Evaluation of the animals' responding during the first, drug-free interval of each daily session provided a measure of cocaine-seeking behaviour, independent of other pharmacological effects of the self-administered drug. Thus, a dose-response study (dose range: 0.083, 0.25 and 0.50 mg/infusion) revealed that responding under this schedule during the initial, drug-free interval changed monotonically with dose, whereas an inverse relationship between cocaine dose and response level tended to appear during the rest of the session, after rats had self-administered the drug. Responding under this schedule was also shown to occur under the control of the CS, which had acquired conditioned reinforcing properties. Thus, a decrease in responding and an increase in the latency to initiate responding followed the omission of the CS for 3 consecutive days. In addition, extinction of cocaine-seeking behaviour was slower when contingent CS presentations occurred compared to extinction when the CS was not present. Furthermore, the reinstatement of responding for cocaine, which followed a brief period of non-contingent CS presentations, was retarded when this conditioned reinforcer had been extinguished together with cocaine. Finally, cocaine-seeking behaviour decreased markedly for the first 6 h that followed a 12-h period of continuous access to cocaine, when compared to responding 6 h after a 90-min session of limited access to the drug. Responding subsequently increased to baseline levels within 72 h. These results emphasise the utility of second-order schedules for studying drug-seeking behaviour and the importance of drug-associated cues in maintaining such responding for cocaine.

Excitotoxic lesions of the mediodorsal thalamic nucleus attenuate intravenous cocaine self-administration

The present experiments investigated the effects of excitotoxic, axon-sparing lesions of the mediodorsal nucleus of the thalamus (MD) on locomotor activity and i.v. cocaine self-administration. Infusion of quinolinic acid into the MD using a glass micropipette produced well-defined neuronal loss restricted to medial and lateral portions of the MD, sparing adjacent areas such as the lateral habenula and paraventricular thalamic nucleus. MD lesions resulted in delayed habituation to activity cages. In addition, lesioned rats self-administered significantly smaller amounts of cocaine than controls during a 14-day acquisition period, and showed attenuated responding for cocaine doses on the descending limb of the dose-effect function. Since typical titrating patterns of responding were maintained in lesioned rats, and responding on the inactive lever did not differ from sham-operated animals, these present results indicate an enhanced sensitivity to the reinforcing effects of response-contingent cocaine in rats with excitotoxic lesions of the MD.

Basal forebrain cholinergic lesions enhance conditioned approach responses to stimuli predictive of food

This study examined the effects of lesions to different neuronal populations within the basal forebrain on reward-related learning. Rats received bilateral alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) or quinolinate lesions that preferentially destroy the cholinergic nucleus basalis magnocellularis (NBM) or noncholinergic ventral pallidal neurons, respectively. Both lesions enhanced conditioned approach responses to stimuli predictive of food but did not increase the locomotor stimulating effect of d-amphetamine. Although both lesions disrupted the discriminative control over behavior by a conditioned stimulus, they did not impair the subsequent acquisition of instrumental responding with conditioned reinforcement (CR). Indeed, both lesions were associated with an increased responding with CR following intra-accumbens infusions of d-amphetamine (0, 1, 3, 10, and 20 microg). Quinolinate lesions also increased responses on an inactive control lever. Neither lesion altered consummatory responses to food or sucrose. Results suggest that NBM lesions may disrupt the balance between cortical and subcortical dopamine levels, and/or produce a deficit in attentional mechanisms that is manifested as increased responding to specific stimuli.

3-Nitropropionic acid-induced changes in the expression of metabolic and astrocyte mRNAs

Systemic administration of 3-nitropropionic acid (3NPA) in rats produces bilateral striatal lesions which are similar to those seen in Huntington's disease (HD). We examined the effects of systemic 3NPA on the expression of cytochrome oxidase (COX-II and COX-IV), succinate dehydrogenase (SDH) and astrocytic glial fibrillary acidic protein (GFAP) mRNAs and on the activity of COX and SDH as assessed by the density of histochemical staining. COX-II and COX-IV mRNA was reduced in rats with 3NPA-induced lesions, but not in those without, whereas SDH, but not COX, staining was significantly and dose-dependently reduced in both 3NPA treated groups. GFAP mRNA expression was increased in both intact striatum and cortex but was absent from the lesion core.

Neural systems underlying arousal and attention. Implications for drug abuse

The monoaminergic and cholinergic systems are implicated in different forms of behavioral arousal that can be dissected in terms of their forebrain targets and the nature of the behavioral processes they modulate in distinct regions. Thus, evidence in rats with selective neurochemical manipulations tested behaviorally using an analog of an attentional task developed for human subjects indicates that the coeruleo-cortical noradrenergic system is implicated in divided and selective attention, the basal forebrain cholinergic system in stimulus detection, the mesostriatal and mesolimbic dopaminergic systems in response speed and vigor, and the mesencephalic serotoninergic or 5-HT systems in response inhibition. Our recent studies have focused on fractionating, in the same task, the differential contributions of the dorsal and median raphé 5-HT systems as well as elucidating the functions of the mesocortical dopaminergic system, each of which may be relevant to understanding the behavioral and cognitive sequelae of cocaine administration in human subjects as well as in experimental animals.

Neural Systems Underlying Arousal and Attention: Implications for Drug Abusea

The monoaminergic and cholinergic systems are implicated in different forms of behavioral arousal that can be dissected in terms of their forebrain targets and the nature of the behavioral processes they modulate in distinct regions. Thus, evidence in rats with selective neurochemical manipulations tested behaviorally using an analog of an attentional task developed for human subjects indicates that the coeruleo-cortical noradrenergic system is implicated in divided and selective attention, the basal forebrain cholinergic system in stimulus detection, the mesostriatal and mesolimbic dopaminergic systems in response speed and vigor, and the mesencephalic serotoninergic or 5-HT systems in response inhibition. Our recent studies have focused on fractionating, in the same task, the differential contributions of the dorsal and median raphé 5-HT systems as well as elucidating the functions of the mesocortical dopaminergic system, each of which may be relevant to understanding the behavioral and cognitive sequelae of cocaine administration in human subjects as well as in experimental animals.

Basal forebrain cholinergic lesions enhance conditioned approach responses to stimuli predictive of food

This study examined the effects of lesions to different neuronal populations within the basal forebrain on reward-related learning. Rats received bilateral alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) or quinolinate lesions that preferentially destroy the cholinergic nucleus basalis magnocellularis (NBM) or noncholinergic ventral pallidal neurons, respectively. Both lesions enhanced conditioned approach responses to stimuli predictive of food but did not increase the locomotor stimulating effect of d-amphetamine. Although both lesions disrupted the discriminative control over behavior by a conditioned stimulus, they did not impair the subsequent acquisition of instrumental responding with conditioned reinforcement (CR). Indeed, both lesions were associated with an increased responding with CR following intra-accumbens infusions of d-amphetamine (0, 1, 3, 10, and 20 microg). Quinolinate lesions also increased responses on an inactive control lever. Neither lesion altered consummatory responses to food or sucrose. Results suggest that NBM lesions may disrupt the balance between cortical and subcortical dopamine levels, and/or produce a deficit in attentional mechanisms that is manifested as increased responding to specific stimuli.

Effects of lesions of the nucleus basalis magnocellularis on the acquisition of cocaine self-administration in rats

The nucleus basalis magnocellularis (NBM) is one element in the limbic cortical-ventral striatal circuitry that has been implicated in reinforcement processes. The present study examined the involvement of the cholinergic neurons of the NBM in mediating aspects of cocaine reinforcement. Lesions of the NBM were made by injecting 0.01 M AMPA into the subpallidal basal forebrain. Following 4 days' recovery, rats were implanted chronically with catheters in the jugular vein. In three separate experiments, rats were trained to acquire cocaine self-administration under a FR1 schedule of reinforcement at doses of 0.25, 0.083 and 0.028 mg/injection. A dose-effect function was also determined at the end of the acquisition experiments using five different doses of cocaine (0.009, 0.028, 0.083, 0.25, 0.50 mg/injection) and saline which were presented once daily in a Latin square design. There were no significant differences between groups in the acquisition of cocaine self-administration at any of the three doses studied (0.028, 0.083 and 0.25 mg/injection), although at the lowest dose, lesioned animals responded at greater levels on both active and inactive levers. However, a shift to the left in the cocaine dose-response function was observed revealing that the lesioned group self-administered significantly higher amounts of low doses of cocaine than control rats. These data suggest that the integrity of the NBM is not a critical determinant of the reinforcing effects of cocaine during the acquisition of self-administration of the drug, but that NBM-dependent cholinergic mechanisms may nevertheless interact with the neural substrates mediating the reinforcing properties of cocaine. The data are relevant to recent hypotheses of functional interactions between the dopaminergic system and the cholinergic NBM.

Perseveration and strategy in a novel spatial self-ordered sequencing task for nonhuman primates: effects of excitotoxic lesions and dopamine depletions of the prefrontal cortex

Damage to the prefrontal cortex disrupts the performance of self-ordered sequencing tasks, although the precise mechanisms by which this effect occurs is unclear. Active working memory, inhibitory control, and the ability to generate and perform a sequence of responses are all putative cognitive abilities that may be responsible for the impaired performance that results from disruption of prefrontal processing. In addition, the neurochemical substrates underlying prefrontal cognitive function are not well understood, although active working memory appears to depend upon an intact mesocortical dopamine system. The present experiments were therefore designed to evaluate explicitly the contribution of each of these abilities to successful performance of a novel spatial self-ordered sequencing task and to examine the contribution of the prefrontal cortex and its dopamine innervation to each ability in turn. Excitotoxic lesions of the prefrontal cortex of the common marmoset profoundly impaired the performance of the self-ordered sequencing task and induced robust perseverative responding. Task manipulations that precluded perseveration ameliorated the effect of this lesion and revealed that the ability to generate and perform sequences of responses was unaffected by excitotoxic damage to prefrontal cortex. In contrast, large dopamine and noradrenaline depletions within the same areas of prefrontal cortex had no effect on any aspect of the self-ordered task but did impair the acquisition of an active working memory task, spatial delayed response, to the same degree as the excitotoxic lesion. These results demonstrate that a lesion of the ascending monoamine projections to the prefrontal cortex is not always synonymous with a lesion of the prefrontal cortex itself and thereby challenge existing concepts concerning the neuromodulation of prefrontal cognitive function.