Regional and temporal differences in real-time dopamine efflux in the nucleus accumbens during free-choice novelty

Frequency of dopamine concentration transients increases in dorsal and ventral striatum of male rats during introduction of conspecifics

Transient, elevated concentrations of extracellular dopamine were characterized in the dorsal and ventral striatum of male rats during solitude, brief interaction with a conspecific, and copulation. Conspecific rats were systematically presented to male rats and allowed to interact for 30 sec; the males were kept in solitude between each presentation. During these episodes, 125 dopamine concentration transients from 17 rats were detected with fast-scan cyclic voltammetry at carbon-fiber microelectrodes (peak amplitude, 210 +/- 10 nm; duration, 530 +/- 20 msec). The frequency of dopamine transients increased sixfold during conspecific episodes compared with solitude. However, the phasic dopamine activity habituated on the second presentation of the conspecifics. When males were allowed to copulate with receptive females, additional dopamine transients were observed at frequencies approximately 20% of those during the previous interaction episodes. A subset of these transients immediately preceded intromission. Overall, phasic dopamine activity appeared to be associated with input from multiple sensory modalities and was followed by a variety of approach and appetitive behaviors, consistent with electrophysiological observations of dopaminergic neuron burst-firing. In summary, (1) dopamine concentration transients occur in awake rats during solitude, in the absence of overt external cues; (2) dopamine transients are significantly more frequent in the presence of a conspecific, although this effect habituates; and (3) dopamine transients are less frequent during copulation than during brief conspecific episodes. These results establish for the first time that transient dopamine fluctuations occur throughout the dorsal and ventral striatum and demonstrate that they are more frequent with salient stimuli that elicit a response behavior.

Modeling fast dopamine neurotransmission in the nucleus accumbens during behavior

Recent advances in electrophysiology and voltammetry permit monitoring of dopamine (DA) neuronal activity in real time in the brain of awake animals. Studies using these approaches demonstrate that behaviorally relevant events elicit characteristic patterns of electrical activity in midbrain DA neurons as well as large, transient changes in extracellular DA in the nucleus accumbens (NAc). In addition to providing insight into the role of the DA system in the processing of motor, motivational, and sensory information, the new findings also shed light on fast DA neurotransmission in a behavioral context. This report, (1). summarizes the information obtained by electrophysiological and real-time voltammetric approaches and (2). describes a general model of phasic DA signaling in the NAc that links the observed changes in DA electrical activity and extracellular dynamics. The analysis demonstrates that the behaviorally evoked DA transients are governed by similar mechanisms as those produced by short trains of electrical stimulation. Thus, action potential-dependent release and presynaptic uptake are primary determinants of functional DA levels in the brain during behavior. Interestingly, the model predicts that the same burst of electrical activity generated at DA cell bodies produces markedly different DA dynamics in forebrain projection fields. The distinct changes result from heterogeneous release and uptake rates and may underlie region-specific effects of DA. Auto- and heteroreceptors, as well as other sites of presynaptic control, could further modulate the DA transients.

Dissociation in the involvement of dopaminergic neurons innervating the core and shell subregions of the nucleus accumbens in latent inhibition and affective perception

Mesencephalic dopaminergic neurons have been found to be involved in affective processes. Their implication in cognitive processes appears less well understood. The use of latent inhibition paradigms is a means of studying these kinds of processes. In this study, we investigated the involvement of dopaminergic projections in the core, the dorsomedial shell and the ventromedial shell of the nucleus accumbens, in latent inhibition in olfactory aversive learning. Variations in extracellular dopamine levels induced by an aversively conditioned olfactory stimulus were monitored in the three parts of the nucleus accumbens in the left hemisphere, after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. The parallel between dopamine changes and place preference or aversion toward the stimulus were analyzed in pre-exposed and non-pre-exposed animals. Results showed that dopaminergic neurons innervating the nucleus accumbens are differentially involved in the latent inhibition phenomenon. Dopaminergic neurons innervating the core and the dorsomedial shell subregions of the nucleus accumbens appeared to be involved in latent inhibition processes, unlike those reaching the ventromedial shell. Nonetheless dopamine in the ventromedial shell was found to be involved in affective perception of the stimulus.The present data suggest that dopaminergic neurons innervating the three nucleus accumbens subregions are functionally related to networks involved in parallel processing of the cognitive and affective values of environmental information, and that interaction between these systems, at some levels, may lead to a given behavioral output. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

Concurrent autoreceptor-mediated control of dopamine release and uptake during neurotransmission: an in vivo voltammetric study

Receptor-mediated feedback control plays an important role in dopamine (DA) neurotransmission. Recent evidence suggests that release and uptake, key mechanisms determining brain extracellular levels of the neurotransmitter, are governed by presynaptic autoreceptors. The goal of this study was to investigate whether autoreceptors regulate both mechanisms concurrently. Extracellular DA in the caudate-putamen and nucleus accumbens, evoked by electrical stimulation of the medial forebrain bundle, was monitored in the anesthetized rat by real-time voltammetry. Effects of the D2 antagonist haloperidol (0.5 mg/kg, i.p.) on evoked DA levels were measured to evaluate autoreceptor control mechanisms. Two strategies were used to resolve individual contributions of release and uptake to the robust increases in DA signals observed after acute haloperidol challenge in naive animals: pretreatment with 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), an irreversible inhibitor of the DA transporter, and kinetic analysis of extracellular DA dynamics. RTI-76 effectively removed the uptake component from recorded signals. In RTI-76-pretreated rats, haloperidol induced only modest increases in DA elicited by low frequencies and had little or no effect at high frequencies. These results suggest that D2 antagonism alters uptake at all frequencies but only release at low frequencies. Kinetic analysis similarly demonstrated that haloperidol decreased V(max) for DA uptake and increased DA release at low (10-30 Hz) but not high (40-60 Hz) stimulus frequencies. We conclude that presynaptic DA autoreceptors concurrently downregulate release and upregulate uptake, and that the mechanisms are also independently controlled during neurotransmission.

Transient changes in mesolimbic dopamine and their association with 'reward'

Mesolimbic dopaminergic neurons modulate complex circuitry in the ventral forebrain involved in reward processing, although the precise function of the dopaminergic input is debated. Electrophysiological measurements have revealed that mesolimbic dopaminergic neurons can fire in either tonic or phasic modes, and that phasic firing accompanies the alerting or anticipatory phases of reward. However, the neurochemical relevance of this rapid neuronal discharge within the reward processing circuitry is not yet clear, in part because of difficulty in interpretation of extracellular dopamine measurements. Herein, the nature of the information provided by different neurochemical techniques is critically discussed. Classical methods of monitoring dopamine reveal changes in extracellular dopamine resulting from tonic neuronal activity, but do not have the temporal resolution to distinguish concentration transients. However, recent advances in dopamine sensors now enable transient dopamine concentrations resulting from phasic firing to be positively identified and followed on a physiologically relevant timescale. This has enabled demonstrations of discrete, phasic dopamine signals accompanying rewarding or alerting stimuli. Thus, enhanced dopamine release at terminals appears to be coincident with phasic electrical activity at cell bodies. These accumulating data promise to help unravel the precise role of phasic dopamine transmission in reward processing.

Time window of autoreceptor-mediated inhibition of limbic and striatal dopamine release

Forebrain dopamine release is under the local control of D2 family (D2 and D3) autoreceptors. In this study, autoreceptor-mediated modulation of forebrain dopamine release was investigated using amperometry in brain slices following local electrical stimulation. 350 microm-thick slices of nucleus accumbens or dorsolateral neostriatum were prepared from male Wistar rats (150-200 g) and superfused with artificial cerebrospinal fluid at 32 degrees C. Dopamine release was evoked by electrical pulses (0.1 ms, 10 mA) across bipolar tungsten stimulating electrodes and measured at carbon fibre microelectrodes using fixed potential amperometry (+300 mV vs. Ag/AgCl). Peak dopamine release on stimulation (single pulse) was 0.75 microM (neostriatum) and 1.37 microM (nucleus accumbens). Metoclopramide (1 microM) had no significant effect on DA efflux from a single pulse in either region. Using paired pulse stimuli, dopamine release on the second pulse varied according to the interval between the two pulses. At very long intervals (>20 sec), dopamine release was similar to that for the first pulse. At shorter intervals, dopamine efflux was attenuated. Metoclopramide had no effect on second pulse dopamine release when the pulse was applied at short (<0.1 sec) or long (>5.0 sec) intervals after the first. At intermediate intervals, metoclopramide significantly increased second pulse dopamine release. The peak dopamine autoreceptor effect occurred at approximately 550 ms in neostriatum and approximately 700 ms in nucleus accumbens. The onset time is due both to diffusion of dopamine from the release sites to the autoreceptors and receptor-effector mechanisms. These findings may have implications for the local control of forebrain dopamine function in physiological and pathological states.

Interactions between environmental stimulation and antipsychotic drug effects on forebrain c-fos activation

The immediate-early gene product Fos is differentially induced in the rat brain by the antipsychotic drugs haloperidol and clozapine. It is often claimed that although both drugs induce Fos in the nucleus accumbens, haloperidol but not clozapine increases Fos-like immunoreactivity in the striatum, whereas clozapine but not haloperidol increases Fos-like immunoreactivity in prefrontal cortex. Investigations of antipsychotic drug effects on Fos have typically administered high doses with pronounced sedative effects to behaviorally naive animals. In the present study, we compared the effects of low doses of haloperidol (0.1 mg/kg) and clozapine (5 mg/kg) on Fos-like immunoreactivity in rats which were either behaviorally naive, exposed to a novel environment or tested for two-way active avoidance. We determined that haloperidol increased Fos in the striatum and nucleus accumbens regardless of testing condition whereas clozapine markedly reduced the induction of Fos by behavioral testing in these regions; moreover, haloperidol dramatically increased prefrontal cortical Fos expression in animals placed in a novel environment, but not in testing-naive controls. From these results we suggest that antipsychotic drug-induced patterns of Fos expression in the rat are highly dependent on animals' concurrent behavioral status, perhaps reflecting neuroanatomically specific interactions between antipsychotic drugs and environmental stressors which also may occur in the schizophrenic condition.

Bed nucleus of the stria terminalis and extended amygdala inputs to dopamine subpopulations in primates

The 'extended amygdala', a forebrain continuum implicated in complex motivational responses, is comprised of the bed nucleus of the stria terminalis and its sublenticular extension into the centromedial amygdala. Dopamine is also involved in motivated behavior, and is increased in several brain regions by emotionally relevant stimuli. To examine how the extended amygdala influences the dopamine cells, we determined the organization of inputs from subdivisions of the bed nucleus of the stria terminalis and sublenticular extended amygdala to the dopamine subpopulations in monkeys. Inputs from the bed nucleus of the stria terminalis and corresponding regions of the sublenticular extended amygdala are differentially organized. The medial bed nucleus of the stria terminalis and its medial sublenticular extension have a mediolateral organization with the densest inputs to the medial substantia nigra, pars compacta, and relatively few inputs to the central and lateral substantia nigra. In contrast, the lateral bed nucleus of the stria terminalis (and its continuation into the sublenticular extended amygdala) projects across the mediolateral extent of the substantia nigra. The subnuclei of the lateral bed nucleus of the stria terminalis also have differential projections to the dopamine cells. While the central core of the lateral bed nucleus of the stria terminalis has restricted inputs, the surrounding dorsolateral, capsular and juxtacapsular subdivisions project strongly to the dorsal tier dopamine neurons. The posterior subdivision of the lateral bed nucleus of the stria terminalis and its continuation into the central sublenticular extended amygdala project more broadly to both the dorsal tier and densocellular region of the ventral tier. From these results we suggest that specific subdivisions of the bed nucleus of the stria terminalis have differential influences on the dopamine subpopulations, influencing dopamine responses in diverse brain regions.

Sub-second changes in accumbal dopamine during sexual behavior in male rats

Transient (200--900 ms), high concentrations (200--500 nM) of dopamine, measured using fast-scan cyclic voltammetry, occurred in the nucleus accumbens core of male rats at the presentation of a receptive female. Additional dopamine signals were observed during subsequent approach behavior. Background-subtracted cyclic voltammograms of the naturally-evoked signals matched those of electrically-evoked dopamine measured at the same recording sites. Administration of nomifensine amplified natural and evoked dopamine release, and increased the frequency of detectable signals. While gradual changes in dopamine concentration during sexual behavior have been well established, these findings dramatically improve the time resolution. The observed dopamine transients, probably resulting from neuronal burst firing, represent the first direct correlation of dopamine with sexual behavior on a sub-second time scale.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

Dopamine D2 receptors in the insular cortex and the personality trait of novelty seeking

Human personality has been considered to have a neurochemical background. We examined the relation between extrastriatal dopamine D2 receptor binding in living human brain and the personality trait of novelty seeking that has been proposed to be related to dopaminergic function in the brain. We measured extrastriatal dopamine D2 receptors of 24 healthy young male subjects using [(11)C]FLB 457 positron emission tomography. The personality trait of each subject was assessed by the Temperament and Character Inventory (TCI). Correlation of dopamine D2 receptor binding with novelty seeking was calculated using region-of-interest analysis and statistical parametric mapping based on the binding potential images generated using a reference tissue model. A significant negative correlation was observed between binding potential values and the novelty seeking scores on TCI in the right insular cortex. No significant correlation was observed in any other region. Our result indicates that there is a significant association between dopamine D2 receptor binding and the human novelty seeking trait in the right insular cortex.

Novelty-evoked elevations of nucleus accumbens dopamine: dependence on impulse flow from the ventral subiculum and glutamatergic neurotransmission in the ventral tegmental area

In vivo microdialysis in freely moving rats was used to monitor novelty-evoked elevations in extracellular dopamine in the nucleus accumbens septi (NAS) and to examine the role of the ventral subiculum of the hippocampus and glutamatergic transmission in the ventral tegmental area (VTA) on these elevations. Exposure to novel stimuli evoked investigatory activity and increased nucleus accumbens dopamine. Unilateral injections of the sodium channel blocker tetrodotoxin (0.16 ng/0.5 microL) into the ventral subiculum ipsilateral to the dialysed NAS abolished novelty-evoked elevations in dopamine. Injections of tetrodotoxin into the contralateral VS did not prevent novelty-evoked elevations in nucleus accumbens dopamine. Unilateral perfusion (via microdialysis) of the ionotropic glutamate receptor antagonists kynurenic acid (1 mM) into the ipsilateral but not the contralateral VTA blocked novelty-evoked elevations in nucleus accumbens dopamine. Neither unilateral injections of tetrodotoxin nor unilateral perfusion of kynurenic acid disrupted investigatory behaviour. These data indicate that phasic elevations in nucleus accumbens dopamine evoked by exposure to unconditioned novel stimuli are dependent on impulse flow from the hippocampus and glutamatergic transmission in the VTA.

Environmental familiarization in rats: differential effects of acute and chronic nicotine

If an environment is familiar, rats will interact more with a novel object than if the environment is unfamiliar. In two experiments we used this behavioral tendency to assess the effects of nicotine on environmental familiarization (i.e., an elevated platform). As expected, rats given 2 min of exposure to the platform on 2 consecutive days (familiarization phase) interacted more with a novel object in a subsequent test than rats that had not experienced the platform until the test day. During the familiarization phase acute pretreatment with nicotine (0.6 and 1.8 mg/kg, subcutaneous) 10 min before platform exposure interfered with familiarization processes, as measured by object interaction on the drug-free test day. Behavioral measures of activity (e.g., turning and midline crosses) eliminated an account based on nicotine-induced motor impairment. Furthermore, this effect of acute nicotine on familiarization was not due to nonspecific effects of nicotine. Controls that received equivalent nicotine exposure temporally separated from platform exposure interacted more with the novel object than similarly treated controls that were unfamiliar with the platform on the test day. Interestingly, rats treated once daily with 0.6 mg/kg nicotine for 14 days before the familiarization phase (chronic condition) did not show a decrease in environmental familiarity. This dissociation extends a growing literature finding that the behavioral and neurobiological effects of nicotine differ, in part, after acute and chronic exposure. Indeed, acute nicotine (0. 2, 0.6, and 1.2 mg/kg) in the present report consistently decreased the amount of time spent with one paw on the edge of the platform; chronic nicotine did not affect this behavior.

Phasic inhibition of dopamine uptake in nucleus accumbens induced by intravenous cocaine in freely behaving rats

A new approach combining fast-scan cyclic voltammetry with iontophoretic dopamine delivery was used in freely behaving rats to evaluate the time-course of dopamine uptake inhibition in nucleus accumbens induced by intravenous cocaine at a dose (1.0mg/kg) known to maintain self-administration behavior. Cocaine significantly increased the decay time of the dopamine response without altering its magnitude or time to peak. An increase in decay time was evident at 2 min, peaked at 6 min (+87%), and decreased to baseline at 18 min after a single cocaine injection. The change in decay time was similar in all rats and remained essentially the same, albeit slightly larger, for subsequent cocaine injections both within a session and over repeated sessions. The change in dopamine decay time did not correlate with cocaine-induced motor activation, which was maximal during the first minute after injection and decreased slowly over the next 20 min. Our data provide direct evidence for a phasic change in dopamine uptake induced by intravenous cocaine under behaviorally relevant conditions. The relatively slow and gradual development of dopamine uptake inhibition, which peaks at times when behaving rats self-inject cocaine, is inconsistent with the suggested role of this mechanism in the acute rewarding (euphoric) effects of self-injected cocaine, but supports its role in the activational and motivational aspects of drug-seeking and drug-taking behavior. Because intravenous cocaine enters the brain rapidly and peaks in neural tissue (1-2 min) long before it effectively inhibits dopamine uptake (6 min), it appears that some of the acute psychoemotional ("rush"), behavioral, autonomic, and neuronal effects of this drug, which are apparently resistant to dopamine receptor blockade, are mediated via rapid central or peripheral mechanisms independent of monoamine uptake.

Differential involvement of dopamine in the shell and core of the nucleus accumbens in the expression of latent inhibition to an aversively conditioned stimulus

Latent inhibition, the process whereby pre-exposure to a conditioned stimulus without consequence impairs subsequent learning of an association between the conditioned stimulus and an unconditioned stimulus, is reportedly disrupted in both amphetamine-treated rats and in acute schizophrenics. This has led to the suggestion that disruptions in latent inhibition model some of the cognitive impairments associated with hyperactive dopamine transmission as it is expressed in schizophrenic patients. Specifically, fluctuations in dopamine neurotransmission within the nucleus accumbens have been implicated in the mediation of latent inhibition; however, it has not been established whether these dopamine-mediated effects occur in the shell or core subregion of the nucleus. In the present study, 48h after conditioned stimulus-pre-exposed and non-pre-exposed animals experienced 10 pairings of tone and footshock, we measured extracellular levels of dopamine in the shell and core during the expression of latent inhibition to an aversively conditioned tone using in vivo microdialysis. Our results show that pre-exposure to the tone eliminated the conditioned release of dopamine in the shell of the nucleus accumbens and resulted in an attenuated conditioned freezing response to the tone conditioned stimulus. In contrast, dopamine release in the core was not affected by pre-exposure to the tone. These data suggest that it is specifically the shell of the nucleus accumbens in which alterations of dopaminergic tone, whether pharmacologically induced in rodents or the result of disease in humans, may act to disrupt latent inhibition.

The central nucleus of the amygdala projection to dopamine subpopulations in primates

The dopamine system plays a major role in responses to potentially rewarding stimuli. An important input to the dopamine neurons is derived from the central nucleus of the amygdala. The central nucleus is a complex structure consisting of several subdivisions with distinct histochemical, morphologic, and connectional features. The central nucleus subdivisions are therefore likely to have specific inputs to the dopamine neurons. The midbrain dopamine cells are divided into dorsal and ventral subpopulations. We determined the organization of inputs from the central nucleus subdivisions to the dopamine subpopulations in monkeys. The dorsal tier neurons receive relatively greater central nucleus input compared to the ventral tier. Within the ventral tier, the central nucleus projects to the densocellular region, but not the cell columns. Furthermore, the midbrain subpopulations receive a differential projection from specific central nucleus subterritories. The medial subdivision of the central nucleus has the greatest input to the dopamine system, and projects throughout the dorsal tier and densocellular regions. This indicates that the medial subdivision influences not only the ventral striatum but also more dorsal striatal areas, through its inputs to these dopamine subpopulations. In contrast, the capsular subdivision of the lateral central nucleus and the amygdalostriatal area project preferentially to the dorsal tier, which selectively modulates the ventral striatum and cortex. The central core of the lateral central nucleus is unique in its restricted projection to the lateral substantia nigra in the region of the nigrotectal pathway. Taken as a whole, the central nucleus-nigral pathway provides a route for affectively significant stimuli to modulate the DA system, influencing the initiation of behavioral responses.

The distribution and origin of the calretinin-containing innervation of the nucleus accumbens of the rat

The nucleus accumbens of the rat consists of several subregions that can be distinguished on the basis of histochemical markers. For example, the calcium-binding protein calbindin D28k is a useful marker of the core compartment of the nucleus accumbens. Calretinin, another calcium-binding protein, is found in a dense fibre plexus in the accumbal shell and septal pole regions. The source of the accumbal calretinin innervation is not known. We examined the distribution of calretinin in the nucleus accumbens and used tract-tracing and lesion methods to determine the source of this calretinin innervation. Intense calretinin immunoreactivity was present in the medial shell, but the density of calretinin axons diminished sharply in the ventrolateral shell. Regions of dense calretinin immunostaining and those areas with calbindin-like immunoreactive cell bodies were generally segregated in the nucleus accumbens, although some overlap in the transition region between the core and shell was seen. Small clusters of calretinin-immunoreactive fibres were seen in the core, where they were restricted to calbindin-negative patches. Injections of the anterograde tracer biotinylated dextran amine into the paraventricular thalamic nucleus labelled fibres in calretinin-rich regions of the accumbens. Conversely, injections of Fluoro-gold into the accumbal shell retrogradely labelled numerous cells in the paraventricular thalamic nucleus that were calretinin-immunoreactive. Electrolytic lesions of the paraventricular thalamic nucleus reduced calretinin levels in the shell by approximately 80%. These data indicate that the calretinin innervation of the nucleus accumbens is derived primarily from the thalamic paraventricular nucleus, and marks accumbal territories that are largely complementary to those defined by calbindin.

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.

Improving data acquisition for fast-scan cyclic voltammetry

Described is an improved data acquisition system for fast-scan cyclic voltammetry (FSCV). The system was designed to significantly diminish noise sources that were identified in previously recorded FSCV measurements for the detection of neurotransmitters. Minimized noise is necessary to observe the low concentrations of neurotransmitters that are physiologically important. The system was based on a high-speed, 16-bit AD/DA acquisition board that allowed high scan rates and better resolved the small faradaic currents which remained after background subtraction. Irregularities that occur when independent timing sources are used for generation of the voltage waveform and collection of the current can create large noise artifacts near the voltage limits during FSCV. These were eliminated by the use of a single acquisition board that generated the voltage waveform and collected the current. Noise from frequency drift of the power line was eliminated through the use of a phase-locked loop. To demonstrate the improved performance of the system, data were collected using carbon-fiber microelectrodes in a flow injection analysis system and in brain slices. This new data acquisition system performed significantly better than another system previously used in our laboratory without these features. The improved detection limits of the new system allowed clearly resolved current spikes featuring pre-release "feet" to be recorded adjacent to individual mast cells following chemical stimulation. When combined with false-color plots, the low-noise system facilitated identification of dopamine release in a freely moving animal.

High versus low reactivity to a novel environment: behavioural, pharmacological and neurochemical assessments

Based on their rearing response to a novel open field, male Wistar rats were divided into two sub-groups with either high or low behavioural activity (high rearings, versus low rearings). These sub-groups were repeatedly exposed to the same open field and tested for behavioural habituation. Since we previously found neurochemical evidence for different cholinergic reactivities in such high rearing and low rearing rats, their behavioural responses to the muscarinic antagonist scopolamine (0.5 mg/kg) were also investigated in the open field. Additionally, they were exposed to the elevated plus-maze to test for possible differences in measures of anxiety. After behavioural testing, tissue concentrations of biogenic amines were determined in the ventral striatum (nucleus accumbens, olfactory tubercle), frontal cortex, striatum, hippocampus and amygdala. The results show that the higher rearing responses of high rearing rats in the novel open field were paralleled by higher locomotor activity. These behavioural differences between groups decreased with repeated open field exposure, an effect which was largely due to between-session habituation in high rearing rats. Thereby, high rearing rats approached the lower levels of low rearing rats, in which locomotor activity and rearings did not habituate between testing. Nevertheless, habituation was also observed in low rearing rats, especially in the measure of thigmotactic scanning, since the levels of scanning declined both between and within test sessions. The anticholinergic challenge with scopolamine induced a general pattern of behavioural activation. Furthermore, scopolamine partly reinstated the behavioural differences between high and low rearing rats that had been observed in the novel open field, since high rearing rats showed more rearing behaviour than low rearing rats under scopolamine. In contrast to the open field, there were no significant differences between high and low rearing rats in the plus-maze. The neurochemical analysis revealed, among others, higher dopamine levels in the ventral striatum of high rearing rats together with lower serotonin levels in the medial frontal cortex. The current findings thus indicate that high and low rearing rats not only differ in their behavioural response to a novel environment, but also in their patterns of behavioural habituation, and with respect to behaviour induced by an anti-cholinergic challenge. These differential behavioural profiles of high and low rearing animals are discussed with respect to the role of dopaminergic mechanisms in the forebrain, and the potential impact of cholinergic mechanisms.

Dopamine antagonism in a novel-object recognition and a novel-object place conditioning preparation with rats

Access to novel objects, similar to drugs of abuse, can enhance a place preference in rats. In the present experiments, the dopamine D1 receptor antagonist SCH-23390 blocked an increase in place preference conditioned by access to novel objects at doses that did not interfere with object interaction (0.01 and 0.03 mg/kg) or produce a place aversion in controls. However, eticlopride, a D2/D3 dopamine receptor antagonist, only blocked the conditioned increase in place preference at a dose (0.3 mg/kg) that impaired object interaction. In contrast, neither SCH-23390 nor eticlopride blocked preference for the novel object in an object recognition task at doses that did not interfere with object interaction. These experiments provide further evidence that the neural processes controlling learned associations between novel stimuli and the environment overlap with drugs of abuse.

Effects of cocaine on dopamine in subregions of the rat prefrontal cortex and their efferents to subterritories of the nucleus accumbens

The present study sought to investigate the contributions of the ventral prelimbic/infralimbic cortices and shell subterritory of the nucleus accumbens as well as the dorsal prelimbic/anterior cingulate cortices and core subregion of the nucleus accumbens to the acute systemic effects of cocaine (20 mg/kg i.p.) on both locomotor activity and simultaneous dialysate dopamine levels using a dual-probe microdialysis design. Basal dopamine levels were significantly higher in the ventral medial prefrontal cortex compared with the dorsal medial prefrontal cortex and higher concentrations of dopamine were also observed in the core of the nucleus accumbens compared with its shell counterpart. Cocaine produced a significant decrease in dopamine levels in both the ventral and dorsal medial prefrontal cortices. In contrast, cocaine significantly increased dialysate dopamine in the shell of the nucleus accumbens, whereas only a slight increase in dopamine was observed in the core subregion of the nucleus accumbens. A significant negative relationship between dopamine levels in the ventral and dorsal medial prefrontal cortices and dialysate dopamine concentrations in the shell and core of the nucleus accumbens was observed. Finally, in both the ventral and dorsal medial prefrontal cortices, the magnitude of the locomotor response to cocaine was inversely related to dialysate dopamine levels. In contrast, the magnitude of the locomotor response to cocaine became progressively larger as dopamine levels increased in the shell of the nucleus accumbens. These results show a dissociation in the pattern of dopamine release in subterritories of both the medial prefrontal cortex and nucleus accumbens in response to the acute systemic administration of cocaine.

Tetrahydropapaveroline injected in the ventral tegmental area shifts dopamine efflux differentially in the shell and core of nucleus accumbens in high-ethanol-preferring (HEP) rats

Since the 1970s tetrahydropapaveroline (THP) and other tetrahydroisoquinoline alkaloids have been implicated in the etiology of alcoholism. When injected into the cerebral ventricle or at specific sites in the mesolimbic system such as the ventral tegmental area (VTA), THP evokes spontaneous and intense intake of alcohol in the nondrinking animal. Further, THP evokes the extracellular efflux of dopamine in the nucleus accumbens (NAC), which comprises, in part, the postulated alcohol drinking "circuit" of neurons. The purpose of this study was to characterize the action of a THP reactive structure, the VTA, on the activity of dopamine and its metabolism in the NAC. In the anesthetized high-ethanol-preferring (HEP) rat, artificial CSF was perfused for 10 min at a rate of 10 microl per min specifically in either the core or shell of the NAC. A microbore push-pull cannula system was selected over a microdialysis probe because of its superior recovery of neurotransmitters and tip exposure of less than 1.0 mm. After a series of 5-min perfusions, a single microinjection of 5.0 microg/microl of THP was made in the ipsilateral VTA while the NAC was perfused simultaneously. Sequential samples of the NAC perfusate were assayed by an HPLC coulometric system to quantitate the concentrations of dopamine and its metabolites, DOPAC and HVA, as well as the 5-HT metabolite, 5-HIAA. The results showed that THP injected in the VTA caused a significant increase by 94 +/- 23% in the efflux of dopamine from the core of the NAC. Conversely, the THP injected identically in the VTA suppressed the efflux of dopamine within the shell of the NAC by 51 +/- 10%. The levels of DOPAC, HVA and 5-HIAA within the core and shell of the NAC generally paralleled the increase and decrease in efflux, respectively, of dopamine. CSF control injections in the VTA as well as injections outside of the VTA failed to alter dopamine or metabolite activity in the NAC. These results demonstrate that the presence of THP in the VTA alters directly the function of the pathway of mesolimbic neurons generally and the dopaminergic system specifically. That such a perturbation could account for the induction of alcohol preference is proposed in relation to a reinforcing mechanism involving opioidergic and dopaminergic elements.

Behavioral neurochemistry reveals a new functional dichotomy in the shell subregion of the nucleus accumbens

1. The behavioral and neurochemical effects produced by the direct infusion of amphetamine by reverse microdialysis into either the core or shell of the nucleus accumbens were studied across the anteroposterior axis of this nucleus. 2. Amphetamine (0.05; 0.10; 0.50; 1.00 microM) produced a dose-dependent increase in locomotor activity after microinfusion into either the rostral shell, caudal shell or core of the nucleus accumbens. However, the amphetamine-induced locomotor activating effect, was significantly higher in the rostral shell of the nucleus accumbens compared with both the caudal shell and core. 3. The lowest concentrations of amphetamine produced an equipotent decrease in dialysate dopamine in either the rostral shell, caudal shell, or core. At 1.0 microM, however, amphetamine selectively increased dopamine in the rostral shell. In contrast, the highest dose of amphetamine significantly increased dialysate serotonin levels over baseline only in the caudal shell of the nucleus accumbens. 4. These results demonstrate the preferential effect of amphetamine on dopamine in the rostral shell and serotonin in the caudal shell subterritory of the nucleus accumbens.