Repeated morphine exposure decreased the nucleus accumbens excitability during short-term withdrawal

It is well known that the nucleus accumbens plays an important role in drug reinforcing effect and relapse. However, the cellular neuroadaptations that take place in accumbens neurons after repeated drug exposure are still not well understood, especially for opioids. Here, we examined how nucleus accumbens neuronal excitability becomes affected in rats exposed to morphine using whole-cell patch-clamp recordings. Medium spiny neurons recorded from brain slices of repeated morphine treated rats exhibited a significant decrease in the intrinsic excitability after 3-4 days withdrawal, compared to that of neurons from saline treated animals, which was indicated by the increase of current to evoke the first spike and the decrease of spike number induced by depolarizing current steps in the morphine group. Moreover, the excitability decrease was accompanied by related membrane property changes, such as resting membrane potential hyperpolarization, input resistance, and membrane time constant decrease, inward rectification increase, and action potential duration decrease. Taken together, repeated morphine exposure and short-term withdrawal may reduce nucleus accumbens activity and output by modulating intrinsic membrane properties of its output neurons, which could be an important neuroadaptation process that mediates morphine addictive effect.     

Projection neurons of the nucleus accumbens: an intracellular labeling study

Projection neurons of nucleus accumbens (NAC) of the rat were identified by either antidromic activation from stimulation of midbrain ventral tegmental area-substantia nigra (VTA-SN) regions, or by tracing axons of intracellularly labeled NAC neurons into the ventral pallidum. The morphology of these NAC projection neurons were determined to be medium spiny neurons similar to those identified in the caudate-putamen.     

Double dissociation of the effects of selective nucleus accumbens core and shell lesions on impulsive-choice behaviour and salience learning in rats

The nucleus accumbens can be subdivided into at least two anatomically distinct subregions: a dorsolateral 'core' and a ventromedial 'shell', and this distinction may extend to a functional dissociation. Here, we contrasted the effects of selective excitotoxic core and medial shell lesions on impulsive-choice behaviour using a delayed reward choice paradigm and a differential reward for low rates of responding (DRL) test, against a form of salience learning known as latent inhibition (LI). Core lesions led to enhanced impulsive choices as evidenced by a more pronounced shift from choosing a continuously reinforced lever to a partially reinforced lever, when a delay between lever press and reward delivery was imposed selectively on the former. The core lesions also impaired performance on a DRL task that required withholding the response for a fixed period of time in order to earn a reward. Medial shell lesions had no effect on these two tasks, but abolished the LI effect, as revealed by the failure of stimulus pre-exposure to retard subsequent conditioning to that stimulus in an active avoidance procedure in the lesioned animals. As expected, selective core lesions spared LI. The double dissociations demonstrated here support a functional segregation between nucleus accumbens core and shell, and add weight to the hypothesis that the core, but not the shell, subregion of the nucleus accumbens is preferentially involved in the control of choice behaviour under delayed reinforcement conditions and in the inhibitory control of goal-directed behaviour.     

Increased volume of the nucleus accumbens in schizophrenia

Summary. The nucleus accumbens, an integral and important part of limbic and prefrontal cortico-striato-pallidal-thalamic circuits, is involved in several cognitive, emotional and psychomotor functions altered in schizophrenia. In animal models, developmental disturbances within the entorhinal cortex and the hippocampus induce a dysregulation of inputs to the nucleus accumbens resulting in behavioral abnormalities which point to psychotic psychopathology. Nonetheless, due to the complex neuroanatomy of the human ventral striatum hardly any morphometric data on the nucleus accumbens are available. A postmortem stereological investigation of the nucleus accumbens was performed in complete brains of 9 male schizophrenics and 9 male controls between the ages of 46 and 64. Complete serial coronal slices of both hemispheres were stained with a modified Nissl-technique. Tissue shrinkage after staining and embedding was corrected for both individual and regional shrinkage. Based on recent precise delimitations of the human ventral striatum, in vivo hemisphere-adjusted volumes of the nucleus accumbens (volume densities) and absolute accumbal volumes were calculated applying the Cavalieri-estimator. In schizophrenics, mean hemisphere-adjusted volumes of the nucleus accumbens were significantly increased on both sides (right accumbens: p = 0,005^**; left accumbens: p = 0,016^*). Hemispherical volumes and volumes of the nucleus accumbens were significantly correlated in both groups (p = 0,02^*). Most likely, this increase in volume of the ventral striatum reflects a decrease in naturally occuring cell death following prenatal cortical neurodevelopmental disturbances. Received May 23, 2000; accepted July 20, 2000     

Projections from the paraventricular nucleus of the thalamus to the rat prefrontal cortex and nucleus accumbens shell: ultrastructural characteristics and spatial relationships with dopamine afferents

The paraventricular nucleus of the thalamus (PVT) participates in the functional integration of limbic cortical and striatal circuitry. In the rat, the PVT projects to the deep layers of the medial prefrontal cortex (PFC) and to the shell of the nucleus accumbens (NAc). However, the synaptic organization of PVT afferents within these regions remains undescribed. Furthermore, although dopamine (DA) modulates excitatory glutamate transmission in both areas, possible anatomic substrates for specific DA modulation of PVT inputs have not yet been investigated. To address these issues, immunoperoxidase labeling for tyrosine hydroxylase (TH) in DA axons was combined with anterograde tract-tracing, either by biotinylated dextran amine (BDA) labeled with immunogold-silver or by degeneration after lesions of the PVT. In both regions, and with either tracing method, PVT terminals formed primarily asymmetric axospinous synapses; in the NAc, a proportion of PVT terminals also synapsed onto dendrites. PVT profiles in both regions were often seen in direct apposition to TH-immunoreactive axons; this association was more evident in the NAc where the DA innervation is denser. Within the PFC, PVT profiles and TH-labeled axons were occasionally apposed to the same dendrites, but synaptic specializations were not typically seen at these seeming points of convergence. Within the NAc, PVT profiles occasionally made synapses onto spines and distal dendrites that received convergent synapses from TH-immunoreactive varicosities. These findings represent the first demonstration of postsynaptic convergence between DA and thalamic afferents to a striatal region and are consistent with direct synaptic modulation of PVT transmission by DA in the NAc but not the PFC.     

The role of the nucleus accumbens in learned approach behavior diminishes with training

Nucleus accumbens dopamine plays a key role in reward‐directed approach. Past findings suggest that dopamine's role in the expression of learned behavior diminishes with extended training. However, little is known about the central substrates that mediate the shift to dopamine‐independent reward approach. In the present study, rats approached and inserted the head into a reward compartment in response to a cue signaling food delivery. On days 4 and 5 of 28‐trial‐per‐day sessions, D1 receptor antagonist R(+)‐7‐chloro‐8‐hydroxy‐3‐methyl‐1‐phenyl‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepine hydrochloride (SCH23390) infused to the NAc core reduced the probability and speed of cued approach. The disruptive effect of D1 receptor blockade was specific to the nucleus accumbens core and not seen with drug infusions to nearby dopamine target regions. In rats that received drug infusions after extended training (days 10 or 11), accumbens core D1 receptor blockade produced little effect on the expression of the same behavior. These results could have been due to a continued accumbens mediation of cued approach even after the behavior had become independent of accumbens D1 receptors. However, accumbens core ionotropic glutamate receptor blockade disrupted cued approach during early but not late stages of training, similar to the effects of D1 antagonist infusions. The results suggest that with extended training, a nucleus accumbens D1‐dependent behavior becomes less dependent not only on nucleus accumbens D1 transmission but also on excitatory transmission in the nucleus accumbens. These findings fill an important gap in a growing literature on reorganization of striatal function over the course of training.     

Involvement of a basolateral amygdala complex-nucleus accumbens pathway in glucocorticoid-induced modulation of memory consolidation

Systemic or intracerebral administration of glucocorticoids modulates memory consolidation in several tasks. Previously, we have shown that these memory-modulatory effects depend on an intact basolateral complex of the amygdala (BLC) and efferents from the BLC that run through the stria terminalis. It is currently unknown, however, what BLC efferent structures mediate these effects. The present experiments were designed to determine whether the nucleus accumbens (NA), which receives BLC efferents through the stria terminalis and is involved in several BLC-dependent behaviours, is involved in glucocorticoid-induced modulation of memory consolidation. In experiment 1, rats with bilateral sham or N-methyl-D-aspartate (NMDA)-induced lesions of the NA were trained on a one-trial, footshock-motivated inhibitory avoidance task, and given immediate post-training injections of either the synthetic glucocorticoid dexamethasone (0.3 or 1.0 mg/kg, s.c.) or vehicle. Testing 48 h later revealed that dexamethasone significantly enhanced retention in sham-lesioned rats but that the enhancing effect was blocked in NA-lesioned rats. An asymmetrical, or crossed-lesion design was employed in experiment 2. Rats with a unilateral NMDA-induced lesion of the BLC and a unilateral lesion of either the ipsilateral or contralateral NA were trained as in experiment 1. Testing 48 h later revealed that dexamethasone enhanced retention in ipsilaterally lesioned rats, but that this effect was blocked in contralaterally lesioned rats. These findings indicate that an intact BLC-NA pathway is critical for the enhancing effects of glucocorticoids on memory consolidation, and are consistent with the view that the BLC regulates memory consolidation in other brain regions.     

Shell and core in monkey and human nucleus accumbens identified with antibodies to calbindin-D28k

The neurochemical division of the rodent nucleus accumbens into shell and core is now a widely accepted concept. However, such divisions in the primate nucleus accumbens have yet to be fully clarified and described. In the present study, the forebrains of three primates—marmoset, rhesus monkey, and human—and a Wistar rat, were immunoreacted with antibodies directed against calbindin-D_28k. The patterns of immunoreactivity in the primates' ventral striatum were mapped and compared to that of rat. Calbindin staining was uneven in all species and there was no evidence of a bicompartmental organization, i.e., striosome/patch and matrix, in central parts of the nucleus. Nucleus accumbens in primates, as in rat, could be divided immunohistochemically into a crescent-shaped outer shell—medially, ventrally and laterally—and an inner core. In general, medial parts of the shell stained less intensely for calbindin than did lateral parts. However, interspecific variation in the intensity of the immunoreactive staining and the mediolateral extent of the shell was obvious. The core, which immunostained unevenly, was consistently more intensely immunoreactive than either medial or lateral shell in all species except the marmoset. These results suggest that the neurochemical subdivisions of shell and core established for nucleus accumbens of rodents are also present in primates. However, further work is needed to establish whether these territories are homologous and, if so, the full extent of that homology. © 1996 Wiley-Liss, Inc.     

Ventral mesopontine projections of the caudomedial shell of the nucleus accumbens and extended amygdala in the rat: double dissociation by organization and development.

The shell of the nucleus accumbens and central division of the extended amygdala are telencephalic structures that influence motor activity and lately have been regarded by some as components of a single functional-anatomic continuum. Each has a highly differentiated internal organization and output system and distinct pharmacologic responses however, and it is thus likely that each subserves distinct contributions to behavior. In this investigation, nucleus accumbens and extended amygdala outputs were compared by using retrograde tracing in adult and postnatal rats. Fluoro-Gold, when injected into the ventral tegmental area, produced substantial retrograde labeling in the adult nucleus accumbens shell, but only trivial amounts in the central division of the extended amygdala. Injection sites in the lateral mesopontine tegmentum produced robust labeling in the central extended amygdala but little in the nucleus accumbens. The projections of extended amygdala were substantially developed by postnatal day 1, whereas those of the caudomedial shell of the nucleus accumbens only reached the ventral tegmental area by approximately postnatal day 6. Few neurons projecting from the caudomedial shell of the accumbens to the ventral tegmental area were observed even at postnatal day 21. In consideration of the reported importance of the nucleus accumbens, particularly the caudomedial shell, in neural processing related to reward and motivation and the central nervous system response to antipsychotic drugs, it may be important to determine whether processes occurring during the protracted postnatal development of the caudomedial shell are vulnerable to destructive circumstances, such as drug intoxication, maternal separation, or social isolation.     

学习和记忆对大鼠背海马结构内C—FOS表达的影响

采用避暗回避反应实验和免疫组织化学相结合的方法，选用五个时间点对Ｃ－ＦＯＳ在大鼠背海马结构的表达进行了观察。结果表明，训练后１５ｍｉｎ，大鼠背海马各区ＦＯＳ样免疫阳性神经元数量开始增加，训练后１小时峰值，记忆唤醒也可诱导大鼠背海马各区Ｃ－ＦＯＳ的表达，提示学习和记忆过程与背海马内Ｃ－ＦＯＳ的表达密切相关。     

Reward-guided learning beyond dopamine in the nucleus accumbens: the integrative functions of cortico-basal ganglia networks

Here we challenge the view that reward-guided learning is solely controlled by the mesoaccumbens pathway arising from dopaminergic neurons in the ventral tegmental area and projecting to the nucleus accumbens. This widely accepted view assumes that reward is a monolithic concept, but recent work has suggested otherwise. It now appears that, in reward-guided learning, the functions of ventral and dorsal striata, and the cortico-basal ganglia circuitry associated with them, can be dissociated. Whereas the nucleus accumbens is necessary for the acquisition and expression of certain appetitive Pavlovian responses and contributes to the motivational control of instrumental performance, the dorsal striatum is necessary for the acquisition and expression of instrumental actions. Such findings suggest the existence of multiple independent yet interacting functional systems that are implemented in iterating and hierarchically organized cortico-basal ganglia networks engaged in appetitive behaviors ranging from Pavlovian approach responses to goal-directed instrumental actions controlled by action-outcome contingencies.     

A dense cluster of D1+ cells in the mouse nucleus accumbens

The striatum is known to be largely composed of intermingled medium‐sized projection neurons expressing either the D₁ or the D₂ dopamine receptors. In the present study, we took advantage of the double BAC Drd1a‐TdTomato/Drd2‐GFP (D₁/D₂) transgenic mice to reveal the presence of a peculiar cluster of densely‐packed D₁+ cells located in the shell compartment of the nucleus accumbens. This spherical cluster has a diameter of 110 µm and is exclusively composed by D₁+ cells, which are all immunoreactive for the neuronal nuclear marker (NeuN). However, in contrast to other D₁+ or D₂+ striatal cells, those that form the accumbens cluster are devoid of calbindin (CB) and DARPP‐32, two faithful markers for striatal projection neurons. Using GAD‐GFP transgenic mice, we confirm the GABAergic nature of the D₁+ clustered neurons. Intracellular injections from fixed brain slices indicate that these neurons are endowed with distinctive morphological features, including a small (5–6 µm), round cell body giving rise to a single primary dendrite that branches into two secondary processes. Single‐neuronal injections combined to electron microscopy reveal the existence of GAP junctions linking these D₁+ cells. Based on their location, morphological characteristics and neurochemical phenotype, we conclude that the D₁+ accumbens cluster form a highly compact group of small neurons distinct from the larger and more diffusely distributed D₁+ or D₂+ striatal projection neurons that surround it. This remarkable nucleus might play a crucial role in the limbic function of the murine striatum.     

Axonal branching patterns of nucleus accumbens neurons in the rat

The patterns of axonal collateralization of nucleus accumbens (Acb) projection neurons were investigated in the rat by means of single-axon tracing techniques using the anterograde tracer biotinylated dextran amine. Seventy-three axons were fully traced, originating from either the core (AcbC) or shell (AcbSh) compartment, as assessed by differential calbindin D28k-immunoreactivity. Axons from AcbC and AcbSh showed a substantial segregation in their targets; target areas were either exclusively or preferentially innervated from AcbC or AcbSh. Axon collaterals in the subthalamic nucleus were found at higher than expected frequencies; moreover, these originated exclusively in the dorsal AcbC. Intercompartmental collaterals were observed from ventral AcbC axons into AcbSh, and likewise, interconnections at pallidal and mesencephalic levels were also observed, although mostly from AcbC axons toward AcbSh targets, possibly supporting crosstalk between the two subcircuits at several levels. Cell somata giving rise to short-range accumbal axons, projecting to the ventral pallidum (VP), were spatially intermingled with others, giving rise to long-range axons that innervated VP and more caudal targets. This anatomical organization parallels that of the dorsal striatum and provides the basis for possible dual direct and indirect actions from a single axon on either individual or small sets of neurons.     

Bursting activation of prefrontal cortex drives sustained up states in nucleus accumbens spiny neurons in vivo

Hippocampal inputs to the nucleus accumbens (NA) have been proposed to implement a gating mechanism by driving NA medium spiny neurons (MSNs) to depolarized up states that facilitate action potential firing in response to brief activation of the prefrontal cortex (PFC). Brief PFC stimulation alone, on the other hand, could not drive NA up states. As these studies were conducted using single‐pulse PFC stimulation, it remains possible that PFC activation with naturalistic, bursty patterns can also drive up states in NA MSNs. Here, we assessed NA responses to PFC stimulation with a pattern similar to what is typically observed in awake animals during PFC‐relevant behaviors. In vivo intracellular recordings from NA MSNs revealed that brief 20–50 Hz PFC stimulus trains evoked depolarizations that were similar to spontaneous up states in NA MSNs and were sustained beyond stimulus offset. Similar train stimulation of corticoaccumbens afferents in a parasagittal slice preparation evoked large amplitude depolarizations in NA MSNs that were sustained during stimulation but decayed rapidly following stimulation offset, suggesting that activation of cortical afferents can drive MSN depolarizations but other mechanisms may contribute to sustaining up states. These data suggest that NA MSNs integrate temporal features of PFC activation and that the NA gating model can be reformulated to include a PFC‐driven gating mechanism during periods of high PFC firing, such as during cognitively demanding tasks. Synapse 63:173–180, 2009. © 2008 Wiley‐Liss, Inc.     

Specificity in the efferent projections of the nucleus accumbens in the rat: Comparison of the rostral pole projection patterns with those of the core and shell

The efferent connections of the rostral pole of the rat accumbens, where distinct core and shell subterritories can not be identified, were examined with the aid of the anterogradely transported plant lectin, Phaseolus vulgaris-leucoagglutinin (PHA-L), for comparison with the previously reported projection patterns of the accumbal core and shell. Injection sites and transported PHA-L were evaluated with the aid of reference to adjacent sections processed to display substance P or calbindin 28 kD immunoreactivities, i.e., markers that demonstrate the core and shell. Lateral parts of the rostral pole gave rise to a “core-like” projection system that involved the rostroventral globus pallidus, subcommissural ventral pallidum, entopeduncular nucleus and an adjacent part of the lateral hypothalamus, lateral ventral tegmental area, dorsal pars compacta, and structures in the lateral mesencephalic tegmentum and central grey. The medial part of the rostral pole gave rise to a “shell-like” innervation of the subcommissural ventral pallidum, lateral preoptic region, lateral hypothalamus, ventral tegmental area, dorsalmost pars compacta, retrorubral field, lateral midbrain tegmentum, and central grey. In contrast to the large numbers of axon varicosities observed through the entire length of lateral hypothalamus following shell injections, dense accumulations of axon collaterals and varicosities in hypothalamus were limited to the levels of origin of the stria medullaris bundle and entopeduncular nucleus and to the posterlateral region following medial injections. The medial part of the rostral pole contributed some projections to preoptic and sublenticular regions, but not to the bed nucleus of the stria terminalis. Noteworthy concentrations of calbindin immunoreactive cells observed in the lateral rostral pole correlate with the origin of the “basal ganglia-like” projection system, provoking the speculation that ventral striatal calbindin immunoreactive cells contribute principally to basal ganglia-like projections while cells lacking calbindin immunoreactivity contribute to the innervation of hypothalamus and midbrain tegmentum. © 1993 Wiley-Liss, Inc.     

Reinstated ethanol‐seeking in rats is modulated by environmental context and requires the nucleus accumbens core

The reinstatement of ethanol (EtOH)‐seeking induced by an EtOH‐predictive light‐tone stimulus is enhanced in an environment associated with prior EtOH self‐administration (SA) compared with a context associated with EtOH unavailability ( Tsiang & Janak, 2006 ). Here we hypothesized that EtOH‐seeking would be elicited by the conditioned sensory stimulus properties of EtOH and that this reinstatement would be similarly modulated by context. We also determined whether pharmacologically inactivating the nucleus accumbens (NAc), a key structure in relapse circuitry, would attenuate reinstated EtOH‐seeking. Rats lever‐pressed for oral EtOH (10% v/v) in operant conditioning chambers distinguished by specific visual, olfactory and tactile stimuli. Responding was then extinguished by withholding EtOH in a different context. EtOH‐seeking, expressed as elevated responding without EtOH delivery, was subsequently tested by presenting an oral EtOH prime (two aliquots of 0.1 mL EtOH) in either the extinction or the prior EtOH‐SA context. Rats received a microinfusion (0.3 μL/hemisphere) of saline or GABA agonists (muscimol/baclofen) into the NAc core or shell immediately before the reinstatement test. Robust EtOH‐seeking was observed in the prior EtOH‐SA but not the extinction context in saline‐pretreated rats. This effect was significantly attenuated by inactivating the NAc core but not shell. Conversely, NAc shell inactivation significantly elevated lever‐pressing in the extinction context. These data suggest that the sensory stimulus properties of oral EtOH can reinstate EtOH‐seeking when experienced in the appropriate context and that functional activity in the NAc core is required for this effect. In contrast, the shell may normally inhibit incorrect behavioral responses.     

Distinct roles of the different ionotropic glutamate receptors within the nucleus accumbens in passive-avoidance learning and memory in mice

Research on the role of the nucleus accumbens in behaviour has been largely focused on the functions of this structure in conditioning to appetitive stimuli. It has been suggested that a network comprising the nucleus accumbens and its convergent inputs might mediate dissociable functions in the acquisition, the consolidation and the retrieval of information. However, findings related to a role of this structure in aversive conditioning are somewhat contradictory, and its involvement in this form of learning is still under debate. Moreover, very little evidence is available on the step of information processing mediated by the accumbens. Thus the purpose of this study was to investigate the effects of the blockade of the AMPA and NMDA glutamate receptors, which have been suggested to mediate the transmission of information from the limbic system to this structure, on a classical aversive conditioning task - the one-trial step through inhibitory avoidance paradigm (24 h interval between training and testing). Intra-accumbens focal injections of AP-5 and DNQX (NMDA and AMPA antagonists, respectively) were performed immediately after training, before training and before testing in mice. The NMDA antagonist (37.5, 75 and 150 ng per side) impaired animal performance only if administered immediately after but not before training or before testing. Conversely, DNQX (0.5, 1.0 and 5.0 ng per side) reduced the step through latencies when administered before training and before testing. These findings suggest that NMDA receptor activation within the accumbens is necessary in formation but not expression of memory for inhibitory avoidance. AMPA receptors, instead, are necessary for the acquisition and the expression but not consolidation of inhibitory avoidance memory.     

Learning,memory and consolidation mechanisms for behavioral control in hierarchically organized cortico‐basal ganglia systems

This article aims to provide a synthesis on the question how brain structures cooperate to accomplish hierarchically organized behaviors, characterized by low‐level, habitual routines nested in larger sequences of planned, goal‐directed behavior. The functioning of a connected set of brain structures—prefrontal cortex, hippocampus, striatum, and dopaminergic mesencephalon—is reviewed in relation to two important distinctions: (a) goal‐directed as opposed to habitual behavior and (b) model‐based and model‐free learning. Recent evidence indicates that the orbitomedial prefrontal cortices not only subserve goal‐directed behavior and model‐based learning, but also code the “landscape” (task space) of behaviorally relevant variables. While the hippocampus stands out for its role in coding and memorizing world state representations, it is argued to function in model‐based learning but is not required for coding of action–outcome contingencies, illustrating that goal‐directed behavior is not congruent with model‐based learning. While the dorsolateral and dorsomedial striatum largely conform to the dichotomy between habitual versus goal‐directed behavior, ventral striatal functions go beyond this distinction. Next, we contextualize findings on coding of reward‐prediction errors by ventral tegmental dopamine neurons to suggest a broader role of mesencephalic dopamine cells, viz. in behavioral reactivity and signaling unexpected sensory changes. We hypothesize that goal‐directed behavior is hierarchically organized in interconnected cortico‐basal ganglia loops, where a limbic‐affective prefrontal‐ventral striatal loop controls action selection in a dorsomedial prefrontal–striatal loop, which in turn regulates activity in sensorimotor‐dorsolateral striatal circuits. This structure for behavioral organization requires alignment with mechanisms for memory formation and consolidation. We propose that frontal corticothalamic circuits form a high‐level loop for memory processing that initiates and temporally organizes nested activities in lower‐level loops, including the hippocampus and the ripple‐associated replay it generates. The evidence on hierarchically organized behavior converges with that on consolidation mechanisms in suggesting a frontal‐to‐caudal directionality in processing control.     

Major coexpression of kappa-opioid receptors and the dopamine transporter in nucleus accumbens axonal profiles.

The behavioral effects of psychostimulants, which are produced at least in part through inhibition of the dopamine transporter (DAT), are modulated by kappa-opioid receptors (KOR) in the nucleus accumbens (Acb). Using electron microscopic immunocytochemistry, we reveal that in the Acb KOR labeling is mainly, and DAT immunoreactivity is exclusively, presynaptic. From 400 KOR-labeled presynaptic structures, including axon terminals, intervaricosities, and small axons, 51% expressed DAT and 29% contacted another population of terminals exclusively labeled for DAT. Within axonal profiles that contained both antigens, DAT and KOR were prominently localized to plasma membrane segments that showed overlapping distributions of the respective immunogold-silver and immunoperoxidase markers. KOR labeling was also localized to membranes of small synaptic vesicles in terminals with or without DAT immunoreactivity. In addition, from 24 KOR-immunoreactive dendritic spines 42% received convergent input from DAT-containing varicosities and unlabeled terminals forming asymmetric, excitatory-type synapses. Our results provide the first ultrastructural evidence that in the Acb, KOR is localized to strategic sites for involvement in the direct presynaptic release and/or reuptake of dopamine. These data also suggest a role for KOR in the presynaptic modulation of other neurotransmitters and in the postsynaptic excitatory responses of single spiny neurons in the Acb. Dual actions on dopamine terminals and their targets in the Acb may account for KOR-mediated attenuation of drug reinforcement and sensitization.