Evidence for a multi-compartmental histochemical organization of the nucleus accumbens in the rat

Role of the striatal dopamine, GABA and opioid systems in mediating feeding and fat intake

Food intake, which is a highly reinforcing behavior, provides nutrients required for survival in all animals. However, when fat and sugar consumption goes beyond the daily needs, it can favor obesity. The prevalence and severity of this health problem has been increasing with time. Besides covering nutrient and energy needs, food and in particular its highly palatable components, such as fats, also induce feelings of joy and pleasure. Experimental evidence supports a role of the striatal complex and of the mesolimbic dopamine system in both feeding and food-related reward processing, with the nucleus accumbens as a key target for reward or reinforcing-associated signaling during food intake behavior. In this review, we provide insights concerning the impact of feeding, including fat intake, on different types of receptors and neurotransmitters present in the striatal complex. Reciprocally, we also cover the evidence for a modulation of palatable food intake by different neurochemical systems in the striatal complex and in particular the nucleus accumbens, with a focus on dopamine, GABA and the opioid system.

Immunolabeling Provides Evidence for Subregions in the Songbird Nucleus Accumbens and Suggests a Context-Dependent Role in Song in Male European Starlings (Sturnus vulgaris)

Birdsong is well known for its role in mate attraction during the breeding season. However, many birds, including European starlings (Sturnus vulgaris), also sing outside the breeding season as part of large flocks. Song in a breeding context can be extrinsically rewarded by mate attraction; however, song in nonbreeding flocks, referred to here as gregarious song, results in no obvious extrinsic reward and is proposed to be intrinsically rewarded. The nucleus accumbens (NAC) is a brain region well known to mediate reward and motivation, which suggests it is an ideal candidate to regulate reward associated with gregarious song. The goal of this study was to provide new histochemical information on the songbird NAC and its subregions (rostral pole, core, and shell) and to begin to determine subregion-specific contributions to gregarious song in male starlings. We examined immunolabeling for tyrosine hydroxylase (TH), neurotensin, and enkephalin (ENK) in the NAC. We then examined the extent to which gregarious and sexually motivated song differentially correlated with immunolabeling for the immediate early genes FOS and ZENK in each subdivision of the NAC. We found that TH and ENK labeling within subregions of the starling NAC was generally similar to patterns seen in the core and shell of NACs in mammals and birds. Additionally, we found that gregarious song, but not sexually motivated song, positively correlated with FOS in all NAC subregions. Our observations provide further evidence for distinct subregions within the songbird NAC and suggest the NAC may play an important role in regulating gregarious song in songbirds.

Brain Structure and Function of Chronic Low Back Pain Patients on Long-Term Opioid Analgesic Treatment: A Preliminary Study

Chronic low back pain (CLBP) is often treated with opioid analgesics (OA), a class of medications associated with a significant risk of misuse. However, little is known about how treatment with OA affect the brain in chronic pain patients. Gaining this knowledge is a necessary first step towards understanding OA associated analgesia and elucidating long-term risk of OA misuse. Here we study CLBP patients chronically medicated with opioids without any evidence of misuse and compare them to CLBP patients not on opioids and to healthy controls using structural and functional brain imaging. CLBP patients medicated with OA showed loss of volume in the nucleus accumbens and thalamus, and an overall significant decrease in signal to noise ratio in their sub-cortical areas. Power spectral density analysis (PSD) of frequency content in the accumbens’ resting state activity revealed that both medicated and unmedicated patients showed loss of PSD within the slow-5 frequency band (0.01–0.027 Hz) while only CLBP patients on OA showed additional density loss within the slow-4 frequency band (0.027–0.073 Hz). We conclude that chronic treatment with OA is associated with altered brain structure and function within sensory limbic areas.

Loss of nucleus accumbens low-frequency fluctuations is a signature of chronic pain

The prevalence of chronic pain has reached epidemic levels. In addition to personal suffering, chronic pain is associated with psychiatric and medical comorbidities, notably substance misuse. Chronic pain does not have a cure or quantitative diagnostic or prognostic tools. Here we show that brain imaging can provide such measures. First, we show that the brain limbic system of patients with subacute back pain at risk for becoming chronic back pain patients exhibits limbic system structural alterations, which predate the onset of chronic pain. Second, we show that the nucleus accumbens activity shows loss of low-frequency fluctuations only when patients transition to the chronic phase, an observation that was reproduced in multiple datasets collected at different sites.

Chronic pain is a highly prevalent disease with poorly understood pathophysiology. In particular, the brain mechanisms mediating the transition from acute to chronic pain remain largely unknown. Here, we identify a subcortical signature of back pain. Specifically, subacute back pain patients who are at risk for developing chronic pain exhibit a smaller nucleus accumbens volume, which persists in the chronic phase, compared to healthy controls. The smaller accumbens volume was also observed in a separate cohort of chronic low-back pain patients and was associated with dynamic changes in functional connectivity. At baseline, subacute back pain patients showed altered local nucleus accumbens connectivity between putative shell and core, irrespective of the risk of transition to chronic pain. At follow-up, connectivity changes were observed between nucleus accumbens and rostral anterior cingulate cortex in the patients with persistent pain. Analysis of the power spectral density of nucleus accumbens resting-state activity in the subacute and chronic back pain patients revealed loss of power in the slow-5 frequency band (0.01 to 0.027 Hz) which developed only in the chronic phase of pain. This loss of power was reproducible across two cohorts of chronic low-back pain patients obtained from different sites and accurately classified chronic low-back pain patients in two additional independent datasets. Our results provide evidence that lower nucleus accumbens volume confers risk for developing chronic pain and altered nucleus accumbens activity is a signature of the state of chronic pain.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Assessing contributions of nucleus accumbens shell subregions to reward-seeking behavior

BACKGROUND

The nucleus accumbens (NAc) plays a key role in brain reward processes including drug seeking and reinstatement. Several anatomical, behavioral, and neurochemical studies discriminate between the limbic-associated shell and the motor-associated core regions. Less studied is the fact that the shell can be further subdivided into a dorsomedial shell (NAcDMS) and an intermediate zone (NAcINT) based on differential expression of transient c-Fos and long-acting immediate-early gene ΔFosB upon cocaine sensitization. These disparate expression patterns suggest that NAc shell subregions may play distinct roles in reward-seeking behavior. In this study, we examined potential differences in the contributions of the NAcDMS and the NAcINT to reinstatement of reward-seeking behavior after extinction.

METHODS

Rats were trained to intravenously self-administer cocaine, extinguished, and subjected to a reinstatement test session consisting of an intracranial microinfusion of either amphetamine or vehicle targeted to the NAcDMS or the NAcINT.

RESULTS

Small amphetamine microinfusions targeted to the NAcDMS resulted in statistically significant reinstatement of lever pressing, whereas no significant difference was observed for microinfusions targeted to the NAcINT. No significant difference was found for vehicle microinfusions in either case.

CONCLUSION

These results suggest heterogeneity in the behavioral relevance of NAc shell subregions, a possibility that can be tested in specific neuronal populations in the future with recently developed techniques including optogenetics.

Blockage of acquisition and expression of morphine-induced conditioned place preference in rats due to activation of glutamate receptors type II/III in nucleus accumbens

Numerous studies have shown that glutamate in the nucleus accumbens (NAc) is an essential neurotransmitter for the extension of morphine-induced place preference. mGlu2/3 glutamate receptors in the NAc have important roles in the reward pathway. However, less is known about the role of this glutamate receptor subtype in morphine-induced conditioned place preference (CPP). In this study, we examined the effects of bilateral intra-accumbal administration of LY379268, an mGlu2/3 receptor agonist on the acquisition and expression of morphine-induced CPP in rats. Adult male Wistar rats (n=136; 220-250g) were evaluated in a CPP paradigm. Doses of LY379268 (0.3, 1 and 3μg/0.5μL saline per side) were administered into the NAc on both sides during the 3days of the conditioning (acquisition) or post-conditioning (expression) phase. The results show that bilateral intra-accumbal administration of LY379268 (0.3, 1 and 3μg) markedly decreased the acquisition of morphine-induced CPP in a dose-dependent manner. In a second series of experiments, we determined that injection of LY379268 into the NAc considerably attenuated the expression of morphine CPP only at the highest dose (3μg). Our findings suggest that activation of mGlu2/3 receptors in the NAc dose-dependently blocked both the establishment and the maintenance of morphine-induced CPP and confirmed the role of this system as a potential therapeutic target for addiction.

Brain BLAQ: Post-hoc thick-section histochemistry for localizing optogenetic constructs in neurons and their distal terminals

Optogenetic constructs have revolutionized modern neuroscience, but the ability to accurately and efficiently assess their expression in the brain and associate it with prior functional measures remains a challenge. High-resolution imaging of thick, fixed brain sections would make such post-hoc assessment and association possible; however, thick sections often display autofluorescence that limits their compatibility with fluorescence microscopy. We describe and evaluate a method we call "Brain BLAQ" (Block Lipids and Aldehyde Quench) to rapidly reduce autofluorescence in thick brain sections, enabling efficient axon-level imaging of neurons and their processes in conventional tissue preparations using standard epifluorescence microscopy. Following viral-mediated transduction of optogenetic constructs and fluorescent proteins in mouse cortical pyramidal and dopaminergic neurons, we used BLAQ to assess innervation patterns in the striatum, a region in which autofluorescence often obscures the imaging of fine neural processes. After BLAQ treatment of 250-350 μm-thick brain sections, axons and puncta of labeled afferents were visible throughout the striatum using a standard epifluorescence stereomicroscope. BLAQ histochemistry confirmed that motor cortex (M1) projections preferentially innervated the matrix component of lateral striatum, whereas medial prefrontal cortex projections terminated largely in dorsal striosomes and distinct nucleus accumbens subregions. Ventral tegmental area dopaminergic projections terminated in a similarly heterogeneous pattern within nucleus accumbens and ventral striatum. Using a minimal number of easily manipulated and visualized sections, and microscopes available in most neuroscience laboratories, BLAQ enables simple, high-resolution assessment of virally transduced optogenetic construct expression, and post-hoc association of this expression with molecular markers, physiology and behavior.

Melanin-concentrating hormone inputs to the nucleus accumbens originate from distinct hypothalamic sources and are apposed to GABAergic and cholinergic cells in the Long-Evans rat brain

Melanin-concentrating hormone [MCH] is a neuropeptide that modulates several behaviors, such as feeding and reward. Because the hedonic and rewarding features of a food also influence feeding behavior, the nucleus accumbens [Acb] has been highlighted as a key area integrating these roles. Functional data confirm that MCH acts on a subdivision of the Acb; however, considering the importance of finding anatomical and neurochemical data that correlate the previously demonstrated function of MCH, we delineated this investigation based on the following points: (1) Is there a pattern of innervation by MCH fibers regarding the subregions within the Acb? (2) Specifically, which hypothalamic nuclei synthesize MCH and innervate the Acb? (3) Finally, what are the neurochemical identities of the accumbal neurons innervated by MCH inputs? We examined the MCH immunoreactivity [MCH-ir] in the Acb in rat brains using the peroxidase technique. Additionally, after injecting retrograde neuronal tracer [Fluoro-Gold® - FG®] into subdivisions of the Acb [shell or core], we mapped single- or double-labeled cells. Moreover, using a double immunoperoxidase protocol, we investigated the MCH-ir fibers for gamma-aminobutyric acid [GABA]-ir and choline acetyltransferase [ChAT]-ir cells in the shell subdivision of the Acb [AcbSh]. We found that the MCH-ir fibers preferentially innervate the medial AcbSh, particularly the septal pole. This innervation originated from the incerto-hypothalamic area [IHy], internuclear area, lateral hypothalamic area, perifornical area, periventricular nucleus and posterior hypothalamus. Moreover, the IHy has the highest relationship between double/single retrogradely labeled cells [n=5.33±0.66/16±0.93, i.e. 33.33%] in the whole hypothalamus. Furthermore, our data suggest that MCH-ir fibers are in apposition to GABAergic and cholinergic cells in the AcbSh. Therefore, we provide anatomical support to the ongoing functional studies investigating the relation among the hypothalamus, MCH transmission into the Acb and the involvement of known neuronal phenotypes within the AcbSh.

The nucleus accumbens beyond the anterior commissure: implications for psychosurgery

INTRODUCTION

The nucleus accumbens (Acc) is a basal forebrain structure integrated in the dopaminergic cerebral rewarding circuits and implicated in some neuropsychiatric disorders. It has become a target for deep brain stimulation for some of these disorders when refractory to medical treatment. However, it is controversial as to which target is the best and similar results have been achieved with the stimulation of neighboring structures such as the bed nucleus of the stria terminalis (BNST). Previous studies have established the stereotactic anatomy of the human Acc, but some difficulties remain concerning its precise posterior limit, which is assumed to be at the level of the anterior commissure (AC). It is our purpose to clarify the anatomy of this zone, given the importance of its exact identification in psychosurgery.

METHODS

A total of 16 Acc were collected by autopsy, fixed, dissected, embedded and cut in coronal 5-µm slices. The slices were stained with hematoxylin and eosin, marked with anti-D1 and anti-D2 antibodies and analyzed under a microscope.

RESULTS

The human Acc has the same cellular structure as the dorsal striatum, except in its posterior subcommissural part where voluminous neurons prevail, similar to and contiguous with the BNST.

CONCLUSIONS

The Acc is longer than previously described, with a sub- and postcommissural extension behind the AC, continuous with the BNST.

Advances in the neurobiological bases for food 'liking' versus 'wanting'

The neural basis of food sensory pleasure has become an increasingly studied topic in neuroscience and psychology. Progress has been aided by the discovery of localized brain subregions called hedonic hotspots in the early 2000s, which are able to causally amplify positive affective reactions to palatable tastes ('liking') in response to particular neurochemical or neurobiological stimulations. Those hedonic mechanisms are at least partly distinct from larger mesocorticolimbic circuitry that generates the incentive motivation to eat ('wanting'). In this review, we aim to describe findings on these brain hedonic hotspots, especially in the nucleus accumbens and ventral pallidum, and discuss their role in generating food pleasure and appetite.

Distribution and compartmental organization of GABAergic medium-sized spiny neurons in the mouse nucleus accumbens

The nucleus accumbens (NAc) is a critical brain region involved in many reward-related behaviors. The NAc comprises major compartments the core and the shell, which encompass several subterritories. GABAergic medium-sized spiny neurons (MSNs) constitute the output neurons of the NAc core and shell. While the functional organization of the NAc core outputs resembles the one described for the dorsal striatum, a simple classification of the NAc shell neurons has been difficult to define due to the complexity of the compartmental segregation of cells. We used a variety of BAC transgenic mice expressing enhanced green fluorescence (EGFP) or the Cre-recombinase (Cre) under the control of the promoter of dopamine D1, D2, and D3 receptors and of adenosine A2a receptor to dissect the microanatomy of the NAc. Moreover, using various immunological markers we characterized in detail the distribution of MSNs in the mouse NAc. In addition, cell-type specific extracellular signal-regulated kinase (ERK) phosphorylation in the NAc subterritories was analyzed following acute administration of SKF81297 (a D1R-like agonist), quinpirole (a D2 receptors (D2R)-like agonist), apomorphine (a non-selective DA receptor agonist), raclopride (a D2R-like antagonist), and psychostimulant drugs, including cocaine and d-amphetamine. Each drug generated a unique topography and cell-type specific activation of ERK in the NAc. Our results show the existence of marked differences in the receptor expression pattern and functional activation of MSNs within the shell subterritories. This study emphasizes the anatomical and functional heterogeneity of the NAc, which will have to be considered in its further study.

Mechanisms of psychostimulant-induced structural plasticity

Psychostimulants robustly induce alterations in neuronal structural plasticity throughout brain reward circuits. However, despite our extensive understanding of how these circuits modulate motivated behavior, it is still unclear whether structural plasticity within these regions drives pathological behavioral responses in addiction. Although these structural changes have been subjected to an exhaustive phenomenological characterization, we still have a limited understanding of the molecular mechanisms regulating their induction and the functional relevance of such changes in mediating addiction-like behavior. Here we have highlighted the known molecular pathways and intracellular signaling cascades that regulate psychostimulant-induced changes in neuronal morphology and synaptic restructuring, and we discuss them in the larger context of addiction behavior.

Transient inactivation of the nucleus accumbens reduces both the expression and acquisition of morphine-induced conditioned place preference in rats

In the present study, the effects of transient inhibition of the shell and core parts of the nucleus accumbens by lidocaine on the expression and acquisition of morphine-induced conditioned place preference in male Wistar rats were investigated. In addition, the number of bouts of sniffing, rearing, and compartment crossing was scored. Lidocaine hydrochloride was injected into different parts of the nucleus accumbens 5 min before each morphine session for the transient inhibition of particular anatomical regions. Subcutaneous (s.c.) injection of morphine (2.5 and 5mg/kg) induced place preference. Transient inhibition of the left and/or right side of the shell part of nucleus accumbens reduced morphine place conditioning. However, when both sides of the nucleus were inhibited, inhibition was weaker when compared to the results when only one side was inhibited. Also, the number of compartment crossings in these animals reduced significantly. Nevertheless, the number of rearing occurrences was reduced only when both sides of the shell part of the nucleus accumbens were inhibited. In contrast, the number of sniffing bouts increased in all three groups. The results for the core part of the nucleus accumbens also indicated that place preference was inhibited after transient inhibition of the left, right, and both sides. However, although the number of total compartment crossings was reduced in all experimental groups, the reduction was not statistically significant. The data obtained was similar to the number of rearings, yet the number of sniffing bouts increased in the experimental groups compared to the control. In conclusion, these results confirmed the involvement of the left and right sides and core and shell parts of the nucleus accumbens in morphine place conditioning.

Regional and cell-type-specific effects of DAMGO on striatal D1 and D2 dopamine receptor-expressing medium-sized spiny neurons

The striatum can be divided into the DLS (dorsolateral striatum) and the VMS (ventromedial striatum), which includes NAcC (nucleus accumbens core) and NAcS (nucleus accumbens shell). Here, we examined differences in electrophysiological properties of MSSNs (medium-sized spiny neurons) based on their location, expression of DA (dopamine) D1/D2 receptors and responses to the μ-opioid receptor agonist, DAMGO {[D-Ala²-MePhe⁴-Gly(ol)⁵]enkephalin}. The main differences in morphological and biophysical membrane properties occurred among striatal sub-regions. MSSNs in the DLS were larger, had higher membrane capacitances and lower Rin (input resistances) compared with cells in the VMS. RMPs (resting membrane potentials) were similar among regions except for D2 cells in the NAcC, which displayed a significantly more depolarized RMP. In contrast, differences in frequency of spontaneous excitatory synaptic inputs were more prominent between cell types, with D2 cells receiving significantly more excitatory inputs than D1 cells, particularly in the VMS. Inhibitory inputs were not different between D1 and D2 cells. However, MSSNs in the VMS received more inhibitory inputs than those in the DLS. Acute application of DAMGO reduced the frequency of spontaneous excitatory and inhibitory postsynaptic currents, but the effect was greater in the VMS, in particular in the NAcS, where excitatory currents from D2 cells and inhibitory currents from D1 cells were inhibited by the largest amount. DAMGO also increased cellular excitability in the VMS, as shown by reduced threshold for evoking APs (action potentials). Together the present findings help elucidate the regional and cell-type-specific substrate of opioid actions in the striatum and point to the VMS as a critical mediator of DAMGO effects.

Parallel associative processing in the dorsal striatum: segregation of stimulus-response and cognitive control subregions

Although evidence suggests that the dorsal striatum contributes to multiple learning and memory functions, there nevertheless remains considerable disagreement on the specific associative roles of different neuroanatomical subregions. We review evidence indicating that the dorsolateral striatum (DLS) is a substrate for stimulus-response habit formation - incremental strengthening of simple S-R bonds - via input from sensorimotor neocortex while the dorsomedial striatum (DMS) contributes to behavioral flexibility - the cognitive control of behavior - via prefrontal and limbic circuits engaged in relational and spatial information processing. The parallel circuits through dorsal striatum interact with incentive/affective motivational processing in the ventral striatum and portions of the prefrontal cortex leading to overt responding under specific testing conditions. Converging evidence obtained through a detailed task analysis and neurobehavioral assessment is beginning to illuminate striatal subregional interactions and relations to the rest of the mammalian brain.

Distribution of tyrosine hydroxylase-expressing interneurons with respect to anatomical organization of the neostriatum

We have recently shown in vitro that striatal tyrosine hydroxylase-expressing interneurons identified in transgenic mice by expression of enhanced green fluorescent protein (TH-eGFP) display electrophysiological profiles that are distinct from those of other striatal interneurons. Furthermore, striatal TH-eGFP interneurons show marked diversity in their electrophysiological properties and have been divided into four distinct subtypes. One question that arises from these observations is whether striatal TH-eGFP interneurons are distributed randomly, or obey some sort of organizational plan as has been shown to be the case with other striatal interneurons. An understanding of the striatal TH-eGFP interneuronal patterning is a vital step in understanding the role of these neurons in striatal functioning. Therefore, in the present set of studies the location of electrophysiologically identified striatal TH-eGFP interneurons was mapped. In addition, the distribution of TH-eGFP interneurons with respect to the striatal striosome–matrix compartmental organization was determined using μ-opioid receptor (MOR) immunofluorescence or intrinsic TH-eGFP fluorescence to delineate striosome and matrix compartments. Overall, the distribution of the different TH-eGFP interneuronal subtypes did not differ in dorsal versus ventral striatum. However, striatal TH-eGFP interneurons were found to be mostly in the matrix in the dorsal striatum whereas a significantly higher proportion of these neurons was located in MOR-enriched domains of the ventral striatum. Further, the majority of striatal TH-eGFP interneurons was found to be located within 100 μm of a striosome–matrix boundary. Taken together, the current results suggest that TH-eGFP interneurons obey different organizational principles in dorsal versus ventral striatum, and may play a role in communication between striatal striosome and matrix compartments.

Extrasynaptic delta-containing GABAA receptors in the nucleus accumbens dorsomedial shell contribute to alcohol intake

Recent findings suggest that extrasynaptic δ-subunit-containing GABA(A) receptors are sensitive to low-to-moderate concentrations of alcohol, raising the possibility that these receptors mediate the reinforcing effects of alcohol after consumption of one or a few drinks. We used the technique of viral-mediated RNAi to reduce expression of the GABA(A) receptor δ-subunit in adult rats in localized regions of the nucleus accumbens (NAc) to test the hypothesis that δ-subunit-containing GABA(A) receptors in the NAc are necessary for oral alcohol consumption. We found that knockdown of the δ-subunit in the medial shell region of the NAc, but not in the ventral or lateral shell or in the core, reduced alcohol intake. In contrast, δ-subunit knockdown in the medial shell did not affect intake of a 2% sucrose solution, suggesting that the effects of GABA(A) receptor δ-subunit reduction are specific to alcohol. These results provide strong evidence that extrasynaptic δ-subunit-containing GABA(A) receptors in the medial shell of the NAc are critical for the reinforcing effects of oral ethanol.

Nr4a1-eGFP is a marker of striosome-matrix architecture, development and activity in the extended striatum

Transgenic mice expressing eGFP under population specific promoters are widely used in neuroscience to identify specific subsets of neurons in situ and as sensors of neuronal activity in vivo. Mice expressing eGFP from a bacterial artificial chromosome under the Nr4a1 promoter have high expression within the basal ganglia, particularly within the striosome compartments and striatal-like regions of the extended amygdala (bed nucleus of the stria terminalis, striatal fundus, central amygdaloid nucleus and intercalated cells). Grossly, eGFP expression is inverse to the matrix marker calbindin 28K and overlaps with mu-opioid receptor immunoreactivity in the striatum. This pattern of expression is similar to Drd1, but not Drd2, dopamine receptor driven eGFP expression in structures targeted by medium spiny neuron afferents. Striosomal expression is strong developmentally where Nr4a1-eGFP expression overlaps with Drd1, TrkB, tyrosine hydroxylase and phospho-ERK, but not phospho-CREB, immunoreactivity in “dopamine islands”. Exposure of adolescent mice to methylphenidate resulted in an increase in eGFP in both compartments in the dorsolateral striatum but eGFP expression remained brighter in the striosomes. To address the role of activity in Nr4a1-eGFP expression, primary striatal cultures were prepared from neonatal mice and treated with forskolin, BDNF, SKF-83822 or high extracellular potassium and eGFP was measured fluorometrically in lysates. eGFP was induced in both neurons and contaminating glia in response to forskolin but SKF-83822, brain derived neurotrophic factor and depolarization increased eGFP in neuronal-like cells selectively. High levels of eGFP were primarily associated with Drd1+ neurons in vitro detected by immunofluorescence; however ∼15% of the brightly expressing cells contained punctate met-enkephalin immunoreactivity. The Nr4a1-GFP mouse strain will be a useful model for examining the connectivity, physiology, activity and development of the striosome system.

Cortico-Basal Ganglia reward network: microcircuitry

Many of the brain's reward systems converge on the nucleus accumbens, a region richly innervated by excitatory, inhibitory, and modulatory afferents representing the circuitry necessary for selecting adaptive motivated behaviors. The ventral subiculum of the hippocampus provides contextual and spatial information, the basolateral amygdala conveys affective influence, and the prefrontal cortex provides an integrative impact on goal-directed behavior. The balance of these afferents is under the modulatory influence of dopamine neurons in the ventral tegmental area. This midbrain region receives its own complex mix of excitatory and inhibitory inputs, some of which have only recently been identified. Such afferent regulation positions the dopamine system to bias goal-directed behavior based on internal drives and environmental contingencies. Conditions that result in reward promote phasic dopamine release, which serves to maintain ongoing behavior by selectively potentiating ventral subicular drive to the accumbens. Behaviors that fail to produce an expected reward decrease dopamine transmission, which favors prefrontal cortical-driven switching to new behavioral strategies. As such, the limbic reward system is designed to optimize action plans for maximizing reward outcomes. This system can be commandeered by drugs of abuse or psychiatric disorders, resulting in inappropriate behaviors that sustain failed reward strategies. A fuller appreciation of the circuitry interconnecting the nucleus accumbens and ventral tegmental area should serve to advance discovery of new treatment options for these conditions.

Prefrontal cortical D1 dopamine receptors modulate subcortical D2 dopamine receptor-mediated stress responsiveness

Increased responsiveness to stress plays an important role in the manifestation of schizophrenia symptoms. Evidence indicates that the prefrontal cortex (PFC), and dopamine neurotransmission in the PFC in particular, is involved in the modulation of stress responsiveness. Decreased dopaminergic activity and loss of dopamine fibres have been reported in PFC in schizophrenia patients. Consequently, it was hypothesized that depletion of dopamine in PFC may facilitate increased stress responsiveness. Adult Sprague-Dawley rats received injections of 6-hydroxydopamine or saline bilaterally into the medial PFC (mPFC) following desipramine pretreatment to selectively deplete dopaminergic fibres. Following a 3-wk recovery period, the lesioned and control rats received injections of a D1 or D2 dopamine receptor agonist or vehicle into the mPFC and were immediately subjected to forced swimming as a stressor. Results showed that frequency of locomotion and rearing, behavioural measures indicative of increased dopaminergic activity in the nucleus accumbens (NAc), were significantly increased following stress in prefrontal cortical dopamine-depleted rats. This effect was significantly ameliorated by infusions of a D1 dopamine receptor agonist directly into the mPFC in a dose-dependent manner but not by infusion of a D2 dopamine receptor agonist. In addition, stress-induced behavioural changes in prefrontal cortical dopamine-depleted rats were significantly reduced following selective discrete infusions of a D2 dopamine receptor antagonist into the NAc shell. The results suggest that dopaminergic transmission via D1 receptors in the mPFC modulates D2 dopamine receptor-mediated stress responsiveness in the NAc, a feature that may be disrupted in schizophrenia patients.

Involvement of dopaminergic mechanisms in the nucleus accumbens core and shell subregions in the expression of fear conditioning

The involvement of dopamine (DA) mechanisms in the nucleus accumbens (NAC) in fear conditioning has been proposed by many studies that have challenged the view that the NAC is solely involved in the modulation of appetitive processes. However, the role of the core and shell subregions of the NAC in aversive conditioning remains unclear. The present study examined DA release in these NAC subregions using microdialysis during the expression of fear memory. Guide cannulae were implanted in rats in the NAC core and shell. Five days later, the animals received 10 footshocks (0.6 mA, 1 s duration) in a distinctive cage A (same context). On the next day, dialysis probes were inserted through the guide cannulae into the NAC core and shell subregions, and the animals were behaviorally tested for fear behavior either in the same context (cage A) or in a novel context (cage B). Dialysates were collected every 5 min for 90 min and analyzed by high-performance liquid chromatography. The rats exhibited a significant fear response in cage A but not in cage B. Moreover, increased DA levels in both NAC subregions were observed 5-25 min after the beginning of the test when the animals were tested in the same context compared with accumbal DA levels from rats tested in the different context. These findings suggest that DA mechanisms in both the NAC core and shell may play an important role in the expression of contextual fear memory.

The role of the striatum in aversive learning and aversive prediction errors

Neuroeconomic studies of decision making have emphasized reward learning as critical in the representation of value-driven choice behaviour. However, it is readily apparent that punishment and aversive learning are also significant factors in motivating decisions and actions. In this paper, we review the role of the striatum and amygdala in affective learning and the coding of aversive prediction errors (PEs). We present neuroimaging results showing aversive PE-related signals in the striatum in fear conditioning paradigms with both primary (shock) and secondary (monetary loss) reinforcers. These results and others point to the general role for the striatum in coding PEs across a broad range of learning paradigms and reinforcer types.

Three-dimensional organization of dendrites and local axon collaterals of shell and core medium-sized spiny projection neurons of the rat nucleus accumbens

Medium-sized spiny projection neurons (MSN) in the head of the primate caudate nucleus are thought to have preferred dendritic orientations that tend to parallel the orientations of the striosomes. Moreover, recurrent axon collaterals of MSN in the rat dorsal striatum have been categorized into two types, i.e., restricted and widespread. The nucleus accumbens (Acb) has a highly complex compartmental organization, and the spatial organization of dendritic and axonal arbors of MSN has not yet been systematically studied. In this study, using single-cell juxtacellular labeling with neurobiotin as well as anterograde neuroanatomical tracing with biotinylated dextran amine, we investigated the three-dimensional (3D) organization of dendrites and axons of MSN of the rat Acb in relation to subregional (shell-core) and compartmental (patch-matrix) boundaries. Our results show that dendritic arbors of MSN in both the Acb shell and core subregions are preferentially oriented, i.e., they are flattened in at least one of the 3D-planes. The preferred orientations are influenced by shell-core and patch-matrix boundaries, suggesting parallel and independent processing of information. Dendritic orientations of MSN of the Acb core are more heterogeneous than those of the shell and the dorsal striatum, suggesting a more complex distribution of striatal inputs within the core. Although dendrites respect the shell-core and patch-matrix boundaries, recurrent axon collaterals may cross these boundaries. Finally, different degrees of overlap between dendritic and axonal arborizations of individual MSN were identified, suggesting various possibilities of lateral inhibitory interactions within and between, functionally distinct territories of the Acb.

Expression of amphetamine sensitization is associated with recruitment of a reactive neuronal population in the nucleus accumbens core

RATIONALE

Repeated exposure to psychostimulant drugs causes a long-lasting increase in the psychomotor and reinforcing effects of these drugs and an array of neuroadaptations. One such alteration is a hypersensitivity of striatal activity such that a low dose of amphetamine in sensitized animals produces dorsal striatal activation patterns similar to acute treatment with a high dose of amphetamine.

OBJECTIVES

To extend previous findings of striatal hypersensitivity with behavioral observations and with cellular activity in the nucleus accumbens and prefrontal cortex in sensitized animals.

MATERIALS AND METHODS

Rats treated acutely with 0, 1, 2.5, or 5 mg/kg i.p. amphetamine and sensitized rats challenged with 1 mg/kg i.p. amphetamine were scored for stereotypy, rearing, and grooming, and locomotor activity recorded. c-fos positive nuclei were quantified in the nucleus accumbens and prefrontal cortex after expression of sensitization with 1 mg/kg i.p. amphetamine.

RESULTS

Intense stereotypy was seen in animals treated acutely with 5 mg/kg amphetamine, but not in the sensitized group treated with 1 mg/kg amphetamine. The c-fos response to amphetamine in the accumbens core was augmented in amphetamine-pretreated animals with a shift in the distribution of optical density, while no effect of sensitization was seen in the nucleus accumbens shell or prefrontal cortex.

CONCLUSIONS

A lack of stereotypy in the sensitized group indicates a dissociation of behavioral responses to amphetamine and striatal immediate-early gene activation patterns. The increase in c-fos positive nuclei and shift in the distribution of optical density observed in the nucleus accumbens core suggests recruitment of a new population of neurons during expression of sensitization.

The role of nitric oxide within the nucleus accumbens on the acquisition and expression of morphine-induced place preference in morphine sensitized rats

In the present study, the effects of intra-accumbal administration of L-arginine, a nitric oxide precursor, and N(G)-nitro-L-arginine methyl-ester (L-NAME), a nitric oxide synthase inhibitor, on the acquisition and expression of morphine-induced place conditioning in morphine-sensitized rats were studied. Subcutaneous (s.c.) administration of morphine (2.5, 5 and 7.5 mg/kg) induced conditioned place preference. Repeated pretreatment of morphine (5 mg/kg, i.p.) followed by 5 days without drug treatment, increased conditioning response induced by morphine (0.25, 0.5 and 0.75 mg/kg). Intra-accumbal (intra-nucleus accumbens; 1 microg/rat) administration of L-arginine (0.3, 1 and 3 microg/rat) significantly increased or reduced the acquisition of morphine place conditioning in non-sensitized and sensitized rats respectively. However, the drug reduced expression of place conditioning by morphine in sensitized animals. Intra-nucleus accumbens injections of L-NAME (0.3, 1 and 3 microg/rat) reduced the acquisition and expression of morphine place conditioning in the sensitized animals. The results indicate that nitric oxide (NO) within the nucleus accumbens is involved in the acquisition and expression of morphine place conditioning in morphine-sensitized rats.

The anatomy of co-morbid neuropsychiatric disorders based on cortico-limbic synaptic interactions

Many brain disorders appear to involve dysfunctions of aminergic systems. Alterations in dopamine activity may underpin both schizophrenia and the establishment and maintenance of drug dependence while disruption of serotonergic signalling may be crucial in both depression and schizophrenia. The co-existence of nicotine and alcohol abuse with depression and schizophrenia is well-documented as is the particular vulnerability of adolescents. At the same time, a common group of brain structures is increasingly implicated in neuropathological studies. For example, depression may involve a lack of serotonin signalling, particularly in the prefrontal cortex, while in schizophrenia there is evidence for reduced dopamine signalling in the same brain region, co-existing with hyperactivity in the mesolimbic dopamine pathway. Increased dopamine release from the mesolimbic dopamine pathway is also a common factor of drugs of abuse. Furthermore, the control of motivational behaviour and dopamine release is apparently modified by hippocampal and amygdala activity, both brain regions showing pathological changes in schizophrenia and depression. Our work has focused on the intricate synaptic interactions of aminergic terminals and cortical and subcortical neurons in order to unravel the anatomical basis for these disorders and their treatments. We show convergence of dopamine and cortical inputs onto single neurons in the nucleus accumbens, and between different cortical inputs to individual neurons, providing a basis for the gating mechanisms attributed to these interactions. We have also examined local and extrinsic connections in the prefrontal cortex and the basis for regulation of both cortical neurons and midbrain dopamine neurons by serotonin from the raph é nucleus. Together with data concerning subcellular receptor distributions, this information provides a detailed synaptic framework for interpreting behavioural, pharmacological and physiological data and enhances our understanding of possible circuitry underlying comorbidity of disorders such as schizophrenia and depression with drug abuse, information invaluable in the introduction of enhanced therapies.

Electrolytic lesions of a discrete area within the nucleus accumbens shell attenuate the long-term expression, but not early phase, of sensitization to cocaine

Repeated exposure to cocaine leads to behavioral sensitization, which is the augmentation of the locomotor response to a subsequent exposure to the drug. The nucleus accumbens (NAc), a major termination site of dopaminergic neurons, is believed to be involved in behavioral sensitization and studies have demonstrated that the NAc shell can be split into five zones of analysis; the vertex, arch, cone, intermediate and ventrolateral zones [Todtenkopf MS, Stellar JR. Assessment of tyrosine hydroxylase immunoreactive innervation in five subregions of the nucleus accumbens shell in rats treated with repeated cocaine. Synapse 2000;38:261-70]. Several reports show cocaine-induced c-fos expression particularly in the intermediate zone after 14, but not 2, drug-free days following repeated cocaine administration, suggesting that this region may be involved in sensitization and particularly in the later phase of expression, versus the earlier phase of sensitization. Bilateral electrolytic lesions of the intermediate zone were made in two groups of rats, which were then repeatedly exposed to cocaine (15 mg/kg, twice/day for 5 days). One group was subsequently given a single cocaine challenge injection (15 mg/kg) after 14 drug-free days, while the other group was challenged after only 2 drug-free days. Two sham surgery groups in which an electrode was lowered but no current was passed served as controls. Results show that lesioned animals as well as sham controls exhibited behavioral sensitization to the drug. However, following a 14-day drug-free period, the lesioned animals showed significant reduction in sensitization, compared to sham controls. Together these findings suggest that the intermediate zone of the NAc shell is indeed involved in the expression phase of behavioral sensitization to cocaine.

Ontogeny of the dopamine innervation in the nucleus accumbens of gerbils

The postnatal maturation of immunohistochemically stained dopamine (DA) fibres was quantitatively examined in the core and shell subareas of the nucleus accumbens (NAC) of gerbils. Animals of different ages, ranging from juvenile [postnatal day (PD) 14, 30] to adolescent (PD70), adult (PD90, PD180, PD360) and ageing (PD540, PD720) were analysed. The timescale of the maturation of the accumbal DA innervation was regionally different, probably due to the different origin of DA fibres in the mesencephalon. Both the accumbal core, with DA afferents arising from the lateral ventral tegmental area (VTA) and the substantia nigra pars compacta, as well as the accumbal shell, with DA afferents arising from the medial VTA, show moderate DA fibre densities at PD14. The core displayed a significant decrease of the DA fibre density up to PD30 and a subsequent significant increase between PD70 and 90, whereas the shell solely showed an augmentation of the DA innervation between PD70 and 90. Our data suggest that the different maturation of the DA innervation in core and shell might reflect differences in the development of motor and limbic functions, mediated by the nigrostriate and the mesolimbic system, respectively.

Noxious heat induces fMRI activation in two anatomically distinct clusters within the nucleus accumbens

Using functional magnetic resonance imaging (fMRI) we found that a noxious thermal stimulus (46 degrees C) to the hand activates the nucleus accumbens (NAc) in humans, while a non-noxious warm stimulus (41 degrees C) does not. Following the noxious stimulus, two distinct foci of decreased activation were observed showing distinct time course profiles. One focus was anterior, superior, and lateral and the second that was more posterior, inferior, and medial. The anatomical segregation may correlate with the functional components of the NAc, i.e., shell and core. The results support heterogeneity of function within the NAc and have implications for the understanding the contribution of NAc function to processing of pain and analgesia.

Anatomical evidence for direct connections between the shell and core subregions of the rat nucleus accumbens

The nucleus accumbens is thought to subserve different aspects of adaptive and emotional behaviors. The anatomical substrates for such actions are multiple, parallel ventral striatopallidal output circuits originating in the nucleus accumbens shell and core subregions. Several indirect ways of interaction between the two subregions and their associated circuitry have been proposed, in particular through striato-pallido-thalamic and dopaminergic pathways. In this study, using anterograde neuroanatomical tracing with Phaseolus vulgaris-leucoagglutinin and biotinylated dextran amine as well as single-cell juxtacellular filling with neurobiotin, we investigated the intra-accumbens distribution of local axon collaterals for the identification of possible direct connections between the shell and core subregions. Our results show widespread intra-accumbens projection patterns, including reciprocal projections between specific parts of the shell and core. However, fibers originating in the core reach more distant areas of the shell, including the rostral pole (i.e. the calbindin-poor part of the shell anterior to the core) and striatal parts of the olfactory tubercle, than those arising in the shell and projecting to the core. The latter projections are more restricted to the border region between the shell and core. The density of the fiber labeling within both the shell and core was very similar. Moreover, specific intrinsic projections within shell and core were identified, including a relatively strong projection from the rostral pole to the rostral shell, reciprocal projections between the rostral and caudal shell, as well as projections within the core that have a caudal-to-rostral predominance. The results of the juxtacellular filling experiments show that medium-sized spiny projection neurons and medium-sized aspiny neurons (most likely fast-spiking) contribute to these intra-accumbens projections. While such neurons are GABAergic, the intrastriatal projection patterns indicate the existence of lateral inhibitory interactions within, as well as between, shell and core subregions of the nucleus accumbens.

The effects of temporary inactivation of the core and the shell subregions of the nucleus accumbens on prepulse inhibition of the acoustic startle reflex and activity in rats

The nucleus accumbens can be dissociated into at least two subregions: a 'core' and a 'shell'. Using temporary chemical inactivation of these subregions, we investigated whether they are differentially involved in the regulation of prepulse inhibition (PPI) of the acoustic startle reflex and activity. For this purpose, rats were bilaterally implanted with guide cannulae aimed at either the core or the shell and infused with the GABA(A) receptor agonist muscimol (0.5 microg/0.2 microl per side). The control group consisted of vehicle infused and unoperated rats. To ascertain the region selectivity of the infusions, 0.2 microl of [3H]muscimol was infused into either the core or the shell of an additional group of rats. The behavioral results demonstrated that in comparison to the control group, inactivation of the core led to a loss of the prepulse intensity dependency of PPI. Moreover, core inactivation resulted in akinesia directly after infusion, but in hyperactivity 24 and 72 h thereafter in contrast to the control group. In both experiments, inactivation of the shell was ineffective compared to controls. Analysis of the autoradiograms revealed that the spread of drug into the other subregion was minimal, supporting the region selectivity of the inactivation. These results lend further support to the existence of a functional dissociation between the core and the shell, with the former being preferentially involved in PPI and locomotion. The persistent hyperactivity after the muscimol infusion into the core could be explained by compensatory mechanisms taking place in the nucleus accumbens.

Reciprocal opioid-opioid interactions between the ventral tegmental area and nucleus accumbens regions in mediating mu agonist-induced feeding in rats

Feeding elicited by the mu-selective agonist, [D-Ala2, M-Phe4, Gly-ol5]-encephalin administered into the nucleus accumbens is blocked by accumbal pre-treatment with mu, delta1, delta2 and kappa, but not mu1 opioid antagonists. Correspondingly, mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by ventral tegmental area pre-treatment with mu and kappa, but not delta opioid antagonists. A bi-directional opioid-opioid feeding interaction has been firmly established such that mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by accumbal naltrexone, and that accumbal mu-agonist-induced feeding is blocked by naltrexone pre-treatment in the ventral tegmental area. To determine which opioid receptor subtypes mediate the regional bi-directional opioid-opioid feeding interactions between these two sites, the present study examined the dose-dependent ability of either general (naltrexone), mu (beta-funaltrexamine), kappa (nor-binaltorphamine) or delta (naltrindole) opioid antagonists administered into one site to block mu-agonist-induced feeding elicited from the other site. General, mu and kappa, but not delta opioid receptor antagonist pre-treatment in the ventral tegmental area dose-dependently reduced mu-agonist-induced feeding elicited from the nucleus accumbens. General, mu and delta, and to a lesser degree kappa, opioid receptor antagonist pre-treatment in the nucleus accumbens dose-dependently reduced mu-agonist-induced feeding elicited from the ventral tegmental area. Thus, multiple, but different opioid receptor subtypes are involved in mediating opioid-opioid feeding interactions between the nucleus accumbens and ventral tegmental area regions.

Differential distribution of parvalbumin immunoreactive neurons in the striatum of cocaine sensitized rats

Intermittent administration of psychostimulants such as cocaine and amphetamine can result in behavioral sensitization, which is believed to model the onset of drug addiction, as well as possible neural adaptations that lead to addictive behaviors. The dorsal striatum and the nucleus accumbens (NAc) have been shown to play an integral role in this phenomenon. However, these structures comprise a complex neuroanatomical organization, and few studies have correlated anatomical differentiation within these brain regions with functional (i.e. behavioral) outcome, particularly after psychostimulant exposure. Parvalbumin (PV)-containing GABAergic interneurons are a key neuronal cell population that can significantly regulate input-output functions in these brain regions. The present study quantified parvalbumin-immunoreactive cells in subterritories of the striatum and NAc in animals behaviorally sensitized to cocaine. Rats received a sensitization-inducing regimen of cocaine (twice-daily injections of 15 mg/kg i.p. for 5 consecutive days). Two or 14 days following the last injection, rats were given a challenge injection of cocaine (15 mg/kg i.p.), and killed 2 h later. Sections through the striatum (including the NAc) were processed for parvalbumin immunoreactivity, and the number of immunoreactive neurons was quantified. Repeated cocaine administration resulted in robust sensitization that correlated with transient increases in the number of PV immunoreactive neurons in the ventrolateral, dorsolateral and dorsomedial striatum. After a 2-week withdrawal period, sensitized animals showed a significant decrease in the number of PV+ neurons in the ventrolateral shell of the NAc and dorsomedial striatum, and no significant difference in any other area examined. These data suggest a dichotomous role for PV interneurons in different subterritories of the striatum and NAc during the short-term (induction) vs. long-term (expression) phases of cocaine sensitization.

Subregion-specific down-regulation of 5-HT3 immunoreactivity in the nucleus accumbens shell during the induction of cocaine sensitization

Repeated exposure to psychostimulants such as cocaine and amphetamine can result in behavioral sensitization, which is believed to model the onset of drug addiction, as well as neural adaptations that occur after repeated drug abuse that lead to addictive behaviors. Dopamine (DA) in the nucleus accumbens (NAc) has been shown to play an integral role in this phenomenon. However, cocaine also acts on the serotonin (5-HT) system, which has been shown to modulate psychostimulant-induced increases in motor behavior and DA release in the NAc. Recently, it has been demonstrated that the shell portion of the NAc can no longer be considered a homogeneous structure and can be subdivided into at least five separate regions. The present study examines 5-HT(3) receptors in the subdivisions of the NAc in cocaine-sensitized rats. Rats received a sensitization-inducing regimen of cocaine (twice-daily injections of 15 mg/kg ip for five consecutive days). Two or 14 days following the last injection, rats were given a challenge injection of cocaine (15 mg/kg ip) and sacrificed 2 h later. Sections of the NAc were processed for 5-HT(3) immunoreactivity (5-HT(3)-IR), and the number of puncta was quantified in each of the subregions of the shell, as well as the core of the accumbens. Repeated cocaine administration resulted in robust sensitization that correlated with a transient decrease in the density of 5-HT(3) immunoreactive puncta in the intermediate zone of the accumbens shell. After a 2-week withdrawal period, sensitized animals no longer showed any differences in any of the areas examined. These data suggest a possible role for 5-HT(3) receptors in the intermediate zone during the induction of cocaine sensitization.

Complementary distribution of vesicular glutamate transporters 1 and 2 in the nucleus accumbens of rat: Relationship to calretinin-containing extrinsic innervation and calbindin-immunoreactive neurons

The caudomedial shell of the rat nucleus accumbens exhibits inhomogeneous distribution patterns of the vesicular glutamate transporters 1 (VGLUT1) and 2 (VGLUT2). This paper focuses on the question of whether patterns of VGLUT1 and VGLUT2 correspond to cytoarchitectonically and cytochemically defined subterritories of the caudomedial shell region. VGLUT2 was shown to be coexpressed with calretinin in the dense axonal plexus known to emanate from the paraventricular thalamic nucleus. In regions termed corridors, which are spared by this paraventricular thalamic innervation, axonal terminals were found to be clustered and VGLUT1-immunoreactive. It is assumed that these fibers originate in the prelimbic cortex and/or in the parvicellular basal amygdaloid nucleus known to project to accumbal shell components. Our findings confirm the existence of two well-separated neuronal circuits in the caudomedial shell that are dominated by two different excitatory input systems originating from either thalamic, cortical, or cortex-like amygdaloid sources. The large lateral corridors-which resemble the accumbal core not only in respect to their VGLUT1 immunolabeling but also concerning their content of calbindin-positive cells-may represent a component of the anatomically weakly defined accumbal shore region.

Lack of intersite GABA receptor subtype antagonist effects upon mu opioid receptor agonist-induced feeding elicited from either the ventral tegmental area or nucleus accumbens shell in rats

Pretreatment with the GABA(A) receptor antagonist, bicuculline or the GABA(B) receptor antagonist, saclofen, into the nucleus accumbens (Nacc) shell, respectively, potentiates and reduces feeding elicited by the mu opioid agonist, [D-Ala(2), Nme(4), Gly-ol(5)]-enkephalin (DAMGO), administered into the same site. DAMGO-induced feeding elicited from the ventral tegmental area (VTA) region is significantly reduced by pretreatment with saclofen into the same site indicating local GABA mediation of opioid-induced feeding in each site. Given the neuroanatomical and functional connections between the two sites, the present study evaluated the dose-dependent actions of bicuculline and saclofen pretreatment in one site upon DAMGO-induced feeding elicited from the second site. Pretreatment of either bicuculline (7.5-75 ng) or saclofen (1.5-10 microg) into the Nacc shell failed to alter the time course or magnitude of DAMGO-induced feeding elicited from the VTA region. DAMGO-induced feeding elicited from the Nacc shell was unaffected by VTA region pretreatment with either bicuculline (7.5-75 ng) or saclofen (1.5-5 microg). A higher (10 microg) saclofen dose prevented significant DAMGO-induced feeding after 1 and 4 h. Thus, although GABA receptor subtype antagonists are capable of differentially modulating DAMGO-induced feeding when both drugs are applied locally in either the VTA region or the Nacc shell, it appears that any effects between the VTA region and the Nacc shell in modulating DAMGO-induced feeding do not depend upon a GABAergic synapse in the other site.

Withdrawal duration differentially affects c-fos expression in the medial prefrontal cortex and discrete subregions of the nucleus accumbens in cocaine-sensitized rats

Intermittent administration of cocaine can result in behavioral sensitization, which is indicated by an augmented behavioral response to a subsequent administration of cocaine. This increase in behavior can be seen after various periods of abstinence from the drug, and is believed to model the cravings of drug users and the onset of drug addiction. It is believed that behavioral sensitization is mediated by activity of the mesocorticolimbic dopamine system. In particular, the nucleus accumbens and prefrontal cortex have been shown to play integral roles in this phenomenon. Recently, it has been demonstrated that the shell portion of the nucleus accumbens can no longer be considered a homogeneous structure, and can be subdivided into five separate regions. The present study was designed to assess the activation of key neuronal populations in subdivisions of the accumbens and subdivisions of the medial prefrontal cortex in cocaine-sensitized rats, using the expression of the immediate early gene, c-fos, as a marker of neuronal activation. Repeated cocaine administration resulted in robust sensitization that correlated with a significant decrease in the density of c-fos nuclei in all three subdivisions of the medial prefrontal cortex, and two subdivisions of the nucleus accumbens only in animals challenged after a 2-day withdrawal period. After a 2-week withdrawal period, sensitized animals no longer showed any differences in the density of c-fos nuclei in any of the areas examined, with the exception of a significant increase in the intermediate zone of the shell. The results indicate that distinct adaptations in neural activation take place in cocaine-sensitized rats that have been drug-free for various lengths of time. Furthermore, while specific subregions of brain areas known to play a role in drug abuse can be uniquely involved in the manifestations of cocaine sensitization, the functional roles of these subregions may differ depending on the time at which the behavior is assessed.

Compartment-specific changes in striatal neuronal activity during expression of amphetamine sensitization are the result of drug hypersensitivity

Repeated exposure to drugs of abuse induces behavioural sensitization, i.e. a persistent hypersensitivity to the psychomotor stimulant effects of these drugs. This may be the result of increased responsiveness, to drugs, of mesostriatal dopamine systems and their projections, but it has also been suggested that acute and sensitized behavioural responses to psychostimulant drugs involve activation of distinct neuronal circuits. In order to distinguish between these possibilities, we studied amphetamine-induced c-fos immunoreactivity in subregions of rat striatum (patch and matrix compartments of caudate-putamen and nucleus accumbens core and shell) in drug-naive rats, as well as during long-term expression of amphetamine sensitization. We found that, in sensitized animals, amphetamine (1.0 mg/kg) evoked an increase in the ratio of c-fos-immunopositive cells in striatal patch and matrix compartments, suggesting a preferential involvement of striatal patches in the sensitized response to amphetamine. In drug-naive rats, amphetamine (0.5-5.0 mg/kg) dose-dependently increased c-fos expression in all striatal subregions. Remarkably, the highest dose of amphetamine also evoked an increase in patch : matrix ratio of c-fos immunoreactivity. In nucleus accumbens core and shell of amphetamine- and saline-pretreated animals, amphetamine (1.0 mg/kg) evoked comparable increases in c-fos expression. These data indicate that distinct striatal compartments display a differential sensitivity to amphetamine in both drug-naive and amphetamine-sensitized animals. In addition, they suggest that the shift in amphetamine-induced c-fos expression from striatal matrix to patches in sensitized animals is the consequence of a change in the sensitivity to amphetamine, rather than a long-term circuitry reorganization that is exclusive to the sensitized state.

A differential involvement of the shell and core subterritories of the nucleus accumbens of rats in attentional processes

The nucleus accumbens comprises of two anatomically distinct subterritories: an inner core and an outer shell region. The distinct pattern of the core and shell input and output targets suggests that these two regions may mediate different behavioral processes. Using N-methyl-D-aspartate excitotoxic lesions in either the core or shell region, we investigated whether we can dissociate functionally these two subterritories. N-Methyl-D-aspartate-lesioned, sham-lesioned and non-operated animals were tested for locomotor activity in an open field and in two behavioral paradigms known to evaluate attentional deficits, namely the pre-pulse inhibition of the acoustic startle reflex and latent inhibition, measured in a two-way active avoidance paradigm. The shell-lesioned animals showed a small but significant hyperactivity in the open field when compared to the core-lesioned and to control animals. In the pre-pulse inhibition paradigm, core-lesioned animals demonstrated reduced pre-pulse inhibition to the two high pre-pulse intensities (80 dB[A], 84 dB[A]). In the active avoidance paradigm, whereas no lesion effects were detected in the non-pre-exposed groups, clear attenuation of latent inhibition was found in the shell-lesioned rats, in comparison to both core-lesioned and control rats, due to improved avoidance performance of the shell-pre-exposed group. From these results we suggest that the two subterritories of the nucleus accumbens are differentially involved in attention-related processes: the core lesion leads to significant disruption of pre-pulse inhibition while the shell lesion leads to heightened activity and significant attenuation of latent inhibition.

Differential modulation of nucleus accumbens synapses

The nucleus accumbens (NAcc) is a brain region involved in functions ranging from motivation and reward to feeding and drug addiction. The NAcc is typically divided into two major subdivisions, the shell and the core. The primary output neurons of both of these areas are medium spiny neurons (MSNs), which are quiescent at rest and depend on the relative input of excitatory and inhibitory synapses to determine when they fire action potentials. These synaptic inputs are, in turn, regulated by a number of neurochemical signaling agents that can ultimately influence information processing in the NAcc. The present study characterized the ability of three major signaling pathways to modulate synaptic transmission in NAcc MSNs and compared this modulation across different synapses within the NAcc. The opioid [Met](5)enkephalin (ME) inhibited excitatory postsynaptic currents (EPSCs) in shell MSNs, an effect mediated primarily by micro-opioid receptors. Forskolin, an activator of adenylyl cyclase, potentiated shell EPSCs. An analysis of miniature EPSCs indicated a primarily presynaptic site of action, although a smaller postsynaptic effect may have also contributed to the potentiation. Adenosine and an adenosine A(1)-receptor agonist inhibited shell EPSCs, although no significant tonic inhibition by endogenous adenosine was detected. The effects of these signaling agents were then compared across four different synapses in the NAcc: glutamatergic EPSCs and GABAergic inhibitory postsynaptic currents (IPSCs) in both the core and shell subregions. ME inhibited all four of these synapses but produced a significantly greater inhibition of shell IPSCs than the other synapses. Forskolin produced an increase in transmission at each of the synapses tested. However, analysis of miniature IPSCs in the shell showed no sign of a postsynaptic contribution to this potentiation, in contrast to the shell miniature EPSCs. Tonic inhibition of synaptic currents by endogenous adenosine, which was not observed in shell EPSCs, was clearly present at the other three synapses tested. These results indicate that neuromodulation can vary between the different subregions of the NAcc and between the different synapses within each subregion. This may reflect differences in neuronal interconnections and functional roles between subregions and may contribute to the effects of drugs acting on these systems.