Immunohistochemical characterization of the shell and core territories of the nucleus accumbens in the rat

The dopamine transporter: comparative ultrastructure of dopaminergic axons in limbic and motor compartments of the nucleus accumbens

The dopamine transporter (DAT) regulates extracellular dopamine concentrations, transports neurotoxins, and acts as a substrate for cocaine reinforcement. These functions are known to differ in the limbic-associated shell and motor-associated core compartments of the nucleus accumbens (NAc). Previous studies have shown differential expression of DAT in the NAc shell and core but were limited in resolution to the regional level. Thus, it is not known whether there are differences in the amount, subcellular localization, or plasmalemmal targeting of DAT within individual dopaminergic axons in the two regions. We used high-resolution electron microscopic immunocytochemistry to investigate these possibilities. We show that in both the shell and core, DAT immunogold labeling is present in tyrosine hydroxylase-immunoreactive varicose axons that form symmetric synapses. Within these labeled axons, most DAT gold particles are located on extrasynaptic plasma membranes, but some are associated with intracellular membranes. Dopaminergic axons in the shell contain lower mean densities of both total DAT gold particles (per square micron) and plasmalemmal DAT gold particles (per micron) than those in the core. Within labeled axons in the NAc shell and core, however, there are no detectable differences in the subcellullar distribution of DAT or the percentage of total DAT gold particles that are located on plasma membranes. These studies are the first to examine and compare the subcellular localization of DAT in the NAc shell and core. As a result, they identify intrinsic, cell-specific differences in the expression of DAT within dopaminergic axons in these functionally distinct striatal compartments.

Neurons in accumbens subterritories of the rat: phasic firing time-locked within seconds of intravenous cocaine self-infusion

Individual neurons were recorded extracellularly in the nucleus accumbens (NAcc) of rats during cocaine self-administration sessions. NAcc neurons exhibited a variety of phasic changes in firing rate within the few seconds before and/or after cocaine self-infusion. Analysis of the topographical distribution of the phasic firing patterns showed that there were no differences between NAcc subterritories in the nature of phasic changes in firing exhibited by neurons in relation to cocaine self-infusion. However, the prevalence of phasic firing was lower in the border regions of the caudal shell and within the caudal shell itself relative to the remainder of the NAcc.

Ultrastructural localization of CART (cocaine- and amphetamine-regulated transcript) peptides in the nucleus accumbens of monkeys

CART (cocaine- and amphetamine-regulated transcript) peptides are proposed to play a role in the action of psychostimulants as neurotransmitters/neuromodulators. In the present study, we demonstrate that the shell of the nucleus accumbens, a brain structure involved in drug reinforcement, is densely innervated by a dense plexus of CART peptide-immunoreactive varicose fibers in register with immunoreactive perikarya in monkeys. At the electron microscopic level, varicosities appeared as immunoreactive axon terminals packed with round electron-lucent vesicles and a variable number of darkly stained dense-core vesicles that formed symmetric synapses with dendrites. These findings suggest that CART peptides may be a cotransmitter with gamma aminobutyric acid (GABA) in intrinsic axon collaterals of striatal projection neurons or interneurons in the primate nucleus accumbens.

GABA in the nucleus accumbens shell participates in the central regulation of feeding behavior

We have demonstrated previously that injections of 6, 7-dinitroquinoxaline-2,3-dione into the nucleus accumbens shell (AcbSh) elicits pronounced feeding in satiated rats. This glutamate antagonist blocks AMPA and kainate receptors and most likely increases food intake by disrupting a tonic excitatory input to the AcbSh, thus decreasing the firing rate of a population of local neurons. Because the application of GABA agonists also decreases neuronal activity, we hypothesized that administration of GABA agonists into the AcbSh would stimulate feeding in satiated rats. We found that acute inhibition of cells in the AcbSh via administration of the GABAA receptor agonist muscimol or the GABAB receptor agonist baclofen elicited intense, dose-related feeding without altering water intake. Muscimol-induced feeding was blocked by coadministration of the selective GABAA receptor blocker bicuculline, but not by the GABAB receptor blocker saclofen. Conversely, baclofen-induced feeding was blocked by coadministration of saclofen, but was not affected by bicuculline. Furthermore, we found that increasing local levels of GABA by administration of a selective GABA-transaminase inhibitor, gamma-vinyl-GABA, elicited robust feeding in satiated rats, suggesting a physiological role for endogenous AcbSh GABA in the control of feeding. A mapping study showed that although some feeding can be elicited by muscimol injections near the lateral ventricles, the ventromedial AcbSh is the most sensitive site for eliciting feeding. These findings demonstrate that manipulation of GABA-sensitive cells in the AcbSh can have a pronounced, but specific, effect on feeding behavior in rats. They also constitute the initial description of a novel and potentially important component of the central mechanisms controlling food intake.

Topographical organization of projections from the entorhinal cortex to the striatum of the rat

The efferent projections of the entorhinal cortex to the striatum were studied with retrograde (horseradish peroxidase wheat germ agglutinin) and anterograde (biocytin and biotinylated dextran amine) tracing methods. The bulk of the entorhinal cortical fibres were found to project to the nucleus accumbens in the ventral striatum, but the caudate putamen is only sparsely and diffusely innervated, rostrally, along its dorsal and medial borders. Fibres arising from neurons in the lateral entorhinal cortex project throughout the rostrocaudal extent of the nucleus accumbens but are most abundant in the core and lateral shell of that nucleus. The rostral neurons of the medial entorhinal cortex were found to project sparsely to the striatum, whereas caudal neurons provide a dense input to the rostral one-third of the nucleus accumbens, especially to the rostral pole, where they concentrate more in the core than in the shell. Contralateral entorhinal projections, which are very sparse, were found in the same parts of the nucleus accumbens and the caudate-putamen as the ipsilateral terminal fields. The present observations that entorhinal inputs to the nucleus accumbens are regionally aligned suggest that disruption of these connections could produce site-specific deficits with, presumably, specific behavioural consequences.

Interconnected parallel circuits between rat nucleus accumbens and thalamus revealed by retrograde transynaptic transport of pseudorabies virus

One of the primary outputs of the nucleus accumbens is directed to the mediodorsal thalamic nucleus (MD) via its projections to the ventral pallidum (VP), with the core and shell regions of the accumbens projecting to the lateral and medial aspects of the VP, respectively. In this study, the multisynaptic organization of nucleus accumbens projections was assessed using intracerebral injections of an attenuated strain of pseudorabies virus, a neurotropic alpha herpesvirus that replicates in synaptically linked neurons. Injection of pseudorabies virus into different regions of the MD or reticular thalamic nucleus (RTN) produced retrograde transynaptic infections that revealed multisynaptic interactions between these areas and the basal forebrain. Immunohistochemical localization of viral antigen at short postinoculation intervals confirmed that the medial MD (m-MD) receives direct projections from the medial VP, rostral RTN, and other regions previously shown to project to this region of the thalamus. At longer survival intervals, injections confined to the m-MD resulted in transynaptic infection of neurons in the accumbens shell but not in the core. Injections that also included the central segment of the MD produced retrograde infection of neurons in the lateral VP and the polymorph (pallidal) region of the olfactory tubercle (OT) and transynaptic infection of a small number of neurons in the rostral accumbens core. Injections in the lateral MD resulted in retrograde infection in the globus pallidus (GP) and in transynaptic infection in the caudate-putamen. Viral injections into the rostroventral pole of the RTN infected neurons in the medial and lateral VP and at longer postinoculation intervals, led to transynaptic infection of scattered neurons in the shell and core. Injection of virus into the intermediate RTN resulted in infection of medial VP neurons and second-order infection of neurons in the accumbens shell. Injections in the caudal RTN or the lateral MD resulted in direct retrograde labeling of cells within the GP and transynaptic infection of neurons in the caudate-putamen. These results indicate that the main output of VP neurons receiving inputs from the shell of the accumbens is heavily directed to the m-MD, whereas a small number of core neurons appear to influence the central MD via the lateral VP. Further segregation in the flow of information to the MD is apparent in the organization of VP and GP projections to subdivisions of the RTN that give rise to MD afferents. Collectively, these data provide a morphological basis for the control of the thalamocortical system by ventral striatal regions, in which parallel connections to the RTN may exert control over activity states of cortical regions.

Dopamine depletion in the rostral nucleus accumbens alters the cerebral metabolic response to cocaine in the rat

The functional consequences of dopamine depletion in the rostral nucleus accumbens were examined using the quantitative 2-[14C]deoxyglucose method for determining rates of local cerebral glucose utilization. Cerebral metabolism was determined in 35 brain structures of Sprague-Dawley rats with unilateral 6-hydroxydopamine or sham lesions of the rostral accumbens. The effect of the lesion was assessed in cocaine-naive animals treated systemically with cocaine or saline. In saline-treated animals, the lesion increased cerebral metabolism in typical basal ganglia regions, such as the globus pallidus and entopeduncular nucleus, as well as portions of the extended amygdala that included the bed nucleus of the stria terminalis and the hypothalamic preoptic area. Cerebral metabolism was affected bilaterally in a subset of all affected structures which demonstrated that the functional consequences of the lesion extended beyond the primary monosynaptic output zones of the rostral accumbens. The lesion also changed the topography of the normal cocaine response such that cocaine effects were blunted in the shell of the nucleus accumbens, globus pallidus and the medial ventral pallidum. Thus, the present study describes functional evidence of the link between the rostral accumbens and the extended amygdala and demonstrates that dopamine in the rostral accumbens plays an important role in the central response to cocaine.

Effects of dopamine depletion in the medial prefrontal cortex on the stress-induced increase in extracellular dopamine in the nucleus accumbens core and shell

In the present study we examined whether depletion of dopamine in the medial prefrontal cortex alters the neurochemical activity of mesoaccumbens dopamine neurons and/or their behavioral correlate, motor behavior. Infusion of 6-hydroxydopamine (1 microgram) into the medial prefrontal cortex of rats pretreated with a norepinephrine uptake blocker produced a 70% loss of tissue dopamine, with relative sparing of the norepinephrine content (-23%) in that region. Using in vivo microdialysis, we monitored basal and evoked extracellular dopamine in the nucleus accumbens core and shell of control and lesioned rats. The concentration of basal extracellular dopamine in the nucleus accumbens core was similar in control and lesioned rats; however, basal dopamine efflux in the nucleus accumbens shell was approximately 30% higher in lesioned rats than in controls. Lesions did not alter the ability of systemic D-amphetamine (1.5 mg/kg, i.p.) to increase extracellular dopamine in the nucleus accumbens shell, in contrast, the dopamine depletion in the medial prefrontal cortex attenuated the amphetamine-induced increase in extracellular dopamine in the nucleus accumbens core, as well as the amphetamine-induced increase in locomotor activity. Lesions did not significantly alter the effects of tail pressure (30 min) on extracellular dopamine in the nucleus accumbens core. However, the depletion of dopamine in the medial prefrontal cortex potentiated the stress-induced increase in extracellular dopamine in the nucleus accumbens shell. These data demonstrate that mesocortical dopamine neurons influence (i) amphetamine-induced dopamine efflux in the nucleus accumbens core and (ii) stress-evoked dopamine efflux in the nucleus accumbens shell. It has been proposed that a disruption in the interaction between cortical and subcortical dopamine neurons is involved in the pathophysiology of schizophrenia. The present data raise the possibility that a disruption in the interaction between mesocortical dopamine neurons and dopamine neurons projecting to the nucleus accumbens shell is involved in those symptoms of schizophrenia that are influenced by stress.

Substantia innominata: a notion which impedes clinical-anatomical correlations in neuropsychiatric disorders

Comparative neuroanatomical investigations in primates and non-primates have helped disentangle the anatomy of the basal forebrain region known as the substantia innominata. The most striking aspect of this region is its subdivision into two major parts. This reflects the fundamental organizational scheme for this portion of the forebrain. According to this scheme, two major subcortical telencephalic structures, i.e. the striatopallidal complex and extended amygdala, form large diagonally oriented bands. The rostroventral extension of the pallidum accounts for a large part of the rostral subcommissural substantia innominata, while the sublenticular substantia innominata is primarily occupied by elements of the extended amygdala. Also dispersed across this region is the basal nucleus of Meynert, which is part of a more or less continuous collection of cholinergic and non-cholinergic corticopetal and thalamopetal cells, which stretches from the septum diagonal band rostrally to the caudal globus pallidus. The basal nucleus of Meynert is especially prominent in the primate, where it is sometimes inappropriately applied as a synonym for the substantia innominata, thereby tacitly ignoring the remaining components. In most mammals, the extended amygdala presents itself as a ring of neurons encircling the internal capsule and basal ganglia. The extended amygdala may be further subdivided, i.e. into the central extended amygdala (related to the central amygdaloid nucleus) and the medial extended amygdala (related to the medial amygdaloid nucleus), which generally form separate corridors both in the sublenticular region and along the supracapsular course of the stria terminalis. The extended amygdala is directly continuous with the caudomedial shell of the accumbens, and to some extent appears to merge with it. Together the accumbens shell and extended amygdala form an extensive forebrain continuum, which establishes specific neuronal circuits with the medial prefrontal-orbitofrontal cortex and medial temporal lobe. This continuum is particularly characterized by a prominent system of long intrinsic association fibers, and a variety of highly differentiated downstream projections to the hypothalamus and brainstem. The various components of the extended amygdala, together with the shell of the accumbens, are ideally structured to generate endocrine, autonomic and somatomotor aspects of emotional and motivational states. Behavioral observations support this proposition and demonstrate the relevance of these structures to a variety of functions, ranging from the various elements of the reproductive cycle to drug-seeking behavior. The neurochemical and connectional features common to the accumbens shell and the extended amygdala are especially relevant to understanding the etiology and treatment of neuropsychiatric disorders. This is discussed in general terms, and also in specific relation to the neurodevelopmental theory of schizophrenia and to the neurosurgical treatment of neuropsychiatric disorders.

The anatomical relationships of the prefrontal cortex with limbic structures and the basal ganglia

This paper briefly discusses the anatomical criteria that have been used to delineate the prefrontal cortex (PFC) from the (pre)motor cortical areas in the frontal lobe. Single anatomical criteria, such as cytoarchitecture, connectivity with the mediodorsal thalamic nucleus or a dopaminergic innervation, are insufficient to unequivocally define the PFC. It is argued that, with respect to a number of structural aspects, the prefrontal and the (pre)motor cortical areas must be viewed as a continuum, whereas a (functional) differentiation is based on the type of information that is being processed in different parts of the frontal lobe. The involvement of the PFC, like the premotor cortex, in a number of basal ganglia-thalamocortical circuits may be interpreted in the same way. The paper also summarizes the organization of the inputs from midline/intralaminar thalamic nuclei, the basal amygdaloid complex and the hippocampus into the PFC-ventral striatal system. The results of tracing studies in rats indicate that these thalamic and limbic inputs both at the level of the PFC and the ventral striatum show various patterns of convergence and segregation. This leads to the conclusion that the PFC-ventral striatal system consists of a number of smaller modules.

Densitometrical analysis of opioid receptor ligand binding in the human striatum--I. Distribution of mu opioid receptor defines shell and core of the ventral striatum

Changes in opioid neurotransmission have been implicated in several basal ganglia-related neurological and psychiatric disorders. To gain a better insight into the opioid receptor distribution in the normal human striatum, we examined in post mortem brain the distribution of the mu opioid receptor using ligand binding of [3H]O-ala2-N-methyl-phe4, gly-ol5-enkephalin. Our results indicate at the regional level the presence of a dorsal-to-ventral high-to-low density gradient in the striatum, with lowest densities in the ventral one-third of the putamen and in the nucleus accumbens. At the subregional level, the nucleus accumbens shows two major types of heterogeneities. First, low vs intermediate binding densities distinguish the core and shell subdivisions, respectively. The low-density core and intermediate-density shell regions extend into the putamen and are therefore characteristic for the entire ventral striatum. The second type of heterogeneity is formed by small areas located along the ventral contours of the nucleus accumbens and putamen that display the highest binding density of the entire striatum. Since these areas can also be recognized in the distribution patterns of other markers and in the cytoarchitecture, they appear to possess a separate identity. To emphasize their special neurochemical characteristics we propose the description "neurochemically unique domains in the accumbens and putamen". The present results, with the difference between core and shell of the ventral striatum as the most prominent outcome, together with the notion that the connectional relationships and neurochemical organization of the striatum are very heterogeneous, suggest a strong regional functional differentiation for mu receptor function in the human striatum.

Cholinergic innervation in the human striatum: a three-compartment model

The mammalian striatum is divided into compartments that are anatomically and neurochemically distinct. The dorsal striatum has been described as containing two compartments, striosomes and matrix, while the ventral striatum is thought to have a more complex, multi-compartmental organization. In this study, we sought to characterize the compartmentalization of the dorsal and ventral portions of the human striatum using choline acetyltransferase as a marker. Image analysis was used to assess relative densities of immunostaining, and three distinct, choline acetyltransferase-immunostained compartments were demonstrated: intensely immunostained, moderately immunostained and weakly immunostained areas. The dorsomedial portion of the striatum was made up of moderately immunostained regions embedded within a densely immunostained background, thus manifesting the characteristic striosome/ matrix organization of the dorsal striatum. However, the ventral and lateral two-thirds of the striatum were made up of a mixture of densely immunostained, moderately immunostained and weakly immunostained areas, with the moderately immunostained region forming the bulk of the background tissue, and smaller, densely immunostained and weakly immunostained regions embedded within it. These compartments were compared to regions defined by distinct levels of acetylcholinesterase immunostaining in adjacent sections; the staining patterns produced by the two cholinergic markers were found to be identical except in some portions of the nucleus accumbens, where acetylcholinesterase immunostaining was found to be more intense than choline acetyltransferase immunostaining. The immunoreactive somata were mapped within sections stained for choline acetyltransferase taken from different rostrocaudal levels of the striatum, and the distributions and densities of immunoreactive somata within these three cholinergic compartments were determined. In general, the densities of cholinergic somata roughly correlated with immunostaining intensity of regions, e.g. the most intensely immunostained compartment also had the highest densities of cholinergic somata. However, in the rostroventral striatum, the densities of cholinergic somata in the weakly immunostained compartment roughly equalled the densities of cholinergic somata in the moderately immunostained compartment, suggesting that local axonal arborizations of cholinergic cells may differ in density or orientation between the two compartments, or, alternatively, that some of the cholinergic cells in the weakly immunostained compartment may project outside of the striatum. The large proportion of striatum displaying ventral striatal characteristics (a complex, multi-compart-mental organization) in humans relative to that observed in other mammals suggests that the role of the ventral striatum may be expanded and more highly differentiated in the human brain.

Ultrastructural immunocytochemical localization of neurotensin and the dopamine D2 receptor in the rat nucleus accumbens

The neuroleptic-like effects of neurotensin (NT) are thought to be due to interactions with dopamine (DA) acting primarily at D2 receptors within the nucleus accumbens septi (Acb). Using electron microscopic dual labeling immunocytochemistry, we sought to demonstrate cellular substrates for functional interactions involving NT and DA D2 receptors in the adult rat Acb. Peroxidase reaction product representing D2 receptor-like immunoreactivity (D2-LI) was seen along membranes of Golgi lamellae and multivesicular bodies of perikarya containing immunogold labeling representing NT-LI. Dually labeled somata usually contained highly indented nuclei, a characteristic of aspiny neurons. Dendrites also occasionally colocalized the two immunomarkers. Other somata, dendrites, and all axon terminals were singly labeled with either NT-LI or D2-LI. In distinct sets of terminals, NT-LI was commonly associated with large, dense-cored vesicles, whereas D2-LI was found along the plasmalemma and over nearby small clear vesicles. Each type of terminal comprised approximately 20% of synaptic input to NT-immunoreactive dendrites. Similar proportions of terminals containing NT-LI or D2-LI contacted unlabeled (approximately 55%) or NT-labeled (approximately 35%) dendrites and, occasionally, were observed converging onto common dendrites. Terminals containing NT-LI or D2-LI also were often closely apposed. These findings provide the first ultrastructural evidence that: (1) NT and D2 receptors are colocalized in aspiny neurons and dendrites, (2) NT may produce a direct postsynaptic effect on neurons receiving input from terminals which are presynaptically modulated by DA via D2 receptors, and (3) NT and DA acting at D2 receptors may interact through presynaptic modulation of common axon terminals.

Contralateral turning elicited by unilateral stimulation of dopamine D2 and D1 receptors in the nucleus accumbens of rats is due to stimulation of these receptors in the shell, but not the core, of this nucleus

The goal of this study was to determine whether dopamine D2 and/or D1 receptors in the shell and the core of the nucleus accumbens of rats have a differential role in turning behaviour. Unilateral injection of a mixture of the dopamine D2 receptor agonist quinpirole (10 micrograms) and the dopamine D1 receptor agonist 1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7, 8-diol (SKF 38393, 5 micrograms) into the shell of the nucleus accumbens produced contralateral turning, when doses which per se were ineffective were injected. This effect was far greater than that found after similar injections into the core of the nucleus accumbens. The effect elicited from the shell was significantly attenuated by prior administration of either the dopamine D2 receptor antagonist l-sulpiride (25 mg/0.5 microliters) or the dopamine D1 receptor antagonist (8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol (SCH 23390, 0.5 micrograms/0.5 microliters) into the same region. These data together with the fact that l-sulpiride is known to be a valid tool to differentiate the involvement of distinct regions within the shell underlie the conclusion that dopamine D2 and D1 receptors in the shell, but not the core, of the nucleus accumbens play a critical role in the contralateral turning induced by unilateral injection of dopamine receptor agonists into this nucleus. The results are discussed in view of the known output pathways of the shell.

Patterns of overlap and segregation between insular cortical, intermediodorsal thalamic and basal amygdaloid afferents in the nucleus accumbens of the rat

Regions of the prefrontal cortex that project to the nucleus accumbens in the rat receive input from midline thalamic and basal amygdaloid nuclei which also project to the same striatal region as their prefrontal cortical target. For example, the prelimbic cortex projects to the medial nucleus accumbens, and receives input from the paraventricular thalamic nucleus and the parvicellular basal amygdala. These latter two areas also project to the medial nucleus accumbens. It has been shown that afferents from the prelimbic cortex, the paraventricular thalamic nucleus and the parvicellular basal amygdala to the nucleus accumbens overlap or are separated in the nucleus accumbens, depending upon their position in the shell and core. The dorsal agranular insular cortex, the intermediodorsal thalamic nucleus and the magnocellular basal amygdaloid nucleus terminate in the lateral part of the nucleus accumbens and adjacent ventral part of the caudate-putamen. The intermediodorsal thalamic nucleus and the magnocellular basal amygdaloid nucleus reach both the dorsal agranular insular cortex and the lateral nucleus accumbens, and thus appear positioned to influence the prefrontal corticostriatal system at cortical and striatal levels. However, all three afferent systems have a heterogeneous distribution within this striatal region, and whether these projections actually reach the same areas is unknown. We investigated the patterns of separation and overlap in the nucleus accumbens between dorsal agranular insular cortical, magnocellular basal amygdaloid and intermediodorsal thalamic afferents with respect to the histochemical features of the nucleus. Techniques allowing the detection of two different anterograde tracers, or a single anterograde tracer and Calbindin-D28k immunoreactivity, in the same tissue sections were used. The results demonstrate that the afferents from the dorsal agranular insular area and the intermediodorsal thalamic nucleus avoid the shell of the lateral nucleus accumbens, which receives strong inputs from the magnocellular basal amygdala. In the matrix of the core and the ventral part of the caudate-putamen, fibers from the superficial layers of the dorsal agranular insular area overlap precisely with afferents from the intermediodorsal nucleus. In the patches, projections from the deep layers of the dorsal agranular insular cortex coincide with those from the magnocellular basal amygdala. The present findings have implications for the compartmental structure of the nucleus accumbens and provide novel insights into the organizational principles of prefrontal corticostriatal circuits.

Burst stimulation of the medial forebrain bundle selectively increase Fos-like immunoreactivity in the limbic forebrain of the rat

The present study was designed to evaluate the postsynaptic functional consequences of different presynaptic activity patterns in midbrain dopamine systems using electrical stimulation of the rat medial forebrain bundle and subsequent determination of c-fos expression, used as a marker for neuronal activation, in dopamine target areas, by means of Fos immunohistochemistry. Nerve terminal dopamine release evoked by electrical stimulation of the medial forebrain bundle was monitored in the same animals using in vivo voltammetry. A 5 Hz stimulation consisting of 60 trains of five pulses and lasting 1 min was applied to the medial forebrain bundle. This stimulation was repeated 15 times every 3 min. Its pattern was defined by the interpulse interval which was either 70 ms or 200 ms for burst or regularly spaced stimulation, respectively. Our results show that burst stimulation of the medial forebrain bundle, which increase release of dopamine in target areas, increases the basal Fos-like immunoreactivity in the stimulated hemisphere, while regular stimulation does not affect expression of this protein. Moreover, the increase in Fos-like immunoreactivity induced by burst stimulation is restricted to limbic related structures, i.e. nucleus accumbens shell and intermediate aspect of the lateral septum, and the major island of Calleja, but is not observed in motor related structures (nucleus accumbens core and striatum). Pretreatment with the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, i.p.), blocked the increase in Fos-like immunoreactivity induced by burst stimulation of the medial forebrain bundle, suggesting a role for these receptors in the observed effects. Pretreatment with the 5-hydroxytryptamine2A/2C receptor antagonist ritanserin (0.4 mg/kg, i.p.) did not affect the increase in Fos-like immunoreactivity induced by burst stimulation in the nucleus accumbens shell or in the lateral septum, although it blocked the stimulated enhancement of Fos-like immunoreactivity in the major island of Calleja. The present data indicate that, rather than the absolute mean discharge rate of midbrain dopamine neurons, the temporal organization of the action potentials they generate conveys information to their target areas.

Prefrontal cortex inputs of the nucleus accumbens-nigro-thalamic circuit

The functional organization of the cortico-nucleus accumbens-substantia nigra pars reticulata circuit was investigated in the rat using combined anatomical and electrophysiological approaches. The nucleus accumbens neurons which project to the substantia nigra pars reticulata are located in a circumscribed region of the core immediately adjacent and extending dorsally to the anterior commissure. As shown by retrograde and anterograde transports of wheatgerm agglutinin conjugated to horseradish peroxidase, the region of the nucleus accumbens related to the substantia nigra was found to receive bilateral inputs from restricted areas of the medial and lateral prefrontal cortex, i.e., prelimbic/medial orbital and dorsal agranular insular areas. The electrical stimulation of these medial and lateral prefrontal cortical areas induced excitatory responses in nucleus accumbens neurons projecting to the dorsomedial substantia nigra pars reticulata. Interestingly, an important proportion (61%) of the nucleus accumbens-nigral cells responding to the stimulation of the lateral prefrontal cortex were also excited by the stimulation of the medial prefrontal cortex, demonstrating the existence of a convergent influence of these cortical areas on single nucleus accumbens cells. Furthermore, the present data also show that the stimulation of the medial prefrontal cortex results in a powerful inhibition of the tonic firing of the substantia nigra pars reticulata neurons. In conclusion, this study reveals the existence of a functional link between the prefrontal cortex (prelimbic/medial orbital and agranular insular areas) and the nucleus accumbens neurons which innervate the dorsomedial region of the substantia nigra pars reticulata. Since the dorsomedial region of substantia nigra pars reticulata is known to project to subfields of the mediodorsal and ventromedial thalamic nuclei related to the prefrontal cortex, the present data further demonstrate the existence of a prefrontal-nucleus accumbens-thalamo-cortical circuit involving the substantia nigra pars reticulata.

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-D28k. 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.

Schedule-induced polydipsia and the nucleus accumbens: electrochemical measurements of dopamine efflux and effects of excitotoxic lesions in the core

The efflux of dopamine (DA) in the nucleus accumbens (NAcc) core during the acquisition of schedule-induced polydipsia (drinking in response to intermittent food presentation) was measured using rapid scan voltammetry. DA efflux increased throughout the SIP sessions, always reaching a peak after the session had terminated. There was, however, no relationship between the acquisition of the drinking response to intermittent food presentation and DA efflux. When water was absent from the test chamber, DA efflux still increased and reached a peak after food delivery was terminated, dissociating drinking and increased DA efflux. Taken in conjunction with previously presented data, these results suggest that the presence of DA in the NAcc core might be necessary for the development of SIP but that its efflux does not bear a systematic relationship to the acquisition of adjunctive behaviour. In a second experiment the effects of NMDA-induced lesions of the NAcc core on the acquisition and performance of SIP were examined. Lesioned rats did not differ to controls in terms of water intake, mean drinking bout length, latency to panel press for food or to begin drinking. The number of drinking bouts/min was reduced in lesioned rats, but did not reach statistical significance; the number of panel presses/min was significantly reduced in lesioned rats. These data demonstrate that the NAcc core is not necessary for the development of SIP but that elements of performance are affected. This suggests that the development of SIP can be fractionated and that different neural elements control different aspects of its expression. These data are used to support the hypothesis that the NAcc core is involved in focusing behaviour and regulating switching between response options.

A light and electron microscopic study of NADPH-diaphorase-, calretinin- and parvalbumin-containing neurons in the rat nucleus accumbens

The rat nucleus accumbens contains medium-sized, spiny projection neurons and intrinsic, local circuit neurons, or interneurons. Sub-classes of interneurons, revealed by calretinin (CR) or parvalbumin (PV) immunoreactivity or reduced nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry, were compared in the nucleus accumbens core, shell and rostral pole. CR, PV and NADPH-diaphorase-containing neurons are shown to form three non-co-localising populations in these three areas. No significant differences in neuronal population densities were found between the subterritories. NADPH-diaphorase-containing neurons could be further separated morphologically into three sub-groups, but CR- and PV-immunoreactive neurons form homogeneous populations. Ultrastructurally, NADPH-diaphorase-, CR- and PV-containing neurons in the nucleus accumbens all possess nuclear indentations. These are deeper and fewer in neurons immunoreactive for PV than in CR- and NADPH-diaphorase-containing neurons. CR-immunoreactive boutons form asymmetrical and symmetrical synaptic specialisations on spines, dendrites and somata, while PV-immunoreactive boutons make only symmetrical synaptic specialisations. Both CR- and PV-immunoreactive boutons form symmetrical synaptic specialisations with medium-sized spiny neurons and contact other CR- and PV-immunoreactive somata, respectively. A novel non-carcinogenic substrate for the peroxidase reaction (Vector Slate Grey, SG) was found to be characteristically electron-dense and may be distinguishable from the diaminobenzidine reaction product. We conclude that the three markers used in this study are localised in distinct populations of nucleus accumbens interneurons. Our studies of their synaptic connections contribute to an increased understanding of the intrinsic circuitry of this area.

Caffeine-induced expression of c-fos mRNA and NGFI-A mRNA in caudate putamen and in nucleus accumbens are differentially affected by the N-methyl-D-aspartate receptor antagonist MK-801

Caffeine (100 mg/kg, i.p.) induces a rapid increase in the expression of mRNA for the immediate early genes (IEGs) c-fos and NGFI-A in rat striatum. We have examined how this response is affected by pretreatment with either the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (1 and 3 mg/kg, i.p.), the competitive NMDA receptor antagonist D-CPP (6 mg/kg, i.p.), or the non-selective excitatory amino acid receptor antagonist kynurenic acid (300 mg/kg, i.p). The two NMDA receptor antagonists significantly reduced the caffeine-induced expression of both c-fos mRNA and NGFI-A mRNA in the medial part of the caudate putamen. The effect was less pronounced in the lateral part of the caudate putamen. MK-801 caused an enhancement of c-fos and NGFI-A mRNA expression in nucleus accumbens. Pretreatment with kynurenic acid caused no marked alterations in the caffeine-induced expression of c-fos mRNA and NGFI-A mRNA in any brain region. These findings suggest that glutamatergic transmission via NMDA receptors contributes to the induction of c-fos mRNA and NGFI-A mRNA by caffeine in striatum. In addition we show that MK-801 can either increase or decrease the caffeine effect on IEGs depending on the region studied.

Influences of neuronal uptake and D2 autoreceptors on regulation of extracellular dopamine in the core, shell and rostral pole of the rat nucleus accumbens

Fast cyclic voltammetry in rat brain slices containing the nucleus accumbens, was used to examine the regulation of the extracellular concentration of electrically stimulated dopamine overflow in the core, shell and rostral pole. One microM (-)-sulpiride, significantly increased dopamine overflow in all 3 regions but only when the duration of stimulation was greater than 500 ms. One microM cocaine, significantly potentiated dopamine overflow in all 3 regions following all patterns of stimulation. In the presence of 1 microM cocaine, superfusion with 1 microM (-)-sulpiride did not result in a further increase in dopamine overflow at any frequency of stimulation in the rostral pole, but significant increases in dopamine overflow were observed after stimulation with 20 pulses at 10 or 20 Hz in the core or shell; the degree of potentiation was greater in the shell than core. Quinpirole inhibited single pulse stimulated dopamine overflow in a concentration dependent manner (maximum inhibition (100%) in all regions) but was significantly less potent in the rostral pole than in the core or shell. Increasing the number of pulses to 2 or 4 pulses at 50 Hz resulted in a shift of the quinpirole dose-response curve to the right in all regions and in the rostral pole, a significant reduction in the maximum inhibition of dopamine overflow to both stimulation parameters. In the shell a significant reduction in maximum inhibition was only seen with 4 pulses at 50 Hz stimulation, whereas in the core there was no change in the maximum inhibitory effect of quinpirole. Neuronal uptake and D2 autoreceptor activity contribute to regulation of the extracellular concentration of dopamine in core, shell and rostral pole. The relative importance of either uptake or autoreceptor control is region and stimulus dependent.

Role of dopamine D1 and D2 receptors in the nucleus accumbens in jaw movements of rats: a critical role of the shell

Given the differences in the dopamine neurotransmission between the shell and the core of the nucleus accumbens, as well as the differential involvement of these two domains in oral behaviour of rats, it was decided to determine whether or not dopamine D1 and/or dopamine D2 receptors differentially direct oral behaviour in these two domains in rats. Intra-accumbens injections of the dopamine D1 receptor agonist (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine (SKF 82958: 5 micrograms/0.2 microliters), the dopamine D2 receptor agonist quinpirole (10 micrograms/0.2 microliters) and their combination were used to assess the role of these accumbens domains in jaw movements of rats. The present study shows that the combined administration of SKF 82958 and quinpirole into the shell, but not the core, of the nucleus accumbens produced a highly significant increase in jaw movements, when doses which per se were nearly ineffective, were injected. This effect was fully inhibited by prior administration of either the dopamine D1 receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390: 0.5 microgram/0.2 micrograms) or the dopamine D2 receptor antagonist (-)-sulpiride (25 ng/0.5 microliter) into the same region. It is concluded that dopamine D1 and D2 receptors in the shell, but not the core, of the nucleus accumbens are involved in jaw movements of the rat, providing the first piece of evidence that dopamine D1 and D2 receptors in the shell of the nucleus accumbens mediate a particular behaviour.

Patterns of convergence and segregation in the medial nucleus accumbens of the rat: relationships of prefrontal cortical, midline thalamic, and basal amygdaloid afferents

In the rat, fibers from the prelimbic cortex terminate in the medial nucleus accumbens. Anterior paraventricular thalamic and parvicellular basal amygdaloid fibers reached both the prelimbic cortex and the medial nucleus accumbens. All three afferent systems have an inhomogenous distribution within the nucleus accumbens, and whether or not these projections actually reach the same areas is unknown. Our aim was to evaluate the relationships of the three afferents with respect to the shell, the core, and the cell clusters of the nucleus accumbens. Double anterograde tracing and single anterograde tracing combined with immunohistochemistry for calbindin (D28k) or Nissl stain was used. Following tracer injections in the prelimbic cortex and the anterior paraventricular thalamus, a complementary (i.e., nonoverlapping) pattern of fibers was found in the shell. Thus, afferents from the prelimbic cortex are associated with cell clusters, whereas those from the anterior paraventricular thalamus avoid these cells but are affiliated with regions exhibiting weak homogeneous calbindin immunoreactivity. In the calbindin-poor patches of the core, the situation is reversed as both sets of fibers overlap. In cases with injections in the prelimbic cortex and the parvicellular basal amygdala, a pattern of overlap was seen in the shell and core. Thus, the fibers in the shell were found together in association with cell clusters, whereas regions of weak homogeneous calbindin immunoreactivity were avoided. In the core, overlap was seen in the patch compartment. Finally, with parvicellular basal amygdala/paraventricular thalamus injections, a complementary fiber organization was present in the shell, but overlap was prominent in the patches of the core. The results demonstrate that the relationships of prelimbic cortical, paraventricular thalamic, and parvicellular basal amygdaloid afferents in the nucleus accumbens vary according to their compartmental (immunohistochemical and cellular) affiliation. Compartmentalization is therefore a possible anatomical substrate for condensation or segregation of neuronal signals passing through the nucleus accumbens.

Prenatal stress in rats facilitates amphetamine-induced sensitization and induces long-lasting changes in dopamine receptors in the nucleus accumbens

Exposure of rats to restraint stress during late pregnancy produces offspring with a variety of behavioral and neurobiological alterations. It has been suggested that prenatal stress leads to long-lasting changes in the hypothalamo-pituitary-adrenal axis in the offspring. One feature of prenatally-stressed rats is a susceptibility to amphetamine self-administration. Since this behavior has been related to amphetamine-induced sensitization and the activity of the mesolimbic dopamine system, we measured dopamine receptor densities and amphetamine-induced sensitization in these animals. The motor response to the first administration of amphetamine was similar in both prestressed and unstressed groups of adult animals, but after repeated drug injections, behavioral sensitization was observed sooner in the prenatally-stressed rats than in the controls. In separate groups of adult animals, densities of D1, D2 and D3 dopamine receptor subtypes in the striatum and nucleus accumbens were measured in prenatally-stressed and control rats by quantitative autoradiography using [3H]SCH23390, [3H]sulpiride and [3H]7-OH-DPAT as ligands respectively. Prenatal stress was found to produce the following alterations in the adult offspring: (i) no significant change in D1 receptor binding in either striatum or nucleus accumbens; (ii) a significant (+24%) increase in D2 receptor binding in the nucleus accumbens; (iii) a significant decrease in D3 receptor binding in both the shell (-16%) and the core (-26%) of the nucleus accumbens. These observations indicate that prenatal stress induces long-lasting changes in the dopamine sensitivity of the nucleus accumbens and in the capacity to develop amphetamine-induced sensitization in adulthood. The possible relationship between an impaired control of corticosterone secretion in prenatally-stressed animals and long-term changes in the mesolimbic dopamine system is discussed.

Reward shifts and motor responses following microinjections of opiate-specific agonists into either the core or shell of the nucleus accumbens

Differences in pharmacology, anatomical connections, and receptor densities between the "core" and "shell" of the nucleus accumbens suggest that behavioral activity normally modulated by the accumbens, such as reward and motor functions, may be differentially regulated across the mediolateral axis. This study investigated the effects of opiate receptor-specific agonists on reward and motor functions in either the accumbens core or shell, using the intracranial self-stimulation (ICSS) rate-frequency curve-shift method. Microinjections of the mu opiate receptor-specific agonist, DAMGO (vehicle, 0.03 nmol, and 0.3 nmol), or the delta opiate receptor-specific agonist DPDPE (vehicle, 0.3 nmol, 3.0 nmol), were administered bilaterally in a random dose order with a minimum of 3 days between injections. Rats were tested over three consecutive 20-min rate-frequency curves immediately following a microinjection to investigate the time course of drug effects. Both opiate agonists decreased the ICSS frequency necessary to maintain half-maximal response rates when injected into the medial and ventral shell region of the accumbens. However, DAMGO microinjections into the lateral accumbens core or the control site of the caudate increased the frequency necessary to elicit half-maximal response rates, while DPDPE microinjections into these regions had no effect. Evaluation of motor effects show that administration of DAMGO resulted in a suppression of activity in all locations. In contrast, DPDPE microinjections resulted in little or no effect on lever pressing activity at any location.

Differential localization of mRNAs encoding dopamine D1 or D2 receptors in cholinergic neurons in the core and shell of the rat nucleus accumbens

By combining immunocytochemistry for ChAT and in situ hybridization for dopamine D1 or D2 receptor mRNA in the striatum, it was found that (1) the percentage of ChAT/D2 mRNA co-localization is higher in the caudate-putamen than in the shell and core of the nucleus accumbens, (2) in the shell the degree of ChAT/D2 mRNA co-localization is higher rostrally than caudally, and 3) no significant regional differences exist in the degree of co-localization of ChAT and D1 mRNA.

Evidence for two neurochemical divisions in the human nucleus accumbens

The neurochemical anatomy of the human nucleus accumbens was studied by comparing the distributional patterns of [3H]DAMGE (mu opioid receptor), [3H]bremazocine (kappa opioid receptor), [3H]SCH-23390 (D1-like dopamine receptor), [3H]7-OH-DPAT (D3 dopamine receptor) binding, preproenkephalin mRNA and acetylcholinesterase activity in sections of post mortem human striatum. Our results demonstrate the presence of at least two neurochemically distinct divisions within the human nucleus accumbens, which may be homologous to the 'shell' and 'core' divisions of the nucleus as found in the rat.

Differential effects of dopamine depletion on the binding and mRNA levels of dopamine receptors in the shell and core of the rat nucleus accumbens

In the present study, using quantitative receptor autoradiography and in situ hybridization histochemistry the effects of unilateral 6-hydroxydopamine lesions on the binding density levels of dopamine D1 and D2 receptors and the levels of mRNA encoding D1 and D2 receptors were investigated in the core and shell territories of the nucleus accumbens (Acb) and in the caudate-putamen (CP). The lesions induced contrasting effects on the D1 binding and D1 mRNA in the Acb and CP, i.e. an increase in binding and a decrease in the mRNA levels. For the D2 receptor an increase in both the binding density and mRNA levels was observed. The lesion-induced effects displayed regional differences. For D1 mRNA and D1 and D2 binding, the lesion effect was more pronounced in the core than in the shell of the Acb. For the D2 mRNA levels an increase was observed in the CP but not in the two territories of the Acb. Furthermore, the decrease in D1 mRNA was greater in the rostral than in the caudal parts of the core and shell of the Acb. These results indicate that the core and shell of the Acb and the CP respond differentially to dopamine depletion.