Heritability of the shape of subcortical brain structures in the general population.

The volumes of subcortical brain structures are highly heritable, but genetic underpinnings of their shape remain relatively obscure. Here we determine the relative contribution of genetic factors to individual variation in the shape of seven bilateral subcortical structures: the nucleus accumbens, amygdala, caudate, hippocampus, pallidum, putamen and thalamus. In 3,686 unrelated individuals aged between 45 and 98 years, brain magnetic resonance imaging and genotyping was performed. The maximal heritability of shape varies from 32.7 to 53.3% across the subcortical structures. Genetic contributions to shape extend beyond influences on intracranial volume and the gross volume of the respective structure. The regional variance in heritability was related to the reliability of the measurements, but could not be accounted for by technical factors only. These findings could be replicated in an independent sample of 1,040 twins. Differences in genetic contributions within a single region reveal the value of refined brain maps to appreciate the genetic complexity of brain structures.

Stimulus-specific and differential distribution of activated extracellular signal-regulated kinase in the nucleus accumbens core and shell during Pavlovian-instrumental transfer.

The ability of reward-predictive cues to potentiate reward-seeking behavior--a phenomenon termed Pavlovian--instrumental transfer (PIT)--depends on the activation of extracellular signal-regulated kinase (ERK) in the nucleus accumbens (NAc). Here, we utilized immunohistochemistry to investigate the subregional pattern of ERK activation during PIT, and the contribution of different elements in the PIT condition to the distribution of ERK signaling in the NAc of rats. We found that the occurrence of reward-seeking behavior (lever pressing) did not affect ERK activation in either the core or the shell of the NAc. In contrast, presentation of the reward-predictive cue (auditory conditioned stimulus) caused a significant increase in ERK activation in both subregions of the NAc, with the effect being slightly more robust in the core than the shell. Different from the pattern evoked by the reward-predictive cue, presentation of the reward itself (food pellets) had no effect on ERK activation in the core but caused a pronounced increase in ERK activation in the shell. Taken together, our results demonstrate that ERK signaling in the NAc during PIT involves both the core and the shell and is driven by the conditioned cue irrespective of whether the situation permits engagement in reward-seeking behavior. Furthermore, our results show that the subregional distribution of ERK signaling in the NAc evoked by rewards differs from that evoked by cues that predict them. The stimulus-specific differential pattern of ERK signaling described here may present the molecular complement to stimulus-specific increases in NAc cell firing reported previously.

Nucleus accumbens, thalamus and insula connectivity during incentive anticipation in typical adults and adolescents.

Reward neurocircuitry links motivation with complex behavioral responses. Studies of incentive processing have repeatedly demonstrated activation of nucleus accumbens (NAc), thalamus, and anterior insula, three key components of reward neurocircuitry. The contribution of the thalamus to this circuitry in humans has been relatively ignored, a gap that needs to be filled, given the central role of this structure in processing and filtering information. This study aimed to understand how these three regions function as a network during gain or loss anticipation in adults and youth. Towards this goal, functional magnetic resonance imaging (fMRI) and dynamic causal modeling (DCM) were used to examine effective connectivity among these three nodes in healthy adults and adolescents who performed the monetary incentive delay (MID) task. Seven connectivity models, based on anatomic connections, were tested. They were estimated for incentive anticipation and underwent Bayesian Model Selection (BMS) to determine the best-fit model for each adult and adolescent group. Connection strengths were extracted from the best-fit model and examined for significance in each group. These variables were then entered into a linear mixed model to test between-group effects on effective connectivity in reward neurocircuitry. The best-fit model for both groups included all possible anatomic connections. Three main findings emerged: (1) Across the task, thalamus and insula significantly influenced NAc; (2) A broader set of significant connections was found for the loss-cue condition than the gain-cue condition in both groups; (3) Finally, between-group comparisons of connectivity strength failed to detect statistical differences, suggesting that adults and adolescents use this incentive-processing network in a similar manner. This study demonstrates the way in which the thalamus and insula influence the NAc during incentive processing in humans. Specifically, this is the first study to demonstrate in humans the key role of thalamus projections onto the NAc in support of reward processing. Our results suggest that anticipation of gain/loss involves an 'alerting' signal (thalamus) that converges with interoceptive information (insula) to shape action selection programs in the ventral striatum.

Expanding applications of deep brain stimulation: a potential therapeutic role in obesity and addiction management.

BACKGROUND: The indications for deep brain stimulation (DBS) are expanding, and the feasibility and efficacy of this surgical procedure in various neurologic and neuropsychiatric disorders continue to be tested. This review attempts to provide background and rationale for applying this therapeutic option to obesity and addiction. We review neural targets currently under clinical investigation for DBS­the hypothalamus and nucleus accumbens­in conditions such as cluster headache and obsessive-compulsive disorder. These brain regions have also been strongly implicated in obesity and addiction. These disorders are frequently refractory, with very high rates of weight regain or relapse, respectively, despite the best available treatments. METHODS: We performed a structured literature review of the animal studies of DBS, which revealed attenuation of food intake, increased metabolism, or decreased drug seeking. We also review the available radiologic evidence in humans, implicating the hypothalamus and nucleus in obesity and addiction. RESULTS: The available evidence of the promise of DBS in these conditions combined with significant medical need, support pursuing pilot studies and clinical trials of DBS in order to decrease the risk of dietary and drug relapse. CONCLUSIONS: Well-designed pilot studies and clinical trials enrolling carefully selected patients with obesity or addiction should be initiated.

Can the chronic administration of the combination of buprenorphine and naloxone block dopaminergic activity causing anti-reward and relapse potential?

Opiate addiction is associated with many adverse health and social harms, fatal overdose, infectious disease transmission, elevated health care costs, public disorder, and crime. Although community-based addiction treatment programs continue to reduce the harms of opiate addiction with narcotic substitution therapy such as methadone maintenance, there remains a need to find a substance that not only blocks opiate-type receptors (mu, delta, etc.) but also provides agonistic activity; hence, the impetus arose for the development of a combination of narcotic antagonism and mu receptor agonist therapy. After three decades of extensive research, the federal Drug Abuse Treatment Act 2000 (DATA) opened a window of opportunity for patients with addiction disorders by providing increased access to options for treatment. DATA allows physicians who complete a brief specialty-training course to become certified to prescribe buprenorphine and buprenorphine/naloxone (Subutex, Suboxone) for treatment of patients with opioid dependence. Clinical studies indicate that buprenorphine maintenance is as effective as methadone maintenance in retaining patients in substance abuse treatment and in reducing illicit opioid use. With that stated, we must consider the long-term benefits or potential toxicity attributed to Subutex or Suboxone. We describe a mechanism whereby chronic blockade of opiate receptors, in spite of only partial opiate agonist action, may ultimately block dopaminergic activity causing anti-reward and relapse potential. While the direct comparison is not as yet available, toxicity to buprenorphine can be found in the scientific literature. In considering our cautionary note in this commentary, we are cognizant that, to date, this is what we have available, and until such a time when the real magic bullet is discovered, we will have to endure. However, more than anything else this commentary should at least encourage the development of thoughtful new strategies to target the specific brain regions responsible for relapse prevention.

High-frequency stimulation of the nucleus accumbens core and shell reduces quinpirole-induced compulsive checking in rats.

Electrical deep brain stimulation (DBS) is currently studied in the treatment of therapy-refractory obsessive compulsive disorders (OCDs). The variety of targeted brain areas and the inconsistency in demonstrating anti-compulsive effects, however, highlight the need for better mapping of brain regions in which stimulation may produce beneficial effects in OCD. Such a goal may be advanced by the assessment of DBS in appropriate animal models of OCD. Currently available data on DBS of the nucleus accumbens (NAc) on OCD-like behavior in rat models of OCD are contradictory and partly in contrast to clinical data and theoretical hypotheses about how the NAc might be pathophysiologically involved in the manifestation of OCD. Consequently, the present study investigates the effects of DBS of the NAc core and shell in a quinpirole rat model of OCD. The study demonstrates that electrical modulation of NAc core and shell activity via DBS reduces quinpirole-induced compulsive checking behavior in rats. We therefore conclude that both, the NAc core and shell constitute potential target structures in the treatment of OCD.

Forebrain activation induced by postoral nutritive substances in rats.

Recent studies have shown the nutrient-sensing systems transmitting nutritive information from the gut to the brain. However neural activity evoked by ingested dietary nutrients has not been investigated adequately. Using functional magnetic resonance imaging, we demonstrated that rat forebrain responded to intragastric administration of glucose, L-glutamate, and NaCl. These dietary nutrients led to a significant activation in the forebrain regions including nucleus accumbens, hypothalamic area, and limbic system with different timings. These data indicate that several forebrain regions have important roles on perception and process of postingestive nutrient information.

Comparative study of the effects of electrical stimulation in the nucleus accumbens, the mediodorsal thalamic nucleus and the bed nucleus of the stria terminalis in rats with schedule-induced polydipsia.

In the schedule-induced polydipsia model, hungry rats receiving a food pellet every minute will display excessive drinking behaviour (compulsive behaviour). We aimed 1) to evaluate if electrical stimulation in the nucleus accumbens (N ACC), the mediodorsal thalamic nucleus (MD) or the bed nucleus of the stria terminalis (BST) can decrease water intake in the schedule-induced polydipsia model; 2) to compare water intake between these groups for different stimulation amplitudes; and 3) to compare the effect of low frequency (2 Hz) with high frequency (100 Hz) stimulation. Rats were randomly divided into four groups: electrode implanted in the 1) N ACC (n=7), 2) MD (n=8), 3) BST (n=8), or 4) a sham-operated control group (n=7). Postoperatively, each rat of group 1, 2 and 3 was randomly tested in the model using pulses with a frequency of 2 Hz and 100 Hz, each at an amplitude of 0.1, 0.2, 0.3, 0.4 and 0.5 mA, or without stimulation. Group 4 was tested 11 times without stimulation. Each day the rats were tested in random order. High-frequency electrical stimulation in all three brain areas decreased water intake significantly at an amplitude of 0.2 mA or higher, however, without differences between the brain areas. Based on these results, we expect a decrease in compulsions in patients suffering from treatment-resistant obsessive-compulsive disorder during electrical stimulation in the N ACC, the MD and the BST. However, we foresee no difference in energy consumption to decrease symptoms during electrical stimulation between these brain areas.

Incentive-elicited brain activation in adolescents: similarities and differences from young adults.

Brain motivational circuitry in human adolescence is poorly characterized. One theory holds that risky behavior in adolescence results in part from a relatively overactive ventral striatal (VS) motivational circuit that readily energizes approach toward salient appetitive cues. However, other evidence fosters a theory that this circuit is developmentally underactive, in which adolescents approach more robust incentives (such as risk taking or drug experimentation) to recruit this circuitry. To help resolve this, we compared brain activation in 12 adolescents (12-17 years of age) and 12 young adults (22-28 years of age) while they anticipated the opportunity to respond to obtain monetary gains as well as to avoid monetary losses. In both age groups, anticipation of potential gain activated portions of the VS, right insula, dorsal thalamus, and dorsal midbrain, where the magnitude of VS activation was sensitive to gain amount. Notification of gain outcomes (in contrast with missed gains) activated the mesial frontal cortex (mFC). Across all subjects, signal increase in the right nucleus accumbens during anticipation of responding for large gains independently correlated with both age and self-rated excitement about the high gain cue. In direct comparison, adolescents evidenced less recruitment of the right VS and right-extended amygdala while anticipating responding for gains (in contrast with anticipation of nongains) compared with young adults. However, brain activation after gain outcomes did not appreciably differ between age groups. These results suggest that adolescents selectively show reduced recruitment of motivational but not consummatory components of reward-directed behavior.

Nucleus accumbens mu-opioids regulate intake of a high-fat diet via activation of a distributed brain network.

Endogenous opioid peptides within the nucleus accumbens, a forebrain site critical for the regulation of reward-related behavior, are believed to play an important role in the control of appetite. In particular, this system is thought to mediate the hedonic aspects of food intake, governing the positive emotional response to highly palatable food such as fat and sugar. Previous work has shown that intra-accumbens administration of the mu-opioid agonist D-Ala2,Nme-Phe4,Glyol5-enkephalin (DAMGO) markedly increases food intake and preferentially enhances the intake of palatable foods such as fat, sucrose, and salt. Using information from recently performed c-fos mapping experiments, we sought to explore the involvement of structures efferent to the nucleus accumbens in this feeding response. Free-feeding rats with dual sets of bilateral cannulas aimed at the nucleus accumbens and one of several output structures were infused with DAMGO (0, 0.25 microg/0.5 microl) in the accumbens, and fat intake was measured over a 2 hr period. Concurrent temporary inactivation with the GABA(A) agonist muscimol (5-20 ng/0.25 microl) of the dorsomedial hypothalamic nucleus, lateral hypothalamus, ventral tegmental area, or the intermediate region of the nucleus of the solitary tract blocked the robust increase in fat intake induced by intra-accumbens DAMGO at doses of muscimol that did not affect general motor activity. Muscimol alone also inhibited and augmented baseline fat intake in the lateral and dorsomedial hypothalamic nuclei, respectively. These results suggest that intake of energy-dense palatable food is controlled by activity in a neural network linking ventral striatal opioids with diencephalic and brainstem structures.

Anticipation of increasing monetary reward selectively recruits nucleus accumbens.

Comparative studies have implicated the nucleus accumbens (NAcc) in the anticipation of incentives, but the relative responsiveness of this neural substrate during anticipation of rewards versus punishments remains unclear. Using event-related functional magnetic resonance imaging, we investigated whether the anticipation of increasing monetary rewards and punishments would increase NAcc blood oxygen level-dependent contrast (hereafter, "activation") in eight healthy volunteers. Whereas anticipation of increasing rewards elicited both increasing self-reported happiness and NAcc activation, anticipation of increasing punishment elicited neither. However, anticipation of both rewards and punishments activated a different striatal region (the medial caudate). At the highest reward level ($5.00), NAcc activation was correlated with individual differences in self-reported happiness elicited by the reward cues. These findings suggest that whereas other striatal areas may code for expected incentive magnitude, a region in the NAcc codes for expected positive incentive value.

Single midline thalamic neurons projecting to both the ventral striatum and the prefrontal cortex in the rat.

The midline thalamic nuclei have been known to send projection fibres to the ventral striatum and the autonomic/limbic-associated areas of the prefrontal cortex. In the present study, we sought to determine whether or not single midline thalamic neurons project both to the ventral striatum and to the cerebral cortical areas. Experiments were performed on chloral hydrate-anaesthetized male Sprague Dawley rats; two fluorescent retrograde tracers were centred on the medial or lateral part of the nucleus accumbens--the major part of the ventral striatum--and the medial or lateral prefrontal viscerolimbic cortex. Our retrograde double-labelling study revealed that a subset of midline thalamic neurons send projection fibres to both the nucleus accumbens and the cerebral cortex. Such neurons projecting to both targets were principally identified in the paraventricular thalamic nucleus. The majority of the dually-labelled neurons in the paraventricular thalamic nucleus projected to the lateral part of the nucleus accumbens and the medial wall of the prefrontal cortex. Dually-labelled neurons were additionally found in other midline nuclei, including the paratenial, intermediodorsal, rhomboid, and reuniens nuclei, as well as in the medial part of the parafascicular thalamic nucleus. Dually-projecting neurons identified in the present study may represent a potential link between the limbic striatum and the viscerolimbic-associated cortex, thus suggesting that non-discriminative information relayed to the prefrontal cortex might exert an influence through the same neurons on the nucleus accumbens implicated in affective behaviour.

The modulation of sensorimotor gating deficits by mesolimbic cholecystokinin.

The effects of cholecystokinin (CCK) in an animal model of sensorimotor-gating deficits with strong face, construct and predictive validity for schizophrenia were investigated. Prepulse inhibition (PPI) occurs when a weak acoustic lead stimulus inhibits the startle response to a loud startling stimulus. Infusions of sulfated CCK-8 in the posterior nucleus accumbens potentiated apomorphine-induced disruption of PPI but had no effect on baseline PPI or the amplitude of acoustic startle reflex itself. The results provide evidence that mesolimbic CCK may play a role in regulating sensorimotor gating deficits but contradict earlier notions that CCK agonists may have antipsychotic properties and upon which clinical trials of CCK agonists in schizophrenia were based. Rather, these results suggest that antagonists of CCK may display neuroleptic-like actions on deficits in PPI and may hold greater promise as antipsychotics.

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.

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.

Participation of the glutamatergic input of the nucleus accumbens in the regulation of the synaptic release of dopamine during associative learning.

The changes in the synaptic release of dopamine in the medial division of the nucleus accumbens in the course of a conditioned emotional response and the influence on this process of the blockade of N-methyl-D-aspartate (NMDA) receptors of this structure were investigated in awake Lister-hooded rats, using the method of intravital intracerebral dialysis in combination with high-pressure liquid chromatography and electrochemical detection. It was established that situational stimuli, previously combined with painful reinforcement, but not acoustic conditional signals, lead to a slow increase in the level of dopamine in the extracellular space of the nucleus accumbens. This process reaches a maximum 40 min after the beginning of testing and lasts 80 min. Dialysis perfusion of the nucleus accumbens with a solution of MK-801 (50 mumole/liter) does not alter the magnitude of the slow rise in the synaptic release of dopamine in this structure in the course of the conditioned emotional response, but completely blocks the late components of the release. It is concluded that the glutamatergic input of the nucleus accumbens participates in the regulation of the late components of the synaptic release of dopamine, governed by the conditioned emotional response, in this structure through NMDA receptors.

Accumbens D2 modulation of sensorimotor gating in rats: assessing anatomical localization.

The normal reduction in acoustic startle amplitude caused by a weak prepulse (prepulse inhibition; PPI) is deficient in schizophrenic patients and in rats after systemic or intraaccumbens treatment with the D2 dopamine agonist quinpirole. We examined the anatomical substrates of the PPI-disruptive effects of intraaccumbens quinpirole. PPI was significantly reduced in a dose-dependent manner by quinpirole infusion into the medial accumbens shell region, the lateral accumbens core region, and an intermediate central region. There was a weak tendency for this quinpirole effect to be more pronounced in core and central accumbens regions than in the medial and anteromedial accumbens. Using the retrograde tracer Nuclear yellow, shell and core regions were verified to receive different patterns of limbic cortical innervation. Although the accumbens appears to have a complex and functionally diversified intrinsic anatomy, the accumbens D2 modulation of sensorimotor gating appears to be distributed across several different accumbens subregions.

Relative accuracy and reproducibility of regional MRI brain volumes for point-counting methods.

Volumetric magnetic resonance imaging (MRI) methods for the measurement of various neuroanatomical regions are of great interest in studies of neuropsychiatric disorders. Both manual and semiautomated methods have been developed. Manual methods include tracing and point counting. Point counting methods are widely used in post-mortem and microscopy studies. Point counting has been well validated for these purposes. In this article, we report in a series of separate studies the accuracy and reproducibility of point-counting methods. Absolute accuracy was evaluated with a spherical phantom. Accuracy and time efficiency were subsequently assessed with an anatomically realistic phantom and various size grids. The point-counting method was also compared to a tracing method. Finally, the reproducibility of the point-counting method for the caudate and putamen was evaluated on four subjects in a test-retest experiment. These studies provide an estimate of the accuracy and time efficiency of point-counting methods. The test-retest reliability was also high for both caudate and putamen. Findings suggest that point counting is a reliable and efficient method for estimating volumes.

Social isolation in the rat produces developmentally specific deficits in prepulse inhibition of the acoustic startle response without disrupting latent inhibition.

A series of experiments examined the effects of 8 weeks of social isolation on spontaneous locomotor activity, prepulse inhibition (PPI) of the acoustic startle response, latent inhibition (LI) in a conditioned suppression paradigm, and basal and d-amphetamine stimulated dopamine (DA) release in the ventral striatum, as measured by in vivo microdialysis. Both isolation-reared animals (those isolated from the weaning age) and isolation-housed animals (those isolated as adults) were hyperactive when placed in a novel environment. Social isolation also led to deficits in PPI of the acoustic startle response that were specific to isolation-reared animals. Isolation rearing was without effect on the expression of LI but did lead to an enhanced response to systemic d-amphetamine in terms of striatal DA release. The data are discussed with respect to the involvement of ventral striatal DA mechanisms in the expression of PPI and LI, differences in the impact of social isolation in young and adult animals, and the utility of social isolation model as a nonlesion, nonpharmacologic means of perturbing ventral striatal DA function.

The patterns of afferent innervation of the core and shell in the "accumbens" part of the rat ventral striatum: immunohistochemical detection of retrogradely transported fluoro-gold.

Recent data have emphasized the neurochemically distinct nature of subterritories in the accumbens part of the rat ventral striatum termed the core, shell, and rostral pole. In order to gain a more comprehensive understanding of how afferents are distributed relative to these subterritories, immunohistochemical detection of retrogradely transported Fluoro-Gold was carried out following iontophoretic injections intended to involve selectively one of the subterritories. The data revealed that a number of cortical afferents of the medial shell and core originate in separate areas, i.e., the dorsal peduncular, infralimbic, and posterior piriform cortices (to medial shell) and the dorsal prelimbic, anterior agranular insular, anterior cingulate, and perirhinal cortices (to core). The lateral shell and rostral pole are innervated by cortical structures that also project either to the medial shell or core. The orbital, posterior agranular insular, and entorhinal cortices, hippocampus, and basal amygdala were observed to innervate the accumbens in a topographic manner. Following core injections, strong bilateral cortical labeling was observed. Few labeled cortical cells were observed contralaterally following injections in the medial shell. Intermediate numbers of labeled neurons were observed in contralateral cortices following lateral shell injections. Robust subcortical labeling in a variety of structures in the ventral forebrain, lateral hypothalamus, deep temporal lobe, and brainstem was observed after shell injections, particularly those that involved the caudal dorsomedial extremity of the shell, i.e., its "septal pole." Selective ipsilateral labeling of subcortical structures in the basal ganglia circuitry was observed following injections in the core and, to a lesser extent, lateral shell. It was concluded that a number of afferent systems exhibit varying degrees of segregation with respect to the accumbal subterritories.