Efferent projections of the nucleus accumbens in the rat with special reference to subdivision of the nucleus: biotinylated dextran amine study

Ethanol-induced Fos immunoreactivity in the extended amygdala and hypothalamus of the rat brain: focus on cholinergic interneurons of the nucleus accumbens

BACKGROUND

The primary goal of this study was to investigate the effects of varying doses of ethanol on cellular activation, as measured by Fos immunoreactivity, in brain areas that have been implicated in the reinforcing and anxiolytic effects of substance abuse and dependence, namely, the extended amygdala and hypothalamus. Specific regions examined included the central nucleus of the amygdala, bed nucleus of the stria terminalis, substantia innominata, and nucleus accumbens of the extended amygdala, as well as the paraventricular nucleus of the hypothalamus. The cholinergic interneurons of the nucleus accumbens were of particular interest, because these cells have recently been reported to play a pivotal role in substance abuse.

METHODS

Adult Sprague-Dawley rats underwent 10 days of handling and 5 days of habituation. Animals then received an injection of saline or 0.5, 1, or 2 g/kg of ethanol. Rats were perfused 2 hr after the injections, and brain sections were processed for single Fos or dual Fos/choline acetyltransferase immunolabeling procedures. The number of Fos-positive neurons was calculated from a 0.45-mm sample area from each of the brain regions examined.

RESULTS

A dose of 2 g/kg of ethanol significantly increased the number of Fos-immunoreactive neurons in the central nucleus of the amygdala by 149%, in the shell nucleus accumbens by 80%, and in the paraventricular nucleus of the hypothalamus by 321%. Additionally, 1 g/kg of ethanol significantly increased the percentage of Fos-immunoreactive cholinergic neurons in the nucleus accumbens by 59%.

CONCLUSIONS

The findings reported in this study reveal region-specific and dose-dependent changes in Fos immunoreactivity in the extended amygdala and hypothalamus and, more specifically, an increase in neuronal activation of cholinergic cells in the shell nucleus accumbens. These findings contribute to our current knowledge of the brain areas and cellular microcircuits involved in the underlying basis of substance abuse and dependence.

Quantification of morphological differences in boutons from different afferent populations to the nucleus accumbens

The nucleus accumbens (Acb) receives convergent glutamatergic inputs from the prefrontal cortex (PFC), central thalamus, basolateral amygdala and the ventral subiculum of the hippocampus. The principal neurons of the nucleus accumbens are modulated by specific sets of convergent afferent inputs, the local circuit neurons also receive a substantial number of glutamatergic inputs, but the full complement of these has yet to be established. The aim of these studies was to define characteristics of the different glutamatergic afferent inputs to the nucleus accumbens that would aid their identification. To enable the characterisation of the glutamatergic inputs to nucleus accumbens neurons we first labelled the four main glutamatergic sources of afferent input to the accumbens with the anterograde tracer biotinylated dextran amine (BDA). Using an unbiased systematic sampling method, the morphological characteristics of their synaptic boutons were measured and assessed at the electron microscopic level. From the criteria assessed, a comparison of the four afferent sources was made, characteristics such as bouton size and vesicle density had significantly different population means, however, the only characteristic that allowed discrimination between the four major glutamatergic afferent to the nucleus accumbens was that of vesicle size. The vesicles in boutons from amygdala were larger than the subiculum which, in turn, were larger than the prefrontal cortex, the thalamus were the smallest in size. The methods used also allow a comparison of the relative frequency of different sized postsynaptic structures targeted, the prefrontal cortex almost exclusively targeted spines whereas the thalamus and the subiculum, in addition to spines, targeted proximal and distal dendrites.

Differences between accumbens core and shell neurons exhibiting phasic firing patterns related to drug-seeking behavior during a discriminative-stimulus task

The habit-forming effects of abused drugs depend on the mesocorticolimbic dopamine system innervating the nucleus accumbens (NAcc). To examine whether different NAcc subterritories (core and medial shell) exhibit a differential distribution of neurons showing phasic firing patterns correlated with drug-seeking behavior, rats were trained to self-administer cocaine, and activity of single NAcc neurons was recorded. In the presence of a discriminative-stimulus (S(D)) tone, a single lever press produced an intravenous infusion of cocaine (0.35 mg/kg), terminated the tone, and started an intertone interval ranging from 3 to 6 min. Lever presses during this intertone interval had no programmed consequences. In addition to evaluating neuronal firing patterns associated with cocaine-reinforced presses, we also evaluated firing patterns associated with unreinforced lever presses to allow interpretation of firing free of factors other than the instrumental response (such as tone-off and onset of the pump signaling drug infusion). Core neurons exhibited a greater change in firing than medial shell neurons both in the seconds preceding the reinforced and unreinforced lever press response and in the seconds following the unreinforced response. Core and medial shell neurons exhibited similar changes in firing during the seconds following the cocaine-reinforced press. The differential distribution of neurons exhibiting phasic changes in firing preceding the lever press suggests that the physiological activity of core neurons may play a greater role than that of medial shell neurons in processes related to the execution of conditioned drug-seeking responses.

Limbic and motor circuitry underlying footshock-induced reinstatement of cocaine-seeking behavior

The role of limbic, cortical, and striatal circuitry in a footshock reinstatement model of relapse to cocaine seeking was evaluated. Transient inhibition of the central extended amygdala [CEA; including the central nucleus of the amygdala (CN), ventral bed nucleus of the stria terminalis (BNSTv), and nucleus accumbens shell (NAshell)], ventral tegmental area (VTA), and motor circuitry [including the dorsal prefrontal cortex (PFCd), nucleus accumbens core (NAcore), and ventral pallidum (VP)] blocked the ability of footshock stress to reinstate lever pressing previously associated with cocaine delivery. However, inhibition of the basolateral amygdala, mediodorsal nucleus of the thalamus, or the ventral prefrontal cortex had no effect on drug-seeking behavior. These data suggest that footshock stress activates limbic circuitry of the CEA that, via the VTA, activates motor output circuitry responsible for producing lever press responding. Consistent with this notion, the D1/D2 dopamine receptor antagonist fluphenazine blocked footshock-induced reinstatement when infused into the PFCd. Further, inhibition of the NAshell blocked a footshock-induced increase in dopamine within the PFC and concomitantly blocked reinstatement responding. Also supporting the idea of a CEA-VTA-motor circuit in stress-induced reinstatement of cocaine seeking, inactivation of the PFCd was shown to block stress-induced glutamate release within the NAcore while concurrently inhibiting reinstatement responding. Taken together, these data suggest that footshock activates limbic circuitry in the CEA, which in turn activates a VTA dopamine projection to the PFCd. The rise in dopamine within the PFCd initiates reinstatement via a glutamatergic projection to the NAcore.

Activation of a subpopulation of orexin/hypocretin-containing hypothalamic neurons by GABAA receptor-mediated inhibition of the nucleus accumbens shell, but not by exposure to a novel environment

Gamma-amino butyric acid (GABA)A receptor stimulation in the nucleus accumbens shell produces intense hyperphagia in rats and increases Fos expression in the lateral hypothalamus. To explore the involvement of hypothalamic orexin/hypocretin- or melanin concentrating hormone-immunoreactive neurons in this effect, the GABAA agonist, muscimol (0, 50 ng), was infused directly into the nucleus accumbens shell of rats; 90 min later, their brains were collected and subsequently processed for immunohistochemistry. A group exposed to a novel environment was included to evaluate the specificity of Fos expression changes with regard to general arousal. Alternating sections through the hypothalamus were double-stained for orexin/hypocretin-Fos or melanin concentrating hormone-Fos combinations. Intra-accumbens shell muscimol treatment significantly increased the percentage of orexin/hypocretin-containing neurons expressing Fos in the lateral, but not medial, portion of the perifornical/lateral hypothalamic area. Regardless of treatment condition, greater percentages of orexin/hypocretin-containing neurons in the medial portion of the hypothalamus expressed Fos relative to cells located more laterally. None of the manipulations increased Fos expression in melanin concentrating hormone-immunoreactive neurons. Muscimol treatment also markedly increased Fos expression in the arcuate nucleus, which connects reciprocally to the lateral/perifornical hypothalamic area. Thus, orexin/hypocretin-containing neurons in lateral sectors of the hypothalamus, along with cells in the arcuate nucleus, display phasic increases in Fos expression after an orexigenic pharmacological manipulation of the nucleus accumbens shell, but to a lesser degree after the heightened arousal associated with exposure to a novel environment.

Dopamine neurons mediate a fast excitatory signal via their glutamatergic synapses

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.

Differential projections of the infralimbic and prelimbic cortex in the rat

The medial prefrontal cortex has been associated with diverse functions including attentional processes, visceromotor activity, decision-making, goal-directed behavior, and working memory. The present report compares and contrasts projections from the infralimbic (IL) and prelimbic (PL) cortices in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris-leucoagglutinin. With the exception of common projections to parts of the orbitomedial prefrontal cortex, olfactory forebrain, and midline thalamus, PL and IL distribute very differently throughout the brain. Main projection sites of IL are: 1) the lateral septum, bed nucleus of stria terminalis, medial and lateral preoptic nuclei, substantia innominata, and endopiriform nuclei of the basal forebrain; 2) the medial, basomedial, central, and cortical nuclei of amygdala; 3) the dorsomedial, lateral, perifornical, posterior, and supramammillary nuclei of hypothalamus; and 4) the parabrachial and solitary nuclei of the brainstem. By contrast, PL projects at best sparingly to each of these structures. Main projection sites of PL are: the agranular insular cortex, claustrum, nucleus accumbens, olfactory tubercle, the paraventricular, mediodorsal, and reuniens nuclei of thalamus, the capsular part of the central nucleus and the basolateral nucleus of amygdala, and the dorsal and median raphe nuclei of the brainstem. As discussed herein, the pattern of IL projections is consistent with a role for IL in the control of visceral/autonomic activity homologous to the orbitomedial prefrontal cortex of primates, whereas those of PL are consistent with a role for PL in limbic-cognitive functions homologous to the dorsolateral prefrontal cortex of primates.

Oral sucrose stimulation increases accumbens dopamine in the rat

Although taste can influence meal size and body weight, the neural substrate for these effects remains obscure. Dopamine, particularly in the nucleus accumbens, has been implicated in both natural and nonnatural rewards. To isolate the orosensory effects of taste from possible postingestive consequences, we investigated the quantitative relationship between sham feeding of sucrose and extracellular dopamine in the nucleus accumbens with microdialysis in rats. Sucrose intake linearly increased as a function of concentration (0.03 M, 18.07 +/- 2.41 ml; 0.1 M, 30.92 +/- 2.60 ml; 0.3 M, 43.28 +/- 2.88 ml). Sham feeding also stimulated accumbens dopamine overflow as a function of sucrose solution concentration (0.03 M, 120.76 +/- 2.6%; 0.1 M, 140.28 +/- 7.8%; 0.3 M, 146.27 +/- 5.05%). A second experiment used the same protocol but clamped the amount of sucrose ingested and revealed a similar, concentration-dependent dopamine activation in the nucleus accumbens. This is the first demonstration of a quantitative relationship between the concentration-dependent rewarding effect of orosensory stimulation by sucrose during eating and the overflow of dopamine in the nucleus accumbens. This finding provides new and strong support for accumbens dopamine in the rewarding effect of sucrose.

Chemical organization of projection neurons in the rat accumbens nucleus and olfactory tubercle

Projection neurons in the ventral striatum, the accumbens nucleus and olfactory tubercle, were examined by combining the retrograde tracing method and immunocytochemistry with antibodies against C-terminals of the preprodynorphin (PPD), preproenkephalin (PPE), preprotachykinin A (PPTA) and preprotachykinin B (PPTB). When the retrograde tracer was injected into the ventral pallidum, about 60% and 40% of retrogradely labeled neurons in the accumbens nucleus were immunoreactive for PPD and PPE, respectively. In contrast, all accumbens nucleus neurons projecting to the ventral mesencephalic regions including the substantia nigra and ventral tegmental area were immunopositive for PPD but not for PPE. Although no olfactory tubercle neurons projected fibers to the mesencephalic regions, 60% and 40% of olfactory tubercle neurons projecting to the ventrolateral portion of the ventral pallidum were immunoreactive for PPD and PPE, respectively, as were the accumbens nucleus neurons. About 70% of accumbens nucleus and olfactory tubercle neurons projecting to the ventral pallidum and all accumbens nucleus neurons projecting to the ventral mesencephalic regions showed PPTA immunoreactivity. A small population (2-12%) of accumbens neurons projecting to the ventral pallidum and mesencephalic regions displayed immunoreactivity for PPTB. Compared with the dorsal striatopallidal projection neurons that were reported to mostly express PPE, it was characteristic of the ventral striatum that only the smaller population (about 40%) of ventral striatopallidal projection neurons expressed PPE. This suggests that the ventral striatopallidal projection system is less specialized than the dorsal striatopallidal system in terms of peptide production, or that the ventral pallidum should be compared with a combined region of the globus pallidus and entopeduncular nucleus in the dorsal system.

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Localization of dopamine D2 receptors on cholinergic interneurons of the dorsal striatum and nucleus accumbens of the rat

Striatal cholinergic interneurons located in the dorsal striatum and nucleus accumbens are amenable to influences of the dopaminergic mesolimbic pathway, which is a pathway involved in reward and reinforcement and targeted by several drugs of abuse. Dopamine and acetylcholine neurotransmission and their interactions are essential to striatal function, and disruptions to these systems lead to a variety of clinical disorders. Dopamine regulates acetylcholine release through dopamine receptors that are localized directly on striatal cholinergic interneurons. The dopamine D2 receptor, which attenuates acetylcholine release, has been implicated in drug relapse and is targeted by therapeutic drugs that are used to treat a variety of neurological disorders including Tourette Syndrome, Parkinson's disease and schizophrenia. The present study provides the first direct evidence for the localization of dopamine D2 receptors on striatal cholinergic interneurons of the rat brain using dual labeling immunocytochemistry procedures. Using light microscopy, dopamine D2 receptors were localized on the cell somata and dendritic and axonal processes of striatal cholinergic interneurons in the dorsal striatum and nucleus accumbens of the rat brain. These findings provide a foundation for understanding the specific roles that cholinergic neuronal network systems and interacting dopaminergic signaling pathways play in striatal function and in a variety of clinical disorders including drug abuse and addiction.

Hippocampal modulation of sensorimotor processes

While the hippocampus makes unique contributions to memory, it has also long been associated with sensorimotor processes, i.e. innate processes involving control of motor responses to sensory stimuli. Moreover, hippocampal dysfunction has been implicated in neuropsychiatric diseases, such as schizophrenia and anxiety disorders, primarily characterized by non-mnemonic deficits in the processing of and responding to sensory information. This review is concerned with the hippocampal modulation of three sensorimotor processes in rats-locomotor activity, prepulse inhibition (PPI) of the startle reflex, and the startle reflex itself-whose alterations are related to human psychosis or anxiety disorders. Its main purpose is to present and discuss the picture emerging from studies examining the effects of pharmacological manipulations of the dorsal and ventral hippocampus by local drug microinfusions. While a role of the hippocampus in regulating locomotor activity, PPI, and startle reactivity has also been suggested based on the effects of hippocampal lesions, the microinfusion studies have revealed additional important details of this role and suggest modifications of notions based on lesion studies. In summary, the microinfusion studies corroborate that hippocampal mechanisms can directly influence locomotor activity, PPI, and startle reactivity, and that aberrant hippocampal function may contribute to neuropsychiatric diseases, in particular psychosis. The relation between different sensorimotor processes and hippocampal neurotransmission, the role of ventral and dorsal hippocampus, and the extrahippocampal mechanisms mediating the hippocampal modulation of different sensorimotor processes can partly be dissociated. Thus, the hippocampal modulation of these sensorimotor processes appears to reflect multiple operations, rather than one unitary operation.

An aversive diet as thiamine-free food blocks food-induced release of excitatory amino acids in the accumbens

AIMS

As the nucleus accumbens shell plays an important role in the control of eating behaviour, the aim of this study was to evaluate the changes in: (a) the level of aspartic and glutamic acids in the accumbens shell of conditioned rats after the presentation of an aversive diet containing thiamine-free food; (b) the temperature of interscapular brown adipose tissue, effector of thermogenesis related to food intake.

METHODS

The concentration of aspartic and glutamic acids in the accumbens shell, and brown adipose tissue temperature were monitored in conditioned male Sprague-Dawley rats before and after the presentation of thiamine-free food or standard laboratory food. The aspartic and glutamic acids were collected using a microdialysis probe and quantified by HPLC. Food intake was also measured.

RESULTS

The results indicated that an intake of standard laboratory food induced an increase in the level of aspartic and glutamic acids, and an elevation in temperature of brown adipose tissue; whereas an intake of thiamine-free food blocks these increases in the conditioned animals.

CONCLUSION

The thiamine-free diet modifies the release of excitatory amino acids in the nucleus accumbens of conditioned animals. This diet also affects thermogenesis.

Pleasures of the brain

How does the brain cause positive affective reactions to sensory pleasure? An answer to pleasure causation requires knowing not only which brain systems are activated by pleasant stimuli, but also which systems actually cause their positive affective properties. This paper focuses on brain causation of behavioral positive affective reactions to pleasant sensations, such as sweet tastes. Its goal is to understand how brain systems generate 'liking,' the core process that underlies sensory pleasure and causes positive affective reactions. Evidence suggests activity in a subcortical network involving portions of the nucleus accumbens shell, ventral pallidum, and brainstem causes 'liking' and positive affective reactions to sweet tastes. Lesions of ventral pallidum also impair normal sensory pleasure. Recent findings regarding this subcortical network's causation of core 'liking' reactions help clarify how the essence of a pleasure gloss gets added to mere sensation. The same subcortical 'liking' network, via connection to brain systems involved in explicit cognitive representations, may also in turn cause conscious experiences of sensory pleasure.

Inhibition of rostral basal forebrain neurons promotes wakefulness and induces FOS in orexin neurons

The present study examined whether the activities of the rostral basal forebrain neurons alter the activities of the orexin (also known as hypocretin) neurons in the tuberal part of the hypothalamus in rats. We performed microdialysis perfusion of the ventromedial portion of the rostral basal forebrain with the GABAA receptor agonist muscimol to inhibit focally the neuronal activities in the rostral basal forebrain. Then, we monitored sleep/wake behaviour and investigated the pattern of activities of orexin neurons by examining the expression of FOS as an indicator of cellular activation. Bilateral perfusion with muscimol (5, 15, and 50 micro m) produced a dose-dependent decrease in the amount of sleep. This perfusion with muscimol at 50 micro m produced FOS-like immunoreactivity in 37% of the orexin neurons located in the tuberal part of the hypothalamus, whereas the FOS-like immunoreactivity was sparse in orexin neurons of the sleeping control rats (P = 0.001 by Mann-Whitney U-test). Unilateral perfusion with muscimol (50 micro m) also suppressed sleep. In this case, FOS-like immunoreactivity was seen in 40% of the orexin neurons on the side ipsilateral to the perfusion site but only in 10% of orexin neurons on the contralateral side (P = 0.018 by Wilcoxon signed rank test). These functional data suggested that a sleep-generating element in the ventromedial part of the rostral basal forebrain provides an inhibitory influence on the activities of the orexin neurons in the tuberal part of the hypothalamus.

Selective stimulation of striatal dopamine receptors of the D1- or D2-class causes opposite changes of fos expression in the rat cerebral cortex

It has been suggested that activation of striatal neurons expressing D1 or D2 dopamine receptors elicits opposite changes in the net output of the basal ganglia circuitry and, consequently, in the functional interactions of the circuit with the cerebral cortex. In particular, it has been recently reported that striatal D1 receptors may regulate cortex function. To further address this issue, we mapped cerebral expression of Fos protein following intrastriatal stimulation of D1- or D2-class receptors in freely moving animals. Using permanent cannulas implanted in the right striatum, Sprague-Dawley rats received intrastriatal microinfusions of SKF 38393 (D1 agonist) or quinpirole (D2 agonist) or saline (controls), combined with systemic administration of D1 antagonist SCH 23390 or D2 antagonist eticlopride or saline. Animals treated with SKF 38393 showed dose-dependent, massive Fos increases in the motor, somatosensory, auditory, visual and limbic regions of the cerebral cortex, ipsilaterally to the injected striatum. Consistent Fos expression was also found in the injected striatum and, bilaterally, in the nucleus accumbens shell. These increases were effectively counteracted by systemic SCH 23390. Conversely, quinpirole did not induce significant cortical or striatal expression of Fos, which was instead observed after the systemic administration of eticlopride. Fos was not detected in any of the other basal ganglia nuclei, regardless of the dopamine agonists or antagonists used. Our results confirm that striatal D1 dopamine receptors play a central role in the modulation of cortical activity, thus providing additional information on the functional interaction between basal ganglia circuitry and cerebral cortex.

Regional and subcellular compartmentation of the dopamine transporter and tyrosine hydroxylase in the rat ventral pallidum

The ventral pallidum (VP) is a major intermediary in the prefrontal cortical circuitry regulating sensorimotor gating and locomotor behavior, both of which are potently modulated by catecholamines. The VP catecholaminergic innervation is derived from midbrain dopaminergic neurons that differ in expression levels of the dopamine transporter (DAT) and from brainstem noradrenergic neurons without DAT. The preferentially low level of DAT in dopaminergic terminals in the prefrontal cortex and in striatal regions projecting more extensively to the VP medial (VPm) compared with VP lateral (VPl) compartment suggests possible region-specific differences in VP axonal distribution of DAT. To test this hypothesis, we examined the electron microscopic localization of DAT and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), in the VPm and VPl of rat brain. In both regions, DAT and TH were localized primarily in small unmyelinated axons and morphologically heterogeneous axon terminals. DAT-immunogold particles were few in number, but mostly located on the plasma membrane. In contrast, TH immunoreactivity was distributed in the cytoplasm of individual profiles, many of which were without detectable DAT. In comparison with TH, the mean area density of DAT-labeled axons was low throughout the VP. The mean area density of DAT-immunogold axon terminals, however, was significantly higher in VPl than in VPm, whereas that of TH-labeled axons was higher in VPm than in VPl. This dissociation suggests that, compared to the VPl, the VPm receives the greatest input from catecholaminergic afferents that are either nondopaminergic or characterized by having low levels or less terminal distributions of DAT.

Preprodynorphin-, preproenkephalin-, preprotachykinin A- and preprotachykinin B-immunoreactive neurons in the accumbens nucleus and olfactory tubercle: double-immunofluorescence analysis

Preprodynorphin (PPD), preproenkephalin (PPE) and preprotachykinins A (PPTA) and B (PPTB) are known to be expressed by neostriatal projection neurons. In the present study, we investigated the distributions and colocalizations of immunoreactivities for those prepropeptides in the ventral striatum, such as the accumbens nucleus (Acb) and olfactory tubercle (OT). Antibodies raised against C-terminal portions of the prepropeptides labeled cell bodies of neurons with diameters of 8-15 microm. PPD-, PPE- and PPTA-immunoreactive neurons were distributed throughout the Acb and concentrated in the dense cell layer of the OT. PPTB-immunoreactive neurons were observed to form cell clusters, which were localized in mu-opioid receptor-immunoreactive patchy regions in the Acb, but were very rarely found in the dense cell layer of the OT. Double-immunofluorescence analysis revealed that PPD, PPE and PPTB immunoreactivities were shown in 69%, 19% and 14% of PPTA-immunoreactive neurons, respectively, in the Acb core region, and in 92%, 7% and 25% of PPTA-immunoreactive neurons, respectively, in the Acb shell region. In the olfactory bulb, 51%, 19% and 3% of PPTA-immunoreactive neurons showed PPD, PPE and PPTB immunoreactivities, respectively. PPD and PPE immunoreactivities were rarely coexpressed in single neurons of all striatal regions. The present results indicated that, although PPTA and PPE were occasionally coexpressed in single neurons of the ventral striatum, the segregated expression of PPD and PPE in the ventral striatum was similar to that in the dorsal striatum. The clustered localization of PPTB-expressing neurons in the Acb and near absence of PPTB-expressing neurons in the dense cell layer of the OT suggests that neurokinin B is a key substance in differentiating between the ventral and dorsal striatal regions.

Origin of the dopaminergic innervation of the central extended amygdala and accumbens shell: a combined retrograde tracing and immunohistochemical study in the rat

The origin of the dopaminergic innervation of the central extended amygdala (EAc; i.e., the lateral bed nucleus of the stria terminalis [BSTl]-central amygdaloid nucleus [Ce] continuum) and accumbens shell (AcSh) was studied in the rat by combining retrograde transport of Fluoro-Gold (FG) with tyrosine hydroxylase (TH) immunofluorescence. Perikaryal profiles (PP) immunoreactive to FG and to both FG and TH were counted in A8-A14 dopaminergic districts. Our results suggest that dopaminergic inputs to the EAc and AcSh arise from the ventral tegmental area-A10, substantia nigra, pars compacta-A9, and retrorubral nucleus-A8 groups as well as from the dorsal raphe nucleus and periaqueductal gray substance, housing the dorsocaudal part of A10 group (A10dc). Quantitative estimates reveal that the A10dc group contains approximately half of the total number of FG/TH double-labeled PP projecting to Ce and BSTl. By using an anti-dopamine serum, DR/PAG projections to Ce were confirmed to be in part dopaminergic. In contrast, modest numbers of FG/TH double-labeled PP were seen in the A10dc group after injections in the sublenticular extended amygdala, interstitial nucleus of the posterior limb of the anterior commissure or AcSh. Ventral mesencephalic projections to the EAc display a crude mediolateral topographic organization, whereas those to the AcSh are topographically organized along a mediolateral and an inverted dorsoventral dimension. The diencephalic dopaminergic groups do not innervate the EAc or AcSh, except for the periventricular gray-A11 which sends light dopaminergic projections to Ce and BSTl. Overall, the present results provide additional details on the organization of the mesolimbic dopaminergic system that critically controls behavioral responsiveness to salient environmental stimuli.

Effects of selective dopamine D1 or D2 receptor blockade within nucleus accumbens subregions on ingestive behavior and associated motor activity

Two anatomically and neurochemically distinguishable regions of the nucleus accumbens (Acb), the core and the shell, have been shown to differentially regulate feeding behavior. Nevertheless, despite the well-known role of Acb dopamine in the modulation of motivated behaviors, there have been no studies directly comparing the effects of acute dopamine receptor blockade in the Acb core versus the Acb shell on feeding. In this study, D1- or D2-selective dopamine receptor antagonists were infused bilaterally into the Acb core or shell of hungry rats, whereupon feeding, drinking, and spontaneous motor activity were monitored. Both the D1 antagonist SCH 23390 (0, 1, and 2 microg/0.5 microl) and the D2 antagonist raclopride (0, 1, and 2 microg/0.5 microl) markedly suppressed ambulation and rearing when infused into either the Acb core or shell. Total food intake and latency to begin feeding were unaffected by either drug in either site. SCH 23390 in the Acb shell, and raclopride in the Acb core or shell, significantly decreased the total number of feeding bouts. In the Acb core, raclopride produced a small but statistically significant increase in overall feeding duration. Dopamine receptor blockade in either site tended to increase mean feeding bout duration. Measures of drinking behavior were generally unaffected. Hence, dopamine receptor blockade in either the Acb core or shell of hungry rats suppressed spontaneous motor activity and shifted the structure of feeding towards longer bout durations, but did not alter the total amount of food consumed. In the Acb shell, the effects of D1 receptor blockade tended to be of greater magnitude than the effects of D2 receptor blockade, although major differences between core and shell effects were not observed. These results are discussed with regard to current theories of dopaminergic control of feeding behavior, and with reference to the functional heterogeneity of Acb subregions.

Ventral striatal anatomy of locomotor activity induced by cocaine, D-amphetamine, dopamine and D1/D2 agonists

The ventral striatum appears to play a critical role in mediating motoric effects (i.e. ambulatory activity and rearing) of psychostimulants such as cocaine. We evaluated whether sub-regions of the ventral striatum play differential roles in locomotion and rearing induced by various dopaminergic drugs. Injections of D-amphetamine and dopamine stimulated locomotion and rearing with a similar potency at each of the sub-regions: the core, medial shell or medial tubercle. However, injections of mixtures of the D(1)- and D(2)-type agonists SKF 38393 and quinpirole or cocaine into the medial olfactory tubercle or the medial shell of the nucleus accumbens induced marked locomotion and rearing, while these injections into the core induced little or no locomotion or rearing. Furthermore, cocaine injections into the lateral or posterior tubercle produced marginal locomotion and rearing, while cocaine injections into regions just dorsal to these tubercle sites, the lateral portion of the shell or the ventral pallidum, did not produce any stimulating effect. We conclude that dopaminergic compounds induce vigorous locomotion and rearing in both core and shell; the relative roles of the core and shell differ depending on chemical compounds. Similar to the nucleus accumbens, the olfactory tubercle, particularly the medial portion, also mediates these behaviors induced by dopaminergic compounds. The medial ventral striatum (i.e. the medial tubercle and medial shell) plays a more important role in cocaine-induced locomotion and rearing than the lateral ventral striatum (i.e. the core, lateral shell and lateral tubercle). Moreover, the differential effects of cocaine between the medial and lateral portions of the shell on locomotion and rearing suggest more than two functional units (the core vs. the shell) within the accumbens.

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

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

Hippocampal and prefrontal cortical inputs monosynaptically converge with individual projection neurons of the nucleus accumbens

Afferents to the nucleus accumbens from different sources innervate specific areas of the central "core" and peripheral "shell" and are related to each other, at the light microscopical level, in an intricate overlapping and nonoverlapping way. This lack of homogeneity suggests that this region consists of circuits involving emsembles of neurons modulated by specific sets of convergent afferent inputs and abnormal regulation of such ensembles has been implicated in mental disorders. Early extracellular studies suggested that individual Acb neurons might respond to activation of afferents from more than one excitatory input: More recent studies of hippocampal and amygdalar or prefrontal cortical afferents suggest that hippocampal afferents gate the input from the prefrontal cortex or amygdala. Electrophysiological evidence for convergence of excitatory afferents in the Acb is strong and suggests that these pathways are monosynaptic. Nevertheless, this convergence has proved difficult to demonstrate anatomically as a result of the spatial distribution of the afferent inputs on the dendritic tree of the target neurons. To establish whether individual accumbens neurons receive monosynaptic input from pairs of afferents, one projection was labelled anterogradely with Phaseolus vulgaris leucoagglutinin and the second with biotinylated dextran amine (BDA) with Vector slate grey and 3,3'-diaminobenzidine tetrahydrochloride as the chromagens. Accumbens neurons possibly postsynaptic to these afferents, labelled by an in vivo focal injection of BDA, were examined using correlated light and electron microscopy to establish the proximal-distal distribution of labelled afferent synaptic inputs on their dendritic arbours. Individual cells were shown to receive monosynaptic afferent input from both ventral subiculum and prefrontal cortex, providing an anatomical framework for the hippocampal gating of other limbic inputs to the accumbens.

Topography of projections from amygdala to bed nuclei of the stria terminalis

A collection of 125 PHAL experiments in the rat has been analyzed to characterize the organization of projections from each amygdalar cell group (except the nucleus of the lateral olfactory tract) to the bed nuclei of the stria terminalis, which surround the crossing of the anterior commissure. The results suggest three organizing principles of these connections. First, the central nucleus, and certain other amygdalar cell groups associated with the main olfactory system, innervate preferentially various parts of the lateral and medial halves of the bed nuclear anterior division, and these projections travel via both the stria terminalis and ansa peduncularis (ventral pathway). Second, in contrast, the medial nucleus, and the rest of the amygdalar cell groups associated with the accessory and main olfactory systems innervate preferentially the posterior division, and the medial half of the anterior division, of the bed nuclei. And third, the lateral and anterior basolateral nuclei of the amygdala (associated with the frontotemporal association cortical system) do not project significantly to the bed nuclei. For comparison, inputs to the bed nuclei from the ventral subiculum, infralimbic area, and endopiriform nucleus are also described. The functional significance of these projections is discussed with reference to what is known about the output of the bed nuclei.

Fear and feeding in the nucleus accumbens shell: rostrocaudal segregation of GABA-elicited defensive behavior versus eating behavior

This study examined localization of positive versus negative motivational functions mediated by GABA circuits within the accumbens shell. Microinjections of a GABA(A) agonist (0, 25, 75, and 225 ng/0.5 microl muscimol) in rostral shell sites elicited appetitive increases in eating behavior. In contrast, microinjections in caudal shell sites elicited defensive burying or paw-treading behavior. Rats whose microinjections landed bilaterally outside of the accumbens shell did not display either behavior. Defensive treading elicited by caudal shell muscimol microinjection appeared to be a negative motivated response to threat (similar in parameters and orientation to normal defensive burying of a threatening electrified shock prod). The nucleus accumbens shell thus appears functionally heterogeneous in coding motivational valence. The demonstration that muscimol elicits positive eating behavior from rostral shell versus negative defensive behavior from caudal shell suggests in particular that GABAergic substrates of positive and negative types of motivated behavior in the nucleus accumbens shell are segregated along a rostrocaudal gradient.

CART peptide-immunoreactive projection from the nucleus accumbens targets substantia nigra pars reticulata neurons in the rat

Cocaine and amphetamine regulated transcript (CART) was originally identified as a mRNA which increases in the striatum after acute cocaine or amphetamine administration in rats. In addition, intra-ventral tegmental (VTA) area injections of CART peptides produce psychostimulant-like behavioral effects. CART peptide immunoreactivity (CARTir) has been localized in discrete nuclei throughout the brain, and, within the striatum, it is located only ventrally in a subpopulation of medium spiny projection neurons in the shell and core of the nucleus accumbens. To better understand the potential role of CART peptides in the mechanism of action of psychomotor stimulants, we analyzed the distribution and synaptic connectivity of CARTir terminals in the ventral midbrain. CARTir terminal-like varicosities were located throughout the rostrocaudal extent of the substantia nigra (SN), VTA, and retrorubral field (RRF). They were particularly abundant in the dorsomedial SN where they overlapped with non-dopaminergic substantia nigra pars reticulata (SNr) neurons and proximal dendrites of dopaminergic substantia nigra pars compacta (SNc) neurons. CARTir terminals were also in register with dopaminergic perikarya in the ventromedial part of the rostral SNc. In many instances, CARTir terminals ensheathed dendrites of SNr neurons. To characterize the postsynaptic targets and potential sources of CARTir terminals in the SN, electron microscopic observations were conducted. Ninety percent of the CARTir terminals examined displayed the ultrastructural features of boutons of striatal origin and 80% of them formed symmetric synapses with distal dendrites of SNr neurons. To further elucidate the source of CARTir terminals in the SN, unilateral excitotoxic lesions directed to the core of the nucleus accumbens (Acc) were produced; this led to a dramatic, almost complete loss of CARTir terminal staining in the ipsilateral SN, whereas the density of CARTir terminals was relatively unchanged in the VTA. In conclusion, this study demonstrates the presence of CART peptides in a direct pathway from the accumbens to the SNr, thus illustrating a unique feature of CART peptides in that they delineate a specific anatomical circuit of the basal ganglia.

Cerebral hemisphere regulation of motivated behavior

The goals of this article are to suggest a basic wiring diagram for the motor neural network that controls motivated behavior, and to provide a model for the organization of cerebral hemisphere inputs to this network. Cerebral projections mediate voluntary regulation of a behavior control column in the ventromedial upper brainstem that includes (from rostral to caudal) the medial preoptic, anterior hypothalamic, descending paraventricular, ventromedial, and premammillary nuclei, the mammillary body, and finally the substantia nigra and ventral tegmental area. The rostral segment of this column is involved in controlling ingestive (eating and drinking) and social (defensive and reproductive) behaviors, whereas the caudal segment is involved in controlling general exploratory or foraging behaviors (with locomotor and orienting components) that are required for obtaining any particular goal object. Virtually all parts of the cerebral hemispheres contribute to a triple descending projection - with cortical excitatory, striatal inhibitory, and pallidal disinhibitory components - to specific parts of the behavior control column. The functional dynamics of this circuitry remain to be established.

Assessment of tyrosine hydroxylase immunoreactive innervation in five subregions of the nucleus accumbens shell in rats treated with repeated cocaine

To explore the effects of behavioral sensitization on the anatomy of the nucleus accumbens shell, we employed a typical cocaine dosing paradigm and assessed tyrosine hydroxylase immunoreactive varicosities in five different areas of the shell, as well as the core of the nucleus accumbens. Rats were given bidaily injections of either saline (1 ml/kg i.p.) or cocaine (15 mg/kg i.p.) for 5 consecutive days, and sacrificed either 2 or 14 days from the last injection. Sections of the nucleus accumbens were processed for tyrosine hydroxylase immunoreactivity and the number of immunoreactive varicosities in contact with neuronal cell bodies was quantified in each of the subregions of the shell, as well as the core of the nucleus accumbens. Compared to saline controls, the cocaine-treated animals showed a significant augmentation in tyrosine hydroxylase immunoreactivity in two of the five subregions after 2 days of withdrawal in the shell, but not in the core. No differences were found in any region tested after 14 days of withdrawal. These data are the first to suggest that increases in nucleus accumbens presynaptic tyrosine hydroxylase may play a role in the development of behavioral sensitization, but not in the long-term expression of this phenomenon.

Opioid site in nucleus accumbens shell mediates eating and hedonic 'liking' for food: map based on microinjection Fos plumes

Microinjection of opioid agonists, such as morphine, into the nucleus accumbens shell produces increases in eating behavior (i.e. 'wanting' for food). This study (1) reports direct evidence that activation of accumbens opioid receptors in rats also augments food 'liking', or the hedonic impact of taste, and (2) identified a neural site that definitely contains receptors capable of increasing food intake. Morphine microinjections (0.5 microgram) into accumbens shell, which caused rats to increase eating, were found also to cause selective increases in positive hedonic patterns of behavioral affective reaction elicited by oral sucrose, using the 'taste reactivity' test of hedonic palatability. This positive shift indicated that morphine microinjections enhanced the hedonic impact of food palatability. The accumbens site mediating morphine-induced increases in food 'wanting' and 'liking' was identified using a novel method based on local expression of Fos induced directly by drug microinjections. The plume-shaped region of drug-induced increase in Fos immunoreactivity immediately surrounding a morphine microinjection site (Fos plume) was objectively mapped. A point-sampling procedure was used to measure the shape and size of 'positive' plumes of Fos expression triggered by microinjections of morphine at locations that caused increases in eating behavior. This revealed a functionally 'positive' neural region, containing receptors directly activated by behaviorally-effective drug microinjections. A subtraction mapping procedure was then used to eliminate all surrounding regions containing any 'negative' Fos plumes that failed to increase food intake. The subtraction produced a conservative map of the positive site, by eliminating regions that gave mixed effects, and leaving only a positive region that must contain receptors capable of mediating increases in food intake. The resulting mapped 'opioid eating site' was contained primarily within the medial caudal subregion of the nucleus accumbens shell, and did not substantially penetrate either into the accumbens core or into other subregions of the shell. Several other structures outside the nucleus accumbens (such as rostral ventral pallidum), immediately medial and adjacent to the shell, also appeared to be included in the functional site. Opioid receptors within this site thus are capable of mediating morphine-induced increases in eating, in part by enhancing the hedonic reward properties of food.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

An integrative neuroanatomical perspective on some subcortical substrates of adaptive responding with emphasis on the nucleus accumbens

Neuroanatomical substrates associated in the literature with adaptive responding are discussed, with a focus on the nucleus accumbens. While it is emphasized that the accumbens exhibits multiple levels of complex organization, a fairly complete list of brief descriptions of recent studies devoted specifically to the accumbens shell and core subterritories is presented in tabular format. The distinct patterns of connectivity of the accumbens core and shell and structures related to them by connections are described. Multiple inputs, outputs and abundant reciprocity of connections within the ventral parts of the basal ganglia are emphasized and the implications for "through-put" of impulses is considered. It is noted, at least on neuroanatomical grounds, that there is ample reason to expect feed forward processing from shell and structures with which it is associated to core and structures with which it is associated. Furthermore, the potential for additional feed forward processing involving several forebrain functional anatomical systems, inlcuding the ventral striatopallidum, extended amygdala and magnocellular basal forebrain complex is considered. It is intended that from the considerations recorded here a conceptual framework will begin to emerge that is amenable to further experimental substantiation as regards how multiple basal forebrain systems and the cortices to which they are related by connections work together to fashion a unitary object--the adaptive response.

Convergence and segregation of ventral striatal inputs and outputs

The ventral striatum, which prominently includes the nucleus accumbens (Acb), is a heterogeneous area. Within the Acb of rats, a peripherally located shell and a centrally situated core can be recognized that have different connectional, neurochemical, and functional identities. Although the Acb core resembles in many respects the dorsally adjacent caudate-putamen complex in its striatal character, the Acb shell has, in addition to striatal features, a more diverse array of neurochemical characteristics, and afferent and efferent connections. Inputs and outputs of the Acb, in particular of the shell, are inhomogeneously distributed, resulting in a mosaical arrangement of concentrations of afferent fibers and terminals and clusters of output neurons. To determine the precise relationships between the distributional patterns of various afferents (e.g., from the prefrontal cortex, the basal amygdaloid complex, the hippocampal formation, and the midline/intralaminar thalamic nuclei) and efferents to the ventral pallidum and mesencephalon, neuroanatomical anterograde and retrograde tracing experiments were carried out. The results of the double anterograde, double retrograde, and anterograde/retrograde tracing experiments indicate that various parts of the shell (dorsomedial, ventromedial, ventral, and lateral) and the core (medial and lateral) have different input-output characteristics. Furthermore, within these Acb regions, various populations of neurons can be identified, arranged in a cluster-like fashion, onto which specific sets of afferents converge and that project to particular output stations, distinct from the input-output relationships of neighboring, cluster-like neuronal populations. These results support the idea that the nucleus accumbens may consist of a collection of neuronal ensembles with different input-output relationships and, presumably, different functional characteristics.