Cholecystokinin innervation of the ventral striatum: a morphological and radioimmunological study

Dopamine: 50 years in perspective

The discovery of dopamine as a neurotransmitter in brain by Arvid Carlsson approximately 50 years ago, and the subsequent insight provided by Paul Greengard into the cellular signalling mechanisms triggered by dopamine, gained these researchers the Nobel Prize for Medicine in 2000. Dopamine research has had a greater impact on the development of biological psychiatry and psychopharmacology than work on any other neurotransmitter. Neuropsychological views of the role of dopamine in the CNS have evolved from that of a simple reward signal to a more complex situation in which dopamine encodes the importance or 'salience' of events in the external world. Hypofunctional dopamine states underlie Parkinson's disease and attention deficit hyperactivity disorder, and there is increasing evidence for dopamine hyperactivity in schizophrenia. Some of the medicines that are most widely used in psychiatry, such as L-DOPA, methylphenidate and neuroleptic drugs, act on dopaminergic mechanisms.

Cellular and subcellular localization of Huntington aggregates in the brain of a rat transgenic for Huntington disease

Huntington disease (HD) is a progressive neurodegenerative disorder characterized by emotional, cognitive, and motor dysfunctions. Aggregation of huntingtin is a hallmark of HD and, therefore, a crucial parameter for the evaluation of HD animal models. We investigated here the regional, cellular, and subcellular distribution of N-terminal huntingtin aggregates and associated neuropathological changes in the forebrain of a rat transgenic for HD (tgHD). The tgHD rat brain showed enormously enlarged lateral ventricles and a similar atrophy of cortical and subcortical areas as known in HD patients. Huntingtin aggregates of varying size and forms were regionally identified in neuronal nuclei, cytoplasm, dendrites, dendritic spines, axons, and synaptic terminals, closely resembling the results described earlier for human HD brains and in established HD mouse models. Huntingtin aggregates in mitochondria support mitochondrial dysfunction as contributing to the disease pathogenesis. Dark cell degeneration was reminiscent of results in HD individuals and HD mouse models. Interestingly, huntingtin aggregates were especially well accumulated in two interacting limbic forebrain systems, the ventral striatopallidum and the extended amygdala, which may contribute to the early onset of emotional changes observed in the tgHD rat. In conclusion, the tgHD rat model reflects to a remarkable extent the cellular and subcellular neuropathological key features as observed in human HD and HD mouse brains and hints of changes in limbic forebrain systems, which may elucidate the emotional dysfunction in the tgHD rat and affective disturbances in HD patients.

Catecholamine mapping within nucleus accumbens: differences in basal and amphetamine-stimulated efflux of norepinephrine and dopamine in shell and core

The nucleus accumbens is believed to play a critical role in mediating the behavioral responses to rewarding stimuli. Although most studies of the accumbens focus on dopamine, it receives afferents from many other nuclei, including noradrenergic cell groups in the brainstem. We used in vivo microdialysis to measure extracellular levels of both norepinephrine and dopamine in the accumbens shell and core. Regional analysis of shell and core and border regions demonstrated that norepinephrine was high in shell and decreased from medial shell to lateral core, where baseline levels were low or undetectable. Conversely, extracellular dopamine in core was twice the level seen in shell. Both catecholamines increased following a single injection of amphetamine (2 mg/kg, i.p.). The norepinephrine response was greater and long-lasting in shell compared with core. The maximal dopamine response was higher in core than in shell, but the duration of the effect was comparable in both regions. The distinct neurochemical characteristics of shell and core are likely to contribute to the functional heterogeneity of the two subregions. Furthermore, norepinephrine may be involved in many of the functions generally attributed to the accumbens, either directly or indirectly via modulation of extracellular dopamine.

Nucleus accumbens subregions: hodological and immunohistochemical study in the domestic chick (Gallus domesticus)

The nucleus accumbens was identified in avian species some time ago. However, the precise localization and extent of this nucleus is still a matter of controversy. We have used immunolabeling against calbindin, neuropeptide Y, and DARPP-32 (dopamine- and adenosine-related phosphoprotein, 32 kDa) for the selective marking of putative accumbens subdivisions and have followed the anterograde transport of biotinylated dextran amine injected to the nucleus tractus solitarii region of 7-day-old domestic chicks. The nucleus accumbens extending between rostrocaudal atlas coordinates A 10.6 and A 8.8 can be subdivided into the core and shell, the core corresponding to the ventromedial and juxtaventricular medial striatum laterodorsal to the bed nucleus of stria terminalis, and the shell representing an arched region situated ventrally and ventrolaterally to the core. Immunoreactivity to both calbindin and neuropeptide Y is more intense in the shell than in the core division. DARPP-32 immunolabeling does not differ in the two divisions but is markedly weaker in the bed nucleus of stria terminalis, enabling the separation of this nucleus from the surrounding accumbens subdivisions. Fibers from the nucleus solitarius predominantly terminate in the shell division, similar to the situation described in mammals. Whereas the suggested core lies entirely within the boundary of the medial striatum, the shell seems partially to overlap the ventral pallidum. We have been unable to subdivide the remaining part of accumbens lying rostral to A 10.6 into a putative shell and core by the methods employed in the present study. This region probably corresponds to the rostral pole of the nucleus accumbens.

Neuroadaptations of Cdk5 in cholinergic interneurons of the nucleus accumbens and prefrontal cortex of inbred alcohol-preferring rats following voluntary alcohol drinking

BACKGROUND

Neurobiological studies have identified brain areas and related molecular mechanisms involved in alcohol abuse and dependence. Specific cell types in these brain areas and their role in alcohol-related behaviors, however, have not yet been identified. This study examined the involvement of cholinergic cells in inbred alcohol-preferring rats following 1 month of alcohol drinking. Cyclin-dependent kinase 5 (Cdk5) immunoreactivity (IR), a marker of neuronal plasticity, was examined in cholinergic neurons of the nucleus accumbens (NuAcc) and prefrontal cortex (PFC) and other brain areas implicated in alcohol drinking, using dual immunocytochemical (ICC) procedures. Single Cdk5 IR was also examined in several brain areas implicated in alcohol drinking.

METHODS

The experimental group self-administered alcohol using a 2-bottle-choice test paradigm with unlimited access to 10% (v/v) alcohol and water for 23 h/d for 1 month. An average of 6 g/kg alcohol was consumed daily. Control animals received identical treatment, except that both bottles contained water. Rats were perfused and brain sections were processed for ICC procedures.

RESULTS

Alcohol drinking resulted in a 51% increase in Cdk5 IR cholinergic interneurons in the shell NuAcc, while in the PFC there was a 51% decrease in the percent of Cdk5 IR cholinergic interneurons in the infralimbic region and a 46% decrease in Cdk5 IR cholinergic interneurons in the prelimbic region. Additionally, single Cdk5 IR revealed a 42% increase in the central nucleus of the amygdala (CNA).

CONCLUSIONS

This study identified Cdk5 neuroadaptation in cholinergic interneurons of the NuAcc and PFC and in other neurons of the CNA following 1 month of alcohol drinking. These findings contribute to our understanding of the cellular and molecular basis of alcohol drinking and toward the development of improved region and cell-specific pharmacotherapeutic and behavioral treatment programs for alcohol abuse and alcoholism.

Specificity in the projections of prefrontal and insular cortex to ventral striatopallidum and the extended amygdala

The basal forebrain functional-anatomical macrosystems, ventral striatopallidum, and extended amygdala are innervated by substantially coextensive distributions of neurons in the prefrontal and insular cortex. This suggests two alternative organizational schemes: convergent, in which a given cortical area projects exclusively to only one of these macrosystems and divergent, in which a given cortical area innervates both forebrain macrosystems. To examine the underlying organization and possibly discriminate between these alternatives, rats were injected with two retrograde tracers in different parts of ventral striatopallidum or extended amygdala (homotypic injection pairs) or with one tracer in each macrosystem (heterotypic). The prefrontal and insular cortex was evaluated microscopically for overlap of retrograde labeling and double labeling of neurons. Homotypic injection pairs in the ventral striatum and extended amygdala produced extensive overlap of retrogradely labeled neurons and significant double labeling, suggesting that cortical projections spread broadly within macrosystems. In contrast, heterotypic injection pairs produced significant overlap of retrograde labeling but negligible double labeling, indicating that ventral striatopallidum and extended amygdala receive inputs from separate sets of prefronto- and insular cortical neurons. The caudomedial shell of the nucleus accumbens, a supposed "transition" zone between striatopallidum and extended amygdala, had extended amygdala-like afferents but produced few double-labeled neurons and these only when paired with ventral striatopallidum. The data suggest that a modular organization of the basal forebrain, with postulated independent information processing by the ventral striatopallidal and extended amygdala macrosystems, is reflected in a corresponding segregation of output neurons in the prefrontal and insular cortices.

The functional divide for primary reinforcement of D-amphetamine lies between the medial and lateral ventral striatum: is the division of the accumbens core, shell, and olfactory tubercle valid?

When projection analyses placed the nucleus accumbens and olfactory tubercle in the striatal system, functional links between these sites began to emerge. The accumbens has been implicated in the rewarding effects of psychomotor stimulants, whereas recent work suggests that the medial accumbens shell and medial olfactory tubercle mediate the rewarding effects of cocaine. Interestingly, anatomical evidence suggests that medial portions of the shell and tubercle receive afferents from common zones in a number of regions. Here, we report results suggesting that the current division of the ventral striatum into the accumbens core and shell and the olfactory tubercle does not reflect the functional organization for amphetamine reward. Rats quickly learned to self-administer D-amphetamine into the medial shell or medial tubercle, whereas they failed to learn to do so into the accumbens core, ventral shell, or lateral tubercle. Our results suggest that primary reinforcement of amphetamine is mediated via the medial portion of the ventral striatum. Thus, the medial shell and medial tubercle are more functionally related than the medial and ventral shell or the medial and lateral tubercle. The current core-shell-tubercle scheme should be reconsidered in light of recent anatomical data and these functional findings.

Dopamine D3 receptor modulation of dopamine efflux in the rat nucleus accumbens

The effect of antipsychotics on electrically evoked dopamine efflux in the rat nucleus accumbens core and shell was investigated, using in vitro fast cyclic voltammetry. In the nucleus accumbens core, the dopamine D2/D3 receptor agonist, (+/-)7-OH-DPAT ((+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene), inhibited dopamine efflux with a pEC50 of 8.1. Clozapine, haloperidol, sulpiride and the selective dopamine D3 receptor antagonist, SB-277011-A, had no effect on dopamine efflux per se but all attenuated the (+/-)7-OH-DPAT-induced-inhibition of dopamine efflux, with pA2 values of 6.6, 7.9, 7.0 and 7.6, respectively. In the nucleus accumbens shell, (+/-)7-OH-DPAT inhibited dopamine efflux with a pEC50 of 8.3. Clozapine and SB-277011-A had no effect on dopamine efflux. In contrast, haloperidol and sulpiride significantly increased dopamine efflux through a D2 receptor-mediated mechanism. Clozapine, haloperidol, sulpiride and SB-277011-A attenuated the (+/-)7-OH-DPAT-induced inhibition with pA2 values of 7.3, 8.6, 7.6 and 8.2, respectively. These data demonstrate that dopamine efflux is modulated by both dopamine D2 and D3 receptors in the rat nucleus accumbens.

Behavioral models of impulsivity in relation to ADHD: translation between clinical and preclinical studies

Impulsivity, broadly defined as action without foresight, is a component of numerous psychiatric illnesses including attention deficit/hyperactivity disorder (ADHD), mania and substance abuse. In order to investigate the mechanisms underpinning impulsive behavior, the nature of impulsivity itself needs to be defined in operational terms that can be used as the basis for empirical investigation. Due to the range of behaviors that the term impulsivity describes, it has been suggested that impulsivity is not a unitary construct, but encompasses a variety of related phenomena that may differ in their biological basis. Through fractionating impulsivity into these component parts, it has proved possible to devise different behavioral paradigms to measure various aspects of impulsivity in both humans and laboratory animals. This review describes and evaluates some of the current behavioral models of impulsivity developed for use with rodents based on human neuropsychological tests, focusing on the five-choice serial reaction time task, the stop-signal reaction time task and delay-discounting paradigms. Furthermore, the contributions made by preclinical studies using such methodology to improve our understanding of the neural and neurochemical basis of impulsivity and ADHD are discussed, with particular reference to the involvement of both the serotonergic and dopaminergic systems, and frontostriatal circuitry.

Ontogeny of the dopamine innervation in the nucleus accumbens of gerbils

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

Preference for cocaine- versus pup-associated cues differentially activates neurons expressing either Fos or cocaine- and amphetamine-regulated transcript in lactating, maternal rodents

We studied the neuronal basis of the motivational response to two powerful but radically different rewards-cocaine and maternal nurturing of pups in the postpartum rat (dam) which is in a unique motivational state. We used a place preference method designed to offer a choice between cues associated with a natural reinforcer (pups) and those associated with a pharmacologic reinforcer (cocaine). Using c-Fos or cocaine- and amphetamine-regulated transcript (CART) immunocytochemistry, we identified the neuronal groups that are activated when the dams expressed a preference for either cues-associated with pups or cues-associated with cocaine. Dams that preferred the cocaine-associated cues had more c-Fos positive neurons in medial prefrontal cortex, nucleus accumbens, and basolateral nucleus of amygdala than pup-associated cue preferring dams or control. Except for the accumbens, there was activation of neurons in these same regions with the pup-associated cue preference. In the nucleus accumbens only CART-immunoreactive (not c-Fos) neurons were activated with pup-cue preference. Notably, the medial preoptic area was the single area where greater activation of neurons was seen with a preference for pup-associated versus cocaine-associated cues. These responses were identified in the absence of the stimuli (cocaine or pups) and are proposed to be, in part, activation of these neurons related to motivational processing. Neither the distribution of neurons responding to pup-associated cue preference nor the demonstration that CART-expressing neurons are responsive to reward-associated cue preference has been previously reported. We hypothesize that the expression of preference for cocaine versus pup-associated cues is made possible by the concerted activity of these regionally distributed networks of neurons that are in part specific to the preference response.

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

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

Comparative analysis of the subcellular and subsynaptic localization of mGluR1a and mGluR5 metabotropic glutamate receptors in the shell and core of the nucleus accumbens in rat and monkey

Group I metabotropic glutamate receptors (mGluRs) play critical roles in synaptic plasticity and drug addiction. To characterize potential sites whereby these receptors mediate their effects in the ventral striatum, we studied the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the shell and core of the nucleus accumbens in rat and monkey. In both species, group I mGluRs are mainly postsynaptic in dendrites and spines, with rare presynaptic labeling in unmyelinated axons. Minor, yet significant, differences in proportions of specific immunoreactive elements were found between the accumbens shell and the accumbens core in monkey. At the subsynaptic level, significant differences were found in the proportion of plasma membrane-bound mGluR5 labeling between species. In dendrites, spines, and unmyelinated axons, a significantly larger proportion of mGluR5 labeling was bound to the plasma membrane in rats (50-70%) than in monkeys (30-50%). Conversely, mGluR1a displayed the same pattern of immunogold labeling in the two species. Electron microscopic colocalization studies revealed 30% colocalization of mGluR1a and mGluR5 in dendrites and as much as 50-65% in spines in both compartments of the rat accumbens. Both group I mGluRs were significantly expressed in D1-immunoreactive dendritic processes (60-75% colocalization) and spines (30-50%) of striatal projection neurons as well as dendrites of cholinergic (30-70%) and parvalbumin-containing (70-85%) interneurons. These findings highlight the widespread expression of group I mGluRs in projection neurons and interneurons of the shell and core of the nucleus accumbens, providing a solid foundation for regulatory and therapeutic functions of group I mGluRs in reward-related behaviors and drug addiction.

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

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

The limbic lobe and its output channels: implications for emotional functions and adaptive behavior

Current dissatisfaction with the limbic system concept reflects a desire to move beyond the limbic system in efforts to explain key facets of emotional functions and motivational behavior. This review promotes an anatomical viewpoint, which originated as a result of histotechnical advances. These improvements paved the way for anatomical discoveries, which in turn led to the concepts of the ventral striatopallidal system and extended amygdala. These two systems, together with the basal nucleus of Meynert and the septum-diagonal band system, serve as output channels for an expanded version of the classic limbic lobe of Broca, which contains all non-isocortical parts of the cortical mantle together with the large laterobasal-cortical amygdaloid complex. Thus defined, the limbic lobe contains all of the major cortical (e.g. orbitofrontal, cingulate and insular cortices in addition to the hippocampal formation) and cortical-like (laterobasal-cortical amygdala) structures known to be especially important for emotional and motivational functions. In their role as output channels for the limbic lobe, the basal forebrain functional-anatomical systems contribute to the establishment of a number of cortico-subcortical circuits, which provide an important part of the anatomical substrate for the elaboration of emotional functions and adaptive behavior.

The evolving theory of basal forebrain functional-anatomical 'macrosystems'

The conceptual basis and continuing development of Alheid and Heimer's [Alheid, G.F., Heimer, L., 1988. New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid and corticopetal components of substantia innominata. Neuroscience 27, 1-39] theory of basal forebrain organization based on the description of basal forebrain functional-anatomical 'macrosytems' is reviewed. It is posed that the macrosystem theory leads to a hypothesis that different macrosystems cooperate and compete to exert distinct influences on motor and cognitive function. Emergent corollaries include, e.g. that the organization of the outputs of different macrosystems should differ. Consistent with these considerations, extant literature and some unpublished data indicate that the input nuclei of macrosystems are not abundantly interconnected and macrosystems systems have distinct neuroanatomical relationships with basal forebrain and brainstem cholinergic and dopaminergic ascending modulatory systems. Furthermore, macrosystem outputs appear to be directed almost exclusively at the reticular formation or structures intimately associated with it. The relative merits of the theory of functional-anatomical macrosystems are discussed in relation to Swanson's model of cerebral hemisphere control of motivated behavior.

Cholecystokinin inhibits evoked inhibitory postsynaptic currents in the rat nucleus accumbens indirectly through gamma-aminobutyric acid and gamma-aminobutyric acid type B receptors

We recently reported that cholecystokinin (CCK) excited nucleus accumbens (NAc) cells and depressed excitatory synaptic transmission indirectly through gamma-aminobutyric acid (GABA), acting on presynaptic GABAB receptors (Kombian et al. [2004] J. Physiol. 555:71-84). The present study tested the hypothesis that CCK modulates inhibitory synaptic transmission in the NAc. Using in vitro forebrain slices containing the NAc and whole-cell patch recording, we examined the effects of CCK on evoked inhibitory postsynaptic currents (IPSCs) recorded at a holding potential of -80 mV throughout CCK-8S caused a reversible inward current accompanied by a concentration-dependent decrease in evoked IPSC amplitude. Maximum IPSC depression was approximately 25% at 10 microM, with an estimated EC50 of 0.1 microM. At 1 microM, CCK-8S induced an inward current of 28.3 +/- 4.8 pA (n=6) accompanied by an IPSC depression of -18.8% +/- 1.6% (n=6). This CCK-induced IPSC depression was blocked by pretreatment with proglumide (100 microM; -3.7% +/- 6.9%; n=4) and by LY225910 (100 nM), a selective CCKB receptor antagonist (4.4% +/- 2.6%; n=4). It was not blocked by SCH23390 (10 microM; -23.5% +/- 1.3%; P < 0.05; n=7) or sulpiride (10 microM; -21.8% +/- 5.1%; P <0.05; n=4), dopamine receptor antagonists. By contrast, it was blocked by CGP55845 (1 microM; -0.4% +/- 3.4%; n=5) a potent GABAB receptor antagonist, and by forskolin (50 microM; 9.9% +/- 5.2%; n=4), an adenylyl cyclase activator, and H-89 (1 microM; 6.9% +/- 3.9%; n=4), a protein kinase A (PKA) inhibitor. These results indicate that CCK acts on CCKB receptors to increase extracellular levels of GABA, which then acts on GABAB receptors to decrease IPSC amplitude.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

An extrahippocampal projection from the dentate gyrus to the olfactory tubercle

BACKGROUND

The dentate gyrus is well known for its mossy fiber projection to the hippocampal field 3 (CA3) and its extensive associational and commissural connections. The dentate gyrus, on the other hand, has only few projections to the CA1 and the subiculum, and none have clearly been shown to extrahippocampal target regions.

RESULTS

Using anterograde and retrograde tracer techniques in the Madagascan lesser hedgehog tenrec (Afrosoricidae, Afrotheria) it was shown in this study that the dentate hilar region gave rise to a faint, but distinct, bilateral projection to the most rostromedial portion of the olfactory tubercle, particularly its molecular layer. Unlike the CA1 and the subiculum the dentate gyrus did not project to the accumbens nucleus. A control injection into the medial septum-diagonal band complex also retrogradely labeled cells in the dentate hilus, but these neurons were found immediately adjacent to the heavily labeled CA3, while the tracer injections into the rostromedial tubercle did not reveal any labeling in CA3.

CONCLUSION

The dentate hilar neurons projecting to the olfactory tubercle cannot be considered displaced cells of CA3 but represent true dentato-tubercular projection neurons. This projection supplements the subiculo-tubercular projection. Both terminal fields overlap among one another as well as with the fiber terminations arising in the anteromedial frontal cortex. The rostromedial olfactory tubercle might represent a distinct ventral striatal target area worth investigating in studies of the parallel processing of cortico-limbic information in tenrec as well as in cat and monkey.

The striatum in the hedgehog tenrec: histochemical organization and cortical afferents

In order to get insight into the striopallidal organization in mammals with little differentiated brain the striatum of the lesser hedgehog tenrec (Afrotheria) was characterized histochemically and analysed with regard to its cortical afferents using axonal tracer substances. The majority of neocortical cells projecting to the striatum were found bilaterally in the layers 2 and 3 of the frontal hemisphere; caudalwards the relative number of cells increased somewhat in the upper layer 5. There was a topographical organization as far as the allocortical projections appeared confined to the ventral striatum, and the efferents from hippocampal, posterior paleocortical, somatosensory and audiovisual areas were distributed in largely different striatal territories. Projections from the anterior frontal cortex, on the other hand, terminated extensively upon the caudate-putamen and also involved the nucleus accumbens and the olfactory tubercle. In the latter region the molecular layer was especially involved. The entorhinal cortex also projected heavily to the olfactory tubercle but unlike other species it scarcely involved the nucleus accumbens. The cortical fibers were distributed in a relatively homogenous fashion within their striatal territory and there was little evidence for patches of high density terminations. Islands of low density labeling, however, were noted occasionally in the caudate-putamen. These islands were partly similar in size as the patches of neuropil staining obtained with anti-calretinin and anti-substance P. There were also hints for the presence of a shell-like region in the nucleus accumbens stained with anti-dopamine transporter and NADPh-diaphorase. The classical striosome-matrix markers such as calbindin, acetylcholinesterase and enkephalin, however, failed to reveal any compartmental organization.

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

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

Neurokinin B-producing projection neurons in the lateral stripe of the striatum and cell clusters of the accumbens nucleus in the rat

Neurons producing preprotachykinin B (PPTB), the precursor of neurokinin B, constitute 5% of neurons in the dorsal striatum and project to the substantia innominata (SI) selectively. In the ventral striatum, PPTB-producing neurons are collected mainly in the lateral stripe of the striatum (LSS) and cell clusters of the accumbens nucleus (Acb). In the present study, we first examined the distribution of PPTB-immunoreactive neurons in rat ventral striatum and found that a large part of the PPTB-immunoreactive cell clusters was continuous to the LSS, but a smaller part was not. Thus, we divided the PPTB-immunoreactive cell clusters into the LSS-associated and non-LSS-associated ones. We next investigated the projection targets of the PPTB-producing ventral striatal neurons by combining immunofluorescence labeling and retrograde tracing. After injection of Fluoro-Gold into the basal component of the SI (SIb) and medial part of the interstitial nucleus of posterior limb of the anterior commissure, many PPTB-immunoreactive neurons were retrogradely labeled in the LSS-associated cell clusters and LSS, respectively. When the injection site included the ventral part of the sublenticular component of the SI(SIsl), retrogradely labeled neurons showed PPTB-immunoreactivity frequently in non-LSS-associated cell clusters. Furthermore, these PPTB-immunoreactive projections were confirmed by the double-fluorescence method after anterograde tracer injection into the ventral striatum containing the cell clusters. Since the dorsalmost part of the SIsl is known to receive strong inputs from PPTB-producing dorsal striatal neurons, the present results indicate that PPTB-producing ventral striatal neurons project to basal forebrain target regions in parallel with dorsal striatal neurons without significant convergence.

Three-dimensional chemoarchitecture of the basal forebrain: spatially specific association of cholinergic and calcium binding protein-containing neurons

The basal forebrain refers to heterogeneous structures located close to the medial and ventral surfaces of the cerebral hemispheres. It contains diverse populations of neurons, including the cholinergic cortically projecting cells that show severe loss in Alzheimer's and related neurodegenerative diseases. The basal forebrain does not display any cytoarchitectural or other structural features that make it easy to demarcate functional boundaries, a problem that allowed different investigators to propose different organizational schemes. The present paper uses novel three-dimensional reconstructions and numerical analyses for studying the spatial organization of four major basal forebrain cell populations, the cholinergic, parvalbumin, calbindin and calretinin containing neurons in the rat. Our analyses suggest that the distribution of these four cell populations is not random but displays a general pattern of association. Within the cholinergic space (i.e. the volume occupied by the cortically projecting cholinergic cell bodies) the three other cell types form twisted bands along the longitudinal axis of a central dense core of cholinergic cells traversing the traditionally defined basal forebrain regions, (i.e. the medial septum, diagonal bands, the substantia innominata, pallidal regions and the bed nucleus of the stria terminalis). At a smaller scale, the different cell types within the cholinergic space occupy overlapping high-density cell clusters that are either chemically uniform or mixed. However, the cell composition of these high-density clusters is regionally specific. The proposed scheme of basal forebrain organization, using cell density or density relations as criteria, offers a new perspective on structure-function relationship, unconstrained by traditional region boundaries.

Activation of brain areas in rat following warm and cold ambient exposure

Environmental thermal stimuli result in specific and coordinated thermoregulatory response in homeothermic animals. Warm exposure activates numerous brain areas within the cortex, hypothalamus, pons and medulla oblongata. We identified these thermosensitive cell groups in the medulla and pons that were suggested but not outlined by previous physiological studies. Using Fos immunohistochemistry, we localized all the nuclei and cell groups in the rat brain that were activated by warm and cold ambient exposure. These neurons located in the hypothalamus and the brainstem, are part of a network responsible for the thermospecific response elicited by thermal stress. Comparison of the distribution of Fos-immunoreactive cells throughout the rat brain revealed topographical differences between the patterns of activated cells following warm and cold environmental exposure. Among several brain regions, warm exposure elicited c-fos expression specifically in the ventrolateral part of the medial preoptic area, the central subdivision of the lateral parabrachial nucleus and the caudal part of the peritrigeminal nucleus, whereas cold stress resulted in c-fos expression in the ventromedial part of the medial preoptic area, the external subdivision of the lateral parabrachial nucleus and the rostral part of the peritrigeminal nucleus. These neurons are part of a network coordinating specific response to warm or cold exposure. The topographical differences suggest that well-defined cell groups and subdivisions of nuclei are responsible for the specific physiological (endocrine, autonomic and behavioral) changes observed in different thermal environment.

Putting a spin on the dorsal-ventral divide of the striatum

Since its conception three decades ago, the idea that the striatum consists of a dorsal sensorimotor part and a ventral portion processing limbic information has sparked a quest for functional correlates and anatomical characteristics of the striatal divisions. But this classic dorsal-ventral distinction might not offer the best view of striatal function. Anatomy and neurophysiology show that the two striatal areas have the same basic structure and that sharp boundaries are absent. Behaviorally, a distinction between dorsolateral and ventromedial seems most valid, in accordance with a mediolateral functional zonation imposed on the striatum by its excitatory cortical, thalamic and amygdaloid inputs. Therefore, this review presents a synthesis between the dorsal-ventral distinction and the more mediolateral-oriented functional striatal gradient.

Selective action of (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268), a group II metabotropic glutamate receptor agonist, on basal and phencyclidine-induced dopamine release in the nucleus accumbens shell

The effect of the group II metabotropic receptor agonist (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268), on basal and phencyclidine-induced dopamine efflux were measured in the shell and core subdivisions of the nucleus accumbens--regions which are associated with limbic and motor functions, respectively. Extracellular levels of dopamine were measured using microdialysis in conscious animals, and LY379268 was delivered locally by inclusion in the artificial cerebrospinal fluid (aCSF) flowing through the microdialysis probe. Local administration of LY379268 in the concentration range 10 nM-10 microM reduced basal levels of dopamine in the nucleus accumbens shell, whilst having no effect in the nucleus accumbens core. In the nucleus accumbens shell, basal levels were reduced to approximately 60% compared to the pre-injection control, with a maximal reduction occurring at concentrations of LY379268 > or =100 nM. The response to LY379268 (100 nM) was reversible, with levels returning to baseline following its removal from the aCSF. In a separate experiment, local perfusion of the nucleus accumbens shell with LY379268 (at both 1 and 10 microM) reduced the magnitude of the response to a subsequent systemic administration of phencyclidine (5 mg/kg i.p.). The reduction in the peak dopamine response was only evident with doses of LY379268 that also reduced basal dopamine efflux--LY379268 being ineffective against PCP at 10 nM. However, in animals pre-treated with LY379268 at 1 or 10 microM, PCP still evoked a dopamine response, and in these animals the relative extent of the response was not significantly different between the respective treatment groups. In contrast, in the nucleus accumbens core the magnitude of the dopamine response to PCP was unaffected by local application of LY379268 (at 1 or 10 microM). Our data suggest that within the nucleus accumbens, there exists a distinct regional difference in the control of dopamine release by group II mGluRs, with the nucleus accumbens shell being preferentially affected. Moreover, the selective action of LY379268 on dopamine levels in the nucleus accumbens shell may have implications for the potential antipsychotic activity of group II mGluR agonists.

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.

The primate basal ganglia: parallel and integrative networks

The basal ganglia and frontal cortex operate together to execute goal directed behaviors. This requires not only the execution of motor plans, but also the behaviors that lead to this execution, including emotions and motivation that drive behaviors, cognition that organizes and plans the general strategy, motor planning, and finally, the execution of that plan. The components of the frontal cortex that mediate these behaviors, are reflected in the organization, physiology, and connections between areas of frontal cortex and in their projections through basal ganglia circuits. This comprises a series of parallel pathways. However, this model does not address how information flows between circuits thereby developing new learned behaviors (or actions) from a combination of inputs from emotional, cognitive, and motor cortical areas. Recent anatomical evidence from primates demonstrates that the neuro-networks within basal ganglia pathways are in a position to move information across functional circuits. Two networks are: the striato-nigral-striatal network and the thalamo-cortical-thalamic network. Within each of these sets of connected structures, there are both reciprocal connections linking up regions associated with similar functions and non-reciprocal connections linking up regions that are associated with different cortical basal ganglia circuits. Each component of information (from limbic to motor outcome) sends both feedback connection, and also a feedforward connection, allowing the transfer of information. Information is channeled from limbic, to cognitive, to motor circuits. Action decision-making processes are thus influenced by motivation and cognitive inputs, allowing the animal to respond appropriate to environmental cues.

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague-Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.

Very mild stress of pregnant rats reduces volume and cell number in nucleus accumbens of adult offspring: some parallels to schizophrenia

Pregnant rats subjected to very mild stress give birth to pups who, when examined as adults, exhibited behavioral and anatomical anomalies that resemble some aspects of schizophrenia. The nucleus accumbens (NAcc) is reduced in volume by 20.7 +/- 3.4% (p = 0.003) in pups born to mothers who were stressed during pregnancy by injections of either saline or amphetamine in saline. The total number of cells is decreased in proportion to the reduction in volume, so the volume cell density of the NAcc is not changed with treatment. The affected volume is localized in the ventral rostral area of the NAcc. Both males and females are affected, but males are slightly more sensitive to the challenge to the mother. Rats born to mothers stressed in mid-pregnancy appear to provide useful parallels to the fetal developmental hypothesis of schizophrenia and to the brain abnormalities seen in this disease.

Distribution and colocalization of cholecystokinin with the prohormone convertase enzymes PC1, PC2, and PC5 in rat brain

During posttranslational processing to generate CCK 8, pro-cholecystokinin (CCK) undergoes endoproteolytic cleavage at three sites. Several studies using endocrine and neuronal tumor cells in culture and recombinant enzymes and synthetic substrates in vitro have pointed to the subtilisin/kexin-like enzymes prohormone convertase (PC) 1, PC2, and PC5 as potential candidates for these endoproteolytic cleavages. In these experimental models, they all appear to be able to cleave pro-CCK to make the correct products. One rodent model has provided information about the role of PC2. PC2 knockout mouse brains had less CCK 8 than wild-type, although a substantial amount of CCK was still present. The degree to which CCK levels were reduced in these mice was regionally specific. These data indicated that PC2 is important for normal production of CCK but that it is not the only endoprotease that is involved in CCK processing. To evaluate whether PC1 and PC5 are possible candidates for the other enzymes involved in CCK processing, the distribution of PC1, PC2, and PC5 mRNA was studied in rat brain. Their colocalization with CCK mRNA was examined using double-label in situ hybridization. PC2 was the most abundant of these enzymes in terms of the intensity and number of cells labeled. It was widely colocalized with CCK. PC1 and PC5 mRNA-positive cells were less abundant, but they were also widely distributed and strongly colocalized with CCK in the cerebral cortex, hippocampus, amygdala, ventral tegmental area, and substantia nigra zona compacta. The degree of colocalization of the enzymes with CCK was regionally specific. It is clear that PC1 and PC5 are extensively colocalized with CCK and could be participating in CCK processing in the rat brain and may be able to substitute for PC2 in its absence. These three enzymes may represent a redundant system to ensure production of biologically active CCK.

Cholecystokinin activates CCKB receptors to excite cells and depress EPSCs in the rat rostral nucleus accumbens in vitro

The peptide cholecystokinin (CCK) is abundant in the rat nucleus accumbens (NAc). Although it is colocalized with dopamine (DA) in afferent terminals in this region, neurochemical and behavioural reports are equally divided as to whether CCK enhances or diminishes DA's actions in this nucleus. To better understand the role of this peptide in the physiology of the NAc, we examined the effects of CCK on excitatory synaptic transmission and tested whether these are dependent on DA and/or other neuromodulators. Using whole-cell recording in rat forebrain slices containing the NAc, we show that sulphated CCK octapeptide (CCK-8S), the endogenously active neuropeptide, consistently depolarized cells and depressed evoked excitatory postsynaptic currents (EPSCs) in the rostral NAc. It caused a reversible, dose-dependent decrease in evoked EPSC amplitude that was accompanied by an increase in the decay constant of the EPSC but with no apparent change in paired pulse ratio. It was mimicked by unsulphated CCK-8 (CCK-8US), a CCK(B) receptor-selective agonist, and blocked by LY225910, a CCK(B) receptor-selective antagonist. Both CCK-8S and CCK-8US induced an inward current with a reversal potential around -90 mV that was accompanied by an increase in input resistance and action potential firing. The CCK-8S-induced EPSC depression was slightly reduced in the presence of SCH23390 but not in the presence of sulpiride or 8-cyclopentyltheophylline. By contrast, it was completely blocked by CGP55845, a potent GABA(B) receptor-selective antagonist. These results indicate that CCK excites NAc cells directly while depressing evoked EPSCs indirectly, mainly through the release of GABA.

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.

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

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

The legacy of the silver methods and the new anatomy of the basal forebrain: implications for neuropsychiatry and drug abuse

The first part of the paper highlights the remarkable legacy of the silver methods, with special emphasis on the travails and opportunities offered by the various Nauta methods and their modifications. When the tracer methods based on axoplasmic flow were introduced in the early 1970s, they were exploited on a backdrop of a basic anatomical framework, which had already been established through the tracing of the major CNS pathways by the aid of the silver methods, especially the widely used Nauta-Gygax methods and their modifications. Some of the silver methods that were developed in the late 1960s for the staining of degenerating boutons (e.g. the Fink-Heimer method and de Olmos cupric silver method) provided the necessary technical improvements that eventually led to a new and more productive way to look at the basal forebrain functional/anatomical organization; if it was not for the silver methods, we would in all likelihood still be promoting the nebulous notion of the substantia innominata rather than the concepts of the ventral striatopallidal system and the extended amygdala. The discovery and elaboration of these two macroanatomical systems symbolize what might deservedly be called the "new anatomy" of the basal forebrain. Following a review of the critical experiments which led to the development of the new anatomy of the basal forebrain, its topography in the human is reviewed in drawings of an abbreviated series of coronal sections. The discovery of the ventral striatopallidal system and its thalamic projection to the mediodorsal thalamus rather than to the ventral anterior-ventral lateral thalamic complex ushered in the idea of parallel cortico-subcortical reentrant circuits, which to a large extent has replaced the limbic system as a theoretical framework for neuropsychiatric disorders. The extended amygdala, which appears as a large ring formation around the internal capsule, is still controversial in some quarters, although it is slowly but surely making its way into the general neuroscience literature, especially in the field of addictive disorders. The ventral striatopallidal system and the extended amygdala are interwoven in a complex fashion with the basal nucleus of Meynert within the basal forebrain. Together, these three systems represent important output channels for so-called "limbic" forebrain regions, especially orbitomedial prefrontal cortex and medial temporal lobe structures, which are increasingly implicated in major neuropsychiatric disorders.

What we know and what we need to know about the role of endogenous CCK in psychostimulant sensitization

The unique distribution of CCK and its receptors and its co-localization with dopamine makes it ideally situated to pay a role in dopamine-mediated reward and psychostimulant sensitization. A number of studies support the hypothesis that CCK acting through the CCK 1 and CCK 2 receptors is an endogenous modulator of dopamine neurotransmission. Behavioral studies with CCK antagonists and CCK 1 receptor mutant rats support a role for endogenous CCK in behavioral sensitization to psychostimulants. CCK microdialysis studies in the nucleus accumbens (NAC) have demonstrated that extracellular CCK is increased in the NAC by psychostimulants, providing neurochemical evidence that CCK could be involved in the behavioral response to psychostimulants. A model for how CCK may be acting in multiple brain regions to foster sensitization is presented and the gaps in our knowledge about the role of CCK in psychostimulant sensitization are described.

Immunohistochemical characterization of cholecystokinin containing neurons in the rat basolateral amygdala

Specific neuronal populations in the basolateral amygdala (ABL) exhibit immunoreactivity for distinct neuropeptides and calcium-binding proteins. In the present study, immunohistochemical techniques were used to analyze neurons in the rat ABL that contain cholecystokinin (CCK). Some pyramidal projection neurons in the anterior subdivision of the basolateral nucleus exhibited low levels of CCK immunoreactivity in rats that received injections of colchicine to interrupt axonal transport; staining was concentrated in the axon initial segments of these cells. High levels of CCK immunoreactivity were observed in two subpopulations of nonpyramidal interneurons in all nuclei of the ABL: (1) type L neurons (characterized by large somata and thick dendrites), and (2) type S neurons (characterized by small somata and thin dendrites). Dual-labeling immunofluorescence studies using confocal laser scanning microscopy revealed that many (30-40%) type L CCK+ interneurons exhibited immunoreactivity for calbindin (CB), but not for parvalbumin (PV), calretinin (CR), or vasoactive intestinal polypeptide (VIP). In contrast, there was extensive colocalization of CR and VIP with CCK in type S neurons, but no significant colocalization with CB or PV. In addition, the majority of CR and VIP interneurons exhibited colocalization of both neurochemicals. Collectively, the results of this and previous studies indicate that there are at least four distinct interneuronal subpopulations in the ABL: (1) PV+ neurons (the great majority of which are CB+); (2) SOM+ neurons (many of which are CB+ and NPY+); (3) large CCK+ neurons (some of which are CB+); and (4) small bipolar/bitufted neurons that exhibit various amounts of colocalization of CCK, VIP, and CR.

Glutamate motivational ensembles in nucleus accumbens: rostrocaudal shell gradients of fear and feeding

This study demonstrates that microinjection of an AMPA/kainate glutamate antagonist elicits motivated fear and feeding behaviour mapped along rostrocaudal gradients of positive-to-negative valence in nucleus accumbens shell (similar to rostrocaudal shell gradients recently reported for GABA agonist microinjections). Rats received rostral or caudal microinjections of the glutamate AMPA/kainate receptor antagonist DNQX (0, 50, 450 or 850 ng in 0.5 micro L) or the NMDA receptor antagonist MK-801 (0, 0.5, 1 or 2 micro g in 0.5 micro L), into medial accumbens shell prior to behavioural tests for fear, feeding or conditioning of place preference or avoidance. Another group received rostral or caudal microinjections of DNQX in nucleus accumbens core. Rostral shell DNQX microinjections potently increased appetitive food intake and established only weak conditioned place avoidance. Caudal shell DNQX microinjections elicited defensive treading behaviour, caused rats to defensively bite the experimenter and emit fearful distress vocalizations when handled, and established strong conditioned place avoidance. By contrast, no rostrocaudal gradients of motivational bivalence were produced by microinjections of the glutamate AMPA/kainate receptor antagonist DNQX into the core, or by microinjections of the NMDA antagonist MK-801 into the shell. Our results indicate that appetitive and aversive motivation is carried in anatomically differentiated channels by mesocorticolimbic glutamate signals to microcircuits in the medial shell. Hyperpolarization of local shell ensembles by AMPA/kainate glutamate receptor blockade elicits fear and feeding behaviours mapped along distinct positive-to-negative rostrocaudal gradients.

Extended amygdala and basal forebrain

The basal forebrain is a confluence of systems that are crucial to understanding some of the most important functions of the brain, including reward and punishment, learning and cognition, and feeding and reproduction. Basic to understanding this broad spectrum of behavior is untangling the interwoven functional systems in basal forebrain. This has been grounded by the appreciation that the major nearby structures, that is, amygdala and basal ganglia, provide a context for interpreting basal forebrain areas that are best viewed as extensions of either of these larger regions. The components of basal forebrain, the ventral striatopallidal system and the medial and central divisions of extended amygdala, are subcortical relays for information garnered from brain stem, thalamus, and cortical areas. With respect to the classically defined amygdala of the temporal lobe, the lateral-basolateral complex, and the superficial amygdaloid nuclei may tentatively be viewed as specialized cortical regions. Their output targets both the striatopallidal complex and the extended amygdala, with some of the most massive basal forebrain efferents originating in the basolateral amygdaloid complex. The subcortical projections of the basolateral nucleus, at least in the rat, appear to be dichotomous, with anterior (or magnocellular) portions of the nucleus preferentially targeting striatum and ventral striatum (including the core of the nucleus accumbens), while the posterior (small-celled) portions of the basolateral nucleus target the extended amygdala as well as the shell of the nucleus accumbens. This divergence represents a particular opportunity for behavioral neuroscientists analyzing basal forebrain functions. Studies exploiting the dual subcortical projection of basolateral amygdala indicate distinct functional roles for striatum versus extended amygdala. These reinforce the identification of extended amygdala as a functional-anatomical entity distinct from the striatopallidal system.

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

The role of the core and the shell subterritories of the nucleus accumbens in conditioned freezing and spatial learning was investigated by means of selective N-methyl-D-aspartate lesions. Shell-lesioned rats showed reduced conditioned freezing to context and a tendency toward reduced freezing to the discrete stimulus compared with controls. However, lesions of the core did not modify the freezing response either to the context or to the discrete stimuli. Although spatial memory, as assessed by a water-maze paradigm, was not disrupted by the lesions, in a 4-arm baited, 4-arm unbaited radial-arm maze paradigm, the shell-lesioned rats showed selective deficits in working memory, but not in reference memory. In contrast, core-lesioned rats showed no memory deficits.

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

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

Ventral striatopallidal oxytocin and vasopressin V1a receptors in the monogamous prairie vole (Microtus ochrogaster)

Oxytocin receptors (OTR) and vasopressin V1a receptors (V1aR) in the ventral forebrain play critical roles in the formation of pair bonds in the monogamous prairie vole. Previous reports have been inconsistent in the identification of the specific brain regions in the ventral forebrain that express these receptors. To delineate more clearly the neuroanatomical boundaries of the OTR and V1aR fields in this species, we compared OTR and V1aR binding in adjacent brain sections and also with markers that delineate neuroanatomical boundaries in the ventral forebrain. OTR binding displayed an overlapping distribution with substance P mRNA and preproenkephalin mRNA, both markers for the shell and core of the nucleus accumbens. V1aR binding was nonoverlapping with each of these markers but colocalized with iron accumulation as shown by Perls' iron stain as well as leucine-enkephalin immunoreactivity, both markers for the ventral pallidum. OTR and V1aR mRNA were also restricted within the nucleus accumbens and ventral pallidum, respectively. Furthermore, destruction of ventral striatal dopaminergic terminals with 6-hydroxydopamine infusions into the nucleus accumbens did not alter OTR binding. Immunocytochemical analysis of oxytocin and vasopressin in the ventral forebrain demonstrated the presence of oxytocin-immunoreactive fibers in the nucleus accumbens and vasopressin-immunoreactive fibers in the ventral pallidum, with males showing a greater density of vasopressin fibers than females, but there was no such sex difference in the oxytocin system. Based on these results, we discuss potential neural mechanisms by which receptors in these brain regions mediate pair bond formation in this monogamous species. J. Comp. Neurol. 468:555-570, 2004.

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.

Some aspects on the anatomy and function of central cholecystokinin systems

The distribution of some cholecystokinin (CCK) systems in the rat brain is reviewed focusing on mesencephalic dopamine neurones which coexpress CCK and, in particular, on cortico-striatal CCK neurones which probably have glutamate as their co-transmitter. Functional studies based on the effect of several CCK(B) antagonists on phencyclidine-induced motility suggest that CCK is involved in locomotor behaviour causing inhibition in phencyclidine-treated habituated rats. In contrast, in unhabituated rats CCK stimulates exploratory behaviour. These effects may be related to the cortico-striatal CCK/glutamatergic pathway. Moreover, these studies provide evidence for endogenous release of a neuropeptide with behavioural consequences.

Comparison of hippocampal, amygdala, and perirhinal projections to the nucleus accumbens: combined anterograde and retrograde tracing study in the Macaque brain

A combination of anterograde and retrograde tracing techniques was used to study the projections to the nucleus accumbens from the amygdala, the hippocampal formation (including the entorhinal cortex), and the perirhinal cortex in two species of macaque monkey. To help identify possible subregions within the nucleus accumbens, the distribution of calbindin was examined in two additional monkeys. Although this revealed evidence of "core"- and "shell"-like regions within the accumbens, these different regions could not consistently be related to cytoarchitectonic features. The rostral amygdala sent nearly equivalent projections to both the medial and the lateral portions of nucleus accumbens, whereas projections arising from the middle and caudal amygdala terminated preferentially in the medial division of nucleus accumbens. The basal nucleus was the major source of these amygdala efferents, and there was a crude topography as parts of the basal and accessory basal nuclei terminated in different parts of nucleus accumbens. The subiculum was the major source of hippocampal projections to the nucleus accumbens, but some hippocampal efferents also originated in the parasubiculum, the prosubiculum, the adjacent portion of CA1, and the uncal portion of CA3. These hippocampal projections, which coursed through the fornix, showed a rostrocaudal gradient as more arose in the rostral hippocampus. Hippocampal efferents terminated most densely in the medial and ventral portions of nucleus accumbens, along with light label in the adjacent olfactory tubercle. The entorhinal projections were more evenly distributed between the medial nucleus accumbens and the olfactory tubercle, whereas the perirhinal projections were primarily to the olfactory tubercle. These cortical inputs were less reliant on the fornix. Amygdala and subicular (hippocampal) projections overlapped most completely in the medial division of nucleus accumbens.

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

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

Differential modulation of nucleus accumbens synapses

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

Cocaine treatment increases extracellular cholecystokinin (CCK) in the nucleus accumbens shell of awake, freely moving rats, an effect that is enhanced in rats that are behaviorally sensitized to cocaine

Cholecystokinin (CCK) is co-localized with dopamine, is known to modulate dopamine neurotransmission and is involved in behavioral sensitization to psychostimulants. To better understand its role, CCK was measured by microdialysis in the nucleus accumbens (NAC) shell in response to cocaine in drug-naive rats and in rats that are behaviorally sensitized to cocaine. Basal extracellular levels of CCK in drug-naive rats were 0.17 pg/20 min fraction, while in cocaine-sensitized rats, they were significantly higher (0.56 pg). Treating drug-naive rats with cocaine caused a significant increase in CCK to 0.58 pg. Cocaine treatment of cocaine-sensitized rats increased CCK to 0.98. When analyzed as a function of time after cocaine treatment, these increases were sustained and were significantly different from CCK levels of saline-treated rats. In cocaine-sensitized rats, CCK levels following cocaine treatment were also significantly higher than levels in drug-naive animals receiving a single injection of cocaine. These results provide evidence for an activation of the mesolimbic and/or cerebral cortical CCK system in response to repeated cocaine administration. These results provide a neurochemical basis for an important role of CCK (via modulation of dopamine neurotransmission) in expression of cocaine sensitization.