Organization of ventral tegmental area projections to the ventral tegmental area-nigral complex in the rat

Distinct contributions of dopamine in the dorsolateral striatum and nucleus accumbens shell to the reinforcing properties of cocaine

Dopaminergic neurotransmission in the dorsal and ventral striatum is thought to be involved in distinct aspects of cocaine addiction. Ventral striatal dopamine mediates the acute reinforcing properties of cocaine, whereas dopamine in the dorsolateral striatum (DLS) is thought to become involved in later stages of the addiction process to mediate well-established cue-controlled drug seeking. However, it is unclear whether the DLS also has a role in the reinforcing properties of cocaine itself. Therefore, we systematically investigated the involvement of dopamine in dorsal and ventral striatal regions in cocaine self-administration, using various schedules of reinforcement in animals with limited drug taking experience. Intra-DLS infusion of the dopamine receptor antagonist α-flupenthixol did not affect the acquisition of cocaine self-administration, increased cocaine self-administration under a fixed ratio-1 (FR-1) schedule of reinforcement, caused a rightward and downward shift of the dose-response curve of cocaine under an FR-1 schedule of reinforcement and decreased responding for cocaine under a progressive ratio (PR) schedule of reinforcement. Infusion of α-flupenthixol into the ventral nucleus accumbens (NAcc) shell inhibited the acquisition of cocaine self-administration, reduced responding for the drug under FR-1 and PR schedules of reinforcement, and caused a downward shift of the dose-response curve of cocaine self-administration under an FR-1 schedule of reinforcement. These data show that dopamine in both the DLS and NAcc shell is involved in cocaine reinforcement. We suggest that the DLS and the NAcc shell mediate somewhat distinct facets of the reinforcing properties of cocaine, related to its rewarding and motivational aspects, respectively.

Roles of D1-like dopamine receptors in the nucleus accumbens and dorsolateral striatum in conditioned avoidance responses

RATIONALE

Aversively motivated learning is more poorly understood than appetitively motivated learning in many aspects, including the role of dopamine receptors in different regions of the striatum.

OBJECTIVES

The present study investigated the roles of the D1-like DA receptors in the nucleus accumbens (NAc) and dorsolateral striatum (DLS) on learning and performance of conditioned avoidance responses (CARs).

METHODS

Adult male Wistar rats received intraperitoneal (i.p.), intra-NAc, or intra-DLS injections of the D1 dopamine receptor agonist SKF 81297 or the D1 receptor antagonist SCH 23390 20 min before or immediately after a training session in the CAR task two-way active avoidance, carried out 24 h before a test session.

RESULTS

Pre-training administration of SCH 23390, but not SKF 81297, caused a significant decrease in the number of CARs in the test, but not in the training session, when injected into the DLS, or in either session when injected into the NAc. It also caused a significant increase in the number of escape failures in the training session when injected into the NAc. Systemic administration caused a combination of these effects. Post-training administrations of these drugs caused no significant effect.

CONCLUSIONS

The results suggest that the D1-like receptors in the NAc and DLS play important, though different, roles in learning and performance of CAR.

Insights to drug addiction derived from ultrastructural views of the mesocorticolimbic system

Drugs of abuse increase the release of dopamine from mesocorticolimbic neurons in the ventral tegmental area. Thus, insights into the cytoarchitecture and the synaptic circuitry affecting the activity of dopaminergic neurons in this area are fundamental for understanding the commonalities produced by mechanistically distinct drugs of abuse. Electron microscopic immunolabeling has provided these insights and also shown the critical relationships between the dopaminergic axon terminals and their targeted neurons in the prefrontal cortex and in the both the dorsal and ventral striatum. These brain regions are among those where dopamine and associated neurotransmitters are most implicated in the transition from recreational to compulsive consumption of reinforcing drugs. Thus, the synaptic circuitry and drug-induced plasticity occurring in the ventral tegmental area and in dopamine-targeted regions are reviewed, as both are essential for understanding the long-lasting changes produced by addictive substances.

Inputs to the midbrain dopaminergic complex in the rat, with emphasis on extended amygdala-recipient sectors

The midbrain dopaminergic neuronal groups A8, A9, A10, and A10dc occupy, respectively, the retrorubral field (RRF), substantia nigra compacta (SNc), ventral tegmental area (VTA), and ventrolateral periaqueductal gray (PAGvl). Collectively, these structures give rise to a mixed dopaminergic and nondopaminergic projection system that essentially permits adaptive behavior. However, knowledge is incomplete regarding how the afferents of these structures are organized. Although the VTA is known to receive numerous afferents from cortex, basal forebrain, and brainstem and the SNc is widely perceived as receiving inputs mainly from the striatum, the afferents of the RRF and PAGvl have yet to be assessed comprehensively. This study was performed to provide an account of those connections and to seek a better understanding of how afferents might contribute to the functional interrelatedness of the VTA, SNc, RRF, and PAGvl. Ventral midbrain structures received injections of retrograde tracer, and the resulting retrogradely labeled structures were targeted with injections of anterogradely transported Phaseolus vulgaris leucoagglutinin. Whereas all injections of retrograde tracer into the VTA, SNc, RRF, or PAGvl produced labeling in many structures extending from the cortex to caudal brainstem, pronounced labeling of structures making up the central division of the extended amygdala occurred following injections that involved the RRF and PAGvl. The anterograde tracing supported this finding, and the combination of retrograde and anterograde labeling data also confirmed reports from other groups indicating that the SNc receives robust input from many of the same structures that innervate the VTA, RRF, and PAGvl.

Discovering a new anatomical structure in the brain: implications for neuropsychiatry and therapy

Describing new brain structures may open research avenues and improve our knowledge of brain functions and of brain disorders. It may also provide new neuroanatomical targets for treatments. A detailed understanding of neuroanatomy is a prerequisite of understanding the pathomechanisms underlying psychiatric disorders on a molecular and cellular level. The tail of the ventral tegmental area (tVTA), also known as rostromedial tegmental nucleus (RMTg), is a recently described region which may be a major inhibitory control centre for the dopaminergic systems. These systems participates to behavioural functions and are implicated in the aetiology, symptoms or treatment of neurological or psychiatric diseases, such as Parkinson's, schizophrenia, mood disorders, attention-deficit hyperactivity-disorder (ADHD) and drug abuse. Belonging to the reticular formation, the tVTA may constitute a major inhibitory GABAergic input to these dopaminergic systems. Moreover, it is sensitive to drugs of abuse, to stimulant or arousing drugs, to aversive stimuli and it could also be the main relay between lateral habenula and VTA. First described in rats, and proposed as a component of the emotional motor system implicated in basic survival behaviours, tVTA appears as a relevant structure for molecular psychiatry, which should foster research to define and study this brain region in the human brain.

Dynamic regulation of midbrain dopamine neuron activity: intrinsic, synaptic, and plasticity mechanisms

Although the roles of dopaminergic signaling in learning and behavior are well established, it is not fully understood how the activity of dopaminergic neurons is dynamically regulated under different conditions in a constantly changing environment. Dopamine neurons must integrate sensory, motor, and cognitive information online to inform the organism to pursue outcomes with the highest reward probability. In this article, we provide an overview of recent advances on the intrinsic, extrinsic (i.e., synaptic), and plasticity mechanisms controlling dopamine neuron activity, mostly focusing on mechanistic studies conducted using ex vivo brain slice preparations. We also hope to highlight some unresolved questions regarding information processing that takes place at dopamine neurons, thereby stimulating further investigations at different levels of analysis.

The inhibitory influence of the lateral habenula on midbrain dopamine cells: ultrastructural evidence for indirect mediation via the rostromedial mesopontine tegmental nucleus

The lateral habenula (LHb) provides an important source of negative reinforcement signals to midbrain dopamine (DA) cells in the substantia nigra and ventral tegmental area (VTA). This profound and consistent inhibitory influence involves a disynaptic connection from glutamate neurons in the LHb to some population of γ-aminobutyric acid (GABA) cells that, in turn, innervates DA neurons. Previous studies demonstrated that the GABA cells intrinsic to the VTA receive insufficient synaptic input from the LHb to serve as the primary source of this intermediate connection. In this investigation, we sought ultrastructural evidence supporting the hypothesis that a newly identified region of the brainstem, the rostromedial mesopontine tegmental nucleus (RMTg), is a more likely candidate for inhibiting midbrain DA cells in response to LHb activation. Electron microscopic examination of rat brain sections containing dual immunoreactivity for an anterograde tracing agent and a phenotypic marker revealed that: 1) more than 55% of the synapses formed by LHb axons in the RMTg were onto GABA-labeled dendrites; 2) more than 80% of the synapses formed by RMTg axons in the VTA contacted dendrites immunoreactive for the DA synthetic enzyme tyrosine hydroxylase; and 3) nearly all RMTg axons formed symmetric synapses and contained postembedding immunoreactivity for GABA. These findings indicate that the newly identified RMTg region is an intermediate structure in a disynaptic pathway that connects the LHb to VTA DA neurons. The results have important implications for understanding mental disorders characterized by a dysregulation of reward circuitry involving LHb and DA cell populations.

Interactions between amygdala central nucleus and the ventral tegmental area in the acquisition of conditioned cue-directed behavior in rats

Rats orient to and approach localizable visual cues paired with food delivery. Previous studies from this laboratory show that the acquisition and expression of these learned cue-directed responses depend on integrity of a system including the central nucleus of the amygdala (CeA), the substantia nigra pars compacta (SNc) and the dorsolateral striatum (DLS). Other investigators have suggested that cue-directed behaviors may also depend on interaction between CeA and the ventral striatum, perhaps via CeA projections to the ventral tegmental area (VTA). In Experiment 1, we examined the effects of unilateral lesions of CeA and/or VTA on rats' acquisition of conditioned responses to visual cues paired with food. Contrary to the results of previous studies that examined interactions of CeA with either SNc or DLS, rats with contralateral disconnection lesions of CeA and VTA were unimpaired in their acquisition of cue-directed responses. By contrast, rats with lesions of both structures in the same hemisphere failed to learn cue-directed responses, but were normal in their acquisition of conditioned responses directed to the food cup. In Experiment 2, we attempted to characterize the influence of VTA on CeA by examining FOS induction in CeA by a visual cue for food in rats with unilateral lesions of VTA. The results suggested an excitatory influence of VTA on CeA in the presence of food cues. Implications of these results for brain circuits involved in learned orienting and incentive motivation are discussed.

Pharmacological recruitment of the GABAergic tail of the ventral tegmental area by acute drug exposure

BACKGROUND AND PURPOSE

The tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, is a newly defined brain structure and a potential control centre for dopaminergic activity. It was identified by the induction of DeltaFosB following chronic cocaine exposure. In this work, we screened 20 drugs for their ability to induce FosB/DeltaFosB in the tVTA.

EXPERIMENTAL APPROACH

Immunohistochemistry following systemic drug administration was used to study FosB/DeltaFosB induction in the tVTA of adult rats. Double-staining was used to determine whether dopamine or GABA neurones are involved in this induction.

KEY RESULTS

The acute injection of the psychostimulant drugs cocaine, D-amphetamine, (+/-)-3,4-methylenedioxymethamphetamine (MDMA), methylphenidate or caffeine, induced the expression of FosB/DeltaFosB in the tVTA GABAergic cells. No induction was observed following exposure to ethanol, diazepam, γ-hydroxybutyric acid (GHB), morphine, ketamine, phencyclidine (PCP), Δ(9)-tetrahydrocannabinol (THC), sodium valproic acid or gabapentin. To evaluate the role of monoamine transporters in the psychostimulant-induced expression of FosB/DeltaFosB, we tested the antidepressant drugs reboxetine, nortriptyline, fluoxetine and venlafaxine (which target the noradrenaline and/or the 5-hydroxytryptamine transporters), the 5-hydroxytryptamine releasing agent dexfenfluramine, and the dopamine transporter inhibitor GBR12909. Only GBR12909 was able to induce FosB/DeltaFosB expression in the tVTA, showing that this induction is mediated by dopamine.

CONCLUSIONS AND IMPLICATIONS

Newly described brain structures may help to increase our knowledge of brain function, pathology and targets for treatments. FosB/DeltaFosB induction in the tVTA is a common feature of drugs sharing psychostimulant properties but not of drugs sharing risk of abuse.

Brain reward circuitry beyond the mesolimbic dopamine system: a neurobiological theory

Reductionist attempts to dissect complex mechanisms into simpler elements are necessary, but not sufficient for understanding how biological properties like reward emerge out of neuronal activity. Recent studies on intracranial self-administration of neurochemicals (drugs) found that rats learn to self-administer various drugs into the mesolimbic dopamine structures-the posterior ventral tegmental area, medial shell nucleus accumbens and medial olfactory tubercle. In addition, studies found roles of non-dopaminergic mechanisms of the supramammillary, rostromedial tegmental and midbrain raphe nuclei in reward. To explain intracranial self-administration and related effects of various drug manipulations, I outlined a neurobiological theory claiming that there is an intrinsic central process that coordinates various selective functions (including perceptual, visceral, and reinforcement processes) into a global function of approach. Further, this coordinating process for approach arises from interactions between brain structures including those structures mentioned above and their closely linked regions: the medial prefrontal cortex, septal area, ventral pallidum, bed nucleus of stria terminalis, preoptic area, lateral hypothalamic areas, lateral habenula, periaqueductal gray, laterodorsal tegmental nucleus and parabrachical area.

Distinct patterns of neuronal inputs and outputs of the juxtaparaventricular and suprafornical regions of the lateral hypothalamic area in the male rat

We have analyzed at high resolution the neuroanatomical connections of the juxtaparaventricular region of the lateral hypothalamic area (LHAjp); as a control and in comparison to this, we also performed a preliminary analysis of a nearby LHA region that is dorsal to the fornix, namely the LHA suprafornical region (LHAs). The connections of these LHA regions were revealed with a coinjection tract-tracing technique involving a retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracer. The LHAjp and LHAs together connect with almost every major division of the cerebrum and cerebrospinal trunk, but their connection profiles are markedly different and distinct. In simple terms, the connections of the LHAjp indicate a possible primary role in the modulation of defensive behavior; for the LHAs, a role in the modulation of ingestive behavior is suggested. However, the relation of the LHAjp and LHAs to potential modulation of these behaviors, as indicated by their neuroanatomical connections, appears to be highly integrative as it includes each of the major functional divisions of the nervous system that together determine behavior, i.e., cognitive, state, sensory, and motor. Furthermore, although a primary role is indicated for each region with respect to a particular mode of behavior, intermode modulation of behavior is also indicated. In summary, the extrinsic connections of the LHAjp and LHAs (so far as we have described them) suggest that these regions have a profoundly integrative role in which they may participate in the orchestrated modulation of elaborate behavioral repertoires.

gamma-Aminobutyric acid cells with cocaine-induced DeltaFosB in the ventral tegmental area innervate mesolimbic neurons

BACKGROUND

The transcription factor DeltaFosB is implicated in the plasticity induced by drugs of abuse. We showed that psychostimulants induce DeltaFosB in gamma-aminobutyric acid (GABA) cells of a caudal subregion of the ventral tegmental area (VTA) that was named tail of the VTA (tVTA). Although tVTA mostly shares VTA inputs, its outputs remain to be characterized.

METHODS

The tVTA efferents were studied by iontophoretic injections of the anterograde tracer biotinylated dextran amine (BDA). To further study VTA inputs arising from tVTA, injections of the retrograde tracer Fluoro-Gold were combined with multiple labeling by immunohistochemistry in rats treated with cocaine. Indirect projections from the tVTA to the nucleus accumbens were assessed with a double-tracing approach, cholera toxin B subunit (CTB) being delivered in the nucleus accumbens and BDA in the tVTA.

RESULTS

Tract-tracing studies showed that tVTA heavily projects to the midbrain dopaminergic system and revealed terminal appositions with dopamine cells in the VTA. Double-labeling studies demonstrated that this tVTA output is mostly GABAergic, includes cells in which cocaine exposure induces DeltaFosB, and displays appositions to dopamine cells projecting to the nucleus accumbens.

CONCLUSIONS

The GABA neurons expressing DeltaFosB in the tVTA after cocaine exposure project to the dopamine mesolimbic neurons.

Cortico-Basal Ganglia reward network: microcircuitry

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

Ultrastructural analysis of local collaterals of rat ventral tegmental area neurons: GABA phenotype and synapses onto dopamine and GABA cells

Local synapses formed by nondopamine cells within the ventral tegmental area (VTA) are thought to provide an important regulatory influence on the activity patterns of dopamine (DA) neurons. However, ultrastructural confirmation of intra-areal synapses formed by VTA neurons is lacking, and the synaptic targets of these connections have not been examined. We performed discrete injections of the specific anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) and used electron microscopy to visualize immunoperoxidase labeling within the local collaterals of VTA cells. The phenotype of target neurons was determined by immunogold-silver labeling for GABA or for tyrosine hydroxylase within DA neurons. Within or immediately adjacent to the VTA injection sites, PHAL was incorporated into the soma and dendrites of both GABA and DA cells. Tracer was also detected within myelinated and unmyelinated axons as well as axon terminals. Some labeled terminals formed identifiable synapses, the majority of which (78%) had symmetric morphology (presumably inhibitory). Both DA and GABA dendrites were contacted by these intrinsic axons. Postembedding immunogold labeling verified that local axon collaterals arose mainly from GABA cells (DA neurons are not known to issue recurrent collaterals). Nevertheless, a few synapses with asymmetric morphology (presumably excitatory) were also noted; whether these derive from local glutamate neurons requires further investigation. Hence, our data provide ultrastructural support for the long standing assumption that GABA VTA neurons synapse locally onto DA cells. The findings also suggest the presence of disinhibitory and possibly excitatory circuitry intrinsic to the VTA.

Lateral habenula projections to dopamine and GABA neurons in the rat ventral tegmental area

Ventral tegmental area (VTA) dopamine (DA) neurons and their forebrain projections are critically involved in reward processing and cognitive functions. Descending projections from the lateral habenula (LHb) play a central role in inhibiting DA cell activity in response to the absence of expected rewards. As LHb efferents are reportedly glutamatergic, their ability to inhibit DA cells would theoretically require a disynaptic connection involving VTA GABA neurons and their local collateral inputs to DA cells. We therefore used anterograde tract-tracing from the LHb to investigate the relative selectivity of LHb synapses onto GABA versus DA VTA neurons. LHb axons were visualized using immunoperoxidase, and DA and GABA cells were marked by immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA, respectively. By ultrastructural analysis, 16% of LHb axons were observed to form synaptic contacts in the VTA, and most of these were of an intermediate morphological type that did not exhibit definitive asymmetric or symmetric character. LHb axons synaptically targeted TH- and GABA-labeled dendrites to a comparable extent (45 and 52% observed incidence, respectively). Pre-embedding immunogold labeling for the vesicular glutamate transporter type 2 and post-embedding immunogold staining for GABA confirmed that approximately 85% of LHb terminals were glutamatergic and not GABAergic. These results suggest that the robust inhibition of DA cells evoked by the LHb is unlikely to arise from a selective innervation of VTA GABA neurons. Moreover, the LHb may mediate a direct excitation of DA cells that is over-ridden by indirect inhibition originating from an extrinsic source.

The mesopontine rostromedial tegmental nucleus: A structure targeted by the lateral habenula that projects to the ventral tegmental area of Tsai and substantia nigra compacta

Prior studies revealed that aversive stimuli and psychostimulant drugs elicit Fos expression in neurons clustered above and behind the interpeduncular nucleus that project strongly to the ventral tegmental area (VTA) and substantia nigra (SN) compacta (C). Other reports suggest that these neurons modulate responses to aversive stimuli. We now designate the region containing them as the "mesopontine rostromedial tegmental nucleus" (RMTg) and report herein on its neuroanatomy. Dense micro-opioid receptor and somatostatin immunoreactivity characterize the RMTg, as do neurons projecting to the VTA/SNC that are enriched in GAD67 mRNA. Strong inputs to the RMTg arise in the lateral habenula (LHb) and, to a lesser extent, the SN. Other inputs come from the frontal cortex, ventral striatopallidum, extended amygdala, septum, preoptic region, lateral, paraventricular and posterior hypothalamus, zona incerta, periaqueductal gray, intermediate layers of the contralateral superior colliculus, dorsal raphe, mesencephalic, pontine and medullary reticular formation, and the following nuclei: parafascicular, supramammillary, mammillary, ventral lateral geniculate, deep mesencephalic, red, pedunculopontine and laterodorsal tegmental, cuneiform, parabrachial, and deep cerebellar. The RMTg has meager outputs to the forebrain, mainly to the ventral pallidum, preoptic-lateral hypothalamic continuum, and midline-intralaminar thalamus, but much heavier outputs to the brainstem, including, most prominently, the VTA/SNC, as noted above, and to medial tegmentum, pedunculopontine and laterodorsal tegmental nuclei, dorsal raphe, and locus ceruleus and subceruleus. The RMTg may integrate multiple forebrain and brainstem inputs in relation to a dominant LHb input. Its outputs to neuromodulatory projection systems likely converge with direct LHb projections to those structures.

Mesolimbic functional magnetic resonance imaging activations during reward anticipation correlate with reward-related ventral striatal dopamine release

The dopaminergic mechanisms that control reward-motivated behavior are the subject of intense study, but it is yet unclear how, in humans, neural activity in mesolimbic reward-circuitry and its functional neuroimaging correlates are related to dopamine release. To address this question, we obtained functional magnetic resonance imaging (fMRI) measures of reward-related neural activity and [(11)C]raclopride positron emission tomography measures of dopamine release in the same human participants, while they performed a delayed monetary incentive task. Across the cohort, a positive correlation emerged between neural activity of the substantia nigra/ventral tegmental area (SN/VTA), the main origin of dopaminergic neurotransmission, during reward anticipation and reward-related [(11)C]raclopride displacement as an index of dopamine release in the ventral striatum, major target of SN/VTA dopamine neurons. Neural activity in the ventral striatum/nucleus accumbens itself also correlated with ventral striatal dopamine release. Additionally, high-reward-related dopamine release was associated with increased activation of limbic structures, such as the amygdala and the hippocampus. The observed correlations of reward-related mesolimbic fMRI activation and dopamine release provide evidence that dopaminergic neurotransmission plays a quantitative role in human mesolimbic reward processing. Moreover, the combined neurochemical and hemodynamic imaging approach used here opens up new perspectives for the investigation of molecular mechanisms underlying human cognition.

The rostromedial tegmental nucleus (RMTg), a GABAergic afferent to midbrain dopamine neurons, encodes aversive stimuli and inhibits motor responses

Separate studies have implicated the lateral habenula (LHb) or amygdala-related regions in processing aversive stimuli, but their relationships to each other and to appetitive motivational systems are poorly understood. We show that neurons in the recently identified GABAergic rostromedial tegmental nucleus (RMTg), which receive a major LHb input, project heavily to midbrain dopamine neurons, and show phasic activations and/or Fos induction after aversive stimuli (footshocks, shock-predictive cues, food deprivation, or reward omission) and inhibitions after rewards or reward-predictive stimuli. RMTg lesions markedly reduce passive fear behaviors (freezing, open-arm avoidance) dependent on the extended amygdala, periaqueductal gray, or septum, all regions that project directly to the RMTg. In contrast, RMTg lesions spare or enhance active fear responses (treading, escape) in these same paradigms. These findings suggest that aversive inputs from widespread brain regions and stimulus modalities converge onto the RMTg, which opposes reward and motor-activating functions of midbrain dopamine neurons.

Circuit projection from suprachiasmatic nucleus to ventral tegmental area: a novel circadian output pathway

The suprachiasmatic nucleus (SCN) is a circadian pacemaker that synchronizes a number of vital processes. Although a great deal of research has focused on input pathways to SCN and on the central clock itself, relatively little is known about SCN output signaling pathways. The ventral tegmental area (VTA) has been extensively studied for its influence in motivated learning and, recently, for a potential role in arousal and sleep-wake regulation. Here we present data that SCN indirectly projects to VTA via the medial preoptic nucleus (MPON). Microinjection of the retrograde, transynaptic tracer pseudorabies virus (PRV) in rat VTA consistently labeled SCN neurons at time points indicative of an indirect circuit projection. To specify intermediate relay nuclei between SCN and VTA, putative relays were lesioned 1 week prior to PRV injections in VTA. Unilateral lesions of MPON reduced PRV labeling in SCN by 81.6% in the ipsilateral hemisphere and 75.8% in the contralateral hemisphere. Bilateral lesions of the caudal-dorsal lateral septum, another putative relay nucleus and dorsal injection control, did not significantly reduce PRV labeling in the SCN. Single-unit extracellular recordings under halothane anesthesia revealed a novel population of VTA neurons that selectively fired during the active circadian phase. These results show that SCN provides an indirect circuit pathway to VTA via MPON, and that VTA neurons exhibit a circadian rhythm in their impulse activity. This pathway may function in the circadian regulation of numerous behavioral processes including arousal and motivation.

Localized low-level re-expression of high-affinity mesolimbic nicotinic acetylcholine receptors restores nicotine-induced locomotion but not place conditioning

High-affinity, beta2-subunit-containing (beta2*) nicotinic acetylcholine receptors (nAChRs) are essential for nicotine reinforcement; however, these nAChRs are found on both gamma-aminobutyric acid (GABA) and dopaminergic (DA) neurons in the ventral tegmental area (VTA) and also on terminals of glutamatergic and cholinergic neurons projecting from the pedunculopontine tegmental area and the laterodorsal tegmental nucleus. Thus, systemic nicotine administration stimulates many different neuronal subtypes in various brain nuclei. To identify neurons in which nAChRs must be expressed to mediate effects of systemic nicotine, we investigated responses in mice with low-level, localized expression of beta2* nAChRs in the midbrain/VTA. Nicotine-induced GABA and DA release were partially rescued in striatal synaptosomes from transgenic mice compared with tissue from beta2 knockout mice. Nicotine-induced locomotor activation, but not place preference, was rescued in mice with low-level VTA expression, suggesting that low-level expression of beta2* nAChRs in DA neurons is not sufficient to support nicotine reward. In contrast to control mice, transgenic mice with low-level beta2* nAChR expression in the VTA showed no increase in overall levels of cyclic AMP response element-binding protein (CREB) but did show an increase in CREB phosphorylation in response to exposure to a nicotine-paired chamber. Thus, CREB activation in the absence of regulation of total CREB levels during place preference testing was not sufficient to support nicotine place preference in beta2 trangenic mice. This suggests that partial activation of high-affinity nAChRs in VTA might block the rewarding effects of nicotine, providing a potential mechanism for the ability of nicotinic partial agonists to aid in smoking cessation.