Midbrain dopamine neurons: projection target determines action potential duration and dopamine D(2) receptor inhibition

Beyond the Classic VTA: Extended Amygdala Projections to DA-Striatal Paths in the Primate

The central extended amygdala (CEA) has been conceptualized as a 'macrosystem' that regulates various stress-induced behaviors. Consistent with this, the CEA highly expresses corticotropin-releasing factor (CRF), an important modulator of stress responses. Stress alters goal-directed responses associated with striatal paths, including maladaptive responses such as drug seeking, social withdrawal, and compulsive behavior. CEA inputs to the midbrain dopamine (DA) system are positioned to influence striatal functions through mesolimbic DA-striatal pathways. However, the structure of this amygdala-CEA-DA neuron path to the striatum has been poorly characterized in primates. In primates, we combined neuronal tracer injections into various arms of the circuit through specific DA subpopulations to assess: (1) whether the circuit connecting amygdala, CEA, and DA cells follows CEA intrinsic organization, or a more direct topography involving bed nucleus vs central nucleus divisions; (2) CRF content of the CEA-DA path; and (3) striatal subregions specifically involved in CEA-DA-striatal loops. We found that the amygdala-CEA-DA path follows macrostructural subdivisions, with the majority of input/outputs converging in the medial central nucleus, the sublenticular extended amygdala, and the posterior lateral bed nucleus of the stria terminalis. The proportion of CRF+ outputs is >50%, and mainly targets the A10 parabrachial pigmented nucleus (PBP) and A8 (retrorubal field, RRF) neuronal subpopulations, with additional inputs to the dorsal A9 neurons. CRF-enriched CEA-DA projections are positioned to influence outputs to the 'limbic-associative' striatum, which is distinct from striatal regions targeted by DA cells lacking CEA input. We conclude that the concept of the CEA is supported on connectional grounds, and that CEA termination over the PBP and RRF neuronal populations can influence striatal circuits involved in associative learning.

Two delta opioid receptor subtypes are functional in single ventral tegmental area neurons, and can interact with the mu opioid receptor

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K⁺ dependent hyperpolarization or a Ca_v2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes. However, in a subset of VTA neurons the DOR antagonist TIPP-Ψ augmented DAMGO responses; we also observed DPDPE or deltorphin II responses augmented by the MOR selective antagonist CTAP. These findings directly support the existence of two independent, stable forms of the DOR, and show that MOR and DOR can interact in some neurons to alter downstream signaling.

Dorsal Raphe Dopamine Neurons Modulate Arousal and Promote Wakefulness by Salient Stimuli

Ventral midbrain dopamine (DA) is unambiguously involved in motivation and behavioral arousal, yet the contributions of other DA populations to these processes are poorly understood. Here, we demonstrate that the dorsal raphe nucleus DA neurons are critical modulators of behavioral arousal and sleep-wake patterning. Using simultaneous fiber photometry and polysomnography, we observed time-delineated dorsal raphe nucleus dopaminergic (DRN^DA) activity upon exposure to arousal-evoking salient cues, irrespective of their hedonic valence. We also observed broader fluctuations of DRN^DA activity across sleep-wake cycles with highest activity during wakefulness. Both endogenous DRN^DA activity and optogenetically driven DRN^DA activity were associated with waking from sleep, with DA signal strength predictive of wake duration. Conversely, chemogenetic inhibition opposed wakefulness and promoted NREM sleep, even in the face of salient stimuli. Therefore, the DRN^DA population is a critical contributor to wake-promoting pathways and is capable of modulating sleep-wake states according to the outside environment, wherein the perception of salient stimuli prompts vigilance and arousal.

Dopamine Inhibition Differentially Controls Excitability of Substantia Nigra Dopamine Neuron Subpopulations through T-Type Calcium Channels

While there is growing appreciation for diversity among ventral tegmental area dopamine neurons, much less is known regarding functional heterogeneity among the substantia nigra pars compacta (SNc) neurons. Here, we show that calbindin-positive dorsal tier and calbindin-negative ventral tier SNc dopaminergic neurons in mice comprise functionally distinct subpopulations distinguished by their dendritic calcium signaling, rebound excitation, and physiological responses to dopamine D2-receptor (D2) autoinhibition. While dopamine is known to inhibit action potential backpropagation, our experiments revealed an unexpected enhancement of excitatory responses and dendritic calcium signals in the presence of D2-receptor inhibition. Specifically, dopamine inhibition and direct hyperpolarization enabled the generation of low-threshold depolarizations that occurred in an all-or-none or graded manner, due to recruitment of T-type calcium channels. Interestingly, these effects occurred selectively in calbindin-negative dopaminergic neurons within the SNc. Thus, calbindin-positive and calbindin-negative SNc neurons differ substantially in their calcium channel composition and efficacy of excitatory inputs in the presence of dopamine inhibition.SIGNIFICANCE STATEMENT Substantia nigra dopaminergic neurons can be divided into two populations: the calbindin-negative ventral tier, which is vulnerable to neurodegeneration in Parkinson's disease, and the calbindin-positive dorsal tier, which is relatively resilient. Although tonic firing is similar in these subpopulations, we find that their responses to dopamine-mediated inhibition are strikingly different. During inhibition, calbindin-negative neurons exhibit increased sensitivity to excitatory inputs, which can then trigger large dendritic calcium transients due to strong expression of T-type calcium channels. Therefore, SNc neurons differ substantially in their calcium channel composition, which may contribute to their differential vulnerability. Furthermore, T-currents increase excitation efficacy onto calbindin-negative cells during dopamine inhibition, suggesting that shared inputs are differentially processed in subpopulations resulting in distinct downstream dopamine signals.

Viral vector strategies for investigating midbrain dopamine circuits underlying motivated behaviors

Midbrain dopamine (DA) neurons have received significant attention in brain research because of their central role in reward processing and their dysfunction in neuropsychiatric disorders such as Parkinson's disease, drug addiction, depression and schizophrenia. Until recently, it has been thought that DA neurons form a homogeneous population whose primary function is the computation of reward prediction errors. However, through the implementation of viral vector strategies, an unexpected complexity and diversity has been revealed at the anatomical, molecular and functional level. In this review, we discuss recent viral vector approaches that have been leveraged to dissect how different circuits involving distinct DA neuron subpopulations may contribute to the role of DA in reward- and aversion-related behaviors. We focus on studies that have used cell type- and projection-specific optogenetic manipulations, discuss the strengths and limitations of each approach, and critically examine emergent organizational principles that have led to a reclassification of midbrain DA neurons.

Heterogeneity in Dopamine Neuron Synaptic Actions Across the Striatum and Its Relevance for Schizophrenia

Brain imaging has revealed alterations in dopamine uptake, release, and receptor levels in patients with schizophrenia that have been resolved on the scale of striatal subregions. However, the underlying synaptic mechanisms are on a finer scale. Dopamine neuron synaptic actions vary across the striatum, involving variations not only in dopamine release but also in dopamine neuron connectivity, cotransmission, modulation, and activity. Optogenetic studies have revealed that dopamine neurons release dopamine in a synaptic signal mode, and that the neurons also release glutamate and gamma-aminobutyric acid as cotransmitters, with striking regional variation. Fast glutamate and gamma-aminobutyric acid cotransmission convey discrete patterns of dopamine neuron activity to striatal neurons. Glutamate may function not only in a signaling role at a subset of dopamine neuron synapses, but also in mediating vesicular synergy, contributing to regional differences in loading of dopamine into synaptic vesicles. Regional differences in dopamine neuron signaling are likely to be differentially involved in the schizophrenia disease process and likely determine the subregional specificity of the action of psychostimulants that exacerbate the disorder, and antipsychotics that ameliorate the disorder. Elucidating dopamine neuron synaptic signaling offers the potential for achieving greater pharmacological specificity through intersectional pharmacological actions targeting subsets of dopamine neuron synapses.

Amygdala Circuits for Fear Memory: A Key Role for Dopamine Regulation

In addition to modulating a number of cognitive functions including reward, punishment, motivation, and salience, dopamine (DA) plays a pivotal role in regulating threat-related emotional memory. Changes in neural circuits of the amygdala nuclei are also critically involved in the acquisition and expression of emotional memory. In this review, we summarize the regulation of amygdala circuits by DA. Specifically, we describe DA signaling in the amygdala, and DA regulation of synaptic transmission and synaptic plasticity of the amygdala neurons. Finally, we discuss a potential contribution of DA-related mechanisms to the pathogenesis of posttraumatic stress disorder.

On the properties of identified dopaminergic neurons in the mouse substantia nigra and ventral tegmental area

We studied the properties of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) in mice expressing the enhanced green fluorescent protein (eGFP) under the control of the tyrosine hydroxylase promoter (TH-GFP). By using a practical map of cell positioning in distinct SNpc and VTA subregions in horizontal midbrain slices we saw that the spontaneous firing, membrane properties, cell body size and magnitude of the hyperpolarization-activated current (I_h ) in TH-GFP-positive neurons (TH-GFP⁺ ) vary significantly among subregions, following a mediolateral gradient. Block of I_h with Zd7288 inhibited firing in the most lateral subregions, but had little effect in the intermediate/medial VTA. In addition, TH-GFP⁺ cells were excited by Met⁵ -Enkephalin. Extracellular recordings from a large neuron number showed that all TH-GFP⁺ cells were inhibited by dopamine, suggesting that this is a reliable approach for identifying dopaminergic neurons in vitro. Simultaneous recordings from dopamine-sensitive and dopamine-insensitive neurons showed that dopamine-insensitive cells (putative non-dopaminergic neurons) are unaffected by Zd7288 but inhibited by Met⁵ -Enkephalin. Under patch-clamp, dopamine generated a quantitatively similar outward current in most TH-GFP⁺ neurons, although medial VTA cells showed reduced dopamine sensitivity. Pargyline prolonged the dopamine current, whereas cocaine enhanced dopamine-mediated responses in both the SNpc and the VTA. Our work provides new insights into the variability in mouse midbrain dopaminergic neurons along the medial-lateral axis and points to the necessity of a combination of different electrophysiological and pharmacological approaches for reliably identifying these cells to distinguish them from non-dopaminergic neurons in the midbrain.

Diversity of Dopaminergic Neural Circuits in Response to Drug Exposure

Addictive substances are known to increase dopaminergic signaling in the mesocorticolimbic system. The origin of this dopamine (DA) signaling originates in the ventral tegmental area (VTA), which sends afferents to various targets, including the nucleus accumbens, the medial prefrontal cortex, and the basolateral amygdala. VTA DA neurons mediate stimuli saliency and goal-directed behaviors. These neurons undergo robust drug-induced intrinsic and extrinsic synaptic mechanisms following acute and chronic drug exposure, which are part of brain-wide adaptations that ultimately lead to the transition into a drug-dependent state. Interestingly, recent investigations of the differential subpopulations of VTA DA neurons have revealed projection-specific functional roles in mediating reward, aversion, and stress. It is now critical to view drug-induced neuroadaptations from a circuit-level perspective to gain insight into how differential dopaminergic adaptations and signaling to targets of the mesocorticolimbic system mediates drug reward. This review hopes to describe the projection-specific intrinsic characteristics of these subpopulations, the differential afferent inputs onto these VTA DA neuron subpopulations, and consolidate findings of drug-induced plasticity of VTA DA neurons and highlight the importance of future projection-based studies of this system.

Voluntary adolescent drinking enhances excitation by low levels of alcohol in a subset of dopaminergic neurons in the ventral tegmental area

Enhanced dopamine (DA) neurotransmission from the ventral tegmental area (VTA) to the ventral striatum is thought to drive drug self-administration and mediate positive reinforcement. We examined neuronal firing rates in slices of mouse midbrain following adolescent binge-like alcohol drinking and find that prior alcohol experience greatly enhanced the sensitivity to excitation by ethanol itself (10-50 mM) in a subset of ventral midbrain DA neurons located in the medial VTA. This enhanced response after drinking was not associated with alterations of firing rate or other measures of intrinsic excitability. In addition, the phenomenon appears to be specific to adolescent drinking, as mice that established a drinking preference only after the onset of adulthood showed no change in alcohol sensitivity. Here we demonstrate not only that drinking during adolescence induces enhanced alcohol sensitivity, but also that this DA neuronal response occurs over a range of alcohol concentrations associated with social drinking in humans.

PPARα modulation of mesolimbic dopamine transmission rescues depression-related behaviors

Depressive disorders cause a substantial burden for the individual and the society. Key depressive symptoms can be modeled in animals and enable the development of novel therapeutic interventions. Chronic unavoidable stress disrupts rats' competence to escape noxious stimuli and self-administer sucrose, configuring a depression model characterized by escape deficit and motivational anhedonia associated to impaired dopaminergic responses to sucrose in the nucleus accumbens shell (NAcS). Repeated treatments that restore these responses also relieve behavioral symptoms. Ventral tegmental area (VTA) dopamine neurons encode reward and motivation and are implicated in the neuropathology of depressive-like behaviors. Peroxisome proliferator-activated receptors type-α (PPARα) acutely regulate VTA dopamine neuron firing via β2 subunit-containing nicotinic acetylcholine receptors (β2*nAChRs) through phosphorylation and this effect is predictive of antidepressant-like effects. Here, by combining behavioral, electrophysiological and biochemical techniques, we studied the effects of repeated PPARα stimulation by fenofibrate on mesolimbic dopamine system. We found decreased β2*nAChRs phosphorylation levels and a switch from tonic to phasic activity of dopamine cells in the VTA, and increased phosphorylation of dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32) in the NAcS. We then investigated whether long-term fenofibrate administration to stressed rats reinstated the decreased DARPP-32 response to sucrose and whether this effect translated into antidepressant-like properties. Fenofibrate restored dopaminergic responses to appetitive stimuli, reactivity to aversive stimuli and motivation to self-administer sucrose. Overall, this study suggests PPARα as new targets for antidepressant therapies endowed with motivational anti-anhedonic properties, further supporting the role of an unbalanced mesolimbic dopamine system in pathophysiology of depressive disorders.

Repeated social defeat stress enhances glutamatergic synaptic plasticity in the VTA and cocaine place conditioning

Enduring memories of sensory cues associated with drug intake drive addiction. It is well known that stressful experiences increase addiction vulnerability. However, it is not clear how repeated stress promotes learning of cue-drug associations, as repeated stress generally impairs learning and memory processes unrelated to stressful experiences. Here, we show that repeated social defeat stress in rats causes persistent enhancement of long-term potentiation (LTP) of NMDA receptor-mediated glutamatergic transmission in the ventral tegmental area (VTA). Protein kinase A-dependent increase in the potency of inositol 1,4,5-triphosphate-induced Ca²⁺ signaling underlies LTP facilitation. Notably, defeated rats display enhanced learning of contextual cues paired with cocaine experience assessed using a conditioned place preference (CPP) paradigm. Enhancement of LTP in the VTA and cocaine CPP in behaving rats both require glucocorticoid receptor activation during defeat episodes. These findings suggest that enhanced glutamatergic plasticity in the VTA may contribute, at least partially, to increased addiction vulnerability following repeated stressful experiences.

DOI:http://dx.doi.org/10.7554/eLife.15448.001

Daily stress increases the likelihood that people who take drugs will become addicted. A very early step in the development of addiction is learning that certain people, places, or paraphernalia are associated with obtaining drugs. These ‘cues’ – drug dealers, bars, cigarette advertisements, etc. – become powerful motivators to seek out drugs and can trigger relapse in recovering addicts. It is thought that learning happens when synapses (the connections between neurons in the brain) that relay information about particular cues become stronger. However, it is not clear how stress promotes the learning of cue-drug associations.

Stelly et al. investigated whether repeated episodes of stress make it easier to strengthen synapses on dopamine neurons, which are involved in processing rewards and addiction. For the experiments, rats were repeatedly exposed to a stressful situation – an encounter with an unfamiliar aggressive rat – every day for five days. Stelly et al. found that these stressed rats formed stronger associations between the drug cocaine and the place where they were given the drug (the cue). Furthermore, a mechanism that strengthens synapses was more sensitive in the stressed rats than in unstressed rats. These changes persisted for 10-30 days after the stressful situation, suggesting that stress might begin a period of time during which the individual is more vulnerable to addiction.

The experiments also show that a hormone called corticosterone – which is released during stressful experiences – is necessary for stress to trigger the changes in the synapses and behavior of the rats. However, corticosterone must work with other factors because giving this hormone to unstressed rats was not sufficient to trigger the changes seen in the stressed rats. Future experiments will investigate what these other stress factors are and how they work together with corticosterone.

DOI:http://dx.doi.org/10.7554/eLife.15448.002

Contemporary approaches to neural circuit manipulation and mapping: focus on reward and addiction

Tying complex psychological processes to precisely defined neural circuits is a major goal of systems and behavioural neuroscience. This is critical for understanding adaptive behaviour, and also how neural systems are altered in states of psychopathology, such as addiction. Efforts to relate psychological processes relevant to addiction to activity within defined neural circuits have been complicated by neural heterogeneity. Recent advances in technology allow for manipulation and mapping of genetically and anatomically defined neurons, which when used in concert with sophisticated behavioural models, have the potential to provide great insight into neural circuit bases of behaviour. Here we discuss contemporary approaches for understanding reward and addiction, with a focus on midbrain dopamine and cortico-striato-pallidal circuits.

Communication in Neural Circuits: Tools, Opportunities, and Challenges

Communication, the effective delivery of information, is fundamental to life across all scales and species. Nervous systems (by necessity) may be most specifically adapted among biological tissues for high rate and complexity of information transmitted, and thus, the properties of neural tissue and principles of its organization into circuits may illuminate capabilities and limitations of biological communication. Here, we consider recent developments in tools for studying neural circuits with particular attention to defining neuronal cell types by input and output information streams--i.e., by how they communicate. Complementing approaches that define cell types by virtue of genetic promoter/enhancer properties, this communication-based approach to defining cell types operationally by input/output (I/O) relationships links structure and function, resolves difficulties associated with single-genetic-feature definitions, leverages technology for observing and testing significance of precisely these I/O relationships in intact brains, and maps onto processes through which behavior may be adapted during development, experience, and evolution.

Specificity and impact of adrenergic projections to the midbrain dopamine system

Dopamine (DA) is a neuromodulator that regulates different brain circuits involved in cognitive functions, motor coordination, and emotions. Dysregulation of DA is associated with many neurological and psychiatric disorders such as Parkinson's disease and substance abuse. Several lines of research have shown that the midbrain DA system is regulated by the central adrenergic system. This review focuses on adrenergic interactions with midbrain DA neurons. It discusses the current neuroanatomy including source of adrenergic innervation, type of synapses, and adrenoceptors expression. It also discusses adrenergic regulation of DA cell activity and neurotransmitter release. Finally, it reviews several neurological and psychiatric disorders where changes in adrenergic system are associated with dysregulation of the midbrain DA system. This article is part of a Special Issue entitled SI: Noradrenergic System.

Ventral Tegmental Area Afferents and Drug-Dependent Behaviors

Drug-related behaviors in both humans and rodents are commonly thought to arise from aberrant learning processes. Preclinical studies demonstrate that the acquisition and expression of many drug-dependent behaviors involves the ventral tegmental area (VTA), a midbrain structure comprised of dopamine, GABA, and glutamate neurons. Drug experience alters the excitatory and inhibitory synaptic input onto VTA dopamine neurons, suggesting a critical role for VTA afferents in mediating the effects of drugs. In this review, we present evidence implicating the VTA in drug-related behaviors, highlight the diversity of neuronal populations in the VTA, and discuss the behavioral effects of selectively manipulating VTA afferents. Future experiments are needed to determine which VTA afferents and what neuronal populations in the VTA mediate specific drug-dependent behaviors. Further studies are also necessary for identifying the afferent-specific synaptic alterations onto dopamine and non-dopamine neurons in the VTA following drug administration. The identification of neural circuits and adaptations involved with drug-dependent behaviors can highlight potential neural targets for pharmacological and deep brain stimulation interventions to treat substance abuse disorders.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

Multiscale single-cell analysis reveals unique phenotypes of raphe 5-HT neurons projecting to the forebrain

Serotonergic neurons of the raphe nuclei exhibit anatomical, neurochemical and elecrophysiological heterogeneity that likely underpins their specific role in multiple behaviors. However, the precise organization of serotonin (5-HT) neurons to orchestrate 5-HT release patterns throughout the brain is not well understood. We compared the electrophysiological and neurochemical properties of dorsal and median raphe 5-HT neurons projecting to the medial prefrontal cortex (mPFC), amygdala (BLA) and dorsal hippocampus (dHP), combining retrograde tract tracing with brain slice electrophysiology and single-cell RT-PCR in Pet1-EGFP mice. Our results show that 5-HT neurons projecting to the dHP and the mPFC and the BLA form largely non-overlapping populations and that BLA-projecting neurons have characteristic excitability and membrane properties. In addition, using an unbiased clustering method that correlates anatomical, molecular and electrophysiological phenotypes, we find that 5-HT neurons with projections to the mPFC and the dHP segregate from those projecting to the BLA. Single-cell gene profiling showed a restricted expression of the peptide galanin in the population of 5-HT neurons projecting to the mPFC. Finally, cluster analysis allowed identifying an atypical subtype of 5-HT neuron with low excitability, long firing delays and preferential expression of the vesicular glutamate transporter type 3. Overall, these findings allow to define correlated anatomical and physiological identities of serotonin raphe neurons that help understanding how discrete raphe cells subpopulations account for the heterogeneous activities of the midbrain serotonergic system.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Intact-Brain Analyses Reveal Distinct Information Carried by SNc Dopamine Subcircuits

Recent progress in understanding the diversity of midbrain dopamine neurons has highlighted the importance--and the challenges--of defining mammalian neuronal cell types. Although neurons may be best categorized using inclusive criteria spanning biophysical properties, wiring of inputs, wiring of outputs, and activity during behavior, linking all of these measurements to cell types within the intact brains of living mammals has been difficult. Here, using an array of intact-brain circuit interrogation tools, including CLARITY, COLM, optogenetics, viral tracing, and fiber photometry, we explore the diversity of dopamine neurons within the substantia nigra pars compacta (SNc). We identify two parallel nigrostriatal dopamine neuron subpopulations differing in biophysical properties, input wiring, output wiring to dorsomedial striatum (DMS) versus dorsolateral striatum (DLS), and natural activity patterns during free behavior. Our results reveal independently operating nigrostriatal information streams, with implications for understanding the logic of dopaminergic feedback circuits and the diversity of mammalian neuronal cell types.

Circuit Architecture of VTA Dopamine Neurons Revealed by Systematic Input-Output Mapping

Dopamine (DA) neurons in the midbrain ventral tegmental area (VTA) integrate complex inputs to encode multiple signals that influence motivated behaviors via diverse projections. Here, we combine axon-initiated viral transduction with rabies-mediated trans-synaptic tracing and Cre-based cell-type-specific targeting to systematically map input-output relationships of VTA-DA neurons. We found that VTA-DA (and VTA-GABA) neurons receive excitatory, inhibitory, and modulatory input from diverse sources. VTA-DA neurons projecting to different forebrain regions exhibit specific biases in their input selection. VTA-DA neurons projecting to lateral and medial nucleus accumbens innervate largely non-overlapping striatal targets, with the latter also sending extensive extra-striatal axon collaterals. Using electrophysiology and behavior, we validated new circuits identified in our tracing studies, including a previously unappreciated top-down reinforcing circuit from anterior cortex to lateral nucleus accumbens via VTA-DA neurons. This study highlights the utility of our viral-genetic tracing strategies to elucidate the complex neural substrates that underlie motivated behaviors.

Mapping neural circuits with CLARITY

The use of whole-brain imaging has shed new light on the organization of the dopamine system.

Marked differences in the number and type of synapses innervating the somata and primary dendrites of midbrain dopaminergic neurons, striatal cholinergic interneurons, and striatal spiny projection neurons in the rat

Elucidating the link between cellular activity and goal-directed behavior requires a fuller understanding of the mechanisms underlying burst firing in midbrain dopaminergic neurons and those that suppress activity during aversive or non-rewarding events. We have characterized the afferent synaptic connections onto these neurons in the rat substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA), and compared these findings with cholinergic interneurons and spiny projection neurons in the striatum. We found that the average absolute number of synapses was three to three and one-half times greater onto the somata of dorsal striatal spiny projection neurons than onto the somata of dopaminergic neurons in the SNpc or dorsal striatal cholinergic interneurons. A similar comparison between populations of dopamine neurons revealed a two times greater number of somatic synapses on VTA dopaminergic neurons than SNpc dopaminergic neurons. The percentage of symmetrical, presumably inhibitory, synaptic inputs on somata was significantly higher on spiny projection neurons and cholinergic interneurons compared with SNpc dopaminergic neurons. Synaptic data on the primary dendrites yielded similar significant differences for the percentage of symmetrical synapses for VTA dopaminergic vs. striatal neurons. No differences in the absolute number or type of somatic synapses were evident for dopaminergic neurons in the SNpc of Wistar vs. Sprague-Dawley rat strains. These data from identified neurons are pivotal for interpreting their electrophysiological responses to afferent activity and for generating realistic computer models of neuronal networks of striatal and midbrain dopaminergic function.

Kappa opioid receptor signaling in the brain: Circuitry and implications for treatment

Kappa opioid receptors (KORs) in the central nervous system have been known to be important regulators of a variety of psychiatry illnesses, including anxiety and addiction, but their precise involvement in these disorders is complex and has yet to be fully elucidated. Here, we briefly review the pharmacology of KORs in the brain, including KOR's involvement in anxiety, depression, and drug addiction. We also review the known neuronal circuitry impacted by KOR signaling, and interactions with corticotrophin-releasing factor (CRF), another key peptide in anxiety-related illnesses, as well as the role of glucocorticoids. We suggest that KORs are a promising therapeutic target for a host of neuropsychiatric conditions.

Dual motor responses elicited by ethanol in the posterior VTA: Consequences of the blockade of μ-opioid receptors

A recent hypothesis, based on electrophysiological and behavioural findings, suggests that ethanol simultaneously exerts opposed effects on the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) through two parallel mechanisms, one promoting and the other reducing the GABA release onto VTA DA neurons. In this sense, the activating effects are mediated by salsolinol, a metabolite of ethanol, acting on the μ-opioid receptors (MORs) located in VTA GABA neurons. The inhibitory effects are, however, triggered by the non-metabolized fraction of ethanol which would cause the GABAA receptors-mediated inhibition of VTA DA neurons. Since both trends tend to offset each other, only the use of appropriate pharmacological tools allows analysis of this phenomenon in depth. Herein, we present new behavioural findings supporting this hypothesis. Motor activity was evaluated in rats after intra-VTA administration of ethanol 35 nmol, an apparently ineffective dose, 24 h after the irreversible blockade of MORs in the VTA with β-FNA. Our results showed that this pre-treatment turned the initially ineffective ethanol dose into a depressant one, confirming that the activating effect of ethanol can be selectively suppressed without affecting the depressant effects mediated by the non-biotransformed fraction of ethanol.

Role of Dopamine Neurons in Reward and Aversion: A Synaptic Plasticity Perspective

The brain is wired to predict future outcomes. Experience-dependent plasticity at excitatory synapses within dopamine neurons of the ventral tegmental area, a key region for a broad range of motivated behaviors, is thought to be a fundamental cellular mechanism that enables adaptation to a dynamic environment. Thus, depending on the circumstances, dopamine neurons are capable of processing both positive and negative reinforcement learning strategies. In this review, we will discuss how changes in synaptic plasticity of dopamine neurons may affect dopamine release, as well as behavioral adaptations to different environmental conditions falling at opposite ends of a saliency spectrum ranging from reward to aversion.

Reducing Ventral Tegmental Dopamine D2 Receptor Expression Selectively Boosts Incentive Motivation

Altered mesolimbic dopamine signaling has been widely implicated in addictive behavior. For the most part, this work has focused on dopamine within the striatum, but there is emerging evidence for a role of the auto-inhibitory, somatodendritic dopamine D2 receptor (D2R) in the ventral tegmental area (VTA) in addiction. Thus, decreased midbrain D2R expression has been implicated in addiction in humans. Moreover, knockout of the gene encoding the D2R receptor (Drd2) in dopamine neurons has been shown to enhance the locomotor response to cocaine in mice. Therefore, we here tested the hypothesis that decreasing D2R expression in the VTA of adult rats, using shRNA knockdown, promotes addiction-like behavior in rats responding for cocaine or palatable food. Rats with decreased VTA D2R expression showed markedly increased motivation for both sucrose and cocaine under a progressive ratio schedule of reinforcement, but the acquisition or maintenance of cocaine self-administration were not affected. They also displayed enhanced cocaine-induced locomotor activity, but no change in basal locomotion. This robust increase in incentive motivation was behaviorally specific, as we did not observe any differences in fixed ratio responding, extinction responding, reinstatement or conditioned suppression of cocaine, and sucrose seeking. We conclude that VTA D2R knockdown results in increased incentive motivation, but does not directly promote other aspects of addiction-like behavior.

Offline reactivation of experience-dependent neuronal firing patterns in the rat ventral tegmental area

In a rest period immediately after a task, neurons in the hippocampus, neocortex, and striatum exhibit spatiotemporal correlation patterns resembling those observed during the task. This reactivation has been proposed as a neurophysiological substrate for memory consolidation. We provide new evidence that rodent ventral tegmental area (VTA) neurons are selective for different types of food stimuli and that stimulus-sensitive neurons strongly reactivate during the rest period following a task that involved those stimuli. Reactivation occurred primarily during slow wave sleep and during quiet awakeness. In these experiments, VTA reactivation patterns were uncompressed and occurred at the firing rate level, rather than on a spike-to-spike basis. Mildly aversive stimuli were reactivated more often than positive ones. The VTA is a pivotal structure involved in the perception and prediction of reward and stimulus salience and is a key neuromodulatory system involved in synaptic plasticity. These results suggest new ways in which dopaminergic signals could contribute to the biophysical mechanisms of selective, system-wide, memory consolidation, and reconsolidation during sleep.

Neuronal circuits for fear and anxiety

Decades of research has identified the brain areas that are involved in fear, fear extinction, anxiety and related defensive behaviours. Newly developed genetic and viral tools, optogenetics and advanced in vivo imaging techniques have now made it possible to characterize the activity, connectivity and function of specific cell types within complex neuronal circuits. Recent findings that have been made using these tools and techniques have provided mechanistic insights into the exquisite organization of the circuitry underlying internal defensive states. This Review focuses on studies that have used circuit-based approaches to gain a more detailed, and also more comprehensive and integrated, view on how the brain governs fear and anxiety and how it orchestrates adaptive defensive behaviours.

Effects of ST1936, a selective serotonin-6 agonist, on electrical activity of putative mesencephalic dopaminergic neurons in the rat brain

The serotonin-6 (5-HT6) receptor is the most recently discovered serotonin receptor, and it represents an increasingly promising target for improving cognition in both normal and disease states. Recently, a new selective 5-HT6 receptor agonist, 2-(5 chloro-2-methyl-1H-indol-3-yl)-N,N-dimethylethanamine (ST1936), with nanomolar affinity for 5-HT6 receptors was described. We performed in-vivo electrophysiological studies to investigate the physiological role of 5-HT6 receptors in the control of the function of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA). Extracellular single-unit recordings were performed from putative dopamine-containing neurons in the SNc and VTA of anesthetised rats. In the SNc, acute systemic administration of ST1936 had no effects on basal firing activity of these dopamine neurons; however, in the VTA, ST1936 induced either dose-related increases (45% of cells) or decreases in basal activity of these dopaminergic neurons. Local application of ST1936 into the VTA caused excitation in all of the dopamine neurons, but had no effects on non-dopamine VTA neurons. Both effects of systemic and microiontophoretic ST1936 were completely reversed by the potent and selective 5-HT6 receptor antagonist 5-chloro-N-(4-methoxy-3-piperazin-1-ylphenyl)-3-methyl-2- benzothiophene-sulfonamide (SB271046). Systemic application of another 5-HT6 agonist, 2-(1-{6-chloroimidazo[2,1-b] [1,3]thiazole-5-sulfonyl}-1H-indol-3-yl)ethan-1-amine (WAY-181187), induced dose-dependent inhibition of these VTA dopaminergic neurons. ST1936 and WAY-181187 appear to have different effects on these VTA dopaminergic neurons, potentially due to different mechanisms of action or to the complexity of 5-HT6 receptor functions. Our data demonstrate the need for further investigations into the use of 5-HT6 receptor agonists to control cognitive disfunction, such as in schizophrenia and depression.

Enhanced serotonin and mesolimbic dopamine transmissions in a rat model of neuropathic pain

In humans, affective consequences of neuropathic pain, ranging from depression to anxiety and anhedonia, severely impair quality of life and are a major disease burden, often requiring specific medications. Depressive- and anxiety-like behaviors have also been observed in animal models of peripheral nerve injury. Dysfunctions in central nervous system monoamine transmission have been hypothesized to underlie depressive and anxiety disorders in neuropathic pain. To assess whether these neurons display early changes in their activity that in the long-term might lead to chronicization, maladaptive plasticity and affective consequences, we carried out in vivo extracellular single unit recordings from serotonin neurons in the dorsal raphe nucleus (DRN) and from dopamine neurons in ventral tegmental area (VTA) in the spared nerve injury (SNI) model of neuropathic pain in rats. Extracellular dopamine levels and the expression of dopamine D1, D2 receptors and tyrosine hydroxylase (TH) were measured in the nucleus accumbens. We report that, two weeks following peripheral nerve injury, discharge rate of serotonin DRN neurons and burst firing of VTA dopamine cells are enhanced, when compared with sham-operated animals. We also observed higher extracellular dopamine levels and reduced expression of D2, but not D1, receptors and TH in the nucleus accumbens. Our study confirms that peripheral neuropathy induces changes in the serotonin and dopamine systems that might be the early result of chronic maladaptation to persistent pain. The allostatic activation of these neural systems, which mirrors that already described as a consequence of stress, might lead to depression and anxiety previously observed in neuropathic animals but also an attempt to cope positively with the negative experience.

Considerations when using cre-driver rodent lines for studying ventral tegmental area circuitry

The use of Cre-driver rodent lines for targeting ventral tegmental area (VTA) cell types has generated important and novel insights into how precise neurocircuits regulate physiology and behavior. While this approach generally results in enhanced cellular specificity, an important issue has recently emerged related to the selectivity and penetrance of viral targeting of VTA neurons using several Cre-driver transgenic mouse lines. Here, we highlight several considerations when utilizing these tools to study the function of genetically defined neurocircuits. While VTA dopaminergic neurons have previously been targeted and defined by the expression of single genes important for aspects of dopamine neurotransmission, many VTA and neighboring cells display dynamic gene expression phenotypes that are partially consistent with both classically described dopaminergic and non-dopaminergic neurons. Thus, in addition to varying degrees of selectivity and penetrance, distinct Cre lines likely permit targeting of partially overlapping, but not identical VTA cell populations. This Matters Arising Response paper addresses the Lammel et al. (2015) Matters Arising paper, published concurrently in Neuron.

Activation of the GLP-1 receptors in the nucleus of the solitary tract reduces food reward behavior and targets the mesolimbic system

The gut/brain peptide, glucagon like peptide 1 (GLP-1), suppresses food intake by acting on receptors located in key energy balance regulating CNS areas, the hypothalamus or the hindbrain. Moreover, GLP-1 can reduce reward derived from food and motivation to obtain food by acting on its mesolimbic receptors. Together these data suggest a neuroanatomical segregation between homeostatic and reward effects of GLP-1. Here we aim to challenge this view and hypothesize that GLP-1 can regulate food reward behavior by acting directly on the hindbrain, the nucleus of the solitary tract (NTS), GLP-1 receptors (GLP-1R). Using two models of food reward, sucrose progressive ratio operant conditioning and conditioned place preference for food in rats, we show that intra-NTS microinjections of GLP-1 or Exendin-4, a stable analogue of GLP-1, inhibit food reward behavior. When the rats were given a choice between palatable food and chow, intra-NTS Exendin-4 treatment preferentially reduced intake of palatable food but not chow. However, chow intake and body weight were reduced by the NTS GLP-1R activation if chow was offered alone. The NTS GLP-1 activation did not alter general locomotor activity and did not induce nausea, measured by PICA. We further show that GLP-1 fibers are in close apposition to the NTS noradrenergic neurons, which were previously shown to provide a monosynaptic connection between the NTS and the mesolimbic system. Central GLP-1R activation also increased NTS expression of dopamine-β-hydroxylase, a key enzyme in noradrenaline synthesis, indicating a biological link between these two systems. Moreover, NTS GLP-1R activation altered the expression of dopamine-related genes in the ventral tegmental area. These data reveal a food reward-suppressing role of the NTS GLP-1R and indicate that the neurobiological targets underlying food reward control are not limited to the mesolimbic system, instead they are distributed throughout the CNS.

A concurrent excitation and inhibition of dopaminergic subpopulations in response to nicotine

Midbrain dopamine (DA) neurons are key players in motivation and reward processing. Increased DA release is thought to be central in the initiation of drug addiction. Whereas dopamine neurons are generally considered to be activated by drugs such as nicotine, we report here that nicotine not only induces excitation of ventral tegmental area (VTA) DA cells but also induces inhibition of a subset of VTA DA neurons that are anatomically segregated in the medial part of the VTA. These opposite responses do not correlate with the inhibition and excitation induced by noxious stimuli. We show that this inhibition requires D2 receptor (D2-R) activation, suggesting that a dopaminergic release is involved in the mechanism. Our findings suggest a principle of concurrent excitation and inhibition of VTA DA cells in response to nicotine. It promotes unexplored roles for DA release in addiction contrasting with the classical views of reinforcement and motivation, and give rise to a new interpretation of the mode of operation of the reward system.

Understanding opioid reward

Opioids are the most potent analgesics in clinical use; however, their powerful rewarding properties can lead to addiction. The scientific challenge is to retain analgesic potency while limiting the development of tolerance, dependence, and addiction. Both rewarding and analgesic actions of opioids depend upon actions at the mu opioid (MOP) receptor. Systemic opioid reward requires MOP receptor function in the midbrain ventral tegmental area (VTA) which contains dopaminergic neurons. VTA dopaminergic neurons are implicated in various aspects of reward including reward prediction error, working memory, and incentive salience. It is now clear that subsets of VTA neurons have different pharmacological properties and participate in separate circuits. The degree to which MOP receptor agonists act on different VTA circuits depends upon the behavioral state of the animal, which can be altered by manipulations such as food deprivation or prior exposure to MOP receptor agonists.

Temporal profiles dissociate regional extracellular ethanol versus dopamine concentrations

In vivo monitoring of dopamine via microdialysis has demonstrated that acute, systemic ethanol increases extracellular dopamine in regions innervated by dopaminergic neurons originating in the ventral tegmental area and substantia nigra. Simultaneous measurement of dialysate dopamine and ethanol allows comparison of the time courses of their extracellular concentrations. Early studies demonstrated dissociations between the time courses of brain ethanol concentrations and dopaminergic responses in the nucleus accumbens (NAc) elicited by acute ethanol administration. Both brain ethanol and extracellular dopamine levels peak during the first 5 min following systemic ethanol administration, but the dopamine response returns to baseline while brain ethanol concentrations remain elevated. Post hoc analyses examined ratios of the dopamine response (represented as a percent above baseline) to tissue concentrations of ethanol at different time points within the first 25-30 min in the prefrontal cortex, NAc core and shell, and dorsomedial striatum following a single intravenous infusion of ethanol (1 g/kg). The temporal patterns of these "response ratios" differed across brain regions, possibly due to regional differences in the mechanisms underlying the decline of the dopamine signal associated with acute intravenous ethanol administration and/or to the differential effects of acute ethanol on the properties of subpopulations of midbrain dopamine neurons. This Review draws on neurochemical, physiological, and molecular studies to summarize the effects of acute ethanol administration on dopamine activity in the prefrontal cortex and striatal regions, to explore the potential reasons for the regional differences observed in the decline of ethanol-induced dopamine signals, and to suggest directions for future research.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Stress and VTA synapses: implications for addiction and depression

While stressful experiences are a part of everyone's life, they can also exact a major toll on health. Stressful life experiences are associated with increased substance abuse, and there exists significant co-morbidity between mental illness and substance use disorders [N.D. Volkow & T.K. Li (2004) Nat. Rev. Neurosci., 5, 963-970; G. Koob & M.J. Kreek (2007) Am. J. Psych., 164, 1149-1159; R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130]. The risk for development of mood or anxiety disorders after stress is positively associated with the risk for substance use disorders [R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130], suggesting that there are common substrates for vulnerability to addictive and affective disorders. Understanding the molecular and physiological substrates of stress may lead to improved therapeutic interventions for the treatment of substance use disorders and mental illnesses.

Heterogeneity of dopamine neuron activity across traits and states

Midbrain dopamine neurons fire irregularly, with interspersed clusters of high-frequency spikes, commonly called 'bursts'. In this review we examine such heterogeneity in activity, and provide insight into how it can participate in psychiatric conditions such as drug addiction. We first describe several techniques used to evaluate dopamine neuron activity, and comment on the different measures that each provides. We next describe the activity of dopamine neurons in 'basal' conditions. Specifically, we discuss how the use of anesthesia and reduced preparations may alter aspects of dopamine cell activity, and how there is heterogeneity across species and regions. We also describe how dopamine cell firing changes throughout the peri-adolescent period and how dopamine neuron activity differs across the population. In the final section, we discuss how dopamine neuron activity changes in response to life events. First, we focus attention on drugs of abuse. Drugs themselves change firing activity through a variety of mechanisms, with effects on firing while drug is present differing from those seen after drug discontinuation. We then review how stimuli that are rewarding, aversive, or salient can evoke changes in firing rate and discharge pattern of dopamine neurons, and provide behavioral relevance of dopamine signaling. Finally, we discuss how stress can modulate dopamine neuron firing and how this may contribute to the role that stressful experiences play in psychiatric disorders such as addiction and depression.

Excitatory drive onto dopaminergic neurons in the rostral linear nucleus is enhanced by norepinephrine in an α1 adrenergic receptor-dependent manner

Dopaminergic innervation of the extended amygdala regulates anxiety-like behavior and stress responsivity. A portion of this dopamine input arises from dopamine neurons located in the ventral lateral periaqueductal gray (vlPAG) and rostral (RLi) and caudal linear nuclei of the raphe (CLi). These neurons receive substantial norepinephrine input, which may prime them for involvement in stress responses. Using a mouse line that expresses eGFP under control of the tyrosine hydroxylase promoter, we explored the physiology and responsiveness to norepinephrine of these neurons. We find that RLi dopamine neurons differ from VTA dopamine neurons with respect to membrane resistance, capacitance and the hyperpolarization-activated current, Ih. Further, we found that norepinephrine increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) on RLi dopamine neurons. This effect was mediated through the α1 adrenergic receptor (AR), as the actions of norepinephrine were mimicked by the α1-AR agonist methoxamine and blocked by the α1-AR antagonist prazosin. This action of norepinephrine on sEPSCs was transient, as it did not persist in the presence of prazosin. Methoxamine also increased the frequency of miniature EPSCs, indicating that the α1-AR action on glutamatergic transmission likely has a presynaptic mechanism. There was also a modest decrease in sEPSC frequency with the application of the α2-AR agonist UK-14,304. These studies illustrate a potential mechanism through which norepinephrine could recruit the activity of this population of dopaminergic neurons.

The heterogeneity of ventral tegmental area neurons: Projection functions in a mood-related context

The ventral tegmental area (VTA) in the brain's reward circuitry is composed of a heterogeneous population of dopamine, GABA, and glutamate neurons that play important roles in mediating mood-related functions including depression. These neurons project to different brain regions, including the nucleus accumbens (NAc), the medial prefrontal cortex (mPFC), and the amygdala. The functional understanding of these projection pathways has been improved since the extensive use of advanced techniques such as viral-mediated gene transfer, cell-type-specific neurophysiology and circuit-probing optogenetics. In this article, we will discuss the recent progress in understanding these VTA projection-specific functions, focusing on mood-related disorders.

Role of nicotinic acetylcholine receptors in regulating dopamine neuron activity

Midbrain dopamine (DA) neurons play a central role in a wide range of behaviors, from attention and motivation to motor control and reinforcement. The release of DA is modulated by a number of factors, and its deregulation has been implicated in multiple psychiatric disorders, such as addiction. In particular, nicotinic acetylcholine receptors (nAChRs) are key modulators of DA cells. Nicotine, the main addictive component in tobacco, strongly interacts with these receptors in the midbrain DA systems, resulting in reinforcing effects that are at the core of tobacco addiction. nAChRs are virtually expressed on every cell of the DA system, both at pre-, post- and extra-synaptic locations. The complex issue of interpreting the role of the large portfolio of different nAChR subtypes expressed on ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) neurons, and especially their role in defining functional DAergic subpopulations, is far from being solved. In this review we will try to provide the reader with an integrative view of the nicotinic modulation of DA neurons and its influence at the cellular, systemic and behavioral levels (exploratory behavior), as well as its implication in the reinforcing effects of nicotine.