Chapter I The organization and circuits of mesencephalic dopaminergic neurons and the distribution of dopamine receptors in the brain

Heterogeneity of mesencephalic dopaminergic neurons: From molecular classifications, electrophysiological properties to functional connectivity

The mesencephalic dopamine (DA) system is composed of neuronal subtypes that are molecularly and functionally distinct, are responsible for specific behaviors, and are closely associated with numerous brain disorders. Existing research has made significant advances in identifying the heterogeneity of mesencephalic DA neurons, which is necessary for understanding their diverse physiological functions and disease susceptibility. Moreover, there is a conflict regarding the electrophysiological properties of the distinct subsets of midbrain DA neurons. This review aimed to elucidate recent developments in the heterogeneity of midbrain DA neurons, including subpopulation categorization, electrophysiological characteristics, and functional connectivity to provide new strategies for accurately identifying distinct subtypes of midbrain DA neurons and investigating the underlying mechanisms of these neurons in various diseases.

Differential Roles of the D1- and D2-Like Dopamine Receptors Within the Ventral Tegmental Area in Modulating the Antinociception Induced by Forced Swim Stress in the Rat

Several preclinical and clinical studies indicate that exposure to acute stress may decrease pain perception and increases pain tolerance. This phenomenon is called stress-induced analgesia (SIA). A variety of neurotransmitters, including dopamine, is involved in the SIA. Dopaminergic neurons in the mesolimbic circuits, originating from the ventral tegmental area (VTA), play a crucial role in various motivational, rewarding, and pain events. The present study aimed to investigate the modulatory role of VTA dopaminergic receptors in the antinociceptive responses evoked by forced swim stress (FSS) in a model of acute pain. One hundred-five adult male albino Wistar rats were subjected to stereotaxic surgery for implanting a unilateral cannula into the VTA. After one week of recovery, separate groups of animals were given different doses of SCH23390 and Sulpiride (0.25, 1, and 4 µg/0.3 µl) as D1- and D2-like receptor antagonists into the VTA, respectively. Then, the animals were exposed to FSS for a 6-min period, and the pain threshold was measured using the tail-flick test over a 60-min time set intervals. Results indicated that exposure to FSS produces a prominent antinociceptive response, diminishing by blocking both dopamine receptors in the VTA. Nonetheless, the effect of a D1-like dopamine receptor antagonist on FSS-induced analgesia was more prominent than that of a D2-like dopamine receptor antagonist. The results demonstrated that VTA dopaminergic receptors contribute to the pain process in stressful situations, and it might be provided a practical approach to designing new therapeutic agents for pain management.

Distinct spatiotemporal brainstem pathways of outcome valence during reward- and punishment-based learning

Learning to seek rewards and avoid punishments, based on positive and negative choice outcomes, is essential for human survival. Yet, the neural underpinnings of outcome valence in the human brainstem and the extent to which they differ in reward and punishment learning contexts remain largely elusive. Here, using simultaneously acquired electroencephalography and functional magnetic resonance imaging data, we show that during reward learning the substantia nigra (SN)/ventral tegmental area (VTA) and locus coeruleus are initially activated following negative outcomes, while the VTA subsequently re-engages exhibiting greater responses for positive than negative outcomes, consistent with an early arousal/avoidance response and a later value-updating process, respectively. During punishment learning, we show that distinct raphe nucleus and SN subregions are activated only by negative outcomes with a sustained post-outcome activity across time, supporting the involvement of these brainstem subregions in avoidance behavior. Finally, we demonstrate that the coupling of these brainstem structures with other subcortical and cortical areas helps to shape participants' serial choice behavior in each context.

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Modulation of Neuron and Astrocyte Dopamine Receptors via Receptor-Receptor Interactions

Dopamine neurotransmission plays critical roles in regulating complex cognitive and behavioral processes including reward, motivation, reinforcement learning, and movement. Dopamine receptors are classified into five subtypes, widely distributed across the brain, including regions responsible for motor functions and specific areas related to cognitive and emotional functions. Dopamine also acts on astrocytes, which express dopamine receptors as well. The discovery of direct receptor-receptor interactions, leading to the formation of multimeric receptor complexes at the cell membrane and providing the cell decoding apparatus with flexible dynamics in terms of recognition and signal transduction, has expanded the knowledge of the G-protein-coupled receptor-mediated signaling processes. The purpose of this review article is to provide an overview of currently identified receptor complexes containing dopamine receptors and of their modulatory action on dopamine-mediated signaling between neurons and between neurons and astrocytes. Pharmacological possibilities offered by targeting receptor complexes in terms of addressing neuropsychiatric disorders associated with altered dopamine signaling will also be briefly discussed.

Structure and function of neocortical layer 6b

Cortical layer 6b is considered by many to be a remnant of the subplate that forms during early stages of neocortical development, but its role in the adult is not well understood. Its neuronal complement has only recently become the subject of systematic studies, and its axonal projections and synaptic input structures have remained largely unexplored despite decades of research into neocortical function. In recent years, however, layer 6b (L6b) has attracted increasing attention and its functional role is beginning to be elucidated. In this review, I will attempt to provide an overview of what is currently known about the excitatory and inhibitory neurons in this layer, their pre- and postsynaptic connectivity, and their functional implications. Similarities and differences between different cortical areas will be highlighted. Finally, layer 6b neurons are highly responsive to several neuropeptides such as orexin/hypocretin, neurotensin and cholecystokinin, in some cases exclusively. They are also strongly controlled by neurotransmitters such as acetylcholine and norepinephrine. The interaction of these neuromodulators with L6b microcircuitry and its functional consequences will also be discussed.

Single-cell and single-nuclei RNA sequencing as powerful tools to decipher cellular heterogeneity and dysregulation in neurodegenerative diseases

Neurodegenerative diseases affect millions of people worldwide and there are currently no cures. Two types of common neurodegenerative diseases are Alzheimer's (AD) and Parkinson's disease (PD). Single-cell and single-nuclei RNA sequencing (scRNA-seq and snRNA-seq) have become powerful tools to elucidate the inherent complexity and dynamics of the central nervous system at cellular resolution. This technology has allowed the identification of cell types and states, providing new insights into cellular susceptibilities and molecular mechanisms underlying neurodegenerative conditions. Exciting research using high throughput scRNA-seq and snRNA-seq technologies to study AD and PD is emerging. Herein we review the recent progress in understanding these neurodegenerative diseases using these state-of-the-art technologies. We discuss the fundamental principles and implications of single-cell sequencing of the human brain. Moreover, we review some examples of the computational and analytical tools required to interpret the extensive amount of data generated from these assays. We conclude by highlighting challenges and limitations in the application of these technologies in the study of AD and PD.

Cognitive enhancement: Effects of methylphenidate, modafinil, and caffeine on latent memory and resting state functional connectivity in healthy adults

Stimulants like methylphenidate, modafinil, and caffeine have repeatedly shown to enhance cognitive processes such as attention and memory. However, brain-functional mechanisms underlying such cognitive enhancing effects of stimulants are still poorly characterized. Here, we utilized behavioral and resting-state fMRI data from a double-blind randomized placebocontrolled study of methylphenidate, modafinil, and caffeine in 48 healthy male adults. The results show that performance in different memory tasks is enhanced, and functional connectivity (FC) specifically between the frontoparietal network (FPN) and default mode network (DMN) is modulated by the stimulants in comparison to placebo. Decreased negative connectivity between right prefrontal and medial parietal but also between medial temporal lobe and visual brain regions predicted stimulant-induced latent memory enhancement. We discuss dopamine's role in attention and memory as well as its ability to modulate FC between large-scale neural networks (e.g., FPN and DMN) as a potential cognitive enhancement mechanism.

Defining the interconnectivity of the medial prefrontal cortex and ventral midbrain

Dysfunction in dopamine (DA) signaling contributes to neurological disorders ranging from drug addiction and schizophrenia to depression and Parkinson’s Disease. How might impairment of one neurotransmitter come to effect these seemingly disparate diseases? One potential explanation is that unique populations of DA-releasing cells project to separate brain regions that contribute to different sets of behaviors. Though dopaminergic cells themselves are spatially restricted to the midbrain and constitute a relatively small proportion of all neurons, their projections influence many brain regions. DA is particularly critical for the activity and function of medial prefrontal cortical (mPFC) ensembles. The midbrain and mPFC exhibit reciprocal connectivity – the former innervates the mPFC, and in turn, the mPFC projects back to the midbrain. Viral mapping studies have helped elucidate the connectivity within and between these regions, which likely have broad implications for DA-dependent behaviors. In this review, we discuss advancements in our understanding of the connectivity between the mPFC and midbrain DA system, focusing primarily on rodent models.

Prefrontal Dopaminergic Regulation of Cue-Guided Risky Decision-Making Performance in Rats

Risky decision-making is the decision made by individuals when they know the probability of each outcome. In order to survive in unpredictable environments, it is necessary for individuals to assess the probability of events occurring to an make appropriate decisions. There are few studies on the neural basis of risky decision-making behavior guided by external cues, which is related to the relative paucity of animal behavioral paradigms. Previous studies have shown that the prefrontal cortex (PFC) plays a key role in risk-based decision-making. The PFC receives projections from the dopamine (DA) system from the ventral tegmental area of the midbrain. The mesocorticolimbic DA system regulates the judgments of reward and value in decision-making. However, the specific receptor mechanism for prefrontal DA regulation of cue-guided risky decision-making behavior remains unclear. Here we established a cue-guided risky decision-making behavioral paradigm (RDM task) to detect the behavior of rats making decisions between a small certain reward and a large uncertain reward in a self-paced manner. The D1 receptor antagonist SCH-23390 (5 mM) or agonist SKF-82958 (5 mM), and the D2 receptor antagonist thioridazine hydrochloride (5 mM) or agonist MLS-1547 (5 mM) was injected into the mPFC, respectively, to investigate how the behavior in the RDM task was changed. The results showed that: (1) rats were able to master the operation of the cue-guided RDM task in a self-paced way; (2) a majority of rats were inclined to choose risk rather than a safe option when the reward expectations were equal; and (3) risk selection was reduced upon inhibition of D1 receptors or stimulation of D2 receptors, but increased upon stimulation of D1 receptors or inhibition of D2 receptors, suggesting that the RDM performance is regulated by D1 and D2 receptors in the mPFC. The present results suggest that DA receptors in the mPFC of rats are involved in regulating cue-guided RDM behavior, with differential involvement of D1 and D2 receptors in the regulation.

New frontiers in Alzheimer's disease diagnostic: Monoamines and their derivatives in biological fluids

Current diagnosis of Alzheimer's disease (AD) relies on a combination of neuropsychological evaluations, biomarker measurements and brain imaging. Nevertheless, these approaches are either expensive, invasive or lack sensitivity to early AD stages. The main challenge of ongoing research is therefore to identify early non-invasive biomarkers to diagnose AD at preclinical stage. Accumulating evidence support the hypothesis that initial degeneration of profound monoaminergic nuclei may trigger a transneuronal spread of AD pathology towards hippocampus and cortex. These studies aroused great interest on monoamines, i.e. noradrenaline (NA), dopamine (D) ad serotonin (5-HT), as early hallmarks of AD pathology. The present work reviews current literature on the potential role of monoamines and related metabolites as biomarkers of AD. First, morphological changes in the monoaminergic systems during AD are briefly described. Second, we focus on concentration changes of these molecules and their derivatives in biological fluids, including cerebrospinal fluid, obtained by lumbar puncture, and blood or urine, sampled via less invasive procedures. Starting from initial observations, we then discuss recent insights on metabolomics-based analysis, highlighting the promising clinical utility of monoamines for the identification of a molecular AD signature, aimed at improving early diagnosis and discrimination from other dementia.

Function and Regulation of ALDH1A1-Positive Nigrostriatal Dopaminergic Neurons in Motor Control and Parkinson's Disease

Dopamine is an important chemical messenger in the brain, which modulates movement, reward, motivation, and memory. Different populations of neurons can produce and release dopamine in the brain and regulate different behaviors. Here we focus our discussion on a small but distinct group of dopamine-producing neurons, which display the most profound loss in the ventral substantia nigra pas compacta of patients with Parkinson's disease. This group of dopaminergic neurons can be readily identified by a selective expression of aldehyde dehydrogenase 1A1 (ALDH1A1) and accounts for 70% of total nigrostriatal dopaminergic neurons in both human and mouse brains. Recently, we presented the first whole-brain circuit map of these ALDH1A1-positive dopaminergic neurons and reveal an essential physiological function of these neurons in regulating the vigor of movement during the acquisition of motor skills. In this review, we first summarize previous findings of ALDH1A1-positive nigrostriatal dopaminergic neurons and their connectivity and functionality, and then provide perspectives on how the activity of ALDH1A1-positive nigrostriatal dopaminergic neurons is regulated through integrating diverse presynaptic inputs and its implications for potential Parkinson's disease treatment.

Behavioral determinants in the expression of the Kamin blocking effect: Implications for associative learning theory

Associative learning makes important contributions to our behavior and decisions. The Kamin blocking effect is an associative learning phenomenon that plays a central role in understanding of the psychological principles underlying associative learning. However, several recent failures to replicate the blocking effect suggest that the conditions necessary for blocking are poorly understood. To understand the conditions necessary for blocking, here we review studies into the expression of blocking in subjects that either approach and interact with the conditioned cue (sign trackers) or approach and interact with the reward location (goal trackers) during appetitive classical conditioning. Psychological theory and the neurophysiological correlates of appetitive classical conditioning make opposing predictions regarding the expression of blocking in sign and goal trackers. We reconcile these opposing predictions in a qualitative model using two parallel learning processes. Such models offer a better framework for understanding the psychological associative structures acquired during learning, their interactions contributing to the conditioned response, and how they affect subsequent learning and the expression of the Kamin blocking effect.

Cholinergic and dopaminergic effects on prediction error and uncertainty responses during sensory associative learning

Navigating the physical world requires learning probabilistic associations between sensory events and their change in time (volatility). Bayesian accounts of this learning process rest on hierarchical prediction errors (PEs) that are weighted by estimates of uncertainty (or its inverse, precision). In a previous fMRI study we found that low-level precision-weighted PEs about visual outcomes (that update beliefs about associations) activated the putative dopaminergic midbrain; by contrast, precision-weighted PEs about cue-outcome associations (that update beliefs about volatility) activated the cholinergic basal forebrain. These findings suggested selective dopaminergic and cholinergic influences on precision-weighted PEs at different hierarchical levels. Here, we tested this hypothesis, repeating our fMRI study under pharmacological manipulations in healthy participants. Specifically, we performed two pharmacological fMRI studies with a between-subject double-blind placebo-controlled design: study 1 used antagonists of dopaminergic (amisulpride) and muscarinic (biperiden) receptors, study 2 used enhancing drugs of dopaminergic (levodopa) and cholinergic (galantamine) modulation. Pooled across all pharmacological conditions of study 1 and study 2, respectively, we found that low-level precision-weighted PEs activated the midbrain and high-level precision-weighted PEs the basal forebrain as in our previous study. However, we found pharmacological effects on brain activity associated with these computational quantities only when splitting the precision-weighted PEs into their PE and precision components: in a brainstem region putatively containing cholinergic (pedunculopontine and laterodorsal tegmental) nuclei, biperiden (compared to placebo) enhanced low-level PE responses and attenuated high-level PE activity, while amisulpride reduced high-level PE responses. Additionally, in the putative dopaminergic midbrain, galantamine compared to placebo enhanced low-level PE responses (in a body-weight dependent manner) and amisulpride enhanced high-level precision activity. Task behaviour was not affected by any of the drugs. These results do not support our hypothesis of a clear-cut dichotomy between different hierarchical inference levels and neurotransmitter systems, but suggest a more complex interaction between these neuromodulatory systems and hierarchical Bayesian quantities. However, our present results may have been affected by confounds inherent to pharmacological fMRI. We discuss these confounds and outline improved experimental tests for the future.

Distinct Connectivity and Functionality of Aldehyde Dehydrogenase 1a1-Positive Nigrostriatal Dopaminergic Neurons in Motor Learning

Parkinson's disease causes the most profound loss of the aldehyde dehydrogenase 1A1-positive (ALDH1A1⁺) nigrostriatal dopaminergic neuron (nDAN) subpopulation. The connectivity and functionality of ALDH1A1⁺ nDANs, however, remain poorly understood. Here, we show in rodent brains that ALDH1A1⁺ nDANs project predominantly to the rostral dorsal striatum, from which they also receive most monosynaptic inputs, indicating extensive reciprocal innervations with the striatal spiny projection neurons (SPNs). Functionally, genetic ablation of ALDH1A1⁺ nDANs causes severe impairments in motor skill learning, along with a reduction in high-speed walking. While dopamine replacement therapy accelerated walking speed, it failed to improve motor skill learning in ALDH1A1⁺ nDAN-ablated mice. Altogether, our study provides a comprehensive whole-brain connectivity map and reveals a key physiological function of ALDH1A1⁺ nDANs in motor skill acquisition, suggesting the motor learning processes require ALDH1A1⁺ nDANs to integrate diverse presynaptic inputs and supply dopamine with dynamic precision.

Strategies for the Treatment of Parkinson's Disease: Beyond Dopamine

Parkinson’s disease (PD) is the second-leading cause of dementia and is characterized by a progressive loss of dopaminergic neurons in the substantia nigra alongside the presence of intraneuronal α-synuclein-positive inclusions. Therapies to date have been directed to the restoration of the dopaminergic system, and the prevention of dopaminergic neuronal cell death in the midbrain. This review discusses the physiological mechanisms involved in PD as well as new and prospective therapies for the disease. The current data suggest that prevention or early treatment of PD may be the most effective therapeutic strategy. New advances in the understanding of the underlying mechanisms of PD predict the development of more personalized and integral therapies in the years to come. Thus, the development of more reliable biomarkers at asymptomatic stages of the disease, and the use of genetic profiling of patients will surely permit a more effective treatment of PD.

Dopamine tunes prefrontal outputs to orchestrate aversive processing

Decades of research suggest that the mesocortical dopamine system exerts powerful control over mPFC physiology and function. Indeed, dopamine signaling in the medial prefrontal cortex (mPFC) is implicated in a vast array of processes, including working memory, stimulus discrimination, stress responses, and emotional and behavioral control. Consequently, even slight perturbations within this delicate system result in profound disruptions of mPFC-mediated processes. Many neuropsychiatric disorders are associated with dysregulation of mesocortical dopamine, including schizophrenia, depression, attention deficit hyperactivity disorder, post-traumatic stress disorder, among others. Here, we review the anatomy and functions of the mesocortical dopamine system. In contrast to the canonical role of striatal dopamine in reward-related functions, recent work has revealed that mesocortical dopamine fine-tunes distinct efferent projection populations in a manner that biases subsequent behavior towards responding to stimuli associated with potentially aversive outcomes. We propose a framework wherein dopamine can serve as a signal for switching mPFC states by orchestrating how information is routed to the rest of the brain.

Putative dopamine neurons in the ventral tegmental area enhance information coding in the prefrontal cortex

It has been proposed that neuronal populations in the prefrontal cortex (PFC) robustly encode task-relevant information through an interplay with the ventral tegmental area (VTA). Yet, the precise computation underlying such functional interaction remains elusive. Here, we conducted simultaneous recordings of single-unit activity in PFC and VTA of rats performing a GO/NoGO task. We found that mutual information between stimuli and neural activity increases in the PFC as soon as stimuli are presented. Notably, it is the activity of putative dopamine neurons in the VTA that contributes critically to enhance information coding in the PFC. The higher the activity of these VTA neurons, the better the conditioned stimuli are encoded in the PFC.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

The novel atypical antipsychotic cariprazine demonstrates dopamine D2 receptor-dependent partial agonist actions on rat mesencephalic dopamine neuronal activity

Aim

Cariprazine, a dopamine D₃‐preferring D₃/D₂ receptor partial agonist, is FDA approved for the treatment of schizophrenia and acute manic or mixed episodes of bipolar disorder. This study used in vivo electrophysiological techniques in anesthetized rats to determine cariprazine's effect on dopaminergic cell activity in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc).

Methods

Extracellular recordings of individual dopaminergic neurons were performed after oral or intravenous administration of cariprazine, the D₃ receptor antagonist SB 277011A, the D₂ receptor antagonist L741,626, and/or the D₃ receptor agonist PD 128,907.

Results

Acute oral treatment with cariprazine significantly increased and chronic cariprazine significantly decreased the number of spontaneously firing dopaminergic neurons in the VTA, but not in the SNc. Intravenous administration of cariprazine partially but significantly inhibited dopaminergic neuronal firing in both regions, which was prevented by L741,626 but not SB 277011A. In both VTA and SNc, cariprazine, SB 277011A, and L741,626 significantly antagonized the suppression of dopamine cell firing elicited by PD 128,907.

Conclusions

Cariprazine significantly modulates the number of spontaneously active VTA dopamine neurons and moderately suppresses midbrain dopamine neuronal activity. The contribution of dopamine D₂ receptors to cariprazine's in vivo effects is prevalent and that of D₃ receptors is less apparent.

Dissociation of reward and effort sensitivity in methcathinone-induced Parkinsonism

Methcathinone‐induced Parkinsonism is a recently described extrapyramidal syndrome characterized by globus pallidus and substantia nigra lesions, which provides a unique model of basal ganglia dysfunction. We assessed motivated behaviour in this condition using a novel cost‐benefit decision‐making task, in which participants decided whether it was worth investing effort for reward. Patients showed a dissociation between reward and effort sensitivity, such that pallidonigral complex dysfunction caused them to become less sensitive to rewards, while normal sensitivity to effort costs was maintained.

The Dopamine Prediction Error: Contributions to Associative Models of Reward Learning

Phasic activity of midbrain dopamine neurons is currently thought to encapsulate the prediction-error signal described in Sutton and Barto’s (1981) model-free reinforcement learning algorithm. This phasic signal is thought to contain information about the quantitative value of reward, which transfers to the reward-predictive cue after learning. This is argued to endow the reward-predictive cue with the value inherent in the reward, motivating behavior toward cues signaling the presence of reward. Yet theoretical and empirical research has implicated prediction-error signaling in learning that extends far beyond a transfer of quantitative value to a reward-predictive cue. Here, we review the research which demonstrates the complexity of how dopaminergic prediction errors facilitate learning. After briefly discussing the literature demonstrating that phasic dopaminergic signals can act in the manner described by Sutton and Barto (1981), we consider how these signals may also influence attentional processing across multiple attentional systems in distinct brain circuits. Then, we discuss how prediction errors encode and promote the development of context-specific associations between cues and rewards. Finally, we consider recent evidence that shows dopaminergic activity contains information about causal relationships between cues and rewards that reflect information garnered from rich associative models of the world that can be adapted in the absence of direct experience. In discussing this research we hope to support the expansion of how dopaminergic prediction errors are thought to contribute to the learning process beyond the traditional concept of transferring quantitative value.

Effects of dorsal hippocampus catecholamine depletion on paired-associates learning and place learning in rats

Growing evidence suggests that the catecholamine (CA) neurotransmitters dopamine and noradrenaline support hippocampus-mediated learning and memory. However, little is known to date about which forms of hippocampus-mediated spatial learning are modulated by CA signaling in the hippocampus. Therefore, in the current study we examined the effects of 6-hydroxydopamine-induced CA depletion in the dorsal hippocampus on two prominent forms of hippocampus-based spatial learning, that is learning of object-location associations (paired-associates learning) as well as learning and choosing actions based on a representation of the context (place learning). Results show that rats with CA depletion of the dorsal hippocampus were able to learn object-location associations in an automated touch screen paired-associates learning (PAL) task. One possibility to explain this negative result is that object-location learning as tested in the touchscreen PAL task seems to require relatively little hippocampal processing. Results further show that in rats with CA depletion of the dorsal hippocampus the use of a response strategy was facilitated in a T-maze spatial learning task. We suspect that impaired hippocampus CA signaling may attenuate hippocampus-based place learning and favor dorsolateral striatum-based response learning.

Pathway-Specific Dopamine Abnormalities in Schizophrenia

In light of the clinical evidence implicating dopamine in schizophrenia and the prominent hypotheses put forth regarding alterations in dopaminergic transmission in this disease, molecular imaging has been used to examine multiple aspects of the dopaminergic system. We review the imaging methods used and compare the findings across the different molecular targets. Findings have converged to suggest early dysregulation in the striatum, especially in the rostral caudate, manifesting as excess synthesis and release. Recent data showed deficit extending to most cortical regions and even to other extrastriatal subcortical regions not previously considered to be "hypodopaminergic" in schizophrenia. These findings yield a new topography for the dopaminergic dysregulation in schizophrenia. We discuss the dopaminergic innervation within the individual projection fields to provide a topographical map of this dual dysregulation and explore potential cellular and circuit-based mechanisms for brain region-dependent alterations in dopaminergic parameters. This refined knowledge is essential to better guide translational studies and efforts in early drug development.

Diversity matters - heterogeneity of dopaminergic neurons in the ventral mesencephalon and its relation to Parkinson's Disease

Dopaminergic neurons in the ventral mesencephalon (the ventral mesencephalic dopaminergic complex) are known for their role in a multitude of behaviors, including cognition, reward, addiction and voluntary movement. Dysfunctions of these neurons are the underlying cause of various neuropsychiatric disorders, such as depression, addiction and schizophrenia. In addition, Parkinson's disease (PD), which is the second most common degenerative disease in developed countries, is characterized by the degeneration of dopaminergic neurons, leading to the core motor symptoms of the disease. However, only a subset of dopaminergic neurons in the ventral mesencephalon is highly vulnerable to the disease process. Indeed, research over several decades revealed that the neurons in the ventral mesencephalic dopaminergic complex do not form a homogeneous group with respect to anatomy, physiology, function, molecular identity or vulnerability/dysfunction in different diseases. Here, we review how the concept of dopaminergic neuron diversity, assisted by the advent and application of new technologies, evolved and was refined over time and how it shaped our understanding of PD pathogenesis. Understanding this diversity of neurons in the ventral mesencephalic dopaminergic complex at all levels is imperative for the development of new and more selective drugs for both PD and various other neuropsychiatric diseases. Several decades of research revealed that the neurons in the ventral mesencephalic dopaminergic complex do not form a homogeneous group in respect to anatomy, physiology, function, molecular identity or vulnerability/dysfunction in diseases like Parkinson's disease (PD). Here, we review how this concept evolved and was refined over time and how it shaped our understanding of the pathogenesis of PD. Source of the midbrain image: www.wikimd.org/wiki/index.php/The_Midbrain_or_Mesencephalon; downloaded 28.01.2016. See also Figures and of the paper. This article is part of a special issue on Parkinson disease.

Models of neuromodulation for computational psychiatry

Psychiatry faces fundamental challenges: based on a syndrome-based nosology, it presently lacks clinical tests to infer on disease processes that cause symptoms of individual patients and must resort to trial-and-error treatment strategies. These challenges have fueled the recent emergence of a novel field-computational psychiatry-that strives for mathematical models of disease processes at physiological and computational (information processing) levels. This review is motivated by one particular goal of computational psychiatry: the development of 'computational assays' that can be applied to behavioral or neuroimaging data from individual patients and support differential diagnosis and guiding patient-specific treatment. Because the majority of available pharmacotherapeutic approaches in psychiatry target neuromodulatory transmitters, models that infer (patho)physiological and (patho)computational actions of different neuromodulatory transmitters are of central interest for computational psychiatry. This article reviews the (many) outstanding questions on the computational roles of neuromodulators (dopamine, acetylcholine, serotonin, and noradrenaline), outlines available evidence, and discusses promises and pitfalls in translating these findings to clinical applications. WIREs Cogn Sci 2017, 8:e1420. doi: 10.1002/wcs.1420 For further resources related to this article, please visit the WIREs website.

Modulation of Postnatal Neurogenesis by Perinatal Asphyxia: Effect of D1 and D2 Dopamine Receptor Agonists

Perinatal asphyxia (PA) is associated to delayed cell death, affecting neurocircuitries of basal ganglia and hippocampus, and long-term neuropsychiatric disabilities. Several compensatory mechanisms have been suggested to take place, including cell proliferation and neurogenesis. There is evidence that PA can increase postnatal neurogenesis in hippocampus and subventricular zone (SVZ), modulated by dopamine, by still unclear mechanisms. We have studied here the effect of selective dopamine receptor agonists on cell death, cell proliferation and neurogenesis in organotypic cultures from control and asphyxia-exposed rats. Hippocampus and SVZ sampled at 1-3 postnatal days were cultured for 20-21 days. At day in vitro (DIV) 19, cultures were treated either with SKF38393 (10 and 100 µM, a D₁ agonist), quinpirole (10 µM, a D₂ agonist) or sulpiride (10 μM, a D₂ antagonist) + quinpirole (10 μM) and BrdU (10 μM, a mitosis marker) for 24 h. At DIV 20-21, cultures were processed for immunocytochemistry for microtubule-associated protein-2 (MAP-2, a neuronal marker), and BrdU, evaluated by confocal microscopy. Some cultures were analysed for cell viability at DIV 20-21 (LIVE/DEAD kit). PA increased cell death, cell proliferation and neurogenesis in hippocampus and SVZ cultures. The increase in cell death, but not in cell proliferation, was inhibited by both SKF38393 and quinpirole treatment. Neurogenesis was increased by quinpirole, but only in hippocampus, in cultures from both asphyxia-exposed and control-animals, effect that was antagonised by sulpiride, leading to the conclusion that dopamine modulates neurogenesis in hippocampus, mainly via D₂ receptors.

Effects of Dopamine and Serotonin Systems on Modulating Neural Oscillations in Hippocampus-Prefrontal Cortex Pathway in Rats

Theta and gamma oscillations are believed to play an important role in cognition and memory, and their phase coupling facilitates the information transmission in hippocampal-cortex network. In a rat model of chronic stress, the phase coupling of both theta and gamma oscillations between ventral hippocampal CA1 (vCA1) and medial prefrontal cortex (mPFC) was found to be disrupted, which was associated with the impaired synaptic plasticity in the pathway. However, little was known about the mechanisms underlying the process. In order to address this issue, both dopamine and serotonin as monoaminergic neurotransmitters were involved in this study, since they were crucial factors in pathological basis of depressive disorder. Local field potentials (LFPs) were recorded simultaneously at both vCA1 and mPFC regions under anesthesia, before and after the injection of dopamine D1 receptor antagonist and 5-HT1A receptor agonist, respectively. The results showed that the blockage of D1 receptor could lead to depression-like decrement on theta phase coupling. In addition, the activation of 5-HT1A receptor enhanced vCA1-mPFC coupling on gamma oscillations, and attenuated CA1 theta-fast gamma cross frequency coupling. These data suggest that the theta phase coupling between vCA1 and mPFC may be modulated by dopamine system that is an underlying mechanism of the cognitive dysfunction in depression. Besides, the serotonergic system is probably involved in the regulation of gamma oscillations coupling in vCA1-mPFC network.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Functional role of Barrington's nucleus in the micturition reflex: relevance in the surgical treatment of Parkinson's disease

The pontine micturition center or Barrington's nucleus (BN) - besides regulating micturition - co-regulates the activity of other pelvic viscera such as the colon and genitals. At present, this issue is gaining particular importance due to: (i) recent findings of α-synuclein in BN, (ii) known urinary dysfunction in parkinsonian patients (part of the so-called non-motor symptoms), other patients with dementia and as in very old individuals; and (iii) its proximity to the pedunculopontine nucleus, a surgical target in deep brain stimulation for Parkinson's disease (PD). The structural and functional organization of the micturition reflex comprises a coordinating action of somatic motor activity with both divisions of the autonomic nervous system, modulated by trunk encephalic and cortical centers that involve the BN as locus coeruleus and periaqueductal gray matter, among other trunk encephalic structures. The involvement of dopaminergic activity (physiologic inhibition of the micturition reflex mediated by dopaminergic D1 activity) that diminishes in Parkinsonism and leads to overactivity of the micturition reflex is also well known. In this review, the integrating role of the BN in the context of vesical and gastrointestinal behavior is revisited, and the principal morpho-functional findings that associate dysfunction with the urinary disorders that appear during the pre-motor stages of PD are summarized.

Dopamine in motivational control: rewarding, aversive, and alerting

Midbrain dopamine neurons are well known for their strong responses to rewards and their critical role in positive motivation. It has become increasingly clear, however, that dopamine neurons also transmit signals related to salient but nonrewarding experiences such as aversive and alerting events. Here we review recent advances in understanding the reward and nonreward functions of dopamine. Based on this data, we propose that dopamine neurons come in multiple types that are connected with distinct brain networks and have distinct roles in motivational control. Some dopamine neurons encode motivational value, supporting brain networks for seeking, evaluation, and value learning. Others encode motivational salience, supporting brain networks for orienting, cognition, and general motivation. Both types of dopamine neurons are augmented by an alerting signal involved in rapid detection of potentially important sensory cues. We hypothesize that these dopaminergic pathways for value, salience, and alerting cooperate to support adaptive behavior.

Chronic atrazine exposure causes disruption of the spontaneous locomotor activity and alters the striatal dopaminergic system of the male Sprague-Dawley rat

The herbicide atrazine (ATR) is widely used around the world, and is a potential toxicant of the dopaminergic systems. Nigrostriatal and mesolimbic systems are the two major dopaminergic pathways of the central nervous system; they play key roles mediating a wide array of critical motor and cognitive functions. We evaluated the effects of exposing male rats for one year to 10 mg ATR/kg B.W. on these systems using motor and cognitive tasks and measuring monoamine content in the striatum, nucleus accumbens, prefrontal cortex, and hypothalamus. ATR administration resulted in impaired motor coordination and greater spontaneous locomotor activity only after 10 to 12 months of exposure. Chronic exposure to 10 mg ATR decreased striatal dopamine, but had no effect on accumbal, hypothalamic or cortical monoamine content. Chronic ATR exposure caused discrete changes in learning tasks that involve either the striatum or the nucleus accumbens. These results indicate that chronic exposure to ATR preferentially targets the nigrostriatal dopaminergic pathway, in comparison to the other dopaminergic pathways evaluated in this study, inducing behavioral and neurochemical alterations. In order to unveil the full extent of atrazine's effects on the nervous system, other neurochemical systems should be considered in future studies.

Functional organization of the basal ganglia: therapeutic implications for Parkinson's disease

The basal ganglia (BG) are a highly organized network, where different parts are activated for specific functions and circumstances. The BG are involved in movement control, as well as associative learning, planning, working memory, and emotion. We concentrate on the "motor circuit" because it is the best understood anatomically and physiologically, and because Parkinson's disease is mainly thought to be a movement disorder. Normal function of the BG requires fine tuning of neuronal excitability within each nucleus to determine the exact degree of movement facilitation or inhibition at any given moment. This is mediated by the complex organization of the striatum, where the excitability of medium spiny neurons is controlled by several pre- and postsynaptic mechanisms as well as interneuron activity, and secured by several recurrent or internal BG circuits. The motor circuit of the BG has two entry points, the striatum and the subthalamic nucleus (STN), and an output, the globus pallidus pars interna (GPi), which connects to the cortex via the motor thalamus. Neuronal afferents coding for a given movement or task project to the BG by two different systems: (1) Direct disynaptic projections to the GPi via the striatum and STN. (2) Indirect trisynaptic projections to the GPi via the globus pallidus pars externa (GPe). Corticostriatal afferents primarily act to inhibit medium spiny neurons in the "indirect circuit" and facilitate neurons in the "direct circuit." The GPe is in a pivotal position to regulate the motor output of the BG. Dopamine finely tunes striatal input as well as neuronal striatal activity, and modulates GPe, GPi, and STN activity. Dopaminergic depletion in Parkinson's disease disrupts the corticostriatal balance leading to increased activity the indirect circuit and reduced activity in the direct circuit. The precise chain of events leading to increased STN activity is not completely understood, but impaired dopaminergic regulation of the GPe, GPi, and STN may be involved. The parkinsonian state is characterized by disruption of the internal balance of the BG leading to hyperactivity in the two main entry points of the network (striatum and STN) and excessive inhibitory output from the GPi. Replacement therapy with standard levodopa creates a further imbalance, producing an abnormal pattern of neuronal discharge and synchronization of neuronal firing that sustain the "off" and "on with dyskinesia" states. The effect of levodopa is robust but short-lasting and converts the parkinsonian BG into a highly unstable system, where pharmacological and compensatory effects act in opposing directions. This creates a scenario that substantially departs from the normal physiological state of the BG.

Galactosylated dopamine enters into the brain, blocks the mesocorticolimbic system and modulates activity and scanning time in Naples high excitability rats

Pathological conditions, such as Parkinson's disease and attention deficit hyperactivity disorder, have been linked to alterations of specific dopamine (DA) pathways. However, since exogenous DA does not cross the blood-brain barrier, DA levels can be modulated e.g. by DA precursors or DA reuptake blockers. Hereby histochemical, analytical and behavioral evidence shows that a galactosylated form of DA (GAL-DA) carries DA into the brain, thus modulating activity and nonselective attention in rats. To this aim adult male rats of the Naples high-excitability (NHE) and random bred controls (NRB) lines were given a single i.p. injection of GAL-DA (10 or 100 mg/kg). Three hours later the behavior was videotaped and analyzed for horizontal activity, orienting frequency and scanning duration. The dose of 100 mglkg of GAL-DA reduced by 25% the horizontal activity in NHE rats, mainly in the first part of the testing period. No effect was observed on orienting frequency or on scanning duration. However, GAL-DA 100 mg/kg was associated with longer rearing episodes in the second part of the testing period in NHE rats. In parallel experiments histochemistry with a galactose-specific lectin showed 10% increase in galactose residues into the striatum between 0.5 and 3.0 h. To quantify the level of GAL-DA, its metabolite DA-succinate and DA in the prefrontal cortex, neostriatum, and cerebellum, rats were killed 2.0 h after the injection of prodrug. Mass high performance liquid chromatography (HPLC) was used for analysis of GAL-DA and DA succinate whereas electrochemical HPLC for DA. Both HPLC techniques demonstrate that GAL-DA carries and releases DA into the brain. Specifically 100 mg/kg of GAL-DA increased DA level in the striatum in the NHE rats only. Moreover, DA in the mesencephalon (MES) was correlated positively with striatal and prefrontal cortex DA in NHE rats. In contrast DA in the MES was negatively correlated with striatal DA in NRB. GAL-DA disrupted these correlations in both rat lines. Thus, this new DA prodrug may modify DA neurotransmission and might have a potential clinical application.

Involvement of adenosine A2A and dopamine receptors in the locomotor and sensitizing effects of cocaine

Recent data indicate that cocaine locomotor responses may be influenced by dopamine (DA) neurotransmission and adenosine neuromodulation involving the A2A receptor (A2AR). Male Wistar rats were injected with MSX-3 (1-25 mg/kg; an antagonist of A2AR), CGS 21680 (0.05-0.2 mg/kg; an agonist of A2AR), SCH 23390 (0.125-0.25 mg/kg; an antagonist of DA D1/5R), raclopride (0.1-0.8 mg/kg; an antagonist of DA D2/3R), nafadotride (0.2-0.4 mg/kg; an antagonist of DA D3R) or 7-OH-PIPAT (0.01-1 mg/kg; an agonist of DA D3R) to verify the hypothesis that adenosine A2AR and DA receptors and their antagonistic interactions may control locomotor and sensitizing effects of cocaine. In well-habituated animals, MSX-3 (5 mg/kg) increased, while raclopride (0.4-0.8 mg/kg) decreased basal locomotor activation; the other drugs were inactive. The locomotor hyperactivation induced by acute cocaine (10 mg/kg) was enhanced by MSX-3 (5-25 mg/kg) or nafadotride (0.4 mg/kg), while CGS 21680 (0.2 mg/kg), SCH 23390 (0.25 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (0.1 mg/kg) decreased this effect of cocaine. Given during the development of sensitization (in combination with 5-daily cocaine, 10 mg/kg, injections), MSX-3 (5-25 mg/kg) increased, but CGS 21680 (0.2 mg/kg) and raclopride (0.8 mg/kg) reduced the locomotor response to a cocaine challenge dose (10 mg/kg) on day 10. When injected acutely with a cocaine challenge dose (on day 10), CGS 21680 (0.2 mg/kg), raclopride (0.2-0.8 mg/kg) or 7-OH-PIPAT (1 mg/kg) reduced, while MSX-3 (5 mg/kg) or nafadotride (0.4 mg/kg) enhanced the expression of cocaine sensitization. The present results show that adenosine A2ARs and DA D3Rs exert inhibitory actions on acute locomotor responses to cocaine and on the expression of cocaine sensitization, while DA D2Rs had an opposing role in such effects. Pharmacological stimulation of adenosine A2ARs protected against both the development and expression of cocaine sensitization, which may offer a therapeutic potential of A2AR agonists in the treatment of cocaine dependence. The results suggest an antagonistic role of A2ARs in D2R-mediated cocaine actions based at least in part on the existence of A2A/D2 heteromeric receptor complexes.