Prolonged and extrasynaptic excitatory action of dopamine mediated by D1 receptors in the rat striatum in vivo

A Basal Ganglia model for understanding working memory functions in healthy and Parkinson's conditions

Working memory (WM) is considered as the scratchpad for reading, writing, and processing information necessary to perform cognitive tasks. The Basal Ganglia (BG) and Prefrontal Cortex are two important parts of the brain that are involved in WM functions, and both structures receive projections from dopaminergic nuclei. In this modelling study, we specifically focus on modelling the WM functions of the BG, the WM deficits in Parkinson's disease (PD) conditions, and the impact of dopamine deficiency on different kinds of WM functions. Though there are many experimental and modelling studies of WM properties, there is a paucity of models of the BG that provide insights into the contributions of the BG in WM functions. The proposed model of BG uses bistable flip-flop neurons to model striatal up-down neurons, a network of nonlinear oscillators to model the oscillations of the Indirect Pathway of BG and race-model for action selection. Five different WM tasks are used to demonstrate the generalisation ability of the proposed model. Experimental data from the four tasks are compared with model performance in both control and PD conditions. The model is extended to predict the response time of subjects and in the PD version of the model, the effect of dopaminergic medication on WM performance is also simulated. The proposed model of BG is a unified model that can explain the WM functions of the BG over a wide variety of tasks in both normal and PD conditions, and can be used to understand why specific WM functions are impaired whereas others remain intact in PD.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11571-023-10056-y.

Standardizing designed and emergent quantitative features in microphysiological systems

Microphysiological systems (MPSs) are cellular models that replicate aspects of organ and tissue functions in vitro. In contrast with conventional cell cultures, MPSs often provide physiological mechanical cues to cells, include fluid flow and can be interlinked (hence, they are often referred to as microfluidic tissue chips or organs-on-chips). Here, by means of examples of MPSs of the vascular system, intestine, brain and heart, we advocate for the development of standards that allow for comparisons of quantitative physiological features in MPSs and humans. Such standards should ensure that the in vivo relevance and predictive value of MPSs can be properly assessed as fit-for-purpose in specific applications, such as the assessment of drug toxicity, the identification of therapeutics or the understanding of human physiology or disease. Specifically, we distinguish designed features, which can be controlled via the design of the MPS, from emergent features, which describe cellular function, and propose methods for improving MPSs with readouts and sensors for the quantitative monitoring of complex physiology towards enabling wider end-user adoption and regulatory acceptance.

Constraints on the subsecond modulation of striatal dynamics by physiological dopamine signaling

Dopaminergic neurons play a crucial role in associative learning, but their capacity to regulate behavior on subsecond timescales remains debated. It is thought that dopaminergic neurons drive certain behaviors by rapidly modulating striatal spiking activity; however, a view has emerged that only artificially high (that is, supra-physiological) dopamine signals alter behavior on fast timescales. This raises the possibility that moment-to-moment striatal spiking activity is not strongly shaped by dopamine signals in the physiological range. To test this, we transiently altered dopamine levels while monitoring spiking responses in the ventral striatum of behaving mice. These manipulations led to only weak changes in striatal activity, except when dopamine release exceeded reward-matched levels. These findings suggest that dopaminergic neurons normally play a minor role in the subsecond modulation of striatal dynamics in relation to other inputs and demonstrate the importance of discerning dopaminergic neuron contributions to brain function under physiological and potentially nonphysiological conditions.

Mechanisms of neuromodulatory volume transmission

A wealth of neuromodulatory transmitters regulate synaptic circuits in the brain. Their mode of signaling, often called volume transmission, differs from classical synaptic transmission in important ways. In synaptic transmission, vesicles rapidly fuse in response to action potentials and release their transmitter content. The transmitters are then sensed by nearby receptors on select target cells with minimal delay. Signal transmission is restricted to synaptic contacts and typically occurs within ~1 ms. Volume transmission doesn't rely on synaptic contact sites and is the main mode of monoamines and neuropeptides, important neuromodulators in the brain. It is less precise than synaptic transmission, and the underlying molecular mechanisms and spatiotemporal scales are often not well understood. Here, we review literature on mechanisms of volume transmission and raise scientific questions that should be addressed in the years ahead. We define five domains by which volume transmission systems can differ from synaptic transmission and from one another. These domains are (1) innervation patterns and firing properties, (2) transmitter synthesis and loading into different types of vesicles, (3) architecture and distribution of release sites, (4) transmitter diffusion, degradation, and reuptake, and (5) receptor types and their positioning on target cells. We discuss these five domains for dopamine, a well-studied monoamine, and then compare the literature on dopamine with that on norepinephrine and serotonin. We include assessments of neuropeptide signaling and of central acetylcholine transmission. Through this review, we provide a molecular and cellular framework for volume transmission. This mechanistic knowledge is essential to define how neuromodulatory systems control behavior in health and disease and to understand how they are modulated by medical treatments and by drugs of abuse.

Pivotal role of phosphodiesterase 10A in the integration of dopamine signals in mice striatal D1 and D2 medium-sized spiny neurones

BACKGROUND AND PURPOSE

Dopamine in the striatum plays a crucial role in reward processes and action selection. Dopamine signals are transduced by D₁ and D₂ dopamine receptors which trigger mirror effects through the cAMP/PKA signalling cascade in D1 and D2 medium-sized spiny neurones (MSNs). Phosphodiesterases (PDEs), which determine the profile of cAMP signals, are highly expressed in MSNs, but their respective roles in dopamine signal integration remain poorly understood.

EXPERIMENTAL APPROACH

We used genetically-encoded FRET biosensors to monitor at the single cell level the functional contribution of PDE2A, PDE4 and PDE10A in the changes of the cAMP/PKA response to transient and continuous dopamine in mouse striatal brain slices.

KEY RESULTS

We found that PDE2A, PDE4 and PDE10A operate on the moderate to high cAMP levels elicited by D₁ or A_2A receptor stimulation. In contrast, only PDE10A is able to reduce cAMP down to baseline in both type of neurones, leading to the dephosphorylation of PKA substrates.

CONCLUSION AND IMPLICATIONS

In both MSN types, PDE10A inhibition blunts the responsiveness to dopamine, whereas PDE2A or PDE4 inhibition reinforces dopamine action.

Spatial and temporal scales of dopamine transmission

Dopamine is a prototypical neuromodulator that controls circuit function through G protein-coupled receptor signalling. Neuromodulators are volume transmitters, with release followed by diffusion for widespread receptor activation on many target cells. Yet, we are only beginning to understand the specific organization of dopamine transmission in space and time. Although some roles of dopamine are mediated by slow and diffuse signalling, recent studies suggest that certain dopamine functions necessitate spatiotemporal precision. Here, we review the literature describing dopamine signalling in the striatum, including its release mechanisms and receptor organization. We then propose the domain-overlap model, in which release and receptors are arranged relative to one another in micrometre-scale structures. This architecture is different from both point-to-point synaptic transmission and the widespread organization that is often proposed for neuromodulation. It enables the activation of receptor subsets that are within micrometre-scale domains of release sites during baseline activity and broader receptor activation with domain overlap when firing is synchronized across dopamine neuron populations. This signalling structure, together with the properties of dopamine release, may explain how switches in firing modes support broad and dynamic roles for dopamine and may lead to distinct pathway modulation.

Different Dopaminergic Dysfunctions Underlying Parkinsonian Akinesia and Tremor

Although the occurrence of Parkinsonian akinesia and tremor is traditionally associated to dopaminergic degeneration, the multifaceted neural processes that cause these impairments are not fully understood. As a consequence, current dopamine medications cannot be tailored to the specific dysfunctions of patients with the result that generic drug therapies produce different effects on akinesia and tremor. This article proposes a computational model focusing on the role of dopamine impairments in the occurrence of akinesia and resting tremor. The model has three key features, to date never integrated in a single computational system: (a) an architecture constrained on the basis of the relevant known system-level anatomy of the basal ganglia-thalamo-cortical loops; (b) spiking neurons with physiologically-constrained parameters; (c) a detailed simulation of the effects of both phasic and tonic dopamine release. The model exhibits a neural dynamics compatible with that recorded in the brain of primates and humans. Moreover, it suggests that akinesia might involve both tonic and phasic dopamine dysregulations whereas resting tremor might be primarily caused by impairments involving tonic dopamine release and the responsiveness of dopamine receptors. These results could lead to develop new therapies based on a system-level view of the Parkinson's disease and targeting phasic and tonic dopamine in differential ways.

Reward-driven changes in striatal pathway competition shape evidence evaluation in decision-making

Cortico-basal-ganglia-thalamic (CBGT) networks are critical for adaptive decision-making, yet how changes to circuit-level properties impact cognitive algorithms remains unclear. Here we explore how dopaminergic plasticity at corticostriatal synapses alters competition between striatal pathways, impacting the evidence accumulation process during decision-making. Spike-timing dependent plasticity simulations showed that dopaminergic feedback based on rewards modified the ratio of direct and indirect corticostriatal weights within opposing action channels. Using the learned weight ratios in a full spiking CBGT network model, we simulated neural dynamics and decision outcomes in a reward-driven decision task and fit them with a drift diffusion model. Fits revealed that the rate of evidence accumulation varied with inter-channel differences in direct pathway activity while boundary height varied with overall indirect pathway activity. This multi-level modeling approach demonstrates how complementary learning and decision computations can emerge from corticostriatal plasticity.

Cognitive process models such as reinforcement learning (RL) and the drift diffusion model (DDM) have helped to elucidate the basic algorithms underlying error-corrective learning and the evaluation of accumulating decision evidence leading up to a choice. While these relatively abstract models help to guide experimental and theoretical probes into associated phenomena, they remain uninformative about the actual physical mechanics by which learning and decision algorithms are carried out in a neurobiological substrate during adaptive choice behavior. Here we present an “upwards mapping” approach to bridging neural and cognitive models of value-based decision-making, showing how dopaminergic feedback alters the network-level dynamics of cortico-basal-ganglia-thalamic (CBGT) pathways during learning to bias behavioral choice towards more rewarding actions. By mapping “up” the levels of analysis, this approach yields specific predictions about aspects of neuronal activity that map to the quantities appearing in the cognitive decision-making framework.

Minimal Circuit Model of Reward Prediction Error Computations and Effects of Nicotinic Modulations

Dopamine (DA) neurons in the ventral tegmental area (VTA) are thought to encode reward prediction errors (RPE) by comparing actual and expected rewards. In recent years, much work has been done to identify how the brain uses and computes this signal. While several lines of evidence suggest the interplay of the DA and the inhibitory interneurons in the VTA implements the RPE computation, it still remains unclear how the DA neurons learn key quantities, for example the amplitude and the timing of primary rewards during conditioning tasks. Furthermore, endogenous acetylcholine and exogenous nicotine, also likely affect these computations by acting on both VTA DA and GABA (γ -aminobutyric acid) neurons via nicotinic-acetylcholine receptors (nAChRs). To explore the potential circuit-level mechanisms for RPE computations during classical-conditioning tasks, we developed a minimal computational model of the VTA circuitry. The model was designed to account for several reward-related properties of VTA afferents and recent findings on VTA GABA neuron dynamics during conditioning. With our minimal model, we showed that the RPE can be learned by a two-speed process computing reward timing and magnitude. By including models of nAChR-mediated currents in the VTA DA-GABA circuit, we showed that nicotine should reduce the acetylcholine action on the VTA GABA neurons by receptor desensitization and potentially boost DA responses to reward-related signals in a non-trivial manner. Together, our results delineate the mechanisms by which RPE are computed in the brain, and suggest a hypothesis on nicotine-mediated effects on reward-related perception and decision-making.

Stress as a Risk Factor for Substance Use Disorders: A Mini-Review of Molecular Mediators

The extant literature supports the role of stress in enhancing the susceptibility of drug abuse progressing to a substance use disorder diagnosis. However, the molecular mediators by which stress enhances the progression from cocaine abuse to cocaine use disorder via the mesolimbic pathway remain elusive. In this mini-review article, we highlight three mechanisms by which glucocorticoids (GCs) and the dopaminergic system interact. First, GCs upregulate tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine (DA) synthesis. Second, GCs downregulate monoamine-oxidase (MAO), an enzyme responsible for DA removal. Lastly, GCs are hypothesized to decrease DA reuptake, subsequently increasing synaptic DA. Based on these interactions, we review preclinical literature highlighting how stress modulates the mesolimbic pathway, including the ventral tegmental area (VTA) and nucleus accumbens (NAcs), to alter cocaine abuse-related effects. Taken together, stress enhances cocaine's abuse-related effects at multiple points along the VTA mesolimbic projection, and uniquely in the NAcs through a positive feedback type mechanism. Furthermore, we highlight future directions to elucidate the interaction between the prefrontal cortex (PFC) and key intermediaries including ΔFosB, cAMP response element binding protein (CREB) and cyclin-dependent kinase 5 (CDK5) to highlight possible mechanisms that underlie stress-induced acceleration of the progression to a cocaine use disorder diagnosis.

Hyperactive Response of Direct Pathway Striatal Projection Neurons to L-dopa and D1 Agonism in Freely Moving Parkinsonian Mice

Dopamine (DA) profoundly stimulates motor function as demonstrated by the hypokinetic motor symptoms in Parkinson's disease (PD) and by the hyperkinetic motor side effects during dopaminergic treatment of PD. Dopamine (DA) receptor-bypassing, optogenetics- and chemogenetics-induced spike firing of striatal DA D1 receptor (D1R)-expressing, direct pathway medium spiny neurons (dSPNs or dMSNs) promotes movements. However, the endogenous D1R-mediated effects, let alone those of DA replacement, on dSPN spike activity in freely-moving animals is not established. Here we show that using transcription factor Pitx3 null mutant (Pitx3Null) mice as a model for severe and consistent DA denervation in the dorsal striatum in Parkinson's disease, antidromically identified striatonigral neurons (D1R-expressing dSPNs) had a lower baseline spike firing rate than that in DA-intact normal mice, and these neurons increased their spike firing more strongly in Pitx3Null mice than in WT mice in response to injection of L-dopa or the D1R agonist, SKF81297; the increase in spike firing temporally coincided with the motor-stimulating effects of L-dopa and SKF81297. Taken together, these results provide the first evidence from freely moving animals that in parkinsonian striatum, identified behavior-promoting dSPNs become hyperactive upon the administration of L-dopa or a D1 agonist, likely contributing to the profound dopaminergic motor stimulation in parkinsonian animals and PD patients.

To Do or Not to Do: Dopamine, Affordability and the Economics of Opportunity

Five years ago, we introduced the thrift hypothesis of dopamine (DA), suggesting that the primary role of DA in adaptive behavior is regulating behavioral energy expenditure to match the prevailing economic conditions of the environment. Here we elaborate that hypothesis with several new ideas. First, we introduce the concept of affordability, suggesting that costs must necessarily be evaluated with respect to the availability of resources to the organism, which computes a value not only for the potential reward opportunity, but also the value of resources expended. Placing both costs and benefits within the context of the larger economy in which the animal is functioning requires consideration of the different timescales against which to compute resource availability, or average reward rate. Appropriate windows of computation for tracking resources requires corresponding neural substrates that operate on these different timescales. In discussing temporal patterns of DA signaling, we focus on a neglected form of DA plasticity and adaptation, changes in the physical substrate of the DA system itself, such as up- and down-regulation of receptors or release probability. We argue that changes in the DA substrate itself fundamentally alter its computational function, which we propose mediates adaptations to longer temporal horizons and economic conditions. In developing our hypothesis, we focus on DA D2 receptors (D2R), arguing that D2R implements a form of “cost control” in response to the environmental economy, serving as the “brain’s comptroller”. We propose that the balance between the direct and indirect pathway, regulated by relative expression of D1 and D2 DA receptors, implements affordability. Finally, as we review data, we discuss limitations in current approaches that impede fully investigating the proposed hypothesis and highlight alternative, more semi-naturalistic strategies more conducive to neuroeconomic investigations on the role of DA in adaptive behavior.

Integrating Brain and Biomechanical Models-A New Paradigm for Understanding Neuro-muscular Control

To date, realistic models of how the central nervous system governs behavior have been restricted in scope to the brain, brainstem or spinal column, as if these existed as disembodied organs. Further, the model is often exercised in relation to an in vivo physiological experiment with input comprising an impulse, a periodic signal or constant activation, and output as a pattern of neural activity in one or more neural populations. Any link to behavior is inferred only indirectly via these activity patterns. We argue that to discover the principles of operation of neural systems, it is necessary to express their behavior in terms of physical movements of a realistic motor system, and to supply inputs that mimic sensory experience. To do this with confidence, we must connect our brain models to neuro-muscular models and provide relevant visual and proprioceptive feedback signals, thereby closing the loop of the simulation. This paper describes an effort to develop just such an integrated brain and biomechanical system using a number of pre-existing models. It describes a model of the saccadic oculomotor system incorporating a neuromuscular model of the eye and its six extraocular muscles. The position of the eye determines how illumination of a retinotopic input population projects information about the location of a saccade target into the system. A pre-existing saccadic burst generator model was incorporated into the system, which generated motoneuron activity patterns suitable for driving the biomechanical eye. The model was demonstrated to make accurate saccades to a target luminance under a set of environmental constraints. Challenges encountered in the development of this model showed the importance of this integrated modeling approach. Thus, we exposed shortcomings in individual model components which were only apparent when these were supplied with the more plausible inputs available in a closed loop design. Consequently we were able to suggest missing functionality which the system would require to reproduce more realistic behavior. The construction of such closed-loop animal models constitutes a new paradigm of computational neurobehavior and promises a more thoroughgoing approach to our understanding of the brain's function as a controller for movement and behavior.

D1, but not D2, receptor blockade within the infralimbic and medial orbitofrontal cortex impairs cocaine seeking in a region-specific manner

Evidence suggests that the infralimbic cortex (IL), a subregion of the ventromedial prefrontal cortex (vmPFC), suppresses cocaine-seeking behavior in a self-administration paradigm, whereas the more anterior vmPFC subregion, the medial orbitofrontal cortex (mOFC), has received very little attention in this regard. Despite the established dopaminergic innervation of the vmPFC, whether dopamine receptor blockade in each subregion alters the reinstatement of cocaine seeking is unclear. To address this issue, male Sprague-Dawley rats underwent 2 weeks of cocaine self-administration, followed by extinction training and reinstatement testing. Immediately prior to each reinstatement test, rats received microinjections of the D1 receptor antagonist SCH 23390, the D2 receptor antagonist sulpiride or their respective vehicles. D1 receptor blockade in the IL reduced cued reinstatement but had no effect on cocaine prime and cue + cocaine-prime reinstatement, whereas D2 receptor blockade in the IL had no effect on reinstatement. For the mOFC, however, D1 receptor blockade reduced cocaine seeking in all reinstatement types, whereas blocking D2 receptors in the mOFC had no effect on any form of cocaine seeking. These findings suggest different roles for D1 receptors in the IL versus the mOFC in regulating cocaine-seeking behavior. Moreover, even as previous work indicates that IL inactivation does not affect reinstatement but, rather, induces cocaine seeking during extinction, the present findings suggest that dopamine receptor activation in the IL is necessary for cocaine seeking under some circumstances.

Detection of phasic dopamine by D1 and D2 striatal medium spiny neurons

KEY POINTS

Brief dopamine events are critical actors of reward-mediated learning in the striatum; the intracellular cAMP-protein kinase A (PKA) response of striatal medium spiny neurons to such events was studied dynamically using a combination of biosensor imaging in mouse brain slices and in silico simulations. Both D1 and D2 medium spiny neurons can sense brief dopamine transients in the sub-micromolar range. While dopamine transients profoundly change cAMP levels in both types of medium spiny neurons, the PKA-dependent phosphorylation level remains unaffected in D2 neurons. At the level of PKA-dependent phosphorylation, D2 unresponsiveness depends on protein phosphatase-1 (PP1) inhibition by DARPP-32. Simulations suggest that D2 medium spiny neurons could detect transient dips in dopamine level.

ABSTRACT

The phasic release of dopamine in the striatum determines various aspects of reward and action selection, but the dynamics of the dopamine effect on intracellular signalling remains poorly understood. We used genetically encoded FRET biosensors in striatal brain slices to quantify the effect of transient dopamine on cAMP or PKA-dependent phosphorylation levels, and computational modelling to further explore the dynamics of this signalling pathway. Medium-sized spiny neurons (MSNs), which express either D1 or D2 dopamine receptors, responded to dopamine by an increase or a decrease in cAMP, respectively. Transient dopamine showed similar sub-micromolar efficacies on cAMP in both D1 and D2 MSNs, thus challenging the commonly accepted notion that dopamine efficacy is much higher on D2 than on D1 receptors. However, in D2 MSNs, the large decrease in cAMP level triggered by transient dopamine did not translate to a decrease in PKA-dependent phosphorylation level, owing to the efficient inhibition of protein phosphatase 1 by DARPP-32. Simulations further suggested that D2 MSNs can also operate in a 'tone-sensing' mode, allowing them to detect transient dips in basal dopamine. Overall, our results show that D2 MSNs may sense much more complex patterns of dopamine than previously thought.

A connectome of a learning and memory center in the adult Drosophila brain

Understanding memory formation, storage and retrieval requires knowledge of the underlying neuronal circuits. In Drosophila, the mushroom body (MB) is the major site of associative learning. We reconstructed the morphologies and synaptic connections of all 983 neurons within the three functional units, or compartments, that compose the adult MB’s α lobe, using a dataset of isotropic 8 nm voxels collected by focused ion-beam milling scanning electron microscopy. We found that Kenyon cells (KCs), whose sparse activity encodes sensory information, each make multiple en passant synapses to MB output neurons (MBONs) in each compartment. Some MBONs have inputs from all KCs, while others differentially sample sensory modalities. Only 6% of KC>MBON synapses receive a direct synapse from a dopaminergic neuron (DAN). We identified two unanticipated classes of synapses, KC>DAN and DAN>MBON. DAN activation produces a slow depolarization of the MBON in these DAN>MBON synapses and can weaken memory recall.

DOI:http://dx.doi.org/10.7554/eLife.26975.001

Dysregulation of striatal projection neurons in Parkinson's disease

The loss of nigrostriatal dopamine (DA) is the primary cause of motor dysfunction in Parkinson's disease (PD), but the underlying striatal mechanisms remain unclear. In spite of abundant literature portraying structural, biochemical and plasticity changes of striatal projection neurons (SPNs), in the past there has been a data vacuum from the natural human disease and its close model in non-human primates. Recently, single-cell recordings in advanced parkinsonian primates have generated new insights into the altered function of SPNs. Currently, there are also human data that provide direct evidence of profoundly dysregulated SPN activity in PD. Here, we review primate recordings that are impacting our understanding of the striatal dysfunction after DA loss, particularly through the analysis of physiologic correlates of parkinsonian motor behaviors. In contrast to recordings in rodents, data obtained in primates and patients demonstrate similar major abnormalities of the spontaneous SPN firing in the alert parkinsonian state. Furthermore, these studies also show altered SPN responses to DA replacement in the advanced parkinsonian state. Clearly, there is yet much to learn about the striatal discharges in PD, but studies using primate models are contributing unique information to advance our understanding of pathophysiologic mechanisms.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Dopamine and opioid systems interact within the nucleus accumbens to maintain monogamous pair bonds

Prairie vole breeder pairs form monogamous pair bonds, which are maintained through the expression of selective aggression toward novel conspecifics. Here, we utilize behavioral and anatomical techniques to extend the current understanding of neural mechanisms that mediate pair bond maintenance. For both sexes, we show that pair bonding up-regulates mRNA expression for genes encoding D1-like dopamine (DA) receptors and dynorphin as well as enhances stimulated DA release within the nucleus accumbens (NAc). We next show that D1-like receptor regulation of selective aggression is mediated through downstream activation of kappa-opioid receptors (KORs) and that activation of these receptors mediates social avoidance. Finally, we also identified sex-specific alterations in KOR binding density within the NAc shell of paired males and demonstrate that this alteration contributes to the neuroprotective effect of pair bonding against drug reward. Together, these findings suggest motivational and valence processing systems interact to mediate the maintenance of social bonds.

Dopamine Rebound-Excitation Theory: Putting Brakes on PTSD

It is not uncommon for humans or animals to experience traumatic events in their lifetimes. However, the majority of individuals are resilient to long-term detrimental changes turning into anxiety and depression, such as post-traumatic stress disorder (PTSD). What underlying neural mechanism accounts for individual variability in stress resilience? Hyperactivity in fear circuits, such as the amygdalar system, is well-known to be the major pathophysiological basis for PTSD, much like a "stuck accelerator." Interestingly, increasing evidence demonstrates that dopamine (DA) - traditionally known for its role in motivation, reward prediction, and addiction - is also crucial in regulating fear learning and anxiety. Yet, how dopaminergic (DAergic) neurons control stress resilience is unclear, especially given that DAergic neurons have multiple subtypes with distinct temporal dynamics. Here, we propose the Rebound-Excitation Theory, which posits that DAergic neurons' rebound-excitation at the termination of fearful experiences serves as an important "brake" by providing intrinsic safety-signals to fear-processing neural circuits in a spatially and temporally controlled manner. We discuss how DAergic neuron rebound-excitation may be regulated by genetics and experiences, and how such physiological properties may be used as a brain-activity biomarker to predict and confer individual resilience to stress and anxiety.

The neurosteroidogenic enzyme 5α-reductase modulates the role of D1 dopamine receptors in rat sensorimotor gating

Neurosteroids exert diverse modulatory actions on dopamine neurotransmission and signaling. We previously documented that the enzyme 5α-reductase, which catalyzes the main rate-limiting step in neurosteroid synthesis, is required for the behavioral responses of Sprague-Dawley rats to non-selective dopaminergic agonists, such as the D1-D2 receptor agonist apomorphine. Specifically, systemic and intra-accumbal administrations of the 5α-reductase inhibitor finasteride countered apomorphine-induced deficits of sensorimotor gating, as measured by the prepulse inhibition (PPI) of the startle reflex; the classes of dopamine receptors involved in these effects, however, remain unknown. Prior rodent studies have revealed that the contributions of dopamine receptors to PPI regulation vary depending on the genetic background; thus, we analyzed the effect of finasteride on the PPI deficits induced by selective dopamine receptor agonists in Long-Evans (a strain exhibiting PPI deficits in response to both D1 and D2 receptor agonists) and Sprague-Dawley rats (which display PPI reductions following treatment with D2, and D3, but not D1 receptor agonists). In Long-Evans rats, finasteride opposed the PPI deficits induced by activation of D1, but not D2 receptors; conversely, in Sprague-Dawley rats, finasteride prevented the reductions in %PPI and accumbal dopamine extracellular levels caused by selective stimulation of D3, but not D2 receptors; however, the effects on %PPI were not confirmed by analyses on absolute PPI values. Our findings suggest that 5α-reductase modulates the effects of D1, but not D2 receptor agonists on sensorimotor gating. These data may help elucidate the role of neurosteroids in neuropsychiatric disorders featuring PPI deficits, including schizophrenia and Tourette syndrome.

Epigenetics, microRNA, and addiction

Drug addiction is characterized by uncontrolled drug consumption and high rates of relapse to drug taking during periods of attempted abstinence. Addiction is now largely considered a disorder of experience-dependent neuroplasticity, driven by remodeling of synapses in reward and motivation relevant brain circuits in response to a history of prolonged drug intake. Alterations in gene expression play a central role in addiction-relevant neuroplasticity, but the mechanisms by which additive drugs remodel brain motivation circuits remains unclear. MicroRNAs (miRNAs) are a class of noncoding RNA that can regulate the expression of large numbers of protein-coding mRNA transcripts by binding to the 3' untranslated region (3' UTR) of target transcripts and blocking their translation into the encoded protein or triggering their destabilization and degradation. Emerging evidence has implicated miRNAs in regulating addiction-relevant neuroplasticity in the brain, and in controlling the motivational properties of cocaine and other drugs of abuse. Here, the role for miRNAs in regulating basic aspects of neuronal function is reviewed. The involvement of miRNAs in controlling the motivational properties of addictive drugs is also summarized. Finally, mechanisms by which miRNAs exert their actions on drug intake, when known, are considered.

Dopamine bioavailability in the mPFC modulates operant learning performance in rats: an experimental study with a computational interpretation

Dopamine encodes reward and its prediction in reinforcement learning. Catechol-O-methyltransferase (COMT) activity in the medial prefrontal cortex (mPFC) has been shown to influence cognitive abilities by modifying dopamine clearance. Nevertheless, it is unknown how COMT in the mPFC influences operant learning. Systemic entacapone (50mg/kg), as well as local entacapone (3 pg) and recombinant COMT (17 μg) in the mPFC were administered to male Long Evans rats prior to training in an operant conditioning task. We found that systemic and local administration of the COMT inhibitor entacapone significantly improves learning performance. Conversely, recombinant COMT administration totally impaired learning. These data have been interpreted through a computational model where the phasic firing of dopaminergic neurons was computed by means of a temporal difference algorithm and dopamine bioavailability in the mPFC was simulated with a gating window. The duration of this window was selected to simulate the effects of inhibited or enhanced COMT activity (by entacapone or recombinant COMT respectively). The model accounts for an improved performance reproducing the entacapone effects, and a detrimental impact on learning when the clearance is increased reproducing the recombinant COMT effects. The experimental and computational results show that learning performance can be deeply influenced by COMT manipulations in the mPFC.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Dopamine D1 receptor blockade impairs alcohol seeking without reducing dorsal striatal activation to cues of alcohol availability

INTRODUCTION

Alcohol-associated cues activate both ventral and dorsal striatum in functional brain imaging studies of heavy drinkers. In rodents, alcohol-associated cues induce changes in neuronal firing frequencies and increase dopamine release in ventral striatum, but the impact of alcohol-associated cues on neuronal activity in dorsal striatum is unclear. We previously reported phasic changes in action potential frequency in the dorsomedial and dorsolateral striatum after cues that signaled alcohol availability, prompting approach behavior.

METHODS

We investigated the hypothesis that dopamine transmission modulates these phasic firing changes. Rats were trained to self-administer alcohol, and neuronal activity was monitored with extracellular electrophysiology during "anticipatory" cues that signaled the start of the operant session. Sessions were preceded by systemic administration of the D1-type dopamine receptor antagonist SCH23390 (0, 10, and 20 μg/kg).

RESULTS

SCH23390 significantly decreased firing rates during the 60 s prior to cue onset without reducing phasic excitations immediately following the cues. While neuronal activation to cues might be expected to initiate behavioral responses, in this study alcohol seeking was reduced despite the presence of dorsal striatal excitations to alcohol cues.

CONCLUSIONS

These data suggest that D1 receptor antagonism reduces basal firing rates in the dorsal striatum and modulates the ability of neuronal activation to "anticipatory" cues to initiate alcohol seeking in rats with an extensive history of alcohol self-administration.

Dopaminergic modulation of striatal networks in health and Parkinson's disease

In the last couple of years, there have been significant advances in our understanding of how dopamine modulates striatal circuits underlying goal-directed behaviors and how therapeutic interventions intended to normalize disordered dopaminergic signaling can go awry. This review summarizes some of the advances in this field with a translational focus on Parkinson's disease.

Synaptic and nonsynaptic plasticity approximating probabilistic inference

Learning and memory operations in neural circuits are believed to involve molecular cascades of synaptic and nonsynaptic changes that lead to a diverse repertoire of dynamical phenomena at higher levels of processing. Hebbian and homeostatic plasticity, neuromodulation, and intrinsic excitability all conspire to form and maintain memories. But it is still unclear how these seemingly redundant mechanisms could jointly orchestrate learning in a more unified system. To this end, a Hebbian learning rule for spiking neurons inspired by Bayesian statistics is proposed. In this model, synaptic weights and intrinsic currents are adapted on-line upon arrival of single spikes, which initiate a cascade of temporally interacting memory traces that locally estimate probabilities associated with relative neuronal activation levels. Trace dynamics enable synaptic learning to readily demonstrate a spike-timing dependence, stably return to a set-point over long time scales, and remain competitive despite this stability. Beyond unsupervised learning, linking the traces with an external plasticity-modulating signal enables spike-based reinforcement learning. At the postsynaptic neuron, the traces are represented by an activity-dependent ion channel that is shown to regulate the input received by a postsynaptic cell and generate intrinsic graded persistent firing levels. We show how spike-based Hebbian-Bayesian learning can be performed in a simulated inference task using integrate-and-fire (IAF) neurons that are Poisson-firing and background-driven, similar to the preferred regime of cortical neurons. Our results support the view that neurons can represent information in the form of probability distributions, and that probabilistic inference could be a functional by-product of coupled synaptic and nonsynaptic mechanisms operating over several timescales. The model provides a biophysical realization of Bayesian computation by reconciling several observed neural phenomena whose functional effects are only partially understood in concert.

Differential dopamine receptor occupancy underlies L-DOPA-induced dyskinesia in a rat model of Parkinson's disease

Dyskinesia is a major side effect of an otherwise effective L-DOPA treatment in Parkinson's patients. The prevailing view for the underlying presynaptic mechanism of L-DOPA-induced dyskinesia (LID) suggests that surges in dopamine (DA) via uncontrolled release from serotonergic terminals results in abnormally high level of extracellular striatal dopamine. Here we used high-sensitivity online microdialysis and PET imaging techniques to directly investigate DA release properties from serotonergic terminals both in the parkinsonian striatum and after neuronal transplantation in 6-OHDA lesioned rats. Although L-DOPA administration resulted in a drift in extracellular DA levels, we found no evidence for abnormally high striatal DA release from serotonin neurons. The extracellular concentration of DA remained at or below levels detected in the intact striatum. Instead, our results showed that an inefficient release pool of DA associated with low D2 receptor binding remained unchanged. Taken together, these findings suggest that differential DA receptor activation rather than excessive release could be the underlying mechanism explaining LID seen in this model. Our data have important implications for development of drugs targeting the serotonergic system to reduce DA release to manage dyskinesia in patients with Parkinson's disease.

Dopaminergic innervation of the human subventricular zone: a comparison between Huntington's chorea and Parkinson's disease

The subventricular zone retains its neurogenic capacity throughout life and, as such, is often considered a potential source for endogenous repair in neurodegenerative disorders. Because dopamine is believed to stimulate adult neurogenesis, we looked for possible variations in the dopaminergic innervation of the subventricular zone between cases of Huntington's chorea and Parkinson's diseases. Antibodies against tyrosine hydroxylase (TH) and proliferating cell nuclear antigen (PCNA) were used as specific markers of dopaminergic axons and cell proliferating activity, respectively. The immunohistochemical approach was applied to postmortem tissue from 2 Parkinson's disease cases, 4 Huntington's disease cases, along with age-matched controls. The immunostaining was revealed with either diaminobenzidine or fluorescent-conjugated secondary antibodies. Optical density measurements were made along the entire dorso-ventral extent of the caudate nucleus. An intense TH+ zone was detected along the ventricular border of the caudate nucleus in Huntington's disease cases, but not in patients with Parkinson's disease or age-matched controls. This thin (287±38 μm) paraventricular zone was composed of numerous small and densely packed dopamine axon varicosities and overlapped the deep layers of the subventricular zone. Its immunoreactivity was 47±8% more intense than that of adjacent striatal areas. The dopamine innervation of the subventricular zone is strikingly massive in Huntington's chorea compared to Parkinson's disease, a finding that concurs with the marked increase in neurogenesis noted in the subventricular zone of Huntington's disease patients. This finding suggests that dopamine plays a crucial role in mechanisms designed to compensate for the massive striatal neuronal losses that occur in Huntington's disease.

Evaluation of animal models of obsessive-compulsive disorder: correlation with phasic dopamine neuron activity

Obsessive compulsive disorder (OCD) is a psychiatric condition defined by intrusive thoughts (obsessions) associated with compensatory and repetitive behaviour (compulsions). However, advancement in our understanding of this disorder has been hampered by the absence of effective animal models and correspondingly analysis of the physiological changes that may be present in these models. To address this, we have evaluated two current rodent models of OCD; repeated injection of dopamine D2 agonist quinpirole and repeated adolescent injection of the tricyclic agent clomipramine in combination with a behavioural paradigm designed to produce compulsive lever pressing. These results were then compared with their relative impact on the state of activity of the mesolimbic dopaminergic system using extracellular recoding of spontaneously active dopamine neurons in the ventral tegmental area (VTA). The clomipramine model failed to exacerbate compulsive lever pressing and VTA dopamine neurons in clomipramine-treated rats had mildly diminished bursting activity. In contrast, quinpirole-treated animals showed significant increases in compulsive lever pressing, which was concurrent with a substantial diminution of bursting activity of VTA dopamine neurons. Therefore, VTA dopamine activity correlated with the behavioural response in these models. Taken together, these data support the view that compulsive behaviours likely reflect, at least in part, a disruption of the dopaminergic system, more specifically by a decrease in baseline phasic dopamine signalling mediated by burst firing of dopamine neurons.

Controlled iontophoresis coupled with fast-scan cyclic voltammetry/electrophysiology in awake, freely moving animals

Simultaneous electrochemical and electrophysiological data were recorded to evaluate the effects of controlled local application of dopaminergic agonists and antagonists in awake rats. Measurements were made with a probe consisting of a carbon-fiber microelectrode fused to three iontophoretic barrels used to introduce the drugs of interest. The probe and the manipulator used to position it in the brain of behaving animals were optimized to improve their performance. The effect of the dopamine autoreceptor on electrically stimulated release was demonstrated. Dopamine inhibited the release of endogenous dopamine whereas raclopride, a D2 antagonist, enhanced it, with similar responses in anesthetized and awake animals. We also examined changes in the firing rate of nucleus accumbens (NAc) neurons in awake animals during and after brief (15 s) iontophoretic ejections of SCH 23390 (D1 receptor antagonist) or raclopride. Changes in response to these antagonists were seen both immediately and on a prolonged time scale. Application of raclopride increased the firing rate in 40% of medium spiny neurons (MSNs), of which half responded immediately. Decreases in firing rate were observed in 46% of MSNs after SCH 23390 application. Only 11% of MSNs responded to both antagonists and one MSN (3%) showed no response to either drug. The same prolonged response in firing rate was seen for electrically stimulated and locally applied dopamine in 75% of MSNs. These results are in agreement with previously reported distributions for dopamine receptor subtypes on MSNs and probe the effects of dopamine on these cell populations.

Striatal neurones have a specific ability to respond to phasic dopamine release

  The cAMP/protein kinase A (PKA) signalling cascade is ubiquitous, and each step in this cascade involves enzymes that are expressed in multiple isoforms. We investigated the effects of this diversity on the integration of the pathway in the target cell by comparing prefrontal cortical neurones with striatal neurones which express a very specific set of signalling proteins. The prefrontal cortex and striatum both receive dopaminergic inputs and we analysed the dynamics of the cAMP/PKA signal triggered by dopamine D1 receptors in these two brain structures. Biosensor imaging in mouse brain slice preparations showed profound differences in the D1 response between pyramidal cortical neurones and striatal medium spiny neurones: the cAMP/PKA response was much stronger, faster and longer lasting in striatal neurones than in pyramidal cortical neurones. We identified three molecular determinants underlying these differences: different activities of phosphodiesterases, particularly those of type 4, which strongly damp the cAMP signal in the cortex but not in the striatum; stronger adenylyl cyclase activity in the striatum, generating responses with a faster onset than in the cortex; and DARPP-32, a phosphatase inhibitor which prolongs PKA action in the striatum. Striatal neurones were also highly responsive in terms of gene expression since a single sub-second dopamine stimulation is sufficient to trigger c-Fos expression in the striatum, but not in the cortex. Our data show how specific molecular elements of the cAMP/PKA signalling cascade selectively enable the principal striatal neurones to respond to brief dopamine stimuli, a critical process in incentive learning.

Phasic-like stimulation of the medial forebrain bundle augments striatal gene expression despite methamphetamine-induced partial dopamine denervation

Methamphetamine-induced partial dopamine depletions are associated with impaired basal ganglia function, including decreased preprotachykinin mRNA expression and impaired transcriptional activation of activity-regulated, cytoskeleton-associated (Arc) gene in striatum. Recent work implicates deficits in phasic dopamine signaling as a potential mechanism linking methamphetamine-induced dopamine loss to impaired basal ganglia function. This study thus sought to establish a causal link between phasic dopamine transmission and altered basal ganglia function by determining whether the deficits in striatal neuron gene expression could be restored by increasing phasic dopamine release. Three weeks after pretreatment with saline or a neurotoxic regimen of methamphetamine, rats underwent phasic- or tonic-like stimulation of ascending dopamine neurons. Striatal gene expression was examined using in situ hybridization histochemistry. Phasic-like, but not tonic-like, stimulation induced immediate-early genes Arc and zif268 in both groups, despite the partial striatal dopamine denervation in methamphetamine-pretreated rats, with the Arc expression occurring in presumed striatonigral efferent neurons. Phasic-like stimulation also restored preprotachykinin mRNA expression. These results suggest that disruption of phasic dopamine signaling likely underlies methamphetamine-induced impairments in basal ganglia function, and that restoring phasic dopamine signaling may be a viable approach to manage long-term consequences of methamphetamine-induced dopamine loss on basal ganglia functions.

Membrane properties of striatal direct and indirect pathway neurons in mouse and rat slices and their modulation by dopamine

D1 and D2 receptor expressing striatal medium spiny neurons (MSNs) are ascribed to striatonigral (“direct”) and striatopallidal (“indirect”) pathways, respectively, that are believed to function antagonistically in motor control. Glutamatergic synaptic transmission onto the two types is differentially affected by Dopamine (DA), however, less is known about the effects on MSN intrinsic electrical properties. Using patch clamp recordings, we comprehensively characterized the two pathways in rats and mice, and investigated their DA modulation. We identified the direct pathway by retrograde labeling in rats, and in mice we used transgenic animals in which EGFP is expressed in D1 MSNs. MSNs were subjected to a series of current injections to pinpoint differences between the populations, and in mice also following bath application of DA. In both animal models, most electrical properties were similar, however, membrane excitability as measured by step and ramp current injections consistently differed, with direct pathway MSNs being less excitable than their counterparts. DA had opposite effects on excitability of D1 and D2 MSNs, counteracting the initial differences. Pronounced changes in AP shape were seen in D2 MSNs. In direct pathway MSNs, excitability increased across experimental conditions and parameters, and also when applying DA or the D1 agonist SKF-81297 in presence of blockers of cholinergic, GABAergic, and glutamatergic receptors. Thus, DA induced changes in excitability were D1 R mediated and intrinsic to direct pathway MSNs, and not a secondary network effect of altered synaptic transmission. DAergic modulation of intrinsic properties therefore acts in a synergistic manner with previously reported effects of DA on afferent synaptic transmission and dendritic processing, supporting the antagonistic model for direct vs. indirect striatal pathway function.

The fine structure of the dopaminergic innervation of area 10 of macaque prefrontal cortex

In common with other areas of the prefrontal cortex, activity in frontopolar area 10 is probably modulated by dopamine. We studied the dopaminergic innervation of monkey prefrontal area 10 by immunostaining with tyrosine hydroxylase (TH) antibodies. TH-positive axons in layer 3 were examined by electron microscopy of series of ultrathin sections. TH-positive boutons containing vesicles were sparse (2 × 10(-4) per μm(3)) and the majority (94%, n = 52) had no identifiable synaptic specialization, which supports the hypothesis that dopamine is released non-synaptically and raises the question of whether the local microenvironment surrounding the boutons is special. Compared with unlabelled boutons TH-positive boutons had a higher proportion of their perimeter in contact with dendritic shafts and were more often in continuous contact with pairs of pre- and postsynaptic structures. However, this may result from exclusion from sites preferred by glutamatergic and GABAergic synapses as the density of all synapses in the closer vicinity was no different from any randomly selected site in the neuropil. This quantitative ultrastructural study presents basic features of the dopaminergic innervation in prefrontal area 10 and provides a more detailed understanding of the structural basis of dopamine signalling in the cortex.

Molecular, cellular, and structural mechanisms of cocaine addiction: a key role for microRNAs

The rewarding properties of cocaine play a key role in establishing and maintaining the drug-taking habit. However, as exposure to cocaine increases, drug use can transition from controlled to compulsive. Importantly, very little is known about the neurobiological mechanisms that control this switch in drug use that defines addiction. MicroRNAs (miRNAs) are small non-protein coding RNA transcripts that can regulate the expression of messenger RNAs that code for proteins. Because of their highly pleiotropic nature, each miRNA has the potential to regulate hundreds or even thousands of protein-coding RNA transcripts. This property of miRNAs has generated considerable interest in their potential involvement in complex psychiatric disorders such as addiction, as each miRNA could potentially influence the many different molecular and cellular adaptations that arise in response to drug use that are hypothesized to drive the emergence of addiction. Here, we review recent evidence supporting a key role for miRNAs in the ventral striatum in regulating the rewarding and reinforcing properties of cocaine in animals with limited exposure to the drug. Moreover, we discuss evidence suggesting that miRNAs in the dorsal striatum control the escalation of drug intake in rats with extended cocaine access. These findings highlight the central role for miRNAs in drug-induced neuroplasticity in brain reward systems that drive the emergence of compulsive-like drug use in animals, and suggest that a better understanding of how miRNAs control drug intake will provide new insights into the neurobiology of drug addiction.

Rapid dopamine dynamics in the accumbens core and shell: learning and action

The catecholamine dopamine (DA) has been implicated in a host of neural processes as diverse as schizophrenia, parkinsonism and reward encoding. Importantly, these distinct features of DA function are due in large part to separate neural circuits involving connections arising from different DA-releasing nuclei and projections to separate afferent targets. Emerging data has suggested that this same principle of separate neural circuits may be applicable within structural subregions, such as the core and shell of the nucleus accumbens (NAc). Further, DA may act selectively on smaller ensembles of cells (or, microcircuits) via differential DA receptor density and distinct inputs and outputs of the microcircuits, thus enabling new learning about Pavlovian cues, instrumental responses, subjective reward processing and decision-making. In this review, by taking advantage of studies using subsecond voltammetric techniques in behaving animals to study how rapid changes in DA levels affect behavior, we examine the spatial and temporal features of DA release and how it relates to both normal learning and similarities to pathological learning in the form of addiction.

Speech-induced striatal dopamine release is left lateralized and coupled to functional striatal circuits in healthy humans: a combined PET, fMRI and DTI study

Considerable progress has been recently made in understanding the brain mechanisms underlying speech and language control. However, the neurochemical underpinnings of normal speech production remain largely unknown. We investigated the extent of striatal endogenous dopamine release and its influences on the organization of functional striatal speech networks during production of meaningful English sentences using a combination of positron emission tomography (PET) with the dopamine D(2)/D(3) receptor radioligand [(11)C]raclopride and functional MRI (fMRI). In addition, we used diffusion tensor tractography (DTI) to examine the extent of dopaminergic modulatory influences on striatal structural network organization. We found that, during sentence production, endogenous dopamine was released in the ventromedial portion of the dorsal striatum, in both its associative and sensorimotor functional divisions. In the associative striatum, speech-induced dopamine release established a significant relationship with neural activity and influenced the left-hemispheric lateralization of striatal functional networks. In contrast, there were no significant effects of endogenous dopamine release on the lateralization of striatal structural networks. Our data provide the first evidence for endogenous dopamine release in the dorsal striatum during normal speaking and point to the possible mechanisms behind the modulatory influences of dopamine on the organization of functional brain circuits controlling normal human speech.

Lesion of the cerebellar paravermis increases dopamine D1 receptor levels in the contralateral striatum

Anatomical and biochemical findings have long suggested that a projection from the cerebellum to the basal ganglia exists, and recent findings proposed that the cerebellum influences glutamatergic striatal activity. We have previously shown that a complete, genetic, lack of Purkinje cells induces an upregulation of dopamine D1 receptors (DRD1) in the output of the basal ganglia, the substantia nigra pars reticulata. In this study, we produced a focal unilateral lesion in the cerebellar paravermal cortex and we studied the levels and distribution of dopamine receptors and transporters, with the use of in vitro receptor autoradiography. The lesion produced a statistically significant increase in DRD1 specific binding in the contralateral medial striatum and a bilateral decrease in dopamine transporter (DAT) levels in the dorsolateral striatum. Our finding of a DRD1 increase after disruption of the cerebellar corticonuclear projection suggests that the cerebellar output modulates the basal ganglia DRD1-mediated pathway.

Disruption of subcellular Arc/Arg 3.1 mRNA expression in striatal efferent neurons following partial monoamine loss induced by methamphetamine

The immediate-early gene Arc (activity-regulated cytoskeleton-associated protein) is provocative in the context of neuroplasticity because of its experience-dependent regulation and mRNA transport to and translation at activated synapses. Normal rats have more preproenkephalin-negative (ppe-neg; presumed striatonigral) neurons with cytoplasmic Arc mRNA than ppe-positive (ppe-pos; striatopallidal) neurons, despite equivalent numbers of these neurons showing novelty-induced transcriptional activation of Arc. Furthermore, rats with partial monoamine loss induced by methamphetamine (METH) show impaired Arc mRNA expression in both ppe-neg and ppe-pos neurons relative to normal animals following response-reversal learning. In this study, Arc expression induced by exposure to a novel environment was used to assess transcriptional activation and cytoplasmic localization of Arc mRNA in striatal efferent neuron subpopulations subsequent to METH-induced neurotoxicity. Partial monoamine depletion significantly altered Arc expression. Specifically, basal Arc expression was elevated, but novelty-induced transcriptional activation was abolished. Without novelty-induced Arc transcription, METH-pre-treated rats also had fewer neurons with cytoplasmic Arc mRNA expression, with the effect being greater for ppe-neg neurons. Thus, METH-induced neurotoxicity substantially alters striatal efferent neuron function at the level of Arc transcription, suggesting a long-term shift in basal ganglia neuroplasticity processes subsequent to METH-induced neurotoxicity. Such changes potentially underlie striatally based learning deficits associated with METH-induced neurotoxicity.

Establishing causality for dopamine in neural function and behavior with optogenetics

Dopamine (DA) is known to play essential roles in neural function and behavior. Accordingly, DA neurons have been the focus of intense experimental investigation that has led to many important advances in our understanding of how DA influences these processes. However, it is becoming increasingly appreciated that delineating the precise contributions of DA neurons to cellular, circuit, and systems-level phenomena will require more sophisticated control over their patterns of activity than conventional techniques can provide. Specifically, the roles played by DA neurons are likely to depend on their afferent and efferent connectivity, the timing and length of their neural activation, and the nature of the behavior under investigation. Recently developed optogenetic tools hold great promise for disentangling these complex issues. Here we discuss the use of light-sensitive microbial opsins in the context of outstanding questions in DA research. A major technical advance offered by these proteins is the ability to bidirectionally modulate DA neuron activity in in vitro and in vivo preparations on a time scale that more closely approximates those of neural, perceptual and behavioral events. In addition, continued advances in rodent genetics and viral-mediated gene delivery have contributed to the ability to selectively target DA neurons or their individual afferent and efferent connections. Further, these tools are suitable for use in experimental subjects engaged in complex behaviors. After reviewing the strengths and limitations of optogenetic methodologies, we conclude by describing early efforts in the application of this valuable new approach that demonstrate its potential to improve our understanding of the neural and behavioral functions of DA. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Contribution of the mesolimbic dopamine system in mediating the effects of leptin and ghrelin on feeding

Feeding behaviour is crucial for the survival of an organism and is regulated by different brain circuits. Among these circuits the mesolimbic dopamine (DA) system is implicated in the anticipation and motivation for food rewards. This system consists of the dopaminergic neurons in the ventral tegmental area (VTA), and their projections to different cortico-limbic structures such as the nucleus accumbens and medial prefrontal cortex. While the importance of this system in motivational drive for different rewards, including drugs of abuse, has been clearly established, its role in energy balance remains largely unexplored. Evidence suggests that peripheral hormones such as leptin and ghrelin are involved in the anticipation and motivation for food and this might be partially mediated through their effects on the VTA. Yet, it remains to be determined whether these effects are direct effects of ghrelin and leptin onto VTA DA neurons, and to what extent indirect effects through other brain areas contribute. Elucidation of the role of leptin and ghrelin signalling on VTA DA neurons in relation to disruptions of energy balance might provide important insights into the role of this neural circuit in obesity and anorexia nervosa.

Dopamine release mediated by the dopamine transporter, facts and consequences

Spontaneous and/or stimulated neural activity of the nigrostriatal dopamine (DA) pathway makes amines run out from the neurons. This DA dynamic follows a rather complex path, running in or out the terminals, and flushing or diffusing into the extracellular space. The location of this leakage is not limited to the axon terminals; it also occurs from the cell bodies and dendrites. This molecular release mechanism was, for a long time, considered as being produced, in part, by the exocytosis of previously stored vesicles. The DA carrier protein (DAT, DA transporter) embedded in the DA cell membrane is known to clear previously released amines through an inward DA influx. The DAT also appears to be an active vector of amine release. Particular local conditions and the presence of numerous psychostimulant substances are able to trigger an outward efflux of DA through the DAT. This process, delivering slowly large amounts of amine could play a major regulatory role in extracellular DA homeostasis.

Role of CAMKII in reinforcement learning: a computational model of glutamate and dopamine signaling pathways

Timely release of dopamine (DA) at the striatum seems to be important for reinforcement learning (RL) mediated by the basal ganglia. Houk et al. (in: Houk et al (eds) Models of information processing in the basal ganglia, (1995) proposed a cellular signaling pathway model to characterize the interaction between DA and glutamate pathways that have a role in RL. The model simulation results, using GENESIS KINETIKIT simulator, point out that there is not only prolongation of duration as proposed by Houk et al. (1995), but also an enhancement in the amplitude of autophosphorylation of CaMKII. Further, the autophosphorylated form of CaMKII may form a basis for the "eligibility trace" condition required in RL. This simulation study is the first of its kind to support the comprehensive theoretical proposal of Houk et al. (1995).

Potential vulnerabilities of neuronal reward, risk, and decision mechanisms to addictive drugs

How do addictive drugs hijack the brain's reward system? This review speculates how normal, physiological reward processes may be affected by addictive drugs. Addictive drugs affect acute responses and plasticity in dopamine neurons and postsynaptic structures. These effects reduce reward discrimination, increase the effects of reward prediction error signals, and enhance neuronal responses to reward-predicting stimuli, which may contribute to compulsion. Addictive drugs steepen neuronal temporal reward discounting and create temporal myopia that impairs the control of drug taking. Tonically enhanced dopamine levels may disturb working memory mechanisms necessary for assessing background rewards and thus may generate inaccurate neuronal reward predictions. Drug-induced working memory deficits may impair neuronal risk signaling, promote risky behaviors, and facilitate preaddictive drug use. Malfunctioning adaptive reward coding may lead to overvaluation of drug rewards. Many of these malfunctions may result in inadequate neuronal decision mechanisms and lead to choices biased toward drug rewards.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.