Disinhibition bursting of dopaminergic neurons

Neuronal implementation of the temporal difference learning algorithm in the midbrain dopaminergic system

The temporal difference learning (TDL) algorithm has been essential to conceptualizing the role of dopamine in reinforcement learning (RL). Despite its theoretical importance, it remains unknown whether a neuronal implementation of this algorithm exists in the brain. Here, we provide an interpretation of the recently described signaling properties of ventral tegmental area (VTA) GABAergic neurons and show that a circuitry of these neurons implements the TDL algorithm. Specifically, we identified the neuronal mechanism of three key components of the TDL model: a sustained state value signal encoded by an afferent input to the VTA, a temporal differentiation circuit formed by two types of VTA GABAergic neurons the combined output of which computes momentary reward prediction (RP) as the derivative of the state value, and the computation of reward prediction errors (RPEs) in dopamine neurons utilizing the output of the differentiation circuit. Using computational methods, we also show that this mechanism is optimally adapted to the biophysics of RPE signaling in dopamine neurons, mechanistically links the emergence of conditioned reinforcement to RP, and can naturally account for the temporal discounting of reinforcement. Elucidating the implementation of the TDL algorithm may further the investigation of RL in biological and artificial systems.

Striatonigrostriatal circuit architecture for disinhibition of dopamine signaling

The basal ganglia operate largely in closed parallel loops, including an associative circuit for goal-directed behavior originating from the dorsomedial striatum (DMS) and a somatosensory circuit important for habit formation originating from the dorsolateral striatum (DLS). An exception to this parallel circuit organization has been proposed to explain how information might be transferred between striatal subregions, for example, from the DMS to the DLS during habit formation. The “ascending spiral hypothesis” proposes that the DMS disinhibits dopamine signaling in the DLS through a tri-synaptic, open-loop striatonigrostriatal circuit. Here, we use transsynaptic and intersectional genetic tools to investigate both closed- and open-loop striatonigrostriatal circuits. We find strong evidence for closed loops, which would allow striatal subregions to regulate their own dopamine release. We also find evidence for functional synapses in open loops. However, these synapses are unable to modulate tonic dopamine neuron firing, questioning the prominence of their role in mediating crosstalk between striatal subregions.

Ambrosi and Lerner investigate a basal ganglia circuit—the ascending spiral—long hypothesized to underlie habit formation by allowing disinhibition of dopamine signaling in the dorsolateral striatum (DLS). They find that synaptic connectivity in the ascending spiral is present, but the circuit cannot support straightforward disinhibition of DLS dopamine signaling.

In vivo patch-clamp recordings reveal distinct subthreshold signatures and threshold dynamics of midbrain dopamine neurons

The in vivo firing patterns of ventral midbrain dopamine neurons are controlled by afferent and intrinsic activity to generate sensory cue and prediction error signals that are essential for reward-based learning. Given the absence of in vivo intracellular recordings during the last three decades, the subthreshold membrane potential events that cause changes in dopamine neuron firing patterns remain unknown. To address this, we established in vivo whole-cell recordings and obtained over 100 spontaneously active, immunocytochemically-defined midbrain dopamine neurons in isoflurane-anaesthetized adult mice. We identified a repertoire of subthreshold membrane potential signatures associated with distinct in vivo firing patterns. Dopamine neuron activity in vivo deviated from single-spike pacemaking by phasic increases in firing rate via two qualitatively distinct biophysical mechanisms: 1) a prolonged hyperpolarization preceding rebound bursts, accompanied by a hyperpolarizing shift in action potential threshold; and 2) a transient depolarization leading to high-frequency plateau bursts, associated with a depolarizing shift in action potential threshold. Our findings define a mechanistic framework for the biophysical implementation of dopamine neuron firing patterns in the intact brain.

Functional Dissection of Basal Ganglia Inhibitory Inputs onto Substantia Nigra Dopaminergic Neurons

Substantia nigra (SNc) dopaminergic neurons respond to aversive stimuli with inhibitory pauses in firing followed by transient rebound activation. We tested integration of inhibitory synaptic inputs onto SNc neurons from genetically defined populations in dorsal striatum (striosome and matrix) and external globus pallidus (GPe; parvalbumin- and Lhx6-positive), and examined their contribution to pause-rebound firing. Activation of striosome projections, which target "dendron bouquets" in the pars reticulata (SNr), consistently quiets firing and relief from striosome inhibition triggers rebound activity. Striosomal inhibitory postsynaptic currents (IPSCs) display a prominent GABA-B receptor-mediated component that strengthens the impact of SNr dendrite synapses on somatic excitability and enables rebounding. By contrast, GPe projections activate GABA-A receptors on the soma and proximal dendrites but do not result in rebounding. Lastly, optical mapping shows that dorsal striatum selectively inhibits the ventral population of SNc neurons, which are intrinsically capable of rebounding. Therefore, we define a distinct striatonigral circuit for generating dopamine rebound.

D-Amphetamine Exposure Differentially Disrupts Signaling Across Ontogeny in the Zebrafish

Background: Prescriptive and illicit amphetamine (AMPH) use continues to increase along with the likelihood that during an individual's lifetime, the drug deleteriously influences the growth and connectivity of behavior circuits necessary for survival. Throughout ontogeny, neural circuits underlying these behaviors grow in complexity, gradually integrating many sensory inputs that trigger higher order coordinated motor responses. In the present study, we examine how AMPH disrupts the establishment of these circuits at critical neurodevelopmental periods, as well as the communication among established survival circuits. Materials and Methods: Zebrafish embryos (from 1 hpf) were raised in AMPH solutions, growth parameters and escape behavior were assessed at 24 and 48 hpf, and spinal cord tissues analyzed for differences in excitatory-inhibitory signaling balance among treatments. Adult fish were fed an acute dosage of AMPH over an 11-day conditioned place preference (PP) paradigm during which behaviors were recorded and brain tissues analyzed for alterations in dopaminergic signaling. Results: AMPH negatively affects embryonic growth and slows the execution of escape behavior, suggesting an imbalance in locomotor signaling. Although local spinal circuits provide primary escape modulation, no differences in inhibitory glycinergic, and excitatory glutamatergic signaling were measured among spinal neurons. AMPH also influenced place preference in adult zebrafish and resulted in the increased expression of dopamine signaling proteins (DRD1) in brain areas governing survival behaviors.

Regional difference in spontaneous firing inhibition by GABAA and GABAB receptors in nigral dopamine neurons

GABAergic control over dopamine (DA) neurons in the substantia nigra is crucial for determining firing rates and patterns. Although GABA activates both GABA_A and GABA_B receptors distributed throughout the somatodendritic tree, it is currently unclear how regional GABA receptors in the soma and dendritic compartments regulate spontaneous firing. Therefore, the objective of this study was to determine actions of regional GABA receptors on spontaneous firing in acutely dissociated DA neurons from the rat using patch-clamp and local GABA-uncaging techniques. Agonists and antagonists experiments showed that activation of either GABA_A receptors or GABA_B receptors in DA neurons is enough to completely abolish spontaneous firing. Local GABA-uncaging along the somatodendritic tree revealed that activation of regional GABA receptors limited within the soma, proximal, or distal dendritic region, can completely suppress spontaneous firing. However, activation of either GABA_A or GABA_B receptor equally suppressed spontaneous firing in the soma, whereas GABA_B receptor inhibited spontaneous firing more strongly than GABA_A receptor in the proximal and distal dendrites. These regional differences of GABA signals between the soma and dendritic compartments could contribute to our understanding of many diverse and complex actions of GABA in midbrain DA neurons.

Midbrain dopaminergic neuron activity across alternating brain states of urethane anaesthetized rat

Midbrain dopaminergic neurons are implicated in the control of motor functions and reward-driven behaviours. The function of this neuronal population is strongly connected with distinct patterns of firing - irregular or bursting, which either maintains basal levels of dopamine (DA) or leads to phasic release, respectively. Heterogeneity of dopaminergic neurons, observed on both structural and functional levels, is also reflected in different responses of DA neurons to changes in global brain states. Preparation of urethane anaesthetized animal is a broadly used model to study brain state dependent activity of neurons. Unfortunately activity of midbrain DA neurons across urethane induced cyclic, spontaneous brain state alternations is poorly described. To fulfil this gap in our knowledge we have performed simultaneous, extracellular recordings of the firing of single putative DA neurons combined with continuous brain state monitoring. We found that during slow wave activity, the firing rate of recorded putative DA neurons was significantly higher compared to firing rates during activated state, both in ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). In the presence of cortical slow waves, putative dopaminergic neurons also intensified bursting activity, but the magnitude of this phenomena differed in respect to the examined region (VTA or SNc). Our results show that activity of DA neurons under urethane anaesthesia is brain-state dependent and emphasize the importance of brain state monitoring during electrophysiological experiments.

Dopamine Neurons Change the Type of Excitability in Response to Stimuli

The dynamics of neuronal excitability determine the neuron's response to stimuli, its synchronization and resonance properties and, ultimately, the computations it performs in the brain. We investigated the dynamical mechanisms underlying the excitability type of dopamine (DA) neurons, using a conductance-based biophysical model, and its regulation by intrinsic and synaptic currents. Calibrating the model to reproduce low frequency tonic firing results in N-methyl-D-aspartate (NMDA) excitation balanced by γ-Aminobutyric acid (GABA)-mediated inhibition and leads to type I excitable behavior characterized by a continuous decrease in firing frequency in response to hyperpolarizing currents. Furthermore, we analyzed how excitability type of the DA neuron model is influenced by changes in the intrinsic current composition. A subthreshold sodium current is necessary for a continuous frequency decrease during application of a negative current, and the low-frequency "balanced" state during simultaneous activation of NMDA and GABA receptors. Blocking this current switches the neuron to type II characterized by the abrupt onset of repetitive firing. Enhancing the anomalous rectifier Ih current also switches the excitability to type II. Key characteristics of synaptic conductances that may be observed in vivo also change the type of excitability: a depolarized γ-Aminobutyric acid receptor (GABAR) reversal potential or co-activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) leads to an abrupt frequency drop to zero, which is typical for type II excitability. Coactivation of N-methyl-D-aspartate receptors (NMDARs) together with AMPARs and GABARs shifts the type I/II boundary toward more hyperpolarized GABAR reversal potentials. To better understand how altering each of the aforementioned currents leads to changes in excitability profile of DA neuron, we provide a thorough dynamical analysis. Collectively, these results imply that type I excitability in dopamine neurons might be important for low firing rates and fine-tuning basal dopamine levels, while switching excitability to type II during NMDAR and AMPAR activation may facilitate a transient increase in dopamine concentration, as type II neurons are more amenable to synchronization by mutual excitation.

Forgetting in Reinforcement Learning Links Sustained Dopamine Signals to Motivation

It has been suggested that dopamine (DA) represents reward-prediction-error (RPE) defined in reinforcement learning and therefore DA responds to unpredicted but not predicted reward. However, recent studies have found DA response sustained towards predictable reward in tasks involving self-paced behavior, and suggested that this response represents a motivational signal. We have previously shown that RPE can sustain if there is decay/forgetting of learned-values, which can be implemented as decay of synaptic strengths storing learned-values. This account, however, did not explain the suggested link between tonic/sustained DA and motivation. In the present work, we explored the motivational effects of the value-decay in self-paced approach behavior, modeled as a series of ‘Go’ or ‘No-Go’ selections towards a goal. Through simulations, we found that the value-decay can enhance motivation, specifically, facilitate fast goal-reaching, albeit counterintuitively. Mathematical analyses revealed that underlying potential mechanisms are twofold: (1) decay-induced sustained RPE creates a gradient of ‘Go’ values towards a goal, and (2) value-contrasts between ‘Go’ and ‘No-Go’ are generated because while chosen values are continually updated, unchosen values simply decay. Our model provides potential explanations for the key experimental findings that suggest DA's roles in motivation: (i) slowdown of behavior by post-training blockade of DA signaling, (ii) observations that DA blockade severely impairs effortful actions to obtain rewards while largely sparing seeking of easily obtainable rewards, and (iii) relationships between the reward amount, the level of motivation reflected in the speed of behavior, and the average level of DA. These results indicate that reinforcement learning with value-decay, or forgetting, provides a parsimonious mechanistic account for the DA's roles in value-learning and motivation. Our results also suggest that when biological systems for value-learning are active even though learning has apparently converged, the systems might be in a state of dynamic equilibrium, where learning and forgetting are balanced.

Dopamine (DA) has been suggested to have two reward-related roles: (1) representing reward-prediction-error (RPE), and (2) providing motivational drive. Role(1) is based on the physiological results that DA responds to unpredicted but not predicted reward, whereas role(2) is supported by the pharmacological results that blockade of DA signaling causes motivational impairments such as slowdown of self-paced behavior. So far, these two roles are considered to be played by two different temporal patterns of DA signals: role(1) by phasic signals and role(2) by tonic/sustained signals. However, recent studies have found sustained DA signals with features indicative of both roles (1) and (2), complicating this picture. Meanwhile, whereas synaptic/circuit mechanisms for role(1), i.e., how RPE is calculated in the upstream of DA neurons and how RPE-dependent update of learned-values occurs through DA-dependent synaptic plasticity, have now become clarified, mechanisms for role(2) remain unclear. In this work, we modeled self-paced behavior by a series of ‘Go’ or ‘No-Go’ selections in the framework of reinforcement-learning assuming DA's role(1), and demonstrated that incorporation of decay/forgetting of learned-values, which is presumably implemented as decay of synaptic strengths storing learned-values, provides a potential unified mechanistic account for the DA's two roles, together with its various temporal patterns.

Contribution of synchronized GABAergic neurons to dopaminergic neuron firing and bursting

In the ventral tegmental area (VTA), interactions between dopamine (DA) and γ-aminobutyric acid (GABA) neurons are critical for regulating DA neuron activity and thus DA efflux. To provide a mechanistic explanation of how GABA neurons influence DA neuron firing, we developed a circuit model of the VTA. The model is based on feed-forward inhibition and recreates canonical features of the VTA neurons. Simulations revealed that γ-aminobutyric acid (GABA) receptor (GABAR) stimulation can differentially influence the firing pattern of the DA neuron, depending on the level of synchronization among GABA neurons. Asynchronous activity of GABA neurons provides a constant level of inhibition to the DA neuron and, when removed, produces a classical disinhibition burst. In contrast, when GABA neurons are synchronized by common synaptic input, their influence evokes additional spikes in the DA neuron, resulting in increased measures of firing and bursting. Distinct from previous mechanisms, the increases were not based on lowered firing rate of the GABA neurons or weaker hyperpolarization by the GABAR synaptic current. This phenomenon was induced by GABA-mediated hyperpolarization of the DA neuron that leads to decreases in intracellular calcium (Ca2+) concentration, thus reducing the Ca2+-dependent potassium (K+) current. In this way, the GABA-mediated hyperpolarization replaces Ca2+-dependent K+ current; however, this inhibition is pulsatile, which allows the DA neuron to fire during the rhythmic pauses in inhibition. Our results emphasize the importance of inhibition in the VTA, which has been discussed in many studies, and suggest a novel mechanism whereby computations can occur locally.

Gonadectomy but not biological sex affects burst-firing in dopamine neurons of the ventral tegmental area and in prefrontal cortical neurons projecting to the ventral tegmentum in adult rats

The mesocortical and mesolimbic dopamine systems regulate cognitive and motivational processes and are strongly implicated in neuropsychiatric disorders in which these processes are disturbed. Sex differences and sex hormone modulation are also known for these dopamine-sensitive behaviours in health and disease. One relevant mechanism of hormone impact appears to be regulation of cortical and subcortical dopamine levels. This study asked whether this regulation of dopamine tone is a consequence of sex or sex hormone impact on the firing modes of ventral midbrain dopamine neurons. To address this, single unit extracellular recordings made in the ventral tegmental area and substantia nigra were compared among urethane-anaesthetized adult male, female, gonadectomized male rats. These comparisons showed that gonadectomy had no effect on nigral cells and no effects on pacemaker, bursty, single-spiking or random modes of dopamine activity in the ventral tegmental area. However, it did significantly and selectively increase burst firing in these cells in a testosterone-sensitive, estradiol-insensitive manner. Given the roles of prefrontal cortex (PFC) in modulating midbrain dopamine cell firing, we next asked whether gonadectomy's effects on dopamine cell bursting had correlated effects on the activity of ventral tegmentally projecting prefrontal cortical neurons. We found that gonadectomy indeed significantly and selectively increased burst firing in ventral tegmentally projecting but not neighbouring prefrontal cells. These effects were also androgen-sensitive. Together, these findings suggest a working model wherein androgen influence over the activity of PFC neurons regulates its top-down modulation of mesocortical and mesolimbic dopamine systems and related dopamine-sensitive behaviours.

Reversal of Alcohol-Induced Dysregulation in Dopamine Network Dynamics May Rescue Maladaptive Decision-making

Alcohol is the most commonly abused substance among adolescents, promoting the development of substance use disorders and compromised decision-making in adulthood. We have previously demonstrated, with a preclinical model in rodents, that adolescent alcohol use results in adult risk-taking behavior that positively correlates with phasic dopamine transmission in response to risky options, but the underlying mechanisms remain unknown. Here, we show that adolescent alcohol use may produce maladaptive decision-making through a disruption in dopamine network dynamics via increased GABAergic transmission within the ventral tegmental area (VTA). Indeed, we find that increased phasic dopamine signaling after adolescent alcohol use is attributable to a midbrain circuit, including the input from the pedunculopontine tegmentum to the VTA. Moreover, we demonstrate that VTA dopamine neurons from adult rats exhibit enhanced IPSCs after adolescent alcohol exposure corresponding to decreased basal dopamine levels in adulthood that negatively correlate with risk-taking. Building on these findings, we develop a model where increased inhibitory tone on dopamine neurons leads to a persistent decrease in tonic dopamine levels and results in a potentiation of stimulus-evoked phasic dopamine release that may drive risky choice behavior. Based on this model, we take a pharmacological approach to the reversal of risk-taking behavior through normalization of this pattern in dopamine transmission. These results isolate the underlying circuitry involved in alcohol-induced maladaptive decision-making and identify a novel therapeutic target.

SIGNIFICANCE STATEMENT

One of the primary problems resulting from chronic alcohol use is persistent, maladaptive decision-making that is associated with ongoing addiction vulnerability and relapse. Indeed, studies with the Iowa Gambling Task, a standard measure of risk-based decision-making, have reliably shown that alcohol-dependent individuals make riskier, more maladaptive choices than nondependent individuals, even after periods of prolonged abstinence. Using a preclinical model, in the current work, we identify a selective disruption in dopamine network dynamics that may promote maladaptive decision-making after chronic adolescent alcohol use and demonstrate its pharmacological reversal in adulthood. Together, these results highlight a novel neural mechanism underlying heightened risk-taking behavior in alcohol-dependent individuals and provide a potential therapeutic target for further investigation.

Modulation of nucleus accumbens connectivity by alcohol drinking and naltrexone in alcohol-preferring rats: A manganese-enhanced magnetic resonance imaging study

The nonselective opioid receptor antagonist naltrexone is now used for the treatment of alcoholism, yet naltrexone's central mechanism of action remains poorly understood. One line of evidence suggests that opioid antagonists regulate alcohol drinking through interaction with the mesolimbic dopamine system. Hence, our goal here was to examine the role of the nucleus accumbens connectivity in alcohol reinforcement and naltrexone's actions using manganese-enhanced magnetic resonance imaging (MEMRI). Following long-term free-choice drinking of alcohol and water, AA (Alko Alcohol) rats received injections of MnCl2 into the nucleus accumbens for activity-dependent tracing of accumbal connections. Immediately after the accumbal injections, rats were imaged using MEMRI, and then allowed to drink either alcohol or water for the next 24h. Naltrexone was administered prior to the active dark period, and the second MEMRI was performed 24h after the first scan. Comparison of signal intensity at 1 and 24h after accumbal MnCl2 injections revealed an ipsilateral continuum through the ventral pallidum, bed nucleus of the stria terminalis, globus pallidus, and lateral hypothalamus to the substantia nigra and ventral tegmental area. Activation was also seen in the rostral part of the insular cortex and regions of the prefrontal cortex. Alcohol drinking resulted in enhanced activation of these connections, whereas naltrexone suppressed alcohol-induced activity. These data support the involvement of the accumbal connections in alcohol reinforcement and mediation of naltrexone's suppressive effects on alcohol drinking through their deactivation.

Closed-Loop Brain Model of Neocortical Information-Based Exchange

Here we describe an “information-based exchange” model of brain function that ascribes to neocortex, basal ganglia, and thalamus distinct network functions. The model allows us to analyze whole brain system set point measures, such as the rate and heterogeneity of transitions in striatum and neocortex, in the context of neuromodulation and other perturbations. Our closed-loop model is grounded in neuroanatomical observations, proposing a novel “Grand Loop” through neocortex, and invokes different forms of plasticity at specific tissue interfaces and their principle cell synapses to achieve these transitions. By implementing a system for maximum information-based exchange of action potentials between modeled neocortical areas, we observe changes to these measures in simulation. We hypothesize that similar dynamic set points and modulations exist in the brain's resting state activity, and that different modifications to information-based exchange may shift the risk profile of different component tissues, resulting in different neurodegenerative diseases. This model is targeted for further development using IBM's Neural Tissue Simulator, which allows scalable elaboration of networks, tissues, and their neural and synaptic components toward ever greater complexity and biological realism.

Persistent Adaptations in Afferents to Ventral Tegmental Dopamine Neurons after Opiate Withdrawal

Protracted opiate withdrawal is accompanied by altered responsiveness of midbrain dopaminergic (DA) neurons, including a loss of DA cell response to morphine, and by behavioral alterations, including affective disorders. GABAergic neurons in the tail of the ventral tegmental area (tVTA), also called the rostromedial tegmental nucleus, are important for behavioral responses to opiates. We investigated the tVTA-VTA circuit in rats after chronic morphine exposure to determine whether tVTA neurons participate in the loss of opiate-induced disinhibition of VTA DA neurons observed during protracted withdrawal. In vivo recording revealed that VTA DA neurons, but not tVTA GABAergic neurons, are tolerant to morphine after 2 weeks of withdrawal. Optogenetic stimulation of tVTA neurons inhibited VTA DA neurons similarly in opiate-naive and long-term withdrawn rats. However, tVTA inactivation increased VTA DA activity in opiate-naive rats, but not in withdrawn rats, resembling the opiate tolerance effect in DA cells. Thus, although inhibitory control of DA neurons by tVTA is maintained during protracted withdrawal, the capacity for disinhibitory control is impaired. In addition, morphine withdrawal reduced both tVTA neural activity and tonic glutamatergic input to VTA DA neurons. We propose that these changes in glutamate and GABA inputs underlie the apparent tolerance of VTA DA neurons to opiates after chronic exposure. These alterations in the tVTA-VTA DA circuit could be an important factor in opiate tolerance and addiction. Moreover, the capacity of the tVTA to inhibit, but not disinhibit, DA cells after chronic opiate exposure may contribute to long-term negative affective states during withdrawal.

SIGNIFICANCE STATEMENT

Dopaminergic (DA) cells of the ventral tegmental area (VTA) are the origin of a brain reward system and are critically involved in drug abuse. Morphine has long been known to affect VTA DA cells via GABAergic interneurons. Recently, GABAergic neurons caudal to the VTA were discovered and named the tail of VTA (tVTA). Here, we show that tVTA GABA neurons lose their capacity to disinhibit, but not to inhibit, VTA DA cells after chronic opiate exposure. The failure of disinhibition was associated with a loss of glutamatergic input to DA neurons after chronic morphine. These findings reveal mechanisms by which the tVTA may play a key role in long-term negative affective states during opiate withdrawal.

Area-specific analysis of the distribution of hypothalamic neurons projecting to the rat ventral tegmental area, with special reference to the GABAergic and glutamatergic efferents

The ventral tegmental area (VTA) is a main regulator of reward and integrates a wide scale of hormonal and neuronal information. Feeding-, energy expenditure-, stress, adaptation- and reproduction-related hypothalamic signals are processed in the VTA and influence the reward processes. However, the neuroanatomical origin and chemical phenotype of neurons mediating these signals to the VTA have not been fully characterized. In this study we have systematically mapped hypothalamic neurons that project to the VTA using the retrograde tracer Choleratoxin B subunit (CTB) and analyzed their putative gamma-aminobutyric acid (GABA) and/or glutamate character with in situ hybridization in male rats. 23.93 ± 3.91% of hypothalamic neurons projecting to the VTA was found in preoptic and 76.27 ± 4.88% in anterior, tuberal and mammillary hypothalamic regions. Nearly half of the retrogradely-labeled neurons in the preoptic, and more than one third in the anterior, tuberal and mammillary hypothalamus appeared in medially located regions. The analyses of vesicular glutamate transporter 2 (VGLUT2) and glutamate decarboxylase 65 (GAD65) mRNA expression revealed both amino acid markers in different subsets of retrogradely-labeled hypothalamic neurons, typically with the predominance of the glutamatergic marker VGLUT2. About one tenth of CTB-IR neurons were GAD65-positive even in hypothalamic nuclei expressing primarily VGLUT2. Some regions were populated mostly by GAD65 mRNA-containing retrogradely-labeled neurons. These included the perifornical part of the lateral hypothalamus where 58.63 ± 19.04% of CTB-IR neurons were GABAergic. These results indicate that both the medial and lateral nuclear compartments of the hypothalamus provide substantial input to the VTA. Furthermore, colocalization studies revealed that these projections not only use glutamate but also GABA for neurotransmission. These GABAergic afferents may underlie important inhibitory mechanism to fine-tune the reward value of specific signals in the VTA.

Mechanisms for multiple activity modes of VTA dopamine neurons

Midbrain ventral segmental area (VTA) dopaminergic neurons send numerous projections to cortical and sub-cortical areas, and diffusely release dopamine (DA) to their targets. DA neurons display a range of activity modes that vary in frequency and degree of burst firing. Importantly, DA neuronal bursting is associated with a significantly greater degree of DA release than an equivalent tonic activity pattern. Here, we introduce a single compartmental, conductance-based computational model for DA cell activity that captures the behavior of DA neuronal dynamics and examine the multiple factors that underlie DA firing modes: the strength of the SK conductance, the amount of drive, and GABA inhibition. Our results suggest that neurons with low SK conductance fire in a fast firing mode, are correlated with burst firing, and require higher levels of applied current before undergoing depolarization block. We go on to consider the role of GABAergic inhibition on an ensemble of dynamical classes of DA neurons and find that strong GABA inhibition suppresses burst firing. Our studies suggest differences in the distribution of the SK conductance and GABA inhibition levels may indicate subclasses of DA neurons within the VTA. We further identify, that by considering alternate potassium dynamics, the dynamics display burst patterns that terminate via depolarization block, akin to those observed in vivo in VTA DA neurons and in substantia nigra pars compacta (SNc) DA cell preparations under apamin application. In addition, we consider the generation of transient burst firing events that are NMDA-initiated or elicited by a sudden decrease of GABA inhibition, that is, disinhibition.

A reinforcement learning mechanism responsible for the valuation of free choice

Humans exhibit a preference for options they have freely chosen over equally valued options they have not; however, the neural mechanism that drives this bias and its functional significance have yet to be identified. Here, we propose a model in which choice biases arise due to amplified positive reward prediction errors associated with free choice. Using a novel variant of a probabilistic learning task, we show that choice biases are selective to options that are predominantly associated with positive outcomes. A polymorphism in DARPP-32, a gene linked to dopaminergic striatal plasticity and individual differences in reinforcement learning, was found to predict the effect of choice as a function of value. We propose that these choice biases are the behavioral byproduct of a credit assignment mechanism responsible for ensuring the effective delivery of dopaminergic reinforcement learning signals broadcast to the striatum.

Sexually dimorphic activation of dopaminergic areas depends on affiliation during courtship and pair formation

For many species, dyadic interaction during courtship and pair bonding engage intense emotional states that control approach or avoidance behavior. Previous studies have shown that one component of a common social brain network (SBN), dopaminergic areas, are highly engaged during male songbird courtship of females. We tested whether the level of activity in dopaminergic systems of both females and males during courtship is related to their level of affiliation. In order to objectively quantify affiliative behaviors, we developed a system for tracking the position of both birds during free interaction sessions. During a third successive daily interaction session, there was a range of levels of affiliation among bird pairs, as quantified by several position and movement parameters. Because both positive and negative social interactions were present, we chose to characterize affiliation strength by pair valence. As a potential neural system involved in regulating pair valence, the level of activity of the dopaminergic group A11 (within the central gray) was selectively reduced in females of positive valence pairs. Further, activation of non-dopaminergic neurons in VTA was negatively related to valence, with this relationship strongest in ventral VTA of females. Together, these results suggest that inhibition of fear or avoidance networks may be associated with development of close affiliation, and highlight the importance of negative as well as positive emotional states in the process of courtship, and in development of long-lasting social bonds.

Food restriction increases glutamate receptor-mediated burst firing of dopamine neurons

Restriction of food intake increases the acquisition of drug abuse behavior and enhances the reinforcing efficacy of those drugs. However, the neurophysiological mechanisms responsible for the interactions between feeding state and drug use are largely unknown. Here we show that chronic mild food restriction increases the burst firing of dopamine neurons in the substantia nigra. Dopamine neurons from food-restricted mice exhibited increased burst firing in vivo, an effect that was enhanced by an injection of the psychomotor stimulant cocaine (10 mg/kg, i.p.). Food restriction also enhanced aspartic acid-induced burst firing of dopamine neurons in an ex vivo brain slice preparation, consistent with an adaptation occurring in the somatodendritic compartment and independent of a circuit mechanism. Enhanced burst firing persisted after 10 d of free feeding following chronic food restriction but was not observed following a single overnight fast. Whole-cell patch-clamp recordings indicated that food restriction also increased electrically evoked AMPAR/NMDAR ratios and increased D2 autoreceptor-mediated desensitization in dopamine neurons. These results identify dopamine neurons in the substantia nigra as a convergence point for the interactions between feeding state and drugs of abuse. Furthermore, increased glutamate transmission combined with decreased autoreceptor inhibition could work in concert to enhance drug efficacy in response to food restriction.

In vitro maturation of dopaminergic neurons derived from mouse embryonic stem cells: implications for transplantation

The obvious motor symptoms of Parkinson's disease result from a loss of dopaminergic neurons from the substantia nigra. Embryonic stem cell-derived neural progenitor or precursor cells, adult neurons and fetal midbrain tissue have all been used to replace dying dopaminergic neurons. Transplanted cell survival is compromised by factors relating to the new environment, for example; hypoxia, mechanical trauma and excitatory amino acid toxicity. In this study we investigate, using live-cell fluorescence Ca²⁺ and Cl⁻ imaging, the functional properties of catecholaminergic neurons as they mature. We also investigate whether GABA has the capacity to act as a neurotoxin early in the development of these neurons. From day 13 to day 21 of differentiation [Cl⁻]_i progressively dropped in tyrosine hydroxylase positive (TH⁺) neurons from 56.0 (95% confidence interval, 55.1, 56.9) mM to 6.9 (6.8, 7.1) mM. At days 13 and 15 TH⁺ neurons responded to GABA (30 µM) with reductions in intracellular Cl⁻ ([Cl⁻]_i); from day 21 the majority of neurons responded to GABA (30 µM) with elevations of [Cl⁻]_i. As [Cl⁻]_i reduced, the ability of GABA (30 µM) to elevate intracellular Ca²⁺ ([Ca²⁺]_i) did also. At day 13 of differentiation a three hour exposure to GABA (30 µM) or L-glutamate (30 µM) increased the number of midbrain dopaminergic (TH⁺ and Pitx3⁺) neurons labeled with the membrane-impermeable nuclear dye TOPRO-3. By day 23 cultures were resistant to the effects of both GABA and L-glutamate. We believe that neuronal susceptibility to amino acid excitotoxicity is dependent upon neuronal maturity, and this should be considered when isolating cells for transplantation studies.

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

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