Firing properties of dopamine neurons in freely moving dopamine-deficient mice: effects of dopamine receptor activation and anesthesia

A mathematical model for the role of dopamine-D2 self-regulation in the production of ultradian rhythms

Many self-motivated and goal-directed behaviours display highly flexible, approximately 4 hour ultradian (shorter than a day) oscillations. Despite lacking direct correspondence to physical cycles in the environment, these ultradian rhythms may be involved in optimizing functional interactions with the environment and reflect intrinsic neural dynamics. Current evidence supports a role of mesostriatal dopamine (DA) in the expression and propagation of ultradian rhythmicity, however, the biochemical processes underpinning these oscillations remain to be identified. Here, we use a mathematical model to investigate D2 autoreceptor-dependent DA self-regulation as the source of ultradian behavioural rhythms. DA concentration at the midbrain-striatal synapses is governed through a dual-negative feedback-loop structure, which naturally gives rise to rhythmicity. This model shows the propensity of striatal DA to produce an ultradian oscillation characterized by a flexible period that is highly sensitive to parameter variations. Circadian (approximately 24 hour) regulation consolidates the ultradian oscillations and alters their response to the phase-dependent, rapid-resetting effect of a transient excitatory stimulus. Within a circadian framework, the ultradian rhythm orchestrates behavioural activity and enhances responsiveness to an external stimulus. This suggests a role for the circadian-ultradian timekeeping hierarchy in governing organized behaviour and shaping daily experience through coordinating the motivation to engage in recurring, albeit not highly predictable events, such as social interactions.

Neurovascular effects of cocaine: relevance to addiction

Cocaine is a highly addictive drug, and its use is associated with adverse medical consequences such as cerebrovascular accidents that result in debilitating neurological complications. Indeed, brain imaging studies have reported severe reductions in cerebral blood flow (CBF) in cocaine misusers when compared to the brains of healthy non-drug using controls. Such CBF deficits are likely to disrupt neuro-vascular interaction and contribute to changes in brain function. This review aims to provide an overview of cocaine-induced CBF changes and its implication to brain function and to cocaine addiction, including its effects on tissue metabolism and neuronal activity. Finally, we discuss implications for future research, including targeted pharmacological interventions and neuromodulation to limit cocaine use and mitigate the negative impacts.

Biochemically functionalized probes for cell-type-specific targeting and recording in the brain

Selective targeting and modulation of distinct cell types and neuron subtypes is central to understanding complex neural circuitry and could enable electronic treatments that target specific circuits while minimizing off-target effects. However, current brain-implantable electronics have not yet achieved cell-type specificity. We address this challenge by functionalizing flexible mesh electronic probes, which elicit minimal immune response, with antibodies or peptides to target specific cell markers. Histology studies reveal selective association of targeted neurons, astrocytes, and microglia with functionalized probe surfaces without accumulating off-target cells. In vivo chronic electrophysiology further yields recordings consistent with selective targeting of these cell types. Last, probes functionalized to target dopamine receptor 2 expressing neurons show the potential for neuron-subtype-specific targeting and electrophysiology.

Biochemically-functionalized probes for cell type-specific targeting and recording in the brain

Selective targeting and modulation of distinct cell types and neuron subtypes is central to understanding complex neural circuitry, and could enable electronic treatments that target specific circuits while minimizing off-target effects. However, current brain-implantable electronics have not yet achieved cell-type specificity. We address this challenge by functionalizing flexible mesh electronic probes, which elicit minimal immune response, with antibodies or peptides to target specific cell markers. Histology studies reveal selective association of targeted neurons, astrocytes and microglia with functionalized probe surfaces without accumulating off-target cells. In vivo chronic electrophysiology further yields recordings consistent with selective targeting of these cell types. Last, probes functionalized to target dopamine 2 receptor expressing neurons show the potential for neuron subtype specific targeting and electrophysiology.

Cocaine's effects on the reactivity of the medial prefrontal cortex to ventral tegmental area stimulation: optical imaging study in mice

BACKGROUND AND AIMS

The prefrontal cortex (PFC) is modulated by dopaminergic and glutamatergic neurons that project from the ventral tegmental area (VTA) and disruption of this modulation might facilitate impulsive behaviors during cocaine intoxication. Here, we assessed the effects of acute cocaine (30 mg/kg, i.p.) on the reactivity of the PFC to VTA stimulation.

METHODS

Using a genetically encoded calcium indicator (GCaMP6f), we optically imaged the neuronal Ca2+ reactance in medial PFC (mPFC) in response to 'tonic-like' (5 Hz) and 'phasic-like' (50 Hz) electrical VTA stimulation. The high temporal and spatial resolutions of our optical system allowed us to capture single Ca2+ neuronal transients from individual stimuli with 'tonic-like' stimulation and to visualize single neuronal activation evoked by 'phasic-like' VTA stimulation.

RESULTS

'Tonic-like' VTA stimulation induced a rapid increase in mean neuronal Ca2+ in mPFC followed by a plateau and recovery upon termination of stimulation. After cocaine, the mPFC sensitivity to 'tonic-like' VTA stimulation was attenuated, with a 50.4% reduction (P = 0.03) in the number of Ca2+ transients corresponding to single electrical stimuli but the recovery time was lengthened (4.30 ± 0.25 sec to 5.41 ± 0.24 sec, P = 0.03). 'Phasic-like' stimulation evoked a rapid Ca2+ fluorescence increase in mPFC with an immediate decay process, and while cocaine did not affect the peak response (7.17 ± 1.07% versus 7.13 ± 0.96%, P = 0.98) it shortened the recovery time to baseline (3.27 ± 0.11 sec versus 2.38 ± 0.23 sec, P = 0.005).

CONCLUSIONS

Acute cocaine impairs reactivity of medial prefrontal cortex (mPFC) to ventral tegmental area stimulation, decreasing its sensitivity to 'tonic-like' stimulation and lengthening the recovery time to return to baseline while shortening it for phasic stimulation. These changes in mPFC might contribute to cocaine binging during intoxication.

The dopamine circuit as a reward-taxis navigation system

Studying the brain circuits that control behavior is challenging, since in addition to their structural complexity there are continuous feedback interactions between actions and sensed inputs from the environment. It is therefore important to identify mathematical principles that can be used to develop testable hypotheses. In this study, we use ideas and concepts from systems biology to study the dopamine system, which controls learning, motivation, and movement. Using data from neuronal recordings in behavioral experiments, we developed a mathematical model for dopamine responses and the effect of dopamine on movement. We show that the dopamine system shares core functional analogies with bacterial chemotaxis. Just as chemotaxis robustly climbs chemical attractant gradients, the dopamine circuit performs ‘reward-taxis’ where the attractant is the expected value of reward. The reward-taxis mechanism provides a simple explanation for scale-invariant dopaminergic responses and for matching in free operant settings, and makes testable quantitative predictions. We propose that reward-taxis is a simple and robust navigation strategy that complements other, more goal-directed navigation mechanisms.

Addiction-related neuroadaptations following chronic nicotine exposure

The addiction-relevant molecular, cellular, and behavioral actions of nicotine are derived from its stimulatory effects on neuronal nicotinic acetylcholine receptors (nAChRs) in the central nervous system. nAChRs expressed by dopamine-containing neurons in the ventral midbrain, most notably in the ventral tegmental area (VTA), contribute to the reward-enhancing properties of nicotine that motivate the use of tobacco products. nAChRs are also expressed by neurons in brain circuits that regulate aversion. In particular, nAChRs expressed by neurons in the medial habenula (mHb) and the interpeduncular nucleus (IPn) to which the mHb almost exclusively projects regulate the "set-point" for nicotine aversion and control nicotine intake. Different nAChR subtypes are expressed in brain reward and aversion circuits and nicotine intake is titrated to maximally engage reward-enhancing nAChRs while minimizing the recruitment of aversion-promoting nAChRs. With repeated exposure to nicotine, reward- and aversion-related nAChRs and the brain circuits in which they are expressed undergo adaptations that influence whether tobacco use will transition from occasional to habitual. Genetic variation that influences the sensitivity of addiction-relevant brain circuits to the actions of nicotine also influence the propensity to develop habitual tobacco use. Here, we review some of the key advances in our understanding of the mechanisms by which nicotine acts on brain reward and aversion circuits and the adaptations that occur in these circuits that may drive addiction to nicotine-containing tobacco products.

Ethanol produces multiple electrophysiological effects on ventral tegmental area neurons in freely moving rats

Although alcohol (i.e., ethanol) is a major drug of abuse, the acute functional effects of ethanol on the reward circuitry are not well defined in vivo. In freely moving rats, we examined the effect of intravenous ethanol administration on neuronal unit activity in the posterior ventral tegmental area (VTA), a central component of the mesolimbic reward system. VTA units were classified as putative dopamine (DA) neurons, fast-firing GABA neurons, and unidentified neurons based on a combination of electrophysiological properties and DA D₂ receptor pharmacological responses. A gradual infusion of ethanol significantly altered the firing rate of DA neurons in a concentration-dependent manner. The majority of DA neurons were stimulated by ethanol and showed enhanced burst firing activity, but a minority was inhibited. Ethanol also increased the proportion of DA neurons that exhibited pacemaker-like firing patterns. In contrast, ethanol mediated a variety of effects in GABA and other unidentified neurons that were distinct from DA neurons, including a nonlinear increase in firing rate, delayed inhibition, and more biphasic activity. These results provide evidence of discrete electrophysiological effects of ethanol on DA neurons compared with other VTA cell types, suggesting a complex role of the VTA in alcohol-induced responses in freely moving animals.

Neurodevelopmental defects and neurodegenerative phenotypes in human brain organoids carrying Parkinson's disease-linked DNAJC6 mutations

Loss-of-function mutations of DNAJC6, encoding HSP40 auxilin, have recently been identified in patients with early-onset Parkinson's disease (PD). To study the roles of DNAJC6 in PD pathogenesis, we used human embryonic stem cells with CRISPR-Cas9-mediated gene editing. Here, we show that DNAJC6 mutations cause key PD pathologic features, i.e., midbrain-type dopamine (mDA) neuron degeneration, pathologic α-synuclein aggregation, increase of intrinsic neuronal firing frequency, and mitochondrial and lysosomal dysfunctions in human midbrain-like organoids (hMLOs). In addition, neurodevelopmental defects were also manifested in hMLOs carrying the mutations. Transcriptomic analyses followed by experimental validation revealed that defects in DNAJC6-mediated endocytosis impair the WNT-LMX1A signal during the mDA neuron development. Furthermore, reduced LMX1A expression during development caused the generation of vulnerable mDA neurons with the pathologic manifestations. These results suggest that the human model of DNAJC6-PD recapitulates disease phenotypes and reveals mechanisms underlying disease pathology, providing a platform for assessing therapeutic interventions.

Loss of Midbrain Dopamine Neurons and Altered Apomorphine EEG Effects in the 5xFAD Mouse Model of Alzheimer's Disease

Cognitive malfunction, synaptic dysfunction, and disconnections in neural networks are core deficits in Alzheimer's disease (AD). 5xFAD mice, a transgenic model of AD, are characterized by an enhanced level of amyloid-β and abnormal neurotransmission. The dopaminergic (DA) system has been shown to be involved in amyloid-β transformations and neuronal plasticity; however, its role in functional network changes in familial AD still remains unclear. In 5xFAD and non-transgenic freely moving mice, electroencephalograms (EEGs) were simultaneously recorded from the secondary motor cortex (MC), superficial layers of the hippocampal CA1 area (HPC), substantia nigra (SN), and ventral tegmental area (VTA). EEGs and their frequency spectra were analyzed before and after systemic injection of a DA receptor agonist, apomorphine (APO). In the baseline EEG from MC and HPC of 5xFAD mice, delta and alpha oscillations were enhanced and beta activity was attenuated, compared to control mice. In VTA and SN of 5xFAD mice, delta-theta activity was decreased and beta oscillations dominated. In control mice, APO suppressed delta activity in VTA to a higher extent than in MC, whereas in 5xFAD mice, this difference was eliminated due to attenuation of the delta suppression in VTA. APO increased beta activity in MC of mice from both groups while significant beta suppression was observed in VTA of 5xFAD mice. These mice were characterized by significant decrease of tyrosine hydroxylase immunopositive cells in both VTA and SN and of DA transporter in MC and hippocampal dentate gyrus. We suggest that the EEG modifications observed in 5xFAD mice are associated with alterations in dopaminergic transmission, resulting in adaptive changes in the cerebral networks in the course of familial AD development.

Rostromedial tegmental nucleus-substantia nigra pars compacta circuit mediates aversive and despair behavior in mice

GABAergic neurons in the rostromedial tegmental nucleus (RMTg) receive major input from the lateral habenula (LHb), which conveys negative reward and motivation related information, and project intensively to midbrain dopamine neurons, including those in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). The RMTg-VTA circuit has been shown to be linked to the affective behavior, but the role of the RMTg-SNc circuit in aversion and depression has not been well understood. This study demonstrated that exciting or inhibiting Vgat^RMTg-SNc neurons was sufficient to increase or decrease immobility time in the forced swim test (FST), respectively. Furthermore, exciting the Vgat^RMTg-SNc pathway caused aversive behavior. Ninety percent of the SNc putative dopamine neurons were inhibited in extracellular recordings. Furthermore, inhibiting the Vgat^RMTg-SNc pathway reversed behavioral despair in chronic restraint stress (CRS) depression model mice. Manipulations of the pathway did not affect the hedonic value of the reward in the sucrose-preference test (SPT) or general motor function. In conclusion, these results indicate that the Vgat^RMTg-SNc pathway regulates aversive and despair behavior, which suggests that the RMTg may mediate the role of LHb in negative behaviors through regulating the activity of SNc neurons.

The antiparkinson drug ropinirole inhibits movement in a Parkinson's disease mouse model with residual dopamine neurons

The dopamine (DA) D2-like receptor (D2R) agonist ropinirole is often used for early and middle stage Parkinson's disease (PD). However, this D2-like agonism-based strategy has a complicating problem: D2-like agonism may activate D2 autoreceptors on the residual DA neurons in the PD brain, potentially inhibiting these residual DA neurons and motor function. We have examined this possibility by using systemic and local drug administration in transcription factor Pitx3 null mutant (Pitx3Null) mice that mimic the DA denervation in early and middle stage PD and in DA neuron tyrosine hydroxylase (TH) gene knockout (KO) mice that mimic the severe DA loss in late stage PD. We found that in Pitx3Null mice with residual DA neurons and normal mice with normal DA system, systemically injected ropinirole inhibited locomotion, whereas bilateral dorsal striatal-microinjected ropinirole stimulated movement in Pitx3Null mice; bilateral microinjection of ropinirole into the ventral tegmental area also inhibited movement in Pitx3Null mice; we further determined that ropinirole inhibited nigral DA neuron spike firing in WT mice. In contrast, both systemically and striatum-locally administered ropinirole increased movements in TH KO mice, but produced relatively more dyskinesia than L-dopa. Although requiring confirmation in non-human primates and PD patients, these data suggest that while activating D2-like receptors in striatal projection neurons and hence stimulating movements, D2-like agonists can inhibit residual DA neurons and cause akinesia when the residual DA neurons and motor functions are still substantial, and this motor-inhibitory effect disappears when almost all DA neurons are lost such as in late stage PD.

Effects of daily L-dopa administration on learning and brain structure in older adults undergoing cognitive training: a randomised clinical trial

Cognitive aging creates major individual and societal burden, motivating search for treatment and preventive care strategies. Behavioural interventions can improve cognitive performance in older age, but effects are small. Basic research has implicated dopaminergic signalling in plasticity. We investigated whether supplementation with the dopamine-precursor L-dopa improves effects of cognitive training on performance. Sixty-three participants for this randomised, parallel-group, double-blind, placebo-controlled trial were recruited via newspaper advertisements. Inclusion criteria were: age of 65–75 years, Mini-Mental State Examination score >25, absence of serious medical conditions. Eligible subjects were randomly allocated to either receive 100/25 mg L-dopa/benserazide (n = 32) or placebo (n = 31) prior to each of twenty cognitive training sessions administered during a four-week period. Participants and staff were blinded to group assignment. Primary outcomes were latent variables of spatial and verbal fluid intelligence. Compared to the placebo group, subjects receiving L-dopa improved less in spatial intelligence (−0.267 SDs; 95%CI [−0.498, −0.036]; p = 0.024). Change in verbal intelligence did not significantly differ between the groups (−0.081 SDs, 95%CI [−0.242, 0.080]; p = 0.323). Subjects receiving L-dopa also progressed slower through the training and the groups displayed differential volumetric changes in the midbrain. No statistically significant differences were found for the secondary cognitive outcomes. Adverse events occurred for 10 (31%) and 7 (23%) participants in the active and control groups, correspondingly. The results speak against early pharmacological interventions in older healthy adults to improve broader cognitive functions by targeting the dopaminergic system and provide no support for learning-enhancing properties of L-dopa supplements in the healthy elderly. The findings warrant closer investigation about the cognitive effects of early dopamine-replacement therapy in neurological disorders. This trial was preregistered at the European Clinical Trial Registry, EudraCT#2016-000891-54 (2016-10-05).

The basolateral amygdala dopaminergic system contributes to the improving effect of nicotine on stress-induced memory impairment in rats

Stress seems to be an important risk factor in the beginning and continuing stages of cigarette tobacco smoking in humans. Considering that both of nicotine administration and stress exposure affect cognitive functions including memory formation, the aim of the present study was 1) to evaluate the effect of subcutaneous (s.c.) administration of nicotine on memory formation under stress and 2) to assess the possible role of the basolateral amygdala (BLA) dopamine D1 and D2 receptors in the effect of nicotine on stress-induced memory retrieval impairment. Adult male wistar rats were bilaterally implanted in the BLA. A step-through type passive avoidance task was used to measure memory retrieval. To induce acute stress, the animals were placed on an elevated platform. The results showed that pre-test exposure to 20 and 30 min stress, but not 10 min, impaired memory retrieval. Nicotine administration (0.05 mg/kg, s.c.) improved stress-induced memory retrieval impairment. The activation of the BLA dopamine receptors via bilateral microinjection of apomorphine (0.025-0.4 μg/rat), a non-selective dopamine receptor agonist, potentiated the effect of nicotine on stress-induced memory retrieval impairment. Interestingly, intra-BLA microinjection of SCH23390 (a selective dopamine D1 receptor antagonist; 0.02-0.5 μg/rat) or sulpiride (a selective dopamine D2 receptor antagonist; 0.02-0.5 μg/rat) dose-dependently inhibited nicotine-induced improvement of the stress amnesic effect. Taken together, it can be concluded that stress-induced impairment of memory retrieval can be improved by nicotine administration. Moreover, the dopaminergic neurotransmission in the BLA through D1 and D2 receptors mediates the improving effect of nicotine on stress-induced memory retrieval impairment.

Ventral Tegmental Area Dopamine Neurons Firing Model Reveals Prenatal Nicotine Induced Alterations

The dopamine (DA) neurons found in the ventral tegmental area (VTA) are widely involved in the addiction and natural reward circuitry of the brain. Their firing patterns were shown to be important modulators of dopamine release and repetitive burst-like firing activity was highlighted as a major firing pattern of DA neurons in the VTA. In the present study we use a state space model to characterize the DA neurons firing patterns, and trace transitions of neural activity through bursting and non-bursting states. The hidden semi-Markov model (HSMM) framework, which we use, offers a statistically principled inference of bursting states and considers VTA DA firing patterns to be generated according to a Gamma process. Additionally, the explicit Gamma-based modeling of state durations allows efficient decoding of underlying neural information. Consequently, we decode and segment our single unit recordings from DA neurons in VTA according to the sequence of statistically discriminated HSMM states. The segmentation is used to study bursting state characteristics in data recorded from rats prenatally exposed to nicotine (6 mg/kg/day starting with gestational day 3) and rats from saline treated dams. Our results indicate that prenatal nicotine exposure significantly alters burst firing patterns of a subset of DA neurons in adolescent rats, suggesting nicotine exposure during gestation may induce severe effects on the neural networks involved in addiction and reward.

Functional Relevance of Different Basal Ganglia Pathways Investigated in a Spiking Model with Reward Dependent Plasticity

The brain enables animals to behaviorally adapt in order to survive in a complex and dynamic environment, but how reward-oriented behaviors are achieved and computed by its underlying neural circuitry is an open question. To address this concern, we have developed a spiking model of the basal ganglia (BG) that learns to dis-inhibit the action leading to a reward despite ongoing changes in the reward schedule. The architecture of the network features the two pathways commonly described in BG, the direct (denoted D1) and the indirect (denoted D2) pathway, as well as a loop involving striatum and the dopaminergic system. The activity of these dopaminergic neurons conveys the reward prediction error (RPE), which determines the magnitude of synaptic plasticity within the different pathways. All plastic connections implement a versatile four-factor learning rule derived from Bayesian inference that depends upon pre- and post-synaptic activity, receptor type, and dopamine level. Synaptic weight updates occur in the D1 or D2 pathways depending on the sign of the RPE, and an efference copy informs upstream nuclei about the action selected. We demonstrate successful performance of the system in a multiple-choice learning task with a transiently changing reward schedule. We simulate lesioning of the various pathways and show that a condition without the D2 pathway fares worse than one without D1. Additionally, we simulate the degeneration observed in Parkinson's disease (PD) by decreasing the number of dopaminergic neurons during learning. The results suggest that the D1 pathway impairment in PD might have been overlooked. Furthermore, an analysis of the alterations in the synaptic weights shows that using the absolute reward value instead of the RPE leads to a larger change in D1.

Transcription factors FOXA1 and FOXA2 maintain dopaminergic neuronal properties and control feeding behavior in adult mice

Midbrain dopaminergic (mDA) neurons are implicated in cognitive functions, neuropsychiatric disorders, and pathological conditions; hence understanding genes regulating their homeostasis has medical relevance. Transcription factors FOXA1 and FOXA2 (FOXA1/2) are key determinants of mDA neuronal identity during development, but their roles in adult mDA neurons are unknown. We used a conditional knockout strategy to specifically ablate FOXA1/2 in mDA neurons of adult mice. We show that deletion of Foxa1/2 results in down-regulation of tyrosine hydroxylase, the rate-limiting enzyme of dopamine (DA) biosynthesis, specifically in dopaminergic neurons of the substantia nigra pars compacta (SNc). In addition, DA synthesis and striatal DA transmission were reduced after Foxa1/2 deletion. Furthermore, the burst-firing activity characteristic of SNc mDA neurons was drastically reduced in the absence of FOXA1/2. These molecular and functional alterations lead to a severe feeding deficit in adult Foxa1/2 mutant mice, independently of motor control, which could be rescued by L-DOPA treatment. FOXA1/2 therefore control the maintenance of molecular and physiological properties of SNc mDA neurons and impact on feeding behavior in adult mice.

Dopamine and norepinephrine receptors participate in methylphenidate enhancement of in vivo hippocampal synaptic plasticity

Attention-deficit hyperactive disorder (ADHD) is the most commonly studied and diagnosed psychiatric disorder in children. Methylphenidate (MPH, e.g., Ritalin) has been used to treat ADHD for over 50 years. It is the most commonly prescribed treatment for ADHD, and in the past decade it was the drug most commonly prescribed to teenagers. In addition, MPH has become one of the most widely abused drugs on college campuses. In this study, we examined the effects of MPH on hippocampal synaptic plasticity, which serves as a measurable quantification of memory mechanisms. Field potentials were recorded with permanently implanted electrodes in freely-moving mice to quantify MPH modulation of perforant path synaptic transmission onto granule cells of the dentate gyrus. Our hypothesis was that MPH affects hippocampal synaptic plasticity underlying learning because MPH boosts catecholamine signaling by blocking the dopamine and norepinephrine transporters (DAT and NET respectively). In vitro hippocampal slice experiments indicated MPH enhances perforant path plasticity, and this MPH enhancement arose from action via D1-type dopamine receptors and β-type adrenergic receptors. Similarly, MPH boosted in vivo initiation of long-term potentiation (LTP). While there was an effect via both dopamine and adrenergic receptors in vivo, LTP induction was more dependent on the MPH-induced action via D1-type dopamine receptors. Under biologically reasonable experimental conditions, MPH enhances hippocampal synaptic plasticity via catecholamine receptors.

Phasic activation of ventral tegmental neurons increases response and pattern similarity in prefrontal cortex neurons

Dopamine is critical for higher neural processes and modifying the activity of the prefrontal cortex (PFC). However, the mechanism of dopamine contribution to the modification of neural representation is unclear. Using in vivo two-photon population Ca²⁺ imaging in awake mice, this study investigated how neural representation of visual input to PFC neurons is regulated by dopamine. Phasic stimulation of dopaminergic neurons in the ventral tegmental area (VTA) evoked prolonged Ca²⁺ transients, lasting ∼30 s in layer 2/3 neurons of the PFC, which are regulated by a dopamine D1 receptor-dependent pathway. Furthermore, only a conditioning protocol with visual sensory input applied 0.5 s before the VTA dopaminergic input could evoke enhanced Ca²⁺ transients and increased pattern similarity (or establish a neural representation) of PFC neurons to the same sensory input. By increasing both the level of neuronal response and pattern similarity, dopaminergic input may establish robust and reliable cortical representation.

DOI:http://dx.doi.org/10.7554/eLife.02726.001

Around 120 years ago, Ivan Pavlov unintentionally sparked a new field of psychology research. He did so by noting that his dogs had learned to associate the sound of the bell that he rang before feeding them with the food itself, such that they would salivate upon hearing the bell even when there was no food present. This form of learning—now known as associative learning—has since been demonstrated in species from honeybees to humans.

For the brain to associate two events, such as the sound of a bell and the delivery of food, it must encode the first event and keep that information available or ‘on-line’ until the occurrence of the second event, at which point the two can be linked together. This process takes place in part of the brain called the prefrontal cortex, but the mechanism by which it occurs is largely unclear.

Now, Iwashita has obtained new insights into the molecular basis of associative learning by studying how the activity of the prefrontal cortex is affected by the activity of a second region of the brain. This second region, called the ventral tegmental area, is part of the brain's reward circuit: it becomes active whenever an animal experiences a desirable event, such as receiving food, and supplies a neurotransmitter called dopamine to its target areas, which include the prefrontal cortex.

Electrodes were used to mimic the changes in brain activity that occur when a mouse learns to associate a visual stimulus with a reward: this involved repeatedly activating the visual cortex in a conscious mouse, followed by activation of the ventral tegmental area. Short-lived increases in calcium levels were seen in the prefrontal cortex, raising the possibility that these ‘calcium transients’ are the signal that enables the brain to link two events. Moreover, blocking proteins called dopamine D1 receptors in the prefrontal cortex reduced the calcium transients, which is consistent with existing evidence that dopamine from the ventral tegmental area is required for associative learning.

Intriguingly, the calcium transients lasted for roughly 30 s, which is also the maximum length of time by which a stimulus and a reward can be separated and still be associated. Given that the calcium transients could not be detected in anesthetized mice, a full understanding of the mechanisms underlying associative learning may require studies of the conscious brain.

DOI:http://dx.doi.org/10.7554/eLife.02726.002

Nicotine exposure increases the complexity of dopamine neurons in the parainterfascicular nucleus (PIF) sub-region of VTA

Background

Recent publications highlight differences within the sub-regions of the ventral tegmental area (VTA) including the parabrachial pigmented nucleus (PBP), parainterfascicular nucleus (PIF) and paranigral nucleus (PN) in the projections to the prefrontal cortex (PFC) and the glutamatergic pathway.

Methods

In order to characterize the effects of prenatal nicotine exposure on the mesocorticolimbic system of the rat offspring, local field potentials were recorded from 27 sites across the VTA of 9 rats aged 40–55 days. The extracellular VTA neural activities were analyzed using Approximate Entropy (ApEn) method. Approximate entropy values were then grouped according to each anatomic location including the PBP, PIF and PN.

Results

Our results have shown that the local field potentials corresponding to the neurons located in the PIF region of the VTA have ApEn values significantly higher (p = 2x10-4) in the maternal nicotine cases when compared to the saline.

Conclusion

Therefore, we speculate that the dopamine neurons located in the PIF sub-region of the VTA are very likely involved with the nicotine addiction.

Control of bursting behavior in neurons by autaptic modulation

Firing properties of biological neurons have long been recognized to be determined by extrinsic synaptic afferents that neurons receive and intrinsic ionic mechanisms that neurons possess, however, previous researches have also demonstrated that firing behavior of single neurons can be modulated by the neurons themselves, realized by the autapses. Thus in this investigation, we argued that how autaptic modulations shape the bursting behavior of biological neurons. We considered the issue from the following two aspects: autaptic-excitation and -inhibition. Our results suggested that for autaptic-excitation, under the condition of relatively weak stimulus, regular bursting was more incline to occur when the autaptic strength was weak, while regular spiking was more likely to appear when the autaptic strength was strong. However, larger stimulus would diminish the portion of bursting, but increase the portion of spiking. For autaptic-inhibition, under relatively weak stimulus, a wide range of regular bursting emerges when the autaptic strength was small, but when stronger stimulus were applied, the range of regular bursting shrinked into a small region. Meanwhile, we observed that synaptic delays have no obvious effects in the case of autaptic-excitation, while a subtle effect of synaptic delays was observed in the case of autaptic-inhibition. These results showed that bursting behavior of neurons could be controlled and modulated by the autaptic mechanisms that biological neurons intrinsically possess, and the final results may further promote the understanding in the generation of various neuronal firing patterns.

Quantitative unit classification of ventral tegmental area neurons in vivo

Neurons in the ventral tegmental area (VTA) synthesize several major neurotransmitters, including dopamine (DA), GABA, and glutamate. To classify VTA single-unit neural activity from freely moving rats, we used hierarchical agglomerative clustering and probability distributions as quantitative methods. After many parameters were examined, a firing rate of 10 Hz emerged as a transition frequency between clusters of low-firing and high-firing neurons. To form a subgroup identified as high-firing neurons with GABAergic characteristics, the high-firing classification was sorted by spike duration. To form a subgroup identified as putative DA neurons, the low-firing classification was sorted by DA D2-type receptor pharmacological responses to quinpirole and eticlopride. Putative DA neurons were inhibited by the D2-type receptor agonist quinpirole and returned to near-baseline firing rates or higher following the D2-type receptor antagonist eticlopride. Other unit types showed different responses to these D2-type receptor drugs. A multidimensional comparison of neural properties indicated that these subgroups often clustered independently of each other with minimal overlap. Firing pattern variability reliably distinguished putative DA neurons from other unit types. A combination of phasic burst properties and a low skew in the interspike interval distribution produced a neural population that was comparable to the one sorted by D2 pharmacology. These findings provide a quantitative statistical approach for the classification of VTA neurons in unanesthetized animals.

Neurophysiology of Nicotine Addiction

Tobacco use is a major health problem, and nicotine is the main addictive component. Nicotine binds to nicotinic acetylcholine receptors (nAChR) to produce its initial effects. The nAChRs subtypes are composed of five subunits that can form in numerous combinations with varied functional and pharmacological characteristics. Diverse psychopharmacological effects contribute to the overall process of nicotine addiction, but two general neural systems are emerging as critical for the initiation and maintenance of tobacco use. Mesocorticolimbic circuitry that includes the dopaminergic pathway originating in the ventral tegmental area and projecting to the nucleus accumbens is recognized as vital for reinforcing behaviors during the initiation of nicotine addiction. In this neural system β2, α4, and α6 are the most important nAChR subunits underlying the rewarding aspects of nicotine and nicotine self-administration. On the other hand, the epithalamic habenular complex and the interpeduncular nucleus, which are connected via the fasciculus retroflexus, are critical contributors regulating nicotine dosing and withdrawal symptoms. In this case, the α5 and β4 nAChR subunits have critical roles in combination with other subunits. In both of these neural systems, particular nAChR subtypes have roles that contribute to the overall nicotine addiction process.

Reward, addiction, withdrawal to nicotine

Nicotine is the principal addictive component that drives continued tobacco use despite users' knowledge of the harmful consequences. The initiation of addiction involves the mesocorticolimbic dopamine system, which contributes to the processing of rewarding sensory stimuli during the overall shaping of successful behaviors. Acting mainly through nicotinic receptors containing the α4 and β2 subunits, often in combination with the α6 subunit, nicotine increases the firing rate and the phasic bursts by midbrain dopamine neurons. Neuroadaptations arise during chronic exposure to nicotine, producing an altered brain condition that requires the continued presence of nicotine to be maintained. When nicotine is removed, a withdrawal syndrome develops. The expression of somatic withdrawal symptoms depends mainly on the α5, α2, and β4 (and likely α3) nicotinic subunits involving the epithalamic habenular complex and its targets. Thus, nicotine taps into diverse neural systems and an array of nicotinic acetylcholine receptor (nAChR) subtypes to influence reward, addiction, and withdrawal.

Neural systems analysis of decision making during goal-directed navigation

The ability to make adaptive decisions during goal-directed navigation is a fundamental and highly evolved behavior that requires continual coordination of perceptions, learning and memory processes, and the planning of behaviors. Here, a neurobiological account for such coordination is provided by integrating current literatures on spatial context analysis and decision-making. This integration includes discussions of our current understanding of the role of the hippocampal system in experience-dependent navigation, how hippocampal information comes to impact midbrain and striatal decision making systems, and finally the role of the striatum in the implementation of behaviors based on recent decisions. These discussions extend across cellular to neural systems levels of analysis. Not only are key findings described, but also fundamental organizing principles within and across neural systems, as well as between neural systems functions and behavior, are emphasized. It is suggested that studying decision making during goal-directed navigation is a powerful model for studying interactive brain systems and their mediation of complex behaviors.

Measuring burstiness and regularity in oscillatory spike trains

The ability of neurons to emit different firing patterns such as bursts or oscillations is important for information processing in the brain. In dopaminergic neurons, prominent patterns include repetitive, oscillatory bursts, regular pacemakers, and irregular spike trains with nonstationary properties. In order to describe and measure the variability of these patterns, we describe burstiness and regularity in a single model framework. We present a doubly stochastic spike train model in which a background oscillation with independent and normally distributed intervals gives rise to either single spikes or bursty spike events with Gaussian firing intensities. Five easily interpretable parameters allow a classification into bursty or single spike and irregularly or regularly oscillating firing patterns. This classification is based primarily on features of the autocorrelation histogram which are usually studied qualitatively by visual inspection. The present model provides a quantitative and objective classification scheme and relates these features directly to the underlying processes. In addition, confidence intervals visualize the uncertainty of parameter estimation and classification precision. We apply the model to a data set obtained from single dopaminergic substantia nigra neurons recorded extracellularly in vivo. The model is able to represent a high variety of discharge patterns observed empirically, and the classification agrees closely with visual inspection. In addition, changes in the parameters can be studied quantitatively, including also the properties related to bursting behavior. Thus, the proposed model can be used for the description of neuronal firing patterns and the investigation of their dynamical changes with cellular and experimental conditions.

Acute in vivo nicotine administration enhances synchrony among dopamine neurons

Altered functional interactions among midbrain dopamine (DA) neurons contribute to the reinforcing properties of environmental stimuli and addictive drugs. To examine correlations among DA neurons, acute nicotine was administrated to rats via an intraperitoneal catheter and unit activity was measured using multi-tetrode in vivo recordings. Nicotine administration enhanced the correlated activity of simultaneously recorded DA neurons from the ventral tegmental area (VTA). The strength of the correlations between DA neuron pairs, as measured by cross covariance among two spike trains, showed dynamic changes over time. Nicotine produced a gradual rise in firing rate and burst activity that reached a stable plateau approximately 20 min after the intraperitoneal nicotine infusion. Shortly after that time the cross correlations measured using 5-ms bins increased significantly above baseline. In addition, nicotine increased the firing rates of DA neurons in the posterior VTA more than in the anterior VTA. Unlike nicotine, eticlopride administration also boosted DA neuron firing activity but did not enhance synchronization, indicating that the cross correlations induced by nicotine were not due to a non-specific increase in firing rate. The overall results show that nicotine induces nearly synchronous firing by a subset of DA neurons, and those changes in correlative firing will enhance the DA signal that contributes to nicotine-induced behavioral reinforcement.

Activation of dopamine neurons is critical for aversive conditioning and prevention of generalized anxiety

Generalized anxiety is thought to result, in part, from impairments in contingency awareness during conditioning to cues that predict aversive or fearful outcomes. Dopamine neurons of the ventral midbrain exhibit heterogeneous responses to aversive stimuli that are thought to provide a critical modulatory signal to facilitate orienting to environmental changes and assignment of motivational value to unexpected events. Here, we describe a mouse model in which activation of dopamine neurons in response to an aversive stimulus is attenuated by conditional genetic inactivation of functional N–methyl–D–aspartate–type glutamate receptors (NMDARs) on dopamine neurons. We discovered that altering the magnitude of excitatory responses by dopamine neurons in response to an aversive stimulus is associated with impaired conditioning to a cue that predicts an aversive outcome. Impaired conditioning by these mice is associated with development of a persistent, generalized anxiety–like phenotype. These data are consistent with a role for dopamine in facilitating contingency awareness that is critical for the prevention of generalized anxiety.

Effects of L-DOPA on nigral dopamine neurons and local field potential: comparison with apomorphine and muscimol

L-DOPA is more effective than direct dopamine (DA) agonists in relieving the motor deficits in Parkinson's disease. Using in vivo recording, we compared the effect of l-DOPA and the direct DA agonist apomorphine on DA neurons in rat substantia nigra (SN). L-DOPA (50-100 mg/kg i.v.) decreased the firing rate as well as the variability and slow oscillation (SO) of firing. All effects were blocked by raclopride and mimicked by quinpirole, suggesting that they are mediated through D2-like receptors. Autoreceptor-selective doses of apomorphine (5-20 μg/kg i.v.) also inhibited all three parameters. The magnitude of the inhibition, however, was significantly greater than that induced by L-DOPA. Neither L-DOPA nor apomorphine had a consistent effect on SN local field potentials (LFPs). The GABA agonist muscimol, known to preferentially inhibit SN non-DA neurons, consistently inhibited the SO in both DA cell firing and LFPs. These results suggest that SN LFPs mainly reflect the synaptic potentials in non-DA neurons, and L-DOPA and apomorphine, unlike muscimol, affect DA neurons primarily through DA autoreceptors. DA autoreceptor activation is known to hyperpolarize DA cells by increasing the membrane conductance to K(+). This increase in membrane conductance would shunt synaptic input to DA neurons, thereby decreasing the variability and SO in DA cell firing. The low potency of L-DOPA to inhibit DA cell firing and reduce their responses to synaptic input may partially account for its superior therapeutic efficacy in Parkinson's disease compared with apomorphine and other direct DA agonists.

Conjunctive processing of locomotor signals by the ventral tegmental area neuronal population

The ventral tegmental area (VTA) plays an essential role in reward and motivation. How the dopamine (DA) and non-DA neurons in the VTA engage in motivation-based locomotor behaviors is not well understood. We recorded activity of putative DA and non-DA neurons simultaneously in the VTA of awake mice engaged in motivated voluntary movements such as wheel running. Our results revealed that VTA non-DA neurons exhibited significant rhythmic activity that was correlated with the animal's running rhythms. Activity of putative DA neurons also correlated with the movement behavior, but to a lesser degree. More importantly, putative DA neurons exhibited significant burst activation at both onset and offset of voluntary movements. These findings suggest that VTA DA and non-DA neurons conjunctively process locomotor-related motivational signals that are associated with movement initiation, maintenance and termination.

A behavioral genetics approach to understanding D1 receptor involvement in phasic dopamine signaling

Dopamine-producing neurons fire with both basal level tonic patterns and phasic bursts. Varying affinities of the five dopamine receptors have led to a hypothesis that higher affinity receptors are primarily activated by basal level tonic dopamine, while lower affinity receptors may be tuned to be sensitive to higher levels caused by phasic bursts. Genetically modified mice provide a method to begin to probe this hypothesis. Here we discuss three mouse models. Dopamine-deficient mice were used to determine which behaviors require dopamine. These behaviors were then analyzed in mice lacking D1 receptors and in mice with reduced phasic dopamine release. Comparison of the latter two mouse models revealed a similar failure to learn about and respond normally to cues that indicate either a positive or negative outcome, giving support to the hypothesis that phasic dopamine release and the D1 receptor act in the same pathway. However, the D1 receptor likely has additional roles beyond those of phasic dopamine detection, because D1 receptor knockout mice have deficits in addition to what has been observed in mice with reduced phasic dopamine release.

Pharmacological modulation of the gating properties of small conductance Ca2+-activated K+ channels alters the firing pattern of dopamine neurons in vivo

Dopamine (DA) neurons are autonomous pacemakers that occasionally fire bursts of action potentials, discharge patterns thought to reflect tonic and phasic DA signaling, respectively. Pacemaker activity depends on the concerted and cyclic interplay between intrinsic ion channels with small conductance Ca(2+)-activated K(+) (SK) channels playing an important role. Bursting activity is synaptically initiated but neither the transmitters nor the specific ion conductances involved have been definitively identified. Physiological and pharmacological regulation of SK channel Ca(2+) sensitivity has recently been demonstrated and could represent a powerful means of modulating the expression of tonic/phasic signaling in DA neurons in vivo. To test this premise, we characterized the effects of intravenous administration of the novel positive and negative SK channel modulators NS309 and NS8593, respectively, on the spontaneous activity of substantia nigra pars compacta DA neurons in anesthetized C57BL/6 mice. NS309, dose-dependently decreased DA cell firing rate, increased the proportion of regular firing cells, and eventually stopped spontaneous firing. By contrast, systemic administration of the negative SK channel modulator NS8593 increased firing rate and shifted the pattern toward increased irregularity/bursting; an effect similar to local application of the pore blocking peptide apamin. The altered firing patterns resulting from inhibiting SK currents persisted independently of changes in firing rates induced by administration of DA autoreceptor agonists/antagonists. We conclude that pharmacological modulation of SK channel Ca(2+)-sensitivity represents a powerful mechanism for switching DA neuron firing activity between tonic and phasic signaling modalities in vivo.

Conjunctive encoding of movement and reward by ventral tegmental area neurons in the freely navigating rodent

As one of the two main sources of brain dopamine, the ventral tegmental area (VTA) is important for several complex functions, including motivation, reward prediction, and contextual learning. Although many studies have identified the potential neural substrate of VTA dopaminergic activity in reward prediction functions during Pavlovian and operant conditioning tasks, less is understood about the role of VTA neuronal activity in motivated behaviors and more naturalistic forms of context-dependent learning. Therefore, VTA neural activity was recorded as rats performed a spatial memory task under varying contextual conditions. In addition to reward- and reward predicting cue-related firing commonly observed during conditioning tasks, the activity of a large proportion of VTA neurons was also related to the velocity and/or acceleration of the animal's movement. It is important to note that movement-related activity was strongest when rats displayed more motivation to obtain reward. Furthermore, many cells displayed a dual code of movement- and reward-related activity. These two modes of firing, however, were differentially regulated by context information, suggesting that movement- and reward-related firing are two independently regulated modes of VTA neuronal activity and may serve separate functions.

Transgenic mice in the study of drug addiction and the effects of psychostimulant drugs

The first transgenic models used to study addiction were based upon a priori assumptions about the importance of particular genes in addiction, including the main target molecules of morphine, amphetamine, and cocaine. This consequently emphasized the importance of monoamine transporters, opioid receptors, and monoamine receptors in addiction. Although the effects of opiates were largely eliminated by mu opioid receptor gene knockout, the case for psychostimulants was much more complex. Research using transgenic models supported the idea of a polygenic basis for psychostimulant effects and has associated particular genes with different behavioral consequences of psychostimulants. Phenotypic analysis of transgenic mice, especially gene knockout mice, has been instrumental in identifying the role of specific molecular targets of addictive drugs in their actions. In this article, we summarize studies that have provided insight into the polygenic determination of drug addiction phenotypes in ways that are not possible with other methods, emphasizing research into the effects of psychostimulant drugs in gene knockouts of the monoamine transporters and monoamine receptors.

Neural bases for addictive properties of benzodiazepines

Benzodiazepines are widely used in clinics and for recreational purposes, but will lead to addiction in vulnerable individuals. Addictive drugs increase the levels of dopamine and also trigger long-lasting synaptic adaptations in the mesolimbic reward system that ultimately may induce the pathological behavior. The neural basis for the addictive nature of benzodiazepines however remains elusive. Here we show that benzodiazepines increase firing of dopamine neurons of the ventral tegmental area through the positive modulation of GABA_A receptors in nearby interneurons. Such disinhibition, which relies on α1-containing GABA_ARs expressed in these cells, triggers drug-evoked synaptic plasticity in excitatory afferents onto dopamine neurons and underlies drug reinforcement. Taken together, our data provide evidence that benzodiazepines share defining pharmacological features of addictive drugs through cell type-specific expression of α1-containing GABA_ARs in the ventral tegmental area. The data also suggest that subunitselective benzodiazepines sparing α1 may be devoid of addiction liability.

Route of nicotine administration influences in vivo dopamine neuron activity: habituation, needle injection, and cannula infusion

Mesolimbic dopamine (DA) systems play a critical role in tobacco addiction driven by nicotine. Nicotine activates midbrain DA neurons and, consequently, elevates DA concentrations in targets, especially in the nucleus accumbens (NAc) of the ventral striatum. The route of drug administration influences the impact of addictive drugs. Here, we examine whether the nature of the administration alters DA neuron activity and DA concentrations in the NAc. Using unhabituated naïve freely moving rats, microdialysis measurements showed that nicotine administered via needle injection caused greater DA release in the NAc than the same dose administered via an implanted chronic cannula. After habituation to the needle injections, however, there was no significant difference in DA signaling between the needle and cannula routes of administration. Consistent with these microdialysis results after habituation, our in vivo tetrode unit recordings showed no significant difference in midbrain DA neuron activity in response to nicotine delivered by needle or cannula as long as predictive cues were avoided

Dopamine enables in vivo synaptic plasticity associated with the addictive drug nicotine

Addictive drugs induce a dopamine signal that contributes to the initiation of addiction, and the dopamine signal influences drug-associated memories that perpetuate drug use. The addiction process shares many commonalities with the synaptic plasticity mechanisms normally attributed to learning and memory. Environmental stimuli repeatedly linked to addictive drugs become learned associations, and those stimuli come to elicit memories or sensations that motivate continued drug use. Applying in vivo recording techniques to freely moving mice, we show that physiologically relevant concentrations of the addictive drug nicotine directly cause in vivo hippocampal synaptic potentiation of the kind that underlies learning and memory. The drug-induced long-term synaptic plasticity required a local hippocampal dopamine signal. Disrupting general dopamine signaling prevented the nicotine-induced synaptic plasticity and conditioned place preference. These results suggest that dopaminergic signaling serves as a functional label of salient events by enabling and scaling synaptic plasticity that underlies drug-induced associative memory.

Controls of tonic and phasic dopamine transmission in the dorsal and ventral striatum

Dopamine (DA) release varies within subregions and local environments of the striatum, suggesting that controls intrinsic and extrinsic to the DA fibers and terminals regulate release. While applying fast-scan cyclic voltammetry and using tonic and phasic stimulus trains, we investigated the regulation of DA release in the dorsolateral to ventral striatum. The ratio of phasic-to-tonic-evoked DA signals varied with the average ongoing firing frequency, and the ratio was generally higher in the nucleus accumbens (NAc) compared with the dorsolateral striatum. At the normal average firing frequency, burst stimulation produces a larger increase in the DA response in the NAc than the dorsolateral striatum. This finding was comparable whether the DA measurements were made using in vitro brain slices or were recorded in vivo from freely moving rodents. Blockade of the dopamine transporters and dopamine D(2) receptors particularly enhanced the tonic DA signals. Conversely, blockade of nicotinic acetylcholine receptors (nAChRs) containing the beta(2) subunit (beta(2)(*)) predominantly suppressed tonic DA signals. The suppression of tonic DA release increased the contrast between phasic and tonic DA signals, and that made the frequency-dependent DA dynamics between the dorsolateral striatum and NAc more similar. The results indicate that intrinsic differences in the DA fibers that innervate specific regions of the striatum combine with (at least) DA transporters, DA receptors, and nAChRs to regulate the frequency dependence of DA release. A combination of mechanisms provides specific local control of DA release that underlies pathway-specific information associated with motor and reward-related functions.

Phasic firing in dopaminergic neurons is sufficient for behavioral conditioning

Natural rewards and drugs of abuse can alter dopamine signaling, and ventral tegmental area (VTA) dopaminergic neurons are known to fire action potentials tonically or phasically under different behavioral conditions. However, without technology to control specific neurons with appropriate temporal precision in freely behaving mammals, the causal role of these action potential patterns in driving behavioral changes has been unclear. We used optogenetic tools to selectively stimulate VTA dopaminergic neuron action potential firing in freely behaving mammals. We found that phasic activation of these neurons was sufficient to drive behavioral conditioning and elicited dopamine transients with magnitudes not achieved by longer, lower-frequency spiking. These results demonstrate that phasic dopaminergic activity is sufficient to mediate mammalian behavioral conditioning.

Dopamine signaling differences in the nucleus accumbens and dorsal striatum exploited by nicotine

The dorsal striatum and the nucleus accumbens (NAc) shell of the ventral striatum have similar cellular components and are both richly innervated by dopamine neurons. Despite similarities that extend throughout the striatum, only the NAc shell has a conspicuous increase in basal dopamine upon the initial administration of psychostimulant drugs such as nicotine. As measured by microdialysis, the elevated dopamine in the NAc shell is considered an identifying functional characteristic of addictive drugs. To examine this general functional difference between nicotine's action on the dorsolateral striatum and NAc shell, we directly monitored dopamine release in rat striatal slices using fast-scan cyclic voltammetry. In addition, we separately monitored the in vivo unit firing activity of putative midbrain dopamine neurons from freely moving rats using chronic multiple tetrodes. Nicotine administration increased the firing frequency of dopamine neurons and specifically increased the number and the length of phasic burst firing. The frequency dependence for dopamine release in the dorsolateral striatum and NAc shell is fundamentally different, enabling mainly the NAc shell to capitalize on the nicotine-induced phasic burst firing by dopamine neurons. Although nicotine decreased low-frequency (tonic) dopamine release in both areas, the increased ratio of phasic bursts relative to tonic firing caused by nicotine boosted the basal dopamine concentration predominantly in the NAc shell. By favoring release from bursts while depressing release from tonic signals, nicotine spreads the range of dopamine signaling and effectively increases the signal-to-noise relationship along dopamine afferents.

Disruption of NMDAR-dependent burst firing by dopamine neurons provides selective assessment of phasic dopamine-dependent behavior

Midbrain dopamine (DA) neurons fire in 2 characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior. However, the inability to selectively disrupt these patterns of activity has hampered the precise definition of the function of these modes of signaling. Here, we addressed the role of phasic DA in learning and other DA-dependent behaviors by attenuating DA neuron burst firing and subsequent DA release, without altering tonic neural activity. Disruption of phasic DA was achieved by selective genetic inactivation of NMDA-type, ionotropic glutamate receptors in DA neurons. Disruption of phasic DA neuron activity impaired the acquisition of numerous conditioned behavioral responses, and dramatically attenuated learning about cues that predicted rewarding and aversive events while leaving many other DA-dependent behaviors unaffected.

Electrophysiological characteristics of dopamine neurons: a 35-year update

This chapter consists of four sections. The first section provides a general description of the electrophysiological characteristics of dopamine (DA) neurons in both the substantia nigra and ventral tegmental area. Emphasis is placed on the differences between DA and neighboring non-DA neurons. The second section discusses the ionic mechanisms underlying the generation of action potential in DA cells. Evidence is provided to suggest that these mechanisms differ not only between DA and non-DA neurons but also between DA cells located in different areas, with different projection sites and at different developmental stages. Some of the differences may play a critical role in the vulnerability of a DA neuron to cell death. The third section describes the firing patterns of DA cells. Data are presented to show that the current "80/160 ms" criteria for burst identification need to be revised and that the burst firing, originally described by Bunney et al., can be described as slow oscillations in firing rate. In the ventral tegmental area, the slow oscillations are, at least partially, derived from the prefrontal cortex and part of prefrontal information is transferred to DA cells indirectly through inhibitory neurons. The final section focuses on the feedback regulation of DA cells. New evidence suggests that DA autoreceptors are coupled to multiple effectors, and both D1 and D2-like receptors are involved in long-loop feedback control of DA neurons. Because of the presence of multiple feedback and nonfeedback pathways, the effect of a drug on a DA neuron can be far more complex than an inhibition or excitation. A better understanding of the intrinsic properties of DA neurons and their regulation by afferent input will, in time, help to point to the way to more effective and safer treatments for disorders including schizophrenia, drug addiction, and Parkinson's disease.

The role of dopamine-dependent negative feedback in the hippocampus-basal ganglia-thalamus-hippocampus loop in the extinction of responses

A mechanism for the extinction of the responses of hippocampal and dopaminergic neurons to repeated sensory stimuli is proposed, based on dopamine-dependent negative feedback in the hippocampus-basal ganglia-thalamus-hippocampus loop. Activation of hippocampal neurons evoked by a new stimulus facilitates the appearance of responses in dopaminergic neurons as a result of disinhibition via striopallidal cells of the nucleus accumbens and ventral pallidum. However, increases in dopamine levels and activation of D2 receptors on striopallidal cells, facilitating depression of hippocampal inputs, prevent disinhibition of dopaminergic neurons, such that their responses start to decline. Subsequent reductions in actions on D1 receptors lead to decreases in the efficiency of excitation both of neurons in hippocampal field CA1 and strionigral cells in the nucleus accumbens. The direct pathway via the basal ganglia mediates disinhibition of the thalamic nucleus reuniens, exciting neurons in field CA1, which leads to extinction of the responses of hippocampal neurons, decreases in disinhibition of dopaminergic cells, and further extinction of their responses.

Dopamine signaling in the dorsal striatum is essential for motivated behaviors: lessons from dopamine-deficient mice

Genetically engineered mice that lack tyrosine hydroxylase in all dopaminergic neurons become hypoactive and aphagic, and they starve by 4 weeks of age. However, they can be rescued by daily treatment with l-dopa, which restores activity and feeding for about 10 hours. Thus, these mice can be examined in both dopamine-depleted and dopamine-replete states. A series of behavioral experiments lead to the primary conclusion that in the dopamine-depleted state these mice are not motivated to engage in goal-directed behaviors. Nevertheless, they still have a preference for sucrose, they can learn the location of food rewards, and they can form a conditioned-place preference for drugs. Dopamine signaling can be restored to the striatum by several different viral gene-therapy procedures. Restoring dopamine signaling selectively to the dorsal striatum is sufficient to allow feeding, locomotion, and reward-based learning. The rescued mice appear to have normal motivation to engage in all goal-directed behaviors that have been tested. The results suggest that dopamine facilitates the output from dorsal striatum, which provides a permissive signal allowing feeding and other goal-directed behaviors.

The role of dopamine in alcohol self-administration in humans: individual differences

OBJECTIVE

To clarify dopamine's role in alcohol self-administration in a heterogeneous sample of drinkers using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Sixteen men with variable drinking histories were characterized on their ethanol-induced cardiac response, a marker previously proposed to index dopamine system reactivity and vulnerability to alcohol abuse. During separate sessions participants were administered (i) a nutritionally balanced (BAL) amino acid (AA) mixture, (ii) a mixture lacking the dopamine precursors, phenylalanine and tyrosine, and (iii) APTD followed by the dopamine precursor, L-DOPA. Five hours after AA administration, participants could earn units of alcohol using a progressive ratio breakpoint task.

RESULTS

Alcohol self-administration was reduced in the APTD and APTD+L-DOPA conditions relative to the BAL condition. In both cases the changes were predicted by ethanol-induced cardiac change.

CONCLUSIONS

The motivation to drink is likely regulated by more than one neurobiological mechanism. Individual differences in cardiac responsivity to ethanol might provide a peripheral marker of responsiveness to pharmacological manipulations of dopamine.

Cocaine-conditioned place preference by dopamine-deficient mice is mediated by serotonin

Rodents learn to associate the rewarding effects of drugs with the environment in which they are encountered and, subsequently, will display a conditioned place preference (CPP) for that environment. Cocaine-induced CPP is generally thought to be mediated through inhibition of the dopamine transporter and the consequent increase in extracellular dopamine. However, here we report that dopamine-deficient (DD) mice formed a CPP for cocaine that was not blocked by a dopamine D1-receptor antagonist. Fluoxetine, a serotonin transporter (SERT) inhibitor, produced CPP in DD, but not control mice, suggesting that serotonin mediates cocaine CPP in DD mice. Inhibition of dopamine neuron firing by pretreatment with quinpirole, a dopamine D2-receptor agonist, blocked both cocaine- and fluoxetine-induced CPP in DD mice. These findings are consistent with the hypothesis that, in the absence of dopamine, cocaine-mediated SERT blockade activates dopamine neurons, which then release some other neurotransmitter that contributes to cocaine reward in DD mice.

Relating mode of action to clinical practice: dopaminergic agents in Parkinson's disease

Most treatment advances in PD have been based on restoring dopaminergic input. The development of levodopa was the first breakthrough and, since then, other compounds have been developed. Each antiparkinsonian medication has its own profile of efficacy and adverse effects, and these can largely be explained by their modes of action. As patients receive a number of different compounds, physicians should be aware of the differences of agents and understand how these differences may relate to clinical practice. This article reviews the three main classes of dopaminergic PD therapy (levodopa, monoamine oxidase inhibitors and dopamine agonists).