Inhibitory action of dopamine involves a subthreshold Cs(+)-sensitive conductance in neostriatal neurons

Repetitive Levodopa Treatment Drives Cell Type-Specific Striatal Adaptations Associated With Progressive Dyskinesia in Parkinsonian Mice

The use of levodopa to manage Parkinson's disease (PD) symptoms leads to levodopa-induced dyskinesia (LID) and other motor fluctuations, which worsen with disease progression and repeated treatment. Aberrant activity of striatal D1- and D2-expressing medium spiny neurons (D1-/D2-MSNs) underlies LID, but the mechanisms driving its progression remain unclear. Using the 6-OHDA mouse model of PD/LID, we combined in vivo and ex vivo recordings to isolate the effect of repeated treatment in LID worsening and other motor fluctuation-related phenotypes. We found that LID worsening is linked to potentiation of levodopa-evoked responses in both D1-/D2-MSNs, independent of changes in dopamine release or MSN intrinsic excitability. Instead, strengthening of glutamatergic synapses onto D1-MSNs emerged as a key driver. Moreover, we found changes in D2-MSN activity that specifically influenced LID duration, potentially contributing to motor fluctuations, which paralleled a reduction in D2R sensitivity. These findings reveal striatal adaptations contributing to worsening of levodopa-related complications.

Updating the striatal-pallidal wiring diagram

The striatal and pallidal complexes are basal ganglia structures that orchestrate learning and execution of flexible behavior. Models of how the basal ganglia subserve these functions have evolved considerably, and the advent of optogenetic and molecular tools has shed light on the heterogeneity of subcircuits within these pathways. However, a synthesis of how molecularly diverse neurons integrate into existing models of basal ganglia function is lacking. Here, we provide an overview of the neurochemical and molecular diversity of striatal and pallidal neurons and synthesize recent circuit connectivity studies in rodents that takes this diversity into account. We also highlight anatomical organizational principles that distinguish the dorsal and ventral basal ganglia pathways in rodents. Future work integrating the molecular and anatomical properties of striatal and pallidal subpopulations may resolve controversies regarding basal ganglia network function.

Dopamine-mediated striatal activity and function is enhanced in GlyRα2 knockout animals

The glycine receptor alpha 2 (GlyRα2) is a ligand-gated ion channel which upon activation induces a chloride conductance. Here, we investigated the role of GlyRα2 in dopamine-stimulated striatal cell activity and behavior. We show that depletion of GlyRα2 enhances dopamine-induced increases in the activity of putative dopamine D1 receptor-expressing striatal projection neurons, but does not alter midbrain dopamine neuron activity. We next show that the locomotor response to d-amphetamine is enhanced in GlyRα2 knockout animals, and that this increase correlates with c-fos expression in the dorsal striatum. 3-D modeling revealed an increase in the neuronal ensemble size in the striatum in response to D-amphetamine in GlyRα2 KO mice. Finally, we show enhanced appetitive conditioning in GlyRα2 KO animals that is likely due to increased motivation, but not changes in associative learning or hedonic response. Taken together, we show that GlyRα2 is an important regulator of dopamine-stimulated striatal activity and function.

Cell specific photoswitchable agonist for reversible control of endogenous dopamine receptors

Dopamine controls diverse behaviors and their dysregulation contributes to many disorders. Our ability to understand and manipulate the function of dopamine is limited by the heterogenous nature of dopaminergic projections, the diversity of neurons that are regulated by dopamine, the varying distribution of the five dopamine receptors (DARs), and the complex dynamics of dopamine release. In order to improve our ability to specifically modulate distinct DARs, here we develop a photo-pharmacological strategy using a Membrane anchored Photoswitchable orthogonal remotely tethered agonist for the Dopamine receptor (MP-D). Our design selectively targets D1R/D5R receptor subtypes, most potently D1R (MP-D1_ago), as shown in HEK293T cells. In vivo, we targeted dorsal striatal medium spiny neurons where the photo-activation of MP-D1_ago increased movement initiation, although further work is required to assess the effects of MP-D1_ago on neuronal function. Our method combines ligand and cell type-specificity with temporally precise and reversible activation of D1R to control specific aspects of movement. Our results provide a template for analyzing dopamine receptors.

Altered excitatory transmission in striatal neurons after chronic ethanol consumption in selectively bred crossed high alcohol-preferring mice

Genetic predisposition to heavy drinking is a risk factor for alcohol misuse. We used selectively bred crossed high alcohol-preferring (cHAP) mice to study sex differences in alcohol drinking and its effect on glutamatergic activity in dorsolateral (DLS) and dorsomedial (DMS) striatum. We performed whole-cell patch-clamp recording in neurons from male and female cHAP mice with 5-week alcohol drinking history and alcohol-naïve controls. In DMS, alcohol-naïve males' neurons displayed lower cell capacitance and higher membrane resistance than females' neurons, both effects reversed by drinking. Conversely, in DLS neurons, drinking history increased capacitance only in males and changed membrane resistance only in females. Altered biophysical membrane properties were accompanied by disrupted glutamatergic transmission. Drinking history increased spontaneous excitatory postsynaptic current (sEPSC) amplitude in DMS and frequency in DLS female neurons, compared to alcohol-naïve females, without effect in males. Acute ethanol differentially impacted DMS and DLS neurons by sex and drinking history. In DMS, acute alcohol significantly increased sEPSC frequency only in neurons from alcohol-naïve females, an effect that disappeared after drinking history. In DLS, acute alcohol had opposing effects in males and females based on drinking history. Estrous cycle also impacted DMS and DLS neurons differently: sEPSC amplitudes were higher in DMS cells from drinking history than alcohol-naïve females, whereas estrous cycle, not drinking history, modified DLS firing rate. Our data show sex differences in cHAP ethanol consumption and neurophysiology, suggesting differential dysregulation of glutamatergic drive onto DMS and DLS after chronic ethanol consumption.

Dopaminergic Transmission Rapidly and Persistently Enhances Excitability of D1 Receptor-Expressing Striatal Projection Neurons

Substantia nigra dopamine neurons have been implicated in the initiation and invigoration of movement, presumably through their modulation of striatal projection neuron (SPN) activity. However, the impact of native dopaminergic transmission on SPN excitability has not been directly demonstrated. Using perforated patch-clamp recording, we found that optogenetic stimulation of nigrostriatal dopamine axons rapidly and persistently elevated the excitability of D1 receptor-expressing SPNs (D1-SPNs). The evoked firing of D1-SPNs increased within hundreds of milliseconds of stimulation and remained elevated for ≥ 10 min. Consistent with the negative modulation of depolarization- and Ca2+-activated K+ currents, dopaminergic transmission accelerated subthreshold depolarization in response to current injection, reduced the latency to fire, and transiently diminished action potential afterhyperpolarization. Persistent modulation was protein kinase A dependent and associated with a reduction in action potential threshold. Together, these data demonstrate that dopaminergic transmission potently increases D1-SPN excitability with a time course that could support subsecond and sustained behavioral control.

Basal Ganglia Neuromodulation Over Multiple Temporal and Structural Scales-Simulations of Direct Pathway MSNs Investigate the Fast Onset of Dopaminergic Effects and Predict the Role of Kv4.2

The basal ganglia are involved in the motivational and habitual control of motor and cognitive behaviors. Striatum, the largest basal ganglia input stage, integrates cortical and thalamic inputs in functionally segregated cortico-basal ganglia-thalamic loops, and in addition the basal ganglia output nuclei control targets in the brainstem. Striatal function depends on the balance between the direct pathway medium spiny neurons (D1-MSNs) that express D1 dopamine receptors and the indirect pathway MSNs that express D2 dopamine receptors. The striatal microstructure is also divided into striosomes and matrix compartments, based on the differential expression of several proteins. Dopaminergic afferents from the midbrain and local cholinergic interneurons play crucial roles for basal ganglia function, and striatal signaling via the striosomes in turn regulates the midbrain dopaminergic system directly and via the lateral habenula. Consequently, abnormal functions of the basal ganglia neuromodulatory system underlie many neurological and psychiatric disorders. Neuromodulation acts on multiple structural levels, ranging from the subcellular level to behavior, both in health and disease. For example, neuromodulation affects membrane excitability and controls synaptic plasticity and thus learning in the basal ganglia. However, it is not clear on what time scales these different effects are implemented. Phosphorylation of ion channels and the resulting membrane effects are typically studied over minutes while it has been shown that neuromodulation can affect behavior within a few hundred milliseconds. So how do these seemingly contradictory effects fit together? Here we first briefly review neuromodulation of the basal ganglia, with a focus on dopamine. We furthermore use biophysically detailed multi-compartmental models to integrate experimental data regarding dopaminergic effects on individual membrane conductances with the aim to explain the resulting cellular level dopaminergic effects. In particular we predict dopaminergic effects on Kv4.2 in D1-MSNs. Finally, we also explore dynamical aspects of the onset of neuromodulation effects in multi-scale computational models combining biochemical signaling cascades and multi-compartmental neuron models.

Dopamine and Acetylcholine, a Circuit Point of View in Parkinson's Disease

Data from the World Health Organization (National Institute on Aging, 2011) and the National Institutes of Health (He et al., 2016) predicts that while today the worldwide population over 65 years of age is estimated around 8.5%, this number will reach an astounding 17% by 2050. In this framework, solving current neurodegenerative diseases primarily associated with aging becomes more pressing than ever. In 2017, we celebrate a grim 200th anniversary since the very first description of Parkinson’s disease (PD) and its related symptomatology. Two centuries after this debilitating disease was first identified, finding a cure remains a hopeful goal rather than an attainable objective on the horizon. Tireless work has provided insight into the characterization and progression of the disease down to a molecular level. We now know that the main motor deficits associated with PD arise from the almost total loss of dopaminergic cells in the substantia nigra pars compacta. A concomitant loss of cholinergic cells entails a cognitive decline in these patients, and current therapies are only partially effective, often inducing side-effects after a prolonged treatment. This review covers some of the recent developments in the field of Basal Ganglia (BG) function in physiology and pathology, with a particular focus on the two main neuromodulatory systems known to be severely affected in PD, highlighting some of the remaining open question from three main stand points:

- Heterogeneity of midbrain dopamine neurons.

- Pairing of dopamine (DA) sub-circuits.

- Dopamine-Acetylcholine (ACh) interaction.

A vast amount of knowledge has been accumulated over the years from experimental conditions, but very little of it is reflected or used at a translational or clinical level. An initiative to implement the knowledge that is emerging from circuit-based approaches to tackle neurodegenerative disorders like PD will certainly be tremendously beneficial.

The serotonin 5-HT2C receptor and the non-addictive nature of classic hallucinogens

Classic hallucinogens share pharmacology as serotonin 5-HT_2A, 5-HT_2B, and 5-HT_2C receptor agonists. Unique among most other Schedule 1 drugs, they are generally non-addictive and can be effective tools in the treatment of addiction. Mechanisms underlying these attributes are largely unknown. However, many preclinical studies show that 5-HT_2C agonists counteract the addictive effects of drugs from several classes, suggesting this pharmacological property of classic hallucinogens may be significant. Drawing from a comprehensive analysis of preclinical behavior, neuroanatomy, and neurochemistry studies, this review builds rationale for this hypothesis, and also proposes a testable, neurobiological framework. 5-HT_2C agonists work, in part, by modulating dopamine neuron activity in the ventral tegmental area-nucleus accumbens (NAc) reward pathway. We argue that activation of 5-HT_2C receptors on NAc shell, GABAergic, medium spiny neurons inhibits potassium Kv1.x channels, thereby enhancing inhibitory activity via intrinsic mechanisms. Together with experiments that show that addictive drugs, such as cocaine, potentiate Kv1.x channels, thereby suppressing NAc shell GABAergic activity, this hypothesis provides a mechanism by which classic hallucinogen-mediated stimulation of 5-HT_2C receptors could thwart addiction. It also provides a potential reason for the non-addictive nature of classic hallucinogens.

BK channels mediate dopamine inhibition of firing in a subpopulation of core nucleus accumbens medium spiny neurons

Dopamine, a key neurotransmitter mediating the rewarding properties of drugs of abuse, is widely believed to exert some of its effects by modulating neuronal activity of nucleus accumbens (NAcc) medium spiny neurons (MSNs). Although its effects on synaptic transmission have been well documented, its regulation of intrinsic neuronal excitability is less understood. In this study, we examined the cellular mechanisms of acute dopamine effects on core accumbens MSNs evoked firing. We found that 0.5 µM A-77636 and 10 µM quinpirole, dopamine D1 (DR1s) and D2 receptor (D2Rs) agonists, respectively, markedly inhibited MSN evoked action potentials. This effect, observed only in about 25% of all neurons, was associated with spike-timing-dependent (STDP) long-term potentiation (tLTP), but not long-term depression (tLTD). Dopamine inhibits evoked firing by compromising subthreshold depolarization, not by altering action potentials themselves. Recordings in voltage-clamp mode revealed that all MSNs expressed fast (IA), slowly inactivating delayed rectifier (Idr), and large conductance voltage- and calcium-activated potassium (BKs) channels. Although A-77636 and quinpirole enhanced IA, its selective blockade by 0.5 µM phrixotoxin-1 had no effect on evoked firing. In contrast, exposing tissue to low TEA concentrations and to 10 µM paxilline, a selective BK channel blocker, prevented D1R agonist from inhibiting MSN firing. This result indicates that dopamine inhibits MSN firing through BK channels in a subpopulation of core accumbens MSNs exclusively associated with spike-timing-dependent long-term potentiation.

Distinct effects of mGlu4 receptor positive allosteric modulators at corticostriatal vs. striatopallidal synapses may differentially contribute to their antiparkinsonian action

Metabotropic glutamate 4 (mGlu4) receptor is a promising target for the treatment of motor deficits in Parkinson's disease (PD). This is due in part to its localization at key basal ganglia (BG) synapses that become hyperactive in this pathology, particularly striatopallidal synapses. In this context, mGlu4 receptor activation using either orthosteric agonists or positive allosteric modulators (PAMs) improves motor symptoms in rodent PD models in certain conditions. However, literature data show that mGlu4 receptor PAMs have no effect at striatopallidal GABAergic synapses (unless combined with an orthosteric agonist) and on the firing activity of pallidal neurons, and fail to provide significant motor improvement in relevant PD models. This questions the mechanistic hypothesis that mGlu4 receptor PAMs should act at striatopallidal synapses to alleviate PD motor symptoms. To shed light on this issue, we performed brain slice electrophysiology experiments. We show that Lu AF21934, an mGlu4 PAM small-molecule probe-compound, was ineffective at striatopallidal synapses at all concentrations tested, while it significantly inhibited corticostriatal synaptic transmission. Similarly, Lu AF21934 did not affect electrophysiology readouts at striatopallidal synapses in the presence of haloperidol or in 6-hydroxydopamine-lesioned rats. Interestingly, co-application of Lu AF21934 with a glutamate transporter inhibitor revealed a significant inhibitory action at striatopallidal synapses. Possibly, this effect could rely on increased level/permanence of glutamate in the synaptic cleft. Such differential efficacy of mGlu4 receptor PAMs at corticostriatal vs. striatopallidal synapses raises several issues regarding the synaptic target(s) of these drugs in the BG, and challenges the mechanisms by which they alleviate motor deficits in experimental PD models.

Dopaminergic modulation of synaptic transmission in cortex and striatum

Among the many neuromodulators used by the mammalian brain to regulate circuit function and plasticity, dopamine (DA) stands out as one of the most behaviorally powerful. Perturbations of DA signaling are implicated in the pathogenesis or exploited in the treatment of many neuropsychiatric diseases, including Parkinson's disease (PD), addiction, schizophrenia, obsessive compulsive disorder, and Tourette's syndrome. Although the precise mechanisms employed by DA to exert its control over behavior are not fully understood, DA is known to regulate many electrical and biochemical aspects of neuronal function including excitability, synaptic transmission, integration and plasticity, protein trafficking, and gene transcription. In this Review, we discuss the actions of DA on ionic and synaptic signaling in neurons of the prefrontal cortex and striatum, brain areas in which dopaminergic dysfunction is thought to be central to disease.

New rules governing synaptic plasticity in core nucleus accumbens medium spiny neurons

The nucleus accumbens is a forebrain region responsible for drug reward and goal-directed behaviors. It has long been believed that drugs of abuse exert their addictive properties on behavior by altering the strength of synaptic communication over long periods of time. To date, attempts at understanding the relationship between drugs of abuse and synaptic plasticity have relied on the high-frequency long-term potentiation model of T.V. Bliss & T. Lømo [(1973) Journal of Physiology, 232, 331-356]. We examined synaptic plasticity using spike-timing-dependent plasticity, a stimulation paradigm that reflects more closely the in vivo firing patterns of mouse core nucleus accumbens medium spiny neurons and their afferents. In contrast to other brain regions, the same stimulation paradigm evoked bidirectional long-term plasticity. The magnitude of spike-timing-dependent long-term potentiation (tLTP) changed with the delay between action potentials and excitatory post-synaptic potentials, and frequency, whereas that of spike-timing-dependent long-term depression (tLTD) remained unchanged. We showed that tLTP depended on N-methyl-d-aspartate receptors, whereas tLTD relied on action potentials. Importantly, the intracellular calcium signaling pathways mobilised during tLTP and tLTD were different. Thus, calcium-induced calcium release underlies tLTD but not tLTP. Finally, we found that the firing pattern of a subset of medium spiny neurons was strongly inhibited by dopamine receptor agonists. Surprisingly, these neurons were exclusively associated with tLTP but not with tLTD. Taken together, these data point to the existence of two subgroups of medium spiny neurons with distinct properties, each displaying unique abilities to undergo synaptic plasticity.

A neurocomputational model of tonic and phasic dopamine in action selection: a comparison with cognitive deficits in Parkinson's disease

The striatal dopamine signal has multiple facets; tonic level, phasic rise and fall, and variation of the phasic rise/fall depending on the expectation of reward/punishment. We have developed a network model of the striatal direct pathway using an ionic current level model of the medium spiny neuron that incorporates currents sensitive to changes in the tonic level of dopamine. The model neurons in the network learn action selection based on a novel set of mathematical rules that incorporate the phasic change in the dopamine signal. This network model is capable of learning to perform a sequence learning task that in humans is thought to be dependent on the basal ganglia. When both tonic and phasic levels of dopamine are decreased, as would be expected in unmedicated Parkinson's disease (PD), the model reproduces the deficits seen in a human PD group off medication. When the tonic level is increased to normal, but with reduced phasic increases and decreases in response to reward and punishment, respectively, as would be expected in PD medicated with L-Dopa, the model again reproduces the human data. These findings support the view that the cognitive dysfunctions seen in Parkinson's disease are not solely either due to the decreased tonic level of dopamine or to the decreased responsiveness of the phasic dopamine signal to reward and punishment, but to a combination of the two factors that varies dependent on disease stage and medication status.

Physiology and pharmacology of striatal neurons

The basal ganglia occupy the core of the forebrain and consist of evolutionarily conserved motor nuclei that form recurrent circuits critical for motivation and motor planning. The striatum is the main input nucleus of the basal ganglia and a key neural substrate for procedural learning and memory. The vast majority of striatal neurons are spiny GABAergic projection neurons, which exhibit slow but temporally precise spiking in vivo. Contributing to this precision are several different types of interneurons that constitute only a small fraction of total neuron number but play a critical role in regulating striatal output. This review examines the cellular physiology and modulation of striatal neurons that give rise to their unique properties and function.

Modulation of horizontal cell function by dopaminergic ligands in mammalian retina

Light responses of rabbit horizontal cell somata (HC) to flickering light stimuli recorded with sharp electrodes consist of a distinctive flicker component superimposed on a sustained hyperpolarisation. Activation of dopamine D1/D5 receptors depolarises HC dark membrane potential and suppresses the flicker component of responses to photopic stimuli without affecting the sustained hyperpolarising response component. Waveforms of responses to scotopic stimuli are preserved. Similar response modulation was observed in depolarising cells of the inner retina, suggesting that activation of D1/D5 receptors of HC causes modification of cone signal transmission to higher order neurons. The impact of dopamine D1/D5 receptor activation on the function of HC in the light stimulated retina is discussed.

Differential tonic GABA conductances in striatal medium spiny neurons

Medium spiny neurons (MSNs) provide the principal output for the dorsal striatum. Those that express dopamine D2 receptors (D2+) project to the globus pallidus external and are thought to inhibit movement, whereas those that express dopamine D1 receptors (D1+) project to the substantia nigra pars reticulata and are thought to facilitate movement. Whole-cell and outside-out patch recordings in slices from bacterial artificial chromosome transgenic mice examined the role of GABA(A) receptor-mediated currents in dopamine receptor D1+ striatonigral and D2+ striatopallidal MSNs. Although inhibitory synaptic currents were similar between the two neuronal populations, D2+ MSNs showed greater GABA(A) receptor-mediated tonic currents. TTX application abolished the tonic current to a similar extent as GABA(A) antagonists, suggesting a synaptic origin of the ambient GABA. Low GABA concentrations produced larger whole-cell responses and longer GABA channel openings in D2+ than in D1+ MSNs. Recordings from MSNs in alpha1-/- mice and pharmacological analysis of tonic currents suggested greater expression of alpha5-containing GABA(A) receptors in D2+ than in D1+ MSNs. As a number of disorders such as Parkinson's disease, Huntington's chorea, and tardive dyskinesia arise from an imbalance between these two pathways, the GABA(A) receptors responsible for tonic currents in D2+ MSNs may be a potential target for therapeutic intervention.

Effects of dopaminergic modulation on the integrative properties of the ventral striatal medium spiny neuron

Dopaminergic modulation produces a variety of functional changes in the principal cell of the striatum, the medium spiny neuron (MSN). Using a 189-compartment computational model of a ventral striatal MSN, we simulated whole cell D1- and D2-receptor-mediated modulation of both intrinsic (sodium, calcium, and potassium) and synaptic currents (AMPA and NMDA). Dopamine (DA) modulations in the model were based on a review of published experiments in both ventral and dorsal striatum. To objectively assess the net effects of DA modulation, we combined reported individual channel modulations into either D1- or D2-receptor modulation conditions and studied them separately. Contrary to previous suggestions, we found that D1 modulation had no effect on MSN nonlinearity and could not induce bistability. In agreement with previous suggestions, we found that dopaminergic modulation leads to changes in input filtering and neuronal excitability. Importantly, the changes in neuronal excitability agree with the classical model of basal ganglia function. We also found that DA modulation can alter the integration time window of the MSN. Interestingly, the effects of DA modulation of synaptic properties opposed the effects of DA modulation of intrinsic properties, with the synaptic modulations generally dominating the net effect. We interpret this lack of synergy to suggest that the regulation of whole cell integrative properties is not the primary functional purpose of DA. We suggest that D1 modulation might instead primarily regulate calcium influx to dendritic spines through NMDA and L-type calcium channels, by both direct and indirect mechanisms.

Coordinated accumbal dopamine release and neural activity drive goal-directed behavior

Intracranial self-stimulation (ICSS) activates the neural pathways that mediate reward, including dopaminergic terminal areas such as the nucleus accumbens (NAc). However, a direct role of dopamine in ICSS-mediated reward has been questioned. Here, simultaneous voltammetric and electrophysiological recordings from the same electrode reveal that, at certain sites, the onset of anticipatory dopamine surges and changes in neuronal firing patterns during ICSS are coincident, whereas sites lacking dopamine changes also lack patterned firing. Intrashell microinfusion of a D1, but not a D2 receptor antagonist, blocks ICSS. An iontophoresis approach was implemented to explore the effect of dopamine antagonists on firing patterns without altering behavior. Similar to the microinfusion experiments, ICSS-related firing is selectively attenuated following D1 receptor blockade. This work establishes a temporal link between anticipatory rises of dopamine and firing patterns in the NAc shell during ICSS and suggests that they may play a similar role with natural rewards and during drug self-administration.

Space, time and dopamine

In recent years, dopamine has emerged as a key neurotransmitter that is crucially involved in incentive motivation and reinforcement learning. Dopamine release is evoked by rewards. The extensive divergence of outputs from a small number of dopaminergic neurons suggests a spatially nonselective action of dopamine, but it reinforces the specific actions that led to reward. How is this achieved? We propose that the selectivity of dopamine effects is achieved by the timing of dopamine release in relation to the activity of glutamatergic synapses, rather than by spatial localization of the dopamine signal to specific synaptic contacts. The synaptic mechanisms of these actions are unknown but reduced levels of dopamine, for example in Parkinson's disease, leads to a paucity of behavioural output, whereas its excess production has been associated with psychiatric problems. Clearly, there are therapeutic imperatives that require a better understanding of how dopamine functions at a synaptic level.

A specialized subclass of interneurons mediates dopaminergic facilitation of amygdala function

The amygdala is under inhibitory control from the cortex through the activation of local GABAergic interneurons. This inhibition is greatly diminished during heightened emotional states due to dopamine release. However, dopamine excites most amygdala interneurons, suggesting that this dopaminergic gate may be mediated by an unknown subpopulation of interneurons. We hypothesized that this gate is mediated by paracapsular intercalated cells, a subset of interneurons that are innervated by both cortical and mesolimbic dopaminergic afferents. Using transgenic mice that express GFP in GABAergic interneurons, we show that paracapsular cells form a network surrounding the basolateral complex of the amygdala. We found that they provide feedforward inhibition into the basolateral and the central amygdala. Dopamine hyperpolarized paracapsular cells through D1 receptors and substantially suppressed their excitability, resulting in a disinhibition of the basolateral and central nuclei. Suppression of the paracapsular system by dopamine provides a compelling neural mechanism for the increased affective behavior observed during stress or other hyperdopaminergic states.

A Reconfiguration of CaV2 Ca2+ Channel Current and Its Dopaminergic D2 Modulation in Developing Neostriatal Neurons

The modulatory effect of D2 dopamine receptor activation on calcium currents was studied in neostriatal projection neurons at two stages of rat development: postnatal day (PD)14 and PD40. D2-class receptor agonists reduced whole cell calcium currents by about 35% at both stages, and this effect was blocked by the D2 receptor antagonist sulpiride. Nitrendipine partially occluded this modulation at both stages, indicating that modulation of CaV1 channels was present throughout this developmental interval. Nevertheless, modulation of CaV1 channels was significantly larger in PD40 neurons. ω-Conotoxin GVIA occluded most of the Ca2+ current modulation in PD14 neurons. However, this occlusion was greatly decreased in PD40 neurons. ω-Agatoxin TK occluded a great part of the modulation in PD40 neurons but had a negligible effect in PD14 neurons. The data indicate that dopaminergic D2-mediated modulation undergoes a change in target during development: from CaV2.2 to CaV2.1 Ca2+ channels. This change occurred while CaV2.2 channels were being down-regulated and CaV2.1 channels were being up-regulated. Presynaptic modulation mediated by D2 receptors reflected these changes; CaV2.2 type channels were used for release in young animals but very little in mature animals, suggesting that changes took place simultaneously at the somatodendritic and the synaptic membranes.

Temporal sequence learning, prediction, and control: a review of different models and their relation to biological mechanisms

In this review, we compare methods for temporal sequence learning (TSL) across the disciplines machine-control, classical conditioning, neuronal models for TSL as well as spike-timing-dependent plasticity (STDP). This review introduces the most influential models and focuses on two questions: To what degree are reward-based (e.g., TD learning) and correlation-based (Hebbian) learning related? and How do the different models correspond to possibly underlying biological mechanisms of synaptic plasticity? We first compare the different models in an open-loop condition, where behavioral feedback does not alter the learning. Here we observe that reward-based and correlation-based learning are indeed very similar. Machine control is then used to introduce the problem of closed-loop control (e.g., actor-critic architectures). Here the problem of evaluative (rewards) versus nonevaluative (correlations) feedback from the environment will be discussed, showing that both learning approaches are fundamentally different in the closed-loop condition. In trying to answer the second question, we compare neuronal versions of the different learning architectures to the anatomy of the involved brain structures (basal-ganglia, thalamus, and cortex) and the molecular biophysics of glutamatergic and dopaminergic synapses. Finally, we discuss the different algorithms used to model STDP and compare them to reward-based learning rules. Certain similarities are found in spite of the strongly different timescales. Here we focus on the biophysics of the different calcium-release mechanisms known to be involved in STDP.

Memory and addiction: shared neural circuitry and molecular mechanisms

An important conceptual advance in the past decade has been the understanding that the process of drug addiction shares striking commonalities with neural plasticity associated with natural reward learning and memory. Basic mechanisms involving dopamine, glutamate, and their intracellular and genomic targets have been the focus of attention in this research area. These two neurotransmitter systems, widely distributed in many regions of cortex, limbic system, and basal ganglia, appear to play a key integrative role in motivation, learning, and memory, thus modulating adaptive behavior. However, many drugs of abuse exert their primary effects precisely on these pathways and are able to induce enduring cellular alterations in motivational networks, thus leading to maladaptive behaviors. Current theories and research on this topic are reviewed from an integrative systems perspective, with special emphasis on cellular, molecular, and behavioral aspects of dopamine D-1 and glutamate NMDA signaling, instrumental learning, and drug cue conditioning.

Repeated cocaine administration suppresses HVA-Ca2+ potentials and enhances activity of K+ channels in rat nucleus accumbens neurons

The nucleus accumbens (NAc) is an important forebrain area involved in sensitization, withdrawal effects, and self-administration of cocaine. However, little is known about cocaine-induced alterations in the neuronal excitability and whole cell neuroplasticity in this region that may affect behaviors. Our recent investigations have demonstrated that repeated cocaine administration decreases voltage-sensitive sodium and calcium currents (VSSCs and VSCCs, respectively) in freshly dissociated NAc neurons of rats. In this study, current-clamp recordings were performed in slice preparations to determine the effects of chronic cocaine on evoked Ca(2+) potentials and voltage-sensitive K(+) currents in NAc neurons. Repeated cocaine administration with 3-4 days of withdrawal caused significant alterations in Ca(2+) potentials, including suppression of Ca(2+)-mediated spikes, increase in the intracellular injected current intensity required for generation of Ca(2+) potentials (rheobase), reduced duration of Ca(2+) plateau potentials, and abolishment of secondary Ca(2+) potentials associated with the primary Ca(2+) plateau potential. Application of nickel (Ni(2+)), which blocks low-voltage activated T-type Ca(2+) channels, had no impact on evoked Ca(2+) plateau potentials in NAc neurons, indicating that these Ca(2+) potentials are high-voltage activated (HVA). In addition, repeated cocaine pretreatment also hyperpolarized the resting membrane potential, increased the amplitude of afterhyperpolarization in Ca(2+) spikes, and enhanced the outward rectification observed during membrane depolarization. These findings indicate that repeated cocaine administration not only suppressed HVA-Ca(2+) potentials but also significantly enhanced the activity of various K(+) channels in NAc neurons. They also demonstrate an integrative role of whole cell neuroplasticity during cocaine withdrawal, by which the subthreshold membrane excitability of NAc neurons is significantly decreased.

Correlating function and gene expression of individual basal ganglia neurons

Functional studies at the level of individual neurons have greatly contributed to our current understanding of basal ganglia function and dysfunction. However, identification of the expressed genes responsible for these distinct neuronal phenotypes is less advanced. Qualitative and quantitative single-cell gene-expression profiling, combined with electrophysiological analysis, allows phenotype-genotype correlations to be made for individual neurons. In this review, progress on gene-expression profiling of individual, functionally characterized basal ganglia neurons is discussed, focusing on ion channels and receptors. In addition, methodological issues are discussed and emerging novel techniques are introduced that will enable a genome-wide comparison of function and gene expression for individual neurons.

Contrast enhancement: a physiological effect of striatal dopamine?

Dopamine functions as an important neuromodulator in the dorsal striatum and ventral striatum/nucleus accumbens. Evidence is accumulating for the idea that striatal neurons compete with each other for control over the animal's motor resources, and that dopamine plays an important modulatory role that allows a particular subset of neurons, encoding a specific behavior, to predominate in this competition. One means by which dopamine could facilitate selection among competing neurons is to enhance the contrast between stronger and weaker excitations (or to increase the "signal to noise ratio" among neurons, where the firing of the most excited neurons is assumed to transmit signal and the firing of the least excited to transmit noise). Here, we review the electrophysiological evidence for this hypothesis and discuss potential cellular mechanisms by which dopamine-mediated contrast enhancement could occur.

Impact of D1-class dopamine receptor on striatal processing of cortical input in experimental parkinsonism in vivo

Recent in vivo electrophysiological studies suggest that chronic dopamine depletion alters profoundly the firing pattern of basal ganglia neurons. These changes may disrupt the processing of cortical information flow from the striatum to the output nuclei, and presumably underlie the clinical manifestations of Parkinson's disease. We have recently reported that chronic nigrostriatal lesions induce changes in the functional state of striatal medium-spiny neurons (MSNs) that could facilitate spreading of cortical synchronous activity (approximately 1 Hz) to striatal target nuclei. Here we show that systemic administration of D1 dopamine agonists was sufficient to restore the changes induced by chronic nigrostriatal lesions on striatal neuronal activity into the normal state. Following systemic administration of SKF38393 or SKF81279 the membrane potential of striatal MSNs was upheld into a more hyperpolarized value and action potential firing probability decreased. D1 agonists also increased the latency to the cortically driven plateau depolarization and reduced the peak potential of the short latency depolarizing postsynaptic response to a more hyperpolarized value. The present study provides in vivo evidence indicating that pharmacological stimulation of D1-class dopamine receptors can modulate the flow of cortical information through the striatum in the parkinsonian state.

Dopaminergic modulation of axon collaterals interconnecting spiny neurons of the rat striatum

Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.

Modulation of striatal single units by expected reward: a spiny neuron model displaying dopamine-induced bistability

Single-unit activity in the neostriatum of awake monkeys shows a marked dependence on expected reward. Responses to visual cues differ when animals expect primary reinforcements, such as juice rewards, in comparison to secondary reinforcements, such as tones. The mechanism of this reward-dependent modulation has not been established experimentally. To assess the hypothesis that direct neuromodulatory effects of dopamine on spiny neurons can account for this modulation, we develop a computational model based on simplified representations of key ionic currents and their modulation by D1 dopamine receptor activation. This minimal model can be analyzed in detail. We find that D1-mediated increases of inward rectifying potassium and L-type calcium currents cause a bifurcation: the native up/down state behavior of the spiny neuron model becomes truly bistable, which modulates the peak firing rate and the duration of the up state and introduces a dependence of the response on the past state history. These generic consequences of dopamine neuromodulation through bistability can account for both reward-dependent enhancement and suppression of spiny neuron single-unit responses to visual cues. We validate the model by simulating responses to visual targets in a memory-guided saccade task; our results are in close agreement with the main features of the experimental data. Our model provides a conceptual framework for understanding the functional significance of the short-term neuromodulatory actions of dopamine on signal processing in the striatum.

Muscarinic receptors involved in the subthreshold cholinergic actions of neostriatal spiny neurons

Administration of the peptide MT-1 (48 nM), a selective agonist of muscarinic M(1)-type receptors, mimicked the subthreshold actions of muscarine (1 microM) on neostriatal neurons, i.e., it produced a reduction in subthreshold inward rectification leading to an enhancement in input resistance (R(N)) and evoked discharge. In all recorded cells, MT-1 effects remained in the presence of the specific peptidergic antagonist of the M(4)-type receptor, MT-3 (10 nM), but were blocked by the specific M(1)-type receptor antagonist MT-7 (5 nM). These results suggest that most muscarinic facilitatory actions in the subthreshold voltage range occur through M(1)-type receptors. However, in a fraction of cells (40%) muscarine produced an excitability enhancement not blocked by MT-7. This additional facilitatory action, not present when using MT-1, was blocked by MT-3, suggesting it was mediated by M(4)-type receptor activation. This facilitation could not be blocked by Cs(+), TTX, or Cd(2+), but only by a reduction in extracellular sodium. This result is the first evidence that M(4)-type receptor activation enhances a cationic inward current in a fraction of neostriatal projection neurons.

Dopamine-dependent plasticity of corticostriatal synapses

Knowledge of the effect of dopamine on corticostriatal synaptic plasticity has advanced rapidly over the last 5 years. We consider this new knowledge in relation to three factors proposed earlier to describe the rules for synaptic plasticity in the corticostriatal pathway. These factors are a phasic increase in dopamine release, presynaptic activity and postsynaptic depolarisation. A function is proposed which relates the amount of dopamine release in the striatum to the modulation of corticostriatal synaptic efficacy. It is argued that this function, and the experimental data from which it arises, are compatible with existing models which associate the reward-related firing of dopamine neurons with changes in corticostriatal synaptic efficacy.

Direct and indirect actions of dopamine on the membrane potential in medium spiny neurons of the mouse neostriatum

Many studies have shown dopamine (DA) to have a modulatory effect on neuronal excitability, which cannot be simply classified as excitatory or inhibitory in the neostriatum. To clarify whether the responses to DA (10-30 microM) are excitatory or inhibitory in the mouse medium spiny neurons, we examined the effects of DA agonists on the synchronous potential trajectory from the resting potential to the subthreshold potential. The DA-induced potential changes, which were estimated at the subthreshold potential (approximately -60 mV), were summarized as the combination of three kinds of responses: an initial hyperpolarization lasting approximately 1 min and a slow depolarization and/or hyperpolarization lasting more than 20 min. A D(1)-like receptor agonist, R(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF81297, 1 microM) mainly induced the initial hyperpolarization and slow depolarization. A D(2)-like receptor agonist, trans-(-)-4aR-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo[3,4-g]quinoline hydrochloride (quinpirole, 1 microM), mainly induced the initial hyperpolarization and slow hyperpolarization. D(1)-like receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390, 1 microM) depressed both the initial hyperpolarization and slow depolarization. D(2)-like receptor antagonist sulpiride (1 microM) depressed all the DA-induced responses except for the slow depolarization. TTX (0.5 microM) abolished all the DA-induced responses. Bicuculline (20 microM) and atropine (1 microM) abolished the DA-induced initial hyperpolarization and slow depolarization, respectively. Either DL-2-amino-5-phosphonopentanoic acid (AP5; 100 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 microM) blocked both the initial hyperpolarization and slow depolarization. The application of exogenous glutamate (Glu) mimicked the initial hyperpolarization and slow depolarization. These results suggest that the initial hyperpolarization is mainly due to GABA release via the cooperative action of D(1)- and D(2)-like receptors and Glu receptors in GABAergic interneurons, whereas the slow depolarization is mediated by acetylcholine (ACh) release via the cooperative action of mainly D(1)-like receptors and Glu receptors in cholinergic interneurons. The potential oscillation was generated at the subthreshold level in a Ba(2+)-, AP5-, CNQX-, bicuculline-, and atropine-containing medium. The oscillation depressed after the addition of TTX, Co(2+), or DA. In DA agonists, quinpirole rather than SKF81297 had a more depressive effect on the potential oscillation. These results indicate that the slow hyperpolarization is due to the suppression of noninactivating Na(+)-Ca(2+) conductances via mainly D(2)-like receptors in the medium spiny neurons. In conclusion, the DA actions on the medium spiny neurons show a transient inhibition by the activation of D(1)- and D(2)-like receptors in mainly GABAergic interneurons and a tonic excitation and/or inhibition by the activation of mainly D(1)-like receptors in cholinergic interneurons and by the activation of mainly D(2)-like receptors in the medium spiny neurons, respectively.

Cortical slow oscillatory activity is reflected in the membrane potential and spike trains of striatal neurons in rats with chronic nigrostriatal lesions

Neurons in the basal ganglia output nuclei display rhythmic burst firing after chronic nigrostriatal lesions. The thalamocortical network is a strong endogenous generator of oscillatory activity, and the striatum receives a massive projection from the cerebral cortex. Actually, the membrane potential of striatal projection neurons displays periodic shifts between a very negative resting potential (down state) and depolarizing plateaus (up states) during which they can fire action potentials. We hypothesized that an increased excitability of striatal neurons may allow transmission of cortical slow rhythms through the striatum to the remaining basal ganglia in experimental parkinsonism. In vivo intracellular recordings revealed that striatal projection neurons from rats with chronic nigrostriatal lesions had a more depolarized membrane potential during both the down and up states and an increased firing probability during the up events. Furthermore, lesioned rats had significantly fewer silent neurons than control rats. Simultaneous recordings of the frontal electrocorticogram and membrane potential of striatal projection neurons revealed that the signals were oscillating synchronously in the frequency range 0.4-2 Hz, both in control rats and rats with chronic nigrostriatal lesions. Spreading of the slow cortical rhythm is limited by the very low firing probability of control rat neurons, but a slow oscillation is well reflected in spike trains of approximately 60% of lesioned rat neurons. These findings provide in vivo evidence for a role of dopamine in controlling the flow of cortical activity through the striatum and may be of outstanding relevance for understanding the pathophysiology of Parkinson's disease.

Modeling functions of striatal dopamine modulation in learning and planning

The activity of midbrain dopamine neurons is strikingly similar to the reward prediction error of temporal difference reinforcement learning models. Experimental evidence and simulation studies suggest that dopamine neuron activity serves as an effective reinforcement signal for learning of sensorimotor associations in striatal matrisomes. In the current study, we simulate dopamine neuron activity with the extended temporal difference model of Pavlovian learning and examine the influences of this signal on medium spiny neurons in striatal matrisomes. The modeled influences include transient membrane effects of dopamine D(1) receptor activation, dopamine-dependent long-term adaptations of corticostriatal transmission, and effects of dopamine on rhythmic fluctuations of the membrane potential between an elevated "up-state" and a hyperpolarized "down-state". The most dominant activity in the striatal matrisomes is assumed to elicit behaviors via projections from the basal ganglia to the thalamus and the cortex. This "standard model" performs successfully when tested for sensorimotor learning and goal-directed behavior (planning). To investigate the contributions of our model assumptions to learning and planning, we test the performance of several model variants that lack one of these mechanisms. These simulations show that the adaptation of the dopamine-like signal is necessary for sensorimotor learning and planning. Sensorimotor learning requires dopamine-dependent long-term adaptation of corticostriatal transmission. Lack of dopamine-like novelty responses decreases the number of exploratory acts, which impairs planning capabilities. The model loses its planning capabilities if the dopamine-like signal is simulated with the original temporal difference model, because the original temporal difference model does not form novel associative chains. Transient membrane effects of the dopamine-like signal on striatal firing substantially shorten the reaction time in the planning task. The capability for planning is improved by influences of dopamine on the durations of membrane potential fluctuations and by manipulations that prolong the reaction time of the model. These results suggest that responses of dopamine neurons to conditioned stimuli contribute to sensorimotor reward learning, novelty responses of dopamine neurons stimulate exploration, and transient dopamine membrane effects are important for planning.

Dopaminergic modulation of neuronal excitability in the striatum and nucleus accumbens

The striatum and its ventral extension, the nucleus accumbens, are involved in behaviors as diverse as motor planning, drug seeking, and learning. Invariably, these striatally mediated behaviors depend on intact dopaminergic innervation. However, the mechanisms by which dopamine modulates neuronal function in the striatum and nucleus accumbens have been difficult to elucidate. Recent electrophysiological studies have revealed that dopamine alters both voltage-dependent conductances and synaptic transmission, resulting in state-dependent modulation of target cells. These studies make clear predictions about how dopamine, particularly via D1 receptor activation, should alter the responsiveness of striatal neurons to extrinsic excitatory synaptic activity.

Action of substance P (neurokinin-1) receptor activation on rat neostriatal projection neurons

Substance P (SP) acts as a neurotransmitter in the neostriatum through the axon collaterals of spiny projection neurons. However, possible direct or indirect actions of SP on the neostriatal output neurons have not been described. Targets of SP terminals within the neostriatum include interneurons, spiny neurons, afferent fibers and boutons. SP induces the release of both dopamine (DA) and acetylcholine (ACh). Since some postsynaptic actions of both DA and ACh on spiny neurons are known, we asked if activation of neostriatal NK1-class receptors is able to reproduce them. The SP NK1-receptor agonist, GR73632 (1 microM), had both excitatory and inhibitory actions on virtually all spiny neurons tested at resting potential. The excitatory action was blocked by atropine and coursed with an increase in firing rate and input resistance (R(N)). The inhibitory action was blocked by haloperidol and coursed with a reduction in firing rate and R(N). Therefore, the release of both DA and ACh induced by NK1-receptor activation modulates indirectly the excitability of the projection neurons. SP facilitates the actions of these transmitters on the spiny neuron. A residual excitatory response to the NK1-receptor agonist was observed in 30% of a sample of neurons tested in the presence of both haloperidol and atropine. The increase in R(N) that accompanied this response could be observed in the presence of 1 microM TTX or 100 microM Cd2+, suggesting a direct effect. Double labeling showed that only SP-immunoreactive neurons were facilitated by NK1-receptor activation in these conditions.

Cholinergic modulation of neostriatal output: a functional antagonism between different types of muscarinic receptors

It is demonstrated that acetylcholine released from cholinergic interneurons modulates the excitability of neostriatal projection neurons. Physostigmine and neostigmine increase input resistance (RN) and enhance evoked discharge of spiny projection neurons in a manner similar to muscarine. Muscarinic RN increase occurs in the whole subthreshold voltage range (-100 to -45 mV), remains in the presence of TTX and Cd2+, and can be blocked by the relatively selective M1,4 muscarinic receptor antagonist pirenzepine but not by M2 or M3 selective antagonists. Cs+ occludes muscarinic effects at potentials more negative than -80 mV. A Na+ reduction in the bath occludes muscarinic effects at potentials more positive than -70 mV. Thus, muscarinic effects involve different ionic conductances: inward rectifying and cationic. The relatively selective M2 receptor antagonist AF-DX 116 does not block muscarinic effects on the projection neuron but, surprisingly, has the ability to mimic agonistic actions increasing RN and firing. Both effects are blocked by pirenzepine. HPLC measurements of acetylcholine demonstrate that AF-DX 116 but not pirenzepine greatly increases endogenous acetylcholine release in brain slices. Therefore, the effects of the M2 antagonist on the projection neurons were attributable to autoreceptor block on cholinergic interneurons. These experiments show distinct opposite functions of muscarinic M1- and M2-type receptors in neostriatal output, i.e., the firing of projection neurons. The results suggest that the use of more selective antimuscarinics may be more profitable for the treatment of motor deficits.

Modulation of synaptic transmission by dopamine and norepinephrine in ventral but not dorsal striatum

Although the ventral striatum (nucleus accumbens; NAc) and dorsal striatum are associated with different behaviors, these structures are anatomically and physiologically similar. In particular, dopaminergic afferents from the midbrain appear to be essential for the normal functioning of both nuclei. Although a number of studies have examined the effects of dopamine on the physiology of NAc or striatal cells, results have varied, and few studies have compared directly the actions of dopamine on both of these nuclei. Here we use slice preparations of the NAc and dorsal striatum to compare how synaptic transmission in these nuclei is modulated by catecholamines. As previously reported, dopamine depressed excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in the NAc. Surprisingly, however, neither EPSPs nor IPSPs in the dorsal striatum were affected by dopamine. Similarly, norepinephrine depressed excitatory synaptic transmission in the NAc by an alpha-adrenergic receptor-dependent mechanism but was without effect on excitatory transmission in the dorsal striatum. Inhibitory synaptic transmission was not affected by norepinephrine in either structure. These results suggest that the functional roles of dopamine and norepinephrine are not the same in the dorsal striatum and the NAc.

D1 receptor activation enhances evoked discharge in neostriatal medium spiny neurons by modulating an L-type Ca2+ conductance

Most in vitro studies of D1 dopaminergic modulation of excitability in neostriatal medium spiny neurons have revealed inhibitory effects. Yet studies made in more intact preparations have shown that D1 receptors can enhance or inhibit the responses to excitatory stimuli. One explanation for these differences is that the effects of D1 receptors on excitability are dependent on changes in the membrane potential occurring in response to cortical inputs that are seen only in intact preparations. To test this hypothesis, we obtained voltage recordings from medium spiny neurons in slices and examined the impact of D1 receptor stimulation at depolarized and hyperpolarized membrane potentials. As previously reported, evoked discharge was inhibited by D1 agonists when holding at negative membrane potentials (approximately -80 mV). However, at more depolarized potentials (approximately -55 mV), D1 agonists enhanced evoked activity. At these potentials, D1 agonists or cAMP analogs prolonged or induced slow subthreshold depolarizations and increased the duration of barium- or TEA-induced Ca2+-dependent action potentials. Both effects were blocked by L-type Ca2+ channel antagonists (nicardipine, calciseptine) and were occluded by the L-type channel agonist BayK 8644-arguing that the D1 receptor-mediated effects on evoked activity at depolarized membrane potential were mediated by enhancement of L-type Ca2+ currents. These results reconcile previous in vitro and in vivo studies by showing that D1 dopamine receptor activation can either inhibit or enhance evoked activity, depending on the level of membrane depolarization.