Inhibiting subthalamic D5 receptor constitutive activity alleviates abnormal electrical activity and reverses motor impairment in a rat model of Parkinson's disease

Estimation of the transmission delays in the basal ganglia of the macaque monkey and subsequent predictions about oscillatory activity under dopamine depletion

The timescales of the dynamics of a system depend on the combination of the timescales of its components and of its transmission delays between components. Here, we combine experimental stimulation data from 10 studies in macaque monkeys that reveal the timing of excitatory and inhibitory events in the basal ganglia circuit, to estimate its set of transmission delays. In doing so, we reveal possible inconsistencies in the existing data, calling for replications, and we propose two possible sets of transmission delays. We then integrate these delays in a model of the primate basal ganglia that does not rely on direct and indirect pathways' segregation and show that extrastriatal dopaminergic depletion in the external part of the globus pallidus and in the subthalamic nucleus is sufficient to generate β-band oscillations (in the high part, 20-35 Hz, of the band). More specifically, we show that D2 and D5 dopamine receptors in these nuclei play opposing roles in the emergence of these oscillations, thereby explaining how completely deactivating D5 receptors in the subthalamic nucleus can, paradoxically, cancel oscillations.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Chlorpromazine, an Inverse Agonist of D1R-Like, Differentially Targets Voltage-Gated Calcium Channel (CaV) Subtypes in mPFC Neurons

The dopamine receptor type 1 (D1R) and the dopamine receptor type 5 (D5R), which are often grouped as D1R-like due to their sequence and signaling similarities, exhibit high levels of constitutive activity. The molecular basis for this agonist-independent activation has been well characterized through biochemical and mutagenesis in vitro studies. In this regard, it was reported that many antipsychotic drugs act as inverse agonists of D1R-like constitutive activity. On the other hand, D1R is highly expressed in the medial prefrontal cortex (mPFC), a brain area with important functions such as working memory. Here, we studied the impact of D1R-like constitutive activity and chlorpromazine (CPZ), an antipsychotic drug and D1R-like inverse agonist, on various neuronal Ca_V conductances, and we explored its effect on calcium-dependent neuronal functions in the mouse medial mPFC. Using ex vivo brain slices containing the mPFC and transfected HEK293T cells, we found that CPZ reduces Ca_V2.2 currents by occluding D1R-like constitutive activity, in agreement with a mechanism previously reported by our lab, whereas CPZ directly inhibits Ca_V1 currents in a D1R-like activity independent manner. In contrast, CPZ and D1R constitutive activity did not affect Ca_V2.1, Ca_V2.3, or Ca_V3 currents. Finally, we found that CPZ reduces excitatory postsynaptic responses in mPFC neurons. Our results contribute to understanding CPZ molecular targets in neurons and describe a novel physiological consequence of CPZ non-canonical action as a D1R-like inverse agonist in the mouse brain.

Constitutive activity of the dopamine (D5 ) receptor, highly expressed in CA1 hippocampal neurons, selectively reduces CaV 3.2 and CaV 3.3 currents

BACKGROUND AND PURPOSE

Ca_V 3.1-3 currents differentially contribute to neuronal firing patterns. Ca_V 3 are regulated by G protein-coupled receptors (GPCRs) activity, but information about Ca_V 3 as targets of the constitutive activity of GPCRs is scarce. We investigate the impact of D₅ recpetor constitutive activity, a GPCR with high levels of basal activity, on Ca_V 3 functionality. D₅ recpetor and Ca_V 3 are expressed in the hippocampus and have been independently linked to pathophysiological states associated with epilepsy.

EXPERIMENTAL APPROACH

Our study models were HEK293T cells heterologously expressing D₁ or D₅ receptor and Ca_V 3.1-3, and mouse brain slices containing the hippocampus. We used chlorpromazine (D₁ /D₅ inverse agonist) and a D₅ receptor mutant lacking constitutive activity as experimental tools. We measured Ca_V 3 currents and excitability parameters using the patch-clamp technique. We completed our study with computational modelling and imaging technique.

KEY RESULTS

We found a higher sensitivity to TTA-P2 (Ca_V 3 blocker) in CA1 pyramidal neurons obtained from chlorpromazine-treated animals compared with vehicle-treated animals. We found that Ca_V 3.2 and Ca_V 3.3-but not Ca_V 3.1-are targets of D₅ receptor constitutive activity in HEK293T cells. Finally, we found an increased firing rate in CA1 pyramidal neurons from chlorpromazine-treated animals in comparison with vehicle-treated animals. Similar changes in firing rate were observed on a neuronal model with controlled Ca_V 3 currents levels.

CONCLUSIONS AND IMPLICATIONS

Native hippocampal Ca_V 3 and recombinant Ca_V 3.2-3 are sensitive to D₅ receptor constitutive activity. Manipulation of D₅ receptor constitutive activity could be a valuable strategy to control neuronal excitability, especially in exacerbated conditions such as epilepsy.

The genesis and functional consequences of cortico-subthalamic beta augmentation and excessive subthalamic burst discharges after dopaminergic deprivation

The cardinal electrophysiological signs in Parkinson's disease (PD) include augmented beta oscillations in the motor cortex-subthalamic nucleus (MC-STN) axis and excessive burst discharges in STN. We have shown that excessive STN burst discharges have a direct causal relation with the locomotor deficits in PD. To investigate the correlation between the two cardinal signs, we characterized the courses of development of the electrophysiological abnormalities in the hemiparkinsonian rat model. The loss of dopaminergic neurons develops fast, and is histologically completed within 4-7 days of the lesion. The increase in STN burst discharges is limited to the lesioned side, and follows a very similar course. In contrast, beta augmentation has a bilateral presentation, and requires 14-21 days for full development. Behaviorally, the gross locomotor deficits in open field test and limb akinesia in stepping test match the foregoing fast and slow time courses, respectively. A further look into the spike entrainment shows that the oscillations in local field potential (LFP) of the MC effectively entrain the multi-unit (MU) spikes of MC, STN and entopeduncular nucleus (EPN), a rat homolog of human globus pallidus interna (GPi), whereas the LFP of STN or EPN (GPi) cannot entrain the spikes in MC. We conclude that excessive STN burst discharges are a direct consequence, whereas beta augmentation is probably a secondary or adaptive changes in the cortico-subcortical re-entrant loops, to dopaminergic deprivation. Beta augmentation is therefore not so consistently present as excessive STN burst discharges, but could signal more delicate derangements at the level of cortical programming in PD.

Constitutive activity of dopamine receptor type 1 (D1R) increases CaV2.2 currents in PFC neurons

Alterations in dopamine receptor type 1 (D1R) density are associated with cognitive deficits of aging and schizophrenia. In the prefrontal cortex (PFC), D1R plays a critical role in the regulation of working memory, which is impaired in these cognitive deficit states, but the cellular events triggered by changes in D1R expression remain unknown. A previous report demonstrated that interaction between voltage-gated calcium channel type 2.2 (Ca_V2.2) and D1R stimulates Ca_V2.2 postsynaptic surface location in medial PFC pyramidal neurons. Here, we show that in addition to the occurrence of the physical receptor-channel interaction, constitutive D1R activity mediates up-regulation of functional Ca_V2.2 surface density. We performed patch-clamp experiments on transfected HEK293T cells and wild-type C57BL/6 mouse brain slices, as well as imaging experiments and cAMP measurements. We found that D1R coexpression led to ∼60% increase in Ca_V2.2 currents in HEK293T cells. This effect was occluded by preincubation with a D1/D5R inverse agonist, chlorpromazine, and by replacing D1R with a D1R mutant lacking constitutive activity. Moreover, D1R-induced increase in Ca_V2.2 currents required basally active Gs protein, as well as D1R-Ca_V2.2 interaction. In mice, intraperitoneal administration of chlorpromazine reduced native Ca_V currents’ sensitivity to ω-conotoxin-GVIA and their size by ∼49% in layer V/VI pyramidal neurons from medial PFC, indicating a selective effect on Ca_V2.2. Additionally, we found that reducing D1/D5R constitutive activity correlates with a decrease in the agonist-induced D1/D5R inhibitory effect on native Ca_V currents. Our results could be interpreted as a stimulatory effect of D1R constitutive activity on the number of Ca_V2.2 channels available for dopamine-mediated modulation. Our results contribute to the understanding of the physiological role of D1R constitutive activity and may explain the noncanonical postsynaptic distribution of functional Ca_V2.2 in PFC neurons.

Korean Red Ginseng Enhances Neurogenesis in the Subventricular Zone of 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Treated Mice

Regulation of adult neurogenesis plays an important role in therapeutic strategies for various neurodegenerative diseases. Recent studies have suggested that the enhancement of adult neurogenesis can be helpful in the treatment of Parkinson’s disease (PD). In this study, we investigated whether Korean red ginseng (KRG) can enhance neurogenesis in the subventricular zone (SVZ) of a PD mouse model. To accomplish this, male 8-week-old C57BL/6 mice were injected with vehicle or 20 mg/kg of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) four times at 2 h intervals. After the final injection, they were administered water or 100 mg/kg of KRG extract and injected intraperitoneally with 50 mg/kg of 5’-bromo-2’-deoxyuridine-monophosphate (BrdU) once a day for 14 consecutive days. After the last pole test, dopaminergic neuronal survival in the striatum and the substantia nigra (SN), cell proliferation in the SVZ and mRNA expression of neurotrophic factors and dopamine receptors in the striatum were evaluated. KRG administration suppressed dopaminergic neuronal death induced by MPTP in the striatum as well as the SN, augmented the number of BrdU- and BrdU/doublecortin (Dcx)-positive cells in the SVZ and enhanced the expression of proliferation cell nuclear antigen, brain derived neurotrophic factor (BDNF), glial cell derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), ciliary neurotrophic factor (CNTF), dopamine receptor D3 (DRD3) and D5 mRNAs. These results suggest that KRG administration augments neurogenesis in the SVZ of the PD mouse model.

Regularizing firing patterns of rat subthalamic neurons ameliorates parkinsonian motor deficits

The subthalamic nucleus (STN) is an effective therapeutic target for deep brain stimulation (DBS) for Parkinson's disease (PD), and histamine levels are elevated in the basal ganglia in PD patients. However, the effect of endogenous histaminergic modulation on STN neuronal activities and the neuronal mechanism underlying STN-DBS are unknown. Here, we report that STN neuronal firing patterns are more crucial than firing rates for motor control. Histamine excited STN neurons, but paradoxically ameliorated parkinsonian motor deficits, which we attributed to regularizing firing patterns of STN neurons via the hyperpolarization-activated cyclic nucleotide-gated channel 2 (HCN2) channel coupled to the H2 receptor. Intriguingly, DBS increased histamine release in the STN and regularized STN neuronal firing patterns under parkinsonian conditions. HCN2 contributed to the DBS-induced regularization of neuronal firing patterns, suppression of excessive β oscillations, and alleviation of motor deficits in PD. The results reveal an indispensable role for regularizing STN neuronal firing patterns in amelioration of parkinsonian motor dysfunction and a functional compensation for histamine in parkinsonian basal ganglia circuitry. The findings provide insights into mechanisms of STN-DBS as well as potential therapeutic targets and STN-DBS strategies for PD.

D5 dopamine receptors control glutamatergic AMPA transmission between the motor cortex and subthalamic nucleus

Corticofugal fibers target the subthalamic nucleus (STN), a component nucleus of the basal ganglia, in addition to the striatum, their main input. The cortico-subthalamic, or hyperdirect, pathway, is thought to supplement the cortico-striatal pathways in order to interrupt/change planned actions. To explore the previously unknown properties of the neurons that project to the STN, retrograde and anterograde tools were used to specifically identify them in the motor cortex and selectively stimulate their synapses in the STN. The cortico-subthalamic neurons exhibited very little sag and fired an initial doublet followed by non-adapting action potentials. In the STN, AMPA/kainate synaptic currents had a voltage-dependent conductance, indicative of GluA2-lacking receptors and were partly inhibited by Naspm. AMPA transmission displayed short-term depression, with the exception of a limited bandpass in the 5 to 15 Hz range. AMPA synaptic currents were negatively controlled by dopamine D5 receptors. The reduction in synaptic strength was due to postsynaptic D5 receptors, mediated by a PKA-dependent pathway, but did not involve a modified rectification index. Our data indicated that dopamine, through post-synaptic D5 receptors, limited the cortical drive onto STN neurons in the normal brain.

Lead-Induced Atypical Parkinsonism in Rats: Behavioral, Electrophysiological, and Neurochemical Evidence for a Role of Noradrenaline Depletion

Background: Lead neurotoxicity is a major health problem known as a risk factor for neurodegenerative diseases, including the manifestation of parkinsonism-like disorder. While lead is known to preferentially accumulate in basal ganglia, the mechanisms underlying behavioral disorders remain unknown. Here, we investigated the neurophysiological and biochemical correlates of motor deficits induced by sub-chronic injections of lead.

Methods: Sprague Dawely rats were exposed to sub-chronic injections of lead (10 mg/kg, i.p.) or to a single i.p. injection of 50 mg/kg N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4), a drug known to induce selective depletion of noradrenaline. Rats were submitted to a battery of behavioral tests, including the open field for locomotor activity and rotarod for motor coordination. Electrophysiological recordings were carried out in three major basal ganglia nuclei, the subthalamic nucleus (STN), globus pallidus (GP), and substantia nigra pars reticulata (SNr). At the end of experiments, post-mortem tissue level of the three monoamines (dopamine, noradrenaline, and serotonin) and their metabolites has been determined using HPLC.

Results: Lead intoxication significantly impaired exploratory and locomotor activity as well as motor coordination. It resulted in a significant reduction in the level of noradrenaline in the cortex and dopamine and its metabolites, DOPAC, and HVA, in the striatum. The tissue level of serotonin and its metabolite 5-HIAA was not affected in the two structures. Similarly, DSP-4, which induced a selective depletion of noradrenaline, significantly decreased exploratory, and locomotor activity as well as motor coordination. L-DOPA treatment did not improve motor deficits induced by lead and DSP-4 in the two animal groups. Electrophysiological recordings showed that both lead and DSP-4 did not change the firing rate but resulted in a switch from the regular normal firing to irregular and bursty discharge patterns of STN neurons. Neither lead nor DSP-4 treatments changed the firing rate and the pattern of GP and SNr neurons.

Conclusions: Our findings provide evidence that lead represents a risk factor for inducing parkinsonism-like deficits. As the motor deficits induced by lead were not improved by L-DOPA, we suggest that the deficits may be due to the depletion of noradrenaline and the parallel disorganization of STN neuronal activity.

The impact of combined administration of paraquat and maneb on motor and non-motor functions in the rat

Paraquat (PQ) and maneb (MB) are potential risk factors for Parkinson's disease. However, their impact on non-motor disorders, monoamine neurotransmission and basal ganglia function is not clearly determined. Here we investigated the effects of combined treatment with PQ/MB on motor behavior, anxiety and "depressive-like" disorders, tissue content of monoamines, and subthalamic nucleus (STN) neuronal activity. Male Sprague-Dawley rats were intoxicated by PQ (10 mg/kg) and MB (30 mg/kg) twice a week. Two weeks later, the majority of animals (group 1, 16/26) showed a severe loss of body weight with tremor and respiratory distress and others (group 2, 6/26) showed only tremor. Animals of group 2 received PQ/MB during four weeks before developing weight loss. A last group (group 3, 4/26) was insensitive to PQ/MB after 6 weeks of injections. Groups 1 and 2 displayed a failure of motor activity and motor coordination. Group 3 showed slight motor deficits only after the last injection of PQ/MB. Moreover, PQ/MB induced anxiety and "depressive-like" behaviors in animals of groups 2 and 3. Biochemical analysis showed that PQ/MB reduced striatal dopamine (DA) tissue content paralleled by changes in the activity of STN neurons without changing the content of norepinephrine and serotonin in the cortex. Our data provide evidence that individuals are not equally sensitive to PQ/MB and show that the motor deficits in vulnerable animals, are not only a result of DA neuron degeneration, but may also be a consequence of peripheral disabilities. Nevertheless, the parkinsonian-like non-motor impairments may be a direct consequence of the bilateral DA depletion.

Functional importance of two conserved residues in intracellular loop 1 and transmembrane region 2 of Family A GPCRs: insights from ligand binding and signal transduction responses of D1 and D5 dopaminergic receptor mutants

For many G protein-coupled receptors (GPCRs), the role of the first intracellular loop (IL1) and its connections with adjacent transmembrane (TM) regions have not been investigated. Notably, these regions harbor several polar residues such as Ser and Thr. To begin uncovering how these polar residues may contribute to the structural basis for GPCR functionality, we have designed human D1-class receptor mutants (hD1-ST1 and hD5-ST1) whereby all Ser and Thr of IL1 and IL1/TM2 juncture have been replaced by Ala and Val, respectively. Both ST1 mutants exhibited a loss of dopamine affinity but similar binding properties for inverse agonists compared to their parent receptors. As well, these mutations diminished receptor activation for both subtypes, as indicated by an ablated constitutive activity and a pronounced decrease in dopamine potency. Interestingly, both mutants exhibited enhanced dopamine-mediated maximal stimulation (Emax) of adenylyl cyclase that was at least two-fold higher than wild-type. Point mutations for hD1R revealed that the loss in dopamine affinity and potency was attributed to Thr59, while the enhanced Emax of adenylyl cyclase was directly influenced by Ser65. These two residues are conserved among many Family A GPCRs and have recurring molecular interactions among crystallized structures. As such, their functional roles for IL1 and its transition into TM2 reported herein may also be applicable to other GPCRs. Our work thus potentially highlights a structural role of Thr59 and Ser65 in the formation of critical intramolecular interactions for ligand binding and signal transduction of D1-class dopaminergic receptors.

Dopaminergic control of the globus pallidus through activation of D2 receptors and its impact on the electrical activity of subthalamic nucleus and substantia nigra reticulata neurons

The globus pallidus (GP) receives dopaminergic afferents from the pars compacta of substantia nigra and several studies suggested that dopamine exerts its action in the GP through presynaptic D2 receptors (D2Rs). However, the impact of dopamine in GP on the pallido-subthalamic and pallido-nigral neurotransmission is not known. Here, we investigated the role of dopamine, through activation of D2Rs, in the modulation of GP neuronal activity and its impact on the electrical activity of subthalamic nucleus (STN) and substantia nigra reticulata (SNr) neurons. Extracellular recordings combined with local intracerebral microinjection of drugs were done in male Sprague-Dawley rats under urethane anesthesia. We showed that dopamine, when injected locally, increased the firing rate of the majority of neurons in the GP. This increase of the firing rate was mimicked by quinpirole, a D2R agonist, and prevented by sulpiride, a D2R antagonist. In parallel, the injection of dopamine, as well as quinpirole, in the GP reduced the firing rate of majority of STN and SNr neurons. However, neither dopamine nor quinpirole changed the tonic discharge pattern of GP, STN and SNr neurons. Our results are the first to demonstrate that dopamine through activation of D2Rs located in the GP plays an important role in the modulation of GP-STN and GP-SNr neurotransmission and consequently controls STN and SNr neuronal firing. Moreover, we provide evidence that dopamine modulate the firing rate but not the pattern of GP neurons, which in turn control the firing rate, but not the pattern of STN and SNr neurons.

Agonist-dependent and -independent dopamine-1-like receptor signalling differentially regulates downstream effectors

De-regulation of energy metabolism by the dopaminergic system is linked to neurological diseases such as schizophrenia and bipolar disorder. Inverse agonists are thought to be more beneficial in treating neurological diseases than neutral antagonists, but only limited experimental data are available regarding the impact of constitutive signalling on energy metabolism. The aim of the present study was to assess the impact of constitutive dopamine-1 receptor (D1R) and dopamine-5 receptor (D5R) signalling on downstream targets in transiently and stably transfected HEK293T cells. The high constitutive activity of D5R was accompanied by increased Na(+)/H(+) exchanger (NHE) activity and accelerated glucose degradation due to increased transcription and translation of the Na, K-ATPase-α3 and NHE-2. Chronic treatment with an agonist increased the mRNA levels of the α2 Na,K-ATPase, NHE-2 and NHE-3. Constitutive D5R activation of a cAMP response element-based reporter was regulated by G protein-coupled receptor kinase 2, but this did not affect the cell-surface abundance of the receptor. Our data suggest that constitutive and agonist-induced activity of D5R differentially regulates the activity and expression of proteins.

Dopamine: a parallel pathway for the modulation of spinal locomotor networks

The spinal cord contains networks of neurons that can produce locomotor patterns. To readily respond to environmental conditions, these networks must be flexible yet at the same time robust. Neuromodulators play a key role in contributing to network flexibility in a variety of invertebrate and vertebrate networks. For example, neuromodulators contribute to altering intrinsic properties and synaptic weights that, in extreme cases, can lead to neurons switching between networks. Here we focus on the role of dopamine in the control of stepping networks in the spinal cord. We first review the role of dopamine in modulating rhythmic activity in the stomatogastric ganglion (STG) and the leech, since work from these preparations provides a foundation to understand its role in vertebrate systems. We then move to a discussion of dopamine’s role in modulation of swimming in aquatic species such as the larval xenopus, lamprey and zebrafish. The control of terrestrial walking in vertebrates by dopamine is less studied and we review current evidence in mammals with a focus on rodent species. We discuss data suggesting that the source of dopamine within the spinal cord is mainly from the A11 area of the diencephalon, and then turn to a discussion of dopamine’s role in modulating walking patterns from both in vivo and in vitro preparations. Similar to the descending serotonergic system, the dopaminergic system may serve as a potential target to promote recovery of locomotor function following spinal cord injury (SCI); evidence suggests that dopaminergic agonists can promote recovery of function following SCI. We discuss pharmacogenetic and optogenetic approaches that could be deployed in SCI and their potential tractability. Throughout the review we draw parallels with both noradrenergic and serotonergic modulatory effects on spinal cord networks. In all likelihood, a complementary monoaminergic enhancement strategy should be deployed following SCI.

Involvement of dopamine loss in extrastriatal basal ganglia nuclei in the pathophysiology of Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by the manifestation of motor symptoms, such as akinesia, muscle rigidity and tremor at rest. These symptoms are classically attributed to the degeneration of dopamine neurons in the pars compacta of substantia nigra (SNc), which results in a marked dopamine depletion in the striatum. It is well established that dopamine neurons in the SNc innervate not only the striatum, which is the main target, but also other basal ganglia nuclei including the two segments of globus pallidus and the subthalamic nucleus (STN). The role of dopamine and its depletion in the striatum is well known, however, the role of dopamine depletion in the pallidal complex and the STN in the genesis of their abnormal neuronal activity and in parkinsonian motor deficits is still not clearly determined. Based on recent experimental data from animal models of Parkinson's disease in rodents and non-human primates and also from parkinsonian patients, this review summarizes current knowledge on the role of dopamine in the modulation of basal ganglia neuronal activity and also the role of dopamine depletion in these nuclei in the pathophysiology of Parkinson's disease.

Localization and function of dopamine receptors in the subthalamic nucleus of normal and parkinsonian monkeys

The subthalamic nucleus (STN) receives a dopaminergic innervation from the substantia nigra pars compacta, but the role of this projection remains poorly understood, particularly in primates. To address this issue, we used immuno-electron microscopy to localize D1, D2, and D5 dopamine receptors in the STN of rhesus macaques and studied the electrophysiological effects of activating D1-like or D2-like receptors in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. Labeling of D1 and D2 receptors was primarily found presynaptically, on preterminal axons and putative glutamatergic and GABAergic terminals, while D5 receptors were more significantly expressed postsynaptically, on dendritic shafts of STN neurons. The electrical spiking activity of STN neurons, recorded with standard extracellular recording methods, was studied before, during, and after intra-STN administration of the dopamine D1-like receptor agonist SKF82958, the D2-like receptor agonist quinpirole, or artificial cerebrospinal fluid (control injections). In normal animals, administration of SKF82958 significantly reduced the spontaneous firing but increased the rate of intraburst firing and the proportion of pause-burst sequences of firing. Quinpirole only increased the proportion of such pause-burst sequences in STN neurons of normal monkeys. In MPTP-treated monkeys, the D1-like receptor agonist also reduced the firing rate and increased the proportion of pause-burst sequences, while the D2-like receptor agonist did not change any of the chosen descriptors of the firing pattern of STN neurons. Our data suggest that dopamine receptor activation can directly modulate the electrical activity of STN neurons by pre- and postsynaptic mechanisms in both normal and parkinsonian states, predominantly via activation of D1 receptors.

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.