Upregulation of D2-class signaling in dopamine-denervated striatum is in part mediated by D3 receptors acting on Ca V 2.1 channels via PIP2 depletion

Dopamine D3 receptor modulates D2 receptor effects on cAMP and GABA release at striatopallidal terminals-Modulation by the Ca2+-Calmodulin-CaMKII system

Dopamine D2 receptor (D2R) is expressed in striatopallidal neurons and decreases forskolin-stimulated cyclic adenine monophosphate (cAMP) accumulation and gamma-aminobutyric acid (GABA) release. Dopamine D3 receptor (D3R) mRNA is expressed in a population of striatal D2R-expressing neurons. Also, D3R protein and binding have been reported in the neuropil of globus pallidus. We explore whether D2R and D3R colocalize in striatopallidal terminals and whether D3R modulates the D2R effect on forskolin-stimulated [3H]cAMP accumulation in pallidal synaptosomes and high K+ stimulated-[3H]GABA release in pallidal slices. Previous reports in heterologous systems indicate that calmodulin (CaM) and CaMKII modulate D2R and D3R functions; thus, we study whether this system regulates its functional interaction. D2R immunoprecipitates with CaM, and pretreatment with ophiobolin A or depolarization of synaptosomes with 15 mM of K+ decreases it. Both treatments increase the D2R inhibition of forskolin-stimulated [3H]cAMP accumulation when activated with quinpirole, indicating a negative modulation of CaM on D2R function. Quinpirole also activates D3R, potentiating D2R inhibition of cAMP accumulation in the ophiobolin A-treated synaptosomes. D2R and D3R immunoprecipitate in pallidal synaptosomes and decrease after the kainic acid striatal lesion, indicating the striatal origin of the presynaptic receptors. CaM-kinase II alfa (CaMKIIα) immunoprecipitates with D3R and increases after high K+ depolarization. In the presence of KN62, a CaMKIIα blocker, D3R potentiates D2R effects on cAMP accumulation in depolarized synaptosomes and GABA release in pallidal slices, indicating D3R function regulation by CaMKIIα. Our data indicate that D3R potentiates the D2R effect on cAMP accumulation and GABA release at pallidal terminals, an interaction regulated by the CaM-CaMKIIα system.

Role of M4 -receptor cholinergic signaling in direct pathway striatal projection neurons during dopamine depletion

Direct pathway striatal projection neurons (dSPNs) are characterized by the expression of dopamine (DA) class 1 receptors (D1 R), as well as cholinergic muscarinic M1 and M4 receptors (M1 R, M4 R). D1 R enhances neuronal firing through phosphorylation of voltage-gate calcium channels (CaV 1 Ca2+ channels) activating Gs proteins and protein kinase A (PKA). Concurrently, PKA suppresses phosphatase PP-1 through DARPP-32, thus extending this facilitatory modulation. M1 R also influences Ca2+ channels in SPNs through Gq proteins and protein kinase C. However, the signaling mechanisms of M4 R in dSPNs are less understood. Two pathways are attributed to M4 R: an inhibitory one through Gi/o proteins, and a facilitatory one via the cyclin Cdk5. Our study reveals that a previously observed facilitatory modulation via CaV 1 Ca2+ channels is linked to the Cdk5 pathway in dSPNs. This result could be significant in treating parkinsonism. Therefore, we questioned whether this effect persists post DA-depletion in experimental parkinsonism. Our findings indicate that in such conditions, M4 R activation leads to a decrease in Ca2+ current and an increased M4 R protein level, contrasting with the control response. Nevertheless, parkinsonian and control actions are inhibited by the Cdk5 inhibitor roscovitine, suggesting Cdk5's role in both conditions. Cdk5 may activate PP-1 via PKA inhibition in DA depletion. Indeed, we found that inhibiting PP-1 restores control M4 R actions, implying that PP-1 is overly active via M4 Rs in DA-depleted condition. These insights contribute to understanding how DA-depletion alters modulatory signaling in striatal neurons. Additional working hypotheses are discussed.

Modulation of D3R Splicing, Signaling, and Expression by D1R through PKA→PTB Phosphorylation

The D1R and D3R receptors functionally and synergistically interact in striatonigral neurons. Dopaminergic denervation turns this interaction antagonistic, which is correlated with a decrement in D3nf isoform and an increment in D3R membranal expression. The mechanisms of such changes in D3R are attributed to the dysregulation of the expression of their isoforms. The cause and mechanism of this phenomenon remain unknown. Dopaminergic denervation produces a decrement in D1R and PKA activity; we propose that the lack of phosphorylation of PTB (regulator of alternative splicing) by PKA produces the dysregulation of D3R splicing and changes D3R functionality. By using in silico analysis, we found that D3R mRNA has motifs for PTB binding and, by RIP, co-precipitates with PTB. Moreover, D1R activation via PKA promotes PTB phosphorylation. Acute and 5-day D1R blockade decreases the expression of D3nf mRNA. The 5-day treatment reduces D3R, D3nf, and PTB protein in the cytoplasm and increases D3R in the membrane and PTB in the nucleus. Finally, the blockade of D1R mimics the effect of dopaminergic denervation in D1R and D3R signaling. Thus, our data indicate that through PKA→PTB, D1R modulates D3R splicing, expression, and signaling, which are altered during D1R blockade or the lack of stimulation in dopaminergic denervation.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use disorders.

Neurobiological and Pharmacological Perspectives of D3 Receptors in Parkinson’s Disease

The discovery of the D3 receptor (D3R) subtypes of dopamine (DA) has generated an understandable increase in interest in the field of neurological diseases, especially Parkinson’s disease (PD). Indeed, although DA replacement therapy with l-DOPA has provided an effective treatment for patients with PD, it is responsible for invalidating abnormal involuntary movements, known as L-DOPA-induced dyskinesia, which constitutes a serious limitation of the use of this therapy. Of particular interest is the finding that chronic l-DOPA treatment can trigger the expression of D1R–D3R heteromeric interactions in the dorsal striatum. The D3R is expressed in various tissues of the central nervous system, including the striatum. Compelling research has focused on striatal D3Rs in the context of PD and motor side effects, including dyskinesia, occurring with DA replacement therapy. Therefore, this review will briefly describe the basal ganglia (BG) and the DA transmission within these brain regions, before going into more detail with regard to the role of D3Rs in PD and their participation in the current treatments. Numerous studies have also highlighted specific interactions between D1Rs and D3Rs that could promote dyskinesia. Finally, this review will also address the possibility that D3Rs located outside of the BG may mediate some of the effects of DA replacement therapy.

Akathisia and Restless Legs Syndrome: Solving the Dopaminergic Paradox

Akathisia is an urgent need to move that is associated with treatment with dopamine receptor blocking agents (DRBAs) and with restless legs syndrome (RLS). The pathogenetic mechanism of akathisia has not been resolved. This article proposes that it involves an increased presynaptic dopaminergic transmission in the ventral striatum and concomitant strong activation of postsynaptic dopamine D₁ receptors, which form complexes (heteromers) with dopamine D₃ and adenosine A₁ receptors. It also proposes that in DRBA-induced akathisia, increased dopamine release depends on inactivation of autoreceptors, whereas in RLS it depends on a brain iron deficiency-induced down-regulation of striatal presynaptic A₁ receptors.

Dopamine D3 Receptor Plasticity in Parkinson's Disease and L-DOPA-Induced Dyskinesia

Parkinson’s Disease (PD) is characterized by primary and secondary plasticity that occurs in response to progressive degeneration and long-term L-DOPA treatment. Some of this plasticity contributes to the detrimental side effects associated with chronic L-DOPA treatment, namely L-DOPA-induced dyskinesia (LID). The dopamine D3 receptor (D3R) has emerged as a promising target in LID management as it is upregulated in LID. This upregulation occurs primarily in the D1-receptor-bearing (D1R) cells of the striatum, which have been repeatedly implicated in LID manifestation. D3R undergoes dynamic changes both in PD and in LID, making it difficult to delineate D3R’s specific contributions, but recent genetic and pharmacologic tools have helped to clarify its role in LID. The following review will discuss these changes, recent advances to better clarify D3R in both PD and LID and potential steps for translating these findings.

Ziconotide-Induced Oro-lingual Dyskinesia: 3 Cases

Background:

Ziconotide (ZCN), a nonopioid analgesic, is first-line intrathecal therapy for patients with severe chronic pain refractory to other management options. We describe three cases of ZCN-induced movement disorders.

Cases:

Case one is a 64-year-old woman who presented with oro-lingual (OL) dyskinesia with dysesthesias and bilateral upper extremity kinetic tremor. Case two is a 43-year-old man with a 20-month history of ZCN treatment who developed OL dyskinesia with dysesthesias, involuntary left hand and neck movements, hallucinations, dysesthesias on his feet, and gait imbalance. Case three is a 70-year-old man with a 4-month history of ZCN use who developed OL dyskinesia with dysesthesias.

Conclusions:

Intrathecal treatment of pain with ZCN may be complicated by a drug-induced movement disorder where OL dyskinesia is characteristic. The movement disorder is likely to be dose related and reversible with ZCN discontinuation, but a chronic movement disorder is also possible.

Coexistence of D3 R typical and atypical signaling in striatonigral neurons during dopaminergic denervation. Correlation with D3 nf expression changes

Dopamine D₃ R are widely expressed in basal ganglia where interact with D₁ R. D₃ R potentiate cAMP accumulation and GABA release stimulated by D₁ R in striatonigral neurons through "atypical" signaling. During dopaminergic denervation, D₃ R signaling changes to a "typical" in which antagonizes the effects of D₁ R, the mechanisms of this switching are unknown. D₃ nf splice variant regulates membrane anchorage and function of D₃ R and decreases in denervation; thus, it is possible that D₃ R signaling switching correlates with changes in D₃ nf expression and increases of membranal D₃ R that mask D₃ R atypical effects. We performed experiments in unilaterally 6-hydroxydopamine lesioned rats and found a decrease in mRNA and protein of D₃ nf, but not of D₃ R in the denervated striatum. Proximity ligation assay showed that D₃ R-D₃ nf interaction decreased after denervation, whereas binding revealed an increased B_max in D₃ R. The new D₃ R antagonized cAMP accumulation and GABA release stimulated by D₁ R; however, in the presence of N-Ethylmaleimide (NEM), to block G_i protein signaling, activation of D₃ R produced its atypical signaling stimulating D₁ R effects. Finally, we investigated if the typical and atypical effects of D₃ R modulating GABA release are capable of influencing motor behavior. Injections of D₃ R agonist into denervated nigra decreased D₁ R agonist-induced turning behavior but potentiated it in the presence of NEM. Our data indicate the coexistence of D₃ R typical and atypical signaling in striatonigral neurons during denervation that correlated with changes in the ratio of expression of D₃ nf and D₃ R isoforms. The coexistence of both atypical and typical signaling during denervation influences motor behavior.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

Reciprocal cross-sensitization of D1 and D3 receptors following pharmacological stimulation in the hemiparkinsonian rat

In the majority of Parkinson's disease (PD) patients, long-term dopamine (DA) replacement therapy leads to dyskinesia characterized by abnormal involuntary movements (AIMs). There are various mechanisms of dyskinesia, such as the sensitization of striatal DA D1 receptors (D₁R) and upregulation of DA D3 receptors (D₃R). These receptors interact physically and functionally in D₁R-bearing medium spiny neurons to synergistically drive dyskinesia. However, the cross-receptor-mediated effects due to D₁R-D₃R cooperativity are still poorly understood. In pursuit of this, we examined whether or not pharmacological D₁R or D₃R stimulation sensitizes the dyskinetic response to the appositional agonist, a process known as cross-sensitization. First, we established D₁R-D₃R behavioral synergy in a cohort of 6-OHDA-lesioned female adult Sprague-Dawley rats. Then, in a new cohort, we tested for cross-sensitization in a between-subject design. Five groups received a sub-chronic regimen of either saline, the D₁R agonist SKF38393 (1.0 mg/kg), or the D₃R agonist PD128907 (0.3 mg/kg). For the final injection, each group received an acute injection of the other agonist. AIMs were monitored following each injection. Sub-chronic administration of both SKF38393 and PD128907 induced the development of dyskinesia. More importantly, cross-agonism tests revealed reciprocal cross-sensitization; chronic treatment with either SKF38393 or PD128907 induced sensitization to a single administration of the other agonist. This reciprocity was not marked by changes to either D₁R or D₃R striatal mRNA expression. The current study provides key behavioral data demonstrating the role of D3R in dyskinesia and provides behavioral evidence of D1R and D3R functional interactions.

Biased G Protein-Independent Signaling of Dopamine D1-D3 Receptor Heteromers in the Nucleus Accumbens

Several studies found in vitro evidence for heteromerization of dopamine D₁ receptors (D1R) and D₃ receptors (D3R), and it has been postulated that functional D1R-D3R heteromers that are normally present in the ventral striatum mediate synergistic locomotor-activating effects of D1R and D3R agonists in rodents. Based also on results obtained in vitro, with mammalian transfected cells, it has been hypothesized that those behavioral effects depend on a D1R-D3R heteromer-mediated G protein-independent signaling. Here, we demonstrate the presence on D1R-D3R heteromers in the mouse ventral striatum by using a synthetic peptide that selectively destabilizes D1R-D3R heteromers. Parallel locomotor activity and ex vivo experiments in reserpinized mice and in vitro experiments in D1R-D3R mammalian transfected cells were performed to dissect the signaling mechanisms of D1R-D3R heteromers. Co-administration of D1R and D3R agonists in reserpinized mice produced synergistic locomotor activation and a selective synergistic AKT phosphorylation in the most ventromedial region of the striatum in the shell of the nucleus accumbens. Application of the destabilizing peptide in transfected cells and in the shell of the nucleus accumbens allowed demonstrating that both in vitro and in vivo co-activation of D3R induces a switch from G protein-dependent to G protein-independent D1R-mediated signaling determined by D1R-D3R heteromerization. The results therefore demonstrate that a biased G protein-independent signaling of D1R-D3R heteromers localized in the shell of the nucleus accumbens mediate the locomotor synergistic effects of D1R and D3R agonists in reserpinized mice.

Disruption of Multiple Distinctive Neural Networks Associated With Impulse Control Disorder in Parkinson's Disease

The phenomenon of impulsivity in Parkinson's disease appears as an arduous side effect of dopaminergic therapy with potentially detrimental consequences for the life of the patients. Although conceptualized as a result of non-physiologic chronic dopaminergic stimulation, recent advances speculate on combined disruption of other networks as well. In the search for neuroanatomical correlates of this multifaceted disturbance, this study employs two distinct, well-defined tasks of close association to motor inhibition and decision-making impulsivity, Go/No Go and Delay discounting. The fMRI and functional connectivity analysis in 21 Parkinson's disease patients, including 8 patients suffering from severe impulse control disorder, and 28 healthy controls, revealed in impulsive Parkinson's disease patients not only decreased fMRI activation in the dorsolateral prefrontal cortex and bilateral striatum, but also vast functional connectivity changes of both caudate nuclei as decreased connectivity to the superior parietal cortex and increased connectivity to the insular area, clearly beyond the commonly stated areas, which indicates that orbitofronto-striatal and mesolimbic functional disruptions are not the sole mechanisms underlying impulse control disorder in Parkinson's disease. Ergo, our results present a refinement and synthesis of gradually developing ideas about the nature of impulsive control disorder in Parkinson's disease—an umbrella term encompassing various behavioral deviations related to distinct neuronal networks and presumably neurotransmitter systems, which greatly exceed the previously envisioned dopaminergic pathways as the only culprit.

The neurobiology of impulse control disorders in Parkinson's disease: from neurotransmitters to neural networks

Impulse control disorders (ICD) are common neuropsychiatric disorders that can arise in Parkinson’s disease (PD) patients after commencing dopamine replacement therapy. Approximately 15% of all patients develop these disorders and many more exhibit subclinical symptoms of impulsivity. ICD is thought to develop due to an interaction between the use of dopaminergic medication and an as yet unknown neurobiological vulnerability that either pre-existed before PD onset (possibly genetic) or is associated with neural alterations due to the PD pathology. This review discusses genes, neurotransmitters and neural networks that have been implicated in the pathophysiology of ICD in PD. Although dopamine and the related reward system have been the main focus of research, recently, studies have started to look beyond those systems to find new clues to the neurobiological underpinnings of ICD and come up with possible new targets for treatment. Studies on the whole-brain connectome to investigate the global alterations due to ICD development are currently lacking. In addition, there is a dire need for longitudinal studies that are able to disentangle the contributions of individual (genetic) traits and secondary effects of the PD pathology and chronic dopamine replacement therapy to the development of ICD in PD.

Dopamine D3 Receptor Modulates l-DOPA-Induced Dyskinesia by Targeting D1 Receptor-Mediated Striatal Signaling

The dopamine D3 receptor (D3R) belongs to the dopamine D2-like receptor family and is principally located in the ventral striatum. However, previous studies reported D3R overexpression in the dorsal striatum following l-DOPA treatment in parkinsonian animals. This fact has drawn attention in the importance of D3R in l-DOPA-induced dyskinesia (LID). Here, we used D3R knockout mice to assess the role of D3R in LID and rotational sensitization in the hemiparkinsonian model. Mice lacking D3R presented a reduction in dyskinesia without interfering with the antiparkinsonian l-DOPA effect and were accompanied by a reduction in the l-DOPA-induced rotations. Interestingly, deleting D3R attenuated important molecular markers in the D1R-neurons such as FosB, extracellular signal-regulated kinase, and histone-3 (H3)-activation. Colocalization studies in D1R-tomato and D2R-green fluorescent protein BAC-transgenic mice indicated that l-DOPA-induced D3R overexpression principally occurs in D1R-containing neurons although it is also present in the D2R-neurons. Moreover, D3R pharmacological blockade with PG01037 reduced dyskinesia and the molecular markers expressed in D1R-neurons. In addition, this antagonist further reduced dyskinetic symptoms in D1R heterozygous mice, indicating a direct interaction between D1R and D3R. Together, our results demonstrate that D3R modulates the development of dyskinesia by targeting D1R-mediated intracellular signaling and suggest that decreasing D3R activity may help to ameliorate LID.

Abnormalities of Dopamine D3 Receptor Signaling in the Diseased Brain

Dopamine D₃ receptors (D₃R) modulate neuronal activity in several brain regions including cortex, striatum, cerebellum, and hippocampus. A growing body of evidence suggests that aberrant D₃R signaling contributes to multiple brain diseases, such as Parkinson’s disease, essential tremor, schizophrenia, and addiction. In line with these findings, D₃R has emerged as a potential target in the treatment of neurological disorders. However, the mechanisms underlying neuronal D₃R signaling are poorly understood, either in healthy or diseased brain. Here, I review the molecular mechanisms involved in D₃R signaling via monomeric D₃R and heteromeric receptor complexes (e.g., D₃R-D₁R, D₃R-D₂R, D₃R-A_2aR, and D₃R-D₃nf). I focus on D₃R signaling pathways that, according to recent reports, contribute to pathological brain states. In particular, I describe evidence on both quantitative (e.g., increased number or affinity) and qualitative (e.g., switched signaling) changes in D₃R that has been associated with brain dysfunction. I conclude with a description of basic mechanisms that modulate D₃R signaling such as desensitization, as disruption of these mechanisms may underlie pathological changes in D₃R signaling. Because several lines of evidence support the idea that imbalances in D₃R signaling alter neural function, a better understanding of downstream D₃R pathways is likely to reveal novel therapeutic strategies toward dopamine-related brain disorders.

Current Concepts on the Physiopathological Relevance of Dopaminergic Receptors

Dopamine (DA) is a key neurotransmitter modulating essential functions of the central nervous system (CNS), like voluntary movement, reward, several cognitive functions and goal-oriented behaviors. The factual relevance of DAergic transmission can be well appreciated by considering that its dysfunction is recognized as a core alteration in several devastating neurological and psychiatric disorders, including Parkinson’s disease (PD) and associated movement disorders, as well as, schizophrenia, bipolar disorder, attention deficit hyperactivity disorder (ADHD) and addiction. Here we present an overview of the current knowledge on the involvement of DAergic receptors in the regulation of key physiological brain activities, and the consequences of their dysfunctions in brain disorders such as PD, schizophrenia and addiction.

Tipping the scales: Lessons from simple model systems on inositol imbalance in neurological disorders

Inositol and inositol-containing compounds have signalling and regulatory roles in many cellular processes, suggesting that inositol imbalance may lead to wide-ranging changes in cellular functions. Indeed, changes in inositol-dependent signalling have been implicated in various diseases and cellular functions such as autophagy, and these changes have often been proposed as therapeutic targets. However, few studies have highlighted the links between inositol depletion and the downstream effects on inositol phosphates and phosphoinositides in disease states. For this research, many advances have employed simple model systems that include the social amoeba D. discoideum and the yeast S. cerevisiae, since these models enable a range of experimental approaches that are not possible in mammalian models. In this review, we discuss recent findings initiated in simple model systems and translated to higher model organisms where the effect of altered inositol, inositol phosphate and phosphoinositide levels impact on bipolar disorder, Alzheimer disease, epilepsy and autophagy.

Pathophysiological signatures of functional connectomics in parkinsonian and dyskinetic striatal microcircuits

A challenge in neuroscience is to integrate the cellular and system levels. For instance, we still do not know how a few dozen neurons organize their activity and relations in a microcircuit or module of histological scale. By using network theory and Ca(2+) imaging with single-neuron resolution we studied the way in which striatal microcircuits of dozens of cells orchestrate their activity. In addition, control and diseased striatal tissues were compared in rats. In the control tissue, functional connectomics revealed small-world, scale-free and hierarchical network properties. These properties were lost during pathological conditions in ways that could be quantitatively analyzed. Decorticated striatal circuits disclosed that corticostriatal interactions depend on privileged connections with a set of highly connected neurons or "hubs". In the 6-OHDA model of Parkinson's disease there was a decrease in hubs number; but the ones that remained were linked to dominant network states. l-DOPA induced dyskinesia provoked a loss in the hierarchical structure of the circuit. All these conditions conferred distinct temporal sequences to circuit activity. Temporal sequences appeared as particular signatures of disease process thus bringing the possibility of a future quantitative pathophysiology at a histological scale.

Dopamine Receptors and Neurodegeneration

Dopamine (DA) is one of the major neurotransmitters and participates in a number of functions such as motor coordination, emotions, memory, reward mechanism, neuroendocrine regulation etc. DA exerts its effects through five DA receptors that are subdivided in 2 families: D1-like DA receptors (D1 and D5) and the D2-like (D2, D3 and D4). All DA receptors are widely expressed in the central nervous system (CNS) and play an important role in not only in physiological conditions but also pathological scenarios. Abnormalities in the DAergic system and its receptors in the basal ganglia structures are the basis Parkinson's disease (PD), however DA also participates in other neurodegenerative disorders such as Huntington disease (HD) and multiple sclerosis (MS). Under pathological conditions reorganization of DAergic system has been observed and most of the times, those changes occur as a mechanism of compensation, but in some cases contributes to worsening the alterations. Here we review the changes that occur on DA transmission and DA receptors (DARs) at both levels expression and signals transduction pathways as a result of neurotoxicity, inflammation and in neurodegenerative processes. The better understanding of the role of DA receptors in neuropathological conditions is crucial for development of novel therapeutic approaches to treat alterations related to neurodegenerative diseases.

A D2-like receptor family agonist produces analgesia in mechanonociception but not in thermonociception at the spinal cord level in rats

The administration of dopaminergic drugs produces analgesia in individuals experiencing different types of pain. Analgesia induced by these drugs at the spinal cord level is mediated by D2-like agonists, which specifically inhibit the detection of nociceptive stimuli by sensory afferents. The extent of the analgesia provided by spinal dopamine agonists remains controversial, and the cellular mechanism of this analgesic process is poorly understood. The objective of this study was to evaluate the analgesic effect of quinpirole, a D2-like agonist, based on two nociceptive tests and at various doses that were selected to specifically activate dopamine receptors. We found that intrathecal quinpirole administration produces analgesia of mechanical but not thermal nociception and that the analgesic effect of quinpirole is reversed by a mix of D2, D3, and D4 receptor-specific antagonists, suggesting that the activation of all D2-like receptors is involved in the analgesia produced by intrathecal quinpirole. The differential effect on thermal and mechanical nociception was also tested upon the activation of μ-opioid receptors. As reported previously, low doses of the μ-opioid receptor agonist DAMGO produced analgesia of only thermonociception. This evidence shows that a D2-like receptor agonist administered at the spinal cord level produces analgesia specific to mechanonociception but not thermonociception.

The Relationship Between the Mechanisms of Action and Safety Profiles of Intrathecal Morphine and Ziconotide: A Review of the Literature

OBJECTIVE

To better characterize safety profiles associated with the intrathecal (IT) administration of morphine and ziconotide and discuss how they relate to mechanisms of action.

METHODS

Published data were evaluated to identify potential relationships between safety profiles of IT morphine and IT ziconotide and their mechanisms of action.

RESULTS

Potentially severe and clinically relevant adverse events (AEs) associated with IT morphine include respiratory depression, tolerance, and granuloma formulation, whereas IT ziconotide is associated with neuropsychiatric AEs, such as cognitive impairment, hallucinations, and changes in mood or consciousness, particularly with high doses and rapid titration. AEs associated with these IT therapies may result from spread of the medication out of the IT space into areas of the central and peripheral nervous systems and systemic circulation. AEs that occur usually can be managed and, in some cases, prevented. To mitigate risk, patients' histories should be reviewed to identify potential complicating factors (e.g., obesity or other risk factors for respiratory dysfunction in patients receiving IT morphine; a history of psychosis in patients receiving IT ziconotide). Also, treatment should be initiated at a low dose, titrated slowly, and patients should be closely monitored during treatment.

CONCLUSIONS

IT morphine and IT ziconotide are approved by the US Food and Drug Administration for patients who do not respond to less invasive treatments, but the safety profiles of each may make them more or less appropriate for certain patient populations.

Dopaminergic denervation switches dopamine D3 receptor signaling and disrupts its Ca(2+) dependent modulation by CaMKII and calmodulin in striatonigral projections of the rat

In striatonigral projections activation of dopamine D3 receptors (D3Rs) potentiates the stimulation of GABA release and cAMP production caused by activation of dopamine D1 receptors (D1Rs). Cytoplasmic [Ca(2+)] in the terminals controls this response by modulating CaMKII, an enzyme that depresses D3R action. To examine the effects of dopamine deprivation on D3R signaling we investigated their function in striatonigral terminals of hemiparkinsonian rats. Denervation switched the signaling cascade initiated by D3R activation. In the non-lesioned side activation of D3R potentiated the stimulatory effects of D1R activation on cAMP production and K(+)-depolarization induced [(3)H] GABA release. In contrast, in the denervated side the stimulatory effects of both D1R activation and forskolin administration were blocked by D3R activation. In non-lesioned slices, D3R responses were inhibited by the activation of CaMKII produced by K(+)-depolarization (via increased Ca(2+) entry). The CaMKII-induced inhibition was blocked by the selective inhibitor KN-62. In denervated tissues the response to D3R stimulation was not modified either by K(+) depolarization or by blocking CaMKII with KN-62. Immunoblotting studies showed that depolarization-induced CaMKII binding to the D3 receptor and CaMKII phosphorylation were suppressed in denervated tissues. We also determined calmodulin expression with PCR and immunoblot techniques. Both techniques showed that calmodulin expression was depressed in the lesioned side. In sum, our studies show that dopaminergic denervation switches the D3R signaling cascade and depresses CaMKII signaling through a process that appears to involve reduced calmodulin levels. Since calmodulin is a major cytoplasmic Ca(2+) buffer our findings suggest that abnormal Ca(2+) buffering may be an important component of the abnormalities observed during dopaminergic denervation.

Depression and impulse control disorders in Parkinson's disease: two sides of the same coin?

Depression and impulse control disorders (ICD) are two common neuropsychiatric features in Parkinson's disease (PD). Studies have revealed that both phenomena are associated with aberrations in ventral striatal dopamine signaling and concomitant dysfunction of the reward-related (limbic) cortico-striatal-thalamocortical (CSTC) circuit. Depression in PD seems associated with decreased activity in the limbic CSTC circuit, whereas ICD seem associated with increased limbic CSTC circuit activity, usually after commencing dopamine replacement therapy (DRT). Not all DRT using PD patients, however, develop symptoms of ICD, suggesting an additional underlying neurobiological susceptibility. Furthermore, the symptoms of depression and ICD frequently coincide even though they are related to seemingly contrasting limbic CSTC circuit activation states. The aim of this review is to provide an overview of the currently available literature on the neurobiology of PD-related depression and ICD and discusses possible susceptibility factors. Finally, we propose a neurobiological model that identifies ventral striatal dopaminergic denervation as a common underlying neurobiological substrate of depression and ICD and subsequent dysfunction of reward and motivation-related brain areas.

Regulation of dopamine D₃ receptor in the striatal regions and substantia nigra in diffuse Lewy body disease

The regulation of D₃ receptor has not been well documented in diffuse Lewy body disease (DLBD). In this study, a novel D₃-preferring radioligand [(3)H]WC-10 and a D₂-preferring radioligand [(3)H]raclopride were used and the absolute densities of the dopamine D₃ and D₂ receptors were determined in the striatal regions and substantia nigra (SN) from postmortem brains from five cases of DLBD, which included dementia with Lewy bodies (DLB, n=4) and Parkinson disease dementia (PDD, n=1). The densities of the dopamine D₁ receptor, vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT) were also measured by quantitative autoradiography using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. The densities of these dopaminergic markers were also measured in the same brain regions in 10 age-matched control cases. Dopamine D₃ receptor density was significantly increased in the striatal regions including caudate, putamen and nucleus accumbens (NAc). There were no significant changes in the dopamine D₁ and D₂ receptor densities in any brain regions measured. VMAT2 and DAT densities were reduced in all the brain regions measured in DLB/PDD, however, the significant reduction was found in the putamen for DAT and in the NAc and SN for VMAT2. The decrease of dopamine pre-synaptic markers implies neuronal loss in the substantia nigra pars compacta (SNpc) in these DLB/PDD cases, while the increase of D₃ receptors in striatal regions could be attributed to dopaminergic medication history and psychiatric states such as hallucinations. Whether it also reflects compensatory regulation upon dopaminergic denervation warrants further confirmations on larger populations.

Learning-related synaptic growth mediated by internalization of Aplysia cell adhesion molecule is controlled by membrane phosphatidylinositol 4,5-bisphosphate synthetic pathway

Long-term facilitation in Aplysia is accompanied by the growth of new synaptic connections between the sensory and motor neurons of the gill-withdrawal reflex. One of the initial steps leading to the growth of these synapses is the internalization, induced by 5-HT, of the transmembrane isoform of Aplysia cell-adhesion molecule (TM-apCAM) from the plasma membrane of sensory neurons (Bailey et al., 1992). However, the mechanisms that govern the internalization of TM-apCAM and how this internalization is coupled to the molecular events that initiate the structural changes are not fully understood. Here, we report that the synthesis of membrane phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)], which is known to be mediated by a signaling cascade through Aplysia Sec7 protein (ApSec7) and phosphatidylinositol-4-phosphate 5-kinase type I α (PIP5KIα) is required for both the internalization of TM-apCAM and the initiation of synaptic growth during 5-HT-induced long-term facilitation. Pharmacological blockade of PI(4,5)P(2) synthesis by the application of the inhibitor phenylarsine oxide blocked the internalization of apCAM. Furthermore, perturbation of the endogenous activation of ApSec7 and its downstream target PIP5KIα also blocked 5-HT-mediated internalization of TM-apCAM and synaptic growth. Finally, long-term facilitation was specifically impaired by blocking the ApSec7 signaling pathway at sensory-to-motor neuron synapses. These data indicate that the ApSec7/PIP5KIα signaling pathway is actively recruited during learning-related 5-HT signaling and acts as a key regulator of apCAM internalization associated with the formation of new synaptic connections during long-term facilitation.