Dominant-negative mutants identify a role for GIRK channels in D3 dopamine receptor-mediated regulation of spontaneous secretory activity

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

HCN4 ion channel function is required for early events that regulate anatomical left-right patterning in a nodal and lefty asymmetric gene expression-independent manner

Laterality is a basic characteristic of all life forms, from single cell organisms to complex plants and animals. For many metazoans, consistent left-right asymmetric patterning is essential for the correct anatomy of internal organs, such as the heart, gut, and brain; disruption of left-right asymmetry patterning leads to an important class of birth defects in human patients. Laterality functions across multiple scales, where early embryonic, subcellular and chiral cytoskeletal events are coupled with asymmetric amplification mechanisms and gene regulatory networks leading to asymmetric physical forces that ultimately result in distinct left and right anatomical organ patterning. Recent studies have suggested the existence of multiple parallel pathways regulating organ asymmetry. Here, we show that an isoform of the hyperpolarization-activated cyclic nucleotide-gated (HCN) family of ion channels (hyperpolarization-activated cyclic nucleotide-gated channel 4, HCN4) is important for correct left-right patterning. HCN4 channels are present very early in Xenopus embryos. Blocking HCN channels (Ih currents) with pharmacological inhibitors leads to errors in organ situs. This effect is only seen when HCN4 channels are blocked early (pre-stage 10) and not by a later block (post-stage 10). Injections of HCN4-DN (dominant-negative) mRNA induce left-right defects only when injected in both blastomeres no later than the 2-cell stage. Analysis of key asymmetric genes' expression showed that the sidedness of Nodal, Lefty, and Pitx2 expression is largely unchanged by HCN4 blockade, despite the randomization of subsequent organ situs, although the area of Pitx2 expression was significantly reduced. Together these data identify a novel, developmental role for HCN4 channels and reveal a new Nodal-Lefty-Pitx2 asymmetric gene expression-independent mechanism upstream of organ positioning during embryonic left-right patterning.

Coordinating heart morphogenesis: A novel role for hyperpolarization-activated cyclic nucleotide-gated (HCN) channels during cardiogenesis in Xenopus laevis

Hyperpolarization-activated cyclic-nucleotide gated channel (HCN) proteins are important regulators of both neuronal and cardiac excitability. Among the 4 HCN isoforms, HCN4 is known as a pacemaker channel, because it helps control the periodicity of contractions in vertebrate hearts. Although the physiological role of HCN4 channel has been studied in adult mammalian hearts, an earlier role during embryogenesis has not been clearly established. Here, we probe the embryonic roles of HCN4 channels, providing the first characterization of the expression profile of any of the HCN isoforms during Xenopus laevis development and investigate the consequences of altering HCN4 function on embryonic pattern formation. We demonstrate that both overexpression of HCN4 and injection of dominant-negative HCN4 mRNA during early embryogenesis results in improper expression of key patterning genes and severely malformed hearts. Our results suggest that HCN4 serves to coordinate morphogenetic control factors that provide positional information during heart morphogenesis in Xenopus.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Alternatively Spliced Isoforms of KV10.1 Potassium Channels Modulate Channel Properties and Can Activate Cyclin-dependent Kinase in Xenopus Oocytes

K_V10.1 is a voltage-gated potassium channel expressed selectively in the mammalian brain but also aberrantly in cancer cells. In this study we identified short splice variants of K_V10.1 resulting from exon-skipping events (E65 and E70) in human brain and cancer cell lines. The presence of the variants was confirmed by Northern blot and RNase protection assays. Both variants completely lacked the transmembrane domains of the channel and produced cytoplasmic proteins without channel function. In a reconstituted system, both variants co-precipitated with the full-length channel and induced a robust down-regulation of K_V10.1 current when co-expressed with the full-length form, but their effect was mechanistically different. E65 required a tetramerization domain and induced a reduction in the overall expression of full-length K_V10.1, whereas E70 mainly affected its glycosylation pattern. E65 triggered the activation of cyclin-dependent kinases in Xenopus laevis oocytes, suggesting a role in cell cycle control. Our observations highlight the relevance of noncanonical functions for the oncogenicity of K_V10.1, which need to be considered when ion channels are targeted for cancer therapy.

Bioelectric signaling regulates size in zebrafish fins

The scaling relationship between the size of an appendage or organ and that of the body as a whole is tightly regulated during animal development. If a structure grows at a different rate than the rest of the body, this process is termed allometric growth. The zebrafish another longfin (alf) mutant shows allometric growth resulting in proportionally enlarged fins and barbels. We took advantage of this mutant to study the regulation of size in vertebrates. Here, we show that alf mutants carry gain-of-function mutations in kcnk5b, a gene encoding a two-pore domain potassium (K⁺) channel. Electrophysiological analysis in Xenopus oocytes reveals that these mutations cause an increase in K⁺ conductance of the channel and lead to hyperpolarization of the cell. Further, somatic transgenesis experiments indicate that kcnk5b acts locally within the mesenchyme of fins and barbels to specify appendage size. Finally, we show that the channel requires the ability to conduct K⁺ ions to increase the size of these structures. Our results provide evidence for a role of bioelectric signaling through K⁺ channels in the regulation of allometric scaling and coordination of growth in the zebrafish.

The proportions of an animal can change during its lifetime. This often occurs through the phenomenon of relative growth, or allometry. In humans, for example, the head grows at a lower rate than the body resulting in a change in proportion between children and adults. The regulation of size and proportion is not well understood. We investigated fin growth in zebrafish as a model to understand this phenomenon. The mutant another longfin develops proportionally larger fins and barbels. Here, we show that another longfin mutants carry a mutation in kcnk5b, a gene coding for a potassium channel. Potassium channels control the electrical potential of cells and are known to regulate processes such as heart beat, neurotransmission and hormone secretion. We demonstrate that increased potassium channel activity can drive changes in growth in the zebrafish. Clonal analysis reveals that the channel acts directly in the fins and barbels to increase their size in a coordinated manner. Our work shows that potassium channels are involved in the determination of appendage size in zebrafish and suggests consistent with recent studies, an instructive role for bioelectrical signaling in development.

Postsynaptic ERG potassium channels limit muscle excitability to allow distinct egg-laying behavior states in Caenorhabditis elegans

Caenorhabditis elegans regulates egg laying by alternating between an inactive phase and a serotonin-triggered active phase. We found that the conserved ERG [ether-a-go-go (EAG) related gene] potassium channel UNC-103 enables this two-state behavior by limiting excitability of the egg-laying muscles. Using both high-speed video recording and calcium imaging of egg-laying muscles in behaving animals, we found that the muscles appear to be excited at a particular phase of each locomotor body bend. During the inactive phase, this rhythmic excitation infrequently evokes calcium transients or contraction of the egg-laying muscles. During the serotonin-triggered active phase, however, these muscles are more excitable and each body bend is accompanied by a calcium transient that drives twitching or full contraction of the egg-laying muscles. We found that ERG-null mutants lay eggs too frequently, and that ERG function is necessary and sufficient in the egg-laying muscles to limit egg laying. ERG K(+) channels localize to postsynaptic sites in the egg-laying muscle, and mutants lacking ERG have more frequent calcium transients and contractions of the egg-laying muscles even during the inactive phase. Thus ERG channels set postsynaptic excitability at a threshold so that further adjustments of excitability by serotonin generate two distinct behavioral states.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Identification and characterization of a novel class of atypical dopamine receptor agonists

PURPOSE

The D3 dopamine receptor exhibits tolerance and slow response termination (SRT) properties that are not exhibited by the closely-related D2 dopamine receptor. We previously demonstrated that the induction of tolerance elicits a unique conformational change in the D3 receptor. Here we tested the hypothesis that the tolerance and SRT properties of the D3 receptor are ligand-dependent.

METHODS

We used pharmacophore modeling and in silico screening approaches coupled with electrophysiological and biochemical methods to identify and functionally characterize the novel dopamine receptor agonists.

RESULTS

We identified cis-8-OH-PBZI (PBZI), FAUC73 and an additional novel compound, ES609, which although they are full D3 receptor agonists, do not induce the tolerance and SRT properties of the D3 receptor. In addition, PBZI has full intrinsic activity at D2L, is a partial agonist at D2S and exhibits functional selectivity at D4.2 dopamine receptors. ES609 is a partial agonist at D2S, D2L and D4.2 receptors, and exhibits functional selectivity at D2L and D4.2 dopamine receptors.

CONCLUSION

We have discovered a novel class of atypical dopamine receptor agonists that include three structurally dissimilar compounds. These new agonists will help determine the physiological and pathophysiological relevance of D3 receptor tolerance and SRT properties.

Regulation of striatal dopamine release by presynaptic auto- and heteroreceptors

Striatal dopamine neurotransmission is critical for normal voluntary movement, affect and cognition. Dysfunctions of its regulation are implicated in a broad range of behaviors and disorders including Parkinson's disease, schizophrenia and drug abuse. Extracellular dopamine levels result from a dynamic equilibrium between release and reuptake by dopaminergic terminals. Both processes are regulated by multiple mechanisms. Here we review data characterizing how dopamine levels are regulated by presynaptic autoreceptors and heteroreceptors, an area intensively investigated due to advances in real time electrochemical detection of extracellular dopamine, i.e., fast-scan cyclic voltammetry and amperometry, and the development of mutant mouse lines with deletions for specific receptors.

Molecular characterization of individual D3 dopamine receptor-expressing cells isolated from multiple brain regions of a novel mouse model

Among dopamine receptors, the expression and function of the D3 receptor subtype is not well understood. The receptor has the highest affinity for dopamine and many drugs that target dopamine receptors.In this paper, we examined, at the single cell level, the characteristics of D3 receptor-expressing cells isolated from different brain regions of male and female mice that were either 35 or 70 days old. The brain regions included nucleus accumbens, Islands of Calleja, olfactory tubercle,retrosplenial cortex, dorsal subiculum, mammillary body,amygdala and septum. The expression analysis was done in the drd3-enhanced green fluorescent protein transgenic mice that report the endogenous expression of D3 receptor mRNA. Using single cell reverse transcriptase PCR, we determined if the D3 receptor-expressing fluorescent cells in these mice were neurons or glia and if they were glutamatergic, GABAergic or catecholaminergic. Next, we determined if the fluorescent cells co-expressed the four other dopamine receptor subtypes, adenylate cyclase V(ACV) isoform, and three different isoforms of G protein coupled inward rectifier potassium (GIRK) channels. The results suggest that D3 receptor is expressed in neurons,with region-specific expression in glutamatergic and GABAergic neurons. The D3 receptor primarily coexpressed with D1 and D2 dopamine receptors with regional, sex and age-dependent differences in the coexpression pattern. The percentage of cells co-expressing D3 receptor and ACV or GIRK channels varied significantly by brain region, sex and age. The molecular characterization of D3 receptor-expressing cells in mouse brain reported here will facilitate the characterization of D(3) receptor function in physiology and pathophysiology.

Control of firing patterns through modulation of axon initial segment T-type calcium channels

Spontaneously active neurons typically fire either in a regular pattern or in bursts. While much is known about the subcellular location and biophysical properties of conductances that underlie regular spontaneous activity, less is known about those that underlie bursts. Here, we show that T-type Ca(2+) channels localized to the site of action potential initiation in the axon initial segment play a pivotal role in spontaneous burst generation. In auditory brainstem interneurons, axon initial segment Ca(2+) influx is selectively downregulated by dopaminergic signalling. This regulation has marked effects on spontaneous activity, converting the predominant mode of spontaneous activity from bursts to regular spiking. Thus, the axon initial segment is a key site, and dopamine a key regulator, of spontaneous bursting activity.

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Do caveolae have a role in the fidelity and dynamics of receptor activation of G-protein-gated inwardly rectifying potassium channels?

In atrial and nodal cardiac myocytes, M2 muscarinic receptors activate inhibitory G-proteins (G_i/o), which in turn stimulate G-protein-gated inwardly rectifying K⁺ channels through direct binding of the Gβγ subunit. Despite also releasing Gβγ, G_s-coupled receptors such as the β-adrenergic receptor are not able to prominently activate this current. An appealing hypothesis would be if components were sequestered in membrane domains such as caveolae/rafts. Using biochemical fractionation followed by Western blotting and/or radioligand binding experiments, we examined the distribution of the components in stable HEK293 and HL-1 cells, which natively express the transduction cascade. The channel, M2 muscarinic, and A1 adenosine receptors were located in noncaveolar/nonraft fractions. G_iα_1/2 was enriched in both caveolar/raft and noncaveolar/nonraft fractions. In contrast, G_sα was only enriched in caveolar/raft fractions. We constructed YFP-tagged caveolin-2 (YFP-Cav2) and chimeras with the M2 (M2-YFP-Cav2) and A1 (A1-YFP-Cav2) receptors. Analysis of gradient fractions showed that these receptor chimeras were now localized to caveolae-enriched fractions. Microscopy showed that M2-YFP and A1-YFP had a diffuse homogenous membrane signal. YFP-Cav2, M2-YFP-Cav2, and A1-YFP-Cav2 revealed a more punctuate pattern. Finally, we looked at the consequences for signaling. Activation via M2-YFP-Cav2 or A1-YFP-Cav2 revealed substantially slower kinetics compared with M2-YFP or A1-YFP and was reversed by the addition of methyl-β-cyclodextrin. Thus the localization of the channel signal transduction cascade in non-cholesterol rich domains substantially enhances the speed of signaling. The presence of G_sα solely in caveolae may account for signaling selectivity between G_i/o and G_s-coupled receptors.

Development of tolerance in D3 dopamine receptor signaling is accompanied by distinct changes in receptor conformation

The D3 but not D2 dopamine receptors exhibit a tolerance property in which agonist-induced D3 receptor response progressively decreases upon repeated agonist stimulation. We have previously shown that the D3 receptor tolerance property is not mediated by receptor internalization, persistent agonist binding or a decrease in receptor binding affinity. In this paper, we test the hypothesis that alterations in D3 receptor conformation underlie the tolerance property. Structural models of wild type and mutant human D3 receptors were generated using the beta adrenergic receptor crystal structure as a template. These models suggested that the agonist-bound D3 receptor undergoes conformational changes that could underlie its tolerance property. To experimentally assess changes in receptor conformation, we measured the accessibility of native cysteine residues present in the extracellular and transmembrane regions of the human D3 receptor to two different thiol-modifying biotinylating reagents. The accessibilities of the native cysteine residues present in the D3 receptor were assessed under control conditions, in the presence of agonist and under conditions that induced receptor tolerance. By comparing the accessibility of D3 receptor cysteine residues to hydrophobic and hydrophilic thiol-modifying biotinylating reagents, we show that the alteration of D3 receptor conformation during tolerance involves the net movement of cysteine residues into a hydrophobic environment. Our results show that the conformation state of the D3 receptor during tolerance is distinct from the conformation under basal and agonist-bound conditions. The results suggest that the D3 receptor tolerance property is mediated by conformational changes that may uncouple the receptor from G-protein signaling.

Evaluation of D2 and D3 dopamine receptor selective compounds on L-dopa-dependent abnormal involuntary movements in rats

A panel of novel D2 and D3 dopamine receptor selective antagonists, partial agonists and full agonists have been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in 6-hydroxydopamine (6-OHDA) unilaterally lesioned male Sprague Dawley rats, which is an animal model of L-dopa-induced dyskinesia (LID). LID is often observed in patients with Parkinson's Disease following chronic treatment with L-dopa. The intrinsic activity of these dopaminergic compounds was determined using a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype. For the initial experiments the 5-HT1A receptor selective partial agonist buspirone was used to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Two D2 dopamine receptor selective antagonists, SV 156 and SV 293, were evaluated and found to minimally attenuate AIM scores in these animals. Four members of our WC series of D3 dopamine receptor selective compounds of varying intrinsic activity at the D3 dopamine receptor subtype, WC 10, WC 21, WC 26 and WC 44, were also evaluated and found to attenuate AIM scores in a dose dependent manner. The in vivo efficacy of the compounds increased when they were administered simultaneously with L-dopa, as compared to when the compounds were administered 60 min prior to the L-dopa/benserazide. It was also found that the D3 receptor antagonist WC 10 could inhibit the involuntary movements after they had achieved maximum intensity. Unlike the D1-like dopamine receptor selective agonist SKF 81297 and the D2-like dopamine receptor agonist bromocriptine which can precipitate abnormal involuntary movements in these unilaterally lesioned animals, abnormal involuntary movements were not observed after administration of our D3 receptor selective agonist WC 44. In addition, we evaluated the effect of these four D3 dopamine receptor selective compounds for their effect on a) spontaneous locomotion and b) coordination and agility using a rotarod apparatus. We also used a cylinder test to assess the effect of L-dopa on spontaneous and independent use of each of the rat's forelimbs in the presence or absence of test compound. The results of these studies suggest that substituted phenylpiperazine D3 dopamine receptor selective compounds are potential pharmacotherapeutic agents for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Over-expression of the potassium channel Kir2.3 using the dopamine-1 receptor promoter selectively inhibits striatal neurons

Dysfunction of basal ganglia circuits underlies a variety of movement disorders and neuropsychiatric conditions. Selective control of the electrical activity of striatal outflow pathways by manipulation of ion channel function presents a novel therapeutic approach. Toward this end, we have constructed and studied in vitro an adenoviral gene transfer vector that employs the promoter region of the dopamine-1 receptor to drive expression of the inward rectifier K(+) channel Kir2.3. The use of this neuronal promoter confers cell-type specificity and a physiological level of trans-gene expression in rat primary striatal cultures. The electrophysiological properties were confirmed in transfected human embryonic kidney cells, in which an inwardly-rectifying, Cs(+)-sensitive current was measured by voltage clamp. Current clamp studies of transduced striatal neurons demonstrated an increase in the firing threshold, latency to first action potential and decrease in neuronal excitability. Neurotoxin-induced activation of c-Fos, a marker of neuronal activity, was blocked in transduced neurons indicating that the decrease in electrical excitability was physiologically significant. When used in vivo, this strategy may have the potential to positively impact movement disorders by selectively changing activity of neurons belonging to the direct striatal pathway, characterized by the expression of dopamine-1 receptors.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.

p75 neurotrophin receptor mediates neuronal cell death by activating GIRK channels through phosphatidylinositol 4,5-bisphosphate

The pan neurotrophin receptor p75(NTR) signals programmed cell death both during nervous system development and after neural trauma and disease in the adult. However, the molecular pathways by which death is mediated remain poorly understood. Here, we show that this cell death is initiated by activation of G-protein-coupled inwardly rectifying potassium (GIRK/Kir3) channels and a consequent potassium efflux. Death signals stimulated by neurotrophin-mediated cleavage of p75(NTR) activate GIRK channels through the generation and binding of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2/PIP2] to GIRK channels. Both GIRK channel activity and p75(NTR)-mediated neuronal death are inhibited by sequestration of PtdIns(4,5)P2 and application of GIRK channel inhibitors, whereas pertussis toxin treatment has no effect. Thus, p75(NTR) activates GIRK channels without the need for G(i/o)-proteins. Our results demonstrate a novel mode of activation of GIRK channels, representing an early step in the p75(NTR)-mediated cell death pathway and suggesting a function for these channels during nervous system development.

Effects of the dopamine D3 receptor (DRD3) gene polymorphisms on risperidone response: a pharmacogenetic study

Previous observations of the anatomical distribution and pharmacological profile of the dopamine D(3) receptor (DRD3) have indicated its potential role in antipsychotic drug action. Risperidone, an effective first-line atypical antipsychotic agent, exhibits a relatively high affinity for this receptor. Recent studies have reported an association of the Ser9Gly polymorphism in the DRD3 gene with therapeutic response to risperidone, but the results were inconsistent. We therefore postulated that the Ser9Gly polymorphism might be in linkage disequilibrium with an undetected variant that exerts a direct influence on risperidone efficacy. The present study genotyped eight single nucleotide polymorphisms (SNPs) distributed throughout the DRD3 gene and examined five of these for association with treatment outcome, following an 8-week period of risperidone monotherapy in 130 schizophrenic patients from mainland China. Clinical symptoms were assessed before and after the treatment period, using the Brief Psychiatry Rating Scale (BPRS). The confounding effects of non-genetic factors were estimated and the baseline symptom score was included as a covariate for adjustment. Neither was any association observed between the five polymorphisms and improvement in total BPRS scores nor was any combined effect of these variants detected in the haplotype analysis. The current results indicate that genetic variations within the DRD3 gene may not contribute significantly to interindividual differences in the therapeutic efficacy of risperidone.

The tolerance property of human D3 dopamine receptor is determined by specific amino acid residues in the second cytoplasmic loop

The D2 and D3 dopamine receptor subtypes are structurally homologous and couple to the same signal transduction pathways. Nevertheless, their evolutionary conservation suggests that the two subtypes might exhibit unique signaling characteristics. We previously determined that D3 but not D2S dopamine receptor exhibits a tolerance property in which the D3 receptor-activated G-protein coupled inward rectifier potassium currents progressively decreases upon repeated agonist stimulation. In this paper, using AtT-20 neuroendocrine cells stably expressing either human D3 or D2S receptor, we show that the tolerance property is also observed in the D3 receptor-adenylyl cyclase and D3 receptor-mitogen-activated protein kinase signaling pathways. We have previously shown that the second cytoplasmic loop of D3 receptor is required for tolerance. Here, using site-directed mutagenesis, we identified the specific amino acids in the D3 second cytoplasmic loop involved in the tolerance property. The results show that substitution of a non-conserved cysteine residue at position 147 with positively-charged lysine or arginine residues abolishes tolerance. Interestingly, the cysteine 147 residue is embedded in a putative phosphorylation site adjacent to two serine residues. Mutation of these serine residues to alanine also attenuates tolerance. Taken together, these structural studies suggest a role for phosphorylation in D3 receptor tolerance property.

The dopaminergic system in hypertension

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport, vascular smooth muscle contractility and production of reactive oxygen species and by interacting with the renin–angiotensin and sympathetic nervous systems. Dopamine receptors are classified into D1-like (D1 and D5) and D2-like (D2, D3 and D4) subtypes based on their structure and pharmacology. Each of the dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central and/or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. This review summarizes the physiology of the different dopamine receptors in the regulation of blood pressure, and the relationship between dopamine receptor subtypes and hypertension.

Ca2+-activated IK1 channels associate with lipid rafts upon cell swelling and mediate volume recovery

Restoration of cell volume in the continued presence of osmotic stimuli is essential, particularly in hepatocytes, which swell upon nutrient uptake. Responses to swelling involve the Ca2+-dependent activation of K+ channels, which promote fluid efflux to drive volume recovery; however, the channels involved in hepatocellular volume regulation have not been identified. We found that hypotonic exposure of HTC hepatoma cells evoked the opening of 50 pS K+-permeable channels, consistent with intermediate conductance (IK) channels. We isolated from rat liver and HTC cells a cDNA with sequence identity to the coding region of IK1. Swelling-activated currents were inhibited by transfection with a dominant interfering IK1 mutant. The IK channel blockers clotrimazole and TRAM-34 inhibited whole cell swelling-activated K+ currents and volume recovery. To determine whether IK1 underwent volume-sensitive localization, we expressed a green fluorescent protein fusion of IK1 in HTC cells. The localization of IK1 was suggestive of distribution in lipid rafts. Consistent with this, there was a time-dependent increase in colocalization between IK1 and the lipid raft ganglioside GM1 on the plasma membrane, which subsequently decreased with volume recovery. Pharmacological disruption of lipid rafts altered the plasma membrane distribution of IK1 and inhibited volume recovery after hypotonic exposure. Collectively, these findings support the hypothesis that IK1 regulates compensatory responses to hepatocellular swelling and suggest that regulation of cell volume involves coordination of signaling from lipid rafts with IK1 function.

Convergent analysis of cDNA and short oligomer microarrays, mouse null mutants and bioinformatics resources to study complex traits

Gene expression data sets have recently been exploited to study genetic factors that modulate complex traits. However, it has been challenging to establish a direct link between variation in patterns of gene expression and variation in higher order traits such as neuropharmacological responses and patterns of behavior. Here we illustrate an approach that combines gene expression data with new bioinformatics resources to discover genes that potentially modulate behavior. We have exploited three complementary genetic models to obtain convergent evidence that differential expression of a subset of genes and molecular pathways influences ethanol-induced conditioned taste aversion (CTA). As a first step, cDNA microarrays were used to compare gene expression profiles of two null mutant mouse lines with difference in ethanol-induced aversion. Mice lacking a functional copy of G protein-gated potassium channel subunit 2 (Girk2) show a decrease in the aversive effects of ethanol, whereas preproenkephalin (Penk) null mutant mice show the opposite response. We hypothesize that these behavioral differences are generated in part by alterations in expression downstream of the null alleles. We then exploited the WebQTL databases to examine the genetic covariance between mRNA expression levels and measurements of ethanol-induced CTA in BXD recombinant inbred (RI) strains. Finally, we identified a subset of genes and functional groups associated with ethanol-induced CTA in both null mutant lines and BXD RI strains. Collectively, these approaches highlight the phosphatidylinositol signaling pathway and identify several genes including protein kinase C beta isoform and preproenkephalin in regulation of ethanol- induced conditioned taste aversion. Our results point to the increasing potential of the convergent approach and biological databases to investigate genetic mechanisms of complex traits.

Subunit stoichiometry of the Kir1.1 channel in proton-dependent gating

Kir1.1 channel regulates membrane potential and K+ secretion in renal tubular cells. This channel is gated by intracellular protons, in which a lysine residue (Lys80) plays a critical role. Mutation of the Lys80 to a methionine (K80M) disrupts pH-dependent channel gating. To understand how an individual subunit in a tetrameric channel is involved in pH-dependent channel gating, we performed these studies by introducing K80M-disrupted subunits to tandem tetrameric channels. The pH sensitivity was studied in whole-cell voltage clamp and inside-out patches. Homomeric tetramers of the wild-type (wt) and K80M-disrupted channels showed a pH sensitivity almost identical to that of their monomeric counterparts. In heteromeric tetramers and dimers, pH sensitivity was a function of the number of wt subunits. Recruitment of the first single wt subunit shifts the pK(a) greatly, whereas additions of any extra wt subunit had smaller effects. Single-channel analysis revealed that the tetrameric channel with two or more wt subunits showed one substate conductance at approximately 40% of the full conductance, suggesting that four subunits act as two pairs. However, three and four substates of conductance were seen in the tetrameric wt-3K80M and 4K80M channels. Acidic pH increased long-time closures when there were two or more wt subunits. Disruption of more than two subunits led to flicking activity with appearance of a new opening event and loss of the long period of closures. Interestingly, the channel with two wt subunits at diagonal and adjacent configurations showed the same pH sensitivity, substate conductance, and long-time closure. These results thus suggest that one functional subunit is sufficient to act in the pH-dependent gating of the Kir1.1 channel, the channel sensitivity to pH increases with additional subunits, the full pH sensitivity requires contributions of all four subunits, and two subunits may be coordinated in functional dimers of either trans or cis configuration.

D3 dopamine receptor activates phospholipase D through a pertussis toxin-insensitive pathway

Within the dopamine receptor family, the D(3) dopamine receptor's function remains inadequately described. The D(3) receptor has been shown to couple to inhibition of adenylyl cyclase, stimulation of mitogenesis, and regulation of K(+) and Ca(2+) currents, all in a pertussis toxin (PTX)-sensitive manner. Here we report D(3) receptor activation of the phospholipase D (PLD) enzyme in HEK 293 cells heterologously expressing the human D(3) receptor. Activation by agonist is dose dependent and displays the pharmacology expected of the D(3) receptor. The D(3) receptor specific antagonists AJ-76 and U99194A ablated the increase in activity by the preferring D(3) agonist (+) 7-OH DPAT. In addition, the D(3) receptor-mediated activation of PLD is not mediated by G-proteins of the G(i)/G(o) family, as pretreatment with PTX had no effect. PLD activation is a novel finding for the D(3) receptor, and is the first example of an effector system where D(3) signals without G(i)/G(o) protein intermediates.

Signaling Mechanisms of the D3Dopamine Receptor

A substantial body of evidence shows the capacity of the dopamine D3 receptor to couple functionally to G proteins when expressed in an appropriate milieu in heterologous expression systems. In these systems, activation of D3 receptors inhibits adenylate cyclase, modulates ion flow through potassium and calcium channels, and activates kinases, most notably mitogen-activated protein kinase. Coupling to Gi/Go is implicated in many of these effects, but other G proteins may contribute. Studies with chimeric receptors implicate the third intracellular loop in the mediation of agonist-induced signal transduction. Finally, D3-preferring drugs modulate expression of c-fos in neuronal cultures and brain. Signaling mechanisms of the D3 receptor in brain, however, remain to be definitively determined.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Functional selectivity of D2 receptor ligands in a Chinese hamster ovary hD2L cell line: evidence for induction of ligand-specific receptor states

There are now several examples of single G protein-coupled receptors to which binding of specific agonists causes differential effects on the associated signaling pathways. The dopamine D(2) receptor is of special importance because the selective activation of functional pathways has been shown both in vitro and in situ. For this reason, the present work characterized a series of rigid D(2) agonists in Chinese hamster ovary cells transfected with the human D(2L) receptor using three distinct functional endpoints: inhibition of cAMP synthesis, stimulation of mitogen-activated protein (MAP) kinase phosphorylation, and activation of G protein-coupled inwardly rectifying potassium channels (GIRKs). In this system, S-propylnorapomorphine (SNPA), R-propylnorapomorphine (RNPA), dihydrexidine (DHX), dinapsoline (DNS), and dinoxyline (DNX) all inhibited forskolin-stimulated adenylate cyclase activity to the same extent as the prototypical D(2) agonist quinpirole (QP). The rank order of potency was the following: RNPA >> QP = DNX > SNPA > DHX = DNS. For MAP kinase phosphorylation, DHX, DNS, DNX, and RNPA had efficacy similar to QP, whereas SNPA was a partial agonist. The rank order of potency for MAP kinase phosphorylation was RNPA >> QP = DNX > DHX > DNS = SNPA. DNX activated GIRK channels to the same extent as QP, whereas DHX and DNS were partial agonists, and RNPA and SNPA caused no appreciable activation. These findings indicate that DHX, DNS, RNPA, and SNPA have atypical functional properties at the hD(2L) receptor and display different patterns of functional selectivity. We hypothesize that this functional selectivity may be a result of ligand induction of specific conformations of the D(2L) receptor that activate only selected signaling pathways.

Identification and characterization of novel properties of the human D3 dopamine receptor

Among dopamine receptors, the function and properties of the D3 receptor subtype are poorly understood. Here we report the identification and characterization of two unique properties of the human D3 receptor. The D3 receptor exhibits a tolerance property wherein the magnitude of the second agonist-induced response is reduced by 60% compared to the first response and progressively decreases upon repeated agonist application. In addition, unlike the D2 dopamine receptor, the D3 receptor response terminates 15-fold more slowly upon agonist removal. Using D3/D2S chimeric receptors, we demonstrate that D3 receptor tolerance property is mediated by a novel conformational mechanism involving the D3 receptor second cytoplasmic region. The slow response termination rate property requires the third cytoplasmic region and is due to the high affinity of the D3 receptor for ligand as well as its unique G-protein signaling mechanism.

Dopamine D 1 Receptor Augmentation of D 3 Receptor Action in Rat Aortic or Mesenteric Vascular Smooth Muscles

Dopamine is an important modulator of blood pressure, in part, by regulating vascular resistance. To test the hypothesis that D 1 and D 3 receptors interact in vascular smooth muscle cells, we studied A10 cells, a rat aortic smooth muscle cell line, and rat mesenteric arteries that express both dopamine receptor subtypes. Fenoldopam, a D 1 -like receptor agonist, increased both D 1 and D 3 receptor protein in a time-dependent and a concentration-dependent manner in A10 cells. The effect of fenoldopam was specific because a D 1 -like receptor antagonist, SCH23390 (10 −7 M/24 h), completely blocked the stimulatory effect of fenoldopam (10 −7 M/24 h) (D 3 receptor: control=21±1 density units [DU]); SCH23390=23±2 DU; fenoldopam=33±2 DU; fenoldopam+SCH23390=23±2 DU; n=10). D 1 and D 3 receptors physically interacted with each other because fenoldopam (10 −7 M/24 h) increased D 1 /D 3 receptor coimmunoprecipitation (35±5 versus 65±5 DU; n=8). A D 3 receptor agonist, PD128907, relaxed mesenteric arterial rings independent of the endothelium, effects that were blocked by a D 3 receptor antagonist, U99194A. Costimulation of D 1 and D 3 receptors led to additive vasorelaxation. We conclude that the D 1 receptor regulates the D 3 receptor by physical interaction and receptor expression. D 1 receptor stimulation augments D 3 receptor vasorelaxant effects. An interaction of D 1 and D 3 receptors may be involved in the regulation of blood pressure.

Distribution and functional properties of human KCNH8 (Elk1) potassium channels

The Elk subfamily of the Eag K+ channel gene family is represented in mammals by three genes that are highly conserved between humans and rodents. Here we report the distribution and functional properties of a member of the human Elk K+ channel gene family, KCNH8. Quantitative RT-PCR analysis of mRNA expression patterns showed that KCNH8, along with the other Elk family genes, KCNH3 and KCNH4, are primarily expressed in the human nervous system. KCNH8 was expressed at high levels, and the distribution showed substantial overlap with KCNH3. In Xenopus oocytes, KCNH8 gives rise to slowly activating, voltage-dependent K+ currents that open at hyperpolarized potentials (half-maximal activation at -62 mV). Coexpression of KCNH8 with dominant-negative KCNH8, KCNH3, and KCNH4 subunits led to suppression of the KCNH8 currents, suggesting that Elk channels can form heteromultimers. Similar experiments imply that KCNH8 subunits are not able to form heteromultimers with Eag, Erg, or Kv family K+ channels.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

D3 dopamine autoreceptors do not activate G-protein-gated inwardly rectifying potassium channel currents in substantia nigra dopamine neurons

Substantia nigra (SN) dopamine neurons express D2 and D3 dopamine autoreceptors. A physiological role for the D3 receptor has not been identified, but an activation of G-protein-gated inwardly rectifying potassium (GIRK; also known as Kir3) channels is strongly implicated because D3 receptors activate channels composed of GIRK2 subunits in cell lines. We confirmed that acutely dissociated SN dopamine neurons indeed contain D3 and GIRK2 subunit mRNA using single-cell RT-PCR. We then tested whether D3 receptors activate GIRK currents in SN dopamine neurons by comparing acutely dissociated neurons from D2-/- receptor knock-out and congenic wild-type mice. In nearly all (14 of 15) wild-type SN dopamine neurons, the D2/D3 agonist quinpirole activated GIRK currents that were blocked by cesium. Quinpirole, however, elicited no GIRK currents in any SN dopamine neuron (0 of 13) derived from D2-/- receptor knock-out mice. The absence of quinpirole response was not caused by a lack of GIRK activity, because the GABAB receptor agonist baclofen continued to elicit these currents in the mutant neurons. Thus, it appears that D3 activation of GIRK currents in SN neurons does not occur or is exceedingly rare.

D3 dopamine autoreceptors do not activate G-protein-gated inwardly rectifying potassium channel currents in substantia nigra dopamine neurons

Substantia nigra (SN) dopamine neurons express D2 and D3 dopamine autoreceptors. A physiological role for the D3 receptor has not been identified, but an activation of G-protein-gated inwardly rectifying potassium (GIRK; also known as Kir3) channels is strongly implicated because D3 receptors activate channels composed of GIRK2 subunits in cell lines. We confirmed that acutely dissociated SN dopamine neurons indeed contain D3 and GIRK2 subunit mRNA using single-cell RT-PCR. We then tested whether D3 receptors activate GIRK currents in SN dopamine neurons by comparing acutely dissociated neurons from D2-/- receptor knock-out and congenic wild-type mice. In nearly all (14 of 15) wild-type SN dopamine neurons, the D2/D3 agonist quinpirole activated GIRK currents that were blocked by cesium. Quinpirole, however, elicited no GIRK currents in any SN dopamine neuron (0 of 13) derived from D2-/- receptor knock-out mice. The absence of quinpirole response was not caused by a lack of GIRK activity, because the GABAB receptor agonist baclofen continued to elicit these currents in the mutant neurons. Thus, it appears that D3 activation of GIRK currents in SN neurons does not occur or is exceedingly rare.

Classic D1 dopamine receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) directly inhibits G protein-coupled inwardly rectifying potassium channels

R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) is a widely used, highly selective antagonist of D1 dopamine receptors. While investigating the crosstalk between D1 and D3 dopamine receptor signaling pathways, we discovered that in addition to being a D1 receptor antagonist, SCH23390 and related compounds inhibit G protein-coupled inwardly rectifying potassium (GIRK) channels. We present evidence that SCH23390 blocks endogenous GIRK currents induced by either somatostatin or D3 dopamine receptors in AtT-20 cells (IC50, 268 nM). The inhibition is receptor-independent because constitutive GIRK currents in Chinese hamster ovary cells expressing only GIRK channels are also blocked by SCH23390. The inhibition of GIRK channels is somewhat selective because members of the closely related Kir2.0 family of inwardly rectifying potassium channels, as well as various endogenous cationic currents present in AtT-20 cells, are not affected. In addition, in current clamp recordings, SCH23390 can depolarize the membrane potential and induce AtT-20 cells to fire action potentials, indicating potential physiological significance of the GIRK channel inhibition. To identify the chemical features that contribute to GIRK channel block, we tested several structurally related compounds [SKF38393, R-(+)-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (nor-methyl-SCH23390), and R-(+)-2,3,4,5-tetrahydro-8-iodo-3-methyl-5-phenyl-1H-3-benzazepin-7-ol hydrochloride (iodo-SCH23390)], and our results indicate that the halide atom is critical for blocking GIRK channels. Taken together, our results suggest that SCH23390 and related compounds might provide the basis for designing novel GIRK channel-selective blockers. Perhaps more importantly, some studies that have exclusively used SCH23390 to probe D1 receptor function or as a diagnostic of D1 receptor involvement may need to be reevaluated in light of these results.

The expression of G-protein-gated inwardly rectifying K+ channels GIRK1 and GIRK2 mRNAs in the supraoptic nucleus of the rat and possible role involved

The expression of G-protein-gated inwardly rectifying K channels subunits GIRK1 and GIRK2 mRNAs in the supraoptic nucleus (SON) was investigated in the rat by in situ hybridization with non-radioactive dig-labeled cRNA probes. Double-labeled methods were used to study the co-localization of GIRK1 and 2 and oxytocin (OT) and vasopressin (AVP) in the SON. The present study revealed wide and intense expression of GIRK1 and GIRK2 mRNAs with high overlapping in the SON, indicating the heterologous channel of GIRK1/GIRK2 was a major functional channel in the SON. Given that 100% of OT-positive and 95% of (AVP)-positive neurons in the SON expressed GIRK1/GIRK2 mRNAs, it is possible that GIRK1/GIRK2 channel, activated through G-protein coupled receptors, may be involved in the inhibitory regulation of the release of OT and AVP from the SON.