Dopamine D2 receptor stimulation of Na+/H+ exchange assessed by quantification of extracellular acidification

Unveiling the Differences in Signaling and Regulatory Mechanisms between Dopamine D2 and D3 Receptors and Their Impact on Behavioral Sensitization

Dopamine receptors are classified into five subtypes, with D2R and D3R playing a crucial role in regulating mood, motivation, reward, and movement. Whereas D2R are distributed widely across the brain, including regions responsible for motor functions, D3R are primarily found in specific areas related to cognitive and emotional functions, such as the nucleus accumbens, limbic system, and prefrontal cortex. Despite their high sequence homology and similar signaling pathways, D2R and D3R have distinct regulatory properties involving desensitization, endocytosis, posttranslational modification, and interactions with other cellular components. In vivo, D3R is closely associated with behavioral sensitization, which leads to increased dopaminergic responses. Behavioral sensitization is believed to result from D3R desensitization, which removes the inhibitory effect of D3R on related behaviors. Whereas D2R maintains continuous signal transduction through agonist-induced receptor phosphorylation, arrestin recruitment, and endocytosis, which recycle and resensitize desensitized receptors, D3R rarely undergoes agonist-induced endocytosis and instead is desensitized after repeated agonist exposure. In addition, D3R undergoes more extensive posttranslational modifications, such as glycosylation and palmitoylation, which are needed for its desensitization. Overall, a series of biochemical settings more closely related to D3R could be linked to D3R-mediated behavioral sensitization.

Decreased Brain pH and Pathophysiology in Schizophrenia

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.

Dopamine Receptors and the Kidney: An Overview of Health- and Pharmacological-Targeted Implications

The dopaminergic system can adapt to the different physiological or pathological situations to which the kidneys are subjected throughout life, maintaining homeostasis of natriuresis, extracellular volume, and blood pressure levels. The role of renal dopamine receptor dysfunction is clearly established in the pathogenesis of essential hypertension. Its associations with other pathological states such as insulin resistance and redox balance have also been associated with dysfunction of the dopaminergic system. The different dopamine receptors (D1-D5) show a protective effect against hypertension and kidney disorders. It is essential to take into account the various interactions of the dopaminergic system with other elements, such as adrenergic receptors. The approach to therapeutic strategies for essential hypertension must go through the blocking of those elements that lead to renal vasoconstriction or the restoration of the normal functioning of dopamine receptors. D1-like receptors are fundamental in this role, and new therapeutic efforts should be directed to the restoration of their functioning in many patients. More studies will be needed to allow the development of drugs that can be targeted to renal dopamine receptors in the treatment of hypertension.

Dopamine D3 receptor agonists as pharmacological tools

Dysregulation of the dopaminergic innervation in the central nervous system plays a key role in different neurological disorders like Parkinson´s disease, restless legs syndrome, schizophrenia etc. Although dopamine D3 receptors have been recognized as an important target in these diseases, their full pharmacological properties need further investigations. With focus on dopamine D3 receptor full agonists, this review has divided the ergoline and non-ergoline ligands in dissimilar chemical subclasses describing their pharmacodynamic properties on different related receptors, on species differences and their functional properties on different signaling mechanism. This is combined with a short description of structure-activity relationships for each class. Therefore, this overview should support the rational choice for the optimal compound selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies.

A novel microphysiometer based on high sensitivity LAPS and microfluidic system for cellular metabolism study and rapid drug screening

This study presents a novel microphysiometer for studying the mechanism of cellular metabolism and drug effect. Based on the photocurrent amplification of light-addressable potentiometric sensor (LAPS), the constant voltage detection mode was introduced to enhance the detection sensitivity to replace the conventional constant current mode with the slow feedback rate. The photocurrent amplification of LAPS was improved by developing the sensor structure and fabrication processes. The sensor unit with microfluidic system was designed to detect the concentration change of cellular acidic metabolites in the extracellular microenvironment rapidly. Characteristic test experiments and cellular metabolism experiments were carried out to determine the performance of microphysiometer. The result showed that sensitivity of microphysiometer is significantly enhanced to sense the fluctuation of cellular metabolism rapidly and sensitively in real-time detection of living cells under physiological condition. With these improvements, the novel microphysiometer holds promise as a utility platform for studying cellular metabolism and evaluating drug effect.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Characterization of functional roles of DRY motif in the 2nd intracellular loop of dopamine D2 and D3 receptors

Dopamine D(2)R and D(3)R (D(2)R, D(3)R) show very high sequence homology and employ virtually identical signaling pathways even though D(2)R is 2 approximately 5 times more active. Among the structural motifs identified, a triplet sequence, Asp-Arg-Tyr (DRY motif), plays critical roles in the determination of receptor conformations for signaling and intracellular trafficking of G protein-coupled receptors by forming intramolecular interactions. Thus, it is possible that different signaling efficiencies of D(2)R and D(3)R might be caused by the receptor activation levels stabilized by their own DRY motifs. In this study, the Arg and Asp residues of D(2)R and D(3)R were mutated, and resulting changes in their signaling and intracellular trafficking properties were comparatively studied. Mutation of the Arg residues of D(2)R and D(3)R abolished their signaling but differently affected their intracellular localizations. The wildtype and R132H-D(2)R were expressed mainly on the plasma membrane. On the other hand, compared with the wildtype D(3)R, a substantial amount of R128H-D(3)R was localized intracellularly. The expression of receptor proteins on the plasma membrane and their signaling efficiencies were more drastically affected by the mutation of the Asp residue of D(3)R than D(2)R. Therefore, it was concluded that the different levels of conformational strain exerted by the DRY motif might partly determine the quantitative differences in the signaling efficiencies between D(2)R and D(3)R.

Synthesis and characterization of selective dopamine D2 receptor antagonists

A series of indole compounds have been prepared and evaluated for affinity at D2-like dopamine receptors using stably transfected HEK cells expressing human D2, D3, or D4 dopamine receptors. These compounds share structural elements with the classical D2-like dopamine receptor antagonists, haloperidol, N-methylspiperone, and benperidol. The compounds that share structural elements with N-methylspiperone and benperidol bind non-selectively to the D2 and D3 dopamine receptor subtypes. However, several of the compounds structurally similar to haloperidol were found to (a) bind to the human D2 receptor subtype with nanomolar affinity, (b) be 10- to 100-fold selective for the human D2 receptor compared to the human D3 receptor, and (c) bind with low affinity to the human D4 dopamine receptor subtype. Binding at sigma (sigma) receptor subtypes, sigma1 and sigma2, were also examined and it was found that the position of the methoxy group on the indole was pivotal in both (a) D2 versus D3 receptor selectivity and (b) affinity at sigma1 receptors. Adenylyl cyclase studies indicate that our indole compounds with the greatest D2 receptor selectivity are neutral antagonists at human D2 dopamine receptor subtypes. With stably transfected HEK cells expressing human D2 (hD2-HEK), these compounds (a) have no intrinsic activity and (b) attenuated quinpirole inhibition of adenylyl cyclase. The D2 receptor selective compounds that have been identified represent unique pharmacological tools that have potential for use in studies on the relative contribution of the D2 dopamine receptor subtypes in physiological and behavioral situations where D2-like dopaminergic receptor involvement is indicated.

Algorithmically designed peptides ameliorate behavioral defects in animal model of ADHD by an allosteric mechanism

This study exemplifies the use of three ADHD-relevant methodological innovations. (1) The use of novel, patented, computational peptide design techniques to generate peptides targeting the extra-cellular and para-transmembrane amino acid loops of the putatively ADHD-involved, D(2) dopamine receptor, D(2)DAR; (2) experimental evidence that these peptides in L-amino acid/ortho ordered or D-amino acid/reverse ordered (retro-inverso), D(2)DAR, hydrophobic eigenmode matched forms, evoked positive allosteric and indirect agonist influences on in vitro stably receptor transfected CHO and LtK cells and on in vivo, brain mediated activity; (3) a representative 15 residue all-D-amino acid, D(2) mode matched peptide, given parenterally, was found to "repair" a key aberrant ADHD behavioral characteristic in a standard animal model of ADHD, the Spontaneously Hypertensive Rat, SHR, relative to its progenitor species control, the Wistar-Kyoto rat, WKY. The representative, retro-inverso peptide, all-D-LLYKNKPRYPKRNRE, reversed SHR's relative deficiency in sensory motor gating (pre-pulse inhibition, PPI) while leaving SHR's nonselective attention (rearings), impulsive behavior (time in center), and activity level (timed total motor behavior) unchanged. Amphetamine also reversed SHRs sensory gating defect, but with significant increases in nonselective attention, impulsivity and hyperactivity. These preliminary results suggest the possibility of a new, "softer" pharmacological approach to ADHD: hydrophobic mode matched peptide allosteric augmentation of the activity of indigenous dopamine with respect to D(2)DAR mediated function, in place of stimulant drug-induced presynaptic dopamine release or impairment of dopamine uptake.

Characterization of the B2 receptor and activity of bradykinin analogs in SHP-77 cell line by Cytosensor microphysiometer

The Cytosensor microphysiometer device (Molecular Devices, Sunnyvale, CA) is capable of measuring the rate at which cells acidify their environment in response to ligand-receptor binding. By measuring the extracellular acidification response (ECAR) we characterized some aspects of ligand-B2 receptor interaction in SHP-77 cell line. SHP-77 cells maximally acidified their environment within 30 s after the exposure to bradykinin (BK) or the BK agonist, B9972, with the maximum effect seen at a ligands concentration of 1 microM. Fetal bovine serum (FBS) modulated the binding of BK or B9972, showing that B9972 is a partial agonist. In addition, the binding of BK agonist or antagonist to the B2 receptor showed different ECAR and different interaction with other intracellular and plasma membrane proteins. Our microphysiometrical results showed that two parameters, antagonist binding affinity (pD2) and antagonist potency (pIC50) are required to characterize BK antagonist activity for the B2 receptor in the SHP-77 cell line. The previously used parameter of B2 antagonist activity, pA2, had high variation and poor correlation with the inhibition of SHP-77 cell growth in vitro and suppression of tumor growth when SHP-77 cells were injected to mice. Our results permit us to conclude that BK agonists and antagonists differ in their interactions with the B2 receptor and consequently elicit different cell responses. Based on our results, we have developed a new microphysiometrical assay for analyzing the activity of BK agonists and antagonist in SHP-77 cells, which may facilitate the discovery of new potent anticancer drugs.

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.

Direct and biochemical interaction between dopamine D3 receptor and elongation factor-1Bbetagamma

Novel signaling components of dopamine D3 receptor (D3R) were searched using yeast two-hybrid system, and the gamma subunit of elongation Factor-1B (eEF1Bgamma) was found to interact with D3R. This interaction was observed specifically between eEF1Bgamma and D3R but not with D2R or D4R. Immunocytochemical studies showed that D3R and eEF1Bgamma form clusters on the plasma membrane and their co-localization was evident in these clusters. The beta subunit of eEF1B (eEF1Bbeta), which forms a tight complex with eEF1Bgamma, was phosphorylated on serine residues in response to the stimulation of D3R. Phosphorylation of eEF1Bbeta was insensitive to pertussis toxin or wortmannin, however, stimulation of cellular protein kinase C (PKC) directly phosphorylated eEF1Bbeta and depletion of PKC abolished D3R-mediated phosphorylation of eEF1Bbeta. These results suggest the involvement of PKC, but not Gi/o proteins or phosphatidylinositol 3-kinase, in D3R-mediated phosphorylation of eEF1Bbeta. Stimulation of D3R did not activate PKC, but the activation of PKC resulted in the phosphorylation of D3R. These results show that PKC has a permissive role for the D3R-mediated phosphorylation of eEF1Bbeta, and suggest that PKC could modulate the mutual interaction between two protein by phosphorylating both D3R and eEF1Bbeta. Therefore, the cellular PKC level would be important for the D3R-mediated modulation of eEF1B, and for their cellular regulations such as protein synthesis or cellular proliferation.

Pepstatin A induces extracellular acidification distinct from aspartic protease inhibition in microglial cell lines

The extrusion of protons is considered a very general parameter of the activation of many kinds of membrane or intracellular molecules, such as receptors, ion channels, and enzymes. We found that pepstatin A caused a reproducible, concentration-related increase in the extracellular acidification rate in two microglial cell lines, Ra2 and 6-3. Washing abolished pepstatin A-induced acidification immediately. However, pepstatin A did not cause the extracellular acidification in other cell types, such as CHO, C6 glioma, and NIH3T3 cells. These observations strongly suggest that pepstatin A interacts with certain membrane proteins specific to both Ra2 and 6-3 cells from outside. N-methylmaleimide and N,N'-dicyclohexylcarbodiimide, inhibitors of H(+)-ATPase, were found to reduce pepstatin A-induced response strongly, while bafilomycin A1, a vacuolar H(+)-ATPase inhibitor, vanadate, a P-type H(+)-ATPase inhibitor, and NaN3, an F1 ATPase inhibitor, virtually did not. 5-(N-ethyl-N-isopropyl) amiloride, an inhibitor of Na(+)/H(+) exchanger isoform 1, greatly enhanced pepstatin-induced response, while amiloride did not. Zn(2+), a voltage-dependent proton channel blocker, did not affect pepstatin-induced response neither. Staurosporine, a nonspecific inhibitor of protein kinase C, inhibited pepstatin A-induced response, while chelerythrine, more selective inhibitor of protein kinase C, greatly enhanced it. H-7 and H-8 did not affected the response. These findings suggest that pepstatin A induces extracellular acidification in microglia cell lines, Ra2 and 6-3, through an N-methylmaleimide- and N,N'-dicyclohexylcarbodiimide-sensitive, but bafilomycin A1-insensitive, ATPase, which seems to be distinct from protein kinase C-dependent process.

Cellular and behavioral effects of D2 dopamine receptor hydrophobic eigenmode-targeted peptide ligands

Patterns in G-protein-coupled receptors' hydrophobically transformed amino-acid sequences can be computationally characterized as hierarchies of autocorrelation waves, "hydrophobic eigenmodes", using autocovariance matrix decomposition and all poles power spectral and wavelet transformations. L- or D-amino acid (retro-inverso) 12-18 residue peptides targeting these modes can be designed using eigenvector templates derived from these computations. In all, 12 human long-form D(2) dopamine receptor eigenmode-targeted 15 mer peptides were designed, synthesized, and shown to modulate and/or indirectly activate the extracellular acidification response, EAR, in stably receptor-transfected CHO and LtK cells, with an 83% hit rate. Representative L- and D-amino-acid retro-inverso peptides injected bilaterally in the nucleus accumbens demonstrated changes in rat exploratory behavior and prepulse inhibition similar to those observed following parenteral amphetamine. In contrast with geometric models used for ligand design, such as pharmacophores, the hydrophobic eigenmode approach to lead modulatory peptide design targets hydrophobic eigenmode-bearing subsequences, including those not visible from X-ray and NMR studies such as extracellular segments and loops.

Differential regulation of the dopamine D2 and D3 receptors by G protein-coupled receptor kinases and beta-arrestins

The D(2) and D(3) receptors (D(2)R and D(3)R), which are potential targets for antipsychotic drugs, have a similar structural architecture and signaling pathway. Furthermore, in some brain regions they are expressed in the same cells, suggesting that differences between the two receptors might lie in other properties such as their regulation. In this study we investigated, using COS-7 and HEK-293 cells, the mechanism underlying the intracellular trafficking of the D(2)R and D(3)R. Activation of D(2)R caused G protein-coupled receptor kinase-dependent receptor phosphorylation, a robust translocation of beta-arrestin to the cell membrane, and profound receptor internalization. The internalization of the D(2)R was dynamin-dependent, suggesting that a clathrin-coated endocytic pathway is involved. In addition, the D(2)R, upon agonist-mediated internalization, localized to intracellular compartments distinct from those utilized by the beta(2)-adrenergic receptor. However, in the case of the D(3)R, only subtle agonist-mediated receptor phosphorylation, beta-arrestin translocation to the plasma membrane, and receptor internalization were observed. Interchange of the second and third intracellular loops of the D(2)R and D(3)R reversed their phenotypes, implicating these regions in the regulatory properties of the two receptors. Our studies thus indicate that functional distinctions between the D(2)R and D(3)R may be found in their desensitization and cellular trafficking properties. The differences in their regulatory properties suggest that they have distinct physiological roles in the brain.

Ouabain-insensitive acidification by dopamine in renal OK cells: primary control of the Na(+)/H(+) exchanger

The present study was aimed at evaluating the role of D(1)- and D(2)-like receptors and investigating whether inhibition of Na(+) transepithelial flux by dopamine is primarily dependent on inhibition of the apical Na(+)/H(+) exchanger, inhibition of the basolateral Na(+)-K(+)-ATPase, or both. The data presented here show that opossum kidney cells are endowed with D(1)- and D(2)-like receptors, the activation of the former, but not the latter, accompanied by stimulation of adenylyl cyclase (EC(50) = 220 +/- 2 nM), marked intracellular acidification (IC(50) = 58 +/- 2 nM), and attenuation of amphotericin B-induced decreases in short-circuit current (28.6 +/- 4.5% reduction) without affecting intracellular pH recovery after CO(2) removal. These results agree with the view that dopamine, through the activation of D(1)- but not D(2)-like receptors, inhibits both the Na(+)/H(+) exchanger (0.001933 +/- 0.000121 vs. 0.000887 +/- 0.000073 pH unit/s) and Na(+)-K(+)-ATPase without interfering with the Na(+)-independent HCO transporter. It is concluded that dopamine, through the action of D(1)-like receptors, inhibits both the Na(+)/H(+) exchanger and Na(+)-K(+)-ATPase, but its marked acidifying effects result from inhibition of the Na(+)/H(+) exchanger only, without interfering with the Na(+)-independent HCO transporter and Na(+)-K(+)-ATPase.

A method for measurement of muscarinic receptor-mediated responses in dissociated single colon longitudinal smooth muscle cells

We previously reported a simple method of acutely preparing dissociated smooth muscle cells from urinary bladder tissue, but the feasibility of this method has not been well ascertained. In the present study, we assessed whether this method is applicable for measuring muscarinic receptor function in intestinal smooth muscle cells. Single smooth muscle cells were prepared from the longitudinal muscle tissue of guinea pig colon by the enzymatic dissociation with papain and hyaluronidase, followed by collagenase digestion. Muscarinic responses in the isolated smooth muscle cells were measured by intracellular Ca(2+) mobilization and extracellular acidification through Fura-2 fluorometry and Cytosensor microphysiometry, respectively. A single, viable population of colon longitudinal smooth muscle cells (approximately 6 x 10(6) cells/animal) was obtained. In these cells, carbachol (muscarinic agonist) induced Ca(2+) mobilization and extracellular acidification over the concentration range similar to that previously reported to produce contraction of the intact colon muscle strips. Atropine (nonselective muscarinic antagonist) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, M(3)-selective antagonist) inhibited the Ca(2+) mobilization with potencies approximately 3 log units greater than that for methoctramine (M(2)-selective antagonist). For extracellular acidification, the potency differences between these antagonists was approximately 2 log units. In addition, the carbachol-induced extracellular acidification was inhibited by 5-[N-ethyl-N-isopropyl]-amiloride, a selective inhibitor of the Na(+)/H(+) exchanger. These findings indicate that in isolated colonic smooth muscle cells, M(3) receptors are predominantly involved in Ca(2+) mobilization, while a mixed population of M(2) and M(3) receptors seems to contribute to extracellular acidification. Our results further suggest the role of the Na(+)/H(+) exchanger in muscarinic-mediated extracellular acidification. Consequently, our method produces viable isolated colonic smooth muscle cells that display physiologically appropriate responses to muscarinic receptor activation, and the method may be applicable for several types of nonvascular smooth muscle tissues.

Effects of BIBP3226 and BIBP3435 on cytosolic calcium in neuropeptide Y Y1 receptor-transfected Chinese hamster ovary cells and wild type CHO-K1 cells

The NPY Y1-receptor selective antagonist BIBP3226 exerts a dual control on the cytosolic free calcium concentration ([Ca2+]i) in NPY Y1 receptor-transfected Chinese Hamster Ovary Cells (CHO-Y1 cells). It is a potent inhibitor of the NPY-evoked increase in [Ca2+]i. This can be ascribed to its antagonistic properties for the NPY Y, receptor since its less active stereoisomer, BIBP3435, is much less potent. However, when its concentration exceeds 1 microM, BIBP3226 produces a large increase in [Ca2+]i on its own. This effect is mimicked by BIBP3435 and it also occurs in wild type CHO-K1 cells. These latter cells do not contain high affinity binding sites for [3H]NPY and [3H]BIBP3226 and, hence, no endogenous NPY Y1 receptors. It is concluded that, at moderately high concentrations, the NPY Y1 receptor antagonist BIBP3226 and its entantiomer BIBP3435 are able to increase the [Ca2+ ]i in CHO cells either by stimulating another receptor or by directly affecting cellular mechanisms that are involved in calcium homeostasis.

Pharmacological characterization of extracellular acidification rate responses in human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells

This study characterized pharmacologically the functional responses to agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in cloned cells using the cytosensor microphysiometer. Dopaminergic receptor agonists caused increases in extracellular acidification rate in adherent Chinese hamster ovary (CHO) clones expressing hD2, hD3 or hD4 receptors. Acidification rate responses to agonists in other cell lines expressing these receptors were smaller than those in adherent CHO cells. The time courses and maximum increases in acidification rate of the agonist responses in adherent CHO cells were different between the three dopamine receptor clones. Responses were blocked by pretreatment of cells with pertussis toxin or amiloride analogues. Most agonists had full intrinsic activity at each of the dopamine receptor subtypes, as compared to quinpirole, however both enantiomers of UH-232 and (-)3-PPP were partial agonists in this assay system. The functional potency of full agonists at each of the three receptors expressed in CHO cells was either higher than, or similar to, the apparent inhibition constants (Ki) determined in [125I]-iodosulpride competition binding studies. Functional selectivities of the agonists were less than radioligand binding selectivities. The rank orders of agonist potencies and selectivities were similar, but not identical, to the rank orders of radioligand binding affinities and selectivities. The dopamine receptor antagonists, iodosulpride and clozapine, had no effect on basal acidification rates but inhibited acidification responses in CHO cells to quinpirole in an apparently competitive manner. Antagonist potencies closely matched their radioligand binding affinities in these cells.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.

Benzodiazepine modulation of recombinant alpha1beta3gamma2 GABA(A) receptor function efficacy determination using the Cytosensor microphysiometer

Gamma-aminobutyric acid (GABA) dose dependently increased extracellular acidification rate in Ltk cells stably expressing human recombinant alpha1beta3gamma2 GABA(A) receptors but had no effect in non-transfected controls. Cells seeded at 1 x 10(5) cells/cup, with 4-5 days induction, had basal acidification rates of 105+/-2 microVs(-1) at 37 degrees C (mean+/-standard error of mean, n=37). GABA responses had a characteristic time-course with an initial alkalinisation followed by a peak of acidification, which was optimized by increasing agonist exposure from 15 s to 25-30 s. The maximum concentration of GABA tested (100 microM) produced a 40+/-2% increase over basal acidification rate (n=3), with an EC50 of 15.5 microM and a Hill slope of 1.5. Responses were specifically antagonized by bicuculline and could be modulated by benzodiazepine ligands with varying efficacies. Full benzodiazepine agonists flunitrazepam (1 microM) and zolpidem (10 microM) significantly potentiated the response to 10 microM GABA by 124+/-15% (n=7) and 117+/-23% (n=3), respectively. The partial agonist bretazenil (100 nM) produced a 45+/-13% (n=3) potentiation whilst the inverse agonist DMCM (10 microM) (methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate) inhibited the response to 20 microM GABA by 53+/-5%. The microphysiometer offers an alternative functional measure for GABA(A) receptors with the sensitivity to measure subtle modulatory effects of benzodiazepine site ligands and to determine their relative efficacy.

Human neuropeptide YY1 receptors exert unequal control of the extracellular acidification rate in different cell lines

The ability of the human neuropeptide YY1 receptor subtype to increase the extracellular acidification rate in different cell lines was investigated by using the Cytosensor Microphysiometer. In CHO-Y1 cells (Chinese Hamster Ovary cells expressing the cloned human neuropeptide YY1 receptor), neuropeptide Y increased the acidification rate by up to 15% of the basal level with a -Log(EC50) of 7.42. As expected for neuropeptide YY1 receptors, this response was potently inhibited by the neuropeptide YY1-selective non-peptide antagonist BIBP3226 ((R)-N2-(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-D-arginine amide). Its enantiomer BIBP3435 ((S)-N2-(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-D-arginin amide) was less potent. The antagonists themselves did not affect the extracellular acidification rate at concentrations up to 10 microM. In SK-N-MC cells (a neuroblastoma cell line of human origin that expresses the neuropeptide YY1 receptor) no change of the acidification rate could be observed in the presence of neuropeptide Y at concentrations up to 1 microM. For control, the neuropeptide YY1 receptors were also investigated by assessing whole cell radioligand binding and, at the functional level, by assessing their ability to decrease the forskolin-induced accumulation of cAMP. The specific (i.e., neuropeptide Y-displaceable) binding of [3H]neuropeptide Y was to a homogeneous class of high-affinity sites in both SK-N-MC and CHO-Y1 cells. The equilibrium dissociation constants for [3H]neuropeptide Y, the total number of binding sites and the kinetic constants for association and for dissociation were similar. Neuropeptide Y produced a dose-dependent inhibition of forskolin-induced cAMP accumulation in SK-N-MC cells (-log(EC50) = 9.40) but it did not affect cAMP accumulation in CHO-Y1 cells. Non-transfected CHO-K1 cells were used as negative control throughout the study. No binding or response could be observed in these cells. Our data suggest that the signalling mechanisms of neuropeptide YY1 receptors are closely related to the cell type in which they are expressed.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices

The importance of voltage-dependent sodium channels and different types of voltage-sensitive calcium channels for depolarisation-induced release of endogenous dopamine from dendrites and cell bodies in superfused guinea pig substantia nigra slices was investigated. The stimulatory effect of veratridine (10 microM) on dopamine release was only marginally attenuated in Ca(2+)-free medium but was completely blocked by tetrodotoxin (1 microM) and by the dopamine reuptake inhibitor GBR 12909 (10 microM). Low extracellular concentration of Na+ stimulated the dopamine release. Potassium-evoked dopamine release was completely Ca(2+)-dependent, not blocked by GBR 12909 and partially blocked by tetrodotoxin. Nifedipine (20 microM), omega-conotoxin GVIA (0.5 microM), penfluridol (5 microM), and Ni2+ (20 microM) had no effect, amiloride (1 mM) attenuated and neomycin (350 microM), and omega-agatoxin IVA (1 microM) almost totally blocked the potassium-induced dopamine release. The results suggest that veratridine released dopamine mostly by reversing the dopamine transporter. High concentrations of potassium induced release of nigral dopamine by opening of voltage-sensitive calcium channels of P/Q type but not L-type, N-type and probably not T-type. The depolarisation evoked by high concentrations of potassium seems to open voltage-sensitive calcium channels both by the depolarisation induced by potassium per se and by the secondary depolarisation induced by opening of voltage-dependent sodium channels.

5-HT1A receptor activates Na+/H+ exchange in CHO-K1 cells through Gialpha2 and Gialpha3

5-HT1A receptors couple to many signaling pathways in CHO-K1 cells through pertussis toxin-sensitive G proteins. The purpose of this study was to determine which members of the Gi/o/z family mediate 5-HT1A receptor-activated Na+/H+ exchange as measured by microphysiometry of cell monolayers. The method was extensively validated, showing that proton efflux was sodium-dependent, inhibited by amiloride analogs, and activated by growth factors, phorbol ester, calcium ionophore, and hypertonic stress. 5-HT and the specific agonist (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide rapidly stimulated proton efflux that was blocked by a specific receptor antagonist, amiloride analogs or pertussis toxin. The activation by 5-HT depended upon extracellular sodium and could be demonstrated under conditions of imposed intracellular acid load, as well as in the presence and absence of glycolytic substrate. Acceleration of proton efflux was not inhibited by sequestration of G protein betagamma-subunits, a maneuver that blocked 5-HT1A receptor activation of mitogen-activated protein kinase. Transfection of Gzalpha and pertussis toxin-resistant mutants of Goalpha and Gialpha1 did not reverse the blockade induced by pertussis toxin. In contrast, pertussis toxin-resistant mutants of Gialpha2 and Gialpha3 "rescued" the ability of 5-HT to increase proton efflux after pertussis toxin treatment. These experiments demonstrate clearly that Gialpha2 and Gialpha3 can specifically mediate rapid agonist-induced acceleration of Na+/H+ exchange.

Dual effects of endothelin-1 on extracellular acidification by A7r5 smooth muscle cells

The effect of endothelin-1 on metabolic activity of A7r5 rat aortic smooth muscle cells was studied. Endothelin-1 (pEC(50) 7.5) elicited an increase in the rate of extracellular medium acidification of the A7r5 cells. The ETA receptor antagonist BQ-123 blocked the endothelin-1 effect completely (pA(2) 7.6). Ca2+ channel blockers affected the endothelin-1 induced response in different ways: diltiazem and nifedipine partially blocked the endothelin-1 induced response, whereas verapamil did not influence this endothelin-1 induced effect. However, upon removal of verapamil an endothelin-1 dependent rise in extracellular acidification occurred, apparently reflecting the lifting of the verapamil blockade of an endothelin-1 induced process. Thus, this study shows that the complex integrated cellular responses upon ET-1 receptor activation are reflected in metabolic activity.

Signal transduction pathways modulated by the D2 subfamily of dopamine receptors

The D2 subfamily of dopamine receptors includes D2A, D2B, D3, and D4 dopamine receptors. These receptors activate cellular effector systems, principally through pertussis toxin-sensitive G-proteins. Historically, D2-like receptors in brain tissues were recognized as the dopamine receptor subtypes that inhibit adenylyl cyclase. Recent studies, reviewed here, have shown that multiple effector systems can be activated by these receptors, and the potential involvement of these in dopaminergic neutrotransmission is discussed.

Dopamine receptors and brain function

In the central nervous system (CNS), dopamine is involved in the control of locomotion, cognition, affect and neuroendocrine secretion. These actions of dopamine are mediated by five different receptor subtypes, which are members of the large G-protein coupled receptor superfamily. The dopamine receptor subtypes are divided into two major subclasses: the D1-like and D2-like receptors, which typically couple to Gs and Gj mediated transduction systems. In the CNS, the various receptor subtypes display specific anatomical distributions, with D1-like receptors being mainly post-synaptic and D2-like receptors being both pre- and post-synaptic. D1 and D2 dopamine receptors, the most abundant subtypes in the CNS, appear to be expressed largely in distinct neurons. Substance P and dynorphin, which are expressed in D1 receptor-containing neurons, as well as pre-proenkephalin in D2 receptor-containing neurons, have been used as monitors of dopaminergic activity in the CNS. Expression of immediate early genes, in particular fos, has also been found to correlate with dopaminergic transmission. Dopamine released from the hypothalamus controls the synthesis and secretion of prolactin from the anterior pituitary via D2 dopamine receptors. As yet none of the dopamine receptor subtypes have been associated with the etiology of psychotic disorders, such as schizophrenia. However, the recent characterization of D3 and D4 receptors which are, interestingly, expressed in areas of the CNS mediating cognition and affect or showing increased affinity for certain neuroleptics, have renewed the interest and hope of finding effective neuroleptics devoid of side effects. Finally, the recent production of genetically-derived animals lacking several of these receptor genes should help elucidate which specific physiological paradigms the receptors mediate.

Regulation and functional characterization of a rat recombinant dopamine D3 receptor

We stably expressed a rat D3 receptor cDNA in C6 glioma cells (C6-D3 cells), quantifying receptor expression with the radioligands [125I]epidepride (KD = 0.1 nM) and [3H]spiperone (KD = 0.7 nM). As reported previously for D2 receptors, quinpirole induced a 9-16% increase in the rate of extracellular acidification by C6-D3 cells. The acidification was inhibited by epidepride and by the Na+/H+ antiporter inhibitors, amiloride and methylisobutylamiloride, but pertussis toxin treatment had no effect on quinpirole-induced extracellular acidification. These data suggest that D3 receptor stimulation of Na+/H+ exchange in C6 glioma cells is not mediated by the pertussis toxin-sensitive G proteins, Gi or G(o). Overnight treatment of C6-D3 cells with N-propylnorapomorphine, dopamine, or quinpirole resulted in large concentration-dependent increases (up to 500%) in the density of D3 receptors on membranes prepared from the cells. Antagonists had smaller, variable effects on the density of D3 receptors in C6-D3 cells, except for domperidone, which significantly increased the density of D3 receptors. Treatment with pertussis toxin had no effect on the agonist-induced receptor up-regulation, indicating that an interaction with pertussis toxin-sensitive G proteins was not required. Densitometry analysis of Northern blots of RNA prepared from C6-D3 cells showed no significant N-propylnorapomorphine-induced increase in D3 receptor message. Treatment with cycloheximide, however, completely prevented receptor up-regulation by N-propylnorapomorphine. Pretreatment of C6-D2 cells with 10 microM DA resulted in a substantial heterologous sensitization, in which isoproterenol-stimulated adenylyl cyclase activity was enhanced more than twofold.(ABSTRACT TRUNCATED AT 250 WORDS)

Pramipexole binding and activation of cloned and expressed dopamine D2, D3 and D4 receptors

Pramipexole (SND 919; 2-amino-4,5,6,7-tetrahydro-6-propylamino-benzthiazole-dihydrochlor ide) is a potent dopamine autoreceptor agonist. We have carried out an analysis of the binding affinities of dopamine D2L, D2S, D3, and D4 receptors for pramipexole using both [3H]pramipexole and [3H]spiperone as radioligands at cloned and heterologously expressed receptors. Studies were carried out using rat and human D2L, D2S and D3 receptors with equivalent results. When the binding of pramipexole to the high affinity, guanine nucleotide-sensitive state of each receptor was analyzed, pramipexole is most selective for D3 compared to D2 and D4 receptors. These results indicate a 5-fold selectivity of pramipexole for D3 receptors, while quinpirole and bromocriptine are non-selective or more D2/D4 receptor selective. Two measurements of receptor activation for dopamine D2, D3, and D4 receptors also show that pramipexole is most potent for activation of D3 receptors. The dopamine D3 receptor selectivity of pramipexole may explain the previously described properties of this drug, including its potent autoreceptor preference.

Nerve growth factor stimulates rapid metabolic responses in PC12 cells

Research into the effects of nerve growth factor (NGF) has involved study of either the signal transduction process or the morphological result of growth factor treatment (cell proliferation and/or differentiation). The Cytosensor Microphysiometer, a silicon-based biosensor system that allows the continuous and real-time monitoring of extracellular acidification rate changes of cells, was used to study the response of PC12 cells to NGF. Stimulation resulted in a rapid increase in the acidification rate of cells in a concentration-dependent fashion (0.1-200 ng/ml NGF; mean effective concentration value of 153 +/- 54 pM). Inhibition of the NGF receptor-linked protein tyrosine kinase by either genistein or K252a attenuated the acidification rate response to NGF. In addition, the acidification response to NGF could be modified by inhibiting Na+/H+ exchange and, separately, glycolysis. This implicates these processes in the metabolic response of PC12 cells to NGF stimulation.

Direct measurement of extracellular proton flux from isolated gastric glands

We used the microphysiometer, a sensitive extracellular pH sensor, to resolve luminal (or apical) H+ secretion and basolateral release of OH- as well as liberation of acidic metabolites in rabbit gastric glands. Stimulation of glands via the adenosine 3',5'-cyclic monophosphate pathway produced a biphasic change in the extracellular acidification rate (EAR): after an initial transient decrease below the unstimulated baseline (-40.9 +/- 3.4%), the EAR increased to a steady-state maximal plateau (+98.1 +/- 5.3%) within 30 min (n = 37). We interpret the biphasic EAR profile as an initial excess of basolaterally released OH- followed by delayed luminal efflux of simultaneously produced H+. The elevated EAR at steady state reflected liberation of metabolic acid attributed to H(+)-K(+)-ATPase enzymatic activity. The presence of H2-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid prevented OH- release and reduced steady-state EAR. Basolateral OH- release and steady-state EAR were also inhibited by the H(+)-K(+)-ATPase inactivators omeprazole and SCH-28080. Inhibition of Na+/H+ exchange did not reduce steady-state EAR and did not affect apical H+ production, as judged by the accumulation of the weak base aminopyrine. Sodium thiocyanate (1 mM), which short circuits intraluminal H+ accumulation, blocked OH- release, demonstrating its dependence on H(+)-OH- separation at the apical membrane. A computerized model was developed to illustrate how the observed biphasic EAR profile would result from a delayed luminal efflux of H+ due to transitory intraluminal compartmentalization.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Using microphysiometry to study the pharmacology of exogenously expressed m1 and m3 muscarinic receptors

The microphysiometer, an instrument that uses a semiconductor-based sensor to monitor cellular metabolic activity, has been shown to detect the activation of a variety of receptors in living cells, largely irrespective of the signal-transduction mechanism. Using the Cytosensor Microphysiometer, we have studied agonist concentration responses for the activation of CHO-K1 cell lines exogenously expressing rat m1 or m3 receptors. Three levels of receptor expression were investigated for each subtype. Carbachol is more potent for m3 than m1 receptors (0.5 to 1.0 log unit lower EC50); for both, potency correlates positively with receptor density. The results agree well with those obtained by measuring phosphoinositide hydrolysis and intracellular [CA++] in the same cells. We also determined that two subtype-selective antagonists, pirenzepine (for m1) and p-fluoro-hexahydrosila-difenidol (for m3) displayed appropriate differential ability to shift carbachol concentration-response curves in the microphysiometer. This study provides additional evidence that pharmacological results obtained by microphysiometry are consistent with those obtained by more conventional functional assays.