Regulation of anterior pituitary D2 dopamine receptors by magnesium and sodium ions

Discovery, Optimization, and Characterization of ML417: A Novel and Highly Selective D3 Dopamine Receptor Agonist

To identify novel D₃ dopamine receptor (D3R) agonists, we conducted a high-throughput screen using a β-arrestin recruitment assay. Counterscreening of the hit compounds provided an assessment of their selectivity, efficacy, and potency. The most promising scaffold was optimized through medicinal chemistry resulting in enhanced potency and selectivity. The optimized compound, ML417 (20), potently promotes D3R-mediated β-arrestin translocation, G protein activation, and ERK1/2 phosphorylation (pERK) while lacking activity at other dopamine receptors. Screening of ML417 against multiple G protein-coupled receptors revealed exceptional global selectivity. Molecular modeling suggests that ML417 interacts with the D3R in a unique manner, possibly explaining its remarkable selectivity. ML417 was also found to protect against neurodegeneration of dopaminergic neurons derived from iPSCs. Together with promising pharmacokinetics and toxicology profiles, these results suggest that ML417 is a novel and uniquely selective D3R agonist that may serve as both a research tool and a therapeutic lead for the treatment of neuropsychiatric disorders.

Enhancing the Signaling of GPCRs via Orthosteric Ions

G protein-coupled receptors play essential roles in cellular processes such as neuronal signaling, vision, olfaction, tasting, and metabolism. As GPCRs are the most important drug targets, understanding their interactions with ligands is of utmost importance for discovering related new medicines. In many GPCRs, an allosteric sodium ion next to the highly conserved residue D^2.50 has been proposed to stabilize the inactive receptor state by mediating interactions between transmembrane helices. Here, we probed the existence of internal and functionally important sodium ions in the dopamine D2 receptor, using molecular dynamics simulations. Besides a new sodium ion at the allosteric ligand binding site, we discovered an additional sodium ion, located close to the orthosteric ligand binding site. Through cell-based activation assays, the signaling of D2 receptor with site-specific mutations was tested against a series of chemically modified agonists. We concluded an important structural role of this newly discovered orthosteric sodium ion in modulating the receptor signaling: It enables the coordination of a polar residue in the ligand binding site with an appropriately designed agonist molecule. An identical interaction was also observed in a recently released high-resolution crystal structure of mu-opioid receptor, which was reresolved in this work. Probably because of similar interactions, various metal ions have been found to increase the signaling of many other GPCRs. This unique principle and strategy could be used to optimize the drug activity of GPCR. Our findings open a new mechanistic opportunity of GPCR signaling and help design the next generation of drugs targeting GPCRs.

Transient receptor potential melastatin (TRPM) channels mediate clozapine-induced phenotypes in Caenorhabditis elegans

The molecular mechanisms of action of antipsychotic drugs (APDs) are not fully understood. Here, we characterize phenotypes of missense and knockout mutations in the Caenorhabditis elegans transient receptor potential melastatin (TRPM) channel ortholog gtl-2, a candidate APD target identified in a genome-wide RNAi (RNA interference) screen for Suppressors of Clozapine-induced Larval Arrest (scla genes). We then employ the developmental phenotypes of gtl-2(lf) mutants to validate our previous gtl-2(RNAi) result. GTL-2 acts in the excretory canal cell to regulate Mg(2+) homeostasis. Using exc (excretory canal abnormal) gene mutants, we demonstrate that excretory canal cell function is necessary for clozapine-induced developmental delay and lethality. Moreover, cell-specific promoter-driven expression studies reveal that GTL-2 function in the excretory canal cell is important for its role in the SCLA phenotype. We then investigate the mechanism by which GTL-2 function in the excretory canal cell impacts clozapine-induced phenotypes. gtl-2(lf) mutations cause hypermagnesemia, and we show that exposure of the wild-type strain to high Mg(2+) phenocopies gtl-2(lf) with respect to suppression of clozapine-induced developmental delay and lethality. Our results suggest that GTL-2 TRPM channel function in the excretory canal cell is important for clozapine's developmental effects. TRP channels are expressed in mammalian brain and are implicated in the pathogenesis of mental illnesses but have not been previously implicated in APD action.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

Autoradiographic mapping of dopamine-D2/D3 receptor stimulated [35S]GTPgammaS binding in the human brain

Agonist stimulated [35S]guanosine 5'-gamma-thiotriphosphate ([35S]GTPgammaS) binding autoradiography was established for the examination of dopamine-D2/D2 receptors in human brain sections. The distribution of G proteins activated by dopamine-D2/D3 receptors was studied in whole hemisphere cryosections. Dopamine stimulated [35S]GTPgammaS binding in brain regions with high densities of dopamine D2-like receptors, i.e. putamen (23 +/- 2%, mean +/- SEM,% stimulation over basal binding), caudate (20 +/- 0%) and substantia nigra (22 +/- 2%), but also in regions with lower receptor densities such as amygdala (17 +/- 8%), hippocampus (16 +/- 6%), anterior cingulate (13 +/- 3%), and thalamus (12 +/- 2%). Dopamine stimulated [35S]GTPgammaS binding to significantly higher levels in the dorsal than in the ventral part of the striatum. Dopamine caused low or very low stimulation in all cortical areas. Raclopride, a selective D2/D3 receptor antagonist, potently inhibited dopamine stimulated [35S]GTPgammaS binding, whereas R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390), a selective D1 antagonist, did not block the [35S]GTPgammaS binding response stimulated by dopamine. Hence, the stimulatory effect of dopamine was primarily mediated by D2/D3 receptors. Quinpirole stimulated [35S]GTPgammaS binding in the same regions as dopamine. The maximal level of stimulation induced by dopamine and quinpirole was not significantly different. The present study demonstrates that agonist stimulated [35S]GTPgammaS binding autoradiography could be a suitable technique for the examination of dopamine-D2/D3 receptors in the human brain. This functional assay could provide useful new information about dopamine receptor/G protein coupling in the postmortem human brain, and reveal possible disease related alterations of the interaction between D2/D3 receptors and G proteins.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Mechanisms of inverse agonism of antipsychotic drugs at the D(2) dopamine receptor: use of a mutant D(2) dopamine receptor that adopts the activated conformation

The antipsychotic drugs have been shown to be inverse agonists at the D(2) dopamine receptor. We have examined the mechanism of this inverse agonism by making mutations in residue T343 in the base of the sixth transmembrane spanning region of the receptor. T343R, T343S and T343K mutant D(2) dopamine receptors were made and the T343R mutant characterized in detail. The T343R mutant D(2) dopamine receptor exhibits properties of a receptor that resides more in the activated state, namely increased agonist binding affinity (independent of G-protein coupling and dependent on agonist efficacy), increased agonist potency in functional tests (adenylyl cyclase inhibition) and increased inverse agonist effects. The binding of agonists to the mutant receptor also shows sensitivity to sodium ions, unlike the native receptor, so that isomerization of the receptor to its inactive state may be driven by sodium ions. The binding of inverse agonists to the receptor is, however, unaffected by the mutation. We conclude that inverse agonism at this receptor is not achieved by the inverse agonist binding preferentially to the non-activated state of the receptor over the activated state. Rather the inverse agonist appears to bind to all forms of the receptor but then renders the receptor inactive.

Endogenous activation of dopamine D2 receptors regulates dopamine release in the fish retina

In the fish retina, horizontal cell electrical coupling and light responsiveness is regulated by activation of dopamine D1 receptors that are located on the horizontal cells themselves. The effects of dopamine and dopamine D2 receptor agonists and antagonists on cone horizontal cell light responses were studied in in vitro superfused goldfish retinas. Horizontal cell light responses and electrical coupling were assessed by monitoring responses to full-field stimuli and to small, centered (0.4 mm diam) spots of light, respectively. Dopamine (0.2-10 microM) application uncoupled horizontal cells and decreased their responses to full-field stimuli. Application of the D2 antagonist eticlopride (10-50 microM) produced similar effects, whereas quinpirole (0.1-10 microM), a D2 agonist, had the opposite effects. The uncoupling effect of eticlopride was blocked by prior application of SCH23390 (10 microM), a D1 receptor antagonist, and was eliminated after destruction of dopaminergic neurons by prior treatment of the retinas with 6-hydroxydopamine. The effects of these D2 drugs were observed following flickering light stimulation, but were not observed following sustained light stimulation. Application of the D2 antagonists sulpiride (0.5-20 microM) and spiperone (0.25-10 microM) uncoupled horizontal cells when the total concentration of divalent cations (Mg2+ and Ca2+) in the Ringer solution was 1.1 mM. However, when the concentration of divalent cations was 0.2 mM, spiperone had no effect on the horizontal cells and sulpiride increased coupling. In contrast, eticlopride uncoupled the cells and decreased their light responsiveness irrespective of the concentration of divalent cations. The effects of quinpirole also depended on the concentration of divalent cations; its coupling effect was reduced when the divalent cation concentration was increased from 0.2 to 1.0 mM. The results suggest that activation of D2 receptors in the fish retina by endogenous dopamine decreases dopamine release and is greater after flickering compared with sustained light stimulation. These D2 receptors thus function as presynaptic autoreceptors that inhibit dopamine release from dopaminergic cells. In addition, the results also indicate that the effectiveness of some D2 drugs at these receptors is dependent on the concentration of divalent cations.

Deriving the therapeutic concentrations for clozapine and haloperidol: the apparent dissociation constant of a neuroleptic at the dopamine D2 or D4 receptor varies with the affinity of the competing radioligand

The apparent dissociation constant, Ki, for a neuroleptic at the dopamine D2 or D4 receptor was consistently higher when competed against [3H]nemonapride than against [3H]spiperone which was in turn higher than that against [3H]raclopride. This finding obtained for all four types of dopamine receptors studied, including the native dopamine D2 receptor in the anterior pituitary tissue, the human D2long receptor, the human D2short receptor and the human D4.4 receptor. Some neuroleptics revealed a difference of over 10-fold between the Ki using [3H]nemonapride and the Ki using [3H]raclopride. The KD values of the three 3H-ligands and the neuroleptic Ki values were lower when using a much lower concentration of tissue, indicating that depletion of ligand presumably accounted for the phenomenon. The Ki values of each neuroleptic were related to the the tissue/buffer partition coefficients of the three 3H-ligands. Extrapolating the neuroleptic Ki value down to a tissue/buffer partition coefficient of unity or zero led to a Ki value for competition versus a water-soluble ligand such as dopamine. Clozapine extrapolated to a Ki value of 1.3 nM. Direct measurement gave a Ki value of 1.6 nM for [3H]clozapine at the dopamine D4 receptor. When competing versus endogenous dopamine, this clozapine value of 1.6 nM would rise to 20 nM for the blockade of 75% of dopamine D4 receptors, matching the observed therapeutic concentration of 18 nM. These data also explain why clozapine occupies 48% of the D2 receptors in patients when measured with [11C]raclopride, but between 0% and 22% when measured with [18F]methylspiperone or [18F]fluoroethylspiperone.

Dopamine D2 receptors are unevenly distributed in the rat hippocampus and are modulated differently than in striatum

The characteristics of dopamine (DA) D2 receptors were studied in rat hippocampus and for comparison in striatum using the [3H]spiperone radioreceptor assay in vitro. D2 receptors exhibit a bimodal distribution along the length of the hippocampus. Heterogeneity also exists in the transverse axis with high densities in the molecular layers of the subiculum and CA1 plus CA2. As in the striatum, D2 receptors in both dorsal and ventral hippocampus display high and low affinity states for agonists, but all three regions differ with regard to the percentage of receptors in these states. The modulation of these affinity states by GTP, Mg2+ and the iron-chelator, 1,1-O-phenanthroline, differs markedly between dorsal and ventral hippocampus, and between these regions and the striatum. A new model for the modulation of the affinity of D2 receptors for agonists is presented. These data suggest the presence of regional differences in the composition of the D2 receptor-regulatory protein complex.

Dopamine D1 and D2 receptors in the caudate nucleus of spontaneously hypertensive rats and normotensive Wistar-Kyoto rats

A series of studies was carried out to characterize the binding properties of dopamine D1 and D2 receptors in membrane homogenates of the caudate nucleus of spontaneously hypertensive rats (SHR). Binding in SHR was studied at the age of 4 weeks when the rats were still in the prehypertensive phase, and at the age of 8 weeks, during the phase in which blood pressure is increasing dramatically; age-matched normotensive Wistar-Kyoto rats (WKY) were used as controls. Binding to dopamine D1 receptors was studied using [3H]SCH 23390. Antagonist binding of dopamine D2 receptors was performed with [3H]spiperone. At both ages no differences were found between SHR and WKY in affinity (Kd) or concentration (Bmax) of dopamine D1 and D2 receptors. Binding to the high affinity state of the dopamine D2 receptor was measured using the agonist [3H]N-n-propylnorapomorphine (NPA). No differences in Bmax or Kd were found between SHR and WKY at both ages studied, indicating that the ratio between dopamine D2 receptors in the high and in the low affinity state is not altered in spontaneous hypertension. Although the results do not reveal differences in affinities or concentrations of dopamine D1 or D2 receptors in the caudate nucleus between SHR and WKY, a role in the development of hypertension for the here described lack of receptor up-regulation in connection with our previous observation of lower release of dopamine in the caudate nucleus of SHR, cannot be excluded.

Coupling of D2 dopamine receptors to G-proteins in solubilized preparations of bovine caudate nucleus

1. The coupling of D2 dopamine receptors and G-proteins has been examined in cholate-solubilized preparations of bovine caudate nucleus. 2. No receptor-G-protein coupling could be detected in solubilized preparations obtained in 0.3% cholate, but if this preparation is diluted 5-fold, coupling is re-established. 3. The dilution process was examined, and it was shown that the change in ionic strength was an important factor in modulating the observed receptor-G-protein interaction. 4. Ionic strength was shown, however, not to be the primary determinant of receptor-G-protein coupling. This is likely to be the formation, upon dilution of the preparation, of vesicles in which receptor and G-protein reassociate. 5. The formation of vesicles upon dilution was examined by a variety of techniques, including thermal-stability studies, gel filtration, centrifugation and electron microscopy.

Modulation of dopamine receptor agonist binding sites by cations and estradiol in intact pituitary and 7315a tumors

The effects of Na+, K+, Mg2+ and Ca2+ on the agonist binding sites of D2 dopamine (DA) receptors were studied in 7315a pituitary tumors. The agonist high and low affinity states of the D2 receptors were investigated with apomorphine competition for [3H]spiperone binding at 25 degrees. In the tumor, all cations markedly increased the affinity of the high affinity binding site, while the affinity of the low affinity binding site was increased only by Na+. The proportion of high to low affinity states was not affected significantly by K+ and Ca2+, whereas it was decreased by Na+ and increased by Mg2+; none of these cations affected the total density of the D2 receptors. The in vitro regulation of D2 receptors by 17 beta-estradiol (E2) was next studied in 7315a tumors and bovine intact adenohypophysis. In intact anterior pituitary, a partial conversion of the high to the low affinity state was obtained in the presence of GTP, while in tumoral pituitary, a complete conversion was observed. Addition of 1 nM E2 to the in vitro incubation mixture prevented these conversions and resulted in a partial return of the high affinity state of the D2 receptors to their control values in both normal and tumoral pituitary. In another experiment, using increasing concentrations of E2 (0.01 to 100 nM) and GTP (10(-8) to 10(-3) nM) on [3H]n-propylnorapomorphine [( 3H]NPA) binding to the D2 receptors in bovine intact adenohypophysis, 1 and 10 nM E2 doubled the IC50 of GTP to decrease [3H]NPA binding. The results show that agonist high and low affinity states of D2 receptors in 7315a tumors are regulated normally by cations. In addition, E2 inhibited the effect of GTP on the agonist sites of the D2 receptors in both intact anterior pituitary tissue and 7315a tumors.

Dopamine receptors labelled by [3H]quinpirole

Since quinpirole (or LY171555) has a high affinity for dopamine D2 receptors, and since the high-affinity state of D2 appears to be the functional state of D2, we prepared [3H]quinpirole to investigate its suitability for labelling the high-affinity state of the D2 receptor. The dissociation constant of [3H]quinpirole binding to canine striatum homogenate was 3.9 nM in the absence of NaCl and 6.8 nM in the presence of NaCl. Only 50% of the total binding was specifically displaced by 10 microM S-sulpiride. The data are consistent with the conclusion that much or most of the [3H]quinpirole binds to the high-affinity state of the D2 receptor, since dopamine D2 agonists and antagonists were the most potent in inhibiting the binding of this ligand, because the density of binding sites was 8-9 pmol/g, about half that for [3H]spiperone, and because the density was reduced by 70% in the presence of guanylylimidodiphosphate. Since quinpirole has a reported Ki value of 5.1 nM for dopamine D3 receptors, similar to the quinpirole Ki value of 4.8 nM for the high-affinity state of the dopamine D2 receptor, it appears that [3H]quinpirole with its Kd of 3.9-6.8 nM could label both these two dopamine receptors. However, since the spiperone and haloperidol Ki values against [3H]quinpirole were the same as their values at dopamine D2 receptors rather than dopamine D3 receptors, it appears that [3H]quinpirole predominantly labels dopamine D2 receptors in the canine striatum. The guanine nucleotide-insensitive component of [3H]quinpirole binding (about 30%) may be to dopamine D3 receptors.

Modulation of dopamine receptors by cations in 7315a, MtTW15 and estradiol-induced pituitary tumors

Modulation of dopamine (DA) receptors by cations (Na+, K+, Mg2+, Ca2+) was compared in 7315a, MtTW15, and estradiol valerate-induced (EV-T) pituitary tumors, and intact adenohypophysis. In 7315a tumors, the affinity of [3H]spiperone binding measured at 25 degrees remained unchanged in the presence of each cation individually or all these cations together (IONS) compared to the affinity obtained using a buffer without ions; the density (Bmax) was not affected by monovalent cations or Mg2+ and was decreased by Ca2+ or IONS. When binding experiments were done at 37 degrees, monovalent cations increased affinity whereas divalent cations or IONS did not modify it, and none of these cations affected Bmax values. In MtTW15 tumors, the affinity of [3H]spiperone binding measured at 25 degrees was not changed by Na+ or IONS and was decreased by K+ or divalent cations; the density was decreased by K+ and unchanged by all the other cations. When binding experiments were done at 37 degrees, Na+ increased the affinity, whereas all the other cations did not affect it: the density was unaffected by all the cations studied. In EV-T assayed at 37 degrees, the affinity was increased by monovalent cations or Mg2+ and was unchanged by Ca2+; monovalent cations did not affect the density of [3H]spiperone binding and divalent cations increased it. In binding experiments performed at 25 degrees and 37 degrees, choline chloride did not change the affinity or the density of [3H]spiperone binding to DA receptors in the three pituitary tumors investigated, suggesting that the effect of cations was specific and not due to differences in ionic strength. In the rat normal anterior pituitary, Na+ increased the affinity of [3H]spiperone for the DA receptors, whereas the affinity was unchanged by Ca2+; the density of [3H]spiperone binding was unaffected by these cations. Our results suggest that DA receptors in 7315a and MtTW15 tumors are regulated abnormally by sodium, potassium, magnesium and calcium. In contrast, DA receptors in EV-T are regulated normally by monovalent cations and abnormally by divalent cations as compared to these receptors in intact pituitary tissue.

Magnesium-induced conditioned place preference in mice

A conditioned place preference procedure was used in mice to test the hypothesis that magnesium possesses reinforcing properties. Mice were conditioned to the nonpreferred end of a three-compartment straight shuttle box with MgCl2 injections alternating with saline injections on the preferred end. Dose of MgCl2 was varied (0, 15, 30, 125 mg/kg) as well as number of conditioning trials (8 or 16). On the day after the first postconditioning test, animals were given acute injections of 5 mg/kg cocaine, or other test drug, to determine if the conditioned effect on behavior would be potentiated, maintained or blocked by these test drugs. Results demonstrated that 15 mg/kg MgCl2 induced the greatest amount of conditioning and that increasing the number of MgCl2/place pairings did not enhance the amount of conditioning, but rather, it decreased it. Amphetamine potentiated MgCl2-induced place preference; cocaine and pentobarbital maintained it; and haloperidol blocked it. These data indicate that MgCl2 has some primary reinforcing properties in mice and that MgCl2 shares stimulus properties with other stimulants and reinforcing substances.

Enhancement of apomorphine and l-amphetamine-induced behaviors by magnesium

The behavioral effects of magnesium suggest that this divalent cation has psychomotor stimulant-like properties. Because deficiencies of this cation lead to reductions in drug-induced behaviors dependent on the levels of norepinephrine and dopamine, and numerous in vitro studies have demonstrated a relationship between magnesium and catecholamine activity, the present experiments investigate whether administration of magnesium will lead to increases in stereotyped and locomotor behaviors induced by apomorphine and l-amphetamine. Such changes would suggest that magnesium is increasing the activity of catecholamines in vivo. The results demonstrate that magnesium dose dependently increases the potency of these drugs by producing greater behavioral effects at certain drug doses, by producing shifts to the left in dose-response functions, and by producing decreases in the ED50 as dose of magnesium increases.

Failure of magnesium to maintain self-administration in cocaine-naive rats

Previous research has shown that magnesium interacts with cocaine in such a way that it potentiates its action in a variety of behavioral situations. More recently, it has been demonstrated that magnesium will dose dependently substitute for cocaine self-administration and reduce the intake of cocaine. It is of considerable interest to determine if magnesium would be self-administered in cocaine-naive animals. The results of two experiments demonstrate that magnesium is not self-administered by cocaine-naive rats since although responding for magnesium chloride is above hypertonic saline control levels on day 1 of access, this responding is not maintained on subsequent days, does not occur in a regularly spaced pattern over time, and is not inversely related to dose. Taken together these data indicate that magnesium is a substitute for cocaine that has low abuse potential.

Comparison of the effects of ions and GTP on substance P binding to membrane-bound and solubilized specific sites

Mg2+ increased but Na+ and GTP decrease [3H]substance P (SP) binding to rat cerebral cortical membranes and to 10 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS)-solubilized membrane fraction. To determine the binding parameters that are modified by the cations and GTP, inhibition experiments of [3H]SP binding by unlabeled SP were performed in both of the preparations. Nonlinear least-squares regression analysis of data in the membrane fraction indicated that optimal fitting of the inhibition curves in the presence of 10 mM MgCl2 was attained with a two-site model, corresponding to a "high-affinity (H)" and a "low-affinity (L)" state. By omitting MgCl2, or by addition of NaCl and GTP, the [3H]SP specific binding was decreased, the H state disappeared, and the L state and a new "super-low affinity (SL)" state observed. The SP/[3H]SP inhibition curves in the cerebral cortical membranes by in vivo treatment with pertussis toxin (islet-activating protein) were similar to that in the presence of GTP in control membranes. The effects of MgCl2, NaCl, and GTP were greater in the CHAPS-solubilized fraction than in the membrane fraction. In contrast to the membrane fraction, the inhibition curves of [3H]SP binding by unlabeled SP in the presence of MgCl2 in the CHAPS-solubilized fraction were best fitted to a one-site model. The KD value was relatively close to that of the low-affinity state in the membrane fraction. Even with the addition of NaCl or GTP, or by reducing MgCl2 concentration to 1 mM, although the inhibition curves consistently fit the one-site model, the KD values changed only slightly.

Magnesium alters the potency of cocaine and haloperidol on mouse aggression

Magnesium has been shown to have certain behavioral effects similar to the stimulants cocaine and amphetamine, particularly on mouse resident-intruder aggression. Consequently, it was hypothesized that magnesium should interact with the indirect agonist cocaine and the antagonist haloperidol to alter their potency in the mouse resident-intruder model. Acute and chronic drug effects were compared. Results demonstrate an enhancement of cocaine potency by 30 and 125 mg/kg MgCl2 and a lowering of cocaine potency by a 15% required-Mg2+ deficient diet as measured by shifts in the dose response to acutely administered cocaine. Following chronic 0.5 mg/kg cocaine for 15 days, a dose of 125 mg/kg acutely administered MgCl2 prevented the disruptive effects of chronic cocaine on mouse aggression. Acutely administered haloperidol was influenced by Mg2+ treatments in a manner opposite from the effects on cocaine, while the chronic effects of haloperidol were affected in the same manner by Mg2+ treatments as those shown for chronic cocaine. Several mechanisms are suggested to explain these interactions.

Guanine nucleotide effects on agonist binding to serotonin 5HT2 receptors in rat frontal cortex

Specific [3H]ketanserin binding to serotonin 5-HT2 receptors of rat frontal cortex tissue is of high affinity, saturable and unaffected by guanine nucleotides. Antagonists displace [3H]ketanserin from a single recognition site (pseudo-Hill coefficients close to unity), which is also unaffected by guanine nucleotides. Agonist displacement of either [3H]ketanserin or [3H]spiperone from three different membrane preparations showed pseudo-Hill coefficients less than one, and may be described in terms of two agonist binding sites with differing agonist affinities. In the presence of guanine nucleotides, overall agonist affinity was lowered slightly, with little or no change in pseudo-Hill coefficient.

Interaction of permanently charged analogs of dopamine with the D-2 dopaminergic receptor

Dopamine can exist in both charged and uncharged forms at physiological pH. At present it is unclear which of these forms is responsible for dopaminergic agonist activity. The purpose of this study was to determine whether permanently charged structural analogs of dopamine containing either a nitrogen, sulfur, or selenium atom in the side chain can bind to and activate the D-2 dopamine receptor. Binding to and activation of the D-2 dopamine receptor were measured by determining the abilities of the permanently charged dopamine analogs to inhibit [3H]spiperone binding to striatal homogenates and to inhibit K+-stimulated [3H]acetylcholine release from striatal slices respectively. The quaternary ammonium, dimethylsulfonium and dimethylselenonium analogs of dopamine were all found to inhibit [3H]spiperone binding to the same extent and in a manner qualitatively similar to the parent amines, dopamine and dimethyldopamine. Thus, [3H]spiperone inhibition curves for dopamine, dimethyldopamine and the permanently charged dopamine analogs were generally shallow and fit best to a two-site binding model as indicated by computer-assisted analyses. The addition of 125 mM NaCl to the incubation medium resulted in a significant decrease in the proportion of high affinity binding sites for both the permanently charged analogs and the parent amines. Similarly, the permanently charged dopamine analogs were found to maximally inhibit the K+-stimulated release of [3H]acetylcholine to the same extent as dopamine and dimethyldopamine. However, the permanently charged analogs were less potent in inhibiting both [3H]spiperone binding and K+-stimulated [3H]acetylcholine release than dopamine and dimethyldopamine. These results show that dopamine analogs possessing a permanent positive charge in the side chain can bind to and activate the D-2 dopamine receptor. The lower potencies of the permanently charged analogs in binding to and activation of the D-2 dopamine receptor suggest that, while the ability of a compound to exist in an uncharged form is not a requirement, both charged and uncharged forms of the agonist molecule appear to play a role in D-2 dopamine agonist activity.

The effects of kainic acid and 6-hydroxydopamine lesions, metal ions and GTP on in vitro binding of the D-2 dopamine agonist, [3H]N-0437, to striatal membranes

The kinetic and pharmacological profiles of the potent and selective D-2 dopamine agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin ([3H]N-0437) have recently been described. This report concerns the effects of chemical lesions and metal ions on the radioreceptor binding of [3H]N-0437. Kainic acid lesions reduced the maximum number of binding sites (Bmax) in the rat striatum by 50%. The affinity of [3H]N-0437 for dopamine receptors was reduced by half. 6-Hydroxydopamine lesions had no measurable effect on the Bmax or on the KD. Of the physiological metal ions tested only Na+ had a significant effect on the binding. Sodium ions reduced the affinity of [3H]N-0437 for striatal receptors from 5.0 +/- 1.1 nM to 8.4 +/- 0.3 nM. In addition GTP lowered the Bmax from 1121 +/- 44 to 868 +/- 84 fmol/mg protein. The trace ions Li+ and Mn2+ had no effect at a concentration of 3.0 mM, while the exogenous ion Hg2+ at the same concentration prevented the specific binding of [3H]N-0437. Together, the results suggest that [3H]N-0437 labels both pre- and postsynaptic receptors, although postsynaptic receptors are labelled preferentially. Moreover, there is an indication that GTP shifts the affinity state of the D-2 receptor from high to low, while Na+ seems to be an allosteric inhibitor.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Dopamine receptors and the dopamine hypothesis of schizophrenia

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked "monoamine receptors" or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)-butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]-dopamine or [3H]-haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase). Based on the fact that dopamine-mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2-selective blockade by all neuroleptics (except the monoamine-depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug-naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low-affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.