Forskolin-induced up-regulation and functional supersensitivity of dopamine D2long receptors expressed by Ltk- cells

Investigating the ligand agonism and antagonism at the D2long receptor by dynamic mass redistribution

The signalling of the D₂ receptor (D₂R), a G protein-coupled receptor (GPCR), is a complex process consisting of various components. For the screening of D₂R ligands, methods quantifying distinct second messengers such as cAMP or the interaction of the receptor with β-arrestin, are commonly employed. In contrast, a label-free biosensor technology like dynamic mass redistribution (DMR), where it is mostly unknown how the individual signalling pathways contribute to the DMR signal, provides a holistic readout of the complex cellular response. In this study, we report the successful application of the DMR technology to CHO-K1 cells stably expressing the human dopamine D_2long receptor. In real-time kinetic experiments, studies of D₂R reference compounds yielded results for agonists and antagonists that were consistent with those obtained by conventional methods and also allowed a discrimination between partial and full agonists. Furthermore, investigations on the signalling pathway in CHO-K1 hD_2longR cells identified the Gα_i/o protein as the main proximal trigger of the observed DMR response. The present study has shown that the DMR technology is a valuable method for the characterisation of putative new ligands and, due to its label-free nature, suggests its use for deorphanisation studies of GPCRs.

D2-receptor upregulation is dependent upon temporal course of D2-occupancy: a longitudinal [11C]-raclopride PET study in cats

Long-term occupancy of dopamine D(2)-receptors, as achieved by chronic treatment with antipsychotics, leads to D(2)-receptor upregulation, and this upregulation is thought to be responsible for loss of efficacy and development of tardive dyskinesia. However, little is known about the parameters of D(2)-receptor blockade (duration and percentage of blockade) that lead to upregulation. In this study, we investigated the effects of different degrees (60 vs >80%) and durations (a transient peak vs 24 h/day) of D(2)-receptor blockade on inducing this upregulation. These different patterns of D(2)-receptor occupancy kinetics were produced in cats using bolus vs constant infusion of haloperidol for 4 weeks. D(2)-receptors were measured using positron emission tomography and Scatchard analyses of [(11)C]raclopride binding, before and after withdrawal of treatment. Continuously high (80% for 24 h/day) D(2)-receptor blockade led to a robust upregulation of striatal D(2)-receptors that was maximal at 1-week withdrawal (35+/-5%) and still detectable at 2-week withdrawal (20+/-3%). This pattern of D(2)-receptor blockade also induced behavioral tolerance to the effect of haloperidol on spontaneous locomotor activity. Continuously moderate (60% for 24 h/day) or transiently high (80% for a few hours/day) D(2)-receptor blockade did not produce any of these effects. The long-term effect of haloperidol on D(2)-receptor density and behavioral tolerance thus appears to be dependent not only on a critical threshold of D(2)-receptor blockade but also on the daily duration of D(2)-receptors blockade. This suggests that as far as antipsychotics are concerned, not only dose but disbursment throughout the day have an impact on eventual pharmacodynamic and behavioral outcomes.

Dopamine D2-like receptor-mediated opening of K+ channels in opossum kidney cells

(1) This study examined the effects of dopamine D(1)- and D(2)-like receptor activation upon basolateral K(+) (I(K)) currents and changes in membrane potential in opossum kidney (OK) cells. (2) The addition of amphotericin B (3 micro g ml(-1)) to the apical side resulted in a rapid increase in I(K), this effect being markedly inhibited by the addition of the K(+) channel blockers barium chloride (1 mM) or glibenclamide (10 micro M), but not apamin (1 micro M). The K(+) channel opener pinacidil increased the amphotericin B-induced I(K). The selective D(2)-like receptor agonist quinerolane increased, in a concentration dependent manner (EC(50)=136 nM), I(K) across the basolateral membrane, this effect being abolished by pre-treatment with pertussis toxin (PTX), S-sulpiride (selective D(2)-like receptor antagonist) and glibenclamide. The selective D(1)-like receptor agonist SKF 38393 did not change I(K). Both H-89 (PKA inhibitor) and chelerythrine (PKC inhibitor) failed to prevent the stimulatory effect of quinerolane upon I(K). (3) Quinerolane did not change basal levels of cyclic AMP and also failed to affect the forskolin-induced increase in cyclic AMP levels. (4) The stimulation of D(2)-like receptor was associated with a rapid hyperpolarizing effect, whereas D(1)-like receptor activation was accompanied by increases in cell membrane potential. The hyperpolarizing effect of quinerolane (EC(50)=129 nM) was prevented by pre-treatment with PTX, S-sulpiride and glibenclamide. (5) It is concluded that stimulation of dopamine D(2)-like, but not D(1)-like, receptors coupled to PTX-sensitive G proteins of the G(i/o) class produce membrane hyperpolarization through opening of K(ATP) channels.

D2-like receptor-mediated inhibition of Na+-K+-ATPase activity is dependent on the opening of K+ channels

This study examined the effects of D2-like dopamine receptor activation on Na+-K+-ATPase activity while apical-to-basal, ouabain-sensitive, amphotericin B-induced increases in short-circuit current and basolateral K+ (I(K)) currents in opossum kidney cells were measured. The inhibitory effect of dopamin on Na+-K+-ATPase activity was completely abolished by either D1- or D2-like receptor antagonists and mimicked by D1- and D2-like receptor agonists SKF-38393 and quinerolane, respectively. Blockade of basolateral K+ channels with BaCl2 (1 mM) or glibenclamide (10 microM), but not apamin (1 microM), totally prevented the inhibitory effects of quinerolane. The K+ channel opener pinacidil decreased Na+-K+-ATPase activity. The inhibitory effect of quinerolane on Na+-K+- ATPase activity was abolished by pretreatment of opossum kidney cells with pertussis toxin (PTX). Quinerolane increased I(K) across the basolateral membrane in a concentration-dependent manner; this effect was abolished by pretreatment with PTX, S-sulpiride, and glibenclamide. SKF-38393 did not change I(K). Both H-89 (protein kinase A inhibitor) and chelerythrine (protein kinase C inhibitor) failed to prevent the stimulatory effect of quinerolane on I(K). The stimulation of the D2-like receptor was associated with a rapid hyperpolarizing effect, whereas D1-like receptor activation was accompanied by increases in cell membrane potential. It is concluded that stimulation of D2-like receptors leads to inhibition of Na+-K+-ATPase activity and hyperpolarization; both effects are associated with the opening of K+ channels.

Long-term effects of postnatal amphetamine treatment on striatal protein kinase A activity, dopamine D(1)-like and D(2)-like binding sites, and dopamine content

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would alter dopaminergic functioning in the dorsal striatum of adult rats. In three experiments, we assessed the effects of repeated amphetamine treatment on striatal protein kinase A (PKA) activity, dopamine (DA) D(1)-like and D(2)-like binding sites, and DA content. Rats were pretreated with saline or amphetamine (2.5 mg/kg, ip) for 7 consecutive days starting on postnatal day (PD) 11. At PD 90, rats were killed and their dorsal striata (i.e., caudate-putamen) were removed and frozen until time of assay. Amphetamine pretreatment produced long-term reductions in both striatal PKA activity and DA content. Early amphetamine exposure also resulted in an upregulation of D(2)-like binding sites, while leaving D(1)-like binding sites unaffected. It is likely that the upregulation of D(2)-like binding sites was stimulated by the persistent decline in striatal DA levels. Although speculative, it is possible that excess striatal D(2)-like receptors were responsible for inhibiting PKA activity through actions on the cAMP signal transduction pathway. The behavioral relevance of these amphetamine-induced neurochemical changes has not yet be determined.

Dopamine activates inward rectifier K+ channel in acutely dissociated rat substantia nigra neurones

The effect of dopamine (DA) was investigated on acutely dissociated rat substantia nigra pars compacta (SNc) neurones by using patch clamp recording. The SNc neurones could be classified into two groups. About 75% of large neurones (>30 microm in diameter) were tyrosine hydroxylase (TH) positive while almost all small neurones (<20 microm) were TH negative. In the large neurones, DA hyperpolarized the membrane, resulting in a reduction of the frequency of spontaneous action potentials in current-clamp mode and induced an inward rectifier K+ current in voltage-clamp mode. Quinpirole, a D2 receptor agonist, mimicked the DA action. S(-)-sulpiride, a D2 receptor antagonist, inhibited the DA-induced current (I(DA)) more effectively than SKF83566, a D1 receptor antagonist. Intracellular application of either guanosine 5'-O-(2-thiodiphosphate) (GDP-betaS) or pertussis toxin (IAP) suppressed I(DA). Guanosine 5'-O-(3-thiotriphosphate) (GTP-gammaS) sustained the DA response. Modulators for cAMP such as forskolin and isobutylmethylxathine, H-89, a protein kinase A inhibitor, and chelerythrine, a protein kinase C inhibitor, had no effect on I(DA). The frequency of DA-induced single channel currents in the inside-out patch configuration, for which the unitary conductance was 56.6pS, was greatly reduced by the replacement of GTP with GDP perfused at the cytosolic side. These results suggest that DA acts on a D2-like receptor and activates directly an IAP-sensitive G protein coupled with inward rectifier K+ channels, resulting in a decrease in the spontaneous firing activities of rat SNc dopaminergic neurones.

Partial agonism at serotonin 5-HT1B and dopamine D2L receptors using a luciferase reporter gene assay

We have used a luciferase reporter gene assay to study the functional responses of two G-protein-coupled receptors in Chinese hamster ovary (CHO) cells. The rank order of potency of drugs for the endogenous 5-HT1B receptor was 5-Hydroxytryptamine (5-HT) > zolmitriptan > dihydroergocristine > (-)lisuride (with no response to bromocriptine). However, only 5-HT and (-)lisuride produced a full functional response, with zolmitriptan and dihydroergocristine achieving 69+/-2% and 50+/-1% of the maximal response. In the same cells stably transfected with the rat dopamine D2L receptor, dopamine and bromocriptine produced a full agonist functional response, whilst (-)lisuride produced a biphasic response curve, indicating activity at both the endogenous 5-HT1B and exogenous dopamine D2L receptors. Using the receptor specific antagonists, pindolol and (+)butaclamol, (-)lisuride was shown to produce 52% of the maximal response at the dopamine D2 receptor relative to dopamine. In comparison to a cAMP accumulation assay, the rank orders of potency and intrinsic activity were the same for all compounds used. These results demonstrate that this reporter gene assay is capable of discriminating both potency and efficacy of drugs and can be used to characterise partial agonists at endogenously and heterologously expressed receptors in CHO cells.

Molecular mechanisms underlying forskolin-mediated up-regulation of human dopamine D2L receptors

1. Human dopamine (DA) D2long (hD2L) receptors, expressed by Ltk- cells, can be up-regulated by treating the cells with forskolin for 16 hr (Johansson and Westlind-Danielsson, 1994). We have examined some of the molecular mechanisms underlying this forskolin-mediated up-regulation. 2. Forskolin (100 microM, 16 hr), but not 1,9-dideoxyforskolin, a forskolin analogue that is unable to activate adenylyl cyclase and raise intracellular cAMP concentrations, up-regulates the hD2L receptor population by 43%. The implication of a cAMP-dependent increase in the receptor up-regulation was further substantiated by treating the cells with 8-bromo-cAMP or prostaglandin E1 (PGE1). The forskolin-mediated rise in receptor number was blocked by cycloheximide or an antisense phosphorothioate oligodeoxynucleotide (ODN) directed toward the hD2L mRNA. KT5720, a specific protein kinase A (PKA) inhibitor, completely blocked the receptor rise, whereas pertussis toxin (PTX) attenuated the increase considerably. Forskolin also produced an increase in the level of the DA hD2short (hD2S) receptor expressed by Ltk- cells. This increase was 2.5-fold higher than that found for the hD2L receptor. 3. The forskolin-mediated hD2L receptor rise is dependent on de novo protein synthesis, a rise in cAMP levels, PKA activation, and, at least partially, PTX-sensitive G proteins. 4. Long-term increases in intracellular cAMP levels may change the sensitivity of a DA receptor expressing cell to DA by increasing D2 receptor density through enhanced cAMP-dependent transcription.

Dopamine D2 receptor upregulation in rat neostriatum following in vivo infusion of forskolin

Intracerebroventricular (i.c.v.) forskolin infusion for 5 days resulted in a concentration-dependent increase in rat striatal dopamine (DA) D2 receptors measured with [3H]raclopride. In animals given 50 nmol/h forskolin, the highest concentration used, raclopride-mediated suppression of spontaneous locomotor activity was attenuated, and (+/-)-7-hydroxy-dipropyl-aminotetralin HBr (7-OH-DPAT)-mediated inhibition of striatal DA synthesis, as estimated by the accumulation of DOPA following inhibition of cerebral decarboxylase, was enhanced. These data suggest that the DA D2 receptor increase comprises receptors localized both post- and presynaptically. The density of striatal DA D1 receptors was also changed with the forskolin treatment, in a concentration-dependent fashion, but in the opposite direction to DA D2 receptors. These findings suggest that striatal DA receptor sensitivity can be changed by manipulation at the second messenger level (e.g. independent of direct neurotransmitter-receptor interactions) in vivo.

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)

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.