Therapeutic Effects of Selenium on Alpha-Synuclein Accumulation in Substantia Nigra Pars Compacta in a Rat Model of Parkinson's Disease: Behavioral and Biochemical Outcomes

Parkinson's disease (PD) is the second most common progressive neurodegenerative disorder characterized by the accumulation of accumulated alpha-synuclein (α-Syn) in substantia nigra. Research has shown that selenium (Se) can protect neural cells through the actions of selenoproteins, including selenoprotein P (SelP) and selenoprotein S (SelS), which participate in endoplasmic reticulum-associated protein degradation (ERAD). In this study, we investigated the potential protective role of Se in a pre-clinical PD rat model.We aimed to evaluate the therapeutic effects of Se administration in the 6-hydroxydopamine (6-OHDA) induced unilateral rat PD model. Male Wistar rats were utilised for unilateral PD animal model which were subjected to stereotaxic surgery and injected with 20 μg 6-OHDA/5 μl 0.2% ascorbate saline. After confirming the model, the rats were intraperitoneally injected with 0.1, 0.2, and 0.3 mg/kg of sodium selenite for 7 days. We then performed behavioral tests, including apomorphine-induced rotation, hanging, and rotarod tests. Following sacrifice, we analysed the substantia nigra area of the brain and serum for protein quantification, element analysis, and gene expression analysis.Our results indicate that the administration of 0.3 mg/kg of Se improved the motor deficiency in hanging, rotarod, and apomorphine-induced rotational tests. While there was no significant improvement in the expression of α-Syn, Se increased the expression of selenoproteins. Additionally, levels of selenoproteins, Se, and α-Syn both brain and serum were re-established by the treatment, suggesting the role of Se on the α-Syn accumulation. Furthermore, Se improved PD-induced biochemical deficits by increasing the levels of SelS and SelP (p<0.005).In conclusion, our findings suggest that Se may have a protective role in PD. 0.3 mg/kg dosage of Se increased the expression of selenoproteins, reduced the accumulation of α-Syn in the brain, and improved PD-induced motor deficits. These results suggest that Se may be a potential therapeutic option for PD treatment.

Apomorphine Reduces A53T α-Synuclein-Induced Microglial Reactivity Through Activation of NRF2 Signalling Pathway

The chiral molecule, apomorphine, is currently used for the treatment of Parkinson’s disease (PD). As a potent dopamine receptor agonist, this lipophilic compound is especially effective for treating motor fluctuations in advanced PD patients. In addition to its receptor-mediated actions, apomorphine has also antioxidant and free radical scavenger activities. Neuroinflammation, oxidative stress, and microglia reactivity have emerged as central players in PD. Thus, modulating microglia activation in PD may be a valid therapeutic strategy. We previously reported that murine microglia are strongly activated upon exposure to A53T mutant α-synuclein. The present study was designed to investigate whether apomorphine enantiomers could modulate this A53T-induced microglial activation. Taken together, the results provided evidence that apomorphine enantiomers decrease A53T-induced microgliosis, through the activation of the NRF2 signalling pathway, leading to a lower pro-inflammatory state and restoring the phagocytic activity. Suppressing NRF2 recruitment (trigonelline exposure) or silencing specifically Nfe2l2 gene (siRNA treatment) abolished or strongly decreased the anti-inflammatory activity of apomorphine. In conclusion, apomorphine, which is already used in PD patients to mimic dopamine activity, may also be suitable to decrease α-synuclein-induced microglial reactivity.

Apomorphine attenuates 6-hydroxydopamine-induced apoptotic cell death in SH-SY5Y cells

Enhanced oxidative stress is implicated in the pathogenesis of Parkinson's disease. The catecholaminergic neurotoxin 6-hydroxydopamine (6-OHDA) induces the production of reactive oxygen species (ROS), leading to neuronal cell death. On the other hand, apomorphine, a dopamine D1/D2 receptor agonist and known as a potent antioxidant, has been reported to have a neuroprotective effect. In the present study, we investigated the effect of apomorphine on 6-OHDA-induced apoptotic cell death using the human dopaminergic neuroblastoma cell line, SH-SY5Y. The co-treatment of cells with apomorphine significantly attenuated 6-OHDA-induced ROS generation, the phosphorylation of c-Jun N-terminal kinase (JNK), DNA fragmentation and subsequent apoptotic cell death. In addition, pretreatment with apomorphine for 24 h and the following concomitant treatment enhanced the protective effects against 6-OHDA-induced toxicity except for the attenuation of JNK phosphorylation. We also demonstrated that pretreatment alone with apomorphine for 24 h prior to the exposure confers resistance against 6-OHDA-induced cell toxicity. These findings suggested that apomorphine acts principally as a radical scavenger to suppress the level of ROS and ROS-stimulated apoptotic signaling pathway, whereas the other mechanisms might be involved in the protective effects.

Mechanisms of G protein activation via the D2 dopamine receptor: evidence for persistent receptor/G protein interaction after agonist stimulation

BACKGROUND AND PURPOSE

The aim of this report is to study mechanisms of G protein activation by agonists.

EXPERIMENTAL APPROACH

The association and dissociation of guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding at G proteins in membranes of CHO cells stably transfected with the human dopamine D(2short) receptor was studied in the presence of a range of agonists.

KEY RESULTS

Binding of [(35)S]GTPgammaS was dissociable in the absence of agonist and dissociation was accelerated both in rate and extent by dopamine, an effect which was blocked by the dopamine D(2) receptor antagonist raclopride and by suramin, which inhibits receptor/G protein interaction. A range of agonists of varying efficacy increased the rate of dissociation of [(35)S]GTPgammaS binding, with the more efficacious agonists resulting in faster dissociation. Agonists were able to dissociate about 70% of the pre-bound [(35)S]GTPgammaS, leaving a component which may not be accessible to the agonist-bound receptor. The dissociable component of the [(35)S]GTPgammaS binding was reduced with longer association times and increased [(35)S]GTPgammaS concentrations.

CONCLUSIONS AND IMPLICATIONS

These data are consistent with [(35)S]GTPgammaS binding being initially to receptor-linked G proteins and then to G proteins which have separated from the agonist bound receptor. Under the conditions used typically for [(35)S]GTPgammaS binding assays, therefore, much of the agonist-receptor complex remains in proximity to G proteins after they have been activated by agonist.

Neuroprotective effects of caffeine in the model of 6-hydroxydopamine lesion in rats

The work shows the effects of caffeine after the intrastriatal injection of 6-OHDA in rats, considered as a model of Parkinson disease (PD). Two weeks after the 6-OHDA lesion, rats exhibit a characteristic rotation behavior as a response to the apomorphine challenge. Our results showed significant increases in the number of apomorphine-induced rotations in 6-OHDA-lesioned rats, as compared to sham-operated animals. A partial recovery was observed in 6-OHDA-lesioned rats, after caffeine (10 and 20 mg/kg, i.p., daily for 14 days) treatment. The stereotaxic injection of 6-OHDA produced loss of striatal neurons, as indicated by the decrease in monoamines levels, in the ipsilateral side (75-85%) when compared to the contralateral side. Significant decreases in noradrenaline levels were seen in the ipsilateral side of 6-OHDA group (62%), and this effect was not significantly reversed in caffeine-treated groups. While significant decreases in dopamine levels were seen in the ipsilateral side of 6-OHDA group (78%), in the caffeine-treated group (10 and 20 mg/kg, i.p.) the decreases were only 53 and 18%, indicating significant recoveries. In conclusion, our data demonstrated beneficial effects of caffeine in this model of PD, suggesting the potential use of A2A antagonists as a novel treatment for this neurodegenerative disease.

Synthesis and binding studies of 2-arylapomorphines

From codeine, four different 2-aryl substituted apomorphines were synthesised in 6 steps each. Oxidation of codeine with IBX followed by acid catalysed rearrangement gave morphothebaine, which was selectively triflylated at the 2-position and subsequently O-acetylated at the 11-position. The resulting triflate was coupled in a Suzuki-Miyaura type reaction with a series of 4-substituted arylboronic esters which, after deprotection, gave the desired 2-aryl apomorphines. The analogues were tested for affinity towards a range of dopaminergic, serotonergic and adrenergic receptors. 2-(4-Hydroxyphenyl)-apomorphine exhibited high affinity for the dopamine D2 receptor. A putative ligand-receptor interaction was put forward.

Mechanisms of inverse agonist action at D2 dopamine receptors

Mechanisms of inverse agonist action at the D2(short) dopamine receptor have been examined. Discrimination of G-protein-coupled and -uncoupled forms of the receptor by inverse agonists was examined in competition ligand-binding studies versus the agonist [3H]NPA at a concentration labelling both G-protein-coupled and -uncoupled receptors. Competition of inverse agonists versus [3H]NPA gave data that were fitted best by a two-binding site model in the absence of GTP but by a one-binding site model in the presence of GTP. K(i) values were derived from the competition data for binding of the inverse agonists to G-protein-uncoupled and -coupled receptors. K(coupled) and K(uncoupled) were statistically different for the set of compounds tested (ANOVA) but the individual values were different in a post hoc test only for (+)-butaclamol. These observations were supported by simulations of these competition experiments according to the extended ternary complex model. Inverse agonist efficacy of the ligands was assessed from their ability to reduce agonist-independent [35S]GTP gamma S binding to varying degrees in concentration-response curves. Inverse agonism by (+)-butaclamol and spiperone occurred at higher potency when GDP was added to assays, whereas the potency of (-)-sulpiride was unaffected. These data show that some inverse agonists ((+)-butaclamol, spiperone) achieve inverse agonism by stabilising the uncoupled form of the receptor at the expense of the coupled form. For other compounds tested, we were unable to define the mechanism.

Measurement of the proportion of D2 receptors configured in state of high affinity for agonists in vivo: a positron emission tomography study using [11C]N-propyl-norapomorphine and [11C]raclopride in baboons

Dopamine D2 receptors are configured in interconvertible states of high (D(2 high)) or low (D(2 low)) affinity for agonists. The in vivo proportion of sites in high-affinity state remains poorly documented. Previous studies have established the D2 agonist [11C]N-propyl-norapomorphine (NPA) as a suitable positron emission tomography radiotracer for imaging D(2 high) in the living brain. To elucidate the proportion of D2 receptors configured in D(2 high) states in vivo, imaging studies were conducted in three baboons with both [11C]NPA and the D2 receptor antagonist [11C]raclopride. These studies were performed under noncarrier- and carrier-added conditions, to compare the Bmax of [11C]NPA and [11C]raclopride in the same animals. [11C]raclopride in vivo KD and Bmax were 1.59 +/- 0.28 nM (n = 3) and 27.3 +/- 3.9 nM (n = 3), respectively. The in vivo KD of [11C]NPA was 0.16 +/- 0.01 nM (n = 3), consistent with its affinity for D(2 high) reported in vitro. The maximal density of sites for [11C]NPA was 21.6 +/- 2.8 nM (n = 3), i.e., 79% of the [11C]raclopride Bmax. This result suggested that 79% of D2 receptors are configured as D(2 high) in vivo. This large proportion of D(2 high) sites might explain the vulnerability of D2 radiotracers to competition by endogenous dopamine, and is consistent with a previous report that the in vivo binding of agonist radiotracer [11C]NPA is more vulnerable to competition by endogenous dopamine than that of antagonist radiotracer [11C]raclopride.

Chiral capillary electrophoretic method for quantification of apomorphine

A new method for chiral determination of apomorphine enantiomers was developed and validated. Seven different neutral and charged cyclodextrins were tested for enantioselectivity on R,S-apomorphine. Sulfobutylether-beta-cyclodextrin was found to offer the best resolution, but with this system, four peaks were detected from a solution of the two enantiomers, which was suggested to be the result of different forms of the complex between the selector and apomorphine. A complexation constant was estimated for a complex of 1:1 ratio for the second and the fourth peak, whereas the other two peaks were fitted to a model ratio of 1:2 (analyte-selector). To avoid this phenomenon, hydroxypropyl-beta-cyclodextrin was then chosen as the chiral selector. An optimisation study was performed on three factors: concentration of the chiral selector, pH of the buffer, and applied voltage. Optimum conditions were: 14 mM of hydroxypropyl-beta-cyclodextrin, pH 3.0, and 16 kV. UV detection was at 200 nm. The method was validated at the chosen conditions, offering a limit of detection of 0.2 microM and a limit of quantification of 0.5 microM. The validated method was applied for the determination of R,S-apomorphine in a transport study with an in vitro cell culture model of the intestinal mucosa (Caco-2).

Sulfation of apomorphine by human sulfotransferases: evidence of a major role for the polymorphic phenol sulfotransferase, SULT1A1

1. The relative roles of various members of the human sulfotransferase (SULT) enzyme family in the metabolism of apomorphine, a dopamine receptor antagonist used in the treatment of Parkinson's disease and, more recently, erectile dysfunction, were examined. In humans, sulfation is the major route of metabolism of this drug. 2. Using recombinant SULTs expressed in Escherichia coli, R(--)-apomorphine sulfation was studied using the universal barium precipitation assay in the presence of [35S] 3'-phosphoadenosine 5'-phosphosulfate and SULTs 1A1, 1A2, 1A3, 1B1, 1C2, 1E1 and 2A1. It was shown that SULTs 1A1, 1A2, 1A3 and 1E1 all sulfated apomorphine to varying extents. Low activity with SULT1B1 was only seen at the highest concentration (100 microM) and no activity with SULT1C2 or SULT2A1 was observed. 3. Kinetic analysis using purified recombinant SULTs showed that 1A1, 1A3 and 1E1 all had similar Vmax/Km values, although SULT1E1 had a slightly lower Km at around 1 microM compared with approximately 4 microM for the other SULTs. 4. By correlating apomorphine sulfation (at 10 microM) in a bank of 28 liver cytosols with SULT activity towards 10 microM 4-nitrophenol (SULT1A1) and 0.2 microM 17beta-oestradiol (SULT1E1), a strong correlation with SULT1A1 activity was clearly demonstrated, suggesting this enzyme was primarily responsible for hepatic apomorphine sulfation. 5. These findings were confirmed using immuno-inhibition experiments with antibodies against SULT1A and SULT1E1, which showed preferential inhibition of apomorphine sulfation in human liver cytosol by anti-SULT1A. 6. The results strongly implicate SULT1A1 as the major enzyme responsible for hepatic apomorphine metabolism. As SULT1A1 is subject to a common functional polymorphism, sulfation phenotype may be an important determinant of susceptibility to side-effects of apomorphine and/or efficacy of treatment.

Subcutaneously administered apomorphine: Pharmacokinetics and metabolism

Apomorphine is a non-narcotic morphine derivative that acts as a potent dopaminergic agonist. Its high first-pass hepatic metabolism prevents effectiveness by the oral route; instead, subcutaneous injection is the usual route, and intranasal, sublingual, rectal, and iontophoretic transdermal delivery has been investigated for the treatment of Parkinson's disease (PD). The rate of uptake after subcutaneous injection is influenced by factors such as location, temperature, depth of injection, and body fat. Studies have shown the latency of onset to clinical effect after s.c. injection ranged from 7.3 to 14 minutes. Cerebrospinal fluid T(max)lags behind plasma T(max) by 10 to 20 minutes. Considerable intersubject variability is found with pharmacokinetic variables; in some studies there are five- to tenfold differences in C(max)and area-under-the-concentration-time-curve seen in PD patients. Apomorphine metabolism occurs through several enzymatic pathways, including N-demethylation, sulfation, glucuronidation, and catechol-O-methyltransferase as well as by nonenzymatic oxidation. The complexities of apomorphine uptake, distribution, and clearance probably contribute to its variability of clinical actions.

Iso-lactam and reduced amide analogues of the peptidomimetic dopamine receptor modulator 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide

An analogue of the highly potent gamma-lactam Pro-Leu-Gly-NH(2) peptidomimetic, 3(R)-[(2(S)-pyrrolidinylcarbonyl) amino]-2-oxo-1-pyrrolidineacetamide (2), 4(R)-[[2(S)-pyrrolidinylcarbonyl]amino]-2-oxo-1-pyrrolidineacetamide (3), in which the lactam carbonyl moiety has been placed in a different position with respect to the 3-amino group was synthesized. Also, a series of analogues of 2, compounds 4-6, were synthesized in which each of the amide bonds of 2 were systematically replaced with a reduced amide bond surrogate. The analogues were tested for their ability to enhance the binding of [3H]N-propylnorapomorphine to dopamine receptors in a functional in vitro assay utilizing bovine striatal membranes. Peptidomimetic 3 was shown to be more potent than 2, while 4 and 5 were significantly less effective than 2. Peptidomimetic 6 had a pharmacological profile similar to that of 2.

Opposite vascular activity of (R)-apomorphine and its oxidised derivatives. Endothelium-dependent vasoconstriction induced by the auto-oxidation metabolite

We have synthetised a series of oxidised apomorphine derivatives (orto and para quinones 2-5), in order to analyse their vascular activity. We have performed radioligand binding assays on rat cortical membranes and functional studies on rat aortic rings. Instead the relaxant activity exhibited by (R)-apomorphine, o-quinones 2, 4, show contractile activity dependent on endothelium in rat aortic rings. Compound 2, the main metabolite of (R)-apomorphine auto-oxidation, was the product which showed enhanced contractile activity by a complex mechanism related to activation of Ca(2+) channels through release and/or inhibition of endothelial factors. Moreover, this compound disrupts the endothelial function as shows the lack of response to acetylcholine observed in vessels pretreated with it.

Quercetin inhibits the sulfation of r(-)-apomorphine in human brain

The first aim of this investigation was to study the sulfation of R(-)-apomorphine in human brain. The second aim was to investigate the inhibition of R(-)-apomorphine sulfation by quercetin in human brain. R(-)-apomorphine is hereafter referred to as apomorphine. Apomorphine sulfation was measured in 5 brain specimens; 3 derived from the frontal cortex and 2 derived from the temporal cortex. The rate of apomorphine sulfation was 5.6 +/- 4.3 pmol/min/mg. The activities of SULT1A1 and SULT1A3, which were also measured in these samples, were 11 +/- 9.1 and 2.6 +/- 1.7 pmol/min/mg, respectively. The rate of apomorphine sulfation correlated with the activity of SULT1A1 (r = 0.989; p = 0.002) and SULT1A3 (r = 0.973; p = 0.005). Apomorphine sulfotransferase followed Michaelis-Menten kinetics, the Km (mean +/- SD) and Vmax values (mean +/- SD) of which, measured in 5 brain samples, were 32 +/- 7.3 microM and 8.9 +/- 7.9 pmol/min/mg, respectively. Quercetin was a potent inhibitor of apomorphine sulfation with an IC50 value, measured in 5 brain samples, of 16 +/- 2.3 nM. The inhibition mechanism of quercetin using apomorphine sulfation in 5 brain samples was mixed, non-competitive with a Ki and Kies (mean +/- SD) of 16 +/- 4.1 and 87 +/- 37 nM, respectively (p = 0.008). The intrinsic clearance value of apomorphine (mean +/- SD) was 247 +/- 170 ml/min/mg(-1) and was decreased to 100 +/- 85 ml/min/mg(-1) (p < 0.01) in the presence of 25 nM quercetin. In conclusion, apomorphine is sulfated in human brain. Sulfation might reduce the level of apomorphine in human brain and be a factor limiting the effect of this drug. Quercetin is a potent inhibitor of apomorphine sulfation and may inhibit the sulfation of apomorphine in human brain in vivo.

Apomorphine induces trophic factors that support fetal rat mesencephalic dopaminergic neurons in cultures

Apomorphine, the catechol-derived dopamine D1/D2 receptor agonist, is currently in use as an antiparkinsonian drug. It has previously been reported that apomorphine was able to elicit expression of the enzyme tyrosine hydroxylase, a marker for DA neurons, in the fetal rat cerebrocortical cultures whilst in the presence of brain-derived neurotrophic factor. The present study demonstrated that treatment of fetal rat ventral mesencephalic cultures with apomorphine caused a marked increase in the number of dopaminergic neurons. The action of apomorphine can be mimicked by dopamine receptor (D1 and D2) agonists or blocked by preincubation with D1/D2 receptor antagonists. Incubation of recipient mesencephalic cultures with the conditioned medium derived from apomorphine-stimulated donor mesencephalic cultures elicited a 3.72-fold increase in the number of TH-positive neurons. Increased mRNA expression levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor were also found in the apomorphine-treated mesencephalic cells along with concomitant protein expression increases in the conditioned medium. Moreover, the trophic activity observed could be partially neutralized by antibodies against either brain-derived neurotrophic factor or glial cell line-derived neurotrophic factor. Cultured fetal striatal cells, but not hippocampal cells, also responded to apomorphine treatment. The membrane filtration studies revealed that both <30 kDa and >50 kDa fractions contained trophic activities. The latter characterization distinguishes them from most known neurotrophic factors. These results suggest that the apomorphine-modulated development of dopaminergic neurons may be mediated by activation of the dopamine receptor subtypes D1 and D2 thereby increasing the production of multiple growth factors.

Sulfation of R(-)-apomorphine in the human liver and duodenum, and its inhibition by mefenamic acid, salicylic acid and quercetin

1. The aims were to study the sulfation of R-(-)-apomorphine (hereafter apomorphine) in the human liver and duodenum, and to study the rate of inhibition of apomorphine sulphation by mefenamic acid, salicylic acid and quercetin also in the human liver and duodenum. 2. A rapid and sensitive method was developed to measure the sulfation rate of apomorphine in the human liver and duodenum. The method was based on the use of 0.4 micro M 3'-phosphoadenosine-5'-phosphosulfate-[(35)S] (PAPS) and 50 micro M apomorphine. The unreacted PAPS was precipitated with barium hydroxide, barium acetate and zinc sulfate. 3. The rate of apomorphine sulfation (mean +/- SD and median) was 261 +/- 82 and 242 pmol min(-1) mg(-1), respectively (liver), and 433 +/- 157 and 443 pmol min(-1) mg(-1), respectively (duodenum). The apomorphine sulfation rate was higher in the duodenum than in the liver (p = 0.0005). 4. Apomorphine sulfation was correlated with SULT1A1 activity in the liver (r(2) = 0.363, p = 0.005) and duodenum (r(2) = 0.494, p = 0.0005), but it did not correlate with SULT1A3 activity both in the liver and duodenum. 5. The K(m) estimate of apomorphine sulfation rate was 20 +/- 3.6 (liver) and 6.5 +/- 0.2 microM (duodenum, p = 0.024), and the V(max) estimate was 248 +/- 99 (liver) and 636 +/- 104 pmol min(-1) mg(-1) (duodenum, p = 0.018). 6. Mefenamic acid, salicylic acid and quercetin were potent inhibitors of apomorphine sulfation rate in the liver, and the IC(50) estimates were 16 +/- 0.2 nM, 54 +/- 8.6 microM and 18 +/- 2.8 nM, respectively. These compounds were poor inhibitors of apomorphine sulfation in the duodenum. 7. Apomorphine is sulfated by the human liver and duodenum, the highest activity being associated with the duodenum. The K(m) of apomorphine sulfotransferase is in the order of micro M both in the liver and duodenum. The non-steroidal anti-inflammatory drug mefenamic acid and the natural flavonoid quercetin inhibit the hepatic sulfation of apomorphine with an IC(50) in the order of nM.

Reversible and irreversible components of [(3)H]-N-propylnorapomorphine interaction with rat striatal membranes

The kinetics of L-(-)-[N-propyl-(3)H(N)]-norapomorphine ([(3)H]NPA) interactions with rat striatal membranes were studied. The analysis revealed that in addition to specific dopaminergic binding a substantial part of the radioligand was bound irreversibly to heterogeneous populations of non-specific binding sites of these membranes. The specific binding of [(3)H]NPA with dopamine receptors, determined from the differences of kinetic curves of total and non-specific binding, was fast, reversible, and revealed high affinity. The irreversible component was heterogeneous and seems to be related to oxidative degradation of the radioligand, as the rate of this process was substantially reduced by antioxidants like ascorbic acid and dithiothreitol.

Differential neurobehavioral deficits induced by apomorphine and its oxidation product, 8-oxo-apomorphine-semiquinone, in rats

Apomorphine is a potent dopamine receptor agonist, which has been used in the therapy of Parkinson's disease. It has been proposed that apomorphine and other dopamine receptor agonists might induce neurotoxicity mediated by their quinone and semiquinone oxidation derivatives. The aim of the present study was to evaluate the possible neurobehavioral effects of apomorphine and its oxidation derivative, 8-oxo-apomorphine-semiquinone (8-OASQ). Adult female Wistar rats were treated with a systemic injection of apomorphine (0.05 or 0.5 mg/kg) or 8-OASQ (0.05 or 0.5 mg/kg) 20 min before behavioral testing. Apomorphine and 8-OASQ induced differential impairing effects on short- and long-term retention of an inhibitory avoidance task. Apomorphine, but not 8-OASQ, dose-dependently impaired habituation to a novel environment. The memory-impairing effects could not be attributed to reduced nociception or other nonspecific behavioral alterations, since neither apomorphine nor 8-OASQ affected footshock reactivity or behavior during exploration of an open field. The results suggest that oxidation products of dopamine or dopamine receptor agonists might induce cognitive deficits.

Specific binding of [(11)C]raclopride and N-[(3)H]propyl-norapomorphine to dopamine receptors in living mouse striatum: occupancy by endogenous dopamine and guanosine triphosphate-free G protein

According to the ternary complex model of G-protein linkage to receptors, agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radioligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should have a greater effect on the binding of the D2,3 partial agonist N-[3H]propylnorapomorphine than on the binding of the D2,3 antagonist [(11)C]raclopride. The baseline binding potential (pB(0)), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine, was simultaneously measured in mouse striatum for [(11)C]raclopride (pB(0) = 8.5) and N-[(3)H]propylnorapomorphine (p'B(0) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice, the pB of N-[(3)H]propylnorapomorphine decreased 52% whereas the pB of [(11)C]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore, amphetamine decreased the pB of N-[(3)H]propylnorapomorphine to a greater extent (73%) than that of [(11)C]raclopride (43%) relative to the reserpine condition. For both radioligands, the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the pharmacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine occupancy and decreased to a value 1.66 times greater for N-[(3)H]propylnorapomorphine than for [(11)C]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist binding is more sensitive than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of dopamine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[(3)H]propylnorapomorphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.

Oxidative behaviour of apomorphine and its metabolites

The metabolism of apomorphine is quite complex due to interactions with proteins and other tissue components that affect its pharmacokinetic profile. The electrochemical oxidation mechanism of apomorphine and of some synthesised apomorphine derivatives was studied. It was found to be related to the reaction of o-diphenol and tertiary amine groups and strongly dependent on pH.

Agonist-independent and -dependent oligomerization of dopamine D(2) receptors by fusion to fluorescent proteins

Oligomerization of the short (D(2S)) and long (D(2L)) isoforms of the dopamine D(2) receptor was explored in transfected Cos-7 cells by their C-terminal fusion to either an enhanced cyan or enhanced yellow fluorescent protein (ECFP or EYFP) and the fluorescent fusion protein interaction was monitored by a fluorescence resonance energy transfer (FRET) assay. The pharmacological properties of the fluorescent fusion proteins, as measured by both displacement of [(3)H]nemonapride binding and agonist-mediated stimulation of [(35)S]GTPgammaS binding upon co-expression with a G(alphao)Cys(351)Ile protein, were not different from the respective wild-type D(2S) and D(2L) receptors. Co-expression of D2S:ECFP+D2S:EYFP in a 1:1 ratio and D2L:ECFP+D2L:EYFP in a 27:1 ratio resulted, respectively, in an increase of 26% and 16% in the EYFP-specific fluorescent signal. These data are consistent with a close proximity of both D(2S) and D(2L) receptor pairs of fluorescent fusion proteins in the absence of ligand. The agonist-independent D(2S) receptor oligomerization could be attenuated by co-expression with either a wild-type, non-fluorescent D(2S) or D(2L) receptor subtype, but not with a distinct beta(2)-adrenoceptor. Incubation with the agonist (-)-norpropylapomorphine dose-dependently (EC(50): 0.23+/-0.06 nM) increased the FRET signal for the co-expression of D2S:ECFP and D2S:EYFP, in support of agonist-dependent D(2S) receptor oligomerization. In conclusion, our data strongly suggest the occurrence of dopamine D(2) receptor oligomers in intact Cos-7 cells.

Receptor crosstalk protein, calcyon, regulates affinity state of dopamine D1 receptors

The recently cloned protein, calcyon, potentiates crosstalk between G(s)-coupled dopamine D1 receptors and heterologous G(q/11)-coupled receptors allowing dopamine D1 receptors to stimulate intracellular Ca(2+) release, in addition to cAMP production. This crosstalk also requires the participating G(q/11)-coupled receptors to be primed by their agonists. We examined the ability of calcyon and priming to regulate the affinity of dopamine D1 receptors for its ligands. Receptor binding assays were performed on HEK293 cell membrane preparations expressing dopamine D1 receptors either alone or in combination with calcyon. Co-expression of dopamine D1 receptor and calcyon affected neither the affinity of this receptor for antagonists nor the affinity of agonist binding to this receptor high and low-affinity states. However, the presence of calcyon dramatically decreased the proportion of the high-affinity dopamine D1 receptor agonist binding sites. This decrease was reversed by carbachol, which primes the receptor crosstalk by stimulating endogenous G(q/11)-coupled muscarinic receptors. Our findings suggest that calcyon regulates the ability of dopamine D1 receptors to achieve the high-affinity state for agonists, in a manner that depends on priming of receptor crosstalk.

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.

Glucuronidation of apomorphine

Apomorphine, a dopaminergic receptor agonist, is largely used in the therapy of Parkinson's disease. In this study, we characterized the glucuronidation of apomorphine and other catechols in rat liver and brain microsomes, using UDP-[U-14C]glucuronic acid and separation of the glucuronides formed by a thin layer chromatographic method. rat liver microsomes glucuronidate apomorphine at a significant rate, that was increased in the presence of dithiothreitol. Two apomorphine glucuronides were separated by high pressure liquid chromatography. We showed by electrospray mass spectrometry that both products were monoglucuronides. Other catechols were also glucuronidated in liver microsomes at various rates, and among them, 4-nitrocatechol was the most efficiently conjugated. in rat brain microsomes, only 4-nitrocatechol was significantly glucuronidated, suggesting that in the liver, several uridine-diphosphate glucuronosyltransferase (UGT) isoforms participate to the conjugation of catechols. To determine which isoforms catalyze apomorphine glucuronidation, two recombinant enzymes expressed in V79 cells were used. The isoform UGT1A6 was unable to glucuronidate apomorphine, but we observed a significant activity catalyzed by the isoform UGT2B1. These results provide, to our knowledge, the first demonstration of apomorphine conjugation by recombinant UGT2B1, and the first evidence of the lack of apomorphine glucuronidation in the rat brain.