Specific [3H]raclopride binding to neostriatal dopamine D2 receptors: role of disulfide and sulfhydryl groups

Serotonin 2A receptor disulfide bridge integrity is crucial for ligand binding to different signalling states but not for its homodimerization

The serotonin 2_A (5-HT_2A) receptor is a G-protein coupled receptor (GPCR) with a conserved disulfide bridge formed by Cys¹⁴⁸ (transmembrane helix 3, TM3) and Cys²²⁷ (extracellular loop 2, ECL-2). We hypothesized that disulfide bridges may determine serotonin 5-HT_2A receptor functions such as receptor activation, functional selectivity and ligand recognition. We used the reducing agent dithiothreitol (DTT) to determine how the reduction of disulfide bridges affects radioligand binding, second messenger mobilization and receptor dimerization. A DTT-induced decrease in the number of binding sites (1190 ± 63.55 fmol/mg protein for control cells compared with 921.2 ± 60.84 fmol/mg protein for DTT-treated cells) as well as in the efficacy of both signalling pathways characterized was observed, although the affinity and potency were unchanged. Bioluminiscence resonance energy transfer (BRET) assays revealed the DTT treatment did not modify the homodimeric nature of serotonin 5-HT_2A receptors. In molecular dynamic simulations, the ECL-2 of the receptor with a broken cysteine bond adopts a wider variety of conformations, some of which protrude deeper into the receptor orthosteric binding pocket leading to collapse of the pocket. A shrunken binding pocket would be incapable of accommodating lysergic acid diethylamide (LSD). Our findings suggest that the decrease of efficacy may be due to disruption of disulfide bridge between TM3 and ECL-2. This reveals the integrity of the ECL-2 epitope, which should be explored in the development of novel ligands acting as allosteric modulators of serotonin 5-HT_2A receptors.

Cdk5 phosphorylates dopamine D2 receptor and attenuates downstream signaling

The dopamine D2 receptor (DRD2) is a key receptor that mediates dopamine-associated brain functions such as mood, reward, and emotion. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase whose function has been implicated in the brain reward circuit. In this study, we revealed that the serine 321 residue (S321) in the third intracellular loop of DRD2 (D2i3) is a novel regulatory site of Cdk5. Cdk5-dependent phosphorylation of S321 in the D2i3 was observed in in vitro and cell culture systems. We further observed that the phosphorylation of S321 impaired the agonist-stimulated surface expression of DRD2 and decreased G protein coupling to DRD2. Moreover, the downstream cAMP pathway was affected in the heterologous system and in primary neuronal cultures from p35 knockout embryos likely due to the reduced inhibitory activity of DRD2. These results indicate that Cdk5-mediated phosphorylation of S321 inhibits DRD2 function, providing a novel regulatory mechanism for dopamine signaling.

Physicochemical modulation of agonist-induced [35s]GTPgammaS binding: implications for coexistence of multiple functional conformations of dopamine D1-like receptors

Dopamine agonist-stimulated [35S]GTPgammaS binding to membrane G proteins was studied in select brain regions under experimental conditions that permit the activation of receptor coupling to the G proteins Gi, Gs, or Gq. Agents studied were agonists known to be effective at various dopamine receptor/effector systems and included quinelorane (D2-like/Gi), SKF38393 (D1-like/Gq, D1-like/Gs), SKF85174 (D1-like/Gs), and SKF83959 (D1-like/Gq). Dopamine and SKF38393 significantly stimulated [35S]GTPgammaS binding to normal striatal membranes by 161% and 67% above controls. Deoxycholate, which enhances agonist-induced phospholipase C (PLC) stimulation, markedly enhanced the agonistic effects of dopamine and SKF38393 to 530% and 637% above controls, respectively. The enhancing effects of deoxycholate were reversed if it was washed off the membranes before agonist addition. The thiol-reducing agent, dithiothreitol, completely abolished the effects of SKF38393 and SKF83959, whereas SKF85174 effects were augmented. Agonist responses were concentration-related, and highest efficacies were obtained in the hippocampus, thus paralleling both the brain regional distribution and agonist efficacies previously observed in phosphoinositide hydrolysis assays. These findings suggest that D1-like receptor conformations that mediate agonist stimulation of Gs/adenylylcyclase may be structurally different from those that mediate Gq/PLC activation. Although the exact mechanism of deoxycholate's effect awaits elucidation, the results are consistent with the emerging concept of functional selectivity whereby deoxycholate could create a membrane environment that facilitates the transformation of the receptor from a conformation that activates Gs/adenylylcyclase to one that favors Gq/PLC signaling.

Thiol Redox State and Lipid and Protein Oxidation in the Mouse Striatum after Pentylenetetrazol-induced Epileptic Seizure

PURPOSE

In the present study, we examined the effects of pentylenetetrazol (PTZ) administration on the thiol redox state (TRS), lipid peroxidation, and protein oxidation in the mouse striatum to (a) quantitate the major components of TRS and relate them to oxidative stress, and (b) investigate whether neuronal activation without synchronization, induced by subconvulsive doses of PTZ, can cause similar qualitative effects on TRS in this brain area. Specifically, we examined the TRS components glutathione (GSH), glutathione disulfide (GSSG), cysteine (CSH), protein thiols (PSH), and the protein (P) and nonprotein (NP/R) disulfides PSSR, NPSSR, NPSSC, and PSSP.

METHODS

TRS components were measured photometrically (GSSG enzymatically) as were lipid peroxidation and protein oxidation.

RESULTS

GSH, GSSG, and NPSSC levels are decreased by 45%, 38% and 26%, respectively, at 15 min after seizure; PSSP and PSSR levels and lipid peroxidation are increased by 47%, 200% and 22%, respectively, whereas CSH, NPSSR, PSH, PSSC, and protein carbonyl levels do not change. At 30 min after seizure, GSH, GSSG, CSH, NPSSC, and protein carbonyl levels are decreased by 26%, 62%, 25%, 40%, and 13%, respectively. PSSP and NPSSR levels are increased by 30% and 42%, respectively, whereas PSH, PSSC, PSSR, and lipid peroxidation remain unchanged. At 24 h after seizure, GSH, NPSSR, PSSR, and lipid-peroxidation levels return to normal; GSSG, CSH, NPSSC, and protein carbonyl levels are decreased by 44%, 22%, 30%, and 27%, respectively.

CONCLUSIONS

The significant decrease in GSH, GSSG, CSH, and NPSSC and the increase in PSSP, NPSSR, PSSR, and lipid peroxidation after PTZ-induced seizure strongly suggest increased oxidative stress in the mouse striatum.

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.

Demonstration of competition between endogenous dopamine and [11C]raclopride binding in in vitro brain slices using a dynamic autoradiography technique

To elucidate the mechanism of in vivo binding competition between radioligand and endogenously released transmitter, we examined the influence of depolarization-induced dopamine (DA) release on [11C]raclopride-specific binding to D2 receptors in slices of living brain tissues using dynamic positron autoradiography. Rat brain slices were incubated in a chamber with [11C]raclopride in oxygenated medium at 34 degrees C for 150 min. Two-dimensional images of radioactivity in the slices were recorded on a storage phosphor screen and dynamic changes were measured. When the brain slices were exposed to the depolarization agents (25 mM K+, 50 mM K+, and 20 microM veratridine), the percentage inhibition of striatal [11C]raclopride-specific binding was 22 +/- 4%, 44 +/- 8% and 54 +/- 7% of the control, respectively. The percentage inhibition of [11C]raclopride-specific binding during each depolarization treatment agreed proportionally with the amount of DA released into the medium. However, preexposure of brain slices to the same depolarization treatment (50 mM K+) did not affect the [11C]raclopride-specific binding, suggesting that the reduction in receptor density and/or affinity was not involved in the decrease of [11C]raclopride-specific binding. [11C]Raclopride-specific binding decreased dose-dependently in the presence of exogenously added DA (range 0.005-3mM). The synaptic DA concentration during each depolarization treatment estimated using three different methods. These results suggest that the decrease of [11C]raclopride-specific binding to striatal slices following evoked DA release is due to competition between endogenous DA and raclopride. It is unlikely that changes in D(2) receptor density or in affinity of the receptors for raclopride are involved. These results provide supportive evidence for in vivo binding competition between radioligand and endogenous neurotransmitter.

[3H]raclopride and [3H]spiroperidol binding to retinal membranes of the teleost Eugerres plumieri: effect of light and dark adaptation

Monoamine and metabolites were determined in the retina of the teleost Eugerres plumieri after dark and light adaptation. Dopamine, homovanillic acid and 3,4-dihydroxyphenylacetic acid increased after light exposure. The results indicate an increase in the turnover rate of dopamine due to light exposure. Dopamine D2 receptors were studied by determining the binding parameters of [3H]spiroperidol and [3H]raclopride to retinal membranes. The results were best fitted to a two-site model, where the high-affinity site may correspond to D2 receptors and the low-affinity site could be D4 receptors, which have been recently described in the retina, although further research is needed to confirm this suggestion. The number of sites labeled with [3H]spiroperidol was lower than with [3H]raclopride. This may indicate the existence of monomer and dimer conformations of D2-like receptors in the retina, as has been shown in the brain. Light exposure increased the number of sites labeled with both ligands. Since D2 receptors are known to modulate the production of melatonin, the augmentation in the capacity of these receptors could contribute to the reduction of melantonin during light exposure.

Modulation of behavioral and neurochemical measures of forebrain dopamine function in mice by species-specific interleukin-2

Interleukin-2 (IL-2) has recently been implicated as a modulator of brain neuronal function and in the pathogenesis of several major neuropsychiatric disorders involving the dopamine system (e.g. schizophrenia and Parkinson's disease). Little is known, however, about the effects of IL-2 on dopamine-mediated behaviors. A series of behavioral experiments were performed in mice to examine the hypothesis that species-specific IL-2 could modify behaviors known to be mediated by forebrain dopamine pathways. IL-2 administered subcutaneously produced a robust increase in locomotor activity in an elevated plus-maze. No effects of the cytokine were evident on measures of acoustic startle, prepulse inhibition of the startle response (PPI), or fearfulness. In complementary in vitro neurochemical experiments, to most closely assess physiologically relevant effects of the cytokine on dopamine release from striatal neurons, species-specific IL-2 as well as high performance liquid chromatography (HPLC) were used to measure endogenous dopamine release from striatal slices. IL-2 dose-dependently modulated veratrine-evoked release of endogenous dopamine in a biphasic pattern, increasing release at lower concentrations and inhibiting release at a high concentration of the cytokine. In radioligand competition binding experiments, IL-2 was not active at striatal binding sites for [3H]spiroperidol (D2-like receptors), [3H]mazindol binding (dopamine uptake sites) and [3H]SCH23390 (D1-like receptors), indicating that the neuromodulatory actions of IL-2 are not the result of direct or allosteric effects on dopamine receptors. Knowledge of the mechanisms by which IL-2 influences brain dopamine function could provide new insight into the pathophysiology of forebrain dopamine neurons seen in disorders such as Parkinson's disease and schizophrenia.