Is the functional interaction between adenosine A2A receptors and metabotropic glutamate 5 receptors a general mechanism in the brain? Differences and similarities between the striatum and the hippocampus

The aim of the present paper was to examine, in a comparative way, the occurrence and the mechanisms of the interactions between adenosine A_2A receptors (A_2ARs) and metabotropic glutamate 5 receptors (mGlu5Rs) in the hippocampus and the striatum. In rat hippocampal and corticostriatal slices, combined ineffective doses of the mGlu5R agonist 2-chloro-5-hydroxyphenylglycine (CHPG) and the A_2AR agonist CGS 21680 synergistically reduced the slope of excitatory postsynaptic field potentials (fEPSPs) recorded in CA1 and the amplitude of field potentials (FPs) recorded in the dorsomedial striatum. The cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway appeared to be involved in the effects of CGS 21680 in corticostriatal but not in hippocampal slices. In both areas, a postsynaptic locus of interaction appeared more likely. N-methyl-D-aspartate (NMDA) reduced the fEPSP slope and FP amplitude in hippocampal and corticostriatal slices, respectively. Such an effect was significantly potentiated by CHPG in both areas. Interestingly, the A_2AR antagonist ZM 241385 significantly reduced the NMDA-potentiating effect of CHPG. In primary cultures of rat hippocampal and striatal neurons (ED 17, DIV 14), CHPG significantly potentiated NMDA-induced lactate dehydrogenase (LDH) release. Again, such an effect was prevented by ZM 241385. Our results show that A_2A and mGlu5 receptors functionally interact both in the hippocampus and in the striatum, even though different mechanisms seem to be involved in the two areas. The ability of A_2ARs to control mGlu5R-dependent effects may thus be a general feature of A_2ARs in different brain regions (irrespective of their density) and may represent an additional target for the development of therapeutic strategies against neurological disorders.     

N-methyl-D-aspartate (NMDA) receptor composition modulates dendritic spine morphology in striatal medium spiny neurons

Dendritic spines of medium spiny neurons represent an essential site of information processing between NMDA and dopamine receptors in striatum. Even if activation of NMDA receptors in the striatum has important implications for synaptic plasticity and disease states, the contribution of specific NMDA receptor subunits still remains to be elucidated. Here, we show that treatment of corticostriatal slices with NR2A antagonist NVP-AAM077 or with NR2A blocking peptide induces a significant increase of spine head width. Sustained treatment with D1 receptor agonist (SKF38393) leads to a significant decrease of NR2A-containing NMDA receptors and to a concomitant increase of spine head width. Interestingly, co-treatment of corticostriatal slices with NR2A antagonist (NVP-AAM077) and D1 receptor agonist augmented the increase of dendritic spine head width as obtained with SKF38393. Conversely, NR2B antagonist (ifenprodil) blocked any morphological effect induced by D1 activation. These results indicate that alteration of NMDA receptor composition at the corticostriatal synapse contributes not only to the clinical features of disease states such as experimental parkinsonism but leads also to a functional and morphological outcome in dendritic spines of medium spiny neurons.     

Regulation of striatal nitric oxide synthesis by local dopamine and glutamate interactions

Nitric oxide (NO) is a key neuromodulator of corticostriatal synaptic transmission. We have shown previously that dopamine (DA) D1/5 receptor stimulation facilitates neuronal NO synthase (nNOS) activity in the intact striatum. To study the impact of local manipulations of D1/5 and glutamatergic NMDA receptors on striatal nNOS activity, we combined the techniques of in vivo amperometry and reverse microdialysis. Striatal NO efflux was monitored proximal to the microdialysis probe in urethane‐anesthetized rats during local infusion of vehicle or drug. NO efflux elicited by systemic administration of SKF‐81297 was blocked following intrastriatal infusion of: (i) the D1/5 receptor antagonist SCH‐23390, (ii) the nNOS inhibitor 7‐nitroindazole, (iii) the non‐specific ionotropic glutamate receptor antagonist kynurenic acid, and (iv) the selective NMDA receptor antagonist 3‐phosphonopropyl‐piperazine‐2‐carboxylic acid. Glycine co‐perfusion did not affect SKF‐81297‐induced NO efflux. Furthermore, intrastriatal infusion of SKF‐81297 potentiated NO efflux evoked during electrical stimulation of the motor cortex. The facilitatory effects of cortical stimulation and SKF‐81297 were both blocked by intrastriatal infusion of SCH‐23390, indicating that striatal D1/5 receptor activation is necessary for the activation of nNOS by corticostriatal afferents. These studies demonstrate for the first time that reciprocal DA‐glutamate interactions play a critical role in stimulating striatal nNOS activity.     

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A–D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA_A receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.     

Involvement of spinal release of α-neo-endorphin on the antinociceptive effect of TAPA

The antinociceptive effect of i.t.-administered Tyr-d-Arg-Phe-β-Ala (TAPA), an N-terminal tetrapeptide analog of dermorphin, was characterized in ddY mice. In the mouse tail-flick test, TAPA administered i.t. produced a potent antinociception. The antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with the κ-opioid receptor antagonist nor-binaltorphimine, as well as by the μ-opioid receptor antagonist β-funaltrexamine and the μ₁-opioid receptor antagonist naloxonazine. TAPA-induced antinociception was also significantly suppressed by co-administration of the μ₁-opioid receptor antagonist Tyr-d-Pro-Phe-Phe-NH₂ (d-Pro²-endomorphin-2) but not by co-administration of the μ₂-opioid receptor antagonists Tyr-d-Pro-Trp-Phe-NH₂ (d-Pro²-endomorphin-1) and Tyr-d-Pro-Trp-Gly-NH₂ (d-Pro²-Tyr-W-MIF-1). In CXBK mice whose μ₁-opioid receptors were naturally reduced, the antinociceptive effect of TAPA was markedly suppressed compared to the parental strain C57BL/6ByJ mice. Moreover, the antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with antiserum against the endogenous κ-opioid peptide α-neo-endorphin but not antisera against other endogenous opioid peptides. In prodynorphin-deficient mice, the antinociceptive effect of TAPA was significantly reduced compared to wild-type mice. These results suggest that the spinal antinociception induced by TAPA is mediated in part through the release of α-neo-endorphin in the spinal cord via activation of spinal μ₁-opioid receptors.     

Further contribution to the study of corticostriatal glutamatergic and nigrostriatal dopaminergic interactions within the striatal network: an in vivo voltammetric investigation

Summary In vivo voltammetry was used in freely moving rats to study the processes whereby striatal dopamine (DA) release is regulated by corticostriatal glutamatergic neurons. Electrical stimulation of the cerebral cortex was found to markedly increase the striatal DA-related voltammetric signal amplitude. Similar enhancements have been observed after intracerebroventricular administration of 10nmoles glutamate, quisqualate and AMPA, whereas NMDA was found to decrease the amplitude of the striatal signals. The NMDA receptor antagonist APV did not significantly affect the voltammetric signal but prevented the NMDA-induced depression of the DA-related signals. These data are in agreement with those obtained in numerous previous studies suggesting that the glutamatergic corticostriatal neurons exert activatory effects on the striatal DA release via non-NMDA receptors. The mechanism involved might be of a presynaptic nature. The role of the NMDA receptors may however consist of modulating the dopaminergic transmission phasically and in a depressive way, which would be consistent with behavioural data suggesting the existence of a functional antagonism between the activity of the corticostriatal glutamatergic and nigrostriatal dopaminergic systems.Abbreviations Glu glutamate - DA dopamine - NMDA N-methyl-D-aspartate - CPP 3-(2-carboxypiperazin-4µl)propyl-1-phosphonic acid - AMPA -amino-3-hydroxy-5-metylisoxazole-4-propionic acid - APV aminophosphonovaleric acid - DOPAC dihydroxyphenylacetic acid - HVA homovanillic acid - DARPP 32 dopamine-cAMP-regulated phosphoprotein 32 - CSF cerebrospinal fluid Laboratory associated with the University of Aix-Marseille II     

Comparative pharmacology of two D1-like dopamine receptors cloned from the silkworm Bombyx mori

Dopamine (DA) is a physiologically important biogenic amine in insect peripheral and nervous tissues. We recently cloned two DA receptors (BmDopR1 and BmDopR2) from the silkworm Bombyx mori and identified them as D1-like receptors, which activate adenylate cyclase to increase intracellular cAMP levels. In this study, these two receptors were stably expressed in HEK-293 cells, and the dose-responsiveness to DA and their pharmacological properties were examined using cAMP assays. BmDopR1 showed a dose-dependent increase in cAMP levels at DA concentrations up to 10^?7 M with EC₅₀ of 3.30 nM, while BmDopR2 required 10^?6 M DA for activation. In BmDopR1-expressing cells, DA at 10^?6–10^?4 M induced 30–50% lower cAMP production than 10^?7 M DA. BmDopR2-expressing cells showed a standard sigmoidal dose–response, with maximum cAMP levels attained with 10^?5–10^?4 M DA and EC₅₀ of 1.30 μM. Both receptors had similar agonist profiles, and the typical vertebrate D1-like receptor agonist SKF-38393 was ineffective. Experiments with antagonists revealed that BmDopR1 exhibits D1-like features. However, the pharmacology of BmDopR2 was distinct from D1-like receptors; the typical vertebrate D1-like receptor antagonist SCH-23390 was less potent than the nonselective antagonist flupenthixol and the D2-like receptor antagonist chlorpromazine. The rank order of activities of several antagonists for BmDopR1 and BmDopR2 was more similar to that of Drosophila melanogaster DA receptors than Apis mellifera DA receptors. These data suggest that DA receptors could be potential targets for specific insecticides or insectistatics.     

Interaction Between Central Opioidergic and Glutamatergic Systems on Food Intake in Neonatal Chicks: Role of NMDA,AMPA and mGLU1 Receptors

The present study was designed to examine the role of opioidergic and glutamatergic systems on feeding behavior in neonatal meat-type chicken. In experiment 1, FD₃ neonatal broilers ICV injected with (A) saline, (B) DAMGO (µ-opioid receptor agonist, 125 pmol), (C) MK-801 (NMDA glutamate receptors antagonist, 15 nmol) and (D) combination of DAMGO plus MK-801. Experiments 2–5 were similar to experiment 1, except FD₃ chicks ICV injected with CNQX (AMPA glutamate receptors antagonist, 390 nmol), AIDA (mGLU₁ receptors antagonist, 2 nmol), LY341495 (mGLU₂ receptors antagonist, 150 nmol) and UBP1112 (mGLU₃ receptors antagonist, 2 nmol) instead of MK-801, respectively. In experiments 6–10, FD₃ chicks ICV injected as the same as procedure to the experiments 1–5, except to inject with DPDPE (δ-opioid receptor agonist, 40 nmol) instead of the DAMGO. The experiments 11–15 were similar to the experiments 1–5, except neonatal broilers ICV injected with U-50488H (κ-opioid receptor agonist, 30 nmol) instead of DAMGO. Then the cumulative food intake measured until 120 min post injection. According to the results, ICV injection of DAMGO, significantly decreased food intake (P?<?0.05) while DPDPE and U-50488H increased feeding behavior compared to the control group (P?<?0.05). Co-injection of the DAMGO?+?MK-801 and DAMGO?+?AIDA, significantly decreased DAMGO-induced hypophagia in neonatal chicks (P?<?0.05). Also, co-injection of the DPDPE?+?CNQX significantly amplified DPDPE induced feeding behavior (P?<?0.05). These results suggested interconnection between central opioidergic and glutamatergic systems on feeding behavior mediates via µ- and δ-opioid receptor with NMDA, AMPA and mGLU₁ receptors in FD₃ neonatal broilers. These findings may shed light on the circuitry underlying interconnection between central opioidergic and glutamatergic systems on feeding behavior.     

Effects of selective PGE2 receptor antagonists in esophageal adenocarcinoma cells derived from Barrett's esophagus

Accumulating evidence suggests that COX-2-derived prostaglandin E₂ (PGE₂) plays an important role in esophageal adenocarcinogenesis. Recently, PGE₂ receptors (EP) have been shown to be involved in colon cancer development. Since it is not known which receptors regulate PGE₂ signals in esophageal adenocarcinoma, we investigated the role of EP receptors using a human Barrett's-derived esophageal adenocarcinoma cell line (OE33). OE33 cells expressed COX-1, COX-2, EP₁, EP₂ and EP₄ but not EP₃ receptors as determined by real time RT-PCR and Western-blot. Treatment with 5-aza-dC restored expression, suggesting that hypermethylation is involved in EP₃ downregulation. Endogenous PGE₂ production was mainly due to COX-2, since this was significantly suppressed with COX-2 inhibitors (NS-398 and SC-58125), but not COX-1 inhibitors (SC-560). Cell proliferation (³H-thymidine uptake) was significantly inhibited by NS-398 and SC-58125, the EP₁ antagonist SC-51322, AH6809 (EP₁/EP₂ antagonist), and the EP₄ antagonist AH23848B, but was not affected by exogenous PGE₂. However, treatment with the selective EP₂ agonist Butaprost or 16,16-dimethylPGE₂ significantly inhibited butyrate-induced apoptosis and stimulated OE33 cell migration. The effect of exogenous PGE₂ on migration was attenuated when cells were first treated with EP₁ and EP₄ antagonists. These findings suggest a potential role for EP selective antagonists in the treatment of esophageal adenocarcinoma.     

Frequency-Dependent Modulation of Dopamine Release by Nicotine and Dopamine D<Subscript>1</Subscript> Receptor Ligands: An In Vitro Fast Cyclic Voltammetry Study in Rat Striatum

Nicotine is a highly addictive drug and exerts this effect partially through the modulation of dopamine release and increasing extracellular dopamine in regions such as the brain reward systems. Nicotine acts in these regions on nicotinic acetylcholine receptors. The effect of nicotine on the frequency dependent modulation of dopamine release is well established and the purpose of this study was to investigate whether dopamine D₁ receptor (D₁R) ligands have an influence on this. Using fast cyclic voltammetry and rat corticostriatal slices, we show that D₁R ligands are able to modulate the effect of nicotine on dopamine release. Nicotine (500 nM) induced a decrease in dopamine efflux at low frequency (single pulse or five pulses at 10 Hz) and an increase at high frequency (100 Hz) electrical field stimulation. The D₁R agonist SKF-38393, whilst having no effect on dopamine release on its own or on the effect of nicotine upon multiple pulse evoked dopamine release, did significantly prevent and reverse the effect of nicotine on single pulse dopamine release. Interestingly similar results were obtained with the D₁R antagonist SCH-23390. In this study we have demonstrated that the modulation of dopamine release by nicotine can be altered by D₁R ligands, but only when evoked by single pulse stimulation, and are likely working via cholinergic interneuron driven dopamine release.     

Functional interaction between purinergic receptors: effect of ligands for A2A and P2Y12 receptors on P2Y1 receptor function

A_2A adenosine receptor (A_2AR), P2Y₁ receptor (P2Y₁R) and P2Y₁₂ receptor (P2Y₁₂R) are predominantly expressed on human platelets. The individual role of each of these receptors in platelet aggregation has been actively reported. Previously, hetero-oligomerization between these three receptors has been shown to occur. Here, we show that Ca²⁺ signaling evoked by the P2Y₁R agonist, 2-methylthioladenosine 5’ diphosphate (2MeSADP) was significantly inhibited by the A_2AR antagonist (ZM241385 and SCH442416) and the P2Y₁₂R antagonist (ARC69931MX) using HEK293T cells expressing the three receptors. It was confirmed that inhibition of P2Y₁R signaling by A_2AR and P2Y₁₂R antagonists was indeed mediated through A_2AR and P2Y₁₂R using 1321N1 human astrocytoma cells which do not express P2Y receptors. We expect that intermolecular signal transduction and specific conformational changes occur among components of hetero-oligomers formed by these three receptors.     

Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis

Glutamate and aspartate are the primary neurotransmitters of projections from motor and premotor cortices to the striatum. Release of glutamate may be modulated by dopamine receptors located on corticostriatal terminals. The present study used microdialysis to investigate the dopaminergic modulation of in vivo striatal glutamate and aspartate release in the striatum of awake-behaving rats. Local perfusion with a depolarizing concentration of K+ through a dialysis probe into the rat striatum produced a significant increase in the release of glutamate, aspartate, and taurine. The D2 agonist LY171555 blocked the K(+)-induced release of glutamate and aspartate, but not taurine, in a concentration-dependent manner. The D1 agonist SKF 38393 did not alter K(+)-induced release of glutamate and taurine, but did significantly decrease aspartate release. Neither agonist had any effect on basal amino acid release. The D2 antagonist (-)-sulpiride reversed the inhibitory effects of LY 171555 on K(+)-induced glutamate release. These results provide in vivo evidence for a functional interaction between dopamine, the D2 receptor, and striatal glutamate release.     

Chemical synthesis,microbial transformation and biological evaluation of tetrahydroprotoberberines as dopamine D1/D2 receptor ligands

Dopamine D1/D2 receptors are important targets for drug discovery in the treatment of central nervous system diseases. To discover new and potential D1/D2 ligands, 17 derivatives of tetrahydroprotoberberine (THPB) with various substituents were prepared by chemical synthesis or microbial transformation using Streptomyces griseus ATCC 13273. Their functional activities on D1 and D2 receptors were determined by cAMP assay and calcium flux assay. Seven compounds showed high activity on D1/D2 receptor with low IC₅₀ values less than 1?µM. Especially, top compound 5 showed strong antagonistic activity on both D1 and D2 receptor with an IC₅₀ of 0.391 and 0.0757?µM, respectively. Five compounds displayed selective antagonistic activity on D1 and D2 receptor. The SAR studies revealed that (1) the hydroxyl group at C-9 position plays an important role in keeping a good activity and small or fewer substituents on ring D of THPBs may also stimulate their effects, (2) the absence of substituents at C-9 position tends to be more selective for D2 receptor, and (3) hydroxyl substitution at C-2 position and the substitution at C-9 position may facilitate the conversion of D1 receptor from antagonist to agonist. Molecular docking simulations found that Asp 103/Asp 114, Ser 107/Cys 118, and Trp 285/ Trp 386 of D1/ D2 receptors are the key residues, which have strong interactions with the active D1/D2 compounds and may influence their functional profiles.     

Dopamine induces inhibitory effects on the circular muscle contractility of mouse distal colon via D1- and D2-like receptors

Dopamine (DA) acts as gut motility modulator, via D1- and D2-like receptors, but its effective role is far from being clear. Since alterations of the dopaminergic system could lead to gastrointestinal dysfunctions, a characterization of the enteric dopaminergic system is mandatory. In this study, we investigated the role of DA and D1- and D2-like receptors in the contractility of the circular muscle of mouse distal colon by organ-bath technique. DA caused relaxation in carbachol-precontracted circular muscle strips, sensitive to domperidone, D2-like receptor antagonist, and mimicked by bromocriptine, D2-like receptor agonist. 7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390), D1-like receptor antagonist, neural toxins, L-NAME (nitric oxide (NO) synthase inhibitor), 2′-deoxy-N₆-methyl adenosine 3′,5′-diphosphate diammonium salt (MRS 2179), purinergic P2Y1 antagonist, or adrenergic antagonists were ineffective. DA also reduced the amplitude of neurally evoked cholinergic contractions. The effect was mimicked by (±)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide (SKF-38393), D1-like receptor agonist and antagonized by SCH-23390, MRS 2179, or L-NAME. Western blotting analysis determined the expression of DA receptor proteins in mouse distal colon. Notably, SCH-23390 per se induced an increase in amplitude of spontaneous and neurally evoked cholinergic contractions, unaffected by neural blockers, L-NAME, MRS 2179, muscarinic, adrenergic, or D2-like receptor antagonists. Indeed, SCH-23390-induced effects were antagonized by an adenylyl cyclase blocker. In conclusion, DA inhibits colonic motility in mice via D2- and D1-like receptors, the latter reducing acetylcholine release from enteric neurons, involving nitrergic and purinergic systems. Whether constitutively active D1-like receptors, linked to adenylyl cyclase pathway, are involved in a tonic inhibitory control of colonic contractility is questioned.     

Investigation of the functional expression of purine and pyrimidine receptors in porcine isolated pancreatic arteries

Receptors for purines and pyrimidines are expressed throughout the cardiovascular system. This study investigated their functional expression in porcine isolated pancreatic arteries. Pancreatic arteries (endothelium intact or denuded) were prepared for isometric tension recording and preconstricted with U46619, a thromboxane A₂ mimetic; adenosine-5′-diphosphate (ADP), uridine-5′-triphosphate (UTP) and MRS2768, a selective P2Y₂ agonist, were applied cumulatively, while adenosine-5′-triphosphate (ATP) and αβ-methylene-ATP (αβ-meATP) response curves were generated from single concentrations per tissue segment. Antagonists/enzyme inhibitors were applied prior to U46619 addition. ATP, αβ-meATP, UTP and MRS2768 induced vasoconstriction, with a potency order of αβ-meATP > MRS2768 > ATP ≥ UTP. Contractions to ATP and αβ-meATP were blocked by NF449, a selective P2X1 receptor antagonist. The contraction induced by ATP, but not UTP, was followed by vasorelaxation. Endothelium removal and DUP 697, a cyclooxygenase-2 inhibitor, had no significant effect on contraction to ATP but attenuated that to UTP, indicating actions at distinct receptors. MRS2578, a selective P2Y₆ receptor antagonist, had no effect on contractions to UTP. ADP induced endothelium-dependent vasorelaxation which was inhibited by MRS2179, a selective P2Y₁ receptor antagonist, or SCH58261, a selective adenosine A_2A receptor antagonist. The contractions to ATP and αβ-meATP were attributed to actions at P2X1 receptors on the vascular smooth muscle, whereas it was shown for the first time that UTP induced an endothelium-dependent vasoconstriction which may involve P2Y₂ and/or P2Y₄ receptors. The relaxation induced by ADP is mediated by P2Y₁ and A_2A adenosine receptors. Porcine pancreatic arteries appear to lack vasorelaxant P2Y₂ and P2Y₄ receptors.     

Differential mediation of cold water swim stress-induced antinociception by δ-opioid receptor subtypes in diabetic mice

The involvement of δ-opioid receptor subtypes in cold water swim stress (CWSS)-induced antinociception in diabetic mice was compared with that in non-diabetic mice. Three-minute swim stress produced significant antinociception in both diabetic and non-diabetic mice as determined by the tail-pinch test. However, the extent of CWSS-induced antinociception in diabetic mice was significantly greater than that in non-diabetic mice. Pretreatment with naltriben, a selective δ₂-opioid receptor antagonist, significantly attenuated CWSS-induced antinociception in both non-diabetic and diabetic mice. In contrast, although 7-benzylidenenaltrexone, a selective δ₁-opioid receptor antagonist, significantly attenuated CWSS-induced antinociception in diabetic mice, it had no effect in non-diabetic mice. These results suggest that CWSS-induced antinociceotion in non-diabetic mice is mediated by δ₂-opioid receptors, whereas CWSS-induced antinociception in diabetic mice is mediated by both δ₁- and δ₂-opioid receptors. Furthermore, the enhanced CWSS-induced antinociception in diabetic mice may be due to the activation of δ₁-opioid receptors.     

Involvement of 5-HT(2A/2B/2C) receptors on memory formation: simple agonism,antagonism, or inverse agonism?

1. The 5-HT₂ receptors subdivision into the 5-HT_2A/2B/2C subtypes along with the advent of the selective antagonists has allowed a more detailed investigation on the role and therapeutic significance of these subtypes in cognitive functions. The present study further analyzed the 5-HT₂ receptors role on memory consolidation.2. The SB-200646 (a selective 5-HT_2B/2C receptor antagonist) and LY215840 (a nonselective 5-HT_2/7 receptor antagonist) posttraining administration had no effect on an autoshaped memory consolidation. However, both drugs significantly and differentially antagonized the memory impairments induced by 1-(3-chlorophenyl)piperazine (mCPP), 1-naphtyl-piperazine (1-NP), mesulergine, or N-(3-trifluoromethylphenyl) piperazine (TFMPP).3. In contrast, SB-200646 failed to modify the facilitatory procognitive effect produced by (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) or ketanserin, which were sensitive to MDL100907 (a selective 5-HT_2A receptor antagonist) and to a LY215840 high dose.4. Finally, SB-200646 reversed the learning deficit induced by dizocilpine, but not that by scopolamine; while SB-200646 and MDL100907 coadministration reversed memory deficits induced by both drugs.5. It is suggested that 5-HT_2B/2C receptors might be involved on memory formation probably mediating a suppressive or constraining action. Whether the drug-induced memory impairments in this study are explained by simple agonism, antagonism, or inverse agonism at 5-HT₂ receptors remains unclear at this time.6. Notably, the 5-HT₂ receptor subtypes blockade may provide some benefit to reverse poor memory consolidation conditions associated with decreased cholinergic, glutamatergic, and/or serotonergic neurotransmission.     

l-trans-Pyrrolidine-2,4-Dicarboxylic Acid-Evoked Striatal Glutamate Levels Are Attenuated by Calcium Reduction, Tetrodotoxin, and Glutamate Receptor Blockade

Abstract: l - trans -Pyrrolidine-2,4-dicarboxylic acid ( l - trans -PDC) reverses plasma membrane glutamate transporters and elevates extracellular glutamate levels in vivo. We investigated the possibility that l - trans -PDC-stimulated glutamate levels are mediated partially by increases in transsynaptic activity. Therefore, the degree to which l - trans -PDC-evoked glutamate levels depend on calcium, sodium-channel activation, and glutamate-receptor activation was investigated by infusing via reverse microdialysis (a) 0.1 m M calcium, (b) 1 µ M tetrodotoxin, a selective blocker of voltage-dependent sodium channels, (c) R (−)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a selective NMDA-receptor antagonist, or (d) LY293558, a selective α-amino-3-hydroxy-5-methylisoxazole-4-propionate antagonist. In separate experimental groups, l - trans -PDC-evoked glutamate levels were reduced significantly by 55% in the presence of 0.1 m M calcium and by 46% in the presence of tetrodotoxin. Additionally, CPP and LY293558 significantly attenuated l - trans -PDC-evoked glutamate levels without altering basal glutamate levels. These data suggest that glutamate transporter reversal by l - trans -PDC initially elevates extracellular glutamate levels enough to stimulate postsynaptic glutamate receptors within the striatum. It is proposed that glutamate-receptor stimulation activates a positive feedback loop within the basal ganglia, leading to further glutamate release from corticostriatal and thalamostriatal afferents. Therefore, either extracellular striatal calcium reduction or tetrodotoxin perfusion leads to decreased action potential-dependent glutamate release from these terminals. In addition, blocking glutamate receptors directly reduces medium spiny neuronal firing and indirectly attenuates corticostriatal and thalamostriatal activity, resulting in an overall depression of l - trans -PDC-stimulated glutamate levels.     

Drug-Induced Up-Regulation of Dopamine D2 Receptors on Cultured Cells

Abstract: Ligand-induced up-regulation of recombinant dopamine D2 receptors was assessed using C₆ glioma cells stably expressing the short (415-amino-acid; D2_S) and long (444-amino-acid; D2_L) forms of the receptor. Overnight treatment of C6-D2_L cells with N-propylnorapomorphine (NPA) caused a time- and concentration-dependent increase in the density of receptors, as assessed by the binding of radioligand to membranes prepared from the cells, with no change in the affinity of the receptors for the radioligand. The effect of 10 µM NPA was maximal after 10 h, at which time the density of D2_L receptors was more than doubled. The agonists dopamine and quinpirole also increased the density of D2_L receptors. The receptor up-regulation was not specific for agonists, because the antagonists epidepride, sulpiride, and domperidone caused smaller (30–60%) increases in receptor density. Prolonged treatment with 10 µM NPA desensitized D2_L receptors, as evidenced by a reduced ability of dopamine to inhibit adenylyl cyclase, whereas treatment with sulpiride was associated with an enhanced responsiveness to dopamine. The magnitude of NPA-induced receptor up-regulation in each of four clonal lines of C6-D2_L cells (mean increase, 80%) was greater than in all four lines of C6-D2_S cells (33%). Inactivation of pertussis toxin-sensitive G proteins had no effect on the basal density of D2_L receptors or on the NPA-induced receptor up-regulation. Treatment with 5 µg/ml of cycloheximide, on the other hand, decreased the basal density of receptors and attenuated, but did not prevent, the NPA-induced increase. Chimeric D1/D2 receptors were used to identify structural determinants of dopamine receptor regulation. Treatment with the D1/D2 agonist NPA decreased the density of D1 and chimeric CH4 and CH3 receptors. The latter two receptors have D1 sequence from the amino-terminus to the amino-terminal end of transmembrane region (TM) VII and VI, respectively. CH2, with D1 sequence up to the amino-terminal end of TM V, and thus the third cytoplasmic loop of the D2 receptor, was up-regulated by NPA or the D2-selective agonist quinpirole. Quinpirole treatment decreased the density of CH3 and had no effect on CH4 or D1 receptors. The different responses of CH2 and CH3 to agonist treatment suggest a role for TM V and the third cytoplasmic loop in the direction of receptor regulation.