Presynaptic GABAB Receptors Regulate Retinohypothalamic Tract Synaptic Transmission by Inhibiting Voltage-Gated Ca2+ Channels

Presynaptic GABAB receptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca2+-sensitive fluorescent probes within RHT terminals were used to examine GABAB-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca2+ channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca2+ channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca2+ channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by ω-conotoxin GVIA (≤72%), mibefradil (≤52%), and ω-agatoxin TK (≤15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca2+ influx and resting Ca2+ concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba2+ nor high extracellular K+ modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca2+ channels, and possibly by activation of 4-AP–sensitive K+ channels, but not by inhibition of R- and L-type Ca2+ channels or by Kir3 channel activation.

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Rosmarinic Acid, a Bioactive Phenolic Compound, Inhibits Glutamate Release from Rat Cerebrocortical Synaptosomes through GABAA Receptor Activation

Biomolecules ◽

10.3390/biom11071029 ◽

2021 ◽

Vol 11 (7) ◽

pp. 1029

Author(s):

Che-Chuan Wang ◽

Pei-Wen Hsieh ◽

Jinn-Rung Kuo ◽

Su-Jane Wang

Keyword(s):

Gabaa Receptor ◽

Gabaa Receptors ◽

Rosmarinic Acid ◽

Glutamate Release ◽

Receptor Activation ◽

Inhibitory Effect ◽

Synapsin I ◽

Dependent Manner ◽

Neuroprotective Effects ◽

Concentration Dependent Manner

Rosmarinic acid, a major component of rosemary, is a polyphenolic compound with potential neuroprotective effects. Asreducing the synaptic release of glutamate is crucial to achieving neuroprotectant’s pharmacotherapeutic effects, the effect of rosmarinic acid on glutamate release was investigated in rat cerebrocortical nerve terminals (synaptosomes). Rosmarinic acid depressed the 4-aminopyridine (4-AP)-induced glutamate release in a concentration-dependent manner. The removal of extracellular calcium and the blockade of vesicular transporters prevented the inhibition of glutamate release by rosmarinic acid. Rosmarinic acid reduced 4-AP-induced intrasynaptosomal Ca2+ elevation. The inhibition of N-, P/Q-type Ca2+ channels and the calcium/calmodulin-dependent kinase II (CaMKII) prevented rosmarinic acid from having effects on glutamate release. Rosmarinic acid also reduced the 4-AP-induced activation of CaMKII and the subsequent phosphorylation of synapsin I, the main presynaptic target of CaMKII. In addition, immunocytochemistry confirmed the presence of GABAA receptors. GABAA receptor agonist and antagonist blocked the inhibitory effect of rosmarinic acid on 4-AP-evoked glutamate release. Docking data also revealed that rosmarinic acid formed a hydrogen bond with the amino acid residues of GABAA receptor. These results suggested that rosmarinic acid activates GABAA receptors in cerebrocortical synaptosomes to decrease Ca2+ influx and CaMKII/synapsin I pathway to inhibit the evoked glutamate release.

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Calcium-sensitivity of inositol 1,4,5-trisphosphate metabolism in exocrine cells from the avian salt gland

Biochemical Journal ◽

10.1042/bj2820703 ◽

1992 ◽

Vol 282 (3) ◽

pp. 703-710 ◽

Cited By ~ 15

Author(s):

J P Hildebrandt ◽

T J Shuttleworth

Keyword(s):

Substrate Concentration ◽

Inositol Phosphates ◽

Receptor Activation ◽

Dependent Manner ◽

Salt Glands ◽

Concentration Dependent Manner ◽

Intact Cells ◽

Exocrine Cells ◽

Tissue Homogenates

The generation of inositol phosphates upon muscarinic-receptor activation was studied in [3H]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular reference to the possible interaction of changes in intracellular [Ca2+] ([Ca2+]i) with the metabolic processes. In intact cells, there was a rapid (within 15 s) generation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, followed by an accumulation of their breakdown products, Ins(1,3,4)P3 and inositol bis- and monophosphates. Ca(2+)-sensitivity of the Ins(1,4,5)P3 3-kinase was demonstrated in tissue homogenates, with the rate of phosphorylation increasing 2-fold at free Ca2+ concentrations greater than 1 microM. However, addition of calmodulin or the presence of the calmodulin inhibitor W-7 (up to 100 microM) had no effect. 3-Kinase activity increased proportionally with the initial Ins(1,4,5)P3 concentration up to 1 microM, but a 10-fold higher substrate concentration produced only a doubling in the phosphorylation rate. Ins(1,3,4,5)P4 was dephosphorylated to Ins(1,3,4)P3, which accumulated in the homogenate assays as well as in intact cells. Depending on its concentration, Ins(1,3,4)P3 was phosphorylated [in part to Ins(1,3,4,6)P4] or dephosphorylated. To investigate the Ca(2+)-sensitivity of the 3-kinase in intact cells, excess quin2 was used to buffer the receptor-mediated transient changes in [Ca2+]i in [3H]inositol-loaded cells. These experiments revealed that increasing [Ca2+]i from less than 100 to approx. 400 nM (i.e. within the physiological range) has no effect on the partitioning of Ins(1,4,5)P3 metabolism (phosphorylation versus dephosphorylation) and on the accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. This indicates that activation of the 3-kinase by physiologically relevant Ca2+ concentrations may not play a major role in the generation of Ins(1,3,4,5)P4 signals upon receptor activation in these cells. The latter are mainly achieved by the receptor-mediated increase in Ins(1,4,5)P3 in the cell and its phosphorylation by the 3-kinase in a substrate-concentration-dependent manner.

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Presynaptic Modulation of Synaptic Transmission by Pregnenolone Sulfate as Studied by Optical Recordings

Journal of Neurophysiology ◽

10.1152/jn.00755.2004 ◽

2005 ◽

Vol 94 (6) ◽

pp. 4131-4144 ◽

Cited By ~ 35

Author(s):

Ling Chen ◽

Masahiro Sokabe

Keyword(s):

Synaptic Transmission ◽

Glutamate Release ◽

Hippocampal Slices ◽

Receptor Activation ◽

Optical Recording ◽

Perforant Path ◽

Dependent Manner ◽

Pregnenolone Sulfate ◽

Voltage Sensitive Dyes ◽

Dose Dependent

The effects of pregnenolone sulfate (PREGS), a putative neurosteroid, on the transmission of perforant path–granule cell synapses were investigated with an optical recording technique in rat hippocampal slices stained with voltage-sensitive dyes. Application of PREGS to the bath solution resulted in an acute augmentation of EPSP in a dose-dependent manner. The PREGS effect was dependent on the extracellular Ca2+ concentration ([Ca2+]o), but independent of NMDA receptor activation. PREGS caused a decrease in paired-pulse facilitation, which implies that PREGS positively modulates presynaptic neurotransmitter releases. Firmer support for this mechanism was that PREGS augmented the synaptically induced glial depolarization (SIGD) that reflects the activity of electrogenic glutamate transporters in glial cells during the uptake of released glutamate. The selective α7nAChR antagonist α-BGT or MLA prevented the SIGD increase by PREGS. Furthermore DMXB, a selective α7nAChR agonist, mimicked the PREGS effect on SIGD and antagonized the effect of PREGS. The presynaptic effect of PREGS was partially attenuated by the L-type Ca2+ channel (VGCC) blocker nifedipine. Based on these findings, we proposed a novel mechanism underlying the facilitated synaptic transmission by PREGS: this neurosteroid sensitizes presynaptic α7nAChR that is followed by an activation of L-type VGCC to increase the presynaptic glutamate release.

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Metabotropic glutamate receptor agonist-induced hyperpolarizations in rat basolateral amygdala neurons: receptor characterization and ion channels

Journal of Neurophysiology ◽

10.1152/jn.1996.76.5.3059 ◽

1996 ◽

Vol 76 (5) ◽

pp. 3059-3069 ◽

Cited By ~ 28

Author(s):

K. H. Holmes ◽

N. B. Keele ◽

V. L. Arvanov ◽

P. Shinnick-Gallagher

Keyword(s):

Dicarboxylic Acid ◽

Glutamate Receptor ◽

Basolateral Amygdala ◽

Metabotropic Glutamate Receptor ◽

Outward Current ◽

Channel Blockers ◽

Dependent Manner ◽

Concentration Dependent Manner ◽

Metabotropic Glutamate ◽

Outward Currents

1. Metabotropic glutamate receptor (mGluR)-agonist-induced hyperpolarizations and corresponding outward currents were analyzed in basolateral amygdala (BLA) neurons in rat brain slice preparations with current-clamp and single-electrode voltage-clamp recording to characterize the mGluR subtype(s) and the ion channel(s) mediating this response. 2. The mGluR agonist (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) induced a membrane hyperpolarization or outward current in BLA neurons in a concentration-dependent manner (median effective concentration = 34 microM; range = 10-200 microM); the 1S,3R-ACPD hyperpolarizations are recorded in 89% of neurons that accommodate or cease firing in response to a 400-ms depolarizing current injection (0.5 nA). 3. mGluR agonists elicited hyperpolarizations or outward currents in a concentration-dependent manner in the following rank order of potency: (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) > 1S,3R-ACPD > (s)-4-carboxyphenylglycine = (RS)-4-carboxy-3-hydroxyphenylglycine (4C3HPG) > L-aminophosphonobutyric acid > (1S,3S)-1-amino-cyclopentane-1,3-dicarboxylic acid. In contrast, the mGluR agonists quisqualate and ibotenate induced only depolarizations in the presence of D-2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2,3-dione in BLA neurons. 4. The 1S,3R-ACPD-induced outward current is mediated through a large-conductance calcium-dependent potassium (BK) conductance. The BK channel blockers iberiotoxin and charybdotoxin blocked the response, as did the potassium channel blockers tetraethylammonium and 4-aminopyridine; the small-conductance calcium-activated potassium channel blocker apamin did not affect the response. 5. The mGluR-agonist-induced hyperpolarization is blocked in amygdala slices from animals pretreated with pertussis toxin (PTX). 1S,3R-ACPD hyperpolarizations were recorded in neurons contralateral but not ipsilateral to the site of PTX injection. 6. The antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG, 500 microM) reduced significantly the 1S,3R-ACPD-induced hyperpolarization. 7. In conclusion, the relative potency of L-CCG-I and 4C3HPG in evoking only hyperpolarizations (outward currents) in accommodating neurons, and the observation that MCPG (500 microM) reduces the hyperpolarization, suggest that a group-II-like mGluR underlies the hyperpolarizing response. The mGluR-induced response is sensitive to iberiotoxin and to pretreatment with PTX, suggesting activation of BK channels through a group II mGluR linked to a PTX-sensitive G protein in BLA neurons.

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Regulation of Na(+)-3HCO3- cotransport in rabbit proximal convoluted tubule via adenosine A1 receptor

AJP Renal Physiology ◽

10.1152/ajprenal.1993.265.4.f511 ◽

1993 ◽

Vol 265 (4) ◽

pp. F511-F519 ◽

Cited By ~ 14

Author(s):

M. Takeda ◽

K. Yoshitomi ◽

M. Imai

Keyword(s):

Basolateral Membrane ◽

Receptor Activation ◽

Proximal Convoluted Tubule ◽

Adenosine A1 Receptor ◽

Intracellular Camp ◽

Dependent Manner ◽

Concentration Dependent Manner ◽

Adenosine A1 ◽

A1 Receptor

We investigated the role of adenosine A1-receptor in the regulation of basolateral Na(+)-3HCO3- cotransporter in the rabbit proximal convoluted tubule (PCT) microperfused in vitro by monitoring basolateral membrane potential and intracellular pH. FK-453, a highly specific A1 antagonist, inhibited basolateral HCO3- conductance in a concentration-dependent manner (10(-10)-10(-5) M). Other A1 antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) at 10(-5) M and theophylline at 10(-3) M, also had similar effects. N6-cyclohexyladenosine (CHA) at 10(-7) M attenuated the effect of low concentration (10(-8) M) of FK-453. Either enhancement of the degradation of adenosine by 0.1 U/ml adenosine deaminase (ADA) or inhibition of adenosine release from the cells by 10(-6) M S-(4-nitrobenzyl)-6-thioinosine (NBTI) mimicked the effects of A1 antagonists. These observations suggest that endogenous adenosine is released from PCT cells and stimulates Na(+)-3HCO3- cotransporter. Both 10(-4) M 8-(4-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPT-cAMP) and 10(-6) M forskolin also inhibited basolateral HCO3- conductance. Both 10(-6) M FK-453 and 10(-4) M CPT-cAMP decreased the initial rate as well as the magnitude of intracellular acidification induced by reduction of peritubular HCO3- concentration from 25 to 0 mM. Neither 10(-6) M FK-453 nor 10(-7) M CHA changed intracellular Ca2+ concentration as measured by fura-2 fluorescence. These results indicate that adenosine might stimulate HCO3- exit across the basolateral membrane through Na(+)-3HCO3- cotransporter by decreasing intracellular cAMP via A1-receptor activation.(ABSTRACT TRUNCATED AT 250 WORDS)

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Effects of estrogen on cerebral blood flow and pial microvasculature in rabbits

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.2000.279.3.h1208 ◽

2000 ◽

Vol 279 (3) ◽

pp. H1208-H1214 ◽

Cited By ~ 20

Author(s):

M. T. Littleton-Kearney ◽

D. M. Agnew ◽

R. J. Traystman ◽

P. D. Hurn

Keyword(s):

Blood Flow ◽

Estrogen Receptor ◽

Cerebral Blood Flow ◽

Receptor Activation ◽

Receptor Subtypes ◽

Dependent Manner ◽

Pial Arteries ◽

Concentration Dependent Manner ◽

Cranial Window ◽

Pial Arterioles

We tested the hypothesis that intracarotid estrogen infusion increases cerebral blood flow (CBF) in a concentration-dependent manner and direct application of estrogen on pial arterioles yields estrogen receptor-mediated vasodilation. Rabbits of both genders were infused with estrogen via a branch of the carotid artery. Estrogen doses of 20 or 0.05 μg · ml−1 · min−1 were used to achieve supraphysiological or physiological plasma estrogen levels, respectively. CBF and cerebral vascular resistance were determined at baseline, during the infusion, and 60-min postinfusion, and effects on pial diameter were assessed via a cranial window. Pial arteriolar response to estrogen alone and to estrogen after administration of tamoxifen (10−7), an antiestrogen drug that binds to both known estrogen receptor subtypes, was tested. No gender differences were observed; therefore, data were combined for both males and females. Systemic estrogen infusion did not increase regional CBF. Estradiol dilated pial arteries only at concentrations ranging from 10−4–10−7 M ( P ≤ 0.05). Pretreatment with tamoxifen alone had no effect on arteriolar diameter but inhibited estrogen-induced vasodilation ( P < 0.001). Our data suggest that estrogen does not increase CBF under steady-state conditions in rabbits. In the pial circulation, topically applied estradiol at micromolar concentrations dilates vessels. The onset is rapid and dependent on estrogen receptor activation.

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Mapping the Protease Activated Receptor 4 (PAR4) Homodimer Interface.

Blood ◽

10.1182/blood.v114.22.773.773 ◽

2009 ◽

Vol 114 (22) ◽

pp. 773-773

Author(s):

Marvin T Nieman

Keyword(s):

Conflicts Of Interest ◽

Specific Interaction ◽

Resonance Energy Transfer ◽

Resonance Energy ◽

Receptor Activation ◽

Dependent Manner ◽

Intracellular Loop ◽

Concentration Dependent Manner ◽

Platelet Surface

Abstract Abstract 773 Thrombin activates platelets by binding and cleaving protease activated receptors 1 and 4 (PAR1 and PAR4). PAR1 and PAR4 communicate with each other to lower the concentration of thrombin required for PAR4 activation (Nieman Biochemistry, 2008). In addition, PAR1 and PAR4 form homo and heterodimers. However, where these receptors interact has not been defined and it is not known if dimerization influences receptor activation, downstream signaling, or both. Since PAR4 activation is important on human and mouse platelets, we sought to characterize the interaction site between PAR4 homodimers. Using bioluminescence resonance energy transfer (BRET), we mapped the PAR4 homodimer interface. The PAR4 homodimers show a specific interaction as indicated by a hyperbolic BRET signal in response to increasing PAR4-GFP expression with a fixed concentration of PAR4-Rluc. The threshold maximum BRET signal was disrupted in a concentration-dependent manner by unlabeled PAR4. In contrast, the unrelated G-protein coupled receptor, rhodopsin, was unable to disrupt the BRET signal indicating that the disruption of the PAR4 homodimer is a specific interaction. We have mapped the region required for PAR4 homodimer formation using chimeras between rhodopsin and PAR4. PAR4 does not interact with rhodopsin in BRET assays. Using a library of rho-PAR4 chimeras that have the junction at the beginning of transmembrane (TM) 2, 3, 4, 5, 6 or 7, we determined where dimer formation is restored. When the junction is placed at the beginning of TM4 or TM5, the chimera does not interact with PAR4-WT. In contrast, when the junction is moved to the end of TM2, the BRET signal is restored. These results indicate that the region on PAR4 required for homodimer formation encompasses a 63 amino acid region that includes the first extracellular loop, TM3 and the second intracellular loop. These studies establish techniques that may be used to define the interactions between other GPCRs found on the platelet surface. These receptor-receptor interactions may be another level of regulation of agonist activity and platelet function in vivo and may provide novel targets for anti-platelet therapies. Disclosures: No relevant conflicts of interest to declare.

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Ca2+-sensitive phosphoinositide hydrolysis is activated in synovial cells but not in articular chondrocytes

Biochemical Journal ◽

10.1042/bj3440545 ◽

1999 ◽

Vol 344 (2) ◽

pp. 545-553 ◽

Cited By ~ 11

Author(s):

Ilaria CAPOZZI ◽

Rossana TONON ◽

Paola d'ANDREA

Keyword(s):

Phospholipase C ◽

Connexin 43 ◽

Articular Chondrocytes ◽

Inositol Phosphate ◽

Receptor Activation ◽

Phosphoinositide Hydrolysis ◽

Functional Coupling ◽

Synovial Cells ◽

Dependent Manner ◽

Concentration Dependent Manner

Cell-to-cell diffusion of second messengers across intercellular channels allows tissues to co-ordinate responses to extracellular stimuli. Intercellular diffusion of inositol 1,4,5-trisphosphate, locally produced by focal stimulations, sustains the propagation of intercellular Ca2+ waves, by stimulating the release of intracellular Ca2+ in neighbouring cells. We previously demonstrated that in cultured articular chondrocytes and HIG-82 synovial cells, studied with digitial fluorescence video imaging, mechanical stimulation of a single cell induced intercellular Ca2+ waves dependent on the presence of gap junctions. In the absence of extracellular Ca2+ the propagating distance of the wave decreased significantly in HIG-82 cells, but appeared unaffected in chondrocytes. We now show that both cells types express connexin 43 and a similar functional coupling, thus suggesting that the different Ca2+ sensitivity of intercellular waves is not due to major differences in gap junction constituent proteins. In HIG-82 synoviocytes, but not in chondrocytes, the Ca2+ ionophore ionomycin stimulated phosphoinositide hydrolysis in a concentration-dependent manner, an effect strictly dependent on the presence of extracellular Ca2+, suggesting the expression, in these cells, of a Ca2+-sensitive phospholipase C activity. Such an activity could be stimulated also by Ca2+ influx induced by P2Y receptor activation and considerably amplifies ATP-induced inositol phosphate (InsP) production. In contrast, Ca2+ influx did not affect considerably the response of chondrocytes to ATP stimulation. In HIG-82 cells, the combined application of ionomycin and ATP maximally stimulated InsP synthesis, suggesting the involvement of two independent mechanisms in inositol phosphate generation. These results suggest that in HIG-82 synovial cells the recruitment of a Ca2+-sensitive phospholipase C activity could amplify the cell response to a focally applied extracellular stimulus, thus providing a positive feedback mechanism for intercellular wave propagation.

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Regulation of electrolyte transport across cultured endometrial epithelial cells by prolactin

Journal of Endocrinology ◽

10.1677/joe-08-0077 ◽

2008 ◽

Vol 197 (3) ◽

pp. 575-582 ◽

Cited By ~ 9

Author(s):

Chatsri Deachapunya ◽

Sutthasinee Poonyachoti ◽

Nateetip Krishnamra

Keyword(s):

Epithelial Cells ◽

Short Circuit ◽

Short Circuit Current ◽

Disulfonic Acid ◽

Maximum Effect ◽

Channel Blockers ◽

Dependent Manner ◽

Concentration Dependent Manner ◽

Anion Secretion ◽

Endometrial Epithelial Cells

The effect of prolactin (PRL) on ion transport across the porcine glandular endometrial epithelial cells was studied in primary cell culture using the short-circuit current technique. Addition of 1 μg/ml PRL either to the apical solution or to the basolateral solution produced a peak followed by a sustained increase in Isc, but with a lesser response when PRL was added apically. Basolateral addition of PRL increased the Isc in a concentration-dependent manner with a maximum effect at 1 μg/ml and an effective concentration value of 120 ng/ml. The PRL-stimulated Isc was significantly reduced by pretreatment with an apical addition of 5-nitro-2-(3-phenylpropylamino) benzoic acid (200 μM), diphenylamine-2-carboxylic acid (1 mM) or 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (200 μM), Cl− channel blockers, but not by amiloride (10 μM), a Na+ channel blocker. In addition, pretreatment with bumetanide (200 μM), a Na+–K+–2Cl− cotransporter inhibitor, in the basolateral solution significantly reduced the PRL-stimulated Isc. Replacement of Cl− or in the bathing solutions also decreased the Isc response to PRL. Pretreatment of the monolayer with AG490 (50 μM), an inhibitor of JAK2 activity significantly inhibited the PRL-induced increase in Isc. Western blot analysis of the porcine endometrial epithelial cells revealed the presence of short isoform of PRL receptor (PRLR-S) that could be regulated by 17β-estradiol. The results of this investigation showed that PRL acutely stimulated anion secretion across the porcine endometrial epithelial cells possibly through PRLR-S present in both apical and basolateral membranes. The PRL response appeared to be mediated by the JAK2-dependent pathway.

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Adenosine Reduces Glutamate Release in Rat Spinal Synaptosomes

Anesthesiology ◽

10.1097/00000542-200511000-00021 ◽

2005 ◽

Vol 103 (5) ◽

pp. 1060-1065 ◽

Cited By ~ 20

Author(s):

Xinhui Li ◽

James C. Eisenach

Keyword(s):

Receptor Antagonist ◽

Adenosine Receptor ◽

Glutamate Release ◽

Receptor Activation ◽

Spinal Nerve ◽

Spinal Nerve Ligation ◽

Dependent Manner ◽

A1 Adenosine Receptor ◽

Adenosine Receptor Antagonist ◽

Adenosine A1

Background A1 adenosine receptor activation reduces hypersensitivity in animal models of chronic pain, but intrathecal adenosine does not produce analgesia to acute noxious stimuli. Here, the authors test whether increased inhibition by adenosine of glutamate release from afferents after injury accounts for this difference. Methods Synaptosomes were prepared from the dorsal half of the lumbar spinal cord of normal rats or those with spinal nerve ligation. Glutamate release evoked by the TRPV-1 receptor agonist, capsaicin, was measured. Adenosine with or without adenosine A1 and A2 receptor antagonists was applied to determine the efficacy and mechanism of adenosine to reduce capsaicin-evoked glutamate release. Results Capsaicin produced a concentration-dependent glutamate release similarly in normal and nerve-injured rats. Capsaicin-evoked glutamate release was inhibited by adenosine or R-PIA (R-N6-(2- phenylisopropyl)-adenosine) in a concentration-dependent manner, with a threshold of 10 nm in both normal and nerve-ligated synaptosomes. Blockade of capsaicin-evoked glutamate release by adenosine was reversed similarly in synaptosomes from normal and spinal nerve-ligated animals by an A1 adenosine receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) but not by an A2 adenosine receptor antagonist DMPX (3'7-dimethyl-1-proparaglyxanthine). Capsaicin-evoked glutamate release, as well as its inhibition by adenosine, did not differ between synaptosomes prepared from tissue ipsilateral and contralateral to spinal nerve ligation. Conclusion These observations confirm previous neurophysiologic studies that presynaptic adenosine A1 receptor activation inhibits glutamate release from primary afferents. This effect is unaltered after peripheral nerve injury and thereby is unlikely to account for the enhanced analgesic efficacy of intrathecal adenosine in this setting.

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