Biomedical subjects

J Sawynok

Publications and source records attributed to J Sawynok.

At least 37 records · Page 2Linked to original sources

PubMed1997

Substance P releases and augments the morphine-evoked release of adenosine from spinal cord.

The effects of substance P on the morphine-evoked release of adenosine were examined. Substance P alone produced a multiphasic effect on release of adenosine, with release occurring at low nanomolar concentrations and at a micromolar concentration, but not at intermediate concentrations. An inactive dose of substance P augmented the morphine-evoked release of adenosine at a nanomolar concentration of morphine. Release of adenosine by substance P alone (1 nM) or substance P/morphine (100 nM/10 nM) was Ca2(+)-dependent and originated from capsaicin-sensitive nerve terminals.

Adenosine↗

PubMed1997

Formalin-induced nociceptive behavior and edema: involvement of multiple peripheral 5-hydroxytryptamine receptor subtypes.

The role of 5-hydroxytryptamine and its receptor subtypes in the development of acute inflammation was investigated using the rat paw formalin test as a model for pain (measured by flinching behavior) and edema formation (measured by plethysmometry). The role of endogenously released 5-hydroxytryptamine was assessed using 5-hydroxytryptamine receptor subtype-selective antagonists co-injected with 2.5% formalin, while the receptor subtypes involved in the inflammatory process were further defined by co-injection of 5-hydroxytryptamine or 5-hydroxytryptamine receptor subtype-selective agonists with 0.5% formalin in anticipation of an augmented response. When co-administered with 2.5% formalin, propranolol, tropisetron or GR113808A, but not ketanserin, effectively blocked nociceptive behavior. In the presence of 0.5% formalin, 5-carboxamidotryptamine, 1-(m-chlorophenyl) biguanide or 5-methoxytryptamine, but not (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane, augmented the flinching response. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in peripheral nociception. There may be some dissociation of nociception and edema formation, since no single 5-hydroxytryptamine receptor antagonist inhibited edema formation with 2.5% formalin; however, with 0.5% formalin, edema formation was enhanced by co-administration of 5-hydroxytryptamine, 5-carboxamidotryptamine, (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane or 5-methoxytryptamine, but not 1-(m-chlorophenyl) biguanide. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine2 and possibly 5-hydroxytryptamine4 receptors in edema formation. These results confirm the involvement of 5-hydroxytryptamine1 and 5-hydroxytryptamine3 receptor subtypes in peripheral nociception associated with acute inflammation and further suggest an involvement of the more recently characterized 5-hydroxytryptamine4 receptor in this process. There appears to be a dissociation in 5-hydroxytryptamine receptors involved in peripheral nociception and edema formation.

Animals↗

PubMed1996

Adenosine kinase inhibitors augment release of adenosine from spinal cord slices.

Inhibitors of adenosine kinase, but not adenosine deaminase, produce antinociception when administered spinally. In this study, we evaluated the relative contribution of adenosine kinase and adenosine deaminase to the regulation of adenosine release into the extracellular space within the spinal cord by determining the effects of the adenosine kinase inhibitors 5'-amino-5'-deoxyadenosine and 5-iodotubercidin, and the adenosine deaminase inhibitor 2'-deoxycoformycin on adenosine release from spinal cord slices in an in vitro perfusion system. Both 5'-amino-5'-deoxyadenosine (5-50 microM) and 5-iodotubercidin (5-50 microM), but not 2'-deoxycoformycin (50 microM), augmented adenosine release. 5-Iodotubercidin was slightly more potent and effective than 5'-amino-5'-deoxyadenosine in augmenting release except at the highest concentration, where it was considerably more effective. Combinations of 2'-deoxycoformycin (50 microM) and minimally active concentrations of 5'-amino-5'-deoxyadenosine and 5-iodotubercidin (5 microM each) produced a synergistic enhancement of release. These results support a predominant involvement of adenosine kinase in regulating extracellular adenosine levels in the spinal cord, but adenosine deaminase also can play a significant role.

Adenosine↗

PubMed1996

Caffeine antinociception: role of formalin concentration and adenosine A1 and A2 receptors.

Caffeine produces antinociception in the formalin test at lower doses than in thermal threshold tests. In this study, we examined (a) the effect of formalin concentration on the action of caffeine, and (b) the relative involvement of adenosine A1 and A2 receptors in the action of caffeine. Formalin 1%, 2% and 5% produces a concentration-dependent increase in flinching behaviour in rats. At 5% formalin, the EC50 for caffeine in reducing phase 2 responses (16-60 min following formalin injection) is 40 mg/kg, but at 2%, this is reduced to 5 mg/kg. There is no further shift using 1% formalin. Caffeine has a more pronounced effect on phase 2A (16-36 min) than on phase 2, with significant effects being observed at lower doses. Both 8-cyclopentyltheophylline (A1 selective) and 3,7-dimethyl-1-propargylxanthine (A2 selective) produce a dose-related inhibition of 1.5% formalin flinching behaviours between 1-10 mg/kg. Both agents also produce locomotor stimulation over this dose range, but significant effects occur only at 10 mg/kg. These results indicate that antinociception by caffeine shows a high dependence on stimulus intensity. At 1-2% formalin, doses of caffeine that produce antinociception are now in the range that corresponds to normal human dietary and therapeutic intake levels. Both antinociception and locomotor stimulation appear to be dependent on activation of adenosine A1 receptors, as effects of 3,7-dimethyl-1-propargylxanthine are observed only at a dose which blocks adenosine A1 receptors in addition to A2 receptors.

Animals↗

PubMed1996

Synergy between mu/delta-opioid receptors mediates adenosine release from spinal cord synaptosomes.

Morphine releases adenosine from the spinal cord and this contributes to spinal antinociception. The present study examined possible interactions between mu- and subclasses of delta-opioid receptors in the release of adenosine. Nanomolar (10(-8), 10(-9) M) concentrations of morphine release adenosine from spinal cord synaptosomes under conditions of partial depolarization with elevated K+, and this component of release is mediated by activation of mu-opioid receptors. Subnanomolar (10(-10), 10(-11) M) concentrations of the mu-opioid receptor agonists morphine, [N-MePhe3,D-Pro4]morphiceptin, and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) have minimal effects on the release of adenosine from the spinal cord. However, [D-Pen2,D-Pen5]enkephalin (DPDPE), a delta 1-opioid receptor agonist, and [D-Ala2,Cys4]deltorphin, a delta 2-opioid receptor agonist, at doses which exhibit no intrinsic effects (10(-8) and 10(-7) M), shifted the dose-response curve for mu-opioid receptor-evoked adenosine release to the left in a dose-dependent manner. DPDPE was more potent than [D-Ala2,Cys4]deltorphin when combined with the highly selective mu-opioid receptor agonist [N-MePhe3,D-Pro4]morphiceptin, but these agents showed similar activity with the less selective agonists DAMGO and morphine. Simultaneous activation of mu- and delta-opioid receptors generates a synergistic release of adenosine from spinal cord synaptosomes. Although agonists for both delta 1- and delta 2-opioid receptor subtypes produce this response, the delta 1-opioid receptor agonist is more potent at eliciting this effect when the most selective mu-opioid receptor ligand is used.

Adenosine↗

PubMed1996

Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception.

The effects of 5-hydroxytryptamine (5-HT) and ligands selective for particular 5-HT receptor subtypes on the transmission of nociceptive information in the spinal cord are complex. In these studies, we have focused on their interactions with two endogenous mediators of pain suppression, noradrenaline (NA) and adenosine. Spinal antinociception by 5-HT is blocked by alpha-adrenoreceptor antagonists and depletion of endogenous NA by 6-hydroxydopamine, while it is potentiated by blockade of NA reuptake with desipramine. These observations provide evidence for a 5-HT receptor-mediated increase in the release of NA from the spinal cord. This action appears to be due to activation of a 5-HT1-like receptor as it is mimicked by some 5-HT1 receptor ligands (mCPP, TFMPP and 5-Me-O-DMT), but not by DOI (5-HT2) or 2-Me-5-HT (5-HT3). An additional component of 5-HT action is via release of adenosine. Antinociception by 5-HT is blocked by the adenosine receptor antagonist 8-phenyltheophylline, and 5-HT has been shown to release adenosine from the spinal cord in in vitro and in vivo paradigms. Methylxanthine-sensitive antinociception is seen with some 5-HT1 receptor ligands (CGS 12066B, mCPP), but not with others or with DOI or 2-Me-5-HT. Further characterization of the 5-HT receptor subtype involved in adenosine release will require the use of additional approaches.

Animals↗

PubMed1996

Neurotoxin-induced lesions to central serotonergic, noradrenergic and dopaminergic systems modify caffeine-induced antinociception in the formalin test and locomotor stimulation in rats.

Our study examined the effects of neurotoxin-induced lesions to central 5-hydroxytryptamine (5-HT), noradrenaline (NA) and dopamine (DA) systems on antinociception by caffeine in the formalin test and on locomotor stimulation by caffeine. Locomotor activity was determined simultaneously with responses to formalin, although in some studies, scores were derived in the absence of formalin. Rats were tested 10 to 12 days after toxin administration, and amine levels in brain regions and the spinal cord determined at the end of the experiment. Intracerebroventricular 5,7-dihydroxytryptamine (+/- desipramine) (which depleted central 5-HT systems) inhibited antinociception by caffeine but had no effect on locomotor stimulation. Intracerebroventricular 6-hydroxydopamine (which depleted NA in brain and spinal cord and DA in brain) augmented both antinociception and locomotor stimulation by caffeine. Both effects were due to NA depletion as they were mimicked by microinjection of 6-hydroxydopamine into the locus ceruleus that selectively depleted NA, not by intracerebroventricular 6-hydroxydopamine with desipramine that selectively depleted DA. In the absence of formalin, locus ceruleus lesions no longer had a significant effect on locomotor stimulation. Phentolamine, an alpha-adrenergic antagonist, inhibited caffeine-induced locomotor stimulation in the absence of formalin. These neurotoxin-induced lesions reveal an involvement of central 5-HT and NA (but not DA) systems in antinociception, and an involvement of central NA (but not 5-HT or DA) systems in locomotor stimulation. There appears to be an interrelationship between noxious stimulation and the expression of locomotor activity.

5,7-Dihydroxytryptamine↗

PubMed1995

Modulation of adenosine release from rat spinal cord by adenosine deaminase and adenosine kinase inhibitors.

Adenosine, a modulator of pain processing in the spinal cord, is metabolized by adenosine kinase and adenosine deaminase. In this study we determined which of these mechanisms is more important for the regulation of endogenous adenosine levels in the rat spinal cord. The effects of the adenosine kinase inhibitors, 5'-amino-5'-deoxyadenosine (NH2dAD) and iodotubercidin (IOT), and the adenosine deaminase inhibitor, 2'-deoxycoformycin (DCF), on adenosine release in a spinal cord superfusion model were studied. DCF markedly increased basal adenosine levels detected in perfusates and was more potent than NH2dAD and IOT in this regard. Coadministration of DCF with NH2dAD produced an enhanced effect compared to the inhibitors alone. NH2dAD, but not DCF, potentiated morphine-evoked adenosine release. These results suggest that adenosine deaminase may be the predominant pathway for adenosine metabolism in this experimental model.

Adenosine↗

PubMed1995

Antinociception by adenosine analogs and an adenosine kinase inhibitor: dependence on formalin concentration.

Spinal administration of adenosine analogs and an adenosine kinase inhibitor produces antinociception in thermal threshold tests. In the present study, we determined the effects of N6-cyclohexyladenosine (adenosine A1 receptor selective), 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethyl-carboxamidoadeno sine (CGS-21680) (adenosine A2A receptor selective), and 5'-N-ethylcarboxamidoadenosine (NECA) (non-selective), on formalin induced nociceptive responses (flinching/lifting and licking/biting) using two concentrations of formalin (2% and 5%). We also examined the antinociceptive effects of 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, and deoxycoformycin, an adenosine deaminase inhibitor, under these conditions. Adenosine A1 receptor agonists, but not the A2A selective agent, produced significant antinociception, as did 5'-amino-5'-deoxyadenosine, but not deoxycoformycin. The extent of antinociception produced was greater with the lower stimulus intensity. The effects of NECA and 5'-amino-5'-deoxyadenosine were inhibited by caffeine, indicating the involvement of cell surface adenosine receptors in their actions. We conclude (a) that the adenosine A1, but not the A2A, receptor is involved in spinally mediated antinociception, (b) that adenosine kinase is more important than adenosine deaminase in regulating endogenous adenosine levels in the spinal cord, and (c) that stimulus intensity is an important determinant of the efficacy of purines in the spinal cord.

Adenosine↗

PubMed1995

Complex role of peripheral adenosine in the genesis of the response to subcutaneous formalin in the rat.

When applied peripherally, adenosine has been shown to be pronociceptive in a number of animal and human models. Recent evidence has implicated adenosine as a significant mediator in the inflammatory process. In this study using rats, we have examined the effect of adenosine and of selective adenosine A1 and A2 receptor agonists and antagonists on the response to a subcutaneous injection of formalin into the rat hindpaw. Adenosine co-injected with formalin 0.5% significantly increased flinching in both phases in a dose-dependent manner. The highest dose of adenosine had no behavioral effect on its own. The adenosine A2 receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS-21680), at a dose of 1.5 nmol, increased flinching associated with 0.5% formalin injection but at higher doses produced depressant effects due to systemic absorption. The adenosine A1 receptor agonist N6-cyclohexyladenosine produced only systemic behavioral effects as determined by contralateral application. The flinching response to 2.5% formalin was significantly decreased by the adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). In contrast, 8-cyclopentyl-1,3-dimethylxanthine (CPT), the selective adenosine A1 receptor antagonist augmented the response to 2.5% formalin. The non-selective adenosine receptor antagonist caffeine had no significant effect over a wide range of doses. In summary, exogenous adenosine enhances nociception in the formalin test, probably via a peripheral A2 receptor-mediated action. Endogenous adenosine, acting at both A1 and A2 receptors, appears to be involved in the formalin-induced inflammatory response.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗

PubMed1995

Pharmacological rationale for the clinical use of caffeine.

Caffeine is widely consumed in beverages to obtain mild CNS stimulant effects. Long term use produces tolerance to some of the pharmacological effects. Withdrawal of caffeine, even from moderate intake levels, can produce symptoms such as headache, fatigue and anxiety. Caffeine is used therapeutically in combination with ergotamine for migraine headaches and in combination with nonsteroidal anti-inflammatory drugs in analgesic formulations. Caffeine alone is used as a somnolytic, to treat various headache conditions, respiratory depression in neonates, postprandial hypotension and obesity, and to enhance seizure duration in electroconvulsive therapy. In some headache and in pain paradigms, caffeine may produce direct adjuvant analgesic properties, while in other headache conditions (perioperative, postdural puncture) caffeine may be effective by alleviating a manifestation of caffeine withdrawal. Other uses, such as to promote wakefulness, for respiratory stimulation and seizure prolongation, rely on central stimulant properties of caffeine. Effects of caffeine on the vasculature may contribute to the relief of some headaches and in postprandial hypotension. Blockade of methylxanthine-sensitive adenosine receptors is the currently accepted mechanism of action of caffeine.

Animals↗

PubMed1995

Spinal opioid receptors and adenosine release: neurochemical and behavioral characterization of opioid subtypes.

Release of adenosine from the spinal cord contributes to spinal antinociception by morphine. Morphine induces a Ca(++)-dependent release of adenosine from dorsal spinal cord synaptosomes, which is augmented under partially depolarizing conditions. The present study examined the opioid receptor subtypes involved in this release, and determined whether adenosine is an important mediator of antinociception induced by the spinal administration of selective opioid agonists in rats. Nanomolar and micromolar concentrations of the selective mu opioid agonists DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin) and PLO17 ([N-MePhe3,D-Pro4]morphiceptin) induced release of adenosine in a biphasic manner in the presence of a partial depolarization (addition of 6 mM K+ to the Krebs' medium). The delta opioid agonists DPDPE ([D-Pen2,D-Pen5]enkephalin) and DELT ([D-Ala2,Cys4]deltorphin) and the kappa opioid agonist U50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-(1-pyrroli-zemeacetamid e) had little effect on the release of adenosine except at high micromolar concentrations. Release of adenosine by mu (nanomolar) and delta (micromolar) ligands is Ca(++)-dependent, whereas the kappa (micromolar) receptor ligand releases adenosine via a Ca(++)-independent mechanism. Behavioral antinociception using the hot-plate threshold test revealed that intrathecal administration of the mu and delta opioid receptor agonists produced dose-dependent antinociception with an order of potency of DAMGO, PLO17 > morphine, DELT > DPDPE. An ED75 dose of morphine, DAMGO or PLO17 was attenuated dose-dependently by intrathecal pretreatment with the adenosine receptor antagonist caffeine. Caffeine did not block the antinociceptive response to delta agonists, but in fact augmented antinociception when combined with DPDPE and DELT. This augmentation was dose-dependent. This study demonstrates that activation of the mu receptor subtype is responsible for the opioid-induced release of adenosine from the spinal cord, that such release contributes to the spinal antinociception by mu agonists and that only release evoked by low doses of opioids is behaviorally relevant.

Adenosine↗

PubMed1994

Spinal supersensitivity to 5-HT1, 5-HT2 and 5-HT3 receptor agonists following 5,7-dihydroxytryptamine.

The present study examined functional supersensitivity to 5-hydroxytryptamine (5-HT) and 5-HT ligands selective for 5-HT1, 5-HT2 and 5-HT3 receptors in two tests for nociception following the spinal administration of 5,7-dihydroxytryptamine (5,7-DHT). Intrathecal pretreatment with 5,7-DHT 30-100 micrograms (following desipramine) produced a selective depletion of spinal cord 5-HT levels of > 80% and augmented the antinociceptive action of 5-HT in the tail flick and hot plate tests. The tail flick test was the more sensitive test for expression of this action. Supersensitivity was observed with the 5-HT1 receptor ligands CGS 12066B (7-trifluoromethyl-4-(4-methyl-1-piperazinyl-pyrrolo[1,2-a] quinoxalinedimaleate), RU 24969 (5-methoxy-3-(1,2,4,6-tetrahydro-4-pyridinyl)1H indole succinate), TFMPP (m-trifluoromethylphenyl-piperazine HCl), mCPP (1-(3-chlorophenyl)piperazine dihydrochloride) and 5-Me-ODMT (5-methoxy-N,N-dimethyltryptamine hydrogen oxalate) but not with the 5-HT2 receptor ligand DOI ((+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane HCl) or the 5-HT3 receptor ligand 2-Me-5-HT (2-methyl-5-hydroxytryptamine maleate) in the tail flick test. In the hot plate test, supersensitivity was observed only with 5-Me-ODMT. Intrathecal pretreatment with fluoxetine, a 5-HT uptake inhibitor, potentiated the action of 5-HT but not any of the other 5-HT1 receptor ligands examined. These results indicate that supersensitivity occurs with 5-HT and 5-HT1 receptor ligands but not with 5-HT2 or 5-HT3 receptor ligands. Both the loss of uptake sites and receptor upregulation may contribute to enhanced activity of 5-HT, but for other ligands, only the latter mechanism appears to occur.

5,7-Dihydroxytryptamine↗

PubMed1993

Morphine-evoked release of adenosine from the spinal cord occurs via a nucleoside carrier with differential sensitivity to dipyridamole and nitrobenzylthioinosine.

We have investigated the potential role of a bi-directional nucleoside carrier in the release of endogenous adenosine from spinal cord synaptosomes by examining the effects of dipyridamole and nitrobenzylthioinosine (NBI) on evoked release of adenosine. When 40 pmol adenosine were added to synaptosomes, only 70 +/- 2% was recovered, suggesting 30% uptake of adenosine. Dipyridamole (0.1-10 microM) reduced this uptake and also increased basal adenosine release, probably due to inhibition of the re-uptake of adenosine derived from released nucleotide. In contrast, NBI (0.1-10 microM) had no effect on either uptake of added adenosine or on basal release of adenosine. Addition of K+ (24 mM) and morphine (10 microM) produced a 50-60% increase in the release of adenosine, and this was reduced 35-98% by both dipyridamole and NBI (0.01-10 microM). Dipyridamole (0.01-1 microM) had no effect on the release of nucleotides (detected as adenosine) induced by noradrenaline, 5-hydroxytryptamine (5-HT) and capsaicin (50 microM each), although 10 microM dipyridamole significantly reduced release evoked by noradrenaline and 5-HT. This latter effect of dipyridamole was determined not to be due to inhibition of ATP release when measured directly. Within the spinal cord, there is a removal system for adenosine which is dipyridamole-sensitive but NBI-insensitive. Release of adenosine, but not nucleotides, appears to occur via this carrier system. The inhibition of release by NBI, but its lack of effect on uptake, suggests the involvement of heterogeneous carrier molecules in adenosine uptake and release from the spinal cord.

Adenosine↗

PubMed1993

ATP release from dorsal spinal cord synaptosomes: characterization and neuronal origin.

The present study determined the Ca(2+)-dependence of the release of adenosine 5'-triphosphate (ATP) from dorsal spinal cord synaptosomes evoked by depolarization with K+ and capsaicin, and the effect of intrathecal capsaicin pretreatment, dorsal rhizotomy and intrathecal pretreatment with 6-hydroxydopamine on such release. Release of ATP evoked by K+ was Ca(2+)-dependent, while that evoked by capsaicin was Ca(2+)-independent. Capsaicin pretreatment (60 micrograms, 7 days), which lesions small diameter afferents, did not alter release of ATP evoked by either K+ or capsaicin. Dorsal rhizotomy, which lesions all afferents, produced a significant reduction in the amount of ATP released from the rhizotomized side compared to the intact side. Pretreatment with 6-hydroxydopamine (100 micrograms, 7 days) to destroy adrenergic nerve terminals, markedly reduced spinal cord noradrenaline levels, but did not alter the K(+)-evoked release of ATP. These results suggest that some K(+)-evoked release of ATP could originate from large but not small diameter afferent nerve terminals in the spinal cord. ATP does not appear to originate from small diameter afferents as, although ATP is released by in vitro exposure to capsaicin, such release occurs only at high concentrations, release is Ca(2+)-independent and it is unaltered by pretreatment with capsaicin. The bulk of the ATP released from the spinal cord does not originate from descending noradrenergic nerve terminals.

Adenosine Triphosphate↗

PubMed1993

Influence of calcium on the release of endogenous adenosine from spinal cord synaptosomes.

Intrathecal administration of Ca2+ has been shown to produce antinociception which is thought to be partially mediated by the release of adenosine. In the present study we have examined directly the effects of varying Ca2+ concentrations on the release of endogenous adenosine, measured by HPLC with fluorescence detection, from rat spinal cord synaptosomes. Although increasing the concentration of extracellular Ca2+ reduces the total amount of adenosine detected extrasynaptosomally, the component derived from the release of adenosine per se is actually augmented. This release of adenosine occurs from dorsal but not ventral spinal cord synaptosomes and appears to originate from capsaicin-sensitive nerve terminals. The Ca2+ ionophore A23187 also releases adenosine, but this release is due to the extrasynaptosomal conversion of released nucleotide(s) to adenosine, as it is markedly reduced by ecto-5'-nucleotidase inhibitors. Release of adenosine by A23187 occurs from both the dorsal and ventral spinal cord, and is not capsaicin-sensitive. Ethanol, used as a vehicle for the ionophore, releases adenosine which is a mixture of adenosine and nucleotide from both dorsal and ventral spinal cord synaptosomes. These observations provide direct support for behavioural studies which demonstrate that methylxanthines block antinociception produced by intrathecal administration of Ca2+.

Adenosine↗

PubMed1993

Involvement of calcium channels in depolarization-evoked release of adenosine from spinal cord synaptosomes.

The potential involvement of L- and N-type voltage-sensitive calcium (Ca2+) channels and a voltage-independent receptor-operated Ca2+ channel in the release of adenosine from dorsal spinal cord synaptosomes induced by depolarization with K+ and capsaicin was examined. Bay K 8644 (10 nM) augmented release of adenosine in the presence of a partial depolarization with K+ (addition of 6 mM) but not capsaicin (1 and 10 microM). This augmentation was dose dependent from 1 to 10 nM and was followed by inhibition of release from 30 to 100 nM. Nifedipine and nitrendipine inhibited the augmenting effect of Bay K 8644 in a dose-dependent manner, but neither antagonist had any effect on release of adenosine produced by K+ (24 mM) or capsaicin (1 and 10 microM). omega-Conotoxin inhibited K(+)-evoked release of adenosine in a dose-dependent manner but had no effect on capsaicin-evoked release. Ruthenium red blocked capsaicin-induced release of adenosine but had no effect on K(+)-evoked release. Although L-type voltage-sensitive Ca2+ channels can modulate release of adenosine when synaptosomes are partially depolarized with K+, N-type voltage-sensitive Ca2+ channels are primarily involved in K(+)-evoked release of adenosine. Capsaicin-evoked release of adenosine does not involve either L- or N-type Ca2+ channels, but is dependent on a mechanism that is sensitive to ruthenium red.

Adenosine↗

PubMed1993

Morphine activates omega-conotoxin-sensitive Ca2+ channels to release adenosine from spinal cord synaptosomes.

Morphine-induced release of adenosine from the spinal cord is believed to contribute to spinal antinociception. Although this release is Ca2+ dependent, little is known of the nature of this dependence. In this study, the effects of the dihydropyridine L-type Ca2+ channel agonist Bay K 8644 and the antagonist nifedipine, the N-type Ca2+ channel antagonist omega-conotoxin, and ruthenium red, a blocker of Ca2+ influx induced by capsaicin, on release of adenosine evoked by morphine were determined. The effect of partial depolarization with a minimally effective concentration of K+ on morphine-evoked release of adenosine also was examined. Morphine 10(-5)-10(-4) M produced a dose-dependent enhancement of adenosine release from dorsal spinal cord synaptosomes. Following the addition of 6 mM K+ (total K+ concentration of 10.7 mM), 10(-6) M morphine also enhanced release, and an additional component of action at 10(-8) M was revealed. Release was Ca(2+)-dependent as it was not observed in the absence of Ca2+ and presence of EGTA. Bay K 8644 (10 nM) and nifedipine (100 nM) had no effect on the release of adenosine evoked by morphine, but omega-conotoxin (100 nM) markedly reduced such release in both the absence and the presence of the additional 6 mM K+. Morphine-evoked adenosine release was not altered in the presence of a partially effective dose of capsaicin, nor by ruthenium red.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗