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

Spinal A3 adenosine receptor activation acutely restores morphine antinociception in opioid tolerant male rats

Opioids are potent analgesics, but their pain-relieving effects diminish with repeated use. The reduction in analgesic potency is a hallmark of opioid analgesic tolerance, which hampers opioid pain therapy. In the central nervous system, opioid analgesia is critically modulated by adenosine, a purine nucleoside implicated in the beneficial and detrimental actions of opioid medications. Here, we focus on the A₃ adenosine receptor (A₃ AR) in opioid analgesic tolerance. Intrathecal administration of the A₃ AR agonist MRS5698 with daily systemic morphine in male rats attenuated the reduction in morphine antinociception over 7 days. In rats with established morphine tolerance, intrathecal MRS5698 partially restored the antinociceptive effects of morphine. However, when MRS5698 was discontinued, these animals displayed a reduced antinociceptive response to morphine. Our results suggest that MRS5698 acutely and transiently potentiates morphine antinociception in tolerant rats. By contrast, in morphine-naïve rats MRS5698 treatment did not impact thermal nociceptive threshold or affect antinociceptive response to a single injection of morphine. Furthermore, we found that morphine-induced adenosine release in cerebrospinal fluid was blunted in tolerant animals, but total spinal A₃ AR expression was not affected. Collectively, our findings indicate that spinal A₃ AR activation acutely potentiates morphine antinociception in the opioid tolerant state.

Chronic Morphine-Induced Changes in Signaling at the A3 Adenosine Receptor Contribute to Morphine-Induced Hyperalgesia, Tolerance, and Withdrawal

Treating chronic pain by using opioids, such as morphine, is hampered by the development of opioid-induced hyperalgesia (OIH; increased pain sensitivity), antinociceptive tolerance, and withdrawal, which can contribute to dependence and abuse. In the central nervous system, the purine nucleoside adenosine has been implicated in beneficial and detrimental actions of morphine, but the extent of their interaction remains poorly understood. Here, we demonstrate that morphine-induced OIH and antinociceptive tolerance in rats is associated with a twofold increase in adenosine kinase (ADK) expression in the dorsal horn of the spinal cord. Blocking ADK activity in the spinal cord provided greater than 90% attenuation of OIH and antinociceptive tolerance through A₃ adenosine receptor (A₃AR) signaling. Supplementing adenosine signaling with selective A₃AR agonists blocked OIH and antinociceptive tolerance in rodents of both sexes. Engagement of A₃AR in the spinal cord with an ADK inhibitor or A₃AR agonist was associated with reduced dorsal horn of the spinal cord expression of the NOD-like receptor pyrin domain-containing 3 (60%-75%), cleaved caspase 1 (40%-60%), interleukin (IL)-1β (76%-80%), and tumor necrosis factor (50%-60%). In contrast, the neuroinhibitory and anti-inflammatory cytokine IL-10 increased twofold. In mice, A₃AR agonists prevented the development of tolerance in a model of neuropathic pain and reduced naloxone-dependent withdrawal behaviors by greater than 50%. These findings suggest A₃AR-dependent adenosine signaling is compromised during sustained morphine to allow the development of morphine-induced adverse effects. These findings raise the intriguing possibility that A₃AR agonists may be useful adjunct to opioids to manage their unwanted effects. SIGNIFICANCE STATEMENT: The development of hyperalgesia and antinociceptive tolerance during prolonged opioid use are noteworthy opioid-induced adverse effects that reduce opioid efficacy for treating chronic pain and increase the risk of dependence and abuse. We report that in rodents, these adverse effects are due to reduced adenosine signaling at the A₃AR, resulting in NOD-like receptor pyrin domain-containing 3-interleukin-1β neuroinflammation in spinal cord. These effects are attenuated by A₃AR agonists, suggesting that A₃AR may be a target for therapeutic intervention with selective A₃AR agonist as opioid adjuncts.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

Opioid elevation of intracellular free calcium: possible mechanisms and physiological relevance

Opioid receptors are seven transmembrane domain Gi/G0 protein-coupled receptors, the activation of which stimulates a variety of intracellular signalling mechanisms including activation of inwardly rectifying potassium channels, and inhibition of both voltage-operated N-type Ca2+ channels and adenylyl cyclase activity. It is now apparent that like many other Gi/G0-coupled receptors, opioid receptor activation can significantly elevate intracellular free Ca2+ ([Ca2+]i), although the mechanism underlying this phenomenon is not well understood. In some cases opioid receptor activation alone appears to elevate [Ca2+]i, but in many cases it requires concomitant activation of Gq-coupled receptors, which themselves stimulate Ca2+ release from intracellular stores via the inositol phosphate pathway. Given the number of Ca2+-sensitive processes known to occur in cells, there are therefore a myriad of situations in which opioid receptor-mediated elevations of [Ca2+](i) may be important. Here, we review the literature documenting opioid receptor-mediated elevations of [Ca2+]i, discussing both the possible mechanisms underlying this phenomenon and its potential physiological relevance.

Increased nociceptive response in mice lacking the adenosine A1 receptor

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

Nonopioid receptor-mediated effects of U-50,488H on [Ca2+]i and extracellular dopamine in PC12 cells

The present studies were carried out to determine the effects of a kappa-opioid receptor agonist on cytosolic Ca(2+) concentration, [Ca(2+)](i), and extracellular dopamine in undifferentiated PC12 cells. The kappa-opioid receptor agonist U-50,488H caused concentration-dependent increases in [Ca(2+)](i) and extracellular dopamine. Neither effect was blocked by the selective kappa-opioid receptor antagonist nor-binaltorphimine. Increases in extracellular dopamine content and [Ca(2+)](i) caused by U-50,488H were correlated positively in the presence of extracellular Ca(2+); however, reduction of extracellular Ca(2+) abolished the increase in [Ca(2+)](i), but not that in dopamine. The latter observation suggests that stimulation of exocytotic release is not the primary mechanism involved in the increase in extracellular dopamine caused by U-50,488H. Effects on dopamine synthesis or catabolism also seem unlikely because the enhancement of extracellular dopamine occurred rapidly, and the amount of a major metabolite of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), was not affected. In any event, neither the increase in [Ca(2+)](i) nor the increase in extracellular dopamine caused by U-50,488H is mediated by the kappa-opioid receptor.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Delta-opioid receptor antagonists as a new concept for central acting antitussive drugs

Our recent findings indicated that mu- and kappa-opioid receptors enhance each other's antitussive processes. However, delta-opioid receptors played an inhibitory role in antitussive processes mediated by the mu- and kappa-opioid receptors. We also concluded that delta(1)-opioid receptors may play an inhibitory role, whereas delta(2)-opioid receptors may play a synergistic role, in antitussive processes mediated by mu-opioid receptors. Furthermore, we clearly demonstrated that delta-opioid receptor antagonists, such as naltrindole and 7-benzylidenenaltrexone, produced potent antitussive effects. These delta-opioid receptor-mediated antitussive effects may be mediated by the antagonism of delta(1)-, but not delta(2)-opioid receptors. In this review, we study the possibility of the delta-opioid receptor antagonist as a new concept for central acting antitussive drugs.

L-type and T-type calcium channel blockade potentiate the analgesic effects of morphine and selective mu opioid agonist, but not to selective delta and kappa agonist at the level of the spinal cord in mice

We examined the effects of amlodipine, a selective L-type voltage dependent Ca(2+) channel (VDCC) blocker, and mibefradil, a selective T-type VDCC blocker on the antinociceptive effects of morphine, and mu, delta and kappa opioid receptor selective agonist-induced antinociception at the spinal level. Intrathecally administered amlodipine and mibefradil potentiated morphine and [D-Ala(2), N mePhe(4), Gly-ol(5)] enkephalin (DAMGO)-induced antinociception by shifting their dose response curves to the left. However, intrathecally administered amlodipine and mibefradil did not affect [D-Pen(2), D-Pen(5)]enkephalin (DPDPE) and [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrolidinyl)cyclohexyl] benzene acetamide (U-50, 488H)-induced antinociception. These data indicate that L-type and T-type VDCC blockers synergistically potentiate the analgesic effects of mu opioid receptor agonists, but not delta and kappa opioid receptor agonists, at the spinal level. Additionally, these data suggest that there is an important functional interaction between L-type and/ or T-type VDCC and mu opioid receptors in the process of analgesia.

Role of calcium channels in the spinal transmission of nociceptive information from the mesentery

Opioids, alpha(2)-adrenoceptor agonists and blockers of voltage-gated calcium channels (VGCCs) have been attributed antinociceptive activity in various experimental set-ups. The present study tested the ability of morphine, clonidine and drugs acting at various VGCCs to inhibit the transmission of noxious stimuli from the mesentery at the level of the spinal cord. In rats under barbiturate anaesthesia traction of 20 g was applied to a bundle of mesenteric blood vessels. This caused immediate transient changes of mean arterial pressure that were taken as indication of nociception. Similar reflexes were elicited by applying 0.6% acetic acid to the same bundle of vessels. The reflexes were dose-dependently reduced by intrathecal administration of morphine or clonidine, but were left unaltered by intrathecal administration of verapamil, Bay-K 8644 or omega-conotoxin MVIIA. Neither verapamil nor Bay-K 8644 influenced clonidine-induced analgesia. Conotoxin markedly enhanced the effectiveness of all doses of clonidine against both types of mesenteric stimuli. Verapamil, Bay-K 8644, as well as conotoxin reduced the ability of morphine to inhibit mechanically evoked reflexes, while there was no statistically significant effect in chemonociception. These data suggest that, at the spinal level, both morphine and clonidine are effective drugs to decrease the cardiovascular changes caused by acute mesenteric pain. In the dorsal spinal cord neither L-type nor N-type VGCCs are responsible on their own for the transmission of noxious stimuli from the mesentery. Inhibition of N-type channels markedly augments the action of clonidine, whereas blocking either VGCC seems to inhibit antinociceptive mechanisms induced by morphine. It is suggested that in patients the combined administration of clonidine with omega-conotoxin MVIIA might lead to effective pain control with reduced side effects.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

Potentiation of transmitter release from NMB human neuroblastoma cells by kappa-opioids is mediated by N-type voltage-dependent calcium channels

The selective kappa-opioid agonist trans-(+/-)-3, 4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzenacetamidemethansulfonate (U50,488) potentiates both basal and depolarization-evoked [3H]dopamine release from NMB cells. The potentiation of dopamine release by U50,488 is mediated by N-type voltage-dependent calcium channels since it is blocked by omega-conotoxin, and is resistant to pertussis toxin (PTX)-treatment. When the stimulation of release by U50,488 is blocked by the N-channel antagonist omega-conotoxin, an inhibitory effect on dopamine release is revealed, suggesting that stimulatory and inhibitory effects of U50,488 are exerted in parallel.

Modulation of formalin-evoked hyperalgesia by intrathecal N-type Ca channel and protein kinase C inhibitor in the rat

1. omega-CgTx attenuated formalin-evoked biphasic flinches, while PKC inhibitor (STU) attenuated phase 2 and was reversed by PDBu. 2. omega-CgTx and STU suppressed the increase in CSF-glutamate after formalin injection. 3. Morphine completely suppressed both increased flinching and CSF glutamate release. 4. Thus, omega-CgTx (N-type Ca channels) may regulate neurotransmitter release evoked by C fiber activation and the formalin-evoked hyperalgesia may possibly be provoked as a result of PKC activation elicited by both presynaptic neurotransmitter release and activation of NMDA receptors in the spinal neurons.

Opioids potentiate transmitter release from SK-N-SH human neuroblastoma cells by modulating N-type calcium channels

Opioids induce dual (inhibitory and excitatory) regulation of depolarization-evoked [3H]dopamine release in SK-N-SH cells through either mu or delta receptors. The potentiation of dopamine release by opioid agonists is mediated by N-type voltage-dependent calcium channels and does not involve Gi/Go proteins. Removal of the excitatory opioid effect by blockade with omega-conotoxin, an N-channel antagonist, reveals the inhibitory effect of opioids on release, thus suggesting that both modulatory effects of opioids are exerted in parallel.

The stimulatory effect of opioids on cyclic AMP production in SK-N-SH cells is mediated by calcium ions

The present study examines the stimulatory effect of opioids on adenosine 3':5'-cyclic monophosphate (cyclic AMP) production in the human neuroblastoma cell line SK-N-SH, and its dependence on calcium. We show that, in this culture, the mu-opioid selective agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin stimulates cyclic AMP production by 30% in a naloxone-reversible manner. This stimulation is completely dependent on calcium and involves the activation of calcium/calmodulin since it is abolished in the presence of EGTA, calcium channel blockers or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7). The results suggest that the activation of calcium/calmodulin dependent adenylyl cyclases by opioids in SK-N-SH cells is secondary to the induction of calcium influx and the consequent elevation of intracellular calcium level.

Stimulatory effects of opioids on transmitter release and possible cellular mechanisms: overview and original results

Opiates and opioid peptides carry out their regulatory effects mainly by inhibiting neuronal activity. At the cellular level, opioids block voltage-dependent calcium channels, activate potassium channels and inhibit adenylate cyclase, thus reducing neurotransmitter release. An increasing body of evidence indicates an additional opposite, stimulatory activity of opioids. The present review summarizes the potentiating effects of opioids on transmitter release and the possible cellular events underlying this potentiation: elevation of cytosolic calcium level (by either activating Ca2+ influx or mobilizing intracellular stores), blockage of K+ channels and stimulation of adenylate cyclase. Biochemical, pharmacological and molecular biology studies suggest several molecular mechanisms of the bimodal activity of opioids, including the coupling of opioid receptors to various GTP-binding proteins, the involvement of different subunits of these proteins, and the activation of several intracellular signal transduction pathways. Among the many experimental preparations used to study the bimodal opioid activity, the SK-N-SH neuroblastoma cell line is presented here as a suitable model for studying the complete chain of events leading from binding to receptors down to regulation of transmitter release, and for elucidating the molecular mechanism involved in the stimulatory effects of opioid agonists.

Omega-agatoxin IVA blocks spinal morphine/clonidine antinociceptive synergism

Involvement of P-type voltage-dependent Ca2+ channels in spinal morphine- or clonidine-induced antinociception and in the synergistic interaction between morphine and clonidine was examined in the present studies. Coadministration of the selective P-type antagonist, omega-agatoxin IVA (25 ng) intrathecally (i.t.) to mice along with morphine or clonidine enhanced the tail flick antinociception of each agonist 5-6-fold. The greater-than-additive (synergistic) interaction that occurred when morphine and clonidine were coadministered i.t. decreased to an additive interaction in the presence of omega-agatoxin IVA. In mice pretreated with pertussis toxin (10 ng) to inactivate G proteins, omega-agatoxin IVA did not alter the morphine/clonidine synergism. Surprisingly, omega-agatoxin IVA reversed the additive morphine/clonidine interaction that occurs in morphine-tolerant mice back to synergism. These results suggest that functional P-type Ca2+ channels play an essential role in the antinociceptive synergism between spinal morphine and clonidine.

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.

Modulation of the activity of midbrain central gray substance neurons by calcium channel agonists and antagonists in vitro

Changes in the background impulse activity of midbrain central gray substance neurons have been studied on slice preparations from the rat midbrain upon application of calcium-free solution, an activator of calcium channels, BAY-K 8644 (10 nM), organic (verapamil, 40 microM; D600, 10 microM; nifedipine, 1-10 microM; amiloride, 1 microM) and inorganic (Co2+, 1.5 mM) calcium channel blockers. Besides BAY-K 8644, all the substances inhibited most of the neurons studied. Verapamil, BAY-K 8644 and Co2+ also revealed facilitatory effects. Facilitatory action of BAY-K was most effective in silent neurons and in those previously inhibited by amiloride. Latent period values of inhibition in calcium-free solution and upon application of organic and inorganic blockers have the following sequence: D600 > amiloride > verapamil > Co2+ > nifedipine > calcium-free solution. Maximum rise time had the following order: amiloride > D600 > nifedipine > verapamil > Co2+ > calcium-free solution. Complete suppression of the neuronal activity induced by amiloride lasted twice as long as that induced by calcium-free solution, Co2+ and nifedipine, and six times as long as verapamil-induced suppression. Preliminary application of calcium channel blockers reduced facilitatory and increased inhibitory effects of serotonin and substance P. Data obtained are discussed with the supposition in mind that inhibition of the function of calcium channels in central gray substance neurons could be one of the mechanisms underlying the analgesic effect of a series of neurotropic agents after their introduction into this structure.

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.

Multiple effects of opiates on intracellular calcium level and on calcium uptake in three neuronal cell lines

The present study examines the modulation by opiates of intracellular calcium levels and calcium entry, using fura-2 imaging and 45Ca2+ uptake, in three neuronal cell lines. We show that opiates (10(-7)-10(-5) M morphine and 10(-9)-10(-7) M etorphine) exert both inhibitory and excitatory effects on KCl-induced elevation in intracellular calcium level in SK-N-SH, NG108-15 and NMB cell lines. In addition, opiates elevate basal (non KCl-stimulated) intracellular calcium level in all three cell cultures. 45Ca2+ uptake is augmented by opiates in SK-N-SH cells and this stimulatory effect is not blocked by pertussis toxin. In NMB cells, an additional inhibitory effect of opiates on basal calcium takes place: opiates reduce intracellular calcium level as measured by fura-2, and decrease calcium influx as detected by 45Ca2+ uptake. The heterogeneity in the opioid regulation of calcium could not be attributed to the type of opioid drug, neither to its concentration nor to the experimental conditions, since neighboring cells within the same culture responded differently.

Adenosine kinase and adenosine deaminase inhibition modulate spinal adenosine- and opioid agonist-induced antinociception in mice

Endogenous purinergic systems mediating antinociception, and their interactions with opioids, were characterized following intrathecal (i.t.) administration of inhibitors of adenosine clearance in mice. 5'-Amino,5'-deoxyadenosine (5'-NH2dAdo), an inhibitor of adenosine kinase, induced significant antinociception after i.t. injection and enhanced antinociception induced by selected opioids (i.t.). Isobolographic analysis of antinociception following coadministration (i.t.) of 5'-NH2dAdo with opioids revealed additive interactions with mu-, and synergistic interactions with delta-, opioid receptor-selective agonists. Inhibitors of adenosine deaminase, deoxycoformycin and erythro-9-(2-hydroxy-3nonyl) adenine (EHNA), generally failed to induce antinociception when administered (i.t.) alone or to enhance opioid (i.t.)-induced antinociception, however, was significantly enhanced by either 5'-NH2dAdo or deoxycoformycin. These results confirm different physiologic roles for adenosine kinase and adenosine deaminase in spinal purinergic systems. 5'-NH2dAdo interactions with opioid receptor-selective agonists demonstrate significant, but heterogeneous interactions between endogenous adenosine and opioid spinal systems mediating antinociception.

Signalling via ATP in the nervous system

Strong evidence has been provided that ATP can act as a transmitter not only in smooth muscle but also in peripheral ganglia and in brain. The cloning and molecular identification of two putative ATP receptors supports the previously established pharmacological receptor classifications. This review places into perspective the evidence for ATP as a neural signalling substance by examining sites of storage, release and hydrolysis, as well as potential actions and targets. The action of ATP is related to that of the nucleoside adenosine, and the potential of additional nucleotides to function as neural messenger is examined briefly.