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

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Is withdrawal hyperalgesia in morphine-dependent mice a direct effect of a low concentration of the residual drug?

Withdrawal of opioid drugs leads to a cluster of unpleasant symptoms in dependent subjects. These symptoms are stimulatory in nature and oppose the acute, inhibitory effects of opiates. The conventional theory that explains the opioid withdrawal syndrome assumes that chronic usage of opioid drugs activates compensatory mechanisms whose stimulatory effects are revealed upon elimination of the inhibitory opioid drug from the body. Based on previous studies that show a dose-dependent dual activity of opiates, including pain perception, we present here an alternative explanation to the phenomenon of withdrawal-induced hyperalgesia. According to this explanation, the residual low concentration of the drug that remains after cessation of its administration elicits the stimulatory withdrawal hyperalgesia. The goal of the present study was to test this hypothesis. In the present study we rendered mice dependent on morphine by a daily administration of the drug. Cessation of morphine application elicited withdrawal hyperalgesia that was completely blocked by a high dose of the opiate antagonist naloxone (100 mg/kg). Similarly, naloxone (2 mg/kg)-induced withdrawal hyperalgesia was also blocked by 100 mg/kg of naloxone. The blockage of withdrawal hyperalgesia by naloxone suggested the involvement of opioid receptors in the phenomenon and indicated that withdrawal hyperalgesia is a direct effect of a residual, low concentration of morphine. Acute experiments that show morphine- and naloxone-induced hyperalgesia further verified our hypothesis. Our findings offer a novel, alternative approach to opiate detoxifications that may prevent withdrawal symptoms by a complete blockage of the opioid receptors using a high dose of the opioid antagonist.

Low-dose morphine induces hyperalgesia through activation of G alphas, protein kinase C, and L-type Ca 2+ channels in rats

Opioids can induce analgesia and also hyperalgesia in humans and in animals. It has been shown that systemic administration of morphine induced a hyperalgesic response at an extremely low dose. However, the exact mechanism(s) underlying opioid-induced hyperalgesia has not yet been clarified. Here, we have investigated cellular events involved in low-dose morphine hyperalgesia in male Wistar rats. The data showed that morphine (0.01 microg i.t.) could elicit hyperalgesia as assessed by the tail-flick test. G(alphas) mRNA and protein levels increased significantly following exposure to the hyperalgesic dose of morphine. Furthermore, morphine at an analgesic dose (20 microg i.t.) significantly decreased cAMP levels in the dorsal half of the lumbar spinal cord, whereas the tissue cAMP levels were not affected by morphine treatment at a hyperalgesic dose. Intrathecal administration of nifedipine, an L-type calcium channel blocker, antagonized the hyperalgesia induced by the low-dose of morphine. Furthermore, pretreatment with the selective protein kinase C (PKC) inhibitor chelerytrine resulted in prevention of the morphine-induced hyperalgesia. KT 5720, a specific inhibitor of protein kinase A (PKA), did not show any effect on low-dose morphine-induced hyperalgesia. These results indicate a role for G(alphas), the PLC-PKC pathway, and L-type calcium channels in intrathecal morphine-induced hyperalgesia in rats. Activation of ordinary G(alphas) signaling through cAMP levels did not appear to play a major role in the induction of hyperalgesia by low-dose of morphine.

The effect of morphine in rat small mesenteric arteries

We investigated the effect of morphine in phenylephrine (PE)- or KCl-precontracted rat small mesenteric arteries. Morphine (10(-6)-10(-4) M) administration caused concentration-dependent relaxation responses in small mesenteric arteries precontracted by PE or KCl. Removal of endothelium did not significantly alter the relaxation responses to morphine. The relaxant responses to morphine were partially inhibited by pre-treatment of tissues with naloxone (NAL, 10(-5) M) for 20 min. The inhibitory effect of NAL on relaxant responses to morphine in PE- or KCl-precontracted arteries did not differ significantly between endothelium-intact and endothelium-denuded preparations. Incubation of endothelium-intact or endothelium-denuded arterial segments with NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or cyclooxygenase (COX) inhibitor indomethacin (10(-5) M) or histamine H(1)-receptor blocker diphenhydramine (10(-6) M), for 20 min did not inhibit the relaxation responses to morphine. Small mesenteric arterial segment contractions induced by stepwise addition of calcium to high KCl solution with no calcium were almost completely inhibited by morphine. These findings suggested that morphine-induced relaxation responses in isolated rat small mesenteric arteries were neither dependent on endothelium nor blocked by NOS or COX inhibition but they rather seem to depend on an interaction of morphine with calcium influx pathways.

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.

Inhibition of calcium uptake via the sarco/endoplasmic reticulum Ca2+-ATPase in a mouse model of Sandhoff disease and prevention by treatment with N-butyldeoxynojirimycin

Gangliosides are found at high levels in neuronal tissues where they play a variety of important functions. In the gangliosidoses, gangliosides accumulate because of defective activity of the lysosomal proteins responsible for their degradation, usually resulting in a rapidly progressive neurodegenerative disease. However, the molecular mechanism(s) leading from ganglioside accumulation to neurodegeneration is not known. We now examine the effect of ganglioside GM2 accumulation in a mouse model of Sandhoff disease (one of the GM2 gangliosidoses), the Hexb-/- mouse. Microsomes from Hexb-/- mouse brain showed a significant reduction in the rate of Ca2+-uptake via the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), which was prevented by feeding Hexb-/- mice with N-butyldeoxynojirimycin (NB-DNJ), an inhibitor of glycolipid synthesis that reduces GM2 storage. Changes in SERCA activity were not due to transcriptional regulation but rather because of a decrease in Vmax. Moreover, exogenously added GM2 had a similar effect on SERCA activity. The functional significance of these findings was established by the enhanced sensitivity of neurons cultured from embryonic Hexb-/- mice to cell death induced by thapsigargin, a specific SERCA inhibitor, and by the enhanced sensitivity of Hexb-/- microsomes to calcium-induced calcium release. This study suggests a mechanistic link among GM2 accumulation, reduced SERCA activity, and neuronal cell death, which may be of significance for delineating the neuropathophysiology of Sandhoff disease.

Kappa-opioid receptor-mediated enhancement of the hyperpolarization-activated current (I(h)) through mobilization of intracellular calcium in rat nucleus raphe magnus

The hyperpolarization-activated current (Ih) is important in the control of resting membrane potential, in the regulation of network firing pattern and in the modulation of presynaptic transmitter release in central neurons. Recent studies on native and cloned Ih channels have demonstrated that the Ih channel is commonly modulated by cAMP through a positive shift in its voltage dependence without a change in its maximum current. The present study demonstrates that activation of kappa-opioid receptors enhances Ih by increasing its maximum current in brainstem neurons in the nucleus raphe magnus. Agents that interfere with the release of intracellular calcium from calcium stores altered the maximum Ih and significantly attenuated the kappa-receptor-mediated enhancement of Ih. These results suggest that kappa-opioid receptors enhance the maximum Ih by mobilizing intracellular calcium from calcium stores. This provides a physiological function for kappa-receptor-stimulated calcium release and may suggest another Ih-regulating mechanism by intracellular calcium in central neurons.

The mu opioid agonist DAMGO stimulates cAMP production in SK-N-SH cells through a PLC-PKC-Ca++ pathway

The mu-opioid agonist DAMGO exerts a dual activity on cAMP production in SK-N-SH neuroblastoma cells. While the classic inhibitory effect was prevented by pretreating the cells with pertussis toxin (PTX), the stimulatory activity was PTX-resistant. The stimulatory effect was abolished by the selective phospholipase C (PLC) blocker U-73122, by the selective protein kinase C (PKC) blocker chelerythrine and by the calcium-channels blockers Ni++, Co++ and Cd++. Hence, it is suggested that the opioid receptor activates PLC (probably through Gq GTP-binding proteins), to mobilize PKC, that positively modulates calcium channels in the plasma membrane; the entry of Ca++ into the cells stimulates calcium-activated adenylyl cyclases to produce cAMP.

CB(1) cannabinoid receptor-G protein association: a possible mechanism for differential signaling

Effects of cannabinoid compounds on neurons are predominantly mediated by the CB(1) cannabinoid receptor. Onset of signaling cascades in response to cannabimimetic drugs is triggered by the interaction of the cannabinoid receptor with G(i/o) proteins. Much work has been done to delineate the cannabinoid agonist-induced downstream signaling events; however, it remains to define the molecular basis of cannabinoid receptor-G protein interactions that stimulate these signaling pathways. In this review, we discuss several signal transduction pathways, focusing on studies that demonstrate the efficacy of CB(1) receptor agonists through G protein mediated pathways.

The effect of nitric oxide synthase blockade on responses to morphine in rat aortic rings

1 It has been suggested that opioids may play an indirect role in the regulation of the peripheral circulation through the control of nitric oxide (NO) release in vascular tissue. The current study was undertaken to investigate the effect of nitric oxide synthase (NOS) blockade on responses to morphine in phenylephrine (PE)- or KCl-precontracted rat aortic rings. 2 Morphine (3 x 10(-8) - 3 x 10(-5) M) administration did not cause any significant effect on basal tonus of endothelium-intact or endothelium-denuded preparations. Morphine produced concentration-dependent relaxation responses in endothelium-intact as well as in endothelium-denuded rat aortic rings precontracted by PE or KCl. Removal of endothelium did not significantly alter the relaxation responses to morphine. 3 The relaxant responses to morphine were significantly and partially inhibited by pretreatment of tissues with naloxone (NAL, 3 x 10(-5) M) for 5 min. The inhibitory effect of NAL on relaxant responses to morphine in PE- or KCl-precontracted rings did not differ significantly between endothelium-intact and endothelium-denuded preparations. 4 Incubation of endothelium-intact or endothelium-denuded rat aortic rings with NOS inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) for 20 min did not cause a significant inhibition on relaxation responses to morphine. 5 These findings confirmed the presence of opiate receptors in rat thoracic aorta, but suggested that mechanisms other than NO release play a role in the relaxant effect of morphine on rat aortic rings.

Sign-reversal during persistent activation in mu-opioid signal transduction

A concept of signal transduction in biological systems specifies that any instantaneous input is appreciated by its departure from the moving average of past activity. The concept provides an adequate account of the occurrence of both the one-directional (e.g. analgesic) effects induced by opioid receptor activation, and of the contra-directional (e.g. hyperalgesic) effects that can be observed when activation is discontinued. Following this transduction concept, the numerical simulations reported here revealed, remarkably, that under some parametric conditions, the input's effect may reverse even as input is maintained at a constant magnitude. In in vitro conditions that are proximal to the signal transduction that occurs when an opioid agonist binds to the G-protein coupled opioid receptor, the effects of opioid receptor activation were monitored by measuring time-dependent Ca(2+) responses in CHO-K1 cells transfected with a mu-opioid receptor and G(alpha 15) protein. The results indicate morphine to produce an initial increase in intracellular Ca(2+) concentration followed by a decrease below basal level. The occurrence of a sign-reversal was confirmed in native conditions of receptor-to-G protein coupling; the continuous in vivo infusion over a 2-week period of 0.31 mg rat(-1)day(-1) of fentanyl initially caused an increase of the mechanical threshold to induce a pain response (i.e. analgesia) that was followed by a decrease (i.e. hyperalgesia). The findings indicate that with opioid signaling systems, transduction mechanisms operate that may cause the sign of the effect to reverse not only when activation is discontinued but also whilst it is maintained at a constant magnitude.

Gi/Go couple met-enkephalin to inhibition of cholinergic and beta-adrenergic stimulation of lacrimal secretion

PURPOSE

To determine whether G-protein-mediated inhibition of secretion by met-enkephalin involves cyclic adenosine monophosphate (cAMP)-dependent events and to identify the G proteins that couple met-enkephalin to inhibition of lacrimal secretion.

METHODS

Secretion of protein was measured in 3-day primary cultures of rabbit lacrimal acini exposed to vehicle, the cholinergic agonist carbachol (Cch), the beta-adrenergic agonist isoproterenol (Isop), vasoactive intestinal peptide (VIP), or forskolin (FSK) with or without the enkephalin analog D-ala2-met-enkephalinamide (DALA). In separate experiments, cells were pretreated with pertussis toxin or polyclonal antibodies against the alpha subunits of Gi/Go to determine the physiologic role of G proteins in met-enkephalin inhibition of the release of lacrimal protein. Adenylyl cyclase (AC) activity was measured by a cAMP-dependent protein kinase binding assay in lacrimal membranes in response to the same agonists used in the secretion studies.

RESULTS

Cch resulted in a significant increase in protein release from cultured lacrimal acini. Increased secretion also occurred with Isop, VIP, and FSK. Cch- and Isop-stimulated secretion was inhibited by DALA to near-basal values. However, DALA did not inhibit VIP- or FSK-stimulated secretion. The inhibitory effect of DALA on Cch and Isop stimulation of secretion was reversed by pertussis toxin. Inhibition of Cch-stimulated secretion was blocked by antibody specific to a common peptide sequence of Gialpha1 and Gialpha2 but was not blocked by anti-Gialpha1 antibody. The inhibitory effect on Cch-stimulated secretion was also blocked by anti-Gialpha3 and anti-Goalpha. Similar experiments resulted in a reversal of DALA inhibition of beta-adrenergic stimulation of secretion by immunoneutralization of Gialpha1/2 and Goalpha but not by immunoneutralization of Gialpha1 or Gialpha3. VIP, Isop, and FSK significantly stimulated AC. However, Cch had no effect on the activity of the enzyme. In addition, DALA had no effect on AC activity under any conditions.

CONCLUSIONS

These results show that enkephalin inhibition of cholinergic and beta-adrenergic stimulation of secretion is mediated by Gi2, Gi3, and Go. The effector coupled by the G proteins is not AC. However, we suggest a role for met-enkephalin in G-protein-coupled modulation of ion channels important for cholinergic and beta-adrenergic stimulation of lacrimal secretion.

Opioid enhancement of calcium oscillations and burst events involving NMDA receptors and L-type calcium channels in cultured hippocampal neurons

Opioid receptor agonists are known to alter the activity of membrane ionic conductances and receptor-activated channels in CNS neurons and, via these mechanisms, to modulate neuronal excitability and synaptic transmission. In neuronal-like cell lines opioids also have been reported to induce intracellular Ca(2+) signals and to alter Ca(2+) signals evoked by membrane depolarization; these effects on intracellular Ca(2+) may provide an additional mechanism through which opioids modulate neuronal activity. However, opioid effects on resting or stimulated intracellular Ca(2+) levels have not been demonstrated in native CNS neurons. Thus, we investigated opioid effects on intracellular Ca(2+) in cultured rat hippocampal neurons by using fura-2-based microscopic Ca(2+) imaging. The opioid receptor agonist D-Ala(2)-N-Me-Phe(4),Gly-ol(5)-enkephalin (DAMGO; 1 microM) dramatically increased the amplitude of spontaneous intracellular Ca(2+) oscillations in the hippocampal neurons, with synchronization of the Ca(2+) oscillations across neurons in a given field. The effects of DAMGO were blocked by the opioid receptor antagonist naloxone (1 microM) and were dependent on functional NMDA receptors and L-type Ca(2+) channels. In parallel whole-cell recordings, DAMGO enhanced spontaneous, synaptically driven NMDA receptor-mediated burst events, depolarizing responses to exogenous NMDA and current-evoked Ca(2+) spikes. These results show that the activation of opioid receptors can augment several components of neuronal Ca(2+) signaling pathways significantly and, as a consequence, enhance intracellular Ca(2+) signals. These results provide evidence of a novel neuronal mechanism of opioid action on CNS neuronal networks that may contribute to both short- and long-term effects of opioids.

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.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

Effects of kappa- and mu-opioid receptor agonists on Ca2+ channels in neuroblastoma cells: involvement of the orphan opioid receptor

The effects of micro-, delta- and kappa-opioid receptor agonists, and orphanin FQ/nociceptin (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), on K+-induced [Ca2+]i increase were examined in SK-N-SH cells. Exposure to K+ (50 mM) resulted in a [Ca2+]i rise, which was blocked (-85%) by furaldipine (1 microM) and increased (63%) by BayK 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethyl-pyridine-5 -carboxylate) (0.5 microM), indicating the involvement of L-type Ca2+ channels. The kappa-opioid receptor agonists 3,4-dichloro-N-Methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U-50488H) (1-50 microM) and 5,7,8-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]benze neacetamide (U-69593) (25 microM), and the mu-opioid receptor agonist sufentanil (100 nM-3 microM) inhibited the amplitude of K+-induced [Ca2+]i increase. The agonist of the orphan opioid receptor, orphanin FQ/nociceptin (1 microM), induced dual excitatory and inhibitory effects on the depolarisation-induced Ca2+ influx. The effects of the opioid receptor agonists were not blocked by the kappa-opioid receptor antagonist nor-binaltorphimine (1 microM), only weakly prevented by naloxone (10-100 microM) and naltrexone (100 microM), and partially prevented by pertussis toxin (100 ng/ml, 24 h). The antagonist of the orphan opioid receptor, [Phe1psi(CH2-NH)Gly2]nociceptin(1-13)NH2 (1 microM), prevented the inhibitory effect of U-50488H, sufentanil and orphanin FQ. The present study provides pharmacological evidence for the presence of L-type Ca2+ channels in SK-N-SH cells, that are modulated by opioids through orphan opioid receptor activation.

Elevation of intracellular glucosylceramide levels results in an increase in endoplasmic reticulum density and in functional calcium stores in cultured neurons

Gaucher disease is a glycosphingolipid storage disease caused by defects in the activity of the lysosomal hydrolase, glucocerebrosidase (GlcCerase), resulting in accumulation of glucocerebroside (glucosylceramide, GlcCer) in lysosomes. The acute neuronopathic type of the disease is characterized by severe loss of neurons in the central nervous system, suggesting that a neurotoxic agent might be responsible for cellular disruption and neuronal death. We now demonstrate that upon incubation with a chemical inhibitor of GlcCerase, conduritol-B-epoxide (CBE), cultured hippocampal neurons accumulate GlcCer. Surprisingly, increased levels of tubular endoplasmic reticulum elements, an increase in [Ca(2+)](i) response to glutamate, and a large increase in [Ca(2+)](i) release from the endoplasmic reticulum in response to caffeine were detected in these cells. There was a direct relationship between these effects and GlcCer accumulation since co-incubation with CBE and an inhibitor of glycosphingolipid synthesis, fumonisin B(1), completely antagonized the effects of CBE. Similar effects on endoplasmic reticulum morphology and [Ca(2+)](i) stores were observed upon incubation with a short-acyl chain, nonhydrolyzable analogue of GlcCer, C(8)-glucosylthioceramide. Finally, neurons with elevated GlcCer levels were much more sensitive to the neurotoxic effects of high concentrations of glutamate than control cells; moreover, this enhanced toxicity was blocked by pre-incubation with ryanodine, suggesting that [Ca(2+)](i) release from ryanodine-sensitive intracellular stores can induce neuronal cell death, at least in neurons with elevated GlcCer levels. These results may provide a molecular mechanism to explain neuronal dysfunction and cell death in neuronopathic forms of Gaucher disease.

Morphine coupling to invertebrate immunocyte nitric oxide release is dependent on intracellular calcium transients

Morphine significantly stimulated invertebrate immunocyte intracellular calcium level increases in a concentration-dependent manner in cells preloaded with Fura 2/AM. Morphine's action was blocked by prior exposure of the cells to the opiate receptor antagonist naloxone. Various opioid peptides did not exhibit this ability, indicating a morphine-mu 3 mediated process. In comparing the sequence of events concerning morphine's action in stimulating both [Ca2+]i and NO production in these cells, we found that the first event precedes the second by 42 +/- 7 s. The opiate stimulation of [Ca2+]i- was attenuated in cells leached of calcium. strongly suggesting that intracellular calcium levels regulate cNOS activity in invertebrate immunocytes.

Morphine and anandamide stimulate intracellular calcium transients in human arterial endothelial cells: coupling to nitric oxide release

Both morphine and anandamide significantly stimulated cultured endothelial intracellular calcium level increases in a concentration-dependent manner in cells pre-loaded with fura 2/AM. Morphine is more potent than anandamide (approximately 275 vs. 135 nM [Ca]i), and the [Ca]i for both ligands was blocked by prior exposure of the cells to their respective receptor antagonist, i.e., naloxone and SR 171416A. Various opioid peptides did not exhibit this ability, indicating a morphine-mu3-mediated process. In comparing the sequence of events concerning morphine's and anandamide's action in stimulating both [Ca]i and nitric oxide production in endothelial cells, we found that the first event precedes the second by 40+/-8 sec. The opiate and cannabinoid stimulation of [Ca]i was attenuated in cells leeched of calcium, strongly suggesting that intracellular calcium levels regulate cNOS activity.

Desensitization of delta-opioid-induced mobilization of Ca2+ stores in NG108-15 cells

Activation of delta-opioid receptors in NG108-15 cells induces the release of calcium from an inositol 1,4,5-trisphosphate- sensitive intracellular store. We used fura-2-based digital imaging to study the effects of prolonged exposure to agonist on opioid-induced increases in [Ca2+]i. Exposure to D-Ala2-E-Leu5 enkephalin (DADLE) (1 microM) for 30 min completely desensitized NG108-15 cells to a second DADLE-induced response. The cells recovered gradually over 25 min following washout of DADLE. The desensitization was not due to depletion of intracellular calcium stores and bradykinin failed to cross-desensitize the DADLE-evoked response, although both agonists mobilized the same Ca2+ store. Desensitization induced by 100 nM DADLE was overcome by a higher concentration of DADLE (100 microM). Treatment with 8-cpt-cAMP (0.1 mM) for 30 min did not influence the DADLE-induced increases in [CA2+]i. Phorbol dibutyrate (PdBu) (1 microM) blocked the response completely. Treatment with the inhibitor of cyclic nucleotide-dependent kinases H8 (1 microM) for 45 min did not prevent DADLE-induced desensitization. Treatment with the protein kinase C (PKC) inhibitors staurosporin (10 nM) and GF-109203X (200 nM) for 45 min reduced desensitization. However, down-regulation of PKC by 24 h exposure to PdBu (1 microM) failed to prevent the DADLE-induced desensitization in NG108-15 cells. Thus, we conclude that multiple pathways participated in desensitization of delta-receptor-mediated Ca2+ mobilization, one of which includes PKC.

Reexamination of opioid stimulation of cGMP formation in cell lines of neuronal origin

1. The present study reexamines a previous notion on opioid stimulation of cyclic GMP (cGMP) formation and the retraction of the original findings. 2. The effect of opioid agonists on cGMP accumulation in two cell lines of neuronal origin was measured. The proportion of cGMP stimulation in NG108-15 neuroblastoma x glioma hybrid cells resembled the proportion of [Ca2+]in elevation by opioids in this culture. The failure of opioids to stimulate cGMP formation in SK-N-SH human neuroblastoma coincided with the lack of cGMP stimulation by other Ca2+ mobilizing agents in these cells. The nitric oxide donor nitroprusside elevated cGMP in both cell lines. 3. The implication of the opioid-Ca(2+)-NO-cGMP cellular pathway for opioid activity in vivo is discussed.

The role of GM1 ganglioside in regulating excitatory opioid effects

Our studies with cultured cells have provided new insight into the particular role of GM1 in regulating excitatory opioid responses. GM1 is significantly elevated in chronic opioid-treated cells via Gs/adenylyl cyclase activation. Such GM1 elevation promotes coupling of opioid receptor with Gs, resulting in attenuation of inhibitory opioid effects and induction of a sustained excitatory response. Application of exogenous GM1, but not other gangliosides, induces excitatory opioid responses not only in neurons and neuroblastoma cells that bear intrinsic opioid receptors but also in nonneuronal cells that are transfected with delta-opioid receptor. The latter system provides evidence that allosteric binding of GM1 changes receptor conformation from a Gi-coupled to a Gs-coupled mode. This is supported by preliminary experiments with a mutated delta-opioid receptor.

Dissociation between the inhibitory and stimulatory effects of opioid peptides on cAMP formation in SK-N-SH neuroblastoma cells

Opioid agonists either potentiate or suppress basal cAMP production in SK-N-SH cells. The inhibitory effect is mediated by PTX-sensitive GTP-binding proteins, while the stimulatory effect involves Ca++ entry and calmodulin activation. Both pathways can be activated simultaneously by opioid agonists. Low (nM) concentrations of either mu (DAMGO) or delta (DPDPE) selective opioids potentiate cAMP formation. At higher (100 nM) concentrations, however, a net suppression takes over; this suppression can be eliminated by PTX, and the underlying stimulatory effect is disclosed. Micromolar concentrations of either mu or delta selective agonists cross-activate the other (delta or mu) receptors, and augment the stimulatory pathway. The overall outcome (either stimulation or inhibition of cAMP production) is dependent on the balance between the two overlapping pathways, and can be modified by blocking either of the two opposing mechanisms.

L-Type calcium channels are required for one form of hippocampal mossy fiber LTP

The requirement of postsynaptic calcium influx via L-type channels for the induction of long-term potentiation (LTP) of mossy fiber input to CA3 pyramidal neurons was tested for two different patterns of stimulation. Two types of LTP-inducing stimuli were used based on the suggestion that one of them, brief high-frequency stimulation (B-HFS), induces LTP postsynaptically, whereas the other pattern, long high-frequency stimulation (L-HFS), induces mossy fiber LTP presynaptically. To test whether or not calcium influx into CA3 pyramidal neurons is necessary for LTP induced by either pattern of stimulation, nimodipine, a L-type calcium channel antagonist, was added during stimulation. In these experiments nimodipine blocked the induction of mossy fiber LTP when B-HFS was given [34 +/- 5% (mean +/- SE) increase in control versus 7 +/- 4% in nimodipine, P < 0.003]; in contrast, nimodipine did not block the induction of LTP with L-HFS (107 +/- 10% in control vs. 80 +/- 9% in nimodipine, P > 0.05). Administration of nimodipine after the induction of LTP had no effect on the expression of LTP. In addition, B- and L-HFS delivered directly to commissural/associational fibers in stratum radiatum failed to induce a N-methyl--aspartate-independent form of LTP, obviating the possibility that the presumed mossy fiber LTP resulted from potentiation of other synapses. Nimodipine had no effect on calcium transients recorded from mossy fiber presynaptic terminals evoked with the B-HFS paradigm but reduced postsynaptic calcium transients. Our results support the hypothesis that induction of mossy fiber LTP by B-HFS is mediated postsynaptically and requires entry of calcium through L-type channels into CA3 neurons.

Mobilization of Ca2+ from intracellular stores in transfected neuro2a cells by activation of multiple opioid receptor subtypes

In neuronal cell lines, activation of opioid receptors has been shown to mobilize intracellular Ca2+ stores. In this report, we describe the excitatory actions of opioid agonists on murine neuroblastoma neuro2a cells stably expressing either delta, mu, or kappa opioid receptors. Fura-2-based digital imaging was used to record opioid-induced increases in intracellular Ca2+ concentration ([Ca2+]i). Repeated challenges of delta, mu, or kappa opioid receptor expressing cells with 100 nM [D-Ala2,D-Leu5]-enkephalin (DADLE), [D-Ala2,N-Me-Phe4,Gly-ol]-enkephalin (DAMGO), or trans-(+/-)-3,4-dichloro N-methyl-N-(2-[1-pyrollidinyl] cyclohexyl) benzene acetamide (U-50488H), respectively, elicited reproducible Ca2+ responses. Non-transfected neuro2a cells did not respond to opioid agonists. Removal of extracellular Ca2+ from the bath prior to and during agonist challenge did not affect significantly the agonist-evoked increase in [Ca2+]i, indicating that the response resulted from the release of Ca2+ from intracellular stores. Naloxone reversibly inhibited responses in all three cell lines, confirming that they were mediated by opioid receptors. Expression of cloned opioid receptors in neuro2a cells, coupled with digital [Ca2+]i imaging, provides a model system for the study of opioid receptors and opioid-activated signaling processes. The fact that all three receptors coupled to the same intracellular signaling mechanism suggests that the primary functional difference between opioid responses in vivo results from their selective localization.

Opioid receptor and calcium channel regulation of adenylyl cyclase, modulated by GM1, in NG108-15 cells: competitive interactions

GM1 ganglioside was previously shown to function as a specific regulator of excitatory opioid activity in dorsal root ganglion neurons and F11 hybrid cells, as seen in its facilitation of opioid-induced activation of adenylyl cyclase and its ability to dramatically reduce the threshold opioid concentration required to prolong the action potential duration. The elevated levels of GM1 resulting from chronic opioid exposure of F11 cells were postulated to cause the ensuing opioid excitatory supersensitivity. We now show that GM1 promotes opioid (DADLE)-induced activation of adenylyl cyclase in NG108-15 cells which possess the delta-type of receptor. In keeping with previous studies of other systems, this can be envisioned as conformational interaction of GM1 with the receptor that results in uncoupling of the receptor from Gi and facilitated coupling to Gs. This would also account for the observation that DADLE-induced attenuation of forskolin-stimulated adenylyl cyclase was reversed by GM1, provided the cells were not pretreated with pertussis toxin. When the cells were so pretreated, GM1 evoked an unexpected attenuation of forskolin-stimulated adenylyl cyclase attributed to GM1-promoted influx of calcium which was postulated to inhibit a calcium-sensitive form of adenylyl cyclase. This is concordant with several studies showing GM1 to be a potent modulator of calcium flux. Pertussis toxin in these experiments exerted dual effects, one being to promote interaction of the delta-opioid receptor with Gs through inactivation of Gi, and the other to enhance the GM1-promoted influx of calcium by inactivation of Go; the latter is postulated to function as constitutive inhibitor of the relevant calcium channel. NG108-15 cells thus provide an interesting example of competitive interaction between two GM1-regulated systems involving enhancement of both opioid receptor excitatory activity and calcium influx.

Perinatal exposure to morphine disrupts brain norepinephrine, ovarian cyclicity, and sexual receptivity in rats

The effect of perinatal exposure to morphine on the development of catecholaminergic and reproductive function in female rats was investigated. Adult rats received morphine intraperitoneally daily for 40 days. The dose of morphine was progressively increased at 10-day intervals from 5, 7.5, 10 to 15 mg/kg body weight until day 40. The rats were mated between days 38 and 45. Administration of morphine at dose rates of 20 and 30 mg/kg continued during pregnancy. The dose was increased to 40 mg/kg for 10 days postpartum. Results showed that morphine disrupted ovarian cyclicity in 52% of the females. Amongst the remaining females, 43% became pregnant when mated. In the female offspring born to such dams, sexual maturation was delayed and body weight was reduced until weaning. At adulthood, lordosis behavior was inhibited when the female offspring were tested against stimulus males. Plasma estradiol and ovarian estradiol and progesterone levels were reduced. Norepinephrine concentration in the hypothalamus was reduced, whereas it remained unchanged in the amygdala. Dopamine concentrations in both hypothalamus and amygdala were not influenced by perinatal morphine exposure. These results suggest that chronic morphine treatment during perinatal life selectively influences the development of noradrenergic mechanisms in the rat brain and this may in turn be responsible for reduced reproductive activity.

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.

Barium elicits reversal of low-concentration etorphine-induced decrease of potassium conductance in cultures of dissociated dorsal root ganglion neurons

Etorphine is an non-selective opioid receptor agonist with very potent analgesic effect. Low concentrations (< nM) of most opioid receptor agonists decrease the K+ conductance (gK) in cultures of dissociated mouse dorsal root ganglion neurons regardless of the presence of Ba2+ However, low concentrations of etorphine, in contrast to all other opioids tested, decreased gK only in the absence of Ba2+. In the presence of Ba2+, pM-nM etorphine elicited dose-dependent increases, instead of decreases in gK. Higher concentrations of etorphine (> nM) not only increased gK but, in addition, appreciably increased a delayed-onset inward Ca2+ current during pulsed depolarization regardless of the presence of Ba2+.

Interaction of the delta-opioid receptor with GM1 ganglioside: conversion from inhibitory to excitatory mode

Previous studies have shown GM1 ganglioside to play a crucial role in regulating excitatory opioid receptor function, which may underlie some aspects of opioid dependence, tolerance, and supersensitivity. To study the mechanism of this receptor modulation we have employed CHO cells containing a single, transfected opioid receptor of the delta-type. When forskolin was employed to elevate cAMP the reduction affected by 10 microM DADLE was counteracted by preincubation of the cells with GM1. No effect was observed with GD1a, GD1b, GT1b GM3, or the GM1 derivative, GM1-OH. In pertussis toxin-treated cells 10 nM DADLE increased basal levels of cAMP after preincubation with as little as 10 nM GM1. The results suggest conformational alteration of the opioid receptor from a form coupled primarily to G(i)/G(o) to one also capable of interacting with G(s).

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.

Etorphine elicits anomalous excitatory opioid effects on sensory neurons treated with GM1 ganglioside or pertussis toxin in contrast to its potent inhibitory effects on naive or chronic morphine-treated cells

The ultra-potent opioid analgesic, etorphine, elicits naloxone-reversible, dose-dependent inhibitory effects, i.e., shortening of the action potential duration (APD) of naive and chronic morphine-treated sensory dorsal root ganglion (DRG) neurons, even at low (pM-nM) concentrations. In contrast, morphine and most other opioid agonists elicit excitatory effects, i.e., APD prolongation, at these low opioid concentrations, require much higher (ca. 0.1-1 microM) concentrations to shorten the APD of naive neurons, and evoke only excitatory effects on chronic morphine-treated cells even at high > 1-10 microM concentrations. In addition to the potent agonist action of etorphine at mu-, delta- and kappa-inhibitory opioid receptors in vivo and on DRG neurons in culture, this opioid has also been shown to be a potent antagonist of excitatory mu-, delta- and kappa-receptor functions in naive and chronic morphine-treated DRG neurons. The present study demonstrates that the potent inhibitory APD-shortening effects of etorphine still occur in DRG neurons tested in the presence of a mixture of selective antagonists that blocks all mu-, delta- and kappa-opioid receptor-mediated functions, whereas addition of the epsilon (epsilon)-opioid-receptor antagonist, beta-endorphin(1-27) prevents these effects of etorphine. Furthermore, after markedly enhancing excitatory opioid receptor functions in DRG neurons by treatment with GM1 ganglioside or pertussis toxin, etorphine shows excitatory agonist action on non-mu-/delta-/kappa-opioid receptor functions in these sensory neurons, in contrast to its usual potent antagonist action on mu-, delta- and kappa-excitatory receptor functions in naive and even in chronic morphine-treated cells which become supersensitive to the excitatory effects of mu-, delta- and kappa-opioid agonists. This weak excitatory agonist action of etorphine on non-mu-/delta-/kappa-opioid receptor functions may account for the tolerance and dependence observed after chronic treatment with extremely high doses of etorphine in vivo.

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.

kappa-opioid receptor expression defines a phenotypically distinct subpopulation of astroglia: relationship to Ca2+ mobilization, development, and the antiproliferative effect of opioids

To assess the role of kappa-opioid receptors in astrocyte development, the effect of kappa-agonists on the growth of astroglia derived from 1-2-day-old mouse cerebra was examined in vitro. kappa-Opioid receptor expression was assessed immunocytochemically (using KA8 and KOR1 antibodies), as well as functionally by examining the effect of kappa-receptor activation on intracellular calcium ([Ca2+]i) homeostasis and DNA synthesis. On days 6-7, as many as 50% of the astrocytes displayed kappa-receptor (KA8) immunoreactivity or exhibited increases in [Ca2+]i in response to kappa-agonist treatment (U69,593 or U50,488H). Exposure to U69,593 (100 nM) for 72 h caused a significant reduction in number and proportion of glial fibrillary acidic protein-immunoreactive astrocytes incorporating bromodeoxyuridine (BrdU) that could be prevented by co-administering the kappa-antagonist, nor-binaltorphimine (300 nM). In contrast, on day 14, only 5 or 14%, respectively, of the astrocytes were kappa-opioid receptor (KA8) immunoreactive or displayed functional increases in [Ca2+]i. Furthermore, U69,593 (100 nM) treatment failed to inhibit BrdU incorporation at 9 days in vitro. Experimental manipulations showed that kappa-receptor activation increases astroglial [Ca2+]i both through influx via L-type channels and through mobilization of intracellular stores (which is an important Ca2+ signaling pathway in cell division). Collectively, these results indicate that a subpopulation of developing astrocytes express kappa-opioid receptors in vitro, and suggest that the activation of kappa-receptors mobilizes [Ca2+]i and inhibits cell proliferation. Moreover, the proportion of astrocytes expressing kappa-receptors was greatest during a period of rapid cell growth suggesting that they are preferentially expressed by proliferating astrocytes.

Determinants of the stimulatory opioid effect on intracellular calcium in SK-N-SH and NG108-15 neuroblastoma

The opiate agonist etorphine elevated [Ca2+]i in two neuroblastoma cell lines. Fura-2 imaging of single cells revealed a small and variable calcium elevation in only 20% of cultures. Three factors were found to increase the probability (up to 70%) and the amplitude of the response to etorphine: (a) synchronization of the cultures; (b) differentiation of the cells; and (c) synergism with other stimulatory agents (carbachol in SK-N-SH and bradykinin in NG108-15 cells). The establishment of a reproducible experimental protocol may facilitate the study of the molecular mechanism(s) underlying the stimulatory activity of opiates.