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

Involvement of ATP-sensitive potassium channels and the opioid system in the anticonvulsive effect of zolpidem in mice

Zolpidem is a hypnotic medication that mainly exerts its function through activating γ-aminobutyric acid (GABA)A receptors. There is some evidence that zolpidem may have anticonvulsive effects. However, the mechanisms underlying this effect have not been elucidated yet. In the present study, we used the pentylentetrazole (PTZ)-induced generalized seizure model in mice to investigate whether zolpidem can affect seizure threshold. We also further evaluated the roles of ATP-sensitive potassium (KATP) channels as well as μ-opioid receptors in the effects of zolpidem on seizure threshold. Our data showed that zolpidem in a dose-dependent manner increased the PTZ-induced seizure threshold. The noneffective (i.e., did not significantly alter the PTZ-induced seizure threshold by itself) doses of KATP channel blocker (glibenclamide) and nonselective opioid receptor antagonist (naloxone) were able to inhibit the anticonvulsive effect of zolpidem. Additionally, noneffective doses of either KATP channel opener (cromakalim) or nonselective μ-opioid receptor agonist (morphine) in combination with a noneffective dose of zolpidem exerted a significant anticonvulsive effect on PTZ-induced seizures in mice. A combination of noneffective doses of naloxone and glibenclamide, which separately did not affect zolpidem effect on seizure threshold, inhibited the anticonvulsive effects of zolpidem. These results suggest a role for KATP channels and the opioid system, alone or in combination, in the anticonvulsive effects of zolpidem.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

The antinociceptive activity of intrathecally administered amiloride and its interactions with morphine and clonidine in rats

In this study, we aimed to evaluate the antinociceptive interaction between intrathecally administered amiloride and morphine or clonidine. Using rats chronically implanted with lumbar intrathecal catheters, we examined the ability of intrathecal amiloride, morphine, clonidine, and mixtures of amiloride-morphine and amiloride-clonidine to alter tail-flick latency. To characterize any interactions, isobolographic analysis was performed. The effects of pretreatment with intrathecally administered naloxone or yohimbine were tested. Intrathecal administration of amiloride (25-150 μg), morphine (.25-10 μg), or clonidine (.5-10 μg) alone produced significant dose-dependent antinociception in the tail-flick test. The median effective dose (ED(50)) values for intrathecally administered amiloride, morphine, and clonidine were 120.5 μg, 5.0 μg, and 4.4 μg, respectively. Isobolographic analysis exhibited a synergistic interaction after coadministration of amiloride-morphine and amiloride-clonidine. Intrathecal pretreatment with naloxone (10 μg) completely blocked the antinociceptive effects of morphine and the amiloride-morphine mixture. Intrathecal pretreatment with yohimbine (20 μg) completely blocked the antinociceptive effect of clonidine and antagonized the effect of the amiloride-clonidine mixture. There was no motor dysfunction or significant change in blood pressure or heart rate after the intrathecal administration of amiloride, amiloride-morphine, and amiloride-clonidine. The synergistic effect observed after the coadministration of amiloride and morphine or clonidine suggests a functional interaction among calcium channels, μ-receptors and α(2)-receptors at the spinal cord level of the nociceptive processing system.

PERSPECTIVE

Although intrathecal morphine and clonidine produces pronounced analgesia, antinociceptive doses of intrathecal morphine and clonidine produce several side effects, including hypotension, bradycardia, sedation, and tolerance. This article presents antinociceptive synergistic interaction between amiloride and morphine, amiloride, and clonidine on thermal nociceptive tests in the rat.

Sympatholytic delta-2 opioid receptors moderate ganglionic vasomotor control

This study tested the hypothesis that enkephalin increases femoral vascular conductance via the delta-2 phenotype of the opioid receptor (DOR-2) within peripheral sympathetic ganglia. Graded pulses of methionine–enkephalin (ME) were administered (0.03–10 μg/kg) into the terminal aorta of anesthetized dogs proximal to lumbar arteries that perfuse vasomotor ganglia regulating femoral blood flow. Femoral vascular conductance increased sharply (ED50 = 2.6 × 10−9 mol/kg) accompanied by declines in arterial pressure and femoral vascular resistance. A dose-related increase in arterial pressure preceded each subsequent fall in pressure. The DOR-2 antagonist, naltriben (NTB), abrogated the hyperemic effect of ME (ID50 = 1.4 × 10−9 mol/kg). DOR-1 blockade (BNTX) was five-fold less effective. The hyperemic effect of ME was also enhanced when sympathetic activity was reflexly increased by bilateral carotid occlusion. The DOR-2 agonist, deltorphin II, produced exaggerated increases in conductance compared with ME that were also reduced by DOR-2 blockade. DOR-1 blockade eliminated the initial pressor responses, exaggerated the subsequent depressor response, increased baseline femoral conductance 10-fold and shifted the ME-mediated hyperemic threshold one dose lower from 0.3 to 0.1 μg/kg, providing indirect support for a competing DOR-1-mediated constriction. Extended exposure to DOR-1 blockade lowered the maximal ME increase in conductance by 30%, suggesting that BNTX reduces the available pool of DOR receptors. In summary, enkephalin mediates a robust hyperemic effect through sympatholytic ganglionic DOR-2 receptors and DOR-1 antagonist studies provide indirect evidence for constituent opposition from a proposed DOR-1-mediated sympathotonic constrictor pathway.

Fentanyl-induced bradykinesia and rigidity after deep brain stimulation in a patient with Parkinson disease

A 58-year-old man with advanced Parkinson disease underwent battery replacement for a deep brain stimulator and experienced severe bradykinesia and rigidity postoperatively for 36 hours. The patient was administered fentanyl as an anesthetic during the procedure and as an analgesic periodically during the day after surgery. The severe bradykinesia and rigidity persisted despite reactivation of the deep brain stimulator and immediate reinstitution of Parkinson disease medications, but resolved completely several hours after discontinuation of fentanyl.

Repeated arterial occlusion, delta-opioid receptor (DOR) plasticity and vagal transmission within the sinoatrial node of the anesthetized dog

Brief interruptions in coronary blood flow precondition the heart, engage delta-opioid receptor (DOR) mechanisms and reduce the damage that typically accompanies subsequent longer coronary occlusions. Repeated short occlusions of the sinoatrial (SA) node artery progressively raised nodal methionine-enkephalin-arginine-phenylalanine (MEAP) and improved vagal transmission during subsequent long occlusions in anesthetized dogs. The DOR type-1 (DOR-1) antagonist, BNTX reversed the vagotonic effect. Higher doses of enkephalin interrupted vagal transmission through a DOR-2 mechanism. The current study tested whether the preconditioning (PC) protocol, the later occlusion or a combination of both was required for the vagotonic effect. The study also tested whether evolving vagotonic effects included withdrawal of competing DOR-2 vagolytic influences. Vagal transmission progressively improved during successive SA nodal artery occlusions. The vagotonic effect was absent in sham animals and after DOR-1 blockade. After completing the PC protocol, exogenously applied vagolytic doses of MEAP reduced vagal transmission under both normal and occluded conditions. The magnitude of these DOR-2 vagolytic effects was small compared to controls and repeated MEAP challenges rapidly eroded vagolytic responses further. Prior DOR-1 blockade did not alter the PC mediated, progressive loss of DOR-2 vagolytic responses. In conclusion, DOR-1 vagotonic responses evolved from signals earlier in the PC protocol and erosion of competing DOR-2 vagolytic responses may have contributed to an unmasking of vagotonic responses. The data support the hypothesis that PC and DOR-2 stimulation promote DOR trafficking, and down regulation of the vagolytic DOR-2 phenotype in favor of the vagotonic DOR-1 phenotype. DOR-1 blockade may accelerate the process by sequestering newly emerging receptors.

Role of glutamate in the rostral ventrolateral medulla in acupuncture-related modulation of visceral reflex sympathoexcitation

Visceral sympathoexcitatory reflexes induced by stimulation of the gallbladder with bradykinin (BK) are attenuated by electroacupuncture (EA) at Neiguan-Jianshi (P5-6) acupoints located over the median nerve. Previous studies have shown that neurons in the rostral ventrolateral medulla (rVLM) receive convergent input from visceral organs and somatic nerves (activated by EA). Glutamate (Glu), an important excitatory neurotransmitter in the rVLM, processes visceral sympathoexcitatory cardiovascular reflexes. In the present study, we determined the relation between EA-mediated opioidergic modulation of visceral cardiovascular responses and Glu. Reflex cardiovascular responses were evoked by application of BK to the gallbladder before and after EA in anesthetized cats. Glu concentrations ([Glu]) were measured by HPLC from samples collected by microdialysis probe(s) inserted unilaterally or bilaterally into the rVLM. BK-induced reflex responses and [Glu] were attenuated by 45% and 70%, respectively, after 30 min of EA ( n = 6). EA alone did not change [Glu] in the rVLM ( n = 6, P > 0.05). However, microdialysis of naloxone (100 mM) into the rVLM reversed EA-related inhibition of blood pressure and [Glu] ( n = 5). Immunohistochemical visualization showed that δ-opioid receptors colocalized with, and were in close apposition to, vesicular Glu transporter 3- and c-Fos-double-labeled perikarya and processes of rVLM neurons after gallbladder stimulation with BK. These data suggest that EA attenuates BK-induced visceral sympathoexcitatory reflexes through opioid-mediated inhibition of Glu's action in the rVLM.

The monosialosyl ganglioside GM-1 reduces the vagolytic efficacy of delta2-opioid receptor stimulation

The cardiac enkephalin, methionine-enkephalin-arginine-phenylalanine (MEAP), alters vagally induced bradycardia when introduced by microdialysis into the sinoatrial (SA) node. The responses to MEAP are bimodal; lower doses enhance bradycardia and higher doses suppress bradycardia. The opposing vagotonic and vagolytic effects are mediated, respectively, by delta(1) and delta(2) phenotypes of the same receptor. Stimulation of the delta(1) receptor reduced the subsequent delta(2) responses. Experiments were conducted to test the hypothesis that the delta-receptor interactions were mediated by the monosialosyl ganglioside GM-1. When the mixed agonist MEAP was evaluated after nodal GM-1 treatment, delta(1)-mediated vagotonic responses were enhanced, and delta(2)-mediated vagolytic responses were reduced. Prior treatment with the delta(1)-selective antagonist 7-benzylidenaltrexone (BNTX) failed to prevent attrition of the delta(2)-vagolytic response or restore it when added afterward. Thus the GM-1-mediated attrition was not mediated by delta(1) receptors or increased competition from delta(1)-mediated vagotonic responses. When GM-1 was omitted, deltorphin produced a similar but less robust loss in the vagolytic response. In contrast, however, to GM-1, the deltorphin-mediated attrition was prevented by pretreatment with BNTX, indicating that the decline in response after deltorphin alone was mediated by delta(1) receptors and that GM-1 effectively bypassed the receptor. Whether deltorphin has intrinsic delta(1) activity or causes the release of an endogenous delta(1)-agonist is unclear. When both GM-1 and deltorphin were omitted, the subsequent vagolytic response was more intense. Thus GM-1, deltorphin, and time all interact to modify subsequent delta(2)-mediated vagolytic responses. The data support the hypothesis that delta(1)-receptor stimulation may reduce delta(2)-vagolytic responses by stimulating the GM-1 synthesis.

Electroacupuncture induces c-Fos expression in the rostral ventrolateral medulla and periaqueductal gray in cats: relation to opioid containing neurons

Our previous studies have shown that electroacupuncture (EA) at the Neiguan-Jianshi (P5-P6) acupoints inhibits sympathetic outflow and attenuates excitatory visceral cardiovascular reflexes through enkephalin- or beta-endorphin-related opioid receptors in the rostral ventrolateral medulla (rVLM). It is not known whether EA at these acupoints activates neurons containing enkephalin or beta-endorphin in the rVLM as well as in the periaqueductal gray (PAG) that are involved in EA-mediated central neural regulation of sympathetic activity. The present study evaluated activated neurons in the rVLM and PAG by detecting c-Fos immunoreactivity, and identified the relationship between c-Fos nuclei and neuronal structures containing enkephalin or beta-endorphin in these regions. To enhance the detection of cell bodies containing enkephalin or beta-endorphin, colchicine (90-100 microg/kg) was injected into the subarachnoid space in anesthetized cats 28-30 h prior to EA or the sham-operated control for EA. Following bilateral barodenervation and cervical vagotomy, EA (1-4 mA, 2 Hz, 0.5 ms) was performed at the P5-P6 acupoints (overlying median nerve; n=7) for 30 min. Identical procedures, with the exception of electrical stimulation, were carried out in five control animals. EA decreased blood pressure (BP) in four of seven cats (5-15 mm Hg) while the sham procedure for EA produced no responses. Perikarya containing enkephalin were found in the rVLM and rarely in the PAG, while no cell bodies labeled with beta-endorphin were identified in either region. Compared to animals in the control group, more c-Fos immunoreactivity, located principally in close proximity to fibers containing enkephalin or beta-endorphin, was observed in the rVLM and ventrolateral PAG (vlPAG) in EA-treated cats. Moreover, neurons double-labeled with c-Fos and enkephalin in the rVLM were significantly increased in cats following EA stimulation (P<0.05). These data indicate that EA at the P5-P6 acupoints activates neurons in the rVLM and vlPAG. These activated neurons contain enkephalin in the rVLM, and most likely interact with nerve fibers containing enkephalin or beta-endorphin in both the rVLM and vlPAG. The results from this study provide the first anatomical evidence showing that EA at the P5-P6 acupoints has the potential to influence neuronal structures (perikarya, axons and/or dendrites) containing enkephalin or beta-endorphin in specific regions of the brain stem. These neurons likely form the substrate for EA's influence on sympathoexcitatory cardiovascular reflexes.

Depressor and bradycardic responses to microinjections of endomorphin-2 into the NTS are mediated via ionotropic glutamate receptors

The presence of endomorphin-like immunoreactivity has been reported in the nucleus tractus solitarius (NTS). It was hypothesized that endomorphins may play a role in cardiovascular regulation in the medial subnucleus of the NTS (mNTS). Endomorphin-2 (E-2, 0.1–4 mmol/l) was microinjected (100 nl) into the mNTS of urethane-anesthetized, artificially ventilated, adult male Wistar rats. E-2 (0.2 mmol/l) elicited decreases in mean arterial pressure (40 ± 3.5 mmHg) and heart rate (50 ± 7.0 beats/min). These responses were blocked by prior microinjections of naloxonazine (1 mmol/l) into the mNTS. Responses to microinjections of E-2 into the mNTS were abolished by prior combined microinjections of d-2-amino-7-phosphonoheptanoic acid (an NMDA receptor antagonist, 5 mmol/l) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[ f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist, 2 mmol/l) into the mNTS. These results were confirmed by extracellular neuronal recordings. Blockade of GABA receptors in the mNTS by prior combined microinjections of gabazine (a GABAA receptor antagonist, 2 mmol/l) and 2-hydroxysaclofen (a GABAB receptor antagonist, 100 mmol/l) also blocked the responses to E-2. It was concluded that 1) the depressor and bradycardic responses to microinjections of E-2 into the mNTS are mediated via μ1-opioid receptors as well as ionotropic glutamate receptors, 2) GABAergic neurons in the mNTS, which may inhibit the release of glutamate from nerve terminals, are inhibited by E-2 via μ1-opioid receptors, and 3) disinhibition caused by the inhibition of GABAergic neurons by E-2 may result in an increase in the glutamate release from nerve terminals, which, in turn, may elicit depressor and bradycardic responses.

Bimodal delta-opioid receptors regulate vagal bradycardia in canine sinoatrial node

Methionine-enkephalin-arginine-phenylalanine (MEAP) introduced into the interstitium of the canine sinoatrial (SA) node by microdialysis interrupts vagal bradycardia. In contrast, raising endogenous MEAP by occluding the SA node artery improves vagal bradycardia. Both are blocked by the same delta-selective antagonist, naltrindole. We tested the hypothesis that vagal responses to intranodal enkephalin are bimodal and that the polarity of the response is both dose- and opioid receptor subtype dependent. Ultralow doses of MEAP were introduced into the canine SA node by microdialysis. Heart rate frequency responses were constructed by stimulating the right vagus nerve at 1, 2, and 3 Hz. Ultralow MEAP infusions produced a 50-100% increase in bradycardia during vagal stimulation. Maximal improvement was observed at a dose rate of 500 fmol/min with an ED50 near 50 fmol/min. Vagal improvement was returned to control when MEAP was combined with the delta-antagonist naltrindole. The dose of naltrindole (500 fmol/min) was previously determined as ineffective vs. the vagolytic effect of higher dose MEAP. When MEAP was later reintroduced in the same animals at nanomoles per minute, a clear vagolytic response was observed. The delta1-selective antagonist 7-benzylidenenaltrexone (BNTX) reversed the vagal improvement with an ED50 near 1 x 10-21 mol/min, whereas the delta2-antagonist naltriben had no effect through 10-9 mol/min. Finally, the improved vagal bradycardia previously associated with nodal artery occlusion and endogenous MEAP was blocked by the selective delta1-antagonist BNTX. These data support the hypothesis that opioid effects within the SA node are bimodal in character, that low doses are vagotonic, acting on delta1-receptors, and that higher doses are vagolytic, acting on delta2-receptors.

Cardiac enkephalins interrupt vagal bradycardia via delta 2-opioid receptors in sinoatrial node

Local cardiac opioids appear to be important in determining the quality of vagal control of heart rate. Introduction of the endogenous opioid methionine-enkephalin-arginine-phenylalanine (MEAP) into the interstitium of the canine sinoatrial node by microdialysis attenuates vagally mediated bradycardia through a delta-opioid receptor mechanism. The following studies were conducted to test the hypothesis that a delta(2)-opiate receptor subtype mediates the interruption of vagal transmission. Twenty mongrel dogs were anesthetized and instrumented with microdialysis probes inserted into the sinoatrial node. Vagal frequency responses were performed at 1, 2, and 3 Hz during vehicle infusion and during treatment with the native agonist MEAP, the delta(1)-opioids 2-methyl-4aa-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aalpha-octahydroquinolino[2,3,3- g]isoquinoline (TAN-67) and [d-pen(2,5)]-enkephalin (DPDPE), and the delta(2) opioid deltorphin II. The vagolytic effects of intranodal MEAP and deltorphin were then challenged with the delta(1)- and delta(2)-opioid receptor antagonists 7-benzylidenenaltrexone (BNTX) and naltriben, respectively. Although the positive control deltorphin II was clearly vagolytic in each experimental group, TAN-67 and DPDPE were vagolytically ineffective in the same animals. In contrast, TAN-67 improved vagal bradycardia by 30-35%. Naltriben completely reversed the vagolytic effects of MEAP and deltorphin. BNTX was ineffective in this regard but did reverse the vagal improvement observed with TAN-67. These data support the hypothesis that the vagolytic effect of the endogenous opioid MEAP was mediated by delta(2)-opioid receptors located in the sinoatrial node. These data also support the existence of vagotonic delta(1)-opioid receptors also in the sinoatrial node.

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.

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.

Different stimulatory opioid effects on intracellular Ca(2+) in SH-SY5Y cells

Present study revealed the stimulatory effects of delta opioid receptor on intracellular Ca(2+) concentration ([Ca(2+)](i)) in SH-SY5Y cells. Fura-2 based single cell fluorescence ratio (F345/F380) was used to monitor the fluctuation of [Ca(2+)](i). Application of the selective delta-opioid receptor agonist alone, [D-Pen(2,5)]-enkephalin (DPDPE), hardly had any effects on cells cultivated for 3-10 days. However, after the cells had been pre-stimulated with cholinoceptor agonist, carbachol, variable calcium elevation was found in 59% of the cultures. The response was naltridole-reversible and dose-dependent, and was abolished completely by thapsigargin (TG) treatment but not by administration of CdCl(2) or 0-Ca(2+) bath solutions. DPDPE-mediated [Ca(2+)](i) elevation was abolished by pertussis toxin (PTX) pretreatment but not cholera toxin (CTX), indicating coupling via G proteins of G(i)/G(o) subfamily. In 17.5% of the responding cells, biphase response was found which may be due to both the stimulatory and the inhibitory effects of opioid. On the other hand, in acutely dissociated cells, DPPDE alone induced [Ca(2+)](i) increase in 50% of the cultures. The probability and the amplitude of the elevation were decreased considerably by application of nifedipine or 0-Ca(2+) bath solution and was little affected by application of TG. DPDPE activated [Ca(2+)](i) increase via a PTX-insensitive and CTX-sensitive pathway suggesting coupling through G(s) subunit. All these indicated the opioid modulated the intracellular Ca(2+) regulation system through different pathways. SH-SY5Y cell line might be a suitable model for the investigation of the complex mechanism which underlies opioid function.

Extremely low frequency magnetic fields can either increase or decrease analgaesia in the land snail depending on field and light conditions

Results of prior investigations with opioid peptide mediated antinociception or analgaesia have suggested that these extremely low frequency (ELF) magnetic field effects are described by a resonance mechanism rather than mechanisms based on either induced currents or magnetite. Here we show that ELF magnetic fields (141-414 microT peak) can, in a manner consistent with the predictions of Lednev's parametric resonance model (PRM) for the calcium ion, either (i) reduce, (ii) have no effect on, or (iii) increase endogenous opioid mediated analgaesia in the land snail, Cepaea nemoralis. When the magnetic fields were set to parameters for the predictions of the PRM for the potassium ion, opioid-peptide mediated analgaesia increased and there was evidence of antagonism by the K(+) channel blocker, glibenclamide. Furthermore, these effects were dependent on the presence of light; the effects were absent in the absence of light. These observed increases and decreases in opioid analgaesia are largely consistent with the predictions of Lednev's PRM.

Naloxone potentiates anxiolytic-like actions of diazepam, pentobarbital and meprobamate but not those of Ro19-8022 in the rat

The elevated plus-maze test was used to determine if the opiate antagonist naloxone could potentiate the anxiolytic-like effects of the benzodiazepine diazepam, the barbiturate pentobarbital, the propanediol carbamate meprobamate and the partial benzodiazepine receptor agonist [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-methanol (Ro19-8022) in the rat. A subeffective dose of each of these compounds was combined with naloxone, 10 mg/kg. Naloxone had no effect by itself, but potentiated all drugs except Ro19-8022. The proportion of entries on the open arm increased while the total number of arms entries was not modified. These results coincide with and extend data previously obtained in the mouse. One possible explanation for naloxone's effect could be that it blocks opioid inhibition of GABAergic (gamma-aminobutyric acid) neurons thereby enhancing the effects of benzodiazepines. Another possibility is that naloxone blocks opioid effects on adenosinergic systems.

Opioid receptor endocytosis and activation of MAP kinase pathway

Opioid receptors, members of the G-protein coupled receptor (GPCR) super family, bind to endogenous opioid peptides or opiate drugs and induce a wide variety of signal transduction processes by inhibiting adenylyl cyclase, modulating cation channels, and activating the mitogen-activated protein (MAP) kinases. Similar to other GPCRs, agonist binding causes rapid internalization and down-regulation of opioid receptors. The interdependence between receptor endocytosis and activation of MAP kinase pathway are increasingly being examined. We have examined these using ligands that exhibit differential extent of endocytosis as well as mutants of mu and delta opioid receptors that are unable to internalize. We find that ligands, including morphine, that do not induce receptor internalization are able to stimulate MAP kinase phosphorylation not only in heterologous cells but also in neuronal cell lines that express endogenous mu and delta receptors. Moreover, mutant receptors that fail to undergo agonist-mediated internalization are able to efficiently phosphorylate MAP kinases. Taken together, these data are consistent with the notion that the activation of MAP kinase pathway is an internalization independent phenomenon in the case of opioid receptors and that GPCR internalization and activation of MAP kinase are governed by complex regulatory mechanisms.

Anxiolytic-like effects of meprobamate. Interactions with an opiate antagonist in Swiss and BALB/c mice

Naloxone has previously been shown to block the effects of benzodiazepines in the Swiss but not in the BALB/c strain. We have also reported that naloxone potentiates subeffective doses of benzodiazepines in Swiss mice. In the present studies we first determined whether naloxone could block anxiolytic-like effects of meprobamate in Swiss and BALB/c mice. Then we evaluated if subeffective doses of meprobamate could be potentiated in Swiss as well as in BALB/c mice. The elevated plus-maze test and the light/dark choice procedure were used. The lowest dose of meprobamate with anxiolytic-like effects was 60 mg/kg in the BALB/c mice. This dose was effective in both the plus-maze and in the light/dark choice procedure. In Swiss mice the same dose was effective in the plus-maze, whereas 120 mg/kg was required in the light/dark choice procedure. When an effective dose of meprobamate was combined with naloxone, 10 mg/kg, no blockade of anxiolytic-like effects was obtained in any strain in any procedure. To the contrary, when a subeffective dose of meprobamate was combined with naloxone, 10 mg/kg, an anxiolytic-like effect was obtained in both strains in both procedures. The present series of experiment shows that the ability of naloxone to block anxiolytic-like drug effects do not apply to meprobamate. However, the naloxone-induced potentiation of subeffective doses previously observed after treatment with benzodiazepines or buspirone was present also after treatment with meprobamate. Moreover, although blockade of anxiolytic-like drug effects with naloxone has not been observed in BALB/c mice, potentiation was as evident in that strain as in the Swiss. This suggests that the mechanisms behind naloxone's blockade of anxiolytic-like effects are independent from those behind its potentiation of such effects.

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolefance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly Gialpha inhibitory to Gbetagamma stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.

Effect of morphine on cholecystokinin and mu-opioid receptor-like immunoreactivities in rat spinal dorsal horn neurons after peripheral axotomy and inflammation

In order to further investigate the interaction between the octapeptide cholecystokinin and opioid analgesia in the spinal cord we used double-colour immunofluorescence to examine the anatomical distribution of cholecystokinin and mu-opioid receptors in the dorsal horn, as well as the effect of morphine on cholecystokinin- and mu-opioid receptor-like immunoreactivities following peripheral nerve injury and inflammation. Mu-opioid receptor-like immunoreactivity was present in 65.6% of cholecystokinin-positive neurons in laminae I and II of rat spinal cord. Conversely, 40.4% of mu-opioid receptor-positive neurons contained cholecystokinin-like immunoreactivity. Systemic application of morphine (1, 3 or 10 mg/kg; i.v.) after sciatic nerve section significantly, but reversibly, decreased mu-Opioid receptor-like immunoreactivity in the medial half of lamina II in segment L5 of the ipsilateral dorsal horn, and cholecystokinin-like immunoreactivity was also markedly reduced in the same region. These effects were dose- and time-dependent and could be prevented by naloxone preadministration. In contrast, no significant change in the pattern of distribution or intensity of mu-opioid receptor- and cholecystokinin-like immunoreactivities was observed in intact rats or during peripheral inflammation. These results provide a cellular basis for the interaction of mu-opioid receptors and cholecystokinin at the spinal level by showing a high degree of co-existence of these two molecules in local interneurons, and also show that morphine can induce rapid and short lasting effects on mu-opioid receptors after peripheral nerve injury. The results contribute to our understanding of how endogenous cholecystokinin reduces the analgesic effect of morphine.

Calcitonin reverts pertussis toxin blockade of the opioid analgesia in mice

The aim of this paper is to study the influence of salmon calcitonin (SCT) on opioid analgesia when opioid transduction pathways are functionally uncoupled from Gi/o proteins by treatment with pertussis toxin (PTX). The antinociceptive effect of morphine and three selective opioid agonists, [D-Ala2,N-Me-Phe2,Gly5-ol]enkephalin (DAMGO) (OP(3-mu receptor agonist), [D-Pen2.5]-enkephalin (OP-1-delta receptor agonist) and trans-( +/- )-3,4-dichloro-N-methyl-N-[2-1(-pyrrolidinyl)-cyclohexyl]-benzene-acetam ide methane sulfonate (U-50, 488H) (OP1-kappareceptor agonist) was evaluated, using the tail flick test, in mice treated with PTX or with PTX and SCT. PTX blocked the antinociceptive effect of the opioids, being the antinociception similar in control animals and in mice treated with PTX and SCT. Thus, SCT prevents the effect of the blockade of Gi/o-proteins. From this it could be suggested that calcitonin activates alternative antinociceptive mechanisms that are not dependent on Gi/o-proteins.

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.

The effects of endomorphin-1 and endomorphin-2 in CHO cells expressing recombinant mu-opioid receptors and SH-SY5Y cells

1 Endomorphin-1 and -2 (E-1/E-2) have been proposed as endogenous ligands for the mu-opioid receptor. The aims of this study are to characterize the binding of E-1/E-2 and the subsequent effects on cyclic AMP formation and [Ca2+]i levels in SH-SY5Y and Chinese hamster ovary (CHO) cells expressing endogenous and recombinant mu-opioid receptors. 2 E-1 displaced [3H]-diprenorphine ([3H]-DPN) binding in CHO micro and SH-SY5Y membranes with pKi values of 8.02+/-0.09 and 8.54+/-0.13 respectively. E-2 displaced [3H]-DPN binding in CHOmu and SH-SY5Y cells with pKi values of 7.82+/-0.11 and 8.43+/-0.13 respectively. E-1/E-2 bound weakly to CHOdelta and CHOkappa membranes, with IC50 values of greater than 10 microM. 3 In CHOmu cells, E-1/E-2 inhibited forskolin (1 microM) stimulated cyclic AMP formation with pIC50 values of 8.03+/-0.16 (Imax = 53.0+/-9. 3%) and 8.15+/-0.24 (Imax = 56.3+/-3.8%) respectively. In SH-SY5Y cells E1/E2 inhibited forskolin stimulated cyclic AMP formation with pIC50 values of 7.72+/-0.13 (Imax=46.9+/-5.6%) and 8.11+/-0.31 (Imax = 40.2+/-2.8%) respectively. 4 E-1/E-2 (1 microM) increased [Ca2+]i in fura-2 loaded CHOmu cell suspensions in a thapsigargin sensitive and naloxone reversible manner. Mean increases observed were 106+/-28 and 69+/-6.7 nM respectively. In single adherent cells E-1/E-2 (1 microM) increased [Ca2+]i with a mean 340/380 ratio change of 0.81+/-0.09 and 0.40+/-0.08 ratio units respectively. E-1/E-2 failed to increase intracellular calcium in CHOdelta, CHOkappa and SH-SY5Y cells. 5 These data show that E-1/E-2 bind with high affinity and selectivity to mu-opioid receptors and modulate signal transduction pathways typical of opioids. This provides further evidence that these two peptides may be endogenous ligands at the mu-opioid receptor.

The effect of C-terminal truncation of the recombinant delta-opioid receptor on Ca2+i signaling

We have previously shown a stimulatory coupling of the recombinant delta-opioid receptor to phospholipase C leading to production of inositol (1,4,5) triphosphate [Ins(1,4,5)P3] that is affected by truncation of the C-terminus of the receptor. Using a C-terminal mutant of the delta-opioid receptor lacking the final 37 amino acids (CHOdelta37), we examined its coupling to intracellular calcium ion concentration ([Ca2+]i) compared to the full length wild type receptor (CHOdeltaWT) in transfected Chinese hamster ovary (CHO) cells. D-[Pen2,5]enkephalin (DPDPE) mediated increases in [Ca2+]i were measured fluorimetrically in fura-2 loaded whole cell suspensions. DPDPE produced time- and concentration-dependent increases in [Ca2+]i in CHOdeltaWT and CHOdelta37. In both cell types the DPDPE simulated increase in [Ca2+]i was naloxone reversible and pertussis toxin and thapsigargin sensitive. Removal of the C-terminus resulted in a rightward shift of the Ca2+ release concentration-response curve [pEC50 = 8.43 +/- 0.13 and 6.08 +/- 0.25 for CHOdeltaWT and CHOdelta37, respectively]. These data indicate that the C-terminus of the recombinant delta-opioid receptor is important in [Ca2+]i coupling and may be attributed to the effect of C-terminus truncation on phospholipase C coupling reported previously.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: II. Activation and involvement of the alpha, epsilon, and zeta isoforms of PKC

Phosphorylation of specific amino acid residues is believed to be crucial for the agonist-induced regulation of several G protein-coupled receptors. This is especially true for the three types of opioid receptors (mu, delta, and kappa), which contain consensus sites for phosphorylation by numerous protein kinases. Protein kinase C (PKC) has been shown to catalyze the in vitro phosphorylation of mu- and delta-opioid receptors and to potentiate agonist-induced receptor desensitization. In this series of experiments, we continue our investigation of how opioid-activated PKC contributes to homologous receptor down-regulation and then expand our focus to include the exploration of the mechanism(s) by which mu-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells. [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin (DAMGO)-induced PKC translocation follows a time-dependent and biphasic pattern beginning 2 h after opioid addition, when a pronounced translocation of PKC to the plasma membrane occurs. When opioid exposure is lengthened to >12 h, both cytosolic and particulate PKC levels drop significantly below those of control-treated cells in a process we termed "reverse translocation." The opioid receptor antagonist naloxone, the PKC inhibitor chelerythrine, and the L-type calcium channel antagonist nimodipine attenuated opioid-mediated effects on PKC and mu-receptor down-regulation, suggesting that this is a process partially regulated by Ca2+-dependent PKC isoforms. However, chronic exposure to phorbol ester, which depletes the cells of diacylglycerol (DAG) and Ca2+-sensitive PKC isoforms, before DAMGO exposure, had no effect on opioid receptor down-regulation. In addition to expressing conventional (PKC-alpha) and novel (PKC-epsilon) isoforms, SH-SY5Y cells also contain a DAG- and Ca2+-independent, atypical PKC isozyme (PKC-zeta), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-zeta is similarly sensitive to activation by mu-opioids. PKC-zeta translocates from the cytosol to the membrane with kinetics similar to those of PKC-alpha and epsilon in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by mu-opioid agonists is involved in the processes that result in mu-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.

Modulation of opioid analgesia, tolerance and dependence by Gs-coupled, GM1 ganglioside-regulated opioid receptor functions

Studies of direct excitatory effects elicited by opioid agonists on various types of neurone have been confirmed and expanded in numerous laboratories following the initial findings reviewed previously by Stanley Crain and Ke-Fei Shen. However, the critical role of the endogenous glycolipid GM1 ganglioside in regulating Gs-coupled, excitatory opioid receptor functions has not been addressed in any of the recent reviews of opioid stimulatory mechanisms. This article by Stanley Crain and Ke-Fei Shen focuses on crucial evidence that the concentration of GM1 in neurones might, indeed, play a significant role in the modulation of opioid receptor-mediated analgesia, tolerance and dependence.

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.

Effect of the peripherally selective kappa-opioid agonist, asimadoline, on adjuvant arthritis

1. Opioids, though widely used as analgesics, have not been seriously considered as therapy for rheumatoid arthritis. The present study evaluated the dose-effect and time-dependence relationships of a new peripherally selective kappa agonist, asimadoline, in rats with adjuvant arthritis. 2. The arthritis was assessed by a pooled severity index combining the comprehensive criteria of oedema, radiography and histological changes, in the hind limbs. Asimadoline was extremely effective in attenuating joint damage (by up to 80%) when administered parenterally (0.5 to 10 mg kg(-1) day(-1), i.p.) throughout the disease or during its early phase; treatment was less successful if confined to the latter stages. Ten fold higher doses were effective orally. 3. Equimolar doses of a peripherally-selective antagonist, naloxone methiodide, and the kappa-selective antagonist, MR2266, fully reversed the peripheral anti-arthritic effects of asimadoline (5 mg kg(-1) day(-1)), indicating that asimadoline acts through peripheral kappa-opioid receptors. However, an equivalent dose of MR2266 did not fully reverse the anti-arthritic effects of the highest dose of asimadoline (40 mg kg(-1) day(-1)), suggesting a loss of kappa-selectivity at this dose. 4. Asimadoline also exhibited analgesic effects (mechanical nociceptive thresholds) in arthritic but not non-arthritic rats, indicating that inflammation is necessary for asimadoline-induced analgesia. 5. These data confirm our previous findings that kappa-opioids possess anti-arthritic properties and that these effects are mediated via peripheral kappa-receptors. The present results are new in showing that the peripherally acting kappa-opioid agonist, asimadoline, is a potent anti-arthritic agent. Such novel drugs, essentially lacking central side effects, herald new treatments for rheumatoid arthritis.

Presynaptic versus postsynaptic localization of mu and delta opioid receptors in dorsal and ventral striatopallidal pathways

Parallel studies have demonstrated that enkephalin release from nerve terminals in the pallidum (globus pallidus and ventral pallidum) can be modulated by locally applied opioid drugs. To investigate further the mechanisms underlying these opioid effects, the present study examined the presynaptic and postsynaptic localization of delta (DOR1) and mu (MOR1) opioid receptors in the dorsal and ventral striatopallidal enkephalinergic system using fluorescence immunohistochemistry combined with anterograde and retrograde neuronal tracing techniques. DOR1 immunostaining patterns revealed primarily a postsynaptic localization of the receptor in pallidal cell bodies adjacent to enkephalin- or synaptophysin-positive fiber terminals. MOR1 immunostaining in the pallidum revealed both a presynaptic localization, as evidenced by punctate staining that co-localized with enkephalin and synaptophysin, and a postsynaptic localization, as evidenced by cytoplasmic staining of cells that were adjacent to enkephalin and synaptophysin immunoreactivities. Injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) or the retrograde tracer Texas Red-conjugated dextran amine (TRD) into the dorsal and ventral striatum resulted in labeling of striatopallidal fibers and pallidostriatal cell bodies, respectively. DOR1 immunostaining in the pallidum co-localized only with TRD and not PHA-L, whereas pallidal MOR1 immunostaining co-localized with PHA-L and not TRD. These results suggest that pallidal enkephalin release may be modulated by mu opioid receptors located presynaptically on striatopallidal enkephalinergic neurons and by delta opioid receptors located postsynaptically on pallidostriatal feedback 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.