Stimulatory effects of opioids

Glycine Transporter 1 Inhibitors: Predictions on Their Possible Mechanisms in the Development of Opioid Analgesic Tolerance

The development of opioid tolerance in patients on long-term opioid analgesic treatment is an unsolved matter in clinical practice thus far. Dose escalation is required to restore analgesic efficacy, but at the price of side effects. Intensive research is ongoing to elucidate the underlying mechanisms of opioid analgesic tolerance in the hope of maintaining opioid analgesic efficacy. N-Methyl-D-aspartate receptor (NMDAR) antagonists have shown promising effects regarding opioid analgesic tolerance; however, their use is limited by side effects (memory dysfunction). Nevertheless, the GluN2B receptor remains a future target for the discovery of drugs to restore opioid efficacy. Mechanistically, the long-term activation of µ-opioid receptors (MORs) initiates receptor phosphorylation, which triggers β-arrestin-MAPKs and NOS-GC-PKG pathway activation, which ultimately ends with GluN2B receptor overactivation and glutamate release. The presence of glutamate and glycine as co-agonists is a prerequisite for GluN2B receptor activation. The extrasynaptic localization of the GluN2B receptor means it is influenced by the glycine level, which is regulated by astrocytic glycine transporter 1 (GlyT1). Enhanced astrocytic glycine release by reverse transporter mechanisms as a consequence of high glutamate levels or unconventional MOR activation on astrocytes could further activate the GluN2B receptor. GlyT1 inhibitors might inhibit this condition, thereby reducing opioid tolerance.

Opioid Analgesic as a Positive Allosteric Modulator of Acid-Sensing Ion Channels

Tafalgin (Taf) is a tetrapeptide opioid used in clinical practice in Russia as an analgesic drug for subcutaneous administration as a solution (4 mg/mL; concentration of 9 mM). We found that the acid-sensing ion channels (ASICs) are another molecular target for this molecule. ASICs are proton-gated sodium channels that mediate nociception in the peripheral nervous system and contribute to fear and learning in the central nervous system. Using electrophysiological methods, we demonstrated that Taf could increase the integral current through heterologically expressed ASIC with half-maximal effective concentration values of 0.09 mM and 0.3 mM for rat and human ASIC3, respectively, and 1 mM for ASIC1a. The molecular mechanism of Taf action was shown to be binding to the channel in the resting state and slowing down the rate of desensitization. Taf did not compete for binding sites with both protons and ASIC3 antagonists, such as APETx2 and amiloride (Ami). Moreover, Taf and Ami together caused an unusual synergistic effect, which was manifested itself as the development of a pronounced second desensitizing component. Thus, the ability of Taf to act as a positive allosteric modulator of these channels could potentially cause promiscuous effects in clinical practice. This fact must be considered in patients' treatment.

Opioid epidemic: lessons learned and updated recommendations for misuse involving prescription versus non-prescription opioids

INTRODUCTION

In the past decades, the opioid crisis has heavily impacted parts of the US society and has been followed by an increase in the use of opioids worldwide. It is of paramount importance that we explore the origins of the US opioid epidemic to develop best practices to tackle the rising tide of opioid overdoses.

AREAS COVERED

In this expert review, we discuss opioid (over)prescription, change in perception of pain, and false advertisement of opioid safety as the leading causes of the US opioid epidemic. Then, we review the evidence about opioid dependence and addiction potential and provide current knowledge about predictors of aberrant opioid-related behavior. Lastly, we discuss different approaches that were considered or undertaken to combat the rising tide of opioid-related deaths by regulatory bodies, pharmaceutical companies, and health-care professionals. For this expert review, we considered published articles relevant to the topic under investigation that we retrieved from Medline or Google scholar electronic database.

EXPERT OPINION

The opioid epidemic is a dynamic process with many underlying mechanisms. Therefore, no single approach may be best suited to combat it. In our opinion, the best way forward is to employ multiple strategies to tackle different underlying mechanisms.

Basic Opioid Pharmacology - An Update

Opioids are a group of analgesic agents commonly used in clinical practice. The three classical opioid receptors are MOP, DOP and KOP. The NOP (N/OFQ) receptor is considered to be a non-opioid branch of the opioid receptor family. Opioid receptors are G-protein-coupled receptors which cause cellular hyperpolarisation when bound to opioid agonists. Opioids may be classified according to their mode of synthesis into alkaloids, semi-synthetic and synthetic compounds. Opioid use disorder (OUD) is an emerging issue and important lessons can be learnt from the United States where opioid epidemic was declared as a national emergency in 2017.

Ventilation and neurochemical changes during µ-opioid receptor activation or blockade of excitatory receptors in the hypoglossal motor nucleus of goats

Neuromodulator interdependence posits that changes in one or more neuromodulators are compensated by changes in other modulators to maintain stability in the respiratory control network. Herein, we studied compensatory neuromodulation in the hypoglossal motor nucleus (HMN) after chronic implantation of microtubules unilaterally ( n = 5) or bilaterally ( n = 5) into the HMN. After recovery, receptor agonists or antagonists in mock cerebrospinal fluid (mCSF) were dialyzed during the awake and non-rapid eye movement (NREM) sleep states. During day studies, dialysis of the µ-opioid inhibitory receptor agonist [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO; 100 µM) decreased pulmonary ventilation (V̇i), breathing frequency ( f), and genioglossus (GG) muscle activity but did not alter neuromodulators measured in the effluent mCSF. However, neither unilateral dialysis of a broad spectrum muscarinic receptor antagonist (atropine; 50 mM) nor unilateral or bilateral dialysis of a mixture of excitatory receptor antagonists altered V̇i or GG activity, but all of these did increase HMN serotonin (5-HT) levels. Finally, during night studies, DAMGO and excitatory receptor antagonist decreased ventilatory variables during NREM sleep but not during wakefulness. These findings contrast with previous dialysis studies in the ventral respiratory column (VRC) where unilateral DAMGO or atropine dialysis had no effects on breathing and bilateral DAMGO or unilateral atropine increased V̇i and f and decreased GABA or increased 5-HT, respectively. Thus we conclude that the mechanisms of compensatory neuromodulation are less robust in the HMN than in the VRC under physiological conditions in adult goats, possibly because of site differences in the underlying mechanisms governing neuromodulator release and consequently neuronal activity, and/or responsiveness of receptors to compensatory neuromodulators. NEW & NOTEWORTHY Activation of inhibitory µ-opioid receptors in the hypoglossal motor nucleus decreased ventilation under physiological conditions and did not affect neurochemicals in effluent dialyzed mock cerebral spinal fluid. These findings contrast with studies in the ventral respiratory column where unilateral [d-Ala², N-MePhe⁴, Gly-ol]enkephalin (DAMGO) had no effects on ventilation and bilateral DAMGO or unilateral atropine increased ventilation and decreased GABA or increased serotonin, respectively. Our data support the hypothesis that mechanisms that govern local compensatory neuromodulation within the brain stem are site specific under physiological conditions.

Acute administration of tramadol and tapentadol at effective analgesic and maximum tolerated doses causes hepato- and nephrotoxic effects in Wistar rats

Tramadol and tapentadol are two atypical synthetic opioid analgesics, with monoamine reuptake inhibition properties. Mainly aimed at the treatment of moderate to severe pain, these drugs are extensively prescribed for multiple clinical applications. Along with the increase in their use, there has been an increment in their abuse, and consequently in the reported number of adverse reactions and intoxications. However, little is known about their mechanisms of toxicity. In this study, we have analyzed the in vivo toxicological effects in liver and kidney resulting from an acute exposure of a rodent animal model to both opioids. Male Wistar rats were intraperitoneally administered with 10, 25 and 50mg/kg tramadol and tapentadol, corresponding to a low, effective analgesic dose, an intermediate dose and the maximum recommended daily dose, respectively, for 24h. Toxicological effects were assessed in terms of oxidative stress, biochemical and metabolic parameters and histopathology, using serum and urine samples, liver and kidney homogenates and tissue specimens. The acute exposure to tapentadol caused a dose-dependent increase in protein oxidation in liver and kidney. Additionally, exposure to both opioids led to hepatic commitment, as shown by increased serum lipid levels, decreased urea concentration, increased alanine aminotransferase and decreased butyrylcholinesterase activities. It also led to renal impairment, as reflected by proteinuria and decreased glomerular filtration rate. Histopathological findings included sinusoidal dilatation, microsteatosis, vacuolization, cell infiltrates and cell degeneration, indicating metabolic changes, inflammation and cell damage. In conclusion, a single effective analgesic dose or the maximum recommended daily dose of both opioids leads to hepatotoxicity and nephrotoxicity, with tapentadol inducing comparatively more toxicity. Whether these effects reflect risks during the therapeutic use or human overdoses requires focused attention by the medical community.

Basic Opioid Pharmacology

Opioids can be classified according to their mode of synthesis into alkaloids, semi-synthetic and synthetic compounds.There are three classical receptors (DOP, KOP and MOP). The novel NOP receptor is considered to be a non-opioid branch of the opioid receptor family.Opioids can either act as agonists, antagonists or partial agonists at these receptors.Opioid agonists bind to G-protein coupled receptors to cause cellular hyperpolarisation.MOP receptor agonists act in the central and peripheral nervous system to elicit analgesia.

The immunomodulation mediated by a delta-opioid receptor for [Met(5)]-enkephalin in oyster Crassostrea gigas

Opioid receptors (OR) are a group of G protein-coupled receptors with opioids as ligands, which play an important role in triggering the second messengers to modulate immune response in vertebrate immunocytes. In the present study, the full length cDNA of a homologue of δ-opioid receptor (DOR) for [Met(5)]-enkaphalin was cloned from oyster Crassostrea gigas (designated as CgDOR), which was 1104 bp encoding a peptide of 367 amino acids containing a conserved 7tm_1 domain. After the stimulation of [Met(5)]-enkephalin, the concentration of second messengers Ca(2+) and cAMP in the HEK293T cells decreased significantly (p <0.05) with the expression of CgDOR. However, this trend was reverted with the addition of DOR antagonist BNTX. The CgDOR transcripts were ubiquitously detected in the tested tissues including haemocytes, gonad, mantle, kidney, gill, adductor muscle and hepatopancreas, with the highest expression level in the hepatopancreas. After LPS stimulation, the expression level of CgDOR mRNA began to increase (4.05-fold, p <0.05) at 6 h, and reached the highest level (5.00-fold, p <0.05) at 12 h. Haemocyte phagocytic and antibacterial activities increased significantly after [Met(5)]-enkephalin stimulation, whereas the increase was repressed with the addition of DOR antagonist BNTX. These results collectively suggested that CgDOR for [Met(5)]-enkephalin could modulate the haemocyte phagocytic and antibacterial functions through the second messengers Ca(2+) and cAMP, which might be requisite for pathogen elimination and homeostasis maintenance in oyster.

Direct bidirectional μ-opioid control of midbrain dopamine neurons

The ventral tegmental area (VTA) is required for the rewarding and motivational actions of opioids and activation of dopamine neurons has been implicated in these effects. The canonical model posits that opioid activation of VTA dopamine neurons is indirect, through inhibition of GABAergic inputs. However, VTA dopamine neurons also express postsynaptic μ-opioid peptide (MOP) receptors. We report here that in Sprague Dawley rat, the MOP receptor-selective agonist DAMGO (0.5-3 μM) depolarized or increased the firing rate of 87 of 451 VTA neurons (including 22 of 110 dopamine neurons). This DAMGO excitation occurs in the presence of GABAA receptor blockade and its EC50 value is two orders of magnitude lower than for presynaptic inhibition of GABA release on to VTA neurons. Consistent with a postsynaptic channel opening, excitations were accompanied by a decrease in input resistance. Excitations were blocked by CdCl2 (100 μM, n = 5) and ω-agatoxin-IVA (100 nM, n = 3), nonselective and Cav2.1 Ca(2+) channel blockers, respectively. DAMGO also produced a postsynaptic inhibition in 233 of 451 VTA neurons, including 45 of 110 dopamine neurons. The mean reversal potential of the inhibitory current was -78 ± 7 mV and inhibitions were blocked by the K(+) channel blocker BaCl2 (100 μM, n = 7). Blockade of either excitation or inhibition unmasked the opposite effect, suggesting that MOP receptors activate concurrent postsynaptic excitatory and inhibitory processes in most VTA neurons. These results provide a novel direct mechanism for MOP receptor control of VTA dopamine neurons.

Basic opioid pharmacology: an update

Opioids are a group of analgesic agents commonly used in clinical practice. There are three classical opioid receptors (DOP, KOP and MOP), while the novel NOP receptor is considered to be a non-opioid branch of the opioid receptor family. Opioids can act at these receptors as agonists, antagonists or partial agonists. Opioid agonists bind to G-protein coupled receptors to cause cellular hyperpolarisation. Most clinically relevant opioid analgesics bind to MOP receptors in the central and peripheral nervous system in an agonist manner to elicit analgesia. Opioids may also be classified according to their mode of synthesis into alkaloids, semi-synthetic and synthetic compounds.

Long-term potentiation in spinal nociceptive pathways as a novel target for pain therapy

Long-term potentiation (LTP) in nociceptive spinal pathways shares several features with hyperalgesia and has been proposed to be a cellular mechanism of pain amplification in acute and chronic pain states. Spinal LTP is typically induced by noxious input and has therefore been hypothesized to contribute to acute postoperative pain and to forms of chronic pain that develop from an initial painful event, peripheral inflammation or neuropathy. Under this assumption, preventing LTP induction may help to prevent the development of exaggerated postoperative pain and reversing established LTP may help to treat patients who have an LTP component to their chronic pain. Spinal LTP is also induced by abrupt opioid withdrawal, making it a possible mechanism of some forms of opioid-induced hyperalgesia. Here, we give an overview of targets for preventing LTP induction and modifying established LTP as identified in animal studies. We discuss which of the various symptoms of human experimental and clinical pain may be manifestations of spinal LTP, review the pharmacology of these possible human LTP manifestations and compare it to the pharmacology of spinal LTP in rodents.

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.

Second-hand exposure to aerosolized intravenous anesthetics propofol and fentanyl may cause sensitization and subsequent opiate addiction among anesthesiologists and surgeons

We hypothesize that aerosolization of anesthetics administered intravenously to patients in the operating room may be an unintended source of exposure to physicians. This may lead to inadvertent sensitization, which is associated with an increased risk for developing addiction. This may contribute to the over-representation of certain specialties among physicians with addiction. We retrospectively reviewed the de-identified demographic information of all licensed physicians treated for substance abuse in the State of Florida since 1980, to determine if medical specialty was associated with addiction in this group of individuals. Then, to identify the potential for exposure, two mass spectrometry assays were developed to detect two intravenously administered drugs, fentanyl and propofol, in air. Since 1980, 7.6% of licensed Florida physicians underwent treatment for addiction. Addiction in anesthesiologists was higher than expected. Opiate abuse was greater in anesthesiologists and surgeons compared to other specialties. Aerosolized fentanyl was detected in the air of the cardiothoracic operating room, in patients' expiratory circuits, and in the headspace above sharps boxes, but not in adjoining hallways. Aerosolized propofol was detected in the expirations of a patient undergoing transurethral prostatectomy. While access and stress may place anesthesiologists and surgeons at greater risk for substance abuse, an additional risk factor may be unintended occupational exposure to addictive drugs. This report provides preliminary evidence of detection of aerosolized intravenous anesthetics using two newly developed analytical methods. We conclude that the potential exists for chronic exposure to low levels of airborne intravenously administered drugs. Further studies are under way to determine the significance of this exposure.

Subcutaneous Po2 as an index of the physiological limits for hemodilution in the rat

This study was designed to test the hypothesis that changes in subcutaneous Po2 (PscO2) during progressive hemodilution will reliably predict a “critical point” at which tissue O2 consumption (V̇o2) becomes dependent on O2 delivery (Q̇o2). Twelve pentobarbital-anesthetized male Sprague-Dawley rats (315–375 g) underwent stepwise exchange of plasma for blood (1.5 ml of plasma for each 1 ml of blood lost). The initial exchange was equal to 25% of the estimated circulatory blood volume, and each subsequent exchange was equal to 10% of the estimated circulatory blood volume. After nine exchanges, the hematocrit (Hct) fell from 42 ± 1 to 6 ± 1%. Cardiac output and O2 extraction rose significantly. PscO2 became significantly reduced ( P < 0.05) after exchange of 45% of the blood volume (Hct = 16 ± 1%). V̇o2 became delivery dependent when Q̇o2 fell below 21 ml·min−1·kg body wt−1 (mean Hct = 13 ± 1%). Eight control rats undergoing 1:1 blood-blood exchange showed no change in PscO2, pH, HCO3−, or hemodynamics. Measurement of PscO2 may be a useful guide to monitor the adequacy of Q̇o2 during hemodilution.

Epidural analgesia in gastrointestinal surgery

BACKGROUND

The ideal perioperative analgesia should provide effective pain relief, avoid the detrimental effects of the stress response, be simple to administer without the need for intensive monitoring, and have a low risk of complications.

METHODS

This review defines the physiological effects of epidural analgesia and assesses whether the available evidence supports its preferential use in gastrointestinal surgery. All papers studied were identified from a Medline search or selected by cross-referencing.

RESULTS

Epidural analgesia is associated with a shorter duration of postoperative ileus, attenuation of the stress response, fewer pulmonary complications, and improved postoperative pain control and recovery. It does not reduce anastomotic leakage, intraoperative blood loss, transfusion requirement, risk of thromboembolism or cardiac morbidity, or hospital stay compared with that after conventional analgesia in unselected patients undergoing gastrointestinal surgery. Thoracic epidural analgesia reduces hospital costs and stay in patients at high risk of cardiac or pulmonary complications.

CONCLUSIONS

Epidural analgesia enhances recovery after gastrointestinal surgery. The results support the development of structured regimens of early postoperative feeding and mobilization to exploit the potential for thoracic epidural analgesia to reduce hospital stay after gastrointestinal surgery.

Substance P and enkephalinergic synapses onto neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats

Our previous studies have demonstrated that neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the pre-Bötzinger Complex (pre-BötC), the hypothesized kernel of respiratory rhythmogenesis, receive both glutamatergic excitatory and GABAergic or glycinergic inhibitory inputs. Neuromodulators, such as substance P (SP) and opioids, play important roles in normal respiratory activity and respiratory disorders. The identification of the relationship between neurotransmitters and NK1R-ir neurons at the cellular level is essential for understanding the synaptic interaction within the pre-BötC network. Using immunofluorescence and immunogold-silver staining, we wished to exploit SP and enkephalin (ENK) immunoreactivity and their relationships with glutamate, GABA, glycine, or NK1R in the pre-BötC in adult Sprague-Dawley rats. The pre-BötC contained a substantial amount of SP-ir and ENK-ir boutons. They were largely colocalized with glutamate and much less so with GABA. Glycine immunoreactivity was rarely found in either SP-ir or ENK-ir boutons. A number of SP-ir boutons were ENK-ir as well. Synapses were commonly found between SP-ir or ENK-ir terminals and NK1R-ir neurons in the pre-BötC. Most of them were asymmetric. Symmetric synapses made up 10% of all synapses examined between SP-ir boutons and NK1R-ir neurons, and 19% of ENK/NK1R synapses. Colocalization of SP and/or ENK with glutamate in boutons in the pre-BötC implies the combined synaptic release of excitatory amino acid and neuropeptides, which may exert combined post-synaptic effects onto NK1R-ir neurons and contribute to respiratory activity.

The Hemodynamic and Metabolic Effects of Shivering During Acute Normovolemic Hemodilution

To assess the hemodynamic and metabolic effects of shivering during extreme normovolemic hemodilution, we anesthetized 16 pigs with fentanyl-midazolam-pancuronium. Mild hypothermia (36.5 degrees +/- 0.1 degrees C) was induced by surface cooling, and the animals were randomized to either a control group (hemoglobin 118 +/- 3 g/L) or a hemodilution group (hemoglobin 52 +/- 2 g/L). In the latter group, blood was replaced with an isotonic Ringer's acetate/dextran 70 solution. Shivering was allowed to occur by a controlled decrease in the infusion rate of pancuronium. Shivering increased oxygen consumption (VO(2)) in both groups (P < 0.001). Initially, this was predominantly compensated for by an increased oxygen extraction ratio (ER), but when VO(2) was 2.3 +/- 0.2 times baseline, critical levels of mixed venous oxygenation (SVO(2) = 18% +/- 2%; PVO(2) = 22.5 +/- 1.5 mm Hg) and ER (82% +/- 3%) were recorded in anemic animals. Control animals did not reach critical levels until VO(2) was maximal (3.7 +/- 0.3 times baseline). Maximal attained VO(2) was less (2.9 +/- 0.1 times baseline) in the anemic animals (P = 0.01), and at this stage two of these pigs had myocardial lactate production, one of which died in ventricular fibrillation. Coronary perfusion pressure was significantly less (P < 0.001) in the anemic animals. We conclude that in this experimental model, maximal shivering as measured by VO(2) was limited in hemodiluted animals, and left ventricular oxygen balance was marginal, as evidenced by a decreased lactate uptake and extraction.

IMPLICATIONS

The effect of acute increases in oxygen consumption (shivering) on severely anemic individuals has not been evaluated. In this experimental model, left ventricular oxygen balance was marginal, as evidenced by decreased lactate extraction.

kappa-Opioid receptor potentiates apoptosis via a phospholipase C pathway in the CNE2 human epithelial tumor cell line

The mechanism by which kappa-opioid receptor (kappaor) modulated apoptosis was investigated in CNE2 human epithelial tumor cells. Induction of these cells to undergo apoptosis with staurosporine was associated with a massive increase in intracellular cAMP level. The inhibition of the increase in cAMP partially inhibited apoptosis as evidenced by a reduction of PARP and caspase-3 cleavage. Accordingly, a low but significant level of apoptosis is induced in these cells by the elevation of cAMP through the addition of forskolin and isobutylmethylxanthine. The existence of a cAMP-dependent and a cAMP-independent apoptotic pathway is therefore suggested. Receptor binding studies, RT-PCR experiments and Western blot analysis demonstrated the presence of type 1 kappaor in the CNE2 cells. Stimulation of kappaor in these cells resulted in the production of inositol (1,4,5)-trisphosphate, reduction of cAMP level and a marked enhancement of staurosporine-induced apoptosis. The potentiation of apoptosis by kappaor was prevented by inhibition of phospholipase C but was slightly enhanced by the presence of the active cAMP analogues, 8-CPT-cAMP and dibutyryl-cAMP. These data demonstrate for the first time that the phospholipase C pathway activated by type 1 kappaor expressed by cancer cells is involved in the potentiation of apoptosis.

Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous mu-opioid receptor agonists

The recently discovered endogenous opioid peptides, endomorphins-1 and -2, appear to have properties consistent with neurotransmitter/neuromodulator actions in mammals. This review surveys the information gained so far from studies of different aspects of the endomorphins. Thus, the endomorphins have been found unequally in the brain; they are stored in neurons and axon terminals, with a heterogeneous distribution; they are released from synaptosomes by depolarization; they are enzymatically converted by endopeptidases; and they interact specifically and with high affinity with mu-opioid receptors. The most outstanding effect of the endomorphins is their antinociceptive action. This depends on both central and peripheral neurons. Additionally, the endomorphins cause vasodilatation by stimulating nitric oxide release from the endothelium. Their roles in different central and peripheral functions, however, have not been fully clarified yet. From a therapeutic perspective, therefore, they may be conceived at present as potent antinociceptive and vasodilator agents.

Cellular actions of nociceptin: transduction mechanisms

The recent identification of the nociceptin receptor-nociceptin system and the description of its role in nociceptive processing has produced numerous investigative studies. A fundamental part of this research is to understand the cellular signaling events (i.e. the building blocks) upon which the pharmacology of this intriguing system is based. As anticipated, nociceptin receptor activation inhibits the formation of cAMP formation via a pertussis toxin-sensitive G-protein. This indicates that nociceptin receptor couples to the G(i)/G(o) class of G-protein(s). However, there is now growing evidence for nociceptin activation of additional signaling pathways, including MAP kinase and phospholipase C/[Ca(2+)](i). These signaling events are discussed in this review.

Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca(2+) oscillations and prolactin release through K(IR) channel activation

Opioid receptors can couple to K(+) and Ca(2+) channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca(2+) signaling and basal prolactin release. The objective of this study was to examine a possible role for K(+) channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) inhibited [Ca(2+)](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca(2+)](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca(2+) stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH(3)MORDOR cells with an IC(50) of 4 nM. DPDPE increased inward K(+) current recorded from GH(3)MORDOR cells but had no significant effect on K(+) currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K(+) currents recorded from GH(3)MOR and GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K(+) current was inwardly rectifying and was inhibited by Ba(2+) but not TEA. DPDPE had no effect on delayed rectifier-, Ca(2+)-, and voltage-activated or A-type K(+) currents, recorded from GH(3)MORDOR cells. Ba(2+) attenuated the inhibition of [Ca(2+)](i) and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K(IR) channels in these actions of the opioid.

Chronic fentanyl treatments induce the up-regulation of mu opioid receptor mRNA in rat pheochromocytoma cells

Chronic activation of adenylate cyclase-cAMP-cAMP-dependent protein kinase (PKA) systems by administration of opioid receptor agonists has been considered as one of the mechanisms of opioid tolerance and dependence. Although analysis of the micro opioid receptor (MOR) gene suggests that cAMP-related signal transduction systems regulate the expression of this gene, which transcription factors affect the MOR gene expression in brain and neural cells has not been clarified. This study deals with the effects of fentanyl on MOR mRNA levels in the rat pheochromocytoma cell line (PC12 cells). PC12 cells were cultured in medium with clinically relevant concentrations of fentanyl. The quantitative reverse transcription and polymerase chain reaction (RT-PCR) method was used for determination of MOR mRNA. Treatment of PC12 cells with fentanyl induced the MOR mRNA up-regulation in a concentration- and time-dependent manner. A cAMP analogue also up-regulated MOR mRNA. The intracellular cAMP level increased after fentanyl treatment. A PKA inhibitor blocked the MOR mRNA up-regulation by fentanyl and the cAMP analogue. Expression of a dominant inhibitory Ras also inhibited the MOR mRNA up-regulation. Fentanyl-induced up-regulation of MOR mRNA via activation of cAMP signaling may be important in compensating for the MOR reduction during long-term treatment of PC12 cells with fentanyl. The present study could be relevant to understanding the molecular mechanisms of opioids in a state of drug tolerance or dependence, and in patients under anesthesia or being treated for pain.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

[Mechanisms of opioid tolerance and opioid dependence]

OBJECTIVE

Prescription of opiates to non cancer chronic pain patients is controversial, partly because of the risk of tolerance and dependence development. The two objectives of that review were: a) to identify the factors which may explain the variability of tolerance and dependence in clinical practice; b) to analyse the cellular mechanisms of occurrence of those phenomenons.

DATA SOURCES AND EXTRACTION

To our own file, we added articles retrieved in the Medline database, using, alone or in combination, following key-words (opiate, tolerance, dependence, opiate receptor, pain treatment, cAMP, cGMP, NO, NMDA, protein kinase, gene). Out of nearly 450 articles, we selected less than 200.

DATA SYNTHESIS

Tolerance, defined as loss of opioid efficacy with time, is extremely variable and depends on pain mechanisms, intrinsic efficacy and administration modality of the opioid, as well as co-administration of other agents. Physical dependence is a consequence of the intrinsic and extrinsic adaptations concerning structures as locus coeruleus, paragigantocellular nucleus, spinal cord. Acute and chronic application of opiates and withdrawal give rise to cellular adaptations which depend on the nature and efficacy of the opiate, the type of receptor and second messengers, as well as the type of cell line under study. These cellular mechanisms have consequences on neuronal excitability and gene expression. They constitute a model of cellular tolerance and dependence, but cannot explain the subtelties encountered in clinical practice.

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.