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

Coincident Regulation of PLCβ Signaling by Gq-Coupled and μ-Opioid Receptors Opposes Opioid-Mediated Antinociception

Pain management is an important problem worldwide. The current frontline approach for pain management is the use of opioid analgesics. The primary analgesic target of opioids is the μ-opioid receptor (MOR). Deletion of phospholipase Cβ3 (PLCβ3) or selective inhibition of Gβγ regulation of PLCβ3 enhances the potency of the antinociceptive effects of morphine suggesting a novel strategy for achieving opioid-sparing effects. Here we investigated a potential mechanism for regulation of PLC signaling downstream of MOR in human embryonic kidney 293 cells and found that MOR alone could not stimulate PLC but rather required a coincident signal from a Gq-coupled receptor. Knockout of PLCβ3 or pharmacological inhibition of its upstream regulators, Gβγ or Gq, ex vivo in periaqueductal gray slices increased the potency of the selective MOR agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt in inhibiting presynaptic GABA release. Finally, inhibition of Gq- G protein-coupled receptor coupling in mice enhanced the antinociceptive effects of morphine. These data support a model where Gq and Gβγ-dependent signaling cooperatively regulate PLC activation to decrease MOR-dependent antinociceptive potency. Ultimately, this could lead to identification of new non-MOR targets that would allow for lower-dose utilization of opioid analgesics. SIGNIFICANCE STATEMENT: Previous work demonstrated that deletion of phospholipase Cβ3 (PLCβ3) in mice potentiates μ-opioid receptor (MOR)-dependent antinociception. How PLCβ3 is regulated downstream of MOR had not been clearly defined. We show that PLC-dependent diacylglycerol generation is cooperatively regulated by MOR-Gβγ and Gq-coupled receptor signaling through PLCβ3 and that blockade of either Gq-signaling or Gβγ signaling enhances the potency of opioids in ex vivo brain slices and in vivo. These results reveal potential novel strategies for improving opioid analgesic potency and safety.

Combination of paeoniflorin and liquiritin alleviates neuropathic pain by lipid metabolism and calcium signaling coordination

Neuropathic pain (NP) affects 7%–10% of the general population and is still hard to cure. Here, we validated the therapeutic effect and demonstrated the mechanism of paeoniflorin and liquiritin combination (PL) on NP from the perspective of integrated lipidomics and transcriptomics for the first time. SwissTargetPrediction indicated that PL mainly targets lipid metabolism. Notably, lipidomics revealed that imbalanced lipid levels in the NP model could be reprogrammed to normal levels by PL treatment. RNA-sequencing showed that PL treatment could also rebalance the lipid metabolism in an indirect manner. Pathway analysis highly enriched the calcium signaling pathway among the most significant categories. Altogether, these findings suggested that PL can not only balance the lipid metabolism in direct and indirect manners but also reverse the dysfunctional activation of the calcium signaling pathway, thereby alleviating NP. This helps to better understand the mechanisms of NP and provides a new important potential therapeutic option for NP.

Behavioral, Neural, and Molecular Mechanisms of Conditioned Mate Preference: The Role of Opioids and First Experiences of Sexual Reward

Although mechanisms of mate preference are thought to be relatively hard-wired, experience with appetitive and consummatory sexual reward has been shown to condition preferences for partner related cues and even objects that predict sexual reward. Here, we reviewed evidence from laboratory species and humans on sexually conditioned place, partner, and ejaculatory preferences in males and females, as well as the neurochemical, molecular, and epigenetic mechanisms putatively responsible. From a comprehensive review of the available data, we concluded that opioid transmission at μ opioid receptors forms the basis of sexual pleasure and reward, which then sensitizes dopamine, oxytocin, and vasopressin systems responsible for attention, arousal, and bonding, leading to cortical activation that creates awareness of attraction and desire. First experiences with sexual reward states follow a pattern of sexual imprinting, during which partner- and/or object-related cues become crystallized by conditioning into idiosyncratic “types” that are found sexually attractive and arousing. These mechanisms tie reward and reproduction together, blending proximate and ultimate causality in the maintenance of variability within a species.

Involvement of T-type calcium channels in the mechanism of low dose morphine-induced hyperalgesia in adult male rats

It has been shown that systemic and local administration of ultra-low dose morphine induced a hyperalgesic response via mu-opioid receptors. However, its exact mechanism(s) has not fully been clarified. It is documented that mu-opioid receptors functionally couple to T-type voltage dependent Ca⁺² channels. Here, we investigated the role of T-type calcium channels, amiloride and mibefradil, on the induction of low-dose morphine hyperalgesia in male Wistar rats. The data showed that morphine (0.01 μg i.t. and 1 μg/kg i.p.) could elicit hyperalgesia as assessed by the tail-flick test. Administration of amiloride (5 and 10 μg i.t.) and mibefradil (2.5 and 5 μg i.t.) completely blocked low-dose morphine-induced hyperalgesia in spinal dorsal horn. Amiloride at doses of 1 and 5 mg/kg (i.p.) and mibefradil (9 mg/kg ip) 10 min before morphine (1 μg/kg i.p.) inhibited morphine-induced hyperalgesia. Our results indicate a role for T-type calcium channels in low dose morphine-induced hyperalgesia in rats.

Evaluation of therapeutic effect of low dose naltrexone in experimentally-induced Crohn's disease in rats

BACKGROUND AND AIM

Crohn's disease is a relapsing inflammatory condition afflicting the digestive tract. Drugs used for treatment of Crohn's disease may be associated with serious side effects. Endogenous opioid peptides modulate inflammatory cytokine production. Opioid antagonists have been shown to play a role in healing and repair of tissues. This work was designed to detect the possible beneficial effects of opioid antagonist naltrexone in indomethacin-induced Crohn's disease in rats.

EXPERIMENTAL APPROACH

Enteritis was induced in male albino rats by two subcutaneous injection of indomethacin in a dose of 7.5mg/kg 24h apart started on day one. Salfasalazine, naltrexone and their combination were administered orally from day one of induction of enteritis to day 10. Disease activity index, serum levels of C-reactive protein and tumor necrosis factor-α, macroscopic and microscopic pathological scores and in vitro motility studies were evaluated.

RESULTS

Induction of enteritis resulted in significant increase of disease activity index, significant elevation of serum levels of C-reactive protein and tumor necrosis factor-α, significant deterioration of pathological scores and significant increase in the mean contractility response of the isolated ileal segments compared with normal untreated rats. Treatment with sulfasalazine, low dose of natrexone or their combination resulted in significant improvement of all measured parameters compared with enteritis group.

CONCLUSION

The current finding could provide new interesting opportunity for developing new therapeutic approaches for treatment of Crohn's disease. Use of naltrexone, especially in small dose, has little side effects making it of interest for treatment of Crohn's disease. Also, it provides the possibility of reduced doses of other drugs if it is used as combined therapy.

Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides

Opioids are the most powerful analgesics. As pain is driven by sensory transmission and opioid receptors couple to inhibitory G proteins, according to the classical concept, opioids alleviate pain by activating receptors on neurons and blocking the release of excitatory mediators (e.g., substance P). Here we show that analgesia can be mediated by opioid receptors in immune cells. We propose that activation of leukocyte opioid receptors leads to the secretion of opioid peptides Met-enkephalin, β-endorphin and dynorphin A (1-17), which subsequently act at local neuronal receptors, to relieve pain. In a mouse model of neuropathic pain induced by a chronic constriction injury of the sciatic nerve, exogenous agonists of δ-, μ- and κ-opioid receptors injected at the damaged nerve infiltrated by opioid peptide- and receptor-expressing leukocytes, produced analgesia, as assessed with von Frey filaments. The analgesia was attenuated by pharmacological or genetic inactivation of opioid peptides, and by leukocyte depletion. This decrease in analgesia was restored by the transfer of wild-type, but not opioid receptor-lacking leukocytes. Ex vivo, exogenous opioids triggered secretion of opioid peptides from wild-type immune cells isolated from damaged nerves, which was diminished by blockade of Gαi/o or Gβγ (but not Gαs) proteins, by chelator of intracellular (but not extracellular) Ca(2+), by blockers of phospholipase C (PLC) and inositol 1,4,5-trisphosphate (IP3) receptors, and was partially attenuated by protein kinase C inhibitor. Similarly, the leukocyte depletion-induced decrease in exogenous opioid analgesia was re-established by transfer of immune cells ex vivo pretreated with extracellular Ca(2+) chelator, but was unaltered by leukocytes pretreated with intracellular Ca(2+) chelator or blockers of Gαi/o and Gβγ proteins. Thus, both ex vivo opioid peptide release and in vivo analgesia were mediated by leukocyte opioid receptors coupled to the Gαi/o-Gβγ protein-PLC-IP3 receptors-intracellular Ca(2+) pathway. Our findings suggest that opioid receptors in immune cells are important targets for the control of pathological pain.

Portraying G protein-coupled receptors with fluorescent ligands

The thermodynamics of ligand–receptor interactions at the surface of living cells represents a fundamental aspect of G protein-coupled receptor (GPCR) biology; thus, its detailed elucidation constitutes a challenge for modern pharmacology. Interestingly, fluorescent ligands have been developed for a variety of GPCRs in order to monitor ligand–receptor binding in living cells. Accordingly, new methodological strategies derived from noninvasive fluorescence-based approaches, especially fluorescence resonance energy transfer (FRET), have been successfully developed to characterize ligand–receptor interactions. Importantly, these technologies are supplanting more hazardous and expensive radioactive binding assays. In addition, FRET-based tools have also become extremely powerful approaches for visualizing receptor–receptor interactions (i.e., GPCR oligomerization) in living cells. Thus, by means of the synthesis of compatible fluorescent ligands these novel techniques can be implemented to demonstrate the existence of GPCR oligomerization not only in heterologous systems but also in native tissues. Finally, there is no doubt that these methodologies would also be relevant in drug discovery in order to develop new high-throughput screening approaches or to identify new therapeutic targets. Overall, herein, we provide a thorough assessment of all technical and biological aspects, including strengths and weaknesses, of these fluorescence-based methodologies when applied to the study of GPCR biology at the plasma membrane of living cells.

μ-Opioid inhibition of Ca2+ currents and secretion in isolated terminals of the neurohypophysis occurs via ryanodine-sensitive Ca2+ stores

μ-Opioid agonists have no effect on calcium currents (I(Ca)) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of I(Ca) is reliably demonstrated using the perforated-patch configuration. This suggests that the MOR-signaling pathway is sensitive to intraterminal dialysis and is therefore mediated by a readily diffusible second messenger. Using the perforated patch-clamp technique and ratio-calcium-imaging methods, we describe a diffusible second messenger pathway stimulated by the MOR that inhibits voltage-gated calcium channels in isolated terminals from the rat neurohypophysis (NH). Our results show a rise in basal intracellular calcium ([Ca(2+)]i) in response to application of [D-Ala(2)-N-Me-Phe(4),Gly5-ol]-Enkephalin (DAMGO), a MOR agonist, that is blocked by D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a MOR antagonist. Buffering DAMGO-induced changes in [Ca(2+)]i with BAPTA-AM completely blocked the inhibition of both I(Ca) and high-K(+)-induced rises in [Ca(2+)]i due to MOR activation, but had no effect on κ-opioid receptor (KOR)-mediated inhibition. Given the presence of ryanodine-sensitive stores in isolated terminals, we tested 8-bromo-cyclic adenosine diphosphate ribose (8Br-cADPr), a competitive inhibitor of cyclic ADP-ribose (cADPr) signaling that partially relieves DAMGO inhibition of I(Ca) and completely relieves MOR-mediated inhibition of high-K(+)-induced and DAMGO-induced rises in [Ca(2+)]i. Furthermore, antagonist concentrations of ryanodine completely blocked MOR-induced increases in [Ca(2+)]i and inhibition of I(Ca) and high-K(+)-induced rises in [Ca(2+)]i while not affecting KOR-mediated inhibition. Antagonist concentrations of ryanodine also blocked MOR-mediated inhibition of electrically-evoked increases in capacitance. These results strongly suggest that a key diffusible second messenger mediating the MOR-signaling pathway in NH terminals is [Ca(2+)]i released by cADPr from ryanodine-sensitive stores.

The effects of κ-opioid receptor on stretch-induced electrophysiological changes in infarcted rat hearts

INTRODUCTION

Kappa-opioid receptors (κ-OR) and mechanoelectric feedback seem to have common pathways that influence electrophysiological changes resulting from acute myocardial infarction (MI). This study aims to determine the effects of the κ-OR on stretch-induced electrophysiological changes after acute MI.

METHODS

Male Sprague-Dawley rats were randomly divided into 4 groups: sham operated, MI, U-50488H (a selective κ-OR agonist) -treated MI (MI+U-50488H) and nor-BNI (a selective κ-OR antagonist) -treated MI (MI+nor-BNI). After Langendorff perfusion to maintain stabilization, a transient stretch (5 seconds) was delivered early in diastole. Electrophysiological changes were recorded for 1 minute before and after stretch. Similarly, the 20%, 50% and 90% monophasic action potential duration (MAPD20, MAPD50 and MAPD90, respectively) and stretch-induced arrhythmias were recorded.

RESULTS

MAPD90 significantly increased in all 4 groups. MAPD90 in the MI and MI+nor-BNI groups increased significantly before stretch (P < 0.05) and after stretch (P < 0.01) but was reversed in the MI+U-50488H group (P > 0.05). MAPD90 in the MI group was increased compared with that of the MI+U-50488H group but decreased compared with that of the MI+ nor-BNI group after stretch (P < 0.01). The arrhythmia score in the MI and MI+nor-BNI groups was higher than that of the sham-operated group (P < 0.01), and the arrhythmia score in the MI+nor-BNI group was higher than that in MI group after stretch (P < 0.01). The arrhythmia score of the MI+U-50488H group was lower than that of MI group after stretch (P < 0.01).

CONCLUSIONS

The κ-OR could influence the stretch-induced electrophysiological changes and play an antiarrhythmic role in stretch-induced arrhythmias after acute MI.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Unidirectional cross-activation of GRPR by MOR1D uncouples itch and analgesia induced by opioids

Spinal opioid-induced itch, a prevalent side effect of pain management, has been proposed to result from pain inhibition. We now report that the μ-opioid receptor (MOR) isoform MOR1D is essential for morphine-induced scratching (MIS), whereas the isoform MOR1 is required only for morphine-induced analgesia (MIA). MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, relaying itch information. We show that morphine triggers internalization of both GRPR and MOR1D, whereas GRP specifically triggers GRPR internalization and morphine-independent scratching. Providing potential insight into opioid-induced itch prevention, we demonstrate that molecular and pharmacologic inhibition of PLCβ3 and IP3R3, downstream effectors of GRPR, specifically block MIS but not MIA. In addition, blocking MOR1D-GRPR association attenuates MIS but not MIA. Together, these data suggest that opioid-induced itch is an active process concomitant with but independent of opioid analgesia, occurring via the unidirectional cross-activation of GRPR signaling by MOR1D heterodimerization.

Alstonia scholaris R. Br. Significantly Inhibits Retinoid-Induced Skin Irritation In Vitro and In Vivo

Topical retinoids inhibit matrix metalloproteinases and accelerate collagen synthesis, thereby triggering antiaging effects in the skin. However, topical retinoids can cause severe skin reactions, including scaling, erythema, papules, and inflammation. The present study demonstrates that the ethanolic bark extract of Alstonia scholaris R. Br. can significantly inhibit all-trans retinoic acid-induced inflammation in human HaCat keratinocyte cells. Furthermore, two representative retinoid-induced proinflammatory cytokines, monocyte chemoattractant protein-1 and interleukin-8, were significantly suppressed by A. scholaris extract (by 82.1% and 26.3% at 100 ppm, and dose-dependently across the tested concentrations) in vitro. In a cumulative irritation patch test, A. scholaris extract decreased retinol-induced skin irritation, while strengthening the ability of retinoids to inhibit matrix metalloproteinase-1 expression, which is strongly associated with aging effects. These results suggest that A. scholaris is a promising compound that may increase the antiaging function of retinoids while reducing their ability to cause skin irritation.

Role of CD38, a cyclic ADP-ribosylcyclase, in morphine antinociception and tolerance

Our previous studies have demonstrated that an increase in intracellular levels of Ca(2+) in neurons is an important component of both the antinociception produced by morphine and morphine's tolerance. The present study tested the hypothesis that the Ca(2+) signaling second messenger, cyclic ADP-ribose (cADPR), derived from CD38 activation participates in morphine antinociception and tolerance. We first showed that morphine's antinociceptive potency was increased by the intracerebroventricular injection of CD38 substrate beta-NAD(+) in mice. Furthermore, morphine tolerance was reversed by intracerebroventricular administration of each of three different inhibitors of the CD38-cADPR-ryanodine receptor Ca(2+) signaling pathway. These inhibitors were the ADP-ribosylcyclase inhibitor nicotinamide, cADPR analog 8-bromo-cADPR, and a large dose of ryanodine (>50 muM) that blocks the ryanodine receptor. In CD38 gene knockout [CD38(-/-)] mice, the antinociceptive action of morphine was found to be less potent compared with wild-type (WT) mice, as measured by tail-flick response, hypothermia assay, and observations of straub tail. However, there was no difference in locomotor activation between CD38(-/-) and WT animals. It was also found that less tolerance to morphine developed in CD38(-/-) mice compared with WT animals. These results indicate that cADRP-ryanodine receptor Ca(2+) signaling associated with CD38 plays an important role in morphine tolerance.

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

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

Agonist-biased signaling at the sst2A receptor: the multi-somatostatin analogs KE108 and SOM230 activate and antagonize distinct signaling pathways

Somatostatin analogs that activate the somatostatin subtype 2A (sst2A) receptor are used to treat neuroendocrine cancers because they inhibit tumor secretion and growth. Recently, new analogs capable of activating multiple somatostatin receptor subtypes have been developed to increase tumor responsiveness. We tested two such multi-somatostatin analogs for functional selectivity at the sst2A receptor: SOM230, which activates sst1, sst2, sst3, and sst5 receptors, and KE108, which activates all sst receptor subtypes. Both compounds are reported to act as full agonists at their target sst receptors. In sst2A-expressing HEK293 cells, somatostatin inhibited cAMP production, stimulated intracellular calcium accumulation, and increased ERK phosphorylation. SOM230 and KE108 were also potent inhibitors of cAMP accumulation, as expected. However, they antagonized somatostatin stimulation of intracellular calcium and behaved as partial agonists/antagonists for ERK phosphorylation. In pancreatic AR42J cells, which express sst2A receptors endogenously, SOM230 and KE108 were both full agonists for cAMP inhibition. However, although somatostatin increased intracellular calcium and ERK phosphorylation, SOM230 and KE108 again antagonized these effects. Distinct mechanisms were involved in sst2A receptor signaling in AR42J cells; pertussis toxin pretreatment blocked somatostatin inhibition of cAMP accumulation but not the stimulation of intracellular calcium and ERK phosphorylation. Our results demonstrate that SOM230 and KE108 behave as agonists for inhibition of adenylyl cyclase but antagonize somatostatin's actions on intracellular calcium and ERK phosphorylation. Thus, SOM230 and KE108 are not somatostatin mimics, and their functional selectivity at sst2A receptors must be considered in clinical applications where it may have important consequences for therapy.

Receptor reserve reflects differential intrinsic efficacy associated with opioid diastereomers

Structure-activity relationships built around receptor binding or cell-based assays are designed to reveal physiochemical differences between ligands. We hypothesized that agonist receptor reserve may provide a unique approach to distinguish structurally-related agonists exhibiting similar functional characteristics. An intracellular calcium activation assay in Chinese Hamster Ovary (CHO) cells expressing cloned human mu-opioid receptors was developed. We examined two isomers exhibiting indistinguishable receptor binding and in vitro potency profiles. Oxymorphone, a clinically-available congener of codeine has at least two active diastereomeric metabolites (6alpha- and 6beta-oxymorphols) found to be similar for mu-opioid receptor binding affinity (K(d) = 15 versus 14 nM) and calcium activation (EC(50) = 22 versus 14 nM). Calcium activation was then inhibited in CHO cells in a concentration-dependent manner using the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Under these conditions, approximately 10-fold greater receptor reserve was found for 6alpha-oxymorphol compared to 6beta-oxymorphol. This difference between the oxymorphols corresponded to a rank order of intrinsic efficacy (Emax): DAMGO > oxymorphone = 6alpha-oxymorphol = oxycodone > 6beta-oxymorphol. In addition, 6alpha-oxymorphol exhibited greater relative potency than the 6beta-oxymorphol in mouse tail-flick, hot-plate and phenylquinone writhing antinociceptive assays, regardless of route of administration. Thus the beta-FNA/calcium model provides a novel, cell-based approach to distinguish structurally related mu-opioid agonists, and in the specific case of the oxymorphols, receptor reserve differences provided a means to bridge functional in vitro and in vivo models.

Expression and localization of the mu-opioid receptor (MOR) in the equine cumulus-oocyte complex and its involvement in the seasonal regulation of oocyte meiotic competence

The micro-opioid receptor (MOR) was identified in equine oocytes, cumulus and granulosa cells. By RT-PCR, a 441bp fragment was observed. By immunoblotting, a 65 kDa band was detected in samples of winter anestrous whereas in cells recovered in breeding season, two bands, 65 and 50 kDa, were found. The 65 kDa band was significantly more intense in winter anestrous specimens. In samples recovered in the breeding season, this band significantly decreased with the raise of follicle size and was heavier in compact oocytes and cumulus cells. The protein was localized on the oolemma and within the cytoplasm of oocytes and cumulus cells. In vitro oocyte maturation rate (MR), analyzed by confocal microscopy for nuclear chromatin, microfilaments and microtubules, was reduced after the addition of 3 x 10(-8) M beta-endorphin in medium without additional hormones. Inhibitory effects of 10(-3) M Naloxone in oocytes collected in anestrous and spring transition were observed, both in presence and absence of hormones added to culture medium. Increased MRs were observed in oocytes collected in anestrous and cultured in presence of 10(-8) M Naloxone. The exposure to 10(-3) M Naloxone induced significant intracellular calcium increases in cumulus cells recovered all over the year. beta-Endorphin 3 x 10(-8) M induced significant calcium increases only in cumulus cells recovered in fall transition and anestrous. Naloxone 10(-8) M did not induce intracellular calcium modifications. We conclude that the MOR is differentially expressed in equine cumulus-oocyte complexes in the different seasons of the year and plays a role in the seasonal regulation of meiotic competence of equine oocytes.

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

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

mu-Opioid agonists inhibit the enhanced intracellular Ca(2+) responses in inflammatory activated astrocytes co-cultured with brain endothelial cells

In order to imitate the in vivo situation with constituents from the blood-brain barrier, astrocytes from newborn rat cerebral cortex were co-cultured with adult rat brain microvascular endothelial cells. These astrocytes exhibited a morphologically differentiated appearance with long processes. 5-HT, synthetic mu-, delta- or kappa-opioid agonists, and the endogenous opioids endomorphin-1, beta-endorphin, and dynorphin induced higher Ca(2+) amplitudes and/or more Ca(2+) transients in these cells than in astrocytes in monoculture, as a sign of more developed signal transduction systems. Furthermore, stimulation of the co-cultured astrocytes with 5-HT generated a pronounced increase in intracellular Ca(2+) release in the presence of the inflammatory or pain mediating activators substance P, calcitonin gene-related peptide (CGRP), lipopolysaccharide (LPS), or leptin. These Ca(2+) responses were restored by opioids so that the delta- and kappa-opioid receptor agonists reduced the number of Ca(2+) transients elicited after incubation in substance P+CGRP or leptin, while the mu- and delta-opioid receptor agonists attenuated the Ca(2+) amplitudes elicited in the presence of LPS or leptin. In LPS treated co-cultured astrocytes the mu-opioid receptor antagonist naloxone attenuated not only the endomorphin-1, but also the 5-HT evoked Ca(2+) transients. These results suggest that opioids, especially mu-opioid agonists, play a role in the control of neuroinflammatory activity in astrocytes and that naloxone, in addition to its interaction with mu-opioid receptors, also may act through some binding site on astrocytes, other than the classical opioid receptor.

Glial-restricted precursors: patterns of expression of opioid receptors and relationship to human immunodeficiency virus-1 Tat and morphine susceptibility in vitro

Recent evidence suggests that human immunodeficiency virus (HIV)-induced pathogenesis is exacerbated by opioid abuse and that the synergistic toxicity may result from direct actions of opioids in immature glia or glial precursors. To assess whether opioids and HIV proteins are directly toxic to glial-restricted precursors (GRPs), we isolated neural stem cells from the incipient spinal cord of embryonic day 10.5 ICR mice. GRPs were characterized immunocytochemically and by reverse transcriptase-polymerase chain reaction (RT-PCR). At 1 day in vitro (DIV), GRPs failed to express mu opioid receptors (MOR or MOP) or kappa-opioid receptors (KOR or KOP); however, at 5 DIV, most GRPs expressed MOR and KOR. The effects of morphine (500 nM) and/or Tat (100 nM) on GRP viability were assessed in GRPs at 5 DIV by examining the apoptotic effector caspase-3 and cell viability (ethidium monoazide exclusion) at 96 h following continuous exposure. Tat or morphine alone or in combination caused significant increases in GRP cell death at 96 h, but not at 24 h, following exposure. Although morphine or Tat caused increases in caspase-3 activity at 4 h, this was not accompanied with increased cleaved caspase-3 immunoreactive or ethidium monoazide-positive dying cells at 24 h. The results indicate that prolonged morphine or Tat exposure is intrinsically toxic to isolated GRPs and/or their progeny in vitro. Moreover, MOR and KOR are widely expressed by Sox2 and/or Nkx2.2-positive GRPs in vitro and the pattern of receptor expression appears to be developmentally regulated. The temporal requirement for prolonged morphine and HIV-1 Tat exposure to evoke toxicity in glia may coincide with the attainment of a particular stage of maturation and/or the development of particular apoptotic effector pathways and may be unique to spinal cord GRPs. Should similar patterns occur in vivo then we predict that immature astroglia and oligodendroglia may be preferentially vulnerable to HIV-1 infection or chronic opiate exposure.

75 years of opioid research: the exciting but vain quest for the Holy Grail

Over the 75-year lifetime of the British Pharmacological Society there has been an enormous expansion in our understanding of how opioid drugs act on the nervous system, with much of this effort aimed at developing powerful analgesic drugs devoid of the side effects associated with morphine--the Holy Grail of opioid research. At the molecular and cellular level multiple opioid receptors have been cloned and characterised, their potential for oligomerisation determined, a large family of endogenous opioid agonists has been discovered, multiple second messengers identified and our understanding of the adaptive changes to prolonged exposure to opioid drugs (tolerance and physical dependence) enhanced. In addition, we now have greater understanding of the processes by which opioids produce the euphoria that gives rise to the intense craving for these drugs in opioid addicts. In this article, we review the historical pathway of opioid research that has led to our current state of knowledge.

Local analgesic efficacy of tramadol following intraplantar injection

Several studies have suggested that systemic tramadol, an opioid, can represent a valuable treatment in severe pain conditions because of their effects on central pain pathways. However, there are not enough studies supporting that tramadol is efficacious when administered locally. Therefore, we studied the potential local analgesic effects of tramadol in peripheral nociception. In addition, we tested the antinociceptive effects of tramadol-CaCl(2) or naloxone combinations after subcutaneous intraplantar injection in a validated rat model of acute thermal nociception. Local analgesic effects of tramadol were compared with those of lidocaine. The effects of tramadol on thermal paw withdrawal latencies were monitored using the plantar test. The antinociceptive potency of tramadol is higher and long-lasting than that of lidocaine. Naloxone was unable to inhibit the increased antinociceptive response produced by tramadol. Ca(2+) modified the effect of tramadol. When Ca(2+) dose was increased in the solution, thermal antinociceptive potency of tramadol, but not lidocaine was prolonged. Thermal nociceptive responses were not affected in the non-injected paws, indicating a lack of systemic effects with doses of tramadol and lidocaine that elicited local analgesia. These results suggest that intraplantar tramadol administration can produce local analgesic effect with a different action mechanism than that of lidocaine. In addition, extracellular Ca(2+) may play an important role in the local analgesic action of tramadol.

Sleep and GABA levels in the oral part of rat pontine reticular formation are decreased by local and systemic administration of morphine

Morphine, a mu-opioid receptor agonist, is a commonly prescribed treatment for pain. Although highly efficacious, morphine has many unwanted side effects including disruption of sleep and obtundation of wakefulness. One mechanism by which morphine alters sleep and wakefulness may be by modulating GABAergic signaling in brain regions regulating arousal, including the pontine reticular nucleus, oral part (PnO). This study used in vivo microdialysis in unanesthetized Sprague-Dawley rat to test the hypothesis that mu-opioid receptors modulate PnO GABA levels. Validation of the high performance liquid chromatographic technique used to quantify GABA was obtained by dialyzing the PnO (n=4 rats) with the GABA reuptake inhibitor nipecotic acid (500 microM). Nipecotic acid caused a 185+/-20% increase in PnO GABA levels, confirming chromatographic detection of GABA and demonstrating the existence of functional GABA transporters in rat PnO. Morphine caused a concentration-dependent decrease in PnO GABA levels (n=25 rats). Coadministration of morphine (100 microM) with naloxone (1 microM), a mu-opioid receptor antagonist, blocked the morphine-induced decrease in PnO GABA levels (n=5 rats). These results show for the first time that mu-opioid receptors in rat PnO modulate GABA levels. A second group of rats (n=6) was used to test the hypothesis that systemically administered morphine also decreases PnO GABA levels. I.v. morphine caused a significant (P<0.05) decrease (19%) in PnO GABA levels relative to control i.v. infusions of saline. Finally, microinjections followed by 2 h recordings of electroencephalogram and electromyogram tested the hypothesis that PnO morphine administration disrupts sleep (n=8 rats). Morphine significantly (P<0.05) increased the percent of time spent in wakefulness (65%) and significantly (P<0.05) decreased the percent of rapid eye movement (REM) sleep (-53%) and non-REM sleep (-69%). The neurochemical and behavioral data suggest that morphine may disrupt sleep, at least in part, by decreasing GABAergic transmission in the PnO.