After chronic opioid exposure sensory neurons become supersensitive to the excitatory effects of opioid agonists and antagonists as occurs after acute elevation of GM1 ganglioside

Proenkephalin 119-159 in Heart Failure: From Pathophysiology to Clinical Implications

Heart failure (HF) is a challenging clinical syndrome with high morbidity and mortality rates. Along the spectrum of cardiovascular diseases, HF constitutes an ever-expanding area of research aiming at combating the associated mortality and improving the prognosis of patients with HF. Although natriuretic peptides have an established role among biomarkers in HF diagnosis and prognosis, several novel biomarkers reflecting the complex pathophysiology of HF are under investigation for their ability to predict adverse clinical outcomes in HF. Proenkephalin 119-159 (PENK119-159) is a non-functional peptide belonging to the enkephalin family of the endogenous opioid system and is considered a surrogate biomarker of the biologically active enkephalin peptides. PENK119-159 has demonstrated promising results in predicting short- and long-term mortality, readmission rates, and worsening renal function in patients with HF. Indeed, in the setting of HF, the levels of both active enkephalins and their surrogate PENK119-159 are elevated and are associated with a dismal prognosis. However, the biological effects of PENK119-159 remain largely unknown. Thus, it is crucial to gain a deeper insight into both the physiology of the enkephalin peptide family and the enkephalin-mediated cardiovascular regulation. In order to elucidate the complex pathophysiological mechanisms that lead to the upregulation of enkephalins in patients with HF, as well as the potential clinical implications of elevated enkephalins and PENK119-159 levels in this patient population, the present review will describe the physiology and distribution of the endogenous opioid peptides and their corresponding opioid receptors, with a particular focus on the action of enkephalins. The effects of the enkephalin peptides will be analyzed in both healthy subjects and patients with HF, especially with regard to their role in the regulation of cardiovascular and renal function. The review will also discuss the findings of recent studies that have explored the prognostic value of PENK119-159 in patients with HF.

Ganglioside GM1 and the Central Nervous System

GM1 is one of the major glycosphingolipids (GSLs) on the cell surface in the central nervous system (CNS). Its expression level, distribution pattern, and lipid composition are dependent upon cell and tissue type, developmental stage, and disease state, which suggests a potentially broad spectrum of functions of GM1 in various neurological and neuropathological processes. The major focus of this review is the roles that GM1 plays in the development and activities of brains, such as cell differentiation, neuritogenesis, neuroregeneration, signal transducing, memory, and cognition, as well as the molecular basis and mechanisms for these functions. Overall, GM1 is protective for the CNS. Additionally, this review has also examined the relationships between GM1 and neurological disorders, such as Alzheimer's disease, Parkinson's disease, GM1 gangliosidosis, Huntington's disease, epilepsy and seizure, amyotrophic lateral sclerosis, depression, alcohol dependence, etc., and the functional roles and therapeutic applications of GM1 in these disorders. Finally, current obstacles that hinder more in-depth investigations and understanding of GM1 and the future directions in this field are discussed.

Positive allosteric modulation of the cannabinoid type-1 receptor (CB1R) in periaqueductal gray (PAG) antagonizes anti-nociceptive and cellular effects of a mu-opioid receptor agonist in morphine-withdrawn rats

Opioid drugs are a first-line treatment for severe acute pain and other chronic pain conditions, but long-term opioid drug use produces opioid-induced hyperalgesia (OIH). Co-administration of cannabinoids with opioid receptor agonists produce anti-nociceptive synergy, but cannabinoid receptor agonists may also produce undesirable side effects. Therefore, positive allosteric modulators (PAM) of cannabinoid type-1 receptors (CB1R) may provide an option reducing pain and/or enhancing the anti-hyperalgesic effects of opioids without the side effects, tolerance, and dependence observed with the use of ligands that target the orthosteric binding sites. This study tested GAT211, a PAM of cannabinoid type-1 receptors (CB1R), for its ability to enhance the anti-hyperalgesic effects of the mu-opioid receptor (MOR) agonist DAMGO in rats treated chronically with morphine (or saline) and tested during withdrawal. We tested the effects of intra-periaqueductal gray (PAG) injections of (1) DAMGO, (2) GAT211, or (3) DAMGO + GAT211 on thermal nociception in chronic morphine-treated rats that were hyperalgesic and also in saline-treated control rats. We used slice electrophysiology to test the effects of DAMGO/GAT211 bath application on synaptic transmission in the vlPAG. Intra-PAG DAMGO infusions dose-dependently reversed chronic morphine-induced hyperalgesia, but intra-PAG GAT211 did not alter nociception at the doses we tested. When co-administered into the PAG, GAT211 antagonized the anti-nociceptive effects of DAMGO in morphine-withdrawn rats. DAMGO suppressed synaptic inhibition in the vlPAG of brain slices taken from saline- and morphine-treated rats, and GAT211 attenuated DAMGO-induced suppression of synaptic inhibition in vlPAG neurons via actions at CB1R. These findings show that positive allosteric modulation of CB1R antagonizes the behavioral and cellular effects of a MOR agonist in the PAG of rats.

Delta Opioid Receptors and Cardioprotection

The opioid receptor family, with associated endogenous ligands, has numerous roles throughout the body. Moreover, the delta opioid receptor (DORs) has various integrated roles within the physiological systems, including the cardiovascular system. While DORs are important modulators of cardiovascular autonomic balance, they are well-established contributors to cardioprotective mechanisms. Both endogenous and exogenous opioids acting upon DORs have roles in myocardial hibernation and protection against ischaemia-reperfusion (I-R) injury. Downstream signalling mechanisms governing protective responses alternate, depending on the timing and duration of DOR activation. The following review describes models and mechanisms of DOR-mediated cardioprotection, the impact of co-morbidities and challenges for clinical translation.

Systemic morphine treatment induces changes in firing patterns and responses of nociceptive afferent fibers in mouse glabrous skin

Patients receiving opioids for pain may experience decreased effectiveness of the drug and even abnormal pain sensitivity-hyperalgesia and/or allodynia. We hypothesized that peripheral nociceptor hyperexcitability contributes to opioid-induced hyperalgesia and tested this using an in vitro mouse glabrous skin-nerve preparation. Mice were injected intraperitoneally with escalating doses of morphine (5, 8, 10, 15 mg/kg) or saline every 12 hours for 48 hours and killed approximately 12 hours after the last injection. Receptive fields of nociceptors were tested for mechanical, heat, and cold sensitivity. Activity was also measured during an initial 2-minute period and during 5-minute periods between stimuli. Aberrant activity was common in fibers from morphine-treated mice but rare in saline-treated mice. Resting background activity was elevated in C-fibers from morphine-treated mice. Both C- and Aδ-fibers had afterdischarge in response to mechanical, heat, and/or cold stimulation of the skin as well as spontaneous, unevoked activity. Compared to saline, morphine treatment increased the proportion of fibers displaying polymodal rather than mechanical-only responses. A significant increase in Aδ-mechanoreceptive fibers responding to cold accounted for most of this change. In agreement with this, morphine-treated mice showed increased sensitivity in the cold tail flick test. In morphine-treated mice, aberrant activity and hyperexcitability of nociceptors could contribute to increased pain sensitivity. Importantly, this activity is likely driving central sensitization, a phenomenon contributing to abnormal sensory processing and chronic pain. If similar changes occur in human patients, aberrant nociceptor activity is likely to be interpreted as pain and could contribute to opioid-induced hyperalgesia.

Tolerance and withdrawal from prolonged opioid use in critically ill children

OBJECTIVE

After prolonged opioid exposure, children develop opioid-induced hyperalgesia, tolerance, and withdrawal. Strategies for prevention and management should be based on the mechanisms of opioid tolerance and withdrawal.

PATIENTS AND METHODS

Relevant manuscripts published in the English language were searched in Medline by using search terms "opioid," "opiate," "sedation," "analgesia," "child," "infant-newborn," "tolerance," "dependency," "withdrawal," "analgesic," "receptor," and "individual opioid drugs." Clinical and preclinical studies were reviewed for data synthesis.

RESULTS

Mechanisms of opioid-induced hyperalgesia and tolerance suggest important drug- and patient-related risk factors that lead to tolerance and withdrawal. Opioid tolerance occurs earlier in the younger age groups, develops commonly during critical illness, and results more frequently from prolonged intravenous infusions of short-acting opioids. Treatment options include slowly tapering opioid doses, switching to longer-acting opioids, or specifically treating the symptoms of opioid withdrawal. Novel therapies may also include blocking the mechanisms of opioid tolerance, which would enhance the safety and effectiveness of opioid analgesia.

CONCLUSIONS

Opioid tolerance and withdrawal occur frequently in critically ill children. Novel insights into opioid receptor physiology and cellular biochemical changes will inform scientific approaches for the use of opioid analgesia and the prevention of opioid tolerance and withdrawal.

Supraspinal Gbetagamma-dependent stimulation of PLCbeta originating from G inhibitory protein-mu opioid receptor-coupling is necessary for morphine induced acute hyperalgesia

Although alterations in micro-opioid receptor (microOR) signaling mediate excitatory effects of opiates in opioid tolerance, the molecular mechanism for the excitatory effect of acute low dose morphine, as it relates to microOR coupling, is presently unknown. A pronounced coupling of microOR to the alpha subunit of G inhibitory protein emerged in periaqueductal gray (PAG) from mice systemically administered with morphine at a dose producing acute thermal hyperalgesia. This coupling was abolished in presence of the selective microOR antagonist d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) administered at the PAG site, showing that the low dose morphine effect is triggered by microOR activated G inhibitory protein at supraspinal level. When Gbetagamma downstream signalling was blocked by intra-PAG co-administration of 2-(3,4,5-trihydroxy-6-oxoxanthen-9-yl)cyclohexane-1-carboxylic acid, a compound that inhibits Gbetagamma dimer-dependent signaling, a complete prevention of low dose morphine induced acute thermal hyperalgesia was obtained. Phospholipase C beta3, an enzyme necessary to morphine hyperalgesia, was revealed to be associated with Gbetagamma in PAG. Although opioid administration induces a shift in microOR-G protein coupling from Gi to Gs after chronic administration, our data support that this condition is not realized in acute treatment providing evidence that a separate molecular mechanism underlies morphine induced acute excitatory effect.

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.

Low doses of cyclic AMP-phosphodiesterase inhibitors rapidly evoke opioid receptor-mediated thermal hyperalgesia in naïve mice which is converted to prominent analgesia by cotreatment with ultra-low-dose naltrexone

Systemic (s.c.) injection in naïve mice of cyclic AMP-phosphodiesterase (cAMP-PDE) inhibitors, e.g. 3-isobutyl-1-methylxanthine [(IBMX) or caffeine, 10 mg/kg] or the more specific cAMP-PDE inhibitor, rolipram (1 mug/kg), rapidly evokes thermal hyperalgesia (lasting >5 h). These effects appear to be mediated by enhanced excitatory opioid receptor signaling, as occurs during withdrawal in opioid-dependent mice. Cotreatment of these mice with ultra-low-dose naltrexone (NTX, 0.1 ng/kg-1 pg/kg, s.c.) results in prominent opioid analgesia (lasting >4 h) even when the dose of rolipram is reduced to 1 pg/kg. Cotreatment of these cAMP-PDE inhibitors in naïve mice with an ultra-low-dose (0.1 ng/kg) of the kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI) or the mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA) also results in opioid analgesia. These excitatory effects of cAMP-PDE inhibitors in naïve mice may be mediated by enhanced release of small amounts of endogenous bimodally-acting (excitatory/inhibitory) opioid agonists by neurons in nociceptive networks. Ultra-low-dose NTX, nor-BNI or beta-FNA selectively antagonizes high-efficacy excitatory (hyperalgesic) Gs-coupled opioid receptor-mediated signaling in naïve mice and results in rapid conversion to inhibitory (analgesic) Gi/Go-coupled opioid receptor-mediated signaling which normally requires activation by much higher doses of opioid agonists. Cotreatment with a low subanalgesic dose of kelatorphan, an inhibitor of multiple endogenous opioid peptide-degrading enzymes, stabilizes endogenous opioid agonists released by cAMP-PDE inhibitors, resulting in conversion of the hyperalgesia to analgesia without requiring selective blockade of excitatory opioid receptor signaling. The present study provides a novel pharmacologic paradigm that may facilitate development of valuable non-narcotic clinical analgesics utilizing cotreatment with ultra-low-dose rolipram plus ultra-low-dose NTX or related agents.

Cholinergic location of δ-opioid receptors in canine atria and SA node

δ-Opioid receptors (DORs) are associated with ischemic preconditioning and vagal transmission in the sinoatrial (SA) node and atria. Although functional studies suggested that DORs are prejunctional on parasympathetic nerve terminals, their precise location remains unconfirmed. DORs were colocalized in tissue slices and synaptosomes from the canine right atrium and SA node along with cholinergic and adrenergic markers, vesicular acetylcholine transporter (VAChT), and tyrosine hydroxylase (TH). Synapsin I immunofluorescence verified the neural character of tissue structures and isolated synaptosomes. Acetylcholine and norepinephrine measurements suggested the presence of both cholinergic and adrenergic synaptosomes. Fluorescent analysis of VAChT and TH signals indicated that >80% of the synapsin-positive synaptosomes were of cholinergic origin and <8% were adrenergic. DORs colocalized 75–85% with synapsin in tissue slices from both atria and SA node. The colocalization was equally strong (85%) for nodal synaptosomes but less so for atrial synaptosomes (57%). Colocalization between DOR and VAChT was 75–85% regardless of the source. Overlap between DOR and TH was uniformly low, ranging from 8% to 17%. Western blots with synaptosomal extracts confirmed two DOR-positive bands at molecular masses corresponding to those reported for DOR monomers and dimers. The abundance of DOR was greater in nodal synaptosomes than in atrial synaptosomes, largely attributable to a greater abundance of monomers in the SA node. The abundant nodal and atrial DORs predominantly associated with cholinergic nerve terminals support the hypothesis that prejunctional DORs regulate vagal transmission locally within the heart.

Cannabinoid-induced tolerance is associated with a CB1 receptor G protein coupling switch that is prevented by ultra-low dose rimonabant

The analgesic effect of opioids is enhanced, and tolerance is attenuated, by ultra-low doses (nanomolar to picomolar) of an opioid antagonist, an effect that is mediated by preventing the receptor from coupling to Gs proteins. Recently, we demonstrated a cannabinoid-opioid interaction at the ultra-low dose level, suggesting that the effect might not be specific to opioid receptors. The purpose of this study was to examine, both behaviorally and mechanistically, whether the cannabinoid CB1 receptor was also sensitive to ultra-low dose effects. Antinociception was tested in rats after an injection of either vehicle, the CB1 receptor agonist WIN 55 212-2 (WIN), an ultra-low dose of the CB1 receptor antagonist rimonabant (SR 141716), or a combination of WIN and the ultra-low-dose rimonabant. In the acute experiment, tail-flick latencies were recorded at 10-min intervals for 90 min; in the chronic experiment, tail-flick latencies were recorded 10 min after a daily injection over 7 days. Ultra-low dose rimonabant extended the duration of WIN-induced antinociception. WIN produced maximal tolerance by day 7, whereas WIN+ultra-low dose rimonabant continued to produce strong antinociception, demonstrating that ultra-low dose rimonabant prevented the development of WIN-induced tolerance. Animals chronically treated with WIN alone had CB1 receptors predominantly coupling to Gs receptors in the striatum, whereas the vehicle, ultra-low dose rimonabant, and WIN+ultra-low dose rimonabant groups had CB1 receptors predominantly coupling to Gi receptors. Cannabinoid-induced tolerance is thus associated with a G protein coupling switch from the inhibitory Gi protein to the excitatory Gs protein, an effect which is prevented by the ultra-low dose rimonabant.

The narcotic bowel syndrome: clinical features, pathophysiology, and management

Narcotic bowel syndrome (NBS) is a subset of opioid bowel dysfunction that is characterized by chronic or frequently recurring abdominal pain that worsens with continued or escalating dosages of narcotics. This syndrome is underrecognized and may be becoming more prevalent. In the United States this may be the result of increases in using narcotics for chronic nonmalignant painful disorders, and the development of maladaptive therapeutic interactions around its use. NBS can occur in patients with no prior gastrointestinal disorder who receive high dosages of narcotics after surgery or acute painful problems, and among patients with functional gastrointestinal disorders or other chronic gastrointestinal diseases who are managed by physicians who are unaware of the hyperalgesic effects of chronic opioids. The evidence for the enhanced pain perception is based on the following: (1) activation of excitatory antianalgesic pathways within a bimodal opioid regulation system, (2) descending facilitation of pain at the rostral ventral medulla and pain facilitation via dynorphin and cholecystokinin activation, and (3) glial cell activation that produces morphine tolerance and enhances opioid-induced pain. Treatment involves early recognition of the syndrome, an effective physician-patient relationship, graded withdrawal of the narcotic according to a specified withdrawal program, and the institution of medications to reduce withdrawal effects.

Naloxone rapidly evokes endogenous kappa opioid receptor-mediated hyperalgesia in naïve mice pretreated briefly with GM1 ganglioside or in chronic morphine-dependent mice

Low-dose naloxone-precipitated withdrawal hyperalgesia is a reliable indicator of physical dependence after chronic morphine treatment. A remarkably similar long-lasting (>3-4 h) hyperalgesia is evoked by injection of a low dose of naloxone (10 microg/kg, s.c.) in naïve mice after acute pretreatment with the glycolipid, GM1 ganglioside (1 mg/kg) (measured by warm-water-immersion tail-flick assays). GM1 treatment markedly increases the efficacy of excitatory Gs-coupled opioid receptor signaling in nociceptive neurons. Co-treatment with an ultra-low-dose (0.1 ng/kg, s.c.) of the broad-spectrum opioid receptor antagonist, naltrexone or the selective kappa opioid receptor antagonist, nor-binaltorphimine, blocks naloxone-evoked hyperalgesia in GM1-pretreated naïve mice and unmasks prominent, long-lasting (>4 h) inhibitory opioid receptor-mediated analgesia. This unmasked analgesia can be rapidly blocked by injection after 1-2 h of a high dose of naltrexone (10 mg/kg) or nor-binaltorphimine (0.1 mg/kg). Because no exogenous opioid is administered to GM1-treated mice, we suggest that naloxone may evoke hyperalgesia by inducing release of endogenous bimodally acting opioid agonists from neurons in nociceptive networks by antagonizing putative presynaptic inhibitory opioid autoreceptors that "gate" the release of endogenous opioids. In the absence of exogenous opioids, the specific pharmacological manipulations utilized in our tail-flick assays on GM1-treated mice provide a novel bioassay to detect the release of endogenous bimodally acting (excitatory/inhibitory) opioid agonists. Because mu excitatory opioid receptor signaling is blocked by ultra-low doses of naloxone, the higher doses of naloxone that evoke hyperalgesia in GM1-treated mice cannot be mediated by activation of mu opioid receptors. Co-treatment with ultra-low-dose naltrexone or nor-binaltorphimine may selectively block signaling by endogenous GM1-sensitized excitatory kappa opioid receptors, unmasking inhibitory kappa opioid receptor signaling, and converting endogenous opioid receptor-mediated hyperalgesia to analgesia. Co-treatment with kelatorphan stabilizes putative endogenous opioid peptide agonists released by naloxone in GM1-treated mice, so that analgesia is evoked rather than hyperalgesia. Acute treatment of chronic morphine-dependent mice with ultra-low-dose naltrexone (0.1 ng/kg) results in remarkably similar rapid blocking of naloxone (10 microg/kg)-precipitated withdrawal hyperalgesia and unmasking of prominent opioid analgesia. These studies may clarify complex mechanisms underlying opioid physical dependence and opioid addiction.

Gbetagamma that interacts with adenylyl cyclase in opioid tolerance originates from a Gs protein

We previously demonstrated that chronic morphine induces a change in G protein coupling by the mu opioid receptor (MOR) from Gi/o to Gs, concurrent with the instatement of an interaction between Gbetagamma and adenylyl cyclase types II and IV. These two signaling changes confer excitatory effects on the cell in place of the typical inhibition by opioids and are associated with morphine tolerance and dependence. Both signaling changes and these behavioral manifestations of chronic morphine are attenuated by cotreatment with ultra-low-dose naloxone. In the present work, using striatum from chronic morphine-treated rats, we isotyped the Gbeta within Gs and Go heterotrimers that coupled to MOR and compared these to the Gbeta isotype of the Gbetagamma that interacted with adenylyl cyclase II or IV after chronic morphine treatment. Isotyping results show that chronic morphine causes a Gs heterotrimer associated with MOR to release its Gbetagamma to interact with adenylyl cyclase. These data suggest that the switch to Gs coupling by MOR in response to chronic morphine, which is attenuated by ultra-low-dose opioid antagonist cotreatment, leads to a two-pronged stimulation of adenylyl cyclase utilizing both Galpha and Gbetagamma subunits of the Gs protein novel to this receptor.

Repeated δ1-opioid receptor stimulation reduces δ2-opioid receptor responses in the SA node

Ultra-low-dose methionine-enkephalin-arginine-phenylalanine improves vagal transmission (vagotonic) and decreases heart rate via δ1-opioid receptors within the sinoatrial (SA) node. Higher doses activate δ2-opioid receptors, interrupt vagal transmission (vagolytic), and reduce the bradycardia. Preconditioning-like occlusion of the nodal artery produced a vagotonic response that was reversed by the δ1-antagonist 7-benzylidenaltrexone (BNTX). The following study tested the hypothesis that extended δ1-opioid receptor stimulation reduces subsequent δ2-receptor responses. The δ2-agonist deltorphin II was introduced in the SA node by microdialysis to evaluate δ2 responses before and after infusion of the δ1-agonist TAN-67. TAN-67 reduced the vagolytic effect of deltorphin by two-thirds. When the δ1-antagonist BNTX was combined with TAN-67, the deltorphin response was preserved, suggesting that attrition of the prior response was mediated by δ1 activity. When TAN-67 was omitted in time control studies, some loss of δ2 responses was apparent in the absence of the δ1 treatment. This loss was also eliminated by BNTX, suggesting that the attenuation of the response after deltorphin alone was also the result of δ1 activity. Additional studies tested TAN-67 alone in the absence of prior deltorphin. When time controls were conducted without the initial deltorphin treatment, a robust vagolytic response was observed. When TAN-67 preceded the delayed deltorphin, the vagolytic response was eroded, indicating an independent effect of TAN-67. BNTX infused afterward was unable to restore the δ2 response. These data support the conclusion that the loss of the δ2 response resulted from reduced δ2 activity mediated by continued δ1-receptor stimulation and not the arithmetic consequence of increased competition from that same δ1 receptor.

Cardioprotective effects of acute and chronic opioid treatment are mediated via different signaling pathways

A 5-day exposure to morphine exerts a profound cardioprotective phenotype in murine hearts. In the present study, we examined mechanisms by which morphine generates this effect, exploring the roles of Gi and Gs proteins, PKA, PKC, and β-adrenergic receptors (β-AR) in acute and chronic opioid preconditioning. Langendorff-perfused hearts from placebo, acute morphine (AM; 10 μmol/l)-, or chronic morphine (CM)-treated mice (75-mg pellet, 5 days) underwent 25-min ischemia and 45-min reperfusion. After reperfusion, placebo-treated hearts exhibited marked contractile and diastolic dysfunction [rate-pressure product (RPP), 40 ± 4% baseline; end-diastolic pressure (EDP), 33 ± 3 mmHg], whereas AM hearts showed significant improvement in recovery of RPP and EDP (60 ± 3% and 23 ± 4 mmHg, respectively; P < 0.05 vs. placebo). Furthermore, CM hearts demonstrated a complete return of diastolic function and significantly greater recovery of contractile function (83 ± 3%, P < 0.05 vs. both placebo and AM). Pretreatment with Gi protein inhibitor pertussis toxin abolished AM protection while partially attenuating CM recovery ( P < 0.05 vs. placebo). Treatment with Gs inhibitor NF-449 did not affect AM preconditioning yet completely abrogated CM preconditioning. Similarly, PKA inhibition significantly attenuated the ischemia-tolerant state afforded by CM, whereas it was ineffective in AM hearts. PKC inhibition with chelerythrine was ineffective in CM hearts while completely abrogating AM preconditioning. Moreover, whereas β1-AR blockade with CGP-20712A failed to alter recovery in CM hearts, the β2-AR antagonist ICI-118,551 significantly attenuated postischemic recovery. These data describe novel findings whereby CM preconditioning is mediated by a PKC-independent pathway involving PKA, β2-AR, and Gs proteins, whereas AM preconditioning is mediated via Gi proteins and PKC.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

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.

Amitriptyline suppresses neuroinflammation and up-regulates glutamate transporters in morphine-tolerant rats

The present study was performed to evaluate the effects of the tricyclic antidepressant amitriptyline on morphine tolerance in rats. Male Wistar rats were implanted with two intrathecal (i.t.) catheters with or without a microdialysis probe, then received a continuous i.t. infusion of saline (control) or morphine (15 microg/h) and/or amitriptyline (15 microg/h) for 5 days. The results showed that amitriptyline alone did not produce an antinociceptive effect, while morphine alone induced antinociceptive tolerance and down-regulation of spinal glutamate transporters (GLAST, GLT-1, and EAAC1) in the rat spinal cord dorsal horn. Co-administration of amitriptyline with morphine attenuated morphine tolerance and up-regulated GLAST and GLT-1 expression. On day 5, morphine challenge (10 microg/10 microl) resulted in a significant increase in levels of the excitatory amino acids (EAAs), aspartate and glutamate, in CSF dialysates in morphine-tolerant rats. Amitriptyline co-infusion not only markedly suppressed this morphine-evoked EAA release, but also preserved the antinociceptive effect of acute morphine challenge at the end of infusion. Glial cells activation and increased cytokine expression (TNFalpha, IL-1beta, and IL-6) in the rat spinal cord were induced by the 5-day morphine infusion and these neuroimmune responses were also prevented by amitriptyline co-infusion. These results show that amitriptyline not only attenuates morphine tolerance, but also preserves its antinociceptive effect. The mechanisms involved may include: (a) inhibition of pro-inflammatory cytokine expression, (b) prevention of glutamate transporter down-regulation, and even up-regulation of glial GTs GLAST and GLT-1 expression, with (c) attenuation of morphine-evoked EAA release following continuous long-term morphine infusion.

Influence of low doses of naltrexone on morphine antinociception and morphine tolerance in male and female rats of four strains

In a recently proposed bimodal opioid receptor model, the inhibitory actions of opioids on action potential duration in dorsal root ganglion neurons have been proposed to produce antinociception, and the excitatory actions of hyperalgesia. Recent studies indicate that selectively blocking these excitatory actions with low doses of opioid antagonists enhances opioid antinociception and attenuates the development of opioid tolerance. To determine if the excitatory actions of opioids contribute to sex as well as strain differences in opioid sensitivity, the effects of morphine alone and in combination with low doses of naltrexone were examined in male and female rats of four strains. The strains examined differed in their sensitivity to opioid antinociception and magnitude of sex differences in opioid sensitivity. All testing was conducted using a thermal tail-flick procedure with the nociceptive stimulus intensity adjusted so that baseline latencies were comparable across strains/sexes. In chronic studies, the morphine dosing regimen was adjusted in each strain/sex to produce comparable levels of tolerance. In each of the strains tested, morphine produced dose-dependent increases in antinociception, with differences in morphine potency observed across strains and sexes. In male and female Sprague-Dawley and Long-Evans rats, naltrexone enhanced morphine antinociception and attenuated the development of morphine tolerance. These effects were not observed in F344 and Lewis rats, even when tests were conducted across a range of morphine and naltrexone doses. These results suggest that the ability of low doses of naltrexone to enhance opioid antinociception does not contribute to sex or rat strain differences in opioid sensitivity.

Ultra-low-dose naloxone suppresses opioid tolerance, dependence and associated changes in mu opioid receptor-G protein coupling and Gbetagamma signaling

Opiates produce analgesia by activating mu opioid receptor-linked inhibitory G protein signaling cascades and related ion channel interactions that suppress cellular activities by hyperpolarization. After chronic opiate exposure, an excitatory effect emerges contributing to analgesic tolerance and opioid-induced hyperalgesia. Ultra-low-dose opioid antagonist co-treatment blocks the excitatory effects of opiates in vitro, as well as opioid analgesic tolerance and dependence, as was demonstrated here with ultra-low-dose naloxone combined with morphine. While the molecular mechanism for the excitatory effects of opiates is unclear, a switch in the G protein coupling profile of the mu opioid receptor and adenylyl cyclase activation by Gbetagamma have both been suggested. Using CNS regions from rats chronically treated with vehicle, morphine, morphine+ultra-low-dose naloxone or ultra-low-dose naloxone alone, we examined whether altered mu opioid receptor coupling to G proteins or adenylyl cyclase activation by Gbetagamma occurs after chronic opioid treatment. In morphine-naïve rats, mu opioid receptors coupled to Go in striatum and to both Gi and Go in periaqueductal gray and spinal cord. Although chronic morphine decreased Gi/o coupling by mu opioid receptors, a pronounced coupling to Gs emerged coincident with a Gbetagamma interaction with adenylyl cyclase types II and IV. Co-treatment with ultra-low-dose naloxone attenuated both the chronic morphine-induced Gs coupling and the Gbetagamma signaling to adenylyl cyclase, while increasing Gi/o coupling toward or beyond vehicle control levels. These findings provide a molecular mechanism underpinning opioid tolerance and dependence and their attenuation by ultra-low-dose opioid antagonists.

Ultra-low-dose naltrexone reduces the rewarding potency of oxycodone and relapse vulnerability in rats

Ultra-low-dose opioid antagonists have been shown to enhance opioid analgesia and alleviate opioid tolerance and dependence. Our present studies in male Sprague-Dawley rats assessed the abuse potential of oxycodone+ultra-low-dose naltrexone (NTX) versus oxycodone alone. The lowest NTX dose (1 pg/kg/infusion), but not slightly higher doses (10 and 100 pg/kg/infusion), enhanced oxycodone (0.1 mg/kg/infusion) intravenous self-administration, suggesting a reduced rewarding potency per infusion. During tests of reinstatement performed in extinction conditions, co-self-administration of any of these three NTX doses significantly reduced drug-seeking precipitated by priming injections of oxycodone (0.25 mg/kg, s.c.), a drug-conditioned cue, or foot-shock stress. During self-administration on a progressive-ratio schedule, animals self-administering oxycodone (0.1 mg/kg/infusion)+NTX (1 pg/kg/infusion) reached a "break-point" sooner and showed a trend toward less responding compared to rats self-administering oxycodone alone (0.1 mg/kg/infusion). In the final experiment, the addition of ultra-low-dose NTX (10 pg/kg, s.c.) enhanced the acute stimulatory effect of oxycodone (1 mg/kg, s.c.), as well as locomotor sensitization produced by repeated oxycodone administration (7 x 1 mg/kg, s.c.). In summary, this work shows that ultra-low-dose NTX co-treatment augments the locomotor effects of oxycodone as it enhances opioid analgesia, but reduces oxycodone's rewarding potency and subsequent vulnerability to relapse.

Adding Ultralow-Dose Naltrexone to Oxycodone Enhances and Prolongs Analgesia: A Randomized, Controlled Trial of Oxytrex

Oxytrex is a novel drug that combines oxycodone with ultralow-dose naltrexone, an opioid antagonist. Ultralow-dose opioid antagonists have been demonstrated to enhance and prolong opiate analgesia and alleviate opioid tolerance and withdrawal in rodents. This 3-week, Phase II clinical trial assessed safety and analgesic efficacy of Oxytrex in patients with moderate to severe pain from osteoarthritis. Patients with a pain score > or =5 received placebo, oxycodone 4 times a day (qid), Oxytrex qid, or Oxytrex twice a day (bid). All active treatment groups received the same total daily dose and dose escalation of oxycodone starting at 10 and ending at 40 mg/day. Importantly, the Oxytrex bid group received a lower daily dose of naltrexone than Oxytrex qid (0.002 vs 0.004 mg/day). Oxytrex bid produced a 39% reduction in pain intensity, which was significantly greater than that of placebo (P < .001), oxycodone qid (P = .006), and Oxytrex qid (P = .003). Oxytrex bid was also superior to placebo in quality of analgesia (P = .002), duration of pain control each day (P = .05), patients' global assessments (P = .04), and the Western Ontario and MacMaster Universities Osteoarthritis Index total score (P = .03). The incidence of side effects was comparable between active treatments. In this Phase II dose-ranging study, Oxytrex bid demonstrated greater pain relief with a more convenient dosing schedule compared to oxycodone qid.

PERSPECTIVE

Preclinical data have shown ultralow-dose opioid antagonists to enhance and prolong opioid analgesia while reducing analgesic tolerance and physical dependence. Recent molecular pharmacology data show a mechanism of action to be the prevention of aberrant G protein coupling by opioid receptors that underlies opioid tolerance and dependence.

Opioid-induced preconditioning: recent advances and future perspectives

Opioids, named by Acheson for compounds with morphine-like actions despite chemically distinct structures, have received much research interest, particularly for their central nervous system (CNS) actions involved in pain management, resulting in thousands of scientific papers focusing on their effects on the CNS and other organ systems. A more recent area which may have great clinical importance concerns the role of opioids, either endogenous or exogenous compounds, in limiting the pathogenesis of ischemia-reperfusion injury in heart and brain. The role of endogenous opioids in hibernation provides tantalizing evidence for the protective potential of opioids against ischemia or hypoxia. Mammalian hibernation, a distinct energy-conserving state, is associated with depletion of energy stores, intracellular acidosis and hypoxia, similar to those which occur during ischemia. However, despite the potentially detrimental cellular state induced with hibernation, the myocardium remains resilient for many months. What accounts for the hypoxia-tolerant state is of great interest. During hibernation, circulating levels of opioid peptides are increased dramatically, and indeed, are considered a "trigger" of hibernation. Furthermore, administration of opioid antagonists can effectively reverse hibernation in mammals. Therefore, it is not surprising that activation of opioid receptors has been demonstrated to preserve cellular status following a hypoxic insult, such as ischemia-reperfusion in many model systems including the intestine [Zhang, Y., Wu, Y.X., Hao, Y.B., Dun, Y. Yang, S.P., 2001. Role of endogenous opioid peptides in protection of ischemic preconditioning in rat small intestine. Life Sci. 68, 1013-1019], skeletal muscle [Addison, P.D., Neligan, P.C., Ashrafpour, H., Khan, A., Zhong, A., Moses, M., Forrest, C.R., Pang, C.Y., 2003. Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction. Am. J. Physiol. Heart Circ. Physiol. 285, H1435-H1443], the CNS [Borlongan, C.V., Wang, Y., Su, T.P., 2005. Delta opioid peptide (d-ala 2, d-leu 5) enkephalin: linking hiberation and neuroprotection. Front Biosci. 9, 3392-3398] and the myocardium [Romano, M.A., Seymour, E.M., Berry, J.A., McNish, R.A., Bolling, S.F., 2004. Relative contribution of endogenous opioids to myocardial ischemic tolerance. J Surg Res. 118, 32-37; Peart, J.N., Gross, G.J., 2004a. Exogenous activation of delta- and kappa-opioid receptors affords cardioprotection in isolated murine heart. Basic Res Cardiol. 99(1), 29-37]. For the purpose of this review, we will focus primarily on the protective effects of opioids against post-reperfusion myocardial stunning and infarction.

Chronic exposure to morphine produces a marked cardioprotective phenotype in aged mouse hearts

Aging is often associated with decreased myocardial ischemic tolerance. We recently reported that chronic preconditioning produced by continuous exposure to morphine affords a profound cardioprotective phenotype in young mice. In this study, we determined if chronic exposure to morphine retained its ability to precondition the myocardium in the young or aged heart. Young (10-14 weeks) or aged (24-26 months) C57/BL6 mice were untreated, administered morphine acutely (30 microM), or implanted with a morphine pellet (75 mg) for 5 days prior to heart isolation and perfusion. Following equilibration, perfused hearts were subjected to 25 min ischemia and 45 min reperfusion. Untreated hearts from both young and aged mice displayed marked contractile dysfunction and LDH release following reperfusion. Acute infusion of morphine improved recovery of end-diastolic pressure and developed pressure in young (P < 0.05 vs. untreated) but not senescent hearts. Hearts from mice exposed to morphine for 5 days displayed a further improvement in post-ischemic contractile function (P < 0.05 vs. acute treatment), and a marked reduction in post-ischemic LDH efflux (P < 0.05 vs. untreated) in both young and senescent hearts. These data demonstrate that aged hearts maintain the ability to be preconditioned by chronic exposure to morphine in the absence of acute protection.

Morphine-Tolerant Mice Exhibit a Profound and Persistent Cardioprotective Phenotype

Background— Morphine and other opioids continue to be used as the major treatment for acute pain both before and after surgery. In this regard, much research has focused on the mechanisms of morphine tolerance and dependence in the central nervous system; however, few studies have examined the effect of morphine on peripheral organs, such as the heart, in morphine-tolerant animals. Here, we examine the effect of tolerance to the analgesic effect of morphine on ischemic tolerance in mice after prolonged morphine exposure and withdrawal.

Methods and Results— Male C57/BL6 mice were implanted subcutaneously with either placebo or morphine pellets (25 or 75 mg). After prolonged exposure to and/or withdrawal from morphine or placebo, the hearts were excised and subjected to 25 minutes of ischemia and 45 minutes of reperfusion. Morphine-tolerant mice exhibited a markedly improved functional recovery compared with placebo and mice subjected to acute morphine. Lactate dehydrogenase release was also significantly reduced. The protection observed was equieffective 48 hours after withdrawal of pellet, whereas the onset of protection preceded analgesic tolerance.

Conclusions— These data demonstrate that chronic exposure to morphine unexpectedly results in a profound and persistent cardioprotective phenotype.

Neuraminidase inhibitor, oseltamivir blocks GM1 ganglioside-regulated excitatory opioid receptor-mediated hyperalgesia, enhances opioid analgesia and attenuates tolerance in mice

The endogenous glycolipid GM1 ganglioside plays a critical role in nociceptive neurons in regulating opioid receptor excitatory signaling demonstrated to mediate "paradoxical" morphine hyperalgesia and to contribute to opioid tolerance/dependence. Neuraminidase (sialidase) increases levels of GM1, a monosialoganglioside, in these neurons by enzymatic removal of sialic acid from abundant polysialylated gangliosides. In this study, acute treatment of mice with the neuraminidase inhibitor, oseltamivir enhanced morphine analgesia. Acute oseltamivir also reversed "paradoxical" hyperalgesia induced by an extremely low dose of morphine, unmasking potent analgesia. In chronic studies, co-administration of oseltamivir with morphine prevented and reversed the hyperalgesia associated with morphine tolerance. These results provide the first evidence indicating that treatment with a neuraminidase inhibitor, oseltamivir, blocks morphine's hyperalgesic effects by decreasing neuronal levels of GM1. The present study further implicates GM1 in modulating morphine analgesia and tolerance, via its effects on the underlying excitatory signaling of Gs-coupled opioid receptors. Finally, this work suggests a remarkable, previously unrecognized effect of oseltamivir-which is widely used clinically as an antiviral agent against influenza-on glycolipid regulation of opioid excitability functions in nociceptive neurons.

Cholera toxin B decreases bicuculline seizures in prenatally morphine- and saline-exposed male rats

Prenatal morphine exposure on gestation days 11-18 alters bicuculline-induced seizures in rats during development and in adulthood. Adult, morphine-exposed male progeny exhibit an increased latency to bicuculline seizures, which can be reversed by administration of the opioid receptor antagonist naloxone. In chronically morphine-treated adult mice, cholera toxin B (CTX-B) can reverse the effects of chronic morphine administration. Therefore, the present study investigated whether prenatally morphine-exposed rats show a similar response to CTX-B as chronically morphine-treated adult rodents. Prenatally morphine-, saline- and unexposed male progeny were tested for seizure susceptibility with a 7.5-mg/kg intraperitoneal injection of bicuculline in adulthood. CTX-B or saline was injected subcutaneously at 24, 12, and 0.5 h before bicuculline injection. CTX-B decreased the occurrence of bicuculline-induced seizures in both prenatally saline- and morphine-exposed but not unexposed rats. Furthermore, three, but not one, saline injections administered at 12-h intervals prior to bicuculline administration reversed the increase in seizure latency in prenatally morphine-exposed adult males, suggesting an altered responsiveness of the stress system. The present study demonstrates that CTX-B can decrease the occurrence of bicuculline seizures in prenatally stressed rats and that increased seizure latencies in prenatally morphine-exposed male rats may be related to stress responses.

Molecular mechanisms of excitatory signaling upon chronic opioid agonist treatment

Opioid receptor agonists mediate their analgesic effects by interacting with Gi/o protein-coupled opioid receptors. Acute treatment with opioid agonists is thought to mediate analgesia by hyperpolarization of presynatic neurons, leading to the inhibition of excitatory (pain) neurotransmitters release. After chronic treatment however, the opioid receptors gradually become less responsive to agonists, and increased drug doses become necessary to maintain the therapeutic effect (tolerance). Analgesic tolerance is the result of two, partially overlapping processes: a gradual loss of inhibitory opioid function is accompanied by an increase in excitatory signaling. Recent data indicate that chronic opioid agonist treatment simultaneously desensitizes the inhibitory-, and augments the stimulatory effects of the opioids. In the present paper we review the molecular mechanisms that may have a role in the augmentation of the excitatory signaling upon chronic opioid agonist treatment. We also briefly review our recent experimental data on the molecular mechanism of chronic opioid agonist-mediated functional sensitization of forskolin-stimulated cAMP formation, in a recombinant Chinese hamster ovary cell line stably expressing the human delta-opioid receptor (hDOR/CHO). To interpret the experimental data, we propose that chronic hDOR activaton leads to activation of multiple redundant signaling pathways that converge to activate the protein kinase, Raf-1. Raf-1 in turn phosphorylates and sensitizes the native adenylyl cyclase VI isoenzyme in hDOR/CHO cells, causing a rebound increase in forskolin-stimulated cAMP formation upon agonist withdrawal.

Potentiation of the excitatory action of NMDA in ventrolateral periaqueductal gray by the mu-opioid receptor agonist, DAMGO

Several lines of evidence have suggested that mu-opioids, generally regarded as inhibitory, also have effects that stimulate neural activity. To look for possible excitatory opioid action in the rat periaqueductal gray (PAG), we first re-examined data from a previous study and found that met-enkephalin could evoke a delayed, sluggish excitation, suggestive of modulation by the opioid on the action of certain excitants. This observation, coupled with other studies that show mu-opioids can modulate NMDA receptor activation, prompted us to perform extracellular recording of the responses of single ventrolateral PAG (vlPAG) neurons in brain slices to DAMGO, a mu-opioid, and to NMDA. When applied alone, DAMGO at nM concentrations, like met-enkephalin, often evoked the delayed excitation and occasionally an inhibition. When applied after a brief exposure to NMDA, DAMGO at doses as low as 0.1 nM potentiated the excitation produced by a subsequent pulse of NMDA. This occurred, depending on cell type, in 23-100% of vlPAG neurons. The potentiating action of DAMGO was blocked by naloxone, suggesting it was mediated by mu-opioid receptors. Characterization of these mu-opioid actions revealed that the potentiation and the delayed excitation, unlike the inhibition, was not blocked by another opioid antagonist, nalmefene, nor by an inhibitor of the G protein of the G(i) class, N-ethylmaleimide. Moreover, the potentiating action was distinct from the inhibition in that it was: (a) enhanced by repeated opioid applications, (b) exhibited low effective doses, (c) had a long time course (minutes to develop and last tens of minutes) and (d) was present in distinct though overlapping cell populations. These data reveal an unconventional action of opioids in PAG neurons, that is, a potentiation of excitation produced by NMDA. This effect appeared mechanistically distinct from opioid inhibition or disinhibition and may be related to established examples of direct opioid excitation. These observations may help understanding behaviorally important mechanisms linked to acute and chronic opioid functions in the vlPAG.

Paradoxical effects of the opioid antagonist naltrexone on morphine analgesia, tolerance, and reward in rats

Opioid agonists such as morphine have been found to exert excitatory and inhibitory receptor-mediated effects at low and high doses, respectively. Ultra-low doses of opioid antagonists (naloxone and naltrexone), which selectively inhibit the excitatory effects, have been reported to augment systemic morphine analgesia and inhibit the development of tolerance/physical dependence. This study investigated the site of action of the paradoxical effects of naltrexone and the generality of this effect. The potential of ultra-low doses of naltrexone to influence morphine-induced analgesia was investigated in tests of nociception. Administration of intrathecal (0.05 and 0.1 ng) or systemic (10 ng/kg i.p.) naltrexone augmented the antinociception produced by an acute submaximal dose of intrathecal (5 microg) or systemic (7.5 mg/kg i.p.) morphine in the tail-flick test. Chronic intrathecal (0.005 and 0.05 ng) or systemic (10 ng/kg) naltrexone combined with morphine (15 microg i.t.; 15 mg/kg i.p.) over a 7-day period inhibited the decline in morphine antinociception and prevented the loss of morphine potency. In animals rendered tolerant to intrathecal (15 microg) or systemic (15 mg/kg) morphine, administration of naltrexone (0.05 ng i.t.; 10 and 50 ng/kg i.p.) significantly restored the antinociceptive effect and potency of morphine. Thus, in ultra-low doses, naltrexone paradoxically enhances morphine analgesia and inhibits or reverses tolerance through a spinal action. The potential of naltrexone to influence morphine-induced reward was also investigated using a place preference paradigm. Systemic administration of ultra-low doses of naltrexone (16.7, 20.0, and 25.0 ng/kg) with morphine (1.0 mg/kg) extended the duration of the morphine-induced conditioned place preference. These effects of naltrexone on morphine-induced reward may have implications for chronic treatment with agonist-antagonist combinations.

Cholera toxin-B subunit blocks excitatory opioid receptor-mediated hyperalgesic effects in mice, thereby unmasking potent opioid analgesia and attenuating opioid tolerance/dependence

In a previous study we demonstrated that injection (i.p.) of low doses of GM1 ganglioside in mice rapidly attenuates morphine's analgesic effects. This result is consonant with our electrophysiologic studies in nociceptive types of dorsal root ganglion (DRG) neurons in culture, which showed that exogenous GM1 rapidly increased the efficacy of excitatory (Gs-coupled) opioid receptor functions. By contrast, treatment of DRG neurons with the non-toxic B-subunit of cholera toxin (CTX-B) which binds selectively to GM1, blocked the excitatory, but not inhibitory, effects of morphine and other bimodally-acting opioid agonists, thereby resulting in a net increase in inhibitory opioid potency. The present study provides more direct evidence that endogenous GM1 plays a physiologic role in regulating excitatory opioid receptor functions in vivo by demonstrating that cotreatment with remarkably low doses of CTX-B (10 ng/kg, s.c.) selectively blocks hyperalgesic effects elicited by morphine or by a kappa opioid agonist, thereby unmasking potent opioid analgesia. These results are comparable to the effects of cotreatment of mice with morphine plus an ultra-low dose of the opioid antagonist, naltrexone (NTX) which blocks opioid-induced hyperalgesic effects, unmasking potent opioid analgesia. Low-dose NTX selectively blocks excitatory opioid receptors at their recognition site, whereas CTX-B binds to, and interferes with, a putative allosteric GM1 regulatory site on excitatory opioid receptors. Furthermore, chronic cotreatment of mice with morphine plus CTX-B attenuates development of opioid tolerance and physical dependence, as previously shown to occur during cotreatment with low-dose NTX.

The Putative Welfare-Reducing Effects of Preventing Equine Stereotypic Behaviour

The common practice of preventing equine stereotypic behaviour in the UK may be of concern, from a welfare perspective, if these behaviours constitute a coping response to a suboptimal environment. The aim of this study was to assess the putative function of these behaviours by measuring behavioural and physiological parameters i) before and after stereotypy prevention; ii) before and after stereotypy performance; and iii) in response to opiate antagonist (naloxone) administration.

The crib-strap significantly (P = 0.05) elevated mean plasma Cortisol levels in crib-biting horses; a similar, although not significant trend (P = 0.07) was also observed for the weaving group during the anti-weave bar treatment. Both crib-strap and anti-weave bar significantly (P < 0.05) elevated plasma Cortisol levels in the control horses. Although the latter result prevented a definite conclusion being drawn about the function of equine stereotypies, the results did indicate that the use of the crib-strap and anti-weave bar is stressful to the horse.

Plasma Cortisol level was significantly (P = 0.04) higher immediately prior to the onset of stereotypy followed by a significant reduction post-stereotypy. This suggested that both crib-biting and weaving have a coping function to reduce stress levels in the animal.

Naloxone significantly reduced crib-biting by 84 per cent (P = 0.05) but it did not reduce weaving behaviour, indicating that crib-biting is a reward behaviour. However, resting behaviour was also significantly (P = 0.02) increased in crib-biting horses, suggesting that the stereotypy reduction was due to a sedative effect of the opiate antagonist. The latter was not measured, however, in control or weaving animals, and thus may be interpreted differently. The welfare implications of these results are discussed.

Acute thermal hyperalgesia elicited by low-dose morphine in normal mice is blocked by ultra-low-dose naltrexone, unmasking potent opioid analgesia

Our previous electrophysiologic studies on nociceptive types of dorsal root ganglion (DRG) neurons in culture demonstrated that extremely low fM-nM concentrations of morphine and many other bimodally-acting mu, delta and kappa opioid agonists can elicit direct excitatory opioid receptor-mediated effects, whereas higher (microM) opioid concentrations evoked inhibitory effects. Cotreatment with pM naloxone or naltrexone (NTX) plus fM-nM morphine blocked the excitatory effects and unmasked potent inhibitory effects of these low opioid concentrations. In the present study, hot-water-immersion tail-flick antinociception assays at 52 degrees C on mice showed that extremely low doses of morphine (ca. 0.1 microg/kg) can, in fact, elicit acute hyperalgesic effects, manifested by rapid onset of decreases in tail-flick latency for periods >3 h after drug administration. Cotreatment with ultra-low-dose NTX (ca. 1-100 pg/kg) blocks this opioid-induced hyperalgesia and unmasks potent opioid analgesia. The consonance of our in vitro and in vivo evidence indicates that doses of morphine far below those currently required for clinical treatment of pain may become effective when opioid hyperalgesic effects are blocked by coadministration of appropriately low doses of opioid antagonists. This low-dose-morphine cotreatment procedure should markedly attenuate morphine tolerance, dependence and other aversive side-effects.

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.

Calmodulin regulation of basal and agonist-stimulated G protein coupling by the mu-opioid receptor (OP(3)) in morphine-pretreated cell

Calmodulin (CaM) has been shown to suppress basal G protein coupling and attenuate agonist-stimulated G protein coupling of the mu-opioid receptor (OP(3)) through direct interaction with the third intracellular (i3) loop of the receptor. Here we have investigated the role of CaM in regulating changes in OP(3)-G protein coupling during morphine treatment, shown to result in CaM release from plasma membranes. Basal and agonist-stimulated G protein coupling by OP(3) was measured before and after morphine pretreatment by incorporation of guanosine 5'-O-(3-[(35)S]thiotriphosphate) into membranes, obtained from HEK 293 cells transfected with human OP(3) cDNA. The opioid antagonist beta-chlornaltrexamine fully suppressed basal G protein coupling of OP(3), providing a direct measure of basal signaling. Pretreatment of the cells with morphine enhanced basal G protein coupling (sensitization). In contrast, agonist-stimulated coupling was diminished (desensitization), resulting in a substantially flattened morphine dose-response curve. To test whether CaM is involved in these changes, we constructed OP(3)-i3 loop mutants with reduced affinity for CaM (K273A, R275A, and K273A/R275A). Basal signaling of these mutant OP(3) receptors was higher than that of the wild-type receptor and, moreover, unaffected by morphine pretreatment, whereas desensitization to agonist stimulation was only slightly attenuated. Therefore, CaM-OP(3) interactions appear to play only a minor role in the desensitization of OP(3). In contrast, release of CaM from the plasma membrane appears to enhance the inherent basal G protein coupling of OP(3), thereby resolving the paradox that OP(3) displays both desensitization and sensitization during morphine treatment.

Enhanced analgesic potency and reduced tolerance of morphine in 129/SvEv mice: evidence for a deficiency in GM1 ganglioside-regulated excitatory opioid receptor functions

10-fold higher doses in SW mice. Furthermore, cotreatment of 129/SvEv mice with morphine plus a low dose of naltrexone (ca. 0.1 microgram/kg) that markedly enhances and prolongs morphine's antinociceptive effects in SW mice did not enhance, and often attenuated6 h. The marked GM1-induced attenuation of morphine's antinociceptive effects in 129/SvEv mice may be due to conversion of some of the opioid receptors in these mice from an inhibitory Gi/Go-coupled to an excitatory Gs-coupled mode. Exogenous GM1 supplementation can, therefore, reverse the anomalous lack of morphine tolerance displayed by this mouse strain in comparison to SW and other mice. The present study may provide insights into factors that regulate the marked variability in nociceptive sensitivity and opioid tolerance/dependence liability among individual humans.

Antagonists of excitatory opioid receptor functions enhance morphine's analgesic potency and attenuate opioid tolerance/dependence liability

Recent preclinical and clinical studies have demonstrated that cotreatments with extremely low doses of opioid receptor antagonists can markedly enhance the efficacy and specificity of morphine and related opioid analgesics. Our correlative studies of the cotreatment of nociceptive types of dorsal-root ganglion neurons in vitro and mice in vivo with morphine plus specific opioid receptor antagonists have shown that antagonism of Gs-coupled excitatory opioid receptor functions by cotreatment with ultra-low doses of clinically available opioid antagonists, e.g. naloxone and naltrexone, markedly enhances morphine's antinociceptive potency and simultaneously attenuates opioid tolerance and dependence. These preclinical studies in vitro and in vivo provide cellular mechanisms that can readily account for the unexpected enhancement of morphine's analgesic potency in recent clinical studies of post-surgical pain patients cotreated with morphine plus low doses of naloxone or nalmefene. The striking consistency of these multidisciplinary studies on nociceptive neurons in culture, behavioral assays on mice and clinical trials on post-surgical pain patients indicates that clinical treatment of pain can, indeed, be significantly improved by administering morphine or other conventional opioid analgesics together with appropriately low doses of an excitatory opioid receptor antagonist.

Dihydroetorphine: physical dependence and stereotypy after continuous infusion in the rat

In a previous study in this laboratory, exposure of rhesus monkeys to intermittent, high doses of dihydroetorphine for 42 days did not evoke behavioral signs of physical dependence on this opioid either after it was abruptly withdrawn or after challenge with a high dose of naloxone. To investigate further the physical dependence capacity of this opioid, it was given by infusion to rats thereby exposing receptors chronically and continuously to this opioid. Abstinence expressed as body weight loss, irritability, and wet-dog shakes was observed after abrupt withdrawal of the low-dose regimen (5,10, 40 and 40 microg/kg per day for 4 days, respectively). The high-dose regimen (10, 20 and 80 microg/kg per day for 3 days, respectively) produced stereotypy and physical dependence. Although many reported molecular events and dependence studies suggest otherwise, dihydroetorphine's propensity to produce physical dependence, an important determinant of opioid abuse, is real.

Novel domain-specific actions of amyloid precursor protein on developing synapses

The effect of the secretory form of amyloid precursor protein (sAPP) on synaptic transmission was examined by using developing neuromuscular synapses in Xenopus cell cultures. The frequency of spontaneous postsynaptic currents (SSCs) was reduced by the addition of sAPP, whereas the amplitude of impulse-evoked postsynaptic currents (ESCs) was increased by sAPP. These opposing effects on spontaneous versus evoked release were separated by using the specific domain of APP. The C-terminal fragment of sAPP (CAPP) only reduced SSC frequency and did not affect ESCs. By contrast, the N-terminal fragment of sAPP (NAPP) did not affect SSC frequency but did increase ESC amplitude. The reduction of SSC frequency by sAPP appears to be mediated by activation of potassium channels through a cGMP-dependent pathway, whereas the increase of ESC amplitude is mediated by a different pathway involving activation of protein kinase(s). These results suggest the potential role of sAPP as a modulator of synaptic activity by two specific domains.

Development of specific synaptic network functions in organotypic central nervous system (CNS) cultures: implications for transplantation of CNS neural cells in vivo

This article provides a broad overview of the significant roles that morphophysiologic analyses of organotypic cultures of neural tissues explanted in vitro-initiated during the 1950s-have played in stimulating the more recent development of techniques for transplantation of neural cells and tissues into specific regions of the central nervous system (CNS) in vivo. The demonstrations by Crain and co-workers in the 1950s and 1960s that fetal rodent and human CNS neurons can continue to develop a remarkable degree of mature structure and function during many months of complete isolation in culture provided crucial evidence that development of many organotypic properties of nerve cells is regulated by epigenetic factors that ensure rather stereotyped expression despite wide variations in environmental conditions. These in vitro studies strongly suggested that fetal neural cells should, indeed, be capable of even more highly organotypic development after transplantation in vivo, as has been elegantly demonstrated by many of the successful CNS transplantation studies reviewed here.

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.

GM1 ganglioside-induced modulation of opioid receptor-mediated functions

Electrophysiologic studies of dorsal-root ganglion (DRG) neurons in culture have demonstrated both excitatory (Gs-coupled) as well as inhibitory (Gi/Go-coupled) opioid receptor-mediated actions. Brief treatment of DRG neurons with cholera toxin-beta which binds specifically to GM1 sites on neuronal membranes, selectively blocks opioid excitatory but not inhibitory effects. Conversely, after brief treatment of DRG neurons with GM1, but not with GM2, GM3, or other related gangliosides, the threshold concentration of opioid agonists for eliciting excitatory effects is markedly decreased from nM to pM-fM levels and opioid antagonists, for example, naloxone (NLX), at low concentrations paradoxically elicit excitatory effects. These studies suggest that the excitatory opioid supersensitivity of GM1-treated DRG neurons is due primarily to increased efficacy of excitatory opioid-receptor activation of Gs. Recent studies of cloned delta opioid receptors transfected into CHO cells suggest that this supersensitivity of GM1-treated DRG neurons may be further augmented by rapid conversion of many opioid receptors from a Gi/Go-coupled inhibitory mode to a Gs-coupled excitatory mode. The opioid excitatory supersensitivity elicited in DRG neurons by acute elevation of exogenous GM1 provides novel insights into mechanisms underlying opioid tolerance and dependence, since remarkably similar supersensitivity occurs in DRG and other neurons after chronic treatment with morphine or other opioid agonists that upregulate endogenous GM1.

Opiate tolerance and dependence: receptors, G-proteins, and antiopiates

Despite the existence of a large body of information on the subject, the mechanisms of opiate tolerance and dependence are not yet fully understood. Although the traditional mechanisms of receptor down-regulation and desensitization seem to play a role, they cannot entirely explain the phenomena of tolerance and dependence. Therefore, other mechanisms, such as the presence of antiopiate systems and the coupling of opiate receptors to alternative G-proteins, should be considered. A further complication of studies of opiate tolerance and dependence is the multiplicity of endogenous opiate receptors and peptides. This review will focus on the endogenous opioid system--peptides, receptors, and coupling of receptors to intracellular signaling via G-proteins--in the context of their roles in tolerance and dependence. Opioid peptides include the recently discovered endomorphins and those encoded by three known genes--pro-opiomelanocortin, pro-enkephalin, and pro-dynorphin. They bind to three types of receptors--mu, delta, and kappa. Each of the receptor types is further divided into multiple subtypes. These receptors are widely known to be coupled to G-proteins of the Gi and Go subtypes, but an increasing body of results suggests coupling to other G-proteins, such as Gs. The coupling of opiate receptors to Gs, in particular, has implications for tolerance and dependence. Alterations at the receptor and transduction level have been the focus of many studies of opiate tolerance and dependence. In these studies, both receptor down-regulation and desensitization have been demonstrated in vivo and in vitro. Receptor down-regulation has been more easily observed in vitro, especially in response to morphine, a phenomenon which suggests that some factor which is missing in vitro prevents receptors from down-regulating in vivo and may play a critical role in tolerance and dependence. We suggest that antiopiate peptides may operate in vivo in this capacity, and we outline the evidence for the antiopiate properties of three peptides: neuropeptide FF, orphanin FQ/nociceptin, and Tyr-W-MIF-1. In addition, we provide new results suggesting that Tyr-W-MIF-1 may act as an antiopiate at the cellular level by inhibiting basal G-protein activation, in contrast to the activation of G-proteins by opiate agonists.

Etorphines: mu-opioid receptor-selective antinociception and low physical dependence capacity

Comparative analgesic studies revealed that dihydroetorphine was more potent than etorphine in the tail-flick and hot-plate tests, respectively and nearly equipotent in the phenylquinone assay. Both compounds were short acting. Studies with selective opioid receptor antagonists beta-funaltrexamine, nor-binaltorphimine and naltrindole revealed that both etorphines were mu-selective agonists. Presumptive evidence for competitive antagonism of these compounds with naloxone was provided by Schild regressions with slopes of near unity. In a suppression test in rhesus monkeys maximally dependent on morphine, dihydroetorphine and etorphine dose-dependently replaced morphine. Drug-naive simians chronically exposed to frequent, intermittent and escalating doses of dihydroetorphine for 42 days showed few withdrawal signs when challenged with large doses of naloxone or were abruptly withdrawn from this drug. The results suggest that these atypical opioids may be useful in the clinical treatment of pain and opiate drug abuse.

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

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