Molecular mechanisms of excitatory signaling upon chronic opioid agonist treatment

Distinct µ-opioid ensembles trigger positive and negative fentanyl reinforcement

Fentanyl is a powerful painkiller that elicits euphoria and positive reinforcement1. Fentanyl also leads to dependence, defined by the aversive withdrawal syndrome, which fuels negative reinforcement2,3 (that is, individuals retake the drug to avoid withdrawal). Positive and negative reinforcement maintain opioid consumption, which leads to addiction in one-fourth of users, the largest fraction for all addictive drugs4. Among the opioid receptors, µ-opioid receptors have a key role5, yet the induction loci of circuit adaptations that eventually lead to addiction remain unknown. Here we injected mice with fentanyl to acutely inhibit γ-aminobutyric acid-expressing neurons in the ventral tegmental area (VTA), causing disinhibition of dopamine neurons, which eventually increased dopamine in the nucleus accumbens. Knockdown of µ-opioid receptors in VTA abolished dopamine transients and positive reinforcement, but withdrawal remained unchanged. We identified neurons expressing µ-opioid receptors in the central amygdala (CeA) whose activity was enhanced during withdrawal. Knockdown of µ-opioid receptors in CeA eliminated aversive symptoms, suggesting that they mediate negative reinforcement. Thus, optogenetic stimulation caused place aversion, and mice readily learned to press a lever to pause optogenetic stimulation of CeA neurons that express µ-opioid receptors. Our study parses the neuronal populations that trigger positive and negative reinforcement in VTA and CeA, respectively. We lay out the circuit organization to develop interventions for reducing fentanyl addiction and facilitating rehabilitation.

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.

Stress activates pronociceptive endogenous opioid signalling in DRG neurons during chronic colitis

AIMS AND BACKGROUND

Psychological stress accompanies chronic inflammatory diseases such as IBD, and stress hormones can exacerbate pain signalling. In contrast, the endogenous opioid system has an important analgesic action during chronic inflammation. This study examined the interaction of these pathways.

METHODS

Mouse nociceptive dorsal root ganglia (DRG) neurons were incubated with supernatants from segments of inflamed colon collected from patients with chronic UC and mice with dextran sodium sulfate (cDSS)-induced chronic colitis. Stress effects were studied by adding stress hormones (epinephrine and corticosterone) to dissociated neurons or by exposing cDSS mice to water avoidance stress. Changes in excitability of colonic DRG nociceptors were measured using patch clamp and Ca²⁺ imaging techniques.

RESULTS

Supernatants from patients with chronic UC and from colons of mice with chronic colitis caused a naloxone-sensitive inhibition of neuronal excitability and capsaicin-evoked Ca²⁺ responses. Stress hormones decreased signalling induced by human and mouse supernatants. This effect resulted from stress hormones signalling directly to DRG neurons and indirectly through signalling to the immune system, leading to decreased opioid levels and increased acute inflammation. The net effect of stress was a change endogenous opioid signalling in DRG neurons from an inhibitory to an excitatory effect. This switch was associated with a change in G protein-coupled receptor excitatory signalling to a pathway sensitive to inhibitors of protein kinase A-protein, phospholipase C-protein and G protein βϒ subunits.

CONCLUSIONS

Stress hormones block the inhibitory actions of endogenous opioids and can change the effect of opioid signalling in DRG neurons to excitation. Targeting these pathways may prevent heavy opioid use in IBD.

Gα(i/o)-coupled receptor-mediated sensitization of adenylyl cyclase: 40 years later

Heterologous sensitization of adenylyl cyclase (also referred to as superactivation, sensitization, or supersensitization of adenylyl cyclase) is a cellular adaptive response first described 40 years ago in the laboratory of Dr. Marshall Nirenberg. This apparently paradoxical cellular response occurs following persistent activation of Gαi/o-coupled receptors and causes marked enhancement in the activity of adenylyl cyclases, thereby increasing cAMP production. Since our last review in 2005, significant progress in the field has led to a better understanding of the relevance of, and the cellular biochemical processes that occur during the development and expression of heterologous sensitization. In this review we will discuss the recent advancements in the field and the mechanistic hypotheses on heterologous sensitization.

Protective effects of the delta opioid peptide [D-Ala2, D-Leu5]enkephalin in an ex vivo model of ischemia/reperfusion in brain slices

INTRODUCTION

The delta opioid peptide [D-Ala2, D-Leu5]enkephalin (DADLE) plays a key role in neuronal protection against both hypoxic and ischemic conditions. However, the cellular mechanisms of action of DADLE under these conditions remain unclear.

METHODS

Ischemia was simulated with perfusing the brain slices with glucose-free artificial cerebrospinal fluid. Apoptosis was examined using an in situ cell death detection kit and expressed as the percentage of positively labeled neurons relative to total number of neurons. PCR was performed by adding cDNA, 5 pm dNTP, 1 μL Taqase, and primers. PCR products were separated with electrophoresis, stained with ethidium bromide, and visualized under ultraviolet light.

AIMS

To investigate the potential effects of DADLE in an ex vivo model of cerebral ischemia/reperfusion.

RESULTS

DADLE attenuated lactic dehydrogenase release and neuronal apoptosis in a concentration-dependent manner. The protective effects of DADLE were attenuated by representative selective delta2, but not delta1 opioid antagonists. Treatment with PD98059, a selective inhibitor of ERK kinase (MEK), also blocked the protective effect of DADLE as well as ERK phosphorylation induced by DADLE.

CONCLUSIONS

Endogenous opioid peptides could promote cell survival via delta2 opioid receptors, possibly through the downstream MEK-ERK pathway.

A trial of methadone tapering schedules in pediatric intensive care unit patients exposed to prolonged sedative infusions

OBJECTIVES

To compare the efficacy of a low-dose methadone tapering schedule to a high-dose methadone tapering schedule in pediatric intensive care unit patients exposed to infusions of fentanyl, with or without infusions of midazolam, for ≥ 5 days.

DESIGN

Prospective, double-blind, randomized trial.

SETTING

Pediatric intensive care unit in a tertiary care children's hospital.

PATIENTS

Seventy-eight patients, 74 of whom had been receiving infusions of both fentanyl and midazolam, were randomized. Forty-one patients were randomized to the low-dose methadone group and 37 were randomized to the high-dose methadone group. Sixty patients successfully completed the trial, 34 were in the low-dose methadone group, and 26 were in the high-dose methadone group.

INTERVENTIONS

Patients were randomized to receive methadone either at a starting dose of 0.1 mg/kg/dose (low-dose methadone group) or at a starting dose based on both the patient's weight and the most recent fentanyl infusion rate (high-dose methadone group). In each group, methadone was administered every 6 hrs for the first 24 hrs and then every 12 hrs for the second 24 hrs. The methadone was then decreased to once daily and tapered off over the next 10 days. Patients were monitored for withdrawal symptoms using the Modified Narcotic Withdrawal Score.

MEASUREMENTS AND MAIN RESULTS

The percentage of patients who successfully completed the 10-day methadone taper was the same in the low-dose methadone group as in the high-dose methadone group (56% vs. 62%; p = .79). Patients that failed to complete the assigned methadone taper had a greater total fentanyl dose and longer pediatric intensive care unit length of stay compared to patients who completed the assigned methadone taper.

CONCLUSIONS

Patients who received infusions of fentanyl for at least 5 days were just as likely to complete a low-dose methadone taper as a high-dose methadone taper. Because of the risks of both withdrawal and oversedation with any fixed methadone schedule, the methadone dose must be adjusted according to each patient's response.

Intrathecal PKA-selective siRNA treatment blocks sustained morphine-mediated pain sensitization and antinociceptive tolerance in rats

Sustained morphine treatment has been shown to produce paradoxical pain sensitization (opioid-induced hyperalgesia) and also causes increase in spinal pain neurotransmitter, such as calcitonin gene related peptide (CGRP), concentration in experimental animals. Studies have also shown that cyclic adenosine-monophosphate (cAMP)-dependent protein kinase (PKA) plays a major role in the regulation of presynaptic neurotransmitter (such as CGRP and substance P) synthesis and release. We have previously shown that in cultured primary sensory dorsal root ganglion (DRG) neurons sustained in vitro opioid agonist treatment upregulates cAMP levels (adenylyl cyclase (AC) superactivation) and augments basal and capsaicin evoked CGRP release in a PKA dependent manner. In the present study, we investigated the in vivo role of PKA in sustained morphine-mediated pain sensitization. Our data indicate that selective knock-down of spinal PKA activity by intrathecal (i.th.) pretreatment of rats with a PKA-selective small interference RNA (siRNA) mixture significantly attenuates sustained morphine-mediated augmentation of spinal CGRP immunoreactivity, thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance. The present findings indicate that sustained morphine-mediated activation of spinal cAMP/PKA-dependent signaling may play an important role in opioid induced hyperalgesia.

Sustained morphine-mediated pain sensitization and antinociceptive tolerance are blocked by intrathecal treatment with Raf-1-selective siRNA

BACKGROUND AND PURPOSE

Long-term morphine treatment enhances pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] levels in the spinal cord. It has been suggested previously that increased spinal CGRP may contribute to sustained morphine-mediated paradoxical pain sensitization and antinociceptive tolerance. Previous in vitro studies from our group indicated that Raf-1 kinase-mediated adenylyl cyclase superactivation played a crucial role in sustained morphine-mediated augmentation of basal and evoked CGRP release from cultured primary sensory neurons. The present study was aimed to evaluate the physiological significance of this molecular mechanism in vivo, in rats.

EXPERIMENTAL APPROACH

Rats were intrathecally (i.th) injected with a Raf-1-selective small interfering RNA (siRNA) mixture for 3 days and were subsequently infused with saline or morphine, s.c. for 7 days. Thermal and mechanical sensory thresholds of the animals were assessed by daily behavioural tests. After final behavioural testing (day 6), spinal cords were isolated from each animal group and spinal CGRP and Raf-1 protein levels were measured using elisa and immunohistochemistry.

KEY RESULTS

Selective knockdown of spinal Raf-1 protein levels by i.th Raf-1-selective siRNA pretreatment significantly attenuated sustained morphine-mediated up-regulation of CGRP immunoreactivity in the spinal cord of rats and prevented the development of thermal hyperalgesia, mechanical allodynia and antinociceptive tolerance.

CONCLUSIONS AND IMPLICATIONS

Raf-1 played a significant role in sustained morphine-mediated paradoxical pain sensitization and antinociceptive tolerance in vivo. These findings suggest novel pharmacological approaches to improve the long-term utility of opioids in the treatment of chronic pain.

Sustained morphine treatment augments prostaglandin E2-evoked calcitonin gene-related peptide release from primary sensory neurons in a PKA-dependent manner

Tissue damage leads to pain sensitization due to peripheral and central release of excitatory mediators such as prostaglandin E₂ (PGE₂). PGE₂ sensitizes spinal pain neurotransmitter such as calcitonin gene-related peptide (CGRP) release via activation of cyclic AMP (cAMP)/protein kinase A (PKA)-dependent signaling mechanisms. Our previous data demonstrate that sustained morphine pretreatment sensitizes adenylyl cyclase(s) (AC) toward the direct stimulator, forskolin, in cultured primary sensory neurons (AC superactivation). In the present work we investigated the hypothesis that morphine pretreatment also sensitizes ACs toward Gs-protein-coupled excitatory modulators (such as PGE₂), leading to augmented PKA-dependent CGRP release from PGE₂-stimulated primary sensory dorsal root ganglion (DRG) neurons. Our results show that sustained morphine treatment potentiated PGE₂-mediated cAMP formation and augmented PGE₂-evoked CGRP release from cultured primary sensory neurons in a PKA-dependent manner. Our data suggest that attenuation of AC superactivation in primary sensory neurons may prevent the development of opioid-induced hyperalgesia.

Sustained cannabinoid agonist treatment augments CGRP release in a PKA-dependent manner

Studies have shown that sustained cannabinoid treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia) and innocuous mechanical stimuli (tactile allodynia). It has been suggested that augmented release of pain neurotransmitters (such as calcitonin gene-related peptide, CGRP) might be responsible for this abnormal pain sensitization. We hypothesize that intracellular adaptations upon sustained cannabinoid treatment causes augmented release of CGRP from primary nociceptors leading to increased pain sensitivity. We show that sustained (24 h) cannabinoid agonist [(+)WIN 55,212-2] treatment of 7-day-old neonatal rat dorsal root ganglion neurons significantly augments basal CGRP release from these cells in a protein kinase A-dependent manner. Our results indicate that these intracellular compensatory adaptations may play a crucial trigger role in further neuronal system adaptations for modulation of pain.

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin). In the present study, we demonstrate that sustained morphine-mediated augmentation of CGRP release from isolated primary sensory dorsal root ganglion neurons is dependent on protein kinase A and Raf-1 kinase. Our data indicate that, in addition to neural system adaptations, sustained opioid agonist treatment also produces intracellular compensatory adaptations in primary sensory neurons, leading to augmentation of evoked pain neurotransmitter release from these cells.

Physiological roles for G protein-regulated adenylyl cyclase isoforms: insights from knockout and overexpression studies

Cyclic AMP is a universal second messenger, produced by a family of adenylyl cyclase (AC) enzymes. The last three decades have brought a wealth of new information about the regulation of cyclic AMP production by ACs. Nine hormone-sensitive, membrane-bound AC isoforms have been identified in addition to a tenth isoform that lacks membrane spans and more closely resembles the cyanobacterial AC enzymes. New model systems for purifying and characterizing the catalytic domains of AC have led to the crystal structure of these domains and the mapping of numerous interaction sites. However, big hurdles remain in unraveling the roles of individual AC isoforms and their regulation in physiological systems. In this review we explore the latest on AC knockout and overexpression studies to better understand the roles of G protein regulation of ACs in the brain, olfactory bulb, and heart.

Sustained cardioprotection: exploring unconventional modalities

Since Murry et al. [Murry, C.E., Jennings, R.B., Reimer, K.A., 1986. Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium. Circulation. 74, 1124-36.] initially reported on the powerful protective effects of ischemic preconditioning (PC), a plethora of experimental investigations have identified varied preconditioning protocols or mimetics to achieve cardioprotection. These stimuli predominantly act via archetypal mediators identified in associated signalling studies (including PI3-K, Akt, PKC, mitochondrial K(ATP) channels). Despite an intense research effort over the last 20 years, there remains a paucity of evidence that this protective paradigm is clinically exploitable. This may arise due to a number of drawbacks to conventional protection, including effects of age, disease, and interactions with other pharmacological agents. This encourages investigation of alternate strategies that trigger protection via unconventional signalling (distinct from conventional PC) and/or mediate sustained shifts in ischemic tolerance in hearts of varying age and disease status. This review considers briefly drawbacks to conventional PC, and focuses on alternate strategies for generating prolonged states of cardiac protection.

Sustained morphine treatment augments basal CGRP release from cultured primary sensory neurons in a Raf-1 dependent manner

Recent studies suggest that sustained morphine-mediated paradoxical pain may play an important role in the development of analgesic tolerance. The intracellular signal transduction pathways involved in sustained opioid mediated augmentation of spinal pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release are not fully clarified. Cyclic AMP (cAMP)-dependent protein kinase (PKA) plays an important role in the modulation of presynaptic neurotransmitter release. Moreover, we have shown earlier that sustained opioid agonist treatment leads to a Raf-1-dependent sensitization of adenylyl cyclase(s) (AC superactivation), augmenting forskolin-stimulated cAMP formation upon opioid withdrawal (cAMP overshoot). Therefore, in the present study we examined the role of Raf-1 in sustained morphine-mediated regulation of cAMP formation and basal CGRP release in vitro, in cultured neonatal rat dorsal root ganglion (DRG) neurons. We found that sustained morphine treatment significantly augments intracellular cAMP production as well as basal CGRP release from cultured neonatal rat DRG neurons. The selective PKA inhibitor, H-89, attenuates the sustained morphine-mediated augmentation of basal CGRP release, indicating that the cAMP/PKA pathway plays an important role in regulation of CGRP release from sensory neurons. Since our present data also demonstrated that selective Raf-1 inhibitor, GW 5074, attenuated both the cAMP overshoot and the augmentation of CGRP release mediated by sustained morphine in neonatal rat DRG neurons, we suggest that Raf-1-mediated sensitization of the intracellular cAMP formation may play an important role in sustained morphine-mediated augmentation of spinal pain neurotransmitter release.

Imaging of opioid receptors in the central nervous system

In vivo functional imaging by means of positron emission tomography (PET) is the sole method for providing a quantitative measurement of mu-, kappa and delta-opioid receptor-mediated signalling in the central nervous system. During the last two decades, measurements of changes to the regional brain opioidergic neuronal activation--mediated by endogenously produced opioid peptides, or exogenously administered opioid drugs--have been conducted in numerous chronic pain conditions, in epilepsy, as well as by stimulant- and opioidergic drugs. Although several PET-tracers have been used clinically for depiction and quantification of the opioid receptors changes, the underlying mechanisms for regulation of changes to the availability of opioid receptors are still unclear. After a presentation of the general signalling mechanisms of the opioid receptor system relevant for PET, a critical survey of the pharmacological properties of some currently available PET-tracers is presented. Clinical studies performed with different PET ligands are also reviewed and the compound-dependent findings are summarized. An outlook is given concluding with the tailoring of tracer properties, in order to facilitate for a selective addressment of dynamic changes to the availability of a single subclass, in combination with an optimization of the quantification framework are essentials for further progress in the field of in vivo opioid receptor imaging.

Raf-1 kinase mediates adenylyl cyclase sensitization by TNF-alpha in human airway smooth muscle cells

Tumor necrosis factor (TNF)-alpha is a potent inflammatory cytokine implicated in the exacerbation of asthma. Chronic exposure to TNF-alpha has been reported to induce G protein-coupled receptor desensitization, but adenylyl cyclase sensitization, in airway smooth muscle cells by an unknown mechanism. Cyclic AMP, which is synthesized by adenylyl cyclases in response to G protein-coupled receptor signals, is an important second messenger involved in the regulation of the airway muscle proliferation, migration, and tone. In other cell types, TNF-alpha receptors transactivate the EGF receptor, which activates raf-1 kinase. Further studies in transfected cells show that raf-1 kinase can phosphorylate and activate some isoforms of adenylyl cyclase. Cultured human airway smooth muscle cells were treated with TNF-alpha in the presence or absence of inhibitors of prostaglandin signaling, protein kinases, or G(i) proteins. TNF-alpha caused a significant dose- (1-10 ng/ml) and time-dependent (24 and 48 h) increase in forskolin-stimulated adenylyl cyclase activity, which was abrogated by pretreatment with GW5074 (a raf-1 kinase inhibitor), was partially inhibited by an EGF receptor inhibitor, but was unaffected by pertussis toxin. TNF-alpha also increased phosphorylation of Ser(338) on raf-1 kinase, indicative of activation. IL-1beta and EGF sensitization of adenylyl cyclase activity was also sensitive to raf-1 kinase inhibition by GW5074. Taken together, these studies link two signaling pathways not previously characterized in human airway smooth muscle cells: TNF-alpha transactivation of the EGF receptor, with subsequent raf-1 kinase-mediated activation of adenylyl cyclase.

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.

Short- and long-term regulation of adenylyl cyclase activity by delta-opioid receptor are mediated by Galphai2 in neuroblastoma N2A cells

Activation of the opioid receptor results in short-term inhibition of intracellular cAMP levels followed by receptor desensitization and subsequent increase of cAMP above the control level (adenylyl cyclase superactivation). Using adenovirus to deliver pertussis toxin-insensitive mutants of the alpha-subunits of G(i/o) that are expressed in neuroblastoma Neuro2A cells (Galpha(i2), Galpha(i3), and Galpha(o)), we examined the identities of the G proteins involved in the short- and long-term action of the delta-opioid receptor (DOR). Pertussis toxin pretreatment completely abolished the ability of [d-Pen(2), d-Pen(5)]-enkephalin (DPDPE) to inhibit forskolin-stimulated intracellular cAMP production. Expression of the C352L mutant of Galpha(i2), and not the C351L mutants of Galpha(i3) or Galpha(o), rescued the short-term effect of DPDPE after pertussis toxin treatment. The ability of Galpha(i2) in mediating DOR inhibition of adenylyl cyclase activity was also reflected in the ability of Galpha(i2), not Galpha(i3) or Galpha(o), to coimmunoprecipitate with DOR. Coincidently, after long-term DPDPE treatment, pertussis toxin treatment eliminated the antagonist naloxone-induced superactivation of adenylyl cyclase activity. Again, only the C352L mutant of Galpha(i2) restored the adenylyl cyclase superactivation after pertussis toxin treatment. More importantly, the C352L mutant of Galpha(i2) remained associated with DOR after long-term agonist and pertussis toxin treatment whereas the wild-type Galpha(i2) did not. These data suggest that Galpha(i2) serves as the signaling molecule in both DOR-mediated short- and long-term regulation of adenylyl cyclase activity.

Is paradoxical pain induced by sustained opioid exposure an underlying mechanism of opioid antinociceptive tolerance?

Opiates are the primary treatment for pain management in cancer patients reporting moderate to severe pain, and are being increasingly used for non-cancer chronic pain. However, prolonged administration of opiates is associated with significant problems including the development of antinociceptive tolerance, wherein higher doses of the drug are required over time to elicit the same amount of analgesia. High doses of opiates result in serious side effects such as constipation, nausea, vomiting, dizziness, somnolence, and impairment of mental alertness. In addition, sustained exposure to morphine has been shown to result in paradoxical pain in regions unaffected by the initial pain complaint, and which may also result in dose escalation, i.e. 'analgesic tolerance'. A concept that has been gaining considerable experimental validation is that prolonged use of opioids elicits paradoxical, abnormal pain. This enhanced pain state requires additional opioids to maintain a constant level of antinociception, and consequently may be interpreted as antinociceptive tolerance. Many substances have been shown to block or reverse antinociceptive tolerance. A non-inclusive list of examples of substances reported to block or reverse opioid antinociceptive tolerance include: substance P receptor (NK-1) antagonists, calcitonin gene-related peptide (CGRP) receptor antagonists, nitric oxide (NO) synthase inhibitors, calcium channel blockers, cyclooxygenase (COX) inhibitors, protein kinase C inhibitors, competitive and non-competitive antagonists of the NMDA (N-methyl-D-aspartate) receptor, AMPA (alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid) antagonists, anti-dynorphin antiserum, and cholecystokinin (CCK) receptor antagonists. Without exception, these substances are also antagonists of pain-enhancing agents. Prolonged opiate administration indeed induces upregulation of substance P (SP) and calcitonin gene-related peptide (CGRP) within sensory fibers in vivo, and this is accompanied by an enhanced release of excitatory neurotransmitters and neuropeptides from primary afferent fibers upon stimulation. The enhanced evoked release of neuropeptides is correlated with the onset of abnormal pain states and opioid antinociceptive tolerance. Importantly, the descending pain modulatory pathway from the brainstem rostral ventromedial medulla (RVM) via the dorsolateral funiculus (DLF) is critical for maintaining the changes observed in the spinal cord, abnormal pain states and antinociceptive tolerance, because animals with lesion of the DLF did not show enhanced evoked neuropeptide release, or develop abnormal pain or antinociceptive tolerance upon sustained exposure to opiates. Microinjection of either lidocaine or a CCK antagonist into the RVM blocked both thermal and touch hypersensitivity as well as antinociceptive tolerance. Thus, prolonged opioid exposure enhances a descending pain facilitatory pathway from the RVM that is mediated at least in part by CCK activity and is essential for the maintenance of antinociceptive tolerance.

Lipopolysaccharide-induced sensitization of adenylyl cyclase activity in murine macrophages

LPS is known to modulate macrophage responses during sepsis, including cytokine release, phagocytosis, and proliferation. Although agents that elevate cAMP reverse LPS-induced macrophage functions, whether LPS itself modulates cAMP and whether LPS-induced decreases in proliferation are modulated via a cAMP-dependent pathway are not known. Murine macrophages (RAW264.7 cells) were treated with LPS in the presence or absence of inhibitors of prostaglandin signaling, protein kinases, CaM, Giproteins, and NF-κB translocation or transcription/translation. LPS effects on CaMKII phosphorylation and the expression of relevant adenylyl cyclase (AC) isoforms were measured. LPS caused a significant dose (5–10,000 ng/ml)- and time (1–8 h)-dependent increase in forskolin-stimulated AC activity that was abrogated by pretreatment with SN50 (an NF-κB inhibitor), actinomycin D, or cycloheximide, indicating that the effect is mediated via NF-κB-dependent transcription and new protein synthesis. Furthermore, LPS decreased the phosphorylation state of CaMKII, and pretreatment with a CaM antagonist attenuated the LPS-induced sensitization of AC. LPS, cAMP, or PKA activation each independently decreased macrophage proliferation. However, inhibition of NF-κB had no effect on LPS-induced decreased proliferation, indicating that LPS-induced decreased macrophage proliferation can proceed via PKA-independent signaling pathways. Taken together, these findings indicate that LPS induces sensitization of AC activity by augmenting the stimulatory effect of CaM and attenuating the inhibitory effect of CaMKII on isoforms of AC that are CaMK sensitive.

High-efficacy 5-hydroxytryptamine 1A receptor activation counteracts opioid hyperallodynia and affective conditioning

Pain may become intractable as tolerance develops to opioids and the opioids, paradoxically, induce pain. We examined the hypothesis that the analgesia produced by the novel analgesic and high-efficacy 5-hydroxytryptamine (5-HT)(1A) receptor agonist (3-chloro-4-fluoro-phenyl)-[4-fluoro-4-[[(5-methyl-pyridin-2-ylmethyl)-amino]methyl]piperidin-1-yl]methanone, fumaric acid salt (F 13640) may counteract opioid-induced pain. In studies of the somatosensory quality of pain in infraorbital nerve-injured rats, morphine infusion (5 mg/day) by means of osmotic pumps initially caused analgesia (i.e., decreased the behavioral response to von Frey filament stimulation), followed by hyperallodynia and analgesic tolerance. Infusion of F 13640 (0.63 mg/day) prevented the development of opioid hyperallodynia and reversed opioid hyperallodynia once established. In studies of the affective/motivational quality of pain, F 13640 both prevented and reversed the conditioned place aversion induced by naloxone (0.04 mg/kg i.p.) in morphine-infused rats; F 13640 also prevented and reversed the conditioned place preference induced by morphine injections (7.5 mg/kg i.p.). The data confirm that opioids produce bidirectional hypo- and proalgesic actions, and offer initial evidence that high-efficacy 5-HT(1A) receptor activation counteracts both the sensory and the affective/motivational qualities of opioid-induced pain. The data also indicate that F 13640 may be effective with opioid-resistant pain. It further is suggested that opioid addiction may represent self-therapy of opioid-induced pathological pain.

Dexras1 blocks receptor-mediated heterologous sensitization of adenylyl cyclase 1

Dexras1/AGS1/RasD1 is a member of the Ras superfamily of monomeric G proteins and has been suggested to disrupt receptor-G protein signaling. We examined the ability of Dexras1 to modulate dopamine D(2L) receptor regulation of adenylyl cyclase (AC) type 1 in HEK293 cells. Acute D(2L) receptor-mediated inhibition of A23187-stimulated AC1 activity (IC50, 4.0+/-1.4 nM; 50+/-3% inhibition) was not altered in the presence of Dexras1 (IC50, 2.4+/-1.3 nM, 50+/-1% inhibition); however, Dexras1 blocked acute D(2L) receptor-mediated activation of ERK 1/2 by approximately 50%. Heterologous sensitization of AC1 induced by persistent activation of D(2L) receptors was completely blocked by Dexras1 under basal and A23187-stimulated conditions. The block of sensitization was concentration-dependent and was not observed with a nucleotide binding-deficient Dexras1G31V mutant. Sensitization of AC1 was Gbetagamma-dependent as demonstrated using the C-terminus of beta-adrenergic receptor kinase (betaARK-ct). These data suggest that Dexras1 selectively regulates receptor-mediated Gbetagamma signaling pathways.

Activation of a novel PKC isoform synergistically enhances D2L dopamine receptor-mediated sensitization of adenylate cyclase type 6

Despite acutely inhibiting adenylate cyclase, prolonged activation of Galpha(i/o)-coupled receptors leads to a subsequent heterologous sensitization of adenylate cyclase responsiveness. Recently, protein kinase signaling and phosphorylation have been implicated in the sensitization of adenylate cyclase type 6 (AC6). To examine the sensitization specifically of AC6, we constructed human embryonic kidney cells (HEK293) cells stably expressing AC6 and the Galpha(i/o)-coupled D2L dopamine receptor. In contrast to observations in delta-opioid-expressing Chinese hamster ovary (CHO) cells that express endogenous AC6 and AC7, neither protein kinase C (PKC) nor tyrosine kinase inhibitors attenuated D2L receptor-mediated sensitization of AC6. Inhibition of Raf1 modestly inhibited the magnitude of D2L receptor-induced sensitization of AC6; however, activation of PKC robustly enhanced D2L receptor-mediated AC6 sensitization in a Raf1-dependent manner. These data indicate that, although PKC and Raf1 are not required for sensitization, activation of the PKC-Raf1 pathway robustly potentiated D2L receptor-mediated sensitization of AC6.

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.

Sensitization of adenylate cyclase by Galpha i/o-coupled receptors

Activation of receptors coupled to inhibitory G proteins (Galpha i/o) has opposing consequences for cyclic AMP accumulation and the activity of cyclic AMP-dependent protein kinase, depending on the duration of stimulation. Acute activation inhibits the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation; in contrast, persistent activation of Galpha i/o-coupled receptors produces a paradoxical enhancement of adenylate cyclase activity, thus increasing cyclic AMP accumulation when the action of the inhibitory receptor is terminated. This heterologous sensitization of cyclic AMP signaling, also called superactivation or supersensitization, likely represents a cellular adaptive response, a mechanism by which the cell compensates for chronic inhibitory input. Recent advances in our knowledge of G protein-mediated signaling, regulation of adenylate cyclase, and other cellular signaling mechanisms have extensively increased our insight into the mechanisms and significance of this phenomenon. In particular, recent evidence points to the Galpha(s)-adenylate cyclase interface as a locus for the expression of the sensitized adenylate cyclase response, and to isoform-specific phosphorylation of adenylate cyclase as one mechanism that can produce sensitization. Galpha i/o-coupled receptor-induced heterologous sensitization may contribute to enhanced Galpha(s)-coupled receptor signaling following neurotransmitter elevations induced by the administration of drugs of abuse and during other types of neuronal function or dysfunction. This review will focus on recent advances in our understanding of signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal function.

From inhibition to excitation: Functional effects of interaction between opioid receptors

Opioids have excitatory effects in multiple regions of the nervous system. Excitation by opioids is generally attributed to inhibition of inhibitory pathways (disinhibition). However, recent studies indicate that opioids can directly excite individual cells. These effects may occur when opioid receptors interact with other G protein coupled receptors, when different subtypes of opioid receptors interact, or when opioids transactivate other receptors such as receptor tyrosine kinases. Changes in the relative level of expression of different receptors in an individual cell may therefore determine its functional response to a given ligand. This phenomenon could represent an adaptive mechanism involved in tolerance, dependence and subsequent withdrawal.

Post-transcriptional gene silencing in neurons

The techniques evolving from the rapidly developing field of small RNAs promise accessible approaches to dissecting cellular and molecular mechanisms of higher brain function. Here, a current overview of the technology is presented, along with an outline of how these approaches might help neuroscientists to more rapidly uncover the cellular and molecular bases of behavior.

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.