Morphine-3beta-D-glucuronide suppresses inhibitory synaptic transmission in rat substantia gelatinosa

Morphine-3-Glucuronide, Physiology and Behavior

Morphine remains the gold standard painkiller available to date to relieve severe pain. Morphine metabolism leads to the production of two predominant metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). This metabolism involves uridine 5'-diphospho-glucuronosyltransferases (UGTs), which catalyze the addition of a glucuronide moiety onto the C3 or C6 position of morphine. Interestingly, M3G and M6G have been shown to be biologically active. On the one hand, M6G produces potent analgesia in rodents and humans. On the other hand, M3G provokes a state of strong excitation in rodents, characterized by thermal hyperalgesia and tactile allodynia. Its coadministration with morphine or M6G also reduces the resulting analgesia. Although these behavioral effects show quite consistency in rodents, M3G effects are much more debated in humans and the identity of the receptor(s) on which M3G acts remains unclear. Indeed, M3G has little affinity for mu opioid receptor (MOR) (on which morphine binds) and its effects are retained in the presence of naloxone or naltrexone, two non-selective MOR antagonists. Paradoxically, MOR seems to be essential to M3G effects. In contrast, several studies proposed that TLR4 could mediate M3G effects since this receptor also appears to be essential to M3G-induced hyperalgesia. This review summarizes M3G's behavioral effects and potential targets in the central nervous system, as well as the mechanisms by which it might oppose analgesia.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

TRPV1 Nociceptor Activity Initiates USP5/T-type Channel-Mediated Plasticity

Peripheral nerve injury and tissue inflammation result in upregulation of the deubiquitinase USP5, thus causing a dysregulation of T-type calcium channel activity and increased pain sensitivity. Here, we have explored the role of afferent fiber activity in this process. Conditioning stimulation of optogenetically targeted cutaneous TRPV1 expressing nociceptors, but not that of non-nociceptive fibers, resulted in enhanced expression of USP5 in mouse dorsal root ganglia and spinal dorsal horn, along with decreased withdrawal thresholds for thermal and mechanical stimuli that abated after 24 hr. This sensitization was drastically reduced by an interfering peptide that prevented USP5-Cav3.2 association. Sensitization was relieved by pharmacological block of TRPV1 afferents, but not of myelinated neurons. In spinal cord slice recordings, we could optogenetically trigger an activity-dependent potentiation of presynaptic neurotransmission in the spinal dorsal horn that relied on Cav3.2 channel activity. This neuronal-activity-induced USP5 upregulation may underlie a protective, transient sensitization of the pain pathway.

[Altered expression of transporter and analgesic of morphine in neuropathic pain mice]

It is known that morphine is less effective for patients with neuropathic pain, accounting for approximately 70% of cancer patients with severe pain. One of the causes of the decline is reported as a decreased function of the μ-opioid receptor, which binds to the active metabolites of morphine in the mesencephalic ventral tegmental area. However, the details of this mechanism are not understood. We hypothesized that a decrease in the concentration of morphine in the brain reduces its analgesic effect on neuropathic pain, and found that the analgesic effect of morphine was correlated with its concentration in the brain. We examined the reason for the decreased concentration of morphine in the brain in case of neuropathic pain. We discovered increased P-glycoprotein (P-gp) expression in the small intestine, increased expression and activity of UGT2B in the liver, and increased P-gp expression in the brain under conditions of neuropathic pain. In this symposium, we argue that low brain morphine concentration is considered one of the causes of lower sensitivity to morphine in neuropathic pain patients.

Plasma morphine and metabolite concentrations are associated with clinical effects of morphine in cancer patients

CONTEXT

Morphine is the opioid of choice for cancer-related pain, but for many patients the benefits of morphine are outweighed by its side effect profile. Morphine is metabolized to morphine-3-glucuronide and morphine-6-glucuronide; however, little is known about the contribution of these metabolites to analgesia and morphine-related side effects.

OBJECTIVES

We investigated the association between plasma morphine and metabolite concentrations and the clinical effects of morphine in cancer patients.

METHODS

A prospective study was performed in cancer patients taking oral morphine for moderate-to-severe cancer pain. Subjects who responded well to morphine (responders) and subjects who failed to respond to morphine because of lack of analgesia and/or the presence of intolerable side effects (nonresponders/switchers) were recruited. Pain and toxicity scores were recorded and blood samples were analyzed for plasma morphine, morphine-3-glucuronide, and morphine-6-glucuronide concentrations.

RESULTS

Results showed that 1) morphine responders have higher plasma morphine and metabolite concentrations compared with nonresponders, 2) lower pain scores are associated with higher plasma morphine and metabolite concentrations, 3) central side effects are associated with a higher metabolite:plasma morphine ratio, and 4) myoclonus is associated with extremely high concentrations of plasma morphine and metabolites.

CONCLUSION

This study has shown that plasma morphine and metabolite concentrations are associated with the clinical effects of morphine therapy. These results are important because they demonstrate the relevance of measuring plasma metabolite concentrations in clinical trials and the potential for metabolite data to deepen our understanding of factors that influence an individual's response to morphine.

A perspective on efflux transport proteins in the liver

Detailed knowledge regarding the influence of hepatic transport proteins on drug disposition has advanced at a rapid pace over the past decade. Efflux transport proteins located in the basolateral and apical (canalicular) membranes of hepatocytes play an important role in the hepatic elimination of many endogenous and exogenous compounds, including drugs and metabolites. This review focuses on the role of these efflux transporters in hepatic drug excretion. The impact of these proteins as underlying factors for disease is highlighted, and the importance of hepatic efflux proteins in the efficacy and toxicity of drugs is discussed. In addition, a brief overview of methodology to evaluate the function of hepatic efflux transport proteins is provided. Current challenges in predicting the impact of altered efflux protein function on systemic, intestinal, and hepatocyte exposure to drugs and metabolites are highlighted.

Neuroexcitatory effects of morphine-3-glucuronide are dependent on Toll-like receptor 4 signaling

BACKGROUND

Multiple adverse events are associated with the use of morphine for the treatment of chronic non-cancer pain, including opioid-induced hyperalgesia (OIH). Mechanisms of OIH are independent of opioid tolerance and may involve the morphine metabolite morphine-3-glucuronide (M3G). M3G exhibits limited affinity for opioid receptors and no analgesic effect. Previous reports suggest that M3G can act via the Toll-like receptor 4 (TLR4)/myeloid differentiation protein-2 (MD-2) heterodimer in the central nervous system to elicit pain.

METHODS

Immunoblot and immunocytochemistry methods were used to characterize the protein expression of TLR4 present in lumbar dorsal root ganglion (DRG). Using in vitro intracellular calcium and current clamp techniques, we determined whether TLR4 activation as elicited by the prototypical agonists of TLR4, lipopolysaccharide (LPS) and M3G, contributed to changes in intracellular calcium and increased excitation. Rodents were also injected with M3G to determine the degree to which M3G-induced tactile hyperalgesia could be diminished using either a small molecule inhibitor of the MD-2/TLR4 complex in rats or TLR4 knockout mice. Whole cell voltage-clamp recordings were made from small- and medium-diameter DRG neurons (25 μm < DRG diameter <45 μm) for both control and M3G-treated neurons to determine the potential influence on voltage-gated sodium channels (NaVs).

RESULTS

We observed that TLR4 immunoreactivity was present in peptidergic and non-peptidergic sensory neurons in the DRG. Non-neuronal cells in the DRG lacked evidence of TLR4 expression. Approximately 15% of assayed small- and medium-diameter sensory neurons exhibited a change in intracellular calcium following LPS administration. Both nociceptive and non-nociceptive neurons were observed to respond, and approximately 40% of these cells were capsaicin-insensitive. Increased excitability observed in sensory neurons following LPS or M3G could be eliminated using Compound 15, a small molecule inhibitor of the TLR4/MD-2 complex. Likewise, systemic injection of M3G induced rapid tactile, but not thermal, nociceptive behavioral changes in the rat, which were prevented by pre-treating animals with Compound 15. Unlike TLR4 wild-type mice, TLR4 knockout mice did not exhibit M3G-induced hyperalgesia. As abnormal pain sensitivity is often associated with NaVs, we predicted that M3G acting via the MD-2/TLR4 complex may affect the density and gating of NaVs in sensory neurons. We show that M3G increases tetrodotoxin-sensitive and tetrodotoxin-resistant (NaV1.9) current densities.

CONCLUSIONS

These outcomes provide evidence that M3G may play a role in OIH via the TLR4/MD-2 heterodimer complex and biophysical properties of tetrodotoxin-sensitive and tetrodotoxin-resistant NaV currents.

Comparative studies of the neuro-excitatory behavioural effects of morphine-3-glucuronide and dynorphin A(2-17) following spinal and supraspinal routes of administration

Morphine-3-glucuronide (M3G) administered centrally produces dose-dependent neuro-excitatory behaviours in rodents via a predominantly non-opioid mechanism. The endogenous opioid peptide, dynorphin A (Dyn A) (1-17), is rapidly cleaved in vivo to the relatively more stable fragment Dyn A(2-17) which also produces excitatory behaviours in rodents via a non-opioid mechanism. This study investigated the possible contribution of Dyn A(2-17) to the neuro-excitatory behaviours evoked by supraspinally and spinally administered M3G in male Sprague-Dawley (SD) rats. Marked qualitative differences in behaviours were apparent following administration of M3G and Dyn A(2-17). Administration of 11 nmol i.c.v. doses of M3G produced intermittent myoclonic jerks, tonic-clonic convulsions, and ataxia, as well as postural changes, whereas i.c.v. Dyn A(2-17) at 15 nmol produced effects on body posture alone. Administration of 11 nmol i.t. doses of M3G produced intermittent explosive motor activity, and touch-evoked agitation, as well as postural changes, whereas i.t. Dyn A(2-17) at 15 nmol produced postural changes, touch-evoked agitation, and paralysis. Pre-treatment with Dyn A antiserum (200 microg) markedly attenuated total behavioural excitation following i.c.v. and i.t. administration of Dyn A(2-17) by approximately 94% and 78%, respectively. However, total behavioural excitation following i.c.v. and i.t. administration of M3G was less markedly attenuated (both approximately 27%) by pre-treatment with Dyn A antiserum, with reductions in tonic-clonic convulsions ( approximately 43%), explosive motor behaviour ( approximately 28%), and touch-evoked agitation ( approximately 22%). The present findings discount a major role for Dyn A in mediating the neuro-excitatory effects of M3G, although it may contribute to maintaining some individual neuro-excitatory behaviours.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Fentanyl-induced neurotoxicity and paradoxic pain

A patient with pain associated with metastatic leiomyosarcoma received escalating doses of opioids. Upon discontinuation of intravenous morphine, transdermal fentanyl was initiated, and after several days, the dose was increased to 200microg/hour for persistent, severe pain. The patient became somnolent, and further dose adjustments and route change were carried out. She then exhibited severe allodynia, myoclonus, and delirium thereafter fentanyl was stopped. All symptoms resolved with discontinuation of fentanyl and subsequent introduction of a weak opioid. Pain was well controlled. Gradually increasing standard doses of fentanyl may lead to severe neurotoxicity, which may respond to opioid discontinuation and/or rotation. Vigilant scrutiny of all possible causes of apparent analgesic failure followed by consideration of opioid reduction and rotation is warranted in cases of neurotoxicity accompanying opioid treatment.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.

Multidrug Resistance Proteins 2 and 3 Provide Alternative Routes for Hepatic Excretion of Morphine-Glucuronides

Glucuronidation is a major hepatic detoxification pathway for endogenous and exogenous compounds, resulting in the intracellular formation of polar metabolites that require specialized transporters for elimination. Multidrug resistance proteins (MRPs) are expressed in the liver and can transport glucuronosyl-conjugates. Using morphine as a model aglycone, we demonstrate that morphine-3-glucuronide (M3G), the predominant metabolite, is transported in vitro by human MRP2 (ABCC2), a protein present in the apical membrane of hepatocytes. Loss of biliary M3G secretion in Mrp2(-/-) mice results in its increased sinusoidal transport that can be attributed to Mrp3. Combined loss of Mrp2 and Mrp3 leads to a substantial accumulation of M3G in the liver, from which it is transported across the sinusoidal membrane at a low rate, resulting in the prolonged presence of M3G in plasma. Our results show that murine Mrp2 and Mrp3 provide alternative routes for the excretion of a glucuronidated substrate from the liver in vivo.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.

Sciatic chronic constriction injury produces cell-type-specific changes in the electrophysiological properties of rat substantia gelatinosa neurons

Peripheral nerve injury increases spontaneous action potential discharge in spinal dorsal horn neurons and augments their response to peripheral stimulation. This "central hypersensitivity, " which relates to the onset and persistence of neuropathic pain, reflects spontaneous activity in primary afferent fibers as well as long-term changes in the intrinsic properties of the dorsal horn (centralization). To isolate and investigate cellular mechanisms underlying "centralization," sciatic nerves of 20-day-old rats were subjected to 13-25 days of chronic constriction injury (CCI; Mosconi-Kruger polyethylene cuff model). Spinal cord slices were then acutely prepared from sham-operated or CCI animals, and whole cell recording was used to compare the properties of five types of substantia gelatinosa neuron. These were defined as tonic, irregular, phasic, transient, or delay according to their discharge pattern in response to depolarizing current. CCI did not affect resting membrane potential, rheobase, or input resistance in any neuron type but increased the amplitude and frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) in delay, transient, and irregular cells. These changes involved alterations in the action potential-independent neurotransmitter release machinery and possible increases in the postsynaptic effectiveness of glutamate. By contrast, in tonic cells, CCI reduced the amplitude and frequency of spontaneous and miniature EPSCs. Such changes may relate to the putative role of tonic cells as inhibitory GABAergic interneurons, whereas increased synaptic drive to delay cells may relate to their putative role as the excitatory output neurons of the substantia gelatinosa. Complementary changes in synaptic excitation of inhibitory and excitatory neurons may thus contribute to pain centralization.

Integration of hepatic drug transporters and phase II metabolizing enzymes: Mechanisms of hepatic excretion of sulfate, glucuronide, and glutathione metabolites

The liver is the primary site of drug metabolism in the body. Typically, metabolic conversion of a drug results in inactivation, detoxification, and enhanced likelihood for excretion in urine or feces. Sulfation, glucuronidation, and glutathione conjugation represent the three most prevalent classes of phase II metabolism, which may occur directly on the parent compounds that contain appropriate structural motifs, or, as is usually the case, on functional groups added or exposed by phase I oxidation. These three conjugation reactions increase the molecular weight and water solubility of the compound, in addition to adding a negative charge to the molecule. As a result of these changes in the physicochemical properties, phase II conjugates tend to have very poor membrane permeability, and necessitate carrier-mediated transport for biliary or hepatic basolateral excretion into sinusoidal blood for eventual excretion into urine. This review summarizes sulfation, glucuronidation, and glutathione conjugation reactions, as well as recent progress in elucidating the hepatic transport mechanisms responsible for the excretion of these conjugates from the liver. The discussion focuses on alterations of metabolism and transport by chemical modulators, and disease states, as well as pharmacodynamic and toxicological implications of hepatic metabolism and/or transport modulation for certain active phase II conjugates. A brief discussion of issues that must be considered in the design and interpretation of phase II metabolite transport studies follows.

Mu-opioid-mediated inhibition of glutamate synaptic transmission in rat central amygdala neurons

The central nucleus of the amygdala (CeA) plays an important role both in stimulus-reward learning for the reinforcing effects of drugs of abuse and in environmental condition-induced analgesia. Both of these two CeA functions involve the opioid system within the CeA. However, the pharmacological profiles of its opioid receptor system have not been fully studied and the synaptic actions of opioid receptors in the CeA are largely unknown. In this study with whole-cell voltage-clamp recordings in brain slices in vitro, we examined actions of opioid agonists on glutamate-mediated excitatory postsynaptic currents (EPSCs) in CeA neurons. Opioid peptide methionine-enkephalin (ME; 10 microM) produced a significant inhibition (38%) in the amplitude of evoked EPSCs, an action mimicked by the mu-opioid receptor agonist [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO; 1 microM, 44%). Both effects of ME and DAMGO were abolished by the mu receptor antagonist CTAP (1 microM), suggesting a mu receptor-mediated effect. Neither delta-opioid receptor agonist [D-Pen(2),D-Pen(5)]-enkephalin (1 microM) nor kappa-opioid receptor agonist U69593 (300 nM) had any effect on the glutamate EPSC. ME significantly increased the paired-pulse ratio of the evoked EPSCs and decreased the frequency of miniature EPSCs without altering the amplitude of miniature EPSCs. Furthermore, the mu-opioid inhibition of the EPSC was blocked by 4-aminopyridine (4AP; 100 microM), a voltage-dependent potassium channel blocker, and by phospholipase A(2) inhibitors AACOCF(3) (10 microM) and quinacrine (10 microM). These results indicate that only the mu-opioid receptor is functionally present on presynaptic glutamatergic terminals in normal CeA neurons, and its activation reduces the probability of glutamate release through a signaling pathway involving phospholipase A(2) and the presynaptic, 4AP-sensitive potassium channel. This study provides evidence for the presynaptic regulation of glutamate synaptic transmission by mu-opioid receptors in CeA neurons.

Mice lacking multidrug resistance protein 3 show altered morphine pharmacokinetics and morphine-6-glucuronide antinociception

Glucuronidation is a major detoxification pathway for endogenous and exogenous compounds in mammals that results in the intracellular formation of polar metabolites, requiring specialized transporters to cross biological membranes. By using morphine as a model aglycone, we demonstrate that multidrug resistance protein 3 (MRP3/ABCC3), a protein present in the basolateral membrane of polarized cells, transports morphine-3-glucuronide (M3G) and morphine-6-glucuronide in vitro. Mrp3(-/-) mice are unable to excrete M3G from the liver into the bloodstream, the major hepatic elimination route for this drug. This results in increased levels of M3G in liver and bile, a 50-fold reduction in the plasma levels of M3G, and in a major shift in the main disposition route for morphine and M3G, predominantly via the urine in WT mice but via the feces in Mrp3(-/-) mice. The pharamacokinetics of injected morphine-glucuronides are altered as well in the absence of Mrp3, and this results in a decreased antinociceptive potency of injected morphine-6-glucuronide.

Subanesthetic ketamine infusion therapy: a retrospective analysis of a novel therapeutic approach to complex regional pain syndrome

Complex Regional Pain Syndrome (CRPS) is a disorder that can be accompanied by severe pain that is often both chronic and resistant to conventional therapy. Harbut and Correll previously reported the successful treatment of a 9-year case of intractable Type I CRPS with an intravenous inpatient infusion of ketamine in an adult female patient.

OBJECTIVE

The purpose of this study was to ascertain if indeed the use of subanesthetic inpatient infusions of ketamine provide meaningful improvements in pain scores, and thus, quality of life, in patients suffering from CRPS. To achieve this objective we focused our analysis on the relief of pain obtained by patients undergoing this novel treatment option developed at Mackay Base Hospital, Queensland, Australia.

METHODS

Case notes of 33 patients whose CRPS pain was treated by the inpatient administration of a continuous subanesthetic intravenous infusion of ketamine were reviewed. The dose and duration of ketamine therapy and the degree and duration of relief obtained were recorded. Notable side effects were also recorded. The degree of relief obtained (immediately after the infusion) was assessed using pre- and posttreatment numeric pain scores. The duration of relief obtained (throughout the follow-up period) was analyzed using a Kaplan-Meier cumulative survival curve analysis.

RESULTS

A total of 33 patients with diagnoses of CRPS who had undergone ketamine treatment at least once were identified. Due to relapse, 12 of 33 patients received a second course of therapy, and two of 33 patients received a third. The degree of relief obtained following the initial course of therapy was impressive (N=33); there was complete pain relief in 25 (76%), partial relief in six (18%), and no relief in two (6%) patients. The degree of relief obtained following repeat therapy (N=12) appeared even better, as all 12 patients who received second courses of treatment experienced complete relief of their CRPS pain. The duration of relief was also impressive, as was the difference between the duration of relief obtained after the first and after the second courses of therapy. In this respect, following the first course of therapy, 54% of 33 individuals remained pain free for >/=3 months and 31% remained pain free for >/=6 months. After the second infusion, 58% of 12 patients experienced relief for >/=1 year, while almost 33% remained pain free for >3 years. The most frequent side effect observed in patients receiving this treatment was a feeling of inebriation. Hallucinations occurred in six patients. Less frequent side effects also included complaints of lightheadedness, dizziness, and nausea. In four patients, an alteration in hepatic enzyme profile was noted; the infusion was terminated and the abnormality resolved thereafter.

CONCLUSION

This retrospective review suggests that limited subanesthetic inpatient infusions of ketamine may offer a promising therapeutic option in the treatment of appropriately selected patients with intractable CRPS. More study is needed to further establish the safety and efficacy of this novel approach.

Opioid-Like Actions of Neuropeptide Y in Rat Substantia Gelatinosa: Y1 Suppression of Inhibition and Y2 Suppression of Excitation

Neuropathic pain that results from injury to the peripheral or CNS responds poorly to opioid analgesics. Y1 and Y2 receptors for neuropeptide Y (NPY) may, however, serve as targets for analgesics that retain their effectiveness in neuropathic pain states. In substantia gelatinosa neurons in spinal cord slices from adult rats, we find that NPY acts via presynaptic Y2 receptors to attenuate excitatory postsynaptic currents (EPSCs) and predominantly on presynaptic Y1 receptors to attenuate glycinergic and GABAergic inhibitory postsynaptic currents (IPSCs). Because NPY attenuates the frequency of TTX-resistant miniature EPSCs and IPSCs, perturbation of the neurotransmitter release process contributes to its actions at both excitatory and inhibitory synapses. These effects, which are reminiscent of those produced by analgesic opioids, provide a cellular basis for previously documented spinal analgesic actions mediated via Y1 and Y2 receptors in neuropathic pain paradigms. They also underline the importance of suppression of inhibition in spinal analgesic mechanisms.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Effects of endomorphin on substantia gelatinosa neurons in rat spinal cord slices

1. Whole-cell patch recordings were made from substantia gelatinosa (SG) neurons in transverse lumbar spinal cord slices of 15- to 30-day-old rats. 2. Endomorphin 1 (EM-1) or EM-2 (<or=10 microM) hyperpolarized or induced an outward current in 26 of the 66 SG neurons. The I-V relationship showed that the peptide activates an inwardly rectifying K+ current. 3. EM-1 or EM-2 (0.3-10 microM) suppressed short-latency excitatory postsynaptic currents (EPSCs) and long-latency inhibitory postsynaptic currents (IPSCs) in nearly all SG neurons tested or short-latency IPSCs in six of the 10 SG neurons. [Met5] enkephalin or [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (1-10 microM) depressed EPSCs and IPSCs. EM-1 or EM-2 depressed synaptic responses without causing a significant change in holding currents or inward currents induced by glutamate. 4. Glutamate also evoked a short-latency outward current in five SG neurons or a biphasic current in two neurons; the outward current was blocked by tetrodotoxin (TTX, 0.3 microM) or bicuculline (10 microM). 5. EM-1 or DAMGO (1 or 5 microM) attenuated the glutamate-evoked outward or biphasic currents in four of the seven SG neurons. EM-1 (1 microm) reduced the frequency, but not the amplitude of miniature EPSCs or miniature IPSCs. 6.. Naloxone (1 microM) or the selective micro-opioid receptor antagonist beta-funaltrexamine (beta-FNA, 25 microM) antagonized the action of EM; EM-induced hyperpolarizations persisted in the presence of the kappa-opioid receptor antagonist (nor-binaltorphimine dihydrochloride, 1 microM) and/or sigma-opioid receptor antagonist (naltrindole hydrochloride, 1 microM). 7. It may be concluded that EM acting on micro-opioid receptors hyperpolarizes a population of SG neurons by activating an inwardly rectifying K+ current, and attenuates excitatory and inhibitory synaptic currents evoked in a population of SG neurons, probably by a presynaptic site of action.

Morphine-3-glucuronide's neuro-excitatory effects are mediated via indirect activation of N-methyl-D-aspartic acid receptors: mechanistic studies in embryonic cultured hippocampal neurones

Indirect evidence indicates that morphine-3-glucuronide (M3G) may contribute significantly to the neuro-excitatory side effects (myoclonus and allodynia) of large-dose systemic morphine. To gain insight into the mechanism underlying M3G's excitatory behaviors, we used fluo-3 fluorescence digital imaging techniques to assess the acute effects of M3G (5-500 microM) on the cytosolic calcium concentration ([Ca(2+)](CYT)) in cultured embryonic hippocampal neurones. Acute (3 min) exposure of neurones to M3G evoked [Ca(2+)](CYT) transients that were typically either (a) transient oscillatory responses characterized by a rapid increase in [Ca(2+)](CYT) oscillation amplitude that was sustained for at least approximately 30 s or (b) a sustained increase in [Ca(2+)](CYT) that slowly recovered to baseline. Naloxone-pretreatment decreased the proportion of M3G-responsive neurones by 10%-25%, implicating a predominantly non-opioidergic mechanism. Although the naloxone-insensitive M3G-induced increases in [Ca(2+)](CYT) were completely blocked by N-methyl-D-aspartic acid (NMDA) antagonists and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate antagonist), CNQX did not block the large increase in [Ca(2+)](CYT) evoked by NMDA (as expected), confirming that M3G indirectly activates the NMDA receptor. Additionally, tetrodotoxin (Na(+) channel blocker), baclofen (gamma-aminobutyric acid(B) agonist), MVIIC (P/Q-type calcium channel blocker), and nifedipine (L-type calcium channel blocker) all abolished M3G-induced increases in [Ca(2+)](CYT), suggesting that M3G may produce its neuro-excitatory effects by modulating neurotransmitter release. However, additional characterization is required.

IMPLICATIONS

Large systemic doses of morphine administered to some patients for cancer pain management have been reported to produce myoclonus and allodynia. Indirect evidence implicates the major morphine metabolite, morphine-3-glucuronide (M3G), in these neuro-excitatory side effects. Hence, this study was designed to gain insight into the cellular mechanism responsible for M3G's neuro-excitatory actions.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Effects and consequences of nerve injury on the electrical properties of sensory neurons

Nociceptive pain alerts the body to potential or actual tissue damage. By contrast, neuropathic or "noninflammatory" pain, which results from injury to the nervous system, serves no useful purpose. It typically continues for years after the original injury has healed. Sciatic nerve lesions can invoke chronic neuropathic pain that is accompanied by persistent, spontaneous activity in primary afferent fibers. This activity, which reflects changes in the properties and functional expression of Na+, K+, and Ca2+ channels, initiates a further increase in the excitability of second-order sensory neurons in the dorsal horn. This change persists for many weeks. The source of origin of the pain thus moves from the peripheral to the central nervous system. We hypothesize that this centralization of pain involves the inappropriate release of peptidergic neuromodulators from primary afferent fibers. Peptides such as substance P, neuropeptide Y (NPY), calcitonin-gene-related peptide (CGRP), and brain-derived neurotrophic factor (BDNF) may promote enduring changes in excitability as a consequence of neurotrophic actions on ion channel expression in the dorsal horn. Findings that form the basis of this hypothesis are reviewed. Study of the neurotrophic control of ion channel expression by spinal peptides may thus provide new insights into the etiology of neuropathic pain.