Topische Applikation von Lokalanästhetika und Opioiden nach elektiver Zahnextraktion

The aim of this study was to provide evidence for the peripheral effect of opioids in oromaxillary procedures and to investigate whether postoperative morphine sulfate plus lidocaine administered as a local spray achieve better analgesic efficacy than lidocaine applied alone. The double-blind randomized study included 60 patients. The patients exhibited neither preoperative pain nor inflammation (tooth extractions) and when pain occurred received as alternative medication diclofenac retard 50 mg or tramadol. Side effects were documented. No differences between the two groups were observed in the demographic data, visual and numeric analog scale in the first 24 h, or in the consumption of analgesics. Since the patients presented in a pain-free state at the time of surgery, apparently no activation of the peripheral opioid receptors occurs. Thus, a peripheral effect of opioids could not be demonstrated in this model.

Rapid upregulation of mu opioid receptor mRNA in dorsal root ganglia in response to peripheral inflammation depends on neuronal conduction

S.c. painful inflammation leads to an increase in axonal transport of opioid receptors from dorsal root ganglia (DRG) toward the periphery, thus causing a higher receptor density and enhanced opioid analgesia at the injured site. To examine whether this increase is related to transcription, the mRNA of Delta- (DOR) and mu-opioid receptor (MOR) in lumbar DRG was quantified by real time Light Cycler polymerase chain reaction (LC-PCR), and correlated to ligand binding in DRG and sciatic nerve. In normal DRG, DOR mRNA was seven times less abundantly expressed than MOR mRNA. After induction of unilateral paw inflammation, mRNA content for DOR remained unchanged, but a bi-phasic upregulation for MOR mRNA with an early peak at 1-2 h and a late increase at 96 h was found in ipsilateral DRG. As no changes were observed in DRG of the non-inflamed side, this effect was apparently not systemically mediated. A significant increase in binding of the MOR ligand DAMGO was detected after 24 h in DRG, and after early and late ligation in the sciatic nerve, indicating an enhanced axonal transport of MOR in response to inflammation. The early increase in MOR mRNA could be completely prevented by local anesthetic blockade of neuronal conduction in sciatic nerve. These data suggest that mRNA of the two opioid receptors DOR and MOR is differentially regulated in DRG during peripheral painful inflammation. The apparently increased axonal transport of MOR in response to this inflammation is preceded by upregulated mRNA-transcription, which is dependent on neuronal electrical activity.

High dose vitamin E therapy in amyotrophic lateral sclerosis as add-on therapy to riluzole: results of a placebo-controlled double-blind study

Increasing evidence has suggested that oxidative stress may be involved in the pathogenesis of amyotrophic lateral sclerosis (ALS). The antioxidant vitamin E (alpha-tocopherol) has been shown to slow down the onset and progression of the paralysis in transgenic mice expressing a mutation in the superoxide dismutase gene found in certain forms of familial ALS. The current study, a double blind, placebo-controlled, randomised, stratified, parallel-group clinical trial, was designed to determine whether vitamin E (5000 mg per day) may be efficacious in slowing down disease progression when added to riluzole.

METHODS

160 patients in 6 German centres with either probable or definite ALS (according to the El Escorial Criteria) and a disease duration of less than 5 years, treated with riluzole, were included in this study and were randomly assigned to receive either alpha-tocopherol (5000 mg per day) or placebo for 18 months. The Primary outcome measure was survival, calculating time to death, tracheostomy or permanent assisted ventilation, according to the WFN-Criteria of clinical trials. Secondary outcome measures were the rate of deterioration of function assessed by the modified Norris limb and bulbar scales, manual muscle testing (BMRC), spasticity scale, ventilatory function and the Sickness Impact Profile (SIP ALS/19). Patients were assessed at entry and every 4 months thereafter during the study period until month 16 and at a final visit at month 18. Vitamin E samples were taken for compliance check and Quality Control of the trial. For Safety, a physical examination was performed at baseline and then every visit until the treatment discontinuation at month 18. Height and weight were recorded at baseline and weight alone at the follow-up visits. A neurological examination as well as vital signs (heart rate and blood pressure), an ECG and VEP's were recorded at each visit. Furthermore, spontaneously reported adverse experiences and serious adverse events were documented and standard laboratory tests including liver function tests performed. For Statistical Analysis, the population to be considered for the primary outcome measure was an "intent-to-treat" (ITT) population which included all randomised patients who had received at least one treatment dose (n = 160 patients). For the secondary outcome measures, a two way analysis of variance was performed on a patient population that included all randomised patients who had at least one assessment after inclusion.

RESULTS

Concerning the primary endpoint, no significant difference between placebo and treatment group could be detected either with the stratified Logrank or the Wilcoxon test. The functional assessments showed a marginal trend in favour of vitamin E, without reaching significance.

CONCLUSION

Neither the primary nor the secondary outcome measures could determine whether a megadose of vitamin E is efficacious in slowing disease progression in ALS as an add-on therapy to riluzol. Larger or longer studies might be needed. However, administration of this megadose does not seem to have any significant side effects in this patient population.

Increased numbers of opioid expressing inflammatory cells do not affect intra-articular morphine analgesia

BACKGROUND

Both locally expressed beta-endorphin (END) and low doses of morphine relieve pain within inflamed knee joints. Here we examined whether enhanced inflammation and END expression within the synovial tissue of patients undergoing arthroscopic knee surgery might shift the analgesic dose-response curve of intra-articular (i.a.) morphine.

METHODS

Following IRB approval and informed consent, patients were randomly assigned to the following i.a. treatments at the end of surgery: group I (n=39), isotonic saline; group II (n=40), 1 mg morphine hydrochloride; group III (n=48), 2 mg morphine hydrochloride; group IV (n=39), 4 mg morphine hydrochloride. Postoperative pain intensity was assessed by the visual analogue scale (VAS), by the time to first analgesic request and by the supplemental piritramide consumption. Synovial specimens from each patient were stained for the presence of inflammatory cells and END and were discriminated into groups with low versus high numbers of these cells. Differences between groups were statistically analyzed by chi(2), anova and mancova where appropiate.

RESULTS

Patient characteristics and VAS scores did not differ between groups. Total postoperative piritramide consumption decreased and the time to first analgesic request increased significantly with increasing doses of i.a. morphine (P<0.05, anova and linear regression). These dose-response relationships were not different between patients with low versus high numbers of inflammatory and END-containing synovial cells (P>0.05, mancova).

CONCLUSIONS

The dose-response relationship of i.a. morphine analgesia is not shifted by enhanced inflammation and END expression within synovial tissue. Thus, the presence of END within inflamed synovial tissue does not seem to interfere with i.a. morphine analgesia.

Painful inflammation-induced increase in mu-opioid receptor binding and G-protein coupling in primary afferent neurons

Opioids mediate their analgesic effects by activating mu-opioid receptors (MOR) not only within the central nervous system but also on peripheral sensory neurons. The peripheral analgesic effects of opioids are best described under inflammatory conditions (e.g., arthritis). The present study investigated the effects of inflammation on MOR binding and G-protein coupling of full versus partial MOR agonists in dorsal root ganglia (DRG) of primary afferent neurons. Our results show that Freund's complete adjuvant (FCA) unilateral hindpaw inflammation induces a significant up-regulation of MOR binding sites (25 to 47 fmol/mg of protein) on DRG membranes without affecting the affinity of either full or partial MOR agonists. In our immunohistochemical studies, the number of MOR-immunoreactive neurons consistently increased. This increase was mostly caused by small-diameter nociceptive DRG neurons. The full agonist DAMGO induced MOR G-protein coupling in DRG of animals without FCA inflammation (EC50 = 56 nM; relative Emax = 100%). FCA inflammation resulted in significant increases in DAMGO-induced MOR G-protein coupling (EC50 = 29 nM; relative Emax = 145%). The partial agonist buprenorphine hydrochloride (BUP) showed no detectable G-protein coupling in DRG of animals without FCA inflammation; however, partial agonist activity of BUP-induced MOR G-protein coupling was detectable in animals with FCA inflammation (EC50 = 1.6 nM; relative Emax = 82%). In behavioral studies, administration of BUP produced significant antinociception only in inflamed but not in noninflamed paws. These findings show that inflammation causes changes in MOR binding and G-protein coupling in primary afferent neurons. They further underscore the important differences in clinical studies testing peripherally active opioids in inflammatory painful conditions.

Peripheral Opioid Analgesia

Opioids have long been thought to act exclusively within the central nervous system. An increasing number of studies recently reported the existence of opioid receptors outside the central nervous system and therefore suggested that opioids are also able to produce analgesic effects in the periphery. Such effects are particularly prominent under painful inflammatory conditions, both in animals and in humans. During inflammatory processes, opioid receptors are transported from dorsal root ganglia towards the peripheral sensory nerve endings. At the same time, immune cells containing endogenous opioid peptides accumulate within the inflamed tissue. Environmental stimuli (e.g. stress) as well as releasing agents (e.g. corticotropin releasing factor, cytokines) can liberate these opioid peptides to interact with the neuronal opioid receptors and elicit local analgesia. The inflammation-induced activation of opioid production and the release of endogenous opioids from immune cells may lead to novel approaches for the development of peripherally acting analgesics. Clinical investigation now focuses on the development of new peripheral opioid agonists as well as on ways to stimulate the endogenous analgesic system in order to induce effective peripheral analgesia with reduced central side effects typically associated with opioids.

[Opioid therapy in chronic non-malignant pain]

In long-term treatment opioids seem to have only minimal side-effects compared with other analgesics and co-analgesics.Nevertheless, some risks have to be considered. While immunosuppression, neurotoxicity, teratogenity, tolerance and addiction are clinically not relevant or very rare, cognitive impairment, sedation and obstipation may have a clinical impact.However, these symptoms can usually be managed by adjuvant medication and patient education. Treatment of non-malignant pain with opioids can only be considered on an individual basis. Scientific evidence for general treatment with opioids, treatment of specific pain syndromes or treatment with certain opioids is not available. In conclusion, only recommendations regarding opioid treatment for certain chronic pain syndromes can be made. In only a minority of patients can a long-term analgesic effect be expected.Therefore, careful evaluation of alternative options of pain management is necessary before opioid therapy is started. With standardized documentation responders may be distinguished from non-responders. For clinical practice of long-term opioid therapy in non-malignant pain a specialized knowledge in pain management is a prerequisite. Future studies with more sophisticated methodology will be necessary to advocate more precise guidelines.However, the therapeutic recommendations from the DGSS consensus conference allow a safer,well structured and validated use of opioids for chronic non-malignant pain.

[Pain and the immune system: friend or foe?]

When tissue is destroyed, pain arises. Tissue destruction as well as wound healing are associated with an inflammatory reaction. This leads to activation of nociceptors ("pain receptors") which can cross-communicate with the inflammatory infiltrate. The following review will concentrate on pain-exaggerating (hyperalgesic) and pain-ameliorating (analgesic) mediators which arise from immune cells or the circulation during the inflammation. In the early stages of inflammation endogenous hyperalgesic mediators are produced, including the proinflammatory cytokines IL-1, IL-6 and TNF-alpha, nerve growth factor as well as bradykinin and prostaglandins. Simultaneously, analgesic mechanisms are activated. Opioid peptides such as endorphins, enkephalins and dynorphins are produced by immune cells and can be released locally in the inflamed tissue on stimulation with IL-1 or corticotropin releasing factor. Analgesia is elicited by binding of the opioid peptides to receptors on peripheral sensory neurons. During the course of an inflammatory process, peripheral opioid-mediated analgesia increases. In parallel, antiinflammatory cytokines such as IL-4, IL-10, IL-13 and IL-1ra are produced and reduce hyperalgesic effects of the proinflammatory cytokines initially produced. Inflammatory pain, therefore, is the result of an interplay between hyperalgesic and analgesic mediators. Drugs such as immunosuppressants influencing this interplay may also impair endogenous hyperalgesic and analgesic mechanisms.

Peripheral analgesic and antiinflammatory effects of opioids

Traditionally, opioids were considered the prototype of centrally acting analgesics. In the past decade a substantial literature has emerged demonstrating that opioids can produce potent and clinically measurable analgesia by activation of opioid receptors on peripheral terminals of primary sensory neurons. In addition, endogenous ligands of these peripheral receptors were discovered in immune cells. Major recent findings in peripheral opioid analgesia include the relative lack of tolerance under inflammatory conditions, tetrapeptides as novel peripherally restricted compounds, the potent antiinflammatory activity of mu- and kappa-agonists and the identification of selectins as important molecules governing the homing of opioid cells to injured tissue. In addition to the extensively documented efficacy of locally applied morphine in post-surgical pain, clinical studies have now moved into the field of chronic arthritic pain.

Opioid peptide-expressing leukocytes: identification, recruitment, and simultaneously increasing inhibition of inflammatory pain

BACKGROUND

Inflammatory pain can be effectively controlled by an interaction of opioid receptors on peripheral sensory nerve terminals with opioid peptides released from immune cells upon stressful stimulation. To define the source of opioid peptide production, we sought to identify and quantify populations of opioid-containing cells during the course of Freund's complete adjuvant-induced hind paw inflammation in the rat. In parallel, we examined the development of stress-induced local analgesia in the paw.

METHODS

At 2, 6, and 96 h after Freund's complete adjuvant inoculation, cells were characterized by flow cytometry using a monoclonal pan-opioid antibody (3E7) and antibodies against cell surface antigens and by immunohistochemistry using a polyclonal antibody to beta-endorphin. After magnetic cell sorting, the beta-endorphin content was quantified by radioimmunoassay. Pain responses before and after cold water swim stress were evaluated by paw pressure thresholds.

RESULTS

In early inflammation, 66% of opioid peptide-producing (3E7+) leukocytes were HIS48+ granulocytes. In contrast, at later stages (96 h), the majority of 3E7+ immune cells were ED1+ monocytes or macrophages (73%). During the 4 days after Freund's complete adjuvant inoculation, the number of 3E7+ cells increased 5.6-fold (P < 0.001, Kruskal-Wallis test) and the beta-endorphin content in the paw multiplied 3.9-fold (P < 0.05, Kruskal-Wallis test). In parallel, cold water swim stress-induced analgesia increased by 160% (P < 0.01, analysis of variance).

CONCLUSIONS

The degree of endogenous pain inhibition is proportional to the number of opioid peptide-producing cells, and distinct leukocyte lineages contribute to this function at different stages of inflammation. These mechanisms may be important for understanding pain in immunosuppressed states such as cancer, diabetes, or AIDS and for the design of novel therapeutic strategies in inflammatory diseases.

Reducing mass degeneracy in SAR by MS by stable isotopic labeling

Mass spectrometry (MS) promises to be an invaluable tool for functional genomics, by supporting low-cost, high-throughput experiments. However, large-scale MS faces the potential problem of mass degeneracy---indistinguishable masses for multiple biopolymer fragments (e.g., from a limited proteolytic digest). This paper studies the tasks of planning and interpreting MS experiments that use selective isotopic labeling, thereby substantially reducing potential mass degeneracy. Our algorithms support an experimental--computational protocol called structure-activity relation by mass spectrometry (SAR by MS) for elucidating the function of protein-DNA and protein-protein complexes. SAR by MS enzymatically cleaves a crosslinked complex and analyzes the resulting mass spectrum for mass peaks of hypothesized fragments. Depending on binding mode, some cleavage sites will be shielded; the absence of anticipated peaks implicates corresponding fragments as either part of the interaction region or inaccessible due to conformational change upon binding. Thus, different mass spectra provide evidence for different structure--activity relations. We address combinatorial and algorithmic questions in the areas of data analysis (constraining binding mode based on mass signature) and experiment planning (determining an isotopic labeling strategy to reduce mass degeneracy and aid data analysis). We explore the computational complexity of these problems, obtaining upper and lower bounds. We report experimental results from implementations of our algorithms.

Analgesic and antiinflammatory effects of two novel kappa-opioid peptides

BACKGROUND

This study investigates two new kappa-agonist tetrapeptides, FE 200665 and FE 200666, with high peripheral selectivity as a result of poor central nervous system penetration.

METHODS

Four days after administration of Freund adjuvant into the hind paw of male Wistar rats, antinociceptive effects of intraplantar and subcutaneous injection of FE 200665 and FE 200666 were measured by paw pressure algesiometry and compared with the kappa-agonist U-69,593. Peripheral and kappa-receptor selectivity was assessed by the antagonists naloxone methiodide (NLXM) and nor-binaltorphimine, respectively. Antiinflammatory effects were evaluated by paw volume plethysmometry and histologic score.

RESULTS

Similar to intraplantar U-69,593, intraplantar FE 200665 (3-100 microg) and FE 200666 (1-30 microg) resulted in significant and dose-related increases of paw pressure thresholds. Higher doses of FE 200665 (0.2-20 mg) and FE 200666 (0.06-6 mg) were required by subcutaneous route to produce similar antinociceptive responses, supporting a peripheral site of action. nor-Binaltorphimine dose-dependently antagonized this effect, implying kappa-opioid selectivity. Analgesic effects of subcutaneous FE 200665 and FE 200666 were abolished by intraplantar nor-binaltorphimine, and both subcutaneous and intraplantar effects were dose-dependently antagonized by subcutaneous NLXM, further demonstrating a peripheral site of action. One to 6 days after Freund adjuvant inoculation, single and repeated intraplantar injections of FE 200665, FE 200666, and U-69,593 significantly reduced paw volume and histologic scores. Both changes were reversed by intraplantar nor-binaltorphimine and subcutaneous NLXM.

CONCLUSION

FE 200665 is a peripherally selective kappa-agonist with potent analgesic and antiinflammatory properties that may lead to improved analgesic-antiinflammatory therapy compared with centrally acting opioids or standard nonsteroidal antiinflammatory drugs.

beta-Endorphin-containing memory-cells and mu-opioid receptors undergo transport to peripheral inflamed tissue

Immunocyte-derived beta-endorphin can activate peripheral opioid receptors on sensory neurons to inhibit pain within inflamed tissue. This study examined mu-opioid receptors (MOR) on sensory nerves and beta-endorphin (END) in activated/memory CD4(+) cells (the predominant population homing to inflamed tissue). We found an upregulation of MOR in dorsal root ganglia, an increased axonal transport of MOR in the sciatic nerve and an accumulation of MOR in peripheral nerve terminals in Freund's adjuvant-induced hindpaw inflammation. A large number of CD4(+) cells containing beta-endorphin, but very few naive cells (CD45RC(+)), were observed in inflamed tissue, suggesting that this opioid is mainly present in activated/memory cells (CD4(+)/CD45RC(-)). Taken together, our results indicate an enhanced transport of both MOR and of the endogenous ligand beta-endorphin to injured tissue. This unique simultaneous upregulation of both receptors and ligands may serve to prevent excessive and/or chronic inflammatory pain.

Efficacy of peripheral morphine analgesia in inflamed, non-inflamed and perineural tissue of dental surgery patients

In a clinical model of dental pain, the analgesic efficacy of local morphine treatment was examined under three different conditions. Patients undergoing dental surgery were randomly assigned to an injection of local anesthetic (articaine) plus 1 mg morphine either into inflamed (n = 14; trial 1) or non-inflamed (n = 24; trial 2) submucous tissue or perineurally n = 19; trial 3). Patients in the control group for each condition (n = 13, trial 1; n = 26, trial 2; n = 16, trial 3) received articaine plus saline. Postoperative pain intensity was assessed by the visual analog scale (VAS) and numeric rating scale (NRS) at 0, 2, 4, 6, 8, 10, 12, 16, 20, and 24 h. In addition, patients recorded the occurrence of side effects and the supplemental consumption of diclofenac. Immediately after the operation, pain scores were reduced to a similar extent in all groups, most likely due to the local anesthetic effect. Thereafter, pain scores and supplemental consumption of diclofenac were significantly lower in patients receiving 1 mg morphine into inflamed submucous tissue than in the control group for up to 24 h. Patients receiving 1 mg morphine into non-inflamed tissue or perineurally did not show any further reduction in pain scores compared to each control group. Our results show in patients undergoing dental surgery that injection of 1 mg of morphine into inflamed tissue results in significant and prolonged postoperative analgesia, whereas administration into non-inflamed tissue or perineurally is not effective. Thus, consistent with experimental studies, the requirement of an inflammatory process for the occurrence of peripheral opioid effects is also found in the clinical setting.

Reducing mass degeneracy in SAR by MS by stable isotopic labeling

Mass spectrometry (MS) promises to be an invaluable tool for functional genomics, by supporting low-cost, high-throughput experiments. However, large-scale MS faces the potential problem of mass degeneracy--indistinguishable masses for multiple biopolymer fragments (e.g. from a limited proteolytic digest). This paper studies the tasks of planning and interpreting MS experiments that use selective isotopic labeling, thereby substantially reducing potential mass degeneracy. Our algorithms support an experimental-computational protocol called Structure-Activity Relation by Mass Spectrometry (SAR by MS), for elucidating the function of protein-DNA and protein-protein complexes. SAR by MS enzymatically cleaves a crosslinked complex and analyzes the resulting mass spectrum for mass peaks of hypothesized fragments. Depending on binding mode, some cleavage sites will be shielded; the absence of anticipated peaks implicates corresponding fragments as either part of the interaction region or inaccessible due to conformational change upon binding. Thus different mass spectra provide evidence for different structure-activity relations. We address combinatorial and algorithmic questions in the areas of data analysis (constraining binding mode based on mass signature) and experiment planning (determining an isotopic labeling strategy to reduce mass degeneracy and aid data analysis). We explore the computational complexity of these problems, obtaining upper and lower bounds. We report experimental results from implementations of our algorithms.

Peripheral opioid analgesia

Major recent findings in peripheral opioid analgesia include the relative lack of tolerance under inflammatory conditions, tetrapeptides as novel peripherally restricted compounds, the potent anti-inflammatory activity of mu and kappa agonists and the identification of selectins as important molecules governing the homing of opioid cells to injured tissue. Clinical studies have now moved into the field of chronic arthritic pain, a problem of major relevance and prevalence.

Chemical modification and structural analysis of the progesterone membrane binding protein from porcine liver membranes

In addition to the classical genomic steroid actions on modulation of transcription and protein synthesis, rapid, nongenomic effects have been described for various steroids. These effects on cellular signaling and function are supposed to be transmitted by membrane binding sites unrelated to the classical intracellular receptors. Recently, a high affinity progesterone membrane binding protein (mPR) has been characterized in porcine liver membranes. In the present study, amino acid residues that are essential for progesterone binding to porcine liver microsomal mPR have been identified by the use of protein modifying reagents. Among all reagents tested, agents with specificity for carboxyl groups, methionine and tryptophan such as N,N'-dicyclohexylcarbodiimide, chloramine T and N-bromosuccinimide induced a reduction in [3H]progesterone binding. To evaluate the presence of essential disulfide bridges, porcine liver microsomes were incubated with the disulfide reducing agent dithiothreitol (DTT) and [3H]progesterone binding was measured. This treatment also resulted in a reduction of binding activity with an IC50 of 20 mM for DTT. Western-blotting analysis in the presence or absence of the reducing agent suggested that mPR--in its binding state--consists of at least two identical subunits with an apparent molecular mass of 28 kDa which are linked by a disulfide bridge. In conclusion, in the present study evidence for an involvement of carboxyl-, tryptophan- and methionine residues in [3H]progesterone binding to porcine liver microsomes is given. In addition, it is shown that mPR can form disulfide-linked homodimers.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.

Why is morphine not the ultimate analgesic and what can be done to improve it?

Although opioids are unsurpassed in the treatment of acute and cancer pain, their use in chronic noncancer pain is clearly limited. This review discusses some open and controversial issues such as the balance between pain relief and side effects, whether all types of pain can be treated with opioids, and current efforts to develop opioids with an improved efficacy-side effect ratio. Whereas respiratory depression or tolerance are usually not major issues in long-term opioid use, it seems questionable whether opioids can produce an analgesic response in certain types of pain when there is a major affective component to the pain or when learned pain behavior is the main problem. Efforts to improve opioids have traditionally aimed at enhancing the selectivity of opioid receptor ligands towards mu-, delta-, and kappa-receptors. Another major strategy has been the search for opioid analgesics acting at opioid receptors outside the central nervous system, with the prospect to avoid centrally mediated side effects.