Opiate analgesia. How central is a peripheral target?

Mouse and Rat Anesthesia and Analgesia

Anesthesia and analgesia play pivotal roles in ethically and humanely using animal models in research, especially concerning mice and rats. These rodent species, extensively utilized in scientific investigations due to their genetic resemblance to humans, serve as invaluable tools for studying diseases and testing treatments. Proper anesthesia and analgesia not only prioritize animal welfare but also heighten experimental validity by minimizing stress-induced physiological responses. Recent years have seen remarkable advancements in anesthesia for mice and rats. The focus has shifted away from the 'one size fits all' toward tailoring anesthesia protocols, considering factors like age, strain, and the nature of the experimental procedure. The use of inhalation agents such as isoflurane and sevoflurane is often preferred due to their rapid induction and recovery characteristics, allowing precise control over anesthesia depth. However, refinements in injectable anesthetic agents also provide researchers the flexibility to select suitable agents based on study requirements. Additionally, progress in analgesic techniques has led to effective pain management strategies for these rodents. Common analgesics such as nonsteroidal anti-inflammatory drugs (NSAIDs), opioids, and local anesthetics are administered to alleviate pain and discomfort. However, standard practice also involves continuous monitoring of animals' behavior and physiological parameters, ensuring timely adjustments in analgesic regimens for optimal pain relief without compromising experimental outcomes. By integrating tailored anesthesia and analgesia protocols into the experimental design, researchers uphold high animal welfare standards while obtaining reliable scientific data. This contributes significantly to advancing medical knowledge and therapeutic interventions with reproducible results. Published 2024. This article is a U.S. Government work and is in the public domain in the USA. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Injectable anesthesia for mouse and rat Basic Protocol 2: Inhalant anesthesia using isoflurane for mouse and rat Basic Protocol 3: Analgesia for mice and rats.

Top-Down Effect of Direct Current Stimulation on the Nociceptive Response of Rats

Transcranial direct current stimulation (tDCS) is an emerging, noninvasive technique of neurostimulation for treating pain. However, the mechanisms and pathways involved in its analgesic effects are poorly understood. Therefore, we investigated the effects of direct current stimulation (DCS) on thermal and mechanical nociceptive thresholds and on the activation of the midbrain periaqueductal gray (PAG) and the dorsal horn of the spinal cord (DHSC) in rats; these central nervous system areas are associated with pain processing. Male Wistar rats underwent cathodal DCS of the motor cortex and, while still under stimulation, were evaluated using tail-flick and paw pressure nociceptive tests. Sham stimulation and naive rats were used as controls. We used a randomized design; the assays were not blinded to the experimenter. Immunoreactivity of the early growth response gene 1 (Egr-1), which is a marker of neuronal activation, was evaluated in the PAG and DHSC, and enkephalin immunoreactivity was evaluated in the DHSC. DCS did not change the thermal nociceptive threshold; however, it increased the mechanical nociceptive threshold of both hind paws compared with that of controls, characterizing a topographical effect. DCS decreased the Egr-1 labeling in the PAG and DHSC as well as the immunoreactivity of spinal enkephalin. Altogether, the data suggest that DCS disinhibits the midbrain descending analgesic pathway, consequently inhibiting spinal nociceptive neurons and causing an increase in the nociceptive threshold. This study reinforces the idea that the motor cortex participates in the neurocircuitry that is involved in analgesia and further clarifies the mechanisms of action of tDCS in pain treatment.

Prevention of propofol injection pain in children: a comparison of pretreatment with tramadol and propofol-lidocaine mixture

BACKGROUND

The pain on propofol injection is considered to be a common and difficult to eliminate problem in children. In this study, we aimed to compare the efficacy of pretreatment with tramadol 1 mg.kg(-1)and propofol-lidocaine 20 mg mixture for prevention of propofol induced pain in children.

METHODS

One hundred and twenty ASA I-II patients undergoing orthopedic and otolaryngological surgery were included in this study and were divided into three groups with random table numbers. Group C (n=39) received normal saline placebo and Group T (n=40) received 1 mg.kg(-1) tramadol 60 sec before propofol (180 mg 1% propofol with 2 ml normal saline) whereas Group L (n=40) received normal saline placebo before propofol-lidocaine mixture (180 mg 1% propofol with 2 ml %1 lidocaine). One patient in Group C was dropped out from the study because of difficulty in inserting an iv cannula. Thus, one hundred and nineteen patients were analyzed for the study. After given the calculated dose of propofol, a blinded observer assessed the pain with a four-point behavioral scale.

RESULTS

There were no significant differences in patient characteristics and intraoperative variables (p>0.05) except intraoperative fentanyl consumption and analgesic requirement one hr after surgery among the groups (p<0.05). Both tramadol 1 mg.kg(-1) and lidocaine 20 mg mixture significantly reduced propofol pain when compared with control group. Moderate and severe pain were found higher in control group (p<0.05). The incidence of overall pain was 79.4% in the control group, 35% in tramadol group, 25% in lidocaine group respectively (p<0.001).

CONCLUSIONS

Pretreatment with tramadol 60 sec before propofol injection and propofol-lidocaine mixture were significantly reduced propofol injection pain when compared to placebo in children.

Prevention of pain on injection of propofol: a comparison of lidocaine with different doses of paracetamol

BACKGROUND AND OBJECTIVE

The aim of this study was to compare the efficacy of pretreatment with paracetamol 0.5 mg kg(-1), 1 mg kg(-1), 2 mg kg(-1) and lidocaine 0.5 mg kg(-1) for prevention of propofol induced pain.

METHODS

In this double-blind, placebo-controlled study, 250 adult patients ASA I or II, scheduled to undergo elective surgery, were randomly assigned into five groups of 50 each. Group P0.5, group P1 and group P2 received 0.5, 1 and 2 mg kg(-1) paracetamol respectively; group L received 0.5 mg kg(-1) lidocaine; and the control group, group C, received isotonic saline pretreatment in the dorsum of the hand, followed by propofol 1 min later. A blinded researcher assessed the patient's pain level via a four-point scale.

RESULTS

There were no significant differences in patient characteristics among the groups. The incidence of propofol injection pain in all treatment groups was significantly lower than in the control group (P < 0.001). When the paracetamol 0.5 mg kg(-1) group was compared with both the paracetamol 1 mg kg(-1) group (P < 0.01) and the paracetamol 2 mg kg(-1) group (P < 0.001), significant differences were observed. In the lidocaine 0.5 mg kg(-1) group propofol injection pain was significantly reduced compared with the paracetamol 0.5 mg kg(-1) group (P < 0.01). However, in the paracetamol 2 mg kg(-1) group pain was more significantly reduced than in the lidocaine 0.5 mg kg(-1) group (P < 0.001). In the paracetamol 2 mg kg(-1) group the incidence of pain was significantly less than in paracetamol 1 mg kg(-1) group (P < 0.001).

CONCLUSION

When given as venous retention pretreatments 1 min before propofol, paracetamol 1 mg kg(-1) and lidocaine 0.5 mg kg(-1) were equally effective in attenuating pain during intravenous (i.v.) injection of propofol whereas pretreatment with paracetamol 2 mg kg(-1) was shown to be the most effective treatment.

Mouse and rat anesthesia and analgesia

Many animal models used in neuroscience research must be surgically created and/or anesthetized for imaging studies. The purpose of this unit is to review the advantages and disadvantages of various anesthetic and analgesic agents in rodents; to discuss state-of-the-art methods for monitoring anesthesia; and to provide tips for troubleshooting problems with anesthesia.

The efficacy of combination analgesic therapy in relieving dental pain

BACKGROUND

An experience of poorly managed pain related to dental treatment can lead patients to avoid or postpone treatment. The development of new pain management strategies equips dental clinicians with additional treatment options that can provide more effective pain relief

LITERATURE REVIEWED

The author reviewed dental and medical literature dealing with the safety, efficacy and mechanisms of action of common analgesic treatments.

CONCLUSIONS

For the treatment of mild to moderate pain, acetaminophen and non-steroidal anti-inflammatory drugs, or NSAIDs, continue to be the most appropriate options. The use of cyclo-oxygenase2-inhibitor NSAIDs should be strongly considered for use with patients at risk of experiencing gastrointestinal toxicity. The pathophysiology of pain is a complex central and peripheral nervous system process, and the use of combination analgesics that act at multiple pain sites can improve pain relief after a dental procedure. For moderate to moderately severe pain, tramadol or combination medications such as tramadol with acetaminophen or codeine with acetaminophen are appropriate. For severe pain, use of opioids or opioid combinations is advised.

CLINICAL IMPLICATIONS

Providing appropriate treatment after dental surgery requires a careful medical history and an educated anticipation of the level of pain the patient may encounter. New analgesic options are available and should be considered, particularly combination analgesics, which can provide faster onset and prolonged duration of action and can combat pain at multiple sites of action.

Treatment of pain in acutely burned children

The child with burns suffers severe pain at the time of the burn and during subsequent treatment and rehabilitation. Pain has adverse physiological and emotional effects, and research suggests that pain management is an important factor in better outcomes. There is increasing understanding of the private experience of pain, and how children benefit from honest preparation for procedures. Developmentally appropriate and culturally sensitive pain assessment, pain relief, and reevaluation have improved, becoming essential in treatment. Pharmacological treatment is primary, strengthened by new concepts from neurobiology, clinical science, and the introduction of more effective drugs with fewer adverse side effects and less toxicity. Empirical evaluation of various hypnotic, cognitive, behavioral, and sensory treatment methods is advancing. Multidisciplinary assessment helps to integrate psychological and pharmacological pain-relieving interventions to reduce emotional and mental stress, and family stress as well. Optimal care encourages burn teams to integrate pain guidelines into protocols and critical pathways for improved care.

The kappa-opioid agonist, asimadoline, alters cytokine gene expression in adjuvant arthritis

OBJECTIVE

We have previously found that the kappa-opioid agonist, asimadoline, attenuates adjuvant arthritis in a dose-dependent, antagonist-reversible manner. To elucidate possible mechanisms, we investigated the effects of asimadoline (5 mg/kg/day i.p.) or vehicle on in vivo cytokine expression and T-cell recruitment in adjuvant arthritis.

METHODS

Arthritis severity was assessed every 3-4 days for 21 days. Rats were killed on days 0, 13 and 21 post-induction and synovial membrane and inguinal lymph nodes were removed for mRNA extraction. Changes in cytokine mRNA expression were measured using reverse transcription-polymerase chain reaction (RT-PCR) and densitometry. T cells in joints were quantified by immunohistochemistry.

RESULTS

Asimadoline significantly decreased arthritis severity at day 13, with a concomitant decrease in synovial membrane expression of cytokines interleukin-17 and transforming growth factor-beta (TGF-beta) mRNA at day 13, and no change in T cell numbers in the joints of arthritic rats. By contrast, in the inguinal lymph nodes, expression of tumour necrosis factor was increased at day 13 and TGF-beta mRNA was increased throughout.

CONCLUSION

An altered balance, therefore, in the pro- and anti-inflammatory effects of TGF-beta by asimadoline might explain its striking anti-arthritic actions.

Ketorolac versus acetaminophen-codeine in the emergency department treatment of acute low back pain

Acute low back pain is a common problem in the emergency department (ED). Effective management of acute pain enhances early rehabilitation and recovery. Given the importance of inflammatory mediators in pain generation and the adverse effects associated with opioids, it is logical to expect that a non-opioid agent with antiinflammatory and analgesic properties would provide excellent analgesia with fewer adverse effects. This double-blind, randomized, multicenter clinical trial, performed in six university and community hospital EDs, compares the analgesic efficacy and adverse effects of ketorolac to those of acetaminophen-codeine in ED patients with acute musculoskeletal low back pain. Our hypothesis was that ketorolac would provide superior analgesia with fewer adverse effects. One hundred twenty-three patients with acute low back pain were randomized to receive ketorolac (KET, N = 63) or acetaminophen-codeine (ACOD, N = 60). Most (79%) were males, and the mean age was 34.5 years. After baseline clinical assessment, patients were treated with ketorolac (10 mg every 4 to 6 h as needed, up to four daily doses) or acetaminophen-codeine (600 mg-60 mg, respectively, every 4 to 6 h as needed, up to six daily doses) and followed for one week. Pain intensity was assessed on visual analogue and categorical scales. Functional capacity, overall pain relief, and overall medication rating were assessed on categorical scales. Adverse events were documented. Primary outcomes included: 1) Pain intensity differences, based on visual analogue scores, for the 0 to 6 h treatment phase. 2) Incidence of adverse events. Secondary outcomes included analgesic efficacy, functional capacity, and overall subjective drug evaluation at one week. Both drugs provided substantial pain relief, with maximal effect 2.2 h after oral dosing. There were no significant differences in analgesic efficacy, functional capacity, or overall pain relief between the two groups. Sixteen patients (10 KET vs. 6 ACOD, NS) withdrew prematurely because of drug inefficacy. Patients in the ACOD group reported significantly more adverse drug events and serious adverse drug events. Seven patients--all in the ACOD group--withdrew from the study because of adverse drug events. Based on comparable efficacy and a superior adverse event profile, ketorolac was preferable to acetaminophen with codeine for the treatment of acute low back pain in the ED.

The efficacy of intra-articular morphine for postoperative knee arthroscopy analgesia

This article describes two prospective, randomized, double-blind clinical trials designed to investigate this. Trial 1 compared a conventional local anaesthetic agent (100 mg bupivacaine) injected intra-articularly (i.a.) with a control (normal saline) and 1 mg of i.a. morphine. No significant difference was noted in the first 4 hours between the groups with respect to visual analogue pain (VAS) scores. However, at 6 and 24 hours, the group of patients who received 1 mg i.a. morphine recorded lower pain scores and required less supplementary analgesia. Trial 2 assessed the dose response relationship for i.a. morphine comparing 5 mg intravenous (i.v.) morphine (control) with 1 mg and 5 mg i.a. morphine. At early time points (1, 2, and 4 hours) similar VAS pain scores were recorded for both 5 mg i.v. morphine and 5 mg i.a. morphine, both significantly lower than the group receiving 1 mg i.a. morphine. At 6 and 24 hours, 5 mg of i.a. morphine produced significantly lower pain scores, less analgesic requirement, and less sleep disturbance on the first postoperative night than the other groups. It can be concluded from these two studies that 5 mg i.a. was the most effective analgesic following knee arthroscopy.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

The effect of intradermal administration of lidocaine and morphine on the response to thermal stimulation

Opioids appear to exert a peripheral effect by gaining access to peripheral opioid receptors. It has been proposed that inflammatory processes and highly osmotic substances could alter the perineural barrier, thereby allowing easy access to opioid receptors. Although local anesthetics do not have osmotic activity, they are highly active on neural tissue and appear to work synergistically with opioids when administered for major conduction blockade. We therefore evaluated, in a double-blind fashion, the combination of lidocaine plus morphine in an attempt to provide a scientific basis for the use of a combination of morphine plus local anesthetics in the periphery. Seven thermal stimuli in 2 degrees C increments (range 40-52 degrees C) were delivered in a random sequence by a computer-controlled thermistor to one of three pretreated sites on 10 volunteers' forearms: reference site (no injection), lidocaine site (0.1-mL intradermal injection of lidocaine 0.5%), or lidocaine plus morphine site (0.1 mL of 0.5 mg of morphine plus lidocaine 1%). Pain responses to the thermal stimuli were rated by the volunteers using the method of magnitude estimation. Pain scores indicated that the combination of lidocaine plus morphine was not more effective than lidocaine alone in attenuating the heat-induced pain. Twenty and 120 min after injection, scores at the lidocaine plus morphine site were 37% and 20% greater than those at the lidocaine site. The addition of morphine to lidocaine did not result in an improvement in the analgesic efficacy and actually had an antianalgesic effect.

The site of anti-arthritic action of the kappa-opioid, U-50, 488H, in adjuvant arthritis: importance of local administration

1. Currently available pharmacological therapies treat arthritis inadequately. We have previously found that the kappa (kappa)-opioid, U-50,488H (trans-(+/-)- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]- benzene-acetamide methane sulphonate), possesses anti-arthritic effects. In light of the finding that opioid receptors in the periphery are upregulated during inflammation, kappa-opioids may represent a novel therapy for arthritis. The primary aim and unique feature of the present study is to investigate whether opioids exert their anti-arthritic effects in the periphery. Thus, the dose-effect relationship of a kappa-opioid agonist, U-50,488H was compared after both local and distant administration. Further, we tested whether the anti-arthritic effects of this drug are stereospecific and receptor-mediated by use of opioid antagonists. 2. Using an adjuvant model of arthritis in male Lewis rats, arthritis was judged by oedema, radiography and histological changes in the contralateral ankle of the hind limb. Treatment with (+/-)-U-50,488H for 3 days during disease onset and 3 days during established disease significantly attenuated arthritis, but the effects of (+/-)-U-50,488H on radiology and histology varied according to treatment time. Administration of (+/-)-U-50,488H during disease onset had a more marked effect on radiography, suggesting that treatment with that drug should be started early to prevent progressive joint destruction. Further, it was found that (+/-)-U-50,488H, administered for 3 days during the disease onset, either by direct subcutaneous injection into the inflamed paw or at a more distant site into the back of the neck, dose-dependently attenuated arthritic damage as measured by an index which pooled all three variables. More importantly however, (+/-)-U-50,488H was approximately fourfold more potent as an 'anti-arthritic' agent after local compared to distant subcutaneous injection (ED50; local vs distant: 5.8 +/- 1.6 vs 19.5 +/- 0.8 mg kg-1). 3. Equivalent doses of the (-)-enantiomer (20 mg kg-1day-1) and the racemate (+/-) of U-50,488H (40 mg kg-1day-1), elicited a similar attenuation of arthritic parameters while the (+/-)-enantiomer exacerbated arthritis, suggesting that the anti-arthritic activity lies solely with the (-)-enantiomer. 4. Both the peripherally selective antagonist, naloxone methiodide, and the kappa-selective antagonist, MR2266 ((-)-5,9 alpha-diethyl-2-(3-furylmethyl)-2'-hydroxy-6,7-benzomorphan), were able to reverse fully the peripheral anti-arthritic effects of U-50,488H, indicating that it exerts its effects through peripheral kappa-opioid receptors. 5. Taken together, these results not only confirm our previous findings that demonstrate anti-arthritic effects of U-50,488H but they indicate that the opioid attenuation of experimental arthritis is mediated via peripheral kappa-receptors in the arthritic joint. Peripherally acting kappa-opioid agonists should lead to new therapies for arthritis.

Approach to the management of nonmalignant pain

Pain is a universal, subjective, unpleasant sensation. It results from a noxious stimulus that causes the body to perceive existing or potential damage to its organs. The biochemical mechanism of pain is based on peripheral nociceptors that preferentially receive noxious stimuli and thereafter cause the primary afferent nociceptor fibers to release endogenous chemicals such as bradykinin, histamine, prostaglandins, serotonin, norepinephrine, and substance P. Additionally, substance P may stimulate prostaglandin and collagenase production, thus providing an explanation for the effectiveness of anti-inflammatory drugs in relieving pain. The interpretation of pain is highly individualized and embodies the entire personality. Thus, no two patients with pain can be treated in the same way. Pain is assessed through medical history, physical examination, and a variety of pain scales. General principles in managing pain call for the physician to (1) respect pain; (2) recognize the psychologic components of pain; and (3) treat the underlying disorder in a timely fashion. Modern management of pain evokes a multidisciplinary approach that includes patient education, pharmacologic intervention, physical medicine, minimally invasive procedures, psychologic counseling, behavioral modification and, in some instances, surgery or a variety of other nonpharmacologic modalities.

Biochemical aspects of chronic pain and its relationship to treatment

This review presents an overview of the neurotransmitters and neuromodulators involved in acute and chronic pain. Although there is little evidence that the neuronal pathways differ in the two types of pain, it is clear that different transmitters or receptor types are involved in hyperalgesia and chronic pain. While most attention has been focussed on spinal processes, it is apparent that some types of chronic pain have both a peripheral and a supraspinal component. The presently available drugs are probably adequate for acute pain, but the treatment of chronic pain may need to be tailored to the individual patient.

Low-dose intra-articular morphine analgesia in day case knee arthroscopy: a randomized double-blinded prospective study

The aim of this study was to demonstrate the effect of intra-articular morphine following knee arthroscopy performed in infiltration analgesia. Fifty-two healthy patients were randomized to receive either 1 mg of morphine or placebo. The pain was assessed 2, 4, 8 and 24 h after the procedure by (1) a VAS scale and (2) the amount of acetaminophen consumed. Demographic data in the 2 groups were similar. The pain scores at 8 and 24 h and the acetaminophen consumption after 8 h were lower in the morphine group (P < 0.05). Our results support the hypothesis of peripherally distributed opioid receptors. Stratifying data in therapeutic versus diagnostic arthroscopy indicated additional effect of morphine in patients undergoing therapy (P < 0.1), an aspect supporting the hypothesis of peripherally administered morphine as a potential suppressor of the substance P-mediated cytokine cascade and the peripheral leukocyte activity. Intra-articular morphine (1 mg) after knee arthroscopy offers efficient analgesia lasting more than 24 h. The method is devoid of side effects and deserves wider recognition.

Local analgesic effect of endogenous opioid peptides

Opioids produce analgesia by interacting with local opioid receptors in peripheral inflamed tissue. This study investigated whether endogenous ligands of these receptors are present in synovia and whether such opioid peptides can inhibit pain by activation of intra-articular opioid receptors. Samples of synovium from 8 patients undergoing arthroscopic knee surgery were examined by immunohistochemistry for the presence of beta-endorphin, met-enkephalin, and dynorphin. All tissue samples showed synovitis. Inflammatory cells stained strongly for beta-endorphin and met-enkephalin but not for dynorphin. To find out whether blockade of intra-articular opioid receptors affected pain, we randomly assigned 22 patients undergoing arthroscopic knee surgery to receive naloxone (0.04 mg) intra-articularly (n = 10) or intravenously (n = 12); each patient received a placebo injection into the other site. Postoperative pain was assessed by visual analogue scale, a numerical rating scale, the McGill pain questionnaire, and supplementary analgesic consumption during the next 24 h. All pain scores were higher in the intra-articular naloxone group than in the intravenous naloxone group. The differences were significant (p < 0.05) during the first 4 h. Supplementary analgesic consumption was significantly higher in the intra-articular group (52.5 [14.0] vs 15.6 [8.0] mg diclofenac, p < 0.05). Opioid peptides are present in inflamed synovial tissue and can inhibit pain after knee surgery through an action specific to intra-articular opioid receptors. These findings expand the gate control theory of pain and suggest new approaches such as the development of peripherally acting opioid analgesics without central side-effects.

Local increases of subcutaneous beta-endorphin immunoactivity at the site of thermal injury

To examine interactions between exogenous opioid analgesia and endogenous opioid generation at a site of burn-induced tissue injury, we measured beta-endorphin (BE) and corticosterone (C) in aliquots of plasma and wound fluid withdrawn from subcutaneous wire mesh chambers beneath the site of a 3-5% surface area burn. After brief inhalational anesthesia at the time of thermal injury, rats received morphine (4 mg/kg, single dose), fentanyl (0.02 mg/kg hourly for 4 h), or no opioid. Systemic hormone responses and behavioral changes were minimal as expected for the minimal percentage burn. In all three groups intrachamber BE and C rose above baseline at 1, 2 and 4 h postburn, then returned to baseline at 24 h. Systemic opioid treatment produced analgesia (by tail flick latency testing) but did not reduce intrachamber hormone responses. Thus local BE and C responses at the site of thermal injury are regulated differently from systemic pituitary-adrenal responses.

Where are peripheral analgesics acting?

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Description of a mechanistic approach to pain management in advanced cancer. Preliminary report

A mechanistic approach to advanced cancer pain management is proposed, based on the clinically perceived anatomical and pathophysiological mechanisms of pain generation. It is an extension of the World Health Organisation (WHO) analgesic ladder in which severity of pain is the principal determinant of analgesic choice. The mechanistic categories are: superficial somatic, deep somatic, visceral and neurogenic (mixed or pure, i.e., nociceptive component present or absent). Allocation of pain to the different categories is based on clinical history, physical findings and investigations to establish the site and extent of active primary or metastatic tumor deposits, and evidence of previous response to medication. Drug choice sequence is determined by the dominant pain mechanism judged to be present and not the severity of the pain. In order to describe this approach, mechanisms of pain, disease distribution and drug treatment have been analysed in the first 20 consecutive patients who consented to enter a longitudinal pain description and evaluation study on admission to an inpatient hospice unit. Despite a high exclusion rate from research standard monitoring due to severity of illness and related factors, the majority of eligible patients approached to enter the study did so, and the pain scoring was well tolerated. The implications of this for future research and clinical practice are discussed. In 6 patients only 1 pain mechanism was identified (visceral 4, deep somatic 2). Two mechanisms were present in 8 patients and 3 mechanisms in 6 patients. The deep somatic mechanism was identified in 15 patients, visceral mechanism in 13, neurogenic in 10 and superficial somatic in 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of opioids in peripheral analgesia

Several pharmacological, neurophysiological and immunohistological studies indicate that exogenous or endogenous opioids can have antinociceptive effects by acting at peripheral sites. Although modulation of mu, delta and kappa receptors can mediate these effects, the nature of the noxious stimulus and the underlying pathological condition may affect the types of opioid receptors involved. Thus, it would be appropriate to develop peripherally-acting opioid analgesics that do not have the untoward central side effects often associated with conventional analgesic drugs. This paper reviews the evidence supportive of a peripheral mechanism of action for opioids.