Pharmacology of spinal opioids

Antagonism of peripheral opioid receptors by methylnaltrexone does not prevent morphine tolerance in rats

Opioids are effective analgesics in the management of severe pain. However, tolerance, leading to dose escalation and adverse effects are significant limiting factors in their use. The role of peripheral opioid receptors in analgesia has been discussed especially under inflammatory conditions. The results from pharmacological and conditional knockout studies together do not provide a clear picture of the contribution of peripheral opioid receptors on antinociceptive tolerance and this needs to be evaluated. Therefore, we studied whether the peripherally restricted opioid receptor antagonist, methylnaltrexone (MNTX), could prevent morphine tolerance without attenuating the antinociceptive effect of morphine. Male Sprague-Dawley rats were treated for 7 days with increasing subcutaneous doses of morphine (5-30 mg/kg) and were coadministered saline, MNTX (0.5 or 2 mg/kg), or naltrexone (NTX; 2 mg/kg). Nociception was assessed with tail-flick, hotplate, and von Frey tests. Morphine, MNTX, and NTX concentrations in the plasma, brain, and spinal cord were measured by liquid chromatography-tandem mass spectrometry. In acute coadministration, NTX, but not MNTX, abolished the acute antinociceptive effects of morphine in all nociceptive tests. The antinociceptive tolerance after repeated morphine administration was also prevented by NTX but not by MNTX. MNTX penetrated to the spinal cord and the brain to some extent after repeated administration. The results do not support the use of MNTX for preventing opioid tolerance and also suggest that morphine tolerance is mediated by central rather than peripheral opioid receptors in the rat.

Efficacy of transverse abdominis plane block in reduction of postoperation pain in laparoscopic cholecystectomy

OBJECTIVE

Transversus abdominis plane (TAP) block is a recently introduced regional anesthesia technique that is used for postoperative pain reduction in some abdominal surgeries. The present study evaluated the efficacy of the TAP block on the post laparoscopic cholecystectomy pain intensity and analgesic consumption.

METHODS

Fifty-four patients were enrolled in three groups: TAP block with normal saline (Group 1, n = 18); TAP block with bupivacaine (Group 2, n = 18); and TAP block with bupivacaine plus sufentanil (Group 3, n = 18). The time to the first fentanyl request, fentanyl consumption in the 24 hours following surgery, and postoperative pain intensity at 30 minutes, 1 hour, 6 hours, 12 hours, and 24 hours following discharge for recovery were measured and recorded.

RESULTS

The total amount of 24-hour fentanyl consumption was higher in Group 1 (877.8 ± 338.8 μg) than either Group 2 (566.7 ± 367.8 μg) or Group 3 (555.5 ± 356.8 μg; p = 0.03). Postoperative pain score was higher in Group 1 than intervention groups (p = 0.006); however, there was no significant difference in intervention groups. The time to the first fentanyl request in Group 1 (79.44 ± 42.2) was significantly lower than Group 3 (206.38 ± 112.7; p = 0.001).

CONCLUSION

The present study demonstrated that bilateral TAP block with 0.5% bupivacaine reduces post laparoscopic cholecystectomy pain intensity and fentanyl request and prolongs time to the first analgesic request. Adding sufentanil to the block solution reduced neither pain intensity nor fentanyl further consumption.

The outcome of thoracic epidural anesthesia in elderly patients undergoing coronary artery bypass graft surgery

BACKGROUND

Thoracic epidural anesthesia (TEA) improves analgesia and outcomes after a cardiac surgery. As aging is a risk factor for postoperative pulmonary complications, TEA is of particular importance in elderly patients undergoing coronary artery bypass graft (CABG).

METHODS

Fifty patients aged 65-75 years; ASA II and III scheduled for elective CABG were included in the study. Patients were randomized to receive either general anesthesia (GA) group alone or GA combined with TEA group. Heart rate (HR), mean arterial pressure (MAP), and central venous pressure were recorded. Total dose of fentanyl μg/kg, aortic cross clamping, cardiopulmonary bypass (CPB) time, time to first awaking and extubation, arterial blood gases, visual analog scale (VAS) score in intensive care unit were reported. Postoperative pulmonary function tests were done.

RESULTS

TEA showed a significant HR and lower MAP compared with the GA group. The total dose of intraoperative fentanyl and nitroglycerine were significantly lower in the TEA. Patients in TEA group have statistically significantly higher PaO(2), lower PaCO(2), increase in Forced Vital Capacity (FVC) and Forced Expiratory Volume in one second (FEV(1))

CONCLUSIONS

TEA reduced severity of postoperative pulmonary function and restoration was faster in TEA group in elderly patients undergoing CABG. Also, it resulted in earlier extubation and awakening, better analgesia, lower VAS.

Autonomic effects of epidural and intravenous fentanyl

BACKGROUND

We tested the hypothesis that there is greater suppression of autonomic reflexes during general anaesthesia when fentanyl is administered epidurally than when it is given intravenously.

METHODS

Ten volunteers were anaesthetized with desflurane. Noxious stimuli of variable intensity were then delivered by tetanic electrical stimuli. Heart rate, arterial pressure, and pupillary dilation in response to these stimuli defined nociception. Seven of these volunteers participated twice using a crossover design: they received i.v. fentanyl on one study day and epidurally on the other. Autonomic responses to alternative tetanic stimuli at L4 and C5 dermatomes were measured every 5 min for 3 h after fentanyl administration.

RESULTS

After a brief redistribution period, plasma fentanyl concentrations were virtually identical on both days. After stimulation of the L4 dermatome only, block of pupillary reflex dilation was greater by 47 (22)% after epidural fentanyl compared with i.v. fentanyl. Time to maximal depression of reflex dilation after L4 stimulation was 41 (13) min. Arterial pressure and heart rate decreased after fentanyl by either route but there were no differences observed between L4 and C5 stimulations.

CONCLUSION

We conclude that during general anaesthesia, epidural fentanyl enhances antinociception by a spinal mechanism which can be detected by pupillary dilation but not by changes in arterial pressure or heart rate.

Antinociceptive and nociceptive actions of opioids

Although the opioids are the principal treatment options for moderate to severe pain, their use is also associated with the development of tolerance, defined as the progressive need for higher doses to achieve a constant analgesic effect. The mechanisms which underlie this phenomenon remain unclear. Recent studies revealed that cholecystokinin (CCK) is upregulated in the rostral ventromedial medulla (RVM) during persistent opioid exposure. CCK is both antiopioid and pronociceptive, and activates descending pain facilitation mechanisms from the RVM enhancing nociceptive transmission at the spinal cord and promoting hyperalgesia. The neuroplastic changes elicited by opioid exposure reflect adaptive changes to promote increased pain transmission and consequent diminished antinociception (i.e., tolerance).

Induction of pain facilitation by sustained opioid exposure: relationship to opioid antinociceptive tolerance

Opioid analgesics are frequently used for the long-term management of chronic pain states, including cancer pain. The prolonged use of opioids is associated with a requirement for increasing doses to manage pain at a consistent level, reflecting the phenomenon of analgesic tolerance. It is now becoming clearer that patients receiving long-term opioid therapy can develop unexpected abnormal pain. Such paradoxical opioid-induced pain, as well as tolerance to the antinociceptive actions of opioids, has been reliably measured in animals during the period of continuous opioid delivery. Several recent studies have demonstrated that such pain may be secondary to neuroplastic changes that result, in part, from an activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM). One mechanism which may mediate such pain facilitation is through the increased activity of CCK in the RVM. Secondary consequences from descending facilitation may be produced. For example, opioid-induced upregulation of spinal dynorphin levels seem to depend on intact descending pathways from the RVM reflecting spinal neuroplasticity secondary to changes at supraspinal levels. Increased expression of spinal dynorphin reflects a trophic action of sustained opioid exposure which promotes an increased pain state. Spinal dynorphin may promote pain, in part, by enhancing the evoked release of excitatory transmitters from primary afferents. In this regard, opioids also produce trophic actions by increasing CGRP expression in the dorsal root ganglia. Increased pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance as manipulations which block abnormal pain also block antinociceptive tolerance. Manipulations that have blocked enhanced pain and antinociceptive tolerance include reversible and permanent ablation of descending facilitation from the RVM. Thus, opioids elicit systems-level adaptations resulting in pain due to descending facilitation, upregulation of spinal dynorphin and enhanced release of excitatory transmitters from primary afferents. Adaptive changes produced by sustained opioid exposure including trophic effects to enhance pain transmitters suggest the need for careful evaluation of the consequences of long-term opioid administration to patients.

Chronic muscle pain induced by repeated acid Injection is reversed by spinally administered mu- and delta-, but not kappa-, opioid receptor agonists

Opioids are commonly used for pain relief clinically and reduce hyperalgesia in most animal models. Two injections of acidic saline into one gastrocnemius muscle 5 days apart produce a long-lasting bilateral hyperalgesia without associated tissue damage. The current study was undertaken to assess the effects of opioid agonists on mechanical hyperalgesia induced by repeated intramuscular injections of acid. Morphine (mu-agonist), [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (mu-agonist; DAMGO), 4-[((alpha)R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (delta-agonist; SNC80), or (1S-trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cylcohexyl]-benzeneacetamide hydrochloride (kappa-agonist; U50,488) were administered intrathecally to activate opioid receptors once hyperalgesia was developed. Mechanical hyperalgesia was assessed by measuring the withdrawal thresholds to mechanical stimuli (von Frey filaments) before the first and second intramuscular injection, 24 h after the second intramuscular injection, and for 1 h after administration of the opioid agonist or vehicle. Morphine, DAMGO, and SNC80 dose dependently increased the mechanical withdrawal threshold back toward baseline responses. The reduction in hyperalgesia produced by morphine and DAMGO was prevented by H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) and that of SNC80 was prevented by naltrindole. U50,488 had no effect on the decreased mechanical withdrawal thresholds. Thus, activation of mu- and delta-, but not kappa-, opioid receptors in the spinal cord reduces mechanical hyperalgesia following repeated intramuscular injection of acid, thus validating the use of this new model of chronic muscle pain.

Definition of and mechanism for opioid-induced sedation

Although sedation is acknowledged to be one of the most common side effects of opioid analgesics, the mechanisms and characteristics of this phenomenon remain elusive, and research in this area is extremely limited. This report integrates research findings on the mechanism of action of opioids with research findings on the phenomenon of consciousness to develop a model of how opioids may act in the central nervous system to produce sedation. Based on this integration, a definition of opioid-induced sedation is proposed to encourage dialogue and research on this perplexing and clinically significant phenomenon.

Pain management in the critically ill child

Children frequently received no treatment, or inadequate treatment, for pain and for painful procedures. The newborn and critically ill children are especially vulnerable to no treatment or under-treatment. Nerve pathways essential for the transmission and perception of pain are present and functioning by 24 weeks of gestation. The failure to provide analgesia for pain results in rewiring the nerve pathways responsible for pain transmission in the dorsal horn of the spinal cord and results in increased pain perception for future painful results. Many children would withdraw or deny their pain in an attempt to avoid yet another terrifying and painful experiences, such as the intramuscular injections. Societal fears of opioid addiction and lack of advocacy are also causal factors in the under-treatment of pediatric pain. False beliefs about addictions and proper use of acetaminophen and other analgesics resulted in the failure to provide analgesia to children. All children even the newborn and critically ill require analgesia for pain and painful procedures. Unbelieved pain interferes with sleep, leads to fatigue and a sense of helplessness, and may result in increased morbidity or mortality.

Antinociceptive interaction between spinal clonidine and lidocaine in the rat formalin test: an isobolographic analysis

Clinical and basic science studies suggest that spinal alpha-2-adrenergic receptor agonists and local anesthetics produce analgesia, but interaction between alpha-2-adrenergic receptor agonists and local anesthetics in the persistent pain model has not been examined. In the present study, using isobolographic analysis, we investigated the antinociceptive interaction of intrathecal clonidine and lidocaine in the rat formalin test. Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters, and were tested for paw flinch by formalin injection. Biphasic painful behavior was counted. Intrathecal clonidine (3-12 nmol) was administered 15 min before formalin, and intrathecal lidocaine (375-1850 nmol) was administered 5 min before formalin. To examine the interaction of intrathecal clonidine and lidocaine, an isobolographic design was used. Spinal administration of clonidine produced dose-dependent suppression of the biphasic responses in the formalin test. Spinal lidocaine resulted in dose-dependent transient motor dysfunction and the motor dysfunction recovered to normal at 10-15 min after administration. Spinal lidocaine produced dose-dependent suppression of phase-2 activity in the formalin test. Isobolographic analysis showed that the combination of intrathecal clonidine and lidocaine synergistically reduced Phase-2 activity. We conclude that intrathecal clonidine synergistically interacts with lidocaine in reducing the nociceptive response in the formalin test.

IMPLICATIONS

Preformalin administration of intrathecal clonidine and lidocaine dose-dependently produced antinociception in the formalin test. The combination of clonidine and lidocaine, synergistically produced suppression of nociceptive response in the persistent pain model.

Subarachnoid Techniques for Cancer Pain Therapy: When, Why, and How?

For cancer patients who obtain inadequate pain relief with conservative treatment, there is a growing list of effective options for subarachnoid therapy. Morphine and bupivacaine have been the most frequently used drugs for intrathecal infusion, and their use has consistently yielded good results. Despite their effectiveness, however, a therapeutic deficit remains, primarily in the treatment of neuropathic cancer pain. Because of this limitation, more recent research has focused on novel compounds for intrathecal therapy such as clonidine, midazolam, ketamine, and SNX-111. In addition to new drug options, there are various catheter delivery systems from which to choose. In reviewing the literature and experience to date with these various medications and delivery systems, we hope to better aid the clinician in tailoring the best treatment for each patient.

Isobolographic analysis of interaction between spinal endomorphin-1, a newly isolated endogenous opioid peptide, and lidocaine in the rat formalin test

Endomorphin-1, a newly isolated endogenous opioid ligand, has a potential affinity with mu-opioid receptor. We investigated antinociception of intrathecal endomorphin-1 and lidocaine in the rat formalin test and examined the interaction between the two agents using isobolographic analysis. Intrathecal endomorphin-1 caused dose-dependent suppression of the formalin-induced biphasic behavioral response. Intrathecal lidocaine produced dose-dependent inhibition of phase-2 behavioral response. Isobolographic analysis confirmed that combination of intrathecal endomorphin-1 and lidocaine, given at a fixed dose ratio, produced synergistic suppression of phase-2 behavioral response. These data demonstrate that spinal endomorphin-1 synergistically interacts with local anesthetic lidocaine in producing antinociception in the formalin test.

Intrathecal morphine delivered via subcutaneous pump for intractable pain in pancreatic cancer

BACKGROUND

Pain secondary to unresectable pancreatic cancer is frequently severe and extremely difficult to control with traditional methods of analgesia. This retrospective study reports the analgesic effects of intrathecal morphine sulfate by implanted infusion pumps in nine patients with unresectable adenocarcinoma of the pancreas.

METHODS

Nine patients were implanted over a 2-year period. Preoperative morphine i.v. equivalents were a mean of 81.51 mg/day, with a range of 20-140 mg/day. Patients were hospitalized for a trial dose of 1-2 mg of intrathecal Duramorph, 1 mg/ml, via lumbar puncture to assess whether adequate pain relief could be achieved and whether there would be drug-related side effects.

RESULTS

All patients who received a trial dose experienced excellent pain relief, and subsequently underwent implantation of a lumbar subarachnoid catheter and infusion pump during the same hospitalization. The mean number of days from diagnosis to pump implant was 119, with a range of 3-587 days. The mean maximum daily dose was 21.28 mg, with a range of 3-73.10 mg. No patient experienced respiratory depression or excess sedation which prevented achievement of pain control. Minor supplemental narcotic use was documented in three of the nine patients. Assessment of pain control was made by the level of activity and the analog pain scale, with 0 being no pain and 10 being the worst pain imaginable. All of the patients experienced good to excellent relief of pain. The mean duration of intrathecal morphine sulfate use until death was 137.3 days, with a range of 52-354 days.

CONCLUSIONS

This series of nine patients indicates that long-term administration of intrathecal morphine via implanted infusion pump in patients with pancreatic cancer is both efficacious and safe. All patients and their families reported an improved quality of life with an increased level of activity.

Effects of GABA and glycine receptor antagonists on the activity and PAG-induced inhibition of rat dorsal horn neurons

The effects of bicuculline and strychnine on the activity and periaqueductal gray (PAG)-induced inhibition of rat dorsal horn neurons of the lumbar spinal cord were tested. Extracellular single unit recordings were from 36 dorsal horn neurons near a microdialysis fiber passed through the spinal cord for drug application. The GABAA receptor antagonist, bicuculline, was tested on 19 cells, whereas the glycine receptor antagonist, strychnine, was tested on 17 cells. Both bicuculline and strychnine increased the background activity and responses to mechanical stimulation (BRUSH, PRESS, and PINCH) of the skin.06 They also significantly blocked the PAG-induced inhibition of responses to peripheral mechanical stimuli. This experiment suggests that the mechanism of PAG-induced descending inhibition of dorsal horn neuron activity involves GABA and/or glycine release in the spinal cord and that there is tonic release of these inhibitory neurotransmitters.

Spinal analgesia in terminal care: risk versus benefit

Cancer pain treatment is well established. The World Health Organization provides clinicians an "analgesic ladder" scheme to optimize cancer pain treatment. At the beginning of the pain treatment, oral analgesic administration is preferred. The analgesic dose must be individualized. Many published papers describe the spinal administration of opioids in combination with various other drugs such as bupivacaine in selected patients with cancer pain. Although complications have been reported to be few, some recent reports debate this idea. We first describe a population of 92 cancer patients, 13 of whom received intrathecal morphine. We then present our experience with a separate group of 33 cancer patients who were also managed using intrathecal morphine. Based on this experience, the generally accepted indications for the technique appeared to be justified. Concern about spinal infection is well considered, however. Three out of those patients developed meningitis, a complication rate that is far too high.

The dilemma of conversion from systemic to epidural morphine: a proposed conversion tool for treatment of cancer pain

There is lack of concensus over what constitutes an appropriate method to affect an equianalgesic conversion from systemic to epidural morphine. A systematic approach to calculate the appropriate starting dose for epidural morphine is needed. A model is proposed here and data from a pilot study are described supporting the concept as well as its utility in the clinical setting. Several key factors may have an impact upon the selection of a starting dose in the opioid-tolerant cancer population such as pain severity, age, previous systemic opioid use, and presence of neuropathic pain. A conversion tool was developed taking these 4 factors into account and was tested in a small number of patients with cancer pain. The rationale for this approach is explored. Four case examples are presented to demonstrate the utilization of the tool and the effectiveness of this formula for clinical practice. Further study is needed to firmly establish the validity and reliability of this tool.

The spinal pharmacology of acutely and chronically administered opioids

Systematic studies on the pharmacology of the antinociceptive activity of spinally administered agents have emphasized the action in animal models of mu and delta opioid receptors. Importantly, aside from receptor selectivity, the opioid agonists differ in the property of efficacy. Agents with high efficacy (large receptor reserves) show smaller rightward shifts in their dose-response curves in the face of a given degree of opioid tolerance or when the stimulus intensity is elevated. With regard to loss of drug effect with long-term exposure (tolerance), multiple mechanisms may be considered, including changes in stimulus intensity, change in afferent processing, or changes in receptor number/coupling. Basic studies are providing insights into these several mechanisms.