Acute tolerance to opioids: Methodological, theoretical and clinical implications

Sustained-release buprenorphine induces acute opioid tolerance in the mouse

Sustained-release buprenorphine is widely used in mice with the intention of providing long-lasting analgesia. Statements about duration of therapeutic efficacy are based on persistence of serum buprenorphine levels over a minimum threshold, but behavioral data demonstrating sustained efficacy is not established. Additionally, chronic opioid exposure can induce tolerance and/or hyperalgesia; mice receiving sustained-release buprenorphine have not been evaluated for these effects. This study assessed clinical efficacy and duration of sustained-release buprenorphine in inflammatory, post-operative, and cancer pain; and screened for centrally-mediated opioid-induced hyperalgesia as well as opioid tolerance. At 1-2 mg/kg sustained-release buprenorphine, statistically significant analgesic efficacy occurred only at time points up to 2 h. These animals showed no changes in von Frey thresholds on the contralateral side, i.e. no centrally-mediated opioid hyperalgesia. To establish whether acute onset opioid tolerance resulted from a single sustained-release buprenorphine administration, we used the tail flick assay, exposing mice to sustained-release buprenorphine or saline on Day 1 and buprenorphine on Day 2. We measured duration and efficacy of 1 mg/kg buprenorphine after 1 mg/kg sustained-release buprenorphine, and also quantified a dose-response curve of buprenorphine (0.1-3 mg/kg) after 2 mg/kg sustained-release buprenorphine. Compared to control animals, mice previously exposed to sustained-release buprenorphine showed diminished analgesic response to buprenorphine; the resultant dose-response curve showed decreased efficacy. Pretreatment with naloxone, an opioid receptor antagonist, blocked sustained-release buprenorphine analgesic action. The short duration of antinociception following administration of sustained-release buprenorphine in mice is caused by the rapid development of tolerance.

Clinical, histological, and biochemical predictors of postsurgical neuropathic pain

Surgical nerve injury sometimes leads to chronic postsurgical neuropathic pain (CPSNP). The risk factors for this condition are not well understood. We prospectively assessed 46 patients scheduled for iliac crest bone harvest, 2 days (D2) and 3 months (M3) after surgery, to determine the time course of nerve fiber degeneration and expression of the TNF-α and NGF genes in skin punch biopsies. Mechanical and thermal detection and pain thresholds were evaluated at D2 and M3, by quantitative sensory testing. Skin punch biopsies were also obtained from the thighs ipsilateral and contralateral to iliac crest bone harvest. Intraepidermal nerve fiber density (IENFD) and cutaneous TNF-α and NGF gene expression were analyzed. Forty-five volunteers matched for age, sex, skin color were examined as controls. Chronic postsurgical neuropathic pain was defined as pain in an area of hypesthesia with a positive Douleur Neuropathique 4 questionnaire score. Overall, 73% (N = 32) of patients developed hypesthesia and 40% (N = 13) of these patients had developed CPSNP at M3. Quantitative sensory testing results, IENFD, and skin TNF-α and NGF gene expression at D2 and M3 did not differ between patients with and without CPSNP. However, in patients with CPSNP, burning, compression, and pain provoked by brushing were correlated with IENFD at M3, suggesting a possible association between partial nerve lesions and more intense CPSNP, than with total nerve lesion. Furthermore, preoperative pain and opioid use were higher in patients who developed CPSNP than in those without CPSNP. These findings suggest that the predictors of CPSNP development are clinical rather than histological or biochemical.

Remifentanil-acute opioid tolerance and opioid-induced hyperalgesia: a systematic review

The use of opioids may seem to be a double-edged sword; they provide straight analgesic and antihyperalgesic effects initially, but subsequently are associated with the expression of acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) that have been reported in experimental studies and clinical observations. It has been suggested that opioids can induce an acute tolerance and hyperalgesia in dose- and/or time-dependent manners even when used within the clinically accepted doses. Recently, remifentanil has been used for pain management in clinical anesthesia and in the intensive care units because of its rapid onset and offset. We reviewed articles analyzing AOT and/or OIH by remifentanil and focused on the following issues: (1) evidence of remifentanil inducing AOT and/or OIH and (2) importance of AOT and/or OIH in considering the reduction of remifentanil dosage or adopting preventive modulations. Twenty-four experimental and clinical studies were identified using electronic searches of MEDLINE (PubMed, Ovid, Springer, and Elsevier). However, the development of AOT and OIH by remifentanil administration remains controversial. There is no sufficient evidence to support or refute the existence of OIH in humans.

Intraoperative use of remifentanil and opioid induced hyperalgesia/acute opioid tolerance: systematic review

INTRODUCTION

The use of opioids has been increasing in operating room and intensive care unit to provide perioperative analgesia as well as stable hemodynamics. However, many authors have suggested that the use of opioids is associated with the expression of acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) in experimental studies and clinical observations in dose and/or time dependent exposure even when used within the clinically accepted doses. Recently, remifentanil has been used for pain management during anesthesia as well as in the intensive care units because of its rapid onset and offset.

OBJECTIVES

Search of the available literature to assess remifentanil AOT and OIH based on available published data.

METHODS

We reviewed articles analyzing remifentanil AOT and OIH, and focused our literature search on evidence based information. Experimental and clinical studies were identified using electronic searches of Medline (PubMed, Ovid, Springer, and Elsevier, ClinicalKey).

RESULTS

Our results showed that the development of remifentanil AOT and OIH is a clinically significant phenomenon requiring further research.

DISCUSSIONS AND CONCLUSIONS

AOT - defined as an increase in the required opioid dose to maintain adequate analgesia, and OIH - defined as decreased pain threshold after chronic opioid treatment, should be suspected with any unexplained pain report unassociated with the disease progression. The clinical significance of these findings was evaluated taking into account multiple methodological issues including the dose and duration of opioids administration, the different infusion mode, the co-administrated anesthetic drug's effect, method assessing pain sensitivity, and the repetitive and potentially tissue damaging nature of the stimuli used to determine the threshold during opioid infusion. Future studies need to investigate the contribution of remifentanil induced hyperalgesia to chronic pain and the role of pharmacological modulation to reverse this process.

No evidence for the development of acute analgesic tolerance during and hyperalgesia after prolonged remifentanil administration in mice

Background

Acute opioid tolerance (AOT) and opioid-induced hyperalgesia (OIH) are undesirable effects of opioids that have been reported in both animals and humans. However, the development of AOT and OIH in cases of potent, short-acting μ-opioid receptor agonist remifentanil administration remains controversial. It has been suggested that the emergence of AOT and OIH by remifentanil could be dose and infusion duration dependent, i.e., low dose and short infusions may lead to negative results. In this study, we determined whether AOT and OIH could be elicited by prolonged, continuous administration of remifentanil at maximally tolerable doses in C57BL/6 mice.

Results

The analgesic effects of continuously administered remifentanil [by short (1 h) and prolonged (4 h) intraperitoneal infusions] were studied. These experiments involved repeated measurements of thermal thresholds during remifentanil administration. Therefore, particular attention was paid to prevent cumulative tissue injury, which could mimic pronociceptive effects of remifentanil. To exclude the possibility of pseudoAOT during infusion, we used brief cooling of all ipsilateral hindpaws that exhibited analgesic response. Thermal thresholds remained steadily elevated over a 1-h period during continuous administration at infusion rates of 120, 180, and 240 mg/kg/h, which indicated no AOT development. To exclude the possibility of pseudoOIH after infusion, intact contralateral hindpaws were used for all postinfusion threshold measurements. Thermal thresholds at each infusion rate returned to the baseline values within 15 min after the termination of the administration. They did not decrease below the baseline values during 1 h following infusion, which indicated no OIH development. Similar threshold dynamics were also observed for thermal and mechanical testing modalities in animals infused at 120 mg/kg/h for 4 h as well as in animals with rapidly attained and maintained maximum analgesia for 3 h.

Conclusions

These results suggest that neither intra-infusion AOT nor postinfusion OIH develops in mice receiving continuous remifentanil when the possibility of cumulative tissue injury mimicking AOT or OIH is carefully avoided.

Chronic inflammatory pain does not attenuate the development of tolerance to chronic morphine in adult male rats

The overall impact of chronic pain on the response to opioids is ambiguous in the literature, and comparisons between human and animal studies are complicated by vast differences between the manner and dosage of opioids given to humans treated for pain in comparison to rodents as well as a lack of healthy participant studies examining the impact of chronic opioids. The purpose of this study was to evaluate the impact of chronic pain on the development of tolerance to morphine and to assess how the concentration of drug affects this process. Twenty-four hours after the injection of CFA or normal saline in the left hind paw, the level of mechanical hypersensitivity was assessed and animals were randomly assigned to a morphine dose (1, 3 or 8 mg/kg or saline). Morphine was administered by subcutaneous injection twice a day for 5 days. On Day 6, animals were challenged with a single dose of 3 mg/kg morphine prior to formalin testing. Evidence of tolerance was mixed, and the results varied widely among the conditions. Analysis of mean paw withdrawal thresholds indicated that the analgesic efficacy of subcutaneous morphine diminished following repeated dosing. The presence of the chronic inflammatory pain condition during the morphine dosing period produced an increase in formalin pain behaviors compared to saline controls, such that animals given any dose of morphine during the 5-day dosing period showed higher responding to formalin following the 3 mg/kg dose than animals that had received saline injections. These results indicate that chronic pain does influence the development of opioid tolerance, but it does not prevent this phenomenon from occurring as suggested by some researchers.

Opioid-induced latent sensitization in a model of non-inflammatory viscerosomatic hypersensitivity

Exposure to opioids can induce a state of "latent sensitization" characterized by long-lasting enhanced responses to subsequent cutaneous injury. Here, we explored the possibility that prior treatment with morphine could induce a state of latent sensitization to visceral pain conditions. Following butyrate enemas to induce non-inflammatory visceral pain, acute morphine administration produced dose-related inhibition of referred viscerosomatic hypersensitivity. Treatment with morphine for a period of six days resulted in a persistent hyperalgesia that resolved many days after termination of drug administration. In morphine pre-exposed rats, butyrate-induced referred hypersensitivity was enhanced and extended in duration. No differences were observed in the morphine dose-response curve in suppression of acute nociception (i.e., the hot-plate assay) when morphine pre-exposed rats were compared to naïve rats indicating that opioid antinociceptive tolerance was not present. However, the morphine dose-response curve to suppress evoked viscerosomatic hypersensitivity was displaced to the right by approximately 4-fold in morphine pre-exposed rats. Induction of viscerosomatic hypersensitivity resulted in an increased labeling of CGRP-, but not substance P-positive cells in the lumbar dorsal root ganglia; increased labeling was not affected by prior exposure to morphine. The data indicate that a period of morphine exposure can induce a state of "latent sensitization" to subsequent visceral pain characterized by extended duration of hypersensitivity. This condition likely reflects enhanced visceral "pain" intensity as a consequence of persistent pronociceptive adaptive changes.