Opioid receptor desensitization contributes to thermal hyperalgesia in infant rats

Intermittent REM sleep deprivation attenuates the development of morphine tolerance and dependence in male rats

Opioid agonists are used in clinic for pain management, however this application is challenged by development of tolerance and dependence following prolonged exposure. Various approaches have been suggested to address this concern, however, there is still no consensus among the researchers. Neural processing of sleep and nociception are co-regulated through shared brain regions having bidirectional interplays. Thus, we aimed to investigate whether application of REM sleep deprivation (REM-SD) could affect morphine analgesic tolerance and dependence. To this end, adult male rats underwent sleep deprivation during light and dark phases (LSD and DSD, respectively) using the inverted flower pot method and then tolerance and dependence was induced by repeated injection of morphine for 7 days (10 mg/kg, daily, i.p.). Results indicated that REM-SD delays the development of tolerance to morphine during both phases; however this effect was more potent following LSD. Moreover, LSD decreased the baseline thermal threshold and total withdrawal score. One possible hypothesis for our observations is REM-SD-induced attenuation of orexin system which is still controversial among the researchers. Other stronger possibilities might be down-regulation of opioid receptors in response to sleep loss experience. Finally, it seems that modification of sleep periods may assist to decrease the severity of opioid tolerance and dependence.

Long-Term Outcomes after Protocolized Sedation versus Usual Care in Ventilated Pediatric Patients

RATIONALE

Whether a nurse-implemented goal-directed sedation protocol resulting in more awake yet calm intubated children affects postdischarge functional status, health-related quality of life, or risk for post-traumatic stress disorder is unknown.

OBJECTIVES

To compare postdischarge outcomes in children with acute respiratory failure cluster-randomized to a sedation protocol or usual care.

METHODS

A stratified random sample of 1,360 patients from 31 centers in the RESTORE (Randomized Evaluation of Sedation Titration for Respiratory Failure) trial was assessed by mail, electronically, and/or telephone 6 months after ICU discharge. In treatment group comparisons, we controlled for age, baseline functional status, and severity of illness.

MEASUREMENTS AND MAIN RESULTS

We used the Pediatric Overall Performance Category and the Pediatric Cerebral Performance Category to characterize functional status, the Infant and Toddler Quality of Life Questionnaire (97-item full-length version) (<2 yr old) or Pediatric Quality of Life Inventory (≥2 yr old), and the Child Post-traumatic Stress Disorder Symptom Scale (≥8 yr old and developmentally able). Functional status worsened from baseline to follow-up in 20%. Decline in functional status did not differ by treatment arm and was more common among those with baseline impairment than those with baseline normal function (27 vs. 18%; P < 0.001). There were no significant differences in health-related quality of life total scores by treatment arm. Scores indicating risk of post-traumatic stress disorder occurred in 30%, with no difference between treatment arms.

CONCLUSIONS

A sedation strategy that allows patients to be more awake and exposes them to fewer sedative and analgesic medications produces no long-term harm. However, postdischarge morbidity after acute respiratory failure is common. Clinical trial registered with www.clinicaltrials.gov (NCT00814099).

Adult spinal opioid receptor μ1 expression after incision is altered by early life repetitive tactile and noxious procedures in rats

Clinical and experimental data suggests that noxious stimulation at critical stages of development results in long‐term changes on nociceptive processing in later life. Here, we use an established, well‐documented rat model of repetitive noxious procedures closely mimicking the clinical situation in the NICU. In order to understand molecular changes underlying the long‐term consequences of repetitive stimulation of the developing nociceptive system the present study aims to analyze the presence of the µ‐opioid‐receptor‐1 (OPRM1). Neonatal rats received either four needle pricks per day in the left hind‐paw from postnatal day 0–7 as a model of procedural pain in infancy. Control pups were handled in the same way but were instead tactile stimulated, or were left undisturbed. At the age of 8 weeks, all animals received an ipsilateral hind‐paw incision as a model for post‐operative pain, and mechanical sensitivity was tested at multiple time‐points. Before, and 1 or 5 days post‐incision, spinal cord tissue was collected for immunostaining of opioid receptor OPRM1. Semi‐quantitative immunocytochemical analysis of superficial laminae in lumbar spinal dorsal horn revealed that: (1) early life repetitive tactile or noxious procedures do not alter baseline levels of OPRM1 staining intensity and (2) early life repetitive tactile or noxious procedures lead to a decrease in OPRM1 staining intensity 5 days after incision in adulthood compared to undisturbed controls. We conclude that early life repetitive tactile or noxious procedures affect the intensity of OPRM1‐immunoreactivity in the lumbar superficial spinal cord dorsal horn after adulthood injury, without affecting baseline intensity. © 2018 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 417–426, 2018

Regulation of opioid receptor signalling: implications for the development of analgesic tolerance

Opiate drugs are the most effective analgesics available but their clinical use is restricted by severe side effects. Some of these undesired actions appear after repeated administration and are related to adaptive changes directed at counteracting the consequences of sustained opioid receptor activation. Here we will discuss adaptations that contribute to the development of tolerance. The focus of the first part of the review is set on molecular mechanisms involved in the regulation of opioid receptor signalling in heterologous expression systems and neurons. In the second part we assess how adaptations that take place in vivo may contribute to analgesic tolerance developed during repeated opioid administration.

Opiate prescription medication dependence and pain perceptions

Our study showed that the perception of pain lessens with detoxification from chronic prescription opiate medications. Thus, removal of opiates resulted in less pain, and chronic administration of opiates actually increased pain perceptions. The underlying pathophysiology of increased pain sensitivity from chronic administration is not well understood. However, the enhanced pain from the development of tolerance and dependence is a common phenomenon. Clinicians are advised to limit their prescribing of opiate drugs to patients on a chronic basis. Further, unresolved pain complaints, and continued complaints of pain despite escalating doses of opiate medications suggest addiction and its adverse consequences. Identification of addiction and detoxification is the proper approach to pain management in chronic opiate administration.

Pharmacological approaches to the management of pain in the neonatal intensive care unit

Effective and consistent management of neonatal pain remains a controversial issue. Premature infants are repeatedly subjected to painful tests and procedures or suffer painful conditions when they are most vulnerable. With different mechanisms transducing various types of pain the practice of 'one-drug fits all' becomes questionable. Clinicians must use the latest non-pharmacologic and pharmacologic therapies for effective management of neonatal pain, distress, or agitation. Pharmacologic strategies for dealing with neonatal pain in the neonatal intensive care unit are described. Opioid therapy, once considered the mainstay for neonatal analgesia, may not be as effective as previously thought. Morphine infusions do not alter the neurological outcomes of preterm neonates and may not be effective against acute pain. Alternative approaches with methadone, ketamine, or local anesthetics should be considered. Clinicians must understand the contextual circumstances underlying pain in individual neonates and tailor therapy accordingly, using the most current evidence related to neonatal pain assessment and management.

Pharmacological therapy for analgesia and sedation in the newborn

Rapid advances have been made in the use of pharmacological analgesia and sedation for newborns requiring neonatal intensive care. Practical considerations for the use of systemic analgesics (opioids, non-steroidal anti-inflammatory agents, other drugs), local and topical anaesthetics, and sedative or anaesthetic agents (benzodiazepines, barbiturates, other drugs) are summarised using an evidence-based medicine approach, while avoiding mention of the underlying basic physiology or pharmacology. These developments have inspired more humane approaches to neonatal intensive care. Despite these advances, little is known about the clinical effectiveness, immediate toxicity, effects on special patient populations, or long-term effects after neonatal exposure to analgesics or sedatives. The desired or adverse effects of drug combinations, interactions with non-pharmacological interventions or use for specific conditions also remain unknown. Despite the huge gaps in our knowledge, preliminary evidence for the use of neonatal analgesia and sedation is available, but must be combined with a clear definition of clinical goals, continuous physiological monitoring, evaluation of side effects or tolerance, and consideration of long-term clinical outcomes.

Effects of opioid prescription medication dependence and detoxification on pain perceptions and self-reports

The purpose of this work is to document whether prescription opioid medications used for pain enhanced or worsened pain syndromes from medical conditions in patients who received a diagnosis of prescription opioid dependence as determined by a diagnosis by Diagnostic and Statistical Manual of Mental Disorders (DSM)-IV criteria. Further, whether detoxification improved or worsened pain perceptions and self-reports in patients who chronically administered prescription opioid medications. Our study consisted of a retrospective sample of patients taken from the Addiction Treatment Unit at St Lawrence Hospital in Lansing, MI. Patients were selected from those who voluntarily sought detoxification from opioid medications in an inpatient setting. Selection criteria for the study consisted of a DSM-IV diagnosis of opioid prescription medication dependence, willingness to undergo medical detoxification, cooperation with self-report scales, and abstinence from opioid medications. Study patients were randomly selected from discharges in patient census for the years 2001 to 2003. The significant findings were that self-reported pain scores improved during the detoxification from admission to discharge, from a mean of 5.5 at admission to mean of 3.4 at discharge (0 is no pain and 10 is the most pain). The detoxification period extended to an average of 5 days. Whereas oxycodone CR (OxyContin) produced higher levels of self-reported pain at admission than at discharge, and these patients experienced significant levels of pain reduction with decremental opioid doses as with other opioid medications. Patients with a DSM-IV diagnosis of prescription opioid dependence reported (self) less pain with detoxification and abstinence from the opioid medications.

Analgesia and local anesthesia during invasive procedures in the neonate

BACKGROUND

Preterm and full-term neonates admitted to the neonatal intensive care unit or elsewhere in the hospital are routinely subjected to invasive procedures that can cause acute pain. Despite published data on the complex behavioral, physiologic, and biochemical responses of these neonates and the detrimental short- and long-term clinical outcomes of exposure to repetitive pain, clinical use of pain-control measures in neonates undergoing invasive procedures remains sporadic and suboptimal. As part of the Newborn Drug Development Initiative, the US Food and Drug Administration and the National Institute of Child Health and Human Development invited a group of international experts to form the Neonatal Pain Control Group to review the therapeutic options for pain management associated with the most commonly performed invasive procedures in neonates and to identify research priorities in this area.

OBJECTIVE

The goal of this article was to review and synthesize the published clinical evidence for the management of pain caused by invasive procedures in preterm and full-term neonates.

METHODS

Clinical studies examining various therapies for procedural pain in neonates were identified by searches of MEDLINE (1980-2004), the Cochrane Controlled Trials Register (The Cochrane Library, Issue 1, 2004), the reference lists of review articles, and personal files. The search terms included specific drug names, infant-newborn, infant-preterm, and pain, using the explode function for each key word. The English-language literature was reviewed, and case reports and small case series were discarded.

RESULTS

The most commonly performed invasive procedures in neonates included heel lancing, venipuncture, IV or arterial cannulation, chest tube placement, tracheal intubation or suctioning, lumbar puncture, circumcision, and SC or IM injection. Various drug classes were examined critically, including opioid analgesics, sedative/hypnotic drugs, nonsteroidal anti-inflammatory drugs and acetaminophen, injectable and topical local anesthetics, and sucrose. Research considerations related to each drug category were identified, potential obstacles to the systematic study of these drugs were discussed, and current gaps in knowledge were enumerated to define future research needs. Discussions relating to the optimal design for and ethical constraints on the study of neonatal pain will be published separately. Well-designed clinical trials investigating currently available and new therapies for acute pain in neonates will provide the scientific framework for effective pain management in neonates undergoing invasive procedures.

Pain activates cortical areas in the preterm newborn brain

To study the patterns of supraspinal pain processing in neonates, we hypothesized that acute pain causes haemodynamic changes associated with activation of the primary somatosensory cortex. Forty preterm neonates at 28-36 weeks of gestation (mean=32.0) and at 25-42 h (mean=30.7) of age were studied following standardized tactile (skin disinfection) and painful (venipuncture) stimuli. Changes in regional cerebral haemodynamics were monitored by near infrared spectroscopy (NIRS) over both somatosensory cortices in 29 newborns, and over the contralateral somatosensory and occipital areas in 11 newborns. Heart rate (HR) and peripheral oxygen saturation (SaO2) were recorded simultaneously with NIRS parameters: oxygenated [HbO2], deoxygenated, and total hemoglobin. Tactile stimulation produced no changes in HR or SaO2. HR increased in the first 20s (p<0.001), while SaO2 decreased during the 40s after venipuncture (p<0.0001). Following tactile or painful stimulation, [HbO2] increased bilaterally regardless of which hand was stimulated (p<0.0001). Pain-induced [HbO2] increases in the contralateral somatosensory cortex (p<0.05) were not mirrored in the occipital cortex (p>0.1). Pain-related [HbO2] increases were more pronounced in male neonates (p<0.05 on left, p<0.001 on right), inversely correlated with gestational age (r=-0.53 on left, p<0.01; r=-0.42 on right, p<0.05) and directly correlated with postnatal age (r=0.75 on left, p<0.0001; r=0.67 on right, p<0.0001). Painful and tactile stimuli elicit specific haemodynamic responses in the somatosensory cortex, implying conscious sensory perception in preterm neonates. Somatosensory cortical activation occurs bilaterally following unilateral stimulation and these changes are more pronounced in male neonates or preterm neonates at lower gestational ages.

Summary proceedings from the neonatal pain-control group

Recent advances in neurobiology and clinical medicine have established that the fetus and newborn may experience acute, established, and chronic pain. They respond to such noxious stimuli by a series of complex biochemical, physiologic, and behavioral alterations. Studies have concluded that controlling pain experience is beneficial with respect to short-term and perhaps long-term outcomes. Yet, pain-control measures are adopted infrequently because of unresolved scientific issues and lack of appreciation for the need for control of pain and its long-term sequelae during the critical phases of neurologic maturation in the preterm and term newborn. The neonatal pain-control group, as part of the Newborn Drug Development Initiative (NDDI) Workshop I, addressed these concerns. The specific issues addressed were (1) management of pain associated with invasive procedures, (2) provision of sedation and analgesia during mechanical ventilation, and (3) mitigation of pain and stress responses during and after surgery in the newborn infant. The cross-cutting themes addressed within each category included (1) clinical-trial designs, (2) drug prioritization, (3) ethical constraints, (4) gaps in our knowledge, and (5) future research needs. This article provides a summary of the discussions and deliberations. Full-length articles on procedural pain, sedation and analgesia for ventilated infants, perioperative pain, and study designs for neonatal pain research were published in Clinical Therapeutics (June 2005).

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Involvement of cAMP/cAMP-dependent protein kinase signaling pathway in regulation of Na+,K+-ATPase upon activation of opioid receptors by morphine

The depolarization of neurons induced by impairment of Na+,K+-ATPase activity after long-term opiate treatment has been shown to involve the development of opioid dependence. However, the mechanisms underlying changes in Na+,K+-ATPase activity after opioid treatment are unclear. The best-established molecular adaptation to long-term opioid exposure is up-regulation of the cAMP/cAMP-dependent protein kinase (PKA) signaling pathway; this study, therefore, was undertaken to investigate the role of up-regulation of cAMP/PKA signaling pathway in alteration of the mouse hippocampal Na+,K+-ATPase activity. The results demonstrated that short-term morphine treatment dose dependently stimulated Na+,K+-ATPase activity. This action could be significantly suppressed by adenylyl cyclase activator 7beta-acetoxy-8,13-epoxy-1alpha,6beta,9alpha-trihydroxylabd-14-en-11-one (forskolin), or the cAMP analog dibutyryl-cAMP. Contrary to short-term morphine treatment, long-term treatment significantly inhibited Na+,K+-ATPase activity. Moreover, an additional decrease in Na+,K+-ATPase activity was observed by naloxone precipitation. The effects of both short- and long-term morphine treatment on Na+,K+-ATPase activity were naltrexone-reversible. The regulation of Na+,K+-ATPase activity by morphine was inversely correlated with intracellular cAMP accumulation. N-[2-(4-Bromocinnamylamino)ethyl]-5-isoquinoline (H89), a specific PKA inhibitor, mimicked the stimulatory effect of short-term morphine but antagonized the inhibitory effect of long-term morphine treatment on Na+,K+-ATPase activity. However, okadaic acid, a protein phosphatase inhibitor, suppressed short-term morphine stimulation but potentiated long-term morphine inhibition of Na+,K+-ATPase activity. The regulation of Na+,K+-ATPase activity by morphine treatment seemed to associate with the alteration in phosphorylation level but not to be relevant to the change in abundance of Na+,K+-ATPase. These findings strongly demonstrate that cAMP/PKA signaling pathway involves regulation of Na+,K+-ATPase activity after activation of opioid receptors.

Social and environmental influences on opioid sensitivity in rats: importance of an opioid's relative efficacy at the mu-receptor

RATIONALE

Evidence indicates that social and environmental enrichment can influence the functional maturation of the central nervous system and may affect an organism's sensitivity to centrally acting drugs.

OBJECTIVE

The purpose of the present study was to examine the effects of social and environmental enrichment on sensitivity to mu-opioids possessing a range of relative efficacies at the mu-receptor.

METHODS

Rats were obtained at weaning (21 days) and divided into two groups immediately upon arrival. Isolated rats were housed individually in opaque laboratory cages with no visual or tactile contact with other rats; enriched rats were housed socially in groups of four in large cages and given various novel objects on a daily basis. After 6 weeks under these conditions, the effects of morphine, levorphanol, buprenorphine, butorphanol, and nalbuphine were examined in the warm-water, tail-withdrawal procedure and the place-conditioning procedure.

RESULTS

In the tail-withdrawal procedure, isolated and enriched rats did not differ in sensitivity to morphine (1.0-30 mg/kg) and levorphanol (0.3-10 mg/kg), but enriched rats were more sensitive to buprenorphine (0.03-3.0 mg/kg), butorphanol (0.3-30 mg/kg), and nalbuphine (0.3-30 mg/kg). In drug combination tests, butorphanol and nalbuphine antagonized the effects of morphine in isolated rats under conditions in which they produced high levels of antinociception in enriched rats. In the place-conditioning procedure, doses of 10 morphine and 3.0 levorphanol established a place preference in both groups of rats, whereas doses of 0.3 buprenorphine, 3.0 butorphanol, and 10 nalbuphine established a place preference only in enriched rats.

CONCLUSIONS

These findings may be taken as evidence that enriched rats are more sensitive than isolated rats to the effects of lower-efficacy mu-opioids and that social and environmental enrichment leads to functional alterations in opioid receptor populations.

Morphine does not provide adequate analgesia for acute procedural pain among preterm neonates

BACKGROUND

Morphine alleviates prolonged pain, reduces behavioral and hormonal stress responses induced by surgery among term neonates, and improves ventilator synchrony and sedation among ventilated preterm neonates, but its analgesic effects on the acute pain caused by invasive procedures remain unclear.

OBJECTIVE

To investigate the analgesic efficacy of intravenously administered morphine on heel stick-induced acute pain among preterm neonates.

DESIGN

This study was nested within a prospective, randomized, double-blind, multicenter, placebo-controlled trial (the NEOPAIN Trial).

SETTING

A tertiary-care NICU in a teaching hospital.

PARTICIPANTS

Forty-two preterm neonates undergoing ventilation.

INTERVENTIONS

Neonates were randomized to either the morphine (loading dose of 100 microg/kg, followed by infusions of 10-30 microg/kg per hour according to gestation, N = 21) or placebo (5% dextrose infusions, N = 21) group. Pain responses to 3 heel sticks were evaluated, ie, before the loading dose (T1), 2 to 3 hours after the loading dose (T2), and 20 to 28 hours after the loading dose (T3).

MAIN OUTCOMES MEASURES

Pain was assessed with the Douleur Aiguë Nouveau-né (DAN) scale (behavioral pain scale) and the Premature Infant Pain Profile (PIPP) (multidimensional pain scale); plasma morphine levels were measured at T3.

RESULTS

Infants in the placebo and morphine groups had similar gestational ages (mean +/- SD: 27.2 +/- 1.7 vs 27.3 +/- 1.8 weeks) and birth weights (972 +/- 270 vs 947 +/- 269 g). Mean +/- SD DAN pain scores at T1, T2, and T3 were 4.8 +/- 4.0, 4.6 +/- 2.9, and 4.7 +/- 3.6, respectively, for the placebo group and 4.5 +/- 3.8, 4.4 +/- 3.7, and 3.1 +/- 3.4 for the morphine group. The within-group factor (pain at T1, T2, and T3) was not statistically different over time. The between-group analysis (infants receiving placebo versus those receiving morphine) showed no significant differences. Mean +/- SD PIPP pain scores at T1, T2, and T3 were 11.5 +/- 4.8, 11.1 +/- 3.7, and 9.1 +/- 4.0, respectively, for the placebo group and 10.0 +/- 3.6, 8.8 +/- 4.9, and 7.8 +/- 3.6 for the morphine group. The within-group factor was statistically different over time. The between-group analysis showed no significant differences. Mean +/- SD plasma morphine levels at T3 were 0.44 +/- 1.79 ng/mL and 63.36 +/- 33.35 ng/mL for the placebo and morphine groups, respectively. There was no correlation between plasma morphine levels and pain scores at T3 (DAN, R = -0.05; PIPP, R = -0.02).

CONCLUSIONS

Despite its routine use in the NICU, morphine given as a loading dose followed by continuous intravenous infusions does not appear to provide adequate analgesia for the acute pain caused by invasive procedures among ventilated preterm neonates.

Does neonatal surgery lead to increased pain sensitivity in later childhood?

Does pain or tissue damage in early life lead to hyperalgesia persisting into childhood? We performed a cross-sectional study in 164 infants to investigate whether major surgery within the first 3 months of life increases pain sensitivity to subsequent surgery and to elucidate whether subsequent surgery in the same dermatome or in a different dermatome leads to differences in pain sensitivity. All infants received standard intraoperative and postoperative pain management, with rescue analgesia guided by a treatment algorithm. Differences in pain sensitivity during surgery were assessed by the intraoperative fentanyl intake and by (nor)epinephrine plasma concentrations. Differences in postoperative pain sensitivity were assessed by the observational pain measures COMFORT and VAS, and by morphine intake and (nor)epinephrine plasma concentrations. Infants previously operated upon in the same dermatome needed more intraoperative fentanyl, had higher COMFORT and VAS scores, had greater (nor)epinephrine plasma concentrations, and needed also more morphine than did infants with no prior surgery. In contrast, infants who previously underwent surgery in another dermatome had only significant higher postoperative analgesic requirements and norepinephrine plasma concentrations in comparison with infants with no prior surgery. These preliminary differences may indicate the occurrence of spinal and supraspinal changes following neonatal surgery. We conclude that the long-term consequences of surgery in early infancy are greater in areas of prior tissue damage and that these effects may portend limited clinical but important neurobiological differences.