Sex differences in opioid analgesia, hyperalgesia, tolerance and withdrawal: central mechanisms of action and roles of gonadal hormones

Lipophilic analogues of D-cysteine prevent and reverse physical dependence to fentanyl in male rats

We examined whether co-injections of the cell-permeant D-cysteine analogues, D-cysteine ethyl ester (D-CYSee) and D-cysteine ethyl amide (D-CYSea), prevent acquisition of physical dependence induced by twice-daily injections of fentanyl, and reverse acquired dependence to these injections in freely-moving male Sprague Dawley rats. Injection of the opioid receptor antagonist, naloxone HCl (NLX, 1.5 mg/kg, IV), elicited a series of withdrawal phenomena that included cardiorespiratory and behavioral responses, and falls in body weight and body temperature, in rats that received 5 or 10 injections of fentanyl (125 μg/kg, IV), and the same number of vehicle co-injections. Regarding the development of physical dependence, the NLX-precipitated withdrawal phenomena were markedly reduced in fentanyl-injected rats that had received co-injections of D-CYSee (250 μmol/kg, IV) or D-CYSea (100 μmol/kg, IV), but not D-cysteine (250 μmol/kg, IV). Regarding reversal of established dependence to fentanyl, the NLX-precipitated withdrawal phenomena in rats that had received 10 injections of fentanyl (125 μg/kg, IV) was markedly reduced in rats that received co-injections of D-CYSee (250 μmol/kg, IV) or D-CYSea (100 μmol/kg, IV), but not D-cysteine (250 μmol/kg, IV), starting with injection 6 of fentanyl. This study provides evidence that co-injections of D-CYSee and D-CYSea prevent the acquisition of physical dependence, and reverse acquired dependence to fentanyl in male rats. The lack of effect of D-cysteine suggests that the enhanced cell-penetrability of D-CYSee and D-CYSea into cells, particularly within the brain, is key to their ability to interact with intracellular signaling events involved in acquisition to physical dependence to fentanyl.

Gender Differences in Pain Threshold, Unpleasantness, and Descending Pain Modulatory Activation Across the Adult Life Span: A Cross Sectional Study

The neurobiological underpinnings of gender differences in pain perception, and how these differences may be modified by age, are incompletely understood, placing patients at risk of suboptimal pain management. Using functional magnetic resonance imaging, we examined brain responses in the descending pain modulatory system (DPMS, specifically, dorsolateral prefrontal cortex, anterior cingulate cortex, insula, hypothalamus, amygdala, and periaqueductal gray, during an evoked pain task. We investigated the interaction of age and gender in our sample of healthy adults (27 females, 32 males, 30-86 years) on DPMS response. In a perceptually matched thermal pain paradigm, we investigated pain unpleasantness and neural responses for 3 heat pain percepts: just noticeable pain, weak pain, and moderate pain (MP). Females reported just noticeable pain at a lower temperature, but reported less unpleasantness at weak pain and MP percepts, compared to males. There was a significant age-by-gender interaction during moderate pain in the right anterior cingulate cortex and bilateral insula, such that, males had a stronger positive relationship between DPMS response and age compared to females in these regions. Our results indicate that differences in DPMS responses may explain some gender differences in pain perception and that this effect may change across the adult lifespan. PERSPECTIVE: Gender differences in pain have been well-documented but the brain mechanisms for these differences are still unclear. This article describes potential differences in brain functioning during different levels of pain that could explain differences in pain responses between men and women across the adult lifespan.

Chronic Morphine Induces Adaptations in Opioid Receptor Signaling in a Thalamostriatal Circuit That Are Location Dependent, Sex Specific, and Regulated by μ-Opioid Receptor Phosphorylation

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) via activity at μ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in the DMS. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation) in male, but not female, mice. At MThal cell bodies, chronic morphine treatment decreased subsequent morphine activation of potassium conductance (morphine tolerance) in both male and female mice. In knock-in mice expressing phosphorylation-deficient MORs, chronic morphine treatment resulted in tolerance to, rather than facilitation of, subsequent morphine signaling at MThal-DMS terminals, suggesting phosphorylation deficiency unmasks adaptations that counter the facilitation observed at presynaptic terminals in wild-type mice. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry, and sex.

Projections from infralimbic medial prefrontal cortex glutamatergic outputs to amygdala mediates opioid induced hyperalgesia in male rats

Repeated use of opioid analgesics may cause a paradoxically exacerbated pain known as opioid-induced hyperalgesia (OIH), which hinders effective clinical intervention for severe pain. Currently, little is known about the neural circuits underlying OIH modulation. Previous studies suggest that laterocapsular division of the central nucleus of amygdala (CeLC) is critically involved in the regulation of OIH. Our purpose is to clarify the role of the projections from infralimbic medial prefrontal cortex (IL) to CeLC in OIH. We first produced an OIH model by repeated fentanyl subcutaneous injection in male rats. Immunofluorescence staining revealed that c-Fos-positive neurons were significantly increased in the right CeLC in OIH rats than the saline controls. Then, we used calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) labeling and the patch-clamp recordings with ex vivo optogenetics to detect the functional projections from glutamate pyramidal neurons in IL to the CeLC. The synaptic transmission from IL to CeLC, shown in the excitatory postsynaptic currents (eEPSCs), inhibitory postsynaptic currents (eIPSCs) and paired-pulse ratio (PPR), was observably enhanced after fentanyl administration. Moreover, optogenetic activation of this IL-CeLC pathway decreased c-Fos expression in CeLC and ameliorated mechanical and thermal pain in OIH. On the contrary, silencing this pathway by chemogenetics exacerbated OIH by activating the CeLC. Combined with the electrophysiology results, the enhanced synaptic transmission from IL to CeLC might be a cortical gain of IL to relieve OIH rather than a reason for OIH generation. Scaling up IL outputs to CeLC may be an effective neuromodulation strategy to treat OIH.

L-cysteine ethyl ester prevents and reverses acquired physical dependence on morphine in male Sprague Dawley rats

The molecular mechanisms underlying the acquisition of addiction/dependence on morphine may result from the ability of the opioid to diminish the transport of L-cysteine into neurons via inhibition of excitatory amino acid transporter 3 (EAA3). The objective of this study was to determine whether the co-administration of the cell-penetrant L-thiol ester, L-cysteine ethyl ester (L-CYSee), would reduce physical dependence on morphine in male Sprague Dawley rats. Injection of the opioid-receptor antagonist, naloxone HCl (NLX; 1.5 mg/kg, IP), elicited pronounced withdrawal phenomena in rats which received a subcutaneous depot of morphine (150 mg/kg) for 36 h and were receiving a continuous infusion of saline (20 μL/h, IV) via osmotic minipumps for the same 36 h period. The withdrawal phenomena included wet-dog shakes, jumping, rearing, fore-paw licking, 360° circling, writhing, apneas, cardiovascular (pressor and tachycardia) responses, hypothermia, and body weight loss. NLX elicited substantially reduced withdrawal syndrome in rats that received an infusion of L-CYSee (20.8 μmol/kg/h, IV) for 36 h. NLX precipitated a marked withdrawal syndrome in rats that had received subcutaneous depots of morphine (150 mg/kg) for 48 h) and a co-infusion of vehicle. However, the NLX-precipitated withdrawal signs were markedly reduced in morphine (150 mg/kg for 48 h)-treated rats that began receiving an infusion of L-CYSee (20.8 μmol/kg/h, IV) at 36 h. In similar studies to those described previously, neither L-cysteine nor L-serine ethyl ester (both at 20.8 μmol/kg/h, IV) mimicked the effects of L-CYSee. This study demonstrates that 1) L-CYSee attenuates the development of physical dependence on morphine in male rats and 2) prior administration of L-CYSee reverses morphine dependence, most likely by intracellular actions within the brain. The lack of the effect of L-serine ethyl ester (oxygen atom instead of sulfur atom) strongly implicates thiol biochemistry in the efficacy of L-CYSee. Accordingly, L-CYSee and analogs may be a novel class of therapeutics that ameliorate the development of physical dependence on opioids in humans.

The use of ketamine infusion to dramatically reduce opioid requirements in a patient whose high-dose intrathecal opioid pump was inadvertently cut during surgery

BACKGROUND

Chronic opioid therapy may lead to high level tolerance development, hyperalgesia, and central sensitization, which further complicates long-term therapeutic management of chronic pain patients. In this case, we encounter a patient who was receiving over 15,000 morphine milligram equivalents through their intrathecal pain pump. Unfortunately, the intrathecal pump was inadvertently cut during a spinal surgery. It was deemed unsafe to delivery IV equivalent opioid therapy in this case; instead, the patient was admitted to the ICU and given a four-day ketamine infusion.

METHOD

The patient was started on a ketamine infusion at a rate of 0.5mg/kg/h, which was continued for three days. On the fourth day, the infusion rate was tapered over 12 h before being completely stopped. No coinciding opioid therapy was given during this time, which was only restarted in the outpatient setting.

RESULTS

Despite chronic high levels of opioid therapy immediately prior to the ketamine infusion, the patient did not experience florid withdrawals during the infusion period. Additionally, the patient experienced remarkable improvement in their subjective pain rating, which decreased from 9 to 3-4 on an 11-point Number Rating Scale, while simultaneously being managed on an MME <100. These results were sustained through a 6-month follow-up period.

CONCLUSION

Ketamine may play an important role in attenuating not only tolerance but also acute withdrawal in a setting where rapid or instant weaning from high dose chronic opioid therapy is needed.

Amygdala Metabotropic Glutamate Receptor 1 Influences Synaptic Transmission to Participate in Fentanyl-Induced Hyperalgesia in Rats

The underlying mechanisms of opioid-induced hyperalgesia (OIH) remain unclear. Herein, we found that the protein expression of metabotropic glutamate receptor 1 (mGluR1) was significantly increased in the right but not in the left laterocapsular division of central nucleus of the amygdala (CeLC) in OIH rats. In CeLC neurons, the frequency and the amplitude of mini-excitatory postsynaptic currents (mEPSCs) were significantly increased in fentanyl group which were decreased by acute application of a mGluR1 antagonist, A841720. Finally, the behavioral hypersensitivity could be reversed by A841720 microinjection into the right CeLC. These results show that the right CeLC mGluR1 is an important factor associated with OIH that enhances synaptic transmission and could be a potential drug target to alleviate fentanyl-induced hyperalgesia.

Modeling spontaneous opioid withdrawal in male and female outbred mice using traditional endpoints and hyperalgesia

Opioid withdrawal significantly impacts drug dependence cycles as hyperalgesia associated with withdrawal is often a reason for continued drug use. Animal models of addiction are important tools for studying how drug dependence and withdrawal impact not only normal neurocircuitry but also the effectiveness of potential treatments for dependence and withdrawal. We conducted a study of the time course of spontaneous morphine withdrawal in outbred male and female mice that can be used to examine sex differences in male and female mice using both traditional somatic endpoints and mechanical hyperalgesia as an endpoint of withdrawal. Male and female national institute of health (NIH) Swiss mice were made dependent upon morphine using an escalating dosing schedule. Injections were stopped after 5 days. Withdrawal behavior was assessed at time intervals up to 106 h after the final injection. Numbers of forepaw tremors, wet-dog shakes, jumps and other behaviors were scored to create a global score. Paw pressure readings were then also taken to track changes in sensitivity to a painful stimulus over time. Male and female mice had approximately similar withdrawal severity peaking at 24 h after the final injection as measured by composite global scores. Females did exhibit an earlier and greater frequency of tremors than males. Although males and females showed similar hyperalgesia during withdrawal, females recovered faster. Spontaneous opioid withdrawal peaking at 24 h was demonstrated in male and female NIH Swiss mice. We also successfully demonstrated that hyperalgesia is an endpoint that varies over the course of withdrawal.

Chronic morphine induces adaptations in opioid receptor signaling in a thalamo-cortico-striatal circuit that are projection-dependent, sex-specific and regulated by mu opioid receptor phosphorylation

Chronic opioid exposure induces tolerance to the pain-relieving effects of opioids but sensitization to some other effects. While the occurrence of these adaptations is well-understood, the underlying cellular mechanisms are less clear. This study aimed to determine how chronic treatment with morphine, a prototypical opioid agonist, induced adaptations to subsequent morphine signaling in different subcellular contexts. Opioids acutely inhibit glutamatergic transmission from medial thalamic (MThal) inputs to the dorsomedial striatum (DMS) and anterior cingulate cortex (ACC) via activity at μ-opioid receptors (MORs). MORs are present in somatic and presynaptic compartments of MThal neurons terminating in both the DMS and ACC. We investigated the effects of chronic morphine treatment on subsequent morphine signaling at MThal-DMS synapses, MThal-ACC synapses, and MThal cell bodies in male and female mice. Surprisingly, chronic morphine treatment increased subsequent morphine inhibition of MThal-DMS synaptic transmission (morphine facilitation), but decreased subsequent morphine inhibition of transmission at MThal-ACC synapses (morphine tolerance) in a sex-specific manner; these adaptations were present in male but not female mice. Additionally, these adaptations were not observed in knockin mice expressing phosphorylation-deficient MORs, suggesting a role of MOR phosphorylation in mediating both facilitation and tolerance to morphine within this circuit. The results of this study suggest that the effects of chronic morphine exposure are not ubiquitous; rather adaptations in MOR function may be determined by multiple factors such as subcellular receptor distribution, influence of local circuitry and sex.

Integrating mechanistic-based and classification-based concepts into perioperative pain management: an educational guide for acute pain physicians

Chronic pain begins with acute pain. Physicians tend to classify pain by duration (acute vs chronic) and mechanism (nociceptive, neuropathic and nociplastic). Although this taxonomy may facilitate diagnosis and documentation, such categories are to some degree arbitrary constructs, with significant overlap in terms of mechanisms and treatments. In clinical practice, there are myriad different definitions for chronic pain and a substantial portion of chronic pain involves mixed phenotypes. Classification of pain based on acuity and mechanisms informs management at all levels and constitutes a critical part of guidelines and treatment for chronic pain care. Yet specialty care is often siloed, with advances in understanding lagging years behind in some areas in which these developments should be at the forefront of clinical practice. For example, in perioperative pain management, enhanced recovery protocols are not standardized and tend to drive treatment without consideration of mechanisms, which in many cases may be incongruent with personalized medicine and mechanism-based treatment. In this educational document, we discuss mechanisms and classification of pain as it pertains to commonly performed surgical procedures. Our goal is to provide a clinical reference for the acute pain physician to facilitate pain management decision-making (both diagnosis and therapy) in the perioperative period.

Oxycodone self-administration and reinstatement in male and female squirrel monkeys: Effects of alternative reinforcer availability

The use of non-drug alternative reinforcers has long been utilized as a component of therapeutic interventions for the management of substance use disorder; however, the conditions under which alternative reinforcers are most effective are not well characterized. This study evaluated the impact of varying the magnitude of an alternative reinforcer on oxycodone self-administration and reinstatement in male and female squirrel monkeys. Subjects (n=4/sex) were trained under concurrent second-order schedules of reinforcement for intravenous oxycodone (0.001-0.1mg/kg/inj) on one lever, and sweetened condensed milk (5, 10, 20, 30% in water) on another. Oxycodone-primed reinstatement was evaluated by administering 0.32mg/kg oxycodone prior to sessions in which saline was available on the drug-paired lever. During oxycodone self-administration sessions, milk availability decreased oxycodone self-administration and preference in a concentration-dependent manner; low milk concentrations were more effective at decreasing oxycodone’s reinforcing potency in males. During reinstatement tests, milk significantly attenuated oxycodone-primed responding in both males and females; low milk concentrations were more effective at decreasing the priming effects of oxycodone in females. That alternative reinforcers differentially impacted self-administration and reinstatement in a sex-dependent manner suggests that treatment strategies that utilize alternative reinforcers may be more effective in males or females depending on when they are implemented.

Spinal microRNA-134-5p targets glutamate receptor ionotropic kainate 3 to modulate opioid induced hyperalgesia in mice

Background: Fentanyl and its analogs are extensively used for pain relief. However, their paradoxically pronociceptive effects often lead to increased opioids consumption and risk of chronic pain. Compared to other synthetic opioids, remifentanil has been strongly linked to acute opioid hyperalgesia after exposure [remifentanil-induced hyperalgesia (RIH)]. The epigenetic regulation of microRNAs (miRNAs) on targeted mRNAs has emerged as an important pathogenesis in pain. The current research aimed at exploring the significance and contributions of miR-134-5p to the development of RIH. Methods: Both the antinociceptive and pronociceptive effects of two commonly used opioids were assessed, and miRNA expression profiles in the spinal dorsal horn (SDH) of mice acutely exposed to remifentanil and remifentanil equianalgesic dose (RED) sufentanil were screened. Next, the candidate miRNA level, cellular distribution, and function were examined by qPCR, fluorescent in situ hybridization (FISH) and Argonaute-2 immunoprecipitation. Furthermore, bioinformatics analysis, luciferase assays, miRNA overexpression, behavioral tests, golgi staining, electron microscopy, whole-cell patch-clamp recording, and immunoblotting were employed to investigate the potential targets and mechanisms underlying RIH. Results: Remifentanil induced significant pronociceptive effects and a distinct miRNA-profile from sufentanil when compared to saline controls. Among top 30 differentially expressed miRNAs spectrum, spinal miR-134-5p was dramatically downregulated in RIH mice but remained comparative in mice subjected to sufentanil. Moreover, Glutamate Receptor Ionotropic Kainate 3 (Grik3) was a target of miR-134-5p. The overexpression of miR-134-5p attenuated the hyperalgesic phenotype, excessive dendritic spine remodeling, excitatory synaptic structural plasticity, and Kainate receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) in SDH resulting from remifentanil exposure. Besides, intrathecal injection of selective KA-R antagonist was able to reverse the GRIK3 membrane trafficking and relieved RIH. Conclusion: The miR-134-5p contributes to remifentanil-induced pronociceptive features via directly targeting Grik3 to modulate dendritic spine morphology and synaptic plasticity in spinal neurons.

An Immunconjugate Vaccine Alters Distribution and Reduces the Antinociceptive, Behavioral and Physiological Effects of Fentanyl in Male and Female Rats

Fentanyl (FEN) is a potent synthetic opioid associated with increasing incidence of opioid use disorder (OUD) and fatal opioid overdose. Vaccine immunotherapy for FEN-associated disorders may be a viable therapeutic strategy. Here, we expand and confirm our previous study in mice showing immunological and antinociception efficacy of our FEN vaccine administered with the adjuvant dmLT. In this study, immunized male and female rats produced significant levels of anti-FEN antibodies that were highly effective at neutralizing FEN-induced antinociception in the tail flick assay and hot plate assays. The vaccine also decreased FEN brain levels following drug administration. Immunization blocked FEN-induced, but not morphine-induced, rate-disrupting effects on schedule-controlled responding. Vaccination prevented decreases on physiological measures (oxygen saturation, heart rate) and reduction in overall activity following FEN administration in male rats. The impact of FEN on these measures was greater in unvaccinated male rats compared to unvaccinated female rats. Cross-reactivity assays showed anti-FEN antibodies bound to FEN and sufentanil but not to morphine, methadone, buprenorphine, or oxycodone. These data support further clinical development of this vaccine to address OUD in humans.

Opioid use for a first-incident upper extremity fracture in 220,440 patients without recent prior use in Ontario, Canada: a retrospective cohort study

To describe opioid use for a first upper extremity fracture in a cohort of patients who did not have recent opioid use.

Design

Descriptive epidemiological study.

Setting

Emergency Department, Hospital.

Patients/Participants

We obtained health administrative data records of adults presenting with a first adult upper extremity fracture from 2013 to 2017 in Ontario, Canada. We excluded patients with previous fractures, opioid prescription in the past 6 months or hospitalization >5 days after the fracture.

Intervention

Opioid prescription.

Main Outcome Measurements

We identified the proportion of patients filling an opioid prescription within 7 days of fracture. We described this based on different upper extremity fractures (ICD-10), Demographics (age, sex, rurality), comorbidity (Charlson Comorbidity Index, Rheumatoid arthritis, Diabetes), season of injury, and social marginalization (Ontario Marginalization Index-a data algorithm that combines a wide range of demographic indicators into 4 distinct dimensions of marginalization). We considered statistical differences (P< .01) that reached a standardized mean difference of 10% as being clinically important (standardized mean difference [SMD] ≥ 0.1).

Results

From 220,440 patients with a first upper extremity fracture (50% female, mean age 50), opioids were used by 34% of cases overall (32% in males, 36% in females, P< .001, SMD ≥ 0.1). Use varied by body region, with those with multiple or proximal fractures having the highest use: multiple shoulder 64%, multiple regions 62%, shoulder 62%, elbow 38%, wrist 31%, and hand 21%; and was higher in patients who had a nerve/tendon injury or hospitalization (P< .01, SMD ≥ 0.1). Social marginalization, comorbidity, and season of injury had clinically insignificant effects on opioid use.

Conclusions

More than one-third of patients who are recent-non-users will fill an opioid prescription within 7 days of a first upper extremity fracture, with usage highly influenced by fracture characteristics.Level of Evidence: Level II.

Opioids alter paw placement during walking, confounding assessment of analgesic efficacy in a postsurgical pain model in mice

Introduction:

Hind paw–directed assays are commonly used to study the analgesic effects of opioids in mice. However, opioid-induced hyperlocomotion can obscure results of such assays.

Objectives:

We aimed to overcome this potential confound by using gait analysis to observe hind paw usage during walking in mice.

Methods:

We measured changes in the paw print area after induction of postsurgical pain (using the paw incision model) and treatment with oxycodone.

Results:

Paw incision surgery reduced the paw print area of the injured hind paw as mice avoided placing the incised section of the paw on the floor. Surprisingly, oxycodone caused a tiptoe-like gait in mice, reducing the paw print area of both hind paws. Further investigation of this opioid-induced phenotype revealed that analgesic doses of oxycodone or morphine dose-dependently reduced the hind paw print area in uninjured mice. The gait changes were not dependent on opioid-induced increases in the locomotor activity; speed and paw print area had no correlation in opioid-treated mice, and other analgesic compounds that alter locomotor activity did not affect the paw print area.

Conclusion:

Unfortunately, the opioid-induced “tiptoe” gait phenotype prevented gait analysis from being a viable metric for demonstrating opioid analgesia in injured mice. However, this work reveals an important, previously uncharacterized effect of treatment with analgesic doses of opioids on paw placement. Our characterization of how opioids affect gait has important implications for the use of mice to study opioid pharmacology and suggests that scientists should use caution when using hind paw–directed nociceptive assays to test opioid analgesia in mice.

Sex-specific role of the circadian transcription factor NPAS2 in opioid tolerance, withdrawal and analgesia

Opioids like fentanyl remain the mainstay treatment for chronic pain. Unfortunately, opioid's high dependence liability has led to the current opioid crisis, in part, because of side-effects that develop during long-term use, including analgesic tolerance and physical dependence. Both tolerance and dependence to opioids may lead to escalation of required doses to achieve previous therapeutic efficacy. Additionally, altered sleep and circadian rhythms are common in people on opioid therapy. Opioids impact sleep and circadian rhythms, while disruptions to sleep and circadian rhythms likely mediate the effects of opioids. However, the mechanisms underlying these bidirectional relationships between circadian rhythms and opioids remain largely unknown. The circadian protein, neuronal PAS domain protein 2 (NPAS2), regulates circadian-dependent gene transcription in structure of the central nervous system that modulate opioids and pain. Here, male and female wild-type and NPAS2-deficient (NPAS2-/-) mice were used to investigate the role of NPAS2 in fentanyl analgesia, tolerance, hyperalgesia and physical dependence. Overall, thermal pain thresholds, acute analgesia and tolerance to a fixed dose of fentanyl were largely similar between wild-type and NPAS2-/- mice. However, female NPAS2-/- exhibited augmented analgesic tolerance and significantly more behavioral symptoms of physical dependence to fentanyl. Only male NPAS2-/- mice had increased fentanyl-induced hypersensitivity, when compared with wild-type males. Together, our findings suggest sex-specific effects of NPAS2 signaling in the regulation of fentanyl-induced tolerance, hyperalgesia and dependence.

Interactions between the κ opioid system, corticotropin-releasing hormone and oxytocin in partner loss

Selective adult social attachments, or ‘pair bonds’, represent central relationships for individuals in a number of social species, including humans. Loss of a pair mate has emotional consequences that may or may not diminish over time, and that often translate into impaired psychological and physical health. In this paper, we review the literature on the neuroendocrine mechanisms for the emotional consequences of partner loss, with a special focus on hypothesized interactions between oxytocin, corticotropin-releasing hormone and the κ opioid system.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

Opioid analgesia alters corticospinal coupling along the descending pain system in healthy participants

Opioids are potent analgesic drugs with widespread cortical, subcortical, and spinal targets. In particular, the central pain system comprising ascending and descending pain pathways has high opioid receptor densities and is thus crucial for opioid analgesia. Here, we investigated the effects of the opioid remifentanil in a large sample (n = 78) of healthy male participants using combined corticospinal functional MRI. This approach offers the possibility to measure BOLD responses simultaneously in the brain and spinal cord, allowing us to investigate the role of corticospinal coupling in opioid analgesia. Our data show that opioids altered activity in regions involved in pain processing such as somatosensory regions, including the spinal cord and pain modulation such as prefrontal regions. Moreover, coupling strength along the descending pain system, that is, between the anterior cingulate cortex, periaqueductal gray, and spinal cord, was stronger in participants who reported stronger analgesia during opioid treatment while participants that received saline showed reduced coupling when experiencing less pain. These results indicate that coupling along the descending pain pathway is a potential mechanism of opioid analgesia and can differentiate between opioid analgesia and unspecific reductions in pain such as habituation.

HPA axis dysfunction during morphine withdrawal in offspring of female rats exposed to opioids preconception

Opioid use and abuse remain a significant public health problem, particularly in the United States. Indeed, it is estimated that up to 10% of youths (age 12-18) have taken opioids illicitly. A growing body of evidence suggests that this level of widespread opioid exposure can have effects that extend to subsequent generations. Utilizing a well-established rodent model of preconception adolescent opioid exposure in females, we found decreased opioid self-administration coupled with increased cocaine self-administration in adult offspring. This bidirectional effect may be related to negative affect associated with opioid withdrawal, including enhanced stress reactivity. In this study, we tested the hypothesis that the adult offspring of females exposed to morphine during adolescence will demonstrate increased signs of opioid withdrawal when compared to offspring of saline controls. Females were administered increasing doses of morphine (5-25 mg/kg s.c.) or saline (1 ml/kg) from postnatal day 30 (PND30)-PND39. They were then maintained drug free for a minimum of 4 weeks and mated with drug-naïve males on or after PND70. As adults, their male and female offspring (referred to as Mor-F1 or Sal-F1) were administered morphine (10 mg/kg s.c.) twice a day for 5 days. They were then tested for spontaneous withdrawal behaviors for the next 4 days (∼PND70). Levels of corticotropin releasing hormone (Crh) and urocortin 3 (Ucn3) were examined in the amygdala at 48 h and 96 h of withdrawal. Circulating corticosterone was measured at 48 h. Results indicate that Mor-F1 males are heavier than Sal-F1 males with no baseline differences in females. However, Mor-F1 females did not gain weight at the same rate as Sal-F1 females during withdrawal. While there were no differences in somatic withdrawal signs, gene expression data revealed a sex-specific and time-dependent effect on Crh as well as increased Ucn3 and corticosterone in females at 48hrs withdrawal. Overall, these data point to differences in withdrawal and stress reactivity in Mor-F1 animals that may contribute to observed differences in addiction-like behaviors.

Sex differences in opioid receptor mediated effects: Role of androgens

An abundance of data indicates there are sex differences in endogenous opioid peptides and opioid receptors, leading to functional differences in sensitivity to opioid receptor mediated behaviors between males and females. Many of these sex differences are mediated by the effects of gonadal hormones on the endogenous opioid system. Whereas much research has examined the role of ovarian hormones on opioid receptor mediated endpoints, comparatively less research has examined the role of androgens. This review describes what is currently known regarding the influence of androgens on opioid receptor mediated endpoints and how androgens may contribute to sex differences in these effects. The review also addresses the clinical implications of androgenic modulation of opioid receptor mediated behaviors and suggests future lines of research for preclinical and clinical investigators. We conclude that further investigation into androgenic modulation of opioid receptor mediated effects may lead to new options for addressing conditions such as chronic pain and substance use disorders.

Central amygdala inflammation drives pain hypersensitivity and attenuates morphine analgesia in experimental autoimmune encephalomyelitis

ABSTRACT

Chronic pain is a highly prevalent symptom associated with the autoimmune disorder multiple sclerosis (MS). The central nucleus of the amygdala plays a critical role in pain processing and modulation. Neuropathic pain alters nociceptive signaling in the central amygdala, contributing to pain chronicity and opioid tolerance. Here, we demonstrate that activated microglia within the central amygdala disrupt nociceptive sensory processing and contribute to pain hypersensitivity in experimental autoimmune encephalomyelitis (EAE), the most frequently used animal model of MS. Male and female mice with EAE exhibited differences in microglial morphology in the central amygdala, which was associated with heat hyperalgesia, impaired morphine reward, and reduced morphine antinociception in females. Animals with EAE displayed a lack of morphine-evoked activity in cells expressing somatostatin within the central amygdala, which drive antinociception. Induction of focal microglial activation in naïve mice via injection of lipopolysaccharide into the central amygdala produced a loss of morphine analgesia in females, similar to as observed in EAE animals. Our data indicate that activated microglia within the central amygdala may contribute to the sexually dimorphic effects of morphine and may drive neuronal adaptations that lead to pain hypersensitivity in EAE. Our results provide a possible mechanism underlying the decreased efficacy of opioid analgesics in the management of MS-related pain, identifying microglial activation as a potential therapeutic target for pain symptoms in this patient population.

Influences of Gender on Intravenous Nalbuphine Actions After Major Abdominal Surgery: A Multicenter Study

INTRODUCTION

Nalbuphine, a synthetic kappa-opioid receptor (KOR) agonist and a partial µ-opioid receptor (MOR) antagonist, has been used for years as an effective analgesic. It has been shown to have a better safety profile than morphine. Considering the long history of use of this drug, it is interesting that only a limited amount of information exists on how gender differences influence nalbuphine responses. In this randomized double-blind comparative trial after major abdominal surgery, the analgesic effects of two doses of continuous intravenous infusion of nalbuphine were evaluated based on gender.

METHODS

Enrolled patients were divided into four groups (two females and two males with 32 patients in each group). Two of them (groups A1 and A2), one male and one female, received postoperative continuous intravenous infusions of nalbuphine at 2 mg/h via patient-controlled analgesia (PCA). Each patient had the potential of receiving a rescue bolus of 1 mg of nalbuphine with a lock out time of 15 min. The other two groups (groups B1 and B2) received half the infusion dose, 1 mg/h, and half the nalbuphine rescue dose with the PCA pump, 0.5 mg maximum every 15 min as needed. Patients' vital signs, numerical pain rating scores, rescue nalbuphine, and incidence of side effects were assessed immediately after the operation, and every 3 h during the first 12 h.

RESULTS

Nalbuphine 2 mg/h dosing led to significantly lower pain scores amongst females compared to males at 6, 9, and 12 h; while the 1 mg/h infusion pain scores were only lower at the 9-h time period. Females receiving the nalbuphine 2 mg dose at 6 h, and the 1 mg dose at 6, 9, and 12-h measurements needed significantly lower doses of rescue nalbuphine. Females on the 1 mg dose experienced significantly more nausea, vomiting, and sedation at the 6-, 9-, and 12-h measurement times. In the multivariate analysis, female gender was a negative predictor at all measurement times.

CONCLUSIONS

The current study supports the hypothesis that although nalbuphine was found to be an effective and well-tolerated analgesic after major abdominal surgery, females were statistically more responsive than males.

TRIAL REGISTRATION

The study was registered at the Pan African Clinical trials Registry PACTR201304000486309, and approved for the Ethical aspects.

Sex and Estrous Cycle Differences in Analgesia and Brain Oxycodone Levels

Sex differences in opioid analgesia occur in rodents and humans, and could be due to differences in drug and metabolite levels. Thus, we investigated the sex and cycle differences in analgesia (nociception) from oxycodone in rats and related these to sex and cycle differences in brain and plasma oxycodone and metabolite levels. Since numerous opioids are CYP2D enzyme substrates and variation in CYP2D alters opioid drug levels and response, we also initiated studies to see if the sex and cycle differences observed might be due to differences in brain CYP2D activity. Across oxycodone doses, females in diestrus had higher analgesia (using tail flick latency) compared to males and females in estrus; we also demonstrated a direct effect of estrous cycle on analgesia within females. Consistent with the analgesia, females in diestrus had highest brain oxycodone levels (assessed using microdialysis) compared to males and females in estrus. Analgesia correlated with brain oxycodone, but not brain oxymorphone or noroxycodone levels, or plasma drug or metabolite levels. Propranolol (a CYP2D mechanism-based inhibitor), versus vehicle pre-treatments, increased brain oxycodone, and decreased brain oxymorphone/oxycodone drug level ratios (an in vivo CYP2D activity phenotype in the brain) in males and females in estrus, but not in females in diestrus. Brain oxymorphone/oxycodone inversely correlated with analgesia. Together, both sex and estrous cycle impact oxycodone analgesia and brain oxycodone levels, likely through regulation of brain CYP2D oxycodone metabolism. As CYP2D6 is expressed in human brain, perhaps similar sex and cycle influences also occur in humans.

Sex differences in neuroimmune and glial mechanisms of pain

ABSTRACT

Pain is the primary motivation for seeking medical care. Although pain may subside as inflammation resolves or an injury heals, it is increasingly evident that persistency of the pain state can occur with significant regularity. Chronic pain requires aggressive management to minimize its physiological consequences and diminish its impact on quality of life. Although opioids commonly are prescribed for intractable pain, concerns regarding reduced efficacy, as well as risks of tolerance and dependence, misuse, diversion, and overdose mortality rates limit their utility. Advances in development of nonopioid interventions hinge on our appreciation of underlying mechanisms of pain hypersensitivity. For instance, the contributory role of immunity and the associated presence of autoimmune syndromes has become of particular interest. Males and females exhibit fundamental differences in innate and adaptive immune responses, some of which are present throughout life, whereas others manifest with reproductive maturation. In general, the incidence of chronic pain conditions, particularly those with likely autoimmune covariates, is significantly higher in women. Accordingly, evidence is now accruing in support of neuroimmune interactions driving sex differences in the development and maintenance of pain hypersensitivity and chronicity. This review highlights known sexual dimorphisms of neuroimmune signaling in pain states modeled in rodents, which may yield potential high-value sex-specific targets to inform future analgesic drug discovery efforts.

Addiction among women and sexual minority groups

Gender-related alcohol and drug abuse problems are related not only to biologic differences but also to social and environment factors, all of which can influence the clinical presentation, consequences of use, and treatment approaches. The number of women becoming addicted to alcohol or drugs of abuse has significantly increased with women becoming the fastest-growing group of substance abusers in the United States. Given that women experience a more rapid progression of their addiction than men, it is important that we understand and address the differences to help develop prevention and treatment programs that are tailored for women, incorporating trauma assessment and management, comorbidities, financial independence, pregnancy, and child care.

Role of β-arrestin-2 in short- and long-term opioid tolerance in the dorsal root ganglia

G-protein-biased agonists with reduced β-arrestin-2 activation are being investigated as safer alternatives to clinically-used opioids. β-arrestin-2 has been implicated in the mechanism of opioid-induced antinociceptive tolerance. Opioid-induced analgesic tolerance is classically considered as centrally-mediated, but recent reports implicate nociceptive dorsal root ganglia neurons as critical mediators in this process. Here, we investigated the role of β-arrestin-2 in the mechanism of opioid tolerance in dorsal root ganglia nociceptive neurons using β-arrestin-2 knockout mice and the G-protein-biased μ-opioid receptor agonist, TRV130. Whole-cell current-clamp electrophysiology experiments revealed that 15-18-h overnight exposure to 10 μM morphine in vitro induced acute tolerance in β-arrestin-2 wild-type but not knockout neurons. Furthermore, in wild-type neurons circumventing β-arrestin-2 activation by overnight treatment with 200 nM TRV130 attenuated tolerance. Similarly, acute morphine tolerance in vivo in β-arrestin-2 knockout mice was prevented in the warm-water tail-withdrawal assay. Treatment with 30 mg/kg TRV130 s.c. also inhibited acute antinociceptive tolerance in vivo in wild-type mice. Alternately, in β-arrestin-2 knockout neurons tolerance induced by 7-day in vivo exposure to 50 mg morphine pellet was conserved. Likewise, β-arrestin-2 deletion did not mitigate in vivo antinociceptive tolerance induced by 7-day exposure to 25 mg or 50 mg morphine pellet in both female or male mice, respectively. Consequently, these results indicated that β-arrestin-2 mediates acute but not chronic opioid tolerance in dorsal root ganglia neurons and to antinociception in vivo. This suggests that opioid-induced antinociceptive tolerance may develop even in the absence of β-arrestin-2 activation, and thus significantly affect the clinical utility of biased agonists.

Sexually dimorphic neuroimmune response to chronic opioid treatment and withdrawal

Opioid use disorder is a leading cause of morbidity and mortality in the United States. Increasing pre-clinical and clinical evidence demonstrates sex differences in opioid use and dependence. However, the underlying molecular mechanisms contributing to these effects, including neuroinflammation, are still obscure. Therefore, in this study, we investigated the effect of oxycodone exposure and withdrawal on sex- and region-specific neuroimmune response. Real-time PCR and multiplex cytokine array analysis demonstrated elevated neuroinflammation with increased pro-inflammatory cytokine levels, and aberrant oligodendroglial response in reward neurocircuitry, following withdrawal from chronic oxycodone treatment. Chronic oxycodone and withdrawal treated male mice had lower mRNA expression of TMEM119 along with elevated protein levels of pro-inflammatory cytokines/chemokines and growth factors (IL-1β, IL-2, IL-7, IL-9, IL-12, IL-15, IL17, M-CSF, VEGF) in the prefrontal cortex (PFC) as compared to their female counterparts. In contrast, reduced levels of pro-inflammatory cytokines/chemokines (IL-1β, IL-6, IL-9, IL-12, CCL11) was observed in the nucleus accumbens (NAc) of oxycodone and withdrawal-treated males as compared to female mice. No treatment specific effects were observed on the mRNA expression of putative microglial activation markers (Iba1, CD68), but an overall sex specific decrease in the mRNA expression of Iba1 and CD68 was found in the PFC and NAc of male mice as compared to females. Moreover, a sex and region-specific increase in the mRNA levels of oligodendrocyte lineage markers (NG2, Sox10) was also observed in oxycodone and withdrawal treated animals. These findings may open a new avenue for the development of sex-specific precision therapeutics for opioid dependence by targeting region-specific neuroimmune signaling.

Fundamental sex differences in morphine withdrawal-induced neuronal plasticity

ABSTRACT

Withdrawal from systemic opioids can induce long-term potentiation (LTP) at spinal C-fibre synapses ("opioid-withdrawal-LTP"). This is considered to be a cellular mechanism underlying opioid withdrawal-induced hyperalgesia, which is a major symptom of the opioid withdrawal syndrome. Opioids can activate glial cells leading to the release of proinflammatory mediators. These may influence synaptic plasticity and could thus contribute to opioid-withdrawal-LTP. Here, we report a sexual dimorphism in the mechanisms of morphine-withdrawal-LTP in adult rats. We recorded C-fibre-evoked field potentials in the spinal cord dorsal horn from deeply anaesthetised male and female rats. In both sexes, we induced a robust LTP through withdrawal from systemic morphine infusion (8 mg·kg-1 bolus, followed by a 1-hour infusion at a rate of 14 mg·kg-1·h-1). This paradigm also induced mechanical hypersensitivity of similar magnitude in both sexes. In male rats, systemic but not spinal application of (-)naloxone blocked the induction of morphine-withdrawal-LTP, suggesting the involvement of descending pronociceptive pathways. Furthermore, we showed that in male rats, the induction of morphine-withdrawal-LTP required the activation of spinal astrocytes and the release of the proinflammatory cytokines tumour necrosis factor and interleukin-1. In striking contrast, in female rats, the induction of morphine-withdrawal-LTP was independent of spinal glial cells. Instead, blocking µ-opioid receptors in the spinal cord was sufficient to prevent a facilitation of synaptic strength. Our study revealed fundamental sex differences in the mechanisms underlying morphine-withdrawal-LTP at C-fibre synapses: supraspinal and gliogenic mechanisms in males and a spinal, glial cell-independent mechanism in females.

Review of addiction risk potential associated with adolescent opioid use

Adolescence is a critical period of development with robust behavioral, morphological, hormonal, and neurochemical changes including changes in brain regions implicated in the reinforcing effects of drugs such as opioids. Here we examine the preclinical and, where appropriate complementary clinical literature, for the behavioral and neurological changes induced by adolescent opioid exposure/use and their long-term consequences during adulthood. Adolescent opioid exposure results in a widened biphasic shift in reinforcement with increased impact of positive rewarding aspects during initial use and profound negative reinforcement during adulthood. Females may have enhanced vulnerability due to fast onset of antinociceptive tolerance and reduced severity of somatic withdrawal symptoms during adolescence. Overall, adolescent opioid exposure, be it legally prescribed protracted intake or illicit consumption, results in significant and prolonged consequences of increased opioid reward concomitant with reduced analgesic efficacy and exacerbated somatic withdrawal severity during opioid use/exposure in adulthood. These findings are highly relevant to physicians, parents, law makers, and the general public as adolescent opioid exposure/misuse results in heightened risk for substance use disorders.

Sex Differences and the Role of Estradiol in Mesolimbic Reward Circuits and Vulnerability to Cocaine and Opiate Addiction

Although both men and women become addicted to drugs of abuse, women transition to addiction faster, experience greater difficulties remaining abstinent, and relapse more often than men. In both humans and rodents, hormonal cycles are associated with females' faster progression to addiction. Higher concentrations and fluctuating levels of ovarian hormones in females modulate the mesolimbic reward system and influence reward-directed behavior. For example, in female rodents, estradiol (E2) influences dopamine activity within the mesolimbic reward system such that drug-directed behaviors that are normally rewarding and reinforcing become enhanced when circulating levels of E2 are high. Therefore, neuroendocrine interactions, in part, explain sex differences in behaviors motivated by drug reward. Here, we review sex differences in the physiology and function of the mesolimbic reward system in order to explore the notion that sex differences in response to drugs of abuse, specifically cocaine and opiates, are the result of molecular neuroadaptations that differentially develop depending upon the hormonal state of the animal. We also reconsider the notion that ovarian hormones, specifically estrogen/estradiol, sensitize target neurons thereby increasing responsivity when under the influence of either cocaine or opiates or in response to exposure to drug-associated cues. These adaptations may ultimately serve to guide the motivational behaviors that underlie the factors that cause women to be more vulnerable to cocaine and opiate addiction than men.

Careful Breakthrough Cancer Pain Treatment through Rapid-Onset Transmucosal Fentanyl Improves the Quality of Life in Cancer Patients: Results from the BEST Multicenter Study

OBJECTIVES

To explore the effect of breakthrough cancer pain (BTcP) treatment on quality of sleep and other aspects of the health-related quality of life (HRQoL) in patients with cancer pain.

METHODS

In an observational, multicenter, cohort study, cancer patients from palliative care units, oncology departments, and pain clinics and affected by BTcP were included. Enrolled patients were assessed at the four visits: T0 (baseline), T7, T14, and T28. Stable chronic background pain (numeric rating scale, NRS ≤ 4) during the whole study period was mandatory. BTcP was treated through transmucosal fentanyl. Three questionnaires were used to measure the HRQoL: EORTC QLQ-C15-PAL, Pittsburgh Sleep Quality Index (PSQI), and the Edmonton Symptom Assessment System (ESAS).

RESULTS

In 154 patients, the HRQoL showed a significant improvement for all physical and emotional characteristics in the EORTC QLQ-C15-PAL, except for nausea and vomiting (linear p-value = 0.1) and dyspnea (Linear p-value = 0.05). The ESAS and PSQI questionnaires confirmed these positive results (p < 0.0001 and p = 0.002, respectively).

CONCLUSIONS

This prospective investigation by an Italian expert group, has confirmed that careful management of BTcP induces a paramount improvement on the HRQoL. Because in cancer patients there is a high prevalence of BTcP and this severe acute pain has deleterious consequences, this information can have an important clinical significance.

Examining Relationships between Gender, Opioid Dependence, and Distress Tolerance among Patients in Substance Use Disorder Treatment

Background: Opioid abuse/dependence is associated with multiple negative outcomes relative to other forms of substance abuse/dependence, including relapse. Research identifying modifiable characteristics associated with opioid dependence and associated negative outcomes may inform the development of targeted interventions for this high-risk population. One factor warranting investigation is low distress tolerance (DT). Purpose/Objectives: In a sample of patients in substance use disorder (SUD) treatment, the present study examined DT levels among patients with current opioid dependence versus no history of opioid dependence, as well as the moderating role of gender. We predicted that patients with opioid dependence would exhibit lower DT than those without a history of opioid dependence, and that women with opioid dependence would exhibit lower levels of DT than men with opioid dependence. Methods: A sample of 203 patients in residential SUD treatment were administered a series of diagnostic interviews and a behavioral measure of DT. Results: DT did not differ significantly as a function of opioid dependence. However, there was a significant opioid dependence by gender interaction, such that men with current opioid dependence exhibited significantly lower levels of DT than women with opioid dependence and men without a history of opioid dependence. Conclusions/Importance: Findings highlight a modifiable characteristic associated with opioid dependence among men that may be targeted in interventions.

Ranking the Harm of Psychoactive Drugs Including Prescription Analgesics to Users and Others-A Perspective of German Addiction Medicine Experts

Background: Over the past 15 years, comparative assessments of psychoactive substance harms to both users and others have been compiled by addiction experts. None of these rankings however have included synthetic cannabinoids or non-opioid prescription analgesics (NOAs, e.g., gabapentinoids) despite evidence of increasing recreational use. We present here an updated assessment by German addiction medicine experts, considering changing Western consumption trends-including those of NOAs. Methods: In an initial survey, 101 German addiction medicine physicians evaluated both physical and psychosocial harms (in 5 dimensions) of 33 psychoactive substances including opioids and NOAs, to both users and others. In a second survey, 36 addiction medicine physicians estimated the relative weight of each health and social harm dimension to determine the overall harm rank of an individual substance. We compared our ranking with the most recent European assessment from 2014. Results: Illicit drugs such as methamphetamine, heroin, cocaine and also alcohol were judged particularly harmful, and new psychoactive drugs (cathinones, synthetic cannabinoids) were ranked among the most harmful substances. Cannabis was ranked in the midrange, on par with benzodiazepines and ketamine-somewhat more favorable compared to the last European survey. Prescribed drugs including opioids (in contrast to the USA, Canada, and Australia) were judged less harmful. NOAs were at the bottom end of the ranking. Conclusion: In Germany, alcohol and illicit drugs (including new psychoactive substances) continue to rank among the most harmful addictive substances in contrast to prescribed agents including opioid analgesics and NOAs. Current laws are incongruent with these harm rankings. This study is the first of its kind to include comparative harm rankings of several novel abused substances, both licit/prescribed and illicit.

Preliminary evidence of different and clinically meaningful opioid withdrawal phenotypes

Opioid use disorder (OUD) is a public health crisis. Differences in opioid withdrawal severity that predict treatment outcome could facilitate the process of matching patients to treatments. This is a secondary analysis of a randomized controlled trial (RCT) that enrolled treatment seeking heroin-users (N = 89, males = 78) into a residential study. Participants maintained on morphine (30 mg, subcutaneous, four-times daily) underwent a naloxone (0.4 mg, IM = intramuscular) challenge session to precipitate withdrawal. Area-under-the-curve (AUC) values from self-reported withdrawal ratings during the challenge session were analyzed using K-means clustering, revealing two phenotype groups. Withdrawal and retention from the subsequent 14-day double-blind, double-dummy RCT comparing three study medications (clonidine, tramadol-ER, and buprenorphine) were evaluated as a function of phenotype. Cluster analyses suggested HIGH (N = 37; mean [SD] subjective opiate withdrawal scale [SOWS]-AUC 123.7 [65.8]) and LOW (N = 52; SOWS-AUC 68.0 [47.7]) withdrawal phenotype groups. HIGH participants were significantly more female and had lower body mass indices than LOW participants; no drug-use variables were significant. Regarding RCT outcomes, HIGH phenotype participants were less likely to be retained in the study (P = 0.02) and had higher mean self-reported withdrawal (P = 0.05) than LOW phenotype participants. A significant interaction in RCT retention was observed between phenotype (P = 0.02) and study medication (P < 0.01). Self-reported withdrawal was significant for phenotype (P = 0.02); study medication trended towards significance (P = 0.07). Results suggest patients have meaningfully different experiences of opioid withdrawal that may predict differential response to opioid pharmacotherapies during supervised withdrawal. Additional prospective research to replicate and more thoroughly evaluate withdrawal phenotype correlates and sex differences is warranted.

Female-male differences in prescription pain reliever dependence levels: Evidence on newly incident adolescent and young adult users in the United States, 2002-2014

BACKGROUND

A comprehensive epidemiology of dependence on prescription opioid pain relievers requires evidence about age-specific female-male differences, possibly manifest during adolescent and early adult years. In this study, we identified newly incident extra-medical users of prescription pain relievers (EMPPR), all observed with onsets before the 22nd birthday. We then quantified female-male differences in clinical features or manifestations of opioid dependence (OD), devised a measurement-equivalent OD dimension, and estimated age-specific female-male differences in OD levels.

METHOD

The population under study included 12-to-21-year-old non-institutionalized civilian community residents of United States sampled for recent nation-scale surveys. Confidential computer-assisted self-interviews identified newly incident EMPPR users (n = 10,188). Analysis-weighted estimation procedures yielded cumulative incidence proportions for each OD feature, evaluated measurement non-equivalence across subgroups, and estimated female-male differences age-by-age.

RESULTS

(1) Tolerance and salience ('spending a lot of time') are most common OD features. (2) Measurement non-equivalence (bias) was found across sex- and onset-age groups. (3) With biasing features removed, we can see elevated OD levels for female new initiates, age-by-age. Subsidiary analyses suggested possibly accelerated progression toward higher OD levels when extra-medical PPR use starts before age 18.

CONCLUSIONS

Dimensional approaches to OD and other drug use disorders have gained popularity but can be fragile when differential measurement biases are left uncontrolled. This study's bias-corrected dimensional view of female-male differences shows elevated OD levels among newly incident female EMPPR users relative to new male initiates. Future studies can check for accelerated progression to higher OD levels when EM use starts before age 18 years.

Individuals with Chronic Pain Who Misuse Prescription Opioids Report Sex-Based Differences in Pain and Opioid Withdrawal

Objective Individuals with chronic pain who misuse prescription opioids are at high risk for developing opioid use disorder and/or succumbing to opioid overdose. The current study conducted a survey to evaluate sex-based differences in pain catastrophizing, opioid withdrawal, and current pain in persons with co-occurring chronic pain and opioid misuse. We hypothesized that women with chronic pain who misused prescription opioids would self-report higher pain ratings compared with men and that the relationship between pain catastrophizing and self-reported current pain would be moderated by symptoms of opioid withdrawal in women only. Design Survey assessment of the relationship between pain and opioid misuse. Setting Online via Amazon Mechanical Turk. Participants Persons with ongoing chronic pain who also misused prescription opioids on one or more days in the last 30 days were eligible (N = 181). Methods Participants completed demographic and standardized assessments including the Brief Pain Inventory (BPI), Pain Catastrophizing Scale (PCS), and Subjective Opiate Withdrawal Scale (SOWS). Results Women reported higher levels of current (P < 0.001), average (P < 0.001), and worst (P = .002) pain in the last 24 hours compared with men. Women also endorsed higher scores on the PCS (P = 0.006) and marginally higher past-30-day SOWS ratings (P = 0.068) compared with men. SOWS ratings moderated the relationship between PCS and BPI Worst Pain in women (ΔR2 < 0.127, ΔF(1, 78) = 12.39, P = 0.001), but not in men (ΔR2 < 0.000, ΔF(1, 98) = 0.003, P = 0.954). Conclusions These data suggest a strong relationship between opioid withdrawal, pain catastrophizing, and the experience of pain in women with chronic pain who misuse opioids.

The Burden of Opioid Adverse Events and the Influence on Cancer Patients' Symptomatology

CONTEXT

Opioids are frequently used for the treatment of moderate-to-severe pain and their use may produce a number of unwanted adverse events (AEs).

OBJECTIVES

The objective of this study was to understand the burden of opioid-induced AEs in cancer patients with pain after the introduction of strong opioids (WHO Step III).

METHODS

This is a cohort study derived from a randomized controlled trial involving 498 cancer patients with pain who received strong opioids. During 28-day follow-up, we analyzed frequency, intensity, and changes over time of the main opioid-induced AEs; the influence of previous pain therapy on AEs; and the relationships between the presence of AEs and analgesic response.

RESULTS

After starting strong opioids, dry mouth, nausea, and vomiting immediately increased and persisted over time, constipation continued to increase, while drowsiness and confusion tended to decrease. Patients previously treated with weak opioids had more frequent and severe AEs. While at all observation points the percentage of patients without AEs was 37%-39%, considering all the five scheduled visits, from Day 3 to Day 28, 17% of patients never experienced any AEs, while 48% of patients had four or more concomitant AEs. Patients with no AEs experienced significantly lower pain intensity.

CONCLUSION

Opioid introduction induces various AEs that persist over time and worse patients' symptomatology. Moreover, there seems to be a different expression of the opioid toxicity among patients, and a possible interaction between AEs and the analgesic response. The balance between the opioids analgesic effect and induced toxicity is fundamental in deciding the best management for pain in cancer patients.

The association between frequency of opioid misuse and opioid use disorder among youth and adults in the United States

BACKGROUND

Studies have shown the frequency of drug misuse is important in the development of a substance use disorder, but little is known about this relationship in the context of opioid misuse and opioid use disorder and withdrawal symptoms.

METHODS

Our study included 2694 civilian respondents from the United States 12 years and older who had misused prescription opioids in the past year from the 2014 National Survey on Drug Use and Health. Logistic regression was used to model the association between frequency of prescription opioid misuse and opioid use disorder and withdrawal symptoms, adjusting for age, sex, race/ethnicity, and income level.

RESULTS

Misusing prescription opioids more frequently was significantly associated with opioid use disorder in a dose-dependent manner (daily misuse: OR = 14.92, 95% confidence interval (CI) 9.29, 23.95; 1-2 days/week: OR = 4.46 95% CI: 2.52, 7.89}; 1-4 times/month: OR = 2.40, 95% CI: 1.37, 4.18) compared to use less than 1 time a month. OR = 2.89, 95% CI 1.90, 4.40, respectively) after controlling for sex, age, race/ethnicity, and income. A dose-dependent relationship was found between frequency of opioid misuse and withdrawal symptoms (daily misuse: OR = 2.89, 95% CI: 1.90, 4.40; 1-2 times/week: OR = 1.91, 95% CI: 1.05, 3.45; and 1-4 times/month: 1.74, 95% CI: 1.90, 4.40) after controlling for all covariates mentioned above.

CONCLUSIONS

Our results provide evidence that higher frequency of opioid misuse is associated with both opioid use disorder and withdrawal symptoms. Frequency of use should be considered in the development of more effective opioid use disorder prevention strategies.

The analgesic efficacy of morphine varies with rat strain and experimental pain model: implications for target validation efforts in pain drug discovery

Background

Translating efficacy of analgesic drugs from animal models to humans remains challenging. Reasons are multifaceted, but lack of sufficiently rigorous preclinical study design criteria and phenotypically relevant models may be partly responsible. To begin to address this fundamental issue, we assessed the analgesic efficacy of morphine in three inbred rat strains (selected based on stress reactivity and affective/pain phenotypes), and outbred Sprague Dawley (SD) rats supplied from two vendors.

Methods

Sensitivity to morphine (0.3–6.0 mg/kg, s.c.) was evaluated in the hot plate test of acute thermal nociception, the Complete Freund's Adjuvant (CFA) model of inflammatory‐induced mechanical hyperalgesia, and in a locomotor motility assay in male rats from the following strains; Lewis (LEW), Fischer (F344), Wistar Kyoto (WKY), and SD's from Envigo and Charles River.

Results

F344 and SD rats were similarly sensitive to morphine in hot plate and CFA‐induced inflammatory hyperalgesia (Minimum Effective Dose (MED) = 3.0 mg/kg). WKY rats developed a less robust mechanical hypersensitivity after CFA injection, and were less sensitive to morphine in both pain tests (MED = 6.0 mg/kg). LEW rats were completely insensitive to morphine in the hot plate test, in contrast to the reversal of CFA‐induced hyperalgesia (MED = 3.0 mg/kg). All strains exhibited a dose‐dependent reduction in locomotor activity at 3.0–6.0 mg/kg.

Conclusion

Sensory phenotyping in response to acute thermal and inflammatory‐induced pain, and sensitivity to morphine in various inbred and outbred rat strains indicates that different pathophysiological mechanisms are engaged after injury. This could have profound implications for translating preclinical drug discovery efforts into pain patients.

Significance

The choice of rat strain used in preclinical pain research can profoundly affect the outcome of experiments in relation to (a) nociceptive threshold responses, and (b) efficacy to analgesic treatment, in assays of acute and tonic inflammatory nociceptive pain.

Time course of copeptin during a model of experimental pain and hyperalgesia: A randomised volunteer crossover trial

BACKGROUND

A reliable biomarker for quantifying pain or hyperalgesia has yet to be found. A surrogate marker of arginine vasopressin, copeptin, is elevated in a number of states of physiological and psychological stress and may have a role in quantifying pain and/or hyperalgesia.

OBJECTIVES

To evaluate copeptin as a biomarker for pain or hyperalgesia developing after 120 min of sustained electrical stimulation.

DESIGN

Secondary analysis of a randomised, double-blinded, crossover trial.

SETTING

Single, tertiary university hospital from September 2014 to January 2015.

PARTICIPANTS

A total of 16 healthy, opioid-naïve white men with no confounding medication or history of pain.

INTERVENTIONS

Copeptin and cortisol were measured five times during an established model of transdermal electrical stimulation designed to assess pain and hyperalgesia.

MAIN OUTCOME MEASURES

The primary outcome was the change in copeptin concentration after 120 min of sustained electrical stimulation. Secondary outcomes were copeptin and cortisol concentrations after a subsequent period of rest and analyses of copeptin and cortisol concentrations were made in high-dose and low-dose fentanyl groups separately.

RESULTS

Total copeptin concentrations were not significantly elevated after 120 min [9.15 pmol l (interquartile ranges (IQR), 3.45 to 35.45 pmol l); P = 0.150] compared with baseline [6.15 pmol l (IQR, 3.60 to 10.62 pmol l)]. In the high-dose fentanyl group, there was a significant increase in copeptin within individuals [P = 0.001; median, 37.9 pmol l (IQR, 8.1 to 62 pmol l)] after 120 min, and in the low-dose fentanyl group a significant decrease in copeptin concentrations within individuals [P = 0.006; median, 4.7 pmol l (IQR, 3.13 to 9.35 pmol l)]. No correlation between copeptin concentration and either the area under the pain curve or area under the hyperalgesia curve could be found, indicating that the observed differences may be due to other fentanyl-mediated effects.

CONCLUSION

Copeptin concentrations do not appear to be associated directly with pain and hyperalgesia. Instead, some fentanyl-mediated effect or effects appear to have greatly increased copeptin concentrations from baseline to 120 min.

TRIAL REGISTRATION

Clinicaltrials.gov identifier: NCT02252458.

Effectiveness of long-term opioid therapy among chronic non-cancer pain patients attending multidisciplinary pain treatment clinics: A Quebec Pain Registry study

OBJECTIVE

The objective of this study was to investigate in a real-life context the effectiveness of long-term opioid therapy for reducing pain intensity and interference and improving health-related quality of life (QOL) in patients with chronic noncancer pain.

METHODS

Participants were 893 patients (age = 52.4 ± 14.1, female = 62.4%) enrolled in the Quebec Pain Registry (2008-2011) who completed questionnaires before their first visit at one of three multidisciplinary pain management clinics and 6 and 12 months thereafter. Based on their opioid use profile (OUP), patients were categorized as nonusers, non-lasting users, or lasting users. Data were analyzed using generalized estimating equations.

RESULTS

More than 60% of patients newly initiated on opioid therapy stopped their medication mainly because of adverse effects and/or lack of pain relief. OUP significantly predicted pain intensity and interference and physical QOL (pQOL; P values < 0.001). Lasting users of opioids reported higher levels of pain intensity and interference and poorer pQOL than nonusers and/or non-lasting users over the 12-month follow-up (P values < 0.001). However, all effect sizes were small, thus questioning the clinical significance of these group differences. Among lasting users, more than 20% of patients experienced a meaningful amelioration in pain intensity and interference as well as mental QOL (mQOL), whereas only 8% exhibited improved pQOL.

DISCUSSION

A significant subgroup of patients may benefit from long-term opioid therapy in terms of pain severity and mQOL but the majority do not. The challenge facing clinicians is how to identify who the responders will be.

A review of factors affecting analgesic selection in large animals undergoing translational research

The widespread physiological effects of pain in experimental animals are likely to reduce the validity of data except when pain itself is studied. Appropriately prescribed analgesics will limit pain and improve the welfare of animals undergoing noxious experimental procedures. However, their injudicious use may also introduce variability in data and limit study reproducibility. Optimizing both animal welfare and the value of scientific data from experimental studies requires the ability to identify, quantify and treat animal pain by applying a knowledge of analgesic pharmacology that is sympathetic to study objectives. This review first examines the reasons for promoting analgesic use in translational animal research and, in focussing on pigs and small ruminants, then identifies factors that should be considered when devising analgesic plans.

Altered Signaling in the Descending Pain-modulatory System after Short-Term Infusion of the μ-Opioid Agonist Remifentanil

μ-Opioid receptor agonists are widely used within the contemporary treatment of pain, but abrupt opioid suspension, even after short-term infusion, can paradoxically increase the sensitivity to noxious stimuli, a phenomenon that has been, for example, reported after application of the fast-acting μ-opioid receptor agonist remifentanil. To investigate the mechanisms underlying the effects of discontinuation of remifentanil application on pain processing in the human CNS, we analyzed neuronal responses to thermal stimuli before and after a short-term infusion of remifentanil (30 min 0.1 μg/kg body weight/min) compared with control in the brain, brainstem, and spinal cord in drug-naive male volunteers using fMRI. Subsequent to remifentanil suspension, we observed reduced heat pain thresholds and increased neuronal responses in pain-encoding as well as in key regions of the descending pain-modulatory system, such as the periaqueductal gray matter, the nucleus cuneiformis, and the rostral ventromedial medulla. Moreover, the spinal pain-related multivoxel activity pattern showed an opioid-specific change after drug suspension. Importantly, remifentanil suspension increased the functional coupling between the nucleus cuneiformis and the rostral anterior cingulate cortex, and the coupling strength between the rostral anterior cingulate cortex and the nucleus cuneiformis correlated negatively with the individual pain threshold after opioid suspension. These findings demonstrate that, already subsequent to a short-term infusion of the μ-opioid receptor agonist remifentanil, signaling in the descending pain-modulatory system is fundamentally altered and that these changes are directly related to the behavioral sensitivity to pain.SIGNIFICANCE STATEMENT Opioids are widely used in modern medicine, but, in addition to their known side effects, it is increasingly recognized that opioids can also increase sensitivity to pain subsequent to their use. Using the fast-acting μ-opioid receptor agonist remifentanil and fMRI in healthy male volunteers, this study demonstrates how signaling changes occur along the entire descending pain-modulatory pathway after opioid discontinuation and how these alterations are closely linked to increased behavioral pain sensitivity. Particularly by revealing modified responses in pain-modulatory brainstem regions that have been previously demonstrated to be causally involved in acute opioid withdrawal effects in rodents, the data provide a plausible neuronal mechanism by which the increased sensitivity to pain after opioid suspension is mediated in humans.

Sex-related differences in effect-site concentration of remifentanil for preventing anesthetic emergence cough in elderly patients

Purpose

Cough on anesthetic emergence should be prevented considering its dangerous complications. Target-controlled infusion (TCI) of remifentanil can reduce emergence cough effectively, and sex-related differences in effect-site concentration (Ce) of remifentanil have been evaluated in young patients. In this study, we determined the Ce of remifentanil for preventing emergence cough following extubation in male and female elderly patients and evaluated the sex-related difference.

Patients and methods

Twenty-three male and 22 female elderly patients aged between 60 and 75 years were enrolled. Anesthesia was maintained with sevoflurane and remifentanil TCI. The Ce of remifentanil for preventing emergence cough was determined for each sex using isotonic regression method with a bootstrapping approach, following Dixon’s up-and-down method.

Results

The Ce of remifentanil for preventing emergence cough in 50% (EC₅₀) and 95% (EC₉₅) of the population was significantly lower in females than in males. Isotonic regression revealed the EC₅₀ (83% confidence interval [CI]) of remifentanil was 1.67 (1.55–1.83) ng/mL in females and 2.60 (2.29–2.91) ng/mL in males. The EC₉₅ (95% CI) of remifentanil was 2.30 (2.02–2.62) ng/mL in females and 3.41 (3.27–3.58) ng/mL in males. Dixon’s up-and-down method indicated that the mean EC₅₀ in females was lower than in males (1.56±0.26 ng/mL vs 2.56±0.37 ng/mL, P<0.001).

Conclusion

The remifentanil requirement for preventing emergence cough was lower in female than in male elderly patients, indicative of sex-related differences in Ce of remifentanil. Sex should be considered when using remifentanil TCI for preventing emergence cough in elderly patients.

Endogenous opioid system: a promising target for future smoking cessation medications

BACKGROUND

Nicotine addiction continues to be a health challenge across the world. Despite several approved medications, smokers continue to relapse. Several human and animal studies have evaluated the role of the endogenous opioid system as a potential target for smoking cessation medications.

METHODS

In this review, studies that have elucidated the role of the mu (MORs), delta (DORs), and kappa (KORs) opioid receptors in nicotine reward, nicotine withdrawal, and reinstatement of nicotine seeking will be discussed. Additionally, the review will discuss discrepancies in the literature and therapeutic potential of the endogenous opioid system, and suggest studies to address gaps in knowledge with respect to the role of the opioid receptors in nicotine dependence.

RESULTS

Data available till date suggest that blockade of the MORs and DORs decreased the rewarding effects of nicotine, while activation of the MORs and DORs decreased nicotine withdrawal-induced aversive effects. In contrast, activation of the KORs decreased the rewarding effects of nicotine, while blockade of the KORs decreased nicotine withdrawal-induced aversive effects. Interestingly, blockade of the MORs and KORs attenuated reinstatement of nicotine seeking. In humans, MOR antagonists have shown benefits in select subpopulations of smokers and further investigation is required to realize their full therapeutic potential.

CONCLUSION

Future work must assess the influence of polymorphisms in opioid receptor-linked genes in nicotine dependence, which will help in both identifying individuals vulnerable to nicotine addiction and the development of opioid-based smoking cessation medications. Overall, the endogenous opioid system continues to be a promising target for future smoking cessation medications.