Does opioid analgesia show a gender preference for females?

Sex Differences in Pain and Its Treatment

Pain is a highly personal experience. Pain is often considered to be a purely neurologic phenomenon, but in actuality, it is a combination of both sensory and emotional experiences. This has sometimes been translated clinically toward a more mechanistic approach to the assessment and treatment of pain instead of one that does not discount pain mechanisms, but also is more inclusive of the need for humanism - considering the individual. In today's medical environment, more than ever before there is a significant amount of attention being paid to educating clinicians to better understand that several physiological, neurophysiological, and psychosocial factors can significantly impact responses to pain. The composition of these factors will be unique to that individual's life narrative, context, sex, and prior life experiences. Thus, the concept that a templated approach to pain assessment and pharmacotherapeutic treatment planning should not be expected to provide optimal patient satisfaction and treatment outcomes in the majority. The hypotheses that there may be sex-based differences in the pain experience in a variety of ways including pain sensitivity, tolerance to pain, threshold at which something becomes painful, and the effectiveness of endogenous pain modulation systems are not new and have been well represented in the literature. This chapter reviews important key findings in the scientific literature with respect to sex-based differences in pain and pain responses to experimentally induced painful stimuli, pain experienced in commonly occurring painful medical conditions, and variations in responses to pain treatments. Possible explanations to account for observed differences or similarities will also be discussed.

Sex, Race, Insurance, and Pain: Do Patient Sociodemographics Influence Postoperative Opioid Prescriptions Among Hand Surgeons?

BACKGROUND

Social and demographic factors may influence patient treatment by physicians. This study analyzes the influence of patient sociodemographics on prescription practices among hand surgeons.

METHODS

We performed a retrospective analysis of all hand surgeries (N = 5278) at a single academic medical center from January 2016 to September 2018. The average morphine milligram equivalent (MME) prescribed following each surgery was calculated and then classified by age, race, sex, type of insurance, and history of substance use or chronic pain. Multivariate linear regression was used to compare MME among groups.

RESULTS

Overall, patients with a history of substance abuse were prescribed 31.2 MME more than those without (P < .0001), and patients with a history of chronic pain were prescribed 36.7 MME more than those without (P < .0001). After adjusting for these variables and the type of procedure performed, women were prescribed 11.2 MME less than men (P = .0048), and Hispanics were prescribed 16.6 MME more than whites (P = .0091) overall. Both Hispanic and black patients were also prescribed more than whites following carpal tunnel release (+19.0 and + 20.0 MME, respectively; P < .001). Patients with private insurance were prescribed 24.5 MME more than those with Medicare (P < .0001), but 25.0 MME less than those with Medicaid (P < .0001). There were no differences across age groups.

CONCLUSIONS

Numerous sociodemographic factors influenced postoperative opioid prescription among hand surgeons at our institution. These findings highlight the importance of establishing more uniform, evidence-based guidelines for postoperative pain management, which may help minimize subjectivity and prevent the overtreatment or undertreatment of pain in certain patient populations.

A diencephalic circuit in rats for opioid analgesia but not positive reinforcement

Mu opioid receptor (MOR) agonists are potent analgesics, but also cause sedation, respiratory depression, and addiction risk. The epithalamic lateral habenula (LHb) signals aversive states including pain, and here we found that it is a potent site for MOR-agonist analgesia-like responses in rats. Importantly, LHb MOR activation is not reinforcing in the absence of noxious input. The LHb receives excitatory inputs from multiple sites including the ventral tegmental area, lateral hypothalamus, entopeduncular nucleus, and the lateral preoptic area of the hypothalamus (LPO). Here we report that LHb-projecting glutamatergic LPO neurons are excited by noxious stimulation and are preferentially inhibited by MOR selective agonists. Critically, optogenetic stimulation of LHb-projecting LPO neurons produces an aversive state that is relieved by LHb MOR activation, and optogenetic inhibition of LHb-projecting LPO neurons relieves the aversiveness of ongoing pain.

Opioids are potent analgesics but also have addiction risk. Here a lateral preoptic area to lateral habenula connection is identified by which opioids relieve ongoing pain but do not produce reward in animals that do not have ongoing pain.

Estrogens as arbiters of sex-specific and reproductive cycle-dependent opioid analgesic mechanisms

The organization of estrogenic signaling in the CNS is exceedingly complex. It is comprised of peripherally and centrally synthesized estrogens, and a plethora of types of estrogen receptor that can localize to both the nucleus and the plasma membrane. Moreover, CNS estrogen receptors can exist independent of aromatase (aka estrogen synthase) as well as oligomerize with it, along with a host of other membrane signaling proteins. This ability of CNS estrogen receptors to either to physically pair or exist separately enables locally produced estrogens to act on multiple spatial levels, with a high degree of gradated regulation and plasticity, signaling either in-phase or out-of phase with circulating estrogens. This complexity explains the numerous contradictory findings regarding sex-dependent pain processing and sexually dimorphic opioid antinociception. This review highlights the increasing awareness that estrogens are major endogenous arbiters of both opioid analgesic actions and the mechanisms used to achieve them. This behooves us to understand, and possibly intercede at, the points of intersection of estrogenic signaling and opioid functionality. Factors that integrate estrogenic actions at subcellular, synaptic, and CNS regional levels are likely to be prime drug targets for novel pharmacotherapies designed to modulate CNS estrogen-dependent opioid functionalities and possibly circumvent the current opioid epidemic.

What factors are associated with a second opioid prescription after treatment of distal radius fractures with a volar locking plate?

PURPOSE

Knowledge of factors associated with patient's requests for a second opioid prescription after volar plate fixation of a fracture of the distal radius might inform better pain management protocols and encourage decreased and safer use of opioids. This study tested the primary null hypothesis that there is no difference in demographics, prior opioid prescriptions, injury characteristics, and psychological factors between patients that do and do not receive a second opioid prescription following treatment volar locking plate after distal radius fracture.

PATIENTS AND METHODS

We used data on 206 patients enrolled in one of two prospective studies. Their mean age was 53 years ± SD 15, and 60 (30 %) were men. Forty-seven (23 %) patients received a second opioid prescription. We recorded additional demographics, AO fracture type, American Society for Anesthesiologists (ASA) classification, radiographic parameters at the time of injury prior to reduction and after surgery, and catastrophic thinking.

RESULTS

Male sex (odds ratio [OR] 2.2, 95 % confidence interval [CI] 1.0-4.6, partial pseudo R (2) = 0.018, P = 0.044) and greater dorsal angulation of the articular surface on the lateral post injury radiograph (OR 0.98, 95 % CI 0.96 to 1.0, partial pseudo R (2) = 0.033, P = 0.040) were associated with a second opioid prescription after surgery (pseudo R (2) 0.12, P = 0.0071).

CONCLUSIONS

One measure of fracture severity (dorsal displacement) was independently associated with a second opioid prescription, but alone it accounted for 3.3 % of the variation. Other factors such as the patient's expectation prior to surgery, in particular the realization that injury and surgery hurt, might be addressed in future research.

LEVEL OF EVIDENCE

Prognostic II.

Sex differences in opioid analgesia and addiction: interactions among opioid receptors and estrogen receptors

Opioids are widely used as the pain reliever and also notorious for being addictive drugs. Sex differences in the opioid analgesia and addiction have been reported and investigated in human subjects and animal models. Yet, the molecular mechanism underlying the differences between males and females is still unclear. Here, we reviewed the literature describing the sex differences in analgesic responses and addiction liabilities to clinically relevant opioids. The reported interactions among opioids, estrogens, opioid receptors, and estrogen receptors are also evaluated. We postulate that the sex differences partly originated from the crosstalk among the estrogen and opioid receptors when stimulated by the exogenous opioids, possibly through common secondary messengers and the downstream gene transcriptional regulators.

Combination of oral clonidine and intravenous low-dose ketamine reduces the consumption of postoperative patient-controlled analgesia morphine after spine surgery

OBJECTIVE

Because ketamine, clonidine, and morphine modulate nociceptive pain, coadministration of these drugs would augment the activity of postoperative analgesic drugs. The purpose of this study was to evaluate the effects of coadministration of ketamine and clonidine on postoperative morphine consumption in patients after spine surgery.

METHODS

The patients undergoing spine surgery were allocated randomly to one of the four study groups, which are as follows: group M (n = 12), intravenously (IV) administered patient-controlled analgesia (PCA) morphine alone; group MK (n = 12), IV-PCA morphine plus intra- and postoperative ketamine; group MC (n = 13), IV-PCA morphine plus oral clonidine premedication; group MCK (n = 12), IV-PCA morphine plus intra- and postoperative ketamine and clonidine premedication. The patients in the MC and MCK groups received 4 μg/kg clonidine orally, whereas those in the MK and MCK groups received IV bolus of ketamine (10 mg) at a rate of 2 mg/kg/hour during anesthesia. Patients were arranged to use IV-PCA mode for administration of drugs, which was programmed to deliver a bolus dose of 2-mg morphine (groups M and MC), or boluses of 2-mg morphine and 2-mg ketamine (groups MK and MCK). Scores of visual analog scale (VAS) for pain, morphine requirement, vital signs, nausea, sedation, and other side effects were followed up to 60 hours after surgery.

RESULTS

Although there were significant differences in VAS pain scores at rest 24-48 hours after the surgery, the VAS pain score at movement was similar among the groups. The number of PCA request and cumulative morphine requirement were significantly lower in the MCK group than in the M group.

CONCLUSION

This study results show that the administration of perioperative low-dose ketamine combined with clonidine premedication could reduce the consumption of postoperative PCA morphine following spine surgery.

Importance of sex to pain and its amelioration; relevance of spinal estrogens and its membrane receptors

Estrogens have a multitude of effects on opioid systems and are thought to play a key role in sexually dimorphic nociception and opioid antinociception. Heretofore, classical genomic actions of estrogens are largely thought to be responsible for the effects of these steroids on nociception and opioid antinociception. The recent discovery that estrogens can also activate estrogen receptors that are located in the plasma membrane, the effects of which are manifest in seconds to minutes instead of hours to days has revolutionized our thinking concerning the ways in which estrogens are likely to modulate pain responsiveness and the dynamic nature of that modulation. This review summarizes parameters of opioid functionality and nociception that are subject to modulation by estrogens, underscoring the added dimensions of such modulation that accrues from rapid membrane estrogen receptor signaling. Implications of this mode of signaling regarding putative sources of estrogens and its degradation are also discussed.

Do sex differences exist in opioid analgesia? A systematic review and meta-analysis of human experimental and clinical studies

Although a contribution of sex in opioid efficacy has garnered much attention, the confirmation and direction of any such difference remain elusive. We performed a systematic review of the available literature on sex differences in μ and mixed μ/κ opioid effect on acute and experimental pain. Fifty unique studies (including three unpublished studies) were included in the analyses. Across the 25 clinical studies on μ-opioids there was no significant sex-analgesia association. Restricting the analysis to patient-controlled analgesia (PCA) studies (irrespective of the opioid) yielded greater analgesia in women (n=15, effect size 0.22, 95% c.i. 0.02-0.42, P=0.028). Further restricting the analysis to PCA morphine studies yielded an even greater effect in women (n=11, effect size=0.36, 95% c.i. 0.17-0.56, P=0.003). Meta-regression indicated that the longer the duration of PCA, the difference in effect between the sexes further increased. Across experimental pain studies on μ-opioids women had greater antinociception from opioids (n=11, effect size=0.35; 95% c.i. 0.01-0.69, P=0.047), which was predominantly due to 6 morphine studies. Female patients had greater μ/κ opioid analgesia (n=7, effect size 0.84; 95% c.i. 0.25-1.43, P=0.005), but no sex-analgesia association was present in experimental studies (n=7). Sex differences exist in morphine-induced analgesia in both experimental pain studies and clinical PCA studies, with greater morphine efficacy in women. The data on non-morphine μ and mixed μ/κ-opioids are less convincing and require further study.

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

This article reviews sex differences in opiate analgesic and related processes as part of a Special Issue in Hormones and Behavior. The research findings on sex differences are organized in the following manner: (a) systemic opioid analgesia across mu, delta and kappa opioid receptor subtypes and drug efficacy at their respective receptors, (b) effects of the activational and organizational roles of gonadal steroid hormones and estrus phase on systemic analgesic responses, (c) sex differences in spinal opioid analgesia, (d) sex differences in supraspinal opioid analgesia and gonadal hormone effects, (e) the contribution of genetic variance to analgesic sex differences, (f) sex differences in opioid-induced hyperalgesia, (g) sex differences in tolerance and withdrawal-dependence effects, and (h) implications for clinical therapies.

Sex and gender differences in pain and analgesia

It is a clinical reality that women make up the large majority of chronic pain patients, and there is now consensus from laboratory experiments that when differences are seen, women are more sensitive to pain than men. Research in this field has now begun to concentrate on finding explanations for this sex difference. Although sex differences in sociocultural, psychological, and experiential factors likely play important roles, evidence largely from animal studies has revealed surprisingly robust and often qualitative sex differences at low levels of the neuraxis. Although not yet able to affect clinical practice, the continued study of sex differences in pain may have important implications for the development of new analgesic strategies.

Sex, gender, and pain: a review of recent clinical and experimental findings

Sex-related influences on pain and analgesia have become a topic of tremendous scientific and clinical interest, especially in the last 10 to 15 years. Members of our research group published reviews of this literature more than a decade ago, and the intervening time period has witnessed robust growth in research regarding sex, gender, and pain. Therefore, it seems timely to revisit this literature. Abundant evidence from recent epidemiologic studies clearly demonstrates that women are at substantially greater risk for many clinical pain conditions, and there is some suggestion that postoperative and procedural pain may be more severe among women than men. Consistent with our previous reviews, current human findings regarding sex differences in experimental pain indicate greater pain sensitivity among females compared with males for most pain modalities, including more recently implemented clinically relevant pain models such as temporal summation of pain and intramuscular injection of algesic substances. The evidence regarding sex differences in laboratory measures of endogenous pain modulation is mixed, as are findings from studies using functional brain imaging to ascertain sex differences in pain-related cerebral activation. Also inconsistent are findings regarding sex differences in responses to pharmacologic and non-pharmacologic pain treatments. The article concludes with a discussion of potential biopsychosocial mechanisms that may underlie sex differences in pain, and considerations for future research are discussed.

PERSPECTIVE

This article reviews the recent literature regarding sex, gender, and pain. The growing body of evidence that has accumulated in the past 10 to 15 years continues to indicate substantial sex differences in clinical and experimental pain responses, and some evidence suggests that pain treatment responses may differ for women versus men.

Sex-specific responses to opiates: animal and human studies

It is widely reported that analgesic drugs acting at mu, kappa, and delta opioid-receptors display quantitative and qualitative differences in effect in males and females. These sex-related differences are not restricted to the analgesic/antinociceptive properties of opioids, but are also present in opioid-induced side effects, such as changes in respiration, locomotor activity, learning/memory, addiction, and changes in the cardiovascular system. An increasing number of well-controlled animal and human studies directly examining the issue of sex in the potency of opioids show that, although sex may affect opioid analgesia, the direction and magnitude of sex differences depend on many interacting variables. These include those specific to the drug itself, such as dose, pharmacology, and route and time of administration, and those particular to the subject, such as species, type of pain, genetics, age, and gonadal/hormonal status. In the current review, we systematically present these animal and human studies and discuss the data in relation to the depending variables. Although the observed sex differences in opioid effect may be clinically relevant, lack of knowledge on other factors involved in the large variability in patient opioid analgesic sensitivity should compel practitioners to customize their dosing regimens based on individual requirements.

Morphine sex-dependently induced place conditioning in adult Wistar rats

The present study was conducted to investigate the potential sex-differences in morphine-induced conditioned place preference. A 3-day unbiased conditioning procedure was used to establish conditioned place preference in adult male and female Wistar rats (weighing 200-250 g). The effect of morphine on locomotor activity of subjects was also studied. Naloxone (0.5-2 mg/kg, i.p.), a selective antagonist of mu-opioid receptor or sulpiride (0.5-2 mg/kg, s.c.), a selective antagonist of dopamine D(2) receptor was administered, during conditioning, to indicate the receptor-mediated mechanisms governing upon possible sex-differences to the opioid response. Results show that morphine (0.5-10 mg/kg, s.c.) differently produced a significant place preference in female and male Wistar rats. Although, the opioid maximum response in both sexes was observed at 7.5 mg/kg, but, it was found that female rats acquired conditioned place preference at a lower dose (0.5 mg/kg, s.c.) of morphine compared to male rats. Moreover, the increase in morphine-induced response at higher doses (5-10 mg/kg, s.c.) was more pronounced in females than the males, indicating that female Wistar rats are more sensitive to the place conditioning induced by morphine. Also, the females were more sensitive to locomotor activation induced by morphine at least at one dose (7.5 mg/kg). Animals' body-weight at 10 mg/kg of opioid was increased, the effect that was not dependent to sex. The results also demonstrate that naloxone (1 and 2 mg/kg, i.p.) induced a significant place preference in two sexes with no significant effect on animals' locomotor activity. The antagonist in males but not in females showed a significant effect on animals' body-weight. Naloxone (0.5-2 mg/kg, i.p.) prior-administration to morphine, during conditioning, attenuated the opioid response in two sexes. The attenuation of the morphine response was more pronounced in males than the other sex at the higher dose (2 mg/kg) of the antagonist. In addition, the preadministration of naloxone, during morphine conditioning, both attenuated the drug-induced hyperactivity in females and decreased the animals' body-weight, albeit more effectively in females than the males. Sulpiride injections (1 and 2 mg/kg s.c.), during the conditioning period, induced a significant aversion in males but not in females with no significant effect either on locomotor activity or body-weight in both sexes. When sulpiride (0.5-2 mg/kg, s.c.), during conditioning, was morphine pre-injected, the antagonist at higher doses significantly attenuated the opioid response in males, reflecting the involvement of dopamine D(2) receptor in sex-dependent morphine-conditioned place preference. Prior-injections of sulpiride to morphine produced a significant effect on locomotor activity of females. The effect of the antagonist preinjections on body-weight was also observed in males. Present results indicate sex-differences both in reinforcing and locomotor activity effects of morphine in Wistar rats.

A Randomized, Double-blind, Placebo-controlled Pilot Study of IV Morphine-6-Glucuronide for Postoperative Pain Relief After Knee Replacement Surgery

OBJECTIVES

To determine the dose-response effect of intravenous morphine-6-glucuronide (M6G) on acute postoperative pain.

METHODS

Patients undergoing knee replacement surgery under spinal anesthesia were randomly assigned to 1 of 4 single intravenous M6G doses, 0 (placebo), 10, 20, or 30 mg/70 kg, administered 150 minutes after the spinal anesthetic was given. Analgesic effects were evaluated by determining the cumulative patient controlled analgesia (PCA) morphine dose, consumed over a 12 and 24 hours period, after the initial dose of M6G. For pain assessments, a 10 cm visual analog scale was used.

RESULTS

Data from 41 patients were evaluated (n=10, 10, 10, and 11 in the 0, 10, 20, and 30 mg M6G groups). Only at the highest M6G dose (30 mg/70 kg), morphine PCA consumption was significantly less compared with placebo: over the first 12 postoperative hours mean PCA morphine consumption was 3.0+/-2.0 mg/h after placebo and 1.4+/-0.5 mg/h after 30 mg M6G (P=0.03); over the first 24 h mean PCA morphine consumption was 2.5+/-2.1 mg after placebo and 1.0+/-0.4 mg after 30 mg M6G (P=0.04) (mean+/-SD). Visual analog scale values were similar across all groups during these time periods.

DISCUSSION

The analgesic effect of M6G in postoperative pain was demonstrated with 30 mg/70 kg M6G superior to placebo. At this dose, M6G has a long duration of action as determined by a reduction in the use of morphine PCA over 12 and 24 hours.

Can personality traits and gender predict the response to morphine? An experimental cold pain study

The aim of the present study was to examine the possible role of personality traits, in accordance with Cloninger's theory, and gender, in the variability of responsiveness to opioids. Specifically, it was intended to test whether or not the three personality dimensions - harm avoidance (HA), reward dependence (RD) and novelty seeking (NS) - as suggested by Cloninger, can predict inter-personal differences in responsiveness to morphine after exposure to experimental cold pain. Thirty-four healthy volunteers (15 females, 19 males) were given the cold pressor test (CPT). Pain threshold, tolerance, and magnitude (VAS) were measured before and after (six measures, 30 min apart) the administration of either 0.5 mg/kg oral morphine sulphate (n=21) or 0.33 mg/kg oral active placebo (diphenhydramine) (n=13) in a randomized, double blind design. Assessment of the three personality traits, according to Cloninger's Tridimensional Personality Questionnaire, was performed before the CPT. A high HA score (but not RD, NS, or baseline values of the three pain parameters) predicted a significantly larger pain relief following the administration of morphine sulphate (but not of the placebo). Women exhibited a larger response in response to both treatments, as indicated by a significantly increased threshold and tolerance following morphine sulphate as well as significantly increased tolerance and decreased magnitude following placebo administration. The present study confirms the existence of individual differences in response to analgesic treatment. It suggests that high HA personality trait is associated with better responsiveness to morphine treatment, and that females respond better than men to both morphine and placebo.

Low efficacy opioids: implications for sex differences in opioid antinociception

It is becoming increasingly evident that the sex of an organism is a critical determinant of responsiveness to opioid analgesics. However, the factors that determine the magnitude and direction of sex differences in opioid antinociception have not been fully elucidated. One factor that has received attention is the relative efficacy of the opioid. This review summarizes recent findings in which opioid efficacy was systematically manipulated as an independent variable to probe underlying sex differences in opioid system function. Overall, in rodents and nonhuman primates, mu and kappa opioids are generally more potent and effective in males than in females. The data indicate that although sex differences in the potency of high efficacy opioids such as morphine are generally less than 3.0-fold, sex differences with lower efficacy opioids can be greater than 90-fold. Moreover, that these drugs can function as full agonists in males while functioning as antagonists in females under identical conditions suggests some fundamental sex difference in opioid system function. In addition to efficacy, a number of other variables can affect the outcomes of these studies, including the drug history, genotype, and nociceptive stimulus modality, duration, and intensity. These factors may interact with opioid efficacy to determine the specific conditions under which sex differences are observed. The testing of low efficacy opioids by other laboratories and under other experimental conditions will determine the extent to which this variable affords a strategic research tool. The potential utility of low efficacy opioids in other domains of behavioral pharmacology is also discussed.

Factors Affecting Patient-controlled Analgesia Requirements

BACKGROUND/PURPOSE

Intravenous patient-controlled analgesia (IVPCA) is one of the most widely used postoperative analgesic methods. Many factors could affect the total analgesic consumption of IVPCA. This retrospective study investigated the relationship between patient characteristics and total morphine consumption during a 3-day course of postoperative IVPCA.

METHODS

Patients receiving surgery under general anesthesia with postoperative IVPCA for 3 days during the period between January 2002 and December 2003 were included. Patient data including age, sex, weight, height, body mass index (BMI), operation type and site were collected. Total morphine consumption was recorded at the end of the 3-day IVPCA course. Stepwise regression analyses were conducted to select factors significantly associated with morphine consumption. Stratified analyses were also conducted among different surgical, BMI and age subgroups.

RESULTS

A total of 1308 patients (646 men, 662 women) were included in the analysis. For all operations, weight, age, procedures involving malignant disease, and surgical sites were significantly associated with total morphine consumption. The R and adjusted R2 values of the selected model were 0.509 and 0.256, respectively. Weight was the only common factor among all stratified analyses (all p < 0.001). Age was negatively correlated with morphine consumption. Gender was not a significant factor except in lower abdominal operations. Height was not associated with total morphine consumption. BMI status was not significantly associated with components of the selected factors.

CONCLUSION

This study demonstrated that weight and surgical sites significantly influence total IVPCA requirements. The effect of surgical sites should be considered when evaluating the influence of demographic characteristics on IVPCA demand.

Pain Medication as an Indicator of Interference With Lifestyle and Oral Function During Recovery After Third Molar Surgery

Purpose This study was designed to assess the impact of taking pain medications, as a more comprehensive indicator of perceived pain, on the extent of interference with lifestyle and oral function during recovery after third molar surgery. Patients and Methods Recovery data after the removal of 4 third molars were available for patients enrolled in an institutional review board-approved, prospective, multicenter clinical trial. A self-administered health-related quality of life instrument, designed to assess a patients perception of recovery for pain, lifestyle, and oral function, was completed each postsurgery day (PSD) for 14 days. Taking pain medications was a proxy for a patients perceived level of pain, adding a more sensitive behavioral component to the report of pain. Each PSD day, the patients who thought that their pain was sufficient to require taking medications (an opioid, a nonsteroidal anti-inflammatory, or the combination) were compared with patients not taking pain medications. The extent of interference in lifestyle (daily activity, social life, recreation, sleep) and in oral function (eating, chewing, mouth opening) as self-reported on a scale of 1 (no trouble) to 5 (lots of trouble) were compared for those taking and not taking medications using Cochran-Mantel-Haenszel row mean statistics ( P < .05). Results The 445 study patients were mostly female (63%) and white (86%). Median age was 20 years (IQ, 18, 24 years). Median surgery time was 30 minutes (IQ, 20, 40 minutes). Both mandibular third molars were below the occlusal plane in 60%. Almost all patients took pain medication on PSD 1. By PSD 7, 48% of patients were taking pain medication, decreasing to 20% by PSD 11. Patients with pain sufficient to take an analgesic reported a greater extent of interference for all lifestyle and oral function measures. Recovery was significantly delayed for PSD 2 through 14 for patients who took medications ( P < 0.01). Recovery for females taking pain medications was significantly delayed compared with that for males. Conclusions Patients with pain sufficient to prompt taking pain medications were likely to report interference with recovery for lifestyle and oral function. A patients choice to take pain medication appears to be a better indicator of a patients perceived pain and the impact of that pain on recovery than numerical pain scales. This method provides more sensitive behavioral information during the period of recovery after third molar removal.

Morphine responses and experimental pain: sex differences in side effects and cardiovascular responses but not analgesia

Sex differences in analgesic responses to mu opioid agonists have been reported, although the direction of these differences varies across studies. To further characterize sex differences in responses to mu opioids, the analgesic effects of intravenous morphine (0.08 mg/kg) were determined in healthy women (n = 61) and men (n = 39) by using 3 experimental pain models, heat pain, pressure pain, and ischemic pain. Each pain procedure was conducted before and after double-blind administration of both morphine and saline, which occurred on separate days in counterbalanced order. Although morphine produced significant analgesic effects for all pain stimuli, no significant sex differences in morphine analgesia emerged. However, morphine attenuated cardiovascular reactivity to the ischemic pain task in men but not women, and women reported significantly more drug-related adverse effects than men. These findings are in contrast with some recent clinical and experimental results suggesting more robust analgesic response to mu opioids among women compared to men, although the data indicate that sex differences in non-analgesic effects of morphine were present. These results suggest that sex differences in responses to morphine might depend on the pain model and/or drug dose as well as the specific end point assessed.

PERSPECTIVE

This study examines morphine responses in women and men by using laboratory pain measures. The results indicate no sex differences in analgesia, but women reported greater side effects, and morphine attenuated cardiovascular responses more strongly among men than women. These results add to the literature regarding sex differences in response to opioids.

Sex-related psychological predictors of baseline pain perception and analgesic responses to pentazocine

Sex differences in pain perception and analgesic responses have garnered increasing attention in recent years. We examined the association of psychological factors to baseline pain perception and pentazocine analgesia among 49 healthy women and 39 men. Subjects completed psychological questionnaires measuring positive and negative affect as well as catastrophizing. Subsequently, responses to experimental pain were assessed before and after double-blind administration of intravenous pentazocine (0.5mg/kg). In correlational analyses, positive affect predicted lower pain sensitivity among men but not women. Negative affect predicted lower baseline pain tolerances among both sexes but predicted poorer analgesia only among men. Catastrophizing was associated with greater pain sensitivity and less analgesia more consistently in men than women. Regression models revealed that positive affect predicted lower overall pain sensitivity and catastrophizing predicted poorer overall analgesic responses among men, while no significant predictors of overall pain or analgesia emerged for women. Moreover, positive affect and catastrophizing were negatively and positively correlated, respectively, with side effects from the medication, but only among men. These findings indicate sex-dependent associations of psychological factors with baseline pain perception, analgesic responses, and medication side effects.

The effects of a cold-water stimulus on butorphanol effects in males and females

Using a crossover, randomized, double-blind, cumulative-dosing procedure, we examined whether a painful stimulus modulated subjective and psychomotor effects of butorphanol in eight male and eight female volunteers. During each session, volunteers received four intravenous injections of either butorphanol (0, 0.5, 1, and 2 mg/70 kg) or saline (placebo) at hourly intervals. Saline and butorphanol were tested in two conditions, forearm immersion 30 min after each injection into either 2 or 37 degrees C water. During the 180-s immersion, volunteers completed a visual analog scale (VAS), psychomotor test, and pain ratings. VAS ratings of "Coasting ('spaced out')", "heavy or sluggish feeling," and "sleepy" were lower in the 2 degrees C than in the 37 degrees C condition during butorphanol administration, but only in females. Modulation by a painful stimulus of sleepy ratings was confined to the third immersion (i.e., a dose effect). The cold-water stimulus significantly decreased butorphanol-induced impairment during the third immersion for males, and females showed a similar trend. Overall, pain ratings were higher in females, and although not significant, males reported a greater degree of analgesia. The differences in pain ratings and degree of analgesia between the sexes are discussed as a possible mechanism for the sex differences in modulatory effects.

Can coadministration of oxycodone and morphine produce analgesic synergy in humans? An experimental cold pain study

AIMS

The coadministration of subantinociceptive doses of oxycodone with morphine has recently been shown to result in a synergistic antinociceptive effect in rats. The present study was aimed to investigate the possibility that coadministration of morphine and oxycodone can produce a similar synergistic effect in humans exposed to an experimental model of cold pressor test (CPT).

METHODS

The enriched enrollment design was used to exclude 'stoic' and 'placebo responders' in a single-blind fashion. 'Nonstoic', placebo 'nonresponder' female volunteers (n = 30) were randomly assigned to receive 0.5 mg kg(-1) oral morphine sulphate, 0.5 mg kg(-1) oral oxycodone hydrochloride, and the combination of 0.25 mg kg(-1) morphine sulphate with 0.25 mg kg(-1) oxycodone hydrochloride, 1 week apart from each other, in a double-blind crossover design. Latency to pain onset (threshold), pain intensity (VAS), and pain tolerance (time until removal of the hand from the water) were measured six times over a 3-h period, subsequent to the administration of each medication, and were used to assess their antinociceptive effect.

RESULTS

The combination produced a significantly higher effect on latency to pain onset than that of morphine alone [difference in mean postbaseline value 2.2; 95% confidence interval (CI) 0.48, 3.9; P = 0.01] but the effect was nonsignificantly smaller that that of oxycodone alone. Similarly, the effect of the combination on pain tolerance was significantly larger than that of morphine alone (combination difference 8.4; 95% CI 2.5, 14.3; P = 0.007), whereas oxycodone alone caused a nonsignificantly larger effect than that of the combination treatment. Comparisons of pain magnitude failed to show any significant differences between the three treatments.

CONCLUSIONS

These results indicate that at the doses tested, morphine and oxycodone do not produce synergistic antinociceptive effects in healthy humans exposed to the CPT.

Effect of oral ketorolac and gender on human cold pressor pain tolerance

1. Although the analgesic effect of opioids on experimental cold pressor (CP) pain has been well demonstrated, the analgesic effect of non-steroidal anti-inflammatory drugs (NSAIDs) on experimental CP pain has been less reliable, a finding complicated by inconsistencies in how CP analgesic effect is measured. 2. In the present study, a clinically relevant CP response of pain tolerance was used to assess the previously undescribed analgesic efficacy of the potent NSAID ketorolac (10 mg, p.o.), on CP pain across gender in a sample of normal subjects (n = 50). 3. Using a placebo-controlled crossover design, neither a main nor interaction effect for ketorolac on CP pain tolerance was detected. When examined by gender, male subjects exhibited a large placebo response to CP pain under study conditions, whereas women (albeit less pain tolerant at baseline) evidenced no placebo effect but a modest-to-good NSAID analgesic response. 4. Findings on the gender-specific placebo and analgesic NSAID response, integrated with the current literature, indicate that the lack of NSAID analgesic efficacy in the CP pain model may be related to unexamined and differential effects of how gender affects NSAID analgesic effect.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Sex differences in opioid antinociception in rhesus monkeys: antagonism of fentanyl and U50,488 by quadazocine

Sex differences have been observed in the antinociceptive effects of opioids in rodents and rhesus monkeys. Sex differences in the affinity of opioid ligands for opioid receptors may contribute to these findings. To test this hypothesis, the relative affinity of the competitive opioid antagonist quadazocine for mu and kappa opioid receptors was determined in rhesus monkeys using in vivo pA(2) analysis. The antinociceptive effects of the mu opioid agonist fentanyl and the kappa opioid agonist U50,488 were determined alone and after pretreatment with quadazocine in 4 females and 4 males using a warm-water tail-withdrawal assay of thermal nociception. The relative potency of quadazocine antagonism of fentanyl and U50,488 in females and males was used to assess sex differences in the relative affinity of quadazocine for mu and kappa receptors. Fentanyl was equipotent in female and male monkeys, and quadazocine was equipotent as an antagonist of fentanyl in females and males. In contrast, U50,488 was significantly less potent in females, and quadazocine was less potent as an antagonist of U50,488 in females. These findings suggest that opioid ligands have similar affinity for mu receptors but lower affinity for at least some kappa receptors in female than in male rhesus monkeys.

Characterizing the subjective, psychomotor, and physiological effects of oral oxycodone in non-drug-abusing volunteers

RATIONALE

The subjective, psychomotor, and physiological effects of a widely prescribed and abused prescription opioid, oxycodone, have not been studied in a population of non-drug-abusing people.

OBJECTIVES

To characterize the effects of oxycodone in non-drug-abusing volunteers.

METHODS

Eighteen volunteers participated in a crossover, randomized, double-blind study in which they received, all p.o., placebo, 10 mg oxycodone, 20 mg oxycodone, 30 mg oxycodone, 40 mg morphine, and 2 mg lorazepam. Measures were assessed before and for 300 min after drug administration. End-of-session and 24-h post-session measures were taken to assess residual drug effects and overall subjects' assessments of the drug effects.

RESULTS

Subjective effects of oxycodone were dose related, with the majority of statistically significant effects limited to the two higher doses tested. Oxycodone produced a profile of subjective effects that included both pleasant and unpleasant effects. Morphine in general produced effects similar in magnitude to those of 10 mg and 20 mg oxycodone. Peak liking and drug-wanting ratings were increased by all doses of oxycodone and by morphine, and trough ratings of liking (dislike) were lower in the 20-mg and 30-mg oxycodone conditions, relative to the placebo condition. Post-session ratings of overall liking and drug wanting were not statistically significant, either at the end of the session or 24 h later. Cognitive and psychomotor impairment were obtained with the higher doses of oxycodone, but to a much lesser degree than that of lorazepam. Miosis and exophoria were increased in a dose-related manner by oxycodone.

CONCLUSIONS

Oxycodone produced effects similar to those of other mu opioid agonists. Although oxycodone produced abuse liability-related subjective effects, it also produced unpleasant effects, a phenomenon we have observed in other opioid studies in non-drug-abusing volunteers.

Differential relationships between anxiety and treatment-associated pain reduction among male and female chronic pain patients

OBJECTIVES

Clinical, epidemiological, and laboratory-based studies have all suggested that female sex and elevated anxiety are associated with greater experience of pain. However, several recent reports have also indicated that sex may moderate the relationship between anxiety and responses to noxious stimuli, with anxiety more strongly related to pain among males. The present study examined whether anxiety differentially impacts outcomes for pain treatment among males and females.

METHODS

Seventy-four chronic pain patients (34 males, 40 females) completed the Pain Anxiety Symptoms Scale and several other psychologic measures before undergoing a variety of treatment procedures including epidural steroids, trigger point injections, and participation in brief, cognitive-behaviorally oriented psychoeducational groups. Patients provided pre- and post-treatment ratings of pain for all interventions.

RESULTS

Consistent with findings from previous investigations, the present study noted stronger relationships between baseline anxiety and pre-treatment pain severity among males relative to females. In addition, while lower levels of baseline anxiety were related to greater treatment-associated pain reduction among females, the reverse pattern emerged for males. These relationships persisted even after controlling for other psychologic factors such as depression, coping style, and hypervigilance.

DISCUSSION

These findings suggest differential relationships between anxiety and pain relief as a function of sex. While we are unable to identify a mechanism for this effect, higher anxiety may have predicted more pain relief among males and less pain relief among females due to sex differences in coping strategies or placebo effects.

Gonadectomy induces site-specific differences in nociception in rats

High prevalence of chronic orofacial pain in women and its relationship with ovarian states suggest that ovarian hormones may be involved in the control of orofacial nociception. Since the interaction between ovarian hormones and nociception seems more evident in the orofacial area than in many other parts of the body, a possible site specificity of an ovarian hormone effect on nociception was tested in rats. Two nociceptive tests were applied to three groups of male rats (n=46) and three groups of female rats (n=46), that were gonadectomised (n=17), sham-operated (n=15) or intact (n=14). Each rat in each group received a local subcutaneous injection of formalin in the upper lip and in the hindpaw. Upper lip injection resulted in an increased occurrence of upper lip rubbing for more than 45 min and hindpaw injection resulted in an increased occurrence of hindpaw licking for about 1h. The duration of the nociceptive behaviours was measured at 3 months after surgery. No significant difference was found between intact and sham-operated animals. A significant increase (54%) in the upper lip rubbing but not the hindpaw licking was observed in gonadectomised females. No difference was observed in castrated males for upper lip rubbing, but a tendency towards an increased duration (102%) of hindpaw licking was noted. The depletion in gonadal hormones was confirmed 3 months after gonadectomy and after the sacrifice of the animals, by the observed decline in the bone mineral density measured on the femur of 40 rats belonging to the six groups. A role of ovarian hormones was also suggested after immunostaining of oestrogen receptors in the lamina II of Caudalis subnucleus of the trigeminal sensory complex and cervical (C1-C2) spinal dorsal horn. The number of cells expressing oestrogen receptors displayed a small (13.6%) but significant (P=0.037) increase in ovariectomised compared with sham-operated rats. These results suggest that the lack of ovarian hormones induces a site-specific increase in the sensitivity to orofacial nociceptive stimulation, and that an up-regulation of oestrogen receptors in the Caudalis subnucleus and C1-C2 dorsal horn may be one of the factors involved in this effect.

Sex differences in analgesic responses: evidence from experimental pain models

BACKGROUND AND OBJECTIVE

Sex-related influences on the experience of pain have received considerable empirical attention. Women are at greater risk for several forms of clinical pain and exhibit greater perceptual responses to experimental pain. In recent years, investigators have turned their attention to the influence of sex-related factors on analgesic responses. The purpose of this review is to examine the literature on sex differences in analgesic responses, emphasizing findings from experimental studies.

METHODS

First, important methodological issues in laboratory pain research are presented, and sex differences in responses to experimentally-induced pain are briefly addressed. Next, previous data from non-human animal research and human experimental and clinical research related to sex differences in analgesia are discussed. Also, preliminary results are presented from an ongoing study in our laboratory examining analgesic responses in women and men.

RESULTS AND CONCLUSIONS

Both previous research and preliminary findings from our laboratory suggests that opioids produce greater analgesic responses in women than men. Potential mechanisms underlying sex differences in analgesia are proposed, and important directions for future research are suggested.

Modification of morphine analgesia and tolerance by flumazenil in male and female rats

This study assessed the effect of the central benzodiazepine receptor antagonist, 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester (flumazenil), on morphine-induced analgesia, locomotor effects, and development of tolerance in rats. The thermally evoked pain (tail flick) response was determined after acute and chronic intraperitoneal (i.p.) administration of morphine and flumazenil, alone and in combination. In acute studies, flumazenil induced weak analgesia unrelated to dose and sex, whereas morphine-induced analgesia was dependent on both dose and sex (male>female). Flumazenil dose-dependently enhanced the analgesic effect of morphine in female but not in male rats. Isobolographic analysis suggested synergism between flumazenil and morphine in female rats, but antagonism in male rats. Flumazenil-induced locomotor changes (alone and with morphine) were related to sex but not dose. Chronic coadministration of flumazenil with morphine enhanced analgesia and attenuated tolerance development in female rats. The findings suggest a possible role for flumazenil as an adjunct with opioids in acute and chronic pain therapy.

Management of opioid side effects in cancer-related and chronic noncancer pain: a systematic review

Side effects can limit opioid dosage and reduce quality of life. The purpose of this systematic review was to assess the management of opioid side effects in the context of cancer pain management or, in the event that no evidence was available for cancer pain, for chronic noncancer pain. The side effects studied were constipation, pruritus, nausea and vomiting, myoclonus, sedation, respiratory depression, and delirium. Opioid rotation to manage side effects was also studied. For each side effect, we searched MEDLINE and the Cochrane Controlled Trials Register and identified 657 possible titles for inclusion. Of these, 67 studies met inclusion criteria for analysis. The lack of well-designed, randomized controlled trials and the heterogeneity of populations and study designs made the drawing of firm conclusions difficult and precluded performance of meta-analysis. The type, strength, and consistency of evidence for available interventions to manage opioid side effects vary from strong (eg, on the use of naloxone to reverse respiratory depression or constipation) to weak (eg, changing from the oral to epidural route of morphine administration to manage sedation). Well-designed trials in the specified populations are required to furnish clinicians with secure evidence on managing opioid side effects successfully.

Agonist/antagonist properties of nalbuphine, butorphanol and (-)-pentazocine in male vs. female rats

To determine whether sex differences in the effects of mixed-action opioids could be due to differential activity at mu or kappa receptors, agonist/antagonist properties of nalbuphine, butorphanol and (-)-pentazocine were compared in male vs. female rats using a diuresis test. In water-loaded rats (2-h test), nalbuphine and (-)-pentazocine dose-dependently increased urination similarly in both sexes, whereas butorphanol increased urination more in females than in males on a ml/kg basis. The diuretic effects of all three opioids were at least partially blocked by the kappa receptor-selective antagonist nor-binaltorphimine (nor-BNI, 5 mg/kg) in both sexes. Kappa receptor-mediated antagonism of diuresis induced by U69,593 (0.56 mg/kg) was only observed with butorphanol in males. In water-loaded rats (1-h test), nalbuphine did not suppress, and butorphanol and (-)-pentazocine significantly suppressed urination in males only; all three mixed-action opioids dose-dependently blocked the antidiuretic effect of the selective mu agonist fentanyl (0.056 mg/kg) in both sexes. The ability of nalbuphine and (-)-pentazocine to block fentanyl-induced antidiuresis was not affected by pretreatment with nor-BNI in either sex. In contrast, the ability of butorphanol to block fentanyl-induced antidiuresis was attenuated by pretreatment with nor-BNI in males but not in females. These results suggest that sex differences in the effects of these mixed-action opioids are primarily due to their greater relative efficacy at the mu receptor in male than in female rats; butorphanol also may have greater efficacy at kappa receptors in females than in males.

Gender differences in drug effects: implications for anesthesiologists

BACKGROUND

The gender aspect in pharmacokinetics and pharmacodynamics of anesthetics has attracted little attention. Knowledge of previous work is required to decide if gender-based differences in clinical practice is justified, and to determine the need for research.

METHODS

Basis for this paper was obtained by Medline searches using the key words 'human' and 'gender' or 'sex,' combined with individual drug names. The reference lists of these papers were further checked for other relevant studies.

RESULTS

Females have 20-30% greater sensitivity to the muscle relaxant effects of vecuronium, pancuronium and rocuronium. When rapid onset of or short duration of action is very important, gender-modified dosing may be considered. Males are more sensitive than females to propofol. It may therefore be necessary to decrease the propofol dose by 30-40% in males compared with females in order to achieve similar recovery times. Females are more sensitive than males to opioid receptor agonists, as shown for morphine as well as for a number of kappa (OP2) receptor agonists. On this basis, males will be expected to require 30-40% higher doses of opioid analgesics than females to achieve similar pain relief. On the other hand, females may experience respiratory depression and other adverse effects more easily if they are given the same doses as males.

CONCLUSION

These examples illustrate that gender should be taken into account as a factor that may be predictive for the dosage of several anesthetic drugs. Moreover, there is an obvious need for more research in this area in order to further optimize drug treatment in anesthesia.

Clinical characteristics of chronic back pain as a function of gender and oral opioid use

STUDY DESIGN

A cross-sectional analysis of data derived from patients with chronic spinal pain undergoing evaluation at a multidisciplinary pain treatment center was conducted.

OBJECTIVE

To determine whether pain severity, psychological status, and physical disability differed as a function of gender and opioid use, and whether the clinical correlates of opioid use differed in women and men with chronic back pain.

SUMMARY OF BACKGROUND DATA

Gender differences in the experience of pain have been widely reported. For example, in the general population, several chronic pain conditions are more prevalent among women than among men, and many experimental studies demonstrate lower pain thresholds and tolerances among women. In addition, recent evidence from studies of experimental and acute clinical pain suggests that responses to analgesic medications may differ in women and men.

METHODS

The sample consisted of 240 patients (35% women) with low back, upper back, or neck pain undergoing evaluation for treatment at a multidisciplinary pain center. The patients were classified as opioid or nonopioid users on the basis of self-report and medical record review. All the patients completed a battery of clinical assessments, including measures of pain severity, psychological adjustment, self-reported disability, functional tasks, and pain tolerance. Analyses were conducted to examine clinical variables as a function of gender and opioid use.

RESULTS

The results indicated that opioid use was associated with greater self-reported disability and poorer function in both women and men. However, the association of opioid use with affective distress differed between women and men. The women using opioids showed lower affective distress, whereas the opioid-using men reported greater affective distress. Opioid use was not associated with pain severity, although the women reported greater pain than men.

CONCLUSIONS

These findings indicate that both opioid use and gender are significant predictors of clinical status of patients with chronic spinal pain. More interesting, these two variables interact because opioid use was associated with increased affective distress among the men, but the reverse was true for the women. In addition, the women reported greater pain severity, which is consistent with some previous findings. Potential explanations for these findings are presented, and the practical implications are discussed.