Sex-specific differences in levels of morphine, morphine-3-glucuronide, and morphine antinociception in rats

Fentanyl activates opposing opioid and non-opioid receptor systems that control breathing

Fentanyl elicits profound disturbances in ventilatory control processes in humans and experimental animals. The traditional viewpoint with respect to fentanyl-induced respiratory depression is that once the effects on the frequency of breathing (Freq), tidal volume (TV), and minute ventilation (MV = Freq × TV) are resolved, then depression of breathing is no longer a concern. The results of the present study challenge this concept with findings, as they reveal that while the apparent inhibitory effects of fentanyl (75 μg/kg, IV) on Freq, TV, and MV in adult male rats were fully resolved within 15 min, many other fentanyl-induced responses were in full effect, including opposing effects on respiratory timing parameters. For example, although the effects on Freq were resolved at 15 min, inspiratory duration (Ti) and end inspiratory pause (EIP) were elevated, whereas expiratory duration (Te) and end expiratory pause (EEP) were diminished. Since the effects of fentanyl on TV had subsided fully at 15 min, it would be expected that the administration of an opioid receptor (OR) antagonist would have minimal effects if the effects of fentanyl on this and other parameters had resolved. We now report that the intravenous injection of a 1.0 mg/kg dose of the peripherally restricted OR antagonist, methyl-naloxone (naloxone methiodide, NLXmi), did not elicit arousal but elicited some relatively minor changes in Freq, TV, MV, Te, and EEP but pronounced changes in Ti and EIP. In contrast, the injection of a 2.5 mg/kg dose of NLXmi elicited pronounced arousal and dramatic changes in many variables, including Freq, TV, and MV, which were not associated with increases in non-apneic breathing events such as apneas. The two compelling conclusions from this study are as follows: 1) the blockade of central ORs produced by the 2.5 mg/kg dose of NLXmi elicits pronounced increases in Freq, TV, and MV in rats in which the effects of fentanyl had apparently resolved, and 2) it is apparent that fentanyl had induced the activation of two systems with counter-balancing effects on Freq and TV: one being an opioid receptor inhibitory system and the other being a non-OR excitatory system.

Influence of a chronic beta-blocker therapy on perioperative opioid consumption - a post hoc secondary analysis

BACKGROUND

Beta-blocker (BB) therapy plays a central role in the treatment of cardiovascular diseases. An increasing number of patients with cardiovascular diseases undergoe noncardiac surgery, where opioids are an integral part of the anesthesiological management. There is evidence to suggest that short-term intravenous BB therapy may influence perioperative opioid requirements due to an assumed cross-talk between G-protein coupled beta-adrenergic and opioid receptors. Whether chronic BB therapy could also have an influence on perioperative opioid requirements is unclear.

METHODS

A post hoc analysis of prospectively collected data from a multicenter observational (BioCog) study was performed. Inclusion criteria consisted of elderly patients (≥ 65 years) undergoing elective noncardiac surgery as well as total intravenous general anesthesia without the use of regional anesthesia and duration of anesthesia ≥ 60 min. Two groups were defined: patients with and without BB in their regular preopreative medication. The administered opioids were converted to their respective morphine equivalent doses. Multiple regression analysis was performed using the morphine-index to identify independent predictors.

RESULTS

A total of 747 patients were included in the BioCog study in the study center Berlin. 106 patients fulfilled the inclusion criteria. Of these, 37 were on chronic BB. The latter were preoperatively significantly more likely to have arterial hypertension (94.6%), chronic renal failure (27%) and hyperlipoproteinemia (51.4%) compared to patients without BB. Both groups did not differ in terms of cumulative perioperative morphine equivalent dose (230.9 (BB group) vs. 214.8 mg (Non-BB group)). Predictive factors for increased morphine-index were older age, male sex, longer duration of anesthesia and surgery of the trunk. In a model with logarithmised morphine index, only gender (female) and duration of anesthesia remained predictive factors.

CONCLUSIONS

Chronic BB therapy was not associated with a reduced perioperative opioid consumption.

TRIAL REGISTRATION

This study was registered at ClinicalTrials.gov ( NCT02265263 ) on the 15.10.2014 with the principal investigator being Univ.-Prof. Dr. med. Claudia Spies.

Central metabolism as a potential origin of sex differences in morphine antinociception but not induction of antinociceptive tolerance in mice

BACKGROUND AND PURPOSE

In rodents, morphine antinociception is influenced by sex. However, conflicting results have been reported regarding the interaction between sex and morphine antinociceptive tolerance. Morphine is metabolised in the liver and brain into morphine-3-glucuronide (M3G). Sex differences in morphine metabolism and differential metabolic adaptations during tolerance development might contribute to behavioural discrepancies. This article investigates the differences in peripheral and central morphine metabolism after acute and chronic morphine treatment in male and female mice.

EXPERIMENTAL APPROACH

Sex differences in morphine antinociception and tolerance were assessed using the tail-immersion test. After acute and chronic morphine treatment, morphine and M3G metabolic kinetics in the blood were evaluated using LC-MS/MS. They were also quantified in several CNS regions. Finally, the blood-brain barrier (BBB) permeability of M3G was assessed in male and female mice.

KEY RESULTS

This study demonstrated that female mice showed weaker morphine antinociception and faster induction of tolerance than males. Additionally, female mice showed higher levels of M3G in the blood and in several pain-related CNS regions than male mice, whereas lower levels of morphine were observed in these regions. M3G brain/blood ratios after injection of M3G indicated no sex differences in M3G BBB permeability, and these ratios were lower than those obtained after injection of morphine.

CONCLUSION

These differences are attributable mainly to morphine central metabolism, which differed between males and females in pain-related CNS regions, consistent with weaker morphine antinociceptive effects in females. However, the role of morphine metabolism in antinociceptive tolerance seemed limited.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

No antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol in male and female rats with persistent inflammatory pain

Studies have demonstrated antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol (THC) in animals, but whether such synergy occurs against all types of pain and in humans is unclear. Because a majority of chronic pain patients are women, and sex differences in morphine and THC potencies have been observed in rodents, the present study examined sex-specific effects of morphine and THC given alone and in combination, in rats with persistent inflammatory pain. On day 1, baseline mechanical and thermal response thresholds, hindpaw weight-bearing, locomotor activity, and hindpaw thickness were determined. Inflammation was then induced via hindpaw injection of complete Freund's adjuvant (CFA). Three days later, morphine (s.c.), THC (i.p) or a morphine-THC combination (1:1, 3:1 and 1:3 dose ratios) was administered, and behavioral testing was conducted at 30-240 min postinjection. Morphine alone was antiallodynic and antihyperalgesic, with no sex differences, but at some doses increased weight-bearing on the CFA-treated paw more in males than females. THC alone reduced mechanical allodynia with similar potency in both sexes, but reduced thermal hyperalgesia and locomotor activity with greater potency in females than males. All morphine-THC combinations reduced allodynia and hyperalgesia, but isobolographic analysis of mechanical allodynia data showed no significant morphine-THC synergy in either sex. Additionally, whereas morphine alone was antinociceptive at doses that did not suppress locomotion, morphine-THC combinations suppressed locomotion and did not increase weight-bearing on the inflamed paw. These results suggest that THC is unlikely to be a beneficial adjuvant when given in combination with morphine for reducing established inflammatory pain.

Physiological properties of pain-modulating neurons in rostral ventromedial medulla in female rats, and responses to opioid administration

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Physiological properties of RVM pain-modulating neurons were described in female rats.

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ON- and OFF-cells in females have fundamental properties comparable to those in males.

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As in males, RVM neuron output is altered in persistent inflammation and by morphine.

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This work provides a foundation for future studies of RVM in females.

Functional pain disorders disproportionately impact females, but most pain research in animals has been conducted in males. While there are anatomical and pharmacological sexual dimorphisms in brainstem pain-modulation circuits, the physiology of pain-modulating neurons that comprise a major functional output, the rostral ventromedial medulla (RVM), has not been explored in female animals. The goal of this study was to identify and characterize the activity of RVM cells in female, compared to male, rats. ON- and OFF-cells were identified within the RVM in females, with firing properties comparable to those described in males. In addition, both ON- and OFF-cells exhibited a sensitized response to somatic stimuli in females subjected to persistent inflammation, and both ON- and OFF-cells responded to systemically administered morphine at a dose sufficient to produce behavioral antinociception. These data demonstrate that the ON-/OFF-cell framework originally defined in males is also present in females, and that as in males, these neurons are recruited in females in persistent inflammation and by systemically administered morphine. Importantly, this work establishes a foundation for the use of female animals in studies of RVM and descending control.

Samidorphan, an opioid receptor antagonist, attenuates drug-induced increases in extracellular dopamine concentrations and drug self-administration in male Wistar rats

Opioid receptors modulate neurochemical and behavioral responses to drugs of abuse in nonclinical models. Samidorphan (SAM) is a new molecular entity that binds with high affinity to human mu- (μ), kappa- (κ), and delta- (δ) opioid receptors and functions as a μ-opioid receptor antagonist with partial agonist activity at κ- and δ-opioid receptors. Based on its in vitro profile, we hypothesized that SAM would block key neurobiological effects of drugs of abuse. Therefore, we assessed the effects of SAM on ethanol-, oxycodone-, cocaine-, and amphetamine-induced increases in extracellular dopamine (DA_ext) in the nucleus accumbens shell (NAc-sh), and ethanol and cocaine self-administration behavior in rats. In microdialysis studies, administration of SAM alone did not result in measurable changes in NAc-sh DA_ext when given across a large range of doses. However, SAM markedly decreased average and maximal increases in NAc-sh DA_ext produced by each of the drugs of abuse tested. In behavioral studies, SAM attenuated fixed-ratio ethanol self-administration and progressive ratio cocaine self-administration. These results highlight the potential of SAM to counteract the neurobiological and behavioral effects of several drugs of abuse with differing mechanisms of action.

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.

A Mechanism-Based Forensic Investigation into the Postmortem Redistribution of Morphine

The interpretation of postmortem drug levels is complicated by the change in drug blood levels during the postmortem period, a phenomenon known as postmortem drug redistribution. We investigated the postmortem redistribution (PMR) of morphine, morphine-3-glucuronide and normorphine in the rat. Morphine (10 mg/kg) was intravenously injected into rats, followed by euthanasia 1 h post-injection. The carcasses were placed in a supine position at room temperature, and tissues including heart blood, femoral blood, liver, lung and brain were collected at different time points: 0, 8, 16 or 24 h postmortem. The samples were analyzed with a validated (following modified Scientific Working Group for Forensic Toxicology (SWGTOX) (20) guidelines) liquid chromatography-tandem mass spectrometry method. The use of a mechanism-based approach (involving the used set doses of drug with the study performed in controlled environment) to assess PMR using systematic and statistical analyses provides important information that has not previously been presented in PMR literature. While previous human studies focus on central to peripheral ratios as well as peripheral to tissue ratio, this work focused on the change in morphine and metabolite concentrations over the course of the postmortem interval in relation to each other in addition to the comparison to additional matrices at each postmortem interval. Postmortem redistribution was identified in several tissues across the postmortem interval; however, there was minimal statistical difference observed among each matrix at a given postmortem interval with the exception of normorphine and morphine-3-glucuronide. Combined, our study provides a valuable resource and reference information that can aide toxicologists, medical examiners or coroners when assessing postmortem drug concentrations of morphine and metabolites when they are making determinations of cause of death.

Sex-dependent influences of morphine and its metabolites on pain sensitivity in the rat

Preclinical studies report that the effective dose for morphine is approximately 2-fold higher in females than males. Following systemic administration, morphine is metabolized via Phase II glucuronidation in the liver and brain into two active metabolites: morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), each possessing distinct pharmacological profiles. M6G binds to μ opioid receptors and acts as a potent analgesic. In contrast, M3G binds to toll-like receptor 4 (TLR4), initiating a neuroinflammatory response that directly opposes the analgesic effects of morphine and M6G. M3G serum concentrations are 2-fold higher in females than males, however, sex-specific effects of morphine metabolites on analgesia and glial activation in vivo remain unknown. The present studies test the hypothesis that increased M3G, and subsequent TLR4-mediated activation of glia, is a primary mechanism driving the attenuated response to morphine in females. We demonstrate that intra-PAG M6G results in a greater analgesic response in females than morphine alone. M6G analgesia was reversed with co-administration of (-)-naloxone, but not (+)-naloxone, suggesting that this effect is μ opioid receptor mediated. In contrast, intra-PAG administration of M3G significantly attenuated the analgesic effects of systemic morphine in males only, increasing the 50% effective dose of morphine two-fold (5.0 vs 10.3mg/kg) and eliminating the previously observed sex difference. An increase in IL-1β, IL-6 and TNF was observed in females following intra-PAG morphine or M6G. In males, only IL-1β levels increased following morphine. Changes in cytokine levels following M3G were limited to TNF in females. Together, these data implicate sex differences in morphine metabolism, specifically M3G, as a contributing factor in the attenuated response to morphine observed in females.

Effects of environmental enrichment on self-administration of the short-acting opioid remifentanil in male rats

BACKGROUND

Opioid abuse is a major problem around the world. Identifying environmental factors that contribute to opioid abuse and addiction is necessary for decreasing this epidemic. In rodents, environmental enrichment protects against the development of low dose stimulant self-administration, but studies examining the effect of enrichment and isolation (compared to standard housing) on the development of intravenous opioid self-administration have not been conducted. The present study investigated the role of environmental enrichment on self-administration of the short-acting μ-opioid remifentanil.

METHODS

Rats were raised in an enriched condition (Enr), standard condition (Std), or isolated condition (Iso) beginning at 21 days of age and were trained to lever press for 1 or 3 μg/kg/infusion remifentanil in young adulthood. Acquisition of self-administration and responding during increasing fixed ratio requirements were assessed, and a dose-response curve was generated.

RESULTS

In all phases, Enr rats lever pressed significantly less than Std and Iso rats, with Enr rats pressing between 9 and 40% the amount of Iso rats. Enr rats did not acquire remifentanil self-administration when trained with 1 μg/kg/infusion, did not increase responding over increasing FR when trained at either dose, and their dose-response curves were flattened compared to Std and Iso rats. When expressed as economic demand curves, Enr rats displayed a decrease in both essential value (higher α) and reinforcer intensity (Q ₀) compared to Std and Iso rats at the 1 μg/kg/infusion training dose.

CONCLUSION

Environmental enrichment reduced remifentanil intake, suggesting that social and environmental novelty may protect against opioid abuse.

Species- and gender-dependent differences in the glucuronidation of a flavonoid glucoside and its aglycone determined using expressed UGT enzymes and microsomes

Flavonoids occur naturally as glucosides and aglycones. Their common phenolic hydroxyl groups may trigger extensive UDP-glucuronosyltransferase (UGT)- catalysed metabolism. Unlike aglycones, glucosides contain glucose moieties. However, the influence of these glucose moieties on glucuronidation of glucosides and aglycones remains unclear. In this study, the flavonoid glucoside tilianin and its aglycone acacetin were used as model compounds. The glucuronidation characteristics and enzyme kinetics of tilianin and acacetin were compared using human UGT isoforms, liver microsomes and intestinal microsomes obtained from different animal species. Tilianin and acacetin were metabolized into different glucuronides, with UGT1A8 produced as the main isoform. Assessment of enzyme kinetics in UGT1A8, human liver microsomes and human intestinal microsomes revealed that compared with tilianin, acacetin displayed lower Km (0.6-, 0.7- and 0.6-fold, respectively), higher Vmax (20-, 60- and 230-fold, respectively) and higher clearance (30-, 80- and 300-fold, respectively). Furthermore, glucuronidation of acacetin and tilianin showed significant species- and gender-dependent differences. In conclusion, glucuronidation of flavonoid aglycones is faster than that of glucosides in the intestine and the liver. Understanding the metabolism and species- and gender-dependent differences between glucosides and aglycones is crucial for the development of drugs from flavonoids.

Sex-based differences in pain perception and treatment

OBJECTIVE

This review highlights research on sex-based differences in pain perception and treatment. We sought to illuminate the complex factors contributing to differences in pain and analgesic responses between males and females, ranging from psychosocial to biological processes.

DESIGN

We reviewed published studies of pain induction by chemical, electric, heat, surgical, or psychological means, and opioid and nonopioid analgesia comparing responses in men and women.

RESULTS

A substantial body of research indicates that women experience greater clinical pain, suffer greater pain-related distress, and show heightened sensitivity to experimentally induced pain compared with men. Research on sex-based differences in the pain experience and treatment is beginning to uncover patterns that may enable tailoring of pain treatment to individual characteristics. The factors underpinning sex differences in the experience of pain are multifactorial and complex; for example, psychosocial factors such as pain-related catastrophizing may explain sex-based differences in reporting certain types of pain, as women tend to use catastrophizing to a greater degree. Gonadal hormone levels in cycling women also have a substantial impact on pain perception and analgesic response. Women perceive more pain during the luteal phase, and estrogen antagonists provide long-term pain relief in certain situations.

CONCLUSIONS

Collectively, greater understanding of the factors that commonly and differentially affect the disparity in pain perception, as well as analgesic response, are beginning to illuminate research targets and promising areas of therapeutic intervention for improved pain management.

Antinociception versus serum concentration relationships following acute administration of intravenous morphine in male and female Sprague-Dawley rats: differences between the tail flick and hot plate nociceptive tests

1. Antinociception versus serum morphine concentration relationships were defined in male and female Sprague-Dawley rats administered single intravenous (i.v.) bolus doses of morphine, using the hot plate (2.1-14 mg/kg) and tail flick tests (1-8 mg/kg). 2. Serum concentrations of morphine and morphine-3-glucuronide (M3G), its major metabolite in the rat, were assayed using high-performance liquid chromatography (HPLC) with electrochemical detection. 3. Significantly higher (P < 0.05) values of peak antinociception (approximately 1.7-fold), as well as the extent and duration of antinociception (approximately fourfold), were observed in male compared with female rats administered 10 mg/kg morphine in the hot plate test. Although there were no significant sex-related differences in the area under the serum morphine concentration versus time curve (AUC) at this dose, systemic exposure to M3G (M3G AUC) was significantly higher (approximately twofold; P < 0.05) in female than male rats. 4. In contrast with most previous studies investigating sex differences in morphine antinociception in rats, the antinociceptive effects of single i.v. doses of morphine (1-8 mg/kg) in the tail flick test did not differ significantly between male and female rats. 5. Morphine ED(50) and EC(50) values (95% confidence intervals) for antinociception in the hot plate test were significantly lower (P < 0.05) in male rats (ED(50) 8.4 mg/kg (7.6-9.2); EC(50) 1.8 nmol/L (1.5-2.1)) compared with female rats (ED(50) 10.6 mg/kg (9.1-12.0); EC(50) 3.7 nmol/L (3.4-4.1)). However, in the tail flick test, there was no significant difference between male and female rats in ED(50) (1.8 (0.4-3.3) and 1.4 mg/kg (0.4-2.5), respectively) or EC(50) (0.5 (0.3-0.6) and 0.4 nmol/L (0.2-0.5), respectively) values. 6. Supraspinal attenuation of morphine antinociception by M3G may account for these differences.

Pharmacokinetic aspects of naloxone-precipitated morphine withdrawal in male and female prepubertal mice

It has been shown that the expression of the morphine (MOR) withdrawal syndrome precipitated by naloxone (NAL) is more intense in male mice than in females, but the reasons for this phenomenon remain uncertain. The purpose of the present study was to evaluate whether this sexual dimorphism might be due to differences in MOR and/or NAL plasma levels after a chronic treatment with MOR. Prepubertal Swiss male and female mice were rendered dependent by intraperitoneal (i.p.) injection of MOR (2 mg/kg), twice daily for 9 days. On day 10 dependent mice received NAL (6 mg/kg, i.p.) 60 min after MOR injection. Blood samples were taken at different times in order to determine MOR and NAL plasma levels by gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC), respectively. Pharmacokinetic analysis showed no differences between male and female mice either for MOR or for NAL. In conclusion, although males and females respond differentially to NAL-precipitated withdrawal, this dimorphic behavior would not be influenced by a pharmacokinetic factor.

Sex differences in morphine-induced analgesia of visceral pain are supraspinally and peripherally mediated

Increasing evidence suggests there is a sex difference in opioid analgesia of pain arising from somatic tissue. However, the existence of a sex difference in visceral pain and opioid analgesia is unclear. This was examined in the colorectal distention (CRD) model of visceral pain in the current study. The visceromotor response (vmr) to noxious CRD was recorded in gonadally intact male and female rats. Subcutaneous injection of morphine dose-dependently decreased the vmr in both groups without affecting colonic compliance. However, morphine was significantly more potent in male rats than females. Because systemic morphine can act at peripheral tissue and in the central nervous system (CNS), the source of the sex difference in morphine analgesia was determined. The peripherally restricted mu-opioid receptor (MOR) antagonist naloxone methiodide dose-dependently attenuated the effects of systemic morphine. Systemic administration of the peripherally restricted MOR agonist loperamide confirmed peripherally mediated morphine analgesia and revealed greater potency in males compared with females. Spinal administration of morphine dose-dependently attenuated the vmr, but there was no sex difference. Intracerebroventricular administration of morphine also dose-dependently attenuated the vmr with significantly greater potency in male rats. The present study documents a sex difference in morphine analgesia of visceral pain that is both peripherally and supraspinally mediated.

Comparison of the antinociceptive response to morphine and morphine-like compounds in male and female Sprague-Dawley rats

Male rats are more sensitive to the antinociceptive effects of morphine than female rats. This difference is seen across several rat strains using a variety of nociceptive stimuli. However, the literature in regard to sex differences in antinociceptive responses to mu-opioids other than morphine is less consistent. The present study was designed to examine whether there is a structure-activity rationale that determines which mu-opioids will show a differential antinociceptive response between male and female rats. A series of morphinans closely related in structure to morphine, namely, codeine, heroin, hydrocodone, hydromorphone, oxymorphone, and oxycodone, were examined for their antinociceptive activity in male and female Sprague-Dawley rats and compared with the structurally unrelated mu-opioid agonists methadone and fentanyl. Antinociception was measured by the warm-water tail-withdrawal assay. The results show that morphine is more potent in males compared with females > hydromorphone = hydrocodone = oxymorphone, but there was no observable sex difference in the antinociceptive potency of codeine, heroin, oxycodone, methadone, or fentanyl. The potency to stimulate guanosine 5'-O-(3-[35 S]thio)triphosphate ([35S]GTPgammaS) binding and binding affinity of the various morphinans was compared in rat glioma C6 cells expressing the rat mu-opioid receptor; relative efficacy was also compared by stimulation of [35S]GTPgammaS binding in slices of rat brain thalamus. The presence of a sex difference in antinociceptive responsiveness was not related to drug potency, efficacy, or affinity. Consequently, it is likely that differential metabolism of the opioid, possibly by glucuronidation, determines the presence or absence of a sex difference.

Age-dependent morphine tolerance development in the rat

In all age groups, the use of opioids to treat chronic pain conditions has increased, yet the impact of age on opioid tolerance development has not been comprehensively addressed. In this study, we investigated age-related differences in morphine tolerance development in rats. Rats aged 3 wk, 3 mo, 6 mo, and 1 yr were used in the study. Morphine (8 mg/kg) was injected subcutaneously twice each day and its analgesic effect assessed by the change in tail-flick latency using a thermal stimulus 5 min before and 30 min after dosing. Tolerance was defined as a 75% reduction in morphine-induced analgesia compared to Day 1. Rats aged 3 wk, 3 mo, 6 mo, and 1 yr developed tolerance on the 4th, 10th, 14th, and 22nd days of morphine treatment, respectively. Plasma levels of morphine and its metabolites showed that pharmacokinetic differences among the groups did not correlate with the differences in tolerance development. This study demonstrates that morphine tolerance occurs more rapidly in younger rats than older rats and is unlikely to be the result of differences in drug metabolism or clearance. Aging may impact molecular processes involved in tolerance development and provide insight into novel therapeutic targets to delay opioid tolerance development.

Gonadal hormone modulation of mu, kappa, and delta opioid antinociception in male and female rats

Previous studies suggest that sex differences in morphine antinociception in rodents might be attributed to the activational effects of gonadal hormones. The present study determined whether hormonal modulation of opioid antinociception in adult rats extends to opioids other than the prototypic mu agonist morphine. Male and female rats were sham-gonadectomized (sham-GDX) or gonadectomized (GDX) and replaced with no hormone, estradiol (E2, females), progesterone (P4, females), E2+P4 (females), or testosterone (males). Approximately 28 days later, nociception was evaluated on the 50 degrees C hot plate and warm water tail withdrawal tests before and after subcutaneous administration of hydromorphone, buprenorphine, U50,488, or SNC 80. In sham-GDX (gonadally intact) rats, the mu agonists and U50,488 were less effective in females than in males in at least one nociceptive test, and the delta agonist SNC 80 was less effective in males than in females. In males, gonadectomy tended to decrease, and testosterone tended to increase antinociception produced by 3 of the 4 agonists. In females, gonadectomy and hormone treatment had more variable effects, although E2 tended to decrease mu opioid antinociception. The present results suggest that activational effects of gonadal hormones are relatively modest and somewhat inconsistent on antinociception produced by various opioid agonists in the adult rat.

PERSPECTIVE

This study demonstrates that reproductive hormones such as testosterone in males and estradiol in females do not consistently modulate sensitivity to the analgesic effects of opioids in the adult organism.

Sex differences in pain and analgesia: the role of gonadal hormones

There is now strong evidence for sex differences in pain and analgesia. These differences imply that gonadal steroid hormones such as estradiol and testosterone modulate sensitivity to pain and analgesia. The goal of this review is to present an overview of gonadal steroid modulation of pain and analgesia in animals and humans, and to describe mechanisms by which males' and females' biology may differentially predispose them to pain and to analgesic effects of drugs and stress. Evidence is presented to demonstrate that sex differences in pain and analgesia may be both quantitative and qualitative in nature. Current research suggests that sex-specific management of clinical pain will be a reality in the not-so-distant future.

Sex differences in opioid analgesia: clinical and experimental findings

Sex differences in analgesic responses to opioids have received increasing attention in recent years. This article examines the literature on sex differences in opioid analgesia, including the results of studies from the authors' own laboratories. In general, nonhuman animal studies suggest more robust opioid analgesic responses in males relative to females; however, the human studies completed to date seem to indicate greater opioid analgesia among females. The most consistent evidence of sex differences in analgesia comes from studies of kappa-agonist-antagonists administered to patients following oral surgery. These data indicate more robust analgesia in females, and dose-response characteristics suggest that these agents possess both analgesic and antianalgesic properties, and the agonists may produce these effects in different proportions for women versus men. In contrast, the data from laboratory pain models in humans suggest greater analgesic effects in women in response mu-opioid agonists but not kappa-agonist-antagonists. Multiple mechanisms may explain sex differences in opioid analgesia, including gonadal hormonal effects, pharmacokinetics and pharmacodynamics, genetic influences, balance of analgesic/antianalgesic processes, and psychological factors. However, the disparity of results obtained from different pain models--animals versus humans and clinical pain versus experimental pain in humans--suggests that the models themselves are mechanistically different. Additional investigation is warranted in order to further explicate the nature of sex differences in opioid analgesia and to elucidate the underlying mechanisms.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Hyperalgesic priming in the rat demonstrates marked sexual dimorphism

In male rats, carrageenan (CAR)-induced inflammation or exposure to a selective protein kinase C epsilon (PKC epsilon ) agonist (psi epsilon RACK) produces prolongation of the hyperalgesia induced by a subsequent exposure to an inflammatory mediator, a phenomenon referred to as hyperalgesic priming. Since many chronic inflammatory conditions are sexually dimorphic, we tested the hypothesis that hyperalgesic priming is sexually dimorphic. Prior injection of CAR or psi epsilon RACK produced a prolongation of the hyperalgesia induced by a subsequent injection of prostaglandin E(2), from less than 3 h to greater than 24 h, but only in male rats. In ovariectomized female rats priming with CAR and psi epsilon RACK produced hyperalgesic priming effects similar to that observed in the male rat, and this effect was reversed by estrogen replacement. While gonadectomy in males had no effect on CAR and psi epsilon RACK induced hyperalgesic priming, female phenotype was observed following implantation of estrogen in males. Thus, mechanisms mediating the development of hyperalgesic priming produced by inflammation are suppressed by estrogen. This regulation of priming by estrogen appears to occur at or downstream of the activation of PKC epsilon.

Postoperative pain and analgesic responses are similar in male and female Sprague-Dawley rats

PURPOSE

Controversy exists concerning the influence of gender on pain sensitivity and response to analgesics both in animal and human studies. The present study compares postoperative pain scores in male and female rats and how they respond to analgesic interventions.

METHODS

Unilateral plantar foot incisions were made in Sprague-Dawley rats of both genders, producing mechanical allodynia in an established model of postoperative pain. Postoperative pain scores were monitored for four days following incision to identify intrinsic differences between the two groups. Animals were tested with analgesics (opioids, alpha(2)-adrenergic agonists, acetylcholine esterase inhibitors, gabapentin) both systemically and intrathecally on the day after incision to assess gender differences in the anti-allodynic effect of these drugs.

RESULTS

In the plantar foot incision model of postoperative pain there was no gender difference in postoperative mechanical hypersensitivity (von Frey filaments) over four days. Morphine (3 mg x kg(-1)) and gabapentin (25 mg x kg(-1)) administered intraperitoneally decreased postoperative mechanical hypersensitivity, but with no gender difference. Intrathecal morphine (1-2 nmol), gabapentin (60-120 nmol), clonidine (45 nmol), and neostigmine (6.6 nmol) also showed no gender difference in analgesic effect.

CONCLUSION

The results of this study demonstrate that in Sprague-Dawley rats there are no gender differences in postoperative pain perception or the response to analgesics, indicating that this strain of rats can be used without introducing gender bias in studies of postoperative pain.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Gonadal steroid hormone modulation of nociception, morphine antinociception and reproductive indices in male and female rats

The purpose of this study was to examine how gonadal steroid hormones modulate basal nociception and morphine antinociception relative to regulating reproduction in the adult rat. Male and female Sprague-Dawley rats were either gonadectomized (GDX) or sham-gonadectomized (sham); GDX males were implanted subcutaneously with capsules containing testosterone (T), estradiol (E2), dihydrotestosterone (DHT), E2 and DHT, or nothing (0). GDX females received E2, T, or empty (0) capsules immediately after surgery, and vehicle or progesterone (P4) injections at 4-day intervals. Basal nociception and morphine antinociception were tested 28 days after surgery on 50 degrees C and 54 degrees C hotplate tests, and reproductive behavior and physiology were assessed shortly thereafter. There were no significant differences in baseline hotplate latencies among the male treatment groups, but morphine was significantly more potent in sham and GDX+T males than in GDX+0 males. The ability of T to increase morphine's potency was approximated by its major metabolites E2 and DHT, given together but not alone. Baseline hotplate latencies were higher in sham females tested during diestrus than in those tested during estrus. Morphine was significantly more potent in sham females tested during proestrus and diestrus than in those tested during estrus. Baseline hotplate latencies were significantly higher, and morphine was significantly less potent in GDX+E2, GDX+E2/P4 and GDX+T females than in GDX+0 females. All group differences in basal nociception and morphine antinociception observed on the 50 degrees C hotplate test were smaller and generally non-significant on the 54 degrees C hotplate test. Steroid manipulations produced the expected changes in reproductive behaviors and steroid-sensitive organs. These results demonstrate that in adult rats, gonadal steroid manipulations, that are physiologically relevant, modulate (1) basal nociception in females but not males, and (2) morphine's antinociceptive potency in both males and females.

Sex differences in drug- and non-drug-induced analgesia

Historically, biomedical research has been conducted almost exclusively with male subjects. A growing number of studies now demonstrate sex differences in analgesia produced both by drugs and by environmental stimuli. This review summarizes the current literature on sex differences in analgesia produced by opioids, cholinergics and other drugs, and by stress, exercise and other environmental manipulations. A brief overview of the physiological mechanisms underlying sex differences in analgesia is provided, as well as suggestions for future research. It is not yet known whether the development of sex-specific analgesia treatment guidelines is warranted.

Sex differences in opioid analgesia: "from mouse to man"

BACKGROUND

Numerous experimental studies, conducted primarily over the past 10 years, show that there are sex differences in opioid analgesia. This review summarizes the published literature on sex differences in analgesia produced by acute administration of drugs acting at mu-, kappa-, and delta-opioid receptors, in animals and humans. Additionally, methodological issues in research into opioid sex differences are discussed.

CONCLUSIONS

Procedural variables that may influence the outcome of studies examining sex differences in opioid analgesia include modality and intensity of the noxious stimulus used in the pain test, opioid type (efficacy and selectivity), and experimental design and data analytic techniques. Subject variables that may be important to consider include subject genotype and gonadal steroid hormone state of the subject at the time of analgesia testing. Evidence is provided for multiple mechanisms underlying sex differences in opioid analgesia, including both pharmacokinetic and pharmacodynamic factors. Future research directions are suggested, such as examining sex differences in opioid tolerance development, sex differences in opioid analgesia using models of acute inflammatory pain and chronic pain, and sex differences in effects of opioids other than analgesia, which may limit their therapeutic use.

Sex-related differences in mechanical nociception and antinociception produced by mu- and kappa-opioid receptor agonists in rats

Previous studies indicate that in antinociceptive procedures employing thermal, chemical and electrical stimuli, opioids are generally more potent in male than female rodents. The purpose of the present study was to examine nociception and opioid antinociception in male and female rats using a mechanical nociceptive stimulus. Results indicated that males had a higher threshold for nociception, and in tests in which a constant pressure was applied to the hindpaw, the paw withdrawal latencies were consistently longer in males. Opioids with activity at the mu receptor, including levorphanol, morphine, dezocine, buprenorphine, butorphanol and nalbuphine, were generally more potent and/or effective in males. In contrast, sex differences were not consistently observed with the kappa-opioid receptor agonists spiradoline, (5,7,8b)-N-methyl-N[2-1(1-pyrrolidinyl),1-oxaspiro[4,5]dec-8-yl benzeneacetamide (U69593), trans-(+/-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50488), enadoline, ethylketocyclazocine, and nalorphine. These findings suggest that males and females differ in their responsiveness to mechanical nociception and that sex differences in sensitivity to kappa-, but not mu-, opioid receptor agonists are specific to certain nociceptive stimulus modalities.