Sex and day-night differences in opiate-induced responses of insular wild deer mice, Peromyscus maniculatus triangularis

Sex Differences in Brain Region-Specific Activation of c-Fos following Kappa Opioid Receptor Stimulation or Acute Stress in Mice

Kappa opioid receptors (KOPr) are involved in the response to stress. KOPr are also targets for the treatment of stress-related psychiatric disorders including depression, anxiety, and addiction although effects of KOPr are often sex-dependent. Here we investigated c-Fos expression in a range of brain regions in male and female mice following an acute stressor, and a single injection of KOPr agonist. Using adult C57BL/6 c-Fos-GFP transgenic mice and quantitative fluorescence microscopy, we identified brain regions activated in response to a challenge with the KOPr agonist U50,488 (20 mg/kg) or an acute stress (15 min forced swim stress, FSS). In male mice, U50,488 increased expression of c-Fos in the prelimbic area of the prefrontal cortex (PFCx), nucleus accumbens (NAcc), and basolateral nuclei of the amygdala (BLA). In contrast, in female mice U50,488 only activated the BLA but not the PFCx or the NAcc. FSS increased activation of PFCx, NAcc, and BLA in males while there was no activation of the PFCx in female mice. In both sexes, the KOPr antagonist norBNI significantly blocked U50,488-induced, but not stress-induced activation of brain regions. In separate experiments, activated cells were confirmed as non-GABAergic neurons in the PFCx and NAcc. Together these data demonstrate sex differences in activation of brain regions that are key components of the 'reward' circuitry. These differential responses may contribute to sex differences in stress-related psychiatric disorders and in the treatment of depression, anxiety, and addiction.

Role of Efficacy as a Determinant of Locomotor Activation by Mu Opioid Receptor Ligands in Female and Male Mice

Mu opioid receptor (MOR) agonists produce locomotor hyperactivity in mice as one sign of opioid-induced motor disruption. The goal of this study was to evaluate the degree of MOR efficacy required to produce this hyperactivity. Full dose-effect curves were determined for locomotor activation produced in male and female ICR mice by (1) eight different single-molecule opioids with high to low MOR efficacy, and (2) a series of fixed-proportion fentanyl/naltrexone mixtures with high to low fentanyl proportions. Data from the mixtures were used to quantify the efficacy requirement for MOR agonist-induced hyperactivity relative to efficacy requirements determined previously for other MOR agonist effects. Specifically, efficacy requirement was quantified as the EP₅₀ value, which is the "Effective Proportion" of fentanyl in a fentanyl/naltrexone mixture that produces a maximal effect equal to 50% of the maximal effect of fentanyl alone. Maximal hyperactivity produced by each drug and mixture in the present study correlated with previously published data for maximal stimulation of GTPɣS binding in MOR-expressing Chinese hamster ovary cells as an in vitro measure of relative efficacy. Additionally, the EP₅₀ value for hyperactivity induced by fentanyl/naltrexone mixtures indicated that opioid-induced hyperactivity in mice has a relatively high efficacy requirement in comparison to some other MOR agonist effects, and in particular is higher than the efficacy requirement for thermal antinociception in mice or fentanyl discrimination in rats. Taken together, these data show that MOR agonist-induced hyperactivity in mice is efficacy dependent and requires relatively high levels of MOR agonist efficacy for its full expression. Significance Statement Mu opioid receptor (MOR) agonist-induced hyperlocomotion in mice is dependent on the MOR efficacy of the agonist and requires a relatively high degree of efficacy for its full expression.

Sex differences in opioid receptor mediated effects: Role of androgens

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

Chemokine receptor antagonists enhance morphine's antinociceptive effect but not respiratory depression

AIMS

We have shown that chemokines injected into the periaqueductal gray region of the brain blocks opioid-induced analgesia in the rat cold-water tail flick test (CWTF). The present experiments tested whether chemokine receptor antagonists (CRAs), in combination with sub-analgesic doses of morphine, would provide maximal analgesia in the CWTF test and the mouse formalin pain assay. The effect of CRAs on respiratory depression was also evaluated.

MAIN METHODS

One, two or four CRAs (AMD3100/CXCR4, maraviroc/CCR5, RS504393/CCR2 orAZD8797/CX3CR1) were used in combination with sub-analgesic doses of morphine, all given systemically. Pain was assessed using the rat CWTF test or formalin injection into the paw of mice scored by licking. Respiration and oxygen saturation were measured in rats using a MouseOX® Plus - pulse oximeter.

KEY FINDINGS

In the CWTF test, a sub-maximal dose of morphine in combination with maraviroc alone, maraviroc plus AMD3100, or with the four chemokine receptor antagonists, produced synergistic increases in antinociception. In the formalin test, the combination of four CRAs plus a sub-maximal dose of morphine resulted in increased antinociception in both male and female mice. AMD3100 had an additive effect with morphine in both sexes. Coadministration of CRAs with morphine did not potentiate the opioid respiratory depressive effect.

SIGNIFICANCE

These results support the conclusion that combinations of CRAs can increase the potency of sub-analgesic doses of morphine analgesia without increasing respiratory depression. The results support an "opioid sparing" strategy for alleviation of pain using reduced doses of opioids in combination with CRAs to achieve maximal analgesia.

Effects of kappa opioid receptor agonists on fentanyl vs. food choice in male and female rats: contingent vs. non-contingent administration

RATIONALE

Strategies are needed to decrease the abuse liability of mu opioid receptor (MOR) agonists. One strategy under consideration is to combine MOR agonists with kappa opioid receptor (KOR) agonists.

OBJECTIVES

The effects of KOR agonists (U50488, nalfurafine) on fentanyl-vs.-food choice were compared under conditions where the KOR agonists were added to the intravenously self-administered fentanyl (contingent delivery) or administered as subcutaneous pretreatments (non-contingent delivery) in male and female rats.

METHODS

Rats were trained to respond under a concurrent schedule of fentanyl (0, 0.32-10 μg/kg/infusion) and food reinforcement. In experiment 1, U50488 and nalfurafine were co-administered with fentanyl as fixed-proportion mixtures (contingent administration). In experiment 2, U50488 (1-10 mg/kg) and nalfurafine (3.2-32 μg/kg) were administered as acute pretreatments (non-contingent administration). The selective KOR antagonist, nor-BNI (32 mg/kg), was administered prior to contingent and non-contingent KOR-agonist treatment in experiment 3.

RESULTS

Both U50488 and nalfurafine decreased fentanyl choice when administered contingently, demonstrating that KOR agonists punish opioid choice. However, evidence for punishment corresponded with an elimination of operant responding in the majority of rats. Non-contingent U50488 and nalfurafine administration only decreased the number of choices made during the behavioral session without altering fentanyl choice. Contingent and non-contingent KOR-agonist effects on fentanyl choice were both attenuated by nor-BNI.

CONCLUSIONS

These results illustrate that the effects of KOR agonists on fentanyl reinforcement are dependent upon the contingencies under which they are administered.

A new hypertonic saline assay for analgesic screening in mice: effects of animal strain, sex, and diurnal phase

PURPOSE

There exists a pressing need for the identification of novel analgesics. We recently reported on a new preclinical assay for rapid analgesic screening based on intraplantar (i.pl.) injection of 10% hypertonic saline (HS) in female outbred (CD-1) mice. Herein, we characterized the HS assay's performance in inbred (C57BL/6) mice, sensitivity to sex differences, and effects of diurnal rhythm phase.

METHODS

In randomized, controlled, blinded in vivo animal experiments, we studied nociceptive responses induced by i.pl. HS in C57BL/6 (vs CD-1) mice of both sexes (n = 240) and determined diurnal rhythm phase effects in female animals. We established the HS assay's sensitivity to morphine by constructing dose-response curves and calculating half-maximal inhibitory doses (ID₅₀s).

RESULTS

The injection of i.pl. HS produced nociceptive (licking and biting) responses in all C57BL/6 mice tested. In both C57BL/6 and CD-1 mice, the mean (95% confidence interval [CI]) response magnitudes were greater in females vs males (C57BL/6: 87 sec [64 to 110] vs 45 sec [29 to 61]; difference in means, 42 sec; 95% CI, 17 to 68; P < 0.001; n = 10/group; CD-1: 110 sec [95 to 126] vs 53 sec [32 to 74]; difference in means, 57 sec; 95% CI, 34 to 79; P < 0.001; n = 10/group). The mean (95% CI) nociceptive responses were greater at 24:00 hr than at 12:00 hr in C57BL/6 mice (64 sec [40 to 88] vs 37 sec [24 to 51]; difference in means, 27 sec; 95% CI, 7 to 47; P = 0.007; n = 10/group), but not in CD-1 mice (P = 0.97). Intravenous morphine dose-dependently attenuated nociceptive responses of both C57BL/6 and CD-1 mice (ID₅₀, 0.6 and 2.5 mg·kg^-1, respectively; P = 0.41).

CONCLUSION

These findings in inbred and outbred mice solidify the utility of the HS assay as an effective, rapid, robust, and versatile preclinical tool for analgesic screening.

Cortical endogenous opioids and their role in facilitating repetitive behaviors in deer mice

Deer mice provide a non-pharmacologically induced model for the study of repetitive behaviors. In captivity, these animals develop frequent jumping and rearing that resemble clinical symptoms of obsessive-compulsive behavior (OCB), autism spectrum disorder (ASD), complex motor stereotypies (CMS), and Tourette's syndrome (TS). In this study, we pursue the mechanism of repetitive behaviors by performing stereological analyses and liquid chromatography/ mass spectrometry (LC-MS/MS) measurements of glutamate (Glut), GABA, 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA), leu-enkephalin (leu-enk), and dynorphin-A (dyn-A) in frontal cortex (FC), prefrontal cortex (PFC), and basal ganglia. The only significant stereological alteration was a negative correlation between repetitive behaviors and the cell count in the ventromedial striatum (VMS). Neurochemical analyses demonstrated a significant negative correlation between repetitive behaviors and endogenous opioids (leu-enk and dyn-A) in the FC - the site of origin of habitual behaviors and cortical projections to striatal MSNs participating in direct and indirect pathways. The precise neurochemical process by which endogenous opioids influence synaptic neurotransmission is unknown. One postulated cortical mechanism, supported by our findings, is an opioid effect on cortical interneuron GABA release and a consequent effect on glutamatergic cortical pyramidal cells. Anatomical changes in the VMS could have a role in repetitive behaviors, recognizing that this region influences goal-directed and habitual behaviors.

Sex Differences in Kappa Opioid Receptor Function and Their Potential Impact on Addiction

Behavioral, biological, and social sequelae that lead to drug addiction differ between men and women. Our efforts to understand addiction on a mechanistic level must include studies in both males and females. Stress, anxiety, and depression are tightly linked to addiction, and whether they precede or result from compulsive drug use depends on many factors, including biological sex. The neuropeptide dynorphin (DYN), an endogenous ligand at kappa opioid receptors (KORs), is necessary for stress-induced aversive states and is upregulated in the brain after chronic exposure to drugs of abuse. KOR agonists produce signs of anxiety, fear, and depression in laboratory animals and humans, findings that have led to the hypothesis that drug withdrawal-induced DYN release is instrumental in negative reinforcement processes that drive addiction. However, these studies were almost exclusively conducted in males. Only recently is evidence available that there are sex differences in the effects of KOR activation on affective state. This review focuses on sex differences in DYN and KOR systems and how these might contribute to sex differences in addictive behavior. Much of what is known about how biological sex influences KOR systems is from research on pain systems. The basic molecular and genetic mechanisms that have been discovered to underlie sex differences in KOR function in pain systems may apply to sex differences in KOR function in reward systems. Our goals are to discuss the current state of knowledge on how biological sex contributes to KOR function in the context of pain, mood, and addiction and to explore potential mechanisms for sex differences in KOR function. We will highlight evidence that the function of DYN-KOR systems is influenced in a sex-dependent manner by: polymorphisms in the prodynorphin (pDYN) gene, genetic linkage with the melanocortin-1 receptor (MC1R), heterodimerization of KORs and mu opioid receptors (MORs), and gonadal hormones. Finally, we identify several gaps in our understanding of “if” and “how” DYN and KORs modulate addictive behavior in a sex-dependent manner. Future work may address these gaps by building on the mechanistic studies outlined in this review. Ultimately this will enable the development of novel and effective addiction treatments tailored to either males or females.

Effects of defeat stress on behavioral flexibility in males and females: modulation by the mu-opioid receptor

Behavioral flexibility is a component of executive functioning that allows individuals to adapt to changing environmental conditions. Independent lines of research indicate that the mu opioid receptor (MOR) is an important mediator of behavioral flexibility and responses to psychosocial stress. The current study bridges these two lines of research and tests the extent to which social defeat and MOR affect behavioral flexibility and whether sex moderates these effects in California mice (Peromyscus californicus). Males and females assigned to social defeat or control conditions were tested in a Barnes maze. In males, defeat impaired behavioral flexibility but not acquisition. Female performance was unaffected by defeat. MOR binding in defeated and control mice in the orbitofrontal cortex (OFC), striatum and hippocampus was examined via autoradiography. Stressed males had reduced MOR binding in the OFC whereas females were unaffected. The MOR antagonist beta-funaltrexamine (1 mg/kg) impaired performance in males naïve to defeat during the reversal phase but had no effect on females. Finally, we examined the effects of the MOR agonist morphine (2.5 and 5 mg/kg) on stressed mice. As expected, morphine improved behavioral flexibility in stressed males. The stress-induced deficits in behavioral flexibility in males are consistent with a proactive coping strategy, including previous observations that stressed male California mice exhibit strong social approach and aggression. Our pharmacological data suggest that a down-regulation of MOR signaling in males may contribute to sex differences in behavioral flexibility following stress. This is discussed in the framework of coping strategies for individuals with mood disorders.

Effects of kappa opioid receptors on conditioned place aversion and social interaction in males and females

The effects of kappa opioid receptors (KOR) on motivated behavior are well established based on studies in male rodents, but relatively little is known about the effects of KOR in females. We examined the effects of KOR activation on conditioned place aversion and social interaction in the California mouse (Peromyscus californicus). Important differences were observed in long-term (place aversion) and short-term (social interaction) effects. Females but not males treated with a 2.5 mg/kg dose of U50,488 formed a place aversion, whereas males but not females formed a place aversion at the 10 mg/kg dose. In contrast the short term effects of different doses of U50,488 on social interaction behavior were similar in males and females. Acute injection with 10 mg/kg of U50,488 (but not lower doses) reduced social interaction behavior in both males and females. The effects of U50,488 on phosphorylated extracellular signal regulated kinase (pERK) and p38 MAP kinase were cell type and region specific. Higher doses of U50,488 increased the number of pERK neurons in the ventrolateral bed nucleus of the stria terminals in males but not females, a nucleus implicated in male aggressive behavior. In contrast, both males and females treated with U50,488 had more activated p38 cells in the nucleus accumbens shell. Unexpectedly, cells expressing activated p38 co-expressed Iba-1, a widely used microglia marker. In summary we found strong sex differences in the effects of U50,488 on place aversion whereas the acute effects on U50,488 induced similar behavioral effects in males and females.

Sex differences in kappa opioid pharmacology

In recent years it has become apparent that sex is a major factor involved in modulating the pharmacological effects of exogenous opioids. The kappa opioid receptor (KOPR) system is a potential therapeutic target for pain, mood disorders and addiction. In humans mixed KOPR/MOPR ligands have been found to produce greater analgesia in women than men. In contrast, in animals, selective KOPR agonists have been found to produce greater antinociceptive effects in males than females. Collectively, the studies indicate that the direction and magnitude of sex differences of KOPR-mediated antinociception/analgesia are dependent on species, strain, ligand and pain model examined. Of interest, and less studied, is whether sex differences in other KOPR-mediated effects exist. In the studies conducted thus far, greater effects of KOPR agonists in males have been found in neuroprotection against stroke and suppression of food intake behavior. On the other hand, greater effects of KOPR agonists were found in females in mediation of prolactin release. In modulation of drugs of abuse, sex differences in KOPR effects were observed but appear to be dependent on the drug examined. The mechanism(s) underlying sex differences in KOPR-mediated effects may be mediated by sex chromosomes, gonadal hormonal influence on organization (circuitry) and/or acute hormonal influence on KOPR expression, distribution and localization. In light of the diverse pharmacology of KOPR we discuss the need for future studies characterizing the sexual dimorphism of KOPR neural circuitry and in examining other behaviors and processes that are modulated by the KOPR.

Ventromedial and medial preoptic hypothalamic ibotenic acid lesions potentiate systemic morphine analgesia in female, but not male rats

Sex differences in systemic morphine analgesia occur with male rodents displaying significantly greater analgesic magnitudes and potencies than females. Neonatal androgenization, and to a lesser degree, adult ovariectomy enhance systemic morphine analgesia in female rats, implicating both organizational and activational effects of gonadal hormones. The neuroanatomical circuits sensitive to sex-related hormones by which females display a smaller opiate analgesic effect is not clear, but the ventromedial (VMH) and medial preoptic (MPOA) hypothalamic nuclei are critical in the monitoring of estradiol and other sex hormone levels. To assess the contribution of these nuclei to sex and adult gonadectomy differences in systemic morphine analgesia, intact male, intact female and adult ovariectomized (OVEX) female rats received bilateral saline (SAL) or ibotenic acid (IBO) microinjections into either the VMH or MPOA. Following surgeries, baseline tail-flick latencies over 120 minutes (min) were assessed over 4 days in all nine groups with intact females tested in the estrus phase of their cycle. All animals then received an ascending series of morphine (1.0, 2.5, 5.0, 7.5, 10.0mg/kg) injections 30min prior to the tail-flick test time course with 8-12 day inter-injection intervals between doses. Baseline latencies failed to differ between SAL-treated intact males and females, but were significantly higher in SAL-treated OVEX females. Both VMH IBO and MPOA IBO lesions increased baseline latencies in intact male and female rats, but not in OVEX females. SAL-treated intact males (ED(50)=4.0mg/kg) and SAL-treated OVEX females (ED(50)=3.5mg/kg) displayed significantly greater potencies of systemic morphine analgesia than SAL-treated intact females (ED(50)=6.3mg/kg), confirming previous gender and gonadectomy differences. Neither VMH IBO (ED(50)=3.7 mg/kg) nor MPOA IBO (ED(50)=4.1mg/kg) males differed from SAL-treated males in the potency of systemic morphine analgesia. In contrast, VMH IBO (ED(50)=4.1mg/kg) and MPOA IBO (ED(50)=3.5mg/kg) intact females displayed significantly greater potencies in systemic morphine analgesia than SAL-treated intact females. However, VMH IBO OVEX (ED(50)=3.5mg/kg) and MPOA IBO OVEX (ED(50)=3.9 mg/kg) failed to differ from SAL-treated OVEX females in the potency of systemic morphine analgesia. The magnitudes of systemic morphine analgesia as measured by Maximum Percentage Effect values displayed similar patterns, but lesser degrees, of effects. These data suggest that VMH and MPOA nuclei act to tonically inhibit endogenous pain-inhibitory circuits in the intact female, but not intact male brain, and that removal of circulating gonadal hormones by OVEX and/or excitotoxic destruction of these estrogen receptor accumulating nuclei disinhibit the female analgesic response to systemic morphine.

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.

Early methylphenidate exposure enhances morphine antinociception and tolerance in adult rats

Methylphenidate (MPH) is often used to reduce the symptoms of Attention-Deficit/Hyperactivity Disorder. Early MPH treatment in rats has been shown to enhance adult morphine-induced antinociception. Although this enhanced antinociception could improve pain treatment, it could also lead to enhanced tolerance to morphine. This hypothesis was tested by examining the effects of MPH administration during the pre-weanling period on morphine-induced antinociception and tolerance in adulthood. Male and female Sprague-Dawley rats received daily IP injections of saline or MPH (2 or 5 mg/kg) for 10 consecutive days beginning on post-natal day (PD) 11. At 60 days of age, morphine (0, 1.8, 3.2, 5.6, 10.0, and 18 mg/kg) antinociception was assessed. Beginning one day later, rats received two daily injections of either saline or morphine (5 mg/kg) for two consecutive days to induce tolerance. On PD 63 cumulative doses of morphine were administered as before to assess the development of tolerance. Rats pretreated with MPH showed enhanced acute morphine antinociception compared to saline pretreated controls. In addition, tolerance to morphine was greater in rats pretreated with MPH early in life. The magnitude of this decrease in morphine potency was dependent on the dose of MPH, such that animals that received 5 mg/kg of MPH from PD 11 to 20 showed the greatest tolerance. These findings demonstrate that MPH exposure during the pre-weanling period has long-lasting effects that include enhanced morphine antinociception and tolerance.

The role of the periaqueductal gray in the modulation of pain in males and females: are the anatomy and physiology really that different?

Anatomical and physiological studies conducted in the 1960s identified the periaqueductal gray (PAG) and its descending projections to the rostral ventromedial medulla (RVM) and spinal cord dorsal horn, as a primary anatomical pathway mediating opioid-based analgesia. Since these initial studies, the PAG-RVM-spinal cord pathway has been characterized anatomically and physiologically in a wide range of vertebrate species. Remarkably, the majority of these studies were conducted exclusively in males with the implicit assumption that the anatomy and physiology of this circuit were the same in females; however, this is not the case. It is well established that morphine administration produces greater antinociception in males compared to females. Recent studies indicate that the PAG-RVM pathway contributes to the sexually dimorphic actions of morphine. This manuscript will review our anatomical, physiological, and behavioral data identifying sex differences in the PAG-RVM pathway, focusing on its role in pain modulation and morphine analgesia.

Estrogen pretreatment modulates morphine-induced conditioned place preference in ovariectomized mice

Estrogen is known to modulate the neurotransmission in the brain. The main aim of this study was to investigate the effects of estrogen on the rewarding properties of morphine using conditioned place preference (CPP) paradigm in adult female mice. The possible rewarding effect of estrogen was also examined in ovariectomized mice. Following a 6-day conditioning procedure, sham operated animals showed a significant preference towards the side previously paired with a range of morphine doses (2, 5 and 10--but not 20--mg/kg, SC). However, ovariectomized mice showed decreased CPP compared to gonadally intact mice with a right shift in their morphine dose-response curve. These effects were reversed by chronic daily administration of estradiol benzoate (EB; 20 microg/kg, SC). Furthermore, in ovariectomized mice, EB per se was able to induce CPP. In conclusion, our findings indicate that estradiol has a facilitating effect on morphine reward while its deficiency increases the threshold dose of morphine to induce CPP.

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.

Sex differences in locomotor effects of morphine in the rat

Sex differences in reinforcing, analgesic and other effects of opioids have been demonstrated; however, the extent to which sex differences in motoric effects of opioids contribute to apparent sex differences in their primary effects is not known. The goal of this study was to compare the effects of the prototypic mu opioid agonist morphine on locomotor activity in male vs. female rats. Saline or morphine (1-10 mg/kg) was administered s.c. to adult Sprague-Dawley rats, which were placed into a photobeam apparatus for 3-5 h to measure activity. Modulation of morphine's effects by gonadal hormones and by handling (either during the test session or for 4 days before the test session) were examined. Morphine initially suppressed and later increased locomotor activity in both sexes relative to their saline-injected controls, but males were more sensitive than females to the initial locomotor suppressant effect of morphine. Intermittent, brief handling during the 3-h test session blunted morphine-induced locomotor activation in both sexes. Females in proestrus were the most sensitive to morphine's locomotor-stimulant effect, with females in estrus showing the least response to morphine. Gonadectomized (GDX) males with or without testosterone were equally sensitive to morphine's effects, whereas GDX females treated with estradiol showed a blunted response to morphine's effects, similar to intact females in estrus. Brief handling on each of 4 consecutive days pre-test attenuated morphine's locomotor suppressant effect in males but had no effect in females, thereby eliminating the sex difference. These data suggest that sex differences in morphine's effects on locomotor activity can be attributed to gonadal hormones in females, and to differential stress-induced modulation of morphine's effects in males vs. females.

Dissociation between sex differences in the immunological, behavioral, and physiological effects of kappa- and delta-opioids in Fischer rats

RATIONALE

The sex of the individual can have a profound effect on sensitivity to the effects of opioids. Recently, our laboratory provided the first evidence that females may be more sensitive to the immune-altering effects of mu-opioids than males. However, it remains unknown whether kappa- and delta-opioids produce sexually dimorphic effects on immune responses.

OBJECTIVE

The present study sought to determine whether kappa- and delta-opioids produce differential immunological effects in males and females using the memory-T-cell-dependent in vivo inflammatory response contact hypersensitivity (CHS). As sex differences in the magnitude of opioid effects can be outcome-specific, additional experiments were conducted to compare the immunological effects of kappa- and delta-opioids with other behavioral and physiological effects.

MATERIALS AND METHODS

Contact hypersensitivity was induced in male and female Fischer rats. Prior to elicitation of CHS, animals were administered selected doses of the kappa-opioid spiradoline (0.2-20 mg/kg), delta-opioid SNC80 (1-10 mg/kg), or vehicle. The antinociceptive and diuretic effects of spiradoline were also assessed in males and females, as were the locomotor effects of SNC80.

RESULTS

Spiradoline produced significantly greater enhancement of CHS in females than males, but produced comparable antinociceptive and diuretic effects in both sexes. By contrast, SNC80 did not significantly affect the course of CHS in either sex, but females were significantly more sensitive to its locomotor stimulatory effects.

CONCLUSIONS

These results demonstrate that females are more sensitive than males to the CHS-altering effects of spiradoline and that sex differences in the magnitude and direction of opioid-induced sex differences are outcome dependent.

Persistent pain model reveals sex difference in morphine potency

Central or systemic administration of agonists directed at the mu or delta opiate receptors generally produce a greater degree of analgesia in males than in females. To date, most studies examining sex-based differences in opioid analgesia have used acute noxious stimuli (i.e., tail-flick and hot plate test); thus the potential dimorphic response of centrally acting opiates in the alleviation of persistent inflammatory pain is not well established. In the present study, right hind paw withdrawal latency (PWL) to radiant thermal stimuli was measured in intact male and cycling female Sprague-Dawley rats before and after unilateral hind paw injection of the inflammatory agent complete Freund's adjuvant (CFA). Control animals received intraplantar injection of saline. Twenty four hours after CFA or saline injection, animals received either saline or morphine bisulfate (0.5-15 mg/kg sc). Separate groups of control or inflamed animals were tested on their responsiveness to morphine at 7, 14, and 21 days post-CFA or saline. No sex differences were noted for baseline PWLs, and females displayed slightly less thermal hyperalgesia at 24 h post-CFA. At all morphine doses administered, both the antihyperalgesic effects of morphine in the inflamed animals and the antinociceptive effects of morphine in control animals were significantly greater in males compared with females. Similarly, in males, the antihyperalgesic effects of morphine increased significantly at 7-21 days post-CFA; no significant shift in morphine potency was noted for females. These studies demonstrate sex-based differences in the effects of morphine on thermal hyperalgesia in a model of persistent inflammatory pain.

Characterization of the antinociceptive effect of oxycodone in male and female rats

A number of investigators have shown that sex plays an important role in the analgesic effects of opioids. Typically, the antinociceptive responsiveness to mu opioid agonists such as morphine is greater in male than in female rats. The effect of sex on kappa opioid analgesia is less known. The present study was conducted to examine sex-related differences in responsiveness to oxycodone (putative kappa/mu opioid agonist). This information is important since oxycodone is widely used clinically for treatment of pain. The present results indicated that oxycodone had a greater antinociceptive response in female rats compared to male rats. This sex specific responsiveness to oxycodone, however, was lost with chronic administration. The greater antinociception in female rats was even more prominent with U50,488H (selective kappa agonist). Further, low (subanalgesic) doses of oxycodone and U50,488H enhanced the sensitivity to pain (hyperalgesia) to a greater extent in male than in female rats. This is in contrast to the previously shown greater hyperalgesic effect of subanalgesic doses of the mu opioid agonist, morphine, in female than in male rats. The present findings suggest that sexual dimorphism in the effect of opioids is related to the opioid receptors on which they predominately act.

Gender effect on the right-left discrimination task in a sample of heroin-dependent patients

RATIONALE

Discriminating right from left is an everyday cognitive ability. Repeated exposure to certain drugs, such as heroin, can produce poor performance on many cognitive tasks. However, it is yet unclear whether drug abuse impairs the ability of right-left discrimination.

OBJECTIVES

The aim of the present study is to examine whether the spatial ability measured by the right-left discrimination task can be affected by heroin abuse and whether such drug effect, if it exists, is gender related.

METHODS

A paper-and-pen test was used. The test consists of line drawings of a person with no arm, one arm, or both arms crossing the vertical body axis of the figure. The line drawings are viewed from the back, from the front, or randomly alternating between the back and front drawings. The subjects' task is to mark which is the right or left hand in the figure as fast as possible.

RESULTS

A main finding in this study was that the ability to discriminate between left and right in visual space was impaired in heroin-dependent patients. Especially, heroin-dependent females performed poorer than control females in all conditions but heroin-dependent males only performed poorly in part of conditions.

CONCLUSIONS

Recent heroin abuse impairs the ability of right-left discrimination and such impairment is gender related: heroin-dependent females demonstrated greater performance deficits than males.

Organizational manipulation of gonadal hormones and systemic morphine analgesia in female rats: effects of adult ovariectomy and estradiol replacement

Previous research has indicated the importance of sex in mediating the larger magnitude of mu-opioid receptor agonist-induced analgesia in male relative to female rodents. Whereas manipulations involving the adult activational effects of gonadal hormones minimally alter these analgesic sex differences, manipulations involving neonatal organizational effects of gonadal hormones have previously been shown to profoundly affect morphine analgesia. Thus, adult male rats neonatally castrated on the first day after birth displayed reductions in morphine analgesia relative to sham-operated males, and adult female rats neonatally treated with testosterone propionate on the first day after birth displayed enhancements in morphine analgesia relative to vehicle-treated females. Because neonatal androgenization in female rats produces an anovulatory syndrome that could change their adult hormonal milieu, the present study examined whether adult ovariectomy altered the magnitude of systemic morphine analgesia (1-5 mg/kg) in neonatal androgenized female rats relative to neonatal vehicle-treated female rats as well as gonadal steroid hormone replacement with estradiol benzoate. Intact male rats displayed significantly greater magnitudes and potencies (2- to 2.3-fold leftward shift) of systemic morphine analgesia than female rats treated neonatally with either vehicle (1-5 mg/kg) or testosterone (1.7-5 mg/kg). In turn, neonatal androgenized female rats displayed significantly greater magnitudes of systemic morphine (1, 5 mg/kg) analgesia than vehicle-treated female rats accompanied by a smaller 20% leftward shift in potency. Adult ovariectomy minimally affected morphine analgesia in neonatal vehicle-treated females, while significantly reducing the magnitude (1 mg/kg), but not the potency of morphine analgesia in neonatal androgenized female rats. Estradiol replacement therapy significantly increased the magnitude of morphine analgesia in both groups at some doses, but only changed the potency (20-30%) in females treated neonatally with vehicle. Taken together, these data suggest a limited organizational-activational gonadal hormone interaction in the mediation of systemic morphine analgesia in female rats.

Comparison of the antinociceptive effect of acute morphine in female and male Sprague-Dawley rats using the long-lasting mu-antagonist methocinnamox

Male rats are more sensitive than female rats to the antinociceptive action of morphine. The present study used age-matched (9-10 weeks old) male and female Sprague-Dawley rats to investigate whether this difference is due to variation in micro-opioid receptor binding and G protein activation. In the warm-water tail-withdrawal assay at both 50 degrees C and 55 degrees C, morphine was 2-3 times more potent in males than females. In contrast, micro-opioid receptor number and the binding affinity of the micro-opioid agonists morphine and DAMGO in membranes from whole brain, cortex, thalamus, and spinal cord were not different between males and females. Similarly, morphine and DAMGO stimulation of G protein, determined using GTPase and [(35)S]GTPgammaS binding assays, did not show a difference between the sexes. The long-lasting micro-opioid receptor antagonist methocinnamox (0.32 mg/kg), given 24 h prior to morphine, reduced micro-opioid receptor number by approximately 50% in thalamic and spinal cord tissue from female and male rats and reduced the antinociceptive potency of morphine. Pretreatment of male rats with 0.32 mg/kg methocinnamox reduced the antinociceptive potency of morphine to that observed in female rats expressing a full complement of micro-opioid receptors. However, with increasing pretreatment doses of methocinnamox, the maximal antinociceptive effect of morphine was decreased in females but not males. The results suggest that pathways downstream of the micro-opioid receptor and G protein are more efficient in male rats than in female rats such that there is a larger receptor reserve for morphine-mediated antinociception.

Sex and estrus cycle differences in the modulatory effects of morphine on seizure susceptibility in mice

PURPOSE

To evaluate the effects of sex and estrus cycle on biphasic anticonvulsant and proconvulsant modulation of seizure threshold by morphine.

METHODS

The threshold for the clonic seizures (CST) induced by acute intravenous administration of gamma-aminobutyric acid (GABA)-antagonist pentylenetetrazole (PTZ) was assessed in male and female mice. Estrus cycle was assessed by vaginal smears. The effect of removing circulating sex hormones was assessed by gonadectomy.

RESULTS

At baseline, diestrus females had a higher CST compared with males and estrus females. Morphine at lower doses (0.5-3 mg/kg) had a significant anticonvulsant effect in males and estrus females compared with that in vehicle-treated controls, whereas female mice in diestrus phase showed a relative subsensitivity to this effect. Morphine at higher doses (30 and 60 mg/kg) significantly decreased CST in males and diestrus females, with less relative effect in estrus mice. In both phases, morphine exerted stronger effects in males compared with females. Ovariectomy brought the baseline CST to the male level and resulted in significant expression of both phases of morphine effect but did not abolish the sex difference in responsiveness to morphine.

CONCLUSIONS

The biphasic modulation of seizure threshold is subject to both constitutive sex differences in sensitivity to morphine and hormonal fluctuations during the estrus cycle.

Qualitative sex differences in kappa-opioid analgesia in mice are dependent on age

The effects of aging on sex differences in analgesia from the kappa-opioid agonist, U50,488H (U50), were examined in C57BL/6J mice. U50 analgesia can be blocked by the N-methyl-d-aspartate receptor antagonist, MK-801 (MK), in male rodents and gonadectomized females, but not hormonally intact or estrogen-replaced females, suggesting the existence of alternate neurochemical mediation in females. We now report that MK antagonism of U50 analgesia is age-dependent in females. That is, reproductively senescent females display MK-sensitive U50 analgesia qualitatively similar to that displayed by males or hormonally deprived young females. Age-related reductions in U50 analgesic magnitude were also observed in females. Thus, age and gender are likely to alter the clinical efficacy of analgesic drugs active at kappa-opioid receptors.

Estrous cycle stage-dependent expression of acute tolerance to morphine analgesia in rats

Both baseline pain sensitivity and the response to antinociceptive treatment are sensitive to an animal's sex and estrous cycle stage. Sex differences are also observed in the development of antinociceptive tolerance induced by repetitive exposure to opiate drugs such as morphine. Conventional tolerance study protocols do not assess the impact of the estrous cycle stage. The present study aimed to compare the development of acute tolerance to morphine-induced antinociception in male and female (cycling and ovariectomized) Wistar rats using the tail-flick test. Acute tolerance was induced by two consecutive subcutaneous injections of morphine (10 mg/kg) or saline separated by an interval of 6 h. It was found that rats pretreated with morphine were tolerant to the second morphine dose. Tolerance was most pronounced in proestrous female rats and, to a lesser degree, in male rats. It was absent in ovariectomized rats as well as during the estrus, metestrus and diestrus phases. Thus, the estrous cycle exerts dramatic effects on the induction of acute tolerance to morphine-induced antinociception. These results suggest that pain management strategies can be optimized through the use of sex- and estrous cycle-specific techniques.

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.

Effects of asimadoline, a kappa-opioid agonist, on satiation and postprandial symptoms in health

AIM

To evaluate the effect of single administrations of asimadoline, a kappa-opioid agonist, on satiation volume, postprandial symptoms and gastric volumes.

METHODS

Healthy subjects received oral placebo, or 0.5 or 1.5 mg asimadoline in a randomized, double-blind fashion 1 h prior to testing. We assessed effects on the volume of Ensure to achieve full satiation and postprandial symptoms 30 min after meal, and on gastric volume (fasting and postprandial) measured by 99mTc-single photon emission tomography (SPECT) imaging.

RESULTS

Thirteen healthy subjects were studied in each treatment arm. Compared to placebo, asimadoline 0.5 mg decreased postprandial fullness (P = 0.027) without affecting the volume ingested at full satiation (P = 0.6). Asimadoline 1.5 mg decreased satiation during meal, allowing increased satiation volumes (P = 0.008) and tended to decrease postprandial fullness (P = 0.067), despite higher volumes ingested. There was a significant treatment-gender interaction in the effect of asimadoline on gastric volumes (P < 0.05). Asimadoline 0.5 mg (not 1.5 mg) increased fasting (P = 0.047) and postprandial (P = 0.009) gastric volumes in females but decreased fasting volumes in males (P = 0.008). The effect of asimadoline on gastric volume did not explain the effect observed on satiation volume (P = 0.371) or postprandial fullness (P = 0.399).

CONCLUSION

A single oral administration of asimadoline decreases satiation and postprandial fullness in humans independently of its effects on gastric volume.

Absence of nalbuphine anti-analgesia in the rat

In humans, kappa agonist-antagonist opioids such as nalbuphine have been proposed to produce both analgesia and anti-analgesia by acting at distinct receptors. The anti-analgesia appears to be greater in men, which may contribute to the greater nalbuphine analgesia observed in women. Kappa agonist-antagonists are also known to produce sexually dimorphic antinociception in nonhuman species but are generally more potent in males; anti-analgesia has not been reported in animals. The aim of the present study was to determine if nalbuphine anti-analgesia can be detected in the rat. Since nalbuphine anti-analgesia is more sensitive to naloxone antagonism than its analgesic effect, low doses of naloxone were combined with nalbuphine. Using the Randall-Selitto paw-withdrawal test, nalbuphine (0.5-10 mg/kg) induced dose-dependent antinociception in the rat. The antinociceptive effect of nalbuphine (0.5 or 1 mg/kg) was not enhanced by lower doses of naloxone but was antagonized by higher doses. These data do not support the hypothesis that the naloxone-sensitive anti-analgesic effect of nalbuphine observed in humans is present in the rat and could explain, at least in part, the opposite direction of the sex differences for kappa agonist-antagonist opioid analgesia observed in these two species.

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 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.

Acute antinociceptive responses in single and combinatorial opioid receptor knockout mice: distinct mu, delta and kappa tones

We have examined responses of mice lacking mu, delta and kappa opioid receptor (MOR, DOR and KOR, respectively) genes, as well as combinatorial mutants, in several pain models. This is the first truly comparative study of all three opioid receptor-deficient mice, with genotypes and gender analysis using mice on the hybrid 50% 129/SV : 50% C57BL/6 genetic background. In the tail-immersion test, only KOR-/- females showed decreased withdrawal latencies. This modification was also found in MOR/KOR and MOR/DOR/KOR, but not MOR/DOR mutants. The hotplate test revealed increased nociceptive sensitivity for MOR-/-, a phenotype which was also observed in double mutants involving the MOR deletion, and in the triple mutants. The tail-pressure test showed increased response for both MOR-/- and DOR-/- mutants, a modification which was enhanced in the triple-mutant mice. In the formalin test, MOR-/- and DOR-/- mice showed increased responses in the early and late phases, respectively, while the triple mutant tended to show enhanced nociception in both phases. Finally, the enhanced response of KOR-/- mice in the writhing test, which we have demonstrated previously, was confirmed in double MOR/KOR- and triple-mutant mice. Together, the data support the existence of an antinociceptive opioid tone. Each receptor presents a distinct pattern of activities, with mu receptors influencing responses to mechanical, chemical and thermal nociception at a supraspinal level, kappa receptors involved in spinally mediated thermal nociception and chemical visceral pain, and delta receptors modulating mechanical nociception and inflammatory pain. Phenotypes of mutant mice were subtle, suggesting a low endogenous opioid tone in the regulation of physiological pain.

Differential dose-dependent effects of central morphine treatment upon food intake in male and female rats receiving neonatal hormone manipulations

Male rats display significantly greater analgesic responses following morphine than female rats, with neonatal gonadal manipulations reversing the sex-dependent pattern. The present study assessed whether dose-dependent (0.0005-5 microg, icv) effects of morphine-induced feeding were sensitive to sex-dependent and neonatal gonadectomy manipulations. Sex differences in morphine-induced feeding varied as a function of morphine dose with males showing greater increases at low (0.0005 microg) doses, and females showing greater increases at high (5 microg) doses. Neonatal castration, respectively, enhanced and reduced morphine-induced feeding at very low (0.0005 microg) and low (0.005 microg) doses. In contrast, neonatal testosterone administered to females enhanced morphine-induced feeding at higher (0.5-5 microg) doses. These data indicate that sex and neonatal gonadectomy differences in morphine-induced feeding are dependent upon the dose of morphine employed.

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.

Reversal of sex differences in morphine analgesia elicited from the ventrolateral periaqueductal gray in rats by neonatal hormone manipulations

Male rats display significantly greater analgesic responses than female rats following systemic, ventricular and intracerebral morphine administration into either the ventrolateral periaqueductal gray (vlPAG) or the rostral ventromedial medulla, and following beta-endorphin administration into the vlPAG. Although adult gonadectomy severely reduces nonopioid forms of swim stress-induced analgesia, the marked sex differences in morphine analgesia were minimally affected by either male or female adult gonadectomy. Since very little is known about neonatal effects of gonadal hormones upon sex differences in morphine analgesia elicited from the vlPAG, the present study evaluated the effects of neonatal (within 1 day of birth) castration in male rat pups relative to sham-operated controls, and systemic androgenization with testosterone propionate in female rat pups relative to vehicle-injected controls upon baseline nociceptive thresholds and morphine analgesia elicited from the vlPAG in rats tested as adults. Significant sex differences in morphine analgesia elicited from the vlPAG were observed with adult males receiving neonatal sham surgeries displaying significantly greater morphine analgesia on two nociceptive measures than adult females tested during the estrous phase and receiving neonatal vehicle injections. Neonatal gonadectomy essentially reversed the pattern of sex difference effects upon morphine analgesia elicited from the vlPAG. Neonatally-castrated male rats tested in adulthood displayed dramatic reductions in morphine analgesia elicited from the vlPAG on both the tail-flick (approximately 15-fold rightward shift) and jump (6-fold rightward shift) tests relative to sham-operated males, and essentially mirrored those of vehicle-treated females. Conversely, neonatally-androgenized female rats tested in adulthood displayed dramatic increases in morphine analgesia elicited from the vlPAG on the tail-flick (5-fold leftward shift) and jump (12-fold leftward shift) tests relative to vehicle-treated females, and approximated those observed in sham-operated males. The potent differences between neonatally-castrated and sham-operated male rats and between neonatally-androgenized and vehicle-treated female rats suggest a possible 'organizational' role of gonadal hormones in mediating sex differences in morphine analgesia elicited from the vlPAG.

Receptor-selective antagonism of opioid antinociception in female versus male rats

This study was conducted to determine whether sex differences in opioid antinociception may be explained by sex differences in opioid receptor activation. The time course, dose-effect and selectivity of antagonists that have been previously shown to be relatively mu (beta-funaltrexamine, beta-FNA), kappa (norbinaltorphimine, norBNI), or delta (naltrindole, NTI) receptor selective in male animals were compared in female and male Sprague-Dawley rats using a 52 degrees C hotplate test. In both sexes, beta-FNA (10 or 20 microg intracerebroventricularly [i.c.v.]) dose-dependently blocked the antinociceptive effects of fentanyl (0.056 mg/kg subcutaneously); antagonism was observed 24 h after beta-FNA, and diminished within 7-14 days. In both sexes, norBNI (1 or 10 microg i.c.v.) dose-dependently blocked the antinociceptive effects of U69,593 (1.0 mg/kg subcutaneously); antagonism was maximal by 1-3 days post-norBNI and lasted longer than 56 days. NTI (1 or 10 microg i.c.v.) dose-dependently blocked the antinociceptive effects of [D-Pen2, D-Pen5]enkephalin (DPDPE, 100 nmol i.c.v.) in both sexes; however, the duration of action of NTI was shorter in females than in males. The antinociceptive effects of the mu receptor-preferring agonists fentanyl, morphine and buprenorphine were significantly and dose-dependently antagonized by beta-FNA, but not by norBNI or NTI, in both sexes. Beta-FNA antagonism was significantly greater in females compared with males given morphine, but not fentanyl or buprenorphine. The antinociceptive effects of the kappa receptor-preferring agonists U69,593 and U50,488 were significantly and dose-dependently antagonized by norBNI; U50,488 but not U69,593 was also antagonized to a lesser extent by NTI and beta-FNA, in both sexes. The antinociceptive effect of the delta receptor-preferring agonist SNC 80 was significantly antagonized by NTI, but not by norBNI or beta-FNA, in both sexes. The sex difference in beta-FNA antagonism of morphine suggests that there may be sex differences in functional mu opioid receptor reserve or signal transduction; however, the lack of consistency across all mu agonists weakens this hypothesis. Overall, the opioids tested had very similar receptor selectivity in male and female subjects.

Sex differences in opioid antinociception: kappa and 'mixed action' agonists

A number of investigators have shown that male animals are more sensitive than females to the antinociceptive effects of mu-opioid agonists. The present study was conducted to examine sex differences in opioid antinociception in the rat using agonists known to differ in selectivity for and efficacy at kappa- versus mu-receptors. Dose- and time-effect curves were obtained for s.c. U69593, U50488, ethylketazocine, (-)-bremazocine, (-)-pentazocine, butorphanol and nalbuphine on the 50 or 54 degrees C hotplate and warm water tail withdrawal assays; spontaneous locomotor activity was measured 32-52 min post-injection in the same rats. On the hotplate assay, only butorphanol (54 degrees C) and nalbuphine (50 degrees C) were significantly more potent in males than females. On the tail withdrawal assay, all agonists were significantly more potent or efficacious in males than females at one or both temperatures. In contrast, no agonist was consistently more potent in one sex or the other in decreasing locomotor activity. Estrous stage in female rats only slightly influenced opioid effects, accounting for an average of 2.6% of the variance in females' antinociceptive and locomotor responses to drug (50 degrees C experiment). These results suggest that (1) sex differences in antinociceptive effects of opioids are not mu-receptor-dependent, as they may occur with opioids known to have significant kappa-receptor-mediated activity; (2) the mechanisms underlying sex differences in kappa-opioid antinociception may be primarily spinal rather than supraspinal; (3) sex differences in antinociceptive effects of opioid agonists are not secondary to sex differences in their sedative effects.

Morphine responses in humans: a retrospective analysis of sex differences

There is increasing evidence that sex modulates the effects of opioid analgesics in nonhumans, but few studies have examined this issue in humans. Over the past seven years we have conducted several studies in which the subjective, psychomotor, and physiological effects of intravenous morphine were examined in healthy volunteers. In a retrospective analysis encompassing six studies, we re-examined the effects of 10 mg/70 kg (iv) morphine in 57 males and 27 females. There were some differences in morphine's subjective effects as a function of sex. Females reported higher ratings of 'coasting (spaced out),' 'heavy or sluggish feeling' and 'dry mouth.' No differences in degree of psychomotor impairment or physiological effects (miosis and respiration rate) of morphine emerged between males and females. Future studies should focus on other doses of morphine and other opioid drugs, assess multiple behavioral and physiological endpoints, and look at different subsamples of humans (e.g. opioid abusers).

Sex differences in relation to conditioned fear-induced enhancement of morphine analgesia

A number of studies have reported that both the immediate and proactive effects of exposure to a shock stressor are less pronounced in female than in male rats. A separate area of research has demonstrated that female rats are less sensitive to the analgesic effects of morphine than males. Experiments from our laboratory, as well as others, have found that exposure to a context associated with shock (i.e., conditioned fear context) at the time of morphine administration, enhances the analgesic effects of morphine. Since previous studies have exclusively employed male rats, the purpose of Experiment 1 was to determine if a sex difference exists to this context conditioned fear-induced enhancement of morphine-induced analgesia. The findings of Experiment 1 showed that females do not appear to exhibit conditioned fear-induced enhancement of morphine analgesia as compared to males. Experiment 2 demonstrated that females exhibited higher levels of conditioned fear-induced enhancement of morphine analgesia during diestrus I than estrous. Experiment 3 demonstrated that females exhibited lower levels of conditioned analgesia compared to males, while both groups exhibited similar freezing levels. The findings of the present experiments suggest that the sex difference observed in Experiment 1 may be due to differences in conditioned analgesia.

Orphanin FQ produces gender-specific modulation of trigeminal nociception: behavioral and electrophysiological observations

The present study aimed to determine if orphanin FQ, an endogenous ligand for the opioid receptor like-1 receptor, produces gender-specific effects in the modulation of N-methyl-D-aspartate (NMDA)-evoked responses of trigeminal nociceptive neurons, and in the NMDA-induced nociceptive behavior. Single-unit extracellular recordings were made from nociceptive-specific and wide dynamic range neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized (1.5 g/kg urethane) rats. In the proestrous female, orphanin FQ applied microiontophoretically produced facilitation of the NMDA-evoked responses in 50% (16/32) of nociceptive neurons, inhibition in 31% (10/32), and biphasic effects in 19% (6/32). In contrast, in the male, it inhibited the responses in 86% (18/21), and facilitated the responses in 14% (4/21). In ovariectomized animals, orphanin FQ inhibited the responses in 75% (9/12) of nociceptive neurons, facilitated the responses in 17% (2/12) and produced biphasic effects in 8% (1/12). In contrast, in estradiol-treated ovariectomized rats, it facilitated the responses in 46% (5/11), inhibited the responses in 36% (4/11) and produced biphasic effects in 18% (2/11). For behavioral studies, NMDA-induced scratching behavior was used to assess the effects of orphanin FQ. Twenty-eight male, ovariectomized and estradiol-treated ovariectomized rats were microinjected with NMDA (2 nmol in 10 microl) alone through a cannula implanted in the medullary region, while another 27 rats were microinjected with orphanin FQ (10 nmol in 10 microl) 10 min prior to giving NMDA. Orphanin FQ reduced the NMDA-induced nociceptive scratching behavior by 92% in the male, and by 96% in ovariectomized rats. In contrast, in estradiol-treated ovariectomized animals, orphanin FQ facilitated the NMDA-induced scratching behavior by 210%. We conclude from these studies that orphanin FQ is primarily pronociceptive in the female and primarily antinociceptive in the male. Furthermore, we suggest that estrogen is involved in generating the gender-specific effects of orphanin FQ.

kappa-opioid receptor-mediated analgesia: hotplate temperature and sex differences

The present experiment was designed to investigate the dose-effect and time-effect functions of the kappa-opioid receptor agonist [5alpha,7alpha, 8beta)-(-)-N-Methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro(4, 5)dec-8-yl]benzeneacetamide] (U69,593) in intact and gonadectomized male and female rats as a function of hotplate temperature (45 degrees C or 51 degrees C). At 45 degrees C baseline lick latencies were longer in female rats than in male rats. Lick latencies increased dose-dependently in all subjects except intact female rats. U69,593 increased lick latencies as a function of time since its administration in all subjects. At 51 degrees C baseline lick latencies did not differ between groups and they increased dose-dependently in all subjects. The effectiveness of U69,593 decreased as a function of time since its administration, but not in castrated male rats. These observations suggest that gonadal hormones could play a role in modulating the behavioral effects of U69,593 when subjects are tested at different hotplate temperatures.

A comparison of morphine analgesic tolerance in male and female mice

Studies comparing morphine tolerance in males and females are rare, and all studies to date have utilized the rat. To generalize from findings with rats morphine tolerance was investigated in male and female mice using the tail-withdrawal test. Three and 7 days of systemic morphine injections produced significant but unequal rightward shifts in the morphine dose-response curve such that females displayed greater increases in analgesic ED(50) values when compared to males. In a separate experiment, males and females displayed similar reductions in morphine analgesic sensitivity when %MPE (maximum possible effect) and %total (area under the curve) were compared after 3 days of morphine. Differences in initial morphine sensitivity between sexes were not observed in either study. The data demonstrate that, in contrast to rats, female mice undergo greater reductions in morphine analgesia relative to males following chronic morphine, but this sex difference may depend on the method of assessing analgesia. Furthermore, the duration and/or cumulative dose of morphine treatment does not affect the expression of sex differences in morphine tolerance.

Gender differences in the cardiovascular responses to morphine and naloxone in spinal rats

Putative gender differences in opiate cardiovascular effects were evaluated in spinal rats. After a 4-h exposure to a single dose of morphine (30 mg/kg, i.v.), abstinence was precipitated by naloxone (0.03-3 mg/kg, i.v.). Morphine produced a long-lasting bradycardia and a transient increase in arterial pressure that was similar in both genders. Thereafter, blood pressure decreased both in males and females. Naloxone precipitated a similar dose-dependent heart rate increase in both sexes and a gender-dependent increase in blood pressure. This sex difference appeared in the shape of the response. Prazosin (0.2 mg/kg), prior to naloxone, reduced the pressor response in all animals, suggesting a similar participation of the noradrenergic system in both genders. The present results extend to acute dependence the notion of a sex-dependent differential effect of morphine. The need to consider gender as a factor when studying the effects of opioids is highlighted.

Sex differences in thermal nociception and morphine antinociception in rodents depend on genotype

It has been appreciated for some time that the sexes can differ in their sensitivity to pain and its inhibition. Both the human and rodent literatures remain quite contentious, with many investigators failing to observe sex differences that others document clearly. Recent data from our laboratory have pointed to an interaction between sex and genotype in rodents, such that sex differences are observed in some strains but not others. However, these studies employed inbred mouse strains and are thus not directly relevant to existing data. We presently examined whether the observation of statistically significant sex differences in nociception and morphine antinociception might depend on the particular outbred rodent population chosen for study. Rats of both sexes and three common outbred strains were obtained from three suppliers (Long Evans, Simonsen; Sprague Dawley, Harlan; Wistar Kyoto, Taconic) and tested for nociceptive sensitivity on the 49 degrees C tail-withdrawal assay, and antinociception following morphine (1-10mg/kg, i.p.). In further studies, three outbred populations of mice (CD-1, Harlan; Swiss Webster, Harlan; Swiss Webster, Simonsen) were bred in our vivarium for several generations and tested for tail-withdrawal sensitivity and morphine antinociception (1-20male, and no significant difference. In a separate study in which the estrous cycle was tracked in female mice, we found evidence for an interaction between genotype and estrous phase relevant to morphine antinociception. However, estrous cyclicity did not explain the observed sex differences. These data are discussed with respect to the existing sex difference and pain literature, and also as they pertain to future investigations of these phenomena.

Antinociceptive and behavioral activation responses elicited by d-Pro(2)-endomorphin-2 in the ventrolateral periaqueductal gray are sensitive to sex and gonadectomy differences in rats

Sex differences have been observed in antinociception after morphine administered into either the lateral ventricles, rostral ventromedial medulla, or ventrolateral periaqueductal gray such that male rats exhibit significantly greater antinociception than female rats. Adult gonadectomy produced small, but significant changes in morphine antinociception relative to same-sex sham-operated controls. The present study examined whether sex and adult gonadectomy differences were observed in antinociceptive responses after D-Pro(2)-Endomorphin-2 (1-50 microg) elicited from the ventrolateral periaqueductal gray (vlPAG) on the tail-flick and jump tests in rats, and compared these effects with morphine antinociception. D-Pro(2)-Endomorphin-2 antinociception in the vlPAG was significantly greater in estrous-phase, sham-operated and ovariectomized female rats relative to sham-operated and castrated male rats on the tail-flick, but not jump test that differed markedly from the greater magnitude of morphine antinociception noted for male rats on both tests. In testing whether D-Pro(2)-Endomorphin-2's antinociceptive sex differences were secondary to alterations in activity, similar decreases in the pattern of total activity were observed after D-Pro(2)-Endomorphin-2 in the vlPAG in male and female rats. In evaluating whether male and female rats differed in their behavioral activation responses after D-Pro(2)-Endomorphin-2 in the vlPAG, significantly more excessive grooming, seizures, barrel rolls and explosive running behaviors were observed after D-Pro(2)-Endomorphin-2 in male, but not female rats during the precise periods of time when they were failing to display robust antinociceptive responses on the tail-flick test. Thus, the different patterns of sex differences after D-Pro(2)-Endomorphin-2 in the vlPAG appear to be attributable to sex-dependent alterations in behavioral activation rather than nociceptive processing per se.

Analysis of sex and gonadectomy differences in beta-endorphin antinociception elicited from the ventrolateral periaqueductal gray in rats

Male rats exhibit significantly greater antinociception following central administration of morphine than female rats. The present study examined potential differences in beta-endorphin (5.2-26 microg) antinociception elicited from the ventrolateral periaqueductal gray in adult sham-operated and gonadectomized male and female rats. Male rats displayed significantly greater peak (30 min) tail-flick latencies across the entire range of beta-endorphin doses administered into the ventrolateral periaqueductal gray than female rats tested during the estrous phase of the estrous cycle. Adult gonadectomy failed to appreciably change the pattern of this effect in either males of females. Thus, antinociception elicited from the ventrolateral periaqueductal gray by beta-endorphin, like morphine, is sensitive to sex differences.