Pleiotropic opioid regulation of spinal endomorphin 2 release and its adaptations to opioid withdrawal are sexually dimorphic

Sex-specific mechanisms of tolerance for the cannabinoid agonists CP55,940 and delta-9-tetrahydrocannabinol (Δ9-THC)

RATIONALE

Tolerance to cannabinoids could limit their therapeutic potential. Male mice expressing a desensitization-resistant form (S426A/S430A) of the type-1 cannabinoid receptor (CB₁R) show delayed tolerance to delta-9-tetrahydrocannabinol (∆⁹-THC) but not CP55,940. With more women than men using medical cannabis for pain relief, it is essential to understand sex differences in cannabinoid antinociception, hypothermia, and resultant tolerance.

OBJECTIVE

Our objective was to determine whether female mice rely on the same molecular mechanisms for tolerance to the antinociceptive and/or hypothermic effects of cannabinoids that we have previously reported in males. We determined whether the S426A/S430A mutation differentially disrupts antinociceptive and/or hypothermic tolerance to CP55,940 and/or Δ⁹-THC in male and female S426A/S430A mutant and wild-type littermates.

RESULTS

The S426A/S430A mutation conferred an enhanced antinociceptive response for ∆⁹-THC and CP55,940 in both male and female mice. While the S426A/S430A mutation conferred partial resistance to ∆⁹-THC tolerance in male mice, disruption of CB₁R desensitization had no effect on tolerance to ∆⁹-THC in female mice. The mutation did not alter tolerance to the hypothermic effects of ∆⁹-THC or CP55,940 in either sex. Interestingly, female mice were markedly less sensitive to the antinociceptive effects of 30 mg/kg ∆⁹-THC and 0.3 mg/kg CP55,940 compared with male mice.

CONCLUSIONS

Our results suggest that disruption of the GRK/βarrestin2 pathway of desensitization alters tolerance to Δ⁹-THC but not CP55,940 in male but not female mice. As tolerance to Δ⁹-THC appears to develop differently in males and females, sex should be considered when assessing the therapeutic potential and dependence liability of cannabinoids.

Sex dependent alteration of epigenetic marks after chronic morphine treatment in mice organs

Epigenetic marks may be also affected by several factors, such as age, lifestyle, early life experiences and exposure to chemicals or drugs, such as opioids. Previous studies have focused on how morphine epigenetically regulates different regions of the brain that are implicated in tolerance, dependence and other psychiatric disorders more related to the physio-pathological effects of opioids. Nevertheless, a significant knowledge gap remains regarding the effect of chronic treatment on other organs and biological systems. Therefore, the aim of this work is to increase our knowledge about the impact of chronic morphine exposure on DNA methylation and histone modification levels in each of the organs of male and female model mice in vivo. Our results reveal, for the first time, that chronic morphine treatment induced changes in DNA methylation/hydroxymethylation and histone modification in-vivo at the systemic level, revealing a potential physiological effect on the regulation of gene expression. Notably, morphine-induced epigenetic modification occurs in a sex-dependent manner, revealing the existence of different underlying mechanisms of epigenetic modification in male and female mice.

Phosphorylation of unique C-terminal sites of the mu-opioid receptor variants 1B2 and 1C1 influences their Gs association following chronic morphine

We recently demonstrated in rat spinal cord that a regimen of escalating doses of systemic morphine, analogous to regimens used clinically for chronic pain management, selectively up-regulates the mu-opioid receptor (MOR) splice variants MOR-1B2 and MOR-1C1 mRNA and functional protein. This study investigated the potential relevance of up-regulating MOR-1B2 and MOR-1C1 to the ability of chronic morphine to shift MOR signaling from predominantly G_i /G_o inhibitory to G_s stimulatory. Specifically, we tested the hypotheses that chronic morphine induces phosphorylation of carboxyl terminal sites unique to MOR-1B2 and MOR-1C1, and that this phosphorylation is causally related to augmented association of these variants with G_s α. Hypotheses were validated by (i) abolition of the chronic morphine-induced increment in MOR-1C1 and MOR-1B2 association with G_s α by inhibitors of protein kinase A and Casein kinase 2, respectively; (ii) failure of chronic morphine to augment MOR variant G_s α interactions in Chinese hamster ovary cells transiently transfected with either rat MOR-1C1 or MOR-1B2 in which targeted protein kinase A and Casein kinase 2 serine phosphorylation sites, respectively, were mutated to alanine; (iii) abrogation of chronic morphine-induced augmented MOR G_s α association in spinal cord of male rats following intrathecal administration of dicer substrate small interfering RNAs targeting MOR-1B2/MOR-1C1 mRNA. The ability of chronic morphine to not only up-regulate-specific MOR variants but also their carboxyl terminal phosphorylation and consequent augmented association with G_s α may represent a novel component of opioid tolerance mechanisms, suggesting novel potential targets for tolerance abatement.

Sex-biased cellular signaling: molecular basis for sex differences in neuropsychiatric diseases

The recognition that there are fundamental biological sex differences that extend beyond those that define sexual behavior and reproductive function has inspired the drive toward inclusion of both sexes in research design. This is supported by an underlying clinical rationale that studying both sexes is necessary to elucidate pathophysiology and develop treatments for the entire population. However, at a more basic level, sex differences, like genetic differences, can be exploited to better understand biology. Here, we discuss how sex differences at the molecular level of cell signaling and protein trafficking are amplified to create a state of vulnerability that under the right conditions can result in symptoms of neuropsychiatry disease. Although this dialogue focuses on the specific example of corticotropin-releasing factor, the potential for analogous sex differences in signaling and/or trafficking of receptors for other neuromodulators has broad biological and therapeutic implications.

Sex-biased cellular signaling: molecular basis for sex differences in neuropsychiatric diseases

The recognition that there are fundamental biological sex differences that extend beyond those that define sexual behavior and reproductive function has inspired the drive toward inclusion of both sexes in research design. This is supported by an underlying clinical rationale that studying both sexes is necessary to elucidate pathophysiology and develop treatments for the entire population. However, at a more basic level, sex differences, like genetic differences, can be exploited to better understand biology. Here, we discuss how sex differences at the molecular level of cell signaling and protein trafficking are amplified to create a state of vulnerability that under the right conditions can result in symptoms of neuropsychiatry disease. Although this dialogue focuses on the specific example of corticotropin-releasing factor, the potential for analogous sex differences in signaling and/or trafficking of receptors for other neuromodulators has broad biological and therapeutic implications.

Chronic opioid treatment augments caveolin-1 scaffolding: relevance to stimulatory μ-opioid receptor adenylyl cyclase signaling

Caveolin-1 is the predominant structural protein of caveolae, a subset of (lipid) membrane rafts that compartmentalize cell signaling. Caveolin-1 binds most to G protein-coupled receptors and their signaling partners, thereby enhancing interactions among signaling cascade components and the relative activation of specific G protein-coupled pathways. This study reveals that chronic opioid exposure of μ-opioid receptor (MOR) expressing Chinese hamster ovary cells (MOR-CHO) and chronic in vivo morphine exposure of rat spinal cord augmented recruitment of multiple components of MOR-adenylyl cyclase (AC) stimulatory signaling by caveolin-1. Strikingly, in MOR-CHO and spinal cord, blocking the caveolin-1 scaffolding domain substantially attenuated the chronic morphine-induced increased interaction of caveolin-1 with MOR, Gsα, protein phosphatase 2A (PP2A), and AC. Chronic morphine treatment also increased interactions among the above signaling proteins, thus enabling sufentanil to stimulate (rather than inhibit) cAMP production within lipid membrane microdomains. The latter finding underscores the functionality of augmented interactions among MOR, G_s α, PP2A, and AC. In the aggregate, our data strongly suggest that augmented caveolin-1 scaffolding undergirds the ability of chronic opioids to recruit an ancillary signaling pathway by acting as an organizing template for MOR-G_s α-AC signaling and delimiting the membrane compartment(s) in which it occurs. Since caveolin-1 binds to a wide spectrum of signaling molecules, altered caveolin-1 scaffolding following chronic opioid treatment is likely to pertain to most, if not all, MOR signaling partners. The chronic morphine-induced trigger that augments caveolin-1 scaffolding could represent a seminal perturbation that initiates the wide spectrum of adaptations thought to contribute to opioid tolerance and dependence.

Sex and gender disparity in pathology, disability, referral pattern, and wait time for surgery in workers with shoulder injury

BACKGROUND

The role of sex as an important biological determinant of vulnerability to sustaining injury and gender as a social determinate of access to resources, referral for medical care and perceived disability remains conflicted in injured workers. The purpose of this study was to examine sex and gender disparity following a compensable work-related shoulder injury.

METHODS

This study involved cross-sectional analyses of data of two independent samples of workers with shoulder injury. Measures of disability and pain were the Quick Disabilities of the Arm, Shoulder and Hand (QuickDASH) and Numerical Pain Rating Scale (NPRS) for patients seen at an Early Shoulder Physician Assessment (ESPA) program and the American Shoulder and Elbow Surgeons (ASES) assessment form and Visual Analogue Scale (VAS) for the sample who underwent surgery.

RESULTS

The files of 1000 (443 females, 557 men) consecutive patients seen at an ESPA program and 150 (44 females, and 106 men) consecutive patients who underwent rotator cuff surgery (repair or decompression) were reviewed. Significant gender disparity was observed in the referral pattern of injured workers seen at the ESPA program who were referred for surgical consultation (22 vs. 78 % for females and males respectively, p < 0.0001). The independent rotator cuff surgical group had a similar gender discrepancy (29 % vs. 71 %, p < 0.0001). The timeframe from injury to surgery was longer in women in the surgical group (p = 0.01). As well, women waited longer from the date of consent to date of surgery (p = 0.04). Women had higher incidence of repetitive injuries (p = 0.01) with men reporting higher incidence of falls (p = 0.01). Women seen at the ESPA program were more disabled than men (p = 0.02). Women in both samples had a higher rate of medication consumption than men (p = 0.01 to <0.0001). Men seen at the ESPA program had a higher prevalence of full thickness rotator cuff tears (p < 0.0001) and labral pathology (p = 0.01). However, these pathologies did not explain gender disparity in the subsample of ESPA who were referred for surgical consultation or those who had surgery.

CONCLUSIONS

Sex and gender disparity exists in workers with shoulder injuries and is evident in the mechanism of injury, perceived disability, medication consumption, referral pattern, and wait time for surgery.

Selective up-regulation of functional mu-opioid receptor splice variants by chronic opioids

We recently reported (Verzillo, et al. J. Neurochem: 130, 790-796, 2014) that chronic systemic morphine selectively up-regulates mRNA encoding two C-terminal μ-opioid receptor (MOR) splice variants, MOR-1C1 and MOR-1B2 (MOR-1B2/-1C1). Given the known disconnects between changes in levels of mRNA and corresponding protein, it is essential to directly demonstrate that chronic opioid treatment elevates functional MOR-1B2/-1C1 protein prior to inferring relevance of these MOR variants to spinal opioid tolerance mechanisms. Accordingly, we investigated the ability of chronic opioid exposure to up-regulate MOR protein in Chinese hamster ovary cells stably transfected with rat MOR variants MOR-1B2, MOR-1C1, or MOR-1 (considered to be the predominant MOR). Findings revealed that chronic treatment with the clinically relevant opioids morphine, oxycodone and hydrocodone substantially up-regulated membrane MOR-1B2/-1C1 protein. This up-regulation was abolished by the protein synthesis inhibitor cycloheximide, eliminating contributions from receptor redistribution. The increment in MOR-1B2/-1C1 protein was paralleled by a significant increment in opioid agonist-stimulated GTPγS-binding (reflective of increased aggregate MOR G protein coupling) indicating that up-regulated MOR-1B2/-1C1 represented functional receptors. Strikingly, these tolerance-associated adaptations of MOR-1B2/-1C1 differed considerably from those of MOR-1. Antithetical regulation of MOR-1B2/-1C1 and MOR-1 by chronic opioids has significant implications for the design of new therapeutic agents to counteract opioid analgesic tolerance and accompanying addiction. Since chronic opioids induce MOR-1B2/-1C1 up-regulation in spinal cord of males, but not females, elucidating cellular compartments and intracellular pathways mediating MOR-1B2/-1C1 up-regulation and defining their unique signaling attributes would enable a precision medicinal approach to pain management and addiction therapy. In the spinal cord of males, but not females, chronic morphine up-regulates mRNA encoding two mu-opioid receptor (MOR) variants, MOR-1B2 and MOR-1C1 (MOR-1B2/-1C1). We now demonstrate that chronic treatment with the clinically relevant opioids morphine, hydrocodone or oxycodone up-regulates MOR-1B2/-1C1 functional protein, which is dependent on de novo protein synthesis. Findings underscore the importance of unique signaling attributes of MOR variants to sexually dimorphic tolerance mechanisms.

Contribution of Endogenous Spinal Endomorphin 2 to Intrathecal Opioid Antinociception in Rats Is Agonist Dependent and Sexually Dimorphic

Interactions between exogenous and endogenous opioids are not commonly investigated as a basis for sexually dimorphic opioid analgesia. We investigated the influence of spinal endomorphin 2 (EM2), an endogenous mu-opioid receptor (MOR) ligand, on the spinal antinociception produced by intrathecally administered opioids. Activation of spinal MORs facilitated spinal EM2 release. This effect was sexually dimorphic, occurring in males but not in females. Although activational effects of testosterone were required for opioid facilitation of spinal EM2 release in males, the absence of this facilitation in females did not result from either insufficient levels of testosterone or mitigating effects of estrogens. Strikingly, in males, the contribution of spinal EM2 to the analgesia produced by intrathecally applied MOR agonists depended on their analgesic efficacy relative to that of EM2. Spinal EM2 released by the higher efficacy MOR agonist sufentanil diminished sufentanil's analgesic effect, whereas EM2 released by the lower efficacy morphine had the opposite effect on spinal morphine antinociception. Understanding antithetical contributions of endogenous EM2 to intrathecal opioid antinociception not only enlightens the selection of opioid medications for pain management but also helps to explain variable sex dependence of the antinociception produced by different opioids, facilitating the acceptance of sexually dimorphic antinociception as a basic tenet.

PERSPECTIVE

The male-specific MOR-coupled enhancement of spinal EM2 release implies a parallel ability to harness endogenous EM2 antinociception. The inferred diminished ability of females to utilize the spinal EM2 antinociceptive system could contribute to their greater frequency and severity of chronic pain syndromes.

Examining the factor structure of the Clinical Opiate Withdrawal Scale: A secondary data analysis from the National Drug Abuse Treatment Clinical Trials Network (CTN) 0003

BACKGROUND

The Clinical Opiate Withdrawal Scale (COWS) is used to assess withdrawal in clinical trials and practice. The aims of this study were to examine the inter-item correlations and factor structure of the COWS in opioid-dependent men and women.

METHODS

This is a secondary data analysis of the National Drug Abuse Treatment Clinical Trials Network 0003, a randomized clinical trial that compared buprenorphine/naloxone tapering strategies. The trial included 11 sites in 10 US cities. Participants were opioid-dependent individuals (n=516) that had data on the COWS. The COWS at study baseline was analyzed in this study.

RESULTS

Inter-item correlations showed weak to moderate relationships among the items. A 1-factor model did not fit the data for men (comparative fit index (CFI)=.801, root mean square error of approximation (RMSEA)=.073, weighted root mean square residual (WRMR)=1.132) or women (CFI=.694, RMSEA=.071, WRMR=.933), where resting pulse rate was not related to withdrawal for men, and yawning and gooseflesh skin was not related to withdrawal for women. A reduced model comprised of only the 8 items that were significantly related to the construct of withdrawal in both men and women, and an exploratory 2-factor model, were also assessed but not retained due to inconsistencies across gender.

CONCLUSIONS

When traditional psychometric models are applied to the COWS, it appears that the scale may not relate to a single underlying construct of withdrawal. Further research testing the hypothesized factor structure in other opioid-dependent samples is needed.

Mu-opioid receptor splice variants: sex-dependent regulation by chronic morphine

The gene encoding the mu-opioid receptor (MOR) generates a remarkable diversity of subtypes, the functional significance of which remains largely unknown. The structure of MOR could be a critical determinant of MOR functionality and its adaptations to chronic morphine exposure. As MOR antinociception has sexually dimorphic dimensions, we determined the influence of sex, stage of estrus cycle, and chronic systemic morphine on levels of MOR splice variant mRNA in rat spinal cord. Chronic systemic morphine influenced the spinal expression of mRNA encoding rMOR-1B2 and rMOR-1C1 in a profoundly sex-dependent fashion. In males, chronic morphine resulted in a twofold increase in expression levels of rMOR-1B2 and rMOR-1C1 mRNA. This effect of chronic morphine was completely absent in females. Increased density of MOR protein in spinal cord of males accompanied the chronic morphine-induced increase in MOR variant mRNA, suggesting that it reflected an increase in corresponding receptor protein. These results suggest that tolerance/dependence results, at least in part, from different adaptational strategies in males and females. The signaling consequences of the unique composition of the C-terminus tip of rMOR-1C1 and rMOR-1B2 could point the way to defining the molecular components of sex-dependent tolerance and withdrawal mechanisms. Chronic systemic morphine increases levels of mRNA encoding two splice variants of mu-opioid receptor (MOR), MOR-1B2 and MOR-1C1, variants differing from rMOR-1 in their C-terminal (and phosphorylation sites therein) and thus possibly signaling sequelae. This adaptation is sex-specific. It occurs in the spinal cord of males, but not females, indicating the importance of sex-specific mechanisms for and treatments of tolerance and addiction.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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).

On the g-protein-coupled receptor heteromers and their allosteric receptor-receptor interactions in the central nervous system: focus on their role in pain modulation

The modulatory role of allosteric receptor-receptor interactions in the pain pathways of the Central Nervous System and the peripheral nociceptors has become of increasing interest. As integrators of nociceptive and antinociceptive wiring and volume transmission signals, with a major role for the opioid receptor heteromers, they likely have an important role in the pain circuits and may be involved in acupuncture. The delta opioid receptor (DOR) exerts an antagonistic allosteric influence on the mu opioid receptor (MOR) function in a MOR-DOR heteromer. This heteromer contributes to morphine-induced tolerance and dependence, since it becomes abundant and develops a reduced G-protein-coupling with reduced signaling mainly operating via β-arrestin2 upon chronic morphine treatment. A DOR antagonist causes a return of the Gi/o binding and coupling to the heteromer and the biological actions of morphine. The gender- and ovarian steroid-dependent recruitment of spinal cord MOR/kappa opioid receptor (KOR) heterodimers enhances antinociceptive functions and if impaired could contribute to chronic pain states in women. MOR1D heterodimerizes with gastrin-releasing peptide receptor (GRPR) in the spinal cord, mediating morphine induced itch. Other mechanism for the antinociceptive actions of acupuncture along meridians may be that it enhances the cross-desensitization of the TRPA1 (chemical nociceptor)-TRPV1 (capsaicin receptor) heteromeric channel complexes within the nociceptor terminals located along these meridians. Selective ionotropic cannabinoids may also produce cross-desensitization of the TRPA1-TRPV1 heteromeric nociceptor channels by being negative allosteric modulators of these channels leading to antinociception and antihyperalgesia.

Sex-specific cell signaling: the corticotropin-releasing factor receptor model

Elucidating the biological basis for sex differences in diseases can reveal their pathophysiology and guide the development of individualized treatments. Here, we review evidence for the novel concept that receptor signaling can be sex biased such that the specific pathways engaged by ligand binding are determined by sex. As an example, this review focuses on the receptor for corticotropin-releasing factor (CRF), a stress-related peptide implicated in diverse psychiatric and medical disorders that are more prevalent in females. There is evidence for sex biases in CRF receptor coupling to G proteins and β-arrestin that render females more sensitive to acute stress and less able to adapt to chronic stress. Taken with evidence for sex biased signaling in other receptor systems, the studies demonstrate the broad potential impact of this characteristic in determining sex differences in disease and therapeutic efficacy and underscore the importance of studying females in medical and pharmacological research.