Differential effects of impaired mitochondrial energy production on the function of mu and delta opioid receptors in neuronal SK-N-SH cells

Sex-dependent mental illnesses and mitochondria

The prevalence of some mental illnesses, including major depression, anxiety-, trauma-, and stress-related disorders, some substance use disorders, and later onset of schizophrenia, is higher in women than men. While the higher prevalence in women could simply be explained by socioeconomic determinants, such as income, social status, or cultural background, extensive studies show sex differences in biological, pharmacokinetic, and pharmacological factors contribute to females' vulnerability to these mental illnesses. In this review, we focus on estrogens, chronic stress, and neurotoxicity from behavioral, pharmacological, biological, and molecular perspectives to delineate the sex differences in these mental illnesses. Particularly, we investigate a possible role of mitochondrial function, including biosynthesis, bioenergetics, and signaling, on mediating the sex differences in psychiatric disorders.

Opioids Resistance in Chronic Pain Management

Chronic pain management represents a serious healthcare problem worldwide. Chronic pain affects approximately 20% of the adult European population and is more frequent in women and older people. Unfortunately, its management in the community remains generally unsatisfactory and rarely under the control of currently available analgesics. Opioids have been used as analgesics for a long history and are among the most used drugs; however, while there is no debate over their short term use for pain management, limited evidence supports their efficacy of long-term treatment for chronic non-cancer pain. Therapy with opioids is hampered by inter-individual variability and serious side effects and some opioids often result ineffective in the treatment of chronic pain and their use is controversial. Accordingly, for a better control of chronic pain a deeper knowledge of the molecular mechanisms underlying resistance to opiates is mandatory.

Activated δ‑opioid receptors inhibit hydrogen peroxide‑induced apoptosis in liver cancer cells through the PKC/ERK signaling pathway

Apoptotic liver cancer cells have important roles in liver tumorigenesis and liver cancer progression. Recent studies have shown that δ‑opioid receptors are highly expressed in human liver and liver cancer cells. The present study aimed to investigate the role of activated δ‑opioid receptors on human liver cancer cell apoptosis and its interrelation with the mitochondria and the protein kinase C/extracellular‑signal‑regulated kinase (PKC/ERK) signaling pathway. H2O2 was used to induce apoptosis in human liver cancer cells. During apoptosis, mitochondrial transmembrane potentials were observed to decrease, cytochrome c expression was found to increase and B cell lymphoma 2 (Bcl‑2) expression decreased. These findings suggested that H2O2‑induced apoptosis was mediated through the mitochondrial pathway. Of note, activated δ‑opioid receptors were observed to inhibit H2O2‑induced apoptosis in human liver cancer cells. Following δ‑opioid receptor activation, the number of apoptotic liver cancer cells decreased, mitochondrial transmembrane potentials were restored, cytoplasmic cytochrome c and Bcl‑2‑associated X protein expression decreased and Bcl‑2 expression increased. These data suggested that δ‑opioid receptor activation inhibited mitochondria‑mediated apoptosis. In addition, activation of δ‑opioid receptors was observed to increase the expression of PKC and ERK in human liver cancer cells. Furthermore, upon inhibition of the PKC/ERK signaling pathway, the protective effect associated with the δ‑opioid receptor on liver cancer cell apoptosis was inhibited, which was not associated with the status of δ‑opioid receptor activation. These findings suggested that the PKC/ERK signaling pathway has an important role in δ‑opioid receptor‑mediated inhibition of apoptosis in human liver cancer cells.

Supraspinal peroxynitrite modulates pain signaling by suppressing the endogenous opioid pathway

Peroxynitrite (PN, ONOO(-)) is a potent oxidant and nitrating agent that contributes to pain through peripheral and spinal mechanisms, but its supraspinal role is unknown. We present evidence here that PN in the rostral ventromedial medulla (RVM) is essential for descending nociceptive modulation in rats during inflammatory and neuropathic pain through PN-mediated suppression of opioid signaling. Carrageenan-induced thermal hyperalgesia was associated with increased 3-nitrotyrosine (NT), a PN biomarker, in the RVM. Furthermore, intra-RVM microinjections of the PN decomposition catalyst Fe(III)-5,10,15,20-tetrakis(N-methyl-pyridinium-4-yl)porphyrin (FeTMPyP(5+)) dose-dependently reversed this thermal hyperalgesia. These effects of FeTMPyP(5+) were abrogated by intra-RVM naloxone, implicating potential interplay between PN and opioids. In support, we identified NT colocalization with the endogenous opioid enkephalin (ENK) in the RVM during thermal hyperalgesia, suggesting potential in situ interactions. To address the functional significance of such interactions, we exposed methionine-enkephalin (MENK) to PN and identified the major metabolite, 3-nitrotyrosine-methionine-sulfoxide (NSO)-MENK, using liquid chromatography-mass spectrometry. Next, we isolated, purified, and tested NSO-MENK for opioid receptor binding affinity and analgesic effects. Compared to MENK, this NSO-MENK metabolite lacked appreciable binding affinity for δ, μ, and κ opioid receptors. Intrathecal injection of NSO-MENK in rats did not evoke antinociception, suggesting that PN-mediated chemical modifications of ENK suppress opioid signaling. When extended to chronic pain, intra-RVM FeTMPyP(5+) produced naloxone-sensitive reversal of mechanical allodynia in rats following chronic constriction injury of the sciatic nerve. Collectively, our data reveal the central role of PN in RVM descending facilitation during inflammatory and neuropathic pain potentially through anti-opioid activity.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Mechanisms underlying striatal vulnerability to 3-nitropropionic acid

The striatum is a cerebral structure particularly susceptible to the metabolic challenge exerted by 3-nitropropionic acid (3-NPA), a toxin that inhibits the respiratory chain at complex II. The striatum, which receives the nerve endings of the nigro-striatal pathway, concentrates the largest amount of 3,4-dihydroxyphenylethylamine or dopamine (DA) in the brain. DA is metabolized to 3,4-dihydroxyphenylacetic acid (DOPAC) by monoamine oxidase (MAO), an enzyme that contains a redox-active disulfide in the active site. In striatum isolated nerve endings exposed to 3-NPA in vitro, DA increased and DOPAC decreased already after 10 min, and after 2 h also an increase in reactive oxygen species and DA-quinone products formation was detected. These 3-NPA-induced effects resulted in an increase in DA release after 2 h. In striatum homogenates from animals presenting motor disturbances in response to 3-NPA in vivo, the DA metabolites homovanillic acid and DOPAC were increased. It is concluded that in the striatum nerve endings where DA is particularly concentrated, the increase in reactive oxygen species induced by 3-NPA, oxidizes DA generating DA-quinones. These DA-quinones may form adducts with the active site of MAO type A reducing its activity. The DA not metabolized to DOPAC is both, used to unchain generation of more of the harmful DA-oxidation products and released to the external medium, where is metabolized by the non-neuronal enzymes MAO type B and catechol-O-methyltransferase.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Oxidative damage and sensitivity to nociceptive stimulus and opioids in aging rats

Oxidative stress contributes to aging and may cause alterations in pain and analgesia. Knowledge about effects of oxidative stress on the opioid system is very limited. This project was designed to determine the relationship between age-related oxidative damage and opioid antinociception. Three age groups of male Fischer 344 rats were tested for pain sensitivity and responses to morphine and fentanyl using the hot plate method. Oxidative stress markers in various brain regions were measured. With advancing age, nociceptive threshold and antinociceptive effects of opioids decreased significantly. There was a significant negative correlation between morphine antinociception and protein oxidation in cortex, striatum, and midbrain (r(2)=0.73, 0.87, and 0.77, respectively), and lipid peroxidation in cerebral cortex, hippocampus, and striatum (r(2)=0.73, 0.61, and 0.71, respectively). Similar correlation was observed between oxidative stress markers and fentanyl antinociception. These findings demonstrate that the age-related increase in oxidative damage in brain is associated with a significant decrease in the antinociceptive effects of opioids.

Changes in opioid receptor proteins during mitochondrial impairment in differentiated SK-N-SH cells

Aging and neurodegenerative diseases are associated with oxidative damage that may contribute to changes in neurosensory processing, including pain. The effects of neuronal oxidation on the opioid receptor system are poorly understood. Earlier, we have reported that 3-nitroproprionic acid (3-NPA)-induced oxidative stress and impairment of mitochondrial energy metabolism significantly reduced the function of mu but not delta opioid receptors [A. Raut, M. Iglewski, A. Ratka, Differential effects of impaired mitochondrial energy production on the function of mu and delta opioid receptors in neuronal SK-N-SH cells, Neurosci. Lett. 404 (2006) 242-246]. In the present study, we studied the effects of 3-NPA-induced oxidative stress on protein levels of the mu, delta, and kappa opioid receptors (MOR, DOR, and KOR, respectively). The opioid-responsive differentiated SK-N-SH neuronal cells were used as an in vitro model. Cells were exposed to 0, 5, 10, and 20mM of 3-NPA for 0, 1, 2, 12, and 24h. After the 3-NPA treatments, plasma membrane preparations were made and used for the Western blot assay. There was a significant reduction in the level of the MOR protein while levels of DOR and KOR proteins remained unaffected after exposure to 3-NPA. These findings demonstrate for the first time that there is a selective impairment of the MOR protein under conditions of mitochondrial oxidative damage at the neuronal level. The reduction in the level of the MOR protein may contribute to the impairment of MOR function under oxidative damage conditions shown in our previous study.