Ultra low concentrations of morphine increase neurite outgrowth in cultured rat spinal cord and cerebral cortical neurons

Opioid modulation of T-type Ca2+ channel-dependent neuritogenesis/neurite outgrowth through the prostaglandin E2/EP4 receptor/protein kinase A pathway in mouse dorsal root ganglion neurons

Irregular regeneration or inappropriate remodeling of the axons of the primary afferent neurons after peripheral nerve trauma could be associated with the development of neuropathic pain. We analyzed the molecular mechanisms for the neuritogenesis and neurite outgrowth caused by prostaglandin E2 (PGE2) in mouse dorsal root ganglion (DRG) neurons, and evaluated their opioid modulation. PGE2 in combination with IBMX, a phosphodiesterase inhibitor, caused neuritogenesis/neurite outgrowth in DRG cells, an effect abolished by a prostanoid EP4, but not EP2, receptor antagonist, and inhibitors of adenylyl cyclase or protein kinase A (PKA). Blockers of T-type Ca2+ channels (T-channels), that are responsible for window currents involving the sustained low-level Ca2+ entry at voltages near the resting membrane potentials and can be functionally upregulated by PKA, inhibited the neuritogenesis/neurite outgrowth caused by PGE2/IBMX or dibutylyl cyclic AMP, a PKA activator, in DRG neurons, an inhibitory effect mimicked by ZnCl2 and ascorbic acid that block Cav3.2, but not Cav3.1 or Cav3.3, T-channels. Morphine and DAMGO, μ-opioid receptor (MOR) agonists, suppressed the neuritogenesis and/or neurite outgrowth induced by PGE2/IBMX in DRG neurons and also DRG neuron-like ND7/23 cells, an effect reversed by naloxone or β-funaltrexamine, a selective MOR antagonist. Our data suggest that the EP4 receptor/PKA/Cav3.2 pathway is involved in the PGE2-induced neuritogenesis/neurite outgrowth in DRG neurons, which can be suppressed by MOR stimulation. We propose that MOR agonists including morphine in the early phase after peripheral nerve trauma might delay the axonal regeneration of the primary afferent neurons but prevent the development of neuropathic pain.

Maternal morphine intake during pregnancy and lactation affects the circadian clock of rat pups

Early-life morphine exposure causes a variety of behavioural and physiological alterations observed later in life. In the present study, we investigated the effects of prenatal and early postnatal morphine on the maturation of the circadian clockwork in the suprachiasmatic nucleus and the liver, and the rhythm in aralkylamine N-acetyltransferase activity in the pineal gland. Our data suggest that the most affected animals were those born to control, untreated mothers and cross-fostered by morphine-exposed dams. These animals showed the highest mesor and amplitude in the rhythm of Per2, Nr1d1 but not Per1 gene expression in the suprachiasmatic nuclei (SCN) and arrhythmicity in AA-NAT activity in the pineal gland. In a similar pattern to the rhythm of Per2 expression in the SCN, they also expressed Per2 in a higher amplitude rhythm in the liver. Five of seven specific genes in the liver showed significant differences between groups in their expression. A comparison of mean relative mRNA levels suggests that this variability was caused mostly by cross-fostering, animals born to morphine-exposed dams that were cross-fostered by control mothers and vice versa differed from both groups of natural mothers raising offspring. Our data reveal that the circadian system responds to early-life morphine administration with significant changes in clock gene expression profiles both in the SCN and in the liver. The observed differences between the groups suggest that the dose, timing and accompanying stress events such as cross-fostering may play a role in the final magnitude of the physiological challenge that opioids bring to the developing circadian clock.

The Pathology of Morphine-Inhibited Nerve Repair and Morphine-Induced Nerve Damage Is Mediated via Endoplasmic Reticulum Stress

Objective

The aim of the present study was to observe the pathological damage in the cerebral cortex of rats under acute morphine exposure (AME) and different durations of morphine dependence (MD), explore whether endoplasmic reticulum stress (ERS) is involved in the damage process, and assess the effect of morphine exposure on the proliferation and differentiation of newborn neurons.

Methods

Rat models of AME and different durations of MD were established. Pathological changes in cortical neurons were assessed by hematoxylin and eosin (H&E) and thionine staining. The expression of nuclear receptor-related factor 1 (NURR1) and that of the ERS-related proteins glucose-regulated protein 78 (GRP78), p-eIF2α, activating transcription factor 6 (ATF6), and CHOP in cortical neurons was assessed by immunohistochemistry. Double immunofluorescence labeling was used to observe the expression of Ki-67.

Results

H&E and thionine staining revealed that AME resulted in pyknotic changes in cortical neurons. With prolonged morphine exposure, the number of pyknotic neurons was significantly increased, the protein expression of Ki-67 and NURR1 was significantly decreased, and the protein levels of GRP78, p-eIF2α, ATF6, and CHOP showed marked dynamic changes.

Conclusion

AME and different durations of MD caused varying degrees of pathological changes in the cortex. Furthermore, the dynamic changes observed in ERS-related protein expression suggested that ERS may be associated with cortical injury. Different durations of MD inhibited the proliferation, differentiation, and migration of newborn neurons, which may affect the nerve repair process after injury.

Update of the Scientific Opinion on opium alkaloids in poppy seeds

Poppy seeds are obtained from the opium poppy (Papaver somniferum L.). They are used as food and to produce edible oil. The opium poppy plant contains narcotic alkaloids such as morphine and codeine. Poppy seeds do not contain the opium alkaloids, but can become contaminated with alkaloids as a result of pest damage and during harvesting. The European Commission asked EFSA to provide an update of the Scientific Opinion on opium alkaloids in poppy seeds. The assessment is based on data on morphine, codeine, thebaine, oripavine, noscapine and papaverine in poppy seed samples. The CONTAM Panel confirms the acute reference dose (ARfD) of 10 μg morphine/kg body weight (bw) and concluded that the concentration of codeine in the poppy seed samples should be taken into account by converting codeine to morphine equivalents, using a factor of 0.2. The ARfD is therefore a group ARfD for morphine and codeine, expressed in morphine equivalents. Mean and high levels of dietary exposure to morphine equivalents from poppy seeds considered to have high levels of opium alkaloids (i.e. poppy seeds from varieties primarily grown for pharmaceutical use) exceed the ARfD in most age groups. For poppy seeds considered to have relatively low concentrations of opium alkaloids (i.e. primarily varieties for food use), some exceedance of the ARfD is also seen at high levels of dietary exposure in most surveys. For noscapine and papaverine, the available data do not allow making a hazard characterisation. However, comparison of the dietary exposure to the recommended therapeutical doses does not suggest a health concern for these alkaloids. For thebaine and oripavine, no risk characterisation was done due to insufficient data. However, for thebaine, limited evidence indicates a higher acute lethality than for morphine and the estimated exposure could present a health risk.

Activation of NPFF2 receptor stimulates neurite outgrowth in Neuro 2A cells through activation of ERK signaling pathway

Neurite outgrowth is an important process in neural regeneration and plasticity, especially after neural injury, and recent evidence indicates that several G_αi/o protein-coupled receptors play an important role in neurite outgrowth. The neuropeptide (NP)FF system contains two G_αi/o protein-coupled receptors, NPFF₁ and NPFF₂ receptors, which are mainly distributed in the central nervous system. The aim of the present study was to determine whether the NPFF system is involved in neurite outgrowth in Neuro 2A cells. We showed that Neuro 2A cells endogenously expressed NPFF₂ receptor, and the NPFF₂ receptor agonist dNPA inhibited cyclic adenosine monophosphate (cAMP) production stimulated by forskolin in Neuro 2A cells. We also demonstrated that NPFF and dNPA dose-dependently induced neurite outgrowth in Neuro 2A cells, which was completely abolished by the NPFF receptor antagonist RF9. Pretreatment with mitogen-activated protein kinase inhibitors PD98059 and U0126 decreased dNPA-induced neurite outgrowth. In addition, dNPA increased phosphorylation of extracellular signal-regulated kinase (ERK) in Neuro 2A cells, which was completely antagonized by pretreatment with U0126. Our results suggest that activation of NPFF₂ receptor stimulates neurite outgrowth in Neuro 2A cells through activation of the ERK signaling pathway. Moreover, NPFF₂ receptor may be a potential therapeutic target for neural injury and degeneration in the future.

Life span extension in Drosophila melanogaster induced by morphine

The influence of morphine on the life span of Drosophila melanogaster fruit flies has been investigated. Morphine hydrochloride (MH) at concentrations of 0.01, 0.05 and 0.25 mg/ml was added to a medium starting from day 5 or 54 of imaginal life. Supplementation with MH starting from day 5 of imaginal life has resulted in significant increases in the mean life span of males at all concentrations studied. In females, a significant increase in life span compared with control was obtained only for those treated with 0.25 mg/ml MH. In flies with MH feeding from day 54, residual life span was significantly increased in both males and females after treatment with 0.05 mg/ml MH. The present data, together with those of our earlier study in mice (Dubiley et al. Probl Aging Longvity 9:331–332, 2000) suggest that morphine supplementation can result in life extension in both vertebrate and invertebrate animal species.

Morphine regulates dopaminergic neuron differentiation via miR-133b

Morphine is one of the analgesics used most to treat chronic pain, although its long-term administration produces tolerance and dependence through neuronal plasticity. The ability of morphine to regulate neuron differentiation in vivo has been reported. However, the detailed mechanisms have not yet been elucidated because of the inability to separate maternal influences from embryonic events. Using zebrafish embryos as the model, we demonstrate that morphine decreases miR-133b expression, hence increasing the expression of its target, Pitx3, a transcription factor that activates tyrosine hydroxylase and dopamine transporter. Using a specific morpholino to knock down the zebrafish μ-opioid receptor (zfMOR) in the embryos and selective mitogen-activated protein kinase inhibitors, we demonstrate that the morphine-induced miR-133b decrease in zebrafish embryos is mediated by zfMOR activation of extracellular signal-regulated kinase 1/2. A parallel morphine-induced down-regulation of miR-133b was observed in the immature but not in mature rat hippocampal neurons. Our results indicate for the first time that zebrafish embryos express a functional μ-opioid receptor and that zebrafish serves as an excellent model to investigate the roles of microRNA in neuronal development affected by long-term morphine exposure.

GRIN1 regulates micro-opioid receptor activities by tethering the receptor and G protein in the lipid raft

The lipid raft location of mu-opioid receptor (MOR) determines the receptor activities. However, the manner in which MOR is anchored within the lipid rafts is undetermined. Using the targeted proteomic approach and mass spectrometry analyses, we have identified GRIN1 (G protein-regulated inducer of neurite outgrowth 1) can tether MOR with the G protein alpha-subunit and subsequently regulate the receptor distribution within the lipid rafts. Glutathione S-transferase fusion pulldown and receptor mutational analyses indicate that GRIN1-MOR interaction involves a receptor sequence (267)GSKEK(271) within the MOR third intracellular loop that is not involved in Galpha interaction. The GRIN1 domains involved in MOR interaction are also distinct from those involved in Galpha interaction. Pertussis toxin pretreatment reduced the amount of GRIN1 co-immunoprecipitated with MOR but not the amount with Galpha. Furthermore, overexpression of GRIN1 significantly enhanced the amount of MOR in lipid raft and the receptor signaling magnitude as measured by Src kinase activation. Such increase in MOR signaling was demonstrated further by determining the GRIN1-dependent pertussis toxin-sensitive neurite outgrowth. In contrast to minimal neurite outgrowth induced by etorphine in control neuroblastoma N2A cells, overexpression of GRIN1 resulted in the increase in etorphine- and non-morphine-induced neurite outgrowth in these cells. Knocking down endogenous GRIN1 by small interfering RNA attenuated the agonist-induced neurite outgrowth. Disrupting lipid raft by methyl-beta-cyclodextrin also blocked neurite outgrowth. Hence, by tethering Galpha with MOR, GRIN1 stabilizes the receptor within the lipid rafts and potentiates the receptor signaling in the neurite outgrowth processes.

Endogenous morphine in SH-SY5Y cells and the mouse cerebellum

Background

Morphine, the principal active agent in opium, is not restricted to plants, but is also present in different animal tissues and cell types, including the mammalian brain. In fact, its biosynthetic pathway has been elucidated in a human neural cell line. These data suggest a role for morphine in brain physiology (e.g., neurotransmission), but this hypothesis remains a matter of debate. Recently, using the adrenal neuroendocrine chromaffin cell model, we have shown the presence of morphine-6-glucuronide (M6G) in secretory granules and their secretion products, leading us to propose that these endogenous alkaloids might represent new neuroendocrine factors. Here, we investigate the potential function of endogenous alkaloids in the central nervous system.

Methodology and Principal Findings

Microscopy, molecular biology, electrophysiology, and proteomic tools were applied to human neuroblastoma SH-SY5Y cells (i) to characterize morphine and M6G, and (ii) to demonstrate the presence of the UDP-glucuronyltransferase 2B7 enzyme, which is responsible for the formation of M6G from morphine. We show that morphine is secreted in response to nicotine stimulation via a Ca²⁺-dependent mechanism involving specific storage and release mechanisms. We also show that morphine and M6G at concentrations as low as 10⁻¹⁰ M are able to evoke specific naloxone-reversible membrane currents, indicating possible autocrine/paracrine regulation in SH-SY5Y cells. Microscopy and proteomic approaches were employed to detect and quantify endogenous morphine in the mouse brain. Morphine is present in the hippocampus, cortex, olfactory bulb, and cerebellum at concentration ranging from 1.45 to 7.5 pmol/g. In the cerebellum, morphine immunoreactivity is localized to GABA basket cells and their termini, which form close contacts on Purkinje cell bodies.

Conclusions/Significance

The presence of morphine in the brain and its localization in particular areas lead us to conclude that it has a specific function in neuromodulation and/or neurotransmission. Furthermore, its presence in cerebellar basket cell termini suggests that morphine has signaling functions in Purkinje cells that remain to be discovered.

Microglia-Mediated Neurotoxicity Is Inhibited by Morphine through an Opioid Receptor-Independent Reduction of NADPH Oxidase Activity

Recent studies have shown that morphine modulates the function of glia cells through both opioid receptor dependent and independent mechanisms. However, the mechanism by which morphine regulates neuronal disorders through the alteration of microglia activity remains unclear. In this study, using rat primary mesencephalic neuron-glia cultures, we report that both l-morphine and its synthetic stereoenantiomer, d-morphine, an ineffective opioid receptor agonist, significantly reduced LPS- or 1-methyl-4-phenylpyridinium-induced dopaminergic neurotoxicity with similar efficacy, indicating a nonopioid receptor-mediated effect. In addition, using reconstituted neuron and glia cultures, subpicomolar concentrations of morphine were found to be neuroprotective only in the presence of microglia, and significantly inhibited the production of inflammatory mediators from LPS-stimulated microglia cells. Mechanistic studies showed that both l- and d- morphine failed to protect dopaminergic neurons in cultures from NADPH oxidase (PHOX) knockout mice and significantly reduced LPS-induced PHOX cytosolic subunit p47(phox) translocation to the cell membrane by inhibiting ERK phosphorylation. Taken together, our results demonstrate that morphine, even at subpicomolar concentrations, exerts potent anti-inflammatory and neuroprotective effects either through the inhibition of direct microglial activation by LPS or through the inhibition of reactive microgliosis elicited by 1-methyl-4-phenylpyridinium. Furthermore, our study reveals that inhibition of PHOX is a novel site of action for the mu-opioid receptor-independent effect of morphine.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Nicotinic acid adenine dinucleotide phosphate potentiates neurite outgrowth

Ca(2+) regulates a spectrum of cellular processes including many aspects of neuronal function. Ca(2+)-sensitive events such as neurite extension and axonal guidance are driven by Ca(2+) signals that are precisely organized in both time and space. These complex cues result from both Ca(2+) influx across the plasma membrane and the mobilization of intracellular Ca(2+) stores. In the present study, using rat cortical neurons, we have examined the effects of the novel intracellular Ca(2+)-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) on neurite length and cytosolic Ca(2+) levels. We show that NAADP potentiates neurite extension in response to serum and nerve growth factor and stimulates increases in cytosolic Ca(2+) from bafilomycin-sensitive Ca(2+) stores. Simultaneous blockade of inositol trisphosphate and ryanodine receptors abolished the effects of NAADP on neurite length and reduced the magnitude of NAADP-mediated Ca(2+) signals. This is the first report demonstrating functional NAADP receptors in a mammalian neuron. Interplay between NAADP receptors and more established intracellular Ca(2+) channels may therefore play important signaling roles in the nervous system.