Agonist-dependent postsynaptic effects of opioids on miniature excitatory postsynaptic currents in cultured hippocampal neurons

Mechanisms Underlying Mu Opioid Receptor Effects on Parallel Fiber-Purkinje Cell Synaptic Transmission in Mouse Cerebellar Cortex

μ-opioid receptors (MOR) are widely expressed in the brain, varying in density in different areas. Activation of MORs underlies analgesia, euphoria, but may lead to tolerance, dependence, and ultimately opioid addiction. The Purkinje cell (PC) is the only efferent neuron in the cerebellar cortex and receives glutamatergic synaptic inputs from the parallel fibers formed by the axons of granule cells. Studies have shown that MORs are expressed during the development of cerebellar cells. However, the distribution of MOR and their effects on PF-PC synaptic transmission remain unclear. To examine these questions, we used whole-cell patch clamp recordings and pharmacological methods to determine the effects and mechanisms of MOR activation on synaptic transmission at PF-PC synapses. The MOR-selective agonist DAMGO significantly reduced the amplitude and area under the curve (AUC) of PF-PC evoked (e) EPSCs, and increased the paired-pulse ratio (PPR).DAMGO-induced inhibitory effects on PF-PC eEPSCs and PPR were abolished by MOR specific blocker CTOP. Further, DAMGO significantly reduced the frequency of PF-PC mEPSCs, but had no obvious effect on their amplitude, suggesting a presynaptic site of action. The DAMGO-induced reduction in the frequency of PF-PC mEPSCs also was blocked by CTOP. A protein kinase A (PKA) inhibitor PKI added in the pipette solution did not affect the inhibitory effects on PF-PC mEPSCs induced by DAMGO. Both the PKA inhibitor K5720 and MEK inhibitor U0126 in artificial cerebrospinal fluid (ACSF) prevented the inhibitory effects of DAMGO on PF-PC mEPSCs. These findings reveal that MORs are expressed in presynaptic PF axon terminals, where DAMGO can activate presynaptic MORs to inhibit PF-PC synaptic transmission by regulating the release of glutamate. G-protein-dependent cAMP-PKA signaling pathway may be involved in this process.

Non-nociceptive roles of opioids in the CNS: opioids' effects on neurogenesis, learning, memory and affect

Mortality due to opioid use has grown to the point where, for the first time in history, opioid-related deaths exceed those caused by car accidents in many states in the United States. Changes in the prescribing of opioids for pain and the illicit use of fentanyl (and derivatives) have contributed to the current epidemic. Less known is the impact of opioids on hippocampal neurogenesis, the functional manipulation of which may improve the deleterious effects of opioid use. We provide new insights into how the dysregulation of neurogenesis by opioids can modify learning and affect, mood and emotions, processes that have been well accepted to motivate addictive behaviours.

Tramadol ameliorates behavioural, biochemical, mitochondrial and histological alterations in ICV-STZ-induced sporadic dementia of Alzheimer's type in rats

Alzheimer disease represents a major public health issue with limited therapeutic interventions. We explored the possibility of therapeutic approach by repurposing of tramadol in a sporadic animal model of Alzheimer's type. Streptozocin (STZ 3 mg/kg; bilaterally) was injected to male SD rats through intracerebroventricular (ICV) route. Drug treatment was started just after streptozocin administration and continued for 3 weeks. The rats were killed on the 21st day following the last behavioral test, and cytoplasmic fractions of the hippocampus and pre-frontal cortex were prepared for the quantification of acetylcholinesterase, oxidative stress parameter, mitochondrial enzymes activity and histological examination. Tramadol (5, 10 and 20 mg/kg, i.p.) was used as a treatment drug, and memantine (10 mg/kg, i.p.) was used as a standard. Tramadol significantly attenuated behavioral, biochemical, mitochondrial and histological alterations at low (5 mg/kg) and intermediate (10 mg/kg) dose, suggesting its neuroprotective potential in ICV-STZ-treated rats. Further, the neuroprotective effect of tramadol (10 mg/kg) was comparable to memantine (10 mg/kg). In conclusion, our results indicate the effectiveness of tramadol in preventing ICV-STZ-induced cognitive impairment as well as mito-oxidative stress. Further, these findings reveal the possibility of MOR agonist as a therapeutic approach for sporadic Alzheimer disease.

Naltrexone Facilitates Learning and Delays Extinction by Increasing AMPA Receptor Phosphorylation and Membrane Insertion

BACKGROUND

The opioid antagonists naloxone/naltrexone are involved in improving learning and memory, but their cellular and molecular mechanisms remain unknown. We investigated the effect of naloxone/naltrexone on hippocampal α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) trafficking, a molecular substrate of learning and memory, as a probable mechanism for the antagonists activity.

METHODS

To measure naloxone/naltrexone-regulated AMPAR trafficking, pHluorin-GluA1 imaging and biochemical analyses were performed on primary hippocampal neurons. To establish the in vivo role of GluA1-Serine 845 (S845) phosphorylation on the behavioral effect induced by inhibition of the endogenous μ-opioid receptor (MOR) by naltrexone, MOR knockout, and GluA1-S845A mutant (in which Ser(845) was mutated to Ala) mice were tested in a water maze after chronic naltrexone administration. Behavioral responses and GluA1 levels in the hippocampal postsynaptic density in wild-type and GluA1-S845A mutant mice were compared using western blot analysis.

RESULTS

In vitro prolonged naloxone/naltrexone exposure significantly increased synaptic and extrasynaptic GluA1 membrane expression as well as GluA1-S845 phosphorylation. In the MOR knockout and GluA1-S845A mutant mice, naltrexone did not improve learning, which suggests that naltrexone acts via inhibition of endogenous MOR action and alteration of GluA1 phosphorylation. Naltrexone-treated wild-type mice had significantly increased phosphorylated GluA1-S845 and GluA1 levels in their hippocampal postsynaptic density on the third day of acquisition, which is the time when naltrexone significantly improved learning.

CONCLUSIONS

The beneficial effect of naltrexone on spatial learning and memory under normal conditions appears to be the result of increasing GluA1-S845 phosphorylation-dependent AMPAR trafficking. These results can be further explored in a mouse model of memory loss.

Biochemical and pharmacological characterization of three toxic phospholipase A2s from Daboia russelii snake venom

Three isoenzymes of phospholipase A2 (PLA2), VRV-PL-IIIc, VRV-PL-VII, and VRV-PL-IX were isolated from Daboia russelii snake venom. The venom, upon gel filtration on Sephadex G-75 column, resolved into six peaks (DRG75 I-VI). The VRV-PL-IIIc was purified by subjecting DRG75II to homogeneity by rechromatography in the presence of 8M urea on Sephadex G-75 column. The other two isoenzymes VRV-PL-VII and VRV-PL-IX were purified by subjecting DRG75III to ion exchange chromatography on CM-Sephadex C-25 column. Mol wt. for the three PLA2s, VRV-PL-IIIc, VRV-PL-VII, and VRV-PL-IX are 13.003kDa, 13.100kDa and 12.531kDa respectively. The VRV-PL-IIIc is not lethal to mice up to 14mg/kg body weight but it affects blood sinusoids and causes necrosis of the hepatocytes in liver. It causes hemorrhage in kidney and shrinkage of renal corpuscles and renal tubules. The LD50s for VRV-PL-VII and VRV-PL-IX are 7 and 7.5mg/kg body weight respectively. They induced neurotoxic symptoms similar to VRV-PL-V. All the three PLA2s are anticoagulant and induced varying degree of edema in the foot pads of mice. VRV-PL-V and VRV-PL-VII are shown to act as pre and post synaptic toxins, while VRV-PL-IX acts as presynaptic toxin. This is evident from experiments conducted on cultured hippocampal neurons by patch clamp electrophysiology.

Sex differences of excitatory synaptic transmission in RA projection neurons of adult zebra finches

Zebra finches are ideal animals to investigate sex difference in songbirds. Only males can sing. The brain nuclei controlling song learning and production in males are considerably larger than in females. The robust nucleus of the arcopallium (RA) is a premotor nucleus, playing a key role in controlling singing. RA receives denser synapse inputs in males than in females. Sex differences of excitatory synaptic transmission in the RA projection neurons (PNs) have not been reported. In the present study, using whole-cell voltage-clamp recording, spontaneous EPSCs (sEPSCs) and miniature EPSCs (mEPSCs) of RA PNs in the intact males and females were recorded. The average frequency and amplitude of sEPSCs/mEPSCs in the intact males were higher than females. The half-width and decay time of sEPSCs/mEPSCs in the intact males were longer than females. In order to verify whether these sex differences related to sex steroids, males were castrated. The average frequency of sEPSCs/mEPSCs in castrated males was lower than intact males and was similar to in females; the amplitude was not changed after castrating. These results demonstrate the sexually dimorphic of the excitatory synaptic transmission in the RA PNs, the RA PNs in males receive more excitatory synaptic transmission and these sex differences were partly affected by sex hormones. These findings contribute to further illuminate the neural mechanisms under the sexually dimorphism in song production of adult zebra finches.

Interactions of HIV and drugs of abuse: the importance of glia, neural progenitors, and host genetic factors

Considerable insight has been gained into the comorbid, interactive effects of HIV and drug abuse in the brain using experimental models. This review, which considers opiates, methamphetamine, and cocaine, emphasizes the importance of host genetics and glial plasticity in driving the pathogenic neuron remodeling underlying neuro-acquired immunodeficiency syndrome and drug abuse comorbidity. Clinical findings are less concordant than experimental work, and the response of individuals to HIV and to drug abuse can vary tremendously. Host-genetic variability is important in determining viral tropism, neuropathogenesis, drug responses, and addictive behavior. However, genetic differences alone cannot account for individual variability in the brain "connectome." Environment and experience are critical determinants in the evolution of synaptic circuitry throughout life. Neurons and glia both exercise control over determinants of synaptic plasticity that are disrupted by HIV and drug abuse. Perivascular macrophages, microglia, and to a lesser extent astroglia can harbor the infection. Uninfected bystanders, especially astroglia, propagate and amplify inflammatory signals. Drug abuse by itself derails neuronal and glial function, and the outcome of chronic exposure is maladaptive plasticity. The negative consequences of coexposure to HIV and drug abuse are determined by numerous factors including genetics, sex, age, and multidrug exposure. Glia and some neurons are generated throughout life, and their progenitors appear to be targets of HIV and opiates/psychostimulants. The chronic nature of HIV and drug abuse appears to result in sustained alterations in the maturation and fate of neural progenitors, which may affect the balance of glial populations within multiple brain regions.

Opiate drug use and the pathophysiology of neuroAIDS

Opiate abuse and HIV-1 have been described as interrelated epidemics, and even in the advent of combined anti-retroviral therapy, the additional abuse of opiates appears to result in greater neurologic and cognitive deficits. The central nervous system (CNS) is particularly vulnerable to interactive opiate-HIV-1 effects, in part because of the unique responses of microglia and astroglia. Although neurons are principally responsible for behavior and cognition, HIV-1 infection and replication in the brain is largely limited to microglia, while astroglia and perhaps glial progenitors can be latently infected. Thus, neuronal dysfunction and injury result from cellular and viral toxins originating from HIV-1 infected/exposed glia. Importantly, subsets of glial cells including oligodendrocytes, as well as neurons, express µ-opioid receptors and therefore can be direct targets for heroin and morphine (the major metabolite of heroin in the CNS), which preferentially activate µ-opioid receptors. This review highlights findings that neuroAIDS is a glially driven disease, and that opiate abuse may act at multiple glial-cell types to further compromise neuron function and survival. The ongoing, reactive cross-talk between opiate drug and HIV-1 co-exposed microglia and astroglia appears to exacerbate critical proinflammatory and excitotoxic events leading to neuron dysfunction, injury, and potentially death. Opiates enhance synaptodendritic damage and a loss of synaptic connectivity, which is viewed as the substrate of cognitive deficits. We especially emphasize that opioid signaling and interactions with HIV-1 are contextual, differing among cell types, and even within subsets of the same cell type. For example, astroglia even within a single brain region are heterogeneous in their expression of µ-, δ-, and κ-opioid receptors, as well as CXCR4 and CCR5, and Toll-like receptors. Thus, defining the distinct targets engaged by opiates in each cell type, and among brain regions, is critical to an understanding of how opiate abuse exacerbates neuroAIDS.

Non-Coding RNAs Regulating Morphine Function: With Emphasis on the In vivo and In vitro Functions of miR-190

Non-coding RNAs (ncRNAs), especially microRNAs, are reported to be involved in a variety of biological processes, including several processes related to drug addiction. It has been suggested that the biological functions of opioids, one typical type of addictive drugs, are regulated by ncRNAs. In the current review, we examine a variety of mechanisms through which ncRNAs could regulate μ-opioid receptor (OPRM1) activities and thereby contribute to the development of opioid addiction. Using miR-23b as an example, we present the possible ways in which ncRNA-mediated regulation of OPRM1 expression could impact opioid addiction. Using miR-190 as an example, we demonstrate the critical roles played by ncRNAs in the signal cascade from receptor to systemic responses, including the possible modulation of adult neurogenesis and in vivo contextual memory. After discussing the possible targets of ncRNAs involved in the development of opioid addiction, we summarize the mechanisms underlying the interaction between ncRNAs and opioid addiction and present suggestions for further study.

Differential modulation of drug-induced structural and functional plasticity of dendritic spines

Drug-induced plasticity of excitatory synapses has been proposed to be the cellular mechanism underlying the aberrant learning associated with addiction. Exposure to various drugs of abuse causes both morphological plasticity of dendritic spines and functional plasticity of excitatory synaptic transmission. Chronic activation of μ-opioid receptors (MOR) in cultured hippocampal neurons causes two forms of synaptic plasticity: loss of dendritic spines and loss of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. With use of live imaging, patch-clamp electrophysiology, and immunocytochemistry, the present study reveals that these two forms of synaptic plasticity are mediated by separate, but interactive, intracellular signaling cascades. The inhibition of Ca(2+)/calmodulin-dependent protein kinase II with 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62) blocks MOR-mediated structural plasticity of dendritic spines, but not MOR-mediated cellular redistribution of GluR1 and GluR2 AMPA receptor subunits. In contrast, the inhibition of calcineurin with tacrolimus (FK506) blocks both cellular processes. These findings support the idea that drug-induced structural and functional plasticity of dendritic spines is mediated by divergent, but interactive, signaling pathways.

Morphine modulation of thrombospondin levels in astrocytes and its implications for neurite outgrowth and synapse formation

Opioid receptor signaling via EGF receptor (EGFR) transactivation and ERK/MAPK phosphorylation initiates diverse cellular responses that are cell type-dependent. In astrocytes, multiple μ opioid receptor-mediated mechanisms of ERK activation exist that are temporally distinctive and feature different outcomes. Upon discovering that chronic opiate treatment of rats down-regulates thrombospondin 1 (TSP1) expression in the nucleus accumbens and cortex, we investigated the mechanism of action of this modulation in astrocytes. TSP1 is synthesized in astrocytes and is released into the extracellular matrix where it is known to play a role in synapse formation and neurite outgrowth. Acute morphine (hours) reduced TSP1 levels in astrocytes. Chronic (days) opioids repressed TSP1 gene expression and reduced its protein levels by μ opioid receptor and ERK-dependent mechanisms in astrocytes. Morphine also depleted TSP1 levels stimulated by TGFβ1 and abolished ERK activation induced by this factor. Chronic morphine treatment of astrocyte-neuron co-cultures reduced neurite outgrowth and synapse formation. Therefore, inhibitory actions of morphine were detected after both acute and chronic treatments. An acute mechanism of morphine signaling to ERK that entails depletion of TSP1 levels was suggested by inhibition of morphine activation of ERK by a function-blocking TSP1 antibody. This raises the novel possibility that acute morphine uses TSP1 as a source of EGF-like ligands to activate EGFR. Chronic morphine inhibition of TSP1 is reminiscent of the negative effect of μ opioids on EGFR-induced astrocyte proliferation via a phospho-ERK feedback inhibition mechanism. Both of these variations of classical EGFR transactivation may enable opiates to diminish neurite outgrowth and synapse formation.

Interactive comorbidity between opioid drug abuse and HIV-1 Tat: chronic exposure augments spine loss and sublethal dendritic pathology in striatal neurons

HIV-1 infection predisposes the central nervous system to damage by opportunistic infections and environmental insults. Such maladaptive plasticity may underlie the exaggerated comorbidity seen with HIV-1 infection and opioid abuse. Although morphine and HIV-1 Tat synergize at high concentrations to increase neuronal death in vitro, we questioned whether chronic low Tat exposure in vivo might contribute to the spectrum of neuropathology through sublethal neuronal injury. We used a doxycycline-driven, inducible, HIV-1 Tat transgenic mouse, in which striatal neuron death was previously shown to be absent, to examine effects of differential Tat expression, alone and combined with morphine. Low constitutive Tat expression caused neurodegeneration; higher levels induced by 7 days of doxycycline significantly reduced dendritic spine numbers. Moreover, Tat expression widely disrupted the endogenous opioid system, altering mu and kappa, but not delta, opioid receptor and proopiomelanocortin, proenkephalin, and prodynorphin transcript levels in cortex, hippocampus, and striatum. In addition to markedly reducing spine density by itself, morphine amplified the effect of higher levels of Tat on spines, and also potentiated Tat-mediated dendritic pathology, thus contributing to maladaptive neuroplasticity at multiple levels. The dendritic pathology and reductions in spine density suggest that sustained Tat +/- morphine exposure underlie key aspects of chronic neurodegenerative changes in neuroAIDS, which may contribute to the exacerbated neurological impairment in HIV patients who abuse opioids.

Modulations of NeuroD activity contribute to the differential effects of morphine and fentanyl on dendritic spine stability

The cellular level of neurogenic differentiation 1 (NeuroD) is modulated differentially by mu-opioid receptor agonists; fentanyl increases NeuroD level by reducing the amount of microRNA-190 (miR-190), an inhibitor of NeuroD expression, whereas morphine does not alter NeuroD level. In the current study, NeuroD activity was demonstrated to be also under agonist-dependent regulation. After 3 d of treatment, morphine and fentanyl decreased the activity of the Ca(2+)/calmodulin-dependent protein kinase II alpha (CaMKIIalpha), which phosphorylates and activates NeuroD. Because NeuroD activity is determined by both the CaMKIIalpha activity and the cellular NeuroD level, the overall NeuroD activity was reduced by morphine, but maintained during fentanyl treatment. The differential effects of agonists on NeuroD activity were further confirmed by measuring the mRNA levels of four NeuroD downstream targets: doublecortin, Notch1, neurogenic differentiation 4, and Roundabout 1. Decreased dendritic spine stability and mu-opioid receptor signaling capability were also observed when NeuroD activity was attenuated by miR-190 overexpression or treatment with KN93, a CaMKIIalpha inhibitor. The decrease could be rescued by NeuroD overexpression, which restored NeuroD activity to the basal level. Furthermore, elevating NeuroD activity attenuated the morphine-induced decrease in dendritic spine stability. Therefore, by regulating NeuroD activity, mu-opioid receptor agonists modulate the stability of dendritic spines.

Agonist-selective signaling of G protein-coupled receptor: mechanisms and implications

Agonist-selective signaling or ligand-biased signaling of G protein-coupled receptor (GPCR) has become the focus of an increasing number of laboratories. The principle of this concept is that agonist possesses different abilities to activate different signaling pathways. Current review summarizes the observations of agonist-selective signaling of various GPCRs, indicating the significance of agonist-selective signaling in biological processes. In addition, current review also provides an overview on how agonist-selective signaling is initiated. Especially, the relationship between GPCR-G protein interaction and GPCR-beta-arrestin interaction is discussed in depth.

Bidirectional effects of fentanyl on dendritic spines and AMPA receptors depend upon the internalization of mu opioid receptors

Fentanyl is a frequently used and abused opioid analgesic and can cause internalization of mu opioid receptors (MORs). Receptor internalization modulates the signaling pathways of opioid receptors. As changes in dendritic spines and synaptic AMPA receptors play important roles in addiction and memory loss, we investigated how fentanyl affects dendritic spines and synaptic AMPA receptors in cultured hippocampal neurons. Fentanyl at low concentrations (0.01 and 0.1 microM) caused the collapse of dendritic spines and decreased the number of AMPA receptor clusters. In contrast, fentanyl at high concentrations (1 and 10 microM) had opposite effects, inducing the emergence of new spines and increasing the number of AMPA receptor clusters. These dose-dependent bidirectional effects of fentanyl were blocked by a selective MOR antagonist CTOP at 5 microM. In neurons that had been transfected with HA-tagged or GFP-tagged MORs, fentanyl at high concentrations induced persistent and robust internalization of MORs, whereas fentanyl at lower concentrations induced little or transient receptor internalization. The blockade of receptor internalization with the expression of dominant-negative Dynamin I (the K44E mutant) reversed the effect of fentanyl at high concentrations, supporting a role of receptor internalization in modulating the dose-dependent effects of fentanyl. In contrast to morphine, the effects of fentanyl on dendritic spines are distinctively bidirectional and concentration dependent, probably due to its ability to induce robust internalization of MORs at high concentrations. The characterization of the effects of fentanyl on spines and AMPA receptors may help us understand the roles of MOR internalization in addiction and cognitive deficits.

Cell-dependent physiological synaptic action of morphine in the rat habenular nucleus: morphine both inhibits and facilitates excitatory synaptic transmission

Although several lines of evidence have suggested that the activity of thalamic neurons is modulated by opioids, the mechanism by which morphine in the thalamus regulates the release of excitatory neurotransmitters remains unclear. In the present study, we investigated the synaptic modulation of morphine to regulate excitatory synaptic transmission, probably glutamatergic transmission, in habenular nucleus (Hb) and centrolateral nucleus (CL) neurons in the rat thalamus. Using the whole-cell patch-clamp technique, we found dual modulation by morphine in Hb neurons: morphine caused either inhibition or facilitation of the miniature excitatory postsynaptic current (mEPSC) frequency in the Hb. In Hb neurons that showed a morphine-induced decrease in the mEPSC frequency, the mEPSC amplitude was also decreased in the presence of morphine. In contrast, the mEPSC amplitude was markedly increased in Hb neurons that showed a morphine-induced increase in the mEPSC frequency. We also observed a significant decrease in the mEPSC frequency with morphine in CL neurons without any change in the mEPSC amplitude, whereas morphine did not facilitate the mEPSC frequency in CL neurons. These results suggest that morphine may induce cell-dependent dual modulation of glutamatergic synaptic transmission in the Hb.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 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.

Distinct subcellular distribution of delta-opioid receptor fused with various tags in PC12 cells

In small dorsal root ganglion neurons, delta-opioid receptors (DORs) have been found to be mainly distributed in the cytoplasm and often associated with the membrane of large dense-core vesicles (LDCVs) that contain neuropeptides. To study the distribution of DORs under various physiological or pharmacological conditions, the receptors fused with different tags are constructed, transfected into cells or animals, and examined with microscopy. In this study, we show that DOR with different tags have distinct patterns of subcellular distribution in neuroendocrine cells, PC12 cells. Both immunostaining and vesicle fraction analysis showed that the native DORs expressed in PC12 cells were mainly associated with LDCVs. In transfected PC12 cells, DOR tagged with Myc or hemagglutinin exhibited LDCV localization. However, DOR fused with GFP at N- or C-terminus was found to be mainly localized on the cell surface, and mediated the function of DOR agonist. Therefore, the distribution of DOR fused with GFP differs from the native DORs. These results suggest that the subcellular distribution of the receptor could be better presented by the fused tag with smaller molecular size.

Distinct effects of individual opioids on the morphology of spines depend upon the internalization of mu opioid receptors

This study has examined the relationship between the effects of opioids on the internalization of mu opioid receptors (MORs) and the morphology of dendritic spines. Several opioids (morphine, etorphine, DAMGO or methadone) were applied to cultured hippocampal neurons. Live imaging and biochemical techniques were used to examine the dynamic changes in MOR internalization and spine morphology. This study reveals that MOR internalization can regulate opioid-induced morphological changes in dendritic spines: (1) Chronic treatment with morphine, which induced minimal receptor internalization, caused collapse of dendritic spines. In contrast, "internalizing" opioids such as DAMGO and etorphine induced the emergence of new spines. It reveals that opioid-induced changes in spines vary greatly depending on how the applied opioid agonist affects MOR internalization. (2) The blockade of receptor internalization by dominant negative mutant of dynamin, K44E, reversed the effects of DAMGO and etorphine. It indicates that receptor internalization is necessary for the distinct effects of DAMGO and etorphine on spines. (3) In neurons that were cultured from MOR knock-out mice and had been co-transfected with DsRed and MOR-GFP, morphine caused collapse of spines whereas DAMGO induced emergence of new spines, indicating that opioids can alter the structure of spines via postsynaptic MORs. (4) Methadone at a low concentration induced minimal internalization and had effects that were similar to morphine. At a high concentration, methadone induced robust internalization and had effects that are opposite to morphine. The concentration-dependent opioid-induced changes in dendritic spines might also contribute to the variation in the effects of individual opioids.