Morphine withdrawal increases glutamate uptake and surface expression of glutamate transporter GLT1 at hippocampal synapses

Interaction between the glutamate transporter GLT1b and the synaptic PDZ domain protein PICK1

Synaptic plasticity is implemented by the interaction of glutamate receptors with PDZ domain proteins. Glutamate transporters provide the only known mechanism of clearance of glutamate from excitatory synapses, and GLT1 is the major glutamate transporter. We show here that GLT1 interacts with the PDZ domain protein PICK1, which plays a critical role in regulating the expression of glutamate receptors at excitatory synapses. A yeast two-hybrid screen of a neuronal library using the carboxyl tail of GLT1b yielded clones expressing PICK1. The GLT1b C-terminal peptide bound to PICK1 with high affinity (K(i) = 6.5 +/- 0.4 microM) in an in vitro fluorescence polarization assay. We also tested peptides based on other variants of GLT1 and other glutamate transporters. GLT1b co-immunoprecipitated with PICK1 from rat brain lysates and COS7 cell lysates derived from cells transfected with plasmids expressing PICK1 and GLT1b. In addition, expression of GLT1b in COS7 cells changed the distribution of PICK1, bringing it to the surface. GLT1b and PICK1 co-localized with each other and with synaptic markers in hippocampal neurons in culture. Phorbol ester, an activator of protein kinase C (PKC), a known PICK1 interactor, had no effect on glutamate transport in rat forebrain neurons in culture. However, we found that exposure of neurons to a myristolated decoy peptide with sequence identical to the C-terminal sequence of GLT1b designed to block the PICK1-GLT1b interaction rendered glutamate transport into neurons responsive to phorbol ester. These results suggest that the PICK1-GLT1b interaction regulates the modulation of GLT1 function by PKC.

Recent progress in the research field of neuropharmacology in China

In recent years, Chinese neuropharmacologists have done a lot of basic and practical work in neuropharmacology, especially in the fields of pain, drug dependence, depression, Alzheimer's disease, schizophrenia, having obtained some exciting results that are of great significance for the development of neuropharmacology. Here I would like to review recent progress in the research fields of neuropharmacology in China.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Glutamate transporters: confining runaway excitation by shaping synaptic transmission

Traditionally, glutamate transporters have been viewed as membrane proteins that harness the electrochemical gradient to slowly transport glutamate from the extracellular space into glial cells. However, recent studies have shown that glutamate transporters on glial and neuronal membranes also rapidly bind released glutamate to shape synaptic transmission. In this Review, we summarize the properties of glutamate transporters that influence synaptic transmission and are subject to regulation and plasticity. We highlight how the diversity of glutamate-transporter function relates to transporter location, density and affinity.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and its dysregulation has been associated with multiple psychiatric and neurological disorders. However, investigation of this molecule has been complicated by its complex pattern of alternative splicing, including three coding isoforms and multiple 5'- and 3'-UTRs that may have a regulatory function. It is likely that these sequences permit modulation of EAAT2 expression with spatial, temporal and or activity-dependent specificity; however, few studies have attempted to delineate the function of these sequences. Additionally, there are problems with the use of antibodies to study protein localization, possibly due to posttranslational modification of critical amino acid residues. This review describes what is currently known about the regulation of EAAT2 mRNA and protein isoforms and concludes with a summary of studies showing dysregulation of EAAT2 in psychiatric and neurological disorders. EAAT2 has been either primarily or secondarily implicated in a multitude of neuropsychiatric diseases in addition to the normal physiology of learning and memory. Thus, this molecule represents an intriguing therapeutic target once we improve our understanding of how it is regulated under normal conditions.

Some recent advances in basic neuroscience research in China

Neuroscience as a distinct discipline or research programme has been a rather recent event in most Chinese universities and in the Chinese Academy of Sciences. However, the last few years have witnessed increased funding and an improved research environment for neuroscience, both of which facilitated an influx of Chinese neuroscientists trained abroad. In this review, we have highlighted some recent research advances made by neuroscientists in China. Based on our own expertise, this review is focused mainly on findings that have contributed to our understanding of the mechanisms underlying brain development, neural plasticity and cognitive processes, and neural degeneration.

Pseudoginsenoside-F11 decreases morphine-induced behavioral sensitization and extracellular glutamate levels in the medial prefrontal cortex in mice

Morphine produces a variety of behavioral and biochemical changes related to its abuse. Our previous studies showed that Pseudoginsenoside-F11 (PF11), an ocotillol-type saponin existing in American ginseng, can antagonize pharmacological effects of morphine. To further investigate the effects of PF11 on morphine abuse and the underlying mechanisms, we tested the effects of PF11 on morphine-induced development of behavioral sensitization and alterations in glutamate levels in the medial prefrontal cortex (mPFC) in freely moving mice by using in vivo microdialysis. As the results shown, PF11 antagonized the development of behavioral sensitization and decrease of glutamate in the mPFC induced by morphine. Therefore, these findings suggest that PF11 may block the development of morphine-induced behavioral sensitization via its effect, at least partially, on the glutamatergic system in the mPFC.

Neurochemical changes in brain induced by chronic morphine treatment: NMR studies in thalamus and somatosensory cortex of rats

To investigate the effects of chronic morphine treatment and its cessation on thalamus and the somatosensory cortex, an ex vivo high resolution (500 MHz) (1)H nuclear magnetic resonance spectroscopy (NMRS), in the present study, was applied to detect multiple alterations of neurochemicals and/or neurometabolites in the rats. Ten days of chronic morphine administration was observed to markedly increase the total amount of lactate (Lac), myo-inositol (my-Ins) (each P < 0.01) and aspartate (Asp) (P < 0.05), and significantly decrease that of glutamate (Glu) and glutamine (Gln) in the rats thalamus (each P < 0.05). In the somatosensory cortex, chronic morphine was shown to increase the level of Lac and my-Ins, and decrease that of Glu (each P < 0.05). Interestingly, the ratio of Glu/GABA was found to decrease in these two brain areas after chronic morphine treatment, and among the detectable neurochemicals in those two cerebral areas, only taurine (Tau) showed to result in a significant increment in thalamus during the process of morphine discontinuation (P < 0.05). Moreover, the alterations of multiple neurochemicals due to chronic morphine exhibited a tendency of recovery to the normal level over the course of morphine withdrawal. The results suggested that, in thalamus and the somatosensory cortex, chronic morphine administration and its cessation could induce multiple neurochemical changes, which may involve in the brain energy metabolism, activity and transition of neurotransmitters.

Morphine Administration Alters the Profile of Hippocampal Postsynaptic Density-associated Proteins

Numerous studies have shown that drugs of abuse induce changes in protein expression in the brain that are thought to play a role in synaptic plasticity. Drug-induced plasticity can be mediated by changes at the synapse and more specifically at the postsynaptic density (PSD), which receives and transduces synaptic information. To date, the majority of studies examining synaptic protein profiles have focused on identifying the synaptic proteome. Only a handful of studies have examined the changes in synaptic profile by drug administration. We applied a quantitative proteomics analysis technique with the cleavable ICAT reagent to quantitate relative changes in protein levels of the hippocampal PSD in response to morphine administration. We identified a total of 102 proteins in the mouse hippocampal PSD. The majority of these were signaling, trafficking, and cytoskeletal proteins involved in synaptic plasticity, learning, and memory. Among the proteins whose levels were found to be altered by morphine administration, clathrin levels were increased to the largest extent. Immunoblotting and electron microscopy studies showed that this increase was localized to the PSD. Morphine treatment was also found to lead to a local increase in two other components of the endocytic machinery, dynamin and AP-2, suggesting a critical involvement of the endocytic machinery in the modulatory effects of morphine. Because alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are thought to undergo clathrin-mediated endocytosis, we examined the effect of morphine administration on the association of the AMPA receptor subunit, GluR1, with clathrin. We found a substantial decrease in the levels of GluR1 associated with clathrin. Taken together, these results suggest that, by causing a redistribution of endocytic proteins at the synapse, morphine modulates synaptic plasticity at hippocampal glutamatergic synapses.

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

Although chronic treatment with morphine is known to alter the function and morphology of excitatory synapses, the effects of other opioids on these synapses are not clear. Here we report distinct effects of several opioids (morphine, [d-ala(2),me-phe(4),gly(5)-ol]enkephalin (DAMGO), and etorphine) on miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons: 1) chronic treatment with morphine for >3 days decreased the amplitude, frequency, rise time and decay time of mEPSCs. In contrast, "internalizing" opioids such as etorphine and DAMGO increased the frequency of mEPSCs and had no significant effect on the amplitude and kinetics of mEPSCs. These results demonstrate that different opioids can have distinct effects on the function of excitatory synapses. 2) mu opioid receptor fused with green fluorescence protein (MOR-GFP) is clustered in dendritic spines in most hippocampal neurons but is concentrated in axon-like processes in striatal and corticostriatal nonspiny neurons. It suggests that MORs might mediate pre- or postsynaptic effects depending on cell types. 3) Neurons were cultured from MOR knock-out mice and were exogenously transfected with MOR-GFP. Chronic treatment with morphine suppressed mEPSCs only in neurons that contained postsynaptic MOR-GFP, indicating that opioids can modulate excitatory synaptic transmission postsynaptically. 4) Morphine acutely decreased mEPSC amplitude in neurons expressing exogenous MOR-GFP but had no effect on neurons expressing GFP. It indicates that the low level of endogenous MORs could only allow slow opioid-induced plasticity of excitatory synapses under normal conditions. 5) A theoretical model suggests that morphine might affect the function of spines by decreasing the electrotonic distance from synaptic inputs to the soma.

Regulation of the glutamate transporter EAAC1 by expression and activation of delta-opioid receptor

Glutamate transporters play an essential role in regulating temporal and spatial glutamate concentration in the synaptic cleft. The excitatory amino acid carrier 1 (EAAC1) is a neuronal-specific transporter. Opioid receptors are found to modulate the glutamatergic system in brain, but the regulatory mechanism remains to be investigated. Here we report that the delta-opioid receptor (DOR) co-expressed with EAAC1 in Xenopus oocytes, but not the micro-opioid receptor, down-regulates EAAC1 function, and that [d-Pen(2,5)]-enkephalin stimulation of DOR can counteract the down-regulation of the EAAC1-mediated uptake. Results from co-immunoprecipitation and immunofluorescence microscopy in both oocytes and rat hippocampal neurons indicate co-localization and suggest direct interaction between DOR and EAAC1. The results suggest that DOR can reduce EAAC1 function by direct protein-protein interaction and that activation of DOR releases the inhibitory interaction. We suggest that this mechanism may be important for modulation of the glutamatergic system following opioid stimulation.

SNAP-25/syntaxin 1A complex functionally modulates neurotransmitter gamma-aminobutyric acid reuptake

Neurotransmitter gamma-aminobutyric acid (GABA) release to the synaptic clefts is mediated by the formation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which includes two target SNAREs syntaxin 1A and SNAP-25 and one vesicle SNARE VAMP-2. The target SNAREs syntaxin 1A and SNAP-25 form a heterodimer, the putative intermediate of the SNARE complex. Neurotransmitter GABA clearance from synaptic clefts is carried out by the reuptake function of its transporters to terminate the postsynaptic signaling. Syntaxin 1A directly binds to the neuronal GABA transporter GAT-1 and inhibits its reuptake function. However, whether other SNARE proteins or SNARE complex regulates GABA reuptake remains unknown. Here we demonstrate that SNAP-25 efficiently inhibits GAT-1 reuptake function in the presence of syntaxin 1A. This inhibition depends on SNAP-25/syntaxin 1A complex formation. The H3 domain of syntaxin 1A is identified as the binding sites for both SNAP-25 and GAT-1. SNAP-25 binding to syntaxin 1A greatly potentiates the physical interaction of syntaxin 1A with GAT-1 and significantly enhances the syntaxin 1A-mediated inhibition of GAT-1 reuptake function. Furthermore, nitric oxide, which promotes SNAP-25 binding to syntaxin 1A to form the SNARE complex, also potentiates the interaction of syntaxin 1A with GAT-1 and suppresses GABA reuptake by GAT-1. Thus our findings delineate a further molecular mechanism for the regulation of GABA reuptake by a target SNARE complex and suggest a direct coordination between GABA release and reuptake.

Gene transfer of GLT-1, a glutamate transporter, into the nucleus accumbens shell attenuates methamphetamine- and morphine-induced conditioned place preference in rats

Several lines of evidence have suggested that the glutamatergic system in the nucleus accumbens (NAc) plays an important role in the conditioned rewarding effect of drugs of abuse. In addition, it is recognized that extracellular glutamate is rapidly removed from the synaptic cleft by Na+-dependent glutamate transporters in neurons and glial cells, thereby maintaining physiological levels of glutamate. We previously reported that activation of glutamate uptake by a glutamate transporter activator attenuated the acquisition of conditioned place preference induced by methamphetamine and morphine in mice. In the present study, we examined the effects of gene transfer of a glial glutamate transporter, GLT-1, into the NAc shell by recombinant adenoviruses on methamphetamine- and morphine-induced conditioned place preference in rats. Bilateral infusion of the recombinant adenoviruses into the NAc shell efficiently increased GLT-1 expression surrounding the infusion site, at least during the period 2-8 days after the infusion. In the conditioned place preference paradigm, animals were conditioned with repeated subcutaneous injections of methamphetamine (2 mg/kg) or morphine (3 mg/kg). Intra-NAc shell overexpression of GLT-1 before the conditioning significantly attenuated the conditioned place preference induced by methamphetamine or morphine, when compared with control. However, it had no effect on the somatic signs of naloxone-precipitated morphine withdrawal. These results suggest that GLT-1 within the NAc shell plays an inhibitory role in the conditioned rewarding effects of methamphetamine and morphine but not the physical dependence on morphine.

Quantitative analysis of glutamate transporter mRNA expression in prefrontal and primary visual cortex in normal and schizophrenic brain

Abnormalities of the glutamatergic system in schizophrenia have been identified in numerous studies, but little is known about the role of glutamate transporters and their messenger RNA (mRNA) expression. In addition, the abundances of the two major isoforms of human excitatory amino acid transporter 2 (EAAT2) or its rat ortholog, glutamate transporter 1, have never been compared in a quantitative manner. Using quantitative reverse transcription-polymerase chain reaction, we established that the expression of the EAAT1, EAAT2a, EAAT2b, and EAAT3 transcripts was not different in the dorsolateral prefrontal and primary visual cortices of persons with schizophrenia relative to matched controls. EAAT2a expression was about 25-fold and 10-fold higher than EAAT2b in human and rat brain, respectively. The data provided no evidence of an effect of antipsychotic medications on the mRNA expression of the glutamate transporters. However, because most of the schizophrenic subjects in the cohort had been treated with antipsychotics for many years, it is still possible that changes in transporter expression were masked by medication effects.

Ascorbic acid decreases morphine self-administration and withdrawal symptoms in rats

Recent studies have indicated that the glutamatergic system is involved in the motivational aspects during the initiation of drug self-administration. Ascorbic acid (AA), an antioxidant vitamin, is released from glutamatergic neurons, and it modulates the synaptic action of dopamine and glutamate. In this study the AA effects on the self-administration of morphine and on the morphine withdrawal syndrome have been investigated. Wistar rats were allowed to self-administer morphine (1 mg/infusion) during 10 consecutive days for 2 h/session. The number of lever pressings was recorded. An intrapritoneal AA injection (500 mg/kg, i.p.), 30 min before morphine self-administration produced a significant decrease in the initiation of morphine self administration during all sessions. After the last test session morphine withdrawal symptom signs (MWS) were recorded after naloxone precipitation. Most of MWS (but not all) were decreased by AA application. In conclusion, AA may change the motivational processes underlying the morphine self-administration.

Ethanol exposure decreases glutamate uptake in the nucleus accumbens

BACKGROUND

An increased level of extracellular glutamate is a key neurochemical feature associated with ethanol exposure and withdrawal.

METHODS

In the current study, extracellular levels of glutamate and glutamate transport in the nucleus accumbens were assayed 24 hr after repeated ethanol exposure (1 g/kg ip daily for 7 days) with use of in vivo no-net-flux microdialysis and in vitro [(3)H]glutamate uptake, respectively.

RESULTS

Microdialysis revealed higher extracellular glutamate concentrations in the nucleus accumbens of rats that were given ethanol. The increase in basal extracellular glutamate levels was accounted for in part by a decrease in the in vivo probe recovery of glutamate. Moreover, an in vitro accumbens slice preparation measuring [(3)H]glutamate uptake revealed that Na(+)-dependent [(3)H]glutamate uptake was significantly reduced 24 hr after 7 days of repeated ethanol exposure. The ethanol-induced deficit in glutamate uptake was not associated with decreased total tissue levels of the transporters GLAST or GLT1. The in vivo and in vitro ethanol-induced changes in glutamate levels and uptake returned to control levels 14 days after discontinuing 7 days of repeated ethanol exposure.

CONCLUSIONS

These results suggest that the previously reported increases in extracellular glutamate induced by ethanol exposure may be due in part to deficits in glutamate transport.

Morphine decreases extracellular levels of glutamate in the anterior cingulate cortex: an in vivo microdialysis study in freely moving rats

In the present study, we investigated the effect of morphine on the extracellular levels of glutamate in the anterior cingulate cortex (ACC) in freely moving rats using in vivo microdialysis coupled to high performance liquid chromatography and electrochemical detection. The results showed that either acute or chronic morphine treatment decreased the extracellular levels of glutamate in the ACC. Naloxone could reverse the decrease induced by chronic morphine treatment. The present study provided the first neurochemical evidence that morphine decreased extracellular levels of glutamate in the ACC, suggesting that glutamate in ACC is involved in the central actions of morphine.

Morphine modulates glutamate release in the hippocampal CA1 area in mice

Opiate abuse is associated with long-lasting neural adaptative changes in the brain. Increasing evidence demonstrates that opiates significantly alter the function of the glutamatergic system, while how the system is regulated still remains elusive. In the present study, we studied the effect of morphine on extracellular glutamate concentration in the hippocampus, a nucleus rich of the glutamatergic neurons. The results showed that glutamate concentration in the extracellular fluid of the hippocampus was decreased following either acute or chronic treatment of morphine. However, naloxone-induced withdrawal increased glutamate concentration significantly. These results suggest an adaptation of the glutamatergic neuronal transmission in the hippocampus after morphine treatment.

Effect of MS-153, a glutamate transporter activator, on the conditioned rewarding effects of morphine, methamphetamine and cocaine in mice

There is a body of evidence implying the involvement of the glutamatergic system in the conditioned rewarding effects of drugs of abuse. It is recognized that the release of extracellular glutamate from nerve terminals is counterbalanced by the functions of neuronal and glial glutamate transporters. In the present study, we investigated the effects of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a glutamate transporter activator, on the induction of the conditioned place preference to morphine, methamphetamine and cocaine in mice. In the conditioned place preference paradigm, mice were conditioned with repeated subcutaneous injections of morphine (5 mg/kg), methamphetamine (2 mg/kg) or cocaine (8 mg/kg) in combination with or without MS-153 (3 and 10 mg/kg). Co-administration of MS-153 at a dose of 10 mg/kg, but not 3 mg/kg, significantly attenuated the induction of conditioned place preference to morphine, methamphetamine and cocaine. However, MS-153 itself produced neither conditioned place preference nor aversion. On the other hand, co-administration of MS-153 (10 mg/kg) did not alter the acute locomotor activation elicited by a single injection of morphine, methamphetamine and cocaine. These results suggest that MS-153, a glutamate transporter activator, has an inhibitory effect on the conditioned rewarding effects of morphine, methamphetamine and cocaine without affecting their acute locomotor responses.

Evidence for a long-term influence on morphine tolerance after previous morphine exposure: role of neuronal glucocorticoid receptors

Opioid analgesic tolerance is a pharmacological phenomenon that overtime diminishes the opioid analgesic effect. However, it remains unknown as to whether a previous opioid exposure would have a long-term influence on opioid tolerance upon subsequent opioid administration. Here, we show that the onset and degree of antinociceptive tolerance to a subsequent cycle of morphine exposure were substantially exacerbated in rats made tolerant to and then recovered from previous morphine administration, indicating a long-term influence from a previous morphine exposure on the development of morphine tolerance. Mechanistically, morphine exposure induced a cyclic AMP and protein kinase A-dependent upregulation of neuronal glucocorticoid receptors (GR) within the spinal cord dorsal horn, which was maintained after discontinuation of morphine administration and significantly enhanced upon a second cycle of morphine exposure. Prevention of the GR upregulation with GR antisense oligonucleotides as well as inhibition of GR activation with the GR antagonist RU38486 effectively prevented the exacerbated morphine tolerance after subsequent cycles of morphine exposure. The results indicate that a previous morphine exposure could induce lasting cellular changes mediated through neuronal GR and influence morphine analgesia upon a subsequent exposure. These findings may have significant implications in clinical opioid therapy and substance abuse.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 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).

Inhibition of SNAP-25 Phosphorylation at Ser187 Is Involved in Chronic Morphine-induced Down-regulation of SNARE Complex Formation

Opiate abuse has been shown to cause adaptive changes in presynaptic release and protein phosphorylation-mediated synaptic plasticity, but the underlying mechanisms remain unclear. Neuronal SNARE proteins serve as important regulatory molecules underlying neural plasticity in view of their major role in the process of neurotransmitter release. In the present study, the expression of SNAP-25, a t-SNARE protein essential for vesicle release, was found to be dramatically regulated in hippocampus after chronic morphine treatment, which was visualized with two-dimensional gel electrophoresis. The spots of SNAP-25 in the gel were shifted along the dimension of isoelectric point, indicating a likely change of the post-transcriptional modification. Immunoblotting analysis with specific antibody to Ser187, a protein kinase C (PKC) phosphorylation site of SNAP-25, revealed that the specific phosphorylation was correspondingly decreased, which was correlated with morphine-induced inhibition of PKC activity. Moreover, the level of ternary complex of SNARE proteins in either synaptosomes or PC12 cells was significantly reduced after chronic morphine treatment. This suggests a causal relationship between the inhibition of PKC-dependent SNAP-25 phosphorylation and the down-regulation of SNARE complex formation after chronic morphine treatment. Further analysis of SNARE complex formed by transfection of the wild-type or Ser187 mutants of SNAP-25 showed that only wild-type-formed complex was inhibited by morphine treatment. Thus, these results indicate that chronic morphine treatment inhibits phosphorylation of SNAP-25 at Ser187 and leads to a down-regulation of SNARE complex formation, which presents a potential molecular mechanism for the alteration of exocytotic process and neural plasticity during opiate abuse.