Single Morphine Exposure Research Articles

The roles of ubiquitin-proteasome system and regulator of G protein signaling 4 in behavioral sensitization induced by a single morphine exposure.

Opioid addiction is a major public health issue, yet its underlying mechanism is still unknown. The aim of this study was to explore the roles of ubiquitin-proteasome system (UPS) and regulator of G protein signaling 4 (RGS4) in morphine-induced behavioral sensitization, a well-recognized animal model of opioid addiction. We explored the characteristics of RGS4 protein expression and polyubiquitination in the development of behavioral sensitization induced by a single morphine exposure in rats, and the effect of a selective proteasome inhibitor, lactacystin (LAC), on behavioral sensitization. Polyubiquitination expression was increased in time-dependent and dose-related fashions during the development of behavioral sensitization, while RGS4 protein expression was not significantly changed during this phase. Stereotaxic administration of LAC into nucleus accumbens (NAc) core inhibited the establishment of behavioral sensitization. UPS in NAc core is positively involved in behavioral sensitization induced by a single morphine exposure in rats. Polyubiquitination was observed during the development phase of behavioral sensitization, while RGS4 protein expression was not significantly changed, indicating that other members of RGS family might be substrate proteins in UPS-mediated behavioral sensitization.

Acute morphine injection persistently affects the electrophysiological characteristics of rat locus coeruleus neurons

Administration of morphine is associated with critical complications in clinic which primarily includes the development of dependence and tolerance even following a single dose (acute) exposure. Behavioral and electrophysiological studies support the significant role of locus coeruleus (LC) neurons in tolerance and dependence following chronic morphine exposure. The current study was designed to explore the electrophysiological properties of the LC neurons following acute morphine exposure. In-vitro whole-cell patch-clamp recordings were performed in LC neurons 24 h after intraperitoneal morphine injection. Acute morphine injection significantly decreased the spontaneous firing rate of LC neurons, the rising and decay slopes of action potentials, and consequently increased the action potential duration. In addition, morphine treatment did not alter the rheobase current and first spike latency while affected the inhibitory postsynaptic currents elicited in response to orexin-A. In fact, single morphine exposure could inhibit the disinhibitory effect of orexin-A on LC neurons.

The development of behavioral sensitization induced by a single morphine exposure in adult zebrafish (Danio rerio)

BackgroundAccumulating evidence suggest that behavioral sensitization is involved in the process of drug addiction. Zebrafish are sensitive to a variety of addictive drugs and are thus suitable for the study of behavioral sensitization. However, in contrast to mature rodent models of behavioral sensitization, how this phenomenon manifests in aquatic organisms, especially zebrafish, is largely unknown. In this study, we developed a morphine-induced behavioral sensitization adult zebrafish model and performed a preliminary investigation of the underlying mechanisms. MethodsBehavioral sensitization was established in zebrafish by observing their behavior after treatment and challenge with morphine. The effect of morphine was evaluated by a behavioral locomotor test. Different doses of morphine and withdrawal times were used to evaluate the establishment of the behavioral sensitization model. ResultsHyperlocomotion was induced after administration of morphine in adult zebrafish. After withdrawing the drug for a period, challenge with low-dose morphine evoked behavioral sensitization in zebrafish acutely pre-treated with morphine. Low-dose morphine failed to induce behavioral sensitization in zebrafish if the withdrawal time was less than 5 days or more than 7 days. Morphine induced behavioral sensitization in zebrafish may involve dopaminergic, glutamatergic and opioid systems. ConclusionA single low-dose of morphine could induce behavioral sensitization in zebrafish acutely pre-treated with morphine, and this phenomenon was highly correlated with drug dose and withdrawal time. These findings suggest that zebrafish is a suitable model for the study of behavioral sensitization.

Mu-opioid receptors in septum mediate the development of behavioural sensitization to a single morphine exposure in male rats.

Behavioural sensitization (BS) is characterized by enhanced psychomotor responses to a dose of substance of abuse after prior repeated exposure. We previously reported that BS can be induced by a single injection of morphine in rats, whereas septal nuclei are specifically involved in the development phase of BS. Here, we demonstrated that intra-LS or intra-MS microinjections also incubated BS to a systemic morphine injection in a cross-sensitization fashion, whereas inactivation of either subdivision of septal nuclei (LS: lateral septum; MS: medial septum) can negate this ability of morphine. Then, non-selective (naloxone) and selective (μ-, δ- and κ-)opioid receptor antagonists were directly delivered into LS or MS, respectively, ahead of a morphine microinjection, whereas only μ-opioid receptors in both LS and MS play indispensable roles in mediating the BS development. Finally, there was a pronounced elevation in the levels of the monoamines (i.e. dopamine, homovanillic acid, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid) in the septum, 8h after a morphine injection detected with a HPLC-ECD method, suggesting that dopaminergi and serotoninergic systems are implicated in the BS formation. Our studies demonstrated that septal nuclei critically participate in the BS development. Essentially, μ- instead of δ- or κ-opioid receptors in LS and MS mediate sensitization to opiates.

Context- and time-dependent neurobiological and behavioral sensitization induced by a single morphine exposure in mice.

Drug-induced sensitization in the mesocorticolimbic systems is thought to play an important role in certain aspects of drug addiction, including the involvement of drug-associated cues and environments in mediating drug-seeking behaviors. Our previous studies have identified the significance of heat shock protein 70 (Hsp70) in the development of a single morphine exposure-induced behavioral sensitization. The present study expands upon these findings by investigating the effect of environment on the expression of behavioral sensitization induced by a single morphine exposure, and the potential involvement of Hsp70 protein levels in these effects. Mice were pretreated with a single morphine injection in test chambers (morphine-paired) or home cages (morphine-unpaired) on day 1 and challenged on day 2 or 8, in test chambers. Hsp70 expression in the nucleus accumbens (NAc) was analyzed after the challenge. The expression of single morphine exposure-induced behavioral sensitization was accompanied by a significant increase in Hsp70 expression in NAc. In contrast, the unpaired morphine-treated group failed to exhibit behavioral sensitization or higher Hsp70 expression. Additionally, by adding a habituation process prior to the challenge, we demonstrated that conditioned hyperactivity, which was not accompanied by an increased expression of Hsp70, is not essential for behavioral sensitization. Behavioral sensitization induced by a single morphine exposure in mice exhibits context and time dependency, with environmental context likely functioning via an inhibitory conditioning mechanism. Furthermore, alterations in Hsp70 expression in the NAc may represent a neurobiological sensitization mechanism mediating context- and time-dependent behavioral sensitization.

Opioid addiction and withdrawal differentially drive long-term depression of inhibitory synaptic transmission in the hippocampus

Addictive behavior is increasingly accepted as a drug-associated pathological memory in which the hippocampus is profoundly engaged. It has been well documented that adaptations of synaptic plasticity of excitatory transmission in the hippocampus may contribute to opioid addiction. However, it remains unknown whether and how adaptive changes of synaptic plasticity of inhibitory transmission in the hippocampus occurs during opioid abuse. Here, we reported that a single in vivo morphine exposure (SM) did not affect inhibitory long-term depression (I-LTD) in the hippocampus, compared with saline control; while repeated morphine exposure (RM) abolished this I-LTD. Interestingly, opioid withdrawal for 3-5 days after repeated (RMW), but not a single morphine exposure (SMW), largely enhanced I-LTD. More importantly, the I-LTD in single morphine treatment is dependent on presynaptic mechanism since it can be blocked by AM251, a selective cannabinoid receptor 1 antagonist. While the large I-LTD in RMW group is dependent on combinatorial presynaptic and postsynaptic mechanisms since it can be blocked by co-application of AM251 and L-type calcium channel blocker LaCl3. Thus, these results demonstrate that opioid use and withdrawal drive the dynamics of presynaptic and postsynaptic I-LTD expression in the hippocampus that may contribute to the persistent behavioral changes during opioid abuse.

Prefrontal cortex gates acute morphine action on dopamine neurons in the ventral tegmental area

Morphine excites dopamine (DA) neurons in the ventral tegmental area (VTA), an effect mediated by both local and systemic mechanisms. While the importance of the prefrontal cortex (PFC) – VTA circuit in opiate addiction is well established, little is known about how the PFC regulates the activity of VTA DA neurons upon morphine stimulation. One major challenge is that VTA DA neurons are highly heterogeneous in terms of projection and regulation, making their responses to PFC manipulations variable. Our previous work has identified a subgroup of VTA DA neurons exhibiting significant slow oscillation in their firing sequence, and demonstrated that most of these neurons are functionally connected with the PFC. In the present study, we focus our efforts only on VTA DA neurons expressing strong slow oscillation, and report that blocking the neuronal activity in the PFC remarkably attenuates the morphine-induced excitation of these neurons. Using in vivo microdialysis, we find that inactivation of the PFC also reduces the morphine-induced elevation of DA levels in the nucleus accumbens (NAc). Furthermore, 24 h after only single morphine exposure, PFC-inactivation failed to prevent subsequent morphine challenge from exciting VTA DA neurons, which is paralleled by altered response of PFC pyramidal neurons to morphine stimulation. Our results indicate that the PFC gates acute morphine action on a subset of VTA DA neurons, which is highly plastic and can be functionally remodeled by morphine exposure.

Chaperone heat shock protein 70 in nucleus accumbens core: a novel biological target of behavioural sensitization to morphine in rats

Drug addiction is a major public health issue, yet the underlying adaptation of neural networks by drugs of abuse is not fully understood. We have previously linked chaperone heat shock protein 70 (Hsp70) to drug-induced adaptations. Focusing on the NAc core and shell, the present study aims to provide further findings for our understanding of the relation between behavioural sensitization to morphine and Hsp70 at transcriptional and functional levels in rats. Firstly, we delineated the characteristics of behavioural sensitization induced by a single morphine exposure (1-10 mg/kg, s.c.). Secondly, Hsp70 protein expression in the NAc core was time- and dose-relatedly induced during the development of behavioural sensitization to a single morphine exposure in rats, and Pearson analysis indicated a positive correlation between behavioural sensitization and Hsp70 expression in NAc core. Thirdly, at the transcriptional level, intra-NAc core injection of the specific heat shock factor-I (HSF-I) inhibitor N-Formyl-3,4-methylenedioxy-benzylidine-γ-butyrolactam (KNK437) suppressed Hsp70 expression and the development of behavioural sensitization, while the HSF-I specific inducer geranylgeranylacetone (GGA) promoted both of them. Interestingly, intra-NAc shell injection of KNK437 or GGA did not affect the development of behavioural sensitization. Finally, both the functional inhibition of Hsp70 ATPase activity by methylene blue (MB), and the antagonism of Hsp70 substrate binding site (SBD) activity by pifithrin-μ (PES) impaired the development of behavioural sensitization when they were microinjected into the NAc core. Taken together, the critical involvement of chaperone Hsp70 in behavioural sensitization to morphine identifies a biological target for long-lasting adaptations with relevance to addiction.

Molecular chaperone heat shock protein 70 participates in the labile phase of the development of behavioural sensitization induced by a single morphine exposure in mice

De-novo protein synthesis is required in the development of behavioural sensitization. A prior screening test from our laboratory has implicated heat shock protein 70 (Hsp70) as one of the proteins required in this behavioural plasticity. Thus, this study was designed to extend our understanding of the role of Hsp70 in the development of behavioural sensitization induced by a single morphine exposure in mice. First, by employing transcription inhibitor actinomycin D (AD) and protein synthesis inhibitor cycloheximide (CHX), we identified a protein synthesis-dependent labile phase (within 4 h after the first morphine injection) in the development of behavioural sensitization to a single morphine exposure. Second, Hsp70 protein expression in the nucleus accumbens correlated positively with locomotor responses of sensitized mice and, more importantly, the expression of Hsp70 increased within 1 h after the first morphine injection. Third, AD and CHX both prevented expression of Hsp70 and disrupted the development of the single morphine induced behavioural sensitization, which further implied Hsp70 was highly associated with behavioural sensitization. Finally, the selective Hsp70 inhibitor pifithrin-μ (PES) i.c.v. injected in mice prevented the development of behavioural sensitization and, critically, this inhibitory effect occurred only when PES was given within 1 h after the first morphine injection, which was within the labile phase of the development period. Taken together, we draw the conclusion that Hsp70 is crucially involved in the labile phase of the development of behavioural sensitization induced by a single morphine exposure, probably functioning as a molecular chaperone.

Protracted manifestations of acute dependence after a single morphine exposure

Acute opiate exposure produces a state of dependence in humans and animals, which is revealed by signs and symptoms of withdrawal precipitated by opioid receptor antagonists. The physiological changes that underlie this state of acute dependence develop rapidly and can persist long after the end of chronic opiate exposure. The purpose of this investigation was to determine the persistence of acute dependence after a single morphine exposure in rodents, focusing on changes in behavior thought to reflect the negative emotional consequences of withdrawal. The acoustic startle reflex and conditioned place aversion were measured following naloxone administration at different time points after a single morphine exposure. Naloxone administration produced significant potentiation of acoustic startle-a form of anxiety-like behavior-for at least 80 days after one exposure to morphine. In contrast, naloxone produced a conditioned place aversion 24 h but not 20 days after one morphine exposure. Together with existing literature, these results suggest acute as well as chronic opiate exposure leave rodents persistently vulnerable to express anxiety-like behavior in response to opioid receptor antagonists or stressful experience. The adaptations in brain function that underlie this protracted state of dependence may provide a foundation for the escalation of withdrawal severity that develops over repeated opiate exposure, and increase the likelihood of progression from casual drug use to compulsive drug abuse.

Effect of the histone deacetylase inhibitors on behavioural sensitization to a single morphine exposure in mice

Behavioural sensitization to a single morphine exposure has been considered to be a long-term form of behavioural plasticity associated with opioid addiction. Accumulated evidence has shown that histone modification plays a key role in behavioural plasticity. Therefore, this study was designed to investigate whether the histone deacetylase inhibitors sodium butyrate (SB) and valproic acid (VPA) could disrupt behavioural sensitization to a single morphine exposure. Mice were pretreated with a single injection of morphine and elicited subsequent behavioural sensitization by a challenge-dosage of morphine after a 7-day drug-free period. At doses that did not affect the locomotor activity, both SB and VPA inhibited the acute morphine induced hyperactivity and significantly attenuated the development of behavioural sensitization to a single morphine exposure. Furthermore, the combination of SB and VPA at the sub-effective doses could additionally reduce the development of morphine sensitization. Western blot analysis revealed that multiple administration with the effective dose of SB (160 mg/kg, i.p.) or VPA (150 mg/kg, i.p.) in the behavioural experiments induced hyperacetylation of histone H3 in the NAc of mice. Taken together, these findings suggest that histone acetylation may be involved in morphine sensitization.

Involvement of Dopaminergic Receptor Signaling in the Effects of Glutamatergic Receptor Antagonists on Conditioned Place Aversion Induced by Naloxone in Single-Dose Morphine-Treated Rats

A better understanding of the neurochemical mechanisms mediating the aversive consequences of drug withdrawal is important for understanding drug addiction. We previously demonstrated that the inhibitory effect of glutamate receptor antagonists on the conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal after a single morphine exposure could be blocked by dopamine receptor antagonists. Thus, a glutamatergic-dopaminergic interaction may participate in this phenomenon. The current study was undertaken to further characterize this interaction by employing both D(1) (SCH 23390) and D(2) (raclopride and eticlopride) dopamine receptor antagonists. The influence of these antagonists on the attenuation of CPA by MK-801 (NMDA receptor antagonist), GYKI 52466 (AMPA receptor antagonist), and MCPG (metabotropic glutamate receptor antagonist) was determined in rats receiving a single dose of morphine. The dopamine antagonists showed either a significant reversal or a tendency to reverse the effects of MK-801 on CPA. The effect of GYKI 52466 was also attenuated by the blockade of either D(1) or D(2) receptors. The effect of MCPG, however, was only blocked by D(2) antagonists and not by the D(1) antagonist SCH 23390. These results add evidence to the hypothesis that a glutamatergic-dopaminergic interaction may be involved in the CPA induced by naloxone-precipitated withdrawal following a single morphine exposure and suggest that both D(1) and D(2) dopamine receptor signaling mechanisms play a role in mediating the aversive aspects of acute dependence.

Episodic Withdrawal Promotes Psychomotor Sensitization to Morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex--a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization--a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

Transcription and protein synthesis inhibitors reduce the induction of behavioural sensitization to a single morphine exposure and regulate Hsp70 expression in the mouse nucleus accumbens

New protein synthesis has been implicated as necessary for long-lasting changes in neuronal function. Behavioural sensitization to a single exposure to addictive drugs is a form of neuroplasticity, but little is known about the importance of new protein synthesis in the underlying mechanism. This study was designed to investigate the effects of the transcription inhibitor actinomycin D (AD) and the protein synthesis inhibitor cycloheximide (CHX) on induction of behavioural sensitization to a single morphine exposure in mice. In combination with behavioural experiments, changes in gene and protein expression in the mouse nucleus accumbens (NAc) were analysed by RT-PCR array and Western blot respectively. Behavioural sensitization was evident in mice pretreated only once with morphine at the doses of 20 and 40 mg/kg, but not 5 and 10 mg/kg. Mice pretreated with morphine (20 mg/kg) and challenged with a lower dose (5 mg/kg) after a period of 4-21 d washout showed sensitized locomotion. At the doses that did not affect locomotion in mice, AD or CHX significantly suppressed hyperactivity induced by acute treatment, but not challenge with morphine, and blocked induction of behavioural sensitization to a single morphine exposure in a dose-related manner. The results from RT-PCR array and Western blot indicated that the changes of Hsp70 expression in the NAc of mice were associated with behavioural sensitization induced by a single morphine exposure. Together, these findings suggest that induction of behavioural sensitization to a single morphine exposure requires new protein synthesis, potentially involving Hsp70 expression in the NAc of mice.

Distinct Profiles of Anxiety and Dysphoria during Spontaneous Withdrawal from Acute Morphine Exposure

The negative motivational aspects of withdrawal include symptoms of both anxiety and depression, and emerge following termination of chronic drug use as well as after acute drug exposure. States of acute withdrawal are an inherent part of intermittent drug use in humans, but the contribution of acute withdrawal to the development of addiction has received limited systematic investigation, due to a lack of preclinical models for withdrawal states that emerge spontaneously after acute drug exposure. Here, we have characterized a spontaneous increase in the magnitude of the acoustic startle reflex (i.e., spontaneous withdrawal-potentiated startle) that emerges following acute morphine administration in rats, and compared the time course of startle potentiation and place conditioning. We find that startle potentiation appears related to a decrease in opiate receptor occupancy and reflects an anxiety-like state with a pharmacological profile similar to other signs of opiate withdrawal. Spontaneous startle potentiation emerges before the rewarding effects of morphine have subsided, even though naloxone administration after a single morphine exposure causes both startle potentiation and conditioned place aversion (CPA). These results demonstrate that negative emotional signs of withdrawal develop following just one exposure to morphine, and are likely a recurrent aspect of intermittent drug use that may contribute to the earliest adaptations underlying the development of addiction. Furthermore, the dissociation between spontaneous startle potentiation and CPA suggests anxiogenic and dysphoric manifestations of opiate withdrawal may be mediated by distinct neural mechanisms that are progressively engaged as withdrawal unfolds.

Involvement of the Amygdala on Place Aversion Induced by Naloxone in Single-Dose Morphine-Treated Rats

Signs characteristic of opiate withdrawal symptoms can be precipitated by an opiate antagonist after short-term infusion or even a single dose of an opiate both in humans and in animals. This phenomenon has been referred to as acute dependence. In contrast to extensive studies on chronic dependence, less is known about the neural mechanisms mediating acute dependence. It will benefit the development of appropriate therapies to facilitate opiate abstinence and reduced craving to better understand the mechanisms underlying acute opiate dependence and to determine whether there are dissociation and similarity between the early and fully developed stages of dependence. In the present study, we examined the influence of c-Fos expression in the amygdala in acquisition of conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal from a single morphine exposure 24 h earlier. The effect of microinjection into the central amygdaloid nucleus (CeA) of various kinds of glutamatergic neurotransmission inhibitors was also investigated. Findings showed that CeA displayed significant increase in c-Fos expression in the acquisition of CPA. Furthermore, CPA was attenuated significantly and dose-dependently by microinjection into CeA of all glutamatergic neurotransmission inhibitors (NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK-801), AMPA receptor antagonist 1-(4-aminophenyl)4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI52466), metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), and glutamate release inhibitor riluzole). These findings suggest that CeA involves the acquisition of CPA induced by naloxone-precipitated withdrawal from a single morphine exposure, and the function of the glutamatergic system projected from the amygdala to nucleus accumbens plays a facilitative role in formation of morphine dependence.

Single Dose of Morphine Produced a Prolonged Effect on Dopamine Neuron Activities

BackgroundClinical observation and experimental studies have indicated that a single exposure to morphine could induce tolerance and dependence. It has become a concern in clinical antinociceptive practice. However, the underling mechanism remains unknown. This study was designed to explore the changes of dopamine (DA) neuron activities in the ventral tegmental area (VTA) by employing a spectral analysis followed single morphine treatment.ResultsAcute morphine treatment significantly increased not only the firing rate and firing population but also the power of slow oscillation of DA neurons in naïve rats. These changes lasted at least for 3 days following the morphine treatment. During this period of time, responses of the DA neurons to subsequent morphine challenge were diminished. We further demonstrated a transient desensitization of opiate receptors as monitored by GTPγS binding to G-proteins. The present study provided first direct evidence for the temporal changes in the VTA DA neuron activities and opiate receptors desensitization.ConclusionProlonged VTA DA neuron activation and opiate receptors desensitization followed single morphine exposure may underlie the development of dependence and tolerance that may associate with the acute analgesic tolerance and acute addiction of morphine.

Evidence for the role of adaptation in opioid tolerance: Persistent heterologous tolerance following a single morphine exposure

The present study evaluated the time course for the development of reduced responsiveness of longitudinal muscle/myenteric plexus (LM/MP) preparations to the neurogenic twitch inhibition produced by opioid agonists and 2-chloroadenosine (CADO). Tissues were obtained at various times (1 to 21 days) after surgery from guinea-pigs implanted with either morphine or placebo pellets (75mg morphine or lactose/100gm body weight). The inhibitory effect of morphine, DAMGO and CADO on neurogenic twitches of the LM/MP were significantly reduced in a time-dependent manner following a single morphine exposure indicating the presence of heterologous tolerance. The delay in the onset of tolerance to all agonists was not observed until day 4 after pellet implantation (a 2.5 – 5 fold reduction in responsiveness) and reached a maximum that began by day 7 (a 5 – 8 fold reduction in responsiveness) and was maintained at that level through 14 days after implantation. The responsiveness of LM/MP preparations to all agonists returned to normal levels by 21 days after surgery indicating that there was a spontaneous return of responsiveness to control levels. These data suggest that a single implantation of morphine pellets leads to persistent heterologous tolerance that occurs over a time course that would be consistent with a modification of intracellular signaling factors that regulate cellular responsiveness. Since the time courses for the development of tolerance to morphine, DAMGO and CADO were similar then the cellular process underlying the change in responsiveness must be one involved in regulating cell physiology.

The glutamate release inhibitor riluzole attenuates the formation of conditioned place aversion induced by naloxone in rats undergoing a single morphine exposure

Acute morphine exposure has been hypothesized to produce long-lasting central changes that contribute to the withdrawal aversion. We have most recently demonstrated that those changes may involve the glutamatergic system, including multiple classes of receptors. The present study was undertaken to further determine the involvement of the glutamatergic system by examining the effect of riluzole, a glutamate release inhibitor, on the motivational component of withdrawal from acute morphine dependence. The role of the amygdala in the action of riluzole was also assessed. We investigated the effects of riluzole on the conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal from a single morphine exposure 24 h before, and on c-Fos expression within the amygdala during the withdrawal period in rats. Riluzole (2, 4, 8 mg/kg) dose-dependently attenuated the CPA without producing place conditioning itself. This result provided further evidence that glutamatergic mechanisms may be recruited in adaptational changes following acute morphine exposure and play a role in withdrawal aversion. In addition, riluzole appeared to produce nonspecific effects on c-Fos expression by itself, without specifically modifying c-Fos expression following naloxone-precipitated withdrawal in any region of the amygdala examined, suggesting that the amygdala may not serve as a specific site of action for riluzole.

Effect of glutamate receptor antagonists on place aversion induced by naloxone in single‐dose morphine‐treated rats

The neurobiological mechanism underlying the negative motivational component of withdrawal from acute opiate dependence is far from understood. Our objectives were to determine whether the glutamatergic system is involved in the motivational component of morphine withdrawal in acutely dependent rats and such an involvement is associated with dopaminergic neurotransmission. We examined the effects of various kinds of glutamate receptor antagonists on conditioned place aversion (CPA) induced by naloxone-precipitated withdrawal from a single morphine exposure 24 h before. Furthermore, the influence of pretreatment with the dopamine receptor antagonist haloperidol on those effects of glutamate receptor antagonists was also investigated. CPA was attenuated in a dose-dependent manner by all glutamate receptor antagonists examined including the NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine maleate (MK-801) and phencyclidine hydrochloride (PCP), AMPA receptor antagonist 1-(4-aminophenyl)4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466), and metabotropic receptor antagonists (+/-)-2-amino-3-phosphonopropionic acid (AP-3) and (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG). The effects of MK-801, GYKI 52466 and MCPG were blocked by haloperidol. These results suggest that the glutamatergic system involving multiple classes of receptors plays a role in the motivational component of withdrawal from acute morphine dependence, and the function of the glutamatergic system would be closely associated with dopaminergic neurotransmission.