Naloxone-precipitated Withdrawal Syndrome Research Articles

The Effects of Bupropion and Varenicline on Morphine Withdrawal Syndrome in Rats

Objective: Opioid dependence is a chronic and complex disorder characterized by relapse and remission. Chronic administration of morphine causes symptoms of physical and psychological dependence. The purpose of the present study was to investigate the effect of anti-addictive drugs such as bupropion and varenicline on morphine dependence and naloxone precipitated withdrawal syndrome in a rat model. Materials and Methods: To assess the physical dependence of morphine, Wistar albino rats were intraperitoneal (i.p.) administered increasing doses of morphine twice daily for 5 days, 4 hours after a single dose of morphine on day 6, and 15 minutes before subcutaneous (s.c.) naloxone (2 mg/kg, s.c.) administration to elicit withdrawal symptoms. Physical dependence was evaluated by giving intraperitoneal bupropion (5, 10 and 20 mg/kg, i.p.) and varenicline (0.5. 1 and 2 mg/kg, s.c) for 15 minutes. Results: The morphine-dependent rats had significantly more withdrawal symptoms than naive control rats. The results demonstrated administration of bupropion and varenicline during induction of morphine dependence attenuated the most of the severity of withdrawal symptoms. Co-administration of bupropion reduced withdrawal symptoms such as jumping, wet dog shaking, weight loss and total withdrawal symptoms. Co-administration of varenicline was found to be effective on withdrawal symptoms such as bupropion, but had no effect on weight loss. Conclusion: These outcomes provide preliminary data that bupropion and varenicline could be used as a candidate drugs to attenuate morphine withdrawal symptoms.

Acute morphine administration, morphine dependence, and naloxone-induced withdrawal syndrome affect the resting-state functional connectivity and Local Field Potentials of the rat prefrontal cortex

Opiates are among the widely abused substances worldwide. Also, the clinical use of opioids can cause unwanted and potentially severe consequences such as developing tolerance and dependence. This study simultaneously measured the changes induced after morphine dependence and naloxone-induced withdrawal syndrome on the resting-state functional connectivity (rsFC) and Local Field Potential (LFP) power in the prefrontal cortex of the rat. The obtained results revealed that acute morphine administration significantly increased the LFP power in all frequency bands, as well as the rsFC strength of the prefrontal cortex, and naloxone injection reversed this effect. In contrast, chronic morphine administration reduced neural activity and general correlation values in intrinsic signals, as well as the LFP power in all frequency bands. In morphine-dependent rats, after each morphine administration, the LFP power in all frequency bands and the rsFC strength of the prefrontal cortex were increased, and these effects were further enhanced after naloxone precipitated withdrawal syndrome. The present study concludes that general correlation merely reflects the field activity of the local cortices imaged.

Antagonism of orexin type-1 receptors (OX1Rs) attenuates naloxone-precipitated morphine withdrawal syndrome in rat dorsal hippocampus

Herein the effect of hippocampal orexin type-1 receptors (OX1Rs) blockade on morphine withdrawal syndrome was studied. Animals were made dependent by subcutaneous (s.c.) administration of morphine sulfate (10mg/kg) at an interval of 12h for 9 consecutive days. Thereafter, on day 10, naloxone hydrochloride (1.5mg/kg, i.p.) was injected and the somatic signs of withdrawal syndrome were monitored during a 25-min period. Two groups of animals received bilateral microinjection of either SB-334867, a selective OX1Rs antagonist, (0.5μg/0.5μl), or its vehicle into the dorsal hippocampus immediately before each morphine injection. Other groups of animals were made dependent at first and only received a single microinjection of SB-334867 or vehicle on day 10 before naloxone injection. The results showed that intra-hippocampal microinjection of SB-334867 before each morphine treatment, significantly decreased the signs of morphine withdrawal, including teeth chattering (dependent: 18.5±2.3, SB treated: 5±1, p<0.001), diarrhea (dependent: 8.7±0.6, SB treated: 4.1±0.6, p<0.001), ptosis (dependent: 33.8±3.7, SB treated: 11.6±1.1, p<0.001), and chewing (dependent: 40±2.3, SB treated: 29±2.4, p<0.01). SB-334867 did not attenuate withdrawal syndrome, when it was microinjected as a single dose immediately before naloxone injection. The present results suggest a role for orexin in naloxone-precipitated withdrawal and thus possibly morphine dependence and this effect is, at least in part, via OX1Rs in the dorsal hippocampus.

Protective effects of atorvastatin against morphine-induced tolerance and dependence in mice

BackgroundIn this study, we evaluated the effects of atorvastatin, a lipid-lowering medication on morphine-induced tolerance and dependence in mice. MethodsTolerance was induced by subcutaneous administration of morphine (20mg/kg) to animals, twice a day for 9days. Atorvastatin was given at the doses of 5, 10 and 20mg/kg, 30min before each morphine administration, once daily for 9days. Hot plate test was employed to assess antinociceptive effect of morphine on days 1, 3, 5, 7 and 9. Dependence was evaluated by naloxone-precipitated withdrawal syndrome. We attempted to verify withdrawal regulation of induced nitric oxide synthase (iNOS), astroglia marker, glial fibrillary acidic protein (GFAP), ionized calcium-binding protein (Iba1), a microglia activation marker, a pro-inflammatory mediator, tumor necrosis alpha (TNF-α) and immune receptor, toll like receptor 4 (TLR-4) genes by real-time polymerase chain reaction (RT-PCR). Lipid peroxidation was estimated by assessing malondialdehyde (MDA) content in the spinal cord of animals. ResultsTolerance to antinociceptive effects of morphine was observed on days 7 and 9. Decrease in morphine-induced antinociception was reversed by concomitant intraperitoneal administration of atorvastatin (10 and 20mg/kg). Atorvastatin (10 and 20mg/kg) mitigated naloxone-induced withdrawal parameters. Brain expression levels of TNF-α, GFAP, Iba1 and iNOS increased in morphine withdrawn animals which were attenuated by nine days treatment with atorvastatin. Increased MDA was also normalized in withdrawn animals received atorvastatin. ConclusionAtorvastatin exhibits meaningful protective effects against both tolerance to antinociceptive effects of morphine and withdrawal-induced behavioral profile. Neuroprotective effects of atorvastatin is further supported via inhibition of glia activity and antioxidant effects.

Effects of hydroalcoholic extract of &lt;i&gt;Borago officinalis&lt;/i&gt; on naloxone-precipitated withdrawal syndrome in morphine-dependent mice

&lt;p class="Abstract"&gt;The aim of the present study was to investigate the effect of hydroalcoholic extract of &lt;em&gt;Borago officinalis&lt;/em&gt; on morphine withdrawal syndrome in mice. Morphine-dependent group received morphine for nine days and then received naloxone via intraperitoneal injection. Control group received saline for nine days. Post-treated group received &lt;em&gt;B. &lt;/em&gt;officinalis extract intraperitoneally (100 mg/kg) on the day 10 before naloxone injection. Co-treated group received &lt;em&gt;B. &lt;/em&gt;officinalis extract intraperitoneally (100 mg/kg) and morphine for nine days and then received naloxone. Extract-treated group received extract for nine days and then received naloxone. Naloxone injection significantly increased the frequency of jumping, blinking, ptosis, defecation, paw trembling, and two-legged standing in comparison to the control group. Co-treatment and post-treatment with &lt;em&gt;B. officinalis&lt;/em&gt; extract significantly decreased the withdrawal symptoms. In conclusion, hydroalcoholic extract of &lt;em&gt;B. &lt;/em&gt;officinalis significantly attenuated the symptoms of morphine withdrawal syndrome.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Video Clip&lt;/strong&gt;:&lt;/p&gt;&lt;p&gt;&lt;a href="https://youtube.com/v/TCNzTxZeGjw"&gt;Part 1&lt;/a&gt;: 8 min 11 sec &lt;/p&gt;

Antinociceptive and antidepressant-like effects of the crude extract of Vitex megapotamica in rats

Ethnopharmacological relevanceVitex megapotamica (Spreng) Moldenke has been used in South American folk medicine to treat inflammatory diseases. However, the effects of V. megapotamica on animal models of nociception and depression have not been evaluated. Aim of the studyThis study investigated whether the crude leaf extract of V. megapotamica exhibits antinociceptive and antidepressant-like effects in a Freund’s adjuvant-induced chronic inflammation and depression model. Materials and methodsChronic inflammation was induced in rats by the intraplantar administration of complete Freund's adjuvant (CFA; 100μl). The effect of oral crude extract of V. megapotamica (VmE; 3–30mg/kg, p.o.) on nociception (thermal hyperalgesia, mechanical allodynia and arthritis score), inflammation (edema, myeloperoxidase activity), immobility (forced swimming test), locomotor activity (open field), gastrointestinal transit, hyperalgesia and naloxone-precipitated morphine withdrawal syndrome was evaluated. Naloxone (0.4mg/kg, i.p.) was used to investigate the involvement of opioid system in the currently described effects of VmE. ResultsCrude extract caused antinociceptive/antidepressant-like effects in the CFA-induced chronic inflammation model, which was prevented by naloxone. The VmE extract (10mg/kg, p.o.) did not alter the locomotor activity, gastrointestinal function and inflammatory parameters and did not cause hyperalgesia. ConclusionV. megapotamica induces opioid-dependent antinociception and antidepressant-like effect, without anti-inflammatory activity. The results support the use of VmE as analgesic and antidepressant.

Baclofen prevents the elevated plus maze behavior and BDNF expression during naloxone precipitated morphine withdrawal in male and female mice

In previous studies we have shown that baclofen, a selective GABAB receptor agonist, prevents the somatic expression and reestablishes the dopamine and μ-opioid receptors levels, modified during naloxone-precipitated morphine withdrawal syndrome in male and female mice. There are no previous reports regarding sex differences in the elevated plus maze (EPM) and the expression of BDNF in morphine-withdrawn mice. The present study analyses the behavioral and biochemical variations during morphine withdrawal in mice of both sexes, and whether these variations are prevented with baclofen. Swiss-Webster albino prepubertal mice received morphine (2 mg/kg, i.p.) twice daily, for 9 consecutive days. On the 10th day, one group of morphine-treated mice received naloxone (opioid receptor antagonist; 6 mg/kg, i.p.) 1 h after the last dose of morphine to precipitate withdrawal. A second group received baclofen (2 mg/kg, i.p.) before naloxone administration. The EPM behavior was measured during 15 min after naloxone injection. The expression of BDNF-positive cells was determined by immunohistochemistry. Withdrawn male mice showed a higher percentage of time spent and number of entries to the open arms compared to withdrawn female mice. Baclofen prevented this behavior in both sexes. BDNF expression decreased in the AcbC, BNST, CeC, and CA3 of the hippocampus while increased in the BLA of morphine withdrawn male. Baclofen pretreatment prevented the BDNF expression observed in morphine withdrawn male mice in all the brain areas studied except in the CeC. Baclofen prevention of the EPM behavior associated to morphine withdrawal could be partially related to changes in BDNF expression.

Pleiotrophin modulates morphine withdrawal but has no effects on morphine-conditioned place preference

Pleiotrophin (PTN) is a neurotrophic factor with important functions in addiction and neurodegenerative disorders. Morphine administration induces an increase in the expression of PTN and Midkine (MK), the only other member of this family of cytokines, in brain areas related with the addictive effects of drug of abuse, like the Ventral Tegmental Area or the hippocampus. In spite of previous studies showing that PTN modulates amphetamine and ethanol rewarding effects, and that PTN is involved in morphine-induced analgesia, it was still unknown if the rewarding effects of morphine may be regulated by endogenous PTN. Thus, we aim to study the role of PTN in the reward and physical dependence induced by morphine.We used the Conditioned Place Preference (CPP) paradigm in PTN genetically deficient (PTN−/−) and wild type (WT) mice to assess the rewarding effects of morphine in absence of endogenous PTN. Second, to study if PTN may be involved in morphine physical dependence, naloxone-precipitated withdrawal syndrome was induced in PTN−/− and WT morphine dependent mice. Although the increase in the time spent in the morphine-paired compartment after conditioning tended to be more pronounced in PTN−/− mice, statistical significance was not achieved. The data suggest that PTN does not exert an important role in morphine reward. However, our results clearly indicate that PTN−/− mice develop a more severe withdrawal syndrome than WT mice, characterized as a significant increase in the time standing and in the total incidences of forepaw licking, forepaw tremors, wet dog shake and writhing. The data presented here suggest that PTN is a novel genetic factor that plays a role in morphine withdrawal syndrome.

Spontaneous withdrawal in intermittent morphine administration in rats and mice: effect of clonidine coadministration and sex-related differences

Treating animals repeatedly with intermittent and increasing morphine doses has been suggested to allow some withdrawal during each dosing interval, which causes repeated stress. The present study aimed to test this hypothesis and assess sex-related differences in withdrawal signs and their suppression by clonidine. Male and female rats and mice were administered with increasing doses of morphine twice daily at different dosing intervals. Rats were given clonidine in drinking water (5 μg/mL). Spontaneous and naloxone-precipitated withdrawal signs and novelty-induced grooming were evaluated. Male rats and male and female rats displayed manifestations of morphine withdrawal at the end of 14-h and 24-h dosing intervals, respectively. Clonidine attenuated the severity of the withdrawal signs. Male but not female mice displayed withdrawal signs at the end of 12-h and 17-h dosing intervals. Female mice exhibited less pronounced naloxone-precipitated withdrawal syndrome. Grooming did not reflect a "stress-like state" in morphine-treated animals. These findings indicate intermittent morphine treatment-induced spontaneous withdrawal in rats and mice and sex-related differences in spontaneous and naloxone-precipitated withdrawal signs in mice. Since the treatment protocol closely parallels the drug use pattern in opioid addicts, further experiments are needed to clarify the stress associated with the treatment and the efficacy of sedatives.

Nitroxidergic modulation of behavioural, cardiovascular and immune responses, and brain NADPH diaphorase activity upon morphine tolerance/dependence in rats

Opioid and non-opioid effects of acute and chronic morphine administration on behaviour, cardiovascular responses, cell proliferation and apoptosis and nitric-oxide synthase (NOS) activity were studied in rats. A novel score-point scale was introduced to quantify the signs of opioid withdrawal syndrome. NOS inhibitor L-NAME (NG-nitro-L-arginine methyl ester) was applied to reveal the role of NOS/NO pathway in the modulation of morphine-induced in vivo and in vitro responses. The obtained data showed that chronic co-administration of L-NAME drastically attenuated naloxone-precipitated withdrawal syndrome and prevented the development of morphine tolerance to cardiovascular action of morphine. The apoptotic process was very much restricted by L-NAME supplementation of chronic morphine treatment, which resulted in few apoptotic cells, less low molecular weight genomic DNA and preservation of high molecular weight non-fragmented genomic DNA. The study provides new data for nitroxidergic modulation of opioid tolerance and dependence.

In vivo and in vitro attenuation of naloxone-precipitated experimental opioid withdrawal syndrome by insulin and selective KATP channel modulator.

Opiate exposure for longer duration develops state of dependence in humans and animals, which is revealed by signs and symptoms of withdrawal precipitated by opioid receptor antagonists. The sudden withdrawal of opioids produces a withdrawal syndrome in opioid-dependent subjects. Insulin and ATP-sensitive potassium (KATP) channel-mediated glucose homeostasis have been shown to modulate morphine withdrawal. Present study has been structured to investigate the role of insulin and pharmacological modulator of KATP channel (gliclazide) in experimental morphine withdrawal syndrome, both invivo and invitro. In this study, naloxone-precipitated morphine withdrawal syndrome in mice (invivo) as well as in rat ileum (invitro) were utilized to assess opioid withdrawal phenomenon. Morphine withdrawal syndromes like jumping and rearing frequency, forepaw licking, circling, fore paw tremor, wet dog shake, sneezing, overall morphine withdrawal severity (OMWS), serum glucose, brain malondialdehyde (MDA), glutathione (GSH), nitrite/nitrate, and calcium (Ca(+2)) were assessed. Naloxone has significantly increased morphine withdrawal syndrome, both invivo and invitro. Insulin and gliclazide have significantly attenuated, naloxone induced behavioral changes like jumping and rearing frequency, forepaw licking, wet dog shake, sneezing, straightening, circling, OMWS, and various biochemical impairments such as serum glucose, brain MDA, GSH, nitrite/nitrate, and Ca(+2) in morphine-dependent animals (invivo). In vitro, insulin and gliclazide have significantly reduced naloxone-induced contraction in morphine-withdrawn rat ileum preparation. Insulin and gliclazide (KATP channel blocker) have attenuated naloxone-precipitated morphine withdrawal syndrome, both invivo and invitro. Thus, insulin and KATP channel modulation may provide new avenues for research in morphine withdrawal.

Coadministration of glycogen-synthase kinase 3 inhibitor with morphine attenuates chronic morphine-induced analgesic tolerance and withdrawal syndrome

BackgroundGlycogen-synthase kinase 3 (GSK3) is involved in many signaling pathways and is associated with a host of high-profile pathophysiological states. However, its role in morphine tolerance, especially naloxone-precipitated withdrawal syndrome, has not been well investigated. The present study was undertaken to study the role of GSK3 in chronic morphine exposure. MethodsAdult male Sprague–Dawley rats were subjected to intraperitoneal (i.p.) injections of morphine (10 mg/kg) twice daily for 6 consecutive days, and tail-flick tests were conducted to evaluate changes of morphine-induced antinociception. GSK3 inhibitor, SB216763 or SB415286, was i.p. injected prior to morphine to investigate the influences on morphine tolerance. There were four groups receiving morphine plus vehicle (2% dimethyl sulfoxide), morphine plus SB216763 (0.6 mg/kg) or SB415286 (1.0 mg/kg), GSK3 inhibitor alone, or dimethyl sulfoxide: as the control group. On Day 7, naloxone (i.p., 1 mg/kg) was administered and naloxone-precipitated withdrawal behaviors were individually compared between groups. ResultsRepeated morphine exposure in this study led to progressive shortening of tail-flick latencies and produced six of nine observed naloxone-precipitated withdrawal behaviors. Coadministration with SB216763 or SB415286 significantly prevented antinociceptive tolerance and alleviated parts of withdrawal syndrome. Both inhibitors could similarly reverse withdrawal behaviors including grooming, chewing, and ptosis, but did not affect withdrawal behaviors of penis licking and defecation. ConclusionThe results demonstrate the importance of GSK3 in reducing chronic morphine-induced tolerance and withdrawal syndrome. Although GSK3 is involved in diverse physiological functions, aiming at GSK3-related pathway could still be a potential tool to improve therapeutic quality in clinical morphine treatment.

A comparison of the effects of digoxin, ouabain and milrinone on naloxone-precipitated withdrawal syndrome in mice

Modulation of Na+, K+-ATPase activity by acute and chronic opiates has been established for many years. However, the effects of digoxin, a putative inhibitor of Na+, K+-ATPase, on naloxone-precipitated morphine withdrawal syndrome are unknown. In the present study, a digoxin dose-response curve was conducted to observe the effects on naloxone-precipitated withdrawal and locomotor activity in mice. Higher doses of digoxin (1.0 and 2.5mg/kg) inhibited locomotor activity and naloxone-precipitated withdrawal jumping and weight loss, while lower doses of digoxin (0.1 and 0.25mg/kg) inhibited withdrawal weight loss precipitated by naloxone without affecting locomotor activity and naloxone-precipitated withdrawal jumping. To explore the possible mechanisms underlying this behavior, another Na+, K+-ATPase inhibitor ouabain, which does not cross the blood brain barrier, and another cardiotonic drug milrinone, a non-inhibitor of Na+, K+-ATPase, were also included in the present study. Both milrinone and ouabain inhibited, in a dose-dependent manner, naloxone-precipitated weight loss while neither affected naloxone-precipitated withdrawal jumping nor locomotor activity in mice. These results indicate that both the cardiotonic effects and central inhibition of Na+, K+-ATPase contribute to the inhibitory effects of digoxin on morphine withdrawal syndrome in mice.

Modulation of histone deacetylase attenuates naloxone-precipitated opioid withdrawal syndrome

The present study has been designed to investigate the effect of selective inhibitors of histone deacetylase and/or N-acetyl-Asp-Glu-Val-Asp-al (Ac-DEVD-CHO), a selective interleukin-1β converting enzyme inhibitor, on the development of naloxone-induced opioid withdrawal syndrome both in vitro and in vivo and the effect of histone deacetylase inhibition on histone H3 acetylation in brain. Sub-acute morphine administration followed by a single injection of naloxone (8mg/kg, i.p.) was used to precipitate opioid withdrawal syndrome in mice. Behavioral observations were made immediately after naloxone treatment. Withdrawal syndrome was quantitatively assessed in terms of withdrawal severity score and frequency of jumping, rearing, fore paw licking and circling. Separately naloxone-induced contraction in morphine-dependent isolated rat ileum was employed as an in vitro model. An isobolographic study design was employed to assess potential synergistic activity between trichostatin A and Ac-DEVD-CHO. Brain histone acetylation status was examined by western blotting. Injection of naloxone precipitated a severe form of abstinence syndrome in morphine-dependent mice along with strong contracture in isolated rat ileum. Administration of tributyrin (1.5, 3 and 6g/kg, p.o.), trichostatin A (0.3, 1.0 and 3.0mg/kg, p.o.) and Ac-DEVD-CHO (0.3, 1.0 and 3.0mg/kg, p.o.) markedly and dose dependently attenuated naloxone-induced morphine withdrawal syndrome in vivo as well as in vitro in rat ileum. Trichostatin A was also observed to exert a synergistic interaction with Ac-DEVD-CHO. Western blot analysis revealed that multiple administration with the effective dose of tributyrin or trichostatin A in the in vivo experiments induced hyperacetylation of histone H3 in the mouse brain. Thus, it is proposed that histone deacetylase activation linked mechanism might be involved in the development of opioid dependence and the precipitation of its withdrawal syndrome.

Effect of acupuncture on naloxone-precipitated withdrawal syndrome in morphine-experienced rats: The mediation of GABA receptors

Repeated morphine administration increases extracellular dopamine levels in the nucleus accumbens, which results in behavioral sensitization that can be suppressed by acupuncture at Shenmen (HT7) points. The present study was conducted to investigate the effects of acupuncture at HT7 on morphine withdrawal syndrome as well as to explore the role of GABA receptors in mediating the effects of HT7 acupuncture. We induced morphine withdrawal by injecting naloxone to rats that self-administer morphine and evaluated the effects of acupuncture and/or GABA receptor antagonists on their withdrawal symptoms. Acupuncture at HT7, but not at the control point LI5, significantly decreased symptoms of morphine withdrawal. HT7 inhibition of the withdrawal syndrome was blocked by pretreatment with either the GABA A receptor antagonist bicuculline or the GABA B antagonist SCH 50911. These findings suggest that the effects of acupuncture on suppression of morphine withdrawal syndrome are mediated, at least in part, through GABA receptors.

Modulation of src-kinase attenuates naloxone-precipitated opioid withdrawal syndrome in mice

This study was designed to investigate the effect of 2,3-dihydro-N, N-dimethyl-2-oxo-3-[(4,5,6,7-tetrahydro-1H-indol-2-yl)methylene]-1H-indole-5-sulfonamide (SU-6656), a selective inhibitor of src family kinase, on the development of naloxone-induced opioid withdrawal syndrome in mice. Subacute morphine administration followed by a single injection of naloxone (8 mg/kg, intraperitoneally) was used to precipitate the opioid withdrawal syndrome in mice. Behavioral observations were made immediately after naloxone treatment. The withdrawal syndrome was quantitatively assessed in terms of withdrawal severity score and frequency of jumping, rearing, forepaw licking, and circling. Daily single administration of SU-6656 was continued during the morphine treatment procedure. Injection of naloxone precipitated severe withdrawal in morphine-dependent mice. However, once-daily administration of SU-6656 (1.5, 3, and 6 mg/kg, intraperitoneally) markedly and dose-dependently attenuated the naloxone-induced morphine withdrawal syndrome. Therefore, it seems that an src family-kinase-linked mechanism is involved in the development of physiological opioid dependence; thus, src family kinase may serve as a potential target to address the pathological condition of physiological dependence and abstinence associated with continuous opioid usage.

Naloxone-precipitated withdrawal enhances ERK phosphorylation in prefrontal association cortex and accumbens nucleus of morphine-dependent mice

Mitogen-activated protein kinases (MAPK) can be activated by opioids such as morphine via opioid receptor, and their activations have been observed in synaptic plasticity, learning, memory and addiction. Long-term exposure to morphine may induce physical dependence, manifested as somatic withdrawal symptoms such as diarrhea, body weight loss, jumping and headshaking, when drug is deprived. Though morphine dependence and withdrawal have been extensively studied, their molecular mechanisms have not been fully elucidated. In the present study, the physical dependence on morphine was developed in mice by an intermittent, escalating procedure of morphine injections, and was measured by the body weight loss and the behavioral signs (jumping and headshaking). We found that the mice with chronic morphine administration experienced dramatic body weight loss, compared with the saline-treated controls. Naloxone-precipitated withdrawal led to more body weight loss, compared with spontaneous withdrawal. Naloxone-precipitated withdrawal mice showed significantly aggravated morphine-withdrawal symptoms (including jumping and heading shaking), compared with spontaneous withdrawal mice. MAPK pathway activities in the frontal association cortex (FrA), accumbens nucleus (Acb) and caudate putamen (CPu) were examined to probe into molecular mechanism for morphine dependence and withdrawal. Compared with saline-treated mice, morphine-dependent mice and spontaneous withdrawal mice, naloxone-precipitated withdrawal mice showed a significantly increased ERK phosphorylation in FrA and Acb, but not in CPu. However, the activities of other protein kinases in the MAPK pathway, including p38 and JNK, showed no changes in FrA, Acb and CPu of the mice during the chronic morphine dependence and withdrawal phases. These results suggest that the ERK phosphorylation in FrA and Acb may be associated with naloxone-precipitated withdrawal syndrome.

Prodynorphin gene deficiency potentiates nalbuphine-induced behavioral sensitization and withdrawal syndrome in mice

Dynorphin is the presumed endogenous ligand for the kappa-opioid receptor. The dynorphin gene may play a role in psychotropic agent-mediated behavioral changes via dopaminergic modulation. Therefore, in this study, possible involvement of the dynorphin gene in nalbuphine-mediated behavioral responses was examined using prodynorphin ( Pdyn) gene knock-out (−/−) mice. Pdyn gene deficiency potentiates nalbuphine-induced behavioral sensitization of locomotor activity and accumbal c-Fos expression. Administration of nalbuphine induced a significant increase in the dialysate dopamine level in the nucleus accumbens. This increase was more pronounced in the Pdyn (−/−) mice than in the wild-type (WT) mice. In addition, Pdyn (−/−) mice were more vulnerable to the naloxone-precipitated withdrawal syndrome (i.e., teeth chattering, wet dog shakes, forepaw tremors, jumping, weight loss, and global withdrawal score) after repeated treatment with nalbuphine than the WT mice. Consistently, nor-binaltorphimine, a kappa-opioid receptor antagonist, significantly potentiated nalbuphine-induced behavioral effects in WT mice, whereas U-50488H, a kappa-opioid receptor agonist, significantly attenuated these changes in Pdyn (−/−) mice in a dose-dependent manner. Our data suggest that the kappa-opioid receptor/dynorphin system is specifically modulated in response to behavioral sensitization and withdrawal signs induced by nalbuphine.

Tropisetron attenuates naloxone-induced place aversion in single-dose morphine-treated rats: Role of α7 nicotinic receptors

We have previously reported that acute dependence can occur when naloxone is administered 24 h after even a single dose of morphine, and that nicotine attenuates this naloxone-precipitated withdrawal syndrome. In the present study, we studied the effect of tropisetron, an α7 nicotinic receptor agonist and 5-hydroxytryptamine 3 (5-HT 3) receptor antagonist, on place aversion induced by naloxone in morphine-treated rats. Place aversion was significantly attenuated by pre-administered tropisetron (1.0 and 2.0 mg/kg, i. p.) in a dose-dependent manner, however tropisetron alone had no effect in a place-conditioning paradigm. This attenuation was completely antagonized by mecamylamine (1.0 mg/kg, s.c.), which is a central nicotinic receptor antagonist, but not by ondansetron (0.3 and 1.0 mg/kg, s.c.), a 5-HT 3 receptor antagonist. Furthermore, methyllycaconitine (1.0 and 2.0 mg/kg, s.c.), an α7 nicotinic acetylcholine receptor antagonist, but not dihydroxy-β-erithroidine (1.0 and 2.0 mg/kg, s.c.), an α4β2 nicotinic acetylcholine receptor antagonist, also antagonized the inhibitory effect of tropisetron. These findings suggest that tropisetron attenuates place aversion induced by naloxone in single-dose morphine-treated rats via α7 nicotinic receptors.

The prolactin-releasing peptide antagonizes the opioid system through its receptor GPR10

Prolactin-releasing peptide (PrRP) and its receptor G protein-coupled receptor 10 (GPR10) are expressed in brain areas involved in the processing of nociceptive signals. We investigated the role of this new neuropeptidergic system in GPR10-knockout mice. These mice had higher nociceptive thresholds and stronger stress-induced analgesia than wild-type mice, differences that were suppressed by naloxone treatment. In addition, potentiation of morphine-induced antinociception and reduction of morphine tolerance were observed in mutants. Intracerebroventricular administration of PrRP in wild-type mice promoted hyperalgesia and reversed morphine-induced antinociception. PrRP administration had no effect on GPR10-mutant mice, showing that its effects are mediated by GPR10. Anti-opioid effects of neuropeptide FF were found to require a functional PrRP-GPR10 system. Finally, GPR10 deficiency enhanced the acquisition of morphine-induced conditioned place preference and decreased the severity of naloxone-precipitated morphine withdrawal syndrome. Altogether, our data identify the PrRP-GPR10 system as a new and potent negative modulator of the opioid system.