Attenuation of morphine tolerance and dependence by aminoguanidine in mice

The effect of aminoguanidine, an inducible nitric oxide synthase (iNOS) inhibitor, on morphine-induced tolerance and dependence in mice was investigated in this study. Acute administration of aminoguanidine (20 mg/kg, p.o.) did not affect the antinociceptive effect of morphine (10 mg/kg, s.c.) as measured by the hot plate test. Repeated administration of aminoguanidine along with morphine attenuated the development of tolerance to the antinociceptive effect of morphine. Also, the development of morphine dependence as assessed by naloxone-precipitated withdrawal manifestations was reduced by co-administration of aminoguanidine. The effect of aminoguanidine on naloxone-precipitated withdrawal was enhanced by concurrent administration of the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, dizocilpine (0.25 mg/kg, i.p.) or the non-specific nitric oxide synthase (NOS) inhibitor, l-N(G)-nitroarginine methyl ester (l-NAME; 5 mg/kg, i.p.) and antagonized by concurrent administration of the nitric oxide (NO) precursor, l-arginine (50 mg/kg, p.o.). Concomitantly, the progressive increase in NO production, but not in brain glutamate level, induced by morphine was inhibited by repeated administration of aminoguanidine along with morphine. Similarly, co-administration of aminoguanidine inhibited naloxone-induced NO overproduction, but it did not inhibit naloxone-induced elevation of brain glutamate level in morphine-dependent mice. The effect of aminoguanidine on naloxone-induced NO overproduction was potentiated by concurrent administration of dizocilpine or l-NAME and antagonized by concurrent administration of l-arginine. These results provide evidence that blockade of NO overproduction, the consequence of NMDA receptor activation, by aminoguanidine, via inhibition of iNOS, can attenuate the development of morphine tolerance and dependence.     

Involvement of glial glutamate transporters in morphine dependence and naloxone-precipitated withdrawal

A body of evidence supports that excitatory amino acid systems, particularly glutamatergic one, participate in morphine dependence and naloxone-precipitated withdrawal. In this study, we examined the involvement of glial glutamate transporters, GLT-1 and GLAST, in them. Rats were rendered morphine-dependent by subcutaneous implantation of two 75 mg morphine pellets for 5 days. Intracerebroventricular administration of DL-threo-beta-benzyloxyaspartate, a glutamate transporter inhibitor significantly facilitated various naloxone-precipitated withdrawal signs. By northern blot analysis, the expression of GLT-1 mRNA was found to decrease significantly in the striatum and thalamus of morphine-dependent rats, and to increase significantly in the striatum 2 hr after the naloxone-precipitated withdrawal. On the other hand, there were no significant changes in GLAST mRNA levels in any brain regions. In vivo microdialysis experiments revealed that the extracellular glutamate levels was elevated in the striatum and nucleus accumbens, in which the changes of GLT-1 mRNA level were observed, during naloxone-precipitated morphine withdrawal. In cultured astrocytes, the expression of GLT-1 mRNA was regulated by agents activating the cAMP pathway, as well as beta-adrenergic agonist and dopamine, but not morphine. These results suggest that the changes of GLT-1 expression, which alter the glutamate uptake and affect the glutamatergic transmission efficiency, play a role in the development of morphine dependence and the expression of morphine withdrawal.     

Opiate physical dependence and N-methyl-d-aspartate receptors

The present review focused the involvement of N-methyl-d-aspartate (NMDA) receptors in morphine physical dependence. The increased levels of extracellular glutamate, NMDA receptor ζ subunit (NR1) mRNA, NMDA receptor 1 subunit (NR2A) protein, phosphorylated Ca²⁺/calmodulin kinase II (p-CaMKII) protein, c-fos mRNA, c-Fos protein, are observed in the specific brain areas of mice and/or rats showing signs of naloxone-precipitated withdrawal. In preclinical and clinical studies, a variety of NMDA receptor antagonists and pretreatment with an antisense oligonucleotide of the NR1 have been reported to inhibit the development, expression and/or maintenance of opiate physical dependence. In contrast to data obtained in adult animals, NMDA receptor antagonists are neither effective in blocking the development of opiate dependence nor the expression of opiate withdrawal in neonatal rats. In the NMDA receptor-deficient mice, the NR2A knockout mice show the marked loss of typical withdrawal abstinence behaviors precipitated by naloxone. The rescue of NR2A protein by electroporation into the nucleus accumbens of NR2A knockout mice reverses the loss of abstinence behaviors. The activation of CaMKII and increased expression of c-Fos protein in the brain of animals with naloxone-precipitated withdrawal syndrome are prevented by NMDA receptor antagonists, whereas the increased levels of extracellular glutamate are not prevented by them. These findings indicate that glutamatergic neurotransmission at the NMDA receptor site contributes to the development, expression and maintenance of opiate dependence, and suggest that NMDA receptor antagonists may be a useful adjunct in the treatment of opiate dependence.     

Opiate physical dependence and N-methyl-D-aspartate receptors

The present review focused the involvement of N-methyl-D-aspartate (NMDA) receptors in morphine physical dependence. The increased levels of extracellular glutamate, NMDA receptor zeta subunit (NR1) mRNA, NMDA receptor epsilon 1 subunit (NR2A) protein, phosphorylated Ca(2+)/calmodulin kinase II (p-CaMKII) protein, c-fos mRNA, c-Fos protein, are observed in the specific brain areas of mice and/or rats showing signs of naloxone-precipitated withdrawal. In preclinical and clinical studies, a variety of NMDA receptor antagonists and pretreatment with an antisense oligonucleotide of the NR1 have been reported to inhibit the development, expression and/or maintenance of opiate physical dependence. In contrast to data obtained in adult animals, NMDA receptor antagonists are neither effective in blocking the development of opiate dependence nor the expression of opiate withdrawal in neonatal rats. In the NMDA receptor-deficient mice, the NR2A knockout mice show the marked loss of typical withdrawal abstinence behaviors precipitated by naloxone. The rescue of NR2A protein by electroporation into the nucleus accumbens of NR2A knockout mice reverses the loss of abstinence behaviors. The activation of CaMKII and increased expression of c-Fos protein in the brain of animals with naloxone-precipitated withdrawal syndrome are prevented by NMDA receptor antagonists, whereas the increased levels of extracellular glutamate are not prevented by them. These findings indicate that glutamatergic neurotransmission at the NMDA receptor site contributes to the development, expression and maintenance of opiate dependence, and suggest that NMDA receptor antagonists may be a useful adjunct in the treatment of opiate dependence.     

Ionotropic glutamatergic neurotransmission in the ventral tegmental area modulates DeltaFosB expression in the nucleus accumbens and abstinence syndrome in morphine withdrawal rats

The present study sought to assess whether the blockade of ionotropic glutamate receptors in the ventral tegmental area could modulate morphine withdrawal in morphine-dependent rats and the expression of stable DeltaFosB isoforms in the nucleus accumbens during morphine withdrawal. Rats were injected (i.p.) with increasing doses of morphine for 1 week to develop physical dependence, and withdrawal was then precipitated by one injection of naloxone (2 mg/kg, i.p.). Abstinence signs such as jumping, wet-dog shake, writhing posture, weight loss, and Gellert-Holtzman scale score were recorded to evaluate naloxone-induced morphine withdrawal. Two ionotropic glutamate receptor antagonists, dizocilpine (MK-801) and 6, 7-dinitroquinnoxaline-2, 3-dione (DNQX), were microinjected unilaterally into the ventral tegmental area 30 min before naloxone precipitation. A second injection of naloxone (2 mg/kg i.p.) was given 1 h after the first naloxone injection to sustain a maximal level of withdrawal so that the expression of stable DeltaFosB isoforms in the nucleus accumbens could be measured. This would enable determination of the correlation between the MK-801 or DNQX-induced decrease in somatic withdrawal signs and the change in neuronal activity in the nucleus accumbens. The results showed that both MK-801 and DNQX significantly alleviated all symptoms of morphine withdrawal except for weight loss and reduced the expression of stable DeltaFosB isoforms within the nucleus accumbens. These data suggest that ionotropic glutamatergic neurotransmission in the ventral tegmental area regulates the levels of stable DeltaFosB isoforms in the nucleus accumbens, which play a very important role in modulating opiate withdrawal.     

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.     

Effects of dextromethorphan on dopamine release in the nucleus accumbens: Interactions with morphine

Dextromethorphan has been reported to decrease the self-administration of several drugs of abuse, including morphine, methamphetamine, cocaine, and nicotine. Most drugs of abuse increase extracellular levels of dopamine (DA) in the shell of the nucleus accumbens. The effects of dextromethorphan on DA release in the nucleus accumbens of nai;ve rats and of rats treated acutely and chronically with morphine were studied using in vivo microdialysis. DA dialysate levels were evaluated by high-performance liquid chromatography with electrochemical detection. Acute morphine (5 mg/kg i.p.) treatment increased the levels of DA in the nucleus accumbens to approximately 175% of basal levels. Chronic morphine (20 mg/kg i.p. daily for 5 days) increased DA release in the nucleus accumbens to 250% of basal levels. Acute treatment with dextromethorphan (20 or 30 mg/kg s.c.) alone did not alter nucleus accumbens DA levels. Pretreatment with dextromethorphan (20 mg/kg s.c., 20 min prior) potentiated the effects of acute morphine, while attenuating the effects of chronic morphine on nucleus accumbens DA levels. These results with dextromethorphan suggest that the mechanism mediating the effects of dextromethorphan on drug self-administration involves modulation of the dopaminergic mesolimbic pathway.     

吗啡依赖对大鼠伏隔核神经甾体和氨基酸递质的影响

目的　利用大鼠吗啡依赖模型,观察吗啡依赖及戒断对大鼠伏隔核中神经甾体和氨基酸类神经递质水平影响。方法　腹腔注射递增剂量的盐酸吗啡使雄性SD大鼠形成吗啡依赖,并用纳洛酮催促戒断。分离吗啡依赖和戒断大鼠的伏隔核。经液液萃取和固相萃取法提取脱氢表雄酮、孕烯醇酮、别孕烯醇酮、脱氢表雄酮硫酸酯和孕烯醇酮硫酸酯,并采用高效液相色谱质谱系统检测其含量。采用柱前衍生-电化学检测-高效液相色谱法测定甘氨酸、谷氨酸和γ氨基丁酸的含量。结果　与对照组相比,纳洛酮催促戒断时,吗啡依赖大鼠伏隔核脱氢表雄酮硫酸酯的水平下降(P<0 .05),孕烯醇酮(P<0 .01 )和谷氨酸(P<0 .05 )水平均升高。结论　吗啡戒断时大鼠伏隔核的谷氨酸系统处于兴奋状态,伏隔核中的内源性神经甾体在吗啡依赖和戒断的形成中发挥作用。     

Modulation of agmatine on calcium signal in morphine-dependent CHO cells by activation of IRAS, a candidate for imidazoline I1 receptor

The present study investigated the effects of agmatine action on imidazoline I1 receptor antisera-selected protein (IRAS), a candidate for imidazoline I1 receptor, on prolonged morphine-induced adaptations of calcium signal and long-lasting alterations in gene expression to further elucidate the role of IRAS in opioid dependence. Two cell lines, Chinese hamster ovary cells expressing mu opioid receptor alone (CHO-mu) and expressing mu opioid receptor and IRAS together (CHO-mu/IRAS), were used. After chronic treatment with morphine for 48 h, naloxone induced a significant elevation of intracellular calcium concentration ([Ca2+]i) in CHO-mu and CHO-mu/IRAS cells. Agmatine (0.01-3 microM) concentration-dependently inhibited the naloxone-precipitated [Ca2+]i elevation when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Efaroxan, an imidazoline I1 receptor-preferential antagonist, completely reversed the effect of agmatine in CHO-mu/IRAS. Agmatine (1-10 microM) administration after chronic morphine exposure for 48 h partially decreased the [Ca2+]i elevation in CHO-mu/IRAS which was entirely antagonized by efaroxan, but not in CHO-mu. In addition, agmatine (1 microM) co-pretreated with morphine attenuated the naloxone-precipitated increases of cAMP-responsive element binding protein and extracellular signal-regulated kinase 1/2 phosphorylations and c-Fos expression in CHO-mu/IRAS. These effects were blocked by efaroxan as well. Taken together, these results indicate that the agmatine-IRAS action system attenuates the up-regulations of Ca2+ signal and its downstream gene expression in morphine-dependent model in vitro, providing additional evidence to support the contribution of IRAS to opioid dependence.     

Effect of glutamate receptor antagonists microinjected into the nucleus accumbens on place aversion induced by naloxone in single-dose, morphine-treated rats

It is well established that acute morphine withdrawal can be observed following opioid receptor antagonism in rodents. Glutamate receptor antagonists can attenuate the conditioning place aversion (CPA) induced by naloxone in single-dose, morphine-treated rats. Anatomically, the nucleus accumbens appears to be involved in opiate dependence. In the present study, we examined the effects of various glutamate receptor antagonists in the nucleus accumbens on naloxone-induced CPA in rats. MK-801 (an NMDA receptor antagonist), GYKI52466 (an AMPA receptor antagonist), and MCPG (a metabotropic glutamate receptor antagonist) significantly attenuated naloxone-induced CPA following microinjection into the accumbens. In contrast, none of the agents showed place conditioning ability on their own in either morphine-exposed or na?ve rats. The present study suggests that glutamate receptors in the nucleus accumbens play a key role in the motivational component of withdrawal during acute morphine dependence.     

胍丁胺对吗啡长期处理引起的NMDA受体蛋白表达改变的影响

目的:观察胍丁胺对吗啡长期处理引起的NMDA受体蛋白改变的影响。方法:采用吗啡递增给药制备大鼠慢性依赖模型,并观察依赖状态下大鼠海马和伏隔核NMDA受体NR1和NR2B亚基蛋白表达量的变化,以及胍丁胺对吗啡作用的影响。结果:与对照组相比,吗啡慢性处理大鼠在纳洛酮催促下能出现典型的戒断综合征,提示依赖模型建立成功。用免疫印记(Western blotting)技术发现,海马部位的NR2B亚基明显下调,而NR1亚基未见显著性变化;吗啡慢性处理不引起伏隔核NR2B亚基的明显变化,但NR1亚基却显著上调。胍丁胺与吗啡伴随给药能逆转吗啡对两脑区NMDA受体蛋白表达的调节作用。结论:胍丁胺调节阿片依赖可能与其逆转吗啡对NMDA受体亚基数量和构成的调节有关。     

In alcohol-treated rats, naloxone decreases extracellular dopamine and increases acetylcholine in the nucleus accumbens: evidence of opioid withdrawal

Withdrawal from ethanol is aversive. The question is why. As with the withdrawal from morphine, nicotine, diazepam and sugar, the ethanol withdrawal state may involve an increase in nucleus accumbens (NAc) acetylcholine (ACh) causing an alteration of the dopamine (DA)-ACh balance in favor of ACh. Therefore the effects of acute and chronic alcohol (1 gm/kg/day i.p.) treatment on extracellular concentrations of NAc ACh and DA were determined before and after naloxone-precipitated withdrawal. Ethanol initially increased DA to 119% of baseline as measured by microdialysis. This was still the case on the 21st day of ethanol injection when DA increased to 126%. There was no effect of ethanol on ACh. However, naloxone (3 mg/kg s.c.) injected the next day decreased extracellular DA to 83% of baseline and caused a significant rise in ACh to 119%. This state of high ACh combined with low DA may contribute to the aversive aspects of alcohol withdrawal.     

Chronic inhibition of intracellular Ca release or protein kinase C activation significantly reduces the development of morphine dependence

We have previously shown that chronic antagonism of metabotropic glutamate receptors in the brain attenuates naloxone-precipitated withdrawal symptoms in rats treated chronically with subcutaneous (s.c.) morphine. Several subtypes of metabotropic glutamate receptors are directly linked, through a guanine nucleotide regulatory protein, to the phosphatidylinositol (PI) second messenger system. In the present investigation, we assessed the effect of inhibiting the products of PI hydrolysis on the development of opioid dependence. Thus, concurrently with subcutaneous morphine, we infused intracerebroventricularly (i.c.v.) in rats, various doses of chelerythrine, which selectively inhibits the activation of protein kinase C, and thapsigargin, which inhibits the release of intracellular Ca²⁺ when given chronically. Both chelerythrine and thapsigargin reduced the severity of naloxone-precipitated abstinence symptoms when infused i.c.v. at a dose of 10 nmol/day. A single injection of either chelerythrine or thapsigargin immediately prior to the precipitation of withdrawal failed to decrease the severity of abstinence symptoms. Our results suggest that by chronically inhibiting activity of the phosphatidyl-inositol system, the development of morphine dependence can be attenuated.     

NMDA antagonists block restraint-induced increase in extracellular DOPAC in rat nucleus accumbens

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonists CPP, TCP, PK 26124 and ifenprodil, and of the minor tranquillizer diazepam on stress-induced changes of dopamine metabolism in the nucleus accumbens were investigated in the rat. Dopamine metabolism was assessed by measuring the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) by means of in vivo differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes. Physical immobilization of the rats for 4 min caused a marked and long-lasting increase in extracellular DOPAC levels in the nucleus accumbens. A similar, though shorter-lasting, augmentation of extracellular DOPAC was observed in the nucleus accumbens after systemic administration of the anxiogenic agent methyl-beta-carboline-3-carboxylate (beta-CCM) (10 mg/kg s.c.). Pretreatment with CPP (1 mg/kg i.p.), TCP (3 mg/kg i.p.), PK 26124 (3 mg/kg i.p.), ifenprodil (3 mg/kg i.p.) or diazepam (2 mg/kg i.p.) totally antagonized the immobilization-induced increase in extracellular DOPAC in the nucleus accumbens. Diazepam and the benzodiazepine (omega 1-2) receptor antagonist flumazenil (30 mg/kg i.p.), but not ifenprodil, also antagonized the beta-CCM-induced activation of dopamine metabolism in the nucleus accumbens. Finally, systemic administration of haloperidol (25 micrograms/kg i.p.) increased the extracellular concentrations of DOPAC in the nucleus accumbens, but pretreatment with ifenprodil (3 mg/kg i.p.) did not modify this response. These data indicate that NMDA receptor antagonists prevent the activation of dopamine metabolism in the nucleus accumbens caused by immobilization stress but not by beta-CCM-induced anxiogenic stimulation. These results suggest that NMDA receptor antagonists may possess an anxiolytic-like action in the rodent, which is exerted via neuroanatomical circuits distinct from those acted upon by diazepam.     

Effect of agmatine on the development of morphine dependence in rats: potential role of cAMP system

Agmatine is an endogenous amine derived from arginine that potentiates morphine analgesia and blocks symptoms of naloxone-precipitated morphine withdrawal in rats. In this study, we sought to determine whether treatment with agmatine during the development of morphine dependence inhibits the withdrawal symptoms and that the effect is mediated by cAMP system. Exposure of rats to morphine for 7 days resulted in marked naloxone-induced withdrawal symptoms and agmatine treatment along with morphine significantly decreasing the withdrawal symptoms. The levels of cAMP were markedly increased in morphine-treated rat brain slices when incubated with naloxone and this increase was significantly reduced in rats treated with morphine and agmatine. The induction of tyrosine hydroxylase after morphine exposure was also reduced in locus coeruleus when agmatine was administered along with morphine. We conclude that agmatine reduces the development of dependence to morphine and that this effect is probably mediated by the inhibition of cAMP signaling pathway during chronic morphine exposure.     

Attenuation of the reinforcing efficacy of morphine by 18-methoxycoronaridine

In previous studies, 18-methoxycoronaridine, a novel iboga alkaloid congener, has been reported to decrease the self-administration of morphine, cocaine, ethanol and nicotine, and to attenuate naltrexone-precipitated signs of morphine withdrawal. In the present study, the nature of the interaction between 18-methoxycoronaridine and morphine was further investigated. Using in vivo microdialysis, 18-methoxycoronaridine pretreatment (40 mg/kg i.p., 19 h beforehand) was found to markedly inhibit morphine-induced (5 mg/kg, i.p.) dopamine release in the nucleus accumbens and striatum; 18-methoxycoronaridine also enhanced morphine-induced increases in extracellular levels of dopamine's metabolites. These effects, which were more prominent in the nucleus accumbens than in the striatum, suggest that 18-methoxycoronaridine selectively interferes with morphine-induced dopamine release, without altering morphine-induced stimulation of dopamine synthesis. In intravenous morphine self-administration experiments, the effects of acute 18-methoxycoronaridine treatment (40 mg/kg, p.o.) were assessed in rats responding for one of several different unit infusion dosages of morphine (0.01-0.16 mg/kg/infusion). 18-Methoxycoronaridine produced a downward shift in the entire morphine dose-response curve without any displacement to the left or right. These results suggest that 18-methoxycoronaridine reduced the reinforcing efficacy of morphine without altering its apparent potency. Together, the microdialysis and self-administration data suggest that 18-methoxycoronaridine profoundly alters mechanisms crucial to the development and maintenance of opioid addiction.     

IRAS, a candidate for I1-imidazoline receptor, mediates inhibitory effect of agmatine on cellular morphine dependence

Agmatine, an endogenous ligand for the I1-imidazoline receptor, has previously been shown to prevent morphine dependence in rats and mice. To investigate the role of imidazoline receptor antisera-selected protein (IRAS), a strong candidate for I1R, in morphine dependence, two CHO cell lines were created, in which mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of morphine (10 microM), naloxone induced a cAMP overshoot in both cell lines, suggesting cellular morphine dependence had been produced. Agmatine (0.1-2.5 microM) concentration-dependently inhibited the naloxone-precipitated cAMP overshoot when co-pretreated with morphine in CHO-mu/IRAS, but not in CHO-mu. Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS. Efaroxan, an I1R-preferential antagonist, completely blocked the effect of agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of agmatine at 5-100 microM. L-type calcium channel blocker nifedipine entirely mimicked the effects of agmatine at high concentrations on forskolin-stimulated cAMP formation in CHO-mu and naloxone-precipitated cAMP overshoot in morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of agmatine to cause attenuation of cellular morphine dependence. An additional effect of agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated calcium channels are not ruled out. This study suggests that IRAS mediates agmatine's high affinity effects on cellular morphine dependence and may play a role in opioid dependence.     

Acute administration of nicotine increases the in vivo extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid and ascorbic acid preferentially in the nucleus accumbens of the rat: Comparison with caudate-putamen

Using in vivo dialysis and voltammetry, the effect of acute administration of (−)-nicotine (0.8 mg/kg free base, s.c.) on extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid, 5-hydroxyindoleacetic acid and ascorbic acid in the nucleus accumbens and caudate-putamen of chloral hydrateanaesthetised rats has been examined. Nicotine stimulated release of dopamine only in the nucleus accumbens, measured using dialysis. After a short time delay levels of 3,4-dihydroxyphenylacetic acid in both the nucleus accumbens and caudate-putamen also increased. In both regions, 5-hydroxyindoleacetic acid was unaffected by nicotine. Using voltammetry the effect of nicotine on extracellular levels of 3,4-dihydroxyphenylacetic acid and ascorbic acid was examined. An increase in 3,4 dihydroxyphenylacetic acid was observed in both regions after nicotine. This increase was blocked by pretreatment with the central nicotinic receptor antagonist mecamylamine (5 mg/kg). Nicotine increased the level of ascorbic acid in the nucleus accumbens and caudate-putamen; while in animals pretreated with mecamylamine, nicotine decreased levels of ascorbate.These results show that acute administration of nicotine stimulated release of dopamine in the nucleus accumbens and increased the levels of DOPAC and ascorbic acid in the nucleus accumbens and caudate-putamen. This effect is probably mediated by nicotinic receptors as it was antagonised by mecamylamine.     

Cocaine-induced reinstatement in rats: evidence for a critical role of cocaine stimulus properties

Rationale While the N-methyl-d-aspartate (NMDA) glutamate receptor has been strongly implicated in chronic opiate dependence, relatively few studies have examined the effects of NMDA receptor antagonists on withdrawal from acute opiate exposure. Objectives The current study examined the effects of memantine, a well-tolerated NMDA receptor antagonist, on acute opiate dependence as assessed by elevations in rodent startle responding (i.e., “withdrawal-potentiated startle”) and increased pain sensitivity (i.e., hyperalgesia). Results Administration of memantine either attenuated (5 mg/kg) or blocked (10 mg/kg) the expression of withdrawal-potentiated startle during naloxone (2.5 mg/kg)-precipitated withdrawal from a single dose of morphine sulfate (10 mg/kg). Pre-treatment with the NMDA receptor antagonist also inhibited the exacerbation of withdrawal-potentiated startle across repeated acute opiate exposures. Memantine blocked the expression of acute dependence, but was less effective in inhibiting its escalation, when hyperalgesia was used as a measure of withdrawal. These doses of memantine did not affect startle responding or nociception in otherwise drug-free animals. Data from additional control groups indicated that the effects of memantine on the expression of withdrawal were not influenced by nonspecific interactions between the NMDA antagonist and either morphine or naloxone. Conclusions These findings suggest that the NMDA receptor may play a key role in the earliest stages of opiate dependence and provide further evidence that memantine may be useful for the treatment of opiate withdrawal.     

Nicotine attenuates place aversion induced by naloxone in single-dose,morphine-treated rats

Rationale Acute physical dependence refers to the withdrawal syndrome precipitated by an opioid antagonist administered several hours after either a single dose or a short-term infusion of an opioid agonist.Objectives We examined the mechanism of nicotine-induced attenuation of naloxone-precipitated withdrawal syndrome when used to produce an aversive motivational state in a place-conditioning paradigm.Methods The effect of nicotine was investigated through place aversion induced by naloxone in morphine-pretreated rats. Additionally, the mechanism of nicotine action in this model was explored specifically in relation to the dopaminergic system through the use of dopamine receptor antagonist and agonist.Results Place avoidance behavior was potently elicited by naloxone (0.5 mg/kg s.c.) 24 h after a single exposure to morphine (10 mg/kg s.c.). Avoidance behavior was attenuated by pretreatment with a 0.2-mg/kg dose of nicotine 15 min prior to naloxone administration. The effect of nicotine was completely blocked by mecamylamine, but not hexamethonium. The dopamine receptor antagonists haloperidol (0.05, 0.1 mg/kg, s.c.), SCH23390 (0.1 mg/kg, s.c.), raclopride (1.0 mg/kg, s.c.) and eticlopride (0.1 mg/kg, s.c.) showed effects similar to mecamylamine. Additionally, the dopamine receptor agonist apomorphine (0.03, 0.1, 0.3 mg/kg, s.c.) inhibited naloxone-induced place aversion in morphine-treated rats.Conclusion The inhibitory effect of nicotine on place aversion induced by naloxone-precipitated morphine withdrawal may involve a dopaminergic portion of the central nervous system.