Reversal of morphine-induced memory impairment in mice by withdrawal in Morris water maze: possible involvement of cholinergic system

The effects of chronic morphine administration on spatial memory and microtubule dynamicity in male mice's brain

The examination of the influence of morphine on behavioral processes, specifically learning and memory, holds significant importance. Additionally, microtubule proteins play a pivotal role in cellular functions, and the dynamics of microtubules contribute to neural network connectivity, information processing, and memory storage. however, the molecular mechanism of morphine on microtubule dynamics, learning, and memory remains uncovered. In the present study, we examined the effects of chronic morphine administration on memory formation impairment and the kinetic alterations in microtubule proteins induced by morphine in mice. Chronic morphine administration at doses of 5 and 10 mg/kg dose-dependently decreased subjects' performance in spatial memory tasks, such as the Morris Water Maze and Y-maze spontaneous alternation behavior. Furthermore, morphine was found to stabilize microtubule structure, and increase polymerization, and total polymer mass. However, it simultaneously impaired microtubule dynamicity, stemming from structural changes in tubulin dimer structure. These findings emphasize the need for careful consideration of different doses when using morphine, urging a more cautious approach in the administration of this opioid medication.

Chronic exposure to methadone impairs memory, induces microgliosis, astrogliosis and neuroinflammation in the hippocampus of adult male rats

Methadone is a centrally-acting synthetic opioid analgesic widely used in methadone maintenance therapy (MMT) programs throughout the world. Given its neurotoxic effects, particularly on the hippocampus, this study aims to address the behavioral and histological alterations in the hippocampus associated with methadone administration. To do so, twenty-four adult male albino rats were randomized into two groups, methadone treatment and control. Methadone was administered subcutaneously (2.5-10 mg/kg) once a day for two consecutive weeks. A comparison was drawn with behavioral and structural changes recorded in the control group. The results showed that methadone administration interrupted spatial learning and memory function. Accordingly, treating rats with methadone not only induced cell death but also prompted the actuation of microgliosis, astrogliosis, and apoptotic biomarkers. Furthermore, the results demonstrated that treating rats with methadone decreased the complexity of astrocyte processes and the complexity of microglia processes. These findings suggest that methadone altered the special distribution of neurons. Also, a substantial increase was observed in the expression of TNF-α due to methadone. According to the findings, methadone administration exerts a neurodegenerative effect on the hippocampus via dysregulation of microgliosis, astrogliosis, apoptosis, and neuro-inflammation.

Combined molecular, structural and memory data unravel the destructive effect of tramadol on hippocampus

Tramadol is a synthetic analogue of codeine and stimulates neurodegeneration in several parts of the brain that leads to various behavioral impairments. Despite the leading role of hippocampus in learning and memory as well as decreased function of them under influence of tramadol, there are few studies analyzing the effect of tramadol administration on gene expression profiling and structural consequences in hippocampus region. Thus, we sought to determine the effect of tramadol on both PC12 cell line and hippocampal tissue, from gene expression changes to structural alterations. In this respect, we investigated genome-wide mRNA expression using high throughput RNA-seq technology and confirmatory quantitative real-time PCR, accompanied by stereological analysis of hippocampus and behavioral assessment following tramadol exposure. At the cellular level, PC12 cells were exposed to 600μM tramadol for 48 hrs, followed by the assessments of ROS amount and gene expression levels of neurotoxicity associated with neurodegenerative pathways such as apoptosis and autophagy. Moreover, the structural and functional alteration of the hippocampus under chronic exposure to tramadol was also evaluated. In this regard, rats were treated with tramadol at doses of 50 mg/kg for three consecutive weeks. In vitro data revealed that tramadol provoked ROS production and caused the increase in the expression of autophagic and apoptotic genes in PC12 cells. Furthermore, in-vivo results demonstrated that tramadol not only did induce hippocampal atrophy, but it also triggered microgliosis and microglial activation, causing upregulation of apoptotic and inflammatory markers as well as over-activation of neurodegeneration. Tramadol also interrupted spatial learning and memory function along with long-term potentiation (LTP). Taken all together, our data disclosed the neurotoxic effects of tramadol on both in vitro and in-vivo. Moreover, we proposed a potential correlation between disrupted biochemical cascades and memory deficit under tramadol administration.

Unconditioned and learned morphine tolerance influence hippocampal-dependent short-term memory and the subjacent expression of GABA-A receptor alpha subunits

BACKGROUND

ɣ-aminobutyric acid (GABA) facilitator valproic acid may be able to curb memory disruption induced by morphine exposure.

OBJECTIVE

The effects of the GABA facilitator valproic acid on the behavioral tolerance induced by morphine were investigated. Then hippocampal-dependent tasks named spatial-working and short-term memory procedures using the Y-maze apparatus were examined in morphine tolerant rats. Finally, the changes in the expression of hippocampal GABA-A receptors underlying morphine tolerance were also examined.

METHODS

Rats were treated with daily morphine injections, with or without distinct contextual pairing. To examine the effect of valproic acid on morphine tolerance expression, valproic acid was pretreated an hour before morphine. Spatial-working and short-term memory procedures using the Y-maze apparatus were examined in morphine tolerant rats. Afterwards the changes in the expression of hippocampal GABAα receptors using the quantitative real-time PCR and western blot techniques to detect GABArα subunits mRNAs and protein level were studied.

RESULTS

Our results showed that both learned and non-associative morphine tolerance influence short-term memory and the subjacent expression of GABArα mRNAs and protein level. Despite its attenuating effects on the development and expression of both learned and non-associative morphine tolerance, only associative morphine tolerance-induced memory dysfunction was ameliorated by valproic acid pretreatment. We also found that the expression of GABArα1, α2, α5 subunits mRNAs and GABAα protein level were affected heavier in associative morphine tolerant rats.

CONCLUSION

Our data supports the hypothesis that unconditioned and learned morphine tolerance influences short-term memory and the expression of GABArα 1, α2, α5 mRNAs and GABArα protein level differently, and adds to our understanding of the behavioral and molecular aspects of the learned tolerance to morphine effects.

Paternal morphine self-administration produces object recognition memory deficits in female, but not male offspring

RATIONALE

Parental drug use around or before conception can have adverse consequences for offspring. Historically, this research has focused on the effects of maternal substance use on future generations but less is known about the influence of the paternal lineage. This study focused on the impact of chronic paternal morphine exposure prior to conception on behavioral outcomes in male and female progeny.

OBJECTIVES

This study sought to investigate the impact of paternal morphine self-administration on anxiety-like behavior, the stress response, and memory in male and female offspring.

METHODS

Adult, drug-naïve male and female progeny of morphine-treated sires and controls were evaluated for anxiety-like behavior using defensive probe burying and novelty-induced hypophagia paradigms. Hypothalamic-pituitary-adrenal (HPA) axis function was assessed by measuring plasma corticosterone levels following a restraint stressor in male and female progeny. Memory was probed using a battery of tests including object location memory, novel object recognition, and contextual fear conditioning.

RESULTS

Paternal morphine exposure did not alter anxiety-like behavior or stress-induced HPA axis activation in male or female offspring. Morphine-sired male and female offspring showed intact hippocampus-dependent memory: they performed normally on the long-term fear conditioning and object location memory tests. In contrast, paternal morphine exposure selectively disrupted novel object recognition in female, but not male, progeny.

CONCLUSIONS

Our findings demonstrate that paternal morphine taking produces sex-specific and selective impairments in object recognition memory while leaving hippocampal function largely intact.

Pig Skin Gelatin Hydrolysates Attenuate Acetylcholine Esterase Activity and Scopolamine-induced Impairment of Memory and Learning Ability of Mice

The protective effect of pig skin gelatin water extracts (PSW) and the low molecular weight hydrolysates of PSW generated via enzymatic hydrolysis with Flavourzyme® 1000L (LPSW) against scopolamine-induced impairment of cognitive function in mice was determined. Seventy male ICR mice weighing 20–25 g were randomly assigned to seven groups: Control (CON); scopolamine (SCO, 1 mg/kg B.W., intraperitoneally (i.p.); tetrahydroaminoacridine 10 [THA 10, tacrine; 10 mg/kg B.W. per oral (p.o.) with SCO (i.p.)]; PSW 10 (10 mg/kg B.W. (p.o.) with SCO (i.p.); PSW 40 (40 mg/kg B.W. (p.o.) with SCO (i.p.); LPSW 100 (100 mg/kg B.W. (p.o.) with SCO (i.p.); LPSW 400 (400 mg/kg B.W. (p.o.) with SCO (i.p.). All treatment groups, except CON, received scopolamine on the day of the experiment. The oxygen radical absorbance capacity of LPSW 400 at 1 mg/mL was 154.14 μM Trolox equivalent. Administration of PSW and LPSW for 15 weeks did not significantly affect on physical performance of mice. LPSW 400 significantly increased spontaneous alternation, reaching the level observed for THA and CON. The latency time of animals receiving LPSW 400 was higher than that of mice treated with SCO alone in the passive avoidance test, whereas it was shorter in the water maze test. LPSW 400 increased acetylcholine (ACh) content and decreased ACh esterase activity (p<0.05). LPSW 100 and LPSW 400 reduced monoamine oxidase-B activity. These results indicated that LPSW at 400 mg/kg B.W. is a potentially strong antioxidant and contains novel components for the functional food industry.

Mitragynine (Kratom) impairs spatial learning and hippocampal synaptic transmission in rats

BACKGROUND

Mitragynine is the major alkaloid of Mitragyna speciosa (Korth.) or Kratom, a psychoactive plant widely abused in Southeast Asia. While addictive effects of the substance are emerging, adverse cognitive effects of this drug and neuropharmacological actions are insufficiently understood.

AIMS

In the present study, we investigated the effects of mitragynine on spatial learning and synaptic transmission in the CA1 region of the hippocampus.

METHODS

Male Sprague Dawley rats received daily (for 12 days) training sessions in the Morris water maze, with each session followed by treatment either with mitragynine (1, 5, or 10 mg/kg; intraperitoneally), morphine (5 mg/kg; intraperitoneally) or a vehicle. In the second experiment, we recorded field excitatory postsynaptic potentials in the hippocampal CA1 area in anesthetized rats and assessed the effects of mitragynine on baseline synaptic transmission, paired-pulse facilitation, and long-term potentiation. Gene expression of major memory- and addiction-related genes was investigated and the effects of mitragynine on Ca²⁺ influx was also examined in cultured primary neurons from E16-E18 rats.

RESULTS/OUTCOMES

Escape latency results indicate that animals treated with mitragynine displayed a slower rate of acquisition as compared to their control counterparts. Further, mitragynine treatment significantly reduced the amplitude of baseline (i.e. non-potentiated) field excitatory postsynaptic potentials and resulted in a minor suppression of long-term potentiation in CA1. Bdnf and αCaMKII mRNA expressions in the brain were not affected and Ca²⁺ influx elicited by glutamate application was inhibited in neurons pre-treated with mitragynine.

CONCLUSIONS/INTERPRETATION

These data suggest that high doses of mitragynine (5 and 10 mg/kg) cause memory deficits, possibly via inhibition of Ca²⁺ influx and disruption of hippocampal synaptic transmission and long-term potentiation induction.

Exploring Morphine-Triggered PKC-Targets and Their Interaction with Signaling Pathways Leading to Pain via TrkA

It is well accepted that treatment of chronic pain with morphine leads to μ opioid receptor (MOR) desensitization and the development of morphine tolerance. MOR activation by the selective peptide agonist, D-Ala2, N-MePhe4, Gly-ol]-enkephalin(DAMGO), leads to robust G protein receptor kinase activation, β-arrestin recruitment, and subsequent receptor endocytosis, which does not occur in an activation by morphine. However, MOR activation by morphine induces receptor desensitization, in a Protein kinase C (PKC) dependent manner. PKC inhibitors have been reported to decrease receptor desensitization, reduce opiate tolerance, and increase analgesia. However, the exact role of PKC in these processes is not clearly delineated. The difficulties in establishing a particular role for PKC have been, in part, due to the lack of reagents that allow the selective identification of PKC targets. Recently, we generated a conformation state-specific anti-PKC antibody that preferentially recognizes the active state of this kinase. Using this antibody to selectively isolate PKC substrates and a proteomics strategy to establish the identity of the proteins, we examined the effect of morphine treatment on the PKC targets. We found an enhanced interaction of a number of proteins with active PKC, in the presence of morphine. In this article, we discuss the role of these proteins in PKC-mediated MOR desensitization and analgesia. In addition, we posit a role for some of these proteins in mediating pain by TrKA activation, via the activation of transient receptor potential cation channel subfamily V member 1 (TRPV1). Finally, we discuss how these new PKC interacting proteins and pathways could be targeted for the treatment of pain.

Effects of medial prefrontal cortex 5-HT7 receptor knockdown on cognitive control after acute heroin administration

Heroin abuse is linked to a deleterious effect on cognitive functioning in the individual. Recent evidences suggest that the serotonin₇ receptor (5-HT₇R) is engaged in the regulation of cognitive control and the drug use-associated behaviors. However, the role of 5-HT₇R in the cognitive control after acute heroin administration has not been studied. The present study aims to investigate whether the knockdown of the 5-HT₇R by virus-mediated gene silencing in the medial prefrontal cortex (mPFC) could ameliorate the acute heroin-induced cognitive impairments. The attentional function, impulsivity and compulsivity were assessed by the 5-choice serial reaction time task (5-CSRTT) in mice. The memory ability and locomotor activity were examined by the novel objects recognition (NOR), Y-maze and open-field test (OFT). Acute heroin administration at 5 mg/kg produced robust disruptions in attention, impulsivity and motivation in mice. 5-HT₇R knockdown in the mPFC did not affect the 5-CSRTT baseline performance, spatial working memory, visual episodic memory and locomotion. However, mPFC 5-HT₇R knockdown selectively ameliorated acute heroin-induced increase in omissions and premature responses under conditions of increased perceptual load. In addition, mPFC 5-HT₇R knockdown induced increases in perseverative responding observed across both saline and heroin-treated animals. Moreover, 5-HT₇R knockdown prevented the heroin-induced decrease in NR1/CaMKII phosphorylation in mPFC, thus suggesting that 5-HT₇R and N-methyl-d-aspartic acid (NMDA) receptor signaling may be involved in the cognitive outcomes of acute heroin administration. Altogether, these observations suggest modest and restricted effects of mPFC 5-HT7R knockdown on cognitive behaviors, both in the presence or absence of acute heroin treatment.

Mitochondrial impairments contribute to spatial learning and memory dysfunction induced by chronic tramadol administration in rat: Protective effect of physical exercise

Despite the worldwide use of tramadol, few studies have been conducted about its effects on memory and mitochondrial function, and controversial results have been reported. Recently, there has been an increasing interest in physical exercise as a protective approach to neuronal and cognitive impairments. Therefore, the aim of this study was to investigate the effects of physical exercise on spatial learning and memory and brain mitochondrial function in tramadol-treated rats. After completion of 2-week (short-term) and 4-week (long-term) treadmill exercise regimens, male Wistar rats received tramadol (20, 40, 80mg/kg/day) intraperitoneally for 30days. Then spatial learning and memory was assessed by Morris water maze test (MWM). Moreover, brain mitochondrial function was evaluated by determination of mitochondrial reactive oxygen species (ROS) level, mitochondrial membrane potential (MMP), mitochondrial swelling and cytochrome c release from mitochondria. Chronic administration of tramadol impaired spatial learning and memory as well as brain mitochondrial function as indicated by increased ROS level, MMP collapse, increased mitochondrial swelling and cytochrome c release from mitochondria. Conversely, treadmill exercise significantly attenuated the impairments of spatial learning and memory and brain mitochondrial dysfunction induced by tramadol. The results revealed that chronic tramadol treatment caused memory impairments through induction of brain mitochondrial dysfunction. Furthermore, pre-exposure to physical exercise markedly mitigated these impairments through its positive effects on brain mitochondrial function.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

Role of dorsal hippocampal orexin-1 receptors in memory restoration induced by morphine sensitization phenomenon

The present study was examined the blockade of CA1 orexin-1 receptors (OX1Rs) of the dorsal hippocampus in the induction or expression phase on morphine sensitization-induced memory restoration using the Morris water maze (MWM) apparatus. Results showed that pre-training administration of morphine (5mg/kg, s.c.) increases escape latency and traveled distance, while does not alter swimming speed. This supports the impairing effect of morphine on the spatial memory acquisition in male adult rats. Also, in the retrieval session (probe trial) this treatment decreased the time spent in the target quadrant. Moreover, morphine-induced sensitization (15 or 20mg/kg, s.c.; once daily for 3days and followed by 5days no drug treatment) restored the memory acquisition/retrieval deficit which had been induced by pre-training administration of morphine (5mg/kg, s.c.). Intra-CA1 microinjection of subthreshold doses of SB-334867 (OX1Rs antagonist; 10, 20 and 40nmol/rat), 5min before morphine (20mg/kg/day×3days, s.c.; induction phase for morphine sensitization) did not alter restoration of memory acquisition/retrieval produced by the morphine sensitization phenomenon. In contrast, microinjection of subthreshold doses of SB-334867 (10, 20 and 40nmol/rat) into the CA1 region in the training session, 5min prior to morphine (5mg/kg, s.c.; expression phase for morphine sensitization) blocked the spatial memory acquisition/retrieval in morphine-sensitized rats. In conclusion, these findings show that morphine sensitization reverses morphine-induced amnesia. Furthermore, the blockade of CA1 OX1Rs in the expression phase, but not in the induction phase, disrupts memory restoration induced by morphine sensitization.

Memory impairment and reduced exploratory behavior in mice after administration of systemic morphine

In the present study, the effects of morphine were examined on tests of spatial memory, object exploration, locomotion, and anxiety in male ICR mice. Administration of morphine (15 or 30 mg/kg, intraperitoneally (i.p.)) induced a significant decrease in Y-maze alternations compared to saline vehicle-treated mice. The reduced Y-maze alternations induced by morphine were completely blocked by naloxone (15 mg/kg) or β-funaltrexamine (5 mg/kg) but not by norbinaltorphimine (5 mg/kg) or naltrindole (5 mg/kg), suggesting that the morphine-induced spatial memory impairment was mediated predominantly by μ-opioid receptors (MOPs). Significant spatial memory retrieval impairments were observed in the Morris water maze (MWM) in mice treated with morphine (15 mg/kg) or scopolamine (1 mg/kg), but not with naloxone or morphine plus naloxone. Reduced exploratory time was observed in mice after administration of morphine (15 mg/kg), in a novel-object exploration test, without any changes in locomotor activity. No anxiolytic-like behavior was observed in morphine-treated mice in the elevated plus maze. A significant reduction in buried marbles was observed in morphine-treated mice measured in the marble-burying test, which was blocked by naloxone. These observations suggest that morphine induces impairments in spatial short-term memory and retrieval, and reduces exploratory behavior, but that these effects are not because of overall changes in locomotion or anxiety.

Beneficial effects of co-treatment with dextromethorphan on prenatally methadone-exposed offspring

Background

Heroin use among young women of reproductive age has drawn much attention around the world. Although methadone is widely used in maintenance therapy for heroin/morphine addiction, the long-term effects of prenatal exposure to methadone and preventative therapy remain unclear. For revealing this question, female pregnant Sprague–Dawley rats were sub-grouped to receive (1) vehicle, (2) methadone 5 mg/kg at embryonic day 3 (E3) and then 7 mg/kg from E4 to E20, (3) dextromethorphan (DM) 3 mg/kg, and (4) methadone + DM (the rats received methadone followed by DM treatment), subcutaneously, twice a day from E3 to E20. The body weight, natural withdrawal, pain sensitivity, ED50, conditioned place preference and water maze were conducted at different postnatal stages (P1 to P79) of offspring. The quantitative real-time RT-PCR and electrophysiology were also used to measure the gene expression of opioid receptors in the spinal cord and changes of LTP/LTD in the hippocampus, separately.

Results

Prenatal exposure to methadone or DM did not affect survival rate, body weight, water maze and LTP or LTD of offspring. However, prenatal methadone significantly increased the withdrawal symptoms, pain sensitivity, addiction liability and decreased the mRNA expression of pain related opioid receptors. Co-administration of DM with methadone in the maternal rats effectively prevented these abnormalities of offspring induced by methadone.

Conclusions

Our study clearly showed that co-administration of dextromethorphan with methadone in the maternal rats prevented the adverse effects induced by prenatal methadone exposure. It implies that dextromethorphan may have a potential to be used in combination with methadone for maintenance treatment in pregnant heroin-addicted women to prevent the adverse effects induced by methadone on offspring.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0126-2) contains supplementary material, which is available to authorized users.

Imbalance of HCN1 and HCN2 expression in hippocampal CA1 area impairs spatial learning and memory in rats with chronic morphine exposure

The hyperpolarization-activated cyclic-nucleotide-gated non-selective cation (HCN) channels play a vital role in the neurological basis underlying nervous system diseases. However, the role of HCN channels in drug addiction is not fully understood. In the present study, we investigated the expression of HCN1 and HCN2 subunits in hippocampal CA1 and the potential molecular mechanisms underlying the modulation of HCN channels in rats with chronic morphine exposure with approaches of electrophysiology, water maze, and Western blotting. We found that chronic morphine exposure (5 mg/kg, sc, for 7 days) caused an inhibition of long-term potentiation (LTP) and impairment of spatial learning and memory, which is associated with a decrease in HCN1, and an increase in HCN2 on cell membrane of hippocampal CA1 area. Additional experiments showed that the imbalance of cell membrane HCN1 and HCN2 expression under chronic morphine exposure was related to an increase in expression of TPR containing Rab8b interacting protein (TRIP8b) (1a-4) and TRIP8b (1b-2), and phosphorylation of protein kinase A (PKA) and adaptor protein 2 μ2 (AP2 μ2). Our results demonstrate the novel information that drug addiction-induced impairment of learning and memory is involved in the imbalance of HCN1 and HCN2 subunits, which is mediated by activation of TRIP8b (1a-4), TRIP8b (1b-2), PKA and AP2 μ2.

Effects of voluntary and treadmill exercise on spontaneous withdrawal signs, cognitive deficits and alterations in apoptosis-associated proteins in morphine-dependent rats

Chronic exposure to morphine results in cognitive deficits and alterations of apoptotic proteins in favor of cell death in the hippocampus, a brain region critically involved in learning and memory. Physical activity has been shown to have beneficial effects on brain health. In the current work, we examined the effects of voluntary and treadmill exercise on spontaneous withdrawal signs, the associated cognitive defects, and changes of apoptotic proteins in morphine-dependent rats. Morphine dependence was induced through bi-daily administrations of morphine (10mg/kg) for 10 days. Then, the rats were trained under two different exercise protocols: mild treadmill exercise or voluntary wheel exercise for 10 days. After exercise training, their spatial learning and memory and aversive memory were examined by a water maze and by an inhibitory avoidance task, respectively. The expression of the pro-apoptotic protein Bax and the anti-apoptotic protein Bcl-2 in the hippocampus were determined by immunoblotting. We found that chronic exposure to morphine impaired spatial and aversive memory and remarkably suppressed the expression of Bcl-2, but Bax expression remained constant. Both voluntary and treadmill exercise alleviated memory impairment, increased the expression of Bcl-2 protein, and only the later suppressed the expression of Bax protein in morphine-dependent animals. Moreover, both exercise protocols diminished the occurrence of spontaneous morphine withdrawal signs. Our findings showed that exercise reduces the spontaneous morphine-withdrawal signs, blocks the associated impairment of cognitive performance, and overcomes morphine-induced alterations in apoptotic proteins in favor of cell death. Thus, exercise may be a useful therapeutic strategy for cognitive and behavioral deficits in addict individuals.

Morphine increases hippocampal viral load and suppresses frontal lobe CCL5 expression in the LP-BM5 AIDS model

Chronic opiate abuse accelerates the development of cognitive deficits in human immunodeficiency virus (HIV)-1 patients. To investigate morphine's effects on viral infection of the central nervous system, we applied chronic morphine treatment to the LP-BM5 murine acquired immunodeficiency syndrome (MAIDS) model. LP-BM5 infection induces proinflammatory cytokine/chemokine production, correlating to increased blood-brain barrier permeability. Morphine treatment significantly increased LP-BM5 viral load in the hippocampus, but not in the frontal lobe. Morphine reduced the chemokine CCL5 to non-infected levels in the frontal lobe, but not in the hippocampus. These data indicate a region-specific mechanism for morphine's effects on virally-induced neurocognitive deficits.

Effect of repeated morphine withdrawal on spatial learning, memory and serum cortisol level in mice

BACKGROUND

One of the serious problems that opioid addicted people are facing is repeated withdrawal syndrome that is accompanying with a significant stress load for addicts. Therefore, the aim of this study was to evaluate the effects of repeated withdrawal on spatial learning, memory and serum cortisol levels in morphine-dependent mice.

MATERIALS AND METHODS

Male NMRI mice received morphine as daily increasing doses for 3 days. After that, the mice underwent one time or repeated spontaneous or pharmacologic (naloxone-precipitated) withdrawal. Then spatial learning and memory were investigated by morris water maze test, and at the end trunk blood samples were collected for measurement of serum cortisol levels.

RESULTS

The results showed that only repeated spontaneous withdrawal significantly increases escape latency (P < 0.05), and in other models of withdrawal, spatial learning and memory were intact. The results of probe trial were intact in all groups. Radioimmunoassay showed that serum cortisol levels were increased significantly in all models of withdrawal (P < 0.05 and P < 0.01) except the repeated spontaneous withdrawal.

CONCLUSION

The results showed that short periods of withdrawal syndrome can increase serum cortisol levels; however they do not affect spatial learning and memory. Nevertheless, repeated spontaneous withdrawal can make learning slow.

Involvement of nitric oxide in pioglitazone memory improvement in morphine-induced memory impaired mice

INTRODUCTION

Pioglitazone, a PPAR-γ agonist, which is clinically used in treating diabetic patients, has been recently reported to have crucial roles in improving cognition and memory performance. Since the mechanisms involved in the neuroprotective effect of pioglitazone are not entirely understood, the current study was designed to investigate the possible interaction of pioglitazone with morphine in memory-impaired mice and the probable role of nitric oxide (NO) in this effect.

MATERIALS AND METHODS

All the experiments were performed in passive avoidance and Y-maze paradigms. To induce memory impairment, mice were administered morphine (1, 3 and 10mg/kg, s.c.) immediately before the training trial. Pioglitazone (20, 40 and 80mg/kg, p.o.) was gavaged 2h prior to the training trial. Further, an NO synthase inhibitor, L-NAME (10mg/kg, i.p.), or an inducible NO synthase inhibitor, aminoguanidine (100mg/kg, i.p.) was administered 30 min before the training trial to determine the possible involvement of NO in the restorative effect of pioglitazone.

RESULTS

1) Morphine dose dependently impaired the acquisition of spatial memory and passive avoidance task. 2) Treatment with pioglitazone significantly improved the memory performance in morphine-treated mice in both tests. 3) In the passive avoidance task, L-NAME, but not aminoguanidine, altered the effect of pioglitazone on morphine-induced memory impairment. 4) In Y-maze discrimination, the memory improving effect of pioglitazone was reversed by both NO synthase inhibitors, L-NAME and aminoguanidine.

DISCUSSION

Our results demonstrate that the pioglitazone improving effect on the morphine-induced impairment of memory acquisition is at least in part through the NO pathway. It is suggested that in short term spatial recognition memory, both inducible and constitutive NO synthases are involved, but in the long term fear memory, only the constitutive NO synthases indicated a prominent role in the anti-amnestic effect of pioglitazone on morphine-induced memory impairment.

Long-term methadone treatment reduces phosphorylation of CaMKII in rat brain

Objectives

To reveal a possible relationship between a previously reported impairment of novelty seeking in rats exposed to methadone and changes in intracellular molecules related to learning and memory.

Methods

Expression of phosphorylated Ca2+-calmodulin kinase II (pCaMKII), extracellular-signal-regulated kinase 2 (pERK2) and cAMP-responsive element binding protein (pCREB), as well as protein kinase A (PKA), was investigated in rat hippocampus one hour, one day and one week after a three-week methadone administration regime. Studies after an equivalent exposure to morphine, and in the frontal pole, were included for comparison.

Key findings

One day after the last methadone injection the hippocampal level of pCaMKII was significantly reduced. This coincides with a previously reported impairment of novelty seeking. At one hour and one week no significant changes were seen. There was no effect on the other proteins. Morphine affected pCaMKII similarly to methadone. Also in the frontal pole the two drugs reduced pCaMKII one day after the last injection.

Conclusion

The impaired novelty seeking previously found in rats administered methadone for three weeks coincides with a reduced level of pCaMKII in the brain. This finding implies that methadone treatment may affect learning and memory processes, and should stimulate further studies in a field with important knowledge gaps.

Voluntary exercise ameliorates cognitive deficits in morphine dependent rats: the role of hippocampal brain-derived neurotrophic factor

Chronic exposure to opiates impairs spatial learning and memory. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we investigated whether voluntary exercise would ameliorate the cognitive deficits that are induced by morphine dependence. If an effect of exercise was observed, we aimed to investigate the possible role of hippocampal brain-derived neurotrophic factor (BDNF) in the exercise-induced enhancement of learning and memory in morphine-dependent rats. The rats were injected with bi-daily doses (10mg/kg, at 12h intervals) of morphine over a period of 10 days of voluntary exercise. Following these injections, a water maze task was performed twice a day for five consecutive days, followed by a probe trial 2 days later. A specific BDNF inhibitor (TrkB-IgG chimera) was used to block the hippocampal BDNF action during the 10 days of voluntary exercise. We found that voluntary exercise blocked the ability of chronic morphine to impair spatial memory retention. A blockade of the BDNF action blunted the exercise-induced improvement of spatial memory in the dependent rats. Moreover, the voluntary exercise diminished the severity of the rats' dependency on morphine. This study demonstrates that voluntary exercise ameliorates, via a TrkB-mediated mechanism, the cognitive deficits that are induced by chronic morphine. Thus, voluntary exercise might be a potential method to ameliorate some of the deleterious behavioral consequences of the abuse of morphine and other opiates.

Long-term methadone treatment impairs novelty preference in rats both when present and absent in brain tissue

Behavioral consequences of long-term methadone treatment have received little attention either in humans or experimental animals. In this work, we show that methadone (2.5-10 mg/kg) administered (sc) once daily for three weeks with repeated withdrawal on Saturday and Sunday impairs the novelty preference in rats. One hour after the last injection, when methadone was still present in brain tissue, the rats were too affected by the sedative effects of the drug to perform the test. This was confirmed by an almost total lack of locomotor activity or exploratory behavior. One day after the last injection, the methadone treated rats showed a 70% reduction (p < 0.05) in novelty preference compared to rats administered saline. No methadone was detected in the brain tissue at this time. Moreover, there were no differences in locomotor activity or total exploratory behavior between the groups, indicating a specific impairment of cognitive functioning. In brain tissue, the methadone concentration versus time profile was shifted to the left after long-term treatment, indicating a change in uptake and distribution of the drug. The area under the two concentration versus time curves was, however, similar. Methadone disappeared completely from the brain within one day. Together, these results suggest that long-term methadone treatment may have a negative impact on cognitive functioning in rats, regardless of whether methadone is present in brain tissue.

Depression-like behavior and mechanical allodynia are reduced by bis selenide treatment in mice with chronic constriction injury: a comparison with fluoxetine, amitriptyline, and bupropion

RATIONALE

Neuropathic pain is associated with significant co-morbidities, including depression, which impact considerably on the overall patient experience. Pain co-morbidity symptoms are rarely assessed in animal models of neuropathic pain. Neuropathic pain is characterized by hyperexcitability within nociceptive pathways and remains difficult to treat with standard analgesics.

OBJECTIVES

The present study determined the effect of bis selenide and conventional antidepressants (fluoxetine, amitriptyline, and bupropion) on neuropathic pain using mechanical allodynic and on depressive-like behavior.

METHODS

Male mice were subjected to chronic constriction injury (CCI) or sham surgery and were assessed on day 14 after operation. Mice received oral treatment with bis selenide (1-5 mg/kg), fluoxetine, amitriptyline, or bupropion (10-30 mg/kg). The response frequency to mechanical allodynia in mice was measured with von Frey hairs. Mice were evaluated in the forced swimming test (FST) test for depression-like behavior.

RESULTS

The CCI procedure produced mechanical allodynia and increased depressive-like behavior in the FST. All of the drugs produced antiallodynic effects in CCI mice and produced antidepressant effects in control mice without altering locomotor activity. In CCI animals, however, only the amitriptyline and bis selenide treatments significantly reduced immobility in the FST.

CONCLUSION

These data demonstrate an important dissociation between the antiallodynic and antidepressant effects in mice when tested in a model of neuropathic pain. Depressive behavior in CCI mice was reversed by bis selenide and amitriptyline but not by the conventional antidepressants fluoxetine and buproprion. Bis selenide was more potent than the other drugs tested for antidepressant-like and antiallodynic effects in mice.

Chronic morphine treatment impaired hippocampal long-term potentiation and spatial memory via accumulation of extracellular adenosine acting on adenosine A1 receptors

Chronic exposure to opiates impairs hippocampal long-term potentiation (LTP) and spatial memory, but the underlying mechanisms remain to be elucidated. Given the well known effects of adenosine, an important neuromodulator, on hippocampal neuronal excitability and synaptic plasticity, we investigated the potential effect of changes in adenosine concentrations on chronic morphine treatment-induced impairment of hippocampal CA1 LTP and spatial memory. We found that chronic treatment in mice with either increasing doses (20-100 mg/kg) of morphine for 7 d or equal daily dose (20 mg/kg) of morphine for 12 d led to a significant increase of hippocampal extracellular adenosine concentrations. Importantly, we found that accumulated adenosine contributed to the inhibition of the hippocampal CA1 LTP and impairment of spatial memory retrieval measured in the Morris water maze. Adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine significantly reversed chronic morphine-induced impairment of hippocampal CA1 LTP and spatial memory. Likewise, adenosine deaminase, which converts adenosine into the inactive metabolite inosine, restored impaired hippocampal CA1 LTP. We further found that adenosine accumulation was attributable to the alteration of adenosine uptake but not adenosine metabolisms. Bidirectional nucleoside transporters (ENT2) appeared to play a key role in the reduction of adenosine uptake. Changes in PKC-alpha/beta activity were correlated with the attenuation of the ENT2 function in the short-term (2 h) but not in the long-term (7 d) period after the termination of morphine treatment. This study reveals a potential mechanism by which chronic exposure to morphine leads to impairment of both hippocampal LTP and spatial memory.

Interactions between morphine, scopolamine and nicotine: schedule-controlled responding in rats

Functional interactions between drugs acting on either opioid or cholinergic systems have been demonstrated for both neurochemical and behavioral measures. This study used schedule-controlled responding and isobolographic analyses to examine interactions between the micro opioid receptor agonist morphine and the muscarinic acetylcholine receptor antagonist scopolamine as well as the nicotinic acetylcholine receptor agonist nicotine. In 8 rats responding under a fixed ratio 5 schedule of food presentation, morphine (3.2-10mg/kg), scopolamine (0.032-1.0mg/kg), and nicotine (0.1-1mg/kg) each dose-dependently decreased responding. Acute injection of scopolamine shifted the morphine dose-response curved leftward and downward and acute injection of morphine shifted the scopolamine and nicotine dose-response curves leftward and downward. The interaction between morphine and nicotine was additive; however, the interaction between morphine and scopolamine was infra-additive or supra-additive, depending on whether scopolamine or morphine was administered first. These results provide quantitative evidence regarding potentially important interactions between drugs acting on either opioid or cholinergic systems, although these interactions are modest and appear to depend on the specific conditions of drug administration.

Forced exercise improves passive avoidance memory in morphine-exposed rats

The aim of this study was to investigate the effect of short-term forced exercise protocol on passive avoidance retention in morphine-exposed rats. Effects of morphine on acquisition and retrieval of retention have been proven in the avoidance paradigms. Twenty four male Wistar rats weighing 250-300 g were used in this study. Animals were randomly divided into four groups including: (1) non-morphine-exposed without exercise (nA.nE) (2) non-morphine-exposed with exercise (nA.E) (3) morphine-exposed without exercise (A.nE) and (4) morphine-exposed with exercise (A.E). Rats ran as forced exercise on the motorized treadmill 1 h daily for ten days. Morphine-exposed animals received intraperitoneal morphine during first 5 days of the exercise period and their dependence to morphine was confirmed by naloxane admistration (10 mg kg(-1), i.p.) and withdrawal test. After 10 days of forced exercise, step down latency was tested and Inflexion Ratio (IR) was evaluated in each rat. Baseline step down latencies before any morphine exposing or exercise have shown no significant alteration in all groups. Inflexion Ratio (IR) ofnA.E group has increased significantly (p<0.001) at 1, 3, 7 and 14 days after receiving shock (learning) compared to nA.nE and A.E groups. Our data showed that short-term forced exercise on treadmill improved retention in both morphine-exposed and non morphine-exposed rats at least up to 7 days and more than 14 days, respectively. Alteration in retention between exercised groups may attribute the release of adrenal stress hormones such as epinephrine and corticosterone because of the emotional arousal.

Involvement of dorsal hippocampal nicotinic receptors in the effect of morphine on memory retrieval in passive avoidance task

The present study evaluated the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus on morphine-induced amnesia and morphine state-dependent memory in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas in the CA1 regions of the dorsal hippocampi, trained in a step-through type passive avoidance task, and tested 24 h after training to measure step-through latency. Results indicate that post-training subcutaneous (s.c.) administration of morphine (2.5-7.5 mg/kg) dose-dependently reduced the step-through latency, showing an amnestic response. Post-training intra-CA1 microinjection of nicotine (0.5-1 microg/rat) decreased significantly the amnesia induced by post-training morphine (7.5 mg/kg). Moreover, co-treatment of mecamylamine (0.5 and 1 microg/rat, intra-CA1) with an ineffective dose of morphine (2.5 mg/kg), immediately after training, caused inhibition of memory retrieval. On the other hand, amnesia produced by post-training morphine (7.5 mg/kg) was reversed by pre-test administration of the opioid that is due to a state-dependent effect. Interestingly, pre-test intra-CA1 microinjection of nicotine (0.25 and 0.5 microg/rat) improved post-training morphine (7.5 mg/kg)-induced retrieval impairment. Moreover, pre-test administration of the same doses of nicotine in combination with a lower dose of morphine (0.5 mg/kg), which had no effects alone, synergistically improved memory performance impaired by post-training morphine. Pre-test injection of mecamylamine (0.5-2 microg/rat) prevented the restoration of memory by pre-test morphine. It is important to note that post-training or pre-test intra-CA1 administration of the same doses of nicotine or mecamylamine, alone did not affect memory retrieval. These results suggest that nicotinic acetylcholine receptors of the hippocampal CA1 regions may play an important role in morphine-induced amnesia and morphine state-dependent memory.

Expression changes of hippocampal energy metabolism enzymes contribute to behavioural abnormalities during chronic morphine treatment

Dependence and impairment of learning and memory are two well-established features caused by abused drugs such as opioids. The hippocampus is an important region associated with both drug dependence and learning and memory. However, the molecular events in hippocampus following exposure to abused drugs such as opioids are not well understood. Here we examined the effect of chronic morphine treatment on hippocampal protein expression by proteomic analyses. We found that chronic exposure of mice to morphine for 10 days produced robust morphine withdrawal jumping and memory impairment, and also resulted in a significant downregulation of hippocampal protein levels of three metabolic enzymes, including Fe-S protein 1 of NADH dehydrogenase, dihydrolipoamide acetyltransferase or E2 component of the pyruvate dehydrogenase complex and lactate dehydrogenase 2. Further real-time quantitative PCR analyses confirmed that the levels of the corresponding mRNAs were also remarkably reduced. Consistent with these findings, lower ATP levels and an impaired ability to convert glucose into ATP were also observed in the hippocampus of chronically treated mice. Opioid antagonist naltrexone administrated concomitantly with morphine significantly suppressed morphine withdrawal jumping and reversed the downregulation of these proteins. Acute exposure to morphine also produced robust morphine withdrawal jumping and significant memory impairment, but failed to decrease the expression of these three proteins. Intrahippocampal injection of D-glucose before morphine administration significantly enhanced ATP levels and suppressed morphine withdrawal jumping and memory impairment in acute morphine-treated but not in chronic morphine-treated mice. Intraperitoneal injection of high dose of D-glucose shows a similar effect on morphine-induced withdrawal jumping as the central treatment. Taken together, our results suggest that reduced expression of the three metabolic enzymes in the hippocampus as a result of chronic morphine treatment contributes to the development of drug-induced symptoms such as morphine withdrawal jumping and memory impairment.

Effects of morphine dependence on the performance of rats in reference and working versions of the water maze

Numerous studies have dealt with the role of opiate system in tasks aimed at measurement of cognitive behavior, but the role of morphine dependence on learning and memory is still controversial. In this study chronic exposure to morphine was employed to evaluate learning ability and spatial short-term memory (working memory) and long-term memory (reference memory) in the water maze task. Male albino rats were made dependent by chronic administration of morphine in drinking water that lasted at least 21 days. In Experiment 1, the performance of animals was evaluated in reference memory version of the water maze. Rats were submitted to a session of 6 trials for 6 consecutive days to find the submerged platform that was located in the center of a quadrant. Latency and traveled distance to find the platform were measured as indexes of learning. Memory retention was tested 24 h after the last training session in a probe trial (60 s) in which there was no platform and the time spent in each quadrant of the water maze was recorded. Results indicated that latency and traveled distance to find the platform were same in control and dependent rats during training days, but during the probe test morphine-dependent group spent significantly less time in the target quadrant. In Experiment 2, training on working memory version of the water maze task was started. Only two trials per day were given until the performance of animals was stabilized (at least 5 days). Final test was done at day 6. Acquisition-retention interval was 75 min. No significant differences were found on acquisition and retention trials between morphine and control groups. Our findings indicate that chronic exposure to morphine did not impair learning ability, but partially impaired retention of spatial long-term (reference) memory. Moreover, dependence on morphine did not affect either acquisition or retention of spatial short (working) memory.

Effects of scopolamine on morphine-induced conditioned place preference in mice

It is well known that the cholinergic system plays a crucial role in learning and memory. Psychopharmacological studies in humans and animals have shown that a systemic cholinergic blockade may induce deficits in learning and memory. Accumulated studies have indicated that learning and memory play an important role in drug addition. In the present study, in order to get a further understanding about the functions of the cholinergic system in drug-related learning and memory, we examined the effects of scopolamine (0.5, 1.0 and 2.0 mg/kg) on morphine-induced conditioned place preference (CPP). Two kinds of morphine exposure durations (4 days and 12 days) were used. The main finding was that all doses of scopolamine enhanced the extinction of morphine-induced CPP in mice treated with morphine for 12 days. However, in mice treated with morphine for 4 days, all doses of scopolamine did not inhibit morphine-induced CPP. The highest dose (2.0 mg/kg) of scopolamine even significantly delayed the extinction of morphine-induced CPP. Our results suggest that the effects of a systemic cholinergic blockade on morphine-induced CPP depend on the morphine exposure time.

Effects of morphine and its withdrawal on Y-maze spatial recognition memory in mice

Effects of morphine on acquisition and retrieval of memory have been proven in the avoidance paradigms. In present study, we used a two-trial recognition Y-maze to test the effects of acute morphine and morphine withdrawal on spatial recognition memory. The Y-maze is based on the innate tendency of rodents to explore novel environments and therefore avoid punishment and reward.

RESULTS

1) Pre-training morphine 10 mg/kg impaired the recognition spatial memory of acquisition after a 1 h inter-trial interval (ITI), whereas morphine 2.5, 5 and 10 mg/kg showed impairment after 2 h ITI. 2) Pre-retention morphine 5, 10 mg/kg disrupted the retrieval of memory after 1 h ITI. 3) Morphine 5 and 10 mg/kg caused hyper-locomotor activity depending on the state. 4) Mice withdrawn from morphine 40 mg/kg but not 10 mg/kg for 3 days showed amnesia in Y-maze. Our data suggested that acute morphine impaired the acquisition and retrieval of spatial recognition memory and increased the locomotor activity in the Y-maze depending on the dose and state. Moreover, withdrawal from chronic morphine also impaired acquisition in the Y-maze depending on the dose and state.

Chronic methadone treatment and repeated withdrawal impair cognition and increase the expression of apoptosis-related proteins in mouse brain

OBJECTIVES

This study analyzes the effects of prolonged administration of methadone and withdrawal on sensorimotor and cognitive performance in mice and explores the associated changes in brain expression of proteins regulating the extrinsic (FasL, Fas, and caspase-8) and the mitochondrial (Bcl-2, Bcl-x(L), Bad, and Bax) apoptotic pathways.

RESULTS

Our findings indicate that, although acute methadone administration impairs some sensorimotor abilities, tolerance to most of the deleterious effects develops after chronic administration. Cognitive abilities in the Morris water maze were impaired by chronic methadone and, to a greater extent, by exposure to precipitated withdrawal every week in the course of methadone treatment. Both the chronic methadone and repeated withdrawal groups showed up-regulation of several pro-apoptotic proteins (FasL, the active fragment of caspase-8, and Bad) in the cortex and hippocampus, indicating activation of both the death-receptor and mitochondrial apoptotic pathways. In contrast, reduced expression of the apoptosis regulatory proteins FasL and Bad was found after a single administration of methadone.

CONCLUSIONS

Our data suggest that neural apoptotic damage could contribute to impairment of the cognitive abilities of mice observed after chronic methadone administration and withdrawal.

Effects of intracerebroventricularly-injected morphine on anxiety, memory retrieval and locomotor activity in rats: involvement of vasopressinergic system and nitric oxide pathway

Morphine has been shown to alter several behavioural processes. We aimed to investigate the effects of intracerebroventricular (i.c.v.) morphine on anxiety, memory retrieval and locomotor activity in rats and to elucidate the possible involvement of the vasopressinergic system and the nitric oxide (NO) pathway in these effects. Rats were pretreated with morphine (0.5, 5, 50 microg/5 microl; i.c.v.) or saline (5 microl; i.c.v.) 30 min before the elevated plus maze test, the probe trial of the Morris water maze and the open field test. Morphine (5 microg/5 microl; i.c.v.) induced significant anxiolytic effects in the elevated plus maze. None of the doses of morphine produced any effects in the probe trial of the Morris water maze and the open field. Pretreatment with an arginine vasopressin (AVP) V(1) receptor antagonist (25, 125 ng/5 microl; i.c.v.), an AVP V(2) receptor antagonist (25, 125 ng/5 microl; i.c.v.), or L-NAME, an NO synthase inhibitor (5, 25 microg/5 microl; i.c.v.) 30 min before morphine significantly prevented the anxiolytic effects of morphine. These results suggest that i.c.v. morphine has significant anxiolytic effects, probably mediated by both vasopressinergic system and NO pathway, but has no effect on memory retrieval or locomotor activity, at least at the applied doses.

The involvement of cholinergic and noradrenergic systems in behavioral recovery following oxotremorine treatment to chronically stressed rats

Chronic stress in rats has been shown to impair learning and memory, and precipitate several affective disorders like depression and anxiety. The mechanisms involved in these stress-induced disorders and the possible reversal are poorly understood, thus limiting the number of drugs available for their treatment. Our earlier studies suggest cholinergic dysfunction as the underlying cause in the behavioral deficits following stress. Muscarinic cholinergic agonist, oxotremorine is demonstrated to have a beneficial effect in reversing brain injury-induced behavioral dysfunction. In this study, we have evaluated the effect of oxotremorine treatment on chronic restraint stress-induced cognitive deficits. Rats were subjected to restraint stress (6 h/day) for 21 days followed by oxotremorine treatment for 10 days. Spatial learning and memory was assessed in a partially baited eight-arm radial maze task. Stressed rats exhibited impairment in performance, with decreased percentage of correct choices and an increase in the number of reference memory errors (RMEs). Oxotremorine treatment (0.1 or 0.2 mg/kg, i.p.) to stressed rats resulted in a significant increase in the percent correct choices and a decrease in the number of RMEs compared with stress as well as the stress+vehicle-treated groups. In the retention test, oxotremorine treated rats committed less RMEs compared with the stress group. Chronic restraint stress decreased acetylcholinesterase (AChE) activity in the hippocampus, frontal cortex and septum, which was reversed by both the doses of oxotremorine. Further, oxotremorine treatment also restored the norepinephrine levels in the hippocampus and frontal cortex. Thus, this study demonstrates the potential of cholinergic muscarinic agonists and the involvement of both cholinergic and noradrenergic systems in the reversal of stress-induced learning and memory deficits.

Antagonistic effects of methanolic extract of Polygala telephioides on morphine responses in mice

The present study was undertaken to investigate the antagonistic effects of the methanolic extract of Polygala telephioides (PT) on morphine responses in mice. Single administration of PT tended to antagonize the morphine-induced analgesia in a hot-plate test. Moreover, PT (300 mg/kg, p.o.) improved the morphine-induced memory impairment in an elevated plus maze test. However, PT alone had no effect on behaviors in the open-field, hot-plate and elevated plus maze tests. We investigated the effects of PT on naloxone-induced jumping (as withdrawal sign) in morphine-dependent mice. To induce dependence, mice were twice daily treated with morphine (10-45 mg/kg, s.c.) for 5 days. Co-administrations of PT (10, 100 and 300 mg/kg, p.o.) during repeated morphine treatments significantly suppressed the naloxone (10 mg/kg, i.p.)-induced jumping. However, the naloxone-induced jumping was not affected by a single large administration of PT on the 5th day. The inhibitory effect of PT on the naloxone-induced jumping was due to the development of dependence rather than expression of withdrawal sign. Moreover, single administration of PT (30 mg/kg, p.o.) decreased the morphine levels in plasma. These results indicate that PT may be useful in facilitating narcotic detoxification.

Effects of systemic administration of beta-casomorphin-5 on learning and memory in mice

The effects of systemic administration of bovine beta-casomorphin-5 (Tyr-Pro-Phe-Pro-Gly), a mu-opioid receptor agonist derived from milk beta-casein, on spontaneous alternation behavior in the Y-maze (spatial short-term memory) and step-down-type passive avoidance response (non-spatial long-term memory) were investigated in mice. Intraperitoneal (i.p.) administration of beta-casomorphin-5 (0.1-20 mg/kg) did not have a significant effect on either spontaneous alternation behavior or passive avoidance response. However, a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improved scopolamine (1 mg/kg, s.c.)-induced impairment of spontaneous alternation behavior and passive avoidance response. Pretreatment with intracerebroventricular injections of beta-funaltrexamine (a mu-opioid receptor antagonist, 0.1 microg/mouse) and naloxonazine (a mu(1)-opioid antagonist, 5 microg/mouse), which did not improve scopolamine-induced impairment, prevented the ameliorating effect of beta-casomorphin-5 on scopolamine-induced impairment of passive avoidance response. These results indicated that systemic administration of a low dose (1 mg/kg, i.p.) of beta-casomorphin-5 improves the disturbance of learning and memory resulting from cholinergic dysfunction through central mediation involving mu(1)-opioid receptors.