The antagonistic effects of naloxone on cycloheximide and anisomycin-induced amnesia

Farnesyl Transferase Inhibitor Lonafarnib Enhances α7nAChR Expression Through Inhibiting DNA Methylation of CHRNA7 and Increases α7nAChR Membrane Trafficking

Inhibition of Ras farnesylation in acute has been found to upregulate the α7 nicotinic acetylcholine receptor (α7nAChR) activity. This study was carried out to investigate the effect of chronic administration for 7 days of farnesyl transferase inhibitor lonafarnib (50 mg/kg, intraperitoneally injected) to male mice on the expression and activity of α7nAChR in hippocampal CA1 pyramidal cells. Herein, we show that lonafarnib dose dependently enhances the amplitude of ACh-evoked inward currents (I_ACh), owning to the increased α7nAChR expression and membrane trafficking. Lonafarnib inhibited phosphorylation of c-Jun and JNK, which was related to DNA methylation. In addition, reduced DNA methyltransferase 1 (DNMT1) expression was observed in lonafarnib-treated mice, which was reversed by JNK activator. Lonafarnib-upregulated expression of α7nAChR was mimicked by DNMT inhibitor, and repressed by JNK activator. However, only inhibited DNA methylation did not affect I_ACh, and the JNK activator partially decreased the lonafarnib-upregulated I_ACh. On the other hand, lonafarnib also increased the membrane expression of α7nAChR, which was partially inhibited by JNK activator or CaMKII inhibitor, without changes in the α7nAChR phosphorylation. CaMKII inhibitor had no effect on the expression of α7nAChR. Lonafarnib-enhanced spatial memory of mice was also partially blocked by JNK activator or CaMKII inhibitor. These results suggest that Ras inhibition increases α7nAChR expression through depressed DNA methylation of CHRNA7 via Ras-c-Jun-JNK pathway, increases the membrane expression of α7nAChR resulting in part from the enhanced CaMKII pathway and total expression of this receptor, and consequently enhances the spatial memory.

Chronic morphine selectively impairs cued fear extinction in rats: implications for anxiety disorders associated with opiate use

Previous studies have shown that opioid transmission plays an important role in learning and memory. However, little is known about the course of opiate-associated learning and memory deficits after cessation of chronic opiate use in a behavioral animal model. In the present study, we examined the effects of chronic morphine on fear extinction, an important preclinical model for behavior therapy of human anxiety disorders. Rats were administrated subcutaneously morphine hydrochloride or saline twice per day for continuous 10 days. Rats received a cued or contextual fear conditioning session 7 days after the last morphine injection. During subsequent days, rats received four cued or contextual extinction sessions (one session per day). Percent freezing was assessed during all phases of training. Chronic morphine did not affect the acquisition of cued fear response or the initial encoding of extinction memory within each session, but produced an impairment in the between-session extinction. However, the same morphine treatment schedule did not affect the acquisition or extinction of contextual fear response. These results suggest that the effects of chronic morphine on memory for fear extinction are complex. Chronic morphine selectively impairs extinction of cued fear response. This deficit in fear extinction may be one of those critical components that contribute to the high prevalence of anxiety disorders in opiate addicts.

Cocaine-but not methamphetamine-associated memory requires de novo protein synthesis

Context-induced drug craving and continuous drug use manifest the critical roles of specific memory episodes associated with the drug use experiences. Drug-induced conditioned place preference (CPP) in C57BL/6J mouse model, in this regard, is an appropriate behavioral paradigm to study such drug use-associated memories. Requirement of protein synthesis in various forms of long-term memory formation and storage has been phylogenetically demonstrated. This study was undertaken to study the requirement of protein synthesis in the learning and memory aspect of the conditioned place preference induced by cocaine and methamphetamine, two abused drugs of choice in local area. Since pCREB has been documented as a candidate substrate for mediating the drug-induced neuroadaptation, the pCREB level in hippocampus, nucleus accumbens, and prefrontal cortex was examined for its potential participation in the formation of CPP caused by these psychostimulants. We found that cocaine (2.5 and 5.0 mg/kg/dose)-induced CPP was abolished by the pretreatment of anisomycin (50 mg/kg/dose), a protein synthesis inhibitor, whereas methamphetamine (0.5 or 1.0 mg/kg/dose)-induced CPP was not affected by the anisomycin pretreatment. Likewise, cocaine-induced CPP was mitigated by another protein synthesis inhibitor, cycloheximide (15 mg/kg/injection) pretreatment, whereas methamphetamine-induced CPP remained intact by such pretreatment. Moreover, anisomycin treatment 2h after each drug-place pairing disrupted the cocaine-induced CPP, whereas the same treatment did not affect methamphetamine-induced CPP. An increase of accumbal pCREB level was found to associate with the learning phase of cocaine, but not with the learning phase of methamphetamine. We further found that intraaccumbal CREB antisense oligodeoxynucleotide infusion diminished cocaine-induced CPP, whereas did not affect the methamphetamine-induced CPP. Taken together, these data suggest that protein synthesis and accumbal CREB phosphorylation are essential for the learning and consolidation of the cocaine-induced CPP, whereas methamphetamine-induced CPP may be unrelated to the synthesis of new proteins.

Post-translational protein modification as the substrate for long-lasting memory

Prevailing models of memory identify mRNA translation as necessary for long-lasting information storage. However, there are enough instances of memory storage in the virtual absence of protein synthesis to prompt consideration of alternative models. A comprehensive review of the protein synthesis literature leads us to conclude that the translational mechanism is exclusively a permissive, replenishment step. Therefore, we propose that post-translational modification (PTM) of proteins already at the synapse is the crucial instructive mechanism underlying long-lasting memory. A novel feature of this model is that non-random spontaneous (or endogenous) brain activity operates as a regulated positive-feedback rehearsal mechanism, updating network configurations by fine-tuning the PTM state of previously modified proteins. Synapses participating in memory storage are therefore supple, a feature required for networks to alter complexity and update continuously. In analogy with codons for amino acids, a long-lasting memory is represented by a 'degenerate code' - a set of pseudo-redundant networks that can ensure its longevity.

Temporally graded, context-specific retrograde amnesia and its alleviation by context preexposure: effects of postconditioning exposures to morphine in the rat

Five experiments studied retrograde impairments in Pavlovian fear conditioning following prolonged exposure to the opioid receptor agonist morphine. Injections of morphine commencing 1-7 days but not 14 days after conditioning produced amnesia for that conditioning episode. This amnesia was (a) selective such that morphine impaired freezing to the conditioning context but not to the auditory conditioned stimulus, (b) independent of the interval between the last injection of morphine and test, and (c) accompanied by a failure of contextual discrimination. Context preexposure protected context conditioning and discrimination from the amnestic effects of morphine. These results show that retrograde deficits in contextual fear conditioning are mediated by failures to consolidate a contextual representation.

Opioid Receptors Regulate the Extinction of Pavlovian Fear Conditioning

Rats received a single pairing of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus (US). The fear (freezing) that had accrued to the CS was then extinguished. Injection of naloxone prior to this extinction significantly impaired the development of extinction. This impairment was mediated by opioid receptors in the brain and was not observed when naloxone was injected after extinction training. Finally, an injection of naloxone on test failed to reinstate extinguished responding that had already accrued to the CS. These experiments show that opioid receptors regulate the development, but not the expression, of fear extinction and are discussed with reference to the roles of opioid receptors in US processing, memory, and appetitive motivation.

Memory retrieval enhancement by kappa opioid agonist and mu, delta antagonists

The present study sought to identify specific opioid receptor subtypes involved in the modulation of reactivation of amnesic or forgotten memory traces by use of a one-trial inhibitory avoidance training procedures in mice. The effects of naloxone, ICI 174,864 (mu and delta opioid receptor antagonists, respectively) and dynorphin (kappa agonist) were investigated. The results indicated that preretention test administration of naloxone (2 mg/kg) or ICI 174,864 (3 mg/kg) attenuated the amnesia and forgetting as indicated by prolongation of step-through latency. On the other hand, the activation of kappa opioid receptors by dynorphin (1 mg/kg) also showed reactivating effects both after amnesia and forgetting. On the basis of the parallelism of the effects for mu and delta opioid receptor antagonists and kappa agonist, and on the finding that all three opioids demonstrated a different degree of reactivation of amnesic and forgotten memory traces, it was concluded that mu, delta, and kappa opioid receptors contribute to the modulation of amnesia and forgetting by independent mechanisms.

Similar ameliorating effects of benzomorphans and 5-HT2 antagonists on drug-induced impairment of passive avoidance response in mice: comparison with acetylcholinesterase inhibitors

Mice were trained to avoid electric shocks by means of step-down type passive avoidance learning tasks, and memory retention was measured 24 h after the training session. Memory impairment (amnesia) was produced by administering either p-chloroamphetamine (PCA), a serotonin (5-HT) releaser or scopolamine (SCOP), a muscarinic cholinoceptor antagonist, 30 min prior to the training session. Benzomorphans, 5-HT2 antagonists and acetylcholinesterase (AChE) inhibitors were administered immediately after the training session. PCA- but not SCOP-induced amnesia was attenuated by the post-training administration of two benzomorphans, (+)N-allylnormetazocine ((+)SKF-10,047) and (+/- )pentazocine ((+/- )PTZ). Similarly, PCA-induced amnesia was reversed by the post-training administration of 5-HT2 antagonists, ritanserin (RIT) and mianserin (MIA), but SCOP-induced amnesia was not. However, the AChE inhibitors, tetrahydroaminoacridine (THA) and physostigmine (PHY) attenuated both PCA- and SCOP-induced amnesia when administered immediately after the training session. These results indicated that benzomorphans and 5-HT2 antagonists have antiamnestic effects in mice, as do AChE inhibitors. In addition, it is interesting that the patterns of ameliorating effect of benzomorphans were similar to those of 5-HT2 antagonists, which differ from those of AChE inhibitors.

Naloxone reduces social locomotor activity in rats

Naloxone, a nonspecific opioid antagonist, has been found to decrease the activity and social behavior of rats tested in pairs but the effects on individual locomotor activity have been equivocal. In the present study, groups of male Long-Evans hooded rats received naloxone (1 or 4 mg/kg, IP) or vehicle alone (isotonic saline) 30 min prior to testing sessions. Individual locomotor activity was measured in two activity boxes (41-cm3) equipped with two infrared photobeams using daily 30-min testing sessions for 5 consecutive days. Following a 1-week washout period (no testing), activity and social attraction (paired distance and contact) were examined in pairs of rats from each group using daily 15-min testing sessions for 4 consecutive days. Locomotor activity and its habituation were not significantly affected by naloxone in rats tested individually. However, both doses of naloxone significantly reduced paired locomotor activity compared to the control group. Measures of social attraction were not significantly affected by naloxone. The present findings suggest that naloxone does not produce nonspecific depressant effects on activity but rather may antagonize opioid release in situational contexts of high arousal (e.g., social activity) with consequent reduction of activity.

Involvement of serotonergic neuronal systems in the anti-amnesic action of naftidrofuryl oxalate

The effects of naftidrofuryl oxalate on cycloheximide- and 5-hydroxytryptophan (5-HTP)-induced amnesia were investigated using a passive avoidance task in mice. Naftidrofuryl oxalate significantly improved the cycloheximide-induced amnesia. This effect of naftidrofuryl oxalate was antagonized by 5-HTP, a serotonin (5-HT) precursor, and by p-chloroamphetamine (PCA), a 5-HT releaser. Single administration of 5-HTP in combination with pargyline, a monoamine oxidase inhibitor, induced amnesia (5-HTP-induced amnesia). This amnesia was attenuated by ritanserin, a 5-HT2-selective antagonist, but not by pindolol, a 5-HT1-selective antagonist. Naftidrofuryl oxalate also attenuated the 5-HTP-induced amnesia. A binding study revealed that naftidrofuryl oxalate inhibited the binding of [3H]ketanserin to 5-HT2 receptors in mouse brain synaptic membrane in a dose-dependent fashion (IC50 = 1.42 x 10(-7) M), but did not inhibit that of [3H]serotonin to 5-HT1 receptors. These results suggest that naftidrofuryl oxalate may attenuate cycloheximide- and 5-HTP-induced amnesia by blocking 5-HT2 receptor subtypes.

Facilitation of memory retrieval by pre-test morphine and its state dependency in the step-through type passive avoidance learning test in mice

Amnesia produced by scopolamine and cycloheximide were reversed by morphine given 30 min before the test trial (pre-test), and pre-test morphine also facilitated the memory retrieval in the animals administered naloxone during the training trial. Similarly, pre-test scopolamine partially reversed the scopolamine-induced amnesia, but not significantly; and pre-test cycloheximide failed to reverse the cycloheximide-induced amnesia. These results suggest that the facilitation of memory retrieval by pre-test morphine might be the direct action of morphine rather than a state dependent effect.

State dependent and/or direct memory retrieval by morphine in mice

Mice were trained in step-down and step-through type passive avoidance learning tasks and given retention tests. Pre-training administration of morphine impaired retention, the effect recovering completely after an additional injection of the same dose of morphine given 30 min before the retention test. Amnesia produced by scopolamine, cycloheximide and electroconvulsive shock was also reversed by pre-test morphine. Pre-test saline also reversed the morphine-induced memory impairment to some extent, indicating that the recovery may partially be due to the state dependent effect. Thus, it is demonstrated that pre-test morphine not only state dependently but also directly reversed memory impairment in mice.

Effect of minaprine on cycloheximide-induced amnesia in mice

The effects of minaprine on cycloheximide-induced amnesia were investigated in a step-down passive avoidance task in mice. Minaprine significantly improved cycloheximide-induced amnesia. This effect was inhibited by scopolamine, but was potentiated by physostigmine. The anti-amnesic effect of minaprine on the cycloheximide-induced memory impairment was also antagonized by a serotonin (5-HT) releaser, p-chloroamphetamine, and by a 5-HT precursor, 5-hydroxytryptophan, whereas a 5-HT1A-selective agonist, 8-hydroxy-2-(di-n-propylamino)tetralin, was inactive. The memory-improving effect of minaprine on cycloheximide-induced amnesia was potentiated by a selective 5-HT2 antagonist, ritanserin. These results suggest that the beneficial effect of minaprine on cycloheximide-induced amnesia may be related not only to cholinergic but also serotonergic neuronal systems (5-HT2 receptors).

Effects of 5-HT2 receptor antagonist on cycloheximide-induced amnesia in mice

A role played by serotonergic neuronal system in cycloheximide (CXM)-induced amnesia was studied in mice using a step-down passive avoidance task. CXM (30 mg/kg SC) given immediately after training caused impairment of memory. Nonselective serotonin (5-HT) antagonist methysergide and selective 5-HT2 antagonist ritanserin significantly attenuated impairment of memory caused by CXM. 5-HT1 antagonist (+/-)-pindolol had no effect on CXM-induced amnesia. The antiamnesic effect of ritanserin on CXM-induced amnesia was antagonized by 5-HT (ICV), but not by nonselective 5-HT agonist 5-methoxy-N,N-dimethyltryptamine and 5-HT1A selective agonist 8-hydroxy-2-(di-n-propylamino)tetralin at the dose level which did not cause the memory disruption. Scopolamine antagonized the antiamnesic effects of methysergide and ritanserin on CXM-induced amnesia. These results suggest that 5-HT2 receptors and cholinergic neuronal system may play an important role in memory formation.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.