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

State-dependent memory and its modulation by different brain areas and neurotransmitters

The state-dependent memory defines as a state that the retrieval of recently obtained information may be potential if the subject exists in a similar physiological situation as for the period of the encoding stage. Studies revealed that exogenous and endogenous compounds could induce state-dependent memory. The state-dependent memory made it probable to differentiate the effects of drugs per se on learning from the effects due to alterations in drug state during the task. Studies proposed the role of regions beyond the limbic formation and illustrated that state-dependent memory produced by various neurotransmitter systems and pharmacological compounds. Our review of the literature revealed that: (a) re-administration of drugs on the same state induce state-dependent memory; (b) many neurotransmitters induce endogenous state-dependent memory; (c) there are cross state-dependent learning and memory between some drugs; (d) some sites of the brain including the CA1 areas of the hippocampus, central nucleus of the amygdala (CeA), septum, ventral tegmental area (VTA), and nucleus accumbens (NAC) are involved in state-dependent memory. See also Figure 1(Fig. 1).

The role of attention in motor control and learning

Performing and learning motor behaviors are fundamental to everyday life. The relations between perceptual input and motor output have been studied and are well understood for simple experimental settings. Recent findings, however, suggest that motor actions also critically depend on cognitive factors; these influences are most notable in complex environments that place high demands on attention and memory. In this review, the role of various aspects of attention in motor control is discussed, focusing on the following points: (1) recent findings concerning interactions between attentional resources and motor skill acquisition, (2) the consistency of attentional states (divided versus focused) and motor memory retrieval, and (3) the locus of attention (internal versus external) and motor performance. These findings collectively highlight the interplay between attention and motor systems, which in turn has practical implications for developing and improving motor training and rehabilitation programs.

Impaired visuomotor generalization by inconsistent attentional contexts

In daily life, people are constantly presented with situations in which they have to learn and acquire new motor skills in complex environments, where attention is often distracted by other events. Being able to generalize and perform the acquired motor action in different environments is a crucial part of visuomotor learning. The current study examined whether attentional distraction impairs generalization of visuomotor adaptation or whether consistent distraction can operate as an internal cue to facilitate generalization. Using a dual-task paradigm combining visuomotor rotational adaptation and an attention-demanding secondary task, we showed that switching the attentional context from training (dual-task) to generalization (single-task) reduced the range of transfer of visuomotor adaptation to untrained directions. However, when consistent distraction was present throughout training and generalization, visuomotor generalization was equivalent to without distractions at all. Furthermore, this attentional context-dependent generalization was evident even when sensory modality of distractions differed between training and generalization. Therefore, the general nature of the dual tasks, rather than the specific stimuli, is associated with visuomotor memory and serves as a critical cue for generalization. Taken together, we demonstrated that attention plays a critical role during sensorimotor adaptation in selecting and associating multisensory signals with motor memory. This finding provides insight into developing learning programs that are generalizable in complex daily environments.NEW & NOTEWORTHY Learning novel motor actions in complex environments with attentional distraction is a critical function. Successful motor learning involves the ability to transfer the acquired skill from the trained to novel environments. Here, we demonstrate attentional distraction does not impair visuomotor adaptation. Rather, consistency in the attentional context from training to generalization modulates the degree of transfer to untrained locations. The role of attention and memory must, therefore, be incorporated into existing models of visuomotor learning.

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.

Paradoxical benefits of dual-task contexts for visuomotor memory

It is generally thought that increased attention helps when one is learning a new task. However, using a dual-task paradigm, we showed that the rate of visuomotor learning was the same regardless of attentional distraction caused by a secondary task. Yet, when participants were tested later, a motor skill learned under distraction was remembered only when a similar distraction was present; when participants were tested without the distracting task, their performance reverted to untrained levels. This paradoxical result, in which the level of performance decreases when more attentional resources are available, suggests that the dual-task context, or the lack thereof, acts as a vital context for learning. This task-context-dependent "savings" was evident even when the specific secondary task or sensory modality differed between learning and recall; thus, the dual tasking, rather than the specific stimuli, provides context. This discovery suggests that the success of learning and rehabilitation programs may be diminished if they are developed without consideration of the role of task contexts.

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.

Effect of naloxone and morphine on arcaine-induced state-dependent memory in rats

RATIONALE

Arcaine is a competitive antagonist of the polyamine binding site at the N-methyl-D-aspartic acid receptor which induces state-dependent recall. However, no study has addressed the involvement of other neurotransmitter/neuromodulators in arcaine-induced state dependency.

OBJECTIVES

The current study investigates whether the opioid system is involved in arcaine-induced state-dependent memory retrieval of the inhibitory avoidance task (IA) in rats.

RESULTS

The systemic administration of arcaine (30 mg/kg, intraperitoneally (i.p.)) or morphine (5 mg/kg, i.p.) 0, 3, 6, or 9 h post-training, reduced step-down latencies at testing. Arcaine (30 mg/kg, i.p.) or morphine (5 mg/kg, i.p.) injection 30 min before testing reversed the performance deficit induced by administration of arcaine or morphine 0, 3 or 6, but not 9 h post-training. The reversal of arcaine-induced impairment of IA performance was completely transferred to morphine and vice versa. The association of low and ineffective doses of morphine and arcaine (10 and 1.5 mg/kg, respectively) were additive and caused state dependency. Naloxone (2 mg/kg, 3 min post-training, or 1 mg/kg, 1 h pre-test, i.p.) reversed the amnesia and the state dependency induced by morphine and arcaine.

CONCLUSION

These results suggest that state dependency induced by arcaine involves the opioid system.

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 NMDA RECEPTORS IN MORPHINE STATE–DEPENDENT LEARNING IN MICE

In the present study, the effects of intracerebroventricular (i.c.v.) injection of NMDA receptor agonist and antagonist on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (5 mg/kg; s.c.) decreased the learning of one-trial passive avoidance task. Pretest administration of morphine (5 mg/kg) induced state-dependent learning acquired under pretraining morphine influence. Pretest administration of NMDA receptor agonist, L-glutamate (0.00001 and 0.0001 and 0.001 microg/mouse, i.c.v.) following pretraining saline treatment did not affect retention. Amnesia induced by pretraining morphine was significantly reversed by pretest administration of L-glutamate (0.0001 and 0.001 microg/mouse, i.c.v.). Pretest administration of noncompetitive NMDA receptor antagonist, MK-801 (0.5, 1, and 2 microg/mouse, i.c.v.) significantly impaired memory formation. Amnesia induced by pretraining morphine was increased by pretest administration of MK-801 (2 microg/mouse, i.c.v.). Pretest coadministration of L-glutamate (0.0001 and 0.001 microg/mouse, i.c.v.) or MK-801 (0.5, 1, and 2 microg/mouse, i.c.v.) with morphine (5 mg/kg, s.c.) increased and decreased morphine state-dependent learning, respectively. The results suggest that NMDA receptors are involved in morphine state-dependent learning in mice.

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.

Facilitation of avoidance behaviour in mice chronically treated with heroin or methadone

Although the repercussion of chronic treatment with large amounts of opioids on cognitive performance is a matter of concern, the effects of opioid drugs on passive avoidance learning have been scarcely studied. Here, we analyzed the effects of prolonged administration of heroin and methadone, as well as the impact of suffering repeated episodes of withdrawal on fear-motivated learning using the passive avoidance test. Mice received chronic treatment (39 days) with methadone (10 mg/kg/24 h), associated or not with repeated withdrawal episodes, or with heroin (5 mg/kg/12 h). Our results show that, regardless of the type of treatment received, all mice displayed similar basal thermal nociceptive thresholds during 25 days of treatment. In the hot plate test, both methadone and heroin induced antinociception 30 min after drug administration. The analgesic effect was absent when measured 4 h after heroin and 12 h after methadone. Pain behavioural responses elicited by growing intensities of electric shock, applied on day 28th of treatment, were similar in all groups of mice. Our results indicate that chronic opioid treatment had promnesic effects on passive avoidance behaviour in mice, unrelated to changes in the nociceptive state.

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.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

Effect of lithium on morphine state-dependent memory of passive avoidance in mice

In the present study, effects of lithium chloride (LiCl) on morphine induced state-dependent memory of passive avoidance task were examined in mice. One-trial step-down paradigm was used for the assessment of memory retention in adult male NMRI mice. Administration of morphine (5 mg/kg) subcutaneously (s.c.) 30 min before training or testing induced impairment of memory performance. Injection of the same dose of the drug 30 min before testing restored memory retention impaired under pre-training morphine effect. Intraperitoneal (i.p.) injection of lithium, 60 min before training or prior to testing also impaired memory performance. Under the pre-training of morphine, the response of the opioid was restored when animals received LiCl (80 and 160 mg/kg) as pre-test injection. Pre-training administration of lower dose of lithium (20 mg/kg), but not the higher doses of the drug (80 and 160 mg/kg) impaired memory retention in passive avoidance test. LiCl-induced impairment of memory retention was restored by pre-test administration of morphine. In the animals receiving pre-training morphine, combined pre-test morphine and LiCl administration increased the restoration of memory by the opioid. It can be concluded that there may be a cross-state dependency between morphine and lithium.

Effect of the GABAergic System on Memory Formation and State-Dependent Learning Induced by Morphine in Rats

In the present study, the effects of intraperitoneal injections of GABA(A) receptor agonist and antagonist on memory formation and morphine state-dependent learning were investigated in rats. Pre-training administration of morphine (1-15 mg/kg) in a step-down passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after the pre-test administration of the same dose of morphine. The pre-test administration of the GABA(A) receptor agonist, muscimol (0.01, 0.05 and 0.1 mg/kg), significantly decreased state-dependent retrieval induced by pre-test morphine (5 mg/kg). The state-dependency effect of morphine (1 mg/kg) was significantly potentiated by the pre-test administration of the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 mg/kg). Furthermore, the pre-training injection of muscimol (0.01 mg/kg) impaired memory retrieval which was restored by pre-test morphine (1, 3 and 5 mg/kg) administration. However, the pre-training administration of bicuculline did not affect retention by itself. In addition, amnesia induced by pre-training morphine (5 mg/kg) was significantly reversed in rats which had received pre-test injections of muscimol (0.01, 0.05 and 0.1 mg/kg). Pre-test injections of bicuculline (0.125, 0.25 and 0.5 mg/kg) significantly decreased morphine-induced amnesia. It is concluded that the GABA(A) receptor mechanisms may be involved in the memory formation and it is postulated that these receptors may play an important role in morphine state-dependent learning.

The influence of central administration of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice

In the present study, effects of intracerebroventricular (i.c.v.) injections of dopaminergic and cholinergic agents on morphine-induced amnesia in morphine-sensitized mice were investigated by using a one-trial passive avoidance task. Amnesia induced by pre-training morphine was significantly reversed in morphine-sensitized mice, which had previously received once daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days. Three daily injections of SKF 38393 (1, 2 and 4 g/mouse, i.c.v.) or SCH 23390 (0.25, 0.5, 0.75 and 1 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of quinpirole (0.3, 1 and 3 g/mouse, i.c.v.) or sulpiride (0.03, 0.1, 0.3 and 1 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. Morphine-sensitized mice received similar injections of cholinergic agents. Three daily injections of physostigmine (1, 3 and 5 g/mouse, i.c.v.) or atropine (1, 4 and 7 g/mouse, i.c.v.) before morphine, and during morphine-sensitization, decreased and increased the amnesia induced by pre-training morphine respectively. Three daily injections of nicotine (0.75, 1 and 2 g/mouse, i.c.v.) or mecamylamine (1, 3 and 6 g/mouse, i.c.v.) before morphine, also decreased and increased the amnesia induced by pre-training morphine respectively. The results suggest that morphine sensitization affects the impairment of memory formation and thus it is postulated that central dopaminergic and cholinergic systems may play an important role in this effect.

Effects of morphine on the plus-maze discriminative avoidance task: role of state-dependent learning

RATIONALE

The amnesic effects of morphine may be related to its action on nociception, anxiety, or locomotion. This effect is also suggested to be related to state dependency.

OBJECTIVES

The aims of this study were to verify the effects of morphine on mice tested in the plus-maze discriminative avoidance task (DAT) that uses light and noise as aversive stimuli and allows the concomitant evaluation of learning, memory, anxiety, and locomotion and also to verify the possible role of state-dependent learning in the effects of morphine.

METHODS AND RESULTS

The DAT was conducted in a modified elevated plus-maze. In the training, the aversive stimuli were applied when mice entered in one of the enclosed arms, whereas in the test, no stimuli were applied. The main results showed that (1) pretraining morphine (5-20 mg/kg i.p.) induced retrieval deficits (evaluated by the time spent in the aversive arm in the test) but not acquisition deficits (evaluated by the decrease in aversive arm exploration along the training); (2) pretest morphine (5-10 but not 20 mg/kg) counteracted this deficit; (3) morphine induced hypolocomotion (decreased number of entries in the arms), irrespective of memory alterations; and (4) morphine did not alter anxiety-like behavior (evaluated by the time spent in the open arms) during the training.

CONCLUSIONS

Morphine given before training induces retrieval deficits in mice tested in the DAT, and these deficits could be related to morphine-induced state-dependent learning. Neither the memory deficit induced by pretraining morphine nor the reversal of this deficit by pretest morphine seems to be related to anxiety levels or locomotor alterations.

Morphine State-Dependent Learning Sensitization and Interaction with Nitric Oxide

In the present study, the effects of nitric oxide (NO) precursor L-arginine and L-NAME, a potent inhibitor of NO synthase (NOS), on the expression of sensitization of morphine were investigated. Pre-training administration of morphine (5 mg/kg) impaired memory retrieval compared to pre-training saline-treated animals. Amnesia due to pre-training morphine (5 mg/kg) was restored by pre-test morphine (5 mg/kg). The retrieval impairment was also inhibited in mice which had received once-daily injections of morphine (20 and 30 mg/kg, s.c.) for 3 days, followed by 5 days of no drug treatment before training (in order to induce morphine sensitization). Administration of L-arginine (60 mg/kg/day - 3 days) or L-NAME (20 mg/kg/day - 3 days) before training did not alter morphine state dependency. During acquisition of sensitization, administration of L-arginine (60 mg/kg) 20 min before morphine (10 mg/kg/day, for 3 days) increased, while injection of L-NAME (20 mg/kg) 20 min before morphine (30 mg/kg/day, for 3 days) decreased morphine state dependency. It is concluded that NO is involved in the morphine-induced sensitization.

The influence of NMDA receptor agonist and antagonist on morphine state-dependent memory of passive avoidance in mice

The measurement of step-down latency in passive avoidance has been used to study memory in laboratory animals. The pre-training injection of 5 mg/kg morphine impaired memory, which was restored when 24 h later the same dose of the drug was administered. To explore the possible involvement of NMDA modulators on morphine-induced memory impairment, we have investigated the effects of intracerebroventricular (i.c.v.) administration of NMDA and the competitive NMDA antagonist, DL-AP5, on morphine-induced memory impairment or recall, on the test day. Morphine (5 mg/kg, s.c.) was administered 30 min before training to induce impairment of memory and 24 h later, 30 min before test to improve it. Pre-test administration of NMDA (0.00001, 0.0001 and 0.001 microg/mouse, i.c.v.) did not alter the retention latency compared to the saline-treated animals. But restored the memory impairment induced by pre-training morphine (5 mg/kg, s.c.). Pre-test administration of DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) by itself decreased the retention latencies. The same doses of DL-AP5 increased pre-training morphine-induced memory impairment. Co-administration of NMDA (0.0001 and 0.001 microg/mouse, i.c.v.) and morphine (5 mg/kg, s.c.) on the test day increased morphine memory improvement. Conversely, DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) inhibited morphine-induced memory recall. It is concluded that NMDA receptors may be involved, at least in part, in morphine state-dependent learning in mice.

Involvement of the ventral tegmental area (VTA) in morphine-induced memory retention in morphine-sensitized rats

In the present study, the effects of intra-ventral tegmental area (VTA) injections of morphine on memory retention of a one-trial passive avoidance task have been investigated in morphine-sensitized rats. Retrieval was examined 24h after training and used as memory retention. Sensitization was obtained by subcutaneous (s.c.) injections of morphine, once daily for 3 and 5 days free of the opioid before training. Post-training administration of the both systemic (2.5, 5 and 7.5mg/kg, s.c.) and intra-VTA (5 and 7.5microg/rat) of morphine, dose-dependently decreased memory retention. The response induced by post-training administration of intra-VTA morphine (7.5microg/rat) was significantly reversed in morphine-sensitized rats. The inhibition of morphine-induced amnesia in morphine-sensitized rats was decreased by once daily injections of naloxone (0.5, 1 and 2mg/kg, s.c.), SCH 23390 (0.025, 0.05 and 0.1mg/kg, s.c.) or sulpiride (25, 50 and 100mg/kg, s.c.), during the sensitization. The results suggest that VTA has an important role in morphine-induced amnesia and morphine sensitization affects this process through opioid and dopamine receptors.

Estrogen has mnemonic-enhancing effects in the inhibitory avoidance task

Gonadal hormones, such as estrogen, can alter cognitive performance. The present studies investigated the relationship between performance on the inhibitory avoidance task and endogenous fluctuations in ovarian hormones and estrogen replacement. In Experiment 1, proestrous or diestrous I female, or male, rats were trained in the inhibitory avoidance task. Following a 24-h intertrial delay, when female rats were tested in metestrus or diestrus II, no differences in crossover latencies were observed among groups. In Experiment 2, female rats in proestrus or diestrus I, and male rats, were trained in the inhibitory avoidance task and were tested following a 4-h intertrial delay (so that training and testing were accomplished in the same phase of the cycle). In this paradigm, proestrous rats had significantly longer crossover latencies than did either diestrous I or male rats. Posttraining administration of estrogen, but not progesterone, to ovariectomized rats increased crossover latencies compared to vehicle with a 4-h (Experiment 3) or 24-h (Experiment 4) intertrial delay. In Experiment 5, estrogen administration to ovariectomized rats immediately, but not 1, 2, or 3 h posttraining, increased crossover latencies compared to vehicle. Together, these data suggest that estrogen can have positive mnemonic effects in the inhibitory avoidance task.

Influence of potassium channel modulators on morphine state-dependent memory of passive avoidance

In a step-down passive avoidance task, the pre-training injection of 1.25-10 mg/kg of morphine impaired memory. This was restored when injection of the same dose of morphine (pre-test treatment) was repeated 24 h later (morphine state-dependent learning: morphine St-D). ATP-dependent potassium (K(ATP)) channels have been reported to be involved in several actions of morphine following mu-receptor stimulation. We have studied the effect of K(ATP) modulators and naloxone in the restoration of memory by morphine in mice. To investigate the part played by cholinergic systems in the effects of a K(ATP) antagonist (glibenclamide) on morphine St-D, we administered low doses of atropine before glibenclamide administration. Locomotor activity was also studied. Naloxone (0.06-1 mg/kg) reversed the effect of pre-test morphine administration. The effects of the K(ATP) channel blocker glibenclamide (2-18 mg/kg) were similar to those of the pre-test administration of morphine. Pre-test co-administration of glibenclamide and morphine showed no potentiation of the morphine effect. Glibenclamide alone or in combination with morphine did not affect locomotor activity. Pre-test administration of different doses of diazoxide (15-60 mg/kg), a K(ATP)-channel opener, had no effect on restoration of memory when used alone or in combination with morphine. In both cases, the locomotor activity was significantly reduced. Diazoxide blocked the effect of glibenclamide on memory recall. Low-dose atropine also prevented glibenclamide enhancement of memory recall, suggesting that this action of glibenclamide is through the cholinergic system.

Morphine state-dependent learning: sensitization and interactions with dopamine receptors

In the present study, the effects of morphine sensitization on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (0.5, 2.5 and 5 mg/kg) dose dependently decreased the learning of a one-trial passive avoidance task. Pretest administration of morphine (0.5, 2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under pretraining morphine influence. Pretraining or pretest administration of naloxone (0.25, 0.5 and 1 mg/kg) reversed both responses to morphine (5 mg/kg). Amnesia induced by pretraining morphine was significantly reversed in morphine-sensitized mice which had previously received once daily injections of morphine [20 and 30 mg/kg, subcutaneously (s.c.)] for 3 days. Morphine sensitization tended to reverse but did not significantly affect morphine state-dependent memory. The inhibition of morphine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5, 1 and 2 mg/kg) 30 min prior to injection of morphine (20 mg/kg/day x 3 days). Three-days administration of 1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (SKF 38393; 8, 16 and 32 mg/kg) or SCH 23390; R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (0.01, 0.05 and 0.1 mg/kg) before morphine (for 3 days) and during morphine-sensitization, decreased and increased, the amnesia induced by pretraining morphine, respectively. Similar administration of quinpirole (0.5, 1 and 2 mg/kg) or sulpiride (25, 50 and 100 mg/kg) before morphine also decreased and increased the amnesia induced by pretraining morphine, respectively. The results suggest that morphine sensitization affects the impairment of memory formation, but not the facilitation of retrieval induced by morphine and thus it is postulated that dopamine receptors may play an important role in this effect.

Influence of central administration ATP-dependent K+ channel on morphine state-dependent memory of passive avoidance

Pre-training injection of a moderate dose of morphine (5-10 mg/kg) in a step-down passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after the pre-test administration of the same dose of the drug. We have studied the effect of intracerebroventricular administration of naloxone and K(ATP) channel modulators (glibenclamide and diazoxide) on the test day on restoration of memory by morphine in mice. The effect of scopolamine on restoration of memory on the test-day by glibenclamide was studied as well. Naloxone pretreatment (0.006, 0.025 and 0.1 microg/mouse) reversed the effect of pre-test morphine administration. The K(ATP) channel blocker, glibenclamide (0.1, 0.5 and 1 microg/mouse), showed effects similar to those of pre-test administration of morphine. Glibenclamide tended to potentiate the morphine response. Scopolamine (0.15 and 0.30 microg/mouse) prevented the effect of glibenclamide on the restoration of memory. The pre-test administration of different doses of diazoxide (1.7, 5 and 15 microg/mouse), a K(ATP) channel opener, showed no effect on restoration of memory when used alone but decreased morphine state-dependence. Diazoxide blocked the effects of glibenclamide on memory restoration. It is concluded that K(ATP) channel modulators may be involved, at least in part, in morphine state dependence through a cholinergic system mechanism.

The influence of contextual cue on antinociceptive tolerance and facilitation of memory with morphine

The response latencies of the biting response in the tail-pinch test and step-through response in the passive avoidance test were measured in mice. 1) Scopolamine decreased the latency of the step-through response in the passive avoidance test, but diazepam did not. 2) Morphine (5 mg/kg, i.p.) was given 30 min before the test for 4 consecutive days in novel environments by sessions. The decrease in biting response latency was attenuated and delayed in the groups in which contexts were changed daily compared to the groups that were maintained in the same context throughout the conditioning period. The latency of step-through response was increased with morphine in both groups regardless of the change or lack of change in context. 3) Morphine was repeatedly administered to animals in the same environment and then the context was changed. The decreased latency in the tail-pinch test was significantly reversed by the change in context, but the response in the passive avoidance test maintained a longer latency. These findings indicate that morphine develops associative and nonassociative antinociceptive tolerance, indicating that antinociceptive tolerance to morphine has contextual specificity, but the facilitation of memory does not. The results indicate that the facilitation of memory with morphine may contribute to associative learning in antinociceptive tolerance to morphine.

The effect of L-NAME and L-arginine on impairment of memory formation and state-dependent learning induced by morphine in mice

RATIONALE

Morphine and nitric oxide (NO) have important functional interactions in different neural processes, and both modulate learning and memory although their interaction in cognitive performance has not been elucidated.

OBJECTIVE

To examine the effect of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) and NOS substrate L-arginine on morphine-induced impairment of memory formation and the state-dependent retrieval of a passive avoidance task learned under morphine influence.

METHODS

All drugs were administered intraperitoneally, and a one-trial step-down paradigm was used for the assessment of memory in adult male NMRI mice. Morphine was administered 30 min before training to induce impairment of memory formation and 30 min before test to induce state-dependent retrieval of the memory acquired under pre-training morphine influence. L-NAME or L-arginine was administered either 5 min after training or 45 min before the test.

RESULTS

Pre-training morphine induced impairment of memory formation that was reversible by pre-test morphine but not saline. Post-training administration of L-arginine (200 mg/kg) and L-NAME (3, 10 and 30 mg/kg), respectively, facilitated and impaired the memory consolidation, but their pre-test injections did not affect retention. However, post-training L-arginine at per se non-effective doses of 20 mg/kg and 60 mg/kg reversed the morphine-induced impairment of memory formation. Pre-test administration of L-NAME (3 mg/kg and 10 mg/kg) could restore the memory impairment induced by pre-training morphine, and this effect was blocked by concomitant pre-test L-arginine (60 mg/kg). Concomitant administration of low doses of L-NAME (1 mg/kg) and morphine (0.5 mg/kg) pre-test also revealed an additive effect in restoring the morphine state of memory.

CONCLUSION

These results suggest that the impairment of memory formation and the facilitation of retrieval induced by morphine involves decreased synthesis/release of NO and can be counteracted by NOS substrate.

Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

The interactions of -adrenoceptors and acute restraint stress with morphine state-dependent memory of passive avoidance were examined in mice. Memory acquired following pre-training morphine administration (5 mg/kg, i.p.) was dose- and time-dependently retrieved by pre-test morphine; this effect was reversible by yohimbine (1 mg/kg). Pre-test clonidine (0.005-0.1 mg/kg) was also effective in restoring morphine-induced memory. Pre-training clonidine (2 mg/kg) induced an amnestic effect that was restorable by pre-test clonidine or morphine; this effect was also blocked by yohimbine. Acute pre-training stress for 2 h induced an amnestic effect that was reversible by pre-test morphine (1 and 5 mg/kg) or clonidine (0.01 and 0.1 mg/kg). Finally, acute pre-test stress could restore the impairment of memory induced by pre-training morphine. The data are suggestive of a functional interaction between -opioid, -adrenergic receptors and stress in modulating state-dependent learning and memory.

Endomorphins 1 and 2 induce amnesia via selective modulation of dopamine receptors in mice

The involvement of dopamine receptors in the amnesic effects of the endogenous micro-opioid receptor agonists endomorphins 1 and 2 was investigated by observing step-down type passive avoidance learning in mice. Although the dopamine D1 receptor agonist R(+)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol hydrochloride (R(+)-SKF38393) (0.05 and 0.1 mg/kg), the dopamine D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (R(+)-SCH23390) (2.5 and 5 microg/kg) or the dopamine D2 receptor agonist N-n-phenethyl-N-propylethyl-p-(3-hydroxyphenyl)-ethylamine (RU24213) (0.3 and 1 mg/kg) had no significant effects on the endomorphin-1 (10 microg)- or endomorphin-2 (10 microg)-induced decrease in step-down latency of passive avoidance learning, (-)-sulpiride (10 mg/kg), a dopamine D2 receptor antagonist, significantly reversed the decrease in step-down latency evoked by endomorphin-2 (10 microg), but not by endomorphin-1 (10 microg). Taken together, it is likely that stimulation of dopamine D2 receptors results in the endomorphin-2-but not endomorphin-1-induced impairment of passive avoidance learning.

Behavioral and pharmacological studies on gluten exorphin A5, a newly isolated bioactive food protein fragment, in mice

Central effects of gluten exorphin A5 (Gly-Tyr-Tyr-Pro-Thr), a fragment from wheat gluten, were studied on the pain-inhibitory system, emotionality and learning/memory processes in mice. Orally administered gluten exorphin A5 produced neither an antinociceptive effect nor an effect on morphine analgesia. Intracerebroventricularly (i.c.v.) administered gluten exorphin A5 produced mild but significant antinociception in a dose-depepndent manner, while not affecting the morphine analgesia. On the other hand, oral gluten exorphin A5 suppressed the endogenous pain-inhibitory system, i.e., antinociception induced by socio-psychological- (PSY-) stress (SIA) using a communication box; intraperitoneal gluten exorphin A5 abolished both footshock- (FS-) stress-induced antinociception (SIA) and PSY-SIA; and i.c.v. gluten exorphin A5 suppressed FS-SIA, but rather potentiated PSY-SIA. This peptide given by these routes was without effect on forced swim-SIA. In addition, oral gluten exorphin A5 tended to prolong the retention time on open arms in the elevated plus-maze test. Finally, oral gluten exorphin A5 when given during the post-training period of learning/memory processes significantly increased the latency into the dark compartment in the one-trail step-though type passive avoidance test, indicating that the peptide also facilitates the acquire/consolidation process of learning/memory. Thus, gluten exorphin A5 has been found to produce various effects not only in the peripheral nervous systems but also in the central nervous system.

Quantitative age-related changes in apical dendrites and dendritic spines of CA1 pyramidal neurons among senescence accelerated mice (SAMP1TA/Ngs)

SAMP1TA/Ngs, a substrain of senescence accelerated mouse, is a useful animal model for research on brain dysfunction due to senescence. In a previous study it was reported that the age-related changes in basal dendrites and spines of CA1 pyramidal neurons coincide with the behavioral characteristics found in SAMP1TA/Ngs. The goal of the present study was to investigate morphological changes in apical dendrites and dendritic spines of CA1 pyramidal neurons among 3-, 5-, 7-month-old SAMP1TA/Ngs. Pyramidal neurons of the hippocampus were stained by the rapid Golgi method, and the number of apical dendrites, the number of their spines and the density of the dendritic spines were evaluated. The number and density of the spines of apical dendrites were significantly higher at 5 months than at 3 or 7 months of age. We propose that the low number of dendritic spines in 3-month-old animals was caused by immaturity, while the changes in the density and number of dendritic spines in 7-month-old mice were due to accelerated aging. The data on the morphology of apical dendrites are a useful complement to the results reported previously. The findings of the present study also support the hypothesis that this model mouse demonstrates changes in respective developmental stages, i.e. immaturity, adulthood and senescence. This pattern of postnatal growth has special meaning because it indicates the usefulness of the strain in the study of geriatric disorders in humans.

Age-related changes in basal dendrite and dendritic spine of hippocampal pyramidal neurons (CA1) among SAMP1TA/Ngs--quantitative analysis by the rapid Golgi method

It has been confirmed that a substrain of the senescence-accelerated mouse SAMP1TA/Ngs develops learning disturbance-like behavior at 3 months of age, exhibits almost normal behavior at 5 months, and manifests learning disturbance at 7 months. The changes with age in basal dendrites and dendritic spines of CA1 pyramidal neurons were quantitatively evaluated by the Golgi method using male SAMP1TA/Ngs. The correlation between the change in learning ability and the morphometry was examined. The number of dendritic spines in the 3- and 7-month-old groups was significantly lower than that in the 5-month-old group. It is presumed that the disturbance in acquisition of learning ability at 3 months of age is secondary to the immaturity of neurons, while the learning disturbance at 7 months of age is due to neuronal aging. This substrain, which is characterized by the impairment of acquired learning ability due to senescence, is useful as a model for studies on human brain dysfunction associated with senescence.

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.

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.

Facilitation of memory retrieval by pretest morphine mediated by mu but not delta and kappa opioid receptors

Mice were trained to avoid electric shock (0.6 mA) in a step-through type passive avoidance learning task, retention being measured 24 h after the training trial. Morphine 10 mg/kg administered 30 min before the test trial (pretest) facilitated memory retrieval, and the effect was completely antagonized by 1 mg/kg naloxone, a selective mu-opioid receptor antagonist. On the other hand, pretest administration of 0.01-10 mg/kg DTLET, a selective delta-opioid receptor agonist, did not produce the same effect as morphine. Nor-binaltorphimine, a kappa-opioid receptor antagonist, did not antagonize the effect of pretest morphine, at doses of 1 and 2 mg/kg. These results suggest that the facilitation of memory retrieval by pretest morphine is mediated through mu- but not delta- or kappa-opioid receptors.