Strength of scopolamine-induced amnesia as a function of time between training and testing

Forgetting Unwanted Memories: Active Forgetting and Implications for the Development of Psychological Disorders

Intrusive memories are a common feature of many psychopathologies, and suppression-induced forgetting of unwanted memories appears as a critical ability to preserve mental health. In recent years, biological and cognitive studies converged in revealing that forgetting is due to active processes. Recent neurobiological studies provide evidence on the active role of main neurotransmitter systems in forgetting, suggesting that the brain actively works to suppress retrieval of unwanted memories. On the cognitive side, there is evidence that voluntary and involuntary processes (here termed "intentional" and "incidental" forgetting, respectively) contribute to active forgetting. In intentional forgetting, an inhibitory control mechanism suppresses awareness of unwanted memories at encoding or retrieval. In incidental forgetting, retrieval practice of some memories involuntarily suppresses the retrieval of other related memories. In this review we describe recent findings on deficits in active forgetting observed in psychopathologies, like post-traumatic stress disorder, depression, schizophrenia, and obsessive-compulsive disorder. Moreover, we report studies in which the role of neurotransmitter systems, known to be involved in the pathogenesis of mental disorders, has been investigated in active forgetting paradigms. The possibility that biological and cognitive mechanisms of active forgetting could be considered as hallmarks of the early onset of psychopathologies is also discussed.

Astaxanthin ameliorates scopolamine-induced spatial memory deficit via reduced cortical-striato-hippocampal oxidative stress

Alzheimer's disease is characterized by progressive disruption of cholinergic neurotransmission and impaired cognitive functions. In rodents, scopolamine has been used to induce cholinergic dysfunction resulting in cognitive impairments and an increment of oxidative stress in the brain. Here we tested whether oxidative stress can be attenuated via an antioxidant (astaxanthin) to rescue scopolamine-induced spatial memory. For this purpose, we administered either 0.9% saline (control), or scopolamine (SCP), or scopolamine plus astaxanthin (SCP + AST) to Swiss albino mice (ten weeks old; n = 20) for 28 consecutive days and subsequently examined animals' locomotor activity, spatial learning, and memory performance. The mice were then euthanized and prefrontal cortex (PFC), striatum (ST), hippocampus (HP), and liver tissues were assayed for antioxidant enzymes, glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and nitric oxide (NO). The SCP group exhibited impaired spatial learning and significantly altered levels of antioxidant enzymes and NO in the PFC, ST, and HP. In contrast, SCP + AST treatment did not cause spatial learning deficits. Furthermore, this condition also showed unaltered levels of SOD and NO in the ST and HP. Taken together, our results show that scopolamine may interrupt the striatal-hippocampal cholinergic activity resulting in impaired spatial memory. At the same time, these impairments are extinguished with astaxanthin by preventing oxidative damage in the striatal-hippocampal cholinergic neurons. Therefore, we suggest astaxanthin as a potential treatment to slow the onset or progression of cognitive dysfunctions that are elicited by abnormal cholinergic neurotransmission in Alzheimer's disease.

Nicotine facilitates memory consolidation in perceptual learning

Perceptual learning is a special type of non-declarative learning that involves experience-dependent plasticity in sensory cortices. The cholinergic system is known to modulate declarative learning. In particular, reduced levels or efficacy of the neurotransmitter acetylcholine were found to facilitate declarative memory consolidation. However, little is known about the role of the cholinergic system in memory consolidation of non-declarative learning. Here we compared two groups of non-smoking men who learned a visual texture discrimination task (TDT). One group received chewing tobacco containing nicotine for 1 h directly following the TDT training. The other group received a similar tasting control substance without nicotine. Electroencephalographic recordings during substance consumption showed reduced alpha activity and P300 latencies in the nicotine group compared to the control group. When re-tested on the TDT the following day, both groups responded more accurately and more rapidly than during training. These improvements were specific to the retinal location and orientation of the texture elements of the TDT suggesting that learning involved early visual cortex. A group comparison showed that learning effects were more pronounced in the nicotine group than in the control group. These findings suggest that oral consumption of nicotine enhances the efficacy of nicotinic acetylcholine receptors. Our findings further suggest that enhanced efficacy of the cholinergic system facilitates memory consolidation in perceptual learning (and possibly other types of non-declarative learning). In that regard acetylcholine seems to affect consolidation processes in perceptual learning in a different manner than in declarative learning. Alternatively, our findings might reflect dose-dependent cholinergic modulation of memory consolidation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The Ameliorating Effects of Cognition-enhancing Chinese Herbs on Scopolamine-and MK-801-induced Amnesia in Rats

The study was to investigate the ameliorating effects of three Chinese herbs — Achyranthes bidentata (AB), Ophiopogon japonicus (OJ) and Cnidium monnieri (CM) on scopolamine (SCOP)- and MK-801-induced amnesia by using a passive avoidance task in rats. AB, OJ and CM at 0.1 and 0.3 g/kg prolonged the step-through latency (STL) of the retention trial. In addition, AB, OJ and CM reversed the STL shortened by MK-801, but only AB reversed the STL shortened by SCOP. In conclusion, these Chinese herbs possess cognition-enhancing activities and anti-amnestic effects, but the mechanism of the effect of AB was different from those of OJ and CM.

Petroleum ether extract of Cnidium monnieri ameliorated scopolamine-induced amnesia through adrenal gland-mediated mechanism in male rats

AIM

Our previous study indicated petroleum ether layer of Cnidium monnieri L. Cuss. (CM) and its ingredient osthole could alleviate scopolamine-induced amnesia in female rats.

MATERIALS AND METHODS

Hence, this study was desired to investigate the mechanism of the ameliorating effects of petroleum ether layer of CM on the performance impairment of inhibitory avoidance task and Morris water maze induced by scopolamine in male rats.

RESULTS

CM at 0.1-0.6g/kg orally administered 60 min before the training trial ameliorated the scopolamine-induced performance impairment on inhibitory avoidance learning and water maze in male rats. Only adrenalectomy but not peripheral cholinergic antagonist scopolamine methylbromide and catecholaminergic neurotoxin 6-hydroxydopamine blocked the ameliorating effects of CM on scopolamine-induced performance impairment in rats.

CONCLUSION

Therefore, we demonstrated that the ameliorating effects of CM on scopolamine-induced performance impairment may be related to activating the adrenal gland and central acetylcholingeric neuron, instead of peripheral nervous system.

Effects of luteolin on learning acquisition in rats: Involvement of the central cholinergic system

The study was conducted to investigate the ameliorating effects of luteolin on memory acquisition in rats. The effects of luteolin on scopolamine-induced impairment of passive avoidance response were evaluated primarily, as well as the role of the central nervous system through the use of central neurotoxins and central nervous antagonists. Luteolin was not reversed by scopolamine N-methylbromide (M-SCOP) but blocked the impairment of learning acquisition induced by cholinergic neurotoxin (ethylcholine aziridinium, AF64A) and muscarinic (scopolamine hydrobromide, SCOP) and nicotinic (mecamylamine, MECA) receptor antagonists. However, it did not block dopaminergic neurotoxin (6-hydroxydopamine, 6-OHDA)-induced and serotonergic neurotoxin (5,7-dihydroxytryptamine, 5,7-DHT)-induced impairments. From these results, we suggest that the attenuating effect of luteolin (10 mg/kg, i.p.) on the deficits of passive avoidance performance induced by SCOP may be related to the increases in the activities of central muscarinic and nicotinic receptors.

Osthole improves aspects of spatial performance in ovariectomized rats

The present study was designed to investigate the ameliorating effects of Cnidiuim monnieri L. Cusson (CM) and osthole, a constituent of CM, on the spatial performance deficit in scopolamine (SCOP)-treated or ovariectomized (OVA) rats. CM improved the deficit of spatial performance, and reversed the lower plasma estradiol levels caused by SCOP in female rats. In addition, osthole (3 and 10 mg/kg, s.c.) improved the performance deficit in OVA rats. It (10 and 30 micrograms/brain, icv) also improved the performance deficit caused by SCOP in intact female rats, and at 30 micrograms/brain improved the deficit in OVA rats. However, osthole did not alter the latency swum to reach the visible target in SCOP-treated and OVA rats. Accordingly, we suggested that osthole is an active constituent of CM, and possesses ameliorating effects on the spatial performance deficits in SCOP-treated female rats or OVA rats. The action mechanism of the effects of osthole on performance deficits was related to the estrogen-like properties and activating the central cholinergic neuronal system.

The ameliorating effects of the cognitive-enhancing Chinese herbs on scopolamine-induced amnesia in rats

Ameliorating effects were investigated of the cognitive-enhancing Chinese herbs administered orally for 1 week-Panax ginseng (PG), Panax notoginseng (PNG), Dioscorea opposita (DO), Gastrodia elata (GE), Salvia miltiorrhiza (SM), Acorus gramineus (AG), Coptis chinensis (CC), Polygonum multiflorum (PM), Cyperus rotundus (CR) and Psoralea corylifolia (PC)-on the scopolamine (SCOP)-induced amnesia by using a passive avoidance task in rats. Of ten Chinese herbs, only PG, PNG, GE and CC prolonged the SCOP-shortened STL. These results revealed that PG, PNG GE and CC administered orally for 1 week improved the SCOP-induced learning and memory deficit in rats.

Attenuation of morphine tolerance and dependence in scopolamine-treated rats

Morphine tolerance and dependence were investigated in scopolamine-treated rats. The results showed that scopolamine treatment (up to 2 mg/kg) did not affect basal line or morphine-induced latency in the tail-flick test but significantly increased the escape latency in the Morris water-maze task. Co-administration of scopolamine could considerably attenuate development of morphine tolerance and naloxone-precipitated withdrawal syndrome. Pretreatment of scopolamine for 7 days prior to morphine administration also significantly reduced the tolerance and withdrawal symptoms. Scopolamine treatment was further shown to attenuate morphine-induced conditioned place preference, an indicator of morphine psychological dependence. The present study demonstrated the attenuation of morphine tolerance and dependence in scopolamine-treated animals, probably related to scopolamine-produced learning and memory impairment.

Plus-maze retest profile in mice: importance of initial stages of trail 1 and response to post-trail cholinergic receptor blockade

Recent research has shown that a single undrugged prior experience of the elevated plus-maze produces significant behavioural changes upon 24-h retest in rats and mice. Typically, when reexposed to the maze, animals display an increased avoidance of the open arms and a corresponding preference for the enclosed sections of the apparatus. Using ethological analyses, the present series of experiments sought to further characterize this phenomenon in mice and to determine whether or not it involves cholinergic receptor mechanisms. Results confirmed that behaviour during Trial 2 is markedly different to that seen on initial exposure, and that such changes are independent of the duration of Trial 1 (2 vs. 5 min). Retest behavioural changes included reduced entry latencies, reduced open arm entries, less time on the open arms and centre platform, lower levels of exploratory head-dipping, and increased entries into and time spent in the closed arms. The importance to the retest phenomenon of the first few minutes of initial exposure was further suggested by min-by-min analyses of the behaviour of animals naive to the maze. Results showed that behaviour during the first min is characterized by high levels of risk assessment from the centre platform and relatively low, but equal, levels of open- and closed-arm exploration. From min 2 onwards, however, behaviour showed a marked change with increasing open arm/centre platform avoidance, increasing closed-arm preference, and decreasing levels of risk assessment and exploratory head-dipping. Thus, it would appear that this within-session aversive learning transfers between sessions to account for behavioural profiles on retest. Irrespective of the duration of Trial 1 (2 or 5 min), posttrial administration of the muscarinic antagonist, scopolamine (0.1-1.0 mg/kg), failed to significantly alter the behavioural changes seen between trials. Data are discussed in relation to the apparent sensitization of fear produced by plus-maze exposure, its possible relation to phobia acquisition, and the need for further research on underlying mechanisms.

Effects of scopolamine on learning and memory in monkeys

The effects of scopolamine were evaluated in monkeys responding under operant procedures designed to evaluate drug effects on learning and memory. In one procedure, responding was maintained by food presentation under a multiple schedule. One component of the multiple schedule was a repeated-acquisition task in which the discriminative stimuli for left- and right-key responses changed each session (learning). In the other component, the discriminative stimuli for responses were the same each session (performance). In both components of the multiple schedule, scopolamine produced dose-related decreases in responding; there was little evidence of differential rate-decreasing effects between components. Percent errors in learning were increased in a dose-related manner, whereas percent errors in performance were generally unaffected except at high doses, which also produced substantial decreases in response rate. These results suggest that acquisition is more sensitive to the disruptive effects of scopolamine than is performance. The second procedure utilized repeated acquisition and delayed performance as a technique to study the effects of scopolamine on memory. In this procedure, each session was divided into three phases: acquisition, delay and performance. After a 24-h delay, scopolamine had little or no effect on retention, accuracy or rate of responding. In contrast, after a 60-min delay, scopolamine decreased retention in a dose-related manner. These data suggest that scopolamine produces a greater disruptive effect on short (60-min) versus long (24-h) delays.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Negative and positive effects of intracerebroventricular scopolamine on memory in mice undergoing passive avoidance and escape tests

The effects of intracerebroventricular administration of scopolamine on memory and learning in the conscious, freely moving mouse were evaluated using step-down passive avoidance and water maze tests. A new technique was used that allows convenient injection into the cerebral ventricles without disturbing the animal's behavior. No significant changes in locomotor activity were observed after low doses of scopolamine (0.1 and 1.0 microgram). However, 10 micrograms produced an increase in locomotor activity, while 100 micrograms caused an initial decrease followed by an increase in activity. In the passive avoidance test, scopolamine significantly impaired memory acquisition at doses higher than 1.0 microgram, consolidation at a dose of 100 micrograms, and retrieval at doses of 10 and 100 micrograms. In contrast, a dose of 0.1 microgram significantly improved consolidation and retrieval. In the water maze with a bridge, scopolamine either impaired memory acquisition, consolidation, and retrieval, or had no significant effect in the dose range tested. These results suggest that there are differences in the process of memory formation in the passive avoidance and escape tests.

Time gradient for post-test vulnerability to scopolamine-induced amnesia following the initial acquisition session of a spatial reference memory task in mice

The time course for vulnerability to the amnestic effects of the cholinergic antagonist, scopolamine, during the postacquisition period has been investigated. We have examined the effects of post-test injections of scopolamine (1 mg/kg ip) given at different times from 30 s for up to 6 h following the end of the first acquisition session of a concurrent spatial discrimination (reference memory) protocol in an 8-arm radial maze on subsequent long-term (24 h) retention performance in C57BL/6 mice. Results show that the immediate (30 s) post-test injection of scopolamine-HCl on Day 1 produces marked perturbation (amnesia) of long-term retention as attested to by significant deficits in various indices of spatial discrimination performance gain on Day 2 as compared to control subjects injected either with scopolamine-MBr or saline. The severity of this scopolamine-induced amnesia declines only slightly as a function of the treatment period 30 s-3 h post-test. However, no evidence for amnesia is observed if scopolamine-HCl injections are delayed for 6 h postsession. This important latter observation attests to the absence of any significant proactive effects of scopolamine on the ability of mice to perform the retention test via possible long-term effects on attention, motivation, or locomotor performance. These results thus constitute evidence for the existence of a limited (30 s-3 h) time gradient for vulnerability of the early memory trace to disruption by scopolamine. The present results are discussed in relation to our previous direct neurochemical observations describing the differential time courses of intervention of the ascending septohippocampal and nBM-cortical cholinergic pathways in the postlearning period. In particular, the presently observed time window concerning post-test vulnerability to scopolamine-induced amnesia corresponds more closely to the time course of the acute activation of the nBM-cortical cholinergic pathway, induced by testing with the same spatial memory protocol as used in the present study in mice.

A cerebral ischemia model produced by injection of microspheres via the external carotid artery in freely moving rats

We produced an improved microembolism model of cerebral focal ischemia by injection of 1000-2000 microspheres (50 +/- 5 microns diameter) via a tube retrogradely inserted into the right external carotid artery in freely moving rats. The group injected with 2000 spheres showed a much more severe mortality rate as well as neurological signs than did the 1000-sphere group. Brain water content of the 2000-sphere group was examined and found to show an increase from 4 to 24 h after embolization in the right hemisphere, indicating serious brain edema. Severe neurological signs and individual deaths by embolization were most likely related to the extent of development of brain edema. Examination of learning behavior by shuttle-box avoidance revealed partial but significant impairment of learning in the 1000-sphere group. Autoradiographic studies for muscarinic acetylcholine receptors and protein kinase C binding sites were conducted. Both these binding sites decreased in number, but protein kinase C seems to be more susceptible to ischemic injury than muscarinic acetylcholine receptors. The observation was considered to be closely related with an impairment of learning. The present study suggests that our microembolism model in freely moving rats is useful for investigations of the early phase and late phase of cerebral ischemia.

Acetylcholine, theta-rhythm and activity of hippocampal neurons in the rabbit--IV. Sensory stimulation

Modifications of responses of hippocampal neurons to sensory stimuli at the background of increased endogenous acetylcholine level (injection of physostigmine) and during blocking by scopolamine were analysed in the chronic alert rabbit. A significant decrease of reactivity (about 40%) of hippocampal neurons to sensory stimuli occurred after physostigmine injection, inducing stable theta modulation. Suppression and decrease of inhibitory responses (including initial reset phase) and of some excitatory reactions (including on-effects) were observed. However, a limited group of excitatory responses was augmented and prolonged under physostigmine action. Scopolamine, which blocked electroencephalogram theta-rhythm, did not change the responsiveness of hippocampal neurons. Some of the inhibitory and excitatory effects of sensory stimuli, especially on-responses, were strongly facilitated. Tonic responses were shorter, but they were stably reproduced without typical gradual habituation. All these effects were also present in the hippocampus after basal undercutting of the septum, which eliminates ascending brainstem input. It is suggested that under normal conditions a new or significant sensory stimulus evokes, in the hippocampus, an initial inhibitory reset of neuronal activity with subsequent coordinated triggering of rhythmic theta modulation by the septal input and arrival of the cortical input signal phase-locked to it. During the period of theta triggered by the stimulus, its processing and fixation in memory occurs, while the other, interfering stimuli, which are not phase-locked to the ongoing theta activity, are actively filtered out. Thus, septohippocampal interactions may participate in the organization of selective attention as a necessary condition for memory trace formation.

Scopolamine affects response-to-change test involving 20-min retention interval after locomotor exploration in rats

The tendency to select the T-maze arm that has been changed in brightness between two successive trials (response-to-change) was investigated. Our previous findings indicated that scopolamine injections (1.0 mg/kg) impaired responding to change of brightness in a choice trial (trial II) following a 1-min retention interval, when in the first acquisition trial rats could only inspect the white-black T-maze arms through transparent partitions (the passive test). The drug was ineffective when rats were allowed locomotor exploration of the maze (the active test). The aim of the present experiment was to investigate the effect of the same dose of scopolamine on the active test involving a longer 20-min retention interval between the acquisition trial and the choice trial. The effect of cue salience also was examined by using grey-black arms. Rats injected with scopolamine (Scopo) 20 min before the acquisition trial performed in the white-black maze on the chance level, whereas saline-injected rats (Sal) showed significant preference for the changed arm. Decreasing the cue salience impaired response-to-change in Sal rats (50% of changed arm choices) but had no further effect on performance of Scopo rats, presumably because of a floor effect. The postacquisition injection had a somewhat stronger effect than the injections preceding acquisition, which most probably reflects the state dependency phenomenon. The deficient performance due to scopolamine treatment that appeared in the present study at a longer retention interval could be interpreted in terms of increased forgetting.

Effect of propofol on memory in mice

The amnestic effects of the intravenous hypnotic anesthetic agent 2,6-diisopropylphenol (propofol; Diprivan) were studied in a single-trial passive avoidance task. Mice were injected with propofol 10 min before or immediately after training. Memory was impaired in a dose-dependent fashion when the anesthetic was administered before learning, but no amnesia was apparent with posttraining injections. Examination of the acquisition of passive avoidance using a multitrial task showed that propofol-treated mice learned the response normally but forgot the learning significantly faster than vehicle-treated controls. The anterograde amnesia was not the result of state-dependent learning. Propofol also disrupted extinction of fear conditioning when the anesthetic was given during the extinction session. Propofol-induced amnesia could be attenuated by amphetamine (1 mg/kg) injected 30 min before the retention test.

The action of scopolamine on retrieval and memory storage in rats evaluated in the staircase maze

Rats were trained to run on staircase stopping on the 3rd, 6th, 9th, and 12th steps (correct responses). Stopping on any other step was considered an error. The acute administration of scopolamine (1.5 mg/kg) 20 min before the trial caused a reduction of the correct responses. An interruption of the daily training for 20 days caused, in the controls, a 24% reduction of correct responses. A chronic administration of scopolamine, at doses over 10 mg/kg in the first 15 days of the no-training period, nullified the behavioral deterioration observed in the controls. The interpretation of these results is that scopolamine damages the retrieval process and blocks the spontaneous decay of memory, as was observed in the controls after 20 days of interruption of the daily training.

Bilateral injection of fasciculin into the amygdala of rats: effects on two avoidance tasks, acetylcholinesterase activity, and cholinergic muscarinic receptors

These experiments examined the effects of the bilateral injection of fasciculin-2 (FAS), a natural acetylcholinesterase (AChE) inhibitory peptide, into the amygdala of rats on acquisition and retention of two avoidance behaviors. Intraamygdala injection of FAS (150 ng/amygdala) produced a pronounced and long-lasting inhibition of AChE activity: 85% and 74% on day 2 and day 5, respectively. After 48 hr, FAS-treated animals showed no changes in training or test session performance in a step-down inhibitory avoidance task (training-test interval was 24 hr). In a 2-way shuttle avoidance task, intraamygdala FAS slightly reduced retention test performance without modifying training session scores. Two and five days after FAS injections into the amygdala, the density of muscarinic receptor decreased about 50% as measured by the specific bindings of 3H-quinuclidinyl benzilate and 3H-oxotremorine. No alterations were observed in the apparent dissociation constants. On the other hand, the central-type benzodiazepine receptor population of the amygdala remained unchanged, suggesting that FAS microinjection did not produce damage to neuronal components of these nuclei. In conclusion, the results presented have indicated that a clear-cut and long-lasting inhibition of AChE activity in the amygdala is not accompanied by a facilitation of learning and memory of two different avoidance tasks. Compensation of the increased cholinergic activity by a down-regulation of muscarinic receptors could account for these findings.

A vasopressin metabolite produces qualitatively different effects on memory retrieval depending on the accessibility of the memory

A vasopressin metabolite, AVP4-9, was injected 1 h prior to retention tests at 1, 6, 11, and 16 days after learning to test the hypothesis that the peptide exerts qualitatively different effects on memory retrieval depending on the accessibility of the memory. The findings provided strong support for this hypothesis: At a retention interval associated with excellent recall in control animals, pretest administration of AVP4-9 (3.0 micrograms/kg) significantly impaired memory, while this same treatment significantly improved recall at an interval associated with poor retention in controls. At retention tests associated with intermediate recall in controls, retrieval was not significantly affected by the peptide treatment. This pattern of results indicates that the peptide treatment is interacting with endogenous changes that correspond to the accessibility of the memory.