Prevention of experimental amnesia by peripherally administered cholecystokinin octapeptide in the rat

The effect of subcutaneously injected cholecystokinin octapeptide (CCK-8) on amnesia induced by electroconvulsive shock (ECS), CO₂ inhalation, or cycloheximide injection was investigated in rats. In normal rats, single administration of CCK-8 had no significant effect on the passive avoidance response. Treatment with ECS, CO₂, or cycloheximide markedly decreased the latency of the passive avoidance response, but CCK-8 in doses from 0.1 to 10 μg/kg could prevent the induced amnesia when injected 30 min before the training trials, immediately after foot shock or amnesic treatments, and 30 min before the first retention test. The results indicate that peripheral administration of CCK-8 is effective in preventing amnesia in the rat.     

Investigation of the reported protective effect of cycloheximide on memory

Many findings support the hypothesis that formation of long-term memory requires synthesis of proteins in the nervous system close to the time of learning. This hypothesis has been challenged recently by reports that the protein synthesis inhibitor cycloheximide (CYC) injected 2 hr prior to passive avoidance training in mice or rats attenuated the memory impairment induced by a usually amnestic dose of CYC administered 30 min pretraining. To investigate the reports of a "protective" effect of the prior injection, we attempted to replicate them and test their generality. For replication we administered either paired injections of CYC--120 mg/kg 2 hr prior to training and 30 mg/kg 30 min prior to training--or single injections of CYC (either 120 mg/kg or 30 mg/kg) 30 min pretraining and tested for retention of the passive avoidance habit either 1 or 7 days later. No attenuation of amnesia was observed at 1 day tests. Attenuation of amnesia following the double injection of CYC was observed at 7 day tests. When another protein synthesis inhibitor, anisomycin, was used in the same experimental design, there was no "protective" effect; two injections of anisomycin produced greater memory impairment for the passive avoidance habit than did the single low dose. Also, for active avoidance training, two successive injections of CYC caused significantly greater amnesia than did a single dose; this is the opposite of a "protective" effect. We suggest that the reported "protective" effect of CYC on memory is an as yet unexplained phenomenon that does not generalize to other antibiotic drugs and is specific to the passive avoidance task.     

Inhibition of cerebral protein synthesis: performance at different times after passive avoidance training.

Inhibition of cerebral protein synthesis impairs long-term memory in a variety of species and tasks. Recently it was reported that subcutaneous injection of the protein synthesis inhibitor cycloheximide impaired short-term retention, measured 10 min after training in a passive avoidance task. To examine the possibility that inhibition of cerebral protein synthesis may sometimes disrupt short-term memory, mice were injected subcutaneously with cycloheximide (120 mg/kg) or anisomycin (150 mg/kg), or bitemporally with cycloheximide or anisomycin (100 mug/side) and given one training trial in a passive avoidance box. Subcutaneously injected cycloheximide reduced step-through latencies 10 min after training as reported previously, but anisomycin or bitemporally injected cycloheximide did not. All 4 drug groups exhibited impaired long-term memory. Since the results obtained at short intervals after training varied depending on the drug and route of injection, the impairment produced by subcutaneous cycloheximide at 10 min after training cannot be attributed to inhibition of cerebral protein synthesis. It is suggested that performance at short intervals after training reflects drug side effects on step-through behavior. By contrast, the impairment obtained at long intervals after training is consistent with the hypothesis that cerebral protein synthesis is required for formation of long-term memory.     

Cycloheximide induced amnesia and recovery as a function of training parameters

Mice were given 2 or 3 training trials in a passive avoidance task following an injection of cycloheximide or saline. They were tested 1, 1.5, 3, 24, or 72 hr after training and tested again 72 hr after the first test trial. All the cycloheximide groups except the 1 hr groups were inferior to saline controls on the first test trial, and there was no suggestion of spontaneous recovery over the intervals tested. Test 2 performance was generally inferior to Test 1 performance for all groups, but the cycloheximide groups showed the greatest drop in performance. A second experiment extended train/test intervals to 144, 146, 148, and 192 hr. Spontaneous or test induced recovery again did not occur. The discussion attempts to reconcile these results with prior reports of recovery in terms of differential conditioning of different components of passive avoidance memory by the different training procedures. This results in partial sparing of some components of passive avoidance memory by cycloheximide, which has the appearance of recovery under certain test conditions.     

Comparison in mice of the amnestic effects of cycloheximide and 6-hydroxydopamine in a one-trial passive avoidance task

To test further the hypothesis that cycloheximide (CXM)-induced amnesia is due in part to its effects on the central adrenergic system, a comparison was made in mice of the effects of the antibiotic and of 6-hydroxydopamine (6-OHDA) on memory of a one-trial passive avoidance task. Both drugs produced amnesia 24 hr after training but unlike CXM, 6-OHDA had no effect on memory 20 min after training.     

Evidence for adrenergic neurons in a memory access pathway

Injection of a beta-adrenergic blocker, propranolol, in rats within 5 min after training of a step-down passive avoidance response had no effect on performance either 2 or 6 hr later, however, when testing occurred 1, 3 or 7 days after training and injection a significant performance decrement was observed. If drug injection was postponed until 1 or 3 days after training and testing was conducted 2 hr later, again poor avoidance performance was obtained. No support for a state-dependency explanation [14] of the propranolol amnesia could be found. The amnesia that followed beta-adrenergic receptor block was identical to that previously reported when norepinephrine biosynthesis was reduced [9] and supports the hypothesis of a role for adrenergic neurons in memory formation and retrieval that is different from cholinergic neurons [4,10].     

The effects of chlorpromazine on the expression of an acquired passive avoidance response in mice

Mice given one-trial passive avoidance training were examined 24 hours later for retention of the acquired response. Testing was carried out with subjects given either chlorpromazine or saline injections before the test session. Three chlorpromazine doses (0.5, 2.0, and 3.5 mg/kg) were used, and three injection times (10, 90, or 180 min before testing). Chlorpromazine was found to impair the expression of the acquired response, both by depressing its initial elicitation and also by apparently facilitating extinction. A second experiment confirmed that extinction rate was indeed increased. A clear dosage effect was observed but injection time was not important in determining the drug's effect. Further experiments were undertaken to clarify the interpretation of the drug's action; in particular, the possibility that the effects might have been caused by a dissociation of learning between the training and test situations was examined. It is suggested that the elevated extinction rates observed during testing when subjects were given chlorpromazine represents a temporary effect resulting from the reduced stimulus control of behaviour. Permanent effects of pre-test drug administration were noted on the initial expression of the learned response.     

Centrally administered cycloheximide in rats: behavioural concomitants and modulation of amnesic effects by biogenic amines

The behavioural consequences of centrally administered cycloheximide (400 μg, intraventricularly) were examined at various times after the injection and compared with the degree of protein synthesis inhibition. Operant behaviour (FR3 responding for water reward) was significantly depressed at 1, 2, 4, 6, 8, 10 and 12 hr after the injection but not at 24 hr, while general locomotor activity was significantly depressed at all time points except 1 and 24 hr. Amnesia for a passive avoidance response was observed when the cycloheximide was administered 1, 3, 5, 7, and 9 hr before the training trial but not at 11 or 17 hr. Protein synthesis was found to be maximally inhibited (80%) at 1 and 2 hr, moderately inhibited (60%) at 4, 6, and 8 hr, less but still significantly inhibited (40%) at 12 hr and slightly elevated (15%) at 24 hr after the central injection of cycloheximide. Posttraining administration of 1-tryptophan (100 mg/kg) or corticosterone (5 mg/kg) significantly reversed the amnesia produced by a central injection of cycloheximide given 5 hr before training, while imipramine (5 mg/kg), d-amphetamine (5 mg/kg) and hydrocortisone (5 mg/kg) were without significant effect. These results suggest that the disruption of passive avoidance memory by centrally administered cycloheximide may not be related to the inhibition of synthesis of memory-specific protein, but rather to a depression of central levels of biogenic amines, particularly serotonin.     

DDC-Induced amnesia and norepinephrine: A correlated behavioral-biochemical analysis

Diethyldithiocarbamic acid (DDC), a dopamine-B-hydroxylase inhibitor, when injected into rats 30 min to 6 h before training of a passive avoidance task, impaired formation of long-term memory as indicated by performance on a retention test 24 h later. Performance of the task was at its minimum when injection occurred 2 to 4 h prior to training; recovery was evident in animals trained 5 or 6 h after drug treatment. Catecholamine assay of brains of temporally yoked animals showed that norepinephrine depletion followed a time course paralleling that of the amnesia. These findings support the hypothesis that the degree of memory storage, as reflected in performance following training in a passive avoidance task, can be directly correlated with the level of norepinephrine existing at the time of training.     

Cycloheximide and passive avoidance memory in mice: Time-response,dose-response and short-term memory

The greatest loss of memory shown by mice 24 hr after learning was found to occur with cycloheximide (CXM) (120 mg/kg) administered subcutaneously 30 min before training. With injection at this time the extent of the amnesia was done dependent (30–150 mg/kg) and the resultant amnesia was found to be relatively constant when tested at 1, 7 or 14 days. An attempt was made to follow the development of this amnesia with 100 and 120 mg/kg CXM. However, the saline controls showed an unexpectedly low avoidance 6 hr after training. This was interpreted as a possible interaction between the stress of the injection and the 6 hr interval. An experiment designed to test this possibility showed that mice injected with 0.1 ml of 1% lignocaine gave high avoidance at 6 hr but mice receiving only a needle puncture of the skin gave performances similar to mice receiving saline injections. It was felt that these findings cast doubt on the usefulness of the passive avoidance task in the assessment of drug action on short term memory.     

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.     

A parallel study of the amnesic effects of cycloheximide and ECS under different strengths of conditioning

Summary Delay of punishment 0, 30, 60, 120 and 240 sec after the step-through response in a one trial passive avoidance task was used to obtain in mice conditioned responses of different strengths. Both cycloheximide and ECS given immediately after training had a greater amnesic effect upon the weaker conditioned responses obtained with the longer delays of punishment. A proactive effect of ECS, but not of cycloheximide, was demonstrated with the weaker conditioned responses.     

Memory access pathway: role of adrenergic versus cholinergic neurons

A temporary depletion of brain norepinephrine in rats produced by injection of a dopamine beta-hydroxylase inhibitor, diethyldithiocarbamate (DDC), 30 min prior to testing, prevented performance of a trained passive avoidance response 1, 3, 5 or 7 days after training. Subsequent recovery in performance indicated that the memory itself was not destroyed, but rather that the process of memory retrieval was affected. Anticholinesterase treatment produced a similar retrieval amnesia, but the effect was dependent upon the age of the memory at the time of drug injection [11]. In both cases, when the animals were presented a recall trial prior to injection, the normally observed amnesia was blocked. Animals treated with DDC up to 3 hr before training were capable of learning the passive avoidance task and of avoidance performance for a few minutes after training. However, these animals failed to produce a long-term memory of the trained response. Anticholinesterase treatment had no effect on memory formation. These results suggest different roles for adrenergic and cholinergic neurons in a pathway associated with memory storage and retrieval.     

The effects of selective dopamine agonists on a passive avoidance learning task in the day-old chick

Recent examination of the mixed dopamine agonist apomorphine suggests that dopamine inhibits both passive avoidance and response suppression learning. The present study investigated the effects of selective dopamine agonists on memory consolidation using a passive avoidance task in the day-old chick. The dopamine D1 agonist SKF 38393, the D2 agonist quinpirole, and the D4 agonist PD 168077 all failed to disrupt memory consolidation when injected immediately after training. However, chicks injected with 6.0 mg/kg of the D3 agonist (+)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) displayed memory impairment 180 min after aversive training. A study of the time course of this effect of 7-OH-DPAT showed that it first appeared 90 min after aversive training. Pretreatment with the dopamine D3 antagonist U 99194 eliminated the disturbance of passive avoidance learning induced by 7-OH-DPAT. These results indicate that dopamine is involved in the later stages of the memory formation process and that the D3 receptor is crucially involved in this disruption.     

Opposite effects induced by low and high doses of apomorphine on single-trial passive avoidance learning in mice

The effects of apomorphine (0.0125-1 mg/kg, SC), a dopamine (DA) agonist, on passive avoidance learning were assessed in mice which received brief and long foot-shocks in a training test. At low doses, apomorphine stimulates DA autoreceptors. With a shock of brief duration, apomorphine at a low dose (0.05 mg/kg), enhanced the avoidance learning when it was administered 20 min before the training test or the retention test. At high doses, apomorphine stimulates postsynaptic DA receptors. With a shock of long duration, apomorphine at a high dose (1 mg/kg), impaired the avoidance learning when it was administered 20 min before the training test or the retention test. However, apomorphine (0.05 and 1 mg/kg) given immediately after the training test did not have any effect on the avoidance behavior with shocks of either brief or long durations. Apomorphine-induced enhancement of passive avoidance learning was antagonized by sulpiride, but not by haloperidol. These results show that apomorphine induced the opposite effects on the passive avoidance learning depending on the dose or on the reinforcement intensity and suggest that the central DA system may play an important role in modulating memory processes.     

Some behavioral effects of microinjections of serotonin and noradrenaline into the limbic structures of the rat brain

The effects of microinjections of serotonin (5HT) and noradrenaline (NA) into the limbic structures of the rat brain on the open field behavior and acquisition and retention of a step-down passive avoidance, were examined. Bilateral injections of NA and 5HT into the dorsal hippocampus (DH) produced opposite effects in the open field test, of excitatory and inhibitor nature, respectively. Intrahippocampal injections of 5HT slightly attenuated passive avoidance acquisition and significantly impaired its retention. Bilateral microinjections of NA and 5HT into the basomedial amygdala (BMA) inhibited locomotor activity and exploration in the open field test. In a dose of 40 micrograms, NA and 5HT significantly attenuated also the performance of an avoidance retention, examined 24 h after training. Bilateral injection of NA and 5HT into nucleus accumbens septi (NAS) inhibited 5 min later the locomotor activity and exploration in the open field test. The monoamines administered in various doses into the NAS before training did not consistently influence the rate of acquisition and retention of a passive avoidance reaction. The data are discussed in terms of an involvement of monoaminergic innervation of limbic structures of the brain in the organization of animal behavior.     

Effects of pretraining on subsequent cycloheximide induced amnesia

In the first experiment, mice were trained on a passive avoidance (PA) task, given one extinction trial, and then were injected with cycloheximide or saline shortly before retraining on the PA task. On a subsequent test trial, the performance of the cycloheximide group was inferior to the saline group, but superior to a cycloheximide group not given the pretraining experience. In the second experiment, one group of mice was given cycloheximide before each of two training sessions while another group received cycloheximide before the first training session and saline before the second. The group given cycloheximide before each training session was amnesic for both sessions to an equal degree, while the other group was amnesic for only the first session. The final test performance of the latter group was similar to that of a saline group not given any pretraining experience. These data seem to indicate that pretraining has limited effect on subsequent cycloheximide induced amnesia, and that such amnesia is the result of impaired memory formation rather than impaired memory retrieval.     

Effect of minaprine and other reference drugs on passive avoidance impairment induced by cerebral ischemia in Mongolian gerbils

We examined the characteristics of cerebral ischemia-induced behavioral deficit in the passive avoidance task and the effect of minaprine and other cytoprotective drugs on passive avoidance deficit induced by cerebral ischemia in Mongolian gerbils. Severe impairment of passive avoidance was apparent when the duration of the ischemia exceeded 2 min. Histopathological ischemic neuronal damage in CA1 neurons at 7 days after occlusion was also induced when the ischemia was over 2 min. Otherwise, although cerebral ischemia was carried out at 5 min, 2 hr, 5 hr or 24 hr after the training session, the passive avoidance deficit was produced 24 hr after the training session. When the training session was carried out 24 hr before the occlusion, minaprine, which was administered 30 min before the occlusion, led to a recovery of the response latency. Pentobarbital, diazepam and ethylapovincamine improved the passive avoidance deficit induced by 5-min bilateral carotid artery occlusion. On the other hand, the passive avoidance deficit was not ameliorated by Ca(++)-hopantenate, nicardipine and idebenone. The hippocampal damage at 7 days after occlusion was prevented by the drugs that ameliorated the passive avoidance deficit. The relationship between passive avoidance deficit and CA1 neuronal death in the hippocampus induced by cerebral ischemia warrants further attention.     

DDC-induced retrograde amnesias prevented by injections of dl-DOPS.

Injection of a dopamine beta-hydroxylate inhibitor, diethyldithiocarbamate (DDC) in rats 30 min prior to training of a step-down passive avoidance task impaired performance of the task 24 hr later. Similarly, injection of DDC 30 min prior to testing blocked retrieval of a passive avoidance habit trained in normal rats the previous day. Injection of a direct norepinephrine (NE) precursor, dl-thero 3,4-dihydroxyphenylserine (DOPS) 60 min before DDC prevented both amnesias. These data support the hypothesis that reduced levels of NE are responsible for DDC-induced amnesias.     

Oxiracetam prevents electroshock-induced decrease in brain acetylcholine and amnesia

In the rat, 1 min following electroshock (ECS) a 46 and 39% decrease in acetylcholine levels was found in the hippocampus and cerebral cortex, respectively. The decrease in the hippocampus was still statistically significant 30 min after ECS. The ECS applied 1 min after training also disrupted the performance of a passive avoidance conditioned response ('step down') tested 30 min later. Oxiracetam (100 and 300 mg/kg i.p.) administered 90 min before training prevented, in a dose-dependent manner, the decrease of acetylcholine in the cerebral cortex and hippocampus. Oxiracetam prevented the ECS disruption of the acquisition of a passive avoidance response. At the dose of 300 mg/kg the acetylcholine level 1 min after ECS was significantly higher than in the sham-treated rats. Piracetam at the same doses was inactive. These results support the hypothesis that oxiracetam may prevent the disruption of the conditioned response by acting on cortical and hippocampal cholinergic mechanisms.