Recovery as a function of the degree of amnesia due to protein synthesis inhibition

Retrograde amnesia following inhibition of cerebral protein synthesis has generally been explained as either a failure of consolidation or impairment of a retrieval mechanism. Major evidence for the retrieval hypothesis is provided by studies which utilize a reminder (usually footshock) to attenuate the effect of the protein inhibitor. To examine this question, mice were injected subcutaneously with anisomycin (1 mg/animal, 7 mg/animal, or 1 mg/animal every 2 hr x 7) and given one training trial in a passive avoidance box. All animals received a single retention test on each of four consecutive days, starting either 1, 7, or 21 days after training. One-half of the mice in each group received a footshock reminder 1 hr after their initial test. The footshock reminder did not attenuate the inhibitor-induced amnesia, but multiple testing did produce partial recovery in animals demonstrating some memory of training (both Saline and Anisomycin animals). Animals injected with anisomycin whose testing began 1 day after training demonstrated partial recovery irrespective of drug dosage level. The extent of amnesia and recovery were dependent upon both drug dosage and training-test interval. Implications for the consolidation and retrieval hypotheses are discussed.     

Phencyclidine-induced retrograde amnesia in mice

The amnesic action of phencyclidine (PCP) was investigated in mice using a passive avoidance- and escape-learning method. PCP (10–30 mg/kg) administered immediately after the training test dose-dependently shortened and prolonged the step-down latency and escape latency, respectively in the retention test. There was a significant inverse relationship between the step-down and escape latencies, indicating that PCP had induced amnesia. The amnesic actions of PCP were retrograde, being observed when mice were given PCP within 10 min but not more than 30 min after the training test. The amnesic effects of PCP on both variables were antagonized significantly by physostigmine and naloxone, whereas cyproheptadine and haloperidol had no effect. None of these drugs by themselves affected passive avoidance- or escape-learning performance. These results suggest that the retrograde amnesic actions of PCP were produced via either the cholinergic or the opioidergic systems or both, but not through the serotonergic and the dopaminergic systems. Offprint requests to: T. Nabeshima     

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.     

Alleviation of scopolamine amnesia by different retrieval enhancing treatments

Mice were trained in a one-way active avoidance task to a criterion of 9/10 avoidances. Immediately following training they were injected with scopolamine hydrochloride (1 mg/kg SC) or with saline. Retention was assessed 3 days after training by 5 test trials on which the UCS was not present. Thirty min prior to the test, groups were injected with different doses of arecoline, d-amphetamine sulphate or with saline. Other scopolamine-treated mice were exposed to the CS or the UCS 24 hr prior to the test. The scopolamine-induced amnesia was attenuated by both 0.5 and 1.0 mg/kg arecoline and by 2.0 mg/kg d-amphetamine. Retention was also improved by exposure to the CS and the UCS. These data show that scopolamine amnesia can be alleviated by treatments which activate retrieval processes.     

Recovery of memory following forgetting induced by depletion of biogenic amines

Following depletion of biogenic amines by reserpine, mice were trained to avoid one compartment of a shuttle box by employing the procedures of Pavlovian fear conditioning. Retention was tested one week later using both an active and a passive measure. A robust amnesia was apparent in reserpine-treated animals on both retention measures. Treatment with the mixed dopamine-serotonin agonist lisuride 30 min prior to the test alleviated the memory loss. Since improved retention in the drug treated mice was indexed by increased response latencies in the passive test and decreased latencies in the active test it is unlikely that the improvement in performance was the result of non-specific effects on activity. The results are consistent with the hypothesis that lisuride treatment before testing facilitates retrieval processes     

Effects of nootropic drugs in a scopolamine-induced amnesia model in mice

Scopolamine (3 mg/kg IP) given before an acquisition trial, reduced the retention of a one-trial passive avoidance step through response in mice. A single administration of cholinergic agonists such as oxotremorine, BM-5, or arecoline, antagonized this amnesic effect of scopolamine. A significant anti-amnesic effect was also found with nootropic drugs such as piracetam and ucb L059, whereas ucb L060 (the enantiomer of ucb L059), oxiracetam and rolziracetam were shown to be ineffective. Moreover, ucb L059, administered twice daily for 3 days, counteracted the amnesic effects of scopolamine completely, whereas ucb L060 was again inactive. The results demonstrate that: (a) this model of impaired cognition by scopolamine is able to discriminate between closely related chemical substances and even stereoisomers; and (b) nootropic drugs, such as ucb L059, are more effective after repeated rather than after acute administration.     

Peripheral catecholamines and memory: characteristics of syrosingopine-induced amnesia.

The effect of syrosingopine on retention of a passive avoidance trial in mice was investigated. The drug given in doses of 2.5, 4.0 or 6.0 mg/kg 2 hr before training, but not when given 24 or 0.5 hr or immediately after training, resulted in amnesia 7 days later. Dopamine or norepinephrine administered systemically 15 min before to 10 min after training was able to block the syrosingopine-induced amnesia. The role of peripheral catecholamines in memory formation was discussed.     

Effects of 4-hydroxy, 4-phenyl,caproamide on pentylenetetrazol-induced amnesia

Early reports suggest a relationship between pentylenetetrazol-induced convulsions (PTZ) and amnesia. 4,-Hydroxy, 4-phenyl caproamide (YPCA) is potently anticonvulsant againts PTZ-induced convulsions. The purpose of these experiments is to show the possible role of PTZ in these amnestic effects. Experiment 1 proves YPCA antagonism of PTZ-induced convulsions in mice. Experiment 2 shows how YPCA, injected before PTZ and after animal training, protects against convulsions, leaving memory storage un-affected. However, when YPCA is injected before training (10 and 5 min) and PTZ 15 min afterwards (5 and 10 min after training), a retention impairment was observed. The results are discussed in terms that emphasize the need of convulsions in retrogarde amnesia. Experiment 3 shows that foot shock is necessary for passive avoidance acquisition. Experiment 4 shows that subconvulsive doses of PTZ (5, 10, 20, and 40 mg/kg^-1) have no effect on memory.     

Recovery from amnesia induced by pre-test injections of monoamine oxidase inhibitors

Amnesia was induced in C57BL6J male mice by pre-training injections of cycloheximide (CYC) in a one-trial passive avoidance task. This amnesia was reversed by pre-testing injections of two monoamine oxidase inhibitors (MAOI's) catron and pargyline. The results of 2 non-contingent control groups indicated that mice injected with cycloheximide but given foot shock in a place different from the training apparatus did not show increased latency following treatment with catron and pargyline. This indicates that recovery is specific for training in the passive avoidance task. Depletion of norepinephrine (NE) by diethyldithiocarbamate (DEDTC), a dopamine beta hydroxylase inhibitor, resulted in an amnesia similar to that induced by CYC. DEDTC-induced amnesia was also reversed by catron and pargyline.     

Hippocampal monoamine metabolism and the CO2 induced retrograde amnesia gradient in rats.

It was found that in rats a gradient of retrograde amnesia for a passive avoidance response could be established when carbon dioxide (CO2) was used as the amnesic agent. The extent of passive avoidance increased as the period between application of a mild foot shock and CO2 treatment was increased. The amnesia gradient was found to cover a period of at least 60 min. Changes in hippocampal serotonin metabolism parallelled the amnesia gradient. Thus, the concent increased. The changes in hippocampal noradrenaline and dopamine did not correlate with the amnesia gradient.     

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.     

Temporal and pharmacological parameters of puromycin-induced amnesia

Mice were trained in step-down passive avoidance behavior. Bitemporal injections of puromycin (PM) were given either immediately or delayed until 24 hrs after training. PM produced a marked amnesia in both cases during retention testing 3 days later. The amnesia persisted during a second retention test 6 days after training. Of all the antibiotics, only PM is effective as an amnestic agent when injections are delayed 24 or more hours after training. Cycloheximide (CXM) was also injected bitemporally immediately after training. However, CXM produced a weaker amnestic effect even though it produced a much greater inhibition of cerebral protein synthesis, more rapidly, and of longer duration. In an effort to attenuate the amnesia produced by PM, in separate experiments, the mice were injected with combined injections of PM and CXM (bitemporally); mice were also given combined injections of PM (bitemporally) and amphetamine (subcutaneously). The amnesia produced by immediate injections of PM was not attenuated by either CXM or amphetamine. However, the amnesia produced by delayed injections of PM was attenuated by both CXM and amphetamine. These results suggest that delayed injections of PM (24 hours after training) block the e expression or retrieval of memory. This study also supports the contention that puromycin has two separate effects on memory with different temporal parameters depending on when the drug is injected relative to initial training.     

Changes in brain monoamine metabolism and carbon dioxide induced amnesia in the rat.

The effect of treatment with carbon dioxide (CO2) on the performance of rats 24 hr after receiving a foot shock in a passive avoidance task was studied. Foot shock induced avoidance. Carbon dioxide produced retrograde amnesia for the foot shock induced avoidance response. Changes in brain monoamine metabolism were studied in groups of rats which had been treated with CO2, foot shock or foot shock +CO2. The rats were killed 24 hr after treatment. Changes mainly occurred in the brain stem and hippocampus. In the rats which had received foot shock alone, brain stem and hippocampal serotonin concentration were raised. This rise was not observed when the foot shock was followed by CO2 treatment. Furthermore, it was found that there was an increased release of noradrenaline in those rats subjected to foot shock alone but a decreased release of this amine in the group which received foot shock followed by CO2. It is suggested that the amnesic effect of CO2 parallels changes in brain serotonin and noradrenaline metabolism.     

Effect of pertussis toxin on baclofen- and diphenhydramine-induced amnesia

The effect of pretreatment with pertussis toxin at the doses of 0.25 and 0.50 μg per mouse ICV on the amnesic effect produced by baclofen (0.1–4 mg kg⁻¹ IP), diphenhydramine (15–30 mg kg⁻¹ IP) and scopolamine (0.5–5 mg kg⁻¹ IP) was investigated in the mouse passive avoidance test. Ten days after a single injection of pertussis toxin, baclofen (2–4 mg kg⁻¹ IP) amnesia was prevented. By contrast, pertussis toxin had no effect on diphenhydramine- and scopolamine-induced amnesia. Pretreatment with pertussis toxin at both doses used did not impair motor coordination of the mice, as revealed by the rota-rod test. The present results indicate that the activation of pertussis toxin-sensitive G-proteins represents an important transduction step in memory impairment induced by GABA_B (γ-aminobutyric acid B) agonists, but not by antihistaminic and antimuscarinic drugs. Received: 3 June 1997/Final version: 16 October 1997     

Pentylenetetrazol-induced amnesia: a case for overt seizures

In this study, the possible role of overt convulsions following pentylenetetrazol (PTZ) in retrograde amnesia was investigated. Following a single passive avoidance conditioning, when overt convulsions were blocked with sodium pentobarbital (Nem), the amnesic effect of pentylenetetrazol was also blocked. Subconvulsive doses of the drug did not produce amnesia. Animals that received a typically convulsive dose of the drug but failed to convulse were not amnesic; only animals with overt convulsions were different from saline controls. The data suggest that overt convulsions may be necessary for the development of pentylenetetrazol-induced retrograde amnesia.     

Memory retrieval deficits: Alleviation by etiracetam,a nootropic drug

Etiracetam, a nonanaleptic drug related to the nootropic substance piracetam, was found to facilitate memory retrieval in rats in several experimental situations, when injected 30 min prior to retention testing. The drug was active when memory deficits were induced by electroconvulsive shock, undertraining, or by a long training-to-test interval. The behavioral paradigms included a one-trial inhibitory avoidance task and a complex multitrial, spatially discriminated approach task. The clinical interest of drugs which facilitate retrieval processes is also discussed.These results were reported, in part, at the Conference on Practical Aspects of Memory, Cardiff, 1978     

Studies of memory: A reevaluation in mice of the effects of inhibitors on the rate of synthesis of cerebral proteins as related to amnesia

Tests were made of the postulates stating that the degree of inhibition of protein synthesis either (a) at training or (b) following training is the critical variable that determines the degree of amnesia. As a first step it was found that the concentrations of numerous cerebral amino acids were substantially increased in 2 strains of mice 0.5 hr after treatment with amnesic doses of the inhibitors of protein synthesis, cycloheximide (CXM) and anisomycin. This observation led, in several different experiments, to a comparison of the apparent degree of inhibition of protein synthesis derived from the acid-soluble radioactivity with that derived from the specific radioactivity of tyrosine tagged with L(1-¹⁴C)-tyrosine. In all instances the apparent degree of inhibition was decreased when based upon tyrosine's specific radioactivity. The effect of several treatments with CXM on memory of a 1-trial passive avoidance task provided data for analysis of the relationship between the degrees of amnesia and those of the more accurate estimates of inhibition of protein synthesis based upon the specific radioactivity of tyrosine. The results failed to support the views that the level of inhibition of protein synthesis at or after training are entirely sufficient to account for the behavioral results. The results also suggest that CXM produces with time some direct or indirect change in the brain that antagonizes the amnesic effects of the antibiotic and that consequently contributes to the survival of memory in mice trained 2 hr after a large amnesic dose of CXM.     

Time-dependent effects of metrazol on memory

The effects of Metrazol induced seizures on memory were studied in two experiments. In Experiment 1 it was found that Metrazol given at different intervals before a single passive avoidance training trial produced graded performance impairments 24 hr later. The degree of the resulting amnesia was shown to depend on the temporal proximity of the Metrazol seizures to the training trial. In Experiment 2 Metrazol was given immediately following the passive avoidance trial but amnesia was found to develop only after several hr following the Metrazol treatment. The data are similar to those obtained with ECS.     

Dissociation of cycloheximide's effects on activity from its effects on memory

Two doses of cycloheximide (CYC) were used to dissociate the effect of this drug on locomotor activity from its effect on memory. Results indicated significant short-term (30′–40′) and long-term (24 hr) increases in locomotor activity occur at both CYC doses, whereas significant short- and long-term decreases in step-out latencies in a passive-avoidance task occur primarily at the higher CYC dose. Pargyline, a monoamine oxidase inhibitor, significantly increases locomotor activity but does not decrease step-out latencies in the passive-avoidance task. It is concluded that the decreases in step-out latencies shown by CYC-treated animals in the passive avoidance task are the result of an amnesic effect of CYC rather than its effect on locomotor activity.     

Memory modulation with peripherally acting cholinergic drugs

Physostigmine (PHYSO), in doses as low as 0.003 mg/kg IP, antagonized scopolamine (SCOP, 3 mg/kg) induced amnesia of step-through passive avoidance in mice. The peripherally acting acetylcholinesterase (AChE) inhibitor neostigmine (NEO) was also found to reliably, though less strongly, antagonize the SCOP induced amnesia at a dose of 0.03 mg/kg. The NEO antagonism of the SCOP amnesia could be reversed with SCOP (0.3, 1, and 3 mg/kg) and mecamylamine (MECA, 1, 3, and 10 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active both peripherally and centrally, as well as with M-SCOP (0.3 and 1 mg/kg) and hexamethonium (HEX, 1 and 3 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active only in the periphery. In contrast to the ability of these four compounds to attenuate the SCOP amnesia, only the centrally acting compounds SCOP (3 mg/kg) and MECA (10 mg/kg) induced an amnesia when administered alone. These findings suggest that the induction of amnesia of passive avoidance involves central cholinergic systems, whereas the NEO, and possibly PHYSO, reversal of the SCOP induced amnesia is mediated peripherally by both muscarinic and nicotinic receptors. It is hypothesized that the release of adrenal catecholamines, the influence of which on memory processes is well known, and secondarily glucose, may be responsible for the NEO antagonism of the SCOP amnesia.